Magnetically separable humic acid-functionalized magnetite for reductive adsorption of tetrachloroaurate(III) ion in aqueous solution

Recently, it is obvious that the recovery of gold from secondary resources has been of great interest due to its high profitability and vast applications. Among many methods available, reductive adsorption is the easiest, cheapest and eco-friendly way for gold recovery. In the present study, an effective and magnetically separable adsorbent, i.e., humic acid-functionalized magnetite (Fe3O4-HA), had been prepared and used for the reductive adsorption of Au(III) ion. The as-prepared Fe3O4-HA was characterized by using a Fourier transform infrared (FTIR) spectrometer, an X-ray diffractometer (XRD), a scanning electron microscopy with an energy dispersive X-ray spectroscopy (SEM-EDS) and a vibrating sample magnetometer (VSM). Quantitative analyses of the total acidity and the content of carboxyl and phenolic –OH groups of HA and Fe3O4-HA were also performed. While the experiment results showed that Fe3O4-HA was optimum in adsorbing Au(III) ion at pH 3.5 with adsorption capacity (b) according to Langmuir isotherm model was 200 mg g−1, the adsorption process followed pseudo-second order with rate constant (k2p) 3.03 × 10−4 g/(mg min). It was found that the phenolic –OH group of HA was the one responsible for the reduction of Au(III) ion into metallic gold on the surface of the adsorbent, while the carboxyl group was the active site for the adsorption of Au(III) ion. It was revealed that the presence of radical scavenger 2-propanol decreased the adsorption of Au(III) ion and subsequent reduction to Au. Meanwhile, the adsorbent was easily separated from the medium by an application of an external magnetic field. © 2021 Elsevier B.V.