Chapter 15
Evaluation of Organophosphorus Insecticide Hydrolysis by Conventional Means and Reactive Ion Exchange 1
Downloaded by COLUMBIA UNIV on August 2, 2012 | http://pubs.acs.org Publication Date: October 30, 1992 | doi: 10.1021/bk-1992-0510.ch015
Kathryn C. Dowling and Ann T. Lemley
Graduate Field of Environmental Toxicology, Cornell University, Ithaca,NY14853 Different methods of hydrolyzing the organophosphorus insecticide methyl parathion were compared for effectiveness. The aqueous base hydrolysis rate is second order in the insecticide and sodium hydroxide. In a system incorporating sodium perborate, hydrolysis rates are accelerated by two orders of magnitude in comparison with simple aqueous base hydrolysis. Pseudo-first order rates are linear with sodium perborate concentrations for a given sodium hydroxide concentration. A macroporous hydroxide-presenting resin employed in methyl parathion batch studies catalyzes hydrolysis at somewhat less than the rate of simple base hydrolysis. Reactive ion exchange with this resin in a dynamic flow-through column system degrades four organophosphates: methyl parathion, malathion, chlorpyrifos, and methamidophos. Experimental solutions prepared in tap water instead of distilled water are significantly less degraded (intact insecticide appeared in the column effluent). Ions present in tap water may interfere with resin/insecticide interactions, decreasing degradative capacity. The need for development of pesticide treatment systems easily accessible for field use stems from the large amount of insecticide application equipment, including mixing apparati, sprayers, and fumigation airplanes, in use today. Pesticide-contaminatedrinsatesgenerated from emptying and cleaning such equipment can contain pesticide concentrations in the range of 100 to 1000 mg L" (7). These rinsates, often quite toxic, are not always collected for further use or treatment. Since discardedrinsatecan contaminate surface waters and groundwater, field-accessible treatment systems are desirable. A number of hydrolysis treatment schemes that ultimately could be engineered for field use were evaluated for their ability to degrade organophosphorus insecticides. Four organophosphates widely used in 1
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Corresponding author. Current address: College of Human Ecology, 202 MVR Hall, Cornell University, Ithaca, NY 14853-4401
0097-6156/92/0510-0177$06.00/0 © 1992 American Chemical Society In Pesticide Waste Management; Bourke, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
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agriculture were chosen for study: methyl parathion, malathion, methamidophos, and chlorpyrifos. In an attempt to represent actual field conditions, experiments were carried out in aqueous solution at concentrations reflecting insecticide water solubilities and common rinsate levels. The first method investigated, aqueous hydrolysis by hydroxide, is often used to degrade organophosphates. Hydroxide nucleophilic attack on the central organophosphate phosphoester leads to insecticide cleavage. Hydrolysis rate con stants can be calculated easily for a range of pH values. Most previous work on organophosphate base hydrolysis has been done in organic solvents, but one aque ous study can be compared to the present work. Ketelaar (2) found methyl parathion's second order rate constant (15°C) to be 9.2 χ 10" M" min" and its temper ature coefficient (for each 10°C increment) to be 2.57. Thus the expected hydroly sis rate constant at 25°C is 2.36 χ 10" M" min" . Organophosphates are also sus ceptible to hydrolysis by the perhydroxyl anion, a fifty-fold stronger nucleophile than the hydroxyl anion (3,4). Sodium perborate forms the perhydroxyl anion and boric acid in water under alkaline conditions. Higher pH values favor perhydroxyl anion formation, which in turn heightens hydrolysis rates. Boric acid is slightly toxic to mammals (with L D values of 1000-6000 mg kg" in various species); boron, although commonly present in water and soil and essential to plant growth, is of ecological concern and has a drinking water concentration limit of 1 mg L" (5). Janauer et al. (6) and Lemley et al. (7) modified various ion exchange res ins and gels by loading with hydroxyl or other ions; degradation was effected by passing aqueous solutions of organophosphate or carbamate compounds through a resin-packed column. The hydroxide-modified resin was found to be particularly effective at degrading organophosphates. The insecticide hydrolysis product was found to be retained in its anionic form by the resin; this was a welcome concen tration effect for degradation products. Products could be discarded through resin elution/regeneration or disposed of while still bound to the resin. Hydroxyl ion, perhydroxyl ion, and reactive ion exchange (RIEX) hydrolysis of organophosphates will be described and evaluated.
Downloaded by COLUMBIA UNIV on August 2, 2012 | http://pubs.acs.org Publication Date: October 30, 1992 | doi: 10.1021/bk-1992-0510.ch015
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Materials and Methods Chemicals. Methyl parathion, 0,0-dimethyl 0-4-(nitrophenyl) phosphorothioate (98.0% labeled/98.0% confirmed purity), was purchased from Chem Service, Inc., West Chester, PA. Chlorpyrifos, 0,0-diethyl 0-(3,5,6-rrichloro-2-pyridyl) phos phorothioate (99.9% labeled/100.0% confirmed purity), along with its hydrolysis product 3,5,6-trichloro-2-pyridinol (99% labeled/100% tested purity), was donated by Dow Chemical Company, Midland, ML Malathion,
