Aldicarb, chemical hydrolysis

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... 

Relation to Microbial Activity. Breakdown of aldicarb residues by biodegradation is presumably superimposed upon chemical hydrolysis. Results of examination of soil and water samples removed aseptically from depths of 20, 75 and 230 cm below the... 

Residue persistence in groundwater appears to be associated with low pH and alkalinity. Groundwater pH and alkalinity increase with depth in the aquifer. High CO2 contents in the groundwater, especially in summer, tend to increase water contents of Ca and Mg by dissolution of dolomitic limestone residues from the aquifer sediments, increasing pH and alkalinity above laboratory-determined values Such conditions apparently promote chemical hydrolysis of aldicarb residues, reducing their persistence in deeper ground-water. 

Phenylcarbamates, or carbanilates, generally exhibit low water solubilities, and thus they are almost immobile in soil systems. Chlorpropham and Propham are readily volatilized from soil systems, but Terbutol and Carbaryl (Fig. 10, Table 3) are not. Ester- and amide-hydrolysis, N-dealkylation and hydroxylation are among the chemical reactions that carbamates undergo. The N-methylcar-bamate insecticides (Fig. 10, Table 3) commonly used in soils are Carbaryl, Methiocarb,Aldicarb,and Carbofuran [74,173]. 

Aldicarb (A) is both chemically and biologically oxidized to aldicarb sulfoxide (A-SO), which is then further oxidized by similar processes to aldicarb sulfone (A-S02). These compounds are simultaneously subject to other degradative chemical processes dominated by hydrolysis. These reactions have been successfully described by first-order kinetics (J ), and can be generally summarized as ... 

Experiments 1-2, 3-4, and 5-6 represent paired cases of investigation of A, A-SO and A-S02 in Palmyra soil. Sterilized soil was used in these cases to isolate chemical oxidation and hydrolysis from biological conversion processes, and to determine whether the curve-fitted values of K and k depend on water velocity. The only difference within a pair of experiments was in the flow velocity (v). Between pairs, the difference was the aldicarb metabolite applied. 

Transformation of aldicarb and its derivatives was also studied in three experiments in unsterilized Palymra soil (Experiments 7 9) to determine the biological component of each transformation process. The value of kx in unsterilized soil ranged from 0.009-0.010 hr-1, compared to 0.009 hr 1 in the sterilized soil of experiment 2. This result implies that aldicarb hydrolysis is primarily a chemical process. The value of kx was approximately four times the value in unsterilized soil (0.015-0.018 hr 1) than in sterilized soil (0.004... 

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... 

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