Biochar Modification Methods for Augmenting Sorption of Contaminants

Abstract

Biochar is a stable carbonaceous material obtained on pyrolysing biomass. Although it possesses crucial properties of high surface area, porosity, surface functionality, and sorption capacity, there is immense scope to augment these properties for effective contaminant sorption. Physical and chemical modifications enhance surface area, porosity, and contents of oxygen-containing functional groups. While acidic modifications augment surface functional groups and cation exchange capacity, alkaline modifications increase aromaticity, hydrophobicity, and π-π interactions. Impregnation with metals amplifies magnetic properties, availability of active sites, chemisorption, electrostatic attraction, and complexation. These modifications assist in sorption of cationic and anionic contaminants. Accordingly, the present study reviews modified biochars which promote eco-friendly contaminant removal. Moreover, various biomass and modification methods utilised for modified biochar production have been elaborated along with the changes in physico-chemical properties. Importantly, mechanistic insights into the functional role of modified biochars for removal of contaminants have been provided. Further, the impact of ageing on modified biochars and their contaminant adsorption performance have been discussed. Lastly, the feasibility and limitations of various biochar modification methods in addition to different research gaps have been presented to create a road map for future investigations. Waste management and contaminant remediation are the need of the hour for planet survivability, which could be achieved by precise biochar modification.

Graphical abstract

Introduction

In the purview of increasing pollution levels, resulting from anthropogenic activities, and the consequent need to remove organic and inorganic contaminants, sorption has gained enormous attention primarily because of low operational costs, unsophisticated treatment, minimal chemical use, and safe handling of waste [1]. Previously, metal oxides, biomasses, and activated carbons have been used as sorbents with activated carbon being the most widely utilised sorbent in industries because of its high porosity, large surface area (SA), and abundant oxygen (O)-containing surface functional groups [2]. However, activated carbons are expensive while metal oxides could be toxic to the ecosystem which triggers the need for identifying potential alternatives. Lately, biochar has been reported to possess vital physicochemical properties such as high porosity, enhanced SA, alkaline pH, and O-containing functional groups, which enable sorption of contaminants [3,4,5,6]. Biochar is a stable carbon-rich material obtained on subjecting biomass to thermal energy in oxygen-deprived conditions [7,8,9]. It is used for applications ranging from agricultural amendment to industrial manufacturing and from energy production to waste management enabled by its vital properties such as alkaline pH, high SA, surplus porosity, enhanced water holding capacity, commendable sorption capacity (Qe), and abundant nutrients, which are dependent on biomass variation and preparatory conditions [7, 10,11,12].

Although biochar possesses crucial properties, there is huge scope to augment its properties for effective contaminant sorption to reduce pollution [2]. Biochar could be treated with steam for physical modification, while it could be treated with acid/alkali for chemical modification [13, 14]. Alternatively, biochar could be impinged with different materials such as metals, metal oxides, nanomaterials, or other waste materials to fabricate modified biochars [2, 10, 11, 15]. These modifications could be performed before or after thermal treatment of biomass to improve the surface morphology, physico-chemical property, and contaminant removal efficiency (RE) of modified biochar [2, 14]. Previously, reviews have been published for modified biochars mostly focussing on their removal efficiencies for anionic contaminants [2, 16,17,18,19], but holistic studies on contaminant removal potential are limited. Therefore, the present review focuses on (i) describing the various types of modification of biochars which remove organic and inorganic contaminants; (ii) elaborating different methods used for fabrication of modified biochars prepared using various biomass materials; (iii) providing a framework to understand the improvement in physico-chemical properties of modified biochars prepared via different modification techniques; (iv) facilitating insights into the mechanistic aspects of contaminant removal by modified biochars; and (v) presenting different limitations, challenges, and research gaps to provide a direction to future research investigations.

Physical Modification

Physical modifications are performed after biochar production by ball milling it or treating it with steam or heat or purged gas (Fig. 1; Table 1). Although physical modifications increase SA and porosity, they are not regarded to be highly effective in augmenting contaminant removal (especially anionic contaminants like As, F, nitrate, and phosphate), when compared to chemical modifications [20].

Fig. 1

Biochar modifications and their effects

Table 1 Physical modification of biochar

Steam-Modified Biochar

For steam treatment, superheated steam is passed (at 2.2–5 mL/min) through porous biochar at temperature range of 650–950 °C for 30–180 min [21, 22]. During the treatment, oxygen from steam is added to active sites on surface, while hydrogen forms complexes. Steam treatment initiates devolatilisation and removes particles trapped during incomplete combustion, which enhances its porosity and SA [19]. Steam reacts with biochar, thereby converting fixed carbon and volatile matter into CO and CO₂ resulting in pore formation [23]. Further, it oxidises the surface and increases O-containing functional groups (e.g. hydroxyl, carboxyl, carbonyl, phenol, and ether). Although steam is a weak oxidant, it increases hydrophilicity of biochar, which augments electrostatic repulsion with anionic contaminants and electrostatic attraction with cationic contaminants. Longer activation times enable larger contaminant removal. However, steam modification could be associated with limited increase in surface functional groups and repulsive forces between anions and surface, which could be disadvantageous [20, 24,25,26]. Further, steam modification decreases aromaticity, polarity, and H/C, N/C, and O/C ratio, which makes it less preferable to other treatments. Steam treatment augmented the SA of aegle shell (wood apple)-based biochar from 2.3 to 7.1 m²/g, while the biochar removed ibuprofen with 95% efficiency [27]. Rajapaksha et al. [28] treated tea waste biochar with steam, which decreased its pH at 700 °C (from 11.05 to 10.48), eventually enabling a higher removal of sulfamethazine. In another study, Rajapaksha et al. [22] modified Sicyos angulatus-based biochar with steam, where pH at 700 °C decreased from 12.32 to 11.72, eventually facilitating higher sulfamethazine removal (increased from 20.56 to 37.73 mg/g after steam treatment) (Table 2). Lou et al. [25] modified pine sawdust with steam to remove phosphate from aqueous solutions, where steam treatment decreased pH from 4.92 to 4.82 at 300 °C and from 8.16 to 7.46 at 700 °C, which influences surface charge and Qe of biochar.

Table 2 Properties of modified biochars

Purged Gas-Modified Biochar

Biochar is modified by purging CO₂/NH₃ gas, primarily to improve its SA and Qe. While CO₂ modification enhances SA and porosity (especially microporosity), NH₃ modification enriches surface with N-containing compounds. CO₂ purging also increases the activated sites on biochar surface. CO₂ modification increased the SA from 56.9 to 755.3 m²/g in corncob-based biochar [29]. CO₂ reacts with C of biochar to form CO, resulting in formation of a microporous structure, which increases its sorption capacity [23]. The increase in SA, porosity, complexation (between graphitic-N and surface functional groups), and cation-π bonding enables contaminant sorption [30, 31]. Soybean straw–derived biochar was modified with both CO₂ and NH₃ to simultaneously improve the SA (up to 627.15 m²/g) and nitrogen functional groups on surface, which eventually augments sorption capacity [32].

Heat-Modified Biochar

During heat treatment, biochar is exposed to high temperatures (800–900 °C) for 1–2 h mainly to furnish basic surface functional groups to increase hydrocarbon sorption [33, 34]. Removal of hydrophilic groups (such as ether- or carbonyl groups) during heat treatment enhances surface hydrophobicity. Heat-treated biochars could be exposed to gases (such as hydrogen or argon) to form basic functional groups such as pyrone-type groups (arising from hydrophilic ether or carbonyl groups). Moreover, exposure to hydrogen deactivates active sites on surface by forming C–H bonds which stabilises and enhances basicity of biochar [33, 34].

Ball-Milled Biochar

Ball milling is a common technique to augment surface area of any substance by utilising the kinetic energy of moving balls of the instrument to break, grind, mix, and fabricate a material with physical and chemical modifications and an improved surface [35, 36]. It avoids the use of chemicals and is less energy intensive than conventional technologies such as microwave pyrolysis and laser ablation [37]. Ball milling reduces the particle size, increases the pore volume, increases the oxygen-containing surface functional groups, and modifies the surface chemical composition, which eventually augments the adsorption capacity of biochar [38]. The increase in pore volume exposes the graphitic structure of biochar, which enhances the cation-π action [35]. Spheres of dense ceramic materials such as yttria-stabilised zirconia could be utilised as grinding media, due to its chemical inertness, low wear and tear rate, and high reusability [39]. Salt-assisted milling is another ball milling method that utilises sodium chloride crystals along with yttria-stabilised zirconia, where salt crystals in conjunction with biochar is broken down into smaller pieces during milling; salt is later removed by dispersing the milled biochar in an aqueous media followed by centrifugation so that the salt remains in top layer [39]. Bamboo, bagasse, and hickory chip-based biochars were ball milled and high removal of sulfamethoxazole (100.3 mg/g) and sulfapyridine (57.9 mg/g) was observed [40].

Chemical Modification

During chemical treatment, alkali/acid is mixed with feedstock/biochar to enhance properties like porosity, SA, and surface functionality (Fig. 1; Table 3) [20, 45]. Exposure to acidic/alkaline solution oxidises the surface and influences ion exchange, Qe, and hydrophobic/hydrophilic properties [1].

Table 3 Chemical modification of biochar

Alkali-Modified Biochar

Typically, feedstock/biochar is soaked/suspended in different concentrations of alkaline reagents, such as sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)₂), at 30–100 °C for 6–24 h (alkali strengths varying from 0.1 to 10 M), where base:biochar ratio is crucial in synthesising high-performance biochars [16, 46]. Alkaline treatments separate ashes and introduce O-containing groups, like –OH, on the surface and augment porosity and SA, which enhances RE [19]. Alkaline modifications facilitate higher H/C (stability) and N/C (basicity) and lower O/C (hydrophilicity), compared to acidic modifications. High N/C suggests greater N-containing surface groups, which increases basic properties of modified biochars [19]. NaOH modifications increase SA and porosity more than other alkaline modifications [16]. However, decrease in SA has also been reported in previous studies, where KOH modification decreased SA of hydrochar (0.4–1.8 m²/g compared to 4.4–9.1 m²/g of unmodified hydrochar), but significantly increased Cd Qe from 13.92–14.52 mg/g in unmodified to 30.40–40.78 mg/g in KOH modified [47]. KOH-modified bamboo/poplar-based biochar showed improved surface area (1555 m²/g) and porosity (0.2950 cm³/g), which facilitated sites for loading iron particles and removing Cr (Qe = 25.68 mg/g) [48]. Alkaline modification (NaOH) was more effective in adsorption of Ni (6.20 mg/g), Pb (44.64 mg/g), and Cd (0.65 mg/g) in comparison to acid modification (sulphuric and oxalic acid) [49]. Similarly, alkali modification in corn stalk-based biochar improved enrofloxacin removal more (54.08%; 58.29 mg/g) than acid modification (27.80%; 41.91 mg/g), primarily assisted by 14 times increase in SA in alkali modification (10 times rise in acid modification) [50].

Acid-Modified Biochar

Generally, feedstock/biochar is soaked/suspended in acidic reagents (with ratios up to 1:10 w:v ratio), like hydrochloric acid (HCl), sulphuric acid (H₂SO₄), and weak acids (e.g., citric acid) at 30–120 °C for several hours/days (acid strengths varying from 0.1 to 2.5 M) [16, 51,52,53]. Acid treatments remove residues of metals and impurities, and increase stability, regeneration, and RE by augmenting SA, micro-porosity, electrostatic interactions, and surface functionality (hydroxylic, carboxylic, carbonylic, phenolic, lactonic, and ketonic). H₂SO₄ and H₃PO₄ activation increased SA by 80% and microporosity by 263% in pork bone–derived biochar [54]. Acid-modification augments SA less than other modifications due to the breakdown of porous structures and expansion of micro-pores into meso-/macropores. Sahin et al. [55] compared the effect of acid treatments on poultry manure–derived biochar before and after pyrolysis conditions. While in pre-treatment, biochar pH decreased from 9.38 to 8.57 and 9.14 for H₃PO₄ and HNO₃ treatments, respectively, in post-treatment biochar pH decreased to 5.39 and 4.46 for H₃PO₄ and HNO₃ treatments, respectively, suggesting that acid treatment after pyrolysis significantly decreased the pH which could be beneficial for alkaline calcareous soils (Table 2). Yakout et al. [56] observed decrease in SA from 71.4 to 56.9 m²/g in rice straw–based biochar after H₂SO₄ modification. Acid treatment could enhance density of bio-sorbents, which could diminish its Qe [1]. The diminished C content and enhanced O content and acidic surface functional groups increase H/C and O/C of acid-modified biochars [33, 57]. Biochars treated with 30% H₃PO₄ (among 0–50%) showed best sorption results, suggesting importance of its optimisation [58]. Spent sorbents could be reused by using 0.1 N HCl for removing contaminants [59]. Rice husk–derived biochar-supported sulphidated nano-zerovalent iron was pre-treated with HCl, NaOH, and H₂O₂ to improve nitrobenzene reduction performance and observed HCl-modified composite to be the most effective sorbent (100% removal at 200 mg/L in 60 min), probably assisted by negative surface charge, increased acidic functional groups, high surface area, and enhanced electron transfer rate [60]. Acid and alkaline treatments were compared by Mahdi et al. [61] using date seed–based biochar (550 °C pyrolysed), who reported acid pre-treatment to be most effective in greater removal of contaminants (Qe of 0.91 mmol/g, 0.71 mmol/g, and 0.69 mmol/g for Pb, Cu, and Ni, respectively).

Impregnated Biochars

Generally, chemical and physical treatments facilitate biochar with low sorption capacities. Correspondingly, biochar composites have been prepared with enhanced properties including new functional groups and high SA [17]. Typically, their preparation involves pre-/post-processing stages including immersion of feedstock/biochar into solutions of metal oxides or metals (e.g., Fe, Al, Mn, Bi, Zn, and others) (Fig. 1; Table 4), which deposits metals in pores and on surface of biochar, provides positive charge, and enhances RE [2]. Generally, these impregnations improve porosity, SA, surface functionality, and H/C, O/C, and N/C ratios [86]. The increase in surface area result from the accumulation of minerals on surface [87]. Metal impregnation could be combined with physical/chemical modifications to harness additional benefits of multiple treatments for improving the contaminant removal efficiency, as demonstrated in several previous studies (Table 4).

Table 4 Metal impregnation

Fe-Modified Biochar

Iron modified biochars are the most studied biochars, primarily due to their magnetic properties (easy recollection) and the strong interaction between Fe and surface functional groups. Fe modification is achieved by soaking biomass/biochar in FeCl₃, Fe(NO₃)₃, FeSO₄, or Fe₃O₄. Pre-treatment is more effective in removing contaminants than post-treatment (due to Fe transformation into more complex crystalline phases) [88]. Pre-treatment with HNO₃ helps increase hydrophilicity, aiding in vertical growth of iron oxide on the surface and shortening the distance needed for contaminant diffusion in biochar, thereby improving its Qe [89]. Qe also depends on the type and/or structure of iron oxide (Qe-amorphous > crystalline) [90]. Fe modification at high pH and high pyrolysis temperature favours higher Qe [91]. Iron oxide is converted to zero-valent iron (ZVI) during pyrolysis/modification (Fe₂O₃ → Fe₃O₄ → FeO → Fe°), affecting biochar’s properties and Qe [92]. Fe-modified biochar oxidises As(III) to As(V), and the removal is assisted by precipitation, surface sorption, electrostatic interaction, and inner-sphere complexation [93]. A rise in magnetic property of biochar could decrease its Qe. Anyika et al. [94] reported FeCl₃-modified palm kernel shell-derived biochar with significant magnetic property (saturation magnetisation—49.5 emu/g) but low contaminant sorption (Qe = 0.054 mg/g), while Lunge et al. [95] suggested FeCl₃-modified tea waste–derived biochar, with low saturation magnetisation (6.9 emu/g) and high Qe. Nevertheless, Dhoble et al. [96] fabricated Fe(NO₃)₃-modified bark-based biochar with high saturation magnetisation (38.6 emu/g) and superior As(III) affinity (enabled by inner- and outer-sphere complexation), compared to As(V) (only inner-sphere complexation). Corn straw–derived biochars were modified with Fe to compare pre-treatment (FeCl₃.6H₂O) and post-treatment (ZVI) and it was observed that pre-modification removed 50.7–98.6% Cr, while post-modification removed 6.6–21.6% Cr [97]. Biochar prepared from pomelo peel mixed with FeCl₃ solution was analysed to remove Cr and phenol, and the sorbent removed Cr (24.37 mg/g) and phenol (39.32 mg/g) effectively enabled by iron oxides (Fe₂O₃, Fe⁰, FeOOH, and Fe₃O₄), π–π interactions and electron donor–acceptor complex [98]. Rice straw–derived biochar was treated with ZnCl₂ and FeCl₃ to analyse removal of 17β-estradiol and Cu, and it was observed that magnetic biochar pyrolysed at 700 °C best removed 17β-estradiol (153.2 mg/g), and that pyrolysed at 300 °C best removed Cu (85.9 mg/g), with removal enabled by larger surface area, pore volume, and O-containing functional groups (–COOH, C–OH, C = O) [99]. Wheat straw–based biochar modified with α-FeOOH removed Cd (62.9 mg/g) and As (78.3 mg/g) enabled by co-precipitation and ion exchange [93]. ZVI/biochar/Ca-alginate composite removed Cr efficiently (Qe = 86.4 mg/g) along with minimal Fe release, suggestive of the resolution of problem of ZVI instability and decreased secondary pollution resulting from Fe leaching [100]. Pine wood–derived biochar ball milled with FeS₂ removed Cr more efficiently (134 mg/g) than ball milled FeS₂ (62 mg/g) and ball milled biochar (20 mg/g), with 92.25% Cr removed through reduction/precipitation and 8.75% removed via adsorption/surface complexation at equilibrium Cr concentration of 15.7 mg/L [101]. Interestingly, addition of oxalic acid promoted Cr removal from 56 to 100% in the same study. Ren et al. [102] removed phosphate using cotton stalk–derived biochar modified by ferric oxides with adsorption capacity of 0.963 mg/g and observed that coating of ferric oxides on biochar alleviated phosphate release (from 0.471 mg/g in unmodified biochar to 0.089 mg/g in modified biochar).

Al-Modified Biochar

Aluminium modification is achieved by immersing biomass in AlCl₃ followed by pyrolysis; stirring biochar in aqueous solution with Al electrode-based electrochemical treatment; or mixing biomass with red mud (achieved during Al production) followed by pyrolysis. Modification with Al results in an enhanced SA and selectivity, which furnishes greater Qe in composites. Additionally, there could be formation of positive charge on surface, which enables it attract anions, thereby augmenting Qe. Mimosa pigra-derived biochar was modified with 2 M AlCl₃ and observed as an effective adsorbent of NO₃⁻ (31.80 mg/g) and PO₄³⁻ (95.05 mg/g) [103]. Al-modified food waste biochar demonstrated high F removal (Qe = 123.4 mg/g) as high as 91.4% at wide pH range (5–11) [104].

Mn-Modified Biochar

Biochar could be modified with Mn oxides to furnish high RE, enabled by metal(loid)s entering amphoteric functional groups of MnO_x via co-precipitation, complexation, adsorption, or oxidation/reduction. For example, Mn-modified biochar is prepared by treating biomass with MnCl₂.4H₂O (for manganosite crystal deposition) or MnO₂.nH₂O or KMnO₄ (for birnessite crystal deposition) followed by pyrolysis [105, 106]. Birnessite (a mineral with oxides of Na, K, and Mn) possesses high contaminant Qe. Birnessite-loaded biochar was reported superior to manganosite-loaded biochar owing to stronger affinity of birnessite toward contaminants and enhanced synergistic reactions between cations and anions. Corn stalk–based biochar modified with KMnO₄ removed Cd efficiently (191.9 mg/g) enabled by greater active sites [107]. Nanomanganese oxide–modified coconut shell–derived biochar removed EDTA-Cu(II) from aqueous solutions (assisted by Fenton reaction) with removal as high as 94.7%, enabled by increased surface area, pore volume, and hydroxyl radicals [108]. FeOx-modified bone-derived biochar (pyrolysed at 450 °C) and MnOx-modified bone-derived biochar (pyrolysed at 600 °C) removed Cd (40.59 mg/g and 53.12 mg/g, respectively), Cu (56.25 mg/g and 74.78 mg/g, respectively), and Pb (178.58 mg/g and 215.03 mg/g, respectively), with greater removal in MnO-modified biochar primarily enabled by electrostatic attraction in negatively charged surface [109]. Zhou et al. [110] reported an increase in pH from 10.0 to 11.0 when corn straw–derived biochar was modified with nano-MnO₂, which eventually improved its removal efficiency (Table 2).

Bi-Modified Biochar

Bismuth is called “the wonder metal” due to easy chemical combinations (facilitated by electrons in p-orbital). Bi-modified biochar is prepared by mixing biomass with Bi₂O₃ followed by pyrolysis. Bi-mixed biomass could also be mixed with HCl (stirred at 80 °C and sonicated) followed by pyrolysis. The obtained sorbent is washed with distilled water and NaHCO₃. Bi-impregnation enhances SA and contaminant Qe. Bi₂O₃ inhibits tar formation, which prevents pore blockage and micropore collapse of the biochar [111]. Bi₂O₃-doped H₂O₂-modified horse manure–based biochar (carbonised at 500 °C) showed a high U(VI) removal efficiency (93.9%) with Qe = 516.5 mg/g, where removal was enabled by precipitation, electrostatic attraction, reduction, ion exchange, and surface complexation [112].

Zn-Modified Biochar

Zinc modification makes surface of biochar positively charged, which facilitates high anion sorption. Zn modification is achieved by treating biomass/biochar with ZnCl₂, Zn(NO₃)₂, or ZnS, which loads ZnO or ZnS nanoparticles on surface and increases SA and porosity of sorbent. ZnCl₂-modified biochar was found superior to KOH, NaOH, AlCl₃, and FeCl₃ modifications, when considering SA, porosity, and RE [113]. ZnO-impregnated sawdust-based biochar possessed newly formed –OH groups along with large surface area (518.54 m²/g), which helped in removal of Pb (150 mg/g) and p-nitrophenol (170 mg/g) from wastewater [114]. Glue residue–based biochar modified with ZnCl₂ was compared with HCl and KOH modification by Shi et al. [115], who reported that although Zn modification increased SA less than KOH modification (85.93 m²/g, 860.45 m²/g, and 694.03 m²/g for HCl, KOH, and ZnCl₂ treatments), it showed Qe higher than HCl and KOH treatments (48 mg/g, 65 mg/g, and 80 mg/g for HCl, KOH, and ZnCl₂ treatments). Water hyacinth–derived biochar modified with ZnO nanoparticles removed 95% Cr (43.48 mg/g), chiefly assisted by precipitation between Cr ions and ZnO and photo-generated electrons generated by ZnO nanoparticles which reduce Cr(VI) to Cr(III) [116]. Gan et al. [117] modified sugarcane bagasse–based biochar with ZnCl₂ which showed pH_zpc (zero point charge) at 1.9 and the maximum Cr removal was observed at pH 2.0, attributed primarily to enabled by the formation of more polymerised chromium oxide species and stronger interaction between negatively charged chromate ions and positively charged functional groups of biochar at lower pH.

La-Modified Biochar

Lanthanum is a non-toxic, low-cost, and chemically stable element, which provides large number of coordination sites and augments affinity for anions. La-modified biochar is prepared by dipping biomass in LaCl₃ solution followed by pyrolysis. Modification could be accompanied with NaOH/ethanol treatment or the biomass could be spun in sodium alginate solution. Lanthanum modification deposits La₂O₃ on surface, which assists in contaminant removal. La-modified walnut shell–based biochar (walnut shell powder, stirred in 0.5 mol L⁻¹ LaCl₃ solution for 2 h and pyrolysed at 400 °C) was reported as good promoter of increasing P adsorption of soil (increased Qe from 0.171 to 0.421 mg/g after amendment) where surface precipitation, ligand exchange, and electrostatic attraction were involved in adsorption [118]. La carbonate-modified and La hydroxide-modified wheat straw–based biochar were compared for P adsorption and similar Qe (64.3 and 65.0 mg/g, respectively) was obtained for both but selectivity of the latter for P (90–100%) was more than the former (73–99%) [119]. Xu et al. [120] used cattail plant–based biochar modified using La to remove phosphate (Qe = 36.06 mg/g) and observed that strong acid or alkali solutions could be effectively used for successful desorption (92% efficiency).

Mg-Modified Biochar

Magnesium is an essential element involved in synthesising chlorophyll, has low toxicity, and sorbs contaminants. It is considered a suitable cation for anion removal (e.g. As) and recovery (e.g. P). Mg-modified biochar is prepared by dipping biomass/biochar in Mg(OH)₂ or MgCl₂ and could be accompanied by acid/base or electrochemical treatment. Mg modification augments RE. Sugarcane-derived biochar coated with MgO removed Cr effectively (54.64 mg/g), and the Qe increased in H₂SO₄-assisted modified biochar (62.89 mg/g), chiefly enabled by chemical interactions between MgO and Cr [121]. Mg-modified corncob-based biochar showed 2.36–9.34 times metal sorption (Qe for Cu, Cd, and Pb is 182.7, 83.5, and 308.2 mg/g) than pristine biochar, chiefly assisted by ion exchange, complexation, cation π-bonding, and surface precipitation [122]. Mg-alginate/chitosan-modified biochar demonstrated high PO₄³⁻ sorption (Qe = 46.56 mg/g) assisted by high SA (116.2 m²/g) [123]. Mg-modified corn-based biochar (300–600 °C pyrolysed) recovered P (Qe = 239 mg/g) from swine wastewater, which released 3.3–4.4% P during desorption in each cycle [124].

Co-Modified Biochar

Cobalt-modified biochar is prepared by treating biomass with Co(NO₃)₂·6H₂O followed by pyrolysis. The modification could be accompanied with HNO₃, HCl, ethanol, or microwave treatment to enhance biochar properties. Co modification enhances surface morphology and boosts spontaneity of contaminant sorption. Magnetic CoFe₂O₄‐modified banana pseudostem fibre-derived biochar demonstrated high amoxicillin sorption (99.99 mg/g) over a wide pH and temperature range with dominant monolayer chemisorption, π-π stacking, H-bonding, and electrostatic interaction [125]. Wang et al. [126] modified bamboo biochar with cobalt to remove Cr and observed a decrease in pH_zpc from 5.8 to 3.1 after modification, primarily resulting from the neutralised or blocked protonated surface functional groups by the coated polymer layer, which eventually increased the Qe from 8.40 to 45.45 mg/g.

Ni-Modified Biochar

Nickel modified biochar is prepared by treating biomass with NiCl₂ followed by pyrolysis. The treatment could be accompanied with HNO₃ addition, microwave heating, or ultra-sonication [127]. Zhu et al. [128] added N-doped carbon nanotube (CNT) to biochar matrix along with Ni and KOH treatment. Ni modification increases surface properties and contaminant sorption. Diazotisation-modified Se-rich fir-based biochar was converted into NiS/NiSe/3D porous biochar (via vulcanisation and in situ competitive hydrogen reduction), which showed excellent As(III) removal performance from water (100% removal within 110 min) assisted by adsorption and photocatalysis [129]. Wang et al. [127] customised bamboo biochar with nickel to remove Pb, where modification decreased the pH_zpc from 5.75 to 4.72, possibly due to abundance of acidic O groups on surface, which eventually augmented the Qe from 25.0 to 142.7 mg/g after modification.

Ca-Modified Biochar

Calcium is essential for plant growth. Ca-modified biochar is prepared by immersing biomass in CaO, CaCO₃, and CaCl₂ followed by pyrolysis. The treatment could be accompanied with Fe₃O₄ or alginate modification [130, 131]. Addition of Ca helps in increasing pH and negative surface charges, which eventually augments contaminant sorption. Indian pokeweed (Phytolacca acinosa)-derived biochar was treated with CaCl₂ and Na₂CO₃ via vacuum impregnation method to deposit carbonate ions on surface, which helped remove ~ 100% Cd (154.54 mg/g), chiefly being enabled by minerals (89–98%) and Cd − π binding (2–10%) [132].

Mo-Modified Biochar

Metal removal could be enhanced by modification of surface using metal hybrids such as MoS₂. Decoration of surface with MoS₂ exposes S atoms and increases surface area and number of active sites [133]. Corn straw–derived biochar was modified with iron nitrate and ammonium tetrathiomolybdate to increase its surface area and functional groups (O-, C-, S-, and Fe-containing), and the composite removed Cd effectively (139 mg/g) > 7 times more than pristine biochar (17.8 mg/g) enabled by electrostatic attraction, Cd-π interaction, and strong Cd–S complex formation [133]. In a later study, they identified Cd–S complexation (61.7%) and Cd–O bonds (38.3%) as major players in Cd removal [134]. Sawdust-based biochar modified with ammonium tetrathiomolybdate using solvothermal reaction was analysed for Pb removal, where the S groups helped increase Qe from 32.6 mg/g in pristine biochar to 189 mg/g in MoS₂-modified biochar [135].

Multi-modified Biochar

Biochar has been modified with multiple organic and inorganic agents to improve its properties. Ca and Mg were loaded on biochar, which helped in sorbing P from biogas fermentation liquid/wastewater (Qe = 326.63 mg/g) (later recovered for fertiliser application) and the presence of CaO and MgO nanoparticles minimises negative influence of coexisting ions on sorption [136]. MgO- and Fe₂O₃-impregnated biochar removed nitrate [137], while chloride (NH₄Cl, ZnCl₂, and HCl)-activated biochar removed mercury [138]. Polydopamine (hydrophilic polymer with self-polymerising property, enhanced interfacial interactions with nano-ZVI, and –OH assists in dispersing nano-ZVI in adsorbent) and nano-ZVI-modified biochar removed tetracycline [139]. Corn straw–derived biochars pre-treated with FeCl₃ and KMnO₄ solutions were found to remove 91.79% Cr (118.03 mg/g) in comparison to 32.17% by pristine biochar, chiefly enabled by 3 times SA [140]. Similarly, Fe–Mn-modified Pennisetum straw-derived biochar efficiently removed Cd (95.23 mg/g; Qe of pristine biochar was 30.58 mg/g), primarily assisted by cation exchange, Cd-π interactions, complexation with surface functional groups, and precipitation with minerals [141]. Fe–Zn-modified Robinia pseudoacacia and durian shell–derived biochar (post-treated with Fe(NO)₃.9H₂O and ZnSO₄.7H₂O) demonstrated 5 times (98.58 mg/g) and 3 times (161.14 mg/g) improved Qe when compared to pristine biochar (18.18 mg/g and 54.11 mg/g for Robinia pseudoacacia and durian shell–derived biochar, respectively) [142].

Moreover, several studies have combined different types of treatments (physical, chemical, or impregnation) to capture the benefits of multiple treatments for enhancing the efficacy of contaminant removal (Table 4). Zhu et al. [143] modified wheat straw–derived biochar with Bi₂O₃ mixed with HCl followed by sonication to remove Cr, P, and As with 12.2 mg/g, 16.2 mg/g, and 125 mg/g adsorption capacities. Hu et al. [144] pyrolysed Camellia oleifera shells with nitrates of cobalt and gadolinium, treated the biochar with 0.1 M HCl, and rinsed it alternately with ethanol and ultrapure water. The modification introduced spongy pore structure in biochar and newly generated sorption sites which enabled removal of ciprofloxacin and tetracycline accomplished with adsorption capacities of 44.44 mg/g and 119.05 mg/g, respectively. Mohan et al. [145] modified oak wood and bark with salts of Fe³⁺/Fe²⁺ followed by NaOH treatment. While the Fe treatment introduced magnetic properties and decreased porosity, NaOH modification helped improve the porosity of the biochar. While oak bark–based biochar showed 30.2 mg/g Pb and 7.4 mg/g Cd adsorption capacity, oak wood–based biochar showed 10.13 mg/g Pb and 2.87 mg/g Cd adsorption capacity. Frišták et al. [13] pre-treated grape seed–based biochar with HNO₃ followed by Fe(NO₃)₃ post-treatment. While the acid treatment augmented external, internal surfaces, and porosity, iron treatment clogged the pores and blocked the inner sphere pores but Fe impregnation also added − NO₃ functional groups, which eventually improved the As adsorption capacity from 5.08 to 34.91 mg/g. Various mixed combination modifications are summarised in Table 4.

Other Modifications

Microwave Pyrolysis Modification

Conventional pyrolytic methods are associated with extended production time, fast firing, outside to inside heating (energy transfer through radiation, conduction, and convection), and high synthesis cost [190]. On the contrary, microwave pyrolysis involves ~ 1/2 production cost, indirect internal heating (uniform energy transfer through dipole rotations and ionic conduction), and high energy efficiency. However, microwave pyrolysis is associated with inefficiencies such as minimal scaling-up of technology, limited penetration capacity of microwaves, and large-scale production costs [191, 192]. Nevertheless, previous investigations have analysed microwave-assisted pyrolysis, where biochar was used for removing contaminants (Table 5).

Table 5 Other modifications of biochar

Electro Modification

Application of electric field enables efficient and homogeneous deposition of metal oxide on biochar surface by evenly depositing electrolyte and electrode (metal) ions. For electro-modification, current density of 93.96 mA/cm² (0–12 A and 0–100 V) is applied to a biomass using a power supply (Al electrode SA, 75.56 cm²; electrode distance, 5 cm) in an electrolyte solution (NaCl or MgCl₂.6H₂O), followed by pyrolysis. Electro-modification generates strong oxidation agents (OCl⁻ and HOCl) or minerals on surface (like MgO, MgAl₂O₄, AlOOH, and Al₂O₃) from electrolyte which increases SA, microporosity, and RE [165, 193].

Ultrasonic Modification

Ultrasonic treatment could be performed before/after pyrolysis to increase porosity and hasten loading of metals or metal oxides. During ultrasonic treatment, biomass/biochar is placed in ultrasonic bath and provided 40–170 kHz of frequency and 250–1000 W of ultrasonic energy at 20–80 °C for 15–120 min [194]. The treatment is generally accompanied with other modifications, such as Ti-impregnation [195], Bi-loading [143], Fe-loading [196], ZVI-complexation [197], or Mn-impregnation [106].

Ozone Modification

Ozonation has been suggested to increase O:C ratio to augment cation exchange capacity and maintain poly-aromaticity for high biochar stability. Ozone-modified biochar is prepared by exposing biochar to ozone through ozone generator flushed with 3 L/min oxygen flow for 5 min at 25 °C [194]. Jimenez-Cordero et al. [198] exposed grape seed–derived biochar (flash pyrolysed at 800 °C) to ozone and elevated temperature (850–950 °C) in inert atmosphere to desorb the formed O-containing groups, increase the SA, and decrease the pore size (increased micro-porosity). Huff et al. [199] treated pine-derived biochar with ozone to improve its cation exchange capacity and observed a reduction in pH from 7.30 to 5.28 after 90 min ozone treatment, possibly due to the addition of acidic oxygen-functional groups on surface, while the cation exchange capacity doubled from 15.39 to 32.69 cmol/kg.

Carbonaceous Modification

Carbonaceous materials (e.g., CNT and graphene) could be utilised for biochar modification, which transfers significant properties of carbonaceous materials to biochar, decreases production cost, and proliferates their contaminant RE. Biochar is modified with graphene oxide by stirring biomass in graphene suspension for 60 min (homogenised via ultrasonic treatment) followed by pyrolysis. Graphite could be used to synthesise graphene oxide nanosheets through modified Hummers method involving H₂SO₄ and KMnO₄ [200]. On the other hand, CNT modification is achieved by stirring biomass in CNT suspension for 60 min (homogenised via ultrasonic treatment of 20 kHz at pulse intervals of 12 min) followed by pyrolysis. These modifications increase the contents of O-containing functional groups and SA. Carbonaceous modification could be accompanied with other treatments (e.g. sodium dodecyl-benzenesulfonate dispersion) to facilitate superior properties. Corn stalk mixed with graphene was pyrolysed at 350 °C to prepare biochar to remove Cd from aqueous solutions and it was observed that addition of graphene increased sorption capacity 1.26–2.36 times compared to pristine biochar, primarily enable by physisorption, complexation, and ion exchange [201]. Sawdust mixed with graphene oxide pyrolysed at 700 °C to fabricate the composite was used to remove Cd and it was observed as an effective sorbent of Cd (Qe = 55.68 mg/g), with adsorption following pseudo-second-order kinetics and Freundlich isotherm and column study following Admas–Bohart model [202]. Inyang et al. [203] modified hickory and bagasse bichar with CNT and sodium dodecylbenzenesulfonate to improve the Qe of lead and sulfapyridine, where the pH decreased from 7.25 to 6.74 in hickory biochar and from 6.94 to 6.72 in bagasse biochar, possibly enabled by the addition of acidic functional groups on surface.

Chitosan Modification

Chitosan is derived from chitin and contains abundant free –NH₂, –OH, and hydrogen bonds (between the main chains in secondary structure), which provides ion exchange, chelation, and contaminant sorption ability. Chitosan-modified biochar is prepared by stirring biochar in 2% acetic acid solution containing chitosan for 30 min at 40 °C followed by addition of glutaraldehyde and suspension into NaOH solution for 1–12 h [204, 205]. The treatment could be accompanied with addition of ZVI to enhance its RE [148]. Walnut shell–derived biochars were modified with beta-cyclodextrin–chitosan to remove Cr from aqueous solutions, and an RE of 93% was achieved (Qe = 206 mg/g), primarily enabled by electrostatic attraction of HCrO₄⁻ and Cr₂O₇²⁻ to positively charged surface and complexation with –NH₂ and –OH functional groups [206]. Rice straw–derived biochar (450 °C pyrolysed) was modified with chitosan and pyromellitic dianhydride to increase amide and carboxyl groups, which could strongly interact with contaminants via electrostatic attraction, ion exchange, and complexation [207]. Modification improved the RE by ~ 10% with Qe of 8.62 mg/g, 25.78 mg/g, and 71.40 mg/g for Pb, Cd, and Cu, respectively. Chitosan-combined magnetic Loofah sponge-based biochar removed Cr (30.14 mg/g) and Cu (54.68 mg/g) efficiently, mostly assisted by ion exchange and surface complexation, despite the decrease in SA (from 337.35 to 96.91 m²/g) [208].

Clay/Silt/Silica Modification

Biochars could be loaded with clay minerals, such as bentonite, montmorillonite, or kaolinite, to modify its composition and properties to increase sorption of oxyanions, like PO₄³⁻, and polyatomic cations, like NH₄⁺. Typically, biomass is mixed with a suspension of ultrasonicated (form homogenisation) clay minerals, stirred for 60 min and pyrolysed to obtain amorphous biochar with crystalline clay minerals. Additionally, higher pyrolytic temperatures evaporate water molecules from clay minerals, which reduces pore volume (PV) (and pore diameter) [17]. The treatment is enhanced by activation under CO₂ flow for greater microporosity. Rawal et al. [209] pyrolysed bamboo biomass, pre-treated with iron sulphate-clay mixture, at 250–550 °C to obtain clay-modified biochar with enhanced PV and mineral infusion into pores (high content of S, Fe, and Al). Yao et al. [210] reported > 15% enhanced green pepper yield, > 1 0% increased vitamin C content, and ~ 1/3rd decreased nitrate content after amendment with wheat straw–derived biochar (pre-treated with urea, minerals, and H₃PO₄) compared to conventional fertilisers. Oiltea camellia shell–derived biochar modified with silicon was observed to possess augmented surface area (45–112%) and porosity (5–12%) and Cd removal was enabled by complexation with –COOH and C–Si–O groups, surface precipitation (CdCO₃, CdSiO₃, or Cd₂SiO₄), coordination with π electrons (C = C), and ion exchange with Na⁺ [211]. Rosin-based bentonite-coated Fe₂O₃ nanoparticle-supported biochar removed 95% Cr(VI) within 1 min (81.7 mg/g) mainly due to the key role played by α-Fe₂O₃ nanoparticles [212]. Cigarette factory waste–derived biochar modified with bentonite and calcite minerals removed Pb efficiently (99%), with bentonite and calcite catalysing changes in yield, carbonisation, minerals, functional groups, texture, and pH [213]. Siltstone-nanomagnetite-modified Eleocharis dulcis–derived biochar demonstrated high Cr Qe (35.57 mg/g), assisted by electrostatic attraction [214]. Montmorillonite-modified wheat straw–based biochar showed high norfloxacin Qe (increased from 10.58 to 25.53 mg/g after modification), enabled by pore filling, H-bonding, and electrostatic interaction [215]. They also stated that presence of humic acid and Cu²⁺ could reduce norfloxacin sorption due to competitive adsorption and pore blockage. Montmorillonite-modified corncob-based biochar removed Pb (139.78 mg/g) and atenolol efficiently (86.86 mg/g) enabled by hydroxyl O atom acting as a possible reaction site and amino N and amide O atom providing lone pair of electrons, which generates H-bond or strong electrostatic interactions with surface functional groups [216].

H₂O₂ Modification

Hydrogen peroxide (H₂O₂) is a clean and less-expensive product proposed to modify biochar for high sorption. Typically, H₂O₂-modified biochar is prepared by placing biochar in H₂O₂ solutions (1–30%) and stirred for 120–180 min followed by filtration, washing, and drying. H₂O₂ modification was compared with H₂SO₄, HNO₃, and KOH modifications, where H₂SO₄ and HNO₃ treatments caused porosity loss and heterogeneous micropore distribution, while H₂O₂ and KOH treatments enhanced homogenous micropore distribution, micro-PV, and SA. All the treatments enhanced acidic surface functional groups, which enabled biochars (rice straw derived) in removing contaminants like methylene blue, phenol, and iodine [56, 217]. Wongrod et al. [45] reported decreased As removal after H₂O₂ and KOH modification of biochar (while H₂O₂ oxidised organic content, KOH dissolved ash). Pig manure–derived biochar was oxidised using H₂O₂ (oxidation occurred at C = C bonds) and thiolated using 3-mercaptopropyltrimethoxysilane (sulphur content increased to 4.43%), where sorption capacity of biochars augmented by 2.5–4.2 times for Cd and Pb with increase in pyrolysis temperature from 300 to 700 °C [218]. Metal removal was enabled by cation exchange, complexation with functional groups, cation-π EDA interaction, and precipitation with minerals.

Organic Solvent/Compound Modification

Biochar could be modified with organic solvents such as methanol and thioglycolic acid to increase the content of surface functional groups (especially carboxyl and carbonyl groups). However, the volatile nature and high costs associated with the organic solvents limit their real-time large-scale applicability for biochar modification. Previously, organic solvent-modified biochar was used to remove contaminants (Table 5) [219, 220]. Organic compounds like thiourea, polyaniline, and poly(vinyl alcohol) (PVA) have been used to modify biochar. Swine sludge–derived biochar was pre-treated with NaOH and HNO₃ and modified with thiourea to augment C–O, C = O, C–S, C = S, and RSO₃⁻ groups, which enabled high Pb removal (Qe = 143.13 mg/g) [221]. Rice husk– and Eucalyptus saw dust–based biochar was modified with polyaniline to graft NH₂ groups of polyaniline directly on surface, where an increase in SA (38.7–41.3 m²/g) and Cr removal (72.31–81.43%) was observed [222]. Rice straw–derived biochar was aminated with poly(vinyl alcohol), epichlorohydrin, and diethylenetriamine to augment surface functional groups (such as –OH) and Cr(VI) removal (Qe = 140.39 mg/g) [223]. KMnO₄-modified corncob-based biochar was entrapped/immobilised in poly(vinyl alcohol)/sodium alginate hydrogel beads to study Cu removal and an increase in Qe was observed (23.70 mg/g in pristine biochar; 87.07 mg/g after modification), probably due to inner-sphere complexation (Cu–O, Mn–O–Cu bonds) [224]. 3-aminopropyltriethoxysilane and polyamidoamine dendrimer-modified magnetic rice straw–derived biochar removed Cu efficiently (251.81 mg/g) mainly due to electrostatic attraction and bridging influences with –NH₂ groups, despite the decrease in SA [225].

Nitrogen Modification

Biochar is modified with N by recapturing N from wastewater (polar functional groups enable sorption in low temperature–synthesised biochars; high SA enables sorption in high temperature–pyrolysed biochars). Biochars could recover nitrates and ammonium from human urine present in wastewater treatment plants [226]. Polyethylenimine and methanol were used to augment N- and O-containing groups (primarily amino groups) in rice husk–derived biochar to sorb Cr (Qe = 435.7 mg/g) [227]. Stillage-derived biochar was loaded with N-containing phosphates (urea phosphate, ammonia polyphosphate, and ammonia phosphate) to augment its porosity (0.464 cm³/g) and SA (798 m²/g), which sorbed toluene effectively (Qe = 496.2 mg/g) [228]. Saw dust–derived biochar was treated with H₂SO₄, HNO₃, NH₄OH, Na₂S₂O₄, and glacial CH₃COOH to furnish amino groups, which helps sorb Cu²⁺ (Qe = 16.11 mg/g) [229]. Rice straw–derived biochar was modified with iron and (3-aminopropyl)-triethoxysilane to enhance Fe and NH₂ radicals to augment Cr (100.59 mg/g) and Zn removal (83.92 mg/g) [230]. Waste-derived biochar (municipal waste, oak wood, anaerobic digestate press cake, and greenhouse waste; pyrolysed at 450–650 °C) sorbed ammonium (Qe = 146.4 mg/g) and phosphate (Qe = 37.1 mg/g) [231]. Takaya et al. [231] also reported minimal desorption of ammonium (5.1 mg/g) and phosphate (8.5 mg/g), which could enable its usability as slow release soil amendments.

Sulphur Modification

Sulphur modified biochar could be prepared by sorbing H₂S produced during anaerobic digestion of biomass for bioenergy generation. Dairy manure–derived biochar adsorbed H₂S generated during anaerobic digestion of organic waste and obtained S-modified biochar (36.5% S) [232]. S modification could also be achieved by surface modification with reagents like thiols, CS₂, or SO₂ [233, 234]. S-modified biochars immobilise Hg in soils, primarily enabled by the strong affinity of S for Hg. Rice husk–derived biochar modified with S (13.04% S) minimised Hg contamination by ~ 73% (Qe = 67.11 mg/g) primarily by forming HgS [235]. Na₂S-modification of corn straw–based biochar enhanced Hg²⁺ sorption by 76.95% and atrazine sorption by 38.66% [76]. Oilseed rape straw–derived biochar (with enhanced S) immobilised methyl-Hg by complexation and reduced methyl-Hg accumulation in rice grains [236]. Corncob biochar modified with Na₂S showed best Ni removal (15.40 mg/g), SA (195.64 m²/g), and PV (0.2340 cm³/g) among modified crayfish shell, cotton stalks, corncob, and peanut shell-based biochars, chiefly enabled by ion exchange [237].

Living Feedstock Modification

There was a unique and interesting study where plants were irrigated with metal-rich water to enhance the contents of essential nutrients in plants which could boost its yield. Yao et al. [238] irrigated tomato plants with 25 mM Mg solutions to obtain Mg-enriched tomato tissues. Later, it was pyrolysed to produce biochar, where pyrolysis augmented Mg concentrations in the sorbent (8.8% Mg). The presence of MgO and Mg(OH)₂ provided greater removal of phosphate (88.5%).

Microbe Modification

The property of high SA enables biochar in facilitating the growth and colonisation of microorganisms on its surface. The colonised microbes develop a biofilm on surface via secretion of adhesives (polymers), which facilitates stronger viability (due to protection provided by biofilm). The inoculation and colonisation of microbes aids in degrading organic contaminants and biosorbing metals [17]. Microorganism-colonised (Streptomyces violarus strain SBP1) H₂O₂-modified wood waste–derived biochar effectively removed ~ 74.8% Mn(II) and oxidised it into lesser toxic Mn(III) and Mn(IV) [239]. Since biofilm-associated biochar could assist in removal of contaminants, it was suggested for application as filters and treatment of polluted water, wastewater, and soils [240].

Mechanistic Insights into Contaminant Removal

Different models have been proposed to identify processes involved in adsorption, where adsorption capacity of contaminants is plotted against initial concentration, contact time, and temperature to diagnose the best fitting isotherm, kinetic, and thermodynamic model. Langmuir and Freundlich are the most used adsorption isotherm models [256,257,258], while Lagergren pseudo-1st order, pseudo-2nd order, intra-particle diffusion, and Elovich are the most used adsorption kinetic models [259,260,261]. Thermodynamic parameters like entropy, enthalpy, and Gibbs free energy are calculated by plotting adsorption capacity versus temperature [262]. Previous studies have proposed the following mechanisms as chiefly involved in removal of contaminants (Fig. 2): (i) precipitation/co-precipitation, (ii) complexation (coordination, surface complexation), (iii) ion exchange (anion and cation exchange), (iv) physical adsorption, (v) specific adsorption (including chemical process), (vi) electrostatic interaction/attraction/attachment, (vii) reduction, (viii) π-bond interaction, and (ix) micro-electrolysis reaction [263]. For example, Cr (which exists as Cr(VI) and Cr(III), with + 6 state being more toxic) removal primarily involved electrostatic interaction, complexation, reduction from Cr(VI) to Cr(III) through surface functional groups such as –OH and –COOH and ion exchange with Al³⁺, H⁺, and Zn²⁺ [222, 264, 265]. Cd (existing as Cd(II) and hydrxo-complex) removal chiefly incorporates complexation with functional groups (O-, C-, S-, and Fe-containing) and ion exchange with Ca²⁺, Na⁺, and K⁺, apart from physical adsorption and electrostatic attraction [133, 141, 201, 265]. As (existing predominantly in H₂AsO₄⁻ and HAsO₄²⁻ at 3–6 pH and AsO₄³⁻ at alkaline pH) removal mainly involves electrostatic interactions (with –OH and –COOH groups on surface) and complexation/precipitation (with metals and metal oxides on surface), apart from redox transformation, ion exchange, and H-bonding [266]. Pb (primarily prevalent as Pb(II)) removal involves complexation with C–O, –OH, –SH, and C–S functional groups (mainly inner-sphere surface complexation), chemisorption with ion-exchange groups (R–SH and –RSO³⁻) and coordination groups (–COO, –C = S, –C–NH₂) [135, 221]. Cu (occurring as Cu(I) and Cu(II), with + 2 being more harmful) removal incorporates complexation with surface functional groups (such as –COOH, –OH, and –NH₂) [229]. Removal of Pb(II) and Cd(II) involved ion exchange, specific adsorption (Pb/Cd–O or hydroxyl binding), and electrostatic attachment [170]. When competitive adsorption of Pb(II), Cd(II), and Cu(II) was investigated, differential role of functional groups was observed, with N–C = O group majorly involved in Pb(II) removal, C = C and N-containing groups involved in Cd(II) removal, and carbonyl and N-containing groups involved in Cu(II) removal [207].

Fig. 2

Contaminant removal by modified biochar

pH is a crucial factor influencing the removal of contaminants by modified biochars. For example, heavy-metal ions precipitate in alkaline environment, thereby affecting the Qe of modified biochars. The contaminants form complexes with OH⁻ or H⁺ under alkaline or acidic environments, which affect the RE of modified biochars [263]. Changes in surface functionality also affect the acidic/alkaline properties of modified biochars, eventually affecting contaminant removal. pH at zero point charge (pH_zpc) affects the surface charge of modified biochars by changing the protonation effect of surface functional groups of modified biochars in acidic conditions. While a pH lower than pH_zpc makes the surface of modified biochars positively charged and attracts anionic contaminants, a pH higher than pH_zpc makes the surface negatively charged, augmenting the electrostatic attraction of cationic contaminants. Solution pH is another factor governing the removal of contaminants by modified biochars. For example, an increase in solution pH from 2 to 8 augmented the removal of Cd by MgCl₂-modified biochar from 47.17 to 98.30% [267]. In general, a decrease in pH of biochar after modification is observed. While Huff et al. [199] observed a reduction in pH (from 7.30 to 5.28) after ozonation, Wang et al. [126] and Wang et al. [127] reported a reduction in pH_zpc of biochar after modification with nickel (from 5.75 to 4.72) and cobalt (from 5.8 to 3.1), respectively, which increased Qe from 25.0 to 142.7 mg/g and from 8.40 to 45.45 mg/g, respectively. However, an increase in pH has also been reported after modification, like an increase from 10 to 11 after modification with nano-MnO₂ eventually improving removal efficiency as reported by Zhou et al. [110].

Surface properties of modified biochar (SA, charge, and functionality) are involved in removal of emerging organic contaminants [18]. Chemical modification generates abundant adsorption sites, making the surface favourable for surface precipitation, surface complexation, and electrostatic attraction, while physical modification influences SA and microporosity, which affects mechanisms like intra-particle diffusion and pore filling [268]. H₃PO₄-modified pig manure–derived biochar removed tetracycline primarily through π-π electron donor–acceptor interaction and hydrogen bonding [269]. Chitosan and Fe/S-modified sludge-based biochar removed tetracycline primarily using π-π stacking, hydrogen bonding, silicate bond interaction, and pore filling, but also involved chelation and ion exchange, which rarely occurs during adsorption of organic contaminants [270]. KOH-treated potato stem and leaf-derived biochar adsorbed ciprofloxacin via π-π interactions, electrostatic interaction, and hydrogen bonding [271]. Magnetised pine sawdust–based biochar adsorbed ethinyl estradiol and sulphamethoxazole primarily through π-π electron donor–acceptor and hydrophobic interaction [272]. Magnetic bagasse biochar nanoparticles removed 17β-estradiol chiefly via simultaneous hydrophobic interactions (dominating at lower pyrolysis temperatures) and π-π electron donor–acceptor interactions (dominating at higher pyrolysis temperatures) [273]. Ca- and Fe–Mn-modified litchi-derived biochars were suggested to effectively control estrone/estrogen in water and soil environments, where immobilisation was primarily achieved through hydrogen bonding interaction [274]. Magnetic Fe2O3–modified banana peel–based biochar effectively degraded bisphenol-A (completely removed within 20 min), without pH adjustment, through hydrogen bonding and involvement of surface functionality [275]. Bamboo-derived biochar supported CuZnFe₂O₄ composite removed bisphenol-A and sulphamethoxazole through hydrogen bond interaction, hydrophobicity, and π-π interactions [276]. Cotton straw–based biochar removed sulphonamide primarily via van der Waals and hydrophobicity [277]. Tylosin removal using novel goethite biochar was assisted by hydrophobic, π-π electron donor–acceptor electrostatic, cation exchange, and H-bonding interactions, with best fitting of Henry and Freundlich models [278]. KOH-treated wild plum–derived biochar modified with microwave treatment (700 W for 12 min) removed naproxen efficiently (73.14 mg/g) with maximum assistance from electrostatic attraction between positively charged –OH groups on surface and negatively charged naproxen [279]. NaOH-activated alfalfa-derived biochar superiorly removed tetracycline from water (302.37 mg/g), with prime involvement of chemisorption interaction and intra-particle diffusion [78]. MgAl-double lamellar hydroxides/bovine bone–based biochar composite completely removed caffeine from water within 20 min (26.22 mg/g), with best fitting of Redlich–Peterson, indicating that both monolayer and multilayer adsorptions are involved in the removal of caffeine [280]. Overall, electrostatic interactions, cation exchange, surface complexation, hydrogen bonding, and non-specific van der Waals interactions are involved in removal of organic contaminants [281].

Modified Biochar and Ageing

Biochars undergo ageing when applied in the environment, primarily under the influence of natural forces including temperature-induced freeze–thaw cycles, rainfall-induced wetting–drying cycles, sunlight-induced photochemical degradation, and mild forms of oxidation (arising from microorganisms, root exudates, or atmospheric oxygen) [282]. Ageing affects physico-chemical properties like aromaticity, elemental composition, surface morphology, and surface area, which could either increase or decrease removal of organic/inorganic contaminants by biochar. In general, chemically/biologically oxidised biochar removes pronounced amounts of inorganic contaminants, aided by higher surface complexation with O-containing functional groups and retained ash content, which furnishes contaminant co-precipitation. On the contrary, physically aged biochars demonstrate diminished contaminant removal because of lower mineral content and O-containing groups, which minimises complexation and co-precipitation. With respect to ageing, contrasting results of decrease in contaminant removal (Cu and Pb removal decreased due to wet-dry ageing, as reported by Shen et al. [283]) and increase in contaminant remediation (Cd and Pb removal increased by KOH-modified biochar, reported by Wang et al. [284]) after application of biochar has been reported previously. Such contrasting reports could be emanating from dissolved organic matter–induced contaminant mobilisation or O-containing group-supported contaminant immobilisation [285, 286]. Biochar ageing favours microbial degradation of organic contaminants, where biochar acts as electron shuttle between organic contaminants and microorganisms colonised on external and internal surface and with progressive ageing, enhanced O-containing groups favour electron shuttle effect.

Huang et al. [287] prepared Ce/Mn-modified wheat straw biochar to remove As(V), where the team assessed the influence of three different ageing processes (natural, freeze–thaw, and dry–wet cycles) on the adsorption performance of modified biochar. Interestingly, freeze–thaw ageing increased the SA of modified biochar (214.98 m²/g) more than the other two ageing processes. However, pH and C contents reduced after ageing, but contents of H and O increased after ageing. Qe augmented by 16.2% and 10.6% for freeze–thaw and dry–wet ageing treatments, respectively. Freeze–thaw and dry–wet ageing processes also activated Ce/Mn oxides in augmenting Mn^2+/3+ and Ce generation, eventually increasing CeAsO₄ precipitation and Ce/Mn–O–As complexation. In an interesting study, Wang et al. [288] prepared aged biochars by treating corn straw–derived biochar with 20% H₂O₂ at 1:20 (w/v) ratio for 24 h. The team observed a decrease in Cd and Pb RE attributable to a reduction in mineral precipitation and cation exchange mechanism. In another study, Cui et al. [289] simulated Fenton-like chemical ageing by oxidising peanut and bush biochar with citric acid/Fe₂O₃ and citric acid/FeCl₃. The team observed that ageing improved surface properties, surface functionality, and elemental content in biochars, which increased 2,4,6-trichlorophenol removal by 1–11% and 7–38% in aged bush and peanut shell biochar, respectively. The contrasting results trigger the need for performing extensive studies to examine the adsorption/degradation performance of modified biochars with respect to ageing. It is also noteworthy that production temperature could affect the adsorption performance of ageing biochar [290]. While high temperature biochars demonstrate lower Qe when exposed to acid rain leaching or oxidation (which remove inorganic alkaline elements and minimises pH), low temperature biochars display an increase in Qe, assisted by augmented O-containing groups (which promotes ion exchange and surface complexation). A major condensed aromatic carbon portion of biochars remain very stable, the labile portion still remains biodegradable [291].

Theoretically, biochars could immobilise contaminants for a very long term, but Shen et al. [283] reported metal immobilisation for a short term and mobilisation with an accelerated ageing simulated by a wet-dry-ageing experiment (for example, metals such as Cd, Cu, Pb, Ni, and Zn). Therefore, utilisation of biochar for immobilising contaminants in soils could pose long-term risks and might necessitate further enquiry for safer application. Martin et al. [292] compared un-aged biochars to 32-month-aged biochars and observed that removal of contaminants like diuron and atrazine minimised significantly in the latter by 47–68%. Similarly, Ren et al. [293] suggested that adsorption performance of biochar amended soils (for contaminants like atrazine and phenanthrene) reduced to levels of untreated soils after 30 months of application. However, Jones et al. [294] reported that 2-year ageing did not affect the RE of contaminants like simazine. A 3-year experiment by Li et al. [295] observed that hardwood-derived biochar minimised Cu and Cd concentrations by 63.8% and 57.9%, respectively in first year in soil, but the biochars increased mobilisation in the following years. On the contrary, corn straw–derived biochar minimised metal concentrations steadily throughout the experiment. Such a variation in results was attributed to the differences in lignin content. These observations signify that it is crucial to choose the appropriate kinds of feedstock to minimise the bio-availability and mobility of contaminants stably for longer periods in contaminated environments.

Feasibility and Limitations of Biochar Modification

The feasibility of modified biochars is dependent on a number of factors inclusive of the biochar properties, the costs of fabrication, the risks associated with their application, and the viability of its scalability (from lab scale to industrial scale). The physico-chemical properties of the modified biochars depend on the type of feedstock used, treatment temperature, and the modification method. An improved understanding of the properties would enable a better utilisation of the fabricated adsorbents for various applications including removal of contaminants. The calculation of costs of fabrication is complicated and depends again on the feedstock, treatment temperature, and the modifying agent. Waste biomasses, which are typically free, could be used to minimise the costs of production. However, assessment of costs becomes difficult, when factors such as treatment cost, transportation tariffs, labour charges, energy consummation, and operating and maintenance costs are included in the cost calculation, as these are typically the most crucial components of biochar preparation on a large scale [2]. Interestingly, Inyang and Dickenson [296] suggested that on an average unmodified biochar cost ~ 1/6th of the production costs of activated carbon (~ $1500/ton). Such a large difference in production costs could be used to advantage for the fabrication of modified biochars, where only an additional expense of the modifying agent or treatment condition would need to be incorporated. Most of the studies reviewed focussed on batch studies using modified biochars for contaminant removal, while the actual performance of the adsorbent needs to be examined with column/continuous setups incorporating real-time contaminated soil/water in addition to subjecting the adsorbent to multiple sorption-regeneration cycles. Additionally, recovery of the adsorbent and its stability after real-time application for removal of contaminants would be crucial in enhancing the feasibility of the fabricated adsorbent. Recovery and regeneration of adsorbent would critically depend on the mechanisms involved in contaminant sorption, where weaker binding of contaminant (via physisorption) to adsorbent would amplify the regeneration and reusability of adsorbent, when compared to a stronger bonding (involving chemisorption) between adsorbent and contaminants [2].

It must also be observed that different studies comparing different biochar modifications have reported a superior performance of one or the other biochar modification, which on the first instance could intricately confuse the research team in selecting the appropriate modification method. For example, while Wang et al. [50] reported alkaline modification to be superior to acid modification, Zhang et al. [60] reported the contrary superiority of acid modification for contaminant removal. However, Liu et al. [297] suggested that acidic modifications persist with issues of non-recyclability, higher pollution, and larger consumption of activation energy, while alkaline modifications are associated with advantages of higher biochar yield, lower activation temperature, and controllable void structure, which could be suggestive of the preferability of alkaline modifications, also re-affirmed by Lima et al. [20] and Hussain et al. [66]. It makes it crucial for the future research teams to compare the different modification methods for a better understanding of the feasibility and the limitations of its applications, especially removal of contaminants. Modifications such as steam and gas treatment, alkaline treatment, and carbonaceous modification improve the surface area and porosity of adsorbent. However, steam, gas, and carbon modification are associated with high costs, while alkaline treatments are much less costly and could favour their usability. Additionally, physical modifications consume huge amounts of energy, which increases the production costs and makes the fabrication non-greener. Different modifications influence the elemental composition of biochars differently, which affects the basic or hydrophilic nature of biochar [16]. For example, nitrogen-to-carbon ratio regulates basic nature of biochar, while oxygen-to-carbon ratio dictates the hydrophilic nature of biochar. Alkaline treatments when compared to acidic treatments increase basicity and aromaticity of biochar and decreased the hydrophilic nature by altering the carbon, oxygen, and nitrogen contents of biochar. Treatments involving alkaline and oxidising agents augment oxygen-containing functional groups on surface. While alkaline treatments augment hydroxyl and carboxylic groups, oxidising treatments increase carboxylic groups. However, it is also important that after modification, the acid/alkali solutions might need further treatment, which could eventually complicate the fabrication of biochar. In due course, acid/alkali solutions could be treated by neutralising it with alkali/acid solutions. Also, the costs of modifying agents need to be kept into perspective, which could limit its applicability. Nonetheless, cyclic utilisation of modifying agents could minimise the costs of fabrication. When compared to other treatments, metal impregnation enhances the active adsorption sites the most, thereby supporting the removal of contaminants the most. Still, previous reports have suggested leakage of metal ions from the adsorbent, which could influence the stability of modified biochars [19]. By and large, the selection of modification methods depends on the costs of fabrication or the feasibility of follow-up treatments required during modification.

Future Perspectives

• With evolving biochar treatment/modification methods, optimisation of feedstock variation, production condition, and modification route have become inevitable. Studies have been limited to laboratory setups and need proliferation for pilot-scale industrial and commercial application, inclusive of actual contaminated soil and water for determining real-time efficacy.

• Fewer studies have focussed on the removal of contaminants from simulated/industrial wastewater using modified biochars. Further, application of modified biochars for contaminant removal from large-scale industrial wastewater has not been reported. Lesser investigative analysis has been performed on mixed multi-contaminant systems and identification of the probable involved mechanisms could be crucial in the future studies.

• There is a need to investigate the stability of modified biochar composites. Long-term experiments would be needed to examine the bonding strength of composite and achieve a composite with long lifetime and high adsorption capacity under changing climatic scenarios [298, 299].

• Contrasting results were obtained in studies performed on the impact of ageing on adsorption performance of biochars. Moreover, minimal studies have analysed the adsorption behaviour of modified biochars exposed to ageing. Such a scenario triggers the need for performing extensive studies to examine the adsorption/degradation performance of ageing-modified biochars.

• Life cycle assessment of modified biochars should be performed to compare its applicability with sorbents, presently used at industrial setups, considering factors like contaminant RE, biochar stability, biochar reuse, fabrication cost (production and modification), and real-time applicability (transportation, labour involved, and maintenance).

Conclusions

Physical and chemical modification of biochar could enhance the contents of O-containing functional moieties and SA, which eventually assists in sorption of organic and inorganic contaminants. In general, acidic modifications enhance surface functional groups and cation exchange capacity, which promotes ion exchange and sorption of cationic and anionic contaminants. Alkaline modifications facilitate high aromaticity, apart from improving hydrophobicity and contaminant RE. Sorption of oxyanions could be escalated by metal impregnation in biochar. Metal-impregnated biochars sorb contaminants effectively with mechanisms involving chemisorption, electrostatic attraction, and complexation. Biochar could be facilitated with magnetic properties (via Fe modification) to ease its recovery. Pb could be effectively removed by metal/metal oxide-impregnated biochars because of the formation of stable inner sphere complexes. Cd could be better removed by Mn-modified or alkali-modified biochars because of the facilitation of surface basicity that favours electrostatic attraction with cations. Cu could be preferably removed by nitrogen-doped or metal/metal oxide-modified biochars because of the formation of N–Cu complexes. Cr could be efficiently removed by Fe-modified biochar because of the mild reduction reactivity possessed by ZVI, while Cr(VI) is reduced to Cr(III) by oxidising Fe to Fe(III) and formation of Cr(III) hydroxides. Hg could be removed by S-impregnated biochar, as it enables HgS precipitate formation apart from O-containing functional groups involvement in Hg sorption. As could be preferably removed by metal-modified biochars, primarily enabled by electrostatic attraction and formation of complexes with metals. Modified biochars (especially metal oxide and metal salt modification) show high removal of organic contaminants enabled by electrostatic interactions, cation exchange, surface complexation, hydrogen bonding, and van der Waals interactions. In view of contaminant removal and waste management, production of modified biochars from diverse agro-ecological waste could be critical for planet survivability. However, it would be beneficial to perform reality check of contaminant removal using modified and tailor-made biochars with real-time wastewaters and actual contaminated soils along with identifying routes of scaling-up the fabrication of low cost environment-friendly modified biochars and determining the influence of ageing on the stability and adsorption performance of the modified biochars.