Release of Electron Donors during Thermal Treatment of Soils

Thermal treatment of soil and groundwater may provide an in situ source of soluble organic compounds and hydrogen (H₂) that could stimulate microbial reductive dechlorination (MRD) at sites impacted by chlorinated solvents. The objectives of this study were to identify and quantify the release of electron donors and fermentable precursors during soil heating and to estimate availability of these compounds following thermal treatment. Fourteen solid materials containing <0.01 to 63.81 wt % organic carbon (OC) were incubated at 30, 60, or 90 °C for up to 180 d, leading to the release of direct electron donors (i.e., H₂ and acetate) and fermentable volatile fatty acids (VFAs). Total VFA release ranged from 5 ± 0 × 10^-9 carbon per gram solid (mol C/g_s) during 30 °C incubation of quartz sand to 820 ± 50 × 10^-6 mol C/g_s during 90 °C incubation of humic acid, and was positively impacted by incubation time, temperature, and solid-phase OC content. H₂ gas was detected at a maximum of 180 ± 50 × 10^-9 mol H₂/g_s, accounting for less than 0.3% of reducing equivalents associated with VFAs released under the same conditions. These findings will allow for more reliable prediction of substrate release during thermal treatment, supporting the implementation of coupled thermal and biological remediation strategies.