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

Abstract

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.