Bioprecipitation Of Heavy Metals From Acidic Wastewater By Sulfate-reducing Bacteria In A Bench-scale Continuous-flow Stirred Tank Reactor

Mining, smelting and processing, plating and waste containing heavy metal detoxification generate a large number of acidic heavy metals-laden wastewater which to some extent restricts the healthy development of industrial enterprises. The bioprecipitation process has been demonstrated to be an effective technology in removing heavy metals from acidic heavy metals-laden wastewater in recent years.The basic principle of bioprecipitation is as bellows:Sulfate-reducing bacteria (SRB) are able to use sulfate as an electron acceptor and to form hydrogen sulfide leading to an increase in pH of the water and the precipitateon of heavy metal sulfides.The method has advantages of thorough and extensive heavy metals removal, as well as simple process. But the novel technology has just started that the bioprecipitation applied in the treatment of acidic heavy metals-laden wastewater more study should be explored. Using the Hungate anaerobic technology, a functional strain of sulfate reducing bacteria named WF83 is isolated from river sediment in this paper. The present study designes one 0.67L continuous stirred tank reactor (CSTR) and a simulated acidic heavy metal-lader wastewater containing 60 mg Zn2+/L,40 mg Cu2+/L,10 mg Cr3+/L, and 4.0-6.2g SO42-/L is treated by SRB under different COD/SO42- and pH values in this reactor. Based on these results, the performance of the system is tested using the acid sedment leachate.The identifying of morphologyical,16Sr RNA show that the strain may belongs to Enterobacter sp. This bacteria is intersexual anaerobic. The strain belongs to mid-thermophilic and can lives in wide pH range (5.0-9.0). It is very acid-resistant relative to the sulfate-reducing bacterium have been reported which has advantages in wastewater treatment.The results show that the bioprecipitation treatment is effective at various COD/SO42- (3.0-0.78) and pH (4.3-2.6) values.The activeity of SRB increases the water pH to neutral and metal removal efficiency is as high as 99% for Zn,99% for Cu,94% for Cr and 70% for SO42-; The nature of effluent precipitation is analysed by SEM, EDS, and XRD methods. ZnS, Fe3S4, CuS2 and (Fe,Cu)S2 detected in the precipitation prove that the high efficiency of heavy metals removal is because of metal sulfides generated by bioprecipitation.This study still finds that the effluent pH and alkalinity showed a slight decreasing trend with a decrease of the influent pH from 4.3 to 2.6 but has no adverse influence on the final efficiencies of metals and sulfate removal; Changing the chemical oxygen demand (COD)/SO42- ratio from 3.0 to 0.78 decreased sharply the percentage of sulfate reduction from-100 to-20%. From a practical consideration, the recommended influent COD/SO42- and pH value used for the future larger-scale bioprecipitation process is 1.8 and 3.5. For the acid sediment leachate treatment, iit is demonstrated effective that the effluent pH is stable at around 6.8, and 60% of sulfate, and more than 97% of the initial concentrations of Zn, Cu, and Cr were removed, respectively.The extracellular polymeric substances (EPS) excreted by SRB may play a positive role in the removal of heavy metals during bioprecipitation processes. The content of EPS is about 26.27mg/gVSS among which polysaccharide accounts for 74.5% and protein 25.5% and the radio of polysaccharide/protein is 2.92.SEM-EDS and Ft-IR results show that the EPS of SRB is effective in sorbing Cu2+. The maximum sorption capacity is up to 2500mg (Cu2+)/g (EPS) when pH value of solution is 5.5.