Adsorption of As(V) and As(III) by nanocrystalline titanium dioxide

This study evaluated the effectiveness of nanocrystalline titanium dioxide (TiO2) in removing arsenate [As(V)] and arsenite [As(III)] and in photocatalytic oxidation of As(III). Batch adsorption and oxidation experiments were conducted with TiO2 suspensions prepared in a 0.04 M NaCl solution and in a challenge water containing the competing anions phosphate, silicate, and carbonate. The removal of As(V) and As(III) reached equilibrium within 4 h and the adsorption kinetics were described by a pseudo-second-order equation. The TiO2 was effective for As(V) removal at pH<8 and showed a maximum removal for As(III) at pH of about 7.5 in the challenge water. The adsorption capacity of the TiO2 for As(V) and As(III) was much higher than fumed TiO2 (Degussa P25) and granular ferric oxide. More than 0.5 mmol/g of As(V) and As(III) was adsorbed by the TiO2 at an equilibrium arsenic concentration of 0.6 mM. The presence of the competing anions had a moderate effect on the adsorption capacities of the TiO2 for As(III) and As(V) in a neutral pH range. In the presence of sunlight and dissolved oxygen, As(III) (26.7 μM or 2 mg/L) was completely converted to As(V) in a 0.2 g/L TiO2 suspension through photocatalytic oxidation within 25 min. The nanocrystalline TiO2 is an effective adsorbent for As(V) and As(III) and an efficient photocatalyst.