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