Abstract

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 10⁻⁴min⁻¹ to 2.6 x 10⁻²min⁻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.

摘要

评估了使用ZnFe ₂ O ₄ / EDTA（乙二胺四乙酸）在紫外线照射下将有毒的Cr（VI）光催化还原为有害的Cr（III）。在紫外线照射下用裸露的ZnFe ₂ O ₄还原Cr（VI）可以忽略不计。但是，引入EDTA后3小时内溶液中的Cr（VI）完全还原。EDTA会消耗价带孔，并可能被空穴氧化成无机产物。因此，光生电子可用于将Cr（VI）还原为Cr（III）。EDTA，ZnFe ₂ O ₄浓度的影响研究了光催化剂的用量以及初始pH值对光催化还原Cr（VI）的影响。结果表明，通过增加EDTA浓度和ZnFe ₂ O ₄剂量可以加速光催化还原Cr（VI）。本还原过程证实了拟一级反应。准还原速率常数从3.5×10增加^-4分钟^-1〜2.6×10 ^-2分钟^-1与从0到1000毫克的L在初始EDTA浓度的增加^-1。酸性溶液优选用于光催化还原Cr（VI）。Cr（VI）的全部还原是在最佳条件下，pH为2、20 mg ZnFe的2小时内完成的₂ O ₄和500 mg L ^-1 EDTA。[Cr-EDTA] ³⁺络合物的形成可能有利于加速Cr（VI）的还原。在此推测了光催化还原Cr（VI）的可能机理。