Reduction of chromate from electroplating wastewater from pH 1 to 2 using fluidized zero valent iron process

摘要

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: Extremely low pH (1-2) for the electroplating wastewater favoring the ZVI reaction. 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. Higher ZVI utilization for fluidized process due to abrasive motion of the ZVI. For influent chromate concentration of 418mg/L as Cr~(6+), pH 2 and ZVI dosage of 3 g (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.