Submitted to: Chemical Geology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/11/2005
Publication Date: 4/16/2006
Citation: Gao, S., Goldberg, S.R., Chalmers, A.T., Herbel, M.J., Fujii, R., Tanji, K.K. 2006. Sorption processes affecting arsenic solubility in Tulare lake bed sediments, California. Chemical Geology, Vol. 228, pgs. 33-43. Interpretive Summary: The presence of arsenic in drinking water is a major problem worldwide because of its carcinogenic effects. Elevated levels of arsenic in ground waters in the Tulare Basin in California have been of great concern to the municipalities using arsenic-contaminated ground water for potable waters. The contribution of arsenic in the sediments to groundwater and the potential of migration into deep aquifers is not well understood. The main purpose of this research was to examine adsorption and desorption processes controlling the solubility of arsenic in Tulare Lake bed sediments, especially the shallow oxidized sediments that contain naturally high concentrations of arsenic. This study determined sorption isotherms, pH effect on soluble arsenic and sorption hysteresis phenomenon, and used a surface complexation model to describe sorption. Adsorption and desorption were determined as the major processes controlling arsenic solubility in the sediments. The results showed that arsenic exhibited differing behavior in sediments that are subjected to different degrees of desorption. The findings from this study contributed scientific information in understanding the processes and major factors affecting As solubility in natural sediments in Tulare Basin.
Technical Abstract: Elevated concentrations of arsenic (As) in shallow groundwater in Tulare Basin pose an environmental risk because of the carcinogenic properties of As and the potential for its migration to deep aquifers that could serve as a future drinking water source. Adsorption and desorption are hypothesized to be the major processes controlling As solubility in oxidized surface sediments where arsenate [As(V)] is dominant. This study examined the relationship between sorption and desorption processes and arsenic solubility in shallow sediments from the dry Tulare Lake bed by determining sorption isotherms, pH effect on soluble As, and sorption hysteresis phenomenon, and by using a surface complexation model to describe sorption. The sediments showed a high capacity to adsorb As(V), the primary As species in this system. Estimates of the maximum adsorption capacity were 92 mg/Kg at pH 7.5 and 70 mg/Kg at pH 8.5 obtained using the Langmuir adsorption isotherm. Soluble arsenic [>97% As(V)] did not increase dramatically until above pH 10. In the native pH range (7.5-8.5), soluble As concentrations were close to the lowest, indicating that As was strongly retained on the sediment. A surface complexation model, the constant capacitance model was able to provide a simultaneous fit to both adsorption isotherms (pH 7.5 and 8.5) and the adsorption envelope (pH effect on soluble As), although the data ranges are one order of magnitude different. Hystereses were observed in the desorption-readsorption processes. Two phases were observed in sediment samples extensively extracted (or desorbed) where As re-added to the system partitioned differently between the solid phase and the solution phases. The findings suggest that diffusion processes may control As adsorption and desorption in the lower As concentration range for extensively leached soils. The significance of the hysteresis phenomenon in affecting As solubility and mobility may be better understood by further microscopic studies of As interaction mechanisms with sediments.