Black carbon is ubiquitous in soils and sediments and can participate in a number of important biogeochemical processes. It has been shown that black carbon such as biochar can possess a considerable electron storage capacity (ESC) and can serve as a reactive medium to support abiotic and biotic redox transformation. To understand how black carbon controls contaminant fate in subsurface environments and to evaluate its potential for remediation, it is important that we understand the accessibility of ESC to contaminants, as well as the kinetics and mechanism of ESC-promoted redox reactions. 3-Nitro-1,2,4-triazole-5-one (NTO) is an insensitive munition constituent in IMX-101 and IMX-104 formulations and may exist in soil and groundwater near military sites. In this study, we investigated the reductive transformation of NTO by a wood-derived black carbon (Soil Reef biochar, or SRB). The ESC of SRB was ~4 mmol/g, measured using dithionite (Eh = –0.43 V vs. SHE at pH 7) and dissolved O2 (+0.80 V vs. SHE at pH 7, PO2 = 0.21 atm). Parallel experiments were performed using both air-oxidized and dithionite-reduced SRB. The results showed that NTO was removed minimally by oxidized SRB because NTO is anionic at circumneutral pH and was hence negligibly adsorbed. In contrast, when exposed to reduced biochar, NTO was continuously removed and 3-amino-1,2,4-triazol-5-one (ATO) was formed quantitatively, confirming that NTO was chemically reduced to ATO by dithionite-reduced SRB. Based on the ATO yield, the quantity of SRB's ESC that was reactive toward NTO was approximately 500 μmol/g, which was only ~13% of the total ESC. While NTO reduction was rapid initially, the reaction continued for up to 600 h. The reduction rate of NTO by SRB was most likely controlled by pore diffusion within SRB particles. Using a shrinking unreacted core model, we were able to fit the observed NTO reduction kinetics and estimate the effective pore diffusivity of NTO in SRB particles. These results have important implications for the fate of NTO in subsurface environments and for the remediation of military sites.

Reductive transformation of 3-nitro-1,2,4-triazole-5-one (NTO) by biochar through its electron storage capacity (ESC)

ACS Spring Meeting 2021

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