The Influence of Residence Time and Organic Acids on the Desorption of Goethite
Glover II, Leslie James
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The Influence of Residence Time and Organic Acids on the Desorption of Trace Metals from Goethite Leslie James Glover II Committee Chair: M.J. Eick Crop and Soil Environmental Sciences (ABSTRACT) Trace metal concentrations in soil solution, and hence trace metal bioavailability and toxicity, are primarily controlled by sorption/desorption reactions at the mineral-water interface. While numerous studies have been conducted to understand the initial adsorption of these metals to soil minerals, less in known about long-term adsorption/ desorption processes. The objective of this study was to examine the influence of residence time and organic acids on the desorption of Pb2+and Cd2+ from goethite. Adsorption experiments were conducted at pH 6.0. Lead adsorption was nearly completed after 4 hours, with very little additional sorption during a 20-week period. In contrast cadmium showed a continuous slight increase in the amount of adsorption over the 20-week period. Desorption experiments were conducted at pH 4.5 and similar to previous studies examining trace metal desorption from oxide surfaces, the desorption kinetics for Pb2+and Cd2+ were slow compared to the sorption reaction. None of the experiments were completely reversible after an eight-hour desorption period. For all experiments except long-term Pb2+ desorption, the quantity of metal desorbed from goethite followed the order salicylate >NaNO3 > oxalate. Based on differences in cation affinity for the iron oxide surface one would expect a greater quantity of Cd2+ to be removed compared to Pb2+, for each of the extracting solutions. However at a pH of 4.5 we did not find a statistically significant trend. We observed a difference between the amount of metal removed for short and long-term experiments in five of six experiments, but these differences were only significant for Pb2+ experiments in the presence of salicylate. Two first order rate equations best fit the kinetics of trace metals desorption, with R2 values greater than 0.910 in all cases. Although our results show a decrease in rate coefficients (expect k1 for oxalate) with increased residence time, statistical analysis indicates that these results were only significant for Pb2+ experiments in the presence of salicylate. However raw and transformed data both suggest that desorption values are diverging as a function of aging time. Similar to other researchers we believe that Pb2+ and Cd2+ are sequestered by the goethite surface with an increase in residence time. These results suggest that residence time effects observed by many researchers are much less prevalent at low pH values. Therefore a reduction in soil pH created by natural anthropogenic processes may reduce the ability of soils to naturally sequester metals over time.
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