The effect of citrate, oxalate, acetate, silicate and phosphate on stability of synthetic arsenic-loaded ferrihydrite and Al-ferrihydrite.

The effect of the organic species oxalate, citrate and acetate and the inorganic species silicate and phosphate on release of As(V) from synthetic arsenic-loaded ferrihydrite and Al-ferrihydrite in the pH range 4-8 was investigated. For ferrihydrite, the organic species increased %As(V) extraction in the order: acetate<oxalate<<citrate. In each case, the %As(V) extraction increased when acidity changed to pH's lower than 6, but remained unchanged when the pH increased from 6 to 8. For Al-ferrihydrite, the trend for %As release as a function of pH in the presence of acetate and oxalate was similar to that for ferrihydrite. However, unlike the ferrihydrite case, in the presence of citrate the %As extraction went through a minimum at pH 6. This is postulated to be related to the solubility equilibria of Al-ferrihydrite as a function of pH. The co-presence of silicate or phosphate with each of the organic species enhanced As(V) extraction in the order SiO3(2-)>PO4(3-). Competitive adsorption of the investigated organic and inorganic species on ferrihydrite and Al-ferrihydrite could explain the enhanced release of As(V) into solution. However, disproportionate adsorption of other species relative to amount of As(V) release occurs, suggesting that mechanisms other than competitive adsorption are also operative. The results could best be explained in terms of combination of competitive adsorption and processes involving pH-dependent dissolution of ferrihydrite/Al-ferrihydrite, formation of various complex ionic species involving Al(III), Fe(III), As(V), SiO3(2-), PO4(3-), OH-, H+ and organic species: acetate, oxalate and citrate.