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The popular broadleaf herbicide atrazine is often found in contaminated groundwater along with other agricultural chemicals, such as nitrate. Mulch biowalls, a passive treatment placed in situ, can inexpensively remediate groundwater by intercepting and treating a contaminant plume. Three types of organic mulch: cedar, cypress, and hardwood were evaluated for their ability to act as supporting materials for a biowall to simultaneously remove atrazine and nitrate from groundwater. Physical and chemical properties of the mulch were characterized. Cedar mulch had the highest organic carbon content, 996 mg/g. The adsorptive capacity of the mulch for atrazine and nitrate, in mono and binary adsorbate systems were evaluated in a series of isotherm experiments. There was no statistical difference in the ratio of qe/Ce (equilibrium concentration on the mulch/equilibrium concentration in solution) for atrazine or nitrate among the three types of mulch, except for atrazine in the pairs of cedar-hardwood and cypress-hardwood in the binary adsorbate system. Atrazine adsorption appeared to exhibit a C-type isotherm, due to the range of concentrations examined; A wider range of atrazine concentrations may show a more distinct L-type isotherm. Atrazine adsorption was not affected by the presence of nitrate. Nitrate adsorption did not clearly exhibit a specific isotherm type and was affected by surface properties of the mulch as well as the presence of atrazine. The adsorption behaviors of atrazine and nitrate were quantified from Langmuir and Freundlich isotherms. Atrazine adsorption was best modeled by the Freundlich isotherm, while nitrate adsorption was best modeled by the Langmuir. Qualitatively, cypress mulch exhibited the greatest sorption capacity for atrazine and nitrate and was selected to examine the feasibility of a mulch biowall using a laboratory-scale biotic column. The cypress column was not able to remove nitrate because the concentration of dissolved oxygen was too high, even after the addition of an external carbon source. The column was not able to remove atrazine because the concentration of nitrate was too high for bacterial degradation of the herbicide to occur.
Advisor: Mohamed Dahab