MICROBIOLOGY - M.H. Serhiichuk - 2008

Class="center">Chapter 12. HUMAN USE OF MICROORGANISMS

Biogeotechnologies

The geochemical activity of microorganisms is utilized in various biotechnologies, including hydrometallurgy (bacterial leaching of metals, sorption of metals from solutions), desulfurization of coal, reduction of methane concentrations in mines, and enhanced oil recovery.

The ability of microorganisms to assimilate hydrocarbon gases has been successfully applied in microbiological prospecting for oil deposits.

Hydrometallurgy. Bacterial leaching involves the extraction and solubilization of specific chemical elements from solid Materials—such as rocks, ores, and concentrates—through the action of Bacteria and their metabolites.

Metals are constituents of sulfide, silicate, and carbonate minerals, while some form oxides, exist in a Native State, or occur as inclusions within minerals. Consequently, the mechanisms of leaching and The Role of microorganisms in metal extraction vary.

The leaching of metals from ores has been practiced by humans for centuries. As early as the 16th century, copper was leached in Hungary, Germany, and Spain, although the underlying mechanism was unknown. Microbiological leaching began to develop and expand significantly following the Isolation of the bacterium Thiobacillus ferrooxidans from acidic mine waters in 1947.

Today, specific microorganisms are employed in hydrometallurgy (Table 12.6).

Table 12.6. Applications of microorganisms in hydrometallurgy

The aerobic bacterium Thiobacillus ferrooxidans is the most widely used microorganism in metal leaching technologies. It is a Gram-negative, motile, non-spore-forming rod with a polar flagellum (Fig. 12.6, a). As an obligate autotroph, its carbon source is carbon dioxide. It derives energy from The oxidation of various sulfur compounds, sulfide ores, and several variable-valence elements: Fe²⁺, Cu⁺, Zn²⁺, Sb³⁺, Se^2-, and U⁴⁺. Optimal growth conditions are pH 1.3–4.5 and a Temperature of 30–35 ⁰С. Other thiobacillus species are incapable of oxidizing ferrous iron, yet they actively oxidize various sulfur compounds (Fig. 12.6, b).

Fig. 12.6. a — Thiobacillus ferrooxidans (x90,000); b — Thiobacillus neapolitanus (x9,000); c — Leptospirillum ferrooxidans (x90,000);

d — Desulfovibrio sp. (x90,000)

Leptospirillum ferrooxidans is an obligate autotrophic bacterium that oxidizes Fe²⁺. It is a small spirillum with a polar flagellum that inhabits aquatic environments and sulfide deposits at pH 1.5–4.0 (Fig. 12.6, c).

Sulfobacillus thermosulfidooxidans and Sulfolobus acidocaldarius are autotrophic, thermophilic archaea with an optimal growth temperature of 45–75 ⁰С and pH 1–4. In addition to iron, they oxidize sulfur and sulfide minerals.

Alongside autotrophic microorganisms, heterotrophic microorganisms are also utilized in hydrometallurgy. Sulfate-reducing bacteria use organic matter and molecular hydrogen as electron Donors, with sulfates serving as electron acceptors that are reduced to hydrogen sulfide. These bacteria are anaerobes exhibiting diverse Cell morphologies (Fig. 12.6, d). They are employed to precipitate metals from solution by binding them with bacterial hydrogen sulfide.

Certain microorganisms can degrade rock-forming minerals across a wide pH range through the secretion of organic acids, Polysaccharides, and other metabolites. For instance, Bacillus mucilaginosus and Bacillus megaterium break down silicate minerals by cleaving Si-O-Si bonds.

Sulfide minerals serve as the primary source for obtaining most metals. Virtually all known sulfide ores are susceptible to microbial oxidation.

Bacterial oxidation of sulfide minerals involves the following stages:

- adsorption of bacteria onto the mineral surface;

- destruction of the crystal lattice;

- transport of oxidizable elements into The Cell;

> intracellular enzymatic oxidation.

Such processes proceed According to the laws of electrochemical corrosion and depend on the Composition and Structure of the mineral. The consequence of bacterial leaching is The formation of soluble metal sulphates, H₂SO₄, and Fe₂(SO₄)₃.

For example, the oxidation of chalcopyrite is carried out according to the reactions

A distinction is made between heap, in-situ, and vat metal leaching. The technological scheme of heap and in-situ leaching is shown in Fig. 12.7.

Fig. 12.7. Scheme of heap and in-situ leaching of metals from ores

An aqueous solution of H₂SO₄ containing Fe³+ and bacteria must permeate the ore body. In the ore, in the presence of bacteria and oxygen, insoluble sulphide minerals are oxidized, forming soluble sulphates. Rare elements are present in sulphide minerals and pass into solution due to the destruction of the crystal lattice.

Metal solutions are collected in specially equipped settling tanks, from which the metals are extracted in various ways, while the solution is fed back onto the ore, forming a closed-loop system.

Heap leaching is carried out in dumps of low-grade ores left over from primary extraction, or when metal recovery by conventional Methods is uneconomical. Such ore is arranged in layers on specially prepared pads to form heaps. The solution is applied by spraying onto the heap surface (Fig. 12.8).

Fig. 12.8. Scheme of the heap leaching process:

1 - heap, 2 - ground surface, 3 - collection pond for solutions, 4 - pump, 5 - cementation trough, 6 - spent solution pond, 7 - pump, 8 - dump irrigation system, 9 - metal

In-situ leaching is carried out directly in the ore body deposit sites. The leaching solution is supplied through boreholes.

The cost of metals obtained by bacterial heap or in-situ leaching is 1.5–2 times lower than that of conventional extraction methods.

Vat leaching is used to process complex sulphide concentrates and precious metal ores (gold, uranium, tin). The process is carried out in special vessels (agitators or pachucas) connected in series. A pulp—finely ground ore mixed with Water—and microorganisms are introduced into the pachucas. Constant mixing and aeration are maintained, and optimal temperature and pH are ensured. A microbial culture adapted to a specific ore is used in an amount of (10¹⁰–10¹¹ Cells/mL).

Leaching from silicate minerals. Some metals are part of silicate minerals. By releasing metabolites, microorganisms are capable of destroying silicate and aluminosilicate minerals, leading to the leaching of various chemical elements. Heterotrophic microorganisms facilitate the extraction of gold, aluminum, nickel, copper, and other metals from rocks and ores.

Microbiological leaching of aluminum from aluminosilicates is carried out using Fungi of the genus Aspergillus niger. The leaching solution is obtained from the culture liquid of Aspergillus niger cultivated on molasses. This microorganism synthesizes a large amount of citric and oxalic acids, the concentration of which in the culture liquid reaches 200 g/L.

The process is carried out in vats within special acid-resistant reactors at a temperature of 90–100 ⁰C. Under such conditions, 90–95% of the aluminum is extracted within 5 hours.

Manganese leaching. In the environment, manganese is present in more than 100 minerals. The main reserves exist in the form of oxides (pyrolusite _MnO2), carbonates (rhodochrosite MnCO₃), and silicates (rhodonite (CaMnO₄)[Si₅O₁₅]).

To extract manganese from these ores, it is necessary to reduce it from the tetravalent to the divalent state. Such reduction can occur via various pathways, but the action of organically synthesized bacterial acids, especially formic and oxalic acids, is of Practical Application:

To reduce Mn⁴⁺, microorganisms belonging to the genera Bacillus, Pseudomonas, Arthrobacter, and Acinetobacter are used. Bacterial leaching yields up to 90–96% of Mn from oxide ores, 60% from carbonate ores, and up to 85% from silicate ores.

Leaching of native gold. Bacterial leaching of native gold is based on the reaction between native gold and Amino Acids in the presence of strong oxidizing agents. Amino Acids and Proteins in an alkaline environment (pH 9–10), in the presence of H₂O₂, can interact with gold to form an Au-N bond. The role of hydrogen peroxide is to convert Au⁰ into an ionic state.

The highest activity in dissolving native gold is exhibited by bacteria of the genus Bacillus: B. megaterium, B. cereus, B. subtilis.

Microbiological metal recovery from solutions. The final stage of hydrometallurgical processes is the recovery of metals from solutions. Along with Chemical methods of metal precipitation from solutions, microbiological methods also exist. These processes are based on the ability of microorganisms to sorb, precipitate metals as sulphides, and reduce and oxidize metals.

Precipitation of metal sulfides. Sulfate-reducing bacteria produce hydrogen sulfide, which almost completely precipitates metals from solution. The cultivation of these bacteria is carried out under anaerobic conditions with continuous stirring. Kerosene, oleic acid, terpineol, and sodium xanthate can serve as sources of organic matter. The recovery efficiency of metals, such as copper, molybdenum, and tungsten, reaches 94–98 %.

Oxidation and reduction of metals. Arsenic in the trivalent state (arsenites) is more soluble than in the pentavalent state (arsenates). To remove arsenic from solutions, it must be oxidized, i.e., converted into the pentavalent form. The culture of Pseudomonas putida is capable of oxidizing As (III) in weakly alkaline and neutral environments.

The reduction of hexavalent chromium is of practical importance. The culture of Pseudomonas dechromaticans under anaerobic conditions reduces hexavalent chromium to trivalent chromium, which precipitates as Cr(OH)₃. The process is carried out at pH 8–9, using domestic wastewater as a source of organic matter.

Biosorption of metals. Biosorption is based on interaction processes with the Surface structures of microbial cells, microbial metabolites, and exopolymers. Bacteria, yeasts, filamentous fungi, and Algae are used for sorption.

Bacterial exopolysaccharides actively sorb metals from solutions mainly due to phosphate and uronate groups. Filamentous fungi sorb metals owing to the presence of Chitin in their cell walls.

Both living and dead biomass, exometabolites, and preparations based on them have found application. The culture of Zoogloea raminera produces a large amount of exopolysaccharides when grown on a medium containing CARBOHYDRATES. It is used for the sorption of copper, cadmium, and uranium.

The Use of dead biomass and sorbents produced on its basis is considered promising. Thus, to obtain biosorbent M (developed in the Czech Republic), the mycelium of Penicillium chrysogenum (antibiotic production waste) is treated with a urea-formaldehyde polycondensate. As a result, a dense product is formed, which is crushed into granules with a size of 0.3–0.8 mm.

Using microorganisms, it is possible to remove up to 100 % of lead, mercury, zinc, copper, nickel, cobalt, manganese, chromium, and uranium, up to 96–98 % of gold and silver, up to 84 % of platinum, and 93 % of selenium from dilute solutions.

Desulfurization of coal. Coal contains up to 10–12 % sulfur. When coal is burned, sulfur turns into sulfur dioxide, which enters the atmosphere and is oxidized to sulfuric acid, leading to acid rain. Thiobacillus ferrooxidans, T. thiooxidans, and T. organoparus are used to remove sulfur from coal. Microorganisms cause the oxidation of sulfur and the formation of sulfuric acid. Within 5 days, almost 100 % of the sulfur is removed from the coal.

Reducing the concentration of methane in coal mines is carried out using microorganisms that oxidize methane. Despite the wide distribution of methane-oxidizing Microorganisms in the environment, they are virtually absent in coal seams. Microbial biomass is grown in fermenters. The bacterial suspension, along with a solution of nitrogen and phosphorus salts, is pumped into the coal seam. The methane concentration is halved using this microbiological method.

Enhanced oil recovery using microbiological methods is applied to heavy, resinous oils or when wells are clogged with heavy oil fractions. A suspension of hydrocarbon-oxidizing and methanogenic microorganisms, along with a solution of nitrogen and phosphorus salts, is pumped into oil wells. Microorganisms begin to develop actively, utilizing petroleum Hydrocarbons. As a result of the growth of hydrocarbon-oxidizing bacteria, carbon dioxide, hydrogen, and low-molecular-weight organic acids are produced. These metabolites enter the anaerobic zone, where methanogens convert them into methane. The breakdown of surface oil layers with the simultaneous generation of gases leads to oil thinning and an increase in gas pressure within the oil reservoir. All these transformations make it possible to increase oil production.