Pre-treatment of real pharmaceutical wastewater by heterogeneous Fenton and persulfate oxidation processes

This study compares the pre-oxidation of pharmaceutical wastewater by hydroxyl radical based advanced oxidation (HR-AOP) and a sulfate radical based advanced oxidation process (SR-AOP). The heterogeneous Fenton process is chosen as a model HR-AOP and persulfate (PS) activation as a model SR-AOP. The pre-treatment efficacy of both processes in terms of TOC, and COD removals using Fe₃O₄-rGO catalyst were considered. Under the investigated experimental conditions, both processes yielded fluctuating COD values with time. The heterogeneous Fenton process discovered to be the most efficient to remove 68.7% TOC in 180 min of treatment, when Fe₃O₄-rGO: H₂O₂ = 300 mg L^-1:150 mM H₂O₂ was used at pH 3. Notably, the heterogeneous Fenton system was not considerably inhibited at the natural pH of pharmaceutical wastewater (6.75), as the process successfully removed 64.6% TOC. On the other hand, in persulfate activation studies, Fe₃O₄-rGO: PS = 400 mg L^-1: 5 mM was the ideal condition for removing 59.5% TOC in 180 min at pH 3. Whereas the natural pH condition significantly inhibited the TOC removal, as only 20.8% TOC removal was feasible. The wastewater characterisation before and after Fenton treatment reveals that Fenton oxidation leads to an increase in inorganics (chlorides: 160 ± 15 mg L^-1, nitrates: 63.14 ± 3.08 mg L^-1, sulfates: 266.31 ± 31.39 mg L^-1) necessitating an additional treatment step to reduce COD and inorganics further.