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Phosphorus

General Information: Phosphorus (P), the 15th element on the periodic table with an atomic weight of 30.974, is an essential nutrient for all life forms. Phosphorus plays a role in deoxyribonucleic acid (DNA), ribonucleic acid (RNA), adenosine diphosphate (ADP), and adenosine triphosphate (ATP). Phosphorus is required for these necessary components of life to occur.

Phosphorus is the eleventh-most abundant mineral in the earth's crust and does not exist in a gaseous state. Natural inorganic phosphorus deposits occur primarily as phosphate in the mineral apatite. Apatite is defined as a natural, variously colored calcium fluoride phosphate (Ca5F(PO4)3) with chlorine, hydroxyl, and carbonate sometimes replacing the fluoride. Apatite is found in igneous and metamorphic rocks, and sedimentary rocks. When released into the environment, phosphate will speciate as orthophosphate according to the pH of the surrounding soil.

Phosphate is usually not readily available for uptake in soils. Phosphate is only freely soluble in acid solutions and under reducing conditions. In the soil it is rapidly immobilized as calcium or iron phosphates. Most of the phosphorus in soils is adsorbed to soil particles or incorporated into organic matter (Smith, 1990; Craig et al., 1988; Holtan et al., 1988).

Phosphorus in freshwater and marine systems exists in either a particulate phase or a dissolved phase. Particulate matter includes living and dead plankton, precipitates of phosphorus, phosphorus adsorbed to particulates, and amorphous phosphorus. The dissolved phase includes inorganic phosphorus (generally in the soluble orthophosphate form), organic phosphorus excreted by organisms, and macromolecular colloidal phosphorus.

The organic and inorganic particulate and soluble forms of phosphorus undergo continuous transformations. The dissolved phosphorus (usually as orthophosphate) is assimilated by phytoplankton and altered to organic phosphorus. The phytoplankton are then ingested by detritivores or zooplankton. Over half of the organic phosphorus taken up by zooplankton is excreted as inorganic P. Continuing the cycle, the inorganic P is rapidly assimilated by phytoplankton (Smith, 1990; Holtan et al., 1988).

Lakes and reservoir sediments serve as phosphorus sinks. Phosphorus-containing particles settle to the substrate and are rapidly covered by sediment. Continuous accumulation of sediment will leave some phosphorus too deep within the substrate to be reintroduced to the water column. Thus, some phosphorus is removed permanently from biocirculation (Smith, 1990; Holtan et al., 1988).

A portion of the phosphorus in the substrate may be reintroduced to the water column. Phosphorus stored in the uppermost layers of the bottom sediments of lakes and reservoirs is subject to bioturbation by benthic invertebrates and chemical transformations by water chemistry changes. For example, the reducing conditions of a hypolimnion often experienced during the summer months may stimulate the release of phosphorus from the benthos. Recycling of phosphorus often stimulates blooms of phytoplankton. Because of this phenomenon, a reduction in phosphorus loading may not be effective in reducing algal blooms for a number of years (Maki et al., 1983).

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Criteria for phosphorus:

The EPA water quality criteria state that phosphates should not exceed .05 mg/l if streams discharge into lakes or reservoirs, .025 mg/l within a lake or reservoir, and .1 mg/l in streams or flowing waters not discharging into lakes or reservoirs to control algal growth (USEPA, 1986). Surface waters that are maintained at .01 to .03 mg/l of total phosphorus tend to remain uncontaminated by algal blooms.

Numerical Categories:

Designated Use Limit

Freshwater
Aesthetics
Federal criteria
streams/rivers: .1 mg/l
streams entering lakes: .05 mg/l
lakes/reservoirs: .025 mg/l
(USEPA, 1986)
example State criteria used:
Reservoirs (CO) chlorophyll a 15 ug/l
Total P .035 mg/l
(Minn.) Total P .015 mg/l
Impoundments (EPA Region 4)
water supply Total P .015 mg/l
aquatic life Total P .025 mg/l
Lakes (NC) chlorophyll a 40 ug/l
Total P .05 mg/l
mountain lakes .02 mg/l
(VT) Total P .014 mg/l
(USEPA, 1994d)
Estuaries (recommended)
Aquatic life support 0.1 ug/l elemental phosphorus (USEPA, 1994d)
maximum diversity 0.01* total phosphorus (and nitrogen < 0.1) mg/l
moderate diversity 0.1* (and nitrogen < 1.0) mg/l


*These figures are recommended; eutrophication is also dependent on freshwater influx, nutrient cycling, dilution, and flushing of a pollutant load in a particular estuary. (NOAA/EPA, 1988)




Health Effects:

  1. Phosphate: Phosphate itself does not have notable adverse health effects. However, phosphate levels greater than 1.0 may interfere with coagulation in water treatment plants. As a result, organic particles that harbor microorganisms may not be completely removed before distribution.
    (See http://www.chemsoc.org/exemplarchem/entries/2001/duncan/page-3.htm)


Environmental Effects:

The growth of macrophytes and phytoplankton is stimulated principally by nutrients such as phosphorus and nitrogen. Nutrient-stimulated primary production is of most concern in lakes and estuaries, because primary production in flowing water is thought to be controlled by physical factors, such as light penetration, timing of flow, and type of substrate available, instead of by nutrients (McCabe et al., 1985).



Sources:

  1. Nonpoint sources:

  2. Point sources: Sewage treatment plants provide most of the available phosphorus to surface water bodies. A normal adult excretes 1.3 - 1.5 g of phosphorus per day. Additional phosphorus originates from the use of industrial products, such as toothpaste, detergents, pharmaceuticals, and food-treating compounds. Primary treatment removes only 10% of the phosphorus in the waste stream; secondary treatment removes only 30%. The remainder is discharged to the water body (Smith, 1990). Tertiary treatment is required to remove additional phosphorus from the water. The amount of additional phosphorus that can be removed varies with the success of the treatment technologies used. Available technologies include biological removal and chemical precipitation (Tchobanoglous 1991).
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    http://www.sewage.net/
Mode of Transport: Phosphates are primarily discharged directly into the water body by sewage treatment plants. Phosphorus that is adsorped to sediment particles may be transported in overland flow (for more information, please see Sediment section).




Analytical techniques:

A. Total Phosphorus and Orthophosphate: Analysis involves two procedural steps: 1) conversion of the phosphorus form into dissolved orthophosphate by a digestion method, and 2) colorimetric evaluation of the dissolved orthophosphate concentration. (APHA, 1992)

Step 1: Digestion methods

  1. Perchloric Acid Digestion: Recommended only for extremely difficult-to-analyze samples, such as sediments.
  2. Nitric Acid-Sulfuric Acid Method Recommended for most samples.
  3. Persulfate Oxidation Method This simple method should be cross-checked with one or more thorough techniques and adopted if results are identical.
Step 2: Colorimetric methods
  1. Ascorbic Acid Method: Ammonium molybdate and potassium antimonyl tartrate react with orthophosphate to form a heteropoly acid that is reduced to molybdenum blue by ascorbic acid. See also The Ascorbic Acid Method at a Glance

  2. Automated Ascorbic Acid Reduction Method: Ammonium molybdate and potassium antimonyl tartrate react with orthophosphate in an acid medium to form an antimony- phosphomolybdate complex that forms a blue color suitable for photometric measurements when reduced by ascorbic acid.

  3. Vanadomolybdophosphoric Acid Colorimetric Method: Ammomium molybdate reacts under acid conditions to form a heteropolyacid. In the presence of vanadium, yellow vanadomolybdophosphoric acid is formed, the intensity of which indicates the amount of orthophosphate present.

  4. Stannous Chloride Method: Molybdophosphoric acid is formed and reduced by stannous chloride, forming an intensely colored molybdenum blue.