Chromium remediated by biological treatment.
Although the process was designed mainly to convert the more toxic Cr(VI) to Cr(III), it has several major uses. It can be used to treat other heavy metals, such as cadmium, ferrous iron, lead, and zinc, and radionuclides, including uranium, plutonium, and americium. The process is, also useful in remediation of metal-contaminated sites, and treatment of metal-contaminated wastewater.
A field demonstration of the new process was done at Baltimore Harbor in Maryland. The chromium at the Baltimore site originated from tailings left from a chromium ore-roasting process. The concentration of Cr(VI) in the solid residue is typically 2,000 ppm.
The cost of the reagents used in the field demonstration ranged from $12 to $20/st soil or residue, which is competitive with other processes. Regardless of the price, chemical treatment methods are not without their problems. Treatment processes for Cr(VI) either require numerous pH adjustments or generate large amounts of sludge.
There are basically two types of chemical remediation processes used today. One method uses ferrous iron is added, and the other sodium bisulphate. The latter requires several pH changes -- from alkaline to acid and back to slightly alkaline -- to permit all the necessary chemical reactions to take place. (pH must be adjusted to 2 or 3 to convert Cr(VI) to Cr(III) and then readjusted to 8 to precipitate the chromium.) In using ferrous ion, large amounts of sludge are formed.
Allied Signal's bioremediation process for groundwater and soils contaminated with chromium and other heavy metals overcomes these shortcomings. According to Louis DeFilippi, research scientist at Allied, "The process uses the unique ability of sulphate-reducing bacteria (SRB) to reduce metals to their lower oxidation states."
Two major obstacles had to be overcome in developing the technology. Finding a bacterial system that would function in high ionic concentrations was the first. Furthermore, Cr(VI)-contaminated groundwater, industrial effluent, soils, and tailings are often extremely acidic. Since bacteria live only in limited pH ranges, it was necessary to adjust the pH before beginning any biological treatment.
The second problem was ensuring that the microbes would survive the toxic Cr(VI) environment while accomplishing the conversion. This was solved using SRB, which produce sharp [H.sub.2]S gradients. The bacterially-produced hydrogen sulphide reduces the toxic Cr(VI) before it can reach the microbial cells. In many cases, the reduced metal has lower solubility and toxicity and, as a result, precipitates a less toxic metal. Subsequent sequestering and immobilization within the soil matrix or in a bioreactor removes the metal from the groundwater or soil. The process uses proprietary bioaugmentation methods which promote the microbial activities of SRB. Advantages of the method are summarized in Table 1.
Allied's process can be applied in-situ, combined in-situ/ex-situ as shown in Figure 1, or as ex-situ.
The advantages were clear from the Baltimore field demonstration where the in-situ/ex-situ approach was used. According to DeFilippi, "The area where the test was done contained soil contaminated due to spillage of sodium chromate. The soil contained about 250 ppm of extractable Cr(VI), and the groundwater contained approximately 150 ppm. After treatment, the Cr(VI) concentration was reduced to under 0.1 ppm in the soil and under 0.0002 ppm in the groundwater."
In the field test, fermented molasses and sulphate were injected into a contaminated shallow aquifer to achieve remediation of the saturated zone. Almost complete remediation of the groundwater took place in an irregularly shaped zone about 6-ft dia around the injection site.
The molasses were added to provide an inexpensive and effective food source for the growth and metabolism of the SRB. The added sulphate enabled the SRB to produce [H.sub.2]S, which reduced the Cr (VI) ion to Cr(III).
Allied's new bioremediation process achieved similar results in treatability studies for a different site containing 1M st of residue from a chrome-roasting process. The residue contained up to 2,000 ppm of Cr(VI), which was reduced to 0.02 ppm. A full scale in-situ/ex-situ approach is currently under development at the site.
Allied's new technology is also being used to treat acid mine drainage. The drainage handles 100 gal/min of waste-water containing about 200 ppm of ferrous iron. In other laboratory tests, the process has shown that radionuclides can be immobilized by SRB and allied bacteria. Allied has recently developed another chemical treatment offering significant advantages for the remediation of radionuclides-contaminated sites.
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|Title Annotation:||Processing News|
|Publication:||E&MJ - Engineering & Mining Journal|
|Date:||Aug 1, 1994|
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