Alternative chromium reduction and heavy metal precipitation methods for industrial wastewater

A study of alternative chromium-reduction and heavy-metal precipitation methods was performed on wastewater generated by a metal-cleaning facility. Four existing chromium-reduction treatment methods were tested on the wastewater samples: treatment with (1) sodium metabisulfite, (2) ferrous sulfate, (3) zero-valent iron, and (4) dimethyl dithio carbamate, ferrous sulfate, and aluminum/chloride. We found that the first two methods are more effective in chrome reduction, while the last method achieved similar reduction results but produced more sludge than any of the other methods. Our results also indicated that when ferrous sulfate was used as a chrome reduction reagent at a low initial pH (2 to 3), the competitive oxidation reaction of ferrous ions with dissolved oxygen in solution is insignificant. When iron powder was used to reduce hexavalent chromium to trivalent chromium, we found that (a) when the sample containing low levels of total chromium (less than or equal to1.54 x 10(-4) mole/liter) was mixed properly with zero-valent iron powders at a low pH (4 to 5), the total chromium concentration could be reduced to 1 mg/l in 3 hours; and (b) the chrome absorption-reduction efficiency is limited by the transport process of Cr(VI)from the bulk of the wastewater to the surface of the iron powder. In a leaching test of used iron powder filtered from the treated wastewater, we also found that most of chromium absorbed on the iron powder was very stable, and only low Cr(VI) was detected in the leachate.