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Review

The Effect of Iron-Modified Biochar on Phosphorus Adsorption and the Prospect of Synergistic Adsorption between Biochar and Iron-Oxidizing Bacteria: A Review

by
Lei Liu
1,2,
Nannan He
1,
Ali Borham
1,3,
Siwen Zhang
1,
Ruqing Xie
1,
Chen Zhao
1,
Jiawei Hu
1 and
Juanjuan Wang
1,2,*
1
College of Environmental Science and Engineering, Yangzhou University, Yangzhou 225127, China
2
Key Laboratory of Arable Land Quality Monitoring and Evaluation, Ministry of Agriculture and Rural Affairs, Yangzhou University, Yangzhou 225127, China
3
Agricultural Botany Department, Faculty of Agriculture, Kafrelsheikh University, Kafr El-Sheikh 33516, Egypt
*
Author to whom correspondence should be addressed.
Water 2023, 15(18), 3315; https://doi.org/10.3390/w15183315
Submission received: 12 August 2023 / Revised: 5 September 2023 / Accepted: 12 September 2023 / Published: 20 September 2023
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Abstract

:
The release of endogenous phosphorus (P) from sediments is the main cause of lake eutrophication, even after the successful control of exogenous P. Among others, the release of iron-bound P is a major source of endogenous P, and it is necessary to reduce the P concentration by enhancing iron–phosphorus binding. Iron (Fe)-modified biochar adsorption is an effective and widely used method for fixing P in sediments. In this paper, the modification method, mechanism, and application effect of Fe-modified biochar are reviewed. It is found that most of the modification methods are realized through a physicochemical pathway. Therefore, the prospect of biochar modification through a biological pathway is presented. In particular, the possible application of iron-oxidizing bacteria (IOB) for promoting iron–phosphorus binding and biochar modifications is discussed. The potential effects of biochar additions on microbial communities in water and sediments are also discussed. In the future research, emphasis should be placed on the adsorption mechanism and effect analysis in simulated polluted environments before large-scale use, to ensure the economic practicability and sustainability of Fe-modified biochar applications.

Graphical Abstract

1. Introduction

With the advancement of industrialization, the eutrophication of water bodies has become a serious ecological problem worldwide [1]. To solve this problem, it is critical to reduce the release of endogenous pollutants, with the successful control of exogenous pollutant discharges. Studies have shown that controlling the phosphorus (P) concentration in water bodies alone is more effective than reducing the nitrogen (N) concentration alone, and the retention of P by sediments is key in the control of lake eutrophication and ecosystem restoration [2,3]. According to the wooden barrel theory and Liebig’s law of minimum [4], P should be selected as the most critical limiting factor, and the most cost-effective way to remove P and to alleviate eutrophication in water bodies should be chosen.
The process of P remobilization in sediments is closely related to the P form as affected by different bound substances, especially iron (Fe). Previous studies have shown that in anoxic environments, P bound to Fe oxides in sediments tends to be released by an Fe oxide reduction in anoxic environments [5]. P in the iron-bound form is usually present as Fe phosphate, which is susceptible to the ambient redox potential [6]. It has been proved that P retention is attributed to the strong adsorption of Fe oxides in surface sediments under oxic conditions, while the reduction of Fe oxides under anoxic conditions leads to the release of P and Fe (II) into the pore water and diffusion into the water column [7,8,9]. Therefore, in recent years, scholars have used Fe oxides as adsorption materials to fix P in sediments through adsorption and chemical precipitation.
The adsorption method is considered to be more convenient, easy to operate, and safe without causing secondary pollution. A number of adsorption materials are used, including biochar [10,11], activated carbon [12], construction and demolition waste [13], fly ash [14], and various natural or modified composite adsorbents [15]. Modified composite adsorbents include Fe-based compounds [16], calcium-based compounds [17], mercury-based compositions [18], merger-based compounds, and so on [19]. Among others, biochar is one of the most widely used adsorbing materials and has become a hotspot worldwide in recent years. The principle of the Fe-oxide modification of biochar by physical or chemical means has been thoroughly explained, and the effects of Fe oxides in combination with different types of biochar have been demonstrated [20,21,22]. In order to increase the P-adsorption efficiency of biochar, researchers are continuously exploring biochar-modifying methods. This can be demonstrated by the distribution of keywords of related publications shown in Figure 1. Microbes also have unique advantages in terms of biochar modification; however, relatively few studies focus on this subject. Iron-oxidizing bacteria (IOB) have a critical effect on the formation of Fe oxides, which determine the P immobilization. The synergistic effects of biochar and microbes, especially IOB, are also worth exploring. We previously inoculated IOB in P-rich eutrophic sediment with the submerged plant Vallisneria natans, and found that the synergistic interaction of IOB and Vallisneria natans not only enhanced the formation of iron-bound P in sediments, but also increased the sedimental bacterial diversity [23]. Therefore, the potential of IOB in P removal cannot be ignored. However, the interaction between IOB and biochar and their synergetic effects on P immobilization remain to be explored.
Therefore, the adsorption effect and existing problems of Fe-modified biochar by physicochemical methods are reviewed in this paper. Furthermore, the prospect of the synergic adsorption of microorganisms and biochar is explored, and the potential of IOB, a kind of functional bacteria, in the transformation, modification, and adsorption between biochar and the Fe-P cycle is introduced for the first time. The objective is to reveal the possibility of applying IOB in biochar modification and synergistic adsorption with biochar, and to provide a reference for the development of novel ecological bioremediation P-removal technologies in eutrophic water.

2. Mechanisms of Fe Oxidation and P Adsorption and the Influencing Factors

In aquatic systems, sediment is an important “source” or “sink” of P in the water column. Studies have shown that when the source of exogenous P input is controlled, the release of P from sediments becomes the main internal source of P pollution in water bodies [24]. A large amount of P is stored in sediments due to the sedimentation of suspended particles and dead aquatic organisms [25]. Once the environmental conditions of the sediment change, the P stored in the sediment is released into the aqueous phase. Among them, the state of iron–phosphorus binding is easily changed and directly affects the release of endogenous P [26,27]. Xu et al. [28] evaluated the P content and release risk in West Lake sediments and found that the concentration of P in sediments was mainly related to the content of reactive iron–aluminum oxides, and was positively correlated with Fe/Al-P, with a correlation coefficient of 0.745. Wang et al. [29] studied the distribution and morphology of P, Fe, and sulfur (S) in the surface sediments of a mildly eutrophic reservoir in southwest China using high-resolution thin-film diffusion gradient (DGT) and conventional sequential extraction techniques, and found that the release of sedimentary P was significantly inhibited at Fe/P > 30 ratios. DGT-P and DGT-Fe were significantly positively correlated, indicating that Fe played an important role in controlling the mobility of deposited P.
The transformation of Fe and P in lake sediments is closely related to seasonal changes, along with water fluctuations and the changes in temperature, pH, redox potential, and dissolved oxygen in the water and sediment [30]. Among which, the change in redox potential is particularly important for the transformation of Fe and P statuses. Redox potential not only reflects the dissolved oxygen status of an aquatic system, but also responds to the ability of a lake or river to purify or decompose organic pollutants by itself. In general, the higher the Eh value, the better the decomposition capacity of the water body and the healthier the water body [31]. Various studies have shown [32,33] that both Fe and P release values are higher in reducing conditions than in oxidizing conditions. Huang et al. [34] selected Shibianyu Reservoir-bottom mud as a representative of nutrient-poor reservoir-bottom mud and simulated the static P-release process of the reservoir-bottom mud under different physicochemical and microbial conditions in the laboratory, and found that the strong reducing conditions with Eh < 0 mV could promote a high release of P from the sediment, accompanied by a high amount of Fe(II) into the overlying water.
The numerical variation of the indicator factor in the water body is closely related. When the pH value decreases, the Eh value increases, and the effect of solution pH on P release is more obvious than that of Fe [30]. In aquatic environments, as the water temperature increases, the pore water oxygen consumption in the sediments increases, which accelerates the reductive dissolution of Fe, leading to the release of P from Fe-P co-precipitates. Thus, the seasonal rise and fall of lake water levels during extreme weather events and increased human activities directly affect the redox conditions of lake-bottom sediments, the deposition of suspended particles, and the type and survival of benthic organisms and microorganisms [35].
The P concentration in sediments is also influenced by many biochemical factors, such as the microbial activity, concentrations of organic matter (OM), Fe minerals, and plant growth [36]. Microbes are constantly involved in the nutrient cycle of sediments, which use nutrients to grow and reproduce and accelerate the degradation of OM in lakes [37]. Among them, the activity of phosphate-solubilizing bacteria is closely related to changes in the P concentration [38]. The phosphate-solubilizing bacteria in sediments can significantly increase the level of soluble P in the overlying water, which provides the necessary conditions for the outbreak of harmful algal blooms [39,40]. The higher the concentration of OM in the lake, the more conducive it is to algal blooms. The concentration of OM is closely related to the P cycle in lakes, especially the degradation of cyanobacterial OM [41]. Algae plants absorb the P from the sediment during their growth; however, when the algae rot, more are produced in the lakes. Many other plants do the same. Therefore, the control of decaying plants is important for the overall eutrophication of the lake. The growth of plants and occurrence of microbial activities in the lake also change the types and activities of microbial communities. OM is also closely related to Fe redox cycles. Fe minerals in sediments can slow down the decomposition of OM and contribute to carbon storage [42]. However, excessive OM leads to Fe(III) reduction [43]. The continuous physicochemical interaction between OM and Fe minerals promotes the formation of a mineral–organic combination [44]. The surface structure of Fe minerals also changes due to the presence of OM. In conclusion, the changes in various physical and biochemical factors in the lake are complex and closely related, and it is necessary to carefully select improved methods to fix the internal P without causing negative pollution effects.
In recent decades, a lot of research has focused on controlling the release of endogenous P caused by environmental and anthropogenic factors to provide favorable conditions for maintaining the Fe oxidation state in water bodies, and many modified composite materials have been widely created and applied, and biochar is one of the most researched and widely used technologies. However, the P-removal effects of direct biological Fe oxide and biologically modified biochar are not explored.

3. Research Progress on the Adsorption of P from Sediments by Fe-Modified Biochar through Physicochemical Pathways

Numerous studies have shown [45,46] that biochar has a high specific surface area, abundant functional groups, and no antibiotic residues, and has a good ability to remove many pollutants from water [47]. However, the surface of biochar is usually negatively charged and has limited active sites; therefore, it has a poor adsorption performance for anions, and the performance of biochar for P adsorption can be improved by modification [48]. In recent years, many scholars have used physical and chemical methods to modify biochar in order to prepare biochar with a better adsorption effect. Fang et al. [49] prepared magnetic biochar by magnetizing the pyrolysis biochar prepared from municipal sludge at 700 °C with an FeCl3 solution. The results showed that the absorption rate of P was 85.33% at 25 °C, pH 6, and 24 h with 50 mg of biochar. Wang et al. [50] mixed crayfish shell biochar with sponge zero-valent Fe for filling a new substrate in a simulated vertical-flow artificial wetland, which achieved a 49.71% removal of phosphate from wastewater after 89 days under an optimal filler ratio and optimal dosing conditions when the hydraulic retention time was 12 h. This indicates that biochar modified by Fe oxide is more effective in removing P from water.

3.1. Preparation of Fe-Modified Biochar

Common modification methods include acid and alkali treatments, metal loading, and multi-material composites [51]. Biochar-loaded Fe oxide composites are generally prepared by four methods: precursor mixed pyrolysis, chemical precipitation, hydrothermal method, and ball milling [52]. The preparation process for these four methods is shown in Figure 2 The precursor mixed pyrolysis method mainly conducts the metal mixing process before the pyrolysis of biochar, impregnates the biomass raw materials with bivalent iron or a trivalent iron salt solution, introduces Fe ions onto the surface or into the interior of the biomass precursor, and then converts Fe ions into Fe oxides in situ using the reducing substances generated during the biomass pyrolysis process [53]. Then, the biochar is placed in the metal ion solution and the chemical reagent is added to precipitate the metal ion on the biochar [54]. This is called chemical precipitation. The hydrothermal method is performed under a high temperature and high pressure, with water or other solvents as the medium, so that the Fe oxide crystallizes and is evenly dispersed on the surface of or inside the biochar. The ball milling method ball mills the biochar and Fe oxide by a mechanical external force to form structural defects, adjust the solid grain size to the nanometer level [55], and generate energy that can accelerate the fracture of bonds, as well as the free radicals, in various ways [56], which improve the adsorptive properties of biochar. The researchers also loaded Fe with other metals (magnesium, calcium, aluminum, copper, zinc, etc.) and materials rich in metal ions onto the biochar to increase the adsorption sites, which could also achieve the the simultaneous adsorption of pollutants.

3.2. Mechanism of Adsorption of P by Fe-Modified Biochar

The adsorption efficiency of P by modified biochar is affected by several mechanisms, including electrostatic adsorption, ion exchange, surface coordination, and chemical precipitation (see Figure 3) [57]. Electrostatic adsorption refers to the electrostatic induction phenomenon caused by the positive surface charge of biochar and the negative charge of phosphate after modification. The surface electricity of biochar plays a key role in the process of N and P adsorption, and positively charged biochar is more likely to adsorb PO43−. pHPZC, zero point of charge, is an important index to reflect the electrostatic action of biochar. The modification can change the pHPZC of biochar and affect the electrostatic action [58]. The electrostatic attraction is mainly controlled by the pH value of the solution. In acidic and neutral solutions, phosphate mainly exists in the forms of H2PO4− and HPO42−. At a relatively low pH value (4–6), the surface groups of the adsorbent are easy to protonate, thus increasing the positive adsorption site of the adsorbent and improving the adsorption capacity of the adsorbent for H2PO4− and HPO42−. However, in the higher pH range, the negatively charged adsorption sites dominate, and the adsorption capacity of biochar for H2PO4− and HPO42− decreases [59]. With the presence of Fe3+, Fe(H2PO4)3, Fe2(HPO4)3, and FePO4 are formed [60]. This process is called chemical precipitation, which refers to the phenomenon of the precipitation reaction on the surface of the adsorbent after the adsorption of phosphate by the modified biochar [61]. The adsorption mechanism depends on the characteristics of biochar and the target pollutants [62]. This characteristic is mainly reflected by three aspects. Firstly, the surface structure and properties of biochar calcined by different raw materials, temperature, and time are different [63], mainly reflected in the ash content, element content, functional group, porosity, and specific surface area [64]. Secondly, when modified by different materials, the surface structure, metal content and distribution, and crystalline phase ratio on the surface of the biochar are also different [65]. Thirdly, in different adsorption environments (initial P concentration, pH, and temperature), where phosphate is present in different forms (PO43−, H2PO4−, and HPO42−), the adsorption mechanism between biochar and P is different. Fe hydroxide-containing adsorbents are easily protonated at low pH levels, resulting in the creation of positively charged adsorbents, while the negatively charged adsorbents can be easily deprotonated in high-pH conditions [66]. The specific mechanisms are described above. The more detailed adsorption mechanism between different adsorbents and P is still being explored, and the clarification of the mechanism of action will be of great help to the restoration process.

3.3. Current Status and Existing Problems of the Adsorption Effect of Fe-Modified Biochar

This paper summarizes the adsorption of phosphate by different adsorbents in eutrophicated water body (see Table 1). Studies have shown that the adsorption efficiency of biochar modified with only Fe is much higher than that of the original biochar; however, there are some constraints, including temperature, pH, coexisting ions, and other factors [67,68,69]. When the pH is 4–5, the electrostatic attraction on the surface of modified biochar is the highest [70,71]. It is critical to choose an optimal modification method in order to achieve high adsorption efficiencies. Ou et al. used both Fe and calcium to modify biochar to solve the problem of the poor adaptability of adsorbents in different pH conditions. The adsorption results showed that the adsorption capacity of ferric chloride/calcium oxide-modified bamboo biochar (ZFCO-BC) was the highest, reaching 58.24 mg/g, which was significantly higher than other treatments. Compared with other adsorbents, ZFCO-BC can remove a high concentration of phosphate (1660 mg/L) with a removal rate of >99.99%. In complex contaminated water containing different ions, ZFCO-BC acts synergistically and simultaneously removes 21 metals, such As Cr, Cu, Co, Cd, AS, Pb, and Y [72]. It can be seen that the modified biochar supported by multiple metals has better adaptability in complex polluted water bodies than that supported by a single metal.
The effect of Fe-modified biochar on lake P adsorption prepared in physical and chemical ways has been verified [73]; however, there is a risk of secondary pollution [74]. Biochar itself has the potential of releasing endogenous nitrogen and P into the water, and the transformation process mixed with other metal elements, such as Fe, Mg, and Al, can produce infiltration toxicity [75]. Harmful components, such as volatile organic compounds and persistent free radicals present in modified biochar, may threaten the normal physiological activity of microorganisms in water bodies and sediments [76]. Biochar’s particulate diameter is relatively small and basically reaches the nanoscale, which increases the difficulty of biochar recycling and may create ecological risks. In addition, some metals are expensive and consume energy during their preparation [77], which is not conducive to their large-scale promotion and application [78]. As an alternative, the effect of biochar modification through biological means is attracting the interests of many scholars.
Table 1. Adsorption of phosphate in wastewater/sediment by different adsorbents.
Table 1. Adsorption of phosphate in wastewater/sediment by different adsorbents.
FeedstockModified MaterialModification MethodInitial Phosphate ConcentrationRemoval RateAdsorption CapacityMechanism of BiocharReference
Hemp rootFeCl3Impregnation 0.795 mg/L50.10%71.00%1, 2[60]
Corn
stalks		Sulfur-doped nano-zero-valent ironBall milling30 mg P/L84.40%-2, 3, 4, 5[79]
Peanut shellSediment metals Ca2+, Fe3+ in the sedimentCapping reactor-Fe/Al-P: 70.9%, OP: 48.4%1.32 mg/g2, 3[80]
Reed strawFeSO4·7H2O, NaBH4Impregnation Total P in sediment is 376 mg/kgSRP: 99%-1, 2, 6, 7, 16[1]
Spent coffee groundsFeCl3·6H2O, LaCl3·7H2OPrecursor mixed pyrolysis, chemical precipitation20 mg/L71.5–97.8%-2, 3, 5, 16[81]
Walnut shell Red mud, Fe2O3, calcitePrecursor mixed pyrolysis, chemical precipitation10 mg/L99.74%0.95 mg/g
at 1 h		2, 3, 4[82]
Spruce sawdustFeCl3·6H2OImpregnation, precursor mixed pyrolysis5 mg/L95%
(1 g/L)		-6, 7[20]
Corn
stalks		FeCl3, NaOH, NaH2PO4, NaNO3Impregnation 2 mg/L-1.78–2.19 mg/g4, 5, 6, 8[83]
Peanut shellFeCl3, FeSO4·7H2O, Fe(NO3)3·9H2OImpregnation 5 mg/LFS-BC: 99.6%, FN-BC: 95.0%-2, 5[84]
Compress-ed sludgeChitosan, ferrous sulfate, sodium sulfideImpregnation 100 mg/L-49.32 mg/g2, 3, 4, 5, 10[85]
Peanut shellFeSO4·7H2OImpregnation 5.0 mg/LKH2PO474%1.11 mg/g2, 3, 5, 11[86]
CorncobFeCl3Impregnation TP: 5.54 mg·P/L94.33%-5, 9[87]
Rice hullFeCl3·6H2OThermal activation 100 mg/L-69.92 mg/g3, 5, 8[88]
Wood and rice husksFe2(SO4)3·nH2O, n = 6–9, FeSO4Co-precipitation25–150 mg/L-25–28 mg/g2, 3, 5[89]
Corn strawFeCl3Impregnation --26.14 mg/g1, 3, 12, 13,
14		[90]
Corn stalkFeCl3Impregnation 10–50 mg/L85%-1, 15, 16[91]
Waste-activated sludgeFeSO4·7H2O, FeCl3Chemical co-precipitation20 mg/L60% (after 5 successive recycles)111.0 mg/g1, 3, 5, 11[92]
Notes: (1) explanation of abbreviations: OP: organic P; SRP: soluble reactive P; FS-BC, FN-BC: two types of iron-modified biochar adsorption materials. (2) Number of different mechanisms of action of biochar: 1: ion exchange, 2: surface chemical precipitation, 3: electrostatic attraction, 4: hydrogen bonding, 5: ligand effect/exchange, 6: physical adsorption, 7: electrostatic exchange, 8: chemisorption, 9: coordination exchange, 10: pore filling, 11: diffusion, 12: intermolecular force, 13: flocculation, 14: functional group reaction, 15: coprecipitation, 16: surface complexation.

4. The Possibility of Adsorption of P by IOB in Combination with Biochar

4.1. The Mechanism of Biochar-Loading Microorganisms

The combination of microorganisms and biochar has been most studied for its loading effect, because different microorganisms have various functions of their own; however, the effect is limited in some ways as environmental conditions change, such as microbial survival, proliferation, mechanical interference, limited nutrient availability, low adaptability, and competition with native microorganisms [93,94]. At present, the most common method to maintain their activity is to immobilize microorganisms on a carrier material, using the carrier as an ideal habitat for their survival in different environments [95]. Biochar, a stable solid material rich in carbon and porous, and produced at high temperatures with a large surface area and pore volume, is more effective in accommodating and immobilizing microbial cells [96,97] and is considered a very promising material for immobilization carriers [98,99].
The mechanisms of immobilization interactions between biochar and microorganisms mainly include surface adsorption and pore filling [100], aggregation [101], gel encapsulation [102], and electrostatic interactions [103]. Adsorption is mainly the adhesion of microorganisms to the surface of biochar by secreting adhesion substances, which can grow on the surface and pores of the carrier material during immobilization [94]. Polymerization is based on the principle of the polymerization of hydrophilic and hydrophobic monomers on the surface and is one of the mechanisms of embedding methods [104]. Gel embedding is a technique used for intercepting and immobilizing microorganisms in water-insoluble gels with a three-dimensional network structure [105], which have a large pore structure suitable for embedding microorganisms in the internal structure. The main mechanism of action of fixed microorganisms in the absorption method is static electrical interaction, and hydrogen clamps and van der Waals also play a role in the process [106]. Usually, the surface of microorganisms is negatively charged and the use of carrier materials with a positive surface charge facilitates the immobilization process [107], and most microorganisms produce glycoproteins and phospholipids, which attach to the surface of the biochar through electrostatic interactions.

4.2. Practical Application of Biochar-Supported Microorganisms in the P-Adsorption Process

Sediment microbial activity has a certain disturbing effect on the sediment; such microorganisms can transform, absorb, and degrade nutrients and pollutants. Sediment environmental conditions also change, which affects the material circulation at the sediment–water interface. In addition, some microorganisms in the sediments directly participate in the cycle of sediment nutrients. Among them, the microorganisms involved in the phosphorus cycle are mainly phosphorus solubilizing bacteria and phosphorus accumulating bacteria. Phosphorus solubilizing bacteria can produce organic acids, such as lactic acid, oxalic acid, and succinic acid, etc., that can dissolve insoluble phosphates. Phosphorus accumulating bacteria mainly absorb a large amount of soluble phosphate and store it in the body to synthesize polyphosphate [108], and then store it in the body in the form of polyphosphate to form polyphosphate sludge, and finally perform the action of removing P from sewage through sludge discharge [109].
However, this method is mostly used for wastewater treatment and is not suitable for P removal from lake sediments. Therefore, in practice, researchers have been exploring the dephosphorization function of other strains. Shao et al. [110] isolated the heterotrophic strains of Sphingomonas sp. PDD-57b-25 and Acinetobacter towneri from wastewater and used them for pollutant remediation. When the strain was immobilized using biochar, the removal rate of P increased by 19%. The combined use of biochar and microorganisms did not pose any threat to the water environment but improved the yield and quality of crabs. Wang et al. [111] selected three common and harmless microbial agents (nitrifying bacteria, yeast, and Bacillus) that are widely used for environmental pollution control to directly repair polluted river sediments, and evaluated their applications at different doses. The results show that 5 mg of nitrifying bacteria and 10 mg of Bacillus are ideal microbial agents for removing TP from the sediments. This shows that the combination of microorganisms and biochar can exert a synergistic effect. After biochar loads microorganisms on the surface, it enters the water and sediment environment together to ensure that the microorganisms reach the destination smoothly and play their role. The process of microbial loading also modifies the surface structure of biochar, making biochar capable of absorbing more pollutants, including nutrients, organic pollutants, and heavy metals [112]. The selection of dephosphorized microorganisms and the mechanism of biochar loading still need to be explored further.

4.3. Study of the Superiority of Biochar in Collaboration with IOB

However, there is a type of microorganism whose potential for P removal has been neglected, and this microorganism is IOB. Fe oxide has become one of the most ideal P adsorbents due to its powerful adsorption capacity. The formation process includes two categories: natural mineralization and artificial synthesis. Artificial synthetic Fe oxides are intended to apply Fe oxide to more absorption scenarios, including adaptation to seasonal changes in lake water and the effects of plant growth. Using chemical methods to improve Fe oxides is efficient; however, it can also create some secondary problems, such as reagent residues, oxidation by-products, etc. The physical aeration method is effective; however, the cost is too high for large-scale applications. Microbes also play an important role in the flow of P in sediments and water [111], and many bacteria can mediate the redox reaction of Fe [113]. This environmentally friendly and simple method should not be ignored.
Schulz-Vogt et al. [114] found that sediment microorganisms could influence the flux of P into and out of sediments by accumulating microorganisms related to the Fe redox transformation, suggesting an important role for microbially mediated Fe oxidation. It was shown [115,116] that in most environments, Fe (III)-reducing and Fe (II)-oxidizing microorganisms mainly control Fe redox reactions. Under oxidation conditions, Fe (II) is oxidized into Fe (III) by non-biological and biological pathways, which are distinguished by the concentration of oxygen [117,118]. Biogenic Fe oxides are superior in terms of their P sorption capacity compared to natural or chemically synthesized Fe oxides. This is due to the involvement of IOB that allow biogenic Fe oxides to possess a larger specific surface area and stronger redox activity [119].
The iron-oxidizing microorganism-mediated mineralization process has better co-precipitation and adsorption removal efficacies for phosphate [120,121]. Its adsorption or encapsulation combined with the biochar surface can considerably enhance its activity and stability in water, and also maintain the high purity and abundance of IOB. IOB, as a functional microorganism, better guarantees ferrous oxidation to gain energy and produce more trivalent iron, thus immobilizing more P. Biochar-based immobilized microorganism technology is playing an increasingly important role in soil and water remediation processes; however, there is limited information on the use of biochar as an iron-oxidizing microbial carrier to remediate P pollution in sediments. Some researchers have begun to try to create and explore the performance of such vectors; however, it is still in the laboratory exploration stage, and there are no specific application cases whose combined effects need to be actually verified. The specific load process and mechanism are shown in Figure 4. The specific adsorption effect can be promoted through this idea.

5. Potential Risks to Sediment Environments from the Use of Modified Biochar

The purpose of using biochar is to fix P in sediments; however, the use of biochar should not affect the growth and reproduction of organisms in water bodies and sediments. Changes in microbial communities and enzyme activity in sediment ecosystems are the two most important factors [122]. Biochar can directly or indirectly affect the local microbial community structure and enzyme activity by changing the physicochemical properties of sediments and the concentration of potentially toxic elements and organic pollutants [123]. According to the results of previous studies, the effects of different types and quantities of biochar on water and sediment are very different. Huang et al. found that low concentrations of biochar increased the abundance of microbial communities in sediments, while high concentrations of biochar decreased some enzyme activities [124]. After using different doses of biochar to repair the water quality, Li et al. found that the pH and conductivity of the water quality increased, the concentrations of N and P decreased, and it could promote the growth of submerged plants. However, the microbial biomass in the sediment decreased and the fungi–bacterial ratio increased [125]. The effects of biochar on microbial abundance and community structure in sediments are complex and mainly depend on the physicochemical properties of the biochar and sediment [126]. The negative effects of biochar on sediments are still highly uncertain, which should not be ignored. Future studies should pay attention to the dynamic changes in sediments and water bodies while paying attention to the restoration effects, so as to prevent threats to the ecological balance of lakes.
The following environmental risk assessments should be conducted prior to the application of modified biochar. (1) It is necessary to simulate the aging process of biochar under different environmental conditions (pH, temperature, time, pollutant concentration, and type of contaminant). Because biochar in water interacts with organic substances and other elements in the environment via the physical, chemical, and biological processes, thus accelerating the aging process. (2) Analyze the toxicity risk of modified biochar and compare the changes in the microbial community and enzyme activity in the water and sediments after the use of modified biochar. (3) Analyze the likelihood of the dissipation of the target pollutant in water after incorporating modified biochar.

6. Conclusions and Outlook

The method, mechanism, and effect of adsorption by Fe-modified biochar were reviewed in this paper. Previous studies have shown that the adsorption efficiency of Fe-modified biochar for P has been greatly improved, and biochar modified with multiple metals can be suitable for increased application environments. On the one hand, we point out that the adsorption performance of Fe oxides modified by IOB is more superior; on the other hand, we emphasize that the use of microbial-modified biochar is a future research prospect. With the development of the research on biochar, the research system of Fe-modified biochar is gradually expanding. However, the long-term use of Fe-modified biochar poses certain risks. A comprehensive study of the nature of Fe-modified biochar and its interaction with P, other pollutants, and co-existing organisms in water and sediments is essential. Future research should focus on the following points:
(1)
First of all, it is necessary to ensure the safety of biochar itself, avoid other forms of pollution to water bodies and sediments, and not to threaten the microbial diversity of water bodies and sediments. Secondly, the tolerance level of microorganisms to the concentration of pollutants in water bodies should also be verified repeatedly through experiments, and the simulated experimental conditions should be adjusted to be infinitely close to the real environment, so as to prepare for better adaptations to different application scenarios;
(2)
Biochars created from different materials have different surface structures, and the characteristics of the microorganisms vary. Therefore, it is necessary to pay attention to whether the diameters match, in order to ensure that the absorption effect of biochar and the bacterial population are compatible;
(3)
As a carrier material, the recovery of biochar should become a key focus to prevent the accumulation of biomass or secondary pollution caused by its combination with other components in water and sediment.

Author Contributions

Conceptualization, J.W. and L.L.; methodology, J.W., L.L. and R.X.; software, L.L., J.W., S.Z. and J.H.; validation, J.H., L.L. and R.X.; formal analysis, L.L., N.H. and R.X.; investigation, L.L., A.B., J.H. and C.Z.; resources, L.L., N.H. and C.Z.; writing—original draft preparation, L.L., N.H. and J.W.; writing—review and editing, L.L., A.B. and J.W.; visualization, L.L., S.Z., J.W. and A.B.; supervision, J.W.; project administration, J.W.; funding acquisition, L.L. and J.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (41701093) and the Postgraduate Research and Practice Innovation Program of Jiangsu Province (Yangzhou University), grant number SJCX22_1777.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Density distribution maps showing occurrences of keywords of publications related to biochar and phosphorus between 2015 and 2023. Based on the keywords, there are many studies on biochar adsorption, mechanism research, nutrient recovery, wastewater treatment, and modification.
Figure 1. Density distribution maps showing occurrences of keywords of publications related to biochar and phosphorus between 2015 and 2023. Based on the keywords, there are many studies on biochar adsorption, mechanism research, nutrient recovery, wastewater treatment, and modification.
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Figure 2. Procedures for preparing modified biochar by precursor mixed pyrolysis, chemical precipitation, hydrothermal, and ball milling methods.
Figure 2. Procedures for preparing modified biochar by precursor mixed pyrolysis, chemical precipitation, hydrothermal, and ball milling methods.
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Figure 3. Mechanism of adsorption of P by Fe-modified biochar.
Figure 3. Mechanism of adsorption of P by Fe-modified biochar.
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Figure 4. Process and mechanism of biochar modification by IOB.
Figure 4. Process and mechanism of biochar modification by IOB.
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MDPI and ACS Style

Liu, L.; He, N.; Borham, A.; Zhang, S.; Xie, R.; Zhao, C.; Hu, J.; Wang, J. The Effect of Iron-Modified Biochar on Phosphorus Adsorption and the Prospect of Synergistic Adsorption between Biochar and Iron-Oxidizing Bacteria: A Review. Water 2023, 15, 3315. https://doi.org/10.3390/w15183315

AMA Style

Liu L, He N, Borham A, Zhang S, Xie R, Zhao C, Hu J, Wang J. The Effect of Iron-Modified Biochar on Phosphorus Adsorption and the Prospect of Synergistic Adsorption between Biochar and Iron-Oxidizing Bacteria: A Review. Water. 2023; 15(18):3315. https://doi.org/10.3390/w15183315

Chicago/Turabian Style

Liu, Lei, Nannan He, Ali Borham, Siwen Zhang, Ruqing Xie, Chen Zhao, Jiawei Hu, and Juanjuan Wang. 2023. "The Effect of Iron-Modified Biochar on Phosphorus Adsorption and the Prospect of Synergistic Adsorption between Biochar and Iron-Oxidizing Bacteria: A Review" Water 15, no. 18: 3315. https://doi.org/10.3390/w15183315

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