Temperature\nand pH Affect the Sorption and Transformation\nof Dissolved Organic Carbon by Birnessite

Soil mineralogy plays\na vital role in carbon storage.\nBirnessite\nis a widely occurring manganese oxide mineral. Despite its widespread\noccurrence, studies of the interactions between natural soil organic\ncarbon and manganese oxide minerals are rare. This study investigated\nthe influence of triclinic birnessite on the transformation of vermicompost-derived\ndissolved organic carbon (DOC) at different pHs (4 and 8) and temperatures\n(25 and 50 °C). Enhanced adsorption and transformation of DOC,\nparticularly at pH 4 and 50 °C, were observed. The use of SUVA<sub>254</sub> and fluorescence spectroscopy showed that DOC sorption\ncaused an increase in carbon aromaticity, with the highest degree\nobserved at pH 4 at 50 °C. In particular, X-ray photoelectron\nspectroscopy and near-edge X-ray absorption fine structure C 1s characterization\nshowed an 8-fold increase in aromaticity at 50 °C compared to\nthe unreacted DOC, with esterification and/or etherification reactions\nalso occurring at pH 4. The impact of DOC sorption on birnessite stability\nand transformations was also assessed via X-ray absorption near-edge\nstructure spectroscopy, which revealed that reacted and unreacted\ntriclinic birnessite showed modest changes to Mn II, III, and IV.\nManganese K-edge extended X-ray absorption fine structure spectroscopy\nshowed the increased formation of hexagonal birnessite at pH 4, the\nformation of manganite at pH 4 and 8, 50 °C, and the formation\nof up to 6% ramsdellite at pH 8, 25 °C. This study provides new\ninsights into the role of birnessite in soil carbon storage and manganese\nmineral transformations under environmentally relevant conditions.