Transformation of the carbonaceous matter in double refractory gold ore by crude lignin peroxidase released from the white-rot fungus

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Highlights

  • Carbonaceous matter was associated with illite in the as-received sample.

  • Crude enzymes converted the carbonaceous matter into humic-like substances.

  • Humic-like substances aided in the formation of large aluminosilicate aggregates.

Abstract

Sulfides and carbonaceous matter in double refractory gold ore (DRGO) were bio-treated sequentially using an iron-oxidizing archaeon Acidianus brierleyi followed by lignin peroxidase-dominating crude enzymes released from the white-rot fungus Phanerochaete chrysosporium to significantly improve gold recovery from 24% to 92%. Transformation of the carbonaceous matter in the sequential bio-treatment was interpreted with Quantitative Evaluation of Materials by Scanning Electron Microscopy (QEMSCAN), Raman spectroscopy and three-dimensional fluorescence spectrometry. Firstly, microbiological sulfide oxidation did not affect carbonaceous matter but decreased the arsenic content in the solid residue, facilitating the following enzymatic reaction. Next, the crude enzymes predominantly decomposed the defect-bearing graphitic carbon into humic-like substances. The humic-like substances were not completely soluble under pH 4 but were instead retained in the solid residue as a part of a newly formed carbonaceous aluminosilicate (C–Si–Al) phase. Due to a wide pKa range of humic-like substances, it is proposed that at pH 4, electrostatic interaction between humic substances and illite, with and without heavy metals, might have enabled the agglomeration of fine aluminosilicate particles. Some gold grains trapped in C–Si–Al agglomerates were released by the dissolution of humic-like substances in 1 M NaOH, resulting in a further increase in gold recovery of approximately 15%.

Introduction

Carbonaceous matter in gold ores can competitively adsorb Au(CN)2- complex ions during extraction, resulting in decreased gold recovery (Jha, 1987; Haque, 1987). As such, it is necessary to diminish the effect of the carbonaceous matter in the ore before cyanidation. Carbonaceous matter can be decomposed either chemically by high-temperature roasting or biologically using hydrolytic and lignin-degrading microbes (Jha, 1987; Amakwah et al., 2005; Amankwah and Pickles, 2009; Ofori-Sarpong et al., 2013). From an environmental standpoint, the use of these microbes, while not as efficient as the roasting, can significantly improve gold yields (Ofori-Sarpong et al., 2013). Previous studies have used the white-rot fungus Phanerochaete chrysosporium for in vivo or in vitro treatments of several substrates because it can produce extracellular lignin-degrading enzymes like manganese peroxidase and lignin peroxidase (Tien and Kirk, 1988; Wariishi et al., 1991). These enzymes utilize hydrogen peroxide as an activator for a two-step electron transfer oxidation reaction. After activation, the enzyme Compound I and Compound II interact with the carbonaceous matter and decompose it into smaller compounds, that may have less gold adsorption ability (Tien and Kirk, 1988; Wariishi et al., 1989; Wariishi and Gold, 1990; Ofori-Sarpong et al., 2013). Also, the fungus produces carbohydrates and organic acids (Moreira et al., 2003; Flemming and Wingender, 2010), which might cover the surface of the carbonaceous matter, reducing gold recovery. Therefore, the solid residues of fungal treatment are complex products requiring extensive analysis to understand.

Carbonaceous matter often makes up less than 7% of most refractory gold ores, and has chemically and physically diverse structures depending on molecular sizes, aromaticity, functional groups, so that characterization of the original substance and products of its bio-treatments is often difficult to accomplish (Zumberge et al., 1978; Yang et al., 2013 ). Some previous studies have tried to simplify the analysis by extracting the graphitic carbon using strong acids to decompose all the other components in the ore, but this process would irrevocably change the chemical properties of the carbonaceous matter from its original state (Abotsi, and Osseo-Asare, 1986; Afenya et al., 1991). Spectroscopic analytical methods like Raman and FTIR spectroscopy have been applied instead of digestion to study the preg-robbing property of some carbonaceous gold ores (Helm et al., 2009; Dimov and Hart, 2017). It was proposed that there might be a linear correlation between adsorbed amounts of Au(CN)2- and the magnitude of chemical or physical disorder in graphitic carbon. However, without an adequate understanding of the minerals associated with the carbonaceous matter; it is often difficult to determine if the sp2 hybridized D-band is due to the chemical and physical environment around the carbonaceous matter in the sample (Ferrari, 2007; Pimenta, 2007).

Another analytical method, that has become invaluable for characterizing the mineralogy of several metal ores including gold and coal samples since the 1980s’ (Butcher et al., 2000), is the Quantitative Evaluation of Minerals by Scanning Electron Microscopy (QEMSCAN) (Goodall et al., 2005; Liu et al., 2005; Pirrie et al., 2004). This technique can provide detailed information of not only the quantitative and qualitative mineralogical determination but also mineral liberation and associations. To the best of our knowledge, it has not been used for the analysis of carbonaceous matters in refractory gold ores and/or its bio-treated residues because of factors like smaller mass abundance of carbon in the ore sample and possible interference from the chemical resin used to make the pellet for analysis of powdery samples (Liu et al., 2005). This limitation of QEMSCAN can be reduced by replacing the chemical resin with carnauba wax to create a sufficient contrast between the carbonaceous matter and the wax background so that analysis of the refractory gold ore can proceed (Pirrie et al., 2004).

Therefore, the present work seeks to provide knowledge about the carbonaceous matter which is in situ in the double refractory gold ore (DRGO) and the bio-product formed after treatment using QEMSCAN. This will encompass; (1) analyzing the carbonaceous matter in its original state in the gold ore sample to understand its association with the other minerals, (2) determining the changes that occur during its decomposition by lignin peroxidase and the related enzymes, which are released from white-rot fungus, in the cell-free spent medium (CFSM) and (3) using this information to aid in the understanding of results collected from Raman spectroscopy and three-dimensional fluorescence spectrometry. This knowledge will improve the understanding of the beneficiation of carbonaceous matter in refractory gold ores.

Section snippets

Sample preparation and characterization

The concentrate after sulfide flotation of the as-received sample (DRGO) and the product after decomposition of sulfides in the concentrate by a thermophilic, iron-oxidizing archaeon Acidianus brierleyi (DA) were subjected to the CFSM treatment in this study (Fig. 1). The flotation concentrate was supplied by a mine in the Prestea Bogoso gold mining region in Ghana. Before using for bio-treatment, the above sample was washed with 70% ethanol to remove surfactants used for sulfide flotation. The

Carbonaceous matter identification in the flotation concentrate

The carbon content in the as-received sample was determined by CHN to be 5.86% (Table 1), which may include both organic and inorganic forms, due to the existence of small amounts of carbonates in the mineralogical analysis (Table 2). Comparatively, the QEMSCAN analysis was only able to detect about a 10% of the carbon in the size fraction of −1000 μm/10 μm, which corresponds to approximately 48.7% of the total mass of the flotation concentrate (Table 1). The difference between the two results

Conclusions

This study applied QEMSCAN analysis to identify the carbonaceous matter in a DRGO sample in its initial state and also characterize the product after enzymatic treatment. It was discovered that the carbonaceous matter was associated with illite and the porosity of the ensuring carbonaceous illite mineral was dependent on the amount of carbon in the structure. It was concluded that the CFSM treatment was able to decompose the carbonaceous matter into humic-like substances by attacking the

Acknowledgement

The authors are very grateful to Dr. Megan Becker and Ms. Gaynor Yorath at Department of Chemical Engineering, University of Cape Town for their help with the QEMSCAN analysis. This work was financially supported to KS by Japan Society for the Promotion of Science JSPS KAKENHI Grant numbers 18K19045 and 18J10835, and Arai Science and Technology Foundation. KTK appreciates the Advanced Graduate Program in Global Strategy for Green Asia in Kyushu University and JSPS DC2 18J10835 for the

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