Catalytic dehydrochlorination of lindane by nitrogen-containing multiwalled carbon nanotubes (N-MWCNTs)

Abstract

This paper reports a nonreductive dechlorination pathway (dehydrochlorination) of lindane in the presence of N-MWCNTs (CNT-N1 and CNTN2) under environmentally relevant conditions. The enhanced catalytic dehydrochlorination at mild pH values (7.0-9.0) was mainly attributed to the Lewis base nature of nitrogen functional groups on the surface of N-MWCNTs, based on FT-IR spectra, X-ray photoelectron spectroscopy and CO₂-TPD profile. Amine and pyridinic nitrogen species were inferred as the main basic sites that were responsible for the dehydrochlorination of lindane. The concentration of lindane was unchanged in homogenous alkaline aqueous solutions at pH7.0 and 8.0 within 6 d, and up to 89.5% of lindane remained at pH9.0. In sharp contrast, the reaction rate constant (k_obs) increased from 0.024 to 0.876 d^-1 with the increasing pH from 7.0 to 9.0 when CNT-N2 catalyst was used. All lindane was transformed in the presence of CNT-N2 at pH9.0, leading to a much greater k_obs as compared to that obtained in homogenous solution (0.876 vs 0.019 d^-1). Even at neutral pH condition, the k_obs obtained in the presence of CNT-N2 was higher than that observed in weakly alkaline solution (0.024 d^-1 at pH7.0 vs 0.019 d^-1 at pH9.0). γ-1,3,4,5,6-pentachlorocyclohexene and trichlorobenzene isomers were detected as the intermediate and final products, respectively. It might be inferred that amine and pyridinic nitrogen species could attack the hydrogen atom attached to the β‑carbon, and the dehydrochlorination of lindane followed a β-elimination mechanism. This study provides an alternative dechlorination way for making lindane less toxic and better biodegradable under mild conditions, which is beneficial for complete mineralization of lindane when coupled with microbial degradation.

Keywords: Dehydrochlorination; Hexachlorocyclohexane; Lindane; Nitrogen-containing multiwalled carbon nanotube; Solid base.