A Gel Probe Equilibrium Sampler for Measuring Arsenic Porewater Profiles and Sorption Gradients in Sediments: I. Laboratory Development

Abstract

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.

Silver Release from Silver Nanoparticles in Natural Waters

“…Because the gel puck preparation requires melting the agarose at 60 °C, a control experiment was carried out to ascertain the effect of the temperature, and no additional silver release was measured due to the preparation step (see S1 of the Supporting Information). − …”

A Gel Probe Equilibrium Sampler for Measuring Arsenic Porewater Profiles and Sorption Gradients in Sediments: II. Field Application to Haiwee Reservoir Sediment

“…The iron content of HFO-doped gels was 2 × 10 -6 mol Fe/gel slab. A complete description of gel synthesis, re-equilibration and analytical methods is provided in Part I (11). Sorption onto HFO-doped gels is normalized to the amount of Fe in each gel slab (mol sorbate/mol Fe).…”

“…Phosphate competes directly with both As(III) and As(V) for surface sites on Fe oxides (2,3,(5)(6)(7)(8)(9)(10), but has a stronger inhibitory effect on As(V) (see Part I,11). Inorganic carbon may also suppress As adsorption when concentrations are elevated in groundwater because of microbial respiration of organic carbon and/or carbonate mineral dissolution (12)(13)(14)(15).…”

Speciation and natural attenuation of arsenic and iron in a tidally influenced shallow aquifer

“…Relative proportions of sulfide, As(III), and As(V) contributions to the XANES spectra were determined by linear combination fits using reference compounds ( Table 2). The XANES fits are semi-quantitative because they have not been standardized against known mixtures of reference compounds to account for differential absorption/fluorescence response for different arsenic species (Campbell et al, 2008). All XAS data were collected at cryogenic temperature and beam-induced sample damage was not observed during data collection.…”