The effect of silicate on the adsorption of arsenate on coprecipitated ferrihydrite

Dissolved silicate competes with the removal of As(V) from aqueous solutions in the ferrihydrite process. It causes an increase in the residual As(V) and Fe(III) in solution. The effect is a function of initial silicate concentration and pH of the solution from which co-adsorption occurs. In the presence of silicate, the residual As(V) in water increases rapidly at pH > 5. For a solution with initial 1 mg/L As and 30 mg/L Fe(III), when silicate is doubled from 20 to 40 mg/L, the coprecipitation/adsorption of As(V) with ferrihydrite at pH 7 results in five times higher residual As(V) (0.05 to 0.28 mg/L). At the same time, the residual Fe(III) in solution is doubled (approximately 4 to 8 mg/L). It is proposed that the silicate effect is due to a combination of complexation reactions between Fe(III), Si(IV) and As(V) species, and competition between As(V) and Si(IV) for adsorption sites on ferrihydrite.