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

• Cl− was not removed adequately from hydrochloric acid by CO3·Mg–Al LDH.
• The removal is due to the reaction between HCl and CO32− in the interlayers of LDH.
• The removal is also due to the reaction between HCO3− produced and Cl− ion from HCl.
• The Dubinin–Radushkevich adsorption model was best suited.

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.

Adsorption isotherms of Cl− adsorbed by LDH 4Mg2Al·CO3 and LDH 8Mg2Al·CO3. Quantity: 0.2 g; initial hydrochloric acid concentration: 0.001–0.1 M; temperature: 30 °C; time: 24 h.Figure optionsDownload as PowerPoint slide