Comparative Evaluation of Mediated Electrochemical Reduction and Chemical Redox Titration for Quantifying the Electron Accepting Capacities of Soils and Redox-Active Soil Constituents

The electron accepting capacity (EAC) of soil plays a pivotal role in the biogeochemical cycling of nutrients and transformation of redox-labile contaminants. Prior EAC studies of soils and soil constituents utilized different methods, reductants, and mediators, making cross-study comparison difficult. This study was conducted to quantify and compare the EACs of two soil constituents (hematite and Leonardite humic acid) and 12 soils of diverse composition, using chemical redox titration (CRT) with dithionite as the reductant and mediated electrochemical reduction (MER) with diquat as the mediator. The EACs of hematite and humic acid measured by CRT (EAC_CRT) and MER (EAC_MER) are similar and close to the theoretical/reported values. For soils, EAC_CRT and EAC_MER increased with iron and organic carbon (TOC) contents, suggesting iron and carbon were the main contributors to soil EAC. EAC_CRT > EAC_MER for all soils, and their difference (ΔEAC = EAC_CRT - EAC_MER) increased with TOC, presumably due to the longer contact time in CRT and thus more complete reduction of carbonaceous redox moieties. We propose an equation that relates EAC_CRT to EAC_MER (ΔEAC = 1796f_TOC + 32) and another that predicts EAC_CRT from dithionite-reducible Fe and TOC (EAC_CRT = 2705 μmol e^-/g C × f_TOC + 17907 μmol e^-/g Fe × f_{Fedithionite-reducible}). Our results suggest that at least 10-15% of soil organic carbon contributed to EAC_CRT.