Synergistic optimization of bio–oil quality and heavy metal solidification during microwave co–pyrolysis of cow dung and red mud

Abstract

To decrease the environmental risks caused by heavy metals (HMs) in red mud (RM) and improve the quality of pyrolysis oil from biomass, high–temperature pretreated RM and cow dung (CD) were microwave co–pyrolyzed. Then, the optimization potential of energy consumption was evaluated and the interaction mechanism between RM and CD was explored. The results showed that the increase in transition metal oxides and specific surface area improved the microwave–absorption and catalytic capacity of the pretreated RM. By optimizing the parameters, a pretreatment temperature of 650 °C resulted in a 21.65% reduction in acid content of bio–oil, higher HMs immobilization rates (>91%) and a 7.44% reduction in energy consumption. The synergistic optimization of bio–oil quality, HMs immobilization and energy consumption was achieved. After microwave co–pyrolysis with cow dung, the larger specific surface area (92.90 m2 g−1) and higher carbon crystallinity (ID/IG = 1.02) of pyrolysis residues enhanced the physical adsorption to HMs. The complexation of HMs with –OH could further enhance the solidification of HMs. This work will provide support to efficient resource utilization of solid waste, and demonstrate the great potential of microwave co–pyrolysis in HMs immobilization.