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Uranium and Fluoride Removal from Aqueous Solution Using Biochar: A Critical Review for Understanding the Role of Feedstock Types, Mechanisms, and Modification Methods

Department of Environmental Science and Technology, Central University of Punjab, V.P.O. Ghudda, Bathinda 151401, Punjab, India
School of Ecology and Environment Studies, Nalanda University, Rajgir 803116, Bihar, India
Instituto Tecnológico Vale (ITV), Rua Boaventura da Silva, 955, Belém 66055-090, PA, Brazil
Author to whom correspondence should be addressed.
Water 2022, 14(24), 4063;
Submission received: 23 October 2022 / Revised: 25 November 2022 / Accepted: 4 December 2022 / Published: 13 December 2022
(This article belongs to the Topic Sustainable Environmental Technologies)


Uranium (U) and fluoride (F) are the major global geogenic contaminants in aquifers and pose serious health issues. Biochar, a potential adsorbent, has been widely applied to remediate geogenic and anthropogenic contaminants. However, there is a lack of research progress in understanding the role of different feedstock types, modifications, adsorption mechanisms on physico-chemical properties of biochar, and factors affecting the adsorption of U and F from aqueous solution. To fill this lacuna, the present review gives insight into the U and F removal from aqueous solution utilizing biochar from various feedstocks. Feedstock type, pyrolysis temperature, modifications, solution pH, surface area, and surface-charge-influenced biochar adsorption capacities have been discussed in detail. Major feedstock types that facilitated U and F adsorption were crop residues/agricultural waste, softwood, grasses, and animal manure. Low-to-medium pyrolyzing temperature yielded better biochar properties for U and F adsorption. Effective modification techniques were mainly acidic and magnetic for U adsorption, while metal oxides, hydroxides, alkali, and magnetic modification were favourable for F adsorption. The major mechanisms of U adsorption were an electrostatic attraction and surface complexation, while for F adsorption, the major mechanisms were ion exchange and electrostatic attraction. Lastly, the limitations and challenges of using biochar have also been discussed.

1. Introduction

Rapid industrialization, rising population, unplanned urbanization, and intense agricultural activities have severely exploited water resources. As a result, the demand for clean water has increased tremendously. According to the United Nations Children’s fund (UNICEF) and World Health Organization (WHO) report, about 2.2 billion people globally and up to 600 million people in underdeveloped nations still lack access to clean water [1]. Goal 6 of the United Nations (UN) Sustainable Development Goals (SDGs) seeks to ensure access to water and sanitation for all, implying the need to improve water quality and protect water-related ecosystems. This goal points to the need for synthesizing technological advances in water research to be economically viable for developing countries [2]. Groundwater serves as the primary supply of drinking water in the majority of the developing nations due to inadequate freshwater sources. However, groundwater contamination has recently increased due to several contaminants (As, U, F, Cd, Cr, Zn, NO3, PO43−, etc.) via geogenic and endless anthropogenic activities [3,4]. Among them, U and F contamination is mainly caused by geogenic sources [5]. The intake of higher levels of U and F can cause serious health issues such as nephrotoxicity, dental and skeletal fluorosis, bone cancer, and brain damage [6,7]; hence, it is essential to mitigate them. Elevated levels of U in groundwater have been reported in many parts of the world, such as Finland, Greece, Germany, Australia, Canada, and the U.S., and Southeast Asian countries such as India, Pakistan, China, and Bangladesh [8]. The WHO guideline for U in drinking water is 30 µg L−1 and by the Atomic Energy Regulatory Board (AERB) is 60 µg L−1. Drinking water above these contamination limits can cause serious health issues. The health risk of U in groundwater is more due to its chemical toxicity than the radiotoxicity. Chemical toxicity causes damage to the kidney (nephrotoxicity), liver, reproductive system, and skeleton, whereas radiotoxicity targets the lungs, bones, and brain [8,9,10]. Globally, many nations, including India, China, Sri Lanka, Argentina, South Africa, UK, Pakistan, etc., have significant F concentrations in their groundwater [11,12]. More than 200 million individuals worldwide drink water with elevated F levels [13]. The World Health Organization states that 1.5 mg L−1 is the permissible limit for F in drinking water. However, excessive F intake causes skeletal and dental fluorosis, bone cancer, and brain damage [7].
Numerous techniques have been developed to mitigate U and F-, namely membrane filtration methods such as nanofiltration and reverse osmosis, ion exchange, lime softening, coagulation by Fe/Al salts, and permeable reactive barriers using zerovalent iron. Adsorbents such as iron oxide, titanium dioxide, precipitation and coagulation, electrolytic defluoridation, adsorption, and electrodialysis have been utilized [14,15,16,17,18,19,20]. However, technologies have certain limitations, such as high installation and maintenance costs, pH dependence, high energy consumption, membrane fouling, and scaling [14,21]. Hence, these technologies are not cost-effective and cannot be applied in developing countries on a large scale. Among these methods, adsorption is the most effective and promising method for removing F, U, and other heavy metals from contaminated water. For F removal, various adsorbents have been used, such as activated alumina [22], bone char [23], activated carbon [24], metal oxides and hydroxides [25,26], zeolite [21], etc. Adsorbents, such as hematite [27], zeolite [28], activated carbon [29], diatomite [30], etc., have been applied for U removal. However, their expensive production cost makes them uneconomical. As a result, there is a huge demand for sustainable and low-cost adsorbent development.
Biochar is a solid, stable, porous, and low-cost adsorbent which is a carbonaceous material obtained from the thermal degradation of biomass, widely used for the remediation of contaminants from polluted/contaminated soil and water ecosystems [31,32]. Biochar has attained significant recognition due to its economic, sustainable, reusable, environmentally safe, and high adsorption efficiency. Evidence shows that biochar can remove various contaminants, including U and F [33,34,35,36,37]. For further enhancement in the adsorption of U and F, biochar has been modified with different techniques and materials, such as MnFe2O4 [38], FeCl3 [39], HNO3 [33], Al(OH)3 [13], H3PO4 [35], LaCl3 [40], etc. However, there is enough literature available on the review of contaminant removal from aqueous solution using biochar, including organic and inorganic contaminants [41,42,43,44,45,46,47,48,49,50,51], heavy metals [52,53,54,55,56,57,58,59,60,61,62,63], emerging contaminants [64,65,66,67], and chemical and microbial pollutants [68]. However, there is no systematic review on the role of feedstocks on U and F removal by biochar from aqueous solution and its comparison with respect to different raw/pristine and modified biochars. Therefore, to the best of our knowledge, this review gives a systematic idea about the role of feedstock types on biochar properties, modification techniques to enhance adsorption capacity, interactions with contaminants, and factors affecting the adsorption efficiency of U and F removal in aqueous solution.
The present review work aims to highlight (i) the application of biochars obtained from different feedstocks for U and F adsorption, (ii) the different production techniques for synthesis of the biochar and modification methods for the enhancement of adsorption efficiency of the biochar, (iii) the factors affecting the adsorption of U and F such as pH, biochar dosage, U and F concentration, co-existing ions, and temperature, (iv) adsorption mechanisms, (v) adsorption isotherms, kinetics, and thermodynamics, and (vi) challenges and limitations for real U and F groundwater treatment. Lastly, this paper concludes with future studies and recommendations for in-depth research on removing U and F using low-cost biochars.

2. Methodology

To address the objectives, extensive research was performed from scholarly databases (Scopus, ScienceDirect, and Web of Science) on U and F removal using biochar. The following keywords and synonyms in different combinations: ‘biochar’, ‘sorption’, ‘water treatment’, ‘uranium’, ‘fluoride removal’, ‘pollutant/contaminant removal’, ‘heavy metal removal’, ‘aqueous solution’, ‘groundwater’, ‘adsorption’, ‘modification’, ‘mechanism’, ‘biomass adsorbent’, ‘crop residue’, ‘agricultural biomass’, ‘reusability’, and ‘regeneration’ were used for the literature search. More than 1100 studies were identified using these keywords from three different databases. Duplicates were removed using Endnote, and studies that reported metal removal other than the ones under investigation were not considered for this review. More than 300 papers were retrieved after excluding duplicates and studies that were irrelevant. Selected articles were further scrutinized based on the abstract, and approximately 89 studies focusing on U and F removal from aqueous solution/wastewater/groundwater were included in this review. The papers on biochar (studies of U and F removal) covered the period from 2011 to 2022. Publications were distributed in order, with 37 studies on F removal and 52 on U removal. Prisma flowchart was used for displaying data collection, exclusion and inclusion criteria (Figure 1).

3. Results and Discussion

3.1. Role of Different Feedstock Types on Biochar Properties

Biochar has been obtained from pyrolyzing different feedstocks to remove U and F, due to their inherent and excellent physiochemical properties. Generally, woody biomass and crop residues are comprised of cellulose, hemicellulose, and lignin. Feedstocks such as crop residues (sugarcane bagasse, rice husk, wheat straw, rice straw, etc.), grasses, and softwood (pine) are composed of a large proportion of cellulose and hemicellulose and degrade faster due to lower thermal resistance [69,70]. As a result, cellulose and hemicellulose degrade at lower temperatures (200–400 °C), while lignin has a wide decomposition range (200–900 °C) [57,69,71]. Hence, softwood and crop residues require low temperatures for pyrolysis. Biochar yield is less in this case because biomass with high cellulose and hemicellulose contain aliphatic carbon phases, which can easily break. Thus, they do not make stable biochar and produce low biochar yield. These feedstocks produce high-oxygen-containing functional groups such as carboxyl, carbonyl, and hydroxyl. [69,70,72,73].
On the other hand, hardwoods (eucalyptus) are composed of a high fraction of lignin; therefore, they require higher temperatures for degradation. These feedstocks make stable biochar because they contain aromatic monomers and high carbon content making them thermally stable and thus producing high biochar yield [69,70]. Some feedstocks which are not plant-based/non-woody, such as animal manure and sludge, also degrade faster. Thus, they require a low temperature range for biochar preparation [70,72,74]. Biochar prepared at high temperatures is suitable for the sorption of organic contaminants, while the sorption of inorganic contaminants requires low temperatures [45,70,75] As a result, crop residues, grasses, and manure-based feedstocks are beneficial for U and F adsorption as they have oxygen-based functional groups [74,76].

3.2. Applications of Biochar in U and F Remediation

Biochar has gained much attention due to its widely available feedstock, making it a low-cost adsorbent, and it possesses high surface area, porosity, and oxygen-rich functional groups [77]. This section describes the application of biochar for removing U and F from the water and their efficiency.
U is a ubiquitous radioactive element, and its presence in water makes it unfit for drinking and causes many toxic effects on kidneys, bones, and the liver [8,9,10]. Various treatment technologies have been developed for U removal, such as reverse osmosis, ion exchange, coagulation, and adsorption [78,79]. Of these, adsorption by biochar is the most cost-effective method [72,80]. Several kinds of research have been performed on the bioremediation of U-contaminated water through biochar, described in this section.
Feedstocks, modification methods, and optimum operating conditions such as pyrolysis temperature, residence time, pH, and biochar dosage were employed to remediate U and F through biochar (Figure 2). For instance, rice husk biochar was employed for U removal, which showed a removal efficiency of 99.8% and adsorption capacity of 138.88 mg g−1 (Table 1) at pH 5.5 and temperature range of 303–353 K with a biochar dose of 0.38 g L−1 and initial U concentration of 3 mg L−1 [81]. Jin et al. [82] examined wheat straw biochar which exhibited a maximum sorption capacity of 355.6 mg g−1 at pH 4.5, temperature of 25 °C, with initial concentration of 10 mg L−1. HNO3-modified rice straw biochar significantly removed U with a sorption capacity of 242.65 mg g−1 (oxidized biochar) and 162.54 mg g−1 (raw biochar) at pH 5.5, a temperature of 25 °C, and dose of 0.01 g L−1. HNO3 oxidation increased the surface area, porosity, and oxygen-containing functional groups [33]. Pine-needles-based biochar successively removed U with a maximum adsorption capacity of 623.7 mg g−1 at pH 6, a temperature of 25 °C, and dosage of 5 g L−1 [37]. Dai et al. [34] reported U removal from corn-cob-based biochar with a sorption capacity of 163.18 mg g−1 at pH 6, temperature of 25 °C, and biochar dose of 5 g L−1. Palm-based biochar was investigated for U removal, which showed a removal efficiency of 99.2% and maximum sorption potential of 488.7 mg g−1 at pH 3 and 25 °C [36]. Guo et al. [83] successfully removed U using sponge gourd biochar with a sorption capacity of 239.21 mg g−1 at pH 5, temperature of 30 °C, and initial concentration of U given 5 mg L−1. Similarly, Ioannou et al. [84] prepared sponge gourd biochar which exhibited an excellent U sorption potential of 904 mg g−1 at pH 3 and 23 °C. Lingamdinne et al. [85] employed magnetically modified watermelon rind biochar which showed a U uptake of 323.56 mg g−1 for modified biochar and 135.86 mg g−1 for pristine biochar at pH 4 and 20 °C. The high sorption capacity was due to the magnetization of biochar by Fe oxide, resulting in increased surface area, porosity, and sorption potential.
Drinking F-contaminated water can cause serious health issues such as skeletal fluorosis, dental fluorosis, bone cancer, and brain damage [86,87,88]. Various research works have been conducted on F removal using biochar, such as Zhou et al. [89] who prepared La/Fe/Al oxide-loaded rice straw biochar with a maximum F sorption capacity of 10.85 mg g−1 for raw biochar while 111.11 mg g−1 for the modified one (Table 1), in the pH range of 3 to 11 with an initial F concentration of 6 mg L−1. Wheat-straw-derived biochar impregnated with Al(OH)3 and La(OH)3 was used for F removal from water. The prepared biochar showed 98.69% removal with a maximum F uptake of 51.28 mg g−1 at pH 7 and a temperature of 25 °C [90]. Nanoscale rice husk biochar was studied for defluoridation with a removal efficiency of 90% and maximum F adsorption capacity of 17.3 mg g−1 at pH 7 and a temperature of 30 °C [24]. Mohan et al. [91] utilized magnetic corn stover biochar, which removed F at pH 2, with a maximum sorption capacity of 4.11 mg g−1. Tamarix hispida-based biochar showed 99.6% removal from synthetic water and 86.69% from real wastewater. The biochar exhibited an adsorption potential of 164.23 mg g−1 at pH 6 and temperature of 25 °C [40]. Coconut-derived biochar was used for defluoridation with a removal efficiency of 82.45% at pH 6.27 and a solution temperature of 30 °C [92]. Seed shells of Camellia oleifera (tea oil plant) were utilized for biochar production to remove F from aqueous solution and exhibited an adsorption capacity of 11.04 mg g−1 at pH 6.8 [93]. Watermelon rind biochar easily removed F at pH 1 with a maximum sorption capacity of 9.5 mg g−1 [94]. Wang et al. [95] prepared pomelo-peel-based biochar for defluoridation, which showed 100% removal efficiency with a sorption potential of 18.52 mg g−1 at pH 6.5. Okra stem biochar effectively removed F with an adsorption capacity of 20 mg g−1 at pH 2, a temperature of 35 °C, and with an initial concentration of 10 mg L−1 [96]. A biochar composite of platanus acerifoli leaves and eggshell showed an excellent removal efficiency of 98.53% with a maximum adsorption capacity of 308 mg g−1 at pH 5 and 25 °C [97].
Discussed above are some of the studies about the application of biochar in U and F remediation. It was observed that the adsorption capacity of biochar depends on various parameters such as feedstock, modification, pH, dose, temperature, initial concentration of U and F in the solution, and functional groups. The effects of these parameters are discussed in Section 3.5 and Section 3.7. Table 1 summarizes the various research works carried out on U and F removal using biochar.

3.3. Synthesis of Biochar

The thermochemical conversion of biomass is the most common method for the production of biochar [98]. It includes pyrolysis, gasification, torrefaction, and hydrothermal carbonization [72]. For better biochar yield, parameters such as feedstock type, carbonization temperature, modification methods, heating rate, residence time, solution pH, adsorption temperature, adsorbent dosage, etc., must be optimum because of their significant impact on the physicochemical properties of the biochar [74,99,100]. Different categories of feedstock have been utilized for the synthesis of biochar, such as crop residues (corn stover, rice straw, rice husk, wheat straw, corn cob, etc.) [73,101,102], woody biomass (pine needles, pine sawdust, bamboo, eucalyptus wood, palm tree fibres, etc.) [103,104,105], fruit waste (watermelon rind and orange peel) [94,106], animal waste (pig manure, horse manure, and dairy manure) [107,108,109], sewage sludge [110], etc. For example, Jin et al. [82] derived biochar from cow manure and wheat straw for U removal from water. It was found that cow-manure-derived biochar exhibited higher removal efficiency than wheat straw-derived biochar. Because of higher ash content, surface oxygen (which bonded with U ions) and Ca2+ occurred on the surface, exchanged with positively charged U ions, and provided new adsorption sites on the surface of cow-manure-derived biochar.
Furthermore, carbonization temperature is also an essential parameter for synthesis as it affects the pore volume and surface area of the biochar. For instance, Hu et al. [111] pyrolyzed bamboo sawdust at different temperatures (300, 450, and 600 °C). It was observed that surface area and pore volume were positively correlated with the pyrolysis temperature, i.e., the pore volume and surface area of the biochar were enhanced with increasing pyrolysis temperature, resulting in higher U uptake. However, Alkurdi et al. [112] pyrolyzed sheep bone to derive bone char for F removal at different temperatures (500, 650, 800, and 900 °C). The surface area of bone char decreased from 120.031 m2 g−1 to 89.06 m2 g−1; as a result, the pore volume decreased from 0.283 m3 g−1 to 0.235 m3 g−1, with increased pyrolyzing temperature from 500 °C to 900 °C due to pore shrinkage and pore breakage at very high temperatures.
Modification is another critical parameter in the synthesis of biochar, which is performed prior to or after the pyrolysis of raw biomass to improve the adsorption capacity, surface morphology, and physiochemical properties of the biochar. For instance, Lingamdinne et al. [85] magnetically modified watermelon rind biochar for U removal. They found that magnetization improved the surface morphology from poorly structured to a porous and ordered structure of the biochar and enhanced the surface area from 52.1 m2 g−1 to 86.35 m2 g−1. U uptake was enhanced from 135.86 mg g−1 to 323.56 mg g−1 after the magnetization. Different biochar modification methods are further discussed in Section 3.5. The various biochar production techniques are as follows:

3.3.1. Pyrolysis

The pyrolysis process commonly produces biochar, and this process involves the thermal degradation of biomass into solid (biochar), liquid (bio-oil), and gas (syngas) in oxygen-limited conditions at high temperatures ranging from 300 to 700 °C [113,114]. Pyrolysis is further classified into slow and fast pyrolysis based on the pyrolysis temperature, heating rate, and residence time. In slow pyrolysis, biochar is the primary product, while the major products in fast pyrolysis are bio-oil and syngas. Compared to fast pyrolysis, slow pyrolysis is better for biochar production because biochar yields decrease with an increase in temperature and heating rate [114,115].

3.3.2. Gasification

Gasification is the thermal decomposition of solid carbonaceous material derived from fossil fuels such as wood or coal into producer gas known as syngas at high temperature (>700 °C) with gasifying agents such as steam, oxygen, air, etc., to partially oxidize the feedstock [72,114,115]. During gasification, the major product is syngas, while biochar is produced as a by-product with a lower yield [72].

3.3.3. Torrefaction

Torrefaction is a pretreatment process before pyrolysis to improve the properties of the biomass. Torrefaction increases the hydrophobicity, further enhancing the biochar’s storage stability, grindability, and carbon content. Reducing oxygen, water, or moisture from biomass increases the biochar yield [116]. Torrefaction is known as mild pyrolysis, operating at low temperatures and heating rates. Biomass is pyrolyzed in the temperature range of 200–300 °C, at a heating rate of <50 °C min−1, having a residence time of less than 30 min under anaerobic conditions and atmospheric pressure [116,117].

3.3.4. Hydrothermal Carbonization

Hydrothermal carbonization is used for making biochar from wet biomass. It helps improve the properties of the biomass, such as hydrophobicity, grindability, increased carbon content, and reduced oxygen content. It is specially employed for feedstock such as sewage sludge, animal and human wastes, compost, municipal wastes, etc. As biomass does not require drying before the treatment, it is better and more economical than pyrolysis and gasification [113,114].

3.4. Characteristics of Biochar

Scanning electron microscopy–electronic dispersive X-ray (SEM-EDX) analyses the biochar’s surface morphology and elemental mapping. For example, Ahmed et al. [38] determined the surface morphology and elemental composition using SEM-EDS. They concluded the fibrous–porous structure of the biochar and the presence of C, O, Fe, Mn, and Na on the biochar surface, which confirmed the MnFe2O4 fabrication of the biochar. The surface area of biochar is analysed using the Brunauer–Emmett–Teller method (BET). For instance, Guilhen et al. [36] performed the BET method and found that the surface area of biochar increased from 0.8320 to 643.12 m2 g−1 after CO2 activation, resulting in a high adsorption capacity of the biochar, which confirmed that physical activation has potential to enhance U uptake using biochar.
Fourier-transform infrared spectroscopy (FTIR) is used to observe the functional groups on biochar surfaces. For example, several previous studies have utilized acid-modified biochar and found that C–O, C=O, O–H, C–H, and –COOH were the main functional groups present on the biochar surface [7,33,35,82,111,118,119,120,121]. Biochars treated with a base showed C=O, N–H, C–H, C–O, O–H, and C–OH as the major functional groups [90,122,123,124]. Magnetized biochars showed different functional groups such as Fe–O, C=O, C–O, O-H, C=C, C–H, C–O–C, and Si–O–Si [39,85,91,125,126,127]. Ahmed et al. [38] and Hu et al. [105] observed that a new peak was detected at 909 cm−1 and 916 cm−1, which corresponded to the stretching vibration of [U=O=U]2+ and confirmed U adsorption on the biochar surface. Similarly, Sadhu et al. [94] observed a new adsorption peak at 997 cm−1, which corresponded to C–F stretching and indicated F adsorption.
X-ray photoelectron spectroscopy (XPS) examines the elemental composition and chemical state of atoms in a produced material and provides information about the adsorption mechanism. For example, Ding et al. (2018) found that adsorbed U was a mixture of 87% U(IV) and 13% U(VI). X-ray diffraction (XRD) is used to analyse the crystallographic structures of prepared biochar and the dominant minerals present on the biochar surface. For instance, Wei et al. [128] performed XRD and found that the crystalline size of CeO2, which was dispersed onto the biochar surface, was 17.07 nm. Similarly, Halder et al. [92] observed several peaks which showed the presence of fluorinated compounds such as K2MgF4 and KFeF3 and confirmed the F sorption onto the biochar surface.

3.5. Modification Methods

Modification is necessary to improve biochar properties such as surface area, pore structure, and functional groups [129]. Modification has four types: physical modification, chemical modification, magnetic modification, and impregnation or coating of the minerals [130]. Modification or activation can be conducted before or after pyrolysis. Figure 3 shows the different modification methods, including physical, chemical, and magnetic modification and Table 1 summarizes the effect of different modification methods on the properties of biochar for U and F removal.
Table 1. Effect of modification on the adsorption capacity of biochar for U and F removal.
Table 1. Effect of modification on the adsorption capacity of biochar for U and F removal.
FeedstockModificationTarget ContaminantSurface Area
(m2 g−1)
Pore StructureAdsorption Capacity (mg g−1)/Removal Efficiency (%)References
Rice strawHydroxyapatite–biochar nanocompositeU157.96Mesoporous110.56428.25 [131]
Rice strawHNO3 oxidationU-Mesoporous162.54242.65 [33]
Wheat strawHNO3 oxidationU290.1-8.7355.6 [82]
Rice huskMagnetization by SideriteU109.65Mesoporous-52.63 [125]
Rice huskSilicon containing biochar-supported iron oxide nanoparticlesU62.88--138.88 [81]
Rice huskMagnetic modification using Fe2+/Fe3+ plus SO42− solutionU109-64118 [132]
Corn cobThermal air treatment at 300 °C U-Mesoporous68.82163.18 [34]
Pine needlesMagnetization of oxidised biochar (HNO3 treated) by FeCl3U---623.7 [37]
Pine sawdustMgO/biochar compositeU51.45Mesoporous-514.72 [133]
Macaúba palmCO2 activationU643.12Microporous-488.7 [36]
Palm tree fibresHNO3 oxidationU---112 [121]
Bamboo sawdustPhytic acidU1298-16.2229.2 [111]
Bamboo biomassPhosphate impregnation biochar cross-linked Mg–Al layered double-hydroxide compositeU445.17Microporous15.869274.15 [134]
Cactus fibreHNO3 oxidationU<5Microporous-214 [118]
Chinese banyan aerial rootKMnO4 modificationU284Mesoporous19.0827.29 [135]
Puncture vineMagnetization by FeCl3U-Mesoporous-17.24 [39]
Hydrophyte biomassMagnetization by FeCl2U92.43-52.3654.35 [136]
HydrophytePhytic acid modificationU433--128.5 [137]
Pig manureKMnO4U--369.9979.3 [107]
Pig manureH2O2U--369.9661.7 [107]
Pig manureNaOHU227.9Mesoporous369.9952.5 [138]
Pig manureHClU36.3Mesoporous369.953.3 [138]
Pig manureNaOHU345.7Microporous45.8221.4 [124]
Pig manureH2O2U189Microporous45.8145.1 [124]
Horse manureBismuth impregnationU--186516.5 [108]
Horse manureMgCl2 modificationU---625.8 [139]
Cow manureHNO3 oxidationU101.5-6473.3 [82]
Carp fish scalesKOH activationU1074.73Microporous71.59291.98 [122]
Sewage sludgeThermal air treatmentU-Mesoporous78.6696.73 [34]
Sewage sludgeAir roasting–oxidationU623.09Mesoporous139.5490.2 [140]
Winery waste (grape peels)Chemically modification by NaOH, Na2CO3U---255 [141]
Winery waste (grape peels)Thermal modification at 650 °C and oxidized with HNO3U165--100 [141]
Malt spent rootlets HNO3 oxidation U540Mesoporous547500 [120]
Coffee espresso residueHNO3 oxidation U700Mesoporous547357 [120]
Olive kernelsHNO3 oxidation U510Mesoporous357381 [120]
FungiSulfide nano zero valent ironU102.7_-427.9 [142]
Green algaeMn impregnationU63.7Mesoporous-100.2 [143]
CyanobacteriaMagnetic modification using Fe3O4 U--58.0552.06 [144]
Sponge gourdZnO-modified biochar hydrogel U-- 239.21 [83]
Sponge gourd fibresMnO2 oxidationU<5Microporous95904 [84]
Sponge gourd spongesHNO3 oxidationU---92 [119]
Sponge gourd spongeSalophen modification U---833 [145]
Sponge gourd residue Functionalization by hummer methodU---382 [146]
Watermelon rindMagnetization by co-precipitationU86.35-135.86323.56 [85]
Watermelon seedsMnFe2O4 modification U-Mesoporous21.2427.61 [38]
Longan shell (fruit)Nano zero valent ironU1168.88Mesoporous-331.13 [77]
Orange peelMnO2 modificationU273.25Mesoporous165.4246.3 [106]
Orange peelHydrogel U---263.2 [147]
Tea wasteIron manganese oxideU12--510.8 [148]
Rice strawLa/Fe/Al oxides impregnationF95.36Mesoporous10.85111.11 [89]
Wheat straw Impregnation of aluminium and lanthanum hydroxide F---51.28 [90]
Rice huskChemical modification by ironF58.98Mesoporous-4.45 [149]
Rice huskNano-scale size reductionF---17.3 [24]
Rice huskMagnetic biochar anchored with Al and MgF114Mesoporous-21.59 [150]
Corn stoverMagnetization by Fe3+/Fe2+ solutionF3.61Microporous6.424.11 [91]
Chir pineCalcium pretreatedF---16.72 [151]
Mongolian scotch pine tree sawdustPhosphoric acid-microwave methodF339Microporous-0.885 [35]
Douglas fir (pine)Magnetization by Fe2O3/Fe3O4 F494Microporous-9.04 [152]
Douglas fir (pine)Iron-titanium biochar composite F576Microporous-36 [153]
Reed biomassCe-loaded biochar beadsF236.84Mesoporous-34.86 [128]
Kashgar tamariskLanthanum chlorideF164.52Mesoporous-164.23 [40]
Tea oil plant (seed shells)Impregnation of zirconium dioxideF---11.04 [93]
SawdustChemical modification via cross-linking and protonation of the chitosan-sawdust biochar beadsF57.97Microporous-4.413 [154]
Pongammia pinnata seed cakeEngineered biochar by HCl solutionF10.1Microporous-1.11 [7]
CoconutSteam activation F1054Mixture of micropores and mesopores-82.45% [92]
Pomelo peelImpregnation of polypyrroleF---18.52 [95]
Peanut shellMgOF182.3Mesoporous-83.05 [155]
Spent mushroom compostAl(OH)3 coating F28.5--36.5 [13]
Food wasteAlCl3 impregnationF20.95--123.4 [156]
Tea wasteChemical modification by H2SO4, NaNO3, KMnO4F11.833Macroporous-52.5 [157]
Red algae seaweedSpent biochar F319.47Microporous-2.1 [158]
Dairy manure Calcium modificationF2.6-0.110.42 [109]
Tea wasteMagnetic modificationF115.65Mesoporous-18.78 [159]
Peanut hullNilF98.2Mesoporous3.665- [160]
PineconeAlCl3F---14.07 [161]
Conocarpus erectusNilF9.88Microporous205.7- [162]
Yak dungFeCl2F---3.928 [163]
U: Uranium; F: Fluoride; -: data not available.

3.5.1. Physical Modification

Physical modification involves the utilization of oxidizing agents such as CO2, air, steam, and ozone at high temperatures above 700 °C. It is generally employed for increasing the surface area and porosity of biochar [164]. For instance, Guilhen et al. [36] removed U from aqueous solution using biochar modified with CO2 as an activation agent at the temperature range of 700–1000 °C. The CO2 activation increased the aromaticity and porosity of the biochar with the increase in surface area from 0.83 to 643 m2 g−1 and enhanced the removal efficiency from 80.5% (primary biochar) to 99.2% (activated biochar). Activation made the adsorption sites more heterogenous and as a result created more pores of biochar. Similarly, Halder et al. [92] employed the steam activation of biochar at 900 °C for F removal from aqueous solution, resulting in enhanced porosity and a surface area of 1054 m2 g−1. Steam activation of the biochar resulted in an increased removal efficiency of 82.45%. Steam activation created more adsorption sites by releasing volatile compounds during the activation process.

3.5.2. Chemical Modification

Chemical modification is carried out via acids, oxidizing agents, and alkaline treatment. Acid modification enhances the surface area, improves surface morphology, pore structure, and enriches the biochar surface with oxygen-containing functional groups such as carboxylic (COO), hydroxyl (OH), carbonyl (CO), etc., resulting in a negative surface charge of the biochar and promoting enhanced adsorption of the cationic species [120,130]. For example, HNO3 modification induced negative charge on the biochar surface due to oxygen-containing functional groups and enhanced the U removal through complexation between positive U species and acidic functional groups. This modification provided high surface area, porosity, and microporous and mesoporous structure, resulting in higher stability and performance over pristine biochar (Table 1) [33,82,118,119,120,121]. Similarly, Guan et al. [35] modified pine tree sawdust biochar with the phosphoric acid–microwave method for F removal. The fabricated biochar showed an increase in surface area from 7.7 to 389.95 m2 g−1, a decrease in pore diameter from 17 to 0.9 Å, and an enhanced adsorption capacity of 0.885 mg g−1. The modification removed some impurities from the pores of pristine biochar resulting in the availability of more active sites for F adsorption. The acid treatment protonated the surface functional groups such as hydroxyl and carboxyl, resulting in electrostatic attraction between F and protonated functional groups. De et al. [7] modified biochar with hydrochloric acid for F removal. This modified biochar showed increased active sites, a graphite-like carbon structure, and high carbon content resulting in higher adsorption than the raw biochar with a removal efficiency of 98.5% and adsorption potential of 1.11 mg g−1.
Oxidizing agents such as manganese oxide (MnO2) [84,106], hydrogen peroxide (H2O2) [107,124], and potassium permanganate (KMnO4) [107,135] were used to modify biochar for U removal, while metal oxides, such as iron oxide, aluminium oxide, and lanthanum oxide [89], were used to modify the biochar surface for F adsorption. These oxidizing agents showed enhanced adsorption due to a rise in surface area, porosity, and oxygenated functional groups on the biochar surface [165].
Alkaline treatment was given by metal hydroxides, such as KOH [122], NaOH [124,138], La(OH)3 [90], and Al(OH)3 [13]. For example, Saikia et al. [123] reported that the removal efficiency of F by perennial-grass-based biochar activated by adding KOH pellets was 24.8%. After activation, they observed a rise in surface area from 5.57 m2 g−1 to 1248.2 m2 g−1. Similarly, Chen et al. [13] observed an increased adsorption capacity of 36.5 mg g−1 and surface area from 3.6 to 28.5 m2 g−1, and Yan et al. [90] reported increased F removal efficiency from 77.97% to 98.69% resulting from biochar modification with aluminium and lanthanum hydroxides. An increase in surface area was found because the coating of Al(OH)3 on the biochar surface was amorphous (the presence of small particles). Amorphous materials have more active sites for sorption [166].

3.5.3. Magnetic Modification

The magnetization of biochar increases the adsorption capacity by enhancing the surface area, pore volume, surface morphology, functional groups, and stability of the biochar. In addition, magnetic biochar can be reused multiple times through the separation of contaminants from biochar using an external magnetic field [61,85]. For instance, Ahmed et al. [39] produced biochar from Tribulus terrestris (puncture vine) and magnetized it using FeCl3 to remove U(VI) from wastewater. The magnetic biochar exhibited a layered porous structure, which provided increased surface area and enhanced adsorption capacity. After the modification, there was an increase in oxygen content resulting in oxygen-containing functional groups. FTIR analysis revealed the presence of Fe–O, C=O, C–O, and –OH functional groups after the magnetization of biochar. Another study was carried out by Philippou et al. [37] for removing U from aqueous solution through magnetic modification using Fe3O4-loaded pine needles biochar, which enhanced the adsorption capacity to 623.7 mg g−1. Ahmed et al. [38] synthesized biochar from Citrullus lanatus L. (watermelon) seeds and used MnFe2O4 to magnetically modify biochar through the co-precipitation method for U(VI) removal from wastewater. It was observed that magnetic biochar exhibited a mesoporous structure, higher stability, enhanced adsorption capacity of 27.61 mg g−1 from 21.24 mg g−1 (pristine biochar), oxygenated functional groups, and increased availability of active sites on the biochar.
The impregnation of mineral salt solution increases the oxygen functional groups on the biochar surface, increasing the sorption capacity of the adsorbent. For instance, Zhou et al. [89] derived biochar from rice straw impregnated with La/Fe/Al oxides through co-precipitation for F removal from drinking water. Impregnation increased the production of hydroxyl groups on the biochar surface. Impregnated biochar enhanced the surface area from 2.59 m2 g−1 to 95.36 m2 g−1 and the adsorption capacity from 10.85 mg g−1 to 111.11 mg g−1 in a wide pH range of 3–11.

3.5.4. Thermal Air Treatment (TAT)

Dai et al. [34] applied the TAT method to modify the biochar to remove U(VI) from the aqueous solution. In this method, the biochar surface was engineered by heating biochar at 300 °C for 30 min to enhance the adsorption performance of the biochar. The adsorption capacity of the biochar increased from 68.82 mg g−1 to 163.18 mg g−1. The resultant biochar exhibited a high O/C ratio resulting in oxygen-containing functional groups, a reduction in the average pore diameter (11.53 nm to 3.62 nm), and increased surface area (360.35 to 362.26 m2 g−1), resulting in the development of a mesoporous structure, which facilitated U(VI) removal from the water. Compared to physical and chemical modification, thermal air treatment (TAT) exhibited a lower carbonization temperature and shorter processing time resulting in elevated product yield and lower production cost and energy consumption.

3.6. Raw vs. Modified Biochar

Biochar (raw as well as modified) has been widely utilized for U and F removal from aqueous solution at several operating parameters such as pH, dose, temperature, initial concentration, etc. The following section discusses the comparative analysis of adsorption capacities with respect to raw and modified biochar at different experimental conditions. For example, Hu et al. [111] employed phytic-acid-modified bamboo sawdust biochar pyrolyzed at 450 °C to remediate U(VI) from synthetic water. The modified biochar showed a higher sorption capacity (229.2 mg g−1) than raw biochar (16.2 mg g−1) (Table 1) at pH 4 and 25 °C. Fabrication enhanced the surface area of the biochar from 8.47 m2 g−1 to 157.96 m2 g−1, nearly nineteen times higher than the pristine biochar. Modification enlarged the pores due to the release of volatile matter during pyrolysis, resulting in enhanced pore volume from 0.015 cm g−1 (raw biochar) to 0.919 cm g−1 (modified biochar). Phytic acid fabrication introduced phosphate-functionalized groups on the biochar surface and improved the pore structure, which enhanced the U(VI) uptake compared with raw biochar. Similarly, Ahmed et al. [33] used HNO3-modified rice straw biochar to remove U(VI) from aqueous solution. Oxidized biochar exhibited an adsorption capacity of 242.65 mg g−1, while raw biochar showed a maximum sorption capacity of 162.54 mg g−1 at pH 5.5 and a temperature of 25 °C. Nitric acid enriched the carbonized surface with acidic functional groups such as carboxyl and carbonyl, thereby enhancing the adsorption ability of the biochar. Similar findings were reported by [82,118,119,120,121]. A novel adsorbent, hydroxyapatite biochar nanocomposite made from rice straw, was developed by Ahmed et al. [131] to remove U(VI) from laboratory water. The fabricated biochar exhibited an adsorption potential of 428.25 mg g−1, while raw biochar showed an adsorption capacity of 110.56 mg g−1. Essentially, hydroxyapatite is a calcium phosphate material which has a high tendency to remediate environmental contaminants [167]. Hence, introducing this biomaterial on the biochar surface enhanced its adsorption potential compared to raw biochar. The highest U(VI) uptake was observed at pH 5.5, a temperature of 25 °C, and initial concentration of 50 mg L−1 in both the adsorbents. Modified biochar showed excellent removal efficiency >90% even after five sorption–desorption cycles. Han et al. [124] examined NaOH-modified pig manure biochar for U immobilization and observed that modified biochar exhibited more significant sorption potential (221.4 mg g−1) than the pristine one (45.8 mg g−1). The greater U uptake was due to the enhanced surface area (from 135.7 m2 g−1 to 345.7 m2 g−1), pore volume (0.032 cm3 g−1 to 0.119 cm3 g−1), and carboxyl and hydroxyl functional group complexation with U. A similar study was performed using alkali (NaOH)-modified pig manure biochar for U removal which supported these results [107].
Zhou et al. [89] employed tri-metallic (La/Fe/Al oxides)-modified biochar derived from rice straw for the defluoridation of aqueous solution. The modified biochar exhibited a maximum adsorption potential of 111.11 mg g−1, while pristine biochar showed 10.85 mg g−1 F uptake. The impregnation of metal oxides on the biochar surface enhanced the surface area (2.59 to 95.36 m2 g−1), pore volume (0.012 to 0.611 cm3 g−1), and pore diameter (12.01 to 12.49 nm). As La/Fe/Al oxides have a positive charge and fluoride has negative, the fabricated biochar showed a higher capacity for F removal through ion exchange and electrostatic interactions. Generally, biochars are negatively charged [168]; hence, the raw biochar exhibited less F uptake than the modified biochar. Maximum removal was observed at pH 3, with a biochar dosage of 1 g L−1, with an initial concentration of 6 mg L−1 in both sorbents. Limited studies have compared raw and modified biochar in the case of F. They have determined the adsorption capacities of only modified/coated/fabricated biochars.
Based on the available literature, it was observed that less acidic or near-neutral pH (discussed in Section 3.7.1), low-to-medium (200–550 °C) pyrolyzing temperature, a solution temperature of 25 °C, and chemical (acidic) and magnetic modification of the biochar yielded better results for U adsorption. In the case of F, alkali pH (see Section 3.7.1), a medium pyrolyzing temperature (450–700 °C), and a solution temperature of 30 °C, metal oxides and hydroxides, magnetization, and chemical (weak acids and alkali) modification might be more favourable for F removal.

3.7. Factors Affecting the Adsorption of U and F in Aqueous Solution

3.7.1. Influence of pH

Among various factors, the pH of the solution is one of the crucial factors which affects the adsorption of contaminants by governing their speciation and surface charge of biochar in varying-pH solution [84,107]. Speciation of U was influenced at varying pH; at pH values less than 6, U (VI) occurred in the form of uranyl species (UO22+) and positively charged hydroxy complexes, such as (UO2)3(OH)42+, (UO2)2(OH)22+, (UO2)3(OH)5+, UO2(OH)+, (UO2)4(OH)7+, (UO2)3(OH)52+, and (UO2)2OH3+, and negatively charged species of U, i.e., (UO2)2CO3 (OH)3 occurred at pH > 6 (Table 2) [169,170]. It was found that at lower pH values (3–6) (less acidic or near neutral), the biochar surface was negatively charged due to the presence of negatively charged functional groups (COO, OH) [171] and positively charged U species were present at these pH values [169]. In addition, when the solution pH > pHZPC, the surface charge on the biochar became negative, which attracted the positive U species through electrostatic attraction and complexation, resulting in a high adsorption capacity [133] (Table 2). Figure 4 summarizes how major parameters affect U and F adsorption.
For example, Ahmed et al. [39] have found that the adsorption of U (VI) increases with a rise in pH up to pH 6 and decreases at pH > 6 using magnetic-modified biochar. This occurred due to repulsion between negatively charged U species and the negatively charged biochar surface at pH > 6. Hu et al. [105] examined the impact of pH on the adsorption capacity of U (VI) using bamboo shoot shell biochar at a pH ranging from 1 to 7 and found the maximum adsorption at pH 4 which decreased further with the increase in pH. This is because at pH < 4, electrostatic repulsion occurred between the positively charged biochar surface (due to protonation of functional groups—carboxyl and hydroxyl groups) and positively charged U species; hence, adsorption is less. Biochar was positively charged when the pH was highly acidic (1–2) due to the protonation of functional groups leading to repulsion between positive U species and the positive biochar surface. It was observed that slightly acidic or near-neutral pH favoured high adsorption capacities for U because of the presence of negatively charged functional groups due to the deprotonation of functional groups and smaller pHZPC values, which led to attraction between positive U species and the negative biochar surface, whereas at basic pH values, the biochar was negatively charged due to the deprotonation of functional groups resulting in repulsion between negative species of U and the negative biochar surface.
Table 2. Influence of solution pH on the adsorption of U.
Table 2. Influence of solution pH on the adsorption of U.
FeedstockSolution pHpHPZCTarget PollutantSpeciation AdsorbedBiochar Surface ChargeAdsorption Capacity
(mg g−1)
Rice straw5.52.5UUO22+, (UO2)3(OH)5+, (UO2)3(OH)42+, (UO2)2(OH)22+negative428.25 [131]
Rice straw5.52.5U negative242.65 [33]
Wheat straw63UUO22+, (UO2)3(OH)5+, (UO2)2(OH)22+, UO2OH+, (UO2)4(OH)7+negative355.6 [82]
Rice husk43.51UUO22+, UO2OH+, (UO2)3(OH)5+negative52.63 [125]
Rice husk5.54.17UUO22+, (UO2)3(OH)5+, (UO2)2(OH)22+negative138.88 [81]
Rice husk73.71UUO2(OH)+, (UO2)2(OH)2, (UO2)3(OH)52+negative118 [132]
Corn cob6_U(UO2)3(OH)5+, (UO2)2(OH)22+, UO2OH+negative163.18 [34]
Pine needles63.8U__623.7 [37]
Pine needles6_U__62.7 [172]
Pine sawdust42.98UUO22+, (UO2)3(OH)5+, (UO2)2(OH)22+, (UO2)4(OH)7+ negative514.72 [133]
Macaúba palm3 U 488.7 [36]
Palm tree fibres6_U__112 [121]
Bamboo sawdust42.73UUO22+, (UO2)3(OH)5+, (UO2)2(OH)22+, UO2OH+negative229.2 [111]
Bamboo biomass44.28UUO22+, (UO2)3(OH)5+, (UO2)2(OH)22+, UO2OH+ 274.15 [134]
Bamboo6_U___ [34]
Bamboo shoot shell4_UUO22+, (UO2)2(OH)22+, UO2OH+, (UO2)4(OH)7+ negative32.3 [105]
Cactus fibre3_U__214 [118]
Camphor tree leaves6.55.76UUO22+, (UO2)2(OH)22+, UO2OH+negative98.29 [173]
Miswak branches42.79UUO22+, (UO2)2(OH)22+, UO2OH+negative85.71 [174]
Chinese banyan aerial root4_U__27.29 [135]
Eucalyptus wood5.5_U(UO2)2OH3+, (UO2)3(OH)5+, (UO2)2(OH)22+, UO2OH+, (UO2)4(OH)7+negative27.2 [175]
Puncture vine64UUO22+, (UO2)3(OH)5+, (UO2)3(OH)42+, (UO2)2(OH)22+negative17.24 [39]
Water hyacinth6_UUO22+, (UO2)2(OH)22+, UO2OH+, (UO2)4(OH)7+negative138.57 [176]
Hydrophyte biomass34.2UUO22+ 54.35 [136]
Hydrophyte42.46UUO22+, (UO2)3(OH)42+, (UO2)2(OH)22+, UO2OH+negative128.5 [137]
Switchgrass5.9_UUO22+, (UO2)3(OH)5+, (UO2)2(OH)22+, UO2OH+negative4 [177]
Pig manure4 U 979.3 [107]
Pig manure4 U 661.7 [107]
Pig manure4 U 952.5 [138]
Pig manure4.5_UUO22+,(UO2)2(OH)22+, UO2OH+221.4 [124]
Horse manure49.05UUO22+ 516.5 [108]
Horse manure4 UUO22+negative625.8 [139]
Cow manure4.53UUO22+, (UO2)3(OH)5+, (UO2)2(OH)22+, UO2OH+, (UO2)4(OH)7+ negative73.3 [82]
Carp fish scales52.87UUO22+, (UO2)3(OH)5+, (UO2)2(OH)22+, UO2OH+ negative291.98 [122]
Sewage sludge6_U(UO2)3(OH)5+, (UO2)2(OH)22+, UO2OH+negative96.73 [34]
Sewage sludge63UUO22+, (UO2)3(OH)5+, (UO2)2(OH)22+, UO2OH+negative490.2 [140]
Winery waste
(grape peels)
4_UUO22+negative255 [141]
Winery waste
(grape peels)
4_UUO22+negative100 [141]
Malt spent rootlets (MSR)3_U__547 [120]
Coffee espresso residue3_U__547 [120]
Olive kernels3_U__357 [120]
Fungi56.41U positive427.9 [142]
Green algae62.62U(UO2)3(OH)5+, (UO2)2(OH)22+, (UO2)4(OH)7+ negative100.2 [143]
Cyanobacteria63.5U(UO2)3(OH)5+, (UO2)2(OH)22+, UO2OH+negative58.05 [144]
Sponge gourd5_U(UO2)3(OH)5+, (UO2)2(OH)22+, UO2OH+negative239.21 [83]
Sponge gourd fibres3 U 904 [84]
Sponge gourd sponges3_U__92 [119]
Sponge gourd sponge5.5_U__833 [145]
Sponge gourd residue 61.8UUO22+, (UO2)3(OH)5+, (UO2)2(OH)22+, UO2OH+negative382 [146]
Watermelon rind45.4U__323.56 [85]
Watermelon seeds42.5U__27.61 [38]
Longan shell (fruit)66.25U 331.13 [77]
Orange peel5.52.6UUO22+, (UO2)3(OH)42+, (UO2)2(OH)22+, UO2OH+, (UO2)2OH3+, UO2(OH)2negative246.3 [106]
U: Uranium; _: data not available; pHZPC = zero-point charge (pH value at which there is no charge).
The interaction between biochar and F is affected by the point of zero charge (PZC). As the solution pH is less than pHPZC, it favours the adsorption of F electrostatically [89]. Table 3 shows the influence of the solution pH on the biochar adsorption capacity for F removal. For instance, Habibi et al. [40] synthesized lanthanum-chloride-activated biochar, which had a pHPZC of 6.6, and reported that F removal increased at pH <6.6 and gradually decreased at pH > 6.6. At varying pH, the protonation or deprotonation of functional groups on the biochar surface occurs. At low pH, the removal of anionic F species is favoured, as the functional groups present on the surface of the biochar are protonated [7]. At high pH, the removal of cationic species is favoured due to the deprotonation of functional groups present on the biochar surface. Sadhu et al. [94] observed a significant impact on the adsorption of F using watermelon rind biochar, which showed a maximum adsorption of F at pH 1, which was found to be 9.5 mg g−1. Furthermore, a sharp decline in adsorption efficiency was observed above pH 2. Another reason for the high adsorption capacity was the electrostatic attraction between the positively charged biochar and F ions (Table 3). However, there are studies where F adsorption has occurred at higher pH values (pH > 4). This is due to ion exchange between F ions and the negatively charged or less positively charged biochar surface [90].

3.7.2. Effect of Biochar Dose on U and F Adsorption

For the optimum remediation of U and F, it is required to optimize the biochar dose by keeping the pH and the initial concentration of U and F constant. With increased biochar dosage, the adsorption capacity and removal percentage of U and F rose to the optimum level as the high dosage of the biochar provided many effective active sites on the biochar surface [107]. Table 4 shows the influence of biochar dosage on the adsorption capacity. For example, Yan et al. [90] observed that the F removal rate increased from 77.97% to 98.69% as the biochar dosage rose from 0.25 to 1 g L−1. Xu et al. [176] identified that the U (VI) removal rate increased from 31.38 to 96.03% with the rise in iochar dose from 0.03 to 0.3 g L−1. At the same time, a further increase in adsorbent doses decreased the adsorption capacity.
Primarily, the sorption capacity of F and U increased with an increase in the biochar doses due to the rise in the saturation level of the active sites. After that, through further increasing the dose, little changes were observed, or the adsorption capacity decreased due to the low concentration of the residual contaminant (F or U) after the adsorption and unsaturation of the active sites [90]. Figure 4 explains the impact of dose on U and F adsorption on the biochar surface.

3.7.3. Influence of Initial Concentration

The initial concentration of U and F in aqueous solution influences the adsorption capacity of biochar. The impact of initial F and U concentration on biochar adsorption capacity is shown in Table 5. With low initial concentrations of U, F, and fixed biochar dose, the adsorption of U and F ions increased due to the availability of active surface sites. Furthermore, as the initial concentration of U and F increased, the adsorption decreased because fewer surface active sites were available, and there was more competition between the ions [13,105,118]. For instance, Goswami and Kumar [24] analysed that the removal rate of F declined from 90% to 68.3% with an initial F concentration increment from 3 to 10 mg L−1, utilizing nanoscale rice husk biochar.
De et al. [7] observed F concentrations of 5–20 mg L−1 with a fixed biochar dose to examine the impact of initial concentration. The highest removal percentage of 98.5% was obtained at 10 mg L−1 F concentration. However, with a further rise in the initial concentration, the adsorption decreased drastically because of the saturation of active surface sites. Mahmoud et al. [182] determined the influence of the initial concentration of uranyl ions in the range of 30–150 mg L−1 at a constant pH, contact time, and dosage and found that the removal percentage increased from 81.3% to 89.5% for 30–80 mg L−1 uranyl ion concentration. However, increased uranyl ion concentration from 80 to 150 mg L−1 decreased the removal efficiency due to more ions in the solution than the biochar surface active sites.

3.7.4. Influence of Co-Existing Ions

Various ions from different sources, including sulphate, chloride, nitrate, carbonate, bicarbonate, phosphate, etc., are generally present in groundwater [13]. These ions were found to regulate the adsorption process by competing with U and F ions for interaction with the active surface sites of biochar. The presence of these anions decreased the adsorption efficiency. Liao et al. [107] reported a significant reduction in U adsorption due to the interference of Ca2+, Al3+, SO42−, CO32−, and PO43− ions. Due to the large radius and high valency of Ca2+ and Al3+, U ions were easily captured by the biochar and occupied the active surface sites, leading to decreased U adsorption efficiency. SO42−, CO32−, and PO43− formed stable complexes with U resulting in the reduction in adsorption efficiency of U. Mei et al. [93] observed the effect of SO42–, NO3, Cl, and HCO3 on the F adsorption, where NO3, Cl, and SO42– had little impact on the F adsorption while HCO3 reduced the F adsorption due to the ion competition.
However, some studies have shown that the co-existing ions (Cl, NO3, PO43−, and SO42−) had little or no significant impact on the adsorption efficiency of U and F due to their selectivity of biochar-based materials and their modification methods [13,85]. It was observed that the major co-existing ions interfering with the adsorption of U and F were phosphate, sulphate, carbonate, and bicarbonate. In addition, it depends on the properties of the feedstock chosen for the biochar and the materials and methods used for its modification which highly affect the adsorption efficiency of the biochar. Hence, it is crucial to know the co-existing ions in natural groundwater to improve the selectivity of adsorbent for the U and F removal experiments.

3.7.5. Influence of Pyrolysis Temperature on U and F Adsorption

Pyrolysis temperature is an essential factor influencing the quality and yield of biochar. The pyrolysis temperature mainly affects the structure and properties of the biochar. Table 6 shows the effect of pyrolysis temperature on U and F adsorption. Higher temperature increases the pH, surface area, and ash content of the biochar but reduces the yield of the biochar [69,70,74,100]. Furthermore, with the rise in temperature, the porosity of the biochar increases due to the removal of volatile matter, and porosity enhances the adsorptive capacity of biochar [69,74]. Biochar produced at higher temperatures possesses a higher stable fraction than those prepared at lower temperatures [69]. Oxygen-containing functional groups, for example, carboxyl, hydroxyl, carbonyl, ether, and lactone, on the biochar decreases with an increase in temperature [69,183].

3.7.6. Influence of Different Feedstocks on U and F Adsorption

Several feedstocks have been utilized to prepare biochar to remove U and F from aqueous solution. For example, magnetically modified rice husk biochar was prepared by Wang et al. [132] to remove U. Magnetization enhanced the biochar surface area from 52.1 m2 g1 to 109 m2 g1 and pore volume from 0.02 cm3 g1 to 0.05 cm3 g1, thereby increasing U adsorption capacity from 64 mg g1 (raw biochar) to 118 mg g1 (modified biochar) (Table 1) at pH 7 and temperature of 55 °C. Similar findings were reported by [37,39,85,125,136]. Similarly, phosphate-impregnated biochar from bamboo biomass was examined for U removal. Biochar fabrication increased the surface area from 10.31 m2 g1 to 445.17 m2 g1 and pore volume from 0.031 m3 g1 to 0.236 m3 g1, which resulted in enhanced U adsorption potential from 15.869 mg g1 to 274.15 mg g1 at pH 4 and a temperature of 25 °C [134]. Ying et al. [106] applied orange-peel-derived biochar modified with MnO2 for U extraction. MnO2 modification increased the surface area of the biochar from 165.01 m2 g1 to 273.25 m2 g1 and improved the U sorption ability of biochar from 165.4 mg g1 to 246.3 mg g1. Similar results were observed by [84]. Dairy-manure-based biochar was used for the defluoridation of water. The prepared biochar was modified with calcium, and modified biochar showed 75% removal efficiency with a F uptake of 0.11mg g1 for pristine biochar and 0.42 mg g1 for modified biochar at pH 8 and a temperature of 25 °C [109].

3.8. Adsorption Mechanism of U and F

The physical and chemical reactions between adsorbate (U, F) and adsorbent (biochar) regulate U and F removal from an aqueous solution. Adsorption mechanisms differ according to the type of biomass or feedstock, the presence of functional groups on biochar surface, the physicochemical properties of biochar such as the pH of the medium, and the type of target contaminants [47,57]. Figure 5 summarizes the major adsorption mechanisms of U and F on biochar. The possible adsorption mechanisms for U and F are ion exchange, surface complexation, electrostatic attraction, and precipitation.
The most probable adsorption mechanisms of F and U are ion exchange, electrostatic interactions, and surface complexation. Ion exchange is the interaction between the oxygen-based functional groups on the biochar surface and the target contaminant, which involves exchanging the same type of ions (cation–cation or anion–anion). In the case of F adsorption, ion exchange is associated with ion replacement in which F ions replace an ion (hydroxyl, carboxylic, and sulphate) from biochar surface [35,82,83,90,127,128,131,132]. For instance, Mei et al. [93] removed the F from water using zirconium dioxide biochar through ion exchange between OH and F on the zirconia particles. Wang et al. [95] used polypyrrole-modified biochar synthesized from pomelo peel to remove F and concluded that ion exchange (anion exchange between F and Cl) was the main adsorption mechanism.
Electrostatic interaction occurs between the oppositely charged surface of biochar and contaminants. Electrostatic interaction is related to the solution pH and the pHZPC of the biochar. When the solution pH < pHPZC, the surface charge of the biochar becomes positive and binds the anionic contaminants. When the pH > pHPZC, the biochar surface is negatively charged and binds the cationic contaminants [51,56]. For instance, Sadhu et al. [94] found that maximum adsorption of F was at pH 1 because the pHPZC of biochar was at pH 2.1. Electrostatic interactions also occurred between the negative- and positive-charged biochar and U, F as a result of ionization of the functional groups, such as –COOH2+, –ROH2+, or –COO [56,94,107]. Surface complexation (inner sphere and outer sphere) involves the adsorption of U and F through the formation of complexes with oxygen functional groups, such as carboxyl, hydroxyl, and carbonyl [31]. Many studies revealed that the dominant mechanism for U adsorption was surface complexation between U(VI) ions and the surface functional groups –COOH, –OH, –CO [33,34,37,38,39,85,119,132]. Metal F precipitation is another mechanism suggested for F adsorption, which involves the presence of metals, such as Ca, Mg, Si, K, Al, Mn, Ba, Fe, and Ti in biochar, and F ions react with these metals to form insoluble fluorides on biochar [94,97,179].

4. Adsorption Isotherms, Kinetics, and Thermodynamics

4.1. Adsorption Isotherms

The Langmuir and Freundlich isotherm models are generally used to evaluate the equilibrium adsorption capacity of U and F. The Langmuir isotherm shows the homogeneous adsorption process and monolayer adsorption, while the Freundlich model shows heterogeneity and multilayer adsorption. For example, Chen et al. [133] assessed the interaction between initial U concentration and equilibrium adsorption capacity. They reported that the Langmuir adsorption isotherm was more favourable for describing the U adsorption than other models and confirmed that the adsorption process is single-layer adsorption. Similarly, Meilani et al. [156] performed an isotherm study to detect the F adsorption and adsorption capacity of aluminium-modified food waste biochar. It was found that sorption occurred by the Langmuir adsorption isotherm, which suggested that adsorption was monolayer, homogeneous, occurred only on localized sites, and there were no interactions between the adsorbed ions. In another study, Dai et al. [34] applied the Langmuir and Freundlich isotherm models to assess the U adsorption capacity of corn cob biochar (CCB) and sewage sludge biochar (SSB). They found that the Langmuir isotherm model better fitted U adsorption by CCB while the Freundlich isotherm model better fitted U adsorption by SSB. Since the CCB was ash-poor biochar, the surface structure was homogeneous, while SSB was ash-rich biochar; thus, the surface structure was heterogeneous.
However, in some cases, the Tempkin and Halsey adsorption isotherm model is also used. For instance, Sadhu et al. [94] applied four adsorption isotherm models (including Langmuir, Freundlich, Tempkin, and Halsey) to depict the F adsorption and found that experimental data were better fitted by the Freundlich model than the other three models. Halsey isotherm is appropriate for multilayer adsorption, and Tempkin isotherm is suitable for the uniform distribution of binding energies.

4.2. Adsorption Kinetics

Adsorption kinetics is used to analyse the adsorption behaviour between biochar and contaminants (U and F) with reference to contact time. Pseudo-first-order and pseudo-second-order kinetic models were used to investigate U and F adsorption on the biochar. For instance, Chen et al. [133] reported that the adsorption behaviour between U and MgO/biochar was chemisorption because the correlation coefficient (R2) of pseudo-second-order kinetics was greater than the first-order kinetics. Similarly, Meilani et al. [156] employed pseudo-first-order and pseudo-second-order models to examine the adsorption mechanism of F on the surface of aluminium-modified food waste biochar. They found that the adsorption process was chemical adsorption, as pseudo-second-order kinetics fitted better with the experimental data due to the higher regression coefficient (R2) than the pseudo-first-order model. In another study, Dai et al. [34] performed adsorption kinetics and concluded that the U adsorption process was both physical and chemical adsorption. It was found that the U adsorption process involved two steps: first, the initial phase, which contained rapid adsorption, and second, it involved a slower adsorption process. In the initial rapid phase, a higher amount of U adsorbed onto the CCB (ash-poor), and a lower amount of U adsorbed on the SSB (ash-rich) in the later slower phase. It was found that pseudo-first-order and pseudo-second-order models both fitted well for U adsorption process.

4.3. Adsorption Thermodynamics

Adsorption thermodynamics is used to analyse the adsorption process with respect to temperature. The thermodynamic factors, including enthalpy, Gibbs free energy, and entropy, provide information about the adsorption process. Negative Gibbs free energy represents that the adsorption process increases with respect to temperature and is spontaneous. Negative enthalpy indicates that adsorption capacity decreases with an increase in temperature, and the process of adsorption is heat-releasing. The positive enthalpy value shows that the adsorption potential increases with the rise in temperature, and the process is heat-absorbing. The positive entropy value represents the rise in randomness at the liquid–solid mass transfer interface. For example, Chen et al. [133] reported that the U adsorption process by MgO/biochar was spontaneous and exothermic because the adsorption capability declined with the rising temperature, which confirmed that the sorption process was exothermic and negative Gibbs free energy described the spontaneity of the process. In another study, Guan et al. [35] reported that F adsorption on modified Mongolian scotch pine tree sawdust biochar increased with respect to temperature (T = 308, 318, and 328 K), which indicated that the adsorption process was endothermic. Similarly, Ahmed et al. [39] examined the influence of temperature on the sorption process of U on the magnetized biochar derived from Tribulus terrestris plant. They found that the sorption process was spontaneous and heat-absorbing because U adsorption increased with rising temperature from 298 to 318 K with negative Gibbs free energy and positive enthalpy and entropy.

5. Regeneration of Biochar

Regeneration of biochar is the reverse of the adsorption process achieved by desorbing the contaminant from the biochar. It improves the reusability and stability of the biochar by performing sorption/desorption cycles. Desorption experiments were performed using desorbing agents, mainly acids and alkaline solutions. For instance, Mishra et al. [175] studied the desorption of the U from loaded biochar using nitric acid. They reported that acids are better eluting agents because they provide H+ ions which tend to protonate the surface of biochar, resulting in the elution of positively charged U ions. In another investigation, Liao et al. [138] examined the reusability of pig-manure-derived biochar using ethanol, HCl, KOH, and deionized water as desorbing agents. Among them, HCl showed the effective desorption of U because, at low pH, hydronium ions have more affinity for active sites than the U(VI) species. The removal efficiency was found to be 85% after five cycles. Pang et al. [142] tested the stability and reusability of nano-zerovalent iron-based biochar. They performed five regeneration cycles using sodium bicarbonate as a desorbing agent to desorb U. They reported that the removal efficiency of biochar was 80.6% in the first cycle, which declined to 52% after five cycles.
Various U(VI) sorption–desorption cycles were performed by Philippou et al. [37], who found that the adsorption capability of the biochar for U decreased after each cycle due to the loss of biochar material. Adsorption % declined from 99.5 to 87.2%, and desorption% reduced from 99.6 to 62.6% after four cycles. Sadhu et al. [94] performed three cycles of sorption–desorption to analyse the desorption of F from watermelon rind biochar and concluded that the adsorption capacity of biochar was 79.54% after the first cycle, 71.99% after the second cycle, and 60.17% after the third cycle. Hence, the prepared biochar was stable and could be reused. Similarly, De et al. [7] examined the reusability of biochar using H2SO4 desorbing agent to desorb F. They performed five cycles of sorption–desorption and reported an adsorption percentage of 98.5% after the first cycle and 68% after five cycles. Ahmed et al. [39] assessed the reusability of biochar by performing five cycles of repeated adsorption of U on magnetic biochar obtained from a Tribulus terrestris plant. It was found that the adsorption potential of biochar slightly declined after five cycles suggesting that the biochar has the potential to extract U from water and can be reused.

6. Challenges and Limitations for Real U and F Groundwater Treatment

The application of biochar for treating real U and F-contaminated groundwater is crucial for understanding the practical applicability and limitations of low-cost novel adsorbents. Lingamdinne et al. [85] have tested real groundwater spiked with 10 mg L−1 of U from Republic of Korea for U treatment using magnetic watermelon rind biochar. In semi-column experiments, more than 90% of U was removed with magnetic biochar in up to three cycles at pH 4. In addition, Sen et al. [81] collected groundwater from West Bengal and Jharkhand, with U concentrations in the range of 38–85 µg L−1. Therefore, iron-modified biochar showed a maximum removal efficiency of 97–99.77% for U at pH 5.5–8.0 at < 1 g of biochar dose. Interestingly, in the USA, Kumar et al. [177] collected groundwater with pH 3.9 and U concentrations of 3.0 mg L−1, which reported an adsorption capacity of 0.52 mg g−1 using switchgrass biochar in column experiments. Through batch sorption experiments, the adsorption capacity for U was observed at 2.12 mg g−1 at pH 3.9, whereas it increased to about 4 mg g−1 with increasing pH up to 5.9 [177]. From the above analysis, it has been clear that U removal from real groundwater is pH-dependent, and the effect of biochar dose, initial U concentration, and other dependent parameters is yet to be investigated.
Recently, Kumar et al. [184] have summarized practical applications and limitations in treating F-contaminated water through raw/modified biochar. It has been investigated that most of the research has been performed at low pH to treat real or spiked groundwater using biochars. For example, Mohan et al. [91] have reported that low adsorption capacity, 4.38 and 5.37 mg L−1, was observed at pH 2 using corn stover pristine biochar and magnetic biochar, respectively, for 10 mg L−1 fluorides-spiked groundwater collected from Ghaziabad, India. In the context of industrial effluents, Sadhu et al. [94] have treated industrial wastewater with F concentrations of 5570 mg L−1 in multiple batch runs with increasing biochar dosage at pH 1 using watermelon rind biochar. In contrast, few studies have also treated F-contaminated groundwater close to neutral pH (5–8) with a removal efficiency of 81–100% [92,185,186]. For example, Zhou et al. [186] observed that 100% F removal efficiency was removed from groundwater having 5 mg L−1 F concentrations using magnetic biochar at pH 8. Similarly, a removal efficiency of 81% was observed for F-contaminated groundwater with 7 mg L−1 using shell-derived activated biochar at pH 6.5 [92]. In addition, in China, 97% of F-contaminated groundwater (9.8 mg L−1) was treated with lanthanum-modified biochar at pH 5.2 [185]. Besides adsorption, the gravity filtration method showed a removal efficiency of 92.5–94.7% for F ions from drinking water using iron-modified biochar [187]. Apart from this, groundwater contains various cations/anions that can significantly influence F removal during the treatment of real groundwater using biochar, which is yet to be investigated. Batch sorption experiments were extensively performed using raw/modified biochar at the laboratory scale; however, very few studies have reported the implication of biochars in column experiments to treat F-contaminated water. The transport and deposition of F were analysed with respect to adsorbent dosage, F concentrations, and flow velocity, as these parameters can impact F adsorption. For example, the retention and transport mechanism for F in column experiments, using various biochars derived, such as pulse straw biochar [96], modified biochar (MgO-biochar) [155], and dairy manure-derived biochar [109].
Till now, biochar research has mostly been carried out at a small scale/laboratory scale. So, to upscale biochar for the treatment of natural groundwater/wastewater/surface water, the major limitations include controlled experimental designs in the laboratory, which is the main limiting factor while implementing biochar at a large scale. Most of the experiments are batch experiments, with limited column studies. The cost of production is one of the major factors limiting biochar application on a large scale. There are limited studies on removing U and F from real aquifers/surface water. There are operational challenges while designing a water treatment plant to remove U and F using biochar. Natural groundwater/surface waters contain co-existing ions with respect to U and F, which interferes with the adsorption. A risk assessment of biochar application needs to be investigated to remove U and F from drinking water. The toxicological impacts of biochar need to be investigated. Biochar recycling and management of the waste biochar need to be taken into consideration. There are studies related to the regeneration and reusability of biochar, but the efficiency of the biochar decreases after a few cycles. Hence, the safe disposal and alternate use of waste biochar are the concern.

7. Conclusions, Research Gaps, and Future Perspectives

The present review has summarized the recent studies for removing U and F from aqueous solution using raw and modified biochar prepared from different feedstocks. Different feedstocks, production techniques, modification methods, adsorption mechanisms, factors influencing the U and F adsorption, and experimental conditions for the optimum removal of U and F were reviewed in this paper. It was found that pyrolysis was the dominant biochar production technique. Low-to-medium pyrolysis temperature, cellulose, and hemicellulose-rich feedstocks such as crop residues, grasses, softwood, and manure-based biochars were effective for U and F removal. Acidic and magnetic modification favoured U adsorption, while metal oxides, hydroxides, and alkali modification aided F adsorption. The dominated adsorption mechanisms for U adsorption were surface complexation (inner-sphere complexation) and electrostatic attraction, whereas ion exchange, electrostatic attraction, and precipitation were adsorptive mechanisms for F ions. Different parameters affected the adsorption process, including pH, biochar dosage, initial concentration, co-existing ions, and pyrolysis temperature. It was found that alkaline pH facilitated F adsorption while slightly acidic and near-neutral pH favoured U adsorption. High pyrolysis temperature raised the pH of biochar and reduced the biochar yield. Consequently, adsorption capacity decreased due to pore shrinking or pore breakage and loss of acidic functional groups. It is crucial to have knowledge about the co-existing ions in natural groundwater to improve the selectivity of adsorbent for the U and F removal experiments. Although several investigations have been carried out to remove U and F with biochar, some research gaps in the literature need to be addressed.
  • Most of the investigations have been carried out at lab scale through batch studies using synthetic water or simulated water, and limited studies have been performed using natural water. In order to scale up, column studies should be conducted for field applications, and future research should be focused on the treatment of natural water.
  • Most studies have focused on the selective removal/extraction of either U or F using biochar. However, actual waters contain multiple contaminants, so future research should focus on using biochar to remove multiple contaminants from water.
  • There is a lack of studies for the treatment of drinking-water sources such as natural waters and groundwater using biochar. Most of the studies have focused on the remediation of wastewater.
  • Most of the modifications are chemical modifications, and there is a need for environmentally friendly green methods/materials for modification.
  • The effect of co-existing ions has not been studied in detail. There is no detailed study on the impact of multiple components in the solution that interferes with the U and F adsorption.
  • Most studies have considered U as a cation (UO22+), but in alkaline solutions, U exists as (UO2)2CO3 (OH)3− (anion). So, there is no study for treating anionic U using biochar from aqueous solution.

Author Contributions

Conceptualization, A.T. and P.K.S.; methodology, A.T.; formal analysis, A.T.; investigation, data curation, A.T.; writing—original draft preparation, A.T.; writing—review and editing, P.K.S. and R.K.; supervision, P.K.S. All authors have read and agreed to the published version of the manuscript.


This research was financially supported by the University Grants Commission (UGC) in the form of Ph.D. fellowship.

Data Availability Statement

Not applicable.


The first author would like to acknowledge the University Grants Commission (UGC), India for providing financial support in the form of Ph.D. fellowship. We acknowledge the DST-FIST (Fund for Improvement of S & T Infrastructure of the Department of Science and Technology) support at the Department of Environmental Science and Technology of the Central University of Punjab for providing support to this work. PKS would also like to acknowledge the SERB Core Research Grant (CRG/2021/002567) from the Department of Science and Technology (DST), India, and the Research Seed Money Grant (GP-25) from the Central University of Punjab for technical support.

Conflicts of Interest

The authors declare no conflict of interest.


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Figure 1. PRISMA flowchart showing data collection, exclusion, and inclusion criteria.
Figure 1. PRISMA flowchart showing data collection, exclusion, and inclusion criteria.
Water 14 04063 g001
Figure 2. Schematic diagram of U and F adsorption on biochar.
Figure 2. Schematic diagram of U and F adsorption on biochar.
Water 14 04063 g002
Figure 3. Schematic diagram representing different modification methods of biochar which affect the different properties of biochar, including surface area, pore structure, functional groups on biochar surface, etc.
Figure 3. Schematic diagram representing different modification methods of biochar which affect the different properties of biochar, including surface area, pore structure, functional groups on biochar surface, etc.
Water 14 04063 g003
Figure 4. Effect of different parameters on the adsorption capacity and removal efficiency of U and F, including biochar dose, initial concentration of U and F, solution pH, pyrolysis temperature, and Co-existing ions.
Figure 4. Effect of different parameters on the adsorption capacity and removal efficiency of U and F, including biochar dose, initial concentration of U and F, solution pH, pyrolysis temperature, and Co-existing ions.
Water 14 04063 g004
Figure 5. Schematic representation of major mechanisms of U and F adsorption on biochar surface.
Figure 5. Schematic representation of major mechanisms of U and F adsorption on biochar surface.
Water 14 04063 g005
Table 3. Effect of solution pH on biochar adsorption capacity for F removal.
Table 3. Effect of solution pH on biochar adsorption capacity for F removal.
FeedstockSolution pHpHPZCTarget PollutantBiochar Surface ChargeAdsorption Capacity (mg g−1)References
Rice straw8 11Fpositive111.11 [89]
Wheat straw 74.8Fnegative51.28 [90]
Rice husk46Fpositive4.45 [178]
Rice husk7_F_17.3 [24]
Rice husk5.3 F 21.59 [150]
Rice husk6_F_1.856 [149]
Black gram straw2_F_16 [96]
Corn stover21.96F 4.11 [91]
Chir pine4.5_F_16.72 [151]
Mongolian scotch pine tree sawdust7_F_0.885 [35]
Pine bark29Fpositive9.77 [179]
Pine wood29Fpositive7.66 [179]
Douglas fir 711Fpositive9.04 [152]
Douglas fir 66.4Fpositive36 [153]
Reed biomass5.58.26Fpositive34.86 [128]
Kashgar tamarisk66.6Fpositive164.23 [40]
Tea oil plant (seed shells)6.84.45Fnegative11.04 [93]
Sawdust72.2Fnegative4.413 [154]
Pongammia pinnata seed cake7_F_1.11 [7]
Coconut6.5_F__ [92]
Cattail__F_1.28 [180]
Pomelo peel6.58.6Fpositive18.52 [95]
Watermelon rind12.1Fpositive9.5 [94]
Okra (lady finger) stem2_F_20 [96]
spent Mushroom compost10_F_4.7 [13]
Food waste7.1_F_123.4 [158]
Tea waste2_F_52.5 [109]
Red algae seaweed56.9Fpositive2.1 [169]
Dairy manure 58.8Fpositive0.42 [109]
Sheep bone__F_2.33 [112]
Bone residues (chicken, cattle, and mixed bones)__F_4.29 [181]
Eggshell and platanus acerifoli leaves (5:1) 5_F_308 [97]
F: Fluoride; _: data not available, pHZPC: zero-point charge (pH value at which there is no charge).
Table 4. Effect of biochar dosage on U and F adsorption.
Table 4. Effect of biochar dosage on U and F adsorption.
FeedstockBiochar Dose (g L−1) Target PollutantAdsorption Capacity (mg g−1)References
Rice straw5 mg/50 mLU428.25 [131]
Rice straw0.01gU242.65 [33]
Wheat straw_U355.6 [82]
Rice husk1U52.63 [125]
Rice husk0.38U138.88 [81]
Rice husk0.4U118 [132]
Corn cob0.25U163.18 [34]
Pine needles5U623.7 [37]
Pine needles0.01 g/50 mLU62.7 [172]
Pine sawdust0.2U514.72 [133]
Macaúba palm10U488.7 [36]
Palm tree fibres0.1 gU112 [121]
Bamboo sawdust0.4U229.2 [111]
Bamboo biomass_U274.15 [134]
Bamboo shoot shell2U32.3 [105]
Cactus fibre0.01U214 [118]
Camphor tree leaves0.25U98.29 [173]
Miswak branches1U85.71 [174]
Chinese banyan aerial root1U27.29 [135]
Eucalyptus the Wood5U27.2 [175]
Puncture vine0.5gU17.24 [39]
Water hyacinth0.2U138.57 [176]
Hydrophyte biomass1U54.35 [136]
Hydrophyte0.4U128.5 [137]
Switchgrass0.1U4 [177]
Pig manure0.3U979.3 [107]
Pig manure0.3U661.7 [107]
Pig manure0.1U952.5 [138]
Pig manure_U221.4 [124]
Horse manure0.1U516.5 [108]
Horse manure0.1U625.8 [139]
Cow manure_U73.3 [82]
Carp fish scales0.1U291.98 [122]
Sewage sludge0.25U96.73 [34]
Sewage sludge0.13U490.2 [140]
Winery waste (grape peels)1U255 [141]
Winery waste (grape peels)1U100 [141]
Malt spent rootlets 0.01U547 [120]
Coffee espresso residue0.01U547 [120]
Olive kernels0.01U357 [120]
Fungi0.05U427.9 [142]
Green algae0.5U100.2 [143]
Cyanobacteria0.5U58.05 [144]
Sponge gourd5 mg/50 mLU239.21 [83]
Sponge gourd fibres_U904 [84]
Sponge gourd sponges0.01 gU92 [119]
Sponge gourd sponge_U833 [145]
Sponge gourd residue 0.4U382 [146]
Watermelon rind1U323.56 [85]
Watermelon seeds1U27.61 [38]
Longan shell (fruit)0.1U331.13 [77]
Orange peel10 mg/50 mLU246.3 [106]
Rice straw1F111.11 [89]
Wheat straw 1F51.28 [90]
Rice husk4F4.45 [178]
Rice husk1F17.3 [24]
Rice husk0.1F21.59 [150]
Rice husk10F1.856 [149]
Black gram straw2.5F16 [96]
Corn stover5F4.11 [91]
Chir pine2F16.72 [151]
Mongolian scotch pine tree sawdust3.6 g/100 mLF0.885 [35]
Pine bark10F9.77 [179]
Pine wood10F7.66 [179]
Douglas fir (pine)0.05 g/25 mLF9.04 [152]
Douglas fir (pine)25 mgF36 [153]
Reed biomass1F34.86 [128]
Kashgar tamarisk5F164.23 [40]
Tea oil plant (seed shells)1.6F11.04 [93]
Sawdust5F4.413 [154]
Coconut7F_ [92]
Cattail_F1.28 [180]
Pongammia pinnata seed cake10F1.11 [7]
Pomelo peel2.5F18.52 [95]
Watermelon rind0.2 gF9.5 [94]
Okra (lady finger) stem2.5F20 [96]
Spent mushroom compost2F4.7 [13]
Food waste0.1 g/30 mLF123.4 [156]
Tea waste10F52.5 [157]
Red algae seaweed0.6 g/100 mLF2.1 [158]
Dairy manure 0.33F0.51 [109]
Sheep bone1F2.33 [112]
Bone residues (chicken, cattle, and mixed bones)1F4.29 [181]
Eggshell and platanus acerifoli leaves (5:1) 1.6F308 [97]
U: Uranium; F: Fluoride; _: data not available.
Table 5. Influence of initial concentration on adsorption of F and U.
Table 5. Influence of initial concentration on adsorption of F and U.
FeedstockInitial Conc. (mg L−1)Target PollutantAdsorption Capacity (mg g−1)References
Rice straw6F111.11 [89]
Wheat straw6F51.28 [90]
Rice husk5F4.45 [178]
Rice husk5F17.3 [24]
Rice husk2F21.59 [150]
Rice husk4F1.856 [149]
Black gram straw10F16 [96]
Corn stover100F4.11 [91]
Chir pine50F16.72 [151]
Mongolian scotch pine tree sawdust20F0.885 [35]
Pine bark100F9.77 [179]
Pine wood100F7.66 [179]
Douglas fir (pine)10F9.04 [152]
Douglas fir (pine)50F36 [153]
Reed biomass10F34.86 [128]
Kashgar tamarisk40F164.23 [40]
Tea oil plant (seed shells)70F11.04 [93]
Sawdust10F4.413 [154]
Coconut10F_ [92]
Cattail20F1.28 [180]
Pongammia pinnata seed cake10F1.11 [7]
Pomelo peel10F18.52 [132]
Watermelon rind50F9.5 [94]
Okra (lady finger) stem10F20 [96]
Spent mushroom compost10F4.7 [13]
Food waste300F123.4 [156]
Tea waste50F52.5 [157]
Red algae seaweed15F2.1 [158]
Dairy manure 5F0.42 [109]
Sheep bone10F2.33 [112]
Bone residues (chicken, cattle and mixed bones)10F4.29 [181]
Eggshell and platanus acerifoli leaves (5:1) 500F308 [97]
Rice straw50U428.25 [131]
Rice straw50U242.65 [33]
Wheat straw10U355.6 [82]
Rice husk10U52.63 [125]
Rice husk3U138.88 [81]
Rice husk80U118 [132]
Corn cob25U163.18 [34]
Pine needles11.9U623.7 [37]
Pine needles50U62.7 [172]
Pine sawdust10U514.72 [133]
Macaúba palm5U488.7 [36]
Palm tree fibres11.9U112 [121]
Bamboo sawdust47.6U229.2 [111]
Bamboo biomass_U274.15 [134]
Bamboo shoot shell50U32.3 [105]
Cactus fibre119U214 [118]
Camphor tree leaves50U98.29 [173]
Miswak branches60U85.71 [174]
Chinese banyan aerial root30U27.29 [135]
Eucalyptus the Wood300U27.2 [175]
Puncture vine50U17.24 [39]
Water hyacinth30U138.57 [176]
Hydrophyte biomass_U54.35 [136]
Hydrophyte47.6U128.5 [137]
Switchgrass10U4 [177]
Pig manure10U979.3 [107]
Pig manure10U661.7 [107]
Pig manure10U952.5 [138]
Pig manure10U221.4 [124]
Horse manure10U516.5 [108]
Horse manure10U625.8 [139]
Cow manure10U73.3 [82]
Carp fish scales40U291.98 [122]
Sewage sludge25U96.73 [34]
Sewage sludge50U490.2 [140]
Winery waste (grape peels)100U255 [141]
Winery waste (grape peels)100U100 [141]
Malt spent rootlets _U547 [120]
Coffee espresso residue_U547 [120]
Olive kernels_U357 [120]
Fungi10U427.9 [142]
Green algae50U100.2 [143]
Cyanobacteria50U58.05 [144]
Sponge gourd5U239.21 [83]
Sponge gourd fibres U904 [84]
Sponge gourd sponges119U92 [119]
Sponge gourd sponge_U833 [145]
Sponge gourd residue 225U382 [146]
Watermelon rind20U323.56 [85]
Watermelon seeds30U27.61 [38]
Longan shell (fruit)23.6U331.13 [77]
Orange peel50U246.3 [106]
U = Uranium, F = Fluoride, _ data not available.
Table 6. Effect of carbonization temperature on U and F adsorption.
Table 6. Effect of carbonization temperature on U and F adsorption.
FeedstockCarbonization Temp. (°C)Surface Area (m2 g−1)Target PollutantAdsorption Capacity
(mg g−1)
Rice straw500157.96U428.25 [131]
Rice straw500_U242.65 [33]
Wheat straw450290.1U355.6 [82]
Rice husk500109.65U52.63 [125]
Rice husk30062.88U138.88 [81]
Rice husk500109U118 [132]
Corn cob800_U163.18 [34]
Pine needles600_U623.7 [37]
Pine needles180_U62.7 [172]
Pine sawdust50051.45U514.72 [133]
Macaúba palm350643.12U488.7 [36]
palm tree fibres650_U112 [121]
Bamboo sawdust4501298U229.2 [111]
Bamboo biomass700445.17U274.15 [134]
Bamboo shoot shell50010.93U32.3 [105]
Cactus fibre600<5U214 [118]
Camphor tree leaves35065.91U98.29 [173]
Miswak branches4009.05U85.71 [174]
Chinese banyan aerial root600284U27.29 [135]
Eucalyptus wood40020U27.2 [175]
Puncture vine500_U17.24 [39]
Water hyacinth40050.545U138.57 [176]
Hydrophyte biomass70092.43U54.35 [136]
Hydrophyte500433U128.5 [137]
Switchgrass3002.9U4 [177]
Pig manure500_U979.3 [107]
Pig manure500_U661.7 [107]
Pig manure500227.9U952.5 [138]
Pig manure250_U221.4 [124]
Horse manure500_U516.5 [108]
Horse manure500_U625.8 [139]
Cow manure450101.5U73.3 [82]
Carp fish scales3301074.73U291.98 [122]
Sewage sludge500_U96.73 [34]
Sewage sludge600623.09U490.2 [140]
Winery waste (grape peels)650_U255 [141]
Winery waste (grape peels)650165U100 [141]
Malt spent rootlets (MSR)850540U547 [120]
Coffee espresso residue850700U547 [120]
Olive kernels850510U357 [120]
Fungi160102.7U427.9 [142]
Green algae18063.7U100.2 [143]
Cyanobacteria200_U58.05 [144]
Sponge gourd400_U239.21 [83]
Sponge gourd fibres650<5U904 [84]
Sponge gourd sponges650_U92 [119]
Sponge gourd sponge650_U833 [145]
Sponge gourd residue 200_U382 [146]
Watermelon rind50086.35U323.56 [85]
Watermelon seeds350_U27.61 [38]
Longan shell (fruit)8001168.88U331.13 [77]
Orange peel650273.25U246.3 [106]
rice straw50095.36F111.11 [89]
Wheat straw __F51.28 [90]
Rice husk70058.98F4.45 [178]
Rice husk600_F17.3 [24]
Rice husk600114F21.59 [150]
Rice husk5002.45F1.856 [149]
Black gram straw5009.27F16 [96]
Corn stover5003.61F4.11 [91]
Chir pine400_F16.72 [151]
Mongolian scotch pine tree sawdust550339F0.885 [35]
Pine bark4501.88F9.77 [179]
Pine wood4502.73F7.66 [179]
Douglas fir (pine)1000494F9.04 [152]
Douglas fir (pine)1000576F36 [153]
Reed biomass600236.84F34.86 [128]
Kashgar tamarisk350164.52F164.23 [40]
Tea oil plant (seed shells)400_F11.04 [93]
Sawdust66057.97F4.413 [154]
Pongammia pinnata seed cake55010.1F1.11 [7]
Coconut7001054F_ [92]
Cattail800733.62F1.28 [180]
Pomelo peel600_F18.52 [132]
Watermelon rind4000.5365F9.5 [94]
Okra (lady finger) stem60023.52F20 [96]
Spent mushroom compost50028.5F4.7 [13]
Food waste60020.95F123.4 [156]
Tea waste40011.833F52.5 [157]
Red algae seaweed450319.47F2.1 [158]
Dairy manure 5002.6F0.42 [109]
Sheep bone650113.874F2.33 [112]
Bone residues (chicken, cattle, and mixed bones)350_F4.29 [181]
Bone residues (chicken, cattle, and mixed bones)700_F2.91 [181]
Eggshell and platanus acerifoli leaves (5:1) 80044.7F308 [97]
U = Uranium, F = Fluoride, _ not available.
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MDPI and ACS Style

Thakur, A.; Kumar, R.; Sahoo, P.K. Uranium and Fluoride Removal from Aqueous Solution Using Biochar: A Critical Review for Understanding the Role of Feedstock Types, Mechanisms, and Modification Methods. Water 2022, 14, 4063.

AMA Style

Thakur A, Kumar R, Sahoo PK. Uranium and Fluoride Removal from Aqueous Solution Using Biochar: A Critical Review for Understanding the Role of Feedstock Types, Mechanisms, and Modification Methods. Water. 2022; 14(24):4063.

Chicago/Turabian Style

Thakur, Anjali, Rakesh Kumar, and Prafulla Kumar Sahoo. 2022. "Uranium and Fluoride Removal from Aqueous Solution Using Biochar: A Critical Review for Understanding the Role of Feedstock Types, Mechanisms, and Modification Methods" Water 14, no. 24: 4063.

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