Chromate reduction by waste iron from electroplating wastewater using plug flow reactor

Waste iron was used to treat high concentration chromate (534 mg/L as Cr) from electroplating wastewater by plug flow reactor (PFR) due to the following reasons: (1) two wastes are treated simultaneously, (2) low pH of the electroplating wastewater (≈2) benefits the reaction between these two wastes, (3) effluent pH is elevated in the PFR, reducing the base requirement to meet the pH discharge standard for wastewater (pH 6-9). Complete chromate reductions were achieved at pH 1.7 for hydraulic retention time (HRT) of 98 min, pH 1.5 for HRT of 40 min and pH 1.3 for HRT of 20 min. Consequently, optimum HRT for complete chromate reduction was obtained for different pHs. Although more acids were used to lower influent pH to reduce HRT, effluent pH was higher due to more hydrogen ion reacting with chromate. Eventually, fewer bases are required to fulfill the discharge pH requirement of wastewater. Effluent pH 3-5 was observed with high turbidity, indicating the precipitations of chromium oxide and hydroxide were enhanced by the dissolved iron coagulation. X-ray diffraction was conducted to examine the remaining species. Other than chromium oxide and hydroxide species, an iron-chromium complex (Cr2FeO4) was also observed.