Chen Shiao-Shing, Cheng Chih-Yu, Li Chi-Wang, Chai Pao-Hsuan, Chang Yu-Min
Institute of Environmental Engineering and Management, National Taipei University of Technology, No. 1, Sec. 3, Chung-Hsiao E. Rd., Taipei 106,Taiwan.
J Hazard Mater. 2007 Apr 2;142(1-2):362-7. doi: 10.1016/j.jhazmat.2006.08.029. Epub 2006 Aug 18.
Fluidized zero valent iron (ZVI) process was conducted to reduce hexavalent chromium (chromate, CrO(4)(2-)) to trivalent chromium (Cr(3+)) from electroplating wastewater due to the following reasons: (1) Extremely low pH (1-2) for the electroplating wastewater favoring the ZVI reaction. (2) The ferric ion, produced from the reaction of Cr(VI) and ZVI, can act as a coagulant to assist the precipitation of Cr(OH)(3(s)) to save the coagulant cost. (3) Higher ZVI utilization for fluidized process due to abrasive motion of the ZVI. For influent chromate concentration of 418 mg/L as Cr(6+), pH 2 and ZVI dosage of 3g (41 g/L), chromate removal was only 29% with hydraulic detention time (HRT) of 1.2 min, but was increased to 99.9% by either increasing HRT to 5.6 min or adjusting pH to 1.5. For iron species at pH 2 and HRT of 1.2 min, Fe(3+) was more thermodynamically stable since oxidizing agent chromate was present. However, if pH was adjusted to 1.5 or 1, where chromate was completely removed, high Fe(2+) but very low Fe(3+) was present. It can be explained that ZVI reacted with chromate to produce Fe(2+) first and the presence of chromate would keep converting Fe(2+) to Fe(3+). Therefore, Fe(2+) is an indicator for complete reduction from Cr(VI) to Cr(III). X-ray diffraction (XRD) was conducted to exam the remained species at pH 2. ZVI, iron oxide and iron sulfide were observed, indicating the formation of iron oxide or iron sulfide could stop the chromate reduction reaction.
采用流化零价铁（ZVI）工艺将电镀废水中的六价铬（铬酸盐，CrO(4)(2-)）还原为三价铬（Cr(3+)），原因如下：（1）电镀废水的pH值极低（1-2），有利于ZVI反应。（2）Cr(VI)与ZVI反应产生的铁离子可作为混凝剂，辅助Cr(OH)(3(s))沉淀，从而节省混凝剂成本。（3）由于ZVI的磨蚀运动，流化过程中ZVI的利用率更高。对于进水铬酸盐浓度为418 mg/L（以Cr(6+)计）、pH为2且ZVI投加量为3g（41 g/L）的情况，水力停留时间（HRT）为1.2 min时，铬酸盐去除率仅为29%，但通过将HRT增加到5.6 min或调整pH至1.5，去除率提高到了99.9%。对于pH为2且HRT为1.2 min时的铁物种，由于存在氧化剂铬酸盐，Fe(3+)在热力学上更稳定。然而，如果将pH调整到1.5或1，此时铬酸盐被完全去除，则存在高浓度的Fe(2+)但极低浓度的Fe(3+)。可以解释为ZVI首先与铬酸盐反应生成Fe(2+)，铬酸盐的存在会持续将Fe(2+)转化为Fe(3+)。因此，Fe(2+)是Cr(VI)完全还原为Cr(III)的指标。进行了X射线衍射（XRD）分析以检测pH为2时残留的物种。观察到了ZVI、氧化铁和硫化铁，表明氧化铁或硫化铁的形成可能会阻止铬酸盐还原反应。
