| Bioleaching is the extraction of metals into soluble forms from solid materials through the use of living organisms and their metabolites. As an effective, economic and environmentally friendly technology, it is also used to recover metals from waste printed circuit boards(PCBs) in recent years. The studies of bioleaching metals from waste PCBs mainly focus on optimization of leaching process conditions, reducing leaching time and improving leaching efficiency. While the study of bioleaching mechanism is not enough, especially the role and mechanism of extracellular polymeric substances(EPS) during the bioleaching process. The present study is based on our preceding work about bioleaching metal from metal concentrates of waste PCBs. The aim of is to study the role and mechanism of EPS in the bioleaching process. At first, the role of EPS, which was as the contact substances between Acidithiobacillus ferrooxidans and metal concentrates of waste PCBs, was verified. Then the synthetic EPS was utilized to examine the effect in bioleaching process of waste PCBs. Finally, different extraction methods were used to extract EPS and explore the role of EPS in bioleaching metal concentrates of waste PCBs. Main results were as follows:(1) These experiments based on pure acid, different initial concentrations of ferrous ions and dialysis bag leaching were conducted to study the role of Acidithiobacillus ferrooxidans in the bioleaching process. The results indicated that copper leaching efficiency of pure acid leaching was 2.3% while the bioleaching was 83.8%. The copper leaching efficiency was more than 95% under high concentration of ferrous ions, which served as the energy source for bacteria. The copper leaching efficiency of bioleaching with dialysis bag that prevented the contact between bacteria and metal powers was 47.9%. It indicated that both non-contact mechanism and contact mechanism contributed to the bioleaching process. EPS play an important role in the bioleaching process.(2) The experiments were conducted to study the effect and role of EPS during the bioleaching process by adding synthetic EPS. The results indicated that copper leaching efficiency was 11.7% higher than the bioleaching control, 41.4% higher than the abiotic leaching with synthetic EPS. It implied that synthetic EPS could enhance the bioleaching process. The bioleaching with protein could achieved 95.2% leaching efficiency of copper after 7 days leaching. In addition, the highest leaching efficiency were obtained under the concentration of 0.5 g/L synthetic EPS and the adding time at the early stage of lag phase.(3) Five extraction methods, including centrifugation, heating, ultrasonication, EDTA and Na OH, were utilized to extract EPS during bioleaching metal concentrates of waste PCBs by Acidithiobacillus ferrooxidans. The results indicated the organic fraction of EPS as the order above was 31.8 mg/g, 37.2 mg/g, 39.6 mg/g, 36.4 mg/g and 37.6 mg/g, respectively. Ultrasonication obtained the largest content among five extraction methods. Polysaccharide was the major constituent of EPS and it accounted for over 62.9% of total EPS. Metal fraction was also detected in the extracted EPS that the content was in the range of 3.6% ~ 23.4%.(4) The bioleaching experiments with adding extracted EPS and EPS-deficient bacteria were conducted to study the role of EPS during bioleaching metal concentrates of waste PCBs. The results showed that the copper leaching efficiency of bioleaching with EPS was 97.1% while the bioleaching control and the EPS-deficient bioleaching were 87.2% and 88.9%,.respectively. EPS could meditate the contact between the bacteria and metal powder and enhance the bioleaching metal concentrates of waste PCBs during the bioleaching process. |
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