A brown-rot fungus, an unknown Polyporales sp. KUC8959, has recently been identified and proven as a prominent fungal species for bioremediation of CCA-treated wood wastes in our Lab. The fungus produced a larger amount of oxalic acid than other fungi tested, and removed 90 % or more of chromium, copper and arsenic from CCA-treated wood sawdust through bioleaching process. The bioleaching process was consisted of two steps comprising fermentation of the fungus for oxalic acid production and then extraction of metals by oxalic acid produced by the fungus. In order to maximize bioleaching efficiency, optimal fermentation conditions for oxalic acid production by the fungus is required. The objective of this study was to optimize oxalic acid production by unknown Polyporales sp. KUC8959. We used a 20-run central composite design (CCD) using response surface methodology (RSM) with three variables, that is, nutrient concentration, fungal biomass, and fermentation period. From the pre-cultured liquid fermentation broth, fungal hyphae were removed by filtering and then washed thoroughly with sterile deionized water. In accordance with the experimental design, fungal biomass (6.99, 18.32, 34.93, 51.55, or 62.88 mg) was inoculated in a flask containing 100 mL of malt extract solution (0.5, 1.11, 2, 2.89, or 3.5 %). Thereafter, the flask was agitated at 150 rpm on a rotary shaker at 27 °C for 48, 96.66, 168, 239.34, or 288 hours. From the fermentation broth, the amount of oxalic acid produced was determined by high performance liquid chromatography. The fitted RSM model investigated showed high regression coefficient between the variables and the response (R2=0.960) indicating that the model can be highly accurate in predicting the oxalic acid production at various conditions. The model suggested that optimum nutrient concentration, fungal biomass, and fermentation period were 2.40 %, 47.84 mg and 228.96 hours, respectively, to produce 4.11 g/L of oxalic acid. This fitted model will further be used for bioleaching of metals from CCA-treated wood.

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