Why is phosphorus stored in water

The average phosphorus content in seawater is around 60 ppm, with the concentrations being very variable and significantly lower in the upper water layers than in the deep sea. River water generally contains phosphorus concentrations of around 20 ppb.
In dissolved inorganic form, the element comes mainly as HPO42-, but also as H2PO4-, PO43- and H3PO4 in front. There is also MgPO in the oceans4- contain, whereas the occurrence of phosphate is very low and affects at most the upper water layers.


How and in which compounds does phosphorus react with water?

As for the reaction of phosphorus with water, it is known that so-called white phosphorus glows in the dark when it comes into contact with moist air.
Many phosphorus compounds hydrolyze in water, which applies, for example, to phosphorus (III) hydrogen, phosphorus oxychloride and phosphorus trichloride. Phosphorus (V) chloride reacts more violently with water to form HCl and H3PO4 or also POCl3 and HCl. Phosphorus (V) oxide reacts under strong heating with water to be corrosive phosphoric acid.
Fig .: White phosphorus under water (wikipedia)

An example of the hydrolysis of metal phosphides is as follows:

Approx3P.2 + 6H2O -> 2PH3 + 3Ca (OH)2.

Phosphorus trifluoride also hydrolyzes in water, albeit more slowly than other trihalides:

PF3 + 3H2O -> H3PO3 + 3HF.


Water solubility of phosphorus and / or its compounds

Elemental phosphorus is insoluble in water under normal conditions. On the other hand, iron (II) phosphate and phosphorous acid have good water solubility. Phosphoric acid is even soluble in water up to 5700 g / L, while phosphoric acid tri (2-chloroethyl) ester is only 5-7 g / L. Phosphines are almost insoluble in water at normal air pressure.

Solubility and how it can be influenced

How can phosphorus get into the water?

Phosphates, the salts of phosphoric acid, occur in nature in many places, also because they are contained in numerous rocks. Phosphorus can be found in the form of various apatites, such as carbonate fluorapatite in phosphorite.
Phosphates do not only find their way into water and other environmental compartments naturally (phosphorus cycle), but also through human activities. Phosphate-containing fertilizers, which are flushed into the water, play an important role here. Around 90% of the phosphorus used commercially is used for this purpose. For example, phosphoric acid can be used to produce this fertilizer. In the past, bones, later rocks containing phosphate, were treated with sulfuric acid and used for these purposes. Natural guano is also an excellent source of phosphate.
Dung from cattle, pigs and horses contains around 1-2 g of phosphorus per kilogram. In 1980, livestock husbandry excreted around seven times as much phosphate worldwide as humans.
In addition, the element is released in agriculture through the use of organophosphorus herbicides, insecticides and fungicides.
Phosphates from detergents and cleaning agents and human excrement find their way into municipal wastewater. The use in the former was very extreme, as phosphates were used, for example, as water softeners in detergents, but has largely declined. They also have the property of holding the dirt removed from the clothing in suspension so that it does not settle on the clothing again.
Other phosphorus compounds are also used commercially. The element is contained in smoke bombs and incendiary bombs and is used for the production of phosphoric acid and other chemicals. The so-called red phosphorus plays an important role in the manufacture of matches. The element is also used to manufacture semiconductors. Organophosphorus compounds can be used as flame retardants, as an additive to lubricating oil, in chemical warfare agents and in metal extraction. Phosphoric acid can be used as a food additive in very diluted form and can serve as a protective layer to protect metal from rust or to remove rust. Other phosphorus compounds are good plasticizers or are used in paints and adhesives. Disodium phosphate is used in the manufacture of glass and ceramics and in the tanning of leather. The element is contained in toothpaste and, in the form of metal phosphides, also in light-emitting diodes.
Black phosphorus conducts electricity and the isotope 32P is used as a tracer in research.


What environmental problems can water pollution with phosphorus cause?

Phosphorus, in the form of phosphate, is an important nutrient for both plants and animals. It is even essential for all types of organisms. Phosphates are therefore generally considered to be non-toxic. The element is needed to build phospholipids, which make up cell membranes, nucleic acids and other compounds. In addition, in the form of ATP (adenosine triphosphate) it is an energy-transferring molecule that plays an important role in numerous biochemical processes.
The phosphorus concentration in normal air-dry soils is around 200-800 ppm, of which usually around half is both inorganically and organically bound. Phosphorus minerals are often too poorly soluble to release phosphate in a plant-available manner, which is why phosphate is often a limiting factor.
The plant-available phosphate ion is strongly bound to the soil. This happens as an adsorbed anion, but above all as insoluble iron or aluminum phosphate at low pH values ​​and as calcium phosphate at higher pH values. In the extreme pH ranges, this bond is so strong that the phosphate is no longer available to the plants. The best availability is achieved at pH 6-7.
Plants consist of around 3% (based on dry matter) phosphorus. If there is a lack of phosphorus, their growth is inhibited and their leaves take on a dark green to blue-violet color. This is the reason why phosphate fertilizers are widely used in agriculture. A positive side effect of phosphate is that traces of potentially harmful metals contained in fertilizers are stabilized by phosphate and cannot be absorbed by plants.
Although phosphates are less mobile than nitrates, when the soil is saturated they can reach deeper soil and groundwater layers and lead to dystrophy. They can also cause serious environmental problems in water bodies. This is mainly due to their special role in the eutrophication of slow-flowing or stagnant waters. Phosphate is the growth-limiting factor for the phytoplankton in natural waters. This function as a minimum factor no longer exists when the phosphate content increases extremely. The availability of nutrients becomes so great that a so-called algae bloom occurs. When the dead algae are broken down, a great deal of oxygen is consumed, which lowers the concentration in this to such an extent that many aquatic organisms die (see also oxygen and water, nitrogen and water).
First and foremost, the increased growth of algae leads to a clouding of the top layer. This reduces the incidence of light, which restricts photosynthesis and thus has a self-regulating effect. The proliferation of zooplankton, which feed on algae, also contributes to this. Nevertheless, there is ultimately excessive oxygen consumption due to the breakdown of dead plant remains and plankton. As insoluble iron (III) phosphate, the phosphorus is withdrawn from the biological cycle.
In the event of one-off increased phosphate inputs, no long-term effects on the ecological status of the water body can be determined. However, if these take place continuously, the oxygen content can drop to almost zero, especially in layers close to the ground, where most of the decomposition processes take place. This also means that aerobic degradation of biomass can no longer take place. Anaerobic bacteria continue the process, but at the same time produce not only methane but also toxic metabolic products such as ammonia. The conversion of iron (III) phosphate to readily soluble iron (II) phosphate, which takes place under the reducing conditions, brings it back into the biological cycle and stimulates biomass production again.
By adding 1 g of phosphorus, around 100 g of biomass can be produced, which in turn consumes 140 g of oxygen when it is decomposed. The organisms living in the water eventually perish.
If phosphates get into rivers, they are usually washed directly into the sea, where they are slowly sedimented. The phosphate concentration in the oceans is very low, which is due to the insolubility of calcium, magnesium and iron phosphates. It also sinks as an organic deposit.
The entry of phosphate from detergents and cleaning agents into bodies of water has decreased significantly, but phosphate substitutes can also have a negative effect on the environment. In addition, insecticides containing phosphorus are still used, which are usually considered to be harmless to the environment, but the bound residues of which are still unclear.
In the animal organism, phosphate not only takes on the functions described above, but is also a component of all animal bones in the form of calcium phosphate. It is also found in teeth.
Phosphate compounds can have a toxic effect on various organisms. Phosphate esters, used as insecticides, are also toxic to many aquatic animals. Elemental phosphorus and phosphorus (III) hydrogen are also considered toxic to fish. They also have a negative effect on the central nervous system of mammals and irritate the lungs.
Some specific values ​​for the toxicity of phosphorus compounds are also known. For example, phosphorus oxychloride has an LD50-Value of 380 mg / kg when taken orally by the rat. This indicates the dose at which half of a population dies. The LD50The value of triphenyl phosphite when taken orally by the rat is 1600 mg / kg and that of phosphoric acid triethyl ester when taken orally by rats or mice is 1.3-1.6 g / kg.
Toxicities in the form of LC are also for aquatic organisms50Values ​​are known that indicate the concentration of a substance in water at which 50% of a population die. For phosphoric acid triethyl ester this is 2.1 g / L and for phosphoric acid tri (2-chloroethyl) ester 200 mg / L for the golden orfe in 48 hours.
Phosphorus (V) is considered to be a weakly water-polluting substance. The toxicity of elemental phosphorus is one LD50-Value of 15 g / kg in rats and in an LC50-Value of 100 mg / L for the zebrafish.
A single stable isotope of phosphorus exists naturally, but there are also seven unstable isotopes.


What health effects can phosphorus cause in water?

Phosphorus is an essential element in the human body, about 1.1% of which is contained in it. The daily intake through food is the equivalent of 1-2 g of phosphorus, while the body only needs around 800 mg. The highest levels of phosphorus are found in animal food. About 60% of the ingested phosphate is absorbed in the small intestine. The rest is eliminated again. Most of the time we absorb phosphate bound to calcium, which means that if there is sufficient calcium intake, adequate phosphorus absorption is almost always guaranteed. Phosphate deficiency does not normally occur, which means that there are no known deficiency symptoms in adults. Very low birth weight newborns who are breast-fed are more likely to be low in phosphorus, which can lead to hypophosphatemic rickets. If the calcium intake is too low, phosphate can have a negative impact on the bones and increase the risk of osteoporosis.
90% of the phosphorus is in the skeleton. It is also the main component of tooth enamel, also in the form of hydroxyapatite. This can bind fluorine ions and to a certain extent convert hydroxyapatite into stronger fluorapatite, which is why fluoride-containing mouthwashes are offered.
Phosphorus is also needed to build up phospholipids, DNA and ATP. The latter has the effect of a chemical battery and can store energy and only release it again when it is needed. More than one kilogram of ATP is produced, consumed and recycled every hour, for which glucose has to be broken down. Phosphorus also plays a role in the transport of substances in the blood and numerous metabolic processes. It is a component of enzymes and is also contained in the coenzyme NADP, which is important for enzyme processes. The DNA, ATP and most of the organophosphates are negatively charged, which is mainly offset by positively charged magnesium ions.
Elemental phosphorus is toxic to humans, which is especially true of white phosphorus. It attacks the liver and thus leads to death within a week of ingestion of a substantial amount. Amounts of 60-100 mg of dissolved phosphorus are considered lethal to an adult human. In small children this is only about 3 mg. In nature, however, it only occurs in the form of phosphate ions and organophosphates.
Inorganic phosphates are considered to be relatively harmless. However, they can decrease zinc absorption. In addition, an excess in the body can cause hyperactivity in children. Phosphates as food additives are considered safe. However, there are also toxic phosphorus compounds.
Inhaling phosphorus vapor can cause changes in the jawbone.


Which water purification technologies can be used to remove phosphorus?

The phosphorus found in wastewater is mostly phosphates and in turn orthophosphate, polyphosphate and organically bound phosphate. However, the proportion of inorganic orthophosphate predominates. The most common methods of phosphate elimination are precipitation treatment and biological phosphate removal. Precipitation can take place as pre-precipitation during primary clarification or in special precipitation and secondary clarification basins. Simultaneous precipitation with biological wastewater treatment in the activated sludge plants is also possible. The biological phosphate removal falls into the third purification stage of a sewage treatment plant.
Effective agents for the precipitation of phosphates are, for example, aluminum sulfate, iron (III) chloride and lime. The precipitation of phosphates can be supported by changes in the pH value, since their solubility is strongly pH-dependent. The more alkaline a solution, the more phosphate ions form poorly soluble precipitates. The precipitation of polyphosphates is only possible after a previous hydrolysis (P.3O105- + 6H2O -> 3 PO43- + 4H3O+).
The phosphate flakes formed can be allowed to sink after precipitation or filtered off with sand filters. There is also the possibility of precipitating phosphate on grains of sand or even grains of magnetite. The latter can be removed from the water with the help of magnets. A disadvantage of using precipitants is that iron and aluminum salts are brought into water in this way.
During biological phosphate removal, microorganisms absorb the phosphate in their biomass. It is a type of eutrophication under controlled conditions. However, if too much phosphate gets into the sewage treatment plant, the water in it also tips over. In addition, a favorable ratio of carbon, nitrogen and phosphorus is important for the effectiveness of biological wastewater treatment. Some bacterial strains can absorb more phosphorus than is necessary for their cell growth and store it in the form of polyphosphates. This happens especially when there is a rapid change from anaerobic and aerobic states, which is why the process can be combined well with nitrification and denitrification during nitrogen removal (see nitrogen and water).
Phosphorus, which is ultimately found in the cell material, is stored in so-called activated sludge flakes. It can later be used again as a fertilizer.
The use of water hyacinths or marsh plants is also suitable for the biological removal of phosphorus and nitrogen.
Another possibility is the removal of phosphate ions with the help of ion exchangers.
Phosphates themselves are also used to soften water, for example.

References


To the periodic table of the elements

To the overview of the elements and water