Simultaneous removal of Cl⁻ and SO₄ ²⁻ from seawater using Mg−Al oxide: kinetics and equilibrium studies

Mg−Al oxide, obtained by the thermal decomposition of a CO₃ ²⁻-intercalated Mg−Al layered double hydroxide (CO₃·Mg−Al LDH), simultaneously absorbed Cl⁻ and SO₄ ²⁻ from seawater and generated a Mg−Al LDH intercalated with Cl⁻ and SO₄ ²⁻. The Mg−Al oxide with a molar ratio Mg/Al = 4 was more superior than the oxide with Mg/Al = 2 for Cl⁻ removal, whereas a reverse phenomenon was observed for SO₄ ²⁻ removal. The removal of Cl⁻ and SO₄ ²⁻ by the Mg−Al oxide with Mg/Al = 4 could be represented by first-order and pseudo second-order reactions, respectively. The removal of both Cl⁻ and SO₄ ²⁻ by the Mg−Al oxide with Mg/Al = 2 could be represented by a pseudo second-order reaction. The removal of both Cl⁻ and SO₄ ²⁻ by the Mg−Al oxides with Mg/Al = 4 and 2 was proceeded under chemical reaction control. The adsorption isotherms for Cl⁻ and SO₄ ²⁻ adsorbed by the Mg−Al oxides could be expressed by Langmuir-type adsorption. These reactions were derived from monolayer adsorption, indicating the intercalation of Cl⁻ and SO₄ ²⁻ in the interlayer space of Mg−Al LDH. The uptake of Cl⁻ and SO₄ ²⁻ from seawater by Mg−Al oxide was proceeded spontaneously.