The Sulfur Cycle: Sources, Acid Deposition and Mining Techniques!
The sulfur cycle includes both a very slow geologic component between land and water, and rapid atmospheric and biologic components. Like the other nutrient cycles, sulfur is also a requirement of life. However, due to its environmental abundance, this element is rarely a limiting one.
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Organisms use sulfur in both its mineral and organic forms. In protein synthesis, sulfate (SO4=) is reduced to sulfhydryl (SH–) groups. The concentration of sulfur in organisms varies from 0.05% to almost 5%. However this concentration of sulfur, along with the sulfur contained in detritus, is still small compared to the amount in aquatic systems. The sulfur in saltwater is 240 times more abundant than in freshwater. The dominant form of sulfur in saltwater appears as sulfate (SO4=). This is the same form that plants take in.
The decomposition of organic matter and detritus, oxidation and reduction by sulfur bacteria, and natural chemical reactions affect the cycling of sulfur as well as other nutrients. In fact, the oxidation of sulfur to sulfuric acid affects the rate at which minerals are leached from rock, thereby affecting the cycles of many minerals as well as nutrients.
In an analogous manner, the cycling of certain nutrients can affect and interact with the sulfur cycle. The presence and reduction of nitrate over sulfate leads to more potential for the formation of sulfuric acid and the lowering of soil pH.
At the present time, the anthropocentric activities of man appear to be the dominant agents of change within the sulfur cycle. This occurs through combustion of fossil fuels, industrial production, and utilization and application of fertilizers.
Much sulfur is transported as a result of leaching and runoff, which continuously drives it toward the sea. Deep ocean volcanic vents appear to be the energy input for an entirely separate food chain. The sulfur bacteria may well be its base in the same capacity as the primary producers.
Sources and Occurrence of Sulfur:
The many sources of sulfur account for its widespread abundance. The important natural source of sulfur is rock. Calcium sulfate (CaSO4) is a common component of sedimentary rock; however, igneous rock contains little sulfur. When rocks are weathered, or when soil containing sulfur comes in contact with water, the sulfur can be oxidized to sulfuric acid.
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If the elemental form of sulfur is present, the following reaction takes place:
2S + 3O2 + 2H2O → 2H2SO4
Sulfur also occurs in rock, soil, and sediments as pyrite or ferrous sulfide (FeS2).
It is oxidized to ferrous sulfate and sulfuric acid as follows:
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FeS2 + 3.5 O2 + H2O → FeSO4 + H2SO4
The geologic forces of uplifting and subsequent weathering are natural recycling mechanisms augmented by volcanic action. This process inputs sulfur to the atmosphere, as well as to the land and water, as SOx and H2S.
The sulfur contained in the sediments and muds of aquatic systems undergoes rapid change due to the activity of microorganisms. It is both oxidized and reduced, and often cycles very fast through precipitation and deposition. Although this biological process is small compared to man’s activities, it is still important.
Hydrogen sulfide (H2S) results from the anaerobic decomposition of compounds containing organic sulfur as well as those containing inorganic sulfates. It occurs in particular from the decomposition of algae. Man’s contribution of sulfides can originate from several industrial processes including tanneries, paper mills, various chemical plants, and manufactured gas plants.
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Sulfide compounds, when dissolved in water, will dissociate as follows:
H2S ⇌ HS– + H+ ⇌ S= + H+
The reaction goes to the left as the pH decreases and to the right as it increases. At a pH of 9, approximately 99% of the sulfide is hydro sulfide (HS–). At pH 5 it is 99% hydrogen sulfide (H2S).
Organic sources of sulfur also can be incorporated in sulfur-bearing rock. Organic forms of sulfur may be mineralized by biological decomposition of dying or dead cells or organic wastes containing sulfur.
Man may have doubled the amount of sulfur in the atmosphere due to the combustion of fossil fuels. Over time, some of the sulfur locked in the organic molecules of plants and animals is converted into oil and coal. Put away deep in the earth’s lithology, most of it might well have stayed there.
However, the combustion of these fuels, as well as other industrial applications that produce waste gases, releases significant amounts of sulfur oxides to the atmosphere. In fact, in industrialized areas, man’s input probably exceeds natural input. Where igneous rocks dominate the landscape, such as in the northeast, lakes receive almost all their sulfur from the atmosphere.
In 1980, the United States emitted approximately 32.9 million tons of sulfur dioxides. Of this figure, 21.8 million tons, or more than 66%, is attributable to the electric utility industry. Other industrial processes account for only about 6.1 tons, or about 28%.
Man also uses sulfur in several forms that can result in its release to both land and water. Much sulfate is carried from the land by runoff from fertilizers not taken up by plants or from leaching of the soils. Soils with low organic content readily leach sulfides because there are few positively charged organic molecules to hold them.
Phosphate mining employs significant amounts of sulfuric acid in the extraction of the phosphate from the phosphate rock. This use accounts for almost half of the production; the rest is used by a multitude of industrial applications. Significant quantities of excess sulfur can result from coal and ore mining operations, which will be discussed separately.
Sulfur also occurs in water, mostly in the form of sulfate, which is the most assimilative state for plants. However, when decomposition occurs, much is released as hydrogen sulfide, which gets quickly oxidized if oxygen is present. In low DO or anoxic conditions, ferrous iron (Fe+2) released from the sediments will react aggressively with any H2S present to form insoluble ferrous sulfide, which will precipitate.
When the ferrous iron is gone, other metals can migrate from the sediments to form metal sulfides, such as copper, zinc, and lead. Much sulfur can be lost to the sediments because these compounds are very insoluble. With decomposition, though, hydrogen sulfide and even some sulfates are released all the time.
The presence of sulfur in water is also increased by the discharge of wastewaters containing sulfur (such as the pulp and paper industry). It should be remembered that a buildup of reduced sulfur in the hypolimnion sediments will take place during periods of stratification. When turnover occurs, however, the hydrogen sulfide will mostly be oxidized to sulfates.
Acid Deposition:
The term “acid rain” is really somewhat inappropriate because precipitation is normally somewhat acidic. Rainwater saturated with carbon dioxide will form weak carbonic acid with a pH typically of 5.6-5.7. Sulfur and nitrogen compounds will form acids as well.
In addition, precipitation should also include dustfall as well as rain, snow, and hail. A more precise term is therefore “acidic deposition.” While detailed discussion of this topic really is most appropriate when addressing the study of air pollution, the impact on aquatic biology can be of such significance that a brief discussion is warranted here.
As an air pollutant, sulfur is one of the most troublesome. It occurs naturally from volcanic activity, geysers, swamps, and shallow bodies of water, among other sources. Many scientists now feel that man contributes more than nature. Emissions from coal piles and the production of kraft paper are notable sources.
However, the combustion of fossil fuels now predominates the anthropocentric sources. It is estimated that 95% of man’s sulfur emissions are sulfur dioxide. This consists of 70% from coal combustion, 16% from combustion of petroleum products, and the rest mostly from petroleum refining and non-ferrous metal smelting.
In very basic terms the following atmospheric conversions can happen:
These generic steps only address sulfur. We should keep in mind that other constituents react and interact to comprise “acidic deposition”. Besides acidic gases and soot, we must be aware of the potential acidity contribution from organic acids, carbonic acid, hydrochloric and nitric acids, and a host of other, less-abundant constituents.
We also should be cognizant of the precursors of the acidic substances that are suspended in rain, dew, fog, hail, clouds, etc. Dry material consisting of gaseous and particulate matter is transferred to natural systems by gravity settling, turbulent exchange, and vegetative uptake. It usually drops out within a few miles of the source.
A large nickel and copper smelting plant in Sudbury, Ontario, was recognized as a significant producer of sulfuric acid. Its impact on the surrounding area was so significant that it has reduced the atmospheric discharge from its 1,250-foot stack from 7,000 tons per day (TPD) to 2,500 TPD and plans to cut back even more. Today, a large coal-fired electric power plant can produce in one year as much sulfur dioxide as Mount St. Helens did—around 400,000 tons.
Because industrial processes can produce such a large volume of sulfur oxides in their combustion gas emissions, a variety of steps/techniques can be used to reduce the amounts.
Four common ways to reduce the sulfur discharged to the atmosphere are:
1. Decrease the use of sulfur-containing fuels by conservation, recycling, alternate technologies, etc.
2. Use low-sulfur or sulfur-free fuels such as natural gas, nuclear, hydro, etc. Algerian fuel oil contains about 0.2% while Mexican oil averages 3.0% sulfur.
3. Remove sulfur from fuel gases through air pollution control measures such as scrubbers.
4. Remove the sulfur from the fuel. This includes cleaning operations at the mine, at processing plants and at the user location. Recovered sulfur may account for more than 50% of the free world’s sulfur output.
Bacteria and the Sulfur Cycle:
The organically bound sulfur in living organisms is mostly in the reduced sulfhydryl (SH~) group from the initial input of sulfates to plants. When biological decomposition occurs anaerobically, hydrogen sulfide is formed. It is also formed from bacterial sulfate reduction and is the type to which toxicity is attributed. These anaerobic sulfur-reducing bacteria are heterotrophic and use sulfur as the hydrogen acceptor while deriving the oxygen from the reduction of sulfate, generating hydrogen sulfide as shown in the following equation-
Following is a list of some of the better-known genera that form hydrogen sulfide and the environment they commonly are associated with.
Acid mine drainage—Desulfovibrio
Oligotrophic lake—Mycobacterium
Mesotrophic lake—Pseudomonas, Bacterium
Eutrophic lake—Pseudomonas, Bacterium
Dystrophic lake—Pseudomonas, Bacillus
Marine waters—Micrococcus, Mycobacterium, Achromobacter, Vibrio
We should note that while no oxygen is produced during the process of making the hydrogen sulfide, it readily oxidizes and uses oxygen. If the hydrogen sulfide is produced in natural environments or treatment systems, odor problems can result. If hydrogen sulfide is produced in a sewer pipe or other closed environment, H2S-oxidizing bacteria can convert the hydrogen sulfide to sulfuric acid. This can lead to crown corrosion of the pipe or other structures.
There are two groups of bacteria that oxidize sulfur. The chemosynthetic Thiothrix, filamentous bacteria, oxidize sulfur to sulfate, while storing some sulfur inside or outside their cells as a sulfur reserve. This genus is associated with low-quality water conditions, high in organic content.
Thiobacillus has many species that can be differentiated by the sulfur compound used as an energy source. One highly aerobic type can convert elemental sulfur to sulfuric acid and can grow well at pH conditions as low as 2. Another species grows anaerobically. The members of this genus are generally associated with unpolluted waters that are low in organic content. The photosynthetic types, i.e., the green and purple bacteria, use hydrogen sulfide as an electron donor while they reduce carbon dioxide in photosynthesis.
All types of sulfur bacteria have very strict requirements in terms of environmental conditions. They tend to develop large localized populations when conditions favor. In certain lakes and at certain times, bacterial photosynthesis can even exceed algal photosynthesis although when looked at over a whole year, their production is low.
The green sulfur bacteria are typically found in a dense layer below the purple sulfur bacteria layer usually located at the aerobic/anaerobic interface in euthrophying lakes. Zooplankton populations, particularly cladocerans (e.g. Daphnia, the water flea) actively feed on these bacteria.
Acid Mine Drainage:
One of the single most significant sources of water degradation in the United States, particularly freshwater, is acidic waters that drain from mining areas. More of the problem results from drainage of abandoned mines, and there are estimated to be 67,000 inactive sites in the country, in contrast to 15,000 active mining sites. Almost 90% of the problem originates in Appalachia and the Ohio basin states. Where coal strip mining takes place, the pH has been found to be as low as 2.5.
The state of Wyoming is number one in the “extraction industry,” with coal seams 150 feet thick. However, un-reclaimed strip-mined lands occupy an area larger than Rhode Island, and each day more than 400,000 pounds of sulfuric acid leaches into streams. In fact, the Office of Surface Mining indicates many thousands of miles of streams have been affected as a result of this problem.
Sulfuric acid is formed when air and water react with sulfur-bearing minerals such as pyrite, FeS2, commonly associated with coal. The acid enters streams from runoff of exposed seams and tailing piles, or from infiltration and percolation through the mines or refuse (overburden, or waste ore piles). The resulting acidic condition kills vertebrates, invertebrates, and most plant life, making the water useless. In addition, the acidic conditions can leach heavy metals and other elements, including aluminum.
In the United States, acid mine drainage pollutes more than 7,000 miles of streams with approximately 3 million tons of acid per year, mostly from abandoned mines. Most of the polluted streams are in Appalachia. The acid conditions also can cause a secondary effect. The low pH conditions can inhibit bacterial decomposition of organic material. This can potentially result in a buildup of this material in lakes and streams.
Pyrite is an important contaminant in coal and also is an intricate part of the problems associated with acid mine drainage. Iron pyrite is ferrous sulfide, FeS2, also known as fool’s gold. In coal mining operations, the coal is washed to remove pyrites by the coal being crushed and sent to holding ponds.
These in turn leach and leak, causing the same problems as mine drainage waters. The ferrous sulfide dissolves in water and the S2 is oxidized to SO4=. As the dissolved oxygen content is decreased, ferrous sulfate, FeSO4, and sulfuric acid are formed.
Some ferrous ions are reduced and become ferric ions, which combine with OH” ions to form ferric hydroxide, Fe(OH)3, called “yellow or red boy.” It is a gelatinous precipitate that coats and seals the bottom, depriving oxygen and food sources and locking in pollutants and organics. As it precipitates, excess hydrogen ions left behind combine with sulfate to form sulfuric acid.
The following reactions describe this:
Mining Techniques:
To understand the acid mine drainage problem, it is helpful to have at least some knowledge of the mining or “extraction” industry. Because the bulk of acid mine drainage problems are from the largest category of mining operations, coal, we will examine the mining techniques in this industry. There are two types of mining operations- surface mining and shaft mining.
With surface or strip mining, there are three basic techniques. “Area mines” are operated in flat or rolling terrain. A trench is constructed or “cut” on one side of the deposit. Overburden from the next cut is placed in the previous cut. Rain washes through the broken rock and dissolves out the pyrites.
With “contour mining” in hilly regions, a shelf or bench is cut into the side of the hill. Contaminants are leached from the spoil (or overburden), which is stacked out on the outer edge of the shelf or pushed off to form slopes of debris. Contaminants are also leached out from the “highwall,” which is the inner edge of the cut.
The last type, “auger mining” is used to obtain coal from a deposit at the back of a highwall that can no longer be profitably contour-mined. The augers are up to 7 feet in diameter and can bore up to 200 feet into the seam of coal in the highwall. The auger holes interfere with normal drainage and expose pyrites to air and water.
The other type of mining operation is the familiar shaft mine. The mine is constructed as rock is removed to get at the seam of coal. The material must be removed and then separated into ore and tailings. As the mine progresses deeper, geological strata are disturbed and fissures appear, drawing water into the mine. Besides the mine drainage water itself, the ore and tailings piles also leach pollutants.
Correcting this problem is very difficult. New mining operations are carefully controlled, regulated, and monitored; however, much work needs to be done.
The Surface Mining Control and Reclamation Act of 1977 (SMCRA) initiated efforts to force strip miners to reclaim the land. It has only had limited success. It also should be remembered that most of the acid drainage problems originate in abandoned mines.
Coal mining is not the only industry with low pH problems; many other industries also have the problem. The metal plating industry requires the removal of grease and oxides prior to plating by “pickling,” which is the passage through an acid bath process. Acid wastes are generated from the milling of uranium and vanadium ores. These are neutralized with ammonium hydroxide and directed to holding ponds. These ponds then can have problems with high concentrations of ammonia.