Remediation of Hexachlorocyclohexanes by Cobalt-mediated Activation of Peroxymonosulfate

Hexachlorocyclohexane isomers (HCHs) are one of the most ubiquitous and most easily detected organochlorine pesticides in environmental samples. This global distribution is enabled by HCH persistence. Moreover, HCH tend to bioaccumulate in human and animal adipose tissue. Since certain HCHs cause central nervous system, reproductive, and other endocrine damage there is necessity of a suitable remediation method to remove HCH from contaminated groundwater and soil. The present study was conducted to evaluate the potential of peroxymonosulfate (Oxone) induced by cobalt salt (Co(NO3)2; Co(II)) to degrade HCHs. Cobalt (II) nitrate has been chosen instead of Cobalt (II) chloride (which presents better activation properties for Oxone (Anipsitakis and Dionysiou, 2003)) in order to avoid an excess chloride interference and more additional chlorination by-products. The optimal conditions (oxidant dose as well as cobalt dose) were determined for spiked water with HCH (0.85 μM of summed HCH isomers). Optimal oxidant dose was established by running a test with three different Oxone concentrations (molar ratio of Oxone:Co(NO3)2 1:0.1): 0.06, 0.6 and 6 mM. Degradation curves of summed HCH isomers were fitted into the pseudo first-order kinetic model and reaction rates shown to be: 0.03, 0.31 and 0.55 min for doses: 0.06, 0.6 and 6 mM respectively. Further experiments were carried out to find out optimal Oxone:Co(NO3)2 molar ratio. Effectiveness of HCHs decontamination by Oxone:Co(NO3)2 in various molar ratios was in order: 1:0.1>1:1>1:0.01>1:0.001. It was observed that surplus of Co(NO3)2 can slow down degradation kinetics. A reaction between sulfate radicals and Co(NO3)2 could be responsible for this phenomenon, that can lead to formation of sulfate ions without HCHs oxidation. In addition, tests with higher HCH dose (close to the solubility limit) were carried out to determine byproducts of decontamination. In these test 2,4,6-trichlorophenol (2,4,6—TCP) was found to be the major intermediate of HCH isomers degradation. Trace levels of the trichlorobenzene (TCB), tetrachlorophenol (TeCP) and pentachlorophenol (PCP) were detected as well. Presented study revealed that Oxone induced by Co(II) shows a high degradation rate with HCH isomers, which can make it an attractive method for HCHs decontamination in the future.