Adsorption of diclofenac from water solution on layered double hydroxide Mg/Al/CO3 synthesized from sulphate salts and calcined

Abstract

The diclofenac (DCF) is an emerging pollutant commonly found in aqueous resources. The synthesis of the double layered hydroxide Mg/Al-CO3 (LDH) was carried out by a coprecipitation procedure. The elimination of DCF in water by adsorption onto LDH calcined at 550 °C (LDH550) was studied in this work. The XRD analysis confirmed the presence of the hydrotalcite crystalline species in the LDHs. In the LDH calcination, most of the interlaminar anions were eliminated, and an oxide of Mg/Al was formed. Likewise, the memory effect of LDH550 was verified when rehydrating LDH550 in an aqueous solution. The materials characterization was accomplished by SEM, N2 physisorption, Zeta potential and FT-IR. At 25 °C and pH = 7, the highest adsorption capacity of LDH550 towards DCF was 1517 mg/g, which significantly exceeded the capacities reported for other LDHs and carbon materials. Hence, the LDH calcination largely facilitated the availability and interaction between the anionic sites of the LDH550 sheets and the anionic species DCF−. If the LDH550 was reconstructed and rehydrated (LDH550Reh) before the DCF adsorption, the adsorption capacity of LDH550Reh was very low, confirming that reconstruction restricted the accessibility of DCF to the anionic sites of LDH550Reh. The capacity of LDH550 to adsorb DCF decreased with increasing solution pH and ionic strength, and this tendency was attributed to electrostatic attractions. The adsorption was endothermic because the uptake of DCF adsorbed on LDH550 decreased by reducing temperature. The influence of the operating conditions on the adsorption revealed that the adsorption mechanism of DCF on LDH550 can be mainly electrostatic interactions. A novel approach of this work is that the electrostatic attraction mechanism was further confirmed by zeta potential determination of LDH550 with adsorbed DCF and noticing that the adsorption of the DCF− anions balanced the positively charged sites in LDH550.