Catalytic Performance of Fe-Rich Sludge in Pyrolysis of Waste Oil Scum as Volatiles and Magnetic Char

Abstract

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.