Degradation kinetics of Endosulfan isomers by micron- and nano-sized zero valent iron particles (MZVI and NZVI)

BACKGROUND: The pesticide Endosulfan is commercially available as a mixture of two conformal isomers, i.e. Endosulfan-1 (Endo-1) and Endosulfan-2 (Endo-2). Endosulfan has been characterized as extremely toxic by USEPA and is persistent in nature. In this study micron- and nano-sized zero-valent iron particles (MZVI and NZVI) were used for rapid degradation of the Endosulfan isomers. RESULTS: Both Endo-1 and Endo-2 undergo abiotic hydrolysis resulting in the formation of equi-molar quantities of Endosulfan alcohol. During interaction with MZVI, the degradation rates of both Endo-1 and Endo-2 were observed to be pseudo-first-order and the surface normalized degradation rate constants (k_s) were 6.33 × 10⁻⁴ L h⁻¹ m⁻² and 6.38 × 10⁻⁴ L h⁻¹ m⁻² for Endo-1 and Endo-2, respectively. During interaction with NZVI, k_s values for Endo-1 and Endo-2 were 2.58 × 10⁻² L h⁻¹m⁻² and 1.21 × 10⁻² L h⁻¹ m⁻², respectively. GC-MS analysis showed evidence of the formation of partially dechlorinated by-products due to Endo-1 and Endo-2 interactions with MZVI and NZVI. CONCLUSIONS: Surface area normalized degradation rate constants (k_s) for Endo-1 and Endo-2 degradation by NZVI were 40 and 19 times higher, respectively, than the corresponding values by MZVI. This indicates higher reactivity of the NZVI surface vis-à-vis Endo-1 and Endo-2 degradation. Partially dechlorinated by-products observed during interaction of Endo-1 and Endo-2 with NZVI and MZVI may be less toxic and more amenable to subsequent biodegradation than the parent compounds.