Using microbiological leaching method to remove heavy metals from sludge

Abstract

Microbial leaching is one of the most effective methods to remove heavy metals fromsludge. In the conducted researches, the sludge samples were processed withThiobacillus ferrooxidans and Thiobacillus thiooxidans obtained via cultivation,extraction and purification processes. Heavy metals such as Pb, Cd, Cu and Ni wereleached from sludge by Thiobacillus ferrooxidans and Thiobacillus thiooxidans withindifferent substrate concentration and pH value conditions. It is defined that from thepoint of view of economy and efficiency the optimal concentration of FeSO4.7H2O andsulfur for bio-leaching process was 0.2 g. The leaching rates of heavy metals such as Pb,Cd, Cu and Ni of the same concentration were 74.72%, 81.54%, 70.46% and 77.35%respectively. However, no significant differences depending on the pH value among theleaching rates were defined, even for the pH value of 1.5. Along with the removal ofheavy metals from sludge, the organic matter, N, P, K were also leached to some extent.The losing rate of phosphorus was the highest and reached 38.44%. However, thecontent of organic matter, N, P, K in the processed sludge were higher in comparisonwith level I of the National Soil Quality Standards of China. Ecological risk of heavymetals in sludge before and after leaching was assessed by Index of Geo-accumulation(Igeo) and comprehensive potential risk (RI). The results of research defined that thecontent of heavy metals in sludge meets the level of low ecological risk after leachingand their contents is lower in comparison with the National Agricultural SludgeStandard of China. Sludge leached by biological methods is possible to use fortreatment for increasing soil fertility.