Competitive adsorption of selenite [Se(IV)], selenate [Se(VI)] and selenocyanate [SeCN–] species onto TiO2: Experimental findings and surface complexation modelling

Competitive adsorption of aqueous Se(IV) (selenite), Se(VI) (selenate) and SeCN – (selenocyanate) species onto titanium dioxide/TiO 2 was investigated under a varying set of mixed binary and tertiary systems. The obtained anionic type adsorption curves for binary Se(IV)/Se(VI) systems indicated that Se(IV) adsorption is not affected by Se(VI) species however Se(VI) adsorption is markedly suppressed in presence of Se(IV). This was explained based on higher relative affinity of TiO 2 surface sites for Se(IV) compared to Se(VI). The Se(VI) species showed a sharp anionic type adsorption curve between pH 2–5 whereas Se(IV) adsorption gradually decreased from pH 2–4 followed by a sharp decrease till pH 7. Unlike the Se(IV)/Se(VI) results the binary Se(IV)/SeCN – and Se(VI)/SeCN – systems did not show competitive adsorption behavior. Furthermore the tertiary systems having mixed Se(IV), Se(VI), and SeCN – species showed adsorption trends that were similar to above noted binary systems observations. The respective studies indicated the following adsorption trend above pH 4: Se(IV) > SeCN – > Se(VI). Adsorption modelling using the Diffuse Layer Model also showed reasonable predictions. For Se(IV) an inner sphere Ti-SeO 3 – complex generally yielded acceptable modelling estimations whereas for Se(VI) and SeCN – outer sphere complexes Ti-H 2 O-SeO 4 – and Ti-H 2 O-SeCN-respectively showed reasonable modelling results.