Mechanistic investigation on the green recovery of ionic, nanocrystalline, and metallic gold by two anionic nanocelluloses

• Potential recovery of gold from anionic nanocellulose.
• Recovery of ionic, nanoparticulate, and metallic Au from dilute solutions.
• Least interference of pH and competing ions on Au recovery.
• A plausible mechanism suggested for adsorptive–reduction pathways.

The modified cellulose (ADAC samples) used as backbone to obtain biodegradable hydrogels, which demonstrated high affinity to concentrate precious gold species. The primary foci of the study were to develop green techniques to recover Au from dilute solutions by providing the mechanistic evidences for the adsorption coupled reduction pathways. The derivatives of anionic nanocellulose were used for recovery of the ionic, nanoparticulate, and metallic forms of Au. The ADAC samples were promising sorbing phase (57 and 60 mg/g capacity) even at low pH for the treatment of the gold-polluted waters. Thermodynamically, ADAC hydrogels were remained active at moderate to higher temperature (295–333 K) in an endothermic way. Reduction of Au occurred at different stages of kinetic curves, facilitating the formation of zerovalent gold (48 and 41 nm particle size). The Au recovery minimally altered by the competing heavy metals (Cd, Co, Cr, Ni, and As), either at equivalent or lower concentrations. A significant amount of Au(III) (83–99%) regenerated by a strong complexing eluent (0.5 M thiourea in 1 M HCl), moreover thermal crystallization yielded metallic gold in an excellent amount (94%). The mechanism of adsorptive–reduction of gold-nanocellulose were investigated and characterized by the spectroscopic studies including X-ray photoelectron spectroscopy. Overall, recovery of gold in desired forms from nanocellulose can be viewed as an effective biomaterial management strategy.