Bio-based nitrogen doped carbon-silica composites from low value sugarcane by-products for the selective adsorption of Au(III) from aqueous systems*

This study investigates the development of carbon-silica composite (CSC) adsorbents with tuneable surface properties for gold adsorption from silica and molasses. Nitrogen modification of the CSC adsorbents was achieved through a facile urea impregnation onto the CSC, followed by pyrolysis at 400 degrees C (400C-5A). Exceptional Au(III) adsorption was demonstrated from acidic solutions, with a maximum adsorption capacity of 416.9 mg g(-1). While CSC with no nitrogen modification (400C) only exhibits an adsorption capacity of 337.9 mg g(-1). Langmuir isotherm model and pseudo-second order kinetic model were found to best describe the adsorption behaviour for Au(III) sorption on both nitrogen-modified and non-modified CSC. Urea modification led to nitrogen containing graphitic, pyridinic and pyrrolic functionalities, of which pyridinic groups were active sites for metal coordination, leading to enhanced adsorption and high selectivity towards Au and Pd. Au adsorptions of greater than 99.8% were revealed using both modified and unmodified CSC, even at low concentrations of gold (5 or 10 ppm). X-ray Photoelectron spectroscopy (XPS) analysis of 400C-5A following adsorption indicated the presence of both Au(0) and Au(III), demonstrating that adsorption took place through both chelation and an adsorption-reduction mechanism. These effective adsorbents derived from abundantly available, low cost, biobased chemicals or low value by-products could create new opportunities for Au and/or Pd adsorption from acidic solutions.