CA1337019C - Biorecovery of selenium - Google Patents
Biorecovery of seleniumInfo
- Publication number
- CA1337019C CA1337019C CA000580256A CA580256A CA1337019C CA 1337019 C CA1337019 C CA 1337019C CA 000580256 A CA000580256 A CA 000580256A CA 580256 A CA580256 A CA 580256A CA 1337019 C CA1337019 C CA 1337019C
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- Canada
- Prior art keywords
- selenium
- biomass
- process according
- metal
- aqueous solution
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
- C22B61/00—Obtaining metals not elsewhere provided for in this subclass
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B19/00—Selenium; Tellurium; Compounds thereof
- C01B19/02—Elemental selenium or tellurium
-
- C—CHEMISTRY; METALLURGY
- C12—BIOCHEMISTRY; BEER; SPIRITS; WINE; VINEGAR; MICROBIOLOGY; ENZYMOLOGY; MUTATION OR GENETIC ENGINEERING
- C12N—MICROORGANISMS OR ENZYMES; COMPOSITIONS THEREOF; PROPAGATING, PRESERVING, OR MAINTAINING MICROORGANISMS; MUTATION OR GENETIC ENGINEERING; CULTURE MEDIA
- C12N1/00—Microorganisms, e.g. protozoa; Compositions thereof; Processes of propagating, maintaining or preserving microorganisms or compositions thereof; Processes of preparing or isolating a composition containing a microorganism; Culture media therefor
-
- C—CHEMISTRY; METALLURGY
- C12—BIOCHEMISTRY; BEER; SPIRITS; WINE; VINEGAR; MICROBIOLOGY; ENZYMOLOGY; MUTATION OR GENETIC ENGINEERING
- C12P—FERMENTATION OR ENZYME-USING PROCESSES TO SYNTHESISE A DESIRED CHEMICAL COMPOUND OR COMPOSITION OR TO SEPARATE OPTICAL ISOMERS FROM A RACEMIC MIXTURE
- C12P1/00—Preparation of compounds or compositions, not provided for in groups C12P3/00 - C12P39/00, by using microorganisms or enzymes
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
- C22B3/00—Extraction of metal compounds from ores or concentrates by wet processes
- C22B3/18—Extraction of metal compounds from ores or concentrates by wet processes with the aid of microorganisms or enzymes, e.g. bacteria or algae
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2006/00—Physical properties of inorganic compounds
- C01P2006/80—Compositional purity
-
- C—CHEMISTRY; METALLURGY
- C12—BIOCHEMISTRY; BEER; SPIRITS; WINE; VINEGAR; MICROBIOLOGY; ENZYMOLOGY; MUTATION OR GENETIC ENGINEERING
- C12R—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES C12C - C12Q, RELATING TO MICROORGANISMS
- C12R2001/00—Microorganisms ; Processes using microorganisms
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/20—Recycling
Abstract
Water insoluble selenium metal is recovered from a substantially neutral aqueous solution comprising soluble selenium by associating the substantially neutral aqueous solution with an environment comprising aerobic biomass, which biomass is able to metabolize the soluble selenium to insoluble selenium metal.
Preferably, the environment comprises raw sewage.
Preferably, the environment comprises raw sewage.
Description
This invention relates to the extraction of metal from aqueous solutions and in particular to the extraction of selenium from aqueous solutions such as natural ground water and weak acid smelter effluents.
Selenium is a Group 6A element of the periodic table which has wide uses in the production of various electrical instruments and apparatus, as an ingredient of toning baths in photography, as a vulcanizing agent in the processing of rubber and as a catalyst in the dehydrogenation of organic compounds, to name a few.
Accordingly, the extraction of selenium from natural ground water and from smelter effluents is commercially beneficial.
Selenium is an essential trace element in animal diets at levels of approximately 0.1 ppm;
however, it is a toxic element at higher concentrations.
For this reason, and from a general environmental standpoint, it is desirable to maintain the levels of selenium in water to below the toxic level.
Several methods are available for purifying water and removing pollutants including gravity sedimentation, floatation, filtration, ion exchange, activated adsorption, reverse osmosis, electrodialysis, distillation and chemical precipitation; however, none are effective at extracting selenium.
It is known that a variety of microorganisms can reduce elements of Group 6A of the periodic table via their intracellular assimilatory sulfate reduction pathway. Elements such as selenium and tellurium are competitive inhibitors of this pathway and, thus, organisms which can transport the oxy salts of these ~ ~ 1 3370 t 9 elements across the cytoplasmic membrane can reduce the salt to the elemental state intracellularly.
Selenite is metabolized by these microorganisms in at least two different ways:
(1) by the enzymatic utilization of seo3 2 - in lieu of sulfur, presumably by converting SeO3 2 - to the divalent selenite (Se2~), and forming therefore such products as selenomethionine and selenocysteine (Cowle D.B. & Cowen G.N.
1957 "Biosynthesis by Escherichia coli of Active Altered Proteins Containing Selenium Instead of Sulfur", Biochem., Biophys. Acta, 26, 252-261). Competitive inhibition by these seleno-amino acids could account for the toxicity of SeO3 2 - to many species.
Selenium is a Group 6A element of the periodic table which has wide uses in the production of various electrical instruments and apparatus, as an ingredient of toning baths in photography, as a vulcanizing agent in the processing of rubber and as a catalyst in the dehydrogenation of organic compounds, to name a few.
Accordingly, the extraction of selenium from natural ground water and from smelter effluents is commercially beneficial.
Selenium is an essential trace element in animal diets at levels of approximately 0.1 ppm;
however, it is a toxic element at higher concentrations.
For this reason, and from a general environmental standpoint, it is desirable to maintain the levels of selenium in water to below the toxic level.
Several methods are available for purifying water and removing pollutants including gravity sedimentation, floatation, filtration, ion exchange, activated adsorption, reverse osmosis, electrodialysis, distillation and chemical precipitation; however, none are effective at extracting selenium.
It is known that a variety of microorganisms can reduce elements of Group 6A of the periodic table via their intracellular assimilatory sulfate reduction pathway. Elements such as selenium and tellurium are competitive inhibitors of this pathway and, thus, organisms which can transport the oxy salts of these ~ ~ 1 3370 t 9 elements across the cytoplasmic membrane can reduce the salt to the elemental state intracellularly.
Selenite is metabolized by these microorganisms in at least two different ways:
(1) by the enzymatic utilization of seo3 2 - in lieu of sulfur, presumably by converting SeO3 2 - to the divalent selenite (Se2~), and forming therefore such products as selenomethionine and selenocysteine (Cowle D.B. & Cowen G.N.
1957 "Biosynthesis by Escherichia coli of Active Altered Proteins Containing Selenium Instead of Sulfur", Biochem., Biophys. Acta, 26, 252-261). Competitive inhibition by these seleno-amino acids could account for the toxicity of SeO3 2 - to many species.
(2) by the reduction of SeO3 2 - to an insoluble product which is deposited intracellularly.
The intracellular deposition of such precipitates from SeO3 2 - has been shown in Corynebacterium diptheriae (Levine, V.E. 1925, "The Reducing Properties of Microorganisms with Special Reference to Selenium Compounds", J. Bacteriol. 10, 217-263), Neurospora crassa (Zolokar, M., 1953, "Reduction of Selenite by Neurospora", Arch. Biochem. Biophys. 44, 330-337), and Candida albicans (Falcone G. &
Nickerson W.J., 1963, "Reduction of Selenite by Intact Yeast Cells and Cell Free Preparations", J. Bacteriol. 85, 763-771.
McCready et al. (Canadian Journal of Microbiology, Vol. 12, 1966) investigated the reduction of selenite using Salmonella heidelberg and found that the reduction is a direct result of the metabolic activity of this bacteria and that the reduction occurs via two two-electron transfer steps:
Se4+ ----- Se2' ----- Se.
The reduction of selenite occurs at different rates depending on the organism used to mediate the reaction.
There are two amorphous forms of selenium, black and red. Vitreous, black selenium, is formed when molten selenium is cooled rapidly. Red, amorphous selenium is formed by the reduction of selenous acid in water.
A few investigators have attempted to exploit the selenium reducing capacity of selected microorganisms to extract selenium from waters.
U.S. Patent No. 4,519,973 to Baldwin et al.
describes a process for the removal of selenium from aqueous waste solutions using strictly anaerobic bacteria of the genus Clostridium. Specifically, the Clostridium are supported under anaerobic conditions on a porous matrix through which the aqueous waste solution must pass. The bacteria must be provided with a constant and controlled amount of nutrients, in particular an organic carbon source in order to ensure proper growth and metabolism, and accordingly to ensure efficient selenium reduction. Furthermore, strict anaerobic conditions in the treatment zone must be maintained for Clostridium to survive. The maintenance of the above conditions is costly and time consuming and therefore results in a generally unfeasible process. In addition, the process disclosed in this patent is only efficient at removing selenium from solutions having a selenium concentration less than 10 ppm. In many water samples, both natural and waste, the selenium concentration is higher than 10 ppm, such levels rendering this process inefficient and commercially unattractive.
U.S. Patent No. 4,519,912 to Kauffman et al., which is a companion to the Baldwin patent described above, provides that the porous matrix contains not only bacteria of the genus Clostridium, but at least one other anaerobic bacteria selected from Desulfovibrio and Desulfotomaculum. The drawbacks to the Baldwin process apply equally well to the process of the Kauffman patent.
It is an object of the present invention to obviate and mitigate the above disadvantages.
The present invention provides a process for the recovery of insoluble selenium metal from a substantially neutral aqueous solution comprising soluble selenium. The substantially neutral aqueous solution is associated with an environment comprising aerobic biomass to allow the biomass to metabolize the soluble selenium to insoluble selenium metal.
This process is most efficient at selenium metal recovery if the "initial" aqueous solution comprising the soluble selenium has a pH of between 6 -8.5, most preferably 7. If the aqueous solution to be treated by the process as described herein has a pH
which is higher or lower than this desired range, it is desirable that the aqueous solution be neutralized prior to treatment with the aerobic biomass. An example of a typical aqueous solution which may be neutralized prior to treatment is weak acid smelter effluent.
The neutralization step accomplishes two ~ ~ 1 3370 1 9 criteria which are important to the success of the process:
(1) toxic heavy metal ions which may be present in the aqueous solution are precipitated as metal hydroxides and may be recovered from the solution, (2) any soluble sulfate ions which are competitive inhibitors of selenium reduction are precipitated as calcium sulfate or gypsum and may be removed from the solution.
Preferably, the biomass comprising the metabolized insoluble selenium metal is separated from the aqueous solution and the insoluble selenium metal is then appropriately recovered from the organic cellular components of the biomass. Accordingly, one aspect of this invention involves a continous process for the recovery of selenium metal from a substantially neutral aqueous solution comprising soluble selenium wherein the organic cellular components once separated from the selenium metal, are recycled back to the substantially neutral aqueous solution comprising soluble selenium.
In a preferred embodiment of this process, the environment with which the soluble selenium-containing solution associates is raw sewage. The use of raw sewage in this process is extremely advantageous as it provides not only a ready supply of the aerobic selenium-reducing bacteria such as Escherichia coli, but it provides a rich carbon and nutrient source for the growth of the bacteria.
By means of the process of the present invention, there can be obtained from an aqueous solution comprising a relatively high concentration of soluble selenium, insoluble selenium in an efficient and economic manner. In contrast to the known microbial processes for selenium removal, this process permits the extraction of selenium from solutions containing relatively high concentrations (lO - 500 ppm) of selenium. The process is economical as the biomass used to reduce the selenium are aerobic and, accordingly, do not require stringent and expensive environmental controls, and in a preferred form, the use of raw sewage in the process:
(1) eliminates the requirement to supply and supplement a growth media for the bacteria, (2) provides a healthy inoculum of selenium reducing bacteria, in particular E. coli.
The invention will now be described by way of illustration only, with respect to the following drawing which is a diagrammatic process flow sheet of a specific preferred process according to the invention.
Referring to the drawing, an acidic aqueous solution, e.g. weak acid smelter effluent, comprising soluble selenium, is neutralized at neutralization zone 10 with an alkaline slurry, e.g. hydrated lime slurry, to precipitate the toxic heavy metal ions as metal hydroxides and the soluble sulfate ions as calcium sulfate or gypsum, and to produce an aqueous solution having a substantially neutral pH. This neutralized solution is then contacted with aerobic selenium-reducing bacteria at mixing zone 12 and the feedstock resulting from the mixing zone is provided to the treatment zone 14. In the treatment zone, the selenium-reducing bacteria metabolize and reduce the soluble selenium from the feedstock to granules of red amorphous selenium. As these granules of selenium accumulate intracellularly, the selenium-reducing cells die and are flushed from the treatment zone in the effluent stream.
1 3370 1 ~
The selenium-containing cells (biomass) are suitably separated from the remainder of the effluent stream at separation zone 16, e.g. by high speed centrifugation, and the selenium metal is recovered from the biomass, e.g., by digestion of the selenium-containing cells with an alkali such as caustic soda, at mixing and settling zone 18. The effluent stream, separated from the cells at separation zone 16, is discarded. The digestion of the cells at zone 18 solubilizes the organic cellular components and simultaneously converts the red amorphous selenium granules to the black crystalline form of selenium metal. The selenium metal thus formed, settles to the bottom of the mixing and settling zone 18 and, therefore, may be isolated from the liquid phase, washed at washing zone 20, and dried at drying zone 22 to produce selenium metal.
In a preferred form, the pH of the aqueous solution comprising soluble selenium is between 6 - 8.5.
Variations in this pH value will effect the efficiency of selenium reduction by the selenium reducing microorganisms. Most preferably, the pH of the solution is 7. If the aqueous solution to be treated with the selenium biorecovery process is not within the indicated pH range, it is necessary to neutralize the solution with an appropriate basic or acidic material such as lime, caustic or phosphoric acid or the like. Other neutralizing agents are well within the purview of the skilled artisan and, therefore, an exhaustive list need not be recited herein.
Many aqueous solutions such as natural ground water sources may have a pH within the desired range and, accordingly, the neutralization step is not strictly necessary.
Although it is preferred to use raw sewage as a source of aerobic selenium-reducing bacteria and as a carbon source, appropriate selenium-reducing aerobic microorganisms may be cultured, mixed with the neutral aqueous solution and provided to the treatment zone as a feedstock. In this instance, appropriate nutrients, in particular a carbon source, should be provided to the growing microorganisms in the treatment zone. The nature and quantity of the nutrients to be added depends upon the bacteria's ability to utilize particular nutrients, including trace nutrients in the surroundings. For the majority of aerobic selenium-reducing bacteria, any suitable carbon source such as pectic substances, inulin, starches, mono, di or trisaccharides, proteins, amino acids, organic acids or derivatives thereof may be used. It is well within the skill of an artisan in this area of science to choose the appropriate growth conditions for any of the aerobic selenium-reducing bacteria of the present invention.
Naturally, if raw sewage is used as the source of the aerobic selenium-reducing bacteria, the provision of extra nutrients for the maintenance of the bacteria may be unnecessary.
Selenium-reducing microorganisms which are suitable for use within the process of the present invention are those which:
(1) function in an aerobic environment, (2) assimilate selenite and/or selenate and reduce it intracellularly to red amorphous selenium.
Examples of suitable microorganisms include those of the genus Corynebacterium (C. diptheriae), Neurospora (N.
crassa), Candida (C. albicans), Salmonella (S.
heidelberg), and in particular, Escherichia (E. coli).
This list should not be considered exhaustive.
~ ~ 3370 1 9 In accordance with the process of the present invention, the treatment zone is an aerobic environment wherein there are matrices adapted to support the selenium-reducing bacteria. As discussed above, these bacteria may be added to the treatment zone in a feedstock with the neutral solution. Alternatively, the selenium-reducing bacteria may be fed directly into the treatment zone independent of the neutral solution. The treatment zone may be any suitable structure or reactor, including but not limited to structures such as, for example, various vessels, tanks, earthen ponds and the like. The treatment zone is maintained under aerobic conditions, i.e. in the presence of free atmospheric oxygen, so that the aerobic selenium-reducing bacteria such as E. coli may metabolically reduce the selenite and selenate ions (selenium in the Se(IV) and Se(VI) oxidation states) to water insoluble selenium metal, which metal is encapsulated in the cellular structure of the bacteria contained on the matrix.
Various materials can be employed as the matrix support for the bacteria within the treatment zone. Preferably, these matrix materials must possess a porosity capable of allowing passage of the substantially neutral aqueous solution through the treatment zone and also must be substantially inert, i.e. neither destroy the bacteria per se, nor interfere with the bacteria's ability to metabolically reduce the water soluble selenium ions to water insoluble selenium metal. Representative, but non-limiting examples of such materials include gravel, sand, cellulose, glass, wool, glass beads and the like. Combinations of these various materials also may be used.
Most preferably, the treatment zone is a rotary biological contactor (RBC), although other g -systems involving continuous stirred tank reactors (CSTR) and immobilized cell, fluidized bed reactors may be used.
Rotary biological contactors have been used successfully in sewage treatment for at least a decade.
A variety of vessel designs are now available; however, the basic concept involves a trough in which a rotating shaft maintaining numerous discs of an inert support medium is slowly rotated. In the present invention, the shaft and disc assembly is half-immersed in the mixture of the substantially neutral aqueous solution and nutrient medium (feedstock from the m; X; ng zone). The biomass attach to the discs and sequentially pass through this "liquid" phase to adsorb nutrients, selenite and/or selenate and then pass through the air to adsorb atmospheric oxygen. The soluble selenium-containing solution is input at one end, traverses the longitudinal axis of the reactor and the effluent, comprising the flushed cells having intracellular selenium, is collected at the other end.
The selenium-cont~ n; ng cells may be separated from the effluent stream at the separation zone by numerous methods. The preferred method is selected from high-speed centrifugation, tangential flow filtration and flocculation and setting tO.l~ w/v potassium alum or 0.1% v/v Superfloc 320*). Each of these separation techniques is well known and applied in many areas of technology and, therefore, need not be expanded upon herein.
There are, within the scope of this invention, two preferred methods of recovering the insoluble selenium from the biomass at the mixing and setting zone:
*trademark 1) dilute alkali digestion of the biomass (solubilization of the cells or biomass), 2) solubilization of the metallic selenium.
The first preferred method involves digesting the biomass with, for example, 0.25 N caustic soda.
Another example of a dilute alkali which may be used within this invention includes KOH. As a general rule, many suitable digesting agents may be provided in the mixing and settling zone as long as the agent:
1) solubilizes the organic cellular components of the cells or biomass, 2) converts the red amorphous granules of selenium to the black crystalline form of selenium metal.
The preferred conditions for the digestion include a solid:liquid ratio of approximately 5g/L, and a temperature of between 105-110 C for a time of approximately 30 minutes. It is to be appreciated that the solid:liquid ratio may be increased or decreased but the digestion time is proportional to the solids content.
The second preferred method involves extracting the selenium from the cells with a metal solubilizing agent such as boiling Na2 S03 . Other suitable solubilizing agents include, but are not limited to SO2 and Na2HSO3. The addition of this agent into the mixing and setting zone, 18 is shown in the Figure. The boiled ex~ract may then be treated with alumina to adsorb the organic cellular components and filtered to remove the organic laden alumina. The selenium may be precipated by acidification of the filtrate with an agent such as H2 S04 . Other suitable precipitating agents include SO2, Na2SO3 and Na2HSO3.
_ _ 1 3370 1 9 The selenium may then be washed extensively with water, collected and dried as a fine powder. At this stage in the process, the organic-laden alumina may be treated with a hot alkali such as hot caustic soda to remove the organic cellular components which may subsequently or concurrently be recycled back to the neutralization zone or the treatment zone as a carbon source. The alumina may be re-used as an organic adsorbent.
During the course of the acidification and re-precipitation of the selenium in this second preferred method, SO2 gas may be evolved. It would be desirable to trap this gas by a gas washing process such as passage as fine bubbles through water or dilute alkali.
Alternatively, the selenium may be recovered from the biomass by oxidizing the biomass using boiling HNO3. Other oxidizing acids such as perchloric acid or aqua regia may be used with extreme caution. Once the biomass is completely oxidized, the selenium metal is re-precipitated by flushing the hot solution with SO2 or by the addition of Na2SO3. At temperatures above 85 C, re-precipitation will be as the black metallic selenium.
If the acid digest solution is cooled to less than 35 C
the selenium will precipitate in the red amorphous form.
Thus, in a preferred form, the process of the present invention is continuous. To that end, the liquid phase from the mixing and settling zone which comprises cellular components is recycled back to the neutralization zone as indicated by the dashed line in the Figure. Alternatively, the liquid phase may be recycled directly back to the treatment zone. This recycling step reduces the costs of the neutralizing agent and simultaneously provides an additional carbon source for the growing cells in the treatment zone.
~ _ 1 3370 1 9 Furthermore, this recycling step returns any organo seleno compounds back to the reductive step of the process (treatment zone) where they may undergo complete reduction to elemental selenium. The continuity of this process will significantly increase the percent recovery of the final product, selenium metal.
Preferably, the black selenium metal is washed for at least several cycles at the washing zone to remove soluble organic material and residual digesting agent, such as caustic soda.
The drying zone is preferably an evaporator.
The selenium metal, once washed and dried, may be heated to a molten state (approximately 218 C) and poured as ingots or sprayed into water for pelletization.
The rate of flow of the aqueous solution to be treated through the treatment zone will vary depending on the reactor capacity and design. In order to fully exemplify the process of this invention, rather than quote flow rates, it is more meaningful to disclose the retention time of the aqueous solution in the reactor or treatment zone. The retention time is the time required for one complete change of the reactor liquid volume.
The retention time may range from 2.5 to 16 hours, preferably 2.5 to 6 hours, and most preferably 2.5-4 hours, depending on the biomass loading within the reactor and the concentration of soluble selenium ions present in the influent solution.
The treatment zone, in which the biomass is retained, is maintained at temperatures ranging from about 0 C to about 45 C, preferably at temperatures from about 15 C to about 40 C and most preferably at temperatures from about 22 C to about 37 C.
The process of the present invention is particularly suited for the recovery of elemental selenium from aqueous solutions such as smelter and roaster weak acid effluents, water from a uranium, copper or molybdenum mining or leaching operation, mine seepage or drainage water or any other aqueous stream which contains selenium ions in the selenite Se(IV) and selenate Se(VI) oxidation states.
Preferred embodiments of this invention will now be described by way of the following, non-limiting examples:
Example 1 Selenite reducing organisms were grown in Trypticase Soy Broth (Baltimore Biological Co. Ltd.) containing 0.1% w/v SeO3 2 - ( 456 ppm Se) at 37 C. The growth, changes in pH, and deposition of intracellular selenium granules were followed over a 32-hour period.
The organisms were capable of reducing selenite to elemental selenium over the pH range of 6.0 to 8.5 with an optimal rate of reduction at pH 7Ø
Simultaneously, the cultures were examined and photographed under a phase contract microscope at various timed intervals. Granule deposition was observed from 4 to 12 hours after inoculation. When the selenite concentration had been substantially reduced, normal logarithmic growth of the cultures was observed.
A laboratory scale rotary biological contactor with 100 sq. ft. of disk surface and a 30 L capacity was ~ 1 3370 1 9 then used in a pilot-plant study for the removal of selenium from a base metal smelter weak acid effluent.
The weak acid effluent which was neutralized to pH 7.0 was combined with the microbial inoculum and a nutrient solution as an input stream to the rotary biological contactor (RBC). Tests were conducted to optimize parameters such as C:Se ratio, flow rate and retention time.
Using the RBC, soluble selenium levels in the effluent could be reduced by >95~ in a retention time of 4 hours, with a Se:BOD (biological oxygen demand) ratio of 1:20. The temperature was generally maintained at 22 - 23 C. The selenium-laden biomass which sloughed off the disks of the RBC was recovered from the RBC effluent by centrifugation.
The biomass-free effluent was discarded. The remaining biomass comprised the red, amorphous selenium granules.
It was found that a continuous inoculum of selenium reducing organisms and attendant nutrient requirements could be supplied by using raw sewage. In particular, a sewage to effluent (v/v) ratio of 1:10 was used.
Example 2 - Recovery of Selenium by Dilute Alkali Digestion of the Biomass A sample of the biomass comprising the red, amorphous selenium granules, prepared as in Example 1, was extracted by boiling in 0.25 N NaOH at a solid:liquid ratio of 5 g/L for 30 minutes.
The sample was cooled, placed in a large separatory funnel fitted with a stop-cock and side-arm 1.25" above the lower stop-cock. The black, elemental selenium settled in the cone, the soluble protein and 0.25 N NaOH were decanted through the side stop-cock, and the sample was repeatedly washed with distilled water, collected and dried. Chemical and FTIR (Fournier Transformed Infra Red Spectroscopy) analysis of this sample indicated a selenium purity of 99.1% with only a trace of organic contamination.
The chemical analysis is presented in Table 1 - hereinbelow:
Table 1 Analysis of the Bio-Recovered Selenium Element Se 99.1 Fe 0.015 Si 0.013 Mg 0.0013 Cu 0.003 Al <0.002 Example 3 - Recovery of Selenium from Biomass by Solubilization of Metallic Selenium Due to the solubility of selenium in hot Na2 S03, a sample of the biomass comprising the red, amorphous selenium granules, prepared as in Example 1, was extracted with boiling 1~ w/v Na2 S03 . The biomass residue was then removed by filtration, the filtrate was cooled and it was then acidified to re-precipitate the selenium. Using this procedure, elemental selenium of 50.6% purity was recovered. FTIR analysis of the sample indicated organic contamination, probably during acid precipitation, as indicated by amide peaks in the FTIR
spectrum. Two additional S-O peaks were also seen.
A two-step extraction with boiling 1~ w/v Na2 S03 resulted in the recovery of selenium of 93.6 purity. The FTIR spectrum for this sample showed a reduction in size of one of the amide peaks, but little change in the other peak. The S-0 peaks were reduced but still present, thereby reducing the metal purity.
Reverting to the one-step boiling 1~ w/v Na2 S03 extraction, the solution, after boiling, was filtered through a column of activated alumina to adsorb the organic cellular components. The filtrate was then acidified with H2 S04 to precipitate the selenium and the alumina was regenerated by extraction with dilute alkali. The alkaline organic solution was recycled to the RBC as a carbon source. Alternatively, the alumina could be thermally regenerated, but the organics will, in this instance, be lost as C02.
Example 4 A barrel of neutralized smelter weak acid effluent was received from a base metal mine and chemically analyzed. The results of this chemical analysis are presented in Table 2 hereinbelow:
Table 2 Chemical Analysis of Neutralized Base Metal Mine Weak Acid Effluent Element Cu 0 3 Pb 0.4 Zn 270 Se 33 0 Te <1.0 Seventy litres of this solution was treated in a 30 L capacity rotary biological contactor (RBC). The solution was combined with raw sewage in a 10:1 ratio and dry milk powder was added as a carbon source to increase the BOD to Se ratio to approximately 20:1.
The biomass was recovered from the remainder of the effluent by centrifugation at 12,000 rpm using a Sharples High Speed Continuous Flow Centrifuge at a rate of 1 L/min.
The biomass (~30 g wet wt) was extracted (or digested) in 1.5 L of 0.25 N NaOH. This resulted in a very viscous solution which retarded the settling of the selenium granules. For rapid settling of the elemental selenium, the biomass may be digested under the following conditions: 5 g of wet biomass per litre of 0.25 N NaOH.
The digested biomass, substantially free of intracellular selenium, was recycled back to the RBC as a nutrient for the continued process. Accordingly, there is no organic sludge to dispose.
Digestion of 5 g wet biomass per litre of 0.25 N NaOH resulted in a product of 99% purity. As seen in Table 3, the amounts of contaminating metals in the product are extremely low, and the organic content can be reduced under the digestion step by controlling the ratio of biomass to volume of caustic soda.
Table 3 Analysis of the Selenium Produced from a Base Metal Mine Effluent Element %
Se 88.6 Al Ca 0.035 Cr 0.010 Cu 0.010 Fe 0.21 Mg 0.006 Ni 0.001 Pb 0-005 Zn 0.005 Organic debris 10.085 The presence of organic components was confirmed by an FTIR spectrum analysis.
The intracellular deposition of such precipitates from SeO3 2 - has been shown in Corynebacterium diptheriae (Levine, V.E. 1925, "The Reducing Properties of Microorganisms with Special Reference to Selenium Compounds", J. Bacteriol. 10, 217-263), Neurospora crassa (Zolokar, M., 1953, "Reduction of Selenite by Neurospora", Arch. Biochem. Biophys. 44, 330-337), and Candida albicans (Falcone G. &
Nickerson W.J., 1963, "Reduction of Selenite by Intact Yeast Cells and Cell Free Preparations", J. Bacteriol. 85, 763-771.
McCready et al. (Canadian Journal of Microbiology, Vol. 12, 1966) investigated the reduction of selenite using Salmonella heidelberg and found that the reduction is a direct result of the metabolic activity of this bacteria and that the reduction occurs via two two-electron transfer steps:
Se4+ ----- Se2' ----- Se.
The reduction of selenite occurs at different rates depending on the organism used to mediate the reaction.
There are two amorphous forms of selenium, black and red. Vitreous, black selenium, is formed when molten selenium is cooled rapidly. Red, amorphous selenium is formed by the reduction of selenous acid in water.
A few investigators have attempted to exploit the selenium reducing capacity of selected microorganisms to extract selenium from waters.
U.S. Patent No. 4,519,973 to Baldwin et al.
describes a process for the removal of selenium from aqueous waste solutions using strictly anaerobic bacteria of the genus Clostridium. Specifically, the Clostridium are supported under anaerobic conditions on a porous matrix through which the aqueous waste solution must pass. The bacteria must be provided with a constant and controlled amount of nutrients, in particular an organic carbon source in order to ensure proper growth and metabolism, and accordingly to ensure efficient selenium reduction. Furthermore, strict anaerobic conditions in the treatment zone must be maintained for Clostridium to survive. The maintenance of the above conditions is costly and time consuming and therefore results in a generally unfeasible process. In addition, the process disclosed in this patent is only efficient at removing selenium from solutions having a selenium concentration less than 10 ppm. In many water samples, both natural and waste, the selenium concentration is higher than 10 ppm, such levels rendering this process inefficient and commercially unattractive.
U.S. Patent No. 4,519,912 to Kauffman et al., which is a companion to the Baldwin patent described above, provides that the porous matrix contains not only bacteria of the genus Clostridium, but at least one other anaerobic bacteria selected from Desulfovibrio and Desulfotomaculum. The drawbacks to the Baldwin process apply equally well to the process of the Kauffman patent.
It is an object of the present invention to obviate and mitigate the above disadvantages.
The present invention provides a process for the recovery of insoluble selenium metal from a substantially neutral aqueous solution comprising soluble selenium. The substantially neutral aqueous solution is associated with an environment comprising aerobic biomass to allow the biomass to metabolize the soluble selenium to insoluble selenium metal.
This process is most efficient at selenium metal recovery if the "initial" aqueous solution comprising the soluble selenium has a pH of between 6 -8.5, most preferably 7. If the aqueous solution to be treated by the process as described herein has a pH
which is higher or lower than this desired range, it is desirable that the aqueous solution be neutralized prior to treatment with the aerobic biomass. An example of a typical aqueous solution which may be neutralized prior to treatment is weak acid smelter effluent.
The neutralization step accomplishes two ~ ~ 1 3370 1 9 criteria which are important to the success of the process:
(1) toxic heavy metal ions which may be present in the aqueous solution are precipitated as metal hydroxides and may be recovered from the solution, (2) any soluble sulfate ions which are competitive inhibitors of selenium reduction are precipitated as calcium sulfate or gypsum and may be removed from the solution.
Preferably, the biomass comprising the metabolized insoluble selenium metal is separated from the aqueous solution and the insoluble selenium metal is then appropriately recovered from the organic cellular components of the biomass. Accordingly, one aspect of this invention involves a continous process for the recovery of selenium metal from a substantially neutral aqueous solution comprising soluble selenium wherein the organic cellular components once separated from the selenium metal, are recycled back to the substantially neutral aqueous solution comprising soluble selenium.
In a preferred embodiment of this process, the environment with which the soluble selenium-containing solution associates is raw sewage. The use of raw sewage in this process is extremely advantageous as it provides not only a ready supply of the aerobic selenium-reducing bacteria such as Escherichia coli, but it provides a rich carbon and nutrient source for the growth of the bacteria.
By means of the process of the present invention, there can be obtained from an aqueous solution comprising a relatively high concentration of soluble selenium, insoluble selenium in an efficient and economic manner. In contrast to the known microbial processes for selenium removal, this process permits the extraction of selenium from solutions containing relatively high concentrations (lO - 500 ppm) of selenium. The process is economical as the biomass used to reduce the selenium are aerobic and, accordingly, do not require stringent and expensive environmental controls, and in a preferred form, the use of raw sewage in the process:
(1) eliminates the requirement to supply and supplement a growth media for the bacteria, (2) provides a healthy inoculum of selenium reducing bacteria, in particular E. coli.
The invention will now be described by way of illustration only, with respect to the following drawing which is a diagrammatic process flow sheet of a specific preferred process according to the invention.
Referring to the drawing, an acidic aqueous solution, e.g. weak acid smelter effluent, comprising soluble selenium, is neutralized at neutralization zone 10 with an alkaline slurry, e.g. hydrated lime slurry, to precipitate the toxic heavy metal ions as metal hydroxides and the soluble sulfate ions as calcium sulfate or gypsum, and to produce an aqueous solution having a substantially neutral pH. This neutralized solution is then contacted with aerobic selenium-reducing bacteria at mixing zone 12 and the feedstock resulting from the mixing zone is provided to the treatment zone 14. In the treatment zone, the selenium-reducing bacteria metabolize and reduce the soluble selenium from the feedstock to granules of red amorphous selenium. As these granules of selenium accumulate intracellularly, the selenium-reducing cells die and are flushed from the treatment zone in the effluent stream.
1 3370 1 ~
The selenium-containing cells (biomass) are suitably separated from the remainder of the effluent stream at separation zone 16, e.g. by high speed centrifugation, and the selenium metal is recovered from the biomass, e.g., by digestion of the selenium-containing cells with an alkali such as caustic soda, at mixing and settling zone 18. The effluent stream, separated from the cells at separation zone 16, is discarded. The digestion of the cells at zone 18 solubilizes the organic cellular components and simultaneously converts the red amorphous selenium granules to the black crystalline form of selenium metal. The selenium metal thus formed, settles to the bottom of the mixing and settling zone 18 and, therefore, may be isolated from the liquid phase, washed at washing zone 20, and dried at drying zone 22 to produce selenium metal.
In a preferred form, the pH of the aqueous solution comprising soluble selenium is between 6 - 8.5.
Variations in this pH value will effect the efficiency of selenium reduction by the selenium reducing microorganisms. Most preferably, the pH of the solution is 7. If the aqueous solution to be treated with the selenium biorecovery process is not within the indicated pH range, it is necessary to neutralize the solution with an appropriate basic or acidic material such as lime, caustic or phosphoric acid or the like. Other neutralizing agents are well within the purview of the skilled artisan and, therefore, an exhaustive list need not be recited herein.
Many aqueous solutions such as natural ground water sources may have a pH within the desired range and, accordingly, the neutralization step is not strictly necessary.
Although it is preferred to use raw sewage as a source of aerobic selenium-reducing bacteria and as a carbon source, appropriate selenium-reducing aerobic microorganisms may be cultured, mixed with the neutral aqueous solution and provided to the treatment zone as a feedstock. In this instance, appropriate nutrients, in particular a carbon source, should be provided to the growing microorganisms in the treatment zone. The nature and quantity of the nutrients to be added depends upon the bacteria's ability to utilize particular nutrients, including trace nutrients in the surroundings. For the majority of aerobic selenium-reducing bacteria, any suitable carbon source such as pectic substances, inulin, starches, mono, di or trisaccharides, proteins, amino acids, organic acids or derivatives thereof may be used. It is well within the skill of an artisan in this area of science to choose the appropriate growth conditions for any of the aerobic selenium-reducing bacteria of the present invention.
Naturally, if raw sewage is used as the source of the aerobic selenium-reducing bacteria, the provision of extra nutrients for the maintenance of the bacteria may be unnecessary.
Selenium-reducing microorganisms which are suitable for use within the process of the present invention are those which:
(1) function in an aerobic environment, (2) assimilate selenite and/or selenate and reduce it intracellularly to red amorphous selenium.
Examples of suitable microorganisms include those of the genus Corynebacterium (C. diptheriae), Neurospora (N.
crassa), Candida (C. albicans), Salmonella (S.
heidelberg), and in particular, Escherichia (E. coli).
This list should not be considered exhaustive.
~ ~ 3370 1 9 In accordance with the process of the present invention, the treatment zone is an aerobic environment wherein there are matrices adapted to support the selenium-reducing bacteria. As discussed above, these bacteria may be added to the treatment zone in a feedstock with the neutral solution. Alternatively, the selenium-reducing bacteria may be fed directly into the treatment zone independent of the neutral solution. The treatment zone may be any suitable structure or reactor, including but not limited to structures such as, for example, various vessels, tanks, earthen ponds and the like. The treatment zone is maintained under aerobic conditions, i.e. in the presence of free atmospheric oxygen, so that the aerobic selenium-reducing bacteria such as E. coli may metabolically reduce the selenite and selenate ions (selenium in the Se(IV) and Se(VI) oxidation states) to water insoluble selenium metal, which metal is encapsulated in the cellular structure of the bacteria contained on the matrix.
Various materials can be employed as the matrix support for the bacteria within the treatment zone. Preferably, these matrix materials must possess a porosity capable of allowing passage of the substantially neutral aqueous solution through the treatment zone and also must be substantially inert, i.e. neither destroy the bacteria per se, nor interfere with the bacteria's ability to metabolically reduce the water soluble selenium ions to water insoluble selenium metal. Representative, but non-limiting examples of such materials include gravel, sand, cellulose, glass, wool, glass beads and the like. Combinations of these various materials also may be used.
Most preferably, the treatment zone is a rotary biological contactor (RBC), although other g -systems involving continuous stirred tank reactors (CSTR) and immobilized cell, fluidized bed reactors may be used.
Rotary biological contactors have been used successfully in sewage treatment for at least a decade.
A variety of vessel designs are now available; however, the basic concept involves a trough in which a rotating shaft maintaining numerous discs of an inert support medium is slowly rotated. In the present invention, the shaft and disc assembly is half-immersed in the mixture of the substantially neutral aqueous solution and nutrient medium (feedstock from the m; X; ng zone). The biomass attach to the discs and sequentially pass through this "liquid" phase to adsorb nutrients, selenite and/or selenate and then pass through the air to adsorb atmospheric oxygen. The soluble selenium-containing solution is input at one end, traverses the longitudinal axis of the reactor and the effluent, comprising the flushed cells having intracellular selenium, is collected at the other end.
The selenium-cont~ n; ng cells may be separated from the effluent stream at the separation zone by numerous methods. The preferred method is selected from high-speed centrifugation, tangential flow filtration and flocculation and setting tO.l~ w/v potassium alum or 0.1% v/v Superfloc 320*). Each of these separation techniques is well known and applied in many areas of technology and, therefore, need not be expanded upon herein.
There are, within the scope of this invention, two preferred methods of recovering the insoluble selenium from the biomass at the mixing and setting zone:
*trademark 1) dilute alkali digestion of the biomass (solubilization of the cells or biomass), 2) solubilization of the metallic selenium.
The first preferred method involves digesting the biomass with, for example, 0.25 N caustic soda.
Another example of a dilute alkali which may be used within this invention includes KOH. As a general rule, many suitable digesting agents may be provided in the mixing and settling zone as long as the agent:
1) solubilizes the organic cellular components of the cells or biomass, 2) converts the red amorphous granules of selenium to the black crystalline form of selenium metal.
The preferred conditions for the digestion include a solid:liquid ratio of approximately 5g/L, and a temperature of between 105-110 C for a time of approximately 30 minutes. It is to be appreciated that the solid:liquid ratio may be increased or decreased but the digestion time is proportional to the solids content.
The second preferred method involves extracting the selenium from the cells with a metal solubilizing agent such as boiling Na2 S03 . Other suitable solubilizing agents include, but are not limited to SO2 and Na2HSO3. The addition of this agent into the mixing and setting zone, 18 is shown in the Figure. The boiled ex~ract may then be treated with alumina to adsorb the organic cellular components and filtered to remove the organic laden alumina. The selenium may be precipated by acidification of the filtrate with an agent such as H2 S04 . Other suitable precipitating agents include SO2, Na2SO3 and Na2HSO3.
_ _ 1 3370 1 9 The selenium may then be washed extensively with water, collected and dried as a fine powder. At this stage in the process, the organic-laden alumina may be treated with a hot alkali such as hot caustic soda to remove the organic cellular components which may subsequently or concurrently be recycled back to the neutralization zone or the treatment zone as a carbon source. The alumina may be re-used as an organic adsorbent.
During the course of the acidification and re-precipitation of the selenium in this second preferred method, SO2 gas may be evolved. It would be desirable to trap this gas by a gas washing process such as passage as fine bubbles through water or dilute alkali.
Alternatively, the selenium may be recovered from the biomass by oxidizing the biomass using boiling HNO3. Other oxidizing acids such as perchloric acid or aqua regia may be used with extreme caution. Once the biomass is completely oxidized, the selenium metal is re-precipitated by flushing the hot solution with SO2 or by the addition of Na2SO3. At temperatures above 85 C, re-precipitation will be as the black metallic selenium.
If the acid digest solution is cooled to less than 35 C
the selenium will precipitate in the red amorphous form.
Thus, in a preferred form, the process of the present invention is continuous. To that end, the liquid phase from the mixing and settling zone which comprises cellular components is recycled back to the neutralization zone as indicated by the dashed line in the Figure. Alternatively, the liquid phase may be recycled directly back to the treatment zone. This recycling step reduces the costs of the neutralizing agent and simultaneously provides an additional carbon source for the growing cells in the treatment zone.
~ _ 1 3370 1 9 Furthermore, this recycling step returns any organo seleno compounds back to the reductive step of the process (treatment zone) where they may undergo complete reduction to elemental selenium. The continuity of this process will significantly increase the percent recovery of the final product, selenium metal.
Preferably, the black selenium metal is washed for at least several cycles at the washing zone to remove soluble organic material and residual digesting agent, such as caustic soda.
The drying zone is preferably an evaporator.
The selenium metal, once washed and dried, may be heated to a molten state (approximately 218 C) and poured as ingots or sprayed into water for pelletization.
The rate of flow of the aqueous solution to be treated through the treatment zone will vary depending on the reactor capacity and design. In order to fully exemplify the process of this invention, rather than quote flow rates, it is more meaningful to disclose the retention time of the aqueous solution in the reactor or treatment zone. The retention time is the time required for one complete change of the reactor liquid volume.
The retention time may range from 2.5 to 16 hours, preferably 2.5 to 6 hours, and most preferably 2.5-4 hours, depending on the biomass loading within the reactor and the concentration of soluble selenium ions present in the influent solution.
The treatment zone, in which the biomass is retained, is maintained at temperatures ranging from about 0 C to about 45 C, preferably at temperatures from about 15 C to about 40 C and most preferably at temperatures from about 22 C to about 37 C.
The process of the present invention is particularly suited for the recovery of elemental selenium from aqueous solutions such as smelter and roaster weak acid effluents, water from a uranium, copper or molybdenum mining or leaching operation, mine seepage or drainage water or any other aqueous stream which contains selenium ions in the selenite Se(IV) and selenate Se(VI) oxidation states.
Preferred embodiments of this invention will now be described by way of the following, non-limiting examples:
Example 1 Selenite reducing organisms were grown in Trypticase Soy Broth (Baltimore Biological Co. Ltd.) containing 0.1% w/v SeO3 2 - ( 456 ppm Se) at 37 C. The growth, changes in pH, and deposition of intracellular selenium granules were followed over a 32-hour period.
The organisms were capable of reducing selenite to elemental selenium over the pH range of 6.0 to 8.5 with an optimal rate of reduction at pH 7Ø
Simultaneously, the cultures were examined and photographed under a phase contract microscope at various timed intervals. Granule deposition was observed from 4 to 12 hours after inoculation. When the selenite concentration had been substantially reduced, normal logarithmic growth of the cultures was observed.
A laboratory scale rotary biological contactor with 100 sq. ft. of disk surface and a 30 L capacity was ~ 1 3370 1 9 then used in a pilot-plant study for the removal of selenium from a base metal smelter weak acid effluent.
The weak acid effluent which was neutralized to pH 7.0 was combined with the microbial inoculum and a nutrient solution as an input stream to the rotary biological contactor (RBC). Tests were conducted to optimize parameters such as C:Se ratio, flow rate and retention time.
Using the RBC, soluble selenium levels in the effluent could be reduced by >95~ in a retention time of 4 hours, with a Se:BOD (biological oxygen demand) ratio of 1:20. The temperature was generally maintained at 22 - 23 C. The selenium-laden biomass which sloughed off the disks of the RBC was recovered from the RBC effluent by centrifugation.
The biomass-free effluent was discarded. The remaining biomass comprised the red, amorphous selenium granules.
It was found that a continuous inoculum of selenium reducing organisms and attendant nutrient requirements could be supplied by using raw sewage. In particular, a sewage to effluent (v/v) ratio of 1:10 was used.
Example 2 - Recovery of Selenium by Dilute Alkali Digestion of the Biomass A sample of the biomass comprising the red, amorphous selenium granules, prepared as in Example 1, was extracted by boiling in 0.25 N NaOH at a solid:liquid ratio of 5 g/L for 30 minutes.
The sample was cooled, placed in a large separatory funnel fitted with a stop-cock and side-arm 1.25" above the lower stop-cock. The black, elemental selenium settled in the cone, the soluble protein and 0.25 N NaOH were decanted through the side stop-cock, and the sample was repeatedly washed with distilled water, collected and dried. Chemical and FTIR (Fournier Transformed Infra Red Spectroscopy) analysis of this sample indicated a selenium purity of 99.1% with only a trace of organic contamination.
The chemical analysis is presented in Table 1 - hereinbelow:
Table 1 Analysis of the Bio-Recovered Selenium Element Se 99.1 Fe 0.015 Si 0.013 Mg 0.0013 Cu 0.003 Al <0.002 Example 3 - Recovery of Selenium from Biomass by Solubilization of Metallic Selenium Due to the solubility of selenium in hot Na2 S03, a sample of the biomass comprising the red, amorphous selenium granules, prepared as in Example 1, was extracted with boiling 1~ w/v Na2 S03 . The biomass residue was then removed by filtration, the filtrate was cooled and it was then acidified to re-precipitate the selenium. Using this procedure, elemental selenium of 50.6% purity was recovered. FTIR analysis of the sample indicated organic contamination, probably during acid precipitation, as indicated by amide peaks in the FTIR
spectrum. Two additional S-O peaks were also seen.
A two-step extraction with boiling 1~ w/v Na2 S03 resulted in the recovery of selenium of 93.6 purity. The FTIR spectrum for this sample showed a reduction in size of one of the amide peaks, but little change in the other peak. The S-0 peaks were reduced but still present, thereby reducing the metal purity.
Reverting to the one-step boiling 1~ w/v Na2 S03 extraction, the solution, after boiling, was filtered through a column of activated alumina to adsorb the organic cellular components. The filtrate was then acidified with H2 S04 to precipitate the selenium and the alumina was regenerated by extraction with dilute alkali. The alkaline organic solution was recycled to the RBC as a carbon source. Alternatively, the alumina could be thermally regenerated, but the organics will, in this instance, be lost as C02.
Example 4 A barrel of neutralized smelter weak acid effluent was received from a base metal mine and chemically analyzed. The results of this chemical analysis are presented in Table 2 hereinbelow:
Table 2 Chemical Analysis of Neutralized Base Metal Mine Weak Acid Effluent Element Cu 0 3 Pb 0.4 Zn 270 Se 33 0 Te <1.0 Seventy litres of this solution was treated in a 30 L capacity rotary biological contactor (RBC). The solution was combined with raw sewage in a 10:1 ratio and dry milk powder was added as a carbon source to increase the BOD to Se ratio to approximately 20:1.
The biomass was recovered from the remainder of the effluent by centrifugation at 12,000 rpm using a Sharples High Speed Continuous Flow Centrifuge at a rate of 1 L/min.
The biomass (~30 g wet wt) was extracted (or digested) in 1.5 L of 0.25 N NaOH. This resulted in a very viscous solution which retarded the settling of the selenium granules. For rapid settling of the elemental selenium, the biomass may be digested under the following conditions: 5 g of wet biomass per litre of 0.25 N NaOH.
The digested biomass, substantially free of intracellular selenium, was recycled back to the RBC as a nutrient for the continued process. Accordingly, there is no organic sludge to dispose.
Digestion of 5 g wet biomass per litre of 0.25 N NaOH resulted in a product of 99% purity. As seen in Table 3, the amounts of contaminating metals in the product are extremely low, and the organic content can be reduced under the digestion step by controlling the ratio of biomass to volume of caustic soda.
Table 3 Analysis of the Selenium Produced from a Base Metal Mine Effluent Element %
Se 88.6 Al Ca 0.035 Cr 0.010 Cu 0.010 Fe 0.21 Mg 0.006 Ni 0.001 Pb 0-005 Zn 0.005 Organic debris 10.085 The presence of organic components was confirmed by an FTIR spectrum analysis.
Claims (27)
1. A process for the recovery of water insoluble selenium metal from an aqueous solution comprising soluble selenium which comprises:
providing a substantially neutral aqueous solution, having a pH of between about 6 and 8.5, comprising 10-500 ppm soluble selenium, and associating said solution with an environment comprising aerobic biomass, wherein, said biomass is characterized by being capable of metabolizing said soluble selenium to water insoluble selenium metal.
providing a substantially neutral aqueous solution, having a pH of between about 6 and 8.5, comprising 10-500 ppm soluble selenium, and associating said solution with an environment comprising aerobic biomass, wherein, said biomass is characterized by being capable of metabolizing said soluble selenium to water insoluble selenium metal.
2. The process according to claim 1, wherein the biomass, after metabolizing the soluble selenium, is separated from the substantially neutral aqueous solution.
3. The process according to claim 2, wherein the water insoluble selenium metal is recovered from the biomass.
4. The process according to claim 1, wherein the environment comprises raw sewage.
5. The process according to claim 4, wherein the raw sewage is contained in a rotary biological contactor.
6. The process according to claim 1, wherein the pH
is about 7.
is about 7.
7. The process according to claim 2, wherein the biomass is separated from the substantially neutral aqueous solution by a technique selected from high-speed centrifugation, tangential flow filtration and flocculation.
8. The process according to claim 3, wherein the water insoluble selenium metal is recovered from the biomass by digesting the biomass to solubilize organic cellular components and separating the organic cellular components from the water insoluble selenium metal.
9. The process according to claim 8, wherein the biomass is digested with an agent selected from NaOH and KOH.
10. The process according to claim 3, wherein the water insoluble selenium metal is recovered from the biomass by solubilizing the selenium metal, separating organic cellular components of the biomass and re-precipitating the selenium metal.
11. The process according to claim 10, wherein the selenium metal is solubilized by the addition of an agent selected from Na2SO3, SO2 and Na2HSO3.
12. The process according to claim 11, wherein the agent is Na2SO3.
13. The process according to claim 10, wherein the organic cellular components are separated by adsorption onto an alumina column.
14. The process according to claim 10, wherein the selenium metal is re-precipitated by the addition of an agent selected from H2SO4, SO2, Na2SO3, and Na2HSO3.
15. The process according to claim 14, wherein the agent is H2SO4.
16. The process according to claim 3, additionally including the steps of washing and drying the water insoluble selenium metal.
17. The process according to any one of claims 1 through 16, wherein the aqueous solution comprises about 10 to 500 ppm soluble selenium.
18. The process according to claim 2, wherein the insoluble selenium metal is encapsulate within the biomass.
19. A continuous process for the recovery of water insoluble selenium metal from an aqueous solution comprising soluble selenium which comprises:
providing a substantially neutral aqueous solution, having pH of between about 6 and 8.5, comprising 10-500 ppm soluble selenium, providing an environment comprising aerobic biomass, wherein said biomass is characterized by being capable of metabolizing said soluble selenium to water insoluble selenium metal, continuously associating said substantially neutral aqueous solution with said environment, continuously separating from said environment an aqueous effluent comprising biomass in which is encapsulated water insoluble selenium metal.
providing a substantially neutral aqueous solution, having pH of between about 6 and 8.5, comprising 10-500 ppm soluble selenium, providing an environment comprising aerobic biomass, wherein said biomass is characterized by being capable of metabolizing said soluble selenium to water insoluble selenium metal, continuously associating said substantially neutral aqueous solution with said environment, continuously separating from said environment an aqueous effluent comprising biomass in which is encapsulated water insoluble selenium metal.
20. The process according to claim 19, including the step of continuously separating the biomass from the remainder of the aqueous effluent.
21. The process according to claim 20, including the step of separating the selenium metal from the biomass.
22. The process according to claim 21, wherein the biomass, thus separated from the selenium metal is recycled back to the environment comprising the aerobic biomass.
23. The process according to one of claims 1 or 19, wherein the biomass is selected from the following genera:
Corynebacterium, Neurospora, Candida, Salmonella and Escherichia.
Corynebacterium, Neurospora, Candida, Salmonella and Escherichia.
24. The process according to claim 19, wherein the environment comprises raw sewage.
25. The process according to claim 24, wherein the raw sewage is contained in a rotary biological contactor.
26. The process according to one of claims 1 or 19, wherein the biomass is E. coli.
27. The process according to one of claims 1 or 19, wherein the aqueous solution comprising soluble selenium is selected from mine tailings, smelter and roaster weak acid effluents, natural selenium containing aquafers and drainage water.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000580256A CA1337019C (en) | 1988-10-14 | 1988-10-14 | Biorecovery of selenium |
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000580256A CA1337019C (en) | 1988-10-14 | 1988-10-14 | Biorecovery of selenium |
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| CA1337019C true CA1337019C (en) | 1995-09-19 |
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2012079201A1 (en) * | 2010-12-17 | 2012-06-21 | General Electric Company (A New York Corporation) | Selenium recovery from bioreactor sludge |
| CN115159475A (en) * | 2022-07-20 | 2022-10-11 | 孙大利 | Method for extracting selenium from suaeda salsa |
-
1988
- 1988-10-14 CA CA000580256A patent/CA1337019C/en not_active Expired - Fee Related
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2012079201A1 (en) * | 2010-12-17 | 2012-06-21 | General Electric Company (A New York Corporation) | Selenium recovery from bioreactor sludge |
| EP2651817A1 (en) * | 2010-12-17 | 2013-10-23 | General Electric Company (a New York Corporation) | Selenium recovery from bioreactor sludge |
| EP2651817A4 (en) * | 2010-12-17 | 2014-06-18 | Gen Electric | Selenium recovery from bioreactor sludge |
| CN115159475A (en) * | 2022-07-20 | 2022-10-11 | 孙大利 | Method for extracting selenium from suaeda salsa |
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