ADSORPTION OF SELECTED PESTICIDES BY ACTIVATED CARBON USING ISOTHERM AND CONTINUOUS FLOW COLUMN SYSTEMS
Abstract
This research was conducted to evaluate the feasibility of using activated carbon for the removal of pesticides from aqueous solutions. Analysis of effluent components from carbon columns treating bisolute pesticide solutions was made to establish expected patterns of pesticide breakthrough for full-scale column systems. A total of thirty representative pesticides were chosen for isotherm studies. Adsorption isotherms were run at two temperatures for a period of 24 hours. Final pesticide concentrations were determined by spectrophotometric analysis. Results were plotted according to the Langmuir isotherm model, and the derived ultimate monolayer capacity values (x(,m)), were used to calculate heats of adsorption. Most of the tested materials showed significant degrees of adsorption. Compounds of similar chemical structure were found to give closely similar x(,m) values. Charged species and small, highly polar neutral molecules were found to be essentially non-adsorbable. Heats of adsorption could be correlated to the diffusivity of the compounds and the extent of approach to equilibrium conditions. Ten compounds were selected for carbon column adsorption studies. In all cases where single component solutions were employed, the carbon capacity at the exhaustion point of the column, chosen where the effluent concentration was equal to 75% of the influent, was greater than the isothermally determined x(,m) value. All of the non-charged pesticides containing a single aromatic structure, propham, ametryne, metribuzin, fluometuron and oxycarboxin, were found to give essentially identical carbon column exhaustion capacities of approximately 350 mg/g, despite a wide range in determined x(,m) values. Charged compounds or those with a large double aromatic ring structure showed significantly lower capacities. Two aliphatic insecticides, monocrotophos and methomyl, showed similar column exhaustion capacities of approximately 300 mg/g. The order and degree of compound breakthrough in columns treating bi-solute pesticide solutions was found to be directly correlated to the water solubilities of the materials. Independent of the concentration of the individual species, the pesticide having the greatest water solubility appeared first and, at any chosen throughput volume, to a greater extent. Breakthrough curves for compounds of similar water solubilities, were found to be essentially identical. The total column adsorption capacities for the bisolute mixtures were less than those achieved in single solute column studies. Some evidence of compound displacement was observed, and the ability of a pesticide to promote desorption was related to the strength of its affinity for carbon.
Degree
Ph.D.
Subject Area
Civil engineering
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