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Abstract

Biooxidation is being increasingly explored as a pretreatment method for refractory gold ores, where gold is encapsulated within sulfide minerals such as pyrite and arsenopyrite. This interest is driven by the need for cost-effective and environmentally friendly alternatives to conventional methods like roasting and pressure oxidation. Biooxidation employs iron- and sulfur-oxidizing bacteria to break down sulfide matrices, liberating gold for subsequent cyanide leaching. However, the slow reaction kinetics, often requiring several days, limit its broader application. The use of organic additives has been explored to accelerate oxidation, but the mechanisms by which they influence biooxidation remain unclear. However, there is a lack of understanding of the underlying mechanisms by which these additives influence biooxidation. This research examines how organic additives affect arsenopyrite biooxidation, particularly focusing on their interactions with arsenic and impacts on microbial growth and gene expression. Cysteine and humic acid were selected as representative additives due to their known effects on sulfide oxidation and distinct chemical properties. Batch reactor leaching tests assessed their influence on arsenopyrite biooxidation, followed by experiments on arsenic speciation and toxicity. The combined effects of arsenic and organic additives on microbial gene expression were analyzed using quantitative reverse transcription PCR (qPCR). Results revealed contrasting effects of cysteine and humic acid on arsenopyrite biooxidation. Cysteine inhibited biooxidation, whereas humic acid accelerated reaction kinetics. These differences were attributed to how each additive interacted with arsenic released during mineral dissolution: cysteine slowed down arsenite oxidation to arsenate, while humic acid facilitated arsenite oxidation to arsenate. As arsenite is more toxic than arsenate, cysteine increased arsenic toxicity, inhibiting microbial growth and iron-oxidizing activity. In contrast, humic acid mitigated arsenic toxicity. Correspondingly, cysteine with arsenite upregulated arsenic resistance genes, whereas humic acid with arsenic reduced their expression. Overall, this study provides insights into how organic additives can either inhibit or enhance arsenopyrite biooxidation depending on their chemical properties and interactions with arsenic species. These findings advance understanding of the biooxidation process and offer guidance for optimizing performance in the treatment of refractory gold ores.