Development of Microbial Mercury-Detoxifying Processes Using Mercury-Resistant Bacteria

• Main Authors: Law, Weng-Shin, 羅文鑫
• Other Authors: Chang Jo-Shu
• Format: Others
• Language: zh-TW
• Published: 1997
• Online Access: http://ndltd.ncl.edu.tw/handle/90832878067784625328

碩士 === 逢甲大學 === 化學工程研究所 === 85 === Mercury-resistant bacteria usually harbor mercury-resistant genes called mer operon, which is often incorporated into plasmids and transposons. The merB gene of mer operon encodes for organomercurial lyase, which degrades organomercurial compounds to form Hg2+. The merA product mercuric reductase catalyzes the reduction of Hg2+ to Hgo, which is very volatile and easily diffuses out of microbial cells. This research utilized the mechanism of enzymatic mercury reduction to develop batch, fed-batch, and continuous bioreactors able to detoxify soluble mercuric ions from wastewater. The mercury-resistant strain used in this study was Pseudomonas aeruginosa PU21, which harbors a mer-operon-containing plasmid Rip64. The strain can tolerate mercury concentrations up to 150 mg/L in LB broth, and up to 20 mg/L in Pseudomonas minimal medium (PMM). Mercuric ions readily interacted with components of LB to form complexes, which were found to reduce the availability of mercury to the microorganisms. As the mercury concentration was below 10 mg/L, the complex formation between PMM and mercuric ions was negligible. Therefore, this study used PMM as the microbial growth medium for the mercury-detoxification experiments. An increase in mercury concentration tended to extend the lag periods of the cell growth, while addition of amino acid complements not only enhanced specific growth rate and maximal cell concentration, but also shortened the lag phase of cell growth. In batch cultivation, the strain was able to rapidly detoxify the mercury content (2-10 mg/L) in the medium within less than 2 h. The optimal specific detoxification rate (RHg) was 1.1 x 10-6 *g Hg/cell/h, which was achieved with the initial mercury concentration of 8 mg/L. The RHg was significantly affected by the growth phase of the cells with the order of lag phase > exponential phase > stationary phase. In the fixed- interval fed-batch (FIFB) operations, mercury solutions containing 3 or 5 mg Hg/L were fed sequentially (every 1 or 1.5 h) into the cell culture. The results showed that the average mercury detoxification efficiency was 2.9 and 3.3 mg/h/L for the fixed-interval feeding of 3 and 5 mg Hg/L, respectively. In continuously fixed-rate fed-batch (CFRFB) operations, optimal feeding rate was found to be 140 ml/hr, which was utilized for the fed-batch operation with initial culture volumn of 400 and 1200 ml. The results showed that the average mercury detoxification efficiency was raised to 5.26 and 5.6 mg/h/L, respectively, in contrast to about 2.9-3.3 mg/L/h for FIFB operations. The CFRFB operation was repeatedly operated for 2-3 cycles (about 40hr), during which the mercury detoxification efficiency was able to maintain at the original level. In continuous chemostat operations, as the dilution rate (D) was 0.18 h-1, and the mercury concentration in the feeding stream ([Hg]f) was 1-6.15 mg/L, the cells appeared to efficiently detoxify mercury from the culture, and the resulting effluent mercury concentration ([Hg]e) was lower than 5 ppb (EPA*s requirement). The mercury detoxification efficiency obtained from the operation was about 0.64 mg/h/L. As D was raised to 0.325 h-1, the [Hg]e could satisfy EPA*s requirement only when [Hg]f was less than 2 mg/L. However, [Hg]e became 0.18 mg/L when [Hg]f was increased to 6.15 mg/L. This operation led to a mercury detoxification efficiency of 1.94 mg/h/L. The mercury recovery process connected to the gaseous effluent of the bioreactor completely remove vapor mercury contentsresulted from mercury detoxification processes. In the batch operation mode, the acid-digestion columns can collect 80.6%, 82.2 %, 80.0 %, and 88.3 % of total added mercury when initial mercury concentrations were 2, 5, 8 and 10 mg/L, respectively. However, the mercury concentration leaving down-stream activated-carbon trap was negligible.