Mercury-Promoted Hydrolysis of Parathion-methyl: Effect of Chloride and Hydrated Species

Mercury salts are commonly used in laboratory and field experiments as biocides. It has been previously reported that Hg(II) can enhance chemical hydrolysis of a number of pesticides. Earlier studies on metal-promoted hydrolysis have reported overall rate constants as a function of total metal concentration. There are three advantages in reporting the relative importance of the different species: (1) results can be extrapolated form one situation to another, (2) rates can be predicted for specific conditions, and (3) greater understanding of the catalysis mechanism can be gained. In this study, mercury-promoted hydrolysis of parathion-methyl ( O, O -dimethyl- O - p -nitrophenyl phosphorothionate), a probe organophosphate compound (OP), was studied as a function of Hg(II) speciation. The observed rate of hydrolysis was a function of specific mercury species rather than of the total mercury in solution. Second-order rate constants were determined experimentally at various pH values. A pH-dependent kinetic expression, k obs = (α Hg 2+ k 1 + α HgOH + k 2 + α Hg(OH) 2 k 3 ) where k i = with K i representing the Hg:OP equilibrium constant, the rate constant for Hg:OP hydrolysis for the different Hg(II) species, and α is the fraction of the total Hg(II) present as specific species, provides a plausible interpretation for the system. Mercury-chloride species proved to have little catalytic power, whereas the contributions for Hg 2+ and Hg(OH) + were significant. Our results also suggest that a mixed mechanism (electrophilic and nucleophilic) may have to be considered for general metal-promoted hydrolysis of OPs.