Abstract
The interaction between silicic acid (H 4SiO 4) 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 H 4SiO 4, 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 H 4SiO 4 on As(III) and As(V) adsorption could be accurately modelled using the DLM. At higher Si (T)/Fe ratios, H 4SiO 4 polymerisation on the ferrihydrite surface appeared to become increasingly important. However, even when Si (T)/Fe=1.8 and there would have been significant H 4SiO 4 polymerisation, the DLM continued to predict almost all of the observed effect of H 4SiO 4 on As(III) and As(V) adsorption while taking only H 4SiO 4 adsorption into account. This suggests that H 4SiO 4 adsorption inhibits As adsorption to a greater degree than H 4SiO 4 polymerisation.
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