Adsorption and polymerisation of silicic acid on ferrihydrite, and its effect on arsenic adsorption

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.