Soil organic matter (SOM) and iron oxides are ubiquitous in the environment and participate in important biogeochemical processes which regulate carbon cycling. Nanophase minerals can behave differently than their bulk counterparts due to an observable increase in surface reactivity with decreasing particle size. Nanophase hematite and goethite are known to adsorb large quantities of organic carbon in soils, however the sensitivity of sorption to climate warming is unknown. We tested the hypothesis that nanophase iron oxides experience a decrease in sorption extent with warming due to increased particle aggregation at higher temperatures which will reduce available surface area for sorption. We measured the sorption of soil humic acid to hematite and goethite in batch experiments at five temperatures and compared the nanophase to the bulk minerals. The sorption of soil humic acid to nanophase hematite and goethite followed the Langmuir model and were determined to be spontaneous and endothermic processes. When comparing iron oxide mineralogy, on a mass basis nanophase goethite had similar sorption capacities (Qmax) to nanophase hematite, but on a surface area basis nanophase goethite had larger Qmax values. The sorption coefficients (K) increased with warming while the Qmax were temperature independent. When compared to bulk hematite, nanophase hematite had a greater Qmax on a mass and surface area basis. These results suggest that changes in soil iron-carbon associations due to warming are more likely to be the result of indirect effects through processes that can alter particle size (e.g., weathering, precipitation and biomineralization reactions) rather than direct temperature effects on sorption. Climate changes that affect the abundance of iron oxides (hematite) and iron (hydr)oxides (goethite), such as shifts in precipitation and moisture content, may have a greater effect on global iron-carbon associations than warming.

Temperature effects on the interactions of soil organic matter with hematite and goethite: Perspectives on bulk and nano-scale sorption

ACS Spring Meeting 2021

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