Removal Of Cr From Tannery Sludge By Bioleaching Technique

Land application represents the most economical way for final disposal of tannery sludge as it combines the recycling of plant nutrients and sludge disposal at the same time. Unfortunately, the presence of high levels of poisonous metal Cr in tannery sludge often limits its use as a fertilizer. The Cr content in tannery sludge is about 1-4% on a dry weight basis, and in some cases, extremely high concentrations of up to 13% of Cr have been reported. A technologically feasible process called bioleaching was used to remove Cr from tannery sludge with Acidithiobacilli species in this study. The objective of the present study is (1) to optimize the key parameters; (2) to determine the possible inhibitory substance(s) present in tannery sludge, and further to evaluate their impacts on iron and sulfur oxidization by Acidithiobacillus ferrooxidans LX5 and Acidithiobacillus thiooxidans TS6, respectively; (3) to exploit the indigenous heterotrophic microorganisms present in tannery sludge to eliminate side effect of inhibitory substance and improve the heavy metal bioleaching efficiency; (4) to investigate a pilot-scale study for bioleaching of Cr from tannery sludge. Ultimately, the expected outcome from this work will contribute to the understanding of bioleaching strategies with an ultimate purpose of developing an appropriate method to facilitate optimization of bioleaching strategies.Batch experiments showed that almost 100% of Cr removal efficiency could be obtained after 8-10 days of bioleaching when 4 g l^-1 of elemental sulfur was added, in the presence of A. ferrooxidans LX5 and A thiooxidans TS6. 4g l^-1 of elemental sulfur; 4%-6% of solid content; 28℃; 0.185 ml sulfuric acid of the initial of addition to per dry sludge and 1/2 2/3 of recycled ratio of bioleached sludge were reported to be the most favorable for Cr bioleaching from tannery sludge. Although the bioleaching of Cr from tannery sludge was strongly influenced by various parameters such as sulfur concentration; temperature; solid content; the initial sulfuric acid addition; the recycled ratio of bioleached sludge etc, it was found that chromium required a sludge pH of 2-3 to initiate its solubilization and further at constant pH, metal solubiliztion was the same regardless of the above parameters. When sludge pH decreased 1.5, almost 100% of Cr could be removed from tannery sludge. Additionnally, the settling capability of decontaminated sludge was improved tremendously, and the nitrogen, phosphors and organic matter values of sludge samples were only marginally affected.Results obtained in batch cultures in flasks using sludges from three different wasterwater plants (tannery sludge from Haining Kasen Co. Ltd (Sludge H), domestic sludge from Nanjing (Sludge N), and municipal sludge from Wuxi (Sludge W), demonstrated that sludge DOM of three origins significantly inhibited ferrous iron and sulfur oxidation by A. ferrooxidans LX5 and A. thiooxidans TS6, respectively. The toxicity of sludge DOM appeared when the concentration was higher than 150 mg DOC L^-1. Among the organic compounds tested, the monocarboxylic organic acids including formic acid, acetic acid, propionic acid, and butyric acid exhibited a marked toxicity to Acidithiobacillus species. Of these organic acids, formic acid was the most toxic one as indicating that iron and sulfur oxidation almost were entirely inhibited at a concentration of 1.67 mM. Moreover, it was found that A. ferrooxidans LX5 was more sensitive to glucose, starch, and citric acid than A. thiooxidans TS6, while the former seemed to be more acetic, propionic, and butyric acid resistant than the latter. In the selected 150 mg DOC L^-1 of DOM derived from Sludge-H, the concentrations of formic acid and acetic acid were 8.94 mM and 2.09 mM, respectively, being a contributing factor causing 95% inhibition of iron oxidation and 70% inhibition of sulfur oxidation.An acidophilic sludge DOM-degrading yeast TS7 was successfully isolated from tannery sludge and it could achieve optimum growth in potato dextrose agar (PDA) liquid media of pH 2-7. When yeast TS7 was inoculated in the medium containing 2007 mg DOC L-1 sludge DOM derived from tannery sludge, about 83% of sludge DOM was assimilated or decomposed within 72 hours. As a result, Cr bioleaching efficiency was enhanced when yeast TS7 was inoculated simultaneously with A. ferrooxidans LX5 and A. thiooxidans TS6 into tannery sludge. Compared with the 10 days required for maximum solubilizaiton of Cr for the control sludge, the bioleaching period was significantly shortened to 6 days for sludge receiving co-inoculation.A pilot-scale study for bioleaching of Cr from tannery sludge in 280L air lift reactor confirmed that the bioleaching process is a feasible and promising technology for removing heavy metals from tannery sludge.