Microwave-assisted transforming of biosolids into engineered activated carbon employed for adsorption from wastewater

Abstract

Biosolids, a prevalent by-product of wastewater treatment plants, contribute ∼350,000 tonnes annually to dry waste solids in Australia. Biosolids are valuable resources due to their high concentrations of carbon and other components. In line with Circular Economy principles, this study explores the potential conversion of biosolids into value-added products, specifically focusing on the feasibility of producing microstructure-engineered powdered activated carbon (AC) for wastewater treatment. Carbonization of biosolids using microwave (MW)-assisted heating technology, compared to traditional heating, presented reduced energy consumption (by 26%) and 38% higher surface area while attaining a similar carbonization yield. After the activation step, MW technology led to a 16% enhancement in surface area, achieving 1507 ± 60 m2/g. Subsequent diluted acid washing effectively reduced the ash content from 58 ± 2.3 to 14.4 ± 1.2 wt%. Then, the microstructure of the AC was tailored to adsorb methylene blue (MB, as a model molecule) and humic acid (HA, as a complex natural organic matter). Microstructure modifications attained an optimal blend of micro- and mesopores for adsorption. One of the ACs was assessed for methylene blue (MB) adsorption, demonstrating a remarkable adsorption capacity of 759 ± 46 mg/g, surpassing a commercial standard sample by 41.5%. The second AC displayed an adsorption capacity of 126.6 ± 5.1 g/mg of HA, a 22% improvement over the commercial sample with an adsorption capacity of 103.5 ± 4.1 mg/g. Kinetics analysis of both adsorption tests was conducted, and they fit well with the pseudo-second-order model. This study underscores upcycling wastewater biosolids using MW-assisted technology into sustainable ACs for the adsorption of different wastewater pollutants, exemplifying a path towards a circular economy and a more environmentally conscious and resource-efficient future.