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Ferox Technical Bulletin - Cr(VI) Reduction with Zero-Valent Iron (Fe0)

Recent data demonstrated the effectiveness of ARS’ Feroxsm powder in treating hexavalent chromium (VI) in sediments and water. In a recently completed study, ARS scientists examined the effect of mixing our Ferox zero valent iron powder with sediment taken from the Newtown Creek tributary connected to New York’s East River.

Results of the study indicated the effectiveness of the zero-valent iron powder in reducing Cr(VI) concentration in the sediment. A 99% reduction of the Cr(VI) in the sediments was observed after thirty days of treatment.

Chromium is a common groundwater and soil contaminant particularly in industrial areas. The US EPA action level for total chromium in drinking water is 0.1 mg/L.

In groundwater, chromium occurs in two stable states Cr(III) and Cr(VI). The ionic state of chromium is strongly dependent on the pH and Eh of its environment. Cr(III) is dominated by CrOH2+ in the pH range of 4-6.5 and exists as an insoluble hydroxide Cr(OH)3 at the pH range of 6.5-10.5. Cr(VI) exists as oxyanios HCrO4- (bichromate) at pH less than 6 and CrO42- (chromate) at pH greater than 6. Since Cr(VI) species are charged negatively, their sorption on most soil particles having the same negative charge is poor and further decreases at higher pH values. These chromate oxyanios are of significant environmental concern due to their toxic and carcinogenic properties and their greatly increased subsurface mobility when compared to the relatively less toxic and immobile Cr(III) species.

The feasibility of the chromate reduction with Fe° using zero valent iron is well demonstrated with a large number of documented laboratory and field studies.

Redox reactions with ARS Feroxsm zero-valent iron, involving Fe° oxidation with dissolved oxygen and chromate ion reduction with Fe(II) cations, are shown in equations (1) and (2).

2Fe0 + O2 +H2O = 2Fe2+ +4OH- (1)
Fe2+ + CrO42- 4H2O = Fe(OH)3 +Cr(OH)3 + 2OH- (2)

These reactions increase pH of the system and generation insoluble Cr(III) hydroxide or solid solution (Crx Fe1-x)(OH)3 as a result of the chromate reduction.

The most documented field research for Cr(VI) reduction using zero-valent iron is a USEPA reactive barrier (PRG), project located at USCG Support City, NC. At this site, it has been reported1 that Cr(VI) reduction in a soil water slurry (aquifer core material from the USCG Support Center, Elizabeth City, NC and simulated aquifer water) was considerably higher than in water alone without the solid phase.

This observation is consistent with the reaction mechanism that ARS has observed in its research with sediments; reaction rates are significantly enhanced in the presence of soils than in a groundwater only environment.

R.M.Powell, R.W.Puls, et al. Environ. Sci. Technol. 29, 1913, 1995

T.Boronina, K.J.Klabunde, G.Sergeev. Environ. Sci. Technol. 29, 1511, 1995

D.W.Blowes, C.J.Ptacek, J.L.Jambor. Environ. Sci. Technol. 31, 3348, 1997

A.R.Pratt, D.W.Blowes, C.J.Ptacek. Environ. Sci. Technol. 31, 2492, 1997

S.M.Ponder, J.G.Darab, T.E.Mallouk. Environ. Sci. Technol. 34, 2564, 2000
tests and analysis of PRB performance are reported and available from NTIS6,7.

R.W.Puls, C.J.Paul, P.J.Clark. The 213th National Meeting of the ACS, Division of Environ. Chem.,37,pp.241-243, San Francisco, Ca,1997. Available as EPA/600/A-97/002

R.W.Puls, D.W.Blowes, R.W.Gillham.. Thesis of Intern. Conf. On Groundwater Quality, Tubingen, Germany. Available EPA/600/A-98/085

Use of Zero-Valent Iron Powder for the Treatment of Contaminated Dredge Material and Marine Sediments” (NJCST Grant Award No. 98-2890-020-13)

 

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