Arsenate adsorption to soils: Modelling the competition from humic substances

There is currently no agreement to what extent humic substances may compete with arsenate and other anions for oxide adsorption sites in soils, and how to model this interaction. In this study, batch experiments were made in which the competition between fulvic acid and arsenate was studied in a spodic Bs horizon. Additional experiments were performed in which the pH and concentration dependence of arsenate adsorption was studied in four soils. The results showed that fulvic acid decreased the adsorption of AsO4, probably because of competition effects. The Kd value of AsO4 adsorption to the four studied soils differed by three orders of magnitude at pH 5. The introduction of an irreversibly sorbed component RO− into the Three-Plane CD-MUSIC model, and the optimisation of RO− using the Kd of AsO4 for one sample from each data set, permitted the use of the model to predict AsO4 adsorption under different conditions. On the whole, the predictions were reasonably close to measured values, but the model failed in 3 of 4 soils at high surface coverage. A strong relationship between the optimised RO− value and the observed pyrophosphate-extractable C value supported the assumption that RO− can be considered as being an adsorbed humic functional group. The large concentrations of RO− compared to those of adsorbed PO4 and SO4 suggest that humic substances may be the most important competitors for anion adsorption sites in many soils.