Sun Jun, Mao J-D, Gong Hui, Lan Yeqing
College of Sciences, Nanjing Agricultural University, Nanjing 210095, PR China.
J Hazard Mater. 2009 Sep 15;168(2-3):1569-74. doi: 10.1016/j.jhazmat.2009.03.049. Epub 2009 Mar 21.
The photochemical reduction of Cr(VI) by four low-molecular-weight organic acids (tartaric acid, citric acid, malic acid, and n-butyric acid) in the presence of either dissolved Fe(III) in dilute aqueous solution or adsorbed Fe(III) on clay mineral surfaces (kaolinite, montmorillonite and illite) was investigated using batch reactors at a pH range from 3.5 to 4.5 at 25 degrees C. The results indicate that Fe(III) photocatalytic reduction of Cr(VI) by organic acids with alpha-OH is extremely fast. During a reaction period when less than 80% initial Cr(VI) was consumed, the reaction can be described as pseudo-first-order with respect to Cr(VI) when organic acid in excess. By plotting ln[Cr(VI)] as a function of reaction time, rate constants of Cr(VI) reduction by organic acids are obtained. The rate constants involving the four acids are in the order: tartaric acid (with 2 carboxylic groups and 2 alpha-OH groups)>citric acid (with 3 carboxylic groups and 1 alpha-OH group) approximately malic acid (with 2 carboxylic groups and 1 alpha-OH group)>>n-butyric acid (with 1 carboxylic group and no alpha-OH group). This order suggests that the number of alpha-OH but not the number of carboxylic groups is an important determinant of kinetics. With light, the reduction of Cr(VI) by citric acid is accelerated by clay minerals. The enhancement of Cr(VI) reduction is attributed to the catalysis of Fe(III) adsorbed on clay mineral surfaces. However, such an acceleration is markedly suppressed by introducing NaF into the reaction system since NaF forms a complex with Fe(III). It is concluded that the complex formation between Fe(III) and organic acid is a key step for the photocatalytic reduction of Cr(VI) in the presence of Fe(III) and organic acids with alpha-OH.
在25℃、pH值为3.5至4.5的条件下，使用间歇式反应器研究了四种低分子量有机酸（酒石酸、柠檬酸、苹果酸和正丁酸）在稀水溶液中溶解的Fe(III)或粘土矿物表面（高岭石、蒙脱石和伊利石）吸附的Fe(III)存在下对Cr(VI)的光化学还原。结果表明，具有α-OH的有机酸对Fe(III)光催化还原Cr(VI)的反应速度极快。在初始Cr(VI)消耗少于80%的反应期间，当有机酸过量时，该反应对于Cr(VI)可描述为准一级反应。通过绘制ln[Cr(VI)]与反应时间的函数关系图，得到了有机酸还原Cr(VI)的速率常数。涉及这四种酸的速率常数顺序为：酒石酸（含2个羧基和2个α-OH基团）>柠檬酸（含3个羧基和1个α-OH基团）≈苹果酸（含2个羧基和1个α-OH基团）>>正丁酸（含1个羧基且无α-OH基团）。该顺序表明，α-OH的数量而非羧基的数量是动力学的重要决定因素。光照条件下，粘土矿物可加速柠檬酸对Cr(VI)的还原。Cr(VI)还原的增强归因于吸附在粘土矿物表面的Fe(III)的催化作用。然而，由于NaF与Fe(III)形成络合物，向反应体系中引入NaF会显著抑制这种加速作用。得出结论，在Fe(III)和具有α-OH的有机酸存在下，Fe(III)与有机酸之间的络合物形成是光催化还原Cr(VI)的关键步骤。
