The phosphorous cycle is considerably different from the nitrogen cycle. Rather than a predominantly biochemical cycle, the phosphorous cycle is basically a geological cycle. This is because the major reservoir is the lithosphere, with the principle inputs from the erosion and weathering of phosphorous deposits.
Large amounts of phosphorous are eroded and washed into the rivers, lakes, and oceans. Runoff carries excess fertilizers containing phosphorous into rivers and lakes. Treatment plants discharge effluents to receiving water bodies that have substantial levels of phosphorous. Waste from animals in feedlots also is carried away by runoff.
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Lastly, droppings from birds that form deposits of guano erode and are washed into surface waters. The majority of these waterborne phosphates eventually settles to the bottom and become incorporated into the sediments along with the fossilized bones of animals.
Small amounts may re-enter the water and be available for biological uptake, but most precipitates and becomes phosphate deposits and phosphate rock. These geological sedimentary deposits can become uplifted and then are exposed to the elements, where physical processes such as weathering slowly release phosphorous, making it available to organisms.
Most of the phosphorous incorporated into the sediments is lost from the cycle. In this respect the phosphorous cycle is more unidirectional than cyclic, and because of this, it is often referred to as a “non-renewable resource.”
The cycle also has both an atmospheric and biological component. The atmospheric part is negligible. A very small amount, in the form of dust or phosphine gas from swamps, is circulated in the atmosphere. The biological part is somewhat more significant, but still small compared to the geological component.
The biological component of the phosphorous cycle consists of uptake by plants with successive transfer through the food chain. Animals get phosphorous from plants, and it becomes incorporated into many biological systems.
Phosphorous leaves the biological portion of the cycle through excretion as well as death and decay. This is accomplished through the action of phosphatizing bacteria, which break down organic compounds containing phosphorous into inorganic dissolved phosphates.
Forms of Phosphorous:
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There are basically three categories of phosphorous: orthophosphates, condensed phosphates, and organically bound phosphorous. No matter what form of phosphorous you have; it will eventually be reduced to orthophosphate (H2PO4-1, HPO4-2, PO4-3). Orthophosphorous is analytically defined as those phosphorous compounds that respond to colorimetric tests without preliminary hydrolysis or oxidative digestion.
This means that if a water sample undergoes preliminary hydrolysis—i.e., digested with acid and boiled, and measured calorimetrically—total inorganic phosphorous will be obtained. The difference before and after hydrolysis represents the condensed phosphates.
If a sample is subjected to oxidative reduction/digestion, e.g., with perchloric acid or persulfate, and then measured colorimetrically, total phosphorous is obtained. The difference between this measured total phosphorous and total inorganic phosphorous is, therefore, the organically bound phosphorous.
The orthophosphates are the type most amendable to biological systems. They are also the type used in fertilizers that can cause significant eutrophication problems. This can occur when they contaminate the runoff from agricultural lands and drain into nearby surface water bodies.
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The condensed phosphates—including pyrophosphates, polyphosphates (from detergents), and metaphosphates such as those used in water treatment (e.g., sodium hexametaphosphate)—are molecularly dehydrated phosphates. These will hydrolyze very slowly in water and revert to the ortho form, from which they were derived.
The organically bound phosphorous is like nitrogen in that it is a necessity of life. It exists in living organisms as phosphate (P04), and is also a component of RNA and DNA. In addition, besides being a component of fats (phospholipids), phosphorous is also a part of ATP (adenosine triphosphate), where its high energy bonds are important in energy transformation.
It can be a limiting element responsible for primary production and eutrophication. Dissolved phosphates are taken up by plants and then passed through the food web. They can reenter an ecosystem through excretion or as a result of death and detrital decay.
Sources of Phosphorous:
Because the phosphorous cycle is generally considered a geological cycle, the most significant reserves are associated with the upper lithosphere. Phosphate rock, called “apatite,” is the largest reserve and is extensively exploited by man for fertilizer production. Phosphate rock consists of calcium and/or aluminum hydroxy-fluorophosphate and contains about 8-12% phosphorous.
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Major reserves occur in the United States; with a major deposit in the tri-state area of Idaho, Montana, and Wyoming; central Florida; and Tennessee. Phosphorite deposits may be a new source, and have been identified as associated with the continental shelf off Georgia and South Carolina. The fossilized remains of animals also can form significant deposits of phosphorous.
A major input of phosphorous to surface waters is from domestic and industrial wastewater discharges. Before the use of laundry detergents became popular, it is estimated that 2 to 3 mg/l of phosphorous were from the inorganic forms, and 0.5 to 1 mg/l were from organic forms. The inorganic forms are the result of the metabolic breakdown of proteins and subsequent waste production.
The average amounts of phosphorous released from this process is about 1.5 grams per person per day. It is estimated that about two or three times as much inorganic phosphorous exists in wastewater now as compared to before the widespread use of modern synthetic detergents.
In domestic wastewater, about half of the phosphorous present is from human waste and about 20-30% is from detergents. Only about 30% of phosphorous is removed by standard methods of wastewater treatment. Like humans, animals also excrete phosphorous into water. Runoff from feedlots and other places where domesticated animals are kept can carry appreciable amounts of phosphorous to receiving waters as non-point source pollution.
The waste from sea birds, called guano, is especially high in phosphorous compounds as a result of their food chains. In fact, before the advent of synthetic fertilizers, guano deposits were often mined for fertilizer use.
Lastly, some phosphorous is part of the benthic sediments from detrital breakdown. Upwelling of deep ocean waters can make this source available to organisms in the water column.
Conversion and Mechanisms of Transfer:
Phosphate rock, as the major source of phosphorous, is exposed to weathering processes as a result of geologic uplifting. When exposed, the elements of nature, such as freeze/thaw effects, begin to break down the deposits. Some phosphorous dissolves in runoff and joins particulate phosphorous that is eroded away. However, phosphorous has a low solubility and tends to precipitate out.
In soil, much phosphate is immobilized due to the presence of abundant calcium, aluminum, and iron which, together with the phosphates, form salts. Phosphates also can be leached out in dissolved forms and eroded in particulate forms, only to be deposited back into the sediments later. Uplifting can bring some phosphorous back, for example as calcium phosphate, but as before, due to low solubility, it tends to settle to the bottom again.
The process of mining greatly accelerates the cycle. There is great loss of phosphorous from washer and flotation processing during mining recovery operations. In addition, the physical loss of fertilizer phosphates to runoff returns much of what was initially mined for use.
Much of the mined phosphorous that is used is made or incorporated into products that often contain less than 0.1 % phosphorous. It has been observed that natural systems tend to control any large input mostly by deposition into soil and sediments.
The small amount of phosphorous that does get recycled is primarily through incorporation into the food chain. Through bacterial decomposition of waste and detritus, including the activity of phosphatizing bacteria, phosphorous is passed through the food chain to fish and then to birds or man. Any excess phosphorous in the diet is excreted as soluble forms, especially salts. Birds create guano deposits, and waste from man is discharged through sewage treatment plants.
Some phosphorous is taken up by plants. Terrestrial plants absorb dissolved phosphates through their roots and incorporate them into all cells in complex molecules. However, with the presence of abundant oxygen, phosphorous will form insoluble compounds not available for uptake by plants, which proceed in the formation of phosphate rock.
The low solubility of phosphorous leads to low concentrations in the oceans, especially in the upper, warm waters where phytoplankton deplete the phosphate. Deeper waters tend to have more phosphorous due to the presence of calcium, through which the calcium phosphates of the upwelling processes are formed.
Effects of Excess Phosphorous:
In its elemental form, phosphorous is toxic and can bioconcentrate in the same way as mercury. However, its primary environmental effects are related to eutrophication, and it is often the limiting element. As pointed out before, natural systems tend to deposit large inputs by precipitation and deposition into the sediments as well as mineralization in soils.
The chemical half-life of tripolyphosphate in distilled water is 4,000 to 5,000 days. In natural lakes and rivers, however, the decay is 100 to 1,000 times as fast. This will occur as the compound hydrolyzes. Organic phosphorous requires both hydrolysis and biochemical oxidation to convert the organic form to orthophosphate.
The phosphatizing bacteria will consume oxygen to accomplish this. About 1 mg of phosphorous in a lake’s cycle will require about 130 mg of oxygen. This can lead to the potential to create anaerobic conditions.
Phosphate rock contains about 45 to 180 grams of uranium per ton. The potential environmental effects of its release are obvious. In addition, it also contains fluorine. The effect of excess fluorine was best demonstrated in Polk and Hillsborough counties in Florida. Between 1953 and 1960, 30,000 cattle died from drinking water in runoff ditches due to fluorosis, which led to softening of bones and teeth and the mineralizing of tendons and ligaments.
One common example of the impact of phosphorous is in the Great Lakes. Most experts agree that phosphorous is still the limiting element to biological production in Lake Superior, Huron and Michigan but not in Lake Erie or Lake Ontario. This is primarily due to the phosphorous present in treated wastewater effluents discharged into these two lakes. Suffolk County, Long Island, has restricted the use of phosphates in detergents because it has been identified as the limiting element in that area.
Eutrophication from phosphorous is largely a freshwater problem in lakes and other shallow water bodies. Most uncontaminated lakes have 0.01-0.03 ppm of phosphorous. Below 0.01 ppm, algae growth is severely limited; above 0.05 ppm, excessive growth of algae can occur. Therefore, small changes in the phosphorous concentration can have dramatic effects.
Detergents and Phosphorous:
Before the 1930s, soaps were used for washing clothes. These were derived from fats and oils and worked acceptably in soft waters. In hard waters, however, the soaps formed insoluble precipitates that left a residual “ring around the tub.” This was the catalyst for the widespread use of synthetic detergents. They were available in the 1930s but really became popular after 1945 when phosphorous (as polyphosphates) was added as a filler or “builder” to laundry detergents.
Modern detergents can contain anywhere from less than 10% to more than 50% polyphosphates. The purpose of adding polyphosphates was to provide buffering through adding alkalinity, and also to assist in the suspension and emulsification of dirt. The builder also binds or sequesters calcium and magnesium ions, which interfere with another detergent component called the “surfactant.”
The surfactant is responsible for lowering surface tension, and enabling dirt particles to link with water and prevent them from redepositing on clothes. A common type of polyphosphate builder in powdered detergents is sodium tripolyphosphate (Na5P3O10)-liquid detergents, it is typically sodium, or potassium phosphate.
The original surfactant was, an alkylbenzene sulfonate (ABS). These are a branched-chain sulfonate group of synthetic detergents. They are very non-biodegradable and caused excessive foaming in rivers, lakes, and even in groundwater.
By 1965, the detergent industry had switched the ABS surfactants with “LAS,” linear alkyl sulfonates which, as the name implies, are a group of straight chain alkylbenzene sulfonates. These were degradable but resulted in the release of the phosphates from the builder. The released phosphate in turn added to the problem of eutrophication.
As a result, some counties and some states have banned or restricted phosphate detergents. Limitations of 8.7% exist for California, Florida, Indiana, Maine, Michigan, and New York. A limit of 0% exists in Minnesota, Vermont, and Wisconsin. Certain counties in Indiana, and Suffolk County on Long Island, also have established a 0% limitation through the implementation of local regulations.
A number of phosphate substitutes have been tried with limited success (and many consumer complaints). The sodium salt of nitrilotriacetic acid, or “NTA,” was one attempt. However, this caused the release of nitrites, and represented a danger to infants due to the potential for methemoglobinemia to develop. Their use was shortly discontinued.
Phosphate-free detergents now exist on the market and generally contain one of the following substitutes:
(i) Carboxymethyl cellulose, which is biodegradable.
(ii) Silicates and carbonates combined with sodium. They sequester calcium and magnesium, but can make phosphates in sediments more soluble.
(iii) Borates such as sodium tetraborate decahydrate or borax. However, these are toxic to plants at concentrations greater than 1 ppm.