Synthesis and characterization of Layered Double Hydroxides aimed at encapsulation of sodium diclofenac: Theoretical and experimental study

The main objective of the paper is to study the encapsulation/adsorption of sodium diclofenac (Na-DIC) on hydrotalcite cationic clays (i.e., Layered Double Hydroxides, LDHs) by ion exchange and adsorbent reconstruction mechanisms. The encapsulation/adsorption method is generally used to modify some drugs' physicochemical characteristics, such as unpleasant odors, low solubility, high volatility, etc. LDHs are synthesized and characterized by different techniques (SEM, DRX, FTIR, BET, and ATG). Na-DIC adsorption capacity onto calcined LDH (CLDH) is determined (with maximum value in between 195 and 211 mg/L in the range 25–45 °C), together with the effect of the main operating parameters (adsorbent mass, pH, initial concentration of Na-DIC, and temperature). A statistical physics derived adsorption model allows a correct interpretation of experimental data and indicates that multiple molecules' vertical adsorption per active site occurs. Kinetic tests showed that the Pseudo First order (PFO) model correctly describes the Na-DIC adsorption kinetics on LDH at different temperatures, indicating the pore diffusion as the primary resistance mechanism and an activation energy of +34.56 kJ/mol (physical adsorption). HOMO and LUMO obtained from quantum chemical calculations confirm the ability of Na-DIC to receive electrons from LDH, indicative of polar adsorption (ΔNmax = 1.9). Adsorption is also simulated by Monte Carlo method, which allows determining the Na-DIC/CLDH configuration corresponding to the lowest total energy. The value of the adsorption energy confirms a strong interaction between the DIC and the CLDH. In conclusion, LDH clays show great potential as adsorption support and encapsulation medium, which could be proficiently used also to remove Na-DIC from wastewater. Na-DIC is adsorbed onto calcined hydrotalcite by the reconstruction method, which is explained by the so-called “memory effect” of hydrotalcite.