Isolation Of Dominant Bacteria And Their Characteristics In The Wastewater Treatment Including Heavy Metal Cd²⁺

Cadmium pollution mainly comes from paint, electroplating, and other industrial production process. It can not only cause water pollution, but also threaten human health by enrichment in the body through food chain. Traditional treatment methods such as chemical precipitation, ion exchange, adsorbtion can remove heavy metal efficiently, but they have defects like that they cost much, shall producing secondary pollution and can’t handle wastewater with low concentration heavy metal. Biological methods have advantages as follows, cheap, adaptable, recycle, so they have a broad application prospect in wastewater treatment including heavy metal. Moreover, the investigation focus on the isolation and culture of dominant bacteria for effectively adsorbing of heavy metals. Therefore,8 strains of dominant bacteria with efficiently removing Cd2+ were screened from soil polluted by the heavy metal in this study.2 strains of bacterium with highest cadmium removal efficiency were identified. Their optimum growth conditions and some impacts such as dosage, pH, temperature and reaction time were investigated. Under the optimal conditions, the removal rate of Cd2+ was assessed in the wastewater treatment including Cu2+、Mn2+、Pb2+ coexisted with Cd2+ respectively. Also the removal rate of the living bacteria and dead bacteria cadmium was compared. Then, the removal mechanism of cadmium by dominant bacteria was preliminarily discussed by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analysis. The main conclusions are summarized as follows:(1) Eight strains of bacteria with cadmium endurance were screened from soil contaminated by heavy metal and 2 strains with highest cadmium removal effect were identified through physio-biochemical characteristics and phylogenetical analysis of 16S rDNA sequence. It showed that strain Ⅰ and strain Ⅷ belonged to Cupriavidus and Enterobacter respectively. Both were gram negative bacteria.(2) The optimum growth conditons of strain I were pH 6～8, load nutrient solution 100mL/150mL and temperature 37℃. While the optimum growth conditions of strain Ⅷ were pH 6.5-8, load nutrient solution 80mL/150mL and temperature 37℃. During wastewater treatment including cadmium, the optimum operation conditions of 2 strains were pH 6.5, temperature 37℃ and reaction time 120 min. Under the optimal conditions, Cd2+ removal rate of strain Ⅰ and strain Ⅷ were 60.02% and 62.14% respectively.(3) In the wastewater treatment including Cu2+/Cd2+ by strain I, the Cd2+ and Cu2+removal rate was 50.99%～64.96% and about 98% with influent Cd2+ and Cu2+ concentration of 0.8mg/L and 0.5-7.5mg/L respectively. Compared with only including Cd2+ wastewater treatment, the removal rate of Cd2+ has little effect. In the wastewater treatment including Mn2+/Cd2+ or Pb2+/Cd2+, the Cd2+ removal rate was below 30% and 13% with influent Cd2+/Mn2+ or Cd2+/Pb2+ concentration of 0.8mg/L/2-15mg/L or 0.8mg/L/0.5-9mg/L respectively. It shows that Mn2+ and Pb2+ obviously inhibit the removal rate of Cd2+.(4) The extracellular part of living Strain I and VIII play a major role during wastewater treatment including Cd2+. Its occupied percent of the extracellular, intracellular and membrane part on cellular level of strain I was 75.9%,22.09% and 2.01% respectively, while strain Ⅷ`s was 73.77%,24.37% and 1.87% respectively. Based on Fourier transform infrared spectroscopy analysis, living Strain I bacteria mainly depended on P-OH, R-OH, COOH secondary amide groups and so on to remove cadmium. According to the XPS and reference standard database analysis, Cd in intracellular part existed mainly as Cd(OH)2.(5) The Cd2+ removal rate of dead strain I and VIII was 87.17% and 100% with influent Cd2+ concentration of 0.6～0.75 mg/L in wastewater respectively. Their removal rates were better than that of living bacteria. The reason was presumed that living bacteria could secrete extracellular polymers to remove part of Cd2+ and resist cadmium entry into the cells. However, most of dead bacteria did’t have this kind of resistance mechanism and the cadmium may enter into the gap structure of cell.