Arsenate adsorption at the sediment–water interface: sorption experiments and modelling

Abstract

Arsenate adsorption was studied in three clastic sediments, as a function of solution pH (4.0–9.0) and arsenate concentration. Using known mineral values, protolytic constants obtained from the literature and Kads values (obtained by fitting experimental adsorption data with empirical adsorption model), the constant capacitance surface complexation model was used to explain the adsorption behavior. The experimental and modelling approaches indicate that arsenate adsorption increases with increased pH, exhibiting a maximum adsorption value before decreasing at higher pH. Per unit mass, sample S3 (smectite–quartz/muscovite–illite sample) adsorbs more arsenate in the pH range 5–8.5, with 98% of sites occupied at pH 6. S1 and S2 have less adsorption capacity with maxima adsorption in the pH ranges of 6–8.5 and 4–6, respectively. The calculation of saturation indices by PHREEQC at different pH reveals that the solution was undersaturated with respect to aluminum arsenate (AlAsO42H2O), scorodite (FeAsO42H2O), brucite and silica, and supersaturated with respect to gibbsite, kaolinite, illite and montmorillonite (for S3 sample). Increased arsenate concentration (in isotherm experiments) may not produce new solid phases, such as AlAsO42H2O and/or FeAsO42H2O.