RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

Competition between selenium (IV) and silicic acid for the hematite (α-Fe₂O₃) surface has been studied during this work. Single batch experiments have been performed to study separately the sorption of selenium (IV) and silicic acid as a function of the pH. With the help of the 2-pK surface complexation model, experimental data have been fitted using the FITEQL 4.0 program. Two monodentate inner-sphere surface complexes have been used to fit selenite ions retention, ≡FeSeO₃^- and ≡FeHSeO₃. In order to fit sorption of silicic acid, the two following surface complexes, namely ≡FeH₃SiO₄, and ≡FeH₂SiO₄^-, have been used. Using the surface complexation constants coming from these two binary systems, prediction curves of the effect of silicic acid on the retention of selenium (IV) onto hematite have been obtained. Finally, performed experiments showed a competition between selenium (IV) and silicic acid for the surface sites of hematite. Experimental data matched DDLM predictions, confirming the ability of the surface complexation model to predict quantitatively and qualitatively the ternary system selenium (IV)/H₄SiO₄/hematite.