**3. Utilization of lignin-degrading enzymes on DRGO**

## **3.1 Decomposition of sulfide minerals**

In DRGO, gold (Au(0)) is mainly confined in sulfide minerals, especially arsenopyrite (FeAsS) and pyrite (FeS2), which must be decomposed to liberate gold. Sulfide minerals in DRGO typically make up 10 ~ 20% after flotation [10–13]. Most of the other major mineral components in are quartz and clay minerals, which do not participate in the oxidative decomposition of sulfide minerals. The standard BIOX treatment of sulfides utilizes chemoautotrophic bacteria *Acidithiobacillus ferrooxidans*, *Acidithiobacillus thiooxidans*, *Leptospirillum ferrooxidans*, heterotrophic bacteria *Sulfolobus acidocaldarius*, *Sulfolobus* sp., *Sulfobacillus* sp., etc., to oxidize and dissolve, or decompose, sulfides by using Fe3+ ions as oxidizing agents by oxidizing Fe2+ ions in strongly acidic solutions to Fe3+ ions [11, 40].

Some white-rot fungi have been shown to improve sulfide oxidation. *P. chrysosporium* produced a mixture of enzymes like LiP and MnP, which needed to be activated by H2O2 [18]. Some studies have shown that the H2O2 produced by the fungi might be involved in a Fenton reaction with pyrite and arsenopyrite in the DRGO [34–36]. The oxidizing conditions lead to the generation of Fe3+ ions as an additional oxidizing agent. However, the competition for H2O2 between the lignin-degrading enzymes and the Fenton reaction leads to the inadequate oxidation of both the carbonaceous matter and sulfides [11]. Also, the pH 4–5 used in this treatment results in precipitation of the Fe and reduces the efficacy of Fe3+ as an oxidizing agent. Therefore, most studies that have achieved very high gold recoveries have used a sequential treatment.
