Gallic and salicylic acid-functionalized Mg/Al hydrotalcite as highly effective materials for reductive adsorption of AuCl4-

Two materials as the functionalization products of magnesium aluminum hydrotalcite (Mg/Al HT) with gallic and salicylic acids have been prepared and their application to effectively remove AuCl₄^- has been examined. Functionalization was carried out through immobilization in which deprotonated carboxyl group of gallic and salicylic acids was the responsible functional group for the immobilization. Immobilization of both acids was optimum at pH 5.0 and matched better in Langmuir isotherm model than the Freundlich one with capacity of immobilization was 7.22 and 7.18 × 10^-4 mol/g for galic and salicylic acids, respectively. In spite of both acids were immobilized at comparable amount, but gallic-acid-functionalized Mg/Al HT (Mg/Al-GA HT) had higher ability in removing AuCl₄^- than the salicylic-acid-functionalized Mg/Al HT (Mg/Al-SA HT). The removal capacity of Mg/Al-GA HT and Mg/Al-SA HT for AuCl₄^- at temperature 25 °C was 3.10 and 2.08 × 10^-4 mol/g and it was 8.16 and 5.47 time-fold higher than that of Mg/Al HT. The removal of AuCl₄^- by Mg/Al-GA HT and Mg/Al-SA HT was exothermic and involved OH group and this functional group was responsible for not only the adsorption of AuCl₄^- but also reduction of AuCl₄^- to Au metal.