Closer Look at As(III) and As(V) Adsorption onto Ferrihydrite under Competitive Conditions

Batch experiments were conducted in order to investigate the competitive interaction of arsenite (As(III)) and arsenate (As(V)) onto ferrihydrite as a function of initial pH, adsorbent dosage, concentration of coexisting ligands, and order of addition. The pH generally had a great impact on adsorption under both single ion and competitive conditions. However, the amount of As(V) in solution was the controlling factor of adsorption behavior, and As(III) more or less outcompeted As(V) across the pH scale from 4 to 10. Under competitive conditions, i.e., both species were present at the same time, As(III) and As(V) were adsorbed almost equally up to a pH of 5 at an adsorbent dosage of 0.5 g/L and up to a pH of 6 at an adsorbent dosage of 1 g/L. This was contrary to the theoretical prediction that As(V) should adsorb more strongly than As(III) at pH values below the point of zero charge (pzc) of ferrihydrite of about 7 to 8. At low pH, As(V) impedes the adsorption of As(III) but to lesser degree than As(III) impedes As(V) adsorption at a pH above 6. The effect of As(III) on the adsorption of As(V) increased with an increase in pH, and the adsorption of As(V) was almost absent at pH 9 at an adsorbent dosage of 1 g/L and at pH 8 at an adsorbent dosage of 0.5 g/L. In the range of ferrihydrite dosages from 0.2 to 1.6 g/L, As(III) was adsorbed preferentially over As(V) under the availability of less adsorbent. The order of anion addition also had significant effects on their competitive adsorption behavior: the first species was always more favored to compete for the adsorbing sites than when the two species were added to the suspensions simultaneously.