Kraft mill lignin – A potential source of bio-adsorbents for gold recovery from acidic chloride solution

• We developed environmentally benign and low cost adsorbent from kraft mill lignin.
• Specific gold selectivity and high gold uptake capacity of the adsorbent.
• Reduction of adsorbed Au(III) to elemental Au and formation of gold aggregates.
• Promising candidate for selective recovery of gold from base metals in aqueous environment.

The potentiality of lignin, obtained as a byproduct of kraft mill industry, for adsorptive preconcentration and separation of Au(III) from acidic chloride media was investigated. Quantitative adsorption of Au(III) from hydrochloric acid medium was achieved using crude lignin while no base metals were adsorbed on it. Adsorption of Au(III) on lignin followed typical Langmuir monolayer adsorption and the maximum Au(III) uptake capacity of the material was found to be approximately 6.0 mol kg−1 at 313 K. Increasing temperature appeared to improve the amount of gold adsorbed in addition to enhancing the sorption kinetics. The computed thermodynamic parameters demonstrated that the adsorption was a spontaneous process and was favorably governed by an endothermic reaction. A pseudo-first-order kinetic model explained the results of kinetic studies conducted at various temperatures and also allowed the estimation of activation energy of the adsorption reaction. The obtained value of activation energy indicated that the sorption of Au(III) on lignin was a chemisorption process. The adsorbed Au(III) species was subsequently reduced to metallic gold by the reducing functional groups present in the lignin matrix. Because of its considerable gold uptake capacity and resultant formation of elemental gold aggregates, kraft mill lignin is a promising candidate for gold recovery from acidic chloride media.