Chemistry: molecular biology and microbiology – Micro-organism – tissue cell culture or enzyme using process... – Preparing element or inorganic compound except carbon dioxide
Reexamination Certificate
2000-01-05
2002-05-14
Marx, Irene (Department: 1651)
Chemistry: molecular biology and microbiology
Micro-organism, tissue cell culture or enzyme using process...
Preparing element or inorganic compound except carbon dioxide
C435S262000, C210S601000, C210S610000, C210S611000
Reexamination Certificate
active
06387669
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to producing hydrogen (bi)sulfide with microorganisms. The hydrogen (bi)sulfide may be used for removal of metals from a waste stream.
As used herein, the term “hydrogen (bi)sulfide” is defined as hydrogen sulfide and/or hydrogen bisulfide.
BACKGROUND OF THE INVENTION
Industrial (e.g. metal finishing and electronics industries) metal-laden waste streams and environmental restoration activities (e.g., mine drainage) are the focus for treatment. Treatment of these waste streams represents a significant economic cost. The current baseline treatment is conventional pH neutralization followed by precipitation of the metal as sludge, such as by the addition of lime or sodium hydroxide to neutralize the waste and precipitate the metal(s). The conventional process produces large volume of difficult-to-handle, hydroxide sludge that must be disposed of or blended with other material to make it suitable for smelting (reuse). Other technologies can be applied to remove metals from waste streams and produce more manageable sludges/secondary wastes, but these technologies are typically more expensive than the conventional process due to higher energy consumption or higher reagent cost. (e.g., reverse osmosis, ion exchange, chemical sulfide precipitation). For example, U.S. Pat. No. 5,338,460 describes removing dissolved heavy metals from an aqueous stream by reacting at least one of the metals with a water-soluble inorganic sulfide at a controlled pH between about 2 and 3.5 within a temperature range of between 100 degrees F. and 212 degrees F. However, this process has not been commercially accepted because chemical sources of sulfides for use in precipitation processes (e.g., sodium sulfide) are expensive such that chemical sulfide precipitation costs are typically higher than costs for baseline technologies.
The use of sulfate-reducing bacteria (SRB) has been proposed as a cost effective means to remove metals from waste streams. In fact, there are several successful applications of this process in operation for environmental restoration efforts (mine drainage treatment and groundwater treatment) de Vegt, A. L., H. Dijkman, and C. J. Buisman. 1998. “Hydrogen Sulfide Produced from Sulfate by Biological Reduction for use in Metallurgical Operations.” In: Proceedings of the Society for Mining, Metallurgy, and Exploration Annual Meeting, Orlando Fla., Mar. 9-11, 1998. SRB can be used to remove metals for waste streams because these bacteria produce sulfides (hydrogen sulfide (H
2
S) and bisulfide (HS—)), that react with metals to form insoluble metal precipitates. Thus, systems can be designed that induce sulfide generation by the SRB and then contact the resulting sulfide with metal-laden waste streams such that the metals precipitate from solution and are removed from the waste stream.
SRB can be found in nature in a number of environments in which some portion is anaerobic (e.g., municipal sewage sludge, river and marine sediments, aquifers). SRB typically live in conjunction with other bacteria that convert complex organic molecules into the more simple organic molecules and hydrogen that SRB can metabolize. SRB link oxidation of an organic or hydrogen to reduction of sulfate (and in some cases other oxidized sulfur compounds) to produce sulfides as a product. During metabolism, some SRB can completely oxidize small organic acids to CO2 and water. Other SRB incompletely oxidize organic acids such as lactate to acetate. SRB can survive some exposure to oxygen, but are strict anaerobes and only reduce sulfate in the absence of oxygen. Sulfate is the primary electron acceptor for SRB, although, some species/strains can reduce other compounds (Widdel, F. 1988. “Microbiology and Ecology of Sulfate- and Sulfur-Reducing Bactieria.” In: A. J. B Zehnder (Ed.).
Biology of Anaerobic Microorganisms
. Wiley Interscience, New York).
As is taught by U.S. Pat. No. 5,587,079, sulfides produced by SRB may be used for metal sulfide precipitation. The reaction chemistry of the precipitation process is:
M
2
++S
2
−→MS
where M
2
+ represents a metal ion having a valence of 2+,
S
2
− represents a sulfide ion with a valence of 2−, and
MS represents a metal sulfide compound (solid precipitate).
Bacteria are used to provide hydrogen sulfide and carbonate compounds for treating solutions containing metal ions. By controlling the addition of hydrogen sulfide and carbonate compounds to the solution, the preferential isolation of particular metal sulfide concentrates may be accomplished in separate precipitation steps, each with a specific pH and sulfide dosage.
U.S. Pat. No. 5,554,290 reports use of microbially-generated (biogenic) sulfide for metals precipitation for in situ environmental restoration. In this method, nutrients that stimulate indigenous bacteria to produce sulfide are injected into the subsurface aquifer. The sulfide precipitates metals present and the precipitate remains in situ.
In U.S. Pat. No. 4,735,723, waste water containing sulfate and organic material is purified by anaerobic biological waste water treatment where at least 80% of the sulfate is converted into hydrogen sulfide in an acidification process and at least 70% of the resulting hydrogen sulfide is removed from the waste stream.
However, although the use of SRBs produce sulfides useful in the removal of heavy metals from process streams, such processes currently known in the art are essentially steady-state processes that have not heretofore been used for “on-demand” production of sulfides for metal removal. Such limitations are a result of the fact that the sulfide production rate of a biological reactor is proportional to the concentration of viable bacteria in the reactor. That is, the overall production rate is the production rate of a single bacterium (or a specific production rate per gram of bacteria) multiplied times the number of bacteria (or total grams) in the reactor. The concentration of bacteria in the reactor is herein after termed the biomass concentration. A material balance for biomass concentration in a reactor is as follows.
Mass rate of change in biomass=rate of change due to growth−rate of change due to decay−rate of change due to outflow.
The rate of change due to growth represents the bacterial growth from metabolism of a substrate, i.e., substrate conversion. The rate of change due to decay is the decrease in biomass concentration due to maintenance energy requirements and endogenous metabolism which is commonly related to the use of cellular materials or extra-cellular materials as a substrate. In a reactor at steady-state operation, the bacterial growth is balanced by the loss of bacteria in the reactor effluent and the rate of bacterial decay. At this steady state condition, the sulfide production rate of the reactor is stable.
For reliable “on-demand” sulfide production, a reactor must be shut down and then restarted at the same conversion rate (i.e., at the same steady state condition). Therefore, the biomass concentration must be maintained at the same concentration during the time intervals between active demand for sulfide. However, currently, when the inflow and outflow to the reactor are stopped and substrate is not added to the reactor, over time, the biomass decay rate will predominate such that the biomass concentration decreases. To restart the reactor thus requires either adding a charge of new bacteria to return the reactor to the previous level of biomass concentration, or allowing sufficient time for the remaining bacteria to multiply to the previous level of biomass concentration.
Hence, for reliable “on-demand” sulfide production, there remains a need for a method to control the process of microbial production of sulfides wherein the rate of sulfide production may be first halted, and then resumed, without the necessity of adding additional bacteria or waiting for the remaining bacteria to multiply and increase the biomass concentration to the prior level. The need additiona
Peyton Brent M.
Toth James J.
Truex Michael J.
Afremova Vera
Battelle (Memorial Institute)
Marx Irene
May Stephen R.
McKinley, Jr. Douglas E.
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