Biochar for Wastewater Treatment—Conversion Technologies and Applications

Biochar as a stable carbon-rich material shows incredible potential to handle water/wastewater contaminants. Its application is gaining increasing interest due to the availability of feedstock, the simplicity of the preparation methods, and their enhanced physico-chemical properties. The efficacy of biochar to remove organic and inorganic pollutants depends on its surface area, pore size distribution, surface functional groups, and the size of the molecules to be removed, while the physical architecture and surface properties of biochar depend on the nature of feedstock and the preparation method/conditions. For instance, pyrolysis at high temperatures generally produces hydrophobic biochars with higher surface area and micropore volume, allowing it to be more suitable for organic contaminants sorption, whereas biochars produced at low temperatures own smaller pore size, lower surface area, and higher oxygen-containing functional groups and are more suitable to remove inorganic contaminants. In the field of water/wastewater treatment, biochar can have extensive application prospects. Biochar have been widely used as an additive/support media during anaerobic digestion and as filter media for the removal of suspended matter, heavy metals and pathogens. Biochar was also tested for its efficiency as a support-based catalyst for the degradation of dyes and recalcitrant contaminants. The current review discusses on the different methods for biochar production and provides an overview of current applications of biochar in wastewater treatment.


Introduction
The world's water resources are being deteriorated due to the continuous discharge of a large number of organic and inorganic contaminants such as dyes, heavy metals, surfactants, pharmaceuticals, pesticides, and personal care products from industries and municipalities into water bodies [1]. Most of these pollutants are highly persistent in nature and are otherwise convert into recalcitrant form [2]. The uncontrolled discharge of these pollutants is a concern because of their suspected negative effects on ecosystems [1,3,4]. Several conventional technologies are applied worldwide for the removal of wastewater pollutants including coagulation-flocculation, adsorption, membrane filtration, reverse osmosis, chemical precipitation, ion-exchange, electrochemical treatment, solvent extraction and flotation for the removal of inorganic pollutants [5][6][7][8]. However, these technologies suffer from a range of disadvantages stretching from inefficiency to remove pollutants at low concentration and to completely convert pollutants into biodegradable or less toxic byproducts, high energy and chemicals consumption, process complexity, high maintenance and operation costs, etc. [9][10][11]. An efficient and viable treatment process should meet both economic and environmental requirements to be marketed Pyrolysis is one of the main thermal processes used for converting organic wastes into carbonaceous materials that can be used for wastewater depollution [28,29]. During pyrolysis, lignin, cellulose, hemicellulose, and fat in the feedstock are thermally broken down in oxygen-free conditions to enrich the carbon content of the starting material by eliminating non-carbon species such as oxygen and hydrogen [30]. Carbonization temperature, heating rate, nitrogen flow rate, and carbonization time are the main factors controlling the process of pyrolysis and profoundly determine the nature and the distribution of carbonization products: biochar (solid fraction), bio-oil (small quantities of condensable liquid), and non-condensable gases (syngas) (e.g., CO, CO 2 , CH 4 , and H 2 ) [31,32]. The removal of different elements (carbon, hydrogen, oxygen) in the form of gases and volatiles results in a decrease in O/C and H/C atomic ratios and correspondingly an increase in aromaticity and carbon content, which enhances the biochar stability [33,34]. This tendency becomes more pronounced as the pyrolysis temperature increases. Increasing pyrolysis temperature (>500 • C) results also in greater hydrophobicity and higher surface area and micropore volume [35], which make of the produced biochar highly amenable for the removal of organic pollutants. Lower pyrolysis temperature (<500 • C), however, allowed for a biochar with smaller pore size, lower surface area, and higher oxygen-containing functional groups [36], which was more suitable for the removal of inorganic pollutants. The increase in pyrolysis temperature also increases the pH of biochar due to the enrichment of ash content [34,37,38]. The heating rate is also an important factor that must be controlled during the process of carbonization. A high heating rate promotes the release of gases and the decrease in the solid yield [39]. Pyrolysis could be classified into slow and fast pyrolysis based on the temperature, heating rate, pressure, and residence time used during the process. Pyrolysis reaction conditions and associated by-products distribution are summarized in Table 1. During slow pyrolysis, moderate temperatures and longer residence time are used in absence of O 2 , which results in high biochar yield (30%) following the increasing cracking reactions that reduce condensable liquid products [40]. Contrary, fast pyrolysis produces biochar with lower mass yield (12%) and promotes the formation of bio-oil (up to 75%) from biomass [41,42]. Table 1. Thermal conversion processes and products distribution [33,[43][44][45][46].

Hydrothermal Carbonization
HTC is one of the most attractive thermochemical conversion methods used for high energy density materials production. This method is well adapted to wet feedstock without need of an energy intensive drying step; it allows high conversion efficiency of biomass to a carbonaceous material with relatively high yield at low operation temperature [47]. During HTC, the biomass in the presence of a liquid is subjected to temperatures ranging from 180 • C to 300 • C at a pressure between 2 MPa to 10 MPa for several hours [48]. Different biomasses were submitted to HTC such as two-phase olive mill waste [49], beet root chips [50], starch and rice grains [51], maize silage [48], and even polyethylene and rubber wastes [52]. Other studies have investigated the HTC of fecal biomass [53], sewage sludge [54] and agricultural residues [55]. During HTC of biomass, a decrease in the pH is typically observed; this is reported to be due to the formation of a variety of organic acids such as acetic, levulinic, formic, and lactic acids, which promotes the acid-catalyzed reaction of organic compounds without the addition of acid [56,57]. The presence of liquid during HTC allows the acceleration of the carbonization process; since water behaves as a solvent and reaction medium that enhances hydrolysis of lignocellulosic biomass [45]. The pretreatment of biomass by HTC follows different mechanistic pathways, resulting in three phases: hydrochar (solid fraction), aqueous phase (mixture of bio-oil and water), and a small volume of gas (mainly CO 2 ). The distribution of HTC byproducts and the properties of the hydrochar is also governed by factors such as process temperature, residence time, pressure and water/biomass ratio [58,59]. The non-soluble carbonaceous hydrochar has generally a structure with spherical particles due to the broken fibrous lignocellulosic chain at some places [60]. Particles contain a hydrophobic part (aromatic ring) and a hydrophilic part, which envelops the first part ( Figure 1). The first part consists of stable oxygen atoms which are in the form of ether, quinone, etc. While the second part contains oxygenated functional groups (hydroxyl, phenolic, carbonyl, carboxylic, ester, etc.) [61]. Depending upon the process conditions used, the major part of carbon contained in the starting feedstock is remained in the final hydrochar, while a considerable amount of inorganics is removed from the hydrochar and dissolved in the liquid phase [45,56]. HTC is considered to be a promising method to convert biomass into materials with abundant oxygen-containing functional groups and porous structure, which is beneficial for the adsorption of contaminants from wastewater [45,56]. Various hydrochar materials, such as switchgrass hydrochar [62], poultry litter and swine solids hydrochars [63], cassava slag hydrochar [64], banana peels hydrochar [65], and fructose and phloroglucinol hydrochars [66] have been reported as excellent adsorbents for the removal of different inorganic and organic contaminants. In the study by Elaigwu et al. [67], the effectiveness of hydrochar obtained from HTC of Prosopis africana shells was compared to that of pyrolytic-biochar to remove Pb and Cd from aqueous solution. Results showed that HTC-hydrochar was more efficient in adsorbing metal ions as compared to biochar from pyrolysis. The same tendency was observed by Liu et al. [68], who reported that hydrochar from pinewood exhibits higher efficiency to adsorb Cu than pyrolysis-biochar prepared from the same feedstock. The higher adsorption of HTC-hydrochar is due to the presence of more functional groups and available activated sites on its surface [67,68]. The adsorptive capacity of hydrochar could be improved by the addition of some chemical agent (acids, alkalis and surfactants) during the hydrothermal process [69,70]. Reza et al. [71] used acetic acid and potassium hydroxide as acidic and basic agent to modify the pH of the process water (from 2 to 12) during HTC of wheat straw. The results showed that hydrochar produced at pH 2 has 2.7 times higher surface area than that produced at pH 12 and larger pore volume and pore size.
Appl. Sci. 2020, 10, x 4 of 19 abundant oxygen-containing functional groups and porous structure, which is beneficial for the adsorption of contaminants from wastewater [45,56]. Various hydrochar materials, such as switchgrass hydrochar [62], poultry litter and swine solids hydrochars [63], cassava slag hydrochar [64], banana peels hydrochar [65], and fructose and phloroglucinol hydrochars [66] have been reported as excellent adsorbents for the removal of different inorganic and organic contaminants. In the study by Elaigwu et al. [67], the effectiveness of hydrochar obtained from HTC of Prosopis africana shells was compared to that of pyrolytic-biochar to remove Pb and Cd from aqueous solution. Results showed that HTC-hydrochar was more efficient in adsorbing metal ions as compared to biochar from pyrolysis. The same tendency was observed by Liu et al. [68], who reported that hydrochar from pinewood exhibits higher efficiency to adsorb Cu than pyrolysis-biochar prepared from the same feedstock. The higher adsorption of HTC-hydrochar is due to the presence of more functional groups and available activated sites on its surface [67,68]. The adsorptive capacity of hydrochar could be improved by the addition of some chemical agent (acids, alkalis and surfactants) during the hydrothermal process [69,70]. Reza et al. [71] used acetic acid and potassium hydroxide as acidic and basic agent to modify the pH of the process water (from 2 to 12) during HTC of wheat straw. The results showed that hydrochar produced at pH 2 has 2.7 times higher surface area than that produced at pH 12 and larger pore volume and pore size.

Gasification
Gasification is a thermochemical process performed by the conversion of biomass or other organic matter, into a gas mixture "syngas" (85%) containing H2, CO, CO2, and probably small hydrocarbons such as CH4, a solid char (10%), and a liquid phase "tar" (5%) [72]. Tar and char are the undesirable byproducts of the gasification process. The properties of char generated from biomass gasification processes vary widely based on the nature of feedstock, reactor design, gasifying agent, and gasification temperature. In a study performed by Hernández et al. [73], the authors reported a low specific surface area (60 g/m 2 ) of char produced from gasification of dealcoholized marc of grape at 1200 °C under air flow, which discourages their application as activated carbon without further activation. However, García-García et al. [74] previously concluded that gasification of pine wood at 800 °C during 4 h under steam flow enabled the production of good quality activated carbon with a micropore volume of 0.263 cm 3 /g and a surface area of 603 m 2 /g. Equally, Galhetas et al. [75] tested the capacity of chars produced from gasification of coal and pine and activated with potassium

Gasification
Gasification is a thermochemical process performed by the conversion of biomass or other organic matter, into a gas mixture "syngas" (85%) containing H 2 , CO, CO 2 , and probably small hydrocarbons such as CH 4 , a solid char (10%), and a liquid phase "tar" (5%) [72]. Tar and char are the undesirable byproducts of the gasification process. The properties of char generated from biomass gasification processes vary widely based on the nature of feedstock, reactor design, gasifying agent, and gasification temperature. In a study performed by Hernández et al. [73], the authors reported a low specific surface area (60 g/m 2 ) of char produced from gasification of dealcoholized marc of grape at 1200 • C under air flow, which discourages their application as activated carbon without further activation. However, García-García et al. [74] previously concluded that gasification of pine wood at 800 • C during 4 h under steam flow enabled the production of good quality activated carbon with a micropore volume of 0.263 cm 3 /g and a surface area of 603 m 2 /g. Equally, Galhetas et al. [75] tested the capacity of chars produced from gasification of coal and pine and activated with potassium carbonate to adsorb acetaminophen and caffeine from aqueous solutions. The authors reported that the highest porosity development was obtained with the char derived from pine after gasification at 850 • C under air flow and activated with potassium carbonate at 800 • C during 2 h. The produced carbon material exhibited a mass yield of about 35% along with a high surface area of 1500 m 2 /g and adsorption capacities of 434.8 mg/g and 476.2 mg/g toward acetaminophen and caffeine, respectively. Contrary to biochar prepared by pyrolysis that have been extensively explored, fewer studies evaluated the characteristics of char from biomass gasification. Nevertheless, a thorough knowledge of the char properties and its correlation with the gasification technology are crucial in the view of its potential valorization in water and wastewater decontamination.

Torrefaction
Torrefaction is a conventional thermal pretreatment of biomass performed to improve the physicochemical and thermochemical properties of biomass; it enables the energy densification and the homogenization of biomass. The torrefaction is performed generally at slow heating rates under atmospheric pressure at temperatures ranging from 200 • C to 300 • C and without or with limited oxygen supplies [76]. Torrefaction is dominantly used for biofuel production [77], while pyrolysis is the dominant process to prepare biochar for water and wastewater depollution. Compared to biochar, torrefaction-char may contain higher amount of oxygen-containing functional groups, attributed to the much lower temperatures used during the thermal process [77]. Thus, torrefaction-char might be used as an efficient adsorbent. Li et al. [78] compared between the physico-chemical properties of char prepared by torrefaction at 200-300 • C and biochar prepared by pyrolysis at 500 • C and their removal efficiencies toward U(VI) and methylene blue. The results showed that the maximum adsorption capacities towards U(VI) and methylene blue were increased, respectively, from 56.2 and 192.7 mg/g for biochar to >100 and >350 mg/g for torrefaction-char, indicating that char from torrefaction is much more efficient than biochar from pyrolysis for water pollution control. Torrefaction-char from barley straw produced at 220 • C during 20 min was also used as an adsorbent and showed enhanced adsorption capacity towards methylene blue [79]. Due to the lower process temperature, simpler preparation method, and higher product yield, the torrefaction-char as an oxygen-rich-char could be considered as a promising adsorbent. More attention should be paid for its application for contaminants removal.

Modification of Biochar
Owing to the little functional groups and the small surface area and pore volume exhibited by the biochar and the fact that in case of dry pyrolysis, the pores formed during carbonization are plugged with tarred material, a subsequent chemical or physical treatment is required to improve the properties of specific surface area, pore volume, and pore structure of the biochar for its subsequent use in different environmental applications.
During physical modification, biochar is subjected to a controlled gasification at high temperature and under activation atmosphere. Significant changes in textural characteristics including changes in surface area, pores volume and pores distribution and in surface chemical properties including surface functional groups, hydrophobicity, and polarity were observed after physical activation [80,81]. The widely oxidizing agent used for the biochar modification using the physical method is water steam. Chemical reactions involved during steam activation can be expressed as follows [82] (Equations (1) and (2)): The steam activation process can selectively eliminate most reactive carbon atoms from the original biochar and then generate porosity and surface area. The reaction between steam and carbon induces an elimination of volatile materials and a decomposition of tar, which leads to the development of new micropores and the further widening of existing pores [83]. This tendency is more pronounced as the temperature increases [83]. However, at high temperatures, a significant decrease in mass yield was also observed, which does not allows the process to be economically favorable. Prolonged residence time leads to the improvement of surface area and porous structure until a limit where further increase may induce a widening of existing pores with little new pore formation, which causes the reduction in surface area and pore volume. Steam flow rate, the nature of the precursor, and its composition could also be considered as factors effecting steam activation, but there still is a luck in information to figure out their in-depth influence mechanisms. The activation with steam generally favors carbon microporosity and leads to a final activated biochar with a well-developed microporosity with a very small contribution of mesoporosity [82]. Similar to steam modification, gas purging modification can also be applied to improve the structure of biochar and increase its surface area, thus enhancing its adsorption capacity. It consists of a first pyrolysis step of feedstock followed by a purging step of the biochar by carbon dioxide or ammonia gas. Carbon dioxide modification can promote the microporous structure of biochar, while ammonia gas modification introduces nitrogen-containing groups on the biochar [84]. The modification of cotton-stalk-derived biochar by a mixture of carbon dioxide and ammonia gas was reported by Zhang et al. [85] as more efficient to increase the biochar surface area compared with single modification using carbon dioxide or ammonia gas.
During chemical activation, biochar is impregnated with chemical agents such as acids (hydrochloric acid, nitric acid, sulfuric acid, and phosphoric acid), bases (potassium hydroxide, sodium hydroxide, and potassium carbonate), and oxidants (hydrogen peroxide and potassium permanganate). The acid modification aimed mainly to introduce acid functional groups on the surface of biochar. Peng et al. [86] modified reed-derived biochar by 1 M hydrochloric acid and reported an enrichment of hydrophobic adsorption sites for the adsorption of pentachlorophenol. Equally, Mahmoud et al. [87] reported that the treatment of kenaf fiber biochar by hydrochloric acid resulted in the increase of surface area of 289.5-346.6 m 2 /g. giving favor to highly porous activated biochar showing a honeycomb with various sizes. Acid modification can also change the surface area and the pore structure of the biochar depending on the type and concentration of the acid [88]. Peng et al. [86] found that the use of 1 M of hydrochloric acid increased the surface area of biochar from 58.8 m 2 /g to 88.4 m 2 /g. While the modification of biochar by a combination of 30% sulfuric acid and oxalic acid induces a significant increase in the surface area from 2.3 m 2 /g to 571 m 2 /g [89]. Alkaline modification increases the surface area and the oxygen-containing functional groups on the surface of biochar. In the study carried out by Enaime et al. [28], authors activated biochar prepared from a mixture of olive solid wastes and olive mill wastewater using potassium hydroxide as an activating agent. The activated biochar showed high surface area (1375 m 2 /g) and micropore volume (0.52 cm 3 /g) and exhibited high adsorption capacity toward indigo carmine (599 mg/g). In another study, Jing et al. [90] modified municipal solid wastes-derived biochar by potassium hydroxide and showed an increase in the surface area from 14.4 m 2 /g to 49.1 m 2 /g and in the oxygen-containing functional groups on the biochar surface, which resulted in an enhancement of the As(V) removal efficiency. Contrary, a decrease in the surface area from 4.4 m 2 /g to 0.69 m 2 /g was reported by Shen and Zhang [91] by using potassium hydroxide for the modification of wheat-straw-derived hydrochar. As compared to potassium hydroxide, sodium hydroxide modification of coconut-derived biochar significantly increased its surface area up to 2885 m 2 /g, which was higher than that reported for coconut shell derived biochar modified by potassium hydroxide (1940 m 2 /g) [92]. The characteristics of the biochar modified by alkaline agents is strongly dependent on the types of feedstock and the preparation method. The ratio between base and biochar was also reported as significant factor affecting the properties of biochar [91].
In comparison between acid and basic treatments, Iriarte et al. [93] studied the effect of different agents (phosphoric acid, sulfuric acid, sodium hydroxide and potassium carbonate) on the properties of pork bone char. Results showed that sulfuric acid treatment produced biochar with high microporosity and the treatment with sodium hydroxide and potassium carbonate increased more the amounts of micropores and mesopores within biochar structure. While the effect of phosphoric acid on biochar was extremely aggressive and leads to a destruction of biochar pore structure. Other than acid and basic modification, the modification using oxidizing agents such as hydrogen peroxide and potassium permanganate was also reported to increase the content of oxygen-containing functional groups on the biochar. Xue et al. [94] treated a hydrothermally produced hydrochar from peanut hull with hydrogen peroxide. The results showed an increase in the oxygen-containing functional groups on the hydrochar surfaces, while no enhancement in the surface area was observed, as also reported by Tan et al. [95]. Contrary, the modification of hickory-wood-derived biochar by potassium permanganate enhanced its surface area from 101 m 2 /g to 205 m 2 /g and then enhanced its adsorption capacity toward Pb, Cu, and Cd [96]. The modification of biochar by organic solvents was also used. Jing et al. [90] modified a municipal solid waste-derived biochar by methanol and reported an esterification between the carbonyl groups and the biochar, which resulted in a significant enhancement of adsorption capacity toward tetracycline [90]. However, the practical application of organic solvent could be limited by its high cost and its volatile characteristic.

Organic Pollutants
Biochar derived from different biomass have been intensively and effectively used as sorbents for organic contaminants in water and wastewater. The greatest concern of organic contaminants in aqueous solutions has been focused on phenols and dyes ( Table 2). For instance, pyrolysis biochar derived from macroalgae have been effectively used to remove textile dyes (malachite green, crystal violet and Congo red), which are hardly degraded due to their stability to light and oxidizing agents, and resistance to aerobic digestion [20]. Other biochars derived from switchgrass at higher than normal pyrolysis temperatures were used to effectively remove methylene blue, orange G, and Congo red [21]. The results indicated that methylene blue (cationic dye) was highly adsorbed than orange G and Congo red (anionic dye), this was reported due to the small molecular weight of methylene blue and its favorable electrostatic property and strong π-π interaction with biochar surface. Adsorption was also high for biochars prepared at 900 • C than those prepared at 600 • C due to the significantly enhanced surface area at the highest pyrolysis temperature. Biochar have also been reported as an effective adsorbent for phenols in wastewater [97,98]. The study of Thang et al. [97] investigated the potential of biochar derived from chicken manure to remove toxic phenol and 2,4-dinitrophenol from aqueous solution. The obtained biochar exhibited higher adsorption capacities; 106.2 m 2 /g for phenol and 148.1 mg/g for 2,4-dinitrophenol and a high stability up to five cycles. According to the authors, the interaction mechanisms such as hydrogen bonding, electrostatic interaction and π-π bonding are possibly interfering during the adsorption of phenols on biochar under various conditions [97]. In the study performed by Oh and Seo [99], polymer/ biomass-derived biochar was used for the removal of ionizable halogenated phenols from aqueous solution. Solution pH was an important factor in controlling the hydrophobicity and deprotonation of compounds, which was confirmed by correlation analysis of the maximum sorption capacity.
The effectiveness of straw-based biochar as compared to that of commercial activated carbon to remove dyes (reactive brilliant blue and rhodamine B) was evaluated by Qiu et al. [100]. Both carbon materials showed high affinity to adsorb reactive brilliant blue and rhodamine B at pH values of 3.0 and 6.5 with slight difference in favor of biochar to adsorb rhodamine B due to the enhancement of the π-π interactions by the surface functional groups of the biochar. In another study performed by Ferreira et al. [101], authors compared the adsorptive performance of biochars from paper mill sludge with commercial activated carbon to remove fish anaesthetics from water in recirculating aquaculture systems. Despite the higher adsorption capacity of the commercial activated carbon (289-631 mg/g) as compared to the biochars (53-109 mg/g), the authors concluded that paper mill sludge-based adsorbents could be considered as a cost-effective alternative for the adsorption of fish anaesthetics considering that the cost associated to the precursor (paper mill sludge) of these adsorbents is null and the production process did not use chemicals and allows the recovery of energy from wastes, which makes the whole process as an environmentally friendly process.

Heavy Metals
Recent studies showed that biochar offers an excellent ability to remove inorganic pollutants such as heavy metals from wastewater (Table 3). It has been widely recognized that the adsorption capacity of biochar toward heavy metals largely depends on biochar characteristics and the nature of the target metals [102][103][104][105]. Biochar can be an effective material for the sorption of heavy metals due to the abundance of functional groups on its surface such as phenolic, hydroxyl, and carboxyl groups, and due to its porous structure and its large surface area [106,107]. Jingjian et al. [108] compared between straws of canola, rice, soybean and peanut derived biochars for the adsorption of trivalent chromium. The adsorption capacities were consistent with the content of acidic functional groups of the biochars and vary in the following order peanut > soybean > canola > rice. In another study performed by Zhao et al. [107], the properties of biochars from rice straw, chicken manure, and sewage sludge and their effect on the removal efficiency of Pb 2+ and Zn 2+ were investigated. The highest carbon and hydrogen contents, the greatest number of functional groups (e.g., O-H and C=C/C=O), the highest pH, the most negative surface charge, and the highest physical stability were exhibited by the rice straw biochar, which also showed the highest adsorption capacity toward Pb 2+ . Precipitation and ion exchange reactions have been widely considered as possible mechanisms for heavy metals adsorption. Park et al. [109] testing rice straw biochar for the removal of Cu 2+ and Zn 2+ reported that ion exchange of native cations with Cu 2+ and Zn 2+ cations is the dominant mechanism controlling the adsorption. Xu et al. [110] separated biochar into organic and inorganic fractions and tested their capacity in removing Pb. The Pb adsorption capacity of the inorganic fraction was higher than 300 mg/g, while the adsorption was only 1 mg/g for the organic fraction; this was reported to be due to the dominance of the cation exchange and Pb precipitation during the adsorption process, while Pb complexation with organic functional groups was limited [110]. In another study performed by Liu and Fan [111], rice straw-derived biochar was tested for the adsorption of Cd 2+ . The results showed that minerals on the surface of the biochar containing Mg and Ca exchanged preliminary their cations with Cd 2+ , which then form a precipitate within the biochar structure.
In comparison to other conventional adsorbents such as activated carbon, biochar could be a promising substitute for the adsorption of heavy metals. Kołodyńska et al. [112] reported that in spite of the lower surface area of biochar produced by gasification (115.5 m 2 /g) in comparison to commercial activated carbon (759.8 m 2 /g), the biochar removed more efficiently heavy metal ions from aqueous solutions than activated carbon. This was reported to be due to the type and the high amount of oxygen-functional groups on the biochar surface. Activation of biochar has been found to significantly improve its adsorption capacity toward heavy metals. Potassium hydroxide activated-biochar allowed for more than three-fold increase in the adsorption capacity of Cu 2+ as compared to no-activated biochar [103]. Hydrochar from HTC of hickory and peanut hull was also physically activated by carbon dioxide, which significantly enhanced its adsorption capacity toward Pb 2+ , Cu 2+ , and Cd 2+ . This was reported to be due to the improvement of the specific area of the hydrochar and to the introduction of acidic functional groups to its surface following the CO 2 activation which promoted the electrostatic attractions [113]. Samsuri et al. [114] studied the adsorption of AsV and AsIII by Fe-coated biochars from empty fruit bunch and rice husk. With Fe coating, the maximum AsIII sorption capacity increased from 19 mg/g to 31 mg/g, while AsV sorption increased from 5.5-7.1 mg/g to 15-16 mg/g. The reported sorption mechanism is through the complexation of AsV and AsIII with Fe on the biochar. The relatively high affinity of AsIII as compared to AsV could be due to the net charge of the biochar surface (negatively charged in the pH range of 2-9) and to the speciation of As in aqueous solution (AsIII mainly exists as H 3 AsO 3 and AsV exists as H 2 AsO 4 − and HAsO 4 2− ), which strongly depend on the pH value used during the adsorption experiment (8 for AsIII and 6 for AsV) [114]. Researchers have also focused their attention to understand the competitive adsorption of heavy metals on biochars and the mobility of single versus multiple heavy metal species [105,115]. In the study performed by Park et al. [105], sesame-straw-based biochar has been reported as more efficient in adsorbing metal in single metal systems than in a multi-metal system. Ni et al. [104] showed that the adsorption of Pb 2+ was more promoted than that of Cd 2+ in a coexisting system, while in a single system Pb 2+ ions are adsorbed on the same adsorption sites as Cd 2+ ions. The hydrated ionic radius of the metal ion relative to the pore size plays an important role in competitive adsorption; as the hydrated ionic radius is small as the species are preferentially adsorbed compared to species with larger ionic radius [116].
Then, in addition to the biochar properties, the adsorption of heavy metals also depends on the nature of metals and their competitive behavior for biochar sorption sites.

Biochar Adsorption Mechanism
The biochar surface is characterized by its heterogeneity allowing for different sorption mechanisms to be occurred. The adsorption mechanism is depending on the nature of the contaminants and the chemical properties of the adsorbent surface [117]. According to Pignatello [118], the major routes of adsorption could be roughly divided into physical route where the adsorbate settles on the surface of the adsorbent, the precipitation route where the adsorbent form layers on the adsorbent surface and the porefilling route characterized by the condensation of the adsorbate into the pores of the adsorbent. For organic contaminants, the adsorption process is facilitated by electrostatic attraction, pore-filling, π-π electron-donor acceptor interaction, hydrogen-bonding, complexes adsorption, and hydrophobic interactions (Figure 2). For instance, the sorption of organic pollutants onto biochar by pore-filling is depending on the total micropore and mesopore volumes; the penetration of the pollutant on the biochar structure is more promoted as its ionic radius is low, which leads to an increase in the adsorption capacity of biochar [117,119]. Soluble pollutants can be attached to hydrophobic biochar when they have hydrophobic functional group or precipitated on alkaline biochar surfaces. The surface of biochar is usually negatively charged, due to the dissociation of oxygen-containing functional groups, which causes electrostatic attraction between biochar and positively charged molecules [119,120]. For biochar prepared at high temperatures the loss of oxygen-containing and hydrogen-containing functional groups make them less polar and more aromatic and then less appropriate for polar organic contaminants removal. However, the adsorption could occur by hydrogen-bonding promoted as a results of the electrostatic repulsion between negatively charged anionic organic compounds and biochar. The absence of hydrogen-bonding between water and oxygen-functional groups makes more pronounced the penetration of non-polar contaminant to hydrophobic sites [119]. For the removal of inorganic pollutants such as heavy metals, multitude mechanisms could interfere including surface precipitation under alkaline conditions, ion exchange and complexation, cationic and anionic electrostatic attraction (Figure 2). Lu et al. [121] studied the relative contribution of various mechanisms to Pb adsorption on sludge-derived biochar and proposed the following mechanisms: (i) electrostatic complexation due to metal exchange with cations (potassium and sodium) available in the biochar, (ii) co-precipitation and complexation with organic matter and mineral oxides of the biochar, (iii) surface complexation with free carboxyl and hydroxyl functional groups of the biochar, and (iv) surface precipitation as lead-phosphate-silicate (5PbO.P 2 O 5 .SiO 2 ).
Appl. Sci. 2020, 10, x 9 of 19 in a single system Pb 2+ ions are adsorbed on the same adsorption sites as Cd 2+ ions. The hydrated ionic radius of the metal ion relative to the pore size plays an important role in competitive adsorption; as the hydrated ionic radius is small as the species are preferentially adsorbed compared to species with larger ionic radius [116]. Then, in addition to the biochar properties, the adsorption of heavy metals also depends on the nature of metals and their competitive behavior for biochar sorption sites.

Biochar Adsorption Mechanism
The biochar surface is characterized by its heterogeneity allowing for different sorption mechanisms to be occurred. The adsorption mechanism is depending on the nature of the contaminants and the chemical properties of the adsorbent surface [117]. According to Pignatello [118], the major routes of adsorption could be roughly divided into physical route where the adsorbate settles on the surface of the adsorbent, the precipitation route where the adsorbent form layers on the adsorbent surface and the porefilling route characterized by the condensation of the adsorbate into the pores of the adsorbent. For organic contaminants, the adsorption process is facilitated by electrostatic attraction, pore-filling, π-π electron-donor acceptor interaction, hydrogenbonding, complexes adsorption, and hydrophobic interactions ( Figure 2). For instance, the sorption of organic pollutants onto biochar by pore-filling is depending on the total micropore and mesopore volumes; the penetration of the pollutant on the biochar structure is more promoted as its ionic radius is low, which leads to an increase in the adsorption capacity of biochar [117,119]. Soluble pollutants can be attached to hydrophobic biochar when they have hydrophobic functional group or precipitated on alkaline biochar surfaces. The surface of biochar is usually negatively charged, due to the dissociation of oxygen-containing functional groups, which causes electrostatic attraction between biochar and positively charged molecules [119,120]. For biochar prepared at high temperatures the loss of oxygen-containing and hydrogen-containing functional groups make them less polar and more aromatic and then less appropriate for polar organic contaminants removal. However, the adsorption could occur by hydrogen-bonding promoted as a results of the electrostatic repulsion between negatively charged anionic organic compounds and biochar. The absence of hydrogen-bonding between water and oxygen-functional groups makes more pronounced the penetration of non-polar contaminant to hydrophobic sites [119]. For the removal of inorganic pollutants such as heavy metals, multitude mechanisms could interfere including surface precipitation under alkaline conditions, ion exchange and complexation, cationic and anionic electrostatic attraction (Figure 2). Lu et al. [121] studied the relative contribution of various mechanisms to Pb adsorption on sludge-derived biochar and proposed the following mechanisms: (i) electrostatic complexation due to metal exchange with cations (potassium and sodium) available in the biochar, (ii) co-precipitation and complexation with organic matter and mineral oxides of the biochar, (iii) surface complexation with free carboxyl and hydroxyl functional groups of the biochar, and (iv) surface precipitation as lead-phosphate-silicate (5PbO.P2O5.SiO2).

Biochar Characteristics
The sorption of an adsorbate by an adsorbent is controlled by its accessible volume of micropores [146,147]. Adsorbent materials contain pores of various sizes that are grouped into micropores, mesopores, and macropores based on the width of opening [148]. The size and distribution of pores is largely dependent on the experimental conditions used during biochar production with temperature having the greatest impact [149]. Micropores are reported as the most abundant within the biochar structure and the responsible for its surface area and its high adsorptive capacity. Zabaniotou et al. [147] reported that biochar produced by pyrolysis at high temperatures contains a high micropores volume that range between 50%-78% of the total pores. The size of the adsorbate is also an important parameter controlling the sorption rate of the biochar. While larger adsorbate size can cause exclusion or blockage of sorption sites, smaller particle sizes reduce the mass transfer limitation and increase the van der Waal force of penetration of the adsorbate into the adsorbent [150]. The adsorption rate of biochar also depends on types and levels of surface functional groups [120]. The distribution of surface functional groups is determined by the chemical composition of the feedstock, the carbonization method and the temperature of carbonization [31]. Gascó et al. [151] compared the properties of biochar and hydrochar obtained from pyrolysis and HTC of pig manure. Results showed that the broad peak at 3400 cm −1 , assigned to -OH stretching vibration in carboxyl and hydroxyl groups, becomes less obvious in biochars compared to the feedstock following the pyrolysis temperature increase. HTC-hydrochars also showed a broad band at 3400 cm −1 with lower intensity than for the feedstock due to dehydration and decarboxylation reactions occurred during the HTC process. Authors concluded that pyrolysis at high temperature (600 • C) produced biochars with high aromatic structure, while HTC process at 200-240 • C during 2 h give favor to biochar with increased amount of aliphatic structures. Qambrani et al. [120] stated that due to pyrolytic conditions, the O-H, -CH 2 , CO=, CC=, and -CH 3 functional groups were altered in biochar, which promotes the biochar hydrophobic interactions. The abundance of oxygen-and nitrogen-containing functional groups determines the hydrophobic nature of biochars, the lower the oxygen-and nitrogen-containing functional groups in the biochar, the more hydrophobic the biochar [152]. Biochars with hydrophobic nature are believed to contribute in the adsorption of insoluble adsorbates, while hydrophylic biochar are reported as less effective due to the sorption of water; the presence of oxygen-containing functional groups on the hydrophilic biochar surface allows the penetration of water via hydrogen-bonding, which leads to a competitiveness between water and adsorbate for the available sites on the biochar surface [153]. In aqueous solutions, the insoluble or less soluble adsorbates are more likely to be adsorbed in the pores of the biochar [153].

Solution pH
The solution pH is an important factor that control the adsorption process by influencing the surface charge of the adsorbent and the speciation, the charge and the degree of ionization of the adsorbate [136]. When the solution pH is greater than the point of zero charge, negative charge exists on the surface of the adsorbent following the deprotonation of phenolic and carboxylic groups on its surface, then the competitiveness between the protons and the cation contaminants decreases. At lower pH, basic functional groups such as amine get protonated and possess a positive charge, which promoted the adsorption of anions [154][155][156]. This implies that the adsorption behavior of biochar is a function of the pH of the medium and the deprotonation of functional groups. The impact of pH on biochar adsorption capacity toward ammonium (NH 4 + -N) was demonstrated by Kizito et al. [157] and Hu et al. [158]. The authors showed that the adsorption of NH 4 + -N over biochar was lower when the pH was lower (e.g., pH = 3 or 4). With the increase in initial solution pH between 4 and 8, the adsorption capacity of NH 4 + -N increased and then decreased again as the pH was higher than 9.

Adsorbent Dosage
The adsorbent dosage also has a profound influence on the sorbent-sorbate equilibrium of an adsorption system. The application of an increased adsorbent dosage increased the removal efficiency of organic and inorganic pollutants due to the availability of more sorption sites [159,160]. A reduction of the biochar adsorption capacity, however, could be observed when the dosage rate is in excess, consequently an overlapping of the adsorption layers could be occurred, which shields the available active sites on the absorbent surface [157,161]. The adsorbent dosage must be optimized to reach the high removal efficiency and to make the process cost-effective.

Temperature
The adsorption capacity of biochar has been reported to also be affected by the temperature of the medium in which it is applied. Most studies reported that the adsorption efficiency increased when the temperature raised showing an endothermic nature of the adsorption process. In the study performed by Enaime et al. [28], the authors reported that the sorption of indigo carmine onto potassium hydroxide activated biochar increases with the increase in the temperature due to the endothermicity of the sorption process. An increase in temperature produces an increase in the mobility of dye molecule and a possible increase in the porosity of the adsorbent. This may be due, also, to the swelling effect on the internal structure of adsorbent with increasing temperature, which allows more dye to penetrate further [162]. In another study, Kizito et al. [157] showed that the increase in the experimental temperature for the adsorption of NH 4 + -N from 15 • C to 45 • C allowed for an increase in the adsorption efficiency. They claimed that the raise of temperature above 30 • C to a maximum of 45 • C is beneficial for a maximum removal efficiency.

As a Support Media during Anaerobic Digestion
Anaerobic digestion is one of the most promising treatment technologies considered as an eco-friendly process for wastes management because of its ability to combine bioremediation and energy recovery. Performing the anaerobic digestion process under stable conditions with high energy recovery and efficient pollutants removal is a major concern for scientists since some constituents of the feedstock or their metabolic intermediate products could inhibit microorganisms and hinder their activity. Different approaches have been proposed so far to counteract the inhibition in anaerobic reactors such as the acclimation of bacterial cells, the adoption of thermophilic operating conditions and the reduction of the concentration of inhibitors either by dilution or by co-digestion with other substrates. The use of packing materials in anaerobic digestion has been proved as an advantageous alternative to reduce the mobility or bioavailability of inhibitors while allowing for the development of a large bacterial biomass within the digester.
Biochar as a porous, bio-stable, and available material has earned an increasing interest as an alternative to adsorbents like activated carbon and zeolite, and its application in anaerobic digestion is gradually increasing [163]. Moreover, the residual biochar in digestate can be directly reused as amendment to improve the soil properties without environmental risk [164]. The sorption capacity of biochar toward different organic and inorganic contaminants has been widely reported in the literature, but a lack of information with regard to its behavior toward inhibitory compounds during anaerobic digestion still exists. Mohan et al. [22] showed that biochar can adsorb NH 4 + and remain stable in ambient air. Equally, Lü et al. [165] reported that biochar mitigates NH 4 + inhibition during anaerobic digestion of glucose solution at an NH 4 + concentration of 7 g/L. Torri and Fabbri [166] performed batch tests to investigate the anaerobic digestion of aqueous pyrolysis liquid. The results showed poor performance of the anaerobic process and underlined the inhibition of biological process even with the nutrient supplementation, whereas it was found that the biochar addition increased the yield of CH 4 and improved the reaction rate. In another study, Sunyoto et al. [167] investigated the effect of biochar addition on H 2 and CH 4 production in a two-phase batch anaerobic digestion set-up. Their results showed that biochar addition increased H 2 and CH 4 yields and enhanced volatile fatty acids generation during H 2 production and VFAs degradation during CH 4 production. Compared to zeolite, Mumme et al. [168] reported that biochar showed less process stabilizing capacity. According to the same authors, further optimization can be assumed for the effective use of chars in anaerobic digestion. The addition of biochar to increase the buffering capacity within digesters is another option that make beneficial the application of biochar during anaerobic digestion process. The acidification of digesters is often prevented by the addition of alkaline agents such as lime. However, some studies showed that the continuous addition of alkaline biochar could increase the buffering capacity of the anaerobic system [169,170]. Luo et al. [164] compared biochar and non-biochar incubation using glucose as a substrate and showed that for the tests performed by biochar, the CH 4 yield increased by about 90% while the acidification is reduced. Equally, Sunyoto et al. [167] reported that in addition to supporting microbial metabolism and growth, the application of biochar also buffered pH during biohydrogen production. The alkalinity of biochar is depending on the biomass source as well as on the carbonization temperature, with the alkalinity of biochar increases as the pyrolysis temperature raised [38,171].
The immobilization of biomass on a packing material that provides a very large surface area for microbial growth has been proved as an advantageous alternative, since it allows developing a large bacterial biomass that can be maintained within the reactors for a very long operation time, facilitating electron transfer between interspecies and reducing the distance between syntrophic bacteria and methanogens [172][173][174][175][176]. This can be beneficial to reduce the process start-up time, to assure better stability and higher tolerance toward high loading rates, and to allow for microorganisms to recover their performance very quickly after a period of starvation [177]. Packing materials such as zeolite, clay, activated carbon, and other plastic materials have been used to support microbial attachment and growth [174,[178][179][180][181]. The application of biochar for cell immobilization is, however, not as extensive as most other adsorbents. In a study performed by Sunyoto et al. [182], the authors reported that biochar promoted the methanogenic biofilm formation, which enhanced the microbial activity, volatile fatty acids degradation and CH 4 production. Luo et al. [164] observed the colonization of Methanosarina on biochar material during anaerobic digestion of glucose. The authors showed that the use of biochar leads to an increase in biogas production by about 86% with a simultaneous enhancement of the degradation of intermediate acids as compared to the non-biochar study. The different studies performed so far, showed that the addition of biochar increases microbial metabolism and growth and provides favorable support for microorganisms [165,167]. However, no specific description on the special distribution of microorganisms on the biochar surface and the relationship between biochar structure and the amount of the immobilized microorganisms is provided.

As a Filtration Support Media
Biochar can be used in various water treatment designs. For instance, it can be incorporated in a biochar layer/column to a slow sand filter or bio-sand filter systems for the treatment of wastewater. The use of biochar filtration systems has received greater attention for its potential to remove particulate matter, heavy metals and reduce pathogen loads (Table 4). In a study performed by Kaetzl et al. [17], a higher removal efficiency of a Miscanthus biochar filter was observed, which was better or equal compared to a sand filter. Mean removal of chemical oxygen demand of biochar filter (74 ± 18%) was significantly higher than that of sand filter (61 ± 12%). The biochar filter also showed higher removal of E. coli with a mean reduction of 1.35 ± 0.27 log-units compared to that of sand filters with a mean reduction of 1.18 ± 0.31 log-units. Reddy et al. [14] evaluated the potential use of biochar as a filter media for the removal of total suspended solids, nutrients, heavy metals, and E. coli from synthetic storm water. The filtration on biochar was able to remove about 86% of suspended solids, 86% of nitrate and 47% of phosphate from the storm water effluent. After filtration, the concentration of Cd, Cr, Cu, Pb, Ni, and Zn (heavy metals) were also reduced by 18,19,65,75,17, and 24%, respectively, while only 27% removal efficiency of E. coli was observed. This percentage was much lower than those reported for zeolite [183] and activated carbon [184], showing removal efficiencies of E. coli from storm water of about 53% and 98%, respectively. According to the authors, this difference could be due to the inflow concentration and the antecedent microbial levels as also reported by Chandrasena et al. [185].
The removal of suspended solids and organic matter in an anaerobic biochar filter, could be performed by a successive of steps, as described by Kaetzl et al. [186] (Figure 3), starting with a coagulation of suspended solids in the supernatant water layer followed by the sedimentation and the filtration of larger particles on the filter surface, smaller particles are removed by straining and adsorption in deeper filter zones, while organic matter is hydrolyzed under anaerobic conditions. The efficiency of the biochar filter increases with the increase in the surface area and the attachment sites on the biochar surface, which also support the biofilm establishment and the bacterial deposition within the filter column [186,187]. The reduction of pathogens could be governed by different key mechanisms including the filtration of larger pathogens (e.g., protozoa, amoeba) and the adsorption of negatively charged viral and bacterial cells. Other mechanisms such as the electrostatic attraction of bacteria to the biological film developed on the surface of the biochar filter [188] and the attachment of E. coli to the biochar filter coupled with the increase in the water-holding capacity could interfere. The biochar after filtration becomes enriched with nutrients and therefore can be reused as a fertilizer for soil amendment. In the study performed by Werner et al. [189], the biochar filter showed the same removal of pathogens as a common sand filter (1.4 log units on average). The concentration of P, Mg, and K were reduced during filtration, while N content remained unchanged. The agronomic effects of the biochar filter on spring wheat biomass production on an acidic sandy soil was evaluated. The results showed higher wheat biomass production for biochar filter (37%) treatment (20 t/ha), compared to the unamended control [189].
Appl. Sci. 2020, 10, x 16 of 19 lower than those reported for zeolite [183] and activated carbon [184], showing removal efficiencies of E. coli from storm water of about 53% and 98%, respectively. According to the authors, this difference could be due to the inflow concentration and the antecedent microbial levels as also reported by Chandrasena et al. [185]. The removal of suspended solids and organic matter in an anaerobic biochar filter, could be performed by a successive of steps, as described by Kaetzl et al. [186] (Figure 3), starting with a coagulation of suspended solids in the supernatant water layer followed by the sedimentation and the filtration of larger particles on the filter surface, smaller particles are removed by straining and adsorption in deeper filter zones, while organic matter is hydrolyzed under anaerobic conditions. The efficiency of the biochar filter increases with the increase in the surface area and the attachment sites on the biochar surface, which also support the biofilm establishment and the bacterial deposition within the filter column [186,187]. The reduction of pathogens could be governed by different key mechanisms including the filtration of larger pathogens (e.g., protozoa, amoeba) and the adsorption of negatively charged viral and bacterial cells. Other mechanisms such as the electrostatic attraction of bacteria to the biological film developed on the surface of the biochar filter [188] and the attachment of E. coli to the biochar filter coupled with the increase in the water-holding capacity could interfere. The biochar after filtration becomes enriched with nutrients and therefore can be reused as a fertilizer for soil amendment. In the study performed by Werner et al. [189], the biochar filter showed the same removal of pathogens as a common sand filter (1.4 log units on average). The concentration of P, Mg, and K were reduced during filtration, while N content remained unchanged. The agronomic effects of the biochar filter on spring wheat biomass production on an acidic sandy soil was evaluated. The results showed higher wheat biomass production for biochar filter (37%) treatment (20 t/ha), compared to the unamended control [189].    [193] SSA: Specific surface area; HM: heavy metals; COD: chemical oxygen demand; TSS: total suspended solids; NO 3 -N: nitrate nitrogen; NH 4 -N: ammonia nitrogen; Tot-N: total nitrogen; Tot-P: total phosphorous.

As a Catalyst during Heterogenous Oxidation
Biochar-based catalysts have been widely used in different systems for the degradation of biorecalcitrant compounds and the remediation of contaminants in wastewater, including catalytic ozonation processes, Fenton like reactions, and photocatalytic systems. Moussavi et al. [194] tested the catalytic potential of a biochar prepared from pistachio hull biomass and presented a macroporous structure and hydroxyl and phenolic surface functional groups for the ozonation of a water recalcitrant contaminant (reactive red 198 dye). Higher catalytic efficiency was exhibited by the prepared biochar at a pH of 10.0, with a catalytic potential of about 58% and a dye solution mineralization of 71% after a reaction time of 1 h. The excellent ability of carbon materials to activate various oxidants, such as oxygen and hydrogen peroxide, and form reactive oxygen species for the degradation of refractory organic contaminants has been widely reported in the literature [195][196][197]. Fang et al. [198] demonstrated that biochars originated from pine needles, wheat, and maize straw can induce OH generation in the presence of oxygen. Authors reported that free radicals in biochar transfer electrons to oxygen to form superoxide radical anion and hydrogen peroxide, which reacts further with free radicals to produce · OH, OH generated from biochar suspensions was found very efficient to degrade organic contaminants. Qin et al. [199] found that the hydrochar prepared by HTC enhanced Alachlor degradation in the Fe(III)/hydrogen peroxide Fenton-Like reaction by promoting Fe(III)/Fe(II) cycling via electron transfer from biochar to Fe(III). In another study performed by Izghri et al. [200], hydrochars were prepared by HTC of two-phase olive mill wastes impregnated with ferric chloride and used as catalysts for the degradation of methylene blue. The results showed that the hydrochar prepared at 250 • C HTC temperature during 4 h and with the ratio of ferric chloride to biomass of 1.5 has been effectively used as a catalyst in heterogeneous Fenton-Like oxidation, allowing for a degradation efficiency of methylene blue of about 91%.

Future Perspectives and Environmental Concern of Biochar
Biochar is a renewable resource that has significant potential to address several environmental issues in the field of wastewater depollution. Biochar is utilized as an adsorbent with high adsorbing capacity for a wide range of contaminants, however the detailed understanding of the different mechanisms governing the adsorption process are still lacking [201]. Since the efficient utilization of biochar for wastewater depollution depends on the biochar properties such as functional groups and surface area, on the availability and the nature of feedstock and on the conditions of thermal conversion method adopted for the biochar production, further studies are needed to center around "tuning" the biochar properties for tailored applications. Biochar modification is another important area that has to be exploited to enhance the biochar capacity for the removal of specific contaminants. Further research studies are needed to identify various modification methods and the different mechanisms involved according to the modification agents used. The understanding of the relationship between thermal conversion conditions, biochar properties, and biochar behavior toward wastewater contaminants can encourage its use for very complex compounds that hinder for example biological processes adopted for wastewater treatment. Thus, biochar could be incorporated in the pretreatment processes adopted to remove toxic compounds for subsequent biological treatment. Along with the wide application of biochar in the wastewater treatment field, scientists should pay attention to its potential negative effect on the environment. The stability of biochar is one of the most important properties that has to be taken into consideration when biochar is used. The stability of biochar refers generally to its aromaticity and degree of aromatic condensation [202]. When biochar is used for wastewater decontamination, the potential release of carbon from the biochar can increase the carbon content in the solution to be treated. In addition, a release of heavy metals could also occur, especially for biochar derived from sludge. Huang et al. [203] showed that the dissolution of organic matter from biochar into the aqueous solution is due to the biochar instability. The stability of the biochar was also found usually decreased after several cycles when the biochar is used as a support for catalyst due to the variation in the carbon structure of the biochar. The stability of biochar generally depends on the nature of the starting feedstock and on the experimental conditions used during its thermal conversion. Thus, it is necessary to identify the correlation between these two parameters and the biochar stability. Another important factor is the regeneration and the recovery of biochar after use. The adsorption process is generally consisting of the transformation of pollution from the liquid phase to the solid material/adsorbent. Thus, it is important to convert the toxic pollutants that are bound with the biochar into a non-toxic state for their efficient management [159,204].

Conclusions
The low cost of feedstock and the simple preparation process combined with the enhanced physico-chemical properties of biochar make its application more feasible for wastewater treatment. This review systematically presented an overview of different biochar production techniques, adsorption mechanisms toward organic and inorganic contaminates, and prospective applications in the field of wastewater decontamination. The capacity of biochar to remove pollutants in aqueous solutions is directly linked with their physico-chemical properties, which depends on the nature of the starting feedstock, the thermal conversion technique, and the preparation conditions. The modification of biochar through physical and chemical activation methods was reported to adjust the functional groups on the surface of biochar, enhance its surface area and porous structure, and increase its surface oxygen-containing groups. Due to its unique and highly versatile characteristics, biochar has been effectively used in a variety of applications aiming the remediation of contaminated wastewater, including the adsorption of toxic heavy metals and dyes from the aqueous solutions, as a support for catalysts, as an immobilization support media for microorganisms and adsorbent of inhibitive compounds during anaerobic digestion. Overall, it is undoubted that the application of biochar offers several benefits and potential economic and environmental advantages, and its efficiency to remove different contaminants in the lab-scale has been widely reported. However, more in situ experiments should be performed to test the biochar efficiency using real effluents and to examine the real effect of biochar on the environment prior to its large-scale application. The stability of biochar after several cycles of use and its regeneration needs to be further investigated.