The strangely simple effect of silicic acid on arsenic sorption by ferrihydrite

摘要

Silicic acid (H_4SiO_4) adsorbs as monomers and also oligomerizes on ferrihydrite surfaces. In this way H_4SiO_4 can cause elevated dissolved arsenic concentrations in oxic aquatic systems. The H_4SiO_4 adsorption and interfacial oligomerization has a complex dependence on pH and ionic strength. Despite this complexity the surface concentration of Si (Γ_(si)) and the effect of this Si on As(III) and As(V) adsorption by ferrihydrite is, within limits, reasonably accurately described using a model that considers all surface Si as monomeric adsorbed species. The inference is that the silicate oligomerization reaction does not greatly alter the overall stability of the surface Si or result in the occupation of significantly more surface sites. Methods to quantify Γ_(si) on ferrihydrite in natural sediments using EDTA and oxalate extractions are also discussed.