Treatment of hydrochloric acid using Mg–Al layered double hydroxide intercalated with carbonate

Mg–Al layered double hydroxide intercalated with CO₃²⁻ (CO₃·Mg–Al LDH) was found to take up only low amounts of Cl⁻ during the treatment of hydrochloric acid. Low pH promotes the removal of Cl⁻. HCl is considered to react with CO₃²⁻ in the interlayers of Mg–Al LDH as a result of which Cl⁻ is probably intercalated in the interlayers. High pH of the Mg–Al LDH suspension leads to a lower extent of reaction between HCl and Mg–Al LDH, owing to the neutralization of the acid. We can state that the adsorption of Cl⁻ on Mg–Al LDH is best described by the Dubinin–Radushkevich adsorption isotherm. The maximum adsorption amounts were calculated to be 1.4 and 0.6 mmol/g for CO₃·Mg–Al LDH with Mg/Al molar ratios of 2 and 4, respectively, whereas the adsorption energies were calculated to be 82 and 124 kJ/mol, respectively. In summary, the removal of Cl⁻ is caused by both the reaction between hydrochloric acid and CO₃²⁻ in the interlayers of Mg–Al LDH as well as anion exchange between the HCO₃⁻ produced and a different Cl⁻ ion in hydrochloric acid. This explains why the adsorption is best expressed by the D–R adsorption isotherm, which assumes a heterogeneous surface and non-constant adsorption potential.