NL8302069A - METHOD FOR CONVERTING GIPS INTO ELEMENTAL SULFUR, AND FOR PREPARING SULFUR HYDROGEN. - Google Patents

Description

Reg.no. 118769 / Me / EPl.

Process for converting gypsum to elemental sulfur, as well as for preparing hydrogen sulfide.

In Cork, Design and Utilization of a Bench Scale Gas Life Fermentor, 1981, Dev. Ind. Microbiol. 22: 733-728, a gas light fermenter has been described. In Figure 3, a two-step process is described in which sulfate is converted to S and then converted to elemental sulfur. Desulfovibrio desulfuricans is used in the first stage and a Chlorobium microorganism is used in the second stage. A similar system has been described by Cork and Cusanovich in "Continuous Disposal of Sulphate by a Bacteria-rial Mutualism", 1979, Dev. Ind. Microbiol. 20: 591-602. This reference mentions that sulfur removal is a problem in many engineering processes, including those involving gypsum. It is proposed to recycle Chlorobium biomass extract to act as a carbon source at the sulfate reduction stage. However, this extract provides only a small amount (less than 10%) of the reducing power provided by lactic acid.

In Cork and Cusanovich "Sulphate Decomposition: A Microbiological Process", Murr et al eds., Metallurgical 20 Applications of Bacterial Leaching and Related Microbiological Phenomena, biz. 207-222, Academic Press, New York, a Chlorobium / Desulfovibrio system is used to achieve 55% sulfate-sulfur conversions. The sulfate reduction reaction uses lactic acid as the primary energy source and proposes to recycle the chlorobium / biomass (containing lactic acid) to proceed the Desulfovibrio reaction.

In Cork "Acid Waste Gas Bioconversion - An Alternative to the Claus Process", Ind. Microbiol ·. Vol. 23, biz. 379-387, 1982, again describes the use of Chlorobiumthio-30 sulfatifilum to convert hydrogen sulfide from industrial waste gases into elemental sulfur. The used 6302069 I * t * \ · - 2 -

Chlorobium species forms acetic acid in the absence of light.

A detailed summary of the microbiology of sulfur conversions has also been performed by Cork in a 1978 dissertation entitled "Kinetics of Sulphate 5 Conversion to Elementar Sulfur by a Bacterial Mutualism: A Hydrometallurgical Application", University of Arizona. It is also known to use mixed cultures of sulfate-reducing bacteria. See Middleton et al. "Kinetics of Microbial Sulphate Reduction", J. Wat. Poll. Con. Fed. 49: 1659-10 1670, wherein the substrate is acetate. Middleton et al achieve complete oxidation of acetate to CO2 and the sulfate reducing bacteria were found to grow relatively quickly.

Several reactors are known which can be applied to sulfate-sulfur reactions. For example, see the use of fiber glass immobilized whole cells in the response described by Cork et al in a lecture at the Society for Industrial Microbiology Meeting on August 11-17, St. Paul, Minnesota, 1982 (to be published in Dev.

Ind. Microbiol. Volume 24).

With regard to the photolithotropic microorganism of the present invention, Sirevag et al. ("Synthesis, Storage and Degradation of Polyglucose in Chlorobium Thiosulphatophilum", Arch. Microbiol. 111: 239-224) have shown that amounts of synthesized carbon can be stored in the cell as polyglucose. In the absence of light, the polyglucose was used to form fatty acids, including acetic acid. The use of these organisms for the purification of acidic gases was described by Cork in the 1982 1982 article discussed above. In 1971, Pugsley et al (Removal of Heavy Metals from Mine Drainage in Colorado by Precipitation, Chem. Eng.

Ser. 67: 75-89) for a sulfate reduction method to be used in the treatment of mine drains in Colorado.

8302069

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The invention relates to a process for converting gypsum to elemental sulfur using a series of fermentation steps. However, for the fermentation stages it is necessary to solubilize the gypsum, so that the sulfate content is in an aqueous medium and is therefore susceptible to breakdown by microorganisms. As described below, the digestion of gypsum to obtain aqueous sulfate solutions is an important feature of the present invention. In addition to the new digestion process, however, it is possible to easily dissolve gypsum by adjusting the pH to about 11.5, for which purpose the gypsum can be contacted, for example, with an aqueous sodium carbonate solution.

In the first fermentation step, an aqueous gypsum solution is contacted with a sulfate-reducing, acetate-consuming microorganism to reduce sulfate anions to form hydrogen sulfide. In the second stage, the hydrogen sulfide is contacted with a sulfide-reducing photolithotropic acetate-producing microorganism to form elemental sulfur. By the term "photolithotropic" it is meant that the microorganism is capable of performing photosynthesis by converting atmospheric carbon (ie (X ^) into biomass. However, the organism also contains enzymes which function in dim light or in the absence of light under the degradation of the micro-organism and formation of organic compounds, for example fatty acids, such as acetic acid and lactic acid.

The method described above has several important advantages. The amount of sulfate to be converted to hydrogen sulfide is relatively high, ie the molar ratio of SO 2/1 2 S is about 0.5 to I. In addition, the effluent can be recycled from the second stage to the first stage for providing some or all of the organic carbon 8302069 4 ΐ * <Β a> - 4 - organic carbon required in the reduction. Calculated on the organic carbon, which is fed in the form of acetate (to the first stage) including that optionally provided by recycle, as described below, is the weight ratio of biomass (ie D. postgateii cells) to acetate about 0.003 to 0.1, ie, a highly efficient biocatalytic reduction of sulfate to H 2 S is achieved using acetate.

In the first fermentation step, hydrogen sulfide tends to collect in the fermentation broth and this reduces the efficiency of the fermentation reaction. The fermentation broth is therefore blown out with nitrogen onto another inert gas to remove. Due to the anaerobic nature of the reaction, the exhaust gas 15 must not be air or oxygen. Preferably, the fermentation stages utilize the recirculation of nitrogen, ie the nitrogen carrier gas is passed through the second stage reactor and then recycled through the reactor for the first stage.

The acetate-consuming microorganisms used in the invention belong to the genus Desulfo bacter. A strain which has proved to be especially suitable is Desulfobacter postgateii (for example the strain Stickney B. Desulfobacter postgateii). The Stickney BD postgateii organism is deposited with the American Type Culture Collection, 12301 Park Lawn Drive, Rockville, Maryland 20852, under number ATCC 39172. The anaerobic acetate-consuming postgateii organism, first deposited with a Wastewater treatment plant by sequentially transferring a first sludge inoculation into a modification of Starkey's medium with sodium acetate as the sole source of carbon and energy. After several months, the medium was examined microscopically and found to consist of a pure 8302069-5 *. culture of gram-negative, non-moving rods, in morphology similar to those described by Pfenning et al 1981, "Studies on Dissimilatory Sulphate-reducing Bacteria that Decompose Fatty Acids"; Archives -of Microbiology; 129; 5 395-400. Pfenning reported that this organism glowed on acetate and could be used to reduce sulfate.

The stoichiometry of the reaction with the Stickney B strain is as follows; 0

10 CH-C-OH + SO = 2HCO * + H »S

3 4 3 2

Fermentation of the Desulfobacter organism (eg D. postgateii) is accomplished under anaerobic conditions with stirring (eg 50 to 500 revolutions per minute) or other means of agitation. The fermentation is carried out at 25 to 40 ° C and at a pH of 6.5 to 8.0 (preferably 6.8 to 7.2). Diluted mineral acid (e.g. HCl) can be used to adjust the pH of the medium. The fermentation medium includes a carbon source (eg acetic acid, sodium acetate or other alkali metal acetates), a nitrogen source and other elements. Examples of suitable nitrogen sources are urea, ammonium salts of organic acids (for example ammonium acetate and ammonium oxalate and ammonium salts of inorganic acids, for example ammonium nitrate or ammonium chloride. The amounts of the carbon and nitrogen sources in the medium are 0.001 to 20 w / v%. Organic foods (eg corn soaking liquid, peptone, yeast extracts, soybean extracts) and / or inorganic elements (eg potassium phosphate, magnesium phosphate) and vitamins, such as biotin and thiamine, can be added to the medium. The H0S is added together with CO2 was continuously removed from the fermentation broth and contacted (together with the carrier gas) with the chlorobium organisms.

3302069 «- 6 - 9 t

It is preferable, however, to leave a small amount of H ~ S (eg, less than 0.017 w / v%) in the media to aid in maintaining anaerobic conditions and the flow rate of the carrier gas should be controlled accordingly. The acetate / SO4 molar ratio in the fermentation broth is generally kept at about 2 to 1, preferably 1.1 to 1.

The second fermentation step proceeds under anaerobic conditions using a sulfide-reducing organism, which can produce both acetate and elemental sulfur. It is advantageous if the elemental sulfur is secreted by the microorganism and can therefore be collected. Suitable organisms are available from the genus Chlorobium and in particular Chlorobium thiosulfoto-15 filum. A suitable strain of V. thiosulfatofilum is registered with the American Type Culture Collection, 12301 Park Lawn Drive, Rockville, Maryland, under ATCC number 17092

Fermentation of anaerobic, photosynthetic, "green" sulfur bacteria, Chlorobium, such as Chlorobium thio-20 sulfatofilum, can be accomplished under anaerobic conditions, as described in Cork "Acid Waste Gas Biocon-version - an Alternative to the Clause process", volume 23 1982 from Development in Industrial Microbiology and is considered to be fully incorporated in the present application. The 25 green sulfur bacteria can use H2S as their sole source of reducing electrons and CO2 (which are produced by the D. postageii organisms) according to the following equation: 30 2H ,, S + CO2 CH20 + 2S I120

In addition to CO2 and H.7S, the Chlorobium fermentation medium contains a source of nitrogen and other elements. Examples of suitable nitrogen sources are urea, ammonium salts of organic acids (for example ammonium acetate and ammonium oxalate) and ammonium salts of inorganic acids (for example ammonium sulfate, ammonium nitrate or ammonium chloride). The rates of CC ^ and I ^ S used per 24 hours are from about 0.088 to 1.76 w / v% for CO2 and from about 0.17 to 1.36 w / v% for CO2. S. Organic foods (e.g. corn steep liquor, peptone, yeast extract soybean extracts) and / or inorganic elements (e.g. potassium phosphate, magnesium sulfate), vitamins such as biotin and triamine and amino acids can also be added to the medium. For every gram of CO2 used, 1 to 5 grams of cells are formed and for every gram of I ^ S, about 0.9 to 1 g of elemental sulfur is formed, ie the H ^ S / S reaction has an efficiency of about 15 90 to 100.

The second fermentation step is carried out in two sub-steps, hydrogen sulfide is reduced to elemental sulfur and in the second sub-stage polyglucose is converted to acetate. The S reduction reaction has been described above. To initiate the acetate formation, the exposure of the fermentation broth is reduced below 3 and preferably below 0.03 watt / m and kept below this value for about 24 to about 48 hours for each liter of fermentation broth. When the light energy is reduced to the value described, it is believed that the S-reducing microorganism synthesizes enzymes that convert polyglucose into fatty acids, especially acetate. For an initial Chlorobium starch concentration of about 1 g per liter, the potential reducing power per SO 2 reduction in the fermentation broth is assumed to be between about 60 and about 807 (w / v base) after 24 hours at a bolion level of about 0.03 w / m. The "dark *" reaction is discussed in more detail in Sirevag et al 1977, Synthesis, storage and degradation 8302069 V * - 8 - * or polyglucose in Chlorobium thiosulphatophilum, Arch, Microbiol. Ill: 239-246.

If desired, after the "dark" reaction described above, the fermentation broth may be subjected to physical operations (eg centrifugation) to separate biomass from the effluent containing the acetate anions, which can then be recycled to provide a carbon source for the first fermentation stage. The recycled sulfate-reducing ability in the effluent generally has an organic content of 60 to 80% of the starch produced by Chlorobium. The sulfur reducing power / elemental sulfur molar ratio obtained in the second fermentation step is about 0.1 to about 0.5.

The sulfur * 15 formed in the second fermentation step is collected and can be used for many known purposes. For example, the sulfur can be reacted with oxygen and / or water to form sulfuric acid. This method is exothermic and provides usable energy. For example, gypsum, as described above, can be digested by reaction with a highly alkaline aqueous solution. This solution can be obtained by dissolving a sodium carbonate (for example and / or NaHCO4) in water. The sodium carbonate can also be formed by an energy-intensive process in which carbon dioxide, ammonia, water and NaCl are reacted to form sodium bicarbonate and ammonium chloride. The sodium bicarbonate is insoluble in the reaction mixture and precipitates and can be separated and heated to form sodium carbonate. Carbon dioxide is formed as a by-product and can be recycled for reaction with the ammonia, water and sodium chloride. The ammonium chloride can be reacted in May. lime and / or calcium carbonate for the preparation of ammonia for use in the above reaction. Alternatively, the reaction to form sodium bicarbonate may be carried out by passing carbon dioxide into an ammoniacal seawater solution, i.e. the reaction product of ammonia and seawater, whereby sodium bicarbonate precipitates. It is, of course, well known that sodium carbonate can be formed by passing CC 2 in a highly alkaline solution to form sodium bicarbonate, followed by heating to prepare soda (i.e., Na 2 CO 2). This is the well known Solvay process.

Example I - Method for solubilizing plaster •

A 100 mtl Na 2 CO 2 solution was prepared with a pH of 11.4. To this solution was added gypsum in an amount of 20 g / l (equivalent to mM SO4) with stirring at 25 ° C for 1 hour, to obtain a final pH of 10.4. After settling for 15-20 min, the solution was filtered to obtain a clear solution and the pH was adjusted to 7.1 to 7.3. This solution had an SO 2 concentration of 65 to 70 mM.

Example II - Conversion of gypsum to H 2 S

A fermentation medium was prepared with the following composition: 8302069 »- 10 - SL-7 solution ^ 1 ml (2)

Vitamin solution. . 5 ml

CaCO4 (or Na2S04) 3.75 g (3,912 g)

MgS04 1.50 g nh4ci 1.0 g K2HP04 0.496 g

CaCl2 0.08 g

Fe (NH4) 2 (SO4) 2.6H20 0.01 g CH3C00Na. 3H20 5.44 g KCl0.4 g ^ SL-7 solution HCl 7.0 ml

FeCl2.6H20 1.8 g

CoCl2.6H20 0.19 g

MnCl2.4H20 0.10 g

ZnCl2 0.07 g H3B03 0.062 g

Na2MoO4.2H20 0.036 g

NiCl2.6H20 0.024 g

CuC12.2H20 0.017 g

Add H2O to 1 liter, pH to 6.8-7.2 with HCl, Na2CO3 or KOH

8302069 -II- (2) r

Vitamin solution

Biotin '0.02 g PABA 0.02 g

Pantothenate 0.01. g 5 Pyridoxamine HCl 0.01 g

Nicotinic acid 0.005 g

Thiamine HCl 0.005 g

Vitamin B ^ 0.001 g

Adjust distilled water to 1 liter 10 pH to 7.3 and then autoclave when medium is cool. After dithionite add 30 ml

A 50% inoculation of a 48-hour culture of Desulfo-15 bacter posgateii, st. Stickney B., ATCC 39172 was added to the top medium and, while blowing with nitrogen gas, 30 mg / L sodium dithionite was added as a strong reducing agent to maintain anaerobic conditions in the reaction vessel. In addition, continuous nitrogen blowing (8 to 10 ml per minute) and low 1 ^ S content promote keeping oxygen contamination free and ensuring culture purity. The pH of the medium was maintained at 7.1 in certain experiments using a Chemtrix pH controller (model 45 AR) using 1N HCl as neutralizing agent. The culture was continuously stirred with a 200 rpm magnetic stirrer. and all experiments were performed at 30 ° C.

Simple discontinuous tests were performed to determine the conditions under which the organism 30 most efficiently reduced sulfate. In the first runs, round 1-liter flasks were used as discontinuous reactors, initially with N 2 was blown out to ensure anaerobic conditions and the pH was manually adjusted to 7.1 with HCl every 3302069 * - 12 * 12 hours. A 25% inoculum was used with an SO 2 initial concentration of 15.5 mM, but without any magnetic stirring or attempted continuous blow-out.

In some experiments, the pH was adjusted to the optimal value of 7.1 and the BSF-500 flask was switched. Thereafter, the difference in growth parameters was varied until a significant increase in the degree of sulfate production occurred (Table A). All tests were performed at a temperature of 30 ° C and an initial pH of 7.1 with a 1: 1 stoichiometry of sulfate to acetate.

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The most significant increase in the degree of SO 2 reduction occurred in Runs 2 and 3 when the pH was continuously monitored at 7.1, accompanied by stirring and between 15 Runs 5 and 6 where the rate of nitrogen blowout was controlled increased and the SO 2 contents were kept below 20 mM. Simultaneously, desulfobacter was observed to adhere to the sides of the glass reactor, as evidenced by blackening of the walls. A section of laminated fiber glass sheet was placed in a reactor in such a way that a wall was formed along the side of the vessel in an attempt to provide a firm support for immobilizing the bacteria. After inoculating the medium, as described above and with vigorous stirring of the culture liquid (200 rev.

25 per minute), the growth was allowed to proceed in the discontinuous phase until the sulfate values were reduced from an initial concentration of 32 mM to 2 mM after about 90 hours. At that time, the usually dark, cloudy medium was clear and the fiberglass support turned black. A medium stock of 24.5 mM SO4: 25 mM acetate is pumped into the immobilized whole cell reactor at hour 21 of the discontinuous growth to determine if the bound organism could further reduce sulfate.

The results gave a continuous 89.8% conversion of SO4 to 8302069.>> _________________ 9-13-1 'at a medium flow rate of 22.8 ml / h and a conversion of 83.7% at a flow rate of 30 .0 ml / hour. The test was terminated in the chemostat mode after 200 hours.

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Analytical Methods * S was determined by Pachmayr's (1960, Vorkommen und Bestimmung von Schwefelverbindungen in Mineral Wasser, thesis Munich (Univ.), While acetate 5 was determined according to the anxymatic reaction of IA Rose (1955, in SP Colowich & N. Kaplan: Method in Enzymology, Acedemic Press, New York, Vol. 1, Biz 591-593) Sulfate was determined by adding BaC solutions to acidified samples of the above culture and the turbidity obtained. of BaSO4 was determined spectrophotometrically according to MA

Tabatabai (1974, Determination of sulfate in water samples. Sulfur Inst. Journal. 10: 10-13). Glucose was determined by the Dow Diognostest method. After digestion and neutralization, a 100 ml sample is placed in the diagnostics flask, boiled for 10 min and the optical density is measured at 595 nm. Protein was determined by the Coomassie Brilliant Blue protein test, M. Bradford (1976. Analytical Biochemistry, 72: 248-254).

Example III - Conversion of gypsum to H 2 S in a continuous thermostat reactor

A 50% inoculation of Desulfobacter post-gateii was performed in a Virtis-pyrex glass BSF 500 reactor containing the medium of Example II and a fiber glass support. The pH was adjusted to 7.1 with an initial sulfate and acetate concentration of 13.5 mM each. The test was performed at 30 ° C with continuous nitrogen gas blowing at 10 ml per minute. The growth was discontinuous for about 72 hours. At that time, the sulfate chat was 1.1 mM and the bacteria were found to be bound to the fiber glass. A medium stock of 40 mM S0. and 80 mM acetate was then pumped into the 4 reactor at a rate of 20 ml per hour. After an initial rise in SO 2 to 3.6 mM, the reactor concentration dropped to 2.4 mM and remained at this value for more than 100 hours. During this time, the reactor liquid level remained constant at 600 ml. A net reduction of more than 57 mmoles of SO4 was achieved. At hour 276, the concentration of sulfate in the medium reservoir was increased to 60 mM with the addition of Na2SO4 crystals without interrupting the flow of the medium. The sulfate content in the reactor then rose from 1.1 mM to 1.8 mM and remained at this value for 46 hours, during which time 1.16 mmoles of SO2 were reduced per hour (97% conversion). The sulfate concentration in the medium stock was then increased to 80 mM at hour 336, resulting in a subsequent rise in SO 2 in the reactor to 8.4 mM. The concentration gradually decreased to 6.7 mM at 15 hours 462 and remained at this value for 115 hours. For this period, the reduction rate was 1.47 mmoles per hour, a conversion of 91.6%.

The acetate in the medium stock was increased to 180 mM at hour 602 in preparation for the cascading increase in SO 2 as previously described. However, the SO 2 content quickly increased to 24.5 mM and therefore the medium feed pump was turned off to allow the organism to grow discontinuously. The value of SO2 again became 2.5 mM at hour 708.

Example IV - Conversion of H 2 S into sulfur

A fermentation medium of the following composition was obtained: K ^ PO ^ 0.4; NH4 Cl 0.4; MgCl 4-6 O 2.0;

NaCl 1.6; CaCl0 0.016; FeS0 .7H_0 2 ml of 5% FeSO.ILO in 0.4N

Δ a L 4 2

30 HCl. Then the sponronolomonton leading edge solution A

• (1.0 ml / 1) and leading edge solution B (0.8 ml / 1) added to the minimum salt media. Stock solution Λ is a modification of Larsen's trace elements (1952) and consists of (g / l): FeCl ^ 1.6; 8302069 »·« - 17 -

Na2B407.10H2O 0.80; ZnSO 4 .7H 2 O 0.44; CoSO 4 <7H 2 O 0.24;

CuC12.2H20 0.135; MnSO3.H20 0.0165; Fe-EDTA solution 110 ml (Fe-EDTA solution is achieved by adding 24.9 g FeSO4.7H20 and 59 g EDTA to a total volume of 1000 ml and adjusting the 5 pH to 9 with 0.1N NaOH, 7). Trace element stock solution B, as described by Orsol et * al (1973) consists of (g / l): H3B04 2.90; MnCl2.4H2 1.80; ZnSO 4 .7H 2 O 0.22; CuSO 4 .5H 2 O 0.079; NaMoO4.2Ho0 0.25; NH4 VO3 0.023; Co (NO) 3) 2 · 6H 2 O 0.049. Finally, 33 ml of a 0.4M NaH2 PO4 stock solution and 67 ml of 0.4M Na2 HPO4 stock solution were added and the liquid volume was adjusted to 400 ml with deionized water. The preparation and composition of the above-existing fermentation medium is described in Cork "Acid Waste" Gas Bioconversion - an Alternative to the Clause Process ", 1982, Dev. Ind. Microbiol., Volume 23.

400 ml of a culture of Chlorobium thiosulfatofilum ATCC 17092 was added to 400 ml of the above medium. The resulting mixture was incubated at 37 ° C for about 24 hours. Then, H 2 S, formed as in Example 1, was introduced into the medium in an amount of 1.67 mmoles per hour. This can be accomplished by blowing with N2 in the form of a "Closed-Recycling" N2 blowing system. CO2 was also introduced at the same rate. This mixture was incubated at 30 ° C for about 24 hours with stirring (about 150 rpm). The reactor was kept under anaerobic conditions. A 250 watt lamp was placed 6 cm from the 1 liter Virtis reactor used for this reaction and exposed throughout the 24 hour run. Elemental sulfur precipitated in the medium and was collected, dried and weighed. After 12 and 24 hours, 18 and 36 millimoles of elemental sulfur were collected, respectively.

8302069

Claims (23)

1. Process for converting gypsum to elemental sulfur, characterized in that an aqueous gypsum solution is formed, this solution is contacted with a sulfate-reducing, acetate-consuming bacterium to reduce sulfate ions to hydrogen sulfide and contacting the H 2 S with an anaerobic, "green", sulfide-reducing photolithotropic, acetate-producing, to form elemental sulfur. « 2. Process according to claim 1, characterized in that elemental sulfur is collected from the H 2 S fermentation liquid. 3. Process according to claim 1, characterized in that the elemental sulfur is converted into sulfuric acid. Process according to claim 1, characterized in that the acetate-containing H 2 S fermentation liquid is recycled again to bring it into contact with the gypsum fermentation liquid. Method according to claim I, characterized in that the acetate-producing microorganism produces acetate in the absence of ultraviolet radiation. 6. A method according to claim 1, characterized in that the acetate-using microorganism is of the genus Desulfobacter. Method according to claim 6, characterized in that the microorganism is Desuifobacter postgateii. Method according to claim 7, characterized in that the microorganism is DesuiTobacter postgateii ATCC 39172. Process according to claim I, characterized in that the acetate-producing microorganism is of the genus Chlorobium. 10. A method according to claim 9, characterized in 8302069 - 19 w. * fr that the microorganism is Chlorobium thiosulfatfilum. 11. Process according to claim I, characterized in that the microorganism is Chlorobium thiosulfatofilum ATCC 17092. 12. A method according to claim 1, characterized in that the gypsum solution is formed by reacting gypsum with an aqueous sodium carbonate solution. 13. Process according to claim 12, characterized in that the sodium carbonate is formed by the reaction of water, CO2, NaCl and NH4. 14. Process according to claim 12, characterized in that sodium carbonate is formed by the reaction of carbon dioxide, seawater and ammonia. A method according to claim 12, characterized in that at least part of the energy requirement for carrying out the method is provided by oxidation of elemental sulfur to prepare sulfuric acid. 16. A method of solubilizing gypsum, characterized in that the gypsum is reacted with an aqueous sodium carbonate solution. 17. Process according to claim 16, characterized in that the sodium carbonate is formed by the reaction of water, CO2, NaCl and NH4. 18. Process according to claim 16, characterized in that the sodium carbonate is formed by the reaction of carbon dioxide, seawater and ammonia. 19. Process according to claim 16, characterized in that at least part of the energy requirement for carrying out the process is provided by oxidation of elemental sulfur to yield sulfuric acid. 20. A process for preparing hydrogen sulfide, characterized in that an aqueous gypsum solution is formed, and this solution is contacted with a sulfate-reducing 8302069 V; - 2ο - acetate-consuming microorganism to reduce sulfate anions and form hydrogen sulfide. 21. A method according to claim 20, characterized in that the microorganism is selected from the genus Desulfobacter. Method according to claim 20, characterized in that the microorganism consists of Desulfobacter postgateii. 23. A method according to claim 20, characterized in that the microorganism consists of Desulfobacter postgatii ATCC 39172. 8302069