The Research On Treatment Of Copper,Zinc,Lead In Mined Soils By Bioleaching

The mining and metallurgical activities that were particularly intensive during the last century resulted in the generation of huge amounts of mine tailings, including acid-generating sulfidic tailings. Most of the tailings have been left without management. Their improper management in the past resulted in the mobilization of heavy metals to the surrounding environment , contributing to soil substrates contamination,soil texture destruction,short of nutrient,ecological landscape destruction,groundwater pollution,biological diversity decrease etc. The presence of toxic heavy metals in contaminated soils caused lots of serious environmental problems. In order to resolve the above problems,it is important to develop a suitable and economical technology in heavy metal contaminated soils.In this work I want to carry out bioleaching expentment of heavy metal(Pb-Zn-Cu)contaminated soils from Shuikoushan tailing dam by indigenous sulfur-oxidizing bacteria. Bioleaching can reduce the concentration of heavy metals in soil and change the chemical form of heavy metals.This proved that it was feasible to remove heavy metals from contaminated soils with the use of the bioleaching remediation method.The bioleaching process may be defined as the solubilization of metals from solid substrates either directly by the metabolism of leaching bacteria or indirectly by the products of metabolism. Bioleaching processes are based on the ability of microorganisms to transform solid compounds and result in soluble and extractable elements which can be recovered. Metal solubilization from solid wastes or other solids is achieved through a variety of acidophilic and chemoautolithotrophic bacteria such as Acidithiobacillus thiooxidans and Acidithiobacillus ferrooxidans. The bacterium Acidithiobacillus thiooxidans (AT. Thiooxidans) is active at low pH and can endure harsh conditions that exist in concentrated solutions of heavy metals. It can catalyze the oxidation of elemental sulfur or reduced sulfur compounds to sulfuric acid and obtain energy from the oxidation of elemental sulfur or reduced sulfur compound , and cause bioacidification and solubilization of heavy metals.The oxidation and acid producing activity of sulfur-oxidizing bacteria are the primary mode of solubilization of heavy metals in the bioleaching process. The solubilization mechanism of bioleaching by AT thiooxidans can be direct and indierct. In the direct mechanism,metal sulfides can be oxidized into sulfates by these acidophilic bacteria. In the indirect mechanism,H2SO4 generated from the oxidation of elemental sulfur or reduced sulfur compounds by AT. thiooxidans.Because of the advantages of low cost and environment friendliness and better efficiency,bioleaching technology has been a great success for the mining industry. The use of microorganism or its biomass for the recovery of metals from waste as well as the employment of plants for landfill applications have achieved growing attention. In recent years,bioleaching has proved to be a possible way to remove heavy metals from metal contaminated materials such as anaerobically digested sewage sludge,contaminated river sediment,spent nickel–cadmium batteries, tannery sludge,and incinerator fly ash.The effectiveness of bioleaching is highly dependent on the physical,chemical and biological factors,which included nature of contaminated material,solids concentration,temperature,oxygen,pH,oxidation-reduction potential (ORP), composition of the medium,bacterial strain and cell concentration. A complete understanding of the factors that affect the bioleaching process was very important to optimize the bioleaching process. The results of solids concentration experiment indicated that solid concentration 1% was found to be best to bacterial activity and metal solubilization of the five solids concentration tested (1%,2%,5%,8% and 10%) under the chosen experimental conditions. Under the chosen experimental conditions.Except for Pb,solubilization efficiencies of total extractable Zn and Cu were very high(>96%) at this solid concentration,while that of Pb was 43.52%.The results of substrate concentration experiment indicated that substrate concentration 2% was found to be best to bacterial activity and metal solubilization of the five substrate concentration tested(0.5%, 1%, 2%,3% and 5%) under the chosen experimental conditions. Acidic initial pH of the system did not have any specific influence on bioleaching of copper,lead and zinc. These results suggest that initial acidification of the bioleaching system is not a prerequisite for growth and acid production for At. thiooxidans. From the foregoing, it can be concluded that the initial pH(acidic)of the medium does not have any beneficial effect on the biological solubilization of heavy metals from soil. In addition to the total metal content,the distribution of metals among various fractions in the soil is a useful measure as partitioning information determines the behavior of the metal in the environment and it allows an investigation of which metals,bound to which fractions,are solubilized during bioleaching. So it was important to describe the partitioning of heavy metals into different fractions before and after their biological removal from soil. Four binding fraction(sF1:exchangeable, F2: Reducible fraction or fraction associated with Fe and Mn oxides,F3: oxidizable- fraction associated with sulfides and organic matter,F4:Residual fraction)of heavy metals (Cu,Zn and Pb) were analyzed using the Four-step procedure recommended by Community Bureau of Reference(BCR). Metals in exchangeable fractions are considered to be more mobile,dangerous and bioavailable. The residual metals are considered to be more stable and nonbioavailable than metals in exchangeable fractions. After the bioleaching process,metals remaining in the mine tailings were mainly found in the stable fractions.In our experiments,the removal of Cu,Zn and Pb from aqueous solution was investigated in a batch system by sorption on the dead cells of Bacillus licheniformis The results showed that the dead cells of Bacillus licheniformis were an efficient adsorbent of Cu,Zn and Pb in dilute solutions.