Refractory organic pollutants removal in industrial wastewater by a pilot-scale tubular mixed reactor at short hydraulic retention time: Effect and mechanism

This study investigates the efficiency of incomplete oxidation-coagulation/flocculation-separation (IOCS) process for industrial wastewater advanced treatment using pilot-scale tubular mixed reactor and micro/nano air flotation equipment. The underlying mechanism was explored through variations in chemical oxygen demand (COD), pH and H₂O₂ concentration, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV–Vis), gel permeation chromatography (GPC), and gas chromatography/tandem quadrupole time-of-flight mass spectrometry (GC/QTOF-MS) analyses. The IOCS process achieved an average COD removal rate exceeding 50 % and reduced total phosphorus to extremely low levels (< 0.1 mg/L), when the initial pH was elevated to 5.0 and the oxidation hydraulic retention time (HRT) was shortened to 5–10 min without the introduction of pre-prepared catalysts. Mineralization removal only contributed at most 26.3 %–34.4 % of total COD, whereas IOCS removal accounted for at most 73.1 %–86.7 %. A sharp pH drop from 5.0 to approximately 3.5 and rapid H₂O₂ consumption by >95 % were observed within the first 2–5 min, associated with iron hydrolysis and subsequently enhanced oxidation and adsorption. Furthermore, FTIR and GC/QTOF-MS analyses confirmed the incomplete mineralization of refractory organic pollutants, with their intermediates accumulating in the IOCS sludge. In-situ generated Fe(III) likely played an indispensable role in phosphorus and organic pollutants removal. This study proposes a novel approach to wastewater advanced treatment, emphasizing rapid processing, functional integration, and minimal chemical input.