Degradation of HCHs by thermally activated persulfate in soil system: Effect of temperature and oxidant concentration

Soils contaminated with hexachlorocyclohexanes (HCHs) are a legacy of the heavy use of lindane during the 20th century. This pesticide production generated large amounts of residues of other HCH isomers, which were habitually mismanaged. The sediments treated in this work (coming from an old lindane landfill) are an example of this environmental problem of global dimensions. The efficient remediation of these sediments, mainly contaminated with β-HCH (99 mg kg−1) and α-HCH (254 mg kg−1) isomers, is still a challenge. The thermal activation of persulfate (PS) required excessive reaction times, which could be reduced by optimizing the variables of the process. This study aims to explore the effect of using higher reaction temperatures (35, 45, and 55 ºC) and initial oxidant concentrations (10, 20, 40, 60, and 80 g L−1). Batch experiments (liquid/soil = 2/1, 100 rpm) were performed to evaluate the influence of these two variables in HCHs abatement, dechlorination degree, generation of intermediate compounds, and oxidant consumption. An increase in the reaction temperature accelerates the pollutant oxidation and dechlorination rate and decreases the concentration of chlorinated byproducts. The initial PS concentration also plays a crucial role in HCHs degradation. At selected operating conditions (55 ºC, PS = 80 g L−1), a conversion of HCHs around 83% was achieved, without chlorinated byproducts (chlorine balance ≈ 100%) at only 9 days of treatment. A plausible pathway for the degradation of HCHs-polluted soils by thermally activated PS has been proposed from the identified species, involving a scheme of series-parallel reactions.