Hydrolysis and degradation of dazomet with pyrite: Implications for persistence in produced waters in the Marcellus Shale

Hydraulic fracturing and horizontal drilling in the Marcellus Shale present a novel use of chemical additives at unprecedented volumes. Reuse of produced water has become a popular option in Pennsylvania, complicating our understanding of the fate of chemical additives due to the variability of produced water chemistry. This study investigates the effect of pH, temperature, ionic strength and the presence of pyrite on the kinetics of degradation of dazomet, a commonly-used biocide, under a range of conditions expected during hydraulic fracturing. The results show that the degradation rate of dazomet is highly dependent on many of the variables tested. The hydrolysis is base-catalyzed over the pH range of interest which results in half-lives decreasing from 8.5 h to 3.4 h as the pH is increased from 4.1 to 8.2. Dissolved Fe^II ions catalyze dazomet degradation kinetics with solutions of 0.8 mM Fe^II causing degradation rates to increase by 190% over iron-free water. Increasing temperatures from 34 °C to 57 °C quadrupled hydrolysis rates (estimated activation energy of 60 kJ/mol). Reaction with oxygen-exposed pyrite surface led to accelerated degradation of dazomet, but unoxidized pyrite had no effect on the degradation rate of dazomet. The key hydrolysis products of dazomet degradation are formaldehyde and methyl isothiocyanate which are shown to be significantly more toxic than the parent compound. The study points to the need to assess the specific environmental conditions and any toxic by-products in conducting risk assessments for geological applications.