Oily sludge is a hazardous waste containing emulsified petroleum hydrocarbons (i.e. hazardous organic substances such as polycyclic aromatic hydrocarbon, phenol and benzene), water, heavy metals and solid particles. Statistics have it that over 60 million tons of oily sludge is generated annually by oil refineries worldwide; this makes it the one of the most significant solid wastes generated in the petroleum industry. The presence of hazardous components necessitates the effective disposal of oily sludge, which nonetheless poses challenges owing to the recalcitrant nature of the substances. Persistent research has revealed pyrolysis as a promising approach for oily sludge management due to its energy recovery (pyrolysis oil and gas), solid waste reduction, and less secondary pollution. Better still, solid residue char is generated from pyrolysis. Recent publications have shown that the oily sludge-derived char (OSDC) can be used to remove heavy metals from wastewater. It is therefore reasonable to hypothesize that the mineral-rich OSDC could remove heavy metals through a complex mechanism, involving surface adsorption, precipitation and ion exchange.
Unfortunately, vital information regarding the roles of different sorption mechanisms for OSDC is still missing. Further, it can be anticipated that investigation and even quantification of metal sorption mechanisms of OSDC will aid the design of future field-scale remediation projects, and estimation of the long-term performance of OSDC in field conditions. In light of this, Canadian researchers from the University of Northern British Columbia: Dr. Yuan Tian, Dr. Jianbing Li, Dr. Todd W. Whitcombe, Dr. William B. McGill and Dr. Ronald W. Thring investigated the physicochemical properties of oily sludge-derived char produced from pyrolysis at 500 °C (OS500) and its sorption behavior towards lead (Pb2+) and cadmium (Cd2+) in aqueous solution. Their work is currently published in the Chemical Engineering Journal.
In their approach, the sorption kinetics and isotherm of Pb2+ and Cd2+ sorption on OS500 were determined. The results were fitted with four kinetic (pseudo first order, pseudo second order, Elovich equation, and intraparticle diffusion) and six isotherm (Langmuir, Freundlich, Sips, Redlich-Peterson, Temkin, and DubininRadushkevich) models. Additionally, the mechanisms of lead and cadmium sorption on OS500 and their quantitative contributions were further studied by a variety of techniques such as Fourier transform infrared spectroscopy (FTIR), X-ray diffraction and sequential extraction test.
The authors reported that the maximum sorption capacity (QL) of Pb2+ obtained from the Langmuir model to be 373.2 mg/g, while QL of Cd2+ was 23.19 mg/g. Interestingly, results indicated the chemical characteristics but not microstructure of OS500 controlled its Pb2+ and Cd2+ sorption. Further, new mineral precipitates (i.e., hydrocerussite (Pb3(CO3)2(OH)2) and cerussite (PbCO3)) were reported to form during Pb2+ sorption. Moreover, Pb2+-π interaction and complexation of Cd2+ with hydroxyl functional groups were confirmed by FTIR.
In summary, the study presented the characterization of the microstructure and chemical properties of oily sludge-derived char. Their findings showed that both Pb2+ and Cd2+ sorption fit the Elovich equation and Redlich-Peterson isotherm model. Also, precipitation was seen to dominate Pb2+ sorption on OSDC. Further, it was shown that complexation and cation exchange were mainly responsible for Cd2+ removal by OSDC. In a statement to Advances in Engineering, Professor Jianbing Li, the corresponding author pointed out that despite the remarkable achievements reported in their study, further research will still be required to drive new applications of OS500.
Yuan Tian, Jianbing Li, Todd W. Whitcombe, William B. McGill, Ronald W. Thring. Application of oily sludge-derived char for lead and cadmium removal from aqueous solution. Chemical Engineering Journal, volume 384 (2020) 123386.