Development of the Diffusive Gradients in Thin Films Technique for the Measurement of Inorganic Arsenic Speciation in Water and the Investigation of Arsenic Sediment Biogeochemistry

Abstract

A novel diffusive gradients in thin films (DGT) technique for the measurement of inorganic arsenic speciation in situ was developed and evaluated. In addition, a diffusive equilibration in thin films (DET) technique was optimized for the high-resolution, colorimetric measurement of iron(II) in sediment porewaters. These techniques were then utilized to investigate processes of arsenic mobilization in sediments and the effect of changing redox conditions on the cycling of arsenic in freshwater and marine sediments. A titanium dioxide-based adsorbent, Metsorb™, was evaluated as a DGT adsorbent for measuring total inorganic arsenic (AsIII and AsV), as well as selenite (SeIV). Performance characteristics such as diffusion coefficients, linear accumulation over time, the effect of pH and ionic strength, and the capacity of the adsorbent were tested and found to be acceptable. The method was also evaluated in the field and found to accurately and precisely measure environmentally relevant arsenic concentrations. The speciation of inorganic arsenic was achieved by developing a second DGT technique that selectively measured AsIII in the presence of AsV. This new technique utilized a mercaptopropylfunctionalized silica gel as a selective adsorbent for AsIII, allowing the speciation of total inorganic arsenic to be calculated by deploying both sampler types simultaneously and calculating the AsV concentration by difference. This new approach to in situ arsenic speciation was successfully evaluated at a variety of pH levels and ionic strengths, and in the challenging matrix of seawater. The newly developed DGT techniques for arsenic speciation were applied to the investigation of arsenic mobility in sediments. To aid in this, a recently described colorimetric DET technique was optimized for the measurement of iron(II) concentrations in sediment porewater. The diffusive gel located at the front of the DGT samplers was utilized as the DET gel for colorimetric analysis of iron(II), enabling co-distributions of these analytes to be measured in situ. The combined DET – DGT approach was applied to freshwater, estuarine and marine sediment mesocosms to investigate the relationship between the reductive dissolution of iron(III) (hydr)oxide minerals and the release of arsenic to sediment porewaters. The effect of induced anoxia and subsequent reoxygenation on arsenic and iron cycling within freshwater and marine sediments was also investigated using sediment mesocosm incubations and the newly developed DET and DGT techniques. These techniques, used in conjunction with sediment mesocosms, proved to be a powerful new approach for the investigation of arsenic biogeochemistry and the mechanisms of arsenic mobilization in the environment.