The sorption of organic carbon onto differing clay minerals in the presence and absence of hydrous iron oxide

Sorption of organic carbon onto phyllosilicate clays and hydrous iron oxides influences the accumulation and stabilisation of organic carbon in soils. However, the effects of interactions between hydrous iron oxides and phyllosilicate clays on the sorption of dissolved organic carbon (DOC) are poorly understood. We carried out a batch experiment to examine the effects of goethite coatings on kaolinite, illite, and smectite on DOC sorption. The effect of coating illitic clay with different hydrous iron oxides (haematite, goethite, ferrihydrite) on DOC sorption was studied in another experiment. Organic matter extracted from dried medic (Medicago truncatula cv. Praggio) shoot residue was reacted with minerals at DOC concentrations ranging from 0 to 200 mg C L− 1 at pH 6.0. The maximum adsorption capacity (Qmax) of phyllosilicate clays, as determined from fits to the Langmuir equation, increased in the order kaolinite < illite < smectite on a mass basis and illite < smectite < kaolinite on a surface area basis. The sorption capacity of kaolinitic clay increased significantly with goethite coating, whereas the sorption capacity of illitic and smectitic clays was not affected by goethite coating. Ferrihydrite coating increased the sorption capacity of the illitic clays, while haematite coating decreased the sorption capacity; goethite-coated illitic clays had a sorption capacity similar to pure illitic clays. Desorption experiments resulted in the removal of 6-14% of the sorbed DOC. The presence of goethite reduced desorption from kaolinitic clays but did not influence desorption from illitic and smectitic clays. The results suggest that interactions of hydrous iron oxides and phyllosilicate clays can modify DOC sorption and desorption, probably by affecting the surface charges. Therefore, sorption and desorption of organic matter from soils may vary with mineral assemblage, with increasing suppression of the contribution of hydrous oxides at circumneutral to slightly alkaline soil reaction.