Comparative Evaluation\nof Mediated Electrochemical\nReduction and Chemical Redox Titration for Quantifying the Electron\nAccepting Capacities of Soils and Redox-Active Soil Constituents

The electron accepting capacity (EAC) of soil plays a\npivotal role\nin the biogeochemical cycling of nutrients and transformation of redox-labile\ncontaminants. Prior EAC studies of soils and soil constituents utilized\ndifferent methods, reductants, and mediators, making cross-study comparison\ndifficult. This study was conducted to quantify and compare the EACs\nof two soil constituents (hematite and Leonardite humic acid) and\n12 soils of diverse composition, using chemical redox titration (CRT)\nwith dithionite as the reductant and mediated electrochemical reduction\n(MER) with diquat as the mediator. The EACs of hematite and humic\nacid measured by CRT (EAC<sub>CRT</sub>) and MER (EAC<sub>MER</sub>) are similar and close to the theoretical/reported values. For soils,\nEAC<sub>CRT</sub> and EAC<sub>MER</sub> increased with iron and organic\ncarbon (TOC) contents, suggesting iron and carbon were the main contributors\nto soil EAC. EAC<sub>CRT</sub> > EAC<sub>MER</sub> for all soils,\nand their difference (ΔEAC = EAC<sub>CRT</sub> <b>–</b> EAC<sub>MER</sub>) increased with TOC, presumably due to the longer\ncontact time in CRT and thus more complete reduction of carbonaceous\nredox moieties. We propose an equation that relates EAC<sub>CRT</sub> to EAC<sub>MER</sub> (ΔEAC = 1796<i>f</i><sub>TOC</sub> + 32) and another that predicts EAC<sub>CRT</sub> from dithionite-reducible\nFe and TOC (EAC<sub>CRT</sub> = 2705 μmol e<sup>–</sup>/g C × <i>f</i><sub>TOC</sub> + 17907 μmol e<sup>–</sup>/g Fe × <i>f</i><sub>Fe<sub>dithionite‑reducible</sub></sub>). Our results suggest that at least 10−15% of soil\norganic carbon contributed to EAC<sub>CRT</sub>.