Formation of Metal-Arsenate Precipitates at the Goethite-Water Interface

Environ. Sci. Technol., 38 (24), 6561 -6570, 2004. 10.1021/es035166p S0013-936X(03)05166-6
Web Release Date: October 26, 2004

Formation of Metal-Arsenate Precipitates at the Goethite-Water Interface

Markus Gräfe,* Maarten Nachtegaal, and Donald L. Sparks

Environmental Soil Chemistry Group, Department of Plant and Soil Sciences, 152 Townsend Hall, University of Delaware, Newark, Delaware 19717-1303

Received for review October 20, 2003

Revised manuscript received July 26, 2004

Accepted August 16, 2004

Abstract:

Little information is available concerning cosorbing oxyanion and metal contaminants in the environment, yet in most metal-contaminated areas, cocontamination by arsenate [AsO₄, As(V)] is common. This study investigated the cosorption of As(V) and Zn on goethite at pH 4 and 7 as a function of final solution concentration. Complimentary extended X-ray absorption fine structure (EXAFS) spectroscopic data were collected at the As and Zn K-edges in order to glean information about the coordination environment of As and Zn at the goethite-water interface. Macroscopic sorption studies revealed that As(V) and Zn sorption on goethite increased in cosorption experiments beyond that suggested by single sorption isotherms. At pH 4 and 7, As(V) surface saturation was 3.2 and 2.2 mol m^-², respectively, and Zn surface saturation was absent at pH 4 and ~ 1.0 mol m^-² at pH 7. Arsenate sorption on goethite increased in the presence of Zn by 29% and by more than 500% at pH 4 and 7, respectively. In the presence of As(V), Zn sorption on goethite increased by 800 and 1300% at pH 4 and 7, respectively. More As(V) than Zn sorbed on goethite below surface saturation at pH 7. Above surface saturation, the Zn:As surface density ratio (SDR) remained constant at 0.91 ± 0.03. At pH 4, the Zn:As SDR was less than 1 throughout the concentration range. Below As(V) surface saturation on goethite, As(V) formed bidentate binuclear bridging complexes on Fe and/or Zn octahedra, while Zn mainly formed edge-sharing complexes with Fe at the goethite surface. Above surface saturation, Zn was increasingly complexed by AsO₄, gradually forming an adamite-like [Zn₂(AsO₄)OH] surface precipitate on goethite. Precipitated contaminants are more stable due to the limited dissolution kinetics of their solid phase. This study may therefore prove useful in remediation strategies of sites knowingly contaminated with oxyanions and metals.

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