Aldicarb sulfone, degradation

Carbamates such as Aldicarb undergo degradation under both aerobic and anaerobic conditions. Indeed the oxidation of the sulfur moiety to the sulfoxide and sulfone is part of the activation of the compound to its most potent form. Subsequent aerobic metaboHsm can completely mineralize the compound, although this process is usually relatively slow so that it is an effective iasecticide, acaricide and nematocide. Anaerobically these compounds are hydrolyzed, and then mineralized by methanogens (61). 

Rajagopal et al. (1984) used numerous compounds to develop a proposed pathway of degradation of aldicarb in soil. These compounds included aldicarb oxime, A-hydroxymethyl aldicarb, A-hydroxymethyl aldicarb sulfoxide, A-demethyl aldicarb sulfoxide, A-demethyl aldicarb sulfone, aldicarb sulfoxide, aldicarb sulfone, A-hydroxymethyl aldicarb sulfone, aldicarb oxime sulfone, aldicarb sulfone aldehyde, aldicarb sulfone alcohol, aldicarb nitrile sulfone, aldicarb sulfone amide, aldicarb sulfone acid, aldicarb oxime sulfoxide, aldicarb sulfoxide aldehyde, aldicarb sulfoxide alcohol, aldicarb nitrile sulfoxide, aldicarb sulfoxide amide, aldicarb sulfoxide acid, elemental sulfur, carbon dioxide, and water. Mineralization was more rapid in aerobic surface soils than in either aerobic or anaerobic subsurface soils. In surface soils (30 cm depth) under aerobic conditions, half-lives ranged from 20 to 361 d. In subsurface soils (20 and 183 cm depths), half-lives under aerobic and anaerobic conditions were 131-233 and 223-1,130 d, respectively (Ou et al, 1985). The reported half-lives in soil ranged from approximately 70 d (Jury et ah, 1987) to several months (Jones et al, 1986). Bromilow et al. (1980) reported the half-life for aldicarb in soil to be 9.9 d at 15 °C and pH 6.34-7.0. 

Groundwater. In Florida groundwater, aldicarb was converted to aldicarb sulfoxide under aerobic conditions. Conversely, under anaerobic conditions (pH 7.7), oxidative metabolites (aldicarb sulfoxide and aldicarb sulfone) reverted back to the parent compound (aldicarb). Half-lives in unfiltered and filtered groundwater were 635 and 62 d, respectively (Miles and Delfino, 1985). In sterile anaerobic groundwater at pH 8.2, aldicarb slowly hydrolyzed to the aldicarb oxime. In a microorganism-enriched groundwater at pH 6.8, aldicarb rapidly degraded to... 

Aldicarb degrades rapidly in the chlorination of drinking water forming aldicarb sulfoxide which subsequently degrades to aldicarb sulfone, (chloromethyl)sulfonyl species and A-chloro-aldicarb sulfoxide (Miles, 1991). 

Miles, C.J. Degradation of aldicarb, aldicarb sulfoxide, and aldicarb sulfone in chlorinated water, Environ. Sci Technol, 25(10) 1774-1779, 1991. 

Lightfoot E. N., Thome P. S., Jones R. L., Hansen J. L., and Romine R. R. (1987) Laboratory studies on mechanisms for the degradation of aldicarb, aldicarb sulfoxide and aldicarb sulfone. Environ. Toxicol. Chem. 6, 377-394. 

N-methyl carbamates do not need activation to inhibit ChEs. However, at least in the case of aldicarb, inhibition increases with metabolism. Aldicarb is rapidly oxidized to the relatively stable aldicarb sulfoxide, which in turn is more slowly metabolized to aldicarb sulfone, a stronger AChE inhibitor. These products are then detoxified by conversion to oximes and nitriles, which in turn are degraded to aldehydes, acids, and alcohols. Procarbamate derivatives were... 

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 ... 

Saturated Zone Degradation. Because of the rapid oxidation process in the root zone, parent aldicarb is rarely detected in the saturated zone. In rare instances where transport from the soil surface is rapid, aldicarb may be present at less than five percent of the total residues found. In the saturated zone, residues are usually a mixture of aldicarb sulfoxide and aldicarb sulfone in an average ratio of 3 2. 

Samples of [S- CH ] aldicarb, aldicarb sulfoxide and aldicarb sulfone were incubated with samples of groundwater and aquifer sediments to examine the potential for microbial degradation in situ in groundwater. The [S - XH3] aldicarb was provided by Union Carbide Agricultural Products Company the labeled sulfoxide was prepared from this by oxidation with hydrogen peroxide, the sulfone from the aldicarb by oxidation with peroxyacetic acid. These oxidants afforded better yields of cleaner product than the nr chloroperoxybenzoic acid previously used (18). 

The analytical solution to Equation 2 for a range of boundary conditions is a model of pesticide fate that has been used under a variety of laboratory situations to study the basic principles of soil-water-pesticide interaction. It is in fact limited to such laboratory cases, as steady state water flow is an assumption used in deriving the equation. As a modeling approach it is useful in those research studies in which careful control of water and solute fluxes can be used to study degradation and adsorption. For example, Zhong et al. (11) present a study of aldicarb in which the adsorption and degradation of aldicarb, aldicarb sulfone and aldicarb-sulfoxide were simultaneously determined from laboratory soil column effluent data. The solution to a set of equations of the form of Equation 2 was used. A number of similar studies for other chemicals could be cited that have provided useful basic information on pesticide behavior in soil (4,12,13). Yet, these equations are not useful in the field unless re-formulated to describe transient water and solute fluxes rather than steady ones. Early models of pesticide fate based upon Equation 2 (14) were constrained by such assumptions, but were... 

In this paper, aldicarb refers to the sum of parent aldicarb and degradates sulfoxide and sulfone. The physical parameters for the three sites are given in Table I. 

Rajagopal et al. (1989) used numerous compounds to develop a proposed pathway of degradation of aldicarb in soil. These componnds inclnded aldicarb oxime, A-hydroxym-ethyl aldicarb, A-hydroxymethyl aldicarb snUbxide, A-demethyl aldicarb sulfoxide, A-demethyl aldicarb sulfone, aldicarb snUbxide, aldicarb sirlfone, A-hydroxymethyl aldicarb sulfone, aldicarb oxime snUbne, aldicarb snUbne aldehyde, aldicarb snUbne alcohol, aldicarb nitrile sulfone, aldicarb snUbne amide, aldicarb snUone acid, aldicarb oxime suUoxide, aldicarb suUoxide aldehyde, aldicarb suUoxide alcohol, aldicarb nitrile snUoxide, aldicarb... 

Oxime carbamates are generally applied either directly to the tilled soil or sprayed on crops. One of the advantages of oxime carbamates is their short persistence on plants. They are readily degraded into their metabolites shortly after application. However, some of these metabolites have insecticidal properties even more potent than those of the parent compound. For example, the oxidative product of aldicarb is aldicarb sulfoxide, which is observed to be 10-20 times more active as a cholinesterase inhibitor than aldicarb. Other oxime carbamates (e.g., methomyl) have degradates which show no insecticidal activity, have low to negligible ecotoxicity and mammalian toxicity relative to the parent, and are normally nondetectable in crops. Therefore, the residue definition may include the parent oxime carbamate (e.g., methomyl) or parent and metabolites (e.g., aldicarb and its sulfoxide and sulfone metabolites). The tolerance or maximum residue limit (MRL) of pesticides on any food commodity is based on the highest residue concentration detected on mature crops at harvest or the LOQ of the method submitted for enforcement purposes if no detectable residues are found. For example, the tolerances of methomyl in US food commodities range from 0.1 to 6 mg kg for food items and up to 40 mg kg for feed items. ... 

Aldicarb is rapidly oxidized to the sulfone and the hydrolysis half-hfe, at 15°C, decreases from 412 to 16 day at pH 8, consequently, the compound would be degraded before there was time for it to move through the soil profile to ground water. 

---