Functionalisation of Mg/Al hydrotalcite and calcined Mg/Al hydrotalcite with gallic acid for reductive adsorption of tetrachloroaurate(III) ion in aqueous solution

Gold recovery from secondary resources has been an interesting topic due to its high market prices and vast industrial applications. Reductive adsorption is a new promising technique that provides a cost-effective, efficient and eco-friendly way for gold recovery. In this study, we prepared a gallic acid-functionalised Mg/Al hydrotalcite (Mg/Al HT-GA) as well as first and second refunctionalised calcined Mg/Al hydrotalcite (Mg/Al CHT-GA_1 and Mg/Al CHT-GA_2) for reductive adsorption of AuCl4− in aqueous solution. The as-prepared adsorbents were characterised using a Fourier transform infrared (FTIR) spectrometer, an X-ray diffractometer (XRD) and a scanning electron microscope (SEM). Adsorption test showed that Mg/Al HT-GA, Mg/Al CHT-GA_1 and Mg/Al CHT-GA_2 were effective in adsorbing AuCl4 – at pH 3 with adsorption capacities (b) according to Langmuir isotherm model were 175.43, 112.36 and 86.20 mg g–1, respectively. Kinetics study showed that all adsorbents followed pseudo-second order with rate constant (kp2) of 9.81 × 10–4, 33.17 × 10–4 and 9.80 × 10–4 g/(mg.min) for Mg/Al HT-GA, Mg/Al CHT-GA_1 and Mg/Al CHT-GA_2, respectively. The characterisation of the spent adsorbent by using an XRD showed that Au(0) was formed. It was revealed that phenolic – OH groups of gallic acid played a key role for the reduction mechanism of Au(III).