Complexation mechanisms in arsenic and phosphorus adsorption onto iron-coated cork granulates

The complexation mechanisms involved in As(III), As(V) and P(V) adsorption by iron-coated cork granulates, at environmentally relevant conditions (C-i = 1 mg L-1 and presence of background electrolyte), are analysed in this work. Adsorption kinetics and potentiometric titration data were acquired for the three species. It was found that the adsorption rate was faster at 0.01 mol L-1 than 0.1 mol L-1 IS. Kinetic data were well fitted by the pseudo-first-order model for As(V) and P(V) and by the pseudo-second-order and Elovich models for As(III). The adsorbed amount at equilibrium and the adsorption edges showed that As(III) adsorption was favoured at lower IS (especially at low pH), while As(V) and P(V) were favoured at higher IS (especially at higher pH). It is likely that all three species are adsorbed by inner-sphere complexation, although uptake of neutral As(III) is more affected by ion pair formation. In the case of As(V) and P(V), higher electrolyte uptake led to a decrease of the repulsive interactions and increased adsorption. The surface charge estimation and modelling using a quasi-Gaussian Sips distribution function of affinity constants confirmed the likelihood of inner-sphere complexation. It is presumed that phosphate competes with hydroxyl ions for adsorption sites, leading to pH increase due to OH- release in ligand exchange reactions. It was verified that most adsorption mechanisms responsible for uptake onto pure iron oxides are also applicable in iron-coated adsorbents.