Semester

Spring

Date of Graduation

2001

Document Type

Dissertation

Degree Type

PhD

College

Davis College of Agriculture, Natural Resources and Design

Department

Horticulture

Committee Chair

Devinder K. Bhumbla.

Abstract

High concentration of trace elements such as arsenic (As) and lead (Pb) represents one of the potentially costly threats to water and soil resources as well as serious threats to humans. Clay minerals are potential binding agents for pollutants due to their high cation exchange capacity and large surface area. Laboratory investigations were undertaken to evaluate the effect of Fe and Al hydrous oxide coatings on the sorption of Pb and As. Four clay minerals attapulgite, illite, kaolinite and montmorillonite with a range of surface characteristics were selected to investigate the Pb and As sorption. The minerals were coated with three different levels of each of Fe and Al hydrous oxides. Adsorption and desorption studies were performed for each of these heavy metals at seven different pH levels. Coating with both Fe and Al hydrous oxide increased the retention of Pb and As. Lead desorption was decreased when minerals were coated with highest levels of Fe and Al. In order to fully understand the fate and transport of metal in the environment, there is a need to accurately describe and model the complex natural system. The ability of surface complexation models (SCMs) to fit sets of titration data as a function of changes in model parameters was evaluated using FITEQL and acid base titration of clays coated with Fe and Al hydrous oxide. Three SCMs were evaluated: the constant capacitance model, the diffuse-double layer and triple layer model. For all the models evaluated, increasing the value of total number of sites resulted in a decrease in the FITEQL best fit equilibrium log K value. Generally better F value fits were obtained when the site density (Ns) value was between 1 to 10 sites/nm 2.;Sequestering heavy metals in insoluble phosphate minerals has been suggested as an in situ remediation technique for Pb contaminated soils. Lead sequestered in apatite minerals has great durability and leaching resistance, significantly exceeding other chemically stabilized forms. Sequential extractions of Pb have been used to determine the suitability of phosphates to immobilize Pb. Laboratory studies were conducted to investigate whether the lead pyromorphite formation is the artifact of the sequential extraction or it is actually formed. It was found that formation of pyromorphite might be due to the artifact of extraction process. There are concerns about the use of phosphate in soils contaminated with both Pb and As because As and phosphate compete for the same sorption sites. Greenhouse experiments were conducted on soils contaminated with both Pb and As. These soils were treated with four rates of phosphates and seeded with Japanese millet (Echinochloa crusgalli) and red clover (Trifolium pratense). Results showed that when soils were contaminated with arsenic (As) only, application of phosphates resulted in increased concentrations of As in soil solution, however when soils were contaminated with both Pb and As, application of phosphorus did not increase As concentration in soil solution. Laboratory investigations were conducted to determine the mechanism responsible for reduced As mobility in soils that are co-contaminating with Pb. The results suggested that As in Pb contaminated soils coprecipitates with lead phosphates.

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