Adsorption of norfloxacin in aqueous solution by Mg–Al layered double hydroxides with variable metal composition and interlayer anions

Abstract

Mg–Al layered double hydroxides (Mg–Al-LDHs) were synthesized and used for adsorption on norfloxacin (NOR) in aqueous solution. Batch adsorption experiment was conducted to study the adsorption kinetics and isotherms of norfloxacin on Mg–Al-LDHs. Adsorption of NOR by Mg–Al-LDHs with variable Mg2+/Al3+ molar ratios (2:1, 3:1 and 4:1), with Sn4+ incorporation and different interlayer anions (CO32-andCl-) were investigated to explore the effect of metal composition and interlayer anions on the adsorbing performance of LDHs. LDHs with higher Al3+ content and Cl− as interlayer anion exhibited better performance for NOR adsorption both in adsorption rate and adsorption capacity. The incorporation of Sn4+ resulted in the deterioration on the adsorption ability of Mg–Al-LDHs. The pseudo-second rate model correlated the adsorption kinetics data well. Freundlich model fitted well with the adsorption equilibrium data. The negative value of ΔG° and ΔH° observed for Mg–Al–CO3-LDHs and Mg–Al–Cl-LDHs proved the spontaneity and exothermic nature of the adsorption process, while ΔS° value was negative for Mg–Al–Cl-LDHs and positive for Mg–Al–CO3-LDHs, which indicated an opposite trend in the entropy of the solid/liquid system. pH-dependent experiments showed both Mg–Al–CO3-LDHs and Mg–Al–Cl-LDHs had high pH buffering ability and the adsorption amount of NOR insignificantly changed within a wide initial pH range (pH 3.0–11.0). The electrostatic attraction was suggested to control NOR adsorption on Mg–Al–CO3-LDHs, while the dominating anion exchange process along with electrostatic attraction contributed to the adsorption by Mg–Al–Cl-LDHs.