REMOVAL OF HCHs IN GROUNDWATER POLLUTED WITH LINDANE WASTES BY PHOTO-ASSISTED TECHNOLOGIES USING LED VISIBLE LIGHT

The occurrence of chlorinated organic compounds (COCs) from dense non-aqueous phase liquids (DNAPLs) in groundwater is a major concern for the scientific community due to the hazardous associated to this type of compounds. DNAPLs come from accidental spills, leakages, or uncontrolled discharges in different industrial activities^[1]. As a particular case, groundwater contaminated by chlorinated compounds in two industrial landfills (Bailin and Sardas) in Sabiñanigo (Spain) is considered here. High amounts of DNAPLs from lindane (g-hexachlorocyclohexane, g-HCH) production were dumped in unlined landfills for several decades in the twentieth century^[2]. This DNAPL contains 28 different COCs, including different isomers of hexachlorocyclohexane (HCH), being these compounds solubilized in groundwater^[3]. For this reason, it is necessary to develop clean and efficient technologies that allow to remove COCs from water bodies.

In this context, this work focuses on the removal of hexachlorocyclohexanes (HCHs) in polluted groundwater with dense non-aqueous phase liquids (DNAPLs) by photo-oxidation with hydrogen peroxide or persulfate using LED visible light and ferrioxalate as catalyst. Results show that it is possible to attain the degradation of HCHs up to 83 % in 420 min with persulfate whereas percentages lower than 40 % are obtained when using hydrogen peroxide. The use of both oxidants in the presence of ferrioxalate and LED visible light promotes the generation of hydroxyl and sulfate radicals under circumneutral pH values, which are the main responsible species for HCHs removal. HCH removal is favored by activated persulfate due to the sulfate radicals formation. Furthermore, these species are more selective than hydroxyl radicals and favor the C-Cl bond cleavage by electron transfer in HCHs. On the other hand, DNAPL produced as liquid residuum of lindane production contains other chlorinated organic compounds (COCs) which are susceptible to be oxidized by hydroxyl and sulfate radicals, generating competitive oxidation reactions. Aromatic COCs are easy degraded by both radicals, however, HCHs are only completely removed when persulfate is used as oxidant. This better performance points out that the photo-oxidation of DNAPL polluted groundwater with LED-vis light should be carried out with persulfate to ensure the complete removal of COCs. This confirms the excellent ability of sulfate radicals for C-Cl bond breakdown.