Porous carbon endows mayenite with high activity to achieve closed-loop removal of chloride ions from desulfurization wastewater

The high concentration of chloride ions (Cl−) in flue gas desulfurization (FGD) wastewater not only corrodes equipment but also reduces the quality of desulfurization gypsum. The mayenite (Ca12Al14O33) has a notable capacity for adsorbing Cl−, but its efficacy is hindered by its inherently low specific surface area and long reaction time. To improve the effectiveness of mayenite in Cl− removal, the porous carbon derived from sewage sludge (SSC) was employed to modify the mayenite-based Ca/Al oxides (CAO). The resultant SSC/CAO composites featured a predominant mayenite phase with rough surface and exposed flake structure, leading to an increase in surface area from 2 m2/g for CAO to 54 m2/g for SSC/CAO-1 (mass ratio of sewage sludge: CAO = 1: 1). Within 2 h of reaction, SSC/CAO-1 achieved a Cl− removal efficiency of 68 % and a high Cl− adsorption capacity of 97 mg/g. The Cl− removal mechanism is related to the presence of a higher percentage of AlO45-, more oxygen vacancies, and larger surface area in SSC/CAO-1 compared to that in CAO, which facilitated the dissolution and reconstruction of the [Ca − Al − O] framework, resulting in the formation of Ca2Al(OH)6Cl·2H2O with high crystallinity. The negative Cl embedding energy indicates that Ca12Al14O33 is favorable for the embedding of Cl, and its transformation into Ca2Al(OH)6Cl·2H2O occurs spontaneously. The final product after Cl− removal consisted of efficient components to obtain high catalytic hydrolysis efficiency of carbonyl sulfide, which holds great importance to address the issue of handling the Cl– removal product during backend treatment.