Factors affecting sorption of nitro explosives to biochar: pyrolysis temperature, surface treatment, competition, and dissolved metals.

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.