The feasibility of using biochar as a sorbent to remove nitro explosives and metals from contaminated water was investigated through batch experiments. Biochar, synthesized using various biomasses, showed a porous structure and a high surface area and includes embedded carbonate minerals. Compared with granular activated carbon, biochar was competitive as a sorbent for removing Cd, Cu, Pb, and Zn from water according to the maximum sorption capacities of the metals. Some biochars also effectively sorbed nitro explosives from water. Correlation analysis between maximum sorption capacities and properties of biochar showed that the sorption capacity of biochar for cationic toxic metals is related to cation exchange capacity and that the sorption capacity of explosives is proportional to surface area and carbon content. Results from X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy analyses and laboratory experiments suggest that surface functional groups may be responsible for the sorption of cationic metals to the biochar surface. In contrast, carbon contents may account for the sorption of explosives, possibly through π-π electron donor-acceptor interactions. Our results suggest that biochar can be an attractive and alternative option in environmental remediation of nitro explosives and metals through sorption and immobilization and that appropriate selection of biochar may be necessary according to the types of contaminant and the properties of biochar.

The feasibility of using biochar as a sorbent to remove nine halogenated phenols (2,4-dichlorophenol, 2,4-dibromophenol, 2,4-difluorophenol, 2-chlorophenol, 4-chlorophenol, 2-bromophenol, 4-bromophenol, 2-fluorophenol, and 4-fluorophenol) and two pharmaceuticals (triclosan and ibuprofen) from water was examined through a series of batch experiments. Types of biochar, synthesized using various biomasses including fallen leaves, rice straw, corn stalk, used coffee grounds, and biosolids, were evaluated. Compared to granular activated carbon (GAC), most of the biochar samples did not effectively remove halogenated phenols or pharmaceuticals from water. The increase in pH and deprotonation of phenols in biochar systems may be responsible for its ineffectiveness at this task. When pH was maintained at 4 or 7, the sorption capacity of biochar was markedly increased. Considering maximum sorption capacity and properties of sorbents and sorbates, it appears that the sorption capacity of biochar for halogenated phenols is related to the surface area and carbon content of the biochar and the hydrophobicity of halogenated phenols. In the cases of triclosan and ibuprofen, the sorptive capacities of GAC, graphite, and biochars were also significantly affected by pH, according to the point of zero charge (PZC) of sorbents and deprotonation of the pharmaceuticals. Pyrolysis temperature did not affect the sorption capacity of halogenated phenols or pharmaceuticals. Based on the experimental observations, some biochars are good candidates for removal of halogenated phenols, triclosan, and ibuprofen from water and soil.

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The application of rice straw-derived biochar for removing nitro explosives, including 2,4-dinitrotoluene (DNT), 2,4,6-trinitrotoluene (TNT), and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), from contaminated water was investigated through batch experiments. An increase in the pyrolysis temperature from 250 to 900°C in general led to higher pH, surface area, cation exchange capacity (CEC), point of zero charge, and C:O ratio of biochar. The maximum sorption capacity estimated by a mixed sorption-partition model increased when pyrolysis temperatures were elevated from 250 to 900°C, indicating that C content and aromaticity of biochar were strongly related to the sorption of nitro explosives to biochar. Surface treatment with acid or oxidant increased the sorption capacity of biochar for the two strong π-acceptor compounds (DNT and TNT) but not for RDX. However, the enhancement of sorption capacity was not directly related to increased surface area and CEC. Compared with single-sorption systems, coexistence of explosives or cationic metals resulted in decreased sorption of each explosive to biochar, suggesting that sorption of nitro explosives and cationic metals to electron-rich portions in biochar was competitive. Our results suggest that π-π electron donor acceptor interactions are main sorption mechanisms and that changing various conditions can enhance or reduce the sorption of nitro explosives to biochar.