Enhanced Gold(III) adsorption using glutaraldehyde-crosslinked chitosan beads: Effect of crosslinking degree on adsorption selectivity, capacity, and mechanism

Recovery of gold from electronic waste (e-waste) has received significant attention due to its high economic value. This study reports glutaraldehyde-crosslinked chitosan (GCC) beads as highly efficient bioadsorbents for the separation of auric ions (Au(III)) from acidic e-waste leaching solutions. With increasing extent of glutaraldehyde-crosslinking, the GCC beads showed enhanced adsorption selectivity and capacity toward Au(III). The Au(III) adsorption capacity, however, was saturated with a further increase in the crosslinking time due to increasing self-oligomerization of glutaraldehyde. High adsorption selectivity was also observed for other precious metals such as Pd(II), Pt(IV), Ir(III), Os(IV), and Ag(I), but negligible adsorption was found for most other metals. The high Au(III) adsorption capacity and selectivity of GCC beads was attributed to multiple mechanisms such as electrostatic attraction, chelation, and reduction of Au(III), based on a range of spectroscopic evidence (FTIR, XRD, XPS, SEM, and TEM) and Au(III) adsorption behavior. The adsorbed Au(III) was reduced to Au(I) and then to Au(0), forming Au-nanoparticles as the main Au species with increasing adsorption time. The GCC beads showed a good reusability in repeated Au(III) adsorption-desorption cycle tests and were used to successfully separate high purity gold (96.7%) from an acidic e-waste leaching solution.