You are here:
ARSENIC INTERACTION WITH IRON (II, III) HYDROXYCARBONATE GREEN RUST: IMPLICATIONS FOR ARSENIC REMEDIATION
Su, C. AND R T. Wilkin*. ARSENIC INTERACTION WITH IRON (II, III) HYDROXYCARBONATE GREEN RUST: IMPLICATIONS FOR ARSENIC REMEDIATION. Presented at 4th Int'l. Battelle Conf, Monterey, CA, May 24 - 27, 2004.
To inform the public.
Zerovalent iron is being used in permeable reactive barriers (PRBs) to remediate groundwater arsenic contamination. Iron(II, III) hydroxycarbonate green rust is a major corrosion product of zerovalent iron under anaerobic conditions. The interaction between arsenic and this green rust is important in determining the fate of arsenic in groundwater. To optimize the design of iron barriers, it is essential to evaluate the influence of geochemical parameters such as arsenic concentration, pH, and time on the interactions of arsenic with iron corrosion products. We synthesized iron(II, III) hydroxycarbonate green rust in the laboratory by neutralizing ferrous sulfate solution with sodium hydroxide and sodium carbonate or sodium bicarbonate followed by air sparging. The synthetic products were characterized with X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, Fourier transform infrared spectroscopy, and wet chemical analysis.
We conducted batch arsenic sorption experiments with arsenate and arsenite in an anaerobic glovebox. The pH ranged from 8 to10.5. Both arsenate and arsenite sorption increased with increasing time up to 30 days. Arsenite showed much higher sorption than arsenate. Adsorbed arsenite (up to 90 mg kg-1) was partially oxidized on solid surface at pH near 10.5. Oxidation of sorbed arsenite could be advantageous because arsenate is generally less toxic and less mobile than arsenite.
Differential scanning calorimetry patterns show one main endothermic peak that ranges from about 110 to 130 oC. Arsenate and arsenite bearing green rusts decompose at a higher temperature compared to As-free green rust. Arsenic tends to stablize the green rust structure via stablization of the surface bonding environment. Examination of the mass spectra data shows that the endothermic decomposition corresponds to the release of H2O and CO2 molecules. Green rust that has been allowed to age (without As) or that forms as a consequence of zerovalent iron corrosion shows more complicated thermal behavior due to mineral transformation and formation of other carbonate minerals (siderite, iron hydroxy carbonate, calcite). Combination of thermogravimetric, wet chemical, and solid phase carbonate analyses confirm the green rust stoichiometry.
These results should have positive implications for field applications of zerovalent iron based PRBs to remediate arsenic in groundwater. Uptake of either arsenate or arsenite by carbonate green rust will likely keep the iron corrosion product stable in the subsurface. Oxidation of arsenite by carbonate green rust reduces the toxicity of this species. Naturally occurring carbonate green rust may be more common than previously thought and it can be an important sink for arsenic. Its contribution to natural attenuation of arsenic in the subsurface merits further study.