What Is Phosphate? A Detailed Explanation of Wastewater-Treatment Terminology
Structure of the phosphate functional group in acidic solution. In alkaline solution, the group releases two hydrogen atoms and dissociates into phosphate carrying a -2 formal charge. The phosphate ion is a polyatomic ion containing one phosphorus atom surrounded by four oxygen atoms, forming a tetrahedron. The phosphate ion carries a -3 formal charge and is the conjugate base of the hydrogen phosphate ion; the hydrogen phosphate ion is in turn the conjugate base of the dihydrogen phosphate ion; and the dihydrogen phosphate ion is the conjugate base of phosphoric acid. It is a hypervalent molecule (the phosphorus atom has 10 electrons in its valence shell). Phosphate is also an organophosphorus compound with the chemical formula OP(OR)3.
Except for some alkali metals, most phosphates are insoluble in water under standard conditions.
In dilute aqueous solution, phosphate exists in four forms. In strongly alkaline environments, phosphate ions predominate; in weakly alkaline environments, hydrogen phosphate ions predominate. In weakly acidic environments, dihydrogen phosphate ions are more common; and in strongly acidic environments, water-soluble phosphoric acid is the main existing form. [1]
Phosphate is prepared mainly via three process routes: the thermal process, the wet process, and the electrochemical process.
Prepared by heating the reaction of phosphoric acid and hydrochloric acid, it is energy-intensive (14,000-15,000 kWh per ton) and mainly applied in yellow phosphorus production; currently only a few enterprises in Europe and America retain this production line.
Dominated by the sulfuric acid process (80% of capacity), it is prepared through phosphate-rock acidolysis, defluorination and other steps, offering the advantage of continuous production. The defluorination rate must reach above 95%, the key being control of roasting temperature (800-950 degC) and the phosphoric-acid ratio. The key technology for breaking through an 85% phosphorus-recovery rate was developed by the Sichuan Lomon Group, and 10,000-ton-class pilot units have been built in Shanghai, Lanzhou and elsewhere, but large-scale application has not yet been realized due to the requirement for high-grade phosphate rock.
Produced by electrolysis of the reaction between phosphoric acid and hydrochloric acid; currently little industrial application. China has achieved a breakthrough in particulate-product preparation technology, raising single-line capacity from 2,000 t/year to the 500,000-ton class, replacing imported products.
Phosphate is generally used in detergents as a water softener, but because the algal bloom-decay cycle affects phosphate discharge in watersheds, phosphate detergents are regulated in some regions.
In agriculture, phosphate is one of the three major plant nutrients and the main component of fertilizer. Phosphate rock powder is mined from sedimentary phosphate layers. Previously it could be used without processing after mining, but now unprocessed phosphate is used only in organic farming. Generally it is chemically processed into superphosphate, triple superphosphate, or monoammonium phosphate, which have higher concentrations than phosphate and are more readily soluble in water, so plants can absorb them faster.
The three numbers on a fertilizer bag indicate the grade; in order, the first number is the percentage of elemental nitrogen (N), the second is available phosphate (P2O5), and the third is soluble potash (K2O). [6]
Phosphate runoff from over-fertilized farmland can be the cause of eutrophication, red tides, and the subsequent hypoxia. Like phosphate detergents, this causes hypoxia in fish and other aquatic organisms. [3]
Phosphate is used as a binder in refractory materials. A phosphate binder is a refractory binder with cementitious properties whose main compound is an acidic orthophosphate or a condensed phosphate. The binding form of phosphate binders is chemical-reaction binding or polymerization binding. Binders made by reacting phosphoric acid with alkali-metal or alkaline-earth-metal oxides and their hydroxides are mostly air-hardening binders, which set and harden at room temperature without heating. Binders made by reacting phosphoric acid with amphoteric oxides and hydroxides or acidic oxides are mostly heat-hardening binders, which must be heated to a certain temperature before a reaction occurs to produce setting and hardening. Phosphate used as a binder for refractory materials has strong binding strength in the middle- and low-temperature range before ceramic bonding is produced, so it is widely used as a binder for unshaped refractories and unfired refractories. [3]
Phosphorus is an essential and important mineral element for the human body. The main sources of phosphorus intake are natural foods or food phosphate additives; phosphate is one of the natural components of almost all foods. Because phosphate can improve or impart a series of excellent properties to food, it has been used in food processing since over a hundred years ago, with large-scale use beginning after the 1970s. Phosphate is one of the most widely used and largest-volume classes of food additives, applied extensively as an important food ingredient and functional additive in the processing of meat products, poultry products, seafood, fruits, vegetables, dairy products, baked goods, beverages, potato products, seasonings, convenience foods, etc. The phosphates used in food processing are usually sodium, calcium, and potassium salts, as well as iron and zinc salts used as nutritional fortifiers. There are more than thirty commonly used food-grade phosphate varieties; sodium phosphates are the main consumption category of food phosphates in China, and with the development of food-processing technology, potassium phosphate consumption is also rising year by year. [4-5]
Phosphates are divided into orthophosphates and condensed phosphates.
Orthophosphoric acid is a tribasic acid with three kinds of orthophosphates:
1. Dihydrogen phosphate MH2PO4, also called first-generation phosphate, all soluble in water;
Metaphosphate is usually a compound polymerized into a ring, with the general formula (MPO3)n. Common ones are dimeric metaphosphate (six-membered ring) and tetrameric metaphosphate (eight-membered ring). Polyphosphate has no definite crystal structure and is also called phosphate glass. Sodium hexametaphosphate is the most common phosphate glass; it has no fixed melting point, its solubility in water is variable, and the pH of its aqueous solution is between 5.5 and 6.4; it is in fact a long-chain compound with 20-100 PO3 units. Chain phosphates can be used as boiler-water treatment agents, pigment dispersants, slime dispersants, and metal anticorrosives.
3. Orthophosphate M3PO4, also called third-generation phosphate.
The latter two, except for sodium, potassium, and ammonium salts, are generally insoluble in water. M can be a metal of valence other than monovalent. Sodium dihydrogen phosphate is used to control the hydrogen-ion concentration of a solution; disodium hydrogen phosphate is used in water treatment as a precipitant for multivalent metals; trisodium phosphate is used to manufacture soap and detergents.
Pyrophosphoric acid is a tetrabasic acid with four kinds of pyrophosphates, of which the M2H2P2O7 and M4P2O7 types are common.
2. Hydrogen phosphate M2HPO4, also called second-generation phosphate;
Phosphate ions can form the characteristic yellow precipitate ammonium phosphomolybdate, which can be used for analysis and identification. [1]
Phosphate is the naturally occurring form of elemental phosphorus, found in various phosphate minerals. Elemental phosphorus or phosphides are very rare (only trace amounts are found in meteorites). In mineralogy and geology, phosphate refers to stone or ore containing phosphate ions.
Structure of the phosphate functional group in acidic solution
In North America, the largest phosphate-rock deposits are located in central Florida, Soda Springs in Idaho, and the coastal region of North Carolina. Next are Montana, Tennessee, Georgia, and near Charleston in South Carolina. The tiny island nation of Nauru once had large quantities of high-quality phosphate minerals, but they have now been heavily mined. Phosphate rock powder can also be found in Navassa Island, Morocco, Tunisia, Israel, Togo, and Jordan, all of which also have large phosphate-mining industries.
In organisms, phosphorus appears in the form of free phosphate ions in solution, called inorganic phosphate, which must be distinguished from phosphate in phosphate esters. Inorganic phosphate is denoted Pi and can be obtained by hydrolysis of pyrophosphate (denoted PPi).
However, phosphate most commonly appears in the form of adenosine monophosphate (AMP), adenosine diphosphate (ADP), adenosine triphosphate (ATP), deoxyribonucleic acid (DNA), and ribonucleic acid (RNA), and can be released by hydrolysis of ADP or ATP. Similar reactions occur for other di- and tri-phosphonucleosides. The phosphoanhydride bonds in ADP and ATP, or other di- and tri-phosphonucleosides, contain large amounts of energy and therefore play an important role in organisms. They are generally called high-energy phosphates, like phosphocreatine in muscle tissue. Some compounds such as phosphines are also used in organic chemistry, but they seem to have no natural counterparts.
Because of the importance of phosphate to organisms, it is heavily mined ecologically. Therefore, it is often a limiting reagent in the environment, and its availability determines the rate of biological growth. Adding large amounts of phosphate to a phosphate-deficient environment or microbial environment has a major impact on the ecology. For example, the explosion of one organism can cause the death of others, and the reduction of one organism's numbers can cause a lack of resources such as oxygen (see eutrophication). Under pollution problems, phosphate is a major component of total dissolved solids (a principal water-quality indicator). [2]
