The reduction of zinc metal in the paint sludge, a hazardous waste, was investigated using Acidithiobacillus thiooxidans by a two-stage bioleaching process. This process was performed using the response surface methodology (RSM) method based on the central composite design (CCD). Four variables, a temperature range of 32-34.5-37 °C, rotation speed of shaker 120-150-180 rpm, pH of 4.2-3.2-2.2, and particle sizes of 1-2-3 mm, were used to optimize the experiments. The results showed that with a constant pulp density of 10 g/L at 32 °C, shaker speed of 120 rpm, a particle size of 1 mm and pH of 4.2, the highest removal predicted by the software (Design Expert version 11) was 22.89%. Repeating the experiments confirmed a decrease in zinc to the nearest predicted point. According to the ANOVA result, the rotation speed of the shaker has the greatest effect on the bioleaching process, followed by the two variables of the rotation speed of shaker and pH together affects. After the bioleaching process, energy dispersive X-ray (EDX) and mapping analysis showed quantitative changes in the chemical composition of the paint sludge, and morphological changes of texture were confirmed by scanning electron microscopy (SEM).
Keywords: Acidithiobacillus thiooxidans; Bioleaching; automotive industry; hazardous waste; paint sludge.