COMPARISON OF ARSENATE ASSOCIATIONS WITH ALUMINUM MINERALS, MODEL COMPOUNDS AND TWO SOIL TYPES: A BATCH UPTAKE AND SPECTROSCOPIC STUDY

Aluminum oxyhydroxides and clay minerals are important colloid classes that can increase the mobility of trace elements in soils and aquifers. However, these materials can sometimes be difficult to identify in natural soil samples because of their small size and, in some cases, poor crystallinity. For this study, bayerite, gibbsite and poorly crystalline boehmite were synthesized from supersaturated solutions of sodium aluminate at varying [NaOH]/[Al] ratios and base concentrations. Phases were identified with X-ray diffraction and infrared spectroscopy. These materials, along with natural diaspore, calcined diaspore (corundum), clinochlore, and the <63 µm size fraction of three alfisols from a carbonate-bedrock watershed, were subject to batch uptake experiments in sodium arsenate solutions in the pH range 3 to 9, at 100 µM As/L, and solid:As mass ratios of 267 or 950. Of the synthetic samples, the poorly crystalline boehmite demonstrated the highest As uptake, 3750 ppm (100%) at pH 5-6. Bayerite retained ~500 ppm at pH6 and gibbsite retained ~ 250 ppm. For the natural mineral samples: clinochlore retained the most As (750 ppm) at pH 3 despite partial dissolution. At pH 6, the corundum retained ~130 ppm As and the diaspore retained ~900 ppm. The natural soil samples retained 20-200 ppm As. Raman spectral peaks were shifted -6 to +13 cm^-1 when arsenate was adsorbed to gibbsite or bayerite, but Raman shifts were not observed for boehmite with adsorbed arsenate. As K-edge EXAFS spectroscopy was performed on samples with highest As uptake, and on the <63 µm fraction of samples from a smelter-contaminated inceptisol developed on unconsolidated glacial sediments, with 270-860 ppm As. Both soil types show inner sphere bidentate interactions between As tetrahedra and Al substrates, with As-O distances about 1.68 Å and As-Al distances 3.16-3.20 Å. Arsenate association with the soils developed on carbonate bedrock appears similar to arsenate on gibbsite or bayerite, whereas As EXAFS for the smelter-impacted soil more closely resembles arsenate associated with clinochlore (which was identified in the samples), and is best fit when a contribution from multiple scattering within the arsenate tetrahedron is also included.