Performance of EDTA modified magnetic ZnFe2O4 during photocatalytic reduction of Cr(VI) in aqueous solution under UV irradiation.

The photocatalytic reduction of toxic Cr(VI) to non-pernicious Cr(III) using ZnFe2O4/EDTA (ethylenediaminetetraacetic acid) under UV irradiation was evaluated. The reduction of Cr(VI) with bare ZnFe2O4 under UV irradiation was negligible. However, the Cr(VI) in the solution was completely reduced within 3 h after the introduction of EDTA. EDTA could consume valence band holes and could be oxidized by holes into inorganic products. Therefore, photo-generated electrons could be used to reduce Cr(VI) to Cr(III). The effect of concentration of EDTA, ZnFe2O4 photocatalyst dosages, and initial pH on the photocatalytic reduction of Cr(VI) was investigated. The results revealed that the photocatalytic reduction of Cr(VI) accelerated by increasing EDTA concentration and ZnFe2O4 dosage. The present reduction process confirms the pseudo-first-order kinetic reaction. The quasi reduction rate constant increased from 3.5 x 10-4 min-1 to 2.6 x 10-2 min-1 with the increase in initial EDTA concentration from 0 to 1000 mg L-1. The acidic solution is preferable for the photocatalytic reduction of Cr(VI). The entire reduction of Cr(VI) was carried out within 2 h under the optimum conditions with pH 2, 20 mg ZnFe2O4, and 500 mg L-1 EDTA. The formation of [Cr-EDTA]3+ complex may be advantageous to accelerate the Cr(VI) reduction. A probable mechanism for the photocatalytic Cr(VI) reduction was speculated here.