Hexavalent chromium reduction

Treatment for the removal of chromium and nickel from electroplating wastewater involves neutralization, hexavalent chromium reduction, pH adjustment, hydroxide precipitation, and final solid-liquid separation.15 37 48... 

Hexavalent chromium reduction through the use of sulfur dioxide and sodium metabisulfite has found the widest application in the metal finishing industry. It is not truly a treatment step, but a conversion process in which the hexavalent chromium is converted to trivalent chromium. The hexavalent chromium is reduced through the addition of the reductant at a pH in the range of 2.5-3 with a retention time of approximately 30-40 min (Figure 9.7). 

FIGURE 9.7 Hexavalent chromium reduction. (Adapted from U.S. EPA, Meeting Hazardous Waste Requirements for Metal Finishers, Report EPA/625/4-87/018, U.S. Environmental Protection Agency, Cincinnati, OH, 1987.)... 

Gould, 3.P., 1982. The kinetics of hexavalent chromium reduction by metallic iron. Water Res., 16 871-878. 

Fig. 1. Hexavalent chromium reduction with sulfur dioxide.

The chemical reduction process normally generates only small amounts of sludge due to minor shifts in the solubility of the contaminants. In a chromium reduction process, the reduced chromium and other metal ions are precipitated and removed in the subsequent precipitation-sedimentation process. An exception would be hexavalent chromium reduction with ferrous sulfate, where sludge generation may be significant. In a dechlorination process, there will be no chlorine residue, nor reducing agent residue, if the dechlorination process is operated properly. 

Forsyth, P.G. and J.J. Morrell. Hexavalent Chromium Reduction in Chromated Copper Arsenate-treated Douglas-fir and Western Hemlock. Forest Prod. J. 1990,40(6) 48-50. 

Smith WL. (2001). Hexavalent chromium reduction and precipitation by sulphate-redncing bacterial biofilms. Environmental Geochemistry Health 23(3) 297-300. 

Huang LP, Chai XL, Chen GH, Logan BE. Effect of set potential on hexavalent chromium reduction and electricity generation from biocathode microbial fuel cells. Environ Sci Technol 2011 45 5025-5031. 

In removing excess free chlorine from municipal or industrial water and from wastewater, sodium sulfite competes with bisulfite or sulfur dioxide. Other commercial appHcations of sodium sulfite in wastewater treatment include the reduction of hexavalent chromium to the less toxic Cr " salts as well as the precipitation of silver and mercury. 

The key to hexavalent chromium s mutagenicity and possible carcinogenicity is the abiHty of this oxidation state to penetrate the cell membrane. The Cr(VI) Species promotes DNA strand breaks and initiates DNA—DNA and DNA-protein cross-links both in cell cultures and in vivo (105,112,128—130). The mechanism of this genotoxic interaction may be the intercellular reduction of Cr(VI) in close proximity to the nuclear membrane. When in vitro reductions of hexavalent chromium are performed by glutathione, the formation of Cr(V) and glutathione thiyl radicals are observed, and these are beHeved to be responsible for the formation of the DNA cross-links (112). 

It is the sulfurous acid produced from these reactions that is responsible for the reduction of hexavalent chromium. The reaction is shown in the following equation ... 

FIGURE 6.2 Rate of reduction of hexavalent chromium in the presence of excess S02 at various pH levels. (Taken from Krofta, M. and Wang, L.K., Design of Innovative Flotation-Filtration Wastewater Treatment Systems for a Nickel-Chromium Plating Plant, U.S. Department of Commerce, National Technical Information Service, Springfield, VA, Technical Report PB-88-200522/AS, January 1984.)... 

As a practical example, following the reduction of hexavalent chromium, sodium hydroxide, lime, or sodium hydroxide can be added to the wastewater to neutralize the pH and precipitate the trivalent chromium, nickel, iron, divalent, and other heavy metals. If lime is used, lime will react with heavy metals and with any residual sodium sulfate, sulfurous acid, or sodium bisulfite. The following reactions apply ... 

Ferrous sulfide acts as a reducing agent at pH 8 to 9 for reduction of hexavalent chromium and then precipitates the trivalent chromium as a hydroxide in one step without pH adjustment.5162 So, the hexavalent chromium in the nickel-chromium plating wastewater does not have to be isolated and pretreated by reduction to the trivalent form. The new process is applicable for removal of all heavy metals. All heavy metals other than chromium are removed as insoluble metal sulfides, M(II)S. 

Hexavalent chromium wastes resulting from rinsewater and the concentrated acid bleed accumulate in the chromium waste sump [T-20], The chromium wastes are then pumped into the chromium treatment module [T-21] for reduction to the trivalent form. This pump is activated only if the oxidation-reduction potential (ORP) and pH are at the proper levels and if the level in the chromium wastewater sump [T-20] is sufficiently high. 

Liquid flowing into the chromium treatment module [T-21] is monitored by a pH instrument that controls a feed pump to add the required amount of sulfuric acid from a storage tank. The sulfuric acid is needed to lower the pH to 2.0 to 2.5 for the desired reduction reaction to occur. An ORP instrument controls the injection rate of sodium metabisulfite solution from a metering pump to reduce hexavalent chromium (Cr6+) to the trivalent state (Cr3+). 

The selected treatment option involves the reduction of hexavalent chromium to trivalent chromium either chemically or electrochemically. The reduced chromium can then be removed using a conventional precipitation-solids removal system. Alternative hexavalent chromium treatment techniques include chromium regeneration, electrodialysis, evaporation, and ion exchange.16... 

Method Chemical reduction of hexavalent chromium by sulfur dioxide under acid conditions for the continuous operating system and by sodium bisulfite under acid conditions for the batch operating system. The reduced trivalent form of chromium is subsequently removed by precipitation as the hydroxide. 

Reduction Hexavalent chromium Ferrous sulfate arsenic, and results in precipitation of arsenic-iron-manganese compounds. Reduces Cr(VI) to Cr(III)... 

In a very interesting and innovative study recently, the ultrasound-assisted microbial reduction of chromium [22], Mathur et al. reported the reduction of hexavalent chromium using Bacillus sp, isolated from tannery effluent contaminated site. The optimum reduction was found at pH 7 and 37°C. The percent reduction increased with an increase in biomass concentration and decreased with an increase in the initial concentration of hexavalent chromium. 

Franco DV, Da Silva LM, Jardim WF (2009) Reduction of hexavalent chromium in soil and ground water using zero-valent iron under batch and semi-batch conditions. Water Air Soil Poll 197(4) 49-60... 

James B.R. Hexavalent chromium solubility and reduction in alkaline soils enriched with chromite ore processing residue. J Environ Qual 1994 23 227-233. 

Oliver D.D., Brockman F.J., Bowman R.S., Kieft T.L. Microbial reduction of hexavalent chromium under vadose zone conditions. J Environ Qual 2003 32 317-324. 

---