Stabilisation and destabilisation of amitraz : A formamidine ectoparasitic compound / Charlotte May van Eeden

Abstract

Objective: To develop effective dip vat management and waste disposal strategies, this study focused on establishing ways to stabilise or destabiJise amitraz, an ectoparasitic compound, in solution. Background: The formamidines form a small group of insecticides. Their current value lies in the control of organophosphate and carbamate-resistant pests. The accumulation of this compound in the environment is of concern because amitraz is widely used in South Africa to control ticks in mobile and stationary spray and dip vats of up to 1000 L. Through this process large quantities of semiconcentrated wastes is generated. Formamidine poisoning symptoms are distinctly different from other pesticides. Their proposed action is the inhibition of the enzyme monoamine oxidase, which is responsible for degrading the neurotransmitters norephedrine and serotonin. Methods: The pseudo-first order rate constants, k, of 2 ].Lg/ml Amitraz solutions in seven buffers with a pH range from 3 to 10 and a 0.1 M NaOH solution were determined at 25°C. The decrease in amitraz concentration was determined at 285 nm with a spectrophotometer. Using different concentration buffers at the same pH the effect of buffers on degradation was also tested. The rate of hydrolysis of amitraz a~ temperatures of 50°C and 75°C were also determined. This was compared to the results at 25°C. Hydrolysis tempo in different concentrations of ethanol, propylene glycol and dimethyl sulphoxide (OMSO) was also determined. The hydrolysis rates in surfactant solutions containing sodium lauryl sulphate, cetrimide or Tween 80 were also determined. Mass spectra were used to confirm the hydrolysis products of Amitraz. Results: Amitraz degrade by means of hydrolysis. At low pH values the acid-stable 2,4-dimethylphenylformamide is formed. This can be further hydrolysed to 2,4-dimethyl aniline. The hydrolysis of 2,4-dimethylphenylformamide is faster under basic conditions. Thus, the addition of lime, used to stabilise amitraz, will enhance the hydrolysis of its degradation products to aniline. A pseudofirst order rate process is followed, as described by this equation: In([amitraz]/[amitraz]o) = -kobst, where [amitraz] is the amitraz concentration at time t, [amitraz]o is the initial amitraz concentration and kobs is the apparent pseudo-first order rate constant. At low pH values the hydrolysis of amitraz is very fast. The hydrolysis rate decreased as the pH rose. It was the slowest at neutral to alkali pH values. The hydrolysis rate increased again as the pH values became very high, above pH 10. Hydrolysis was the fastest at 75°C and the slowest at 25°C. The activation energy for the hydrolysis of amitraz rose between pH 4 and 6. From pH 6 to 11 the activation energy decreased at a constant rate. The ionic strength had a slight effect on the hydrolysis of amitraz in the acetate buffer. At higher ionic strength, the reaction became slower. Ionic strength had no effect on the phosphate buffer. The pH rate profile for amitraz hydrolysis was type ABCD, which means that hydrolysis starts fast and the rate decreased at a constant rate between pH 3 and 6. The hydrolysis rate decreased further between pH 6 and 10, but this decrease was slower than that between pH 3 and 6. A small increase in hydrolysis rate took place between pH 10 and 14. The same ABCD pH rate profile was observed at 25 and 50°C, but at 75°C the rate of hydrolysis decreased very slowly between pH 10 and 14. When hydrolysis in three organic solvents were compared with one another, the propylene glycol solution degraded amitraz the fastest, ethanol a bit slower and DMSO the slowest. Overalf degradation was still fastest in water. It is evident for amitraz that anionic micelles enhance and cationic micelles retard the rate of hydrolysis and that the magnitude of micellar effects become less with increasing concentrations of the surfactants. Non-ionic surfactants either decreased or had insignificant effects on the rate constants for hydrolysis of amitraz. At higher detergent concentration the catalysis of amitraz hydrolysis became progressively less pronounced. The maximum rate acceleration occurs in the region of catalyst concentration at which the bulk of the amitraz is incorporated in the micelles and additional surfactant, simply solubilise the nucleophllies in the stem layer, thereby rendering them inactive. Conclusion: It was shown that amitraz hydrolysis is the fastest in acid conditions and slowest in neutral to alkaline conditions. Hydrolysis also increased with an increase in temperature. Very importantly, in anionic surfactant solutions amitraz is solubilised and the hydrolysis rate is increased. This surfactant might therefore be used when trying to dispose of dip vat waste.