Evaluation of performance, extracellular polymeric substances, reactive oxygen species and lactate dehydrogenase in a combined ultraviolet/Fenton-sequencing batch reactor at the co-selective pressure of divalent zinc and diclofenac

Abstract

A combined ultraviolet (UV)/Fenton-sequencing batch reactor (SBR) treating synthetic wastewaters containing diclofenac (DCF) and divalent zinc (Zn2+) was investigated. In a UV/Fenton process at 50 mg L−1 DCF, the chemical oxygen demand (COD) and DCF removal rates were 14.2 % and 78.5 %, respectively, at the optimum dosages of hydrogen peroxide (H2O2) (360 mg L−1) and ferrous iron (Fe2+) (60 mg L−1), and the main DCF removal pathways were hydroxyl radicals (OH) attack and decarboxylation. In a combined UV/Fenton-SBR, the UV/Fenton process at the optimum operated condition was used as the SBR pretreatment, and the COD removal rates at 50 mg L−1 DCF, 5 mg L−1 Zn2+ and 50 mg L−1 DCF +5 mg L−1 Zn2+ as well as the DCF removal rates at 50 mg L−1 DCF and 50 mg L−1 DCF +5 mg L−1 Zn2+ were 88.6 %, 94.6 %, 90.9 %, 80.5 % and 81.8 %, respectively, which were higher than those in a SBR without pretreatments. Protein (PN) and polysaccharide (PS) concentrations in loosely bound extracellular polymeric substances (LB-EPS) and tightly bound extracellular polymeric substances (TB-EPS), reactive oxygen species (ROS) content and lactate dehydrogenase (LDH) release in a SBR with pretreatments were lower at DCF and Zn2+ pressure than at DCF or Zn2+ pressure, while they in a SBR without pretreatments were higher at DCF and Zn2+ stress than those at DCF or Zn2+ stress. LB-EPS and TB-EPS in a SBR without pretreatments had more fluorescence peaks than in a SBR with pretreatments in the three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectrum.