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

• Published in: Environmental science & technology Vol. 58; no. 40; pp. 17674 - 17684
• Main Authors: Rincón-Rodríguez, Juan C., Cárdenas-Hernández, Paula A., Murillo-Gelvez, Jimmy, Di Toro, Dominic M., Allen, Herbert E., Carbonaro, Richard F., Chiu, Pei C.
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 08.10.2024
• Subjects: Acidic soils; Aluminum Silicates - chemistry; Biogeochemical cycles; Biogeochemical Cycling; Carbon; Carbon - chemistry; Chemical composition; Chemical reduction; Contaminants; Diquat; Dithionite; Electrochemical Techniques; Electrochemistry; Electrons; Ferric Compounds - chemistry; Hematite; Humic acids; Humic Substances; Iron constituents; Minerals; Nutrient cycles; Nutrients; Organic carbon; Organic soils; Oxidation-Reduction; Redox properties; Reducing agents; Soil - chemistry; Soil pollution; Soils; Titration; humic acid; electron accepting capacity; redox potential profile; hematite; dithionite; mediated potentiometry
• ISSN: 0013-936X; 1520-5851; 1520-5851
• DOI: 10.1021/acs.est.4c06514

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 (EACCRT) and MER (EACMER) are similar and close to the theoretical/reported values. For soils, EACCRT and EACMER increased with iron and organic carbon (TOC) contents, suggesting iron and carbon were the main contributors to soil EAC. EACCRT > EACMER for all soils, and their difference (ΔEAC = EACCRT – EACMER) 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 EACCRT to EACMER (ΔEAC = 1796f TOC + 32) and another that predicts EACCRT from dithionite-reducible Fe and TOC (EACCRT = 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 EACCRT.