Phosphates in Water and Industry: Speciation, Production Routes and Why the Chemistry Drives Eutrophication
Phosphate chemistry explains why one element is simultaneously a fertiliser, a food additive, a refractory binder and the usual suspect in a eutrophication investigation. The behaviour comes down to which protonation state is present, and that is decided by pH.
One Ion, Four Forms
The phosphate ion is polyatomic: a single phosphorus atom surrounded by four oxygen atoms in a tetrahedron, carrying a -3 formal charge. It is the conjugate base of the hydrogen phosphate ion, which is in turn the conjugate base of the dihydrogen phosphate ion, itself the conjugate base of phosphoric acid. With ten electrons in its valence shell it is a hypervalent molecule. As an organophosphorus compound it takes the chemical formula OP(OR)3. In acidic solution the functional group keeps its hydrogens; in alkaline solution it releases two of them and dissociates into phosphate carrying a -2 formal charge.
In dilute aqueous solution, phosphate exists in four forms. Strongly alkaline environments favour phosphate ions; weakly alkaline environments favour hydrogen phosphate; weakly acidic environments favour dihydrogen phosphate; and in strongly acidic solution the dominant species is water-soluble phosphoric acid. Except for some alkali metals, most phosphates are insoluble in water under standard conditions.
Salts divide into orthophosphates and condensed phosphates. Orthophosphoric acid is tribasic and yields three orthophosphates: dihydrogen phosphate MH2PO4, the first-generation phosphate, all soluble in water; hydrogen phosphate M2HPO4, the second generation; and orthophosphate M3PO4, the third. Apart from the sodium, potassium and ammonium salts, the latter two are generally insoluble - and M here can be any metal of valence other than monovalent. Pyrophosphoric acid is tetrabasic with four pyrophosphates, of which the M2H2P2O7 and M4P2O7 types are common. Metaphosphate polymerises into rings with the general formula (MPO3)n, most often the dimeric six-membered ring or the tetrameric eight-membered ring. Polyphosphate has no definite crystal structure and is called phosphate glass; sodium hexametaphosphate is the familiar example, with no fixed melting point, variable water solubility and an aqueous pH between 5.5 and 6.4 - in reality a long-chain compound of 20-100 PO3 units.
Uses follow the structure. Sodium dihydrogen phosphate controls the hydrogen-ion concentration of a solution; disodium hydrogen phosphate works in water treatment as a precipitant for multivalent metals; trisodium phosphate goes into soap and detergents. Chain phosphates serve as boiler-water treatment agents, pigment dispersants, slime dispersants and metal anticorrosives. Phosphate ions also form the characteristic yellow precipitate ammonium phosphomolybdate, which is used for analysis and identification.
How Phosphate Is Made
Three process routes exist. The thermal process heats the reaction of phosphoric acid and hydrochloric acid and is extremely energy-intensive at 14,000-15,000 kWh per ton; it survives mainly in yellow phosphorus production, and only a few enterprises in Europe and America still run the line. The wet process dominates at about 80% of capacity, preparing phosphate through rock acidolysis, defluorination and related steps, with the advantage of continuous production. The defluorination rate must reach above 95%, and the controlling variables are roasting temperature (800-950 degrees C) and the phosphoric acid ratio. A 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 - though large-scale application has not yet been realised because the route demands high-grade phosphate rock.
The electrochemical process produces phosphate by electrolysis of the reaction between phosphoric acid and hydrochloric acid, and currently has 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 and replacing imported products.
Fertiliser, Detergent, Food
Agriculture is the largest sink. Phosphate is one of the three major plant nutrients and the main component of fertiliser, and phosphate rock powder is mined from sedimentary phosphate layers. It could once be spread without processing after mining; now unprocessed phosphate is used only in organic farming. Generally it is chemically processed into superphosphate, triple superphosphate or monoammonium phosphate, which are more concentrated and more readily soluble in water so plants absorb them faster. The three numbers on a fertiliser bag give the grade in order: the percentage of elemental nitrogen (N), available phosphate (P2O5), and soluble potash (K2O).
In detergents, phosphate is generally used as a water softener, but because the algal bloom-decay cycle is sensitive to phosphate discharge in watersheds, phosphate detergents are regulated in some regions. Phosphate runoff from over-fertilised farmland causes eutrophication, red tides and the subsequent hypoxia, and like phosphate detergents it causes hypoxia in fish and other aquatic organisms.
In food, phosphate has been used in processing since over a hundred years ago, with large-scale use beginning after the 1970s. It is one of the most widely used and largest-volume classes of food additive, applied as an important ingredient and functional additive in meat, poultry, seafood, fruit, vegetable, dairy, baked goods, beverages, potato products, seasonings and convenience foods. Food-grade varieties number more than thirty; the usual forms are sodium, calcium and potassium salts, with iron and zinc salts used as nutritional fortifiers. Sodium phosphates are the main consumption category in China, and potassium phosphate consumption is rising year by year. Phosphate is also one of the natural components of almost all foods, so dietary intake comes from both natural sources and additives.
Refractory Binders and the Biological Side
Phosphate also binds refractories. A phosphate binder is a refractory binder with cementitious properties whose main compound is an acidic orthophosphate or a condensed phosphate, and it sets either by chemical-reaction binding or by polymerization binding. Binders made by reacting phosphoric acid with alkali-metal or alkaline-earth-metal oxides and their hydroxides are mostly air-hardening - they set and harden at room temperature without heating. Those made with amphoteric oxides and hydroxides, or acidic oxides, are mostly heat-hardening and must reach a certain temperature before setting begins. Because phosphate bond strength is high in the middle- and low-temperature range before ceramic bonding develops, it is widely used for unshaped and unfired refractories.
Biologically, phosphorus appears in solution as free phosphate ions, called inorganic phosphate and denoted Pi, which must be distinguished from phosphate in phosphate esters; it can be obtained by hydrolysis of pyrophosphate, denoted PPi. Phosphate is better known as a component of adenosine monophosphate (AMP), adenosine diphosphate (ADP), adenosine triphosphate (ATP), deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), released by hydrolysis of ADP or ATP, with similar reactions for other di- and tri-phosphonucleosides. The phosphoanhydride bonds in those molecules contain large amounts of energy and therefore play an important role in organisms - they are generally called high-energy phosphates, as is phosphocreatine in muscle tissue.
That biological importance is exactly why phosphate is mined so heavily and why it is often a limiting reagent in the environment: its availability determines the rate of biological growth. Adding large amounts to a phosphate-deficient environment or microbial community has a major ecological impact - one organism blooms and others die, and the resulting decay strips oxygen from the water. Under pollution problems, phosphate is a major component of total dissolved solids, a principal water-quality indicator.
On supply, phosphate is the naturally occurring form of elemental phosphorus and is found in various phosphate minerals; elemental phosphorus or phosphides are very rare, with only trace amounts in meteorites. In mineralogy and geology the term refers to stone or ore containing phosphate ions. In North America the largest phosphate-rock deposits lie in central Florida, Soda Springs in Idaho and the coastal region of North Carolina, followed by Montana, Tennessee, Georgia and the area near Charleston in South Carolina. The island nation of Nauru once held large quantities of high-quality phosphate mineral, now heavily mined; deposits and substantial mining industries also exist on Navassa Island and in Morocco, Tunisia, Israel, Togo and Jordan.