ADSORPTION OF ARSENIC ONTO NATURAL RIVER SEDIMENTS

Many field investigations have noted the relationship between shallow groundwater rich in sodium (Na) and bicarbonate (HCO₃) and elevated levels of dissolved arsenic. One commonly cited explanation for this relationship is that HCO₃ competes with arsenic anions for adsorption sites on mineral surfaces. However, most experimental work on arsenic adsorption in the presence of HCO₃ has proven difficult to extrapolate to natural systems because of differences in tested mineral compositions and component concentrations. In this study, we performed experiments to test whether the presence of high levels of HCO₃ in groundwater can substantially influence the extent of adsorption of arsenic onto river sediments. We conducted batch adsorption experiments using 0.5 grams of river sediment and electrolyte solutions of 0.1M and 0.01M of NaCl, NaHCO₃, CaCl₂, or MgCl₂ spiked with 10 mg/L arsenic. We focused on arsenic(V) rather than arsenic (III) because the former tends to dominate in oxidized shallow groundwater environments. Arsenate adsorbed strongly to the sediment under all conditions, but the experiments with Na and HCO₃ greatly increased the mobility of arsenic (i.e., less adsorption was observed) relative to systems with Na and Cl. These results support the interpretation that HCO₃ is a strong competitor with arsenate for mineral surface adsorption sites in natural sediments. Systems with divalent cations (e.g., Ca or Mg) adsorbed substantially more arsenic than systems with monovalent cations (e.g., Na). This may be attributable to the presence of ternary arsenic-divalent cation-mineral surface complexes and/or an electrostatic effect that promotes additional adsorption of arsenic. Future work will include surface complexation modeling and EXAFS analysis to elucidate the adsorbed arsenic species. By understanding the role of water geochemistry in controlling arsenic adsorption, we can better understand where and why elevated concentration of arsenic occur in groundwater.