Rapid sulfonation of lignite for cadmium removal in wastewater and subsequent recycle into photocatalysts

Abstract Addressing global warming requires fundamental solutions to reduce fossil fuel consumption and mitigate its environmental impact. This study explores the potential of lignite, a low-energy coal, for novel applications beyond its conventional use as a fuel source. It aims to improve lignite’s capacity to adsorb cadmium (Cd) from wastewater through sulfonation and subsequently utilize the spent adsorbents for photocatalytic degradation of methylene blue (MB). Treatment of lignite at 150 °C for 30 min with a 1:10 ratio of lignite to H2SO4 significantly enhanced its Cd adsorption capacity to 206.51% of the original. Porosity characterization and functional group analyses demonstrated significant alterations in pore structure, surface area, and the abundance of oxygen-containing functional groups, emphasizing the efficiency of the sulfonation process. Key adsorption mechanisms, including complexation and cation exchange, were pivotal in augmenting Cd adsorption. The Cd-enriched adsorbents were then transformed into lignite-based CdS photocatalysts, achieving a maximum degradation rate of 83.65% and retaining 82.33% of the original efficiency in degrading MB after three cycles. Electron Spin Resonance (ESR) studies indicated that superoxide and hydroxyl radicals were the dominant reactive oxygen species in the degradation mechanism. These findings propose key strategies for repurposing lignite in environmental remediation efforts, contributing to resource sustainability and enhancing the economic efficiency of treatment processes. Graphical Abstract