Study On Microbiology Of Leaching Valuable Metals From Waste Printed Circuit Boards By Acidophil Bacteria

Bioleaching is a newly developed technique in recent decades. It was believed to be oneof the most promising technologies with low cost, high efficiency and environmentallyfriendly. Bioleaching has been applied successfully and commercially in biohydrometallurgyfor extracting metals from low-grade ores for many years. It provides a new way for therecovery of waste printed circuit boards (PCBs). In the preceding work of our project group,microbial recovery technology of valuable metals from PCBs was studied. On that basis,some problems about microbiology in this bioleaching system were studied in this study.Firstly, MCAB was enriched from nature environment and was used to bioleach metals frommetal concentrates of PCBs, and then a series of experiments were conducted to determine theinfluence of non-metallic on the bacterial growth and activity and the bioleaching efficiencyof heavy metals. Then, bacterial community shifts during the bioleaching process had beenevaluated by PCR-DGGE. Finally, experiments were designed to isolate pure culture strainsfrom MCAB. The main conclusions are as follows.(1) Culturing and bioleaching experiments were conducted to investigate inhibitoryeffect of non-metallic component of waste PCBs. The results showed that the more thenon-metallic powder, the more obvious the inhibitory action for bacterial growth, the lowerthe average Fe2+oxidation efficiency of MCAB. The bioleaching experiments were conductedunder the condition of constant mass of metal concentrates with varied non-metallic powder.The results showed that the non-metallic component also slowed down the bioleachingprocess and resulted in longer time to reach maximum copper leaching efficiency. Therefore,non-metallic component significantly inhibited bacterial development and activity andbioleaching process.(2) Denaturing gradient gel electrophoresis (DGGE) analysis of polymerase chainreaction (PCR)-amplified16Sr RNA genes was used to evaluate the changes of the microbialcommunity structure during the bioleaching process. The results showed that the sequences ofthe seven bands (Z01~Z07) selected from the DGGE gel had over99%sequence similaritywith Acidithiobacillus ferrooxidans. It meant that the seven strains were all clustered toAcidithiobacillus ferrooxidans genus. The relative abundance of Z01to Z07in the four samples did not changed significantly. Meanwhile, Z03showed the highest relativeabundance during the whole bioleaching process, which was72.70%,82.90%,79.00%and85.80%, respectively. Therefore, the microbial community structure changed a little bit duringthe bioleaching process, which was always consisted of Acidithiobacillus ferrooxidans strainsand was dominated by strain Z03.(3) An acidophilic bacterium named as Z1was isolated from MCAB. It was identified asAcidithiobacillus ferrooxidans genus by morphology, physiology and near full-length16SrRNA gene analysis.The optimum growth pH value and temperature, and the logarithmicphase of strain Z1was2.25and30℃, and from18h to30h, respectively. The averageferrous oxidation rate of the Z1reached0.2307g/(L·h). After preculture24h, Z1couldbioleach92.57%copper from metal concentrates in78h under the optimized conditions ofinitial pH2.25,9g/L initial Fe2+,10%inoculation quantity,12g/L metal concentrates,60~80mesh particle size. Meanwhile, it could bioleach85.24%aluminum and95.18%zinc in183h. Therefore, the isolated strain Z1could be used as a potential bacterium to bioleach metalsfrom waste printed circuit boards.