Adsorption of arsenic (V) on kaolinite and on kaolinite–humic acid complexes: Role of humic acid nitrogen groups

In order to investigate the influence of organic matter on arsenic retention, we used batch experiments at pH 7 to determine the adsorption of As(V) on three different solids: a crude, purified, Ca-exchanged kaolinite and two kaolinites coated with humic acids (HAs) having different nitrogen contents. We first examined the adsorption of each HA onto kaolinite, and then used the HA–kaolinite complexes to study As(V) adsorption. The results clearly show an influence of the HA coating on As adsorption. For example, with low initial As concentrations the solid/liquid partition coefficient (Rd) for both HA complexes is greater than that for the crude kaolinite. We found that increasing the initial As concentrations decreased the Rd values of the HA-coated kaolinites until finally they were the same as the crude kaolinite Rd values. This suggests that adsorption occurs first on the HA sites and then, once the HA sites are saturated, on the remaining kaolinite sites. We also noted that the more reactive HA–kaolinite complex was the one with the highest N/C ratio. Comparing the amount of amine groups in the HA–kaolinite complexes with the total amount of adsorbed As indicates that the HA amine groups, due to their positive charge at pH 7, play a key role in the adsorption of As onto organic matter.