Period Covered by the Report: 7-1-99 to 6-30-00
Date of Report: 7-30-00
EPA Grant Number: R827015-01-0
Title: Microbial Treatment of Naturally Occurring Radioactive Material (NORM)
Investigators: Lee R. Krumholz
Institutions: University of Oklahoma
EPA Project Officer: Bala Krishnan
Project Period: July 1, 1999 to June 30, 2000
Project Amount: $9,977
Research Category: NORM remediation
Pipe scale is formed as Barite, pyrite and various carbonate minerals precipitate on the inside surface of structures associated with oil production. As these minerals are deposited, radium is co-precipitated. There is much cost associated with resultant radioactive material.
We are developing a biotechnology to dissolve scale material and release the radioactivity. The conversion of sulfate to sulfide, carried out by sulfate-reducing bacteria in conjunction with the oxidation of reduced carbon compounds can effectively draw radium and barium into solution.
We have carried out laboratory studies using microorganisms from anoxic landfill sediments. The action of these microorganisms on NORM containing pipe scale incubated in a mineral solution with ethanol as electron donor resulted in the production of sulfide and the release of Barium into solution. Under simple incubation conditions approximately 0.25 gm/L of sulfate was converted to sulfide and 33 g of barium per liter were released into solution. Under these same conditions, no methane was produced and electrons from ethanol appeared to be used to reduce sulfate exclusively. We have recently shown that the additions of more ethanol during the incubation as well as the addition of Ferrous hydroxide serves to dramatically increase the amount of Barium released into solution (42 mg/L) and most likely also increased sulfide production. However, much of the sulfide produced appears to have been incorporated into pyrite. The relatively small increase in dissolved barium relative to sulfide and pyrite present suggests that the majority of Barium has re-precipitated, perhaps as barium carbonate or barium phosphate. Studies using acid to dissolve the precipitated material at the end of the incubation show that a significant of additional barium is released into solution. This data supports the re-precipitation hypothesis. To determine whether radioactivity is also being released into solution, we measured gamma radiation associated with the dissolved phase. Radioactivity was released in proportion to the amount of dissolved barium assuming that NORM was uniformly distributed throughout the barite within the scale material.
Naturally occuring radioactive material (NORM) has been known to be associated with oil and gas deposits for many years. During oil producing operations, large quantities of scale are formed on surfaces of tubing, casings, separators, drilling equipment and water storage vessels, mainly as a result of barium sulfate precipitation. When scale forms, it also hosts the precipitation of Radium (naturally present in groundwater as a daughter of thorium and uranium) as well as other metals including iron and calcium. As there are no currently effective and economically feasible technologies for dissolving and concentrating the scale material, equipment contaminated with NORM containing scale is cleaned mechanically or stored with the intent of disposal.
It has been estimated that between 300,000 and 1,000,000 tons of radioactive scale are produced in the US each year. Virtually all of this scale exceeds the exemption limit of 30 pCi/gm and about 5% has more than 2000 pCi/gm activity. The presence of NORM at oil and gas producing facilities has recently increased in significance as federal and state regulatory agencies lay out more stringent guidelines for transport and disposal.
Although there are no natural conditions in which barium is highly soluble, there have been several previous studies describing the dissolution of barite (barium sulfate) by sulfate reducing bacteria ((Baldi et al., 1996; Bolze et al., 1974; Fedorak, 1986). In each of these studies, the sulfate reduction process converted sulfate associated with barite to sulfide resulting in the release of Ba2+ into solution. Barite was either prepared synthetically (Bolze et al., 1974) or obtained from Sludges ((Baldi et al., 1996; Fedorak, 1986) In each case, pure cultures released less than 36 M Ba2+ into solution from synthetic powdered Barite. Sewage sludge released only 9 M Ba2+, while sulfate reducers in Uranium mill waste sludges released Ba2+ as well as Ra2+ into solution.
In this study, we demonstrate the feasibility of using a sulfate reducing process for the removal of NORM containing pipe scale. Sulfate reducing bacteria associated with subsurface sediments catalyze the release of Ba2+ and radioactivity into solution and under the appropriate conditions could be applied in the field.
Geochemical analyses: Total reduced sulfur was determined as described by Ulrich et al. (Ulrich, 1997) and acid volatile sulfur was determined in a similar manner except that materials were extracted with anoxic 1 M HCl rather than the mix of Cr(II) and concentrated HCl.
Dissolved Barium was measured by Graphite furnace Atomic Absorbtion spectroscopy (name of instrument tom). Dissolved radioactivity was determined by counting for 100 min in a volume of 2 ml. Gamma spectra were obtained with a Germanium Lithium detector.
Scale dissolution experiments: All incubations were carried out in 120 or 160 ml serum bottles. Microbial inoculum was derived from subsurface sulfate reducing sediments collected at the Norman Landfill. 50 gm of sediments were suspended in 50 ml of mineral solution two times. The first wash was discarded and the second time, sediments were shaken with cells to dislodge them. After pouring the solution off the sediments, the second wash was used as innoculum (2 ml per bottle). Each bottle contained 2 gm of crushed pipe scale and 100 ml of mineral solution (Krumholz & Bryant, 1986).
Geochemical modeling was carried out based on ions present in mineral solutions. Models were generated using PhreeqC, a code generated by the USGS. Figure 1, below shows Ba2+ dissolution as a function of carbohydrates oxidized. In this case the model was generated in
Sediments and a mineral solution were incubated with Barite containing scale from an oilfield in Alberta. Additions were made including 20 mM FeCl2, 20 mM ethanol or a combination of the two. Over time, dissolved barium was monitored (Fig. 1). At the end of the experiment (150 days), solids were removed and suspended in 1 M HCl. Acid soluble barium was then measured to determine the barium component that had been dissolved but had reprecipitated as acid soluble material. As the majority of the scale is barite, which is not acid soluble, it makes sense that barium released due to treatment with acid was in the form of some other mineral.
Significant levels of pipe scale and associated NORM can be dissolved by the action of sediment dwelling sulfate reducing bacteria. A large fraction of the dissolved barium and radioactivity reprecipitates as acid soluble minerals. The presence of ferrous iron increases the level of dissolution as well as the addition on ethanol as electron donor for sulfate reduction.
Baldi, F., Pepi, M., Burrini, D., Kniewald, G., Scali, D. and Lanciotti, E. 1996. Dissolution of barium from barite in sewage sludges and cultures of Desulfovibrio desulfuricans. Appl. Environ. Microbiol. 62, 2398-2404.
Bolze, C. E., Malone, P. G. and Smith, M. J. 1974. Microbial mobilization of barite. Chem. Geol. 13, 141-3.
Fedorak, P. M. 1986. Microbial Release of 226Ra2+ from (Ba,Ra)SO4 Sludges from Uranium Mine Wastes. Applied and Environmental Microbiology, 262-268.
Krumholz, L. R. and Bryant, M. P. 1986. Eubacterium oxidoreducens sp. nov. requiring H2 or formate to degrade gallate, pyrogallol, phloroglucinol and quercetin. Arch. Microbiol. 144, 8-14.
Ulrich, G. A., Krumholz, L. R., Suflita, J. M. 1997. A rapid and simple method for estimating sulfate reduction activity and quantifying inorganic sulfides. Appl. Env. Microbiol. 63, 1627-1630.
Microbial Treatment of Naturally Occurring Radioactive Material (NORM). LEE R. KRUMHOLZ, and Chao-Hsiang Wang. The 7th International Petroleum Environmental Conference. Albuquerque, NM.
Microbial Treatment of Naturally Occurring Radioactive Material (NORM). LEE R. KRUMHOLZ, and MARK HASEGAWA. The 6th International Petroleum Environmental Conference. Houston, TX.
Barite, sulfate-reduction, NORM