Electron Storage Capacity of Biochar for Supporting Transformation of Explosives, Danhui Xin

Biochar is a carbon-rich product from pyrolysis of surplus biomass, e.g., wood residues from the timber industry. In addition to the application of biochar as strong sorbents, chemical properties of biochar are getting increasing attention in the field of water and soil remediation. Electron storage capacity (ESC) is an important property that determines the capacity of biochar to store electrons and reversibly exchange electrons with its surroundings. With considerable ESC, biochar may serve as an effective reactive medium to support abiotic and microbial redox transformations. We developed chemical redox titration methods to quantify the ESC of biochar and demonstrated that ESC is constant and highly reversible with respect to the specified reduction potential range. Using dithionite (–0.43 V vs. standard hydrogen electrode at pH 6.4) and dissolved O2 (+0.80 V at pH 7, PO2 = 0.21 atm), we determined that the reversible ESC of Soil Reef biochar (SRB), model biochar, was 4 mmol/g. This means that each gram of dithionite-reduced SRB can donate up to 4 mmol of electrons to support redox reactions. 3-Nitro-1,2,4-triazol-5-one (NTO) is a major insensitive munition constituent, which may exist in soil and groundwater near military sites. To evaluate the efficacy of biochar as a reactive medium to support NTO transformation, we conducted batch experiments using air-oxidized (ESC-depleted) or dithionite-reduced (ESC-filled) SRB. The results showed that NTO was removed minimally by oxidized SRB; In contrast, NTO was continuously removed by reduced biochar, and 3-amino-1,2,4-triazol-5-one (ATO) was formed quantitatively. This confirms that NTO was chemically reduced to ATO by reduced SRB. Based on the ATO yield, the ESC of SRB that was reactive toward NTO was approximately 500 μmol/g. This study demonstrates the potential utility of biochar to degrade explosives and suggests biochar-based remediation strategies may be developed for DoD sites.