Biosorption is a promising alternative to conventional methods for metal recovery during recycling. An extremophilic microalga Galdieria sulphuraria is suitable for use as a biosorbent with the potential for mass production and naturally high and selective adsorption capacity for gold and palladium without modification or extensive pre-treatment. However, the atomic-level understanding of how the biosorbent collects the precious metals was not known. In this study, adsorption mechanisms for G. sulphuraria-derived biosorbent in highly acidic solvents commonly found in metal wastewater were elucidated. It was indicated that the primary site of gold and palladium adsorption was the nitrogen atom on the biosorbent surface in a chlorine-rich environment where enough chlorine atoms were available for the precious metals to form chloride anionic complexes. By using several acidic solvents exhibiting different adsorption efficiency it was found that the more reduced the gold was, the higher the adsorption efficiency. Progressive gold reduction from trivalent to monovalent, then to zerovalent, which consequently produces nanoparticles with metallic gold, contributes to improved adsorption capacity by changing binding partners on the cell surface in the process, vacating adsorption positions for subsequent ionic gold. Redox reactions, including sulphur oxidation were also suggested to promote sustainable reduction of gold while palladium was shown to remain as divalent cations.
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