Effective adsorption of Au(III) and Cu(II) by chemically treated sheep wool and the binding mechanism

Sheep wool as biosorbent materials was chemically modified by NaOH, Na2S, NaHSO3, and NaBH4 solution. The adsorption amounts of Au(III) were increased after the chemical treatment and the adsorption capacity of the chemically treated wool were performed in the following order; Na2S treated wool > NaBH4 treated wool > NaOH treated wool > NaHSO3 treated wool. Furthermore, sheep wool was modified by Na2S at different concentrations (0.002 - 0.1 M). The surface characterization is analyzed by Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The appropriate concentration of treatment was determined at 0.02 M and 0.05 M concentrations of Na2S. Sheep wool changed to a sheet-like structure after 0.05 M of Na2S treatment; however, it keeps fiber structure after treatment of 0.02 M of Na2S. The adsorption amounts of Cu(II) were increased after the Na2S treatment and amounts were performed in the following order; 0.05 M of Na2S treated > 0.02 M of Na2S treated > none treated wool. XPS analysis results indicated that the binding of copper ion to the sheep wool is attributed through the oxygen of carboxyl groups of the sheep wool. Gold was bound to the nitrogen of amino and sulfur of the thiol group was confirmed by XPS. Adsorption kinetics fitted well with a pseudo-second-order kinetic model. Langmuir isotherm model more fit with the experimental data than Freundlich isotherm model. In summary, Na2S treated sheep wool to be revealed as an effective low-cost biosorbent for removal of Au(III) and Cu(II).