Waste oil scum is commonly discharged during the air flotation process at oil-bearing wastewater treatment plants and disposed as an additive in cement kilns and/or furnaces. Herein, it was mixed with a magnetite-rich waste sludge and then completely recycled as value-added gas/oil and magnetic char via a facile catalytic pyrolysis route. Results showed that the oil scum was a blackish gel and contained 36.2% water, 52.5% tar, and 11.3% inorganics. After direct pyrolysis, the conversion efficiencies of tar to gas, oil, and char were 30.2%, 41.2%, and 28.6%, respectively, and the generated gas/oil was rich in aromatics. By adding Fe-rich sludge, the efficiencies varied to 57.3%, 26.9%, and 15.8%, respectively, and the gas/oil mainly comprised a chain hydrocarbon. During oil scum pyrolysis, the redox reaction of tar to Fe-rich sludge enabled a cycle of Fe/magnetite to accelerate the cleavage of tar as volatiles and to steadily retard the polycondensation of tar as char. In addition, the added Fe-rich sludge not only activated the rest of the char and created more surface functional groups for contaminant adsorption but also endowed the char with a good magnetic response. Such magnetic char showed a maximum adsorption capacity of ciprofloxacin of 63.5 mg/g, higher than that without the Fe-sludge catalyst, and had ability to selectively adsorb ciprofloxacin from benzoic/sulfanilamide-bearing wastewater. In summary, a ‘waste to treat waste’ strategy was developed to recycle waste oil scum as combustible gas/oil and magnetic char with the addition of magnetite-rich sludge.

Organic saline wastewater has become a concern in recent decades due to its resistance to biological treatment and potential harm to municipal wastewater treatment plants. While photocatalytic methods have been used for treatment, they often lead to catalyst deterioration. The use of salt-tolerant catalysts presents a viable solution for treating organic saline wastewater. In this study, a Zn-rich g-C₃N₄ was synthesized, demonstrating excellent performance in removing 2,4-DCP and its derivatives from saline wastewater. More than 75.6% of 2,4-DCP was effectively removed with the addition of Zn-rich g-C₃N₄, nearly doubling the removal rate compared to pure g-C₃N₄ and those doped with Co, Ag, Mo, and Bi. Notably, the removal efficiency of 2,4-DCP slightly increased as salinity rose from 0.1 to 2.3 wt.%. Adding 0.1 g L⁻¹ of Zn-rich g-C₃N₄ resulted in the removal of 2,4-DCP, 2-chlorohydroquinone, chloroacetophenone, and 2-chloropropionic acid by 99.3%, 99.8%, 98.2%, and 99.9%, respectively, from a real saline wastewater sample with 2.2 wt.% salinity, corresponding to a 67.7% removal of TOC. The EPR results indicated that Zn-rich g-C₃N₄ generated more free radicals compared to pure g-C₃N₄, such as·OH and Cl, to degrade organic contaminants. The degradation pathway revealed that 2,4-DCP was first dechlorinated into p-phenol and catechol, which were subsequently degraded into maleic acid/fumaric acid, trihydroxyethylene, acetic acid, oxalic acid, and other products. Furthermore, Zn-rich g-C₃N₄ demonstrated excellent stability and holds promising potential for applications in saline wastewater treatment.