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USE OF HYDROGEN RESPIROMETRY TO DETERMINE METAL TOXICITY TO SULFATE REDUCING BACTERIA
Citation:
Tabak*, H H., E. L. Holder, M. J. Kupferle, AND J R. Haines*. USE OF HYDROGEN RESPIROMETRY TO DETERMINE METAL TOXICITY TO SULFATE REDUCING BACTERIA. Presented at 14th Annual West Coast Conference of Association for Environmental Health Science and International Society Environmental Forensics, San Diego, CA, March 15 - 19, 2004.
Impact/Purpose:
To discuss the development of unique methods for the biotreatment of acid mine water present in the Berkeley Pit, in Butte, Montana.
Description:
Acid mine drainage (AMD), an acidic metal-bearing wastewater poses a severe pollution problem attributed to post-mining activities. The metals (metal sulfates) encountered in AMD and considered of concern for risk assessment are: arsenic, cadmium, aluminum, manganese, iron, zinc and copper. A unique method was developed for the biotreatment of acid mine water present in the Berkeley Pit, Butte, Montana. The biotreatment of AMD recovers valuable resources by separating the biological sulfate reduction step by sulfate reducing bacteria (SRB) and the metal precipitation step. Hydrogen sulfide (H2S) produced in the SRB membrane bioreactor system is used to sequentially precipitate metal sulfides and hydroxides as determined by their electrochemical properties. It is essential to develop methods for determination of metal concentrations that are toxic and/or inhibitory to SRB in order to provide better control of metal recovery processes. A respirometric procedure was developed to determine the toxic and inhibitory effects of heavy metals in AMD on the hydrogen (H2)-consuming SRB by quantitatively measuring H2 utilization by SRB. A mixotrophic enrichment culture of SRB was developed that requires carbon dioxide (CO2) and acetate as carbon sources and H2 as the electron donor for the conversion of sulfate to H2S. The H2S produced by the sulfate reduction activity is removed with a zinc acetate trap, analogous to an alkaline trap in the classical respirometry, but allows for the retention of CO2 in the flask headspace. Respirometry can measure both suppression of hydrogen uptake (toxicity) and/or increased lag time before hydrogen uptake begins (inhibition), relative to control flasks without metals. Most bacteriological media contain components which form metal complexes thus reducing metal bioavailability to SRB. The culture medium formulated by Sani et al. (Adv. Environ. Res. 5 (2001) 269-276) was modified for use. Metal complexation was significantly reduced by replacing orthophosphate with tryptone and PIPES for pH control. Respirometric studies were undertaken to measure toxicity and inhibition of zinc (Zn) and copper (Cu) to SRB. Initial respirometric data on H2 uptake, analytical data on sulfate conversion, and H2S production measurements indicate that Zn is inhibitory to SRB between 10 and 25 ppm and that Cu is inhibitory below 17 ppm. Concentrations of these metals above inhibitory levels, were shown to be toxic to SRB as indicated by complete suppression of H2 uptake in the respirometric culture systems. Iron was not found to be toxic at concentrations up to 400 ppm. An experiment combining 200 ppm iron (Fe) with Cu decreased the sensitivity of the SRB to Cu so that inhibition occurred at 7 ppm instead of 4 ppm. A similar experiment with Zn showed the same result; inhibition occurred at 15 ppm instead of 10 ppm. An experiment with diluted acid mine water showed inhibition with concentrations of the primary metal components at 6 ppm Zn, 5 ppm Fe, 3 ppm AI, 2 ppm Cu, and 2 ppm Mn. Design of a treatment system for AMD will require careful attention to the concentration of metals in the water due to the relatively high concentrations found in AMD. For example, three metals studied in this work exist in Berkeley Pit water at Fe 634 mg/I, Cu 173 mg/I, and Zn at 574 mg/l. Either the AMD must be diluted or the SRB must be developed to be metal tolerant.