Sorption-Desorption of DOC By Sequentially Treated Soil Clay Fractions with Different Mineral Compositions.

Sorption is a very important factor in the stabilization of dissolved organic carbon (DOC) in soils and thus C sequestration. Soil clay minerals, especially in the presence of Fe & Al oxides, significantly influence the accumulation and stabilisation of organic carbon (OC) in soils. However, the effects of interactions among clay minerals, native carbon and Fe/Al oxides on the sorption of DOC are poorly understood. To close this knowledge gap, a set of batch sorption and desorption experiments were conducted using clays extracted from soils dominated by kaolinite, smectite and allophane. These three samples were sequentially treated to remove native (endogenous) C and Fe & Al oxides. Thus, three different sub-samples were obtained for each clay type. All of the 9 clay samples thus obtained were investigated for the sorption of DOC (derived from wheat straw).

The DOC sorption, as measured by the Langmuir maximum sorption capacity (Q_max), was the lowest (Q_max = 9.8 mg gm^-1) in kaolinitic/illitic (C, Fe/Al oxides removed) clay and the highest (Q_max = 117.6 mg gm^-1) in allophanic (C, Fe/Al oxides removed) clay. A positive relationship between DOC sorption and SSA was found. It was also found that the greater SSA was mainly contributed by the presence of Fe/Al oxides and allophanic minerals in the clay samples. Removal of these pedogenic Fe/Al oxides from the kaolinic/illitic and smectitic clays resulted in a decrease in SSA and DOC sorption.

Desorption of the sorbed DOC (in deionised water) from the clay samples followed a different trend as compared to sorption. The clay which sorbed the highest amount of DOC desorbed the least. In summary, removal of native C from soil clay fraction increases the SSA and DOC sorption. On the other hand, removal of Fe/Al oxides causes a decrease in both SSA and DOC sorption. It appears that there is a close correspondence between DOC sorption and SSA.