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Chromium Recovery From Electroplating Industry Wastewaters Using Metal Exchange Process

Congress: 2015
Author(s): Abdelaziz Gherrou (Montreal, Canada)

Keyword(s): Sub-theme 10: Management of water resources,
AbstractElectroplating industries generates large volumes of acidic effluents contaminated with hexavalent chromium. The environmental regulations fix the discharge limits of this toxic metal to 2.5 ppm (example of the city of Montreal, Canada). Hence, the installation of efficient wastewater treatment systems able to solve this problem is required. Large number of facilities worldwide uses a chemical and physical process combining reduction-coagulation-flocculation for chromium recovery. However, this process utilizes large quantities of chemicals and some of them are dangerous. On the other hand, it generates large quantities of hazardous sludge which is often disposed in landfills. A new approach has been developed by some authors who use zero-valent iron to substitute the sodium bisulfite reduction step. We applied this approach at laboratory scale to treat a wastewater sample from an electroplating facility containing 18 ppm of Cr(VI) using a steel wool as a source of iron. The mini-column tests showed that the kinetic of the reaction is very low when treating such concentrated wastewater.

According to Gheju (1), Cr(VI) reduction to Cr(III) is accomplished according to the following reactions : Direct reaction: 2HCrO4- (aq) + 3 Fe0 (s) + 14 H+ (aq) → 3 Fe2+ (aq) + 2 Cr3+ (aq) + 8 H20 (1) Indirect reaction: HCrO4- (aq) + 3 Fe2+ (aq) + 7 H+ (aq) → 3 Fe3+ (aq) + Cr3+ (aq) + 4 H20 (2) Overall reaction: HCrO4- (aq) + Fe0 (s) + 7 H+ (aq) → Fe3+ (aq) + Cr3+ (aq) + 4 H20 (3)

The low kinetic of the reaction (3) is then affected by the quantity of Fe(II) ions generated by reaction (1). However, we supposed that by increasing this quantity we could enhance the yield of Cr(VI) reduction and the kinetics of reaction (3).This could be done by an oxygenation of the sample. In fact, under aerobic conditions, in the absence of other oxidized species, dissolved oxygen is the electron acceptor: 2Fe0 (s) + O2(g) + 2H2O (l) → 2Fe2+ (aq) + 4OH- (aq) (4) Thus, we have dosed the wastewater sample with pure oxygen in such a way to increase the dissolved oxygen concentration. Table 1 shows that when increasing dissolved oxygen concentration, the yield of chromium recovery increased drastically to almost 100% (below the detection limit of the atomic absorption instrument used for total chromium analysis)

Table 1: Total chromium concentration at the outlet column. [Cr tot.]init.= 18 ppm. Flow rate 10 ml/min. pH = 3 1.Gheju, M., Water, Air, Soil Pollut., DOI 10.1007/s11270-011-0812-y.
2.Gheju, M., Iovi, A. and Balcu, I., J. Hazard. Mater., 153, 2008, 655-662.
3.Ritu Singh, Virendra Misra, Rana Pratap Singh, Environmental Monitoring and Assessment, 184 (6), 2012, 3643-3651

2011 IWRA - International Water Resources Association - - Admin