Kinetics and mechanism of sulfate radical- and hydroxyl radical-induced degradation of highly chlorinated pesticide lindane in UV/peroxymonosulfate system

Lindane is a highly persistent chlorinated pesticide and a potent endocrine disruptor. The strong electron withdrawing property of the chlorine atoms results in a relatively low reactivity of lindane with ^[rad]OH in conventional advanced oxidation processes (AOPs). In this study, the degradation of lindane by UV (254 nm)/peroxymonosulfate (UV/PMS), which can generate both ^[rad]OH and SO₄ ^-, was investigated. A second-order rate constant of 1.3 × 10⁹ M⁻¹ s⁻¹ between lindane and SO₄ ^- was determined using competition kinetics, suggesting a strong role of SO₄ ^-. The degree of degradation changed with different initial solution pH, achieving 86, 92 and 55% removal of lindane at pH 4.0, 5.8 and 8.0, respectively, in 180 min, attributable to the varying concentrations of ^[rad]OH and SO₄ ^-. The addition of common water quality constituents, e.g., humic acid or inorganic anions, at pH 5.8 showed a varied inhibition effect with 61% degradation in the presence of 1.0 mg L⁻¹ humic acid, and 45, 60, 88 and 91% degradation in the presence of 1 mM CO₃ ²⁻, HCO₃ ⁻, Cl⁻ and SO₄ ²⁻, respectively, in 180 min. With the kinetics being demonstrated to be feasible, the degradation mechanism of lindane by UV/PMS was also assessed. Based on the detected by-products through GC–MS analysis, plausible reaction pathways were proposed, suggesting dechlorination, chlorination, dehydrogenation and hydroxylation via ^[rad]OH and/or SO₄ ^- attack. Meanwhile, reasonable mineralization efficiency was observed, with a 56% total organic carbon removal in 360 min, at an initial PMS concentration of 500 μM. Results from both degradation kinetics and transformation mechanism indicate that UV/PMS is a potential method for the treatment of water contaminated with lindane.