Ionic liquids removal by sequential photocatalytic and biological oxidation

BACKGROUND The industrial application of ionic liquids (ILs) can induce the generation of wastewater and their release into the aquatic media. ILs can be efficiently removed by advanced oxidation processes (AOPs), which are expensive to implement. An alternative, using solar photocatalysis and biological degradation in combination, was used here to remove 1-hexyl-3-methylimidazolium chloride (HmimCl) and 1-butyl-4-methylpyridinium chloride (BmpyrCl). The chemical pretreatment allowed the ILs to be converted into by-products for easier biodegradation in a sequential batch reactor (SBR). RESULTS Photocatalytic degradation, using 0.25 g TiO2 L-1 and 600 W m(-2) solar radiation for 24 h, allowed a complete ILs removal and a partial mineralization of the organic matter, 28% for Bmpyr(+) and 35% for Hmim(+). A degradation pathway based on the by-products identified was proposed for each IL. The reaction effluents were submitted to a biological treatment in an SBR, using organic loading rates of 0.18-0.2 kg COD kg(-1) VSS d(-1) and a biomass concentration of 3.5 g VSS L-1 in 8 h cycles. The combined treatment allowed highly efficient removal of organic matter [total organic carbon (TOC) conversion exceeded 75% for HmimCl and 78% for BmpyrCl]. CONCLUSION Solar photocatalytic oxidation efficiently removed ILs and produced more biodegradable and less ecotoxic effluents. Biological oxidation increased TOC and chemical oxygen demand (COD) removal to more than 75% for the overall treatment. Combining solar photocatalysis and biological degradation therefore provided an effective system for IL removal. (c) 2019 Society of Chemical Industry