Aldicarb hydrolysis rates

Given, C.J. andDierberg, F.E. Effect of pH on the rate of aldicarb hydrolysis. Bull Environ. Contam. Toxicol, 34(5) 627-633, 1985. 

Chemical/Physical. The hydrolysis of aldicarb in water is both acid and base catalyzed (Banks and Tyrell, 1984). Hansen and Spiegel (1983) studied the hydrolysis rate of aldicarb, aldicarb sulfoxide and aldicarb sulfone in aqueous buffer solutions. At a given temperature, the rate of hydrolysis increases rapidly above pH 7.5. The reported hydrolysis half-lives for aldicarb are as follows 4,580 days at pH 5.5 and 5°C, 3,240 days at pH 5.5 and 15°C, 1,950 days at pH 7.5 and 5°C, 1,900 days at pH 7.5 and 15°C, 1,380 days at pH 8.5 and 5°C and 170 days at pH 8.5 and 15°C (Hansen and Spiegel, 1983). The hydrolysis half-lives for aldicarb in a sterile 1% ethanol/water solution at 25°C and pH... 

Base hydrolysis kinetic data are reported for ppb solutions of carbofuran,3-OH carbofuran, methomyl and oxamyl. The results are compared with those reported previously for aldicarb, aldlcarb sulfoxide, and aldicarb sulfone. Second order reaction rate constants, k, have been calculated and range from 169 liter mln mole for oxamyl to 1.15 liter mln mole for aldicarb. The order for rate of base hydrolysis is as follows oxamyl >3-hydroxycarbofuran >aldicarb sulfone v- carbofuran >aldicarb sulfoxide > methomyl -v aldicarb. The activation energy for the base hydrolysis of carbofuran was measured to be 15.1 +0.1 kcal mole , and is similar to the value previously reported for aldicarb sulfone. Rapid detoxification of aldicarb, a representative oxime carbamate pesticide, by in situ hydrolysis on reactive ion exchange beds is reported. 

If aldicarb residues leach into the saturated zone, or groundwater, degradation of the residues continues, mainly by chemical hydrolysis in cold areas and by both chemical hydrolysis and microbial degradation in warm areas. Factors which tend to increase the degradation rate are high temperature and high pH. Microbial populations may provide a significant contribution to... 

Relation to Alkalinity. Some explanations for this behavior can be offered. Persistent levels appear to be associated with low pH (4.5-6.5) and alkalinity (<10 mg/L) in the groundwater. Residues disappeared faster from Fields 4 and 5, where pH and alkalinities were high, especially at deeper levels (Figure 5) than in Fields 1 and 2, where pH and alkalinity were low (Figure 6). Rates of chemical hydrolysis of aldicarb and its oxides have been extensively studied (26-29). Alkalinity and pH tend to increase with depth in the aquifer under all fields. However, prediction of rates of chemical hydrolysis based on known rate constants are complicated by two phenomena 1. fluctuations in groundwater... 

Table VI shows examples of laboratory-measured and the substantially higher calculated in situ alkalinities for 10 samples of groundwater from Field 6. The values shown in Figs. 5 and 6 are laboratory, not in situ pH and alkalinities. Higher in situ pH and alkalinity would lead to faster rates of aldicarb residue breakdown. Projections of aldicarb residue degradation by alkaline hydrolysis should be based on in situ pH, alkalinity and temperature. These, like the residue concentrations, vary widely, spatially and temporally.

TABLE 9.15 Rates of Hydrolysis of Aldicarb Sulfoxide and Sulfone at 23°C... 

An understanding of the factors that control the redox equilibria for aldicarb and aldicarb-sulfone is important because hydrolysis of aldicarb-sulfoxide will occur at a much faster rate than hydrolysis of aldicarb. 

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