Table 1 Various modification approaches of biochars, production temperature, pollutant removals from the water and soil systems, mechanisms, and their applications.
Biochar Feedstock | Pyrolysis temperature (°C) | Modification method | Target contaminant | Decontamination status | Mechanism involved | References |
|---|---|---|---|---|---|---|
Chemical modification | ||||||
 Peanut hull | 300 | H2O2 treatment | Cd, Ni, Cu, and Pb | increased Pb sorption from 0.88 to 22.82 mg g−1, which was higher than commercial AC | Increased oxygen-rich functional groups on the biochar surfaces | |
 Bamboo | 550 | Chemical oxidation (NaOH, HNO3) | Furfural | suppressed the sorption of furfural | A substantial amount of acidic functional groups on the adsorbent surface. Contrastingly, heat and NaOH modifications raised the basicity of adsorbent | |
 Municipal waste | 400–600 | KOH modification | Arsenic pentoxide | Increased 1.3 times adsorption rate than un-treated biochar | Enhance SSA and alter the porous structure, particularly functional groups on the surface of the modified adsorbent | |
 Pine-chips | 300 | NaOH treatment | Ibuprofen, Naproxen and Diclofenac | showed greater sorption efficiency | Large amounts of oxygen-enrich functional groups introduced on the surface of treated biochar | |
 Rice husk | 400, 500 | Treated by H2SO4 and KOH | Tetracycline | Shown better adsorption efficiency (58.8 mg g−1) compared to other biochars | owned larger SA than those of acidic-modified and pristine biochars | |
 Sawdust | 500 | Amino-treated | Copper (Cu) | Improved the sorption up to 5-folds and 8-folds for fixed-bed and batch experiments | Amino moiety strongly complexes with heavy metals because of the high stability constants of metal complexes | |
 Rice husk | 400, 500 | Methanol-treated | Tetracycline | Almost 45% heightening of removal capacity in 12 h and 17% at equilibrium | Due to alteration in oxygen-comprising functional groups | |
 Buttonwood waste | 400 | Modified by (Mg(OH)2) | Fe2+ | Greater removal capacities for treated biochar (84–99%) than by un-treated biochar (38–97%) | Mineral constituents e.g., silicate Mg(OH)2 and calcite in the biochars stimulate the oxidation of Fe2+ and form a precipitate of Fe3+ hydroxides | |
 Rice husk | 450 and 500 | Polyethylenimine treatment | Chromium | Highest removal capability of (435 mg g−1), it was better than Un-treated biochar (23.09 mg g−1) | The appearance of the amino- group stimulates the chemical reduction of chromium and enhances the removal capacity | |
 Walnut-chips | 600 | Carbon nanotube-coating | Methylene blue | Maximum removal capacity among all contaminants | Coated biochar has well thermal stability, greater SA, and higher pore volume | |
 Rice husk and fruit branches | 600 | Ferric coated | As (III) and As (V) | Enhancement of removal capacities | Interactions with FeOH2 and FeOH groups | |
 Sawdust and pine tree | 550 | H3PO4 modification | Fluoride | Substantial increase in removal performance modification | Increasing Fluoride sorption resulting from chemistry reaction and increased SSA | |
 Rice husk | 600 | Coated with silica | Pb | Improvement of removal capacities | A larger SSA observed after coating | |
 Wheat straw | 450 | Coated with Fecl3 and treated by HCL | phosphate and nitrate | Substantial increase of removal after HCl treatment and coating with Fecl3 | – | |
 Wheat straw | 300, 700 | Acid activation | Sulfamethazine | Noteworthy increase in SA and enhancement in the removal of sulfamethazine | – | |
 Bagasse | 600 | Modified by carbon nanotube | Sulfapyridine and Pb | Maximum sorption capacity observed | – | |
 Bamboo hardwood | 550 | NaOH modification | Cd | Highest cadmium sorption capacity | NaOH-treated adsorbent has more roughness compared to un-treated biochar | |
 Cow manure and wheat straw | 450 | HNO3 treatment | U(VI) | Showed the highest sorption capacities after modification, it was higher than un-modified biochar, Highest removal capacity by the treated wheat straw adsorbent exhibited an enhancement of 40 times | Due to a large number of surface COO groups, a great negative surface charge | |
 Swine manure and rice straw | 700 | H3PO4 modification | Tetracycline | Increased the TC removal capacity | Enhancement of the SSA, higher micropore, and total pore after treatment | |
 Poplar chips | 550 | AlCl3-modification | PO43−, NO3- | PO43−, NO3− removal significantly enhanced on Al-treated biochar | The surface area markedly improved with the Al content of the adsorbent. The C content of Al-treated biochar greatly decreased than pristine biochar | |
 Dairy manure | 300 | NaOH-modification | Cd, Pb | The highest removal capacities were 68.08 and 175.53 mg g−1 for Cd and Pb respectively. The sorption capacities of dairy manure biochar for Cd and Pb improved after modification | NaOH modification increased the SSA, amount of O-enrich functional group, and ion-exchange capacity of biochar | |
 Coconut shell | 800 | HCl + ultra-sonication | Zn, Ni, and Cd | Modified biochar showed the highest sorption capacities for heavy metals | Modified-biochar improved surface functional groups | |
 Corn straw | 500 | KOH | Atrazine, Hg(II) | The sorption capacity of treated biochar for Hg (II) enhanced by 76.95%, while that for atrazine enhanced by 38.66% | After modification enhanced SA which was 59.23 m2 g−1 | |
 Auricularia auricular dreg | 400 | Cetyl trimethyl ammonium bromide | Cr (IV) | The removal rate increased by 40 times more as compared to un-treated biochar | The number of micropores and mesoporous in the unit area enhanced, After treatment, the SA enhanced by 6.1% and the average pore diameter increased by 16.5% | |
 Seaweed | 200 | KOH | V(V) | 12 mg g−1 sorption capacity noticed | Complexation, electrostatic interaction and pore diffusion | |
 Rice straw | 400 | β-cyclodextrin and HCl | Pb2+ | 130 mg g−1 sorption capacity found was higher than unmodified biochar | Complexation, ion exchange, and physisorption | |
 Horse manure | 500 | Bismuth(III) nitrate | U(VI) | 516 mg g−1 adsorption capacity found was higher than un-modified biochar | Reductive reaction, ion exchange, and precipitation | |
Physical modification | ||||||
 Bur cucumber | 300, 700 | Steam activation | Sulfamethazine | Around 55% enhancement in removal capacity | – | |
 Whitewood | 550 | Steam activation | Emission of CH4 | Suppress CH4 emission | – | |
 Maize stover | 350 | Steam activation | Emission of N2O | Suppress N2O emission | – | |
 Tea waste, soybean straw, bagasse, and shrub | 300, 700 | Steam activation | Sulfamethazine | Maximum sulfamethazine sorption among all the biochars | Due to its higher SA and pore volume | |
 Guayule, corn stover and cob, switchgrass, alfalfa stems, and chicken manure | 500 | Steam activation | Cu | Highest sorption capacities observed | Largest SSA and porous structure | |
 Cornstalk | 500, 900 | CO2/NH3 Modification | CO2 | – | NH3 reacts with the biochar surface, introducing the nitrogen functional groups; CO2 modification forms more micropore | |
 Black spruce | 454, 900 | Steam activation | Sulfur dioxide | The sorption capacity of sulfur dioxide was found higher (76 mg g−1) | Surface area (590 m2 g−1) and pore volume increased | |
 Canola straw | 700 | Steam modification | Pb (II) | Removal capacity observed (195 mg g−1) | Due to its higher SA and pore volume | |
 Rice straw | 800 | Steam activation | Naphthalene | The sorption rate was noticed at 76% | Higher surface area (106 m2 g−1) and a large amount of surface functional groups | |
 Poplar wood | 300 | Ball milling | Mercury | Sorption capacity was 320 mg g−1 | Surface area and pore structure improved | |
 Soybean straw | 800 | Steam activation | Zn2+, Ni2+, Cd2+, and Cu2+ | Removal capacity 27.8, 30, 21,95.7 mg g-1 for Zn2+, Ni2+, Cd2+, Cu2+ | Higher surface area (793 m2 g−1) and average pore diameter enhanced | |
 Bamboo | 500 | Activation by steam | Tetracycline and Copper (II) | Adsorption capacity 0.22 and 5.03 mmol g−1 tetracycline and Copper (II), respectively | Due to changes in oxygen-enrich functional groups | |
 Mushroom | 800 | Steam activation | Crystal violet | 1057 mg g−1 adsorption capacity found | Higher surface area (332 m2 g−1) | |
 Invasive plants | 700 | Steam modification | Sulfamethazine | 37.7 mg g−1 adsorption capacity observed | Because of higher SA and pore volume | |
 Dendro | 700 | Ball milling | Cadmium and chromium | Sorption capacity for chromium 922 mg g−1 and cadmium 7.46 mg g−1 | Improved pore structure after modification | |
 Tea waste | 700 | Steam activation | Sulfamethazine | 33.81 mg g−1 adsorption capacity noticed | Higher surface area (576.9 m2 g−1) and a large amount of surface functional groups | |
 Hickory chip | 600 | Ball milling | Reactive red | 34.80 mg g−1 adsorption capacity noticed | Enhanced O-moieties and N-enrich functional groups favored the contaminant elimination by electrostatic interaction | 35 |
 Pine sawdust | 550 | Activation by steam | Reduce emission of greenhouse gases | Reduce the CO2 and N2O emission | Decreased enzyme and microbial activities as well as higher surface area (397 m2 g−1) | |
 Poplar wood | 300 | Ball milling | Enrofloxacin | Removal capacity noticed at 80.20% | The increased photocatalytic performance of ball milled-modified-biochar was owing to the generated radicals | |
 Orange peel waste | 950 | Microwave activation | Congo red | 136 mg g−1 sorption capacity noticed | Surface functionality improved | |
 Hickory, bagasse, and bamboo | 600 | Clay-biochar composites | Methylene blue | Enhancement of removal capacities by around 5 times | Electrostatic attraction (with biochar) and Ion exchange (with clay) | |
 Corn straws | 600 | MnOx-doped biochar | Cu | Highest removal capacity; maximal removal capacity as high about 160 mg g−1 | Formation of the inner-sphere complexes with MnOx and oxygen-comprising groups | |
 Mg-accumulated tomato tissues | 600 | Mg-loaded biochar | Phosphate | Around 88% removal of Phosphate from the solution | Nano-scale Mg(OH)2 and MgO particles as core sorption sites for aqueous | |
 Mg-enriched tomato leaves | 600 | Mg-doped biochar | Phosphorus | Highest removal capacity > 100 mg g−1 | Precipitation of Phosphorus by chemical reaction with Mg-particles and surface deposition of Phosphorus on Mg-crystals on biochar surfaces | |
 Peanut hull, hickory chips, sugarcane bagasse, and bamboo | 600 | Chitosan-loaded biochars | Cd, Cu, and Pb | Increased elimination of metals | Electrostatic interaction | |
 Corn | 300,450,600 | Mg-modified biochar | Phosphorus | Highest removal noticed | – | |
 Sugar beet | 300 | Mg-modified biochar | Phosphorus | Highest removal volume > 100 mg g−1 | The appearance of the nano-sized MgO-particles on the biochar surfaces as active sorption sites for aqueous P | |
 Rice straw | 200–500 | Mineral loaded composite by [Ca(H2PO4)2]), CaCO3, and kaolin | Carbon retention | Three minerals, particularly [Ca(H2PO4)2]) were effective in enhancing C retention and strengthening biochar stabilization | Increased C retention and stability of biochar with mineral loading due to increased formation of aromatic Carbon | |
 Pinewood | 600 | MnO-loaded adsorbent | Pb, As(V) | Removal capacities of As(V) enhanced by around 4 and 5 times, while those of Pb enhanced by around 2 and 20 times | The occurrence of birnessite particles exhibited strong interactions with metals | |
 Soybean straw, peanut straw, and rice straw | 750 | Aluminum-treated | As(V) | Al-treated adsorbents sorbed 445–667 mmol kg−1 at 5 pH, in contrast to slight removal on un-treated biochars | Inner sphere complexes with Al(OH)3 on the surfaces of treated adsorbents | |
 Hickory chips | 600 | Fe-doped biochar | Arsenic | Highest removal capacity of About 2 mg g−1 in contrast to negligible removal on raw biochar | Chemisorption mechanism on Fe-loaded biochar | |
 Rice hull | 350 | Composite with nZVI | Trichloroethylene | The degradation efficiency of Trichloroethylene was around 99% due to the nZVI-biochar composite | Higher SSA and O-enrich functional groups of nZVI-treated biochar increased SO4 generation and induced Trichloroethylene degradation | |
 Rice husk | 300 | Fe and Ca-treated biochar | Chromium and As(V) | Observed more than 90% removal | Electrostatic interactions and heavy metal precipitation | |
 Cotton stalk | 350 | Fe2O3-loading | Phosphate | Enhanced phosphate removal capacity from 0 to 0.963 mg g−1 | Desegregation of porous trait of biochar, maximum removal ability of Fe2O3, and exceptional flow features of granular particles | |
 Orange peel | 250–700 | Fe2+/Fe3+ prepared magnetic biochar | p-nitrotoluene and Naphthalene | The removal rate was higher than un-treated biochar | – | |
 Pinewood | 600 | Magnetic biochar | As (V) | Higher sorption of As(V) from aqueous | γ-Fe2O3 particles on the treated adsorbent surface functioned as sorption sites by electrostatic interactions | |
 Rice hull | 400 | Zinc sulfide loading | Pb | Notably increased removal capacity | – | |
 Oak Bar, Oakwood | 400, 450 | Magnetic composite | Pb and Cd | Removal capacities were higher than fresh and other un-treated adsorbents | Electrostatic interactions | |
 Cottonwood | 600 | Fe2O3-modified | Arsenic | The highest removal capacity of the 3147 mg kg−1 was noticed | Nano-colloidal structures of strong dispersed γ-Fe2O3 particles on both surface and interior of the treated adsorbent matrix | |
 Corn straw | 500 | Na2S-modifed | Atrazine, Hg(II) | After modification, the sorption capacities for Atrazine, Hg(II) comprehensively increased | The sulfur content was markedly enhanced by 101.29% under Na2S treatment | |
 Thalia dealbata | 500 | MgCl2-loaded | Cd and sulfamethoxazole | The addition of treated biochar enhanced the removal of sulfamethoxazole (by 50–58%) and Cadmium (by 24–25%) as compared with pristine biochar | SA of MgCl2 loaded biochar (110.6 m2 g−1) was greater than un-modified biochar (7.1 m2 g−1) | |
 Bamboo | 700 | FeSO4, Chitosan and Fe2(SO4)3 | Cr (VI) | 127 mg g−1 sorption capacity was observed by modified biochar | Electrostatic attraction, reduction, chelation, and complexation | |
 Maize straw | 600 | N-loading | Cd2+ | 197 mg g−1 adsorption capacity observed was higher than untreated biochar | Hydroxyl groups, complexation with graphitic N | |
 Ficus microcarpa | 500 | Chitosan | Sb3+ | 167 mg g−1 adsorption capacity observed | H–bonding, π–π interaction, surface complexation, chelation, and electrostatic interaction | |
 Rapeseed straw | 600 | MnSO4 | Sb(V) | 0.70 mg g−1 adsorption capacity noticed was greater than untreated biochar | Electrostatic interaction, hydroxyl/carboxyl Sb inner-sphere complexation, Sb-O-Mn complex, and physical adsorption | |
 Populus | 600 | FeCl3 | As(V) | 99% adsorption efficacy was found higher than unmodified biochar | Electrostatic interaction and Fe-As precipitation | |
 Glucose | 800 | N-loading | Cr(VI) | 400 mg g−1 adsorption capacity noticed | Reduction, complexation, and physisorption | |
 Corn straw | 800 | S-loading | Fe2+ | 50 mg g−1 | Co-precipitation, ion exchange, and chemical complexation | |
Biological modifications | ||||||
 Peanut shell | 500 | hibiscicola strain L1 | Cu2+ | 45.8% removal capacity | Reduction and precipitation | |
 Peanut shell | 500 | Pseudomonas | Cr(VI) | 38.2% removal capacity, which was higher than un-treated biochar | Ion-exchange and complexation | |
 Peanut shell | 500 | Pseudomonas | Ni2+ | 81% removal capacity was noticed, which was higher than un-treated biochar | Reduction and precipitation | |
 Corn straw | 300 | Vibrio | Diesel oil | 94% | Physical adsorption and biodegradation | |
 Erding | 500 | Bacillus cereus LZ01 | Chlortetracycline | 82% | Biochar adsorption and biodegradation via LZ01 | |
