Insights into chloride sorption and phase transformation of the synthesized Ca-Al bimetallic oxides in flue gas desulfurization wastewater

Sorbents enriched with Ca₃Al₂O₆ (C₃A) and Ca₁₂Al₁₄O₃₃ (C₁₂A₇) were successfully synthesized by a solid-state sintering method for Cl⁻ removal from the flue gas desulfurization (FGD) wastewater. C₃A and C₁₂Al₇-rich sorbents prepared at 1200 and 1100 °C achieved the highest Cl⁻ removal of 83.3% and 73.1%, and the former had a higher aluminum utilization rate of 45.2%. Compositional analyses revealed that sorbents synthesized using α-Al₂O₃ had higher atomic density, higher ratio of absorbed oxygen to lattice oxygen, and higher AlO₄⁵⁻/AlO₆⁹⁻ ratio than those using Al₂O₃, resulting in enhanced reactivity and superior Cl⁻ removal performance. Higher calcination temperature also improved the crystallinity of bimetallic oxides. The sorption isotherms and thermodynamic studies confirmed that the sorption of Cl⁻ on Ca-Al oxides was spontaneous and exothermic, and the maximum sorption capacities were 243.7 mg/g on C₃A and 204.1 mg/g on C₁₂Al₇, respectively. Kinetics and characterizations suggested that the hydration-induced mineral phase transformation led to structural reconstruction, and the sorption mechanism involved a synergistic action of chemisorption (as the primary force) and physisorption. These results suggested that Ca-Al-based sorbents are highly promising alternatives for application in the zero liquid discharge process of FGD wastewater.