RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The interaction between silicic acid (H₄SiO₄) and ferrihydrite involves both adsorption and polymerisation, and has a significant effect on the adsorption of anionic species such as the oxyanions of arsenic. Using the diffuse layer model (DLM), the adsorption constants for H₄SiO₄, As(V), and As(III) adsorption onto ferrihydrite have been determined from experimental adsorption data. Silicic acid could be considered to adsorb as a monomer when the total Si to Fe mole ratio (Si_(T)/Fe) was <0.1, and the inhibitory effect of H₄SiO₄ on As(III) and As(V) adsorption could be accurately modelled using the DLM. At higher Si_(T)/Fe ratios, H₄SiO₄ polymerisation on the ferrihydrite surface appeared to become increasingly important. However, even when Si_(T)/Fe=1.8 and there would have been significant H₄SiO₄ polymerisation, the DLM continued to predict almost all of the observed effect of H₄SiO₄ on As(III) and As(V) adsorption while taking only H₄SiO₄ adsorption into account. This suggests that H₄SiO₄ adsorption inhibits As adsorption to a greater degree than H₄SiO₄ polymerisation.