Campbell, Kate M. and Root, Robert and O'Day, Peggy A. and Hering, Janet G. (2008) A Gel Probe Equilibrium Sampler for Measuring Arsenic Porewater Profiles and Sorption Gradients in Sediments: I. Laboratory Development. Environmental Science and Technology, 42 (2). pp. 497-503. ISSN 0013-936X http://resolver.caltech.edu/CaltechAUTHORS:CAMest08a
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A gel probe equilibrium sampler has been developed to study arsenic (As) geochemistry and sorption behavior in sediment porewater. The gels consist of a hydrated polyacrylamide polymer, which has a 92% water content. Two types of gels were used in this study. Undoped (clear) gels were used to measure concentrations of As and other elements in sediment porewater. The polyacrylamide gel was also doped with hydrous ferric oxide (HFO), an amorphous iron (Fe) oxyhydroxide. When deployed in the field, HFO-doped gels introduce a fresh sorbent into the subsurface thus allowing assessment of in situ sorption. In this study, clear and HFO-doped gels were tested under laboratory conditions to constrain the gel behavior prior to field deployment. Both types of gels were allowed to equilibrate with solutions of varying composition and re-equilibrated in acid for analysis. Clear gels accurately measured solution concentrations (±1%), and As was completely recovered from HFO-doped gels (±4%). Arsenic speciation was determined in clear gels through chromatographic separation of the re-equilibrated solution. For comparison to speciation in solution, mixtures of As(III) and As(V) adsorbed on HFO embedded in gel were measured in situ using X-ray absorption spectroscopy (XAS). Sorption densities for As(III) and As(V) on HFO embedded in gel were obtained from sorption isotherms at pH 7.1. When As and phosphate were simultaneously equilibrated (in up to 50-fold excess of As) with HFO-doped gels, phosphate inhibited As sorption by up to 85% and had a stronger inhibitory effect on As(V) than As(III). Natural organic matter (>200 ppm) decreased As adsorption by up to 50%, and had similar effects on As(V) and As(III). The laboratory results provide a basis for interpreting results obtained by deploying the gel probe in the field and elucidating the mechanisms controlling As partitioning between solid and dissolved phases in the environment.
|Additional Information:||Copyright © 2008 American Chemical Society. Freely available via the ACS AuthorChoice program. Received for review May 13, 2007. Revised manuscript received October 1, 2007. Accepted October 5, 2007. Web Release Date: December 12, 2007. This work was supported by funding from NSF BES-0201943, NSF BES-0201888, and EAR-0525387. We thank Nathan Dalleska for analytical support, Mike Vondrus for gel probe construction, Megan Ferguson for LC-ICP-MS method development, Suvasis Dixit for initial project development, and Nelson Rivera for help at the SSRL beamline and for assistance in fitting the XANES calibration. Portions of this research were carried out at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf-of the U.S. Department of Energy, Office of Basic Energy Sciences. Supporting Information Available: Plots of As recovery from clear and HFO-doped gels, variation in As sorption as a function of Fe concentration, As adsorption kinetics, As adsorption isotherm at pH 8, amount of P adsorbed in competitive phosphate experiments, adsorption of As in presence of SR-NOM. This material is available free of charge via the Internet at http://pubs.acs.org.|
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|Deposited On:||24 Jul 2008 18:29|
|Last Modified:||26 Dec 2012 10:10|
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