Phosphorus is one of the most abundantly available and essential non-metallic element on earth. Phosphorus (white phosphorus) has the property of burning spontaneously with oxygen in air and thus produces light. The name 'phosphorus' is derived from Greek and means "bearer of light" (In Greek, word phos means "light" and phorus means carrier or transporter) (Sommers, 2008). In Latin the word 'phosphorus' means 'morning star'. Elemental phosphorus was not isolated until the late 17th century, when the German chemist, Hennig Brand discovered phosphorus in 1669. He prepared white phosphorus by concentrating and distilling human urine (Beatty, 2001).
When in 19th century, it was realized that the animal bones contained phosphorus; it was then utilized to industrially manufacture phosphorus. Phosphorus forms 0.1% of the Earth's crust by weight but due to the highly reactive nature, phosphorus is naturally found in its oxidised phosphate state. The common natural forms are either hydroxyl or fluorapatites (McKetta, 1991). Therefore, for the past century, the phosphorus extraction has been done using phosphorus ores using sulphuric acid (H2SO4) and carbon in electric furnaces (Beatty, 2001). The phosphorus thus produced is stored under water to prevent any reaction with air.
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Majority of phosphorus used is in the form of ortho-phosphate salts. It forms a vital ingredient in the biological life cycle of all plants and animals (McKetta, 1991). Therefore, a major portion of the total phosphorus is industrially utilized for the manufacture of phosphate fertilizers for agricultural use and animal feed. Apart from this, other application areas utilize small percentages of phosphorus.
Characteristic of Phosphorus
In order to study the effect of phosphorus species in fresh water, it is necessary to understand the physical and chemical characteristics of phosphorus. Basic physical and chemical properties of phosphorus are given in the table below (Lenntech_BV).
Â± 3, 4, 5
1.82 g/ml at 20oC
Vander Waal's radius
10.118 eV, 19.725 eV, 29.141 eV
Phosphorus exists in four allotropic forms; Î±-white, Î²-white, black and red. Of these only the Î±-white and red phosphorus are commercially important (McKetta, 1991). White phosphorus is nearly colourless or pale yellow in colour. It glows in the dark and is spontaneously flammable when exposed to air. On such exposure and burning, it produces dense white smoke. Therefore, it is generally used in the manufacture of artillery and mortars. Red phosphorus varies in color from orange to purple, due to variation in chemical structure. The black phosphorus is made under high pressure and has the property to conduct electricity (Lenntech_BV).
The bonding characteristics of phosphorus are similar to that of nitrogen, as it lies below nitrogen in the periodic table. Phosphorus, like nitrogen, exhibits trivalent characteristic and forms phosphine (PH3).
Phosphorus is an essential element in plants and animals; and its proper balance is maintained through the phosphorus cycle. Phosphorus cycle is the natural biogeochemical cycle that explains and demonstrates the movement of phosphorus through the lithosphere, hydrosphere, and biosphere.
Figure : Phosphorus Cycle (Burkett, 2006)
Phosphorus and most of its compounds are in solid phase at normal temperature and pressure. Although phosphoric acid and other compounds may be transported through the atmosphere in the form of dust particles and rain droplets, but its amounts are not significant enough to cause any variations in the phosphorus cycle (Jahnke, 2000). Therefore, the movement of phosphorus in phosphorus cycle does not appreciably take place through the atmosphere. Phosphorus cycle is dominated by the movement of phosphorus through water, soils and sediments.
Another factor that distinguishes the phosphorus cycle from the other matter cycles is the lack of significance of oxidation/reduction reaction in the distribution of phosphorus in the environment. Majority of the phosphorus in the cycle remain in the +5 state. Also the fact that the phosphorus cycle only has one isotopic form of phosphorus with 15 protons and 16 neutrons, distinguishes it from the other matter cycles.
The phosphorus cycle starts with the weathering and erosion of the phosphate rocks through water or other mediums, to form soil. Therefore, generally the upper layer of soil takes part in the phosphorus cycle. Another medium through which phosphates from the rocks is incorporated into soil is by the conversion of mined phosphates into fertilizers. The form in which the phosphorus is present in the soil is dependent on the pH (BookRags_Staff, 2005). In alkaline conditions, phosphorus is present as hydrogenphosphate (HPO42-). In case of lower pH (acidic condition) phosphorus is encountered as dihydrogenphosphate (H2PO4-). Plants have a better absorption towards dihydrogenphosphate as compared to hydrogenphosphate (BookRags_Staff, 2005). For the phosphorus to be in the form so that it is available to the plants, the pH range of the soil must be in the range of 6 to 7.
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Figure : Soil pH impacts P availability (Charles Hyland, 2005)
The phosphorus from soil or fertilizer is absorbed by the plants and trees into their tissues. Phosphorus remains as phosphate (PO43-) in the plants. In the next tropic level, these plant are consumed by the herbivorous animals, which in turn, are consumed by carnivores, who incorporate this phosphorus into teeth, bones, shells etc. When the plants and animals die, their phosphorus again enters into the soil by the action of decomposers and is made available for plants. Some of this soil erodes and deposits into the water bodies. By the sedimentation and solidification process, the phosphates are again converted back to rock, thereby completing the phosphorus cycle.
Due to the lack of involvement of atmosphere (gaseous) and higher dependence on the lithosphere and hydrosphere, the phosphorus cycle is the slowest cycle as compared to other matter cycles. Naturally, phosphorus is present in the form of phosphate rocks and move slowly to the living organisms and plants are absorption through the soil. Once these plants and animals decay and their available phosphorus enters the marine system, it takes millions of years for it to solidify and form rocks. Due to these long processes, phosphorus cycle is extremely slow (BTNEP, 2006).
Like other natural cycles, phosphorus cycle is extremely important to maintain equilibrium in nature. Any disturbance in this cycle can cause drastic effect and damage to all life forms on Earth. It has been known that phosphorus efficiently promotes the growth and development of algae in water bodies. This condition of excessive nutrition and high plant productivity in the aquatic system is called eutrophication (Smolen, 2004). Aquatic environment responds drastically to significant changes in phosphorus levels. Under such conditions, algae tend to have an advantage over other aquatic plant species and continue growing in excess phosphorus condition. This is known as algae boom (BTNEP, 2006). This increase algae lead to a formation of thick mat on the surface which in turn leads to decreased oxygen content in the lake or water body in two ways; first is due to the decrease in photosynthesis and the second is due to the increase in cellular respiration (BTNEP, 2006).
Human have been effecting the phosphorus cycles through varied activities like the cutting of the tropic forests and excessive use of chemical fertilizers. As the forests are cut down, the phosphorus present in the soil is easily eroded and washed into the water bodies. Similar is the case when excessive fertilizers are washed along with the rain water. This washed phosphorus causes the eutrophication process in the lake or water body. Some time back even detergents contained phosphorus which was also drained into the rivers and lakes contributing significantly to the increase phosphorus percentages.
Phosphorus is the tenth most abundant element on Earth and form nearly 0.1% of the Earth's crust. Due to the global phosphorus cycle, it can be found in various forms and states on the Earth. Due to the reactive nature of phosphorus, naturally it is not found in the Free State. In the mineralogical form there are nearly 300 naturally occurring minerals with phosphate as the required mineral structure (Jahnke, 2000). Most of the phosphorus is naturally present in the form of sedimentary rock. 95% of the all the phosphorus on the Earth's crust exists as apatite (Ca10 (P04)6X2). Different forms of apatite are distinguished by the anion present (X) in its hexagonal crystal structure, for example the apatite of Cl-, OH- and F- are known as chlorapatite, hydroxyapatite and fluorapatite respectively (Jahnke, 2000).
The presence of phosphorus is significant for the growth of aquatic plants and algae. In water, phosphorus is present in three different forms:
Dissolved Inorganic ortho-phosphates (DIP)
Dissolved organic phosphorus compounds (DOP)
Particulate phosphorus (PP)
The particulate phosphorus matter includes living and dead plankton, precipitates of phosphorus, phosphorus adsorbed to the particulates and amorphous phosphorus. The sum total of these three kinds of phosphorus is called the Total Phosphorus Content (TPC), which is used as the parameter to check the phosphorus pollution levels.
The Dissolved phosphorus, a major portion of which consist of ortho-phosphates, is readily available to for the absorption by the plants and aquatic animals. It generally results from the leaching of crop residues, human and animal waste or fertilizers. A small percentage of this dissolved phosphorus exists naturally in the soil and is transferred to the lakes or water bodies by runoff water (Smolen, 2004). Large sources of dissolved phosphorus are the freshly used fertilizers and manure available on the soil surface, which are washed away by the runoff water.
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Particulate phosphorus may be associated with soil particles and minerals containing aluminum, iron, or calcium or with organic matter like plant or animal materials (Smolen, 2004). Typically, particulate forms contain bacteria, algae, detritus, and inorganic particulates such as clays, smaller zooplankton, and occasionally, larger zooplankton, sediments, or large plant material (Carlson & Simpson, 1996). The movement of particulate phosphorus takes place through soil erosion and wash off from the surface of land. This form of phosphorus is only slowly available to aquatic animals before it sediments to the floor of the water body. Once deposited at the lake bed, particulate phosphorus converts to Dissolved Phosphorus in many years process.
The Dissolved Inorganic Phosphorus (DIP) is referred to as the ortho-phosphates. The plants and other flora takes up all the phosphorus in the ortho-phosphate form. Ortho-phosphates refer to the third derivative of the tri-protic phosphoric acid (H3PO4) also called the ortho-phosphoric acid. The concentration of the different phosphate species is dependent on equilibrium constant and the pH value of the media containing them. The final concentration for the ortho-phosphate is dependent on the equilibrium constants of the reactions given below (Jahnke, 2000).
H3PO4 ïƒ H+ + H2PO41-
H2PO41- ïƒ H+ + HPO42-
HPO42- ïƒ H+ + PO43-
From the calculation it has been derived that in the fresh water systems, with pH range of roughly 6 to 7, H2PO41- is the dominant phosphorus species (Jahnke, 2000). In sea water the dissociation parameters of phosphoric acid are significantly altered due to the presence of free cations like Ca2+, Mg2+, Na+ etc which form salts with different phosphate species. So, in case of sea water, which roughly has the pH of 8, HPO42- is the dominant species.
Figure : Calculated speciation of PO43- in seawater of 34.8 parts per thousand salinity, at 20Â°C and a pH of 8.0 (Jahnke, 2000)
Therefore, the amount of phosphorus in the PO43- state is dependent on the cationic species present in the solution and pH of the solution. For the standard sea water, only 0.01% of the total phosphorus is present in the PO43-. It is noteworthy that these acid-base and complex reaction are not only important in seawater systems, but also influence the reactivity of P043- in groundwater and freshwater systems (Jahnke, 2000).
The dissolved ortho-phosphate form of phosphorus can also be found in the soil. Plants consume the ortho-phosphate from the water bodies and when these plants decay, the phosphorus present in them is released into the soil in the organic phosphate form. This organic form is them converted back to ortho-phosphate by bacteria and possibly phytoplankton (Watt & Hayes). But this ortho-phosphate, if not utilized by the plants again is washed off to the water bodies.
Organic Phosphorus Compound
Phosphate is the requirement for the basic functioning of plants, animals as well as human beings. Organic phosphate is the phosphate that is bound to plant and/or animal tissue. Organic phosphates are formed primarily by biological processes (Murphy, 2007).
Organic phosphate compounds consist of long complicated organic molecules with phosphate ester bonds. The essential organic compounds in which phosphate is a necessary constituent are nucleic acids, DNA and RNA. These are long polymeric chains in which phosphate is covalently linked with mononucleotide units (Jahnke, 2000). These are all vital components in animal and human body, and contain necessary genetic information. Apart from this, phosphates play a major role in the formation of adenosine triphosphate (ATP), phospholipids and apatite. ATP controls the chemical energy within the cell through the hydrolysis of the terminal phosphate ester bond, phospholipids are important components of cell membrane and apatite forms structural body parts like teeth and bones (Jahnke, 2000).
Organic phosphate naturally exists in the biological system. They are the part of sewage by the body waste and food residue which may be transformed into ortho-phosphate by the biological treatment by bacteria and possibly phytoplankton (Watt & Hayes). Organic phosphates can also be introduced by the breakdown of organic pesticides that contain phosphates (Murphy, 2007). They are also present in water bodies and aquatic organisms in the form of loose fragments.
Soluble Reactive Phosphorus
In a lake or any other water body phosphorus can be present in three different states, namely as Dissolved Inorganic (Ortho-) phosphates (DIP), Dissolved organic phosphorus compounds (DOP) and Particulate phosphorus (PP). A different form of characterization for phosphorus present in natural waters is done as (Rigler, 1973):
soluble reactive phosphorus (SRP)
soluble un-reactive or soluble organic phosphorus (SUP)
particulate phosphorus (PP)
Therefore in order to characterize the water quality various quantitative tools are used. Total Phosphorus (TP) is the measure of all the three forms of phosphorus present in the water sample, that is, it is the total phosphorus content (Murphy, 2007). The sum of soluble reactive phosphorus (SRP) and soluble un-reactive phosphorus (SUP) is known as the Soluble Phosphorus (SP).
Soluble Reactive Phosphorus (SRP) is largly the measure of the ortho-phosphate, which is the soluble inorganic form of phosphorus, directly taken up by the plants. The constituent fraction of this phosphorus is the index of the amount of phosphorus available for the plant growth, especially algae (Carlson & Simpson, 1996). For a limited phosphorus condition, the concentration of the SRP is extremely low (< 5Î¼g/L). For an increased value of ortho-phosphate it can be inferred that the phosphorus is not needed by the algae or it is being supplied at rates faster than being consumed by the biota (Carlson & Simpson, 1996).
The term 'reactive' in SRP is represents that the phosphorus measured in the SRP fraction is not just the inorganic phosphorus but could be any form of phosphorus, including organic phosphorus, that reacts with the reagent (Carlson & Simpson, 1996). Therefore the actual composition of SRP varies with the nature of the water body. The solubility of phosphorus phase is characterized by the filter used, which typically by standards is a 0.45 micron cellulose filter (Carlson & Simpson, 1996).
The quantitative analysis of SRP fraction is done by spectrophotometry or colorimetry using the principle of Lambert Beer's Law. For the detection, water sample is treated with molybdate reagent, which reacts with orthophosphate producing phosphomolybdic acid (Carlson & Simpson, 1996). Phosphomolybdic acid forms the color molybdenum blue upon reduction with ascorbic acid. This gives a peak absorbance at 885 nm in the infrared region (Carlson & Simpson, 1996) and it is linearly related to concentration by the Lambert Beer's Law. This method can be used to measure phosphate concentration in the range of 5 Î¼g/L and 1300 Î¼g/L.
Causes of Phosphorus Occurrence
Phosphorus is a highly reactive element and therefore does not occur in the free state naturally. In the mineralogical form, it is present as phosphate salt which are sedimentary in nature have deposited in millions of years. Due to the presence of phosphorus cycle apart from the mineralogical state phosphorus is also present in the rivers, lakes, oceans and seas, and groundwater. In the phosphorus cycle, atmosphere does not have a major role therefore all the phosphorus is mainly present in lithosphere and hydrosphere.
A varied number of natural and anthropogenic sources contribute to the net phosphorus present in the river or stream through varied number of pathways. Phosphorus is naturally added into the rivers by weathering of soil surface, migratory fish that return to their spawning grounds, riparian vegetation and river bank corrosion (Withersa & Jarvieb, 2008). Such sources generally add very small amount of phosphorus to the rivers and are generally in the particulate form.
The anthropogenic sources of phosphorus in rivers and streams are divided into two categories namely, 'point sources' and 'diffuse sources'. Point sources discharge higher concentration of phosphorus from discrete points whereas in diffuse sources, phosphorus arrives from runoffs due to storms. Waste waters from industries and homes, discharged into the rivers and streams are the point source of phosphorus. Even the poorly designed septic tanks in which the waste is dumped into the rivers, are the source of phosphorus The runoffs form the surfaces is another source of phosphorus in rivers. Runoffs from the gardens and farms have higher percentage of phosphorus due to the use of fertilizers (Kreutzweiser, Hazlett, & Gunn, 2008).
Lakes also have the same sources as that of river and streams. An added source of phosphorus in lakes is the presence of aquatic plants at the bottom of the lake surface which on decomposition release phosphorus into the lake. Even the decomposition of dead aquatic animal and their waste leads to increment in total phosphorus but their contribution is very small. Therefore lakes have a higher percentage of phosphorus in dissolved inorganic phosphate or ortho-phosphate form.
Oceans and seas like lakes and rivers have both point sources and diffuse sources. But in case of oceans and seas, the phosphorus content is also contributed by the phosphorus for the river and streams which merge into the sea. River runoffs are the major source of phosphorus in oceans. It delivers annually about 1.5 Mt of dissolved phosphorus and more than 20 Mt of suspended phosphorus into the ocean (Baturin, 2004).
In case of ground water, the phosphorus is made available due the pervious nature of the soil and rocks. This allows the penetration of dissolved phosphorus into the ground. Phosphates and its naturally occurring compounds are mostly either solid or liquid in state, so the source of precipitation is the particulate phosphorus which is present on the soil surface. This particulate phosphorus is carried by the wind in the form of dust particles, which causes precipitation.
Effect of Phosphorus in Water Bodies
Phosphorus is one of the most important elements due to its requirement in all like forms and has a major role in biological metabolism. In the form of phosphates it forms the structural framework which holds DNA and RNA together. They are also an essential component in ATP, which carries cellular energy. Nearly 80% of the phosphorus in human body is found in teeth and bones in the form of apatite (ELC, 2008).
Urbanization and agricultural intensification has led to a phosphorus imbalance in nature. Over the years, surface waters have become enriched in phosphorus leading to various environmental and health concerns. Various water sources have different effects depending upon the amount of total phosphorus present and flora and fauna associated with the water body. The effect of phosphorus on all the surface water bodies is similar.
Phosphorus is the basic nutrient for growth and development of algae and other aquatic plants. Without the presence of phosphorus there would be minimal plant growth. But excess phosphorus in water can promote algae and plant growth extensively. The progressive deterioration of water quality from overstimulation by nutrients is called eutrophication (Lory, 1999). As nutrient concentrations increase, surface water quality is degraded through the process of eutrophication. The following sequence characterizes changes in surface water quality with increasing nutrient concentration (Lory, 1999):
increased algae growth
reduced water clarity
water treatment problems
odor and bad taste
increased filtration costs
disinfectant byproducts with potential human health effects
reduced oxygen in the water
toxins from cyanobacteria (blue-green algae) affecting human and animal health
In rivers, as these algae die they are decomposed by bacteria, which use the dissolved oxygen for the decomposition purpose. Due to this process the dissolved oxygen in the water is reduced and sometimes dropping too low for the fishes to breathe; thus killing the fishes and other aquatic animals (Murphy, 2007).
In lakes, another process takes place parallel to this. Under the high nutrition condition in lakes, algae have a higher growth rate compared to other aquatic species, in a conditions is known as algae boom (BTNEP, 2006). Due to the stagnation of water in lakes, algae grow to form a thick mat on the surface of the lakes. This results in a decrease in the amount of sunlight that reaches the photosynthetic organisms under the mat during the day; thereby decreases the amount of photosynthesis and subsequently the amount of oxygen produced by photosynthesis. It is known that benthic photosynthetic organisms can die if too little sunlight reaches them. Dead organic matter becomes a food source for decomposers causing increased cellular respiration. Cellular respiration requires oxygen and this reduces the dissolved oxygen in the body of water (BTNEP, 2006).
Excessive algae growth also causes oxygen imbalance. During the day, algae take up carbon dioxide for the photosynthesis process, thus resulting in the increased oxygen content during the day. At nights, algae require oxygen for metabolism, resulting in the increased carbon dioxide content. This causes fluctuations in oxygen levels through the day (Shock & Pratt, 2003).
Ground water, lakes and rivers are the sources of fresh water for animal and humans. Due to high phosphorus content in water, it may have taste and odor problems; the treatment of which can result in increased cost of drinking water. Finally, high nutrient concentration cause reduced water clarity, hostile swimming condition, unpleasant odor, increased toxic and non toxic organisms and polluted appearance; thus hampering the aesthetic value of water resources.
Policies for Phosphorus Pollution Reduction
With increasing interference by the human, aquatic phosphorus cycles have been disturbed. Excessive use of fertilizers and cutting of tropical forests have been the main cause for phosphorus level variations in water bodies. Regulations are therefore required to maintain the phosphorus contents within the desired healthy limits.
There has been a serious concern regarding the increasing phosphorus content in the water bodies, therefore the Ministry of Environment in Canada has amended the phosphorus concentration regulations in the Environment Protection Act, 1999. The regulation, (amended in 2009 would come into force from July 1, 2010) list the allowable limit of phosphorus in the cleaning products used for domestic as well as industrial purposes. According to the law, "the concentration of phosphorus in any household laundry detergent must not exceed 1.1% by weight expressed as phosphorus pentoxide or 0.5% by weight expressed as elemental phosphorus". Whereas in case of commercial or industrial laundry detergent, concentration of phosphorus must not exceed 5% by weight expressed as phosphorus pentoxide or 2.2% by weight expressed as elemental phosphorus. Similar measures have also been taken in other countries like United States and European Union.
Phosphorus addition in soil and water bodies is dominated by agricultural use of fertilizers which have high percentage of phosphorus. These concerns about the phosphorus led to the establishment of the Alberta Soil Phosphorus Limits Project in 1999 (Sutton, 2008). The objectives of the project were (Sutton, 2008):
To develop recommendations for phosphorus limits for agricultural land in Alberta;
To determine implications of soil phosphorus limits to the agricultural industry;
To identify management options for soil phosphorus limit implementation; and
To develop recommendations for an action plan and a time line for implementation of limits.
A number of states in the United States including Arkansas, Delaware, Ohio, Oklahoma, Michigan, Texas and Wisconsin have identified maximum soil-test phosphorus levels of between 150 to 200 ppm. These states recognize that soil-test phosphorus levels in excess of 200 ppm have the potential for unacceptable phosphorus losses in runoff that exceed any reasonable crop requirement concerns (Sutton, 2008).
The current need of the hour is to implement the proposed policies so that phosphorus levels in the environment and specially water bodies, is maintained within the safe limits as listed in the regulations.
Phosphorus is the tenth most abundantly available element, forming nearly 0.01% of the Earth's crust. But due to its reactive nature it does not occur in the phosphorus element state, rather it exists as the salts of phosphates, which are present in mineralogical form (as apatite), in water bodies (as phosphates), in bones and teeth of animals and they also for an essential part in DNA, RNA and ATP cells. Phosphorus is essentially in the growth and development of plants and therefore it is used in fertilizers for accelerated growth. Plants absorb phosphorus in the soluble inorganic state also known as the ortho-phosphates.
With years of weathering and sedimentation process, phosphorus cycle has maintained the appropriate level of phosphorus in the environment. But due to increased interference of humans with the environment, this balance of phosphorus has been disturbed. The activities like deforestation, excessive use of fertilizers, dumping waste water from homes etc have been the major cause for this imbalance. Fertilizers and detergents contain phosphorus which is washed off with the water into the water bodies, thus causing an increased phosphorus percentage. This increased phosphorus percentage causes accelerated algae growth, which causes harm to plant and animal species by reducing the dissolved oxygen in the water.
Therefore, it is the need of the hour to design policies which govern the used of phosphorus in fertilizers and detergents so that the flora and fauna associated with the water bodies can be protected.