Bacterial leaching of toxic metals from municipal sludge: influence of sludge characteristics

Abstract

zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA The presence of high concentrations of heavy metals in wastewater sludge is one of the most important factors that limit the application of sludge on agricultural land. In the last few years, the mi crobiological process of heavy metals removal from wastewater sludge has been studied in a batch mode. In the present study, the effects of metals, solids concentration, and initial pH on indigenous sulfuroxidizing thiobacilli growth and metal solubilization were examined. The results obtained indicate that metals removal is possible even in high sludge solids concentration (70 g/L). The rate of sulfuric acid production through the oxidation of added elemental sulfur is increased by the in creased sludge solids concentration. The initial pH of the sludge (7.0 3.0) does not affect the capacity of metal leaching. The high concentration of metallic ions (Cu 2+ and Zn 2+ ) adjusted in the sludge did not greatly affect the microbiological activity of metalleaching bacteria. The leaching tests conducted with metal sulfides as substrate (chalcocite, galena, pyrite, sphalerite) showed that metals solubilization from sulfides occurred by an indirect mechanism (production of acid followed by metal solubili zation due to the action of the acid produced). No evidence of direct oxidation of metal sulfides was observed.zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Water Environ. Res., 65, 196 (1993).

Removal of Cr from tannery sludge by indigenous sulfur-oxidizing bacteria

“…[8,10] When solely evaluated from the rate of sludge pH decreasing, chemical leaching treatment with sulfuric acid seemed to be superior to bioleaching method because it only took 8 hours for this process to reduce the pH from 7.8 to 1.96. However, approximately 100 mL of concentrated sulfuric acid was consumed during this sludge acidification process by chemical leaching.…”

“…[5−7] An alternative way to replace chemical methods in removing Cr is bioleaching with iron-and/or sulfur-oxidizing bacteria. [8,9] This biotechnological process is based on the natural ability of the acidophilic microbes to transform solid compounds to a soluble and extractable form. [7,10] It was reported that the adapted indigenous iron and sulfuroxidizing bacteria effectively reduced sludge pH to 2.0-3.0.…”

“…More than 80% of Cu, Zn, Ni, Cd and Mn could be removed from sewage sludge in 8-10 days. [8,10] The rate and efficiency of bioleaching varies with a number of factors including inoculum density, initial pH, temperature, total solids concentration and substrate source. [11,12] Bioleaching process has been shown to be 80% less expensive than the traditional chemical methods.…”

Effect of Sludge Dissolved Organic Matter on Oxidation of Ferrous Iron and Sulfur by Acidithiobacillus Ferrooxidans and Acidithiobacillus Thiooxidans

“…Therefore, it is of significance to investigate the ability of acidophilic bacteria to bioleach heavy metals in sewage sludge. To date, the effects of many operational parameters on bioleaching of heavy metals from sewage sludge by A. ferrooxidans or A. thiooxidans have been studied under different laboratory conditions: type of substrate (Tyagi et al, 1993); temperature (Tyagi et al, 1994); sulfur concentration (Sreekrishnan et al, 1996); solid content (Tyagi et al, 1997); and dissolved oxygen (Tyagi et al, 1998). These results have proved that the bioleaching process is a feasible and promising technology for removing heavy metals from sewage sludge.…”

“…In comparison with A. ferrooxidans, however, A. thiooxidans may be more suitable for sewage sludge bioleaching reaction than A. ferrooxidans because A. thiooxidans can actively oxidize sulfur at a wide range of pH from 1 to 9 (Suzuki et al, 1999) while sewage sludge pH value is often near to 7.0, and the bioleached sludge pH is generally required to ensure a pH lower than 2.0 for a sufficient dissolution of various metals from sewage sludge (Tyagi et al, 1993;Ryu et al, 2003;Zhou et al, 2005). In addition, it was found that ferric ions generated in the process of iron oxidation by A. ferrooxidans easily form jarosite precipitates, which reduce the concentration of ferric ions as an oxidant, resulting in lower bioleaching efficiency (Harahuc et al, 2000;Meruane and Vargas, 2003).…”