Department of Chemistry, College of Sciences, Nanjing Agricultural University, Nanjing 210095, PR China.
J Hazard Mater. 2013 Sep 15;260:150-6. doi: 10.1016/j.jhazmat.2013.05.010. Epub 2013 May 15.
The effects of Al (III) on the reduction of Cr (VI) by three α-hydroxy acids, tartaric, malic and citric acids, were investigated through batch experiments at pH from 2.5 to 4.0 and temperatures from 25 °C to 35 °C. These reactions could be described as pseudo-zero-order with respect to Cr (VI) when the concentrations of α-hydroxy acids were greatly in excess. The transformation rates of Cr (VI) to Cr (III) in the presence of Al (III) without light were in the decreasing order of tartaric acid>malic acid>citric acid. This order suggested that the two α-hydroxyl groups in tartaric acid could play an important role in the reduction of Cr (VI) by organic acids. The possible mechanism was that the formed complex between organic acids and Al (III) significantly enhanced the reductivity of α-hydroxy acids and further led to the more complicated Cr (VI)-tartaric acid-Al (III) cyclic ester which greatly accelerated the reduction rate. The Cr (VI) reduction reaction rate increased with the decrease of pH but with the increase of Al (III) concentration, tartaric acid concentration, and temperature. As the pH decreased, the increase of [H(+)] led to an increase in {Al(III)H₂Tar₂}(+), the most active species, and thus the enhanced reduction rate.
在 pH 值为 2.5 至 4.0，温度为 25°C 至 35°C 的条件下，通过分批实验研究了 Al(III)对三种 α-羟基酸（酒石酸、苹果酸和柠檬酸）还原 Cr(VI)的影响。当 α-羟基酸的浓度大大过剩时，这些反应可以描述为对 Cr(VI)的伪零级反应。在没有光照的情况下，Al(III)存在时 Cr(VI)向 Cr(III)的转化速率按酒石酸>苹果酸>柠檬酸的顺序递减。这一顺序表明，酒石酸中的两个 α-羟基在有机酸还原 Cr(VI)中起着重要作用。可能的机制是，有机酸与 Al(III)形成的配合物显著增强了 α-羟基酸的还原性，进一步导致更复杂的 Cr(VI)-酒石酸-Al(III)循环酯，从而大大加速了还原速率。Cr(VI)还原反应速率随 pH 值的降低而增加，但随 Al(III)浓度、酒石酸浓度和温度的增加而增加。随着 pH 值的降低，[H(+)]的增加导致最活跃的物种{Al(III)H₂Tar₂}(+)的增加，从而提高了还原速率。
