Factors Affecting Arsenic Mobilization in Experimental Subsurface Systems
Type of DegreeDissertation
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Elevated concentrations of arsenic (As) in groundwater is a widespread problem that affects millions of people around the world. Other than the adverse affects on human health, this problem often raises economic, social and political issues in the affected nations. Understanding processes that dictate the transport of As in the subsurface is the first step towards finding the solution of this problem. Transport of As, hence its concentration in groundwater, is controlled by processes of adsorption/desorption, oxidation/reduction and reduction/dissolution and it depends on both physical (flow) and chemical (pH, presence of competing ions) conditions. Due to their abundance in natural systems, iron oxides are often used in laboratory experiments as a model media for studying processes of adsorption and desorption of As species. Oxides of manganese are used in As oxidation studies. vi Although many conclusions can be drawn from batch experiments, creating simulated dynamic transport of As in experimental subsurface systems gives a great insight into transport processes and better simulate naturally occurring conditions. Even though As(III) is often assumed to be more mobile than As(V), it was shown in laboratory experiments that under certain conditions, As(V) becomes more mobile. The presence of competing ions often influences As concentration in groundwater by competing for adsorptive sites on mineral surfaces, and/or desorbing previously adsorbed species and releasing them to the solution. The ability of phosphate to mobilize As was studied in column experiments. The presence of carbonate in groundwater is often associated with elevated As concentrations and is pointed to as one of the key factors controlling As distribution in subsurface. This is a topic of scientific dispute, as there are also researchers reporting no effect of carbonate presence. Using a novel experimental setup, it was shown that increased carbonate concentrations had relatively little effect on As(V) adsorption on iron oxide surfaces. Comparing the effects of carbonates and phosphates on mobilization of As(V), it was shown that carbonates mobilize less As(V) even when present in much higher concentration then phosphate. Batch experiments often give important information about kinetic and adsorption processes. Using data obtained from batch experiments, an effort was made to scale batch parameters to column level. This resulted in the development of a numerical model that integrated transport and reactions of As during transport in the column. Validity of the model was confirmed by comparing model prediction with the experimentally obtained data. This showed that it was possible to achieve good scaling from batch to column scale and to predict transport of As in column scale experiments.