Table 4.
The mechanisms and enhanced performance of chemically oxidized and base/acid modified biochars.
| Modification | Pollutant | Sorption capacity | Improvement | Functionalities | Mechanism | Reference |
|---|---|---|---|---|---|---|
| NaOH | Chloramphenicol | Around 2100 mg kg−1 | Enhance surface graphitic C and oxygen-enrich groups | Carboxyl and carbonyl groups | Formation of H-bonds among N-comprising groups in adsorbent surface and chloramphenicol, Electron-donor–acceptor π-π interaction | 22 |
| KOH | As (v) | 30.98 mg g−1 | More than about 1.3 times higher removal capacity | Carbonyl group | Ion exchange | 21 |
| KOH | Cd and Cu | 34 and 31 mg g−1 | Approximately 20 times higher removal capacity than other raw biochars | Aromatic carbon | Surface precipitation, ion exchange electrostatic attraction, and surface complexation | 23 |
| H2SO4 | Tetracycline | 23.26 mg g−1 | Larger SSA and porosity, higher oxygen and carbon contents, lower ash content | O-containing functional groups | Π-π electron donor acceptor interaction between the treated adsorbent surface and aromatic ring | 25 |
| H2SO4 + oxalic acids | Sulfamethazine | 183–229 L kg−1 | – | – | Ligand- and proton-enhanced processes; Increase in SA by acid modification | 29 |
| H3PO4 | – | – | Enhance water extractable organic carbon content | – | Hydrolysis of ester groups formed during the pyrolysis | 26 |
| (Oxidation) H2O2 | Pb | 22.82 mg g−1 | Removal for Pb was 20 times greater than un-treated biochar | Carboxyl groups | Enhance carboxyl groups expedite the formation of bound complexes with Pb | 27 |
| (Oxidation) H2O2 | Hg | 1470.5–1347.9 µg g−1 | Substantially higher removal capacity than other un-treated biochars (380–618 ng mg−1) | Ester, Carboxyl, and Carbonyl groups | Higher SSA and total pore volume Surface O-enrich functional groups increase electron transfer, and Hg-oxidation and allow chemisorption centers | 28 |
| Acetic acid + Na2S2O4 + H2SO4 | Cu | 15.97 mg g−1 | Enhancing removal capacity in fixed-bed and batch experiments | Amino groups | Ion exchange | 40 |
| HNO3 + H2SO4 | Cu | 12.47 mg g−1 | Around 4.62% N was present on the treated adsorbent, fivefold enhancement of removal capacity | Amino groups | Foundation of outer-spherecomplexes with amino groups of treated biochar | 41 |