Retention of dissolved organic matter by phyllosilicate and soil clay fractions in relation to mineral properties

The dependence of dissolved organic carbon (DOC) retention on mineral phase properties is uncertain where soil solution pH is neutral. Therefore, ten soil and phyllosilicate clay fractions were used (after carbon oxidation) to investigate relationships between DOC retention at pH 7 and content of dithionite-extractable iron (Fed), specific surface area (SSA) and cation exchange capacity (CEC). DOC sorption sites were identified by blocking reactive hydroxyl groups with phosphate on subsamples. Reversibility of DOC sorption was determined by desorption experiments. Soil clay fractions sorbed about twice as much DOC as phyllosilicates. A positive relationship between DOC sorption and Fed content suggests that this is due to pedogenic oxides in soil clays. Positive trends were also observed with SSA or CEC. Sorption of 9–44 μg DOC m−2 indicates that DOC interacts only with fractions of the mineral surfaces. Phosphate treatment of subsamples reduced DOC sorption by 39–64% indicating a contribution of reactive hydroxyl groups as well as of siloxane surfaces of phyllosilicate clay minerals to DOC sorption. Desorption experiment removed 13–50% of sorbed DOC. Samples with and without phosphate treatment desorbed similar absolute amounts of DOC indicating that DOC associations with siloxane surfaces may be weaker than interactions with hydroxyl groups.