Stereochemistry Of Degradation Of Insecticide Fenamiphos In Soils

When pollutants degrade in the environment, they may form persistent and toxic transformation products, which should be included in the environmental risk assessment of the parent compounds. The successive oxidation of thioether chemicals (e.g., insecticides and drugs) to sulfoxide and sequentially to suflone, commonly referred to metabolic activation, is the general characteristic of metabolism of such compounds in soils. As a large portion of the parents and/or their metabolites are chiral, stereoselective formation or depletion of such chiral chemicals essentially occurs in the soils of microorganism-rich, and hence lead to differing isomeric compositions of these pollutants. Moreover, many pesticides are rich in various electron donor groups, which can complex with metals prevailing in the environment. This interaction probably changes the environmental fate of organic pesticides.In this study, the successive sulfoxidation of insecticide fenamiphos (F) as a model thioether was followed over two months under aerobic conditions in three different soils. Separate experiment about microbial transformation was conducted with fenamiphos parent, sulfoxide (FSO), sulfone (FSO2), and respective stereoisomers. Furthermore, the effect of Cu(II) on degradation of fenamiphos was also determined.First, the overall transformation of fenamiphos was primarily microbial and exhibited bi-phasic characteristic, consisting of initial rapid oxidation to sulfoxide and subsequent slow transformation to sulfone, with concomitant hydrolysis of these three compounds to corresponding phenol intermediates. There was evident enantioselectivity in the successive sulfoxidation of fenamiphos with one chiral center (phosphorus) to create FSO with one additional chiral center (sulfur) and further convert to FSO2with phosphorus center remaining intact and sulfur one disappearing, associated with selective formation or depletion of intermediate stereoisomers. The stereochemistry of this successive reaction, in principle, was transferred in parallel, indicating two separate system of R(+)Fâ†'SRPR(+)/SSPR(-)-FSOâ†'R(+)FSO2and S(-)Fâ†'SRPS(+)/SSPS(-)-FSOâ†'Sr(-)FSO2, and unidirectional intersystem crossing at FSO was evidenced to create a new system of S(-)Fâ†'SRPR(+)/SSPR(-)-FSOâ†'R(+)FSO2, due to internal conversion of its four stereoisomers.Next, revealing the stereochemistry in the successive sulfoxidation can provide in-depth insights into the understanding of environmental fate and risk assessment of these thioether compounds, as major transformation products are commonly considered similar to higher toxic and their stereoisomers differ in toxicity. Hazard quotient of fenamiphos total toxic residues was used to indicate risks of this fenamiphos on the basis of exposure data in shallow ground waters and LC50values of the stereoisomers. Such selective formation or depletion of intermediate stereoisomers with distinct persistence and toxicity was necessarily responsible for risk alterations of fenamiphos parent, associated with the stereoselective successive sulfoxidation of the insecticide.Finally, with the aid of spectrophotometry, the complexation of fenamiphos with Cu(â…¡) was confirmed, and a complex of1:1was formed. Meanwhile, this complexation was pH-dependent and reached the maximum of ca105in magnitude under the neutral condition. The presence of Cu(â…¡) was observed to inhibit the biodegradation of fenamiphos in the soil. This inhibitory effect was attributable to the toxicity of metal to soil microorganisms, rather than the above complexation, as indicated by dramatic reduction of soil dehydrogenase and phosphatase activities in the presence of Cu(â…¡).