Introduction

Currently, the number of materials used in electronic technology for various purposes, amounts to several thousand. According to the most general classification, they are divided into four classes: conductors, semiconductors, dielectrics and magnetic materials. Among the most important and relatively new materials are semiconductor chemical compounds, among which compounds of type A II B VI are of greatest scientific and practical interest. One of the most significant materials of this group is CdS.

CdS is the mainstay of modern IR technology, since its photosensitivity spectrum covers the atmospheric transparency window (8-14 µm), in which all objects emit environment. This allows it to be used in military affairs, ecology, medicine and other industries. human activity. Today, CdS is produced in film form by a hydrochemical method.

The purpose of this course project is to carry out a project for the production of sensitive elements of photoresistors based on CdS using the hydrochemical method with a productivity of 100 thousand units/year, as well as familiarization with the calculation method intended for the preliminary determination of the conditions for the formation of CdS, hydroxide and cadmium cyanamide.

Characteristics of cadmium sulfide

The diagram of the Cd - S system has not been constructed; the system has one compound CdS, existing in two modifications: b (hexagonal) and c (cubic). CdS occurs naturally in the form of the minerals greenockite and howleyite.

Crystal structure

Compounds of type A II B VI usually crystallize in the structure of sphalerite or wurtzite. The structure of sphalerite is cubic, type B-3, space group F4 3m (T d 2). The structure of wurtzite is hexagonal, type B-4, space group P 6 3 mc (C 6v 4). These structures are very similar to each other; they have the same number of atoms in both the first and second coordination spheres - 4 and 12, respectively. Interatomic bonds in tetrahedra of both modifications are very close.

Cadmium sulfide has been obtained with both sphalerite and wurtzite structures.

Thermodynamic and electrophysical properties

Cadmium sulfide is a one-sided phase of variable composition, always possessing an excess of cadmium. When heated to 1350°C, cadmium sulfide sublimates at atmospheric pressure, without melting, in a vacuum at 180? C it is distilled, without melting and without decomposition; under a pressure of 100 atm it melts at a temperature of about 1750? C. The degree of cadmium dissociation at temperatures above 1000°C reaches 85-98%. Heat of formation of CdS D H 298 0 = -34.71 kcal/mol.

Depending on the conditions of preparation and heat treatment, the properties of CdS may be different. Thus, crystals grown in an excess of cadmium vapor have a significantly higher thermal conductivity than crystals grown under conditions of stoichiometric composition. Resistivity of CdS depending on various factors can vary within wide limits (from 10 12 to 10 -3 ohm*m).

Deviations from stoichiometry have a decisive influence on the electrical properties of CdS. The introduction of oxygen into samples leads to a strong decrease in electrical conductivity. The bandgap of CdS, determined from optical data, is 2.4 V. Cadmium sulfide typically has n-type conductivity, which is due to the lack of sulfur relative to the stoichiometric composition.

The solubility of cadmium in water is insignificant: 1.5 * 10 -10 mol/l.

Traditionally, cadmium sulfide was used as a dye. It can be seen in the paintings of such great artists as Van Gogh, Claude Monet, Matisse. IN last years Interest in it is associated with the use of cadmium sulfide as a film coating for solar cells and in photosensitive devices. This connection is characterized by good ohmic contact with many materials. Its resistance does not depend on the magnitude and direction of the current. Thanks to this, the material is promising for use in optoelectronics, laser technology, and LEDs.

general description

Cadmium sulfide is an inorganic compound that occurs naturally in the rare minerals zinc blende and howleyite. They are of no interest to industry. The main source of cadmium sulfide is artificial synthesis.

By appearance this compound is a powder yellow color. Shades can vary from lemon to orange-red. Due to its bright color and high resistance to external influences Cadmium sulfide was used as a high quality dye. The substance became widely available starting in the 18th century.

Chemical formula compounds - CdS. It has 2 structural forms of crystals: hexagonal (wurtzite) and cubic (zinc blende). Under the influence of high pressure, a third form is also formed, like rock salt.

Cadmium sulfide: properties

A material with a hexagonal lattice structure has the following physical and mechanical properties:

• melting point - 1475 °C;
• density - 4824 kg/m3;
• linear expansion coefficient - (4.1-6.5) μK -1;
• hardness on the Mohs scale - 3.8;
• sublimation temperature - 980 °C.

This connection is a direct semiconductor. When irradiated with light, its conductivity increases, which makes it possible to use the material as a photoresistor. When doped with copper and aluminum, a luminescence effect is observed. CdS crystals can be used in solid-state lasers.

The solubility of cadmium sulfide in water is absent, in dilute acids it is weak, and in concentrated hydrochloric and sulfuric acid it is good. Cd also dissolves well in it.

The substance is characterized by the following Chemical properties:

• precipitates when the solution is exposed to hydrogen sulfide or alkali metals;
• when reacting with hydrochloric acid, CdCl 2 and hydrogen sulfide are formed;
• when heated in an atmosphere with excess oxygen, it oxidizes to sulfate or oxide (this depends on the temperature in the firing furnace).

Receipt

Cadmium sulfide is synthesized in several ways:

• during the interaction of cadmium and sulfur vapors;
• in the reaction of organosulfur and cadmium-containing compounds;
• precipitation from solution under the influence of H 2 S or Na 2 S.

Films based on this substance are produced using special methods:

• chemical precipitation using thiocarbamide as a source of sulfide anions;
• atomization followed by pyrolysis;
• the method of molecular beam epitaxy, in which crystals are grown under vacuum conditions;
• as a result of the sol-gel process;
• by ion sputtering method;
• anodizing and electrophoresis;
• by screen printing method.

To make the pigment, the precipitated solid cadmium sulfide is washed and calcined to obtain a hexagonal shape. crystal lattice and then ground to a powder.

Application

Dyes based of this connection have high heat and light resistance. Additives from selenide, cadmium telluride and mercury sulfide allow you to change the color of the powder to green-yellow and red-violet. Pigments are used in the production of polymer products.

There are other areas of application of cadmium sulfide:

• detectors (recorders) elementary particles, including gamma radiation;
• thin film transistors;
• piezoelectric transducers capable of operating in the GHz range;
• production of nanowires and tubes, which are used as luminescent tags in medicine and biology.

Cadmium sulfide solar cells

Thin-film solar panels are one of the latest inventions in alternative energy. The development of this industry is becoming increasingly important, as the reserves of minerals used to generate electricity are rapidly depleting. The advantages of solar cells based on cadmium sulfide are the following:

• lower material costs in their production;
• increasing the efficiency of converting solar energy into electrical energy (from 8% for traditional types of batteries to 15% for CdS/CdTe);
• the possibility of generating energy in the absence of direct rays and using batteries in foggy areas, in places with high dust levels.

The films used to make solar cells are only 15-30 microns thick. They have a granular structure, the size of the elements is 1-5 microns. Scientists believe that thin-film batteries in the future will be able to become an alternative to polycrystalline batteries due to their unpretentious operating conditions and long service life.

Introduction

Currently, the number of materials used in electronic technology for various purposes amounts to several thousand. According to the most general classification, they are divided into four classes: conductors, semiconductors, dielectrics and magnetic materials. Among the most important and relatively new materials are semiconductor chemical compounds, among which compounds of type A II B VI are of greatest scientific and practical interest. One of the most significant materials of this group is CdS.

CdS is the mainstay of modern IR technology, since its photosensitivity spectrum covers the atmospheric transparency window (8-14 µm), in which all environmental objects emit. This allows it to be used in military affairs, ecology, medicine and other branches of human activity. Today, CdS is produced in film form by a hydrochemical method.

The purpose of this course project is to carry out a project for the production of sensitive elements of photoresistors based on CdS using the hydrochemical method with a productivity of 100 thousand units/year, as well as familiarization with the calculation method intended for the preliminary determination of the conditions for the formation of CdS, hydroxide and cadmium cyanamide.

1. Characteristics of cadmium sulfide

The diagram of the Cd - S system has not been constructed; the system contains one compound CdS, existing in two modifications: α (hexagonal) and β (cubic). CdS occurs naturally in the form of the minerals greenockite and howleyite.

1.1 Crystal structure

Compounds of type A II B VI usually crystallize in the structure of sphalerite or wurtzite. The structure of sphalerite is cubic, type B-3, space group F4 3m (T d 2). The structure of wurtzite is hexagonal, type B-4, space group P 6 3 mc (C 6 v 4). These structures are very similar to each other; they have the same number of atoms in both the first and second coordination spheres - 4 and 12, respectively. Interatomic bonds in tetrahedra of both modifications are very close.

Cadmium sulfide has been obtained with both sphalerite and wurtzite structures.

1.2 Thermodynamic and electrophysical properties

Cadmium sulfide is a one-sided phase of variable composition, always possessing an excess of cadmium. When heated to 1350 ᵒC, cadmium sulfide sublimates at atmospheric pressure without melting; in a vacuum at 180 ᵒC it distills without melting and without decomposition; under a pressure of 100 atm it melts at a temperature of about 1750 ᵒC. The degree of cadmium dissociation at temperatures above 1000 ᵒC reaches 85-98%. Heat of formation of CdS Δ H 298 0 = -34.71 kcal/mol.

Depending on the conditions of preparation and heat treatment, the properties of CdS may be different. Thus, crystals grown in an excess of cadmium vapor have a significantly higher thermal conductivity than crystals grown under conditions of stoichiometric composition. The resistivity of CdS, depending on various factors, can vary within wide limits (from 10 12 to 10 -3 ohm*m).

Deviations from stoichiometry have a decisive influence on the electrical properties of CdS. The introduction of oxygen into samples leads to a strong decrease in electrical conductivity. The bandgap of CdS, determined from optical data, is 2.4 V. Cadmium sulfide typically has n-type conductivity, which is due to the lack of sulfur relative to the stoichiometric composition.

The solubility of cadmium in water is insignificant: 1.5 * 10 -10 mol/l.

2. Methods for obtaining metal chalcogenides

Currently, metal chalcogenides are produced both by physical (evaporation in a vacuum and cathode sputtering) and chemical methods (aerosol spraying of the reaction mixture onto a substrate heated to 400-600 K or deposition from an aqueous solution). Let's look at each method in more detail.

Vacuum condensation method

The essence of the method is to heat the substance in a vacuum (P ≥ 10 -3 mm Hg) to a temperature when the pressure exceeds the pressure of the residual vapor by several orders of magnitude, followed by condensation onto the substrate.

Process steps:

Evaporation of a substance;

Flight of atoms of a substance to the substrate;

Deposition (condensation) of vapor on a substrate with subsequent formation of a film structure.

Cathode vacuum sputtering method.

The method is based on the destruction of the cathode when it is bombarded with molecules of the working gas. The material that is to be deposited in the form of a film is used as a cathode. First, air is pumped out from the working area, then working gas (argon or nitrogen) is introduced into the chamber. A voltage (3-5 kV) is applied between the cathode and anode, which causes breakdown of the gas gap. The operation of the installation is based near a plasma discharge.

Types of cathode sputtering:

Physical: no chemical reactions occur in the system;

Reactive: assumes chemical reaction, a reactive gas (oxygen, nitrogen, carbon monoxide) is added to the working gas, with whose molecules the sprayed substance forms chemical compound. By changing the partial pressure of the working gas, the composition of the film can be changed.

It is worth noting that vacuum production of thin-film structures has wide capabilities and versatility. It has a number of significant disadvantages - it requires complex, expensive equipment, and also does not ensure uniformity of properties.

The most attractive method for producing sulfide films in terms of its simplicity and efficiency is the technology of hydrochemical deposition. Currently, there are three main varieties of this method: chemical deposition from solutions, electrochemical deposition, and spraying solutions onto a heated substrate followed by pyrolysis.

Electrochemical deposition involves anodic dissolution of the metal in an aqueous solution of thiourea. The process of sulfide formation occurs in two stages:

formation of metal ions at the anode;

interaction of metal ions with chalcogenizer.

Despite the advantages of the method: controllability and a clear dependence of the film growth rate on the current strength, the method is not sufficiently economical; thin films with uneven properties and amorphous are formed, which prevents the widespread use of this method in practice.

Method of spraying a solution onto a heated substrate (pyrolysis)

A solution containing a metal salt and thiourea is sprayed onto a substrate heated to 180..250 ᵒC. The main advantage of the pyrolysis method is the possibility of obtaining films of mixed composition. The hardware includes a spraying device for solutions and a heater for the substrate. To obtain films with metal sulfide, the stoichiometric metal-sulfur ratio is optimal.

Chemical deposition from aqueous solutions. The hydrochemical deposition method is characterized by high productivity and efficiency, simplicity of technological design, and the possibility of applying films to the surface complex shape and various natures, as well as doping of the layer with organic ions or molecules that do not allow high-temperature heating, and the possibility of “mild chemical” synthesis. The latter allows us to consider this method, as the most promising for the preparation of metal chalcogenide compounds of complex structure that are metastable in nature.

Hydrochemical deposition is carried out in a reaction bath containing a metal salt, an alkaline and complexing agent, and a chalcogenizer. The process of sulfide formation is realized through a colloid-chemical stage and represents a set of topochemical and autocatalytic reactions, the mechanism of which is not fully understood.

3. Application of films based onCdS

Thin-film cadmium sulfides are widely used as photodetectors, photoluminescent materials, thermoelements, solar cells, sensor materials, decorative coatings, and promising nanostructured catalysts.

4. Description of production technologyCdS

The technological scheme for manufacturing sensitive elements of photoresistors includes the following operations:

1. preparation of substrates (cleaning, etching, washing);

Chemical deposition of semiconductor film;

Washing and drying the film;

Heat treatment of the semiconductor layer under the charge layer at 400 ᵒC for 2 hours;

Vacuum application of Au contacts;

Scribing;

Output control of parameters of FR chips.

.1 Preparation of substrates for film deposition

Film deposition is carried out on previously degreased substrates. The substrates are thoroughly degreased with soda, washed with tap water and, after installation in a fluoroplastic device, placed for 20 seconds in a diluted Dash solution to etch the surface in order to increase film adhesion. After treatment in Dash's etchant, the substrates are rinsed with a large amount of heated distilled water and stored in a glass under a layer of distilled water before the start of the process.

The quality of substrate surface preparation is controlled by the degree of its wettability: distilled water spreads in an even layer on a carefully prepared substrate. It is strictly forbidden to handle the fat-free substrate with your hands.

4.2 Chemical deposition of semiconductor film

Glass ceramic is used as a substrate material for deposition of CdS films.

The following chemical reagents are used to synthesize CdS semiconductor films:

cadmium chloride, CdCl 2 ∙H 2 O;

thiourea, CSN 2 H 4, special purity;

aqueous ammonia solution, NH 3 aq, 25%, chemical grade.

The procedure for draining reagents to prepare a working solution is strictly fixed. The need for this is due to the fact that the process of deposition of chalcogenides is heterogeneous, and its rate depends on the initial conditions of the formation of a new phase.

The working solution is prepared by mixing the calculated volumes of the starting substances. Film synthesis is carried out in a 100 ml molybdenum glass reactor. First, the calculated volume of cadmium salt is added to the reactor, then aqueous ammonia is introduced and distilled water is added. Next, thiourea is added. The solution is mixed and the prepared substrate, fixed in a fluoroplastic device, is immediately immersed in it. The substrate is installed in the reactor with the working surface down at an angle of 15 - 20°. From this moment, the time of the synthesis process begins to be counted using a stopwatch. The reactor is tightly closed and placed in a U-10 thermostat. The accuracy of maintaining the synthesis temperature is ±0.01°C. For some time, no changes occur in the solution. Then the solution begins to become cloudy, and a yellow mirror film forms on the surface of the substrate and the walls of the reactor. Its deposition time is 60 minutes. Precipitation is carried out at a temperature of 70 °C.

4.3 Processing of deposited film

After the end of the specified synthesis time, the reactor is removed from the thermostat, the substrate with the holder is removed and they are washed with a large amount (0.5-1.0 l) of heated distilled water. After this, the substrate is removed from the holder, the working surface of the substrate (the one on which the film was deposited) is carefully wiped with cotton wool soaked in distilled water, and the sediment is removed from the back side. Then the substrate with the film is washed again with distilled water and dried on filter paper until visible traces of moisture are removed.

4.4 Heat treatment

Thoroughly washed and dried, the substrates are sent to the next operation: heat treatment. It is carried out in muffle furnaces PM-1.0-7 or PM-1.0-20 to eliminate stress and improve the electrical properties of films. The process lasts 2 hours at a temperature of 400 °C, followed by cooling to room temperature.

4.5 Vacuum application of Au contacts

Metal films are used in production semiconductor devices and microcircuits as non-rectifying (ohmic) contacts, as well as passive components (conductive tracks, resistors, capacitors, inductors). The main method for producing metal films is vacuum deposition (thermal evaporation in a vacuum) of various metals (aluminum, gold, etc.), since it has a number of advantages: purity and reproducibility of deposition processes, high productivity, the possibility of deposition of one or more metals onto semiconductor wafers in one operation and fusion of the sprayed metal film and vacuum to protect it from oxidation, ease of control of the spraying process and the ability to obtain metal films of various thicknesses and configurations when spraying metals using masks.

Spraying is also carried out in a vacuum installation with a residual pressure under the hood of about 6.5∙10 Pa (5∙10 -6 mm Hg). This pressure is chosen so that there are no collisions between the evaporated metal atoms and the molecules of the residual gas under the installation hood, which lead to the formation of films of a disrupted structure.

In the production of semiconductor devices, for the deposition of various films on semiconductor wafers and other substrates, several models of vacuum deposition installations are used, which differ from each other in various design solutions, primarily the under-cap device, as well as a vacuum system, a power supply system for monitoring process parameters and controlling operating modes , transporting and auxiliary devices for evaporation or spraying.

For thermal deposition of films and sputtering, resistive and electron beam devices are used in these installations, respectively; for sputtering by ion bombardment, discharge devices are used. Despite some disadvantages (difficulty in evaporating refractory materials, high inertia, changes in the ratio of components during evaporation of alloys), installations with electron beam and especially resistive evaporators are quite widely used in semiconductor production due to the ease of their operation. Therefore, we will focus on installations with resistive evaporators, the base model of which is the UVN-2M installation.

4.6 Scribing

Chips of a given size are cut from a substrate with a film deposited on it by scribing (time standard 25 minutes per substrate). Semi-automatic scriber LCD 10.11 is designed for applying a grid of marks on semiconductor wafers. The plates with the marks are broken by rolling them with a rubber roller manually or using special machines. The semiautomatic device is installed in a spacesuit fixed to the table, which serves to create a microclimate. They work semi-automatically, wearing rubber gloves built into the front wall of the spacesuit. Illuminated workplace fluorescent lamps installed in the upper part of the spacesuit. The marks are applied using a diamond cutter mounted in a swinging support.

cadmium sulfide electrophysical vacuum

4.7 Output control of “chip” parameters

Initially, the chips are subjected to visual inspection for the quality of the coating. Inhomogeneities in the layer, spots, irregularities, and areas with poor adhesion are noted.

Output control is carried out using K.50.410 installations (time standard 2 minutes per “chip”).

5. Calculation part

.1 Calculation of boundary conditions of formationCdS, Cd(OH) 2 andCdCN 2

It is necessary to find the boundary conditions for the deposition of lead sulfide, hydroxide, and lead cyanamide at the following initial concentrations, mol/l:

0,4

Hydrochemical synthesis is based on the reaction:

CdL x 2+ + N 2 H 4 CS(Se) + 4OH - = CdS+ CN 2 2- + 4H 2 O

In the reaction mixture, the formation of the following complex compounds is possible (Table 1):

Table 1 Initial data for calculating the conditions for hydrochemical deposition of CdS, Cd(OH) 2, CdCN 2

Compound (complex ion)

Let's calculate α Me z + , for this we use the expression:

where α Me z + is the fractional concentration of uncomplexed metal ions; L is the ligand concentration; k 1, k 1.2,…k 1.2… n - instability constants of various complex forms of the metal.

For the ammonia system the expression is:
8,099∙10 -9

Let's plot the graphical dependence pC n =f (pH) (Fig. 2).

Rice. 2. Boundary conditions for the formation of cadmium sulfide, hydroxide and cyanamide.

Based on the graph, we can conclude that in this system the formation of a CdS film is possible at pH = 9.5-14, Cd(OH) 2 at pH = 10.5-14, and CdCN 2 is not formed at all.

The invention can be used in inorganic chemistry. A method for producing crystalline cadmium sulfide involves placing sulfate-reducing bacteria in a synthetic medium containing metals and adding nutrients, including solutions of vitamins, salts, and cofactors. When cultivating, sulfate-reducing bacteria Desulfovibrio sp. are used. A2, and a synthetic medium containing a source of cadmium ions - a solution of cadmium chloride. The concentration of cadmium ions in the synthetic medium is 150 mg/l. Aluminum foil is placed in the cultivation container, and cultivation is carried out at a temperature of 28°C for 18 days. The sediment containing cadmium sulfide crystals collected from the foil and from the bottom of the bottle is dried. The invention makes it possible to obtain cadmium sulfide from wastewater and liquid waste from metallurgical enterprises. 2 ill., 3 tables, 1 ex.

Drawings for RF patent 2526456

The invention relates to a method for producing pure cadmium sulfide (CdS) from solutions containing metals using sulfate-reducing bacteria (SRB).

The proposed method can be used to obtain pure cadmium sulfide from wastewater containing metal ions, including cadmium, and liquid waste from mining and processing metallurgical enterprises. When applying the proposed method, selective precipitation of cadmium in the form of sulfides is possible. This feature allows the use of liquid waste from metallurgical enterprises and wastewater as secondary source raw materials for the production of cadmium sulfides. Cadmium sulfide is used in semiconductor lasers and is a material for the manufacture of photocells, solar cells, photodiodes, LEDs, phosphors, pigments for art paints, glass and ceramics. Cadmium sulfide pigments are valued for their good temperature stability in many polymers, such as engineering plastics. By replacing part of the sulfur atoms with selenium in CdS crystals, a wide variety of dye colors can be obtained from green-yellow to red-violet. Cadmium sulfide is a wide-gap semiconductor. This property of CdS is used in optoelectronics, both in photodetectors and solar cells. Scintillators are made from cadmium sulfide single crystals to detect elementary particles and gamma radiation.

In nature, cadmium sulfide exists in the form of the minerals greenockite and howliite, which occur as yellow plaque on sphalerite (ZnS) and smithsonite. Since these minerals are not widely distributed in nature, cadmium sulfide is obtained by synthesis for industrial use and scientific and technical work.

Cadmium sulfides are obtained by chemical methods - heating sulfur with cadmium or passing hydrogen sulfide over cadmium, cadmium oxide or chloride when heated. There is a known method for producing powdered cadmium and lead sulfides (RF patent, No. 2203855, C01G 11/02, C01G 21/21, 2003). The invention relates to methods for producing powder materials in molten salts. The synthesis is carried out in a molten medium. The molten medium is formed by crystalline thiourea, and includes anhydrous cadmium or lead acetates as a metal-containing component. The synthesis is carried out by mixing powders of one of these salts and thiourea at a 2-4-fold molar excess of thiourea and further holding at 160-180°C for 20-30 minutes. The practical yield of products obtained by the proposed method is over 95%. In addition, they contain an admixture of elemental sulfur (3-4 wt.%), which, depending on the further use of the product, can be removed by washing with an organic solvent (toluene, carbon tetrachloride, etc.). The disadvantages of this method are the energy consumption of production and the need to use special, expensive equipment. In addition, chemical production has a negative impact on the environment.

The formation of cadmium sulfide crystallites on the cell surface by the bacteria Klebsiella pneumonia and Clostridium thermoaceticum is known (Aiking H. et al. Detoxification of mercury, cadmium, and lead in Klebsiella aerogenes NCTC 418 growing in continuous culture // Appi Environ Microbiol. 1985 Nov;50(5 - P.1262-1267; P.R. Smith et al. PHOTOPHYSICAL AND PHOTOCHEMICAL CHARACTERIZATION OF BACTERIAL SEMICONDUCTOR-SULFIDE PARTICLES // Journal of the Chemical Society. - 1998, 94 (9). ).

CdS crystallites synthesized on the surface of the bacterium K. pneumonia effectively absorb UV light, which protects the bacterium from its harmful effects. The deep-sea fluorescent bacterium Pseudomonas aeruginosa removes cadmium from the environment by forming CdS crystallites on the cell wall (Wang C.L. et al. Cadmium removal by a new strain of Pseudomonas aeruginosa in aerobic culture // Appl. Environ. Microbiol. - 1997, 63. - pp .4075-4078). The sizes of cadmium sulfide crystallites vary from tens of microns outside cells to tens of angstroms inside cells or on their surface. Cadmium sulfide crystallites are formed only in certain conditions for transfer by organisms unfavorable conditions environment.

The closest in essence and achieved result to the claimed invention is a method for removing low concentration cadmium ions using a bioreactor with sulfate-reducing bacteria (Hiroshi H. et al. Removal of Low Concentrated Cadmium Ions Using Fixed-bed Sulfate-Reducing Bioreactor with FS Carrier // Journal of the Mining and Materials Processing Institute of Japan. - 2003. - V.119, No. 9. - pp.559-563). The recovery of heavy metal ions from water took place in a bioreactor using sulfate-reducing bacteria immobilized on fibrous slag, which was used as a biocarrier. In this process, sulfate ions in the liquid are biologically converted to hydrogen sulfide (H 2 S), which reacts with metal ions to form ultrafine metal sulfide particles. Then the resulting particles collect on the surface of the carrier in the upper part of the reactor, resulting in the accumulation of heavy metal ions and their sulfides. When continuously treating water contaminated with 6 mg/l of cadmium, almost complete removal was achieved over a period of about 30 days.

The disadvantage of this known method is that its use is possible only at low concentrations of cadmium ions in the medium and crystalline cadmium sulfide is not formed.

The objective of the present invention is to develop a method for producing crystalline cadmium sulfide from solutions with a high content of cadmium ions (up to 150 mg/l), which does not contain impurities of other metal sulfides, using sulfate-reducing bacteria resistant to elevated concentrations of cadmium ions.

The problem is solved by placing SRB, highly resistant to cadmium ions, in a synthetic medium simulating wastewater containing metals, with the addition of nutrients, including solutions of vitamins, salts, cofactors, lactate, sodium sulfide, with further cultivation in a thermostat and drying, but , unlike the prototype, SRB is used that is resistant to cadmium ions, aluminum foil is added to the medium, and cultivation is carried out at a temperature of 28°C for 18 days.

Cultivation is carried out in a synthetic medium (Table 1 - composition of the synthetic medium) with the addition of nutrients that stimulate bacterial growth. Nutrients and divalent cadmium are added to the synthetic medium before sowing the bacterial culture. The composition of nutrients and the sequence of their addition are shown in Table 2. All nutrients, except vitamins, are autoclaved at 1 atm for 30 minutes. Vitamins are sterilized by filtration using a bacterial filter (0.20 microns).

Sowing is carried out in sterile containers with inserted foil, the volume of inoculum (SRB culture) in an amount of 10% of the volume of the container. Containers with inoculum are filled to the top with synthetic medium (with all added nutrients). The pH of the medium is adjusted to 7.0-7.8 with NaHCO 3 solution. The bottles are closed with aluminum caps, sealed and placed in a thermostat at a temperature of 28°C. The formation of cadmium sulfide crystals occurs on the foil and partially at the bottom of the bottle. After cultivation, the precipitate is collected from the foil and from the bottom of the vial by centrifugation and dried in air. Examples of implementation of the invention in laboratory conditions are given below.

A pure culture of SRB Desulfovibrio sp. A2 was cultured on a synthetic medium containing divalent cadmium at a concentration of 150 mgCd/L and aluminum foil. Cadmium sulfide crystals were obtained on the foil and partially at the bottom of a 120 ml bottle. Vials with aluminum foil were sterilized by dry heat in a sterilizer at 160°C for 2.2 hours.

Sowing was carried out in a sterile laminar flow hood, which was previously disinfected with ultraviolet light for 30 minutes. Before sowing, the synthetic medium (Table 1) was brought to a boil and then quickly cooled under running cold water to remove dissolved oxygen. Chilled until room temperature the medium was supplemented with nutrients (table 2) (per 1 liter) in the following sequence: vitamins (2 ml), salt solution (10 ml), cofactor solution (1 ml), organic substrate - lactate (1.6 ml), NaHCO 3 solution (pH adjusted to 7.0-7.8), sodium sulfide solution (2 ml). A stock solution of cadmium (CdCl 2 × 2.5H 2 O 2 g per 100 ml of water) was added in an amount of 16.72 ml per 1 liter of synthetic medium (thus, a cadmium concentration in the medium of 150 mg/l was achieved).

About 50 ml of synthetic medium with additives added to it and 10 ml of inoculum (bacterial culture) were added to vials with foil, after which the medium was added to the top. Rubber stoppers were ground to the edges of the vials using a sterile needle, which reduced the likelihood of penetration of air oxygen. At the end of sowing, the bottles were closed with aluminum caps, the bottle was sealed with a seaming machine and the thermostat was placed at a temperature of 28°C. Crystallization of cadmium sulfide begins after 10 days of cultivation; with cultivation for 18 days, cadmium sulfide crystallizes completely. The formed precipitate was collected from the foil and from the bottom of the vial by centrifugation and dried in air. The mass of the formed sediment is 0.38 g.

The formed sediments were studied using scanning electron microscopy (Philips SEM515 with EDAX ECON IV analyzer). The crystalline phase was determined by X-ray phase analysis using a Shimadzu XRD 6000 diffractometer.

The size of the crystals, determined under a scanning electron microscope, was 50-300 μm, Figure 1 - micrographs (SEM) of sediments obtained during the cultivation of Desulfovibrio sp. A2 in the presence of Cd ions (150 mg/l) for 18 days, and the corresponding emf. Precipitates obtained by cultivating the strain Desulfovibrio sp. A2, contained cadmium, sulfur, iron, oxygen, carbon and sodium, with the carbon and oxygen coming from the carbon substrate on which the sample lay. The ratio of elements is presented in Table 3 - elemental composition of sediments obtained during the cultivation of Desulfovibrio sp. A2 in the presence of Cd ions (150 mg/l) for 18 days (elements C and O come from the substrate on which the sample lay).

When studying sediments using X-ray phase analysis, the formation of crystalline cadmium sulfide was shown within 18 days (Figure 2 - diffraction pattern of sediments obtained by cultivating Desulfovibrio sp. A2 in the presence of an initial concentration of Cd (150 mg/l) for 18 days. Designations on the diffraction pattern : CdS - cadmium sulfide).

In control sediments obtained during incubation without adding inoculum, no crystalline phase was observed and the main elements were cadmium and oxygen. The method we propose includes the possibility of using wastewater and liquid waste from mining and processing metallurgical enterprises as a synthetic medium for the production of cadmium sulfide.

Table 1
Reagent	Concentration, mg/l
Na2SO4	4000
MgCl 2 6H 2 O	400
NaCl (25%)	0,0125*
FeSO 4 *7H 2 O	2,1
N 3 VO 3	0,03
MnCl 2 *4H 2 O	0,1
CoCl 2 *6H 2 O	0,19
NiCl 2 *6H 2 O	0,024
CuCl 2 *2H 2 O	0,002
ZnSO 4 *7H 2 O	0,144
Na 2 MoO 4 *2H 2 O	0,036
CuSO 4 *7H 2 O	750
H2O	1 l
* - ml/l

table 2
Solution (amount applied per 1 liter of synthetic medium)
	Reagent	Concentration
	4-aminobenzoic acid	4 mg/l
	Biotin (vitamin H)	1 mg/l
	Nicotinic acid (vitamin B 5)	10 mg/l
1. Vitamins (2 ml/l)	Calcium pantothenate (vitamin B 3)	5 mg/l
	Pyridoxine dihydrochloride (vitamin B 6)	15 mg/l
	Cyanocobalamin (vitamin B 12)	5 mg/l
	Thiamine (vitamin B 1)	10 mg/l
	Riboflavin (vitamin B 2)	0.5 mg/l
	Folic acid	0.2 mg/l
	KH 2 PO 4	20 g/l
	NH4Cl	25 g/l
2. Salt solution (10 ml/l)	NaCl	100 g/l
	KCl	50 g/l
	CaCl2	11.3 g/l
	H2O	1 l
3. Cofactor solution (1 ml/l)	NaOH	4 g/l
	Na 2 SeO 3 × 5H 2 O	6 mg/l
	Na 2 WO 4 × 2H 2 O	8 mg/l
4. Lactate solution (1.6 ml/l)
	Lactate	40%
5. Na 2 S solution (2 ml/l)
	Na 2 S×9H 2 O	4.8 g

Table 3
Element	Weight fraction (Wt%)	Atomic fraction (At%)
WITH	7,56	15,1
O	2,75	4,1
Na	0,41	0,4
S	23,3	44,5
Cd	64,7	35,4
Fe	1,28	0,5

CLAIM

A method for producing crystalline cadmium sulfide by placing sulfate-reducing bacteria in a synthetic medium containing metals with the addition of nutrients, including solutions of vitamins, salts, cofactors, characterized in that sulfate-reducing bacteria Desulfovibrio sp. are used during cultivation. A2, use a synthetic medium containing a source of cadmium ions - a solution of cadmium chloride, and the concentration of cadmium ions in the synthetic medium is 150 mg/l, while aluminum foil is placed in the cultivation container, cultivation is carried out at a temperature of 28°C for 18 days, and The sediment containing cadmium sulfide crystals collected from the foil and from the bottom of the bottle is dried.