Screening Of Hydrocyanic Acid Producing Pseudomonas Electronic Waste Precious Metal Dissolution Capability Studies

With the rapid development of electronic industry, electronic wastes are rapidly increased. As we all know, electronic waste contains a lot of teratogenic, mutagenic and carcinogenic substances which has been a potential hazard to human health. On the other hand, the purity of precious metals in electronic waste is more than10times higher than that of rich-content minerals. Therefore, recycling of metals from electronic wastes not only effectively minimizes the environmental pollutions but also benefits the sustainable use of resources. The existing processes for recovery of metals from electronic waste are physical method, pyrometallurgical processing, hydrometallurgical processing and biometallurgical processing. With the advantage of low cost and no secondary pollution, it is believed that biotechnology has been one of the most promising technologies in metallurgical processing. Previous studies have shown that cyanide generating bacterium has been used to leach out metals from ore and electronic wastes. Almost every report about bioleaching of metals focused on using chromobacterium violaceum. But some information reported that chromobacterium violaceum is a disease-causing microbe. It is not suitable to be employed in industrial production. With the advantage of wide distribution, high ability to adapt and non-pathogenic, pseudomonas is the fit microbe. So, the work of cyanogenic bacterium screening and the research for metals dissolution capability are very meaningful. The results of this paper are as follows:(1) We screened108strains from plant rhizosphere, among which11strains are cyanogenic.11strains were identified by characteristics of morphological and molecular biology methods. The results showed that physiological and biochemical characteristics of11strains correspond to pseudomonas’s characteristics. Strain H-4was most similarly Pseudomonas aeruginosa. Strain H-6was most similarly Pseudomonas chlororaphis. Strain H-3, H-5, H-7and H-10were most similarly Pseudomonas putida. Strain H-1was most similarly Pseudomonas mosselii. Strain H-2was most similarly Pseudomonas fluorescens. Strain H-8, H-9and H-11were most similarly Pseudomonas sp.(2) Relationship between the growth dynamic and hydrocyanic acid production performance was studied. It showed that strains produce cyanide during mid-to-late logarithmic and in early stationary phase. Prussic acid production by11strains reached1.20mg/L～7.11mg/L after24h. Different influence over cyanide production was also discussed in this paper. It showed that cyanide production by strain H-1increased7.11mg/L to17mg/L under the condition where pH was in7, temperature was at25degree, the shaking speed was at60r/min and the substrate was glycine with methionine.(3) Experiments on bioleaching of strain H-1for dissolving Au, Ag, Cu and their mixed metal powder were performed. The leached liquor of the metals was analyzed by the method of ICP-OES. The result showed that the concentration of gold、silver and copper in solution reached the highest level of0.54mg/L,2.50mg/L and2.91g/L in the bioleaching experiment of pure particles of metals. In the bioleaching experiment of mixed metals, the concentration of gold, silver and copper in solution reached the highest level of0.22mg/L,0.39mg/L and2.52g/L. It can be seen that copper deteriorated the dissolving process of gold in the bioleaching of strain H-1. In addition, microbial adsorption was another problem since bioleaching rate decreased in microbial growth decline phase.(4) Experiments on bioleaching of strain H-6for leaching out Au, Ag and Cu from W-PCB were investigated. The composition of gold and silver was so low that we could not investigate the maximum number of dissolution of gold and silver by strain H-6. Bioleaching was performed in bottles where pure particles of gold and silver were added to the crashed W-PCBs. The result showed that the concentration of gold、silver and copper in solution reached the highest level of0.52mg/L,0.15mg/L and253.2mg/L when pH was7; when pH was9, the concentration of gold、silver and copper in solution reached the highest level of1.8mg/L,0.8mg/L and192.6mg/L respectively. Therefore, different cyanide strains had different capacity of dissolving metals. It seemed that high pH was conducive to formation of gold and silver cyanide complexes. So, the appropriate control of pH was key to solve the problem of copper deteriorated the dissolving process of gold.