Electrochemical oxidation of trace organic contaminants in reverse osmosis concentrate using RuO2/IrO2-coated titanium anodes

During membrane treatment of secondary effluent from wastewater treatment plants, a reverse osmosis concentrate (ROC) containing trace organic contaminants is generated. As the latter are of concern, effective and economic treatment methods are required. Here, we investigated electrochemical oxidation of ROC using Ti/Ru0.7Ir0.3O2 electrodes, focussing on the removal of dissolved organic carbon (DOC), specific ultra-violet absorbance at 254 nm (SUVA254), and 28 pharmaceuticals and pesticides frequently encountered in secondary treated effluents. The experiments were conducted in a continuously fed reactor at current densities (J) ranging from 1 to 250 A m−2 anode, and a batch reactor at J = 250 A m−2. Higher mineralization efficiency was observed during batch oxidation (e.g. 25.1 ± 2.7% DOC removal vs 0% removal in the continuous reactor after applying specific electrical charge, Q = 437.0 A h m−3 ROC), indicating that DOC removal is depending on indirect oxidation by electrogenerated oxidants that accumulate in the bulk liquid. An initial increase and subsequent slow decrease in SUVA254 during batch mode suggests the introduction of auxochrome substituents (e.g. –Cl, NH2Cl, -Br, and -OH) into the aromatic compounds. Contrarily, in the continuous reactor ring-cleaving oxidation products were generated, and SUVA254 removal correlated with applied charge. Furthermore, 20 of the target pharmaceuticals and pesticides completely disappeared in both the continuous and batch experiments when applying J ≥ 150 A m−2 (i.e. Q ≥ 461.5 A h m−3) and 437.0 A h m−3 (J = 250 A m−2), respectively. Compounds that were more persistent during continuous oxidation were characterized by the presence of electrophilic groups on the aromatic ring (e.g. triclopyr) or by the absence of stronger nucleophilic substituents (e.g. ibuprofen). These pollutants were oxidized when applying higher specific electrical charge in batch mode (i.e. 1.45 kA h m−3 ROC). However, baseline toxicity as determined by Vibrio fischeri bioluminescence inhibition tests (Microtox) was increasing with higher applied charge during batch and continuous oxidation, indicating the formation of toxic oxidation products, possibly chlorinated and brominated organic compounds.

Research highlights
► Removal of 28 pharmaceuticals and pesticides by Ti/RuIrO2 electrode was reported.
► Batch and continuous electrolysis of reverse osmosis concentrate was compared.
► Increase in baseline toxicity of electrochemically oxidized concentrate was shown.