Phosphorus

Typical wastewaters

• municipal sewage orthophosphate, polyphosphate and organic phosphorus from excreta and detergents; total phosphorus 4 to 12 mg/L as P
• digester liquor and sludge dewatering sidestreams orthophosphate released in anaerobic digestion with ammonium and magnesium, crystallising as struvite at pH 8.5 to 9
• chemical sector effluent (cooling and heating systems, nutrient dosing) orthophosphate, polyphosphate and phosphonate from cooling and boiler conditioning and phosphate added as nutrient for the biological plant; total phosphorus 0.50 to 3.0 mg/L
• textile dyeing and finishing total phosphorus 0.4 to 2 mg/L BAT-AEL for direct discharge
• reverse osmosis concentrate phosphonate antiscalant (an HEDP derivative dosed at 5 mg/L in the case read) concentrated in the reject, counted as total phosphorus

1 · Identity

Symbol, number

P, 15

Oxidation states in water

+5 only in practice: orthophosphate (H₃PO₄, H₂PO₄⁻, HPO₄²⁻, PO₄³⁻), condensed polyphosphates that hydrolyse back to orthophosphate, organic phosphorus esters, and phosphonates (C-P bonded antiscalants such as HEDP and ATMP). Analysts split total phosphorus into reactive (ortho), acid hydrolysable (poly) and organic fractions.

Note

The element entry covers phosphate rock, phosphoric acid and the eutrophication warning. This chapter is the wastewater engineer's phosphorus: speciation by pH, metal salt and lime precipitation, enhanced biological removal, struvite and the limits that drive them.

2 · Occurrence in water

Natural sources

Weathering of apatite gives low natural orthophosphate; most surface water phosphorus is bound to particles and to iron oxides and is released from sediments under anoxic conditions. Phosphate is the limiting nutrient of most fresh waters (the element entry).

Anthropogenic sources

Sewage (excreta, detergents where not banned), agricultural runoff and manure, phosphate fertiliser and phosphoric acid plants, food and dairy processing, cooling water and boiler conditioning with phosphates and phosphonates (the CWW BAT conclusions note phosphorus originating from heating or cooling systems), and phosphorus added deliberately to industrial biological treatment as a nutrient.

3 · Speciation

Orthophosphate is a triprotic acid with pKa values near 2.1, 7.2 and 12.3, so in natural and waste water it is a mixture of H₂PO₄⁻ and HPO₄²⁻ around neutral pH, shifting to HPO₄²⁻ above pH 7.2; the fully deprotonated PO₄³⁻ only matters at the high pH of lime treatment. Polyphosphates hydrolyse slowly to orthophosphate, faster when warm and acidic; organic phosphorus is released by biological oxidation. Metal cations precipitate orthophosphate: aluminium and iron(III) as AlPO₄ and FePO₄ near neutral pH, calcium as hydroxyapatite above pH 10, magnesium with ammonium as struvite at pH 8.5 to 9. Phosphonates and polyphosphates at a few mg/L hold calcium carbonate and sulfate below their scaling point by threshold inhibition.

Solubility

AlPO₄ and FePO₄ have solubility minima near pH 6 to 7 and hydroxyapatite is very insoluble above pH 10; struvite solubility falls with rising pH to a minimum near pH 9 (Wu et al.). Constants are not quoted because the sources read do not print them.

Hydrolysis

Polyphosphates (tripolyphosphate, hexametaphosphate) hydrolyse to orthophosphate over hours to days, which is why sequestered iron and manganese eventually precipitate and why total phosphorus samples are digested before analysis.

Complexation

Phosphate sorbs on iron and aluminium hydroxides and competes with arsenate on adsorbents (EPA arsenic manual, via the arsenic chapter: each 0.5 mg/L of phosphate above 0.2 mg/L cuts granular ferric hydroxide capacity about 30 percent); phosphonates chelate calcium and magnesium and poison crystal growth.

Precipitates

AlPO₄, FePO₄, hydroxyapatite Ca₅(PO₄)₃OH and other calcium phosphates, struvite MgNH₄PO₄.6H₂O, vivianite Fe₃(PO₄)₂ in iron dosed anaerobic sludge.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Filter 0.45 µm at once for dissolved reactive phosphorus; unfiltered samples convert particulate and polyphosphate to orthophosphate on standing. Acidify or freeze samples for total phosphorus. Phosphate contamination from detergent washed glassware is the classic blank problem; phosphate free detergents and acid rinsed glass are standard.

7 · Regulatory limits

Limits change, and many are set locally. Treat these as the published values to start from, not as your compliance target: check the standard in force at your site and the numbers written into your own permit.

8 · Health and environmental effects

Toxicity

Orthophosphate in water is not a health concern at any level found; a person eats about 1 g of phosphate a day. White phosphorus, 50 mg fatal, is the toxic form and does not occur in water (the element entry).

Bioaccumulation

Phosphorus is taken up by every organism as nutrient; there is no bioaccumulation hazard in the toxicological sense.

Ecotoxicity

Eutrophication is the effect: phosphate is the limiting nutrient of most lakes and rivers, so discharge limits are set on ecology, not toxicity. The US EPA aquatic life table carries only elemental phosphorus (1986) with no criterion values; nutrient criteria are ecoregional and were not read.

Flags

• The 4 to 12 mg/L municipal wastewater range is a Metcalf and Eddy typical composition table cited from memory of chapter 3.
• The phosphate pKa values, the metal to phosphorus dosing ratios, the lime dose rule and the struvite scaling statement are textbook chapters not re-read this session.
• The 2024 UWWTD Article 7 deadlines for tertiary treatment by plant size were not captured; the national load reduction dates are from Article 10 as read.
• The textile BAT-AEL footnotes on when the phosphorus value applies were not read.
• The antiscalant chemistry rests on one case study (a bisphosphonate at 5 mg/L) and a textbook chapter; typical HEDP and ATMP doses and the German drinking water limit seen in search summaries were not read and are not quoted.
• The EBPR effluent figures quoted are from a laboratory study with high influent phosphorus, not a full scale plant.
• Abu Dhabi values cover two media and two parameters (marine phosphate as P 2.0 mg/L, sewer total phosphorus 50 mg/L).

Gaps

• No source read gives phosphorus in groundwater, surface water or specific industrial effluents; the eutrophication threshold concentrations were not sourced.
• EPA Nutrient Control Design Manual (2010) could not be opened (file too large for the fetch tool); Metcalf and Eddy chapters stand in.
• Typical antiscalant doses and phosphonate limits for drinking water production were not read.
• The US ecoregional nutrient criteria and the EPA 1986 Gold Book phosphorus recommendations were not read.
• The Standard Methods 4500-P E detection limit was not read; the ISO 6878 range stands in.
• Other GCC discharge standards (Saudi, Oman, Qatar) were not read.
• No solubility products for AlPO₄, FePO₄, hydroxyapatite, vivianite or struvite were read. The phosphoric acid pKa ladder is from Stumm and Morgan chapter 3 and the alum, vivianite and polyphosphate stoichiometries from Metcalf and Eddy chapters 3 and 6, from the chapter, not re-read.
• No balanced equation is written for enhanced biological phosphorus removal: the anaerobic release and aerobic uptake steps involve acetate, PHB, glycogen and intracellular polyphosphate of no fixed formula, and no source read prints a stoichiometry.

Sources

Identity

Name and symbol

Phosphorus, P

Atomic number

15 protons

Position

group 15 · period 3 · p-block · polyatomic nonmetal

CAS number

7723-14-0

Atomic structure

Atomic mass

30.973 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p³
[Ne] 3s²³p³

Electrons per shell

2, 8, 5

Valence electrons

5 outer shell

Physical properties

State at room temperature

Solid

Melting point

317.3 K (44.15 °C)

Boiling point

553.65 K (280.5 °C)

Density

1.82 g/cm3

Appearance

white, red and violet are waxy, black is metallic-looking

Thermal conductivity

white: 0.236 W/(m·K)

Electrical resistivity

not in sources

Electrical conductivity

not in sources

Crystal structure

body-centred cubic

Molar heat capacity

23.824 J/(mol·K)

Chemical properties

Oxidation states

+5, +3, -3

Electronegativity

2.19 (Pauling Scale)

Ionisation energy

10.487 eV
1st 1,011.8, 2nd 1,907, 3rd 2,914.1 kJ/mol

Electron affinity

0.746 eV

Atomic radius

empirical 107, covalent 107, van der Waals 180 pm

Ionic radius

P³⁺ 44; P⁵⁺ 38 pm

Reactivity

A pnictogen whose allotropes differ sharply in reactivity: white P4 is pyrophoric, red phosphorus is stable in air until about 300 C and black is the least reactive; all forms are too reactive to occur free, and the chemistry runs on the +5, +3 and -3 states.

with water

Does not react; white phosphorus is insoluble in water and is stored under it to keep air away.

with oxygen, air

White phosphorus glows in moist air (chemiluminescence) and ignites spontaneously at about 30 C, burning to phosphorus pentoxide; red phosphorus ignites at about $\ce{300 C: P4 + 5 O2 -> P4O10}$

with acids

Does not react with non-oxidising acids such as hydrochloric acid, but dissolves in oxidising acids such as nitric acid.

with halogens

All the halogens react violently with phosphorus, white phosphorus most dramatically, giving the trihalides PX3 and, with excess chlorine or bromine, the pentahalides: $\ce{P4 + 6 Cl2 -> 4 PCl3}$

Typical compounds

• H₃PO₄ phosphoric acid triprotic acid made by the megatonne for fertilisers
• P₄O₁₀ phosphorus pentoxide anhydride of phosphoric acid; fierce drying agent
• PCl₃ phosphorus trichloride chlorine over molten phosphorus; pesticide intermediate
• Ca₃(PO₄)₂ calcium phosphate impure in phosphate rock (apatite); bone ash
• Na₃PO₄ trisodium phosphate cleaner, water softener, boiler scale control
• PH₃ phosphine hydride, oxidised by air; from phosphides and water

Occurrence, production and use

Crustal abundance

1.05×103 milligrams per kilogram

Oceanic abundance

6×10-2 milligrams per liter

Occurrence and sources

Never found free in nature, it is widely distributed in combination with minerals. Phosphate rock, which contains the mineral apatite, an impure tri-calcium phosphate, is an important source of the element. Large deposits are found in Russia, in Morocco, and in Florida, Tennessee, Utah, Idaho, and elsewhere.

• dissolved in seawater about 0.06 mg/L; crustal estimate 1,050 mg/kg (Jefferson Lab figures via PubChem)
• sedimentary phosphorite (francolite) China, Morocco, United States (Florida, North Carolina, Idaho, Utah), Russia, Jordan, Saudi Arabia, Egypt, Tunisia, Peru, Brazil, Algeria; also on continental shelves and seamounts
• igneous apatite (fluorapatite) Brazil, Canada, Finland, Russia, South Africa

Extraction, production

Wet-process phosphoric acid: phosphate rock digested with sulfuric acid

$\ce{Ca3(PO4)2 + 3 H2SO4 -> 2 H3PO4 + 3 CaSO4}$

The tricalcium phosphate of the rock reacts with concentrated sulfuric acid; the insoluble calcium sulfate (phosphogypsum) is filtered off, and the reaction is slowed by the gypsum layer that forms on each particle (AAF BREF, PDF p242). Fluoride leaves as gas and as fluosilicic acid, which BAT says to market or dispose of, and phosphogypsum is piled with precautions (PDF p7). More than 95 percent of US phosphate rock went this way in 2024, and about 25 percent of the acid was exported as DAP, MAP and merchant acid (USGS, printed pp. 134 to 135).

Electric furnace reduction to elemental phosphorus

$\ce{2 Ca3(PO4)2 + 6 SiO2 + 10 C -> P4 + 10 CO + 6 CaSiO3}$

Phosphate rock, coke as reductant and gravel for slag are heated in an electric resistance furnace; the gas passes an electrofilter to drop dust and the phosphorus vapour is condensed under water (AAF BREF, PDF p279; RSC). The small share of rock not made into acid goes this way, chiefly to glyphosate and other industrial phosphorus chemicals (USGS).

Phosphorus chlorides from white phosphorus

$\ce{P4 + 6 Cl2 -> 4 PCl3}$

Highly exothermic, -1,276 kJ/mol; phosphoryl chloride follows by oxidation, $\ce{2 PCl3 + O2 -> 2 POCl3 (SIC BREF, PDF}$ p217).

Uses

Phosphorus has three main allotropes: white, red and black. White phosphorus is poisonous and can spontaneously ignite when it comes in contact with air. For this reason, white phosphorus must be stored under water and is usually used to produce phosphorus compounds. Red phosphorus is formed by heating white phosphorus to 250°C (482°F) or by exposing white phosphorus to sunlight. Red phosphorus is not poisonous and is not as dangerous as white phosphorus, although frictional heating is enough to change it back to white phosphorus. Red phosphorus is used in safety matches, fireworks, smoke bombs and pesticides. Black phosphorus is also formed by heating white phosphorus, but a mercury catalyst and a seed crystal of black phosphorus are required. Black phosphorus is the least reactive form of phosphorus and has no significant commercial uses.

Phosphoric acid (H3PO4) is used in soft drinks and to create many phosphate compounds, such as triple superphosphate fertilizer (Ca(H2PO4)2·H2O). Trisodium phosphate (Na3PO4) is used as a cleaning agent and as a water softener. Calcium phosphate (Ca3(PO4)2) is used to make china and in the production of baking powder. Some phosphorus compounds glow in the dark or emit light in response to absorbing radiation and are used in fluorescent light bulbs and television sets.

In recent years, concentrated phosphoric acids, which may contain as much as 70% to 75% P2O5 content, have become of great importance to agriculture and farm production. World-wide demand for fertilizers has caused record phosphate production. Phosphates are used in the production of special glasses, such as those used for sodium lamps.

Bone-ash calcium phosphate is used to create fine chinaware and to produce mono-calcium phosphate, used in baking powder.

Phosphorus is also important in the production of steels, phosphor bronze, and many other products. Trisodium phosphate is important as a cleaning agent, as a water softener, and for preventing boiler scale and corrosion of pipes and boiler tubes.

Phosphorus is also an essential ingredient of all cell protoplasm, nervous tissue, and bones.

• Fertilisers: diammonium and monoammonium phosphate, superphosphates and NPK compounds from wet-process acid; the largest use of phosphorus by far; phosphoric acid is the most important use of sulfuric acid world consumption of P2O5 in fertilisers 47.5 million tonnes in 2024, up from 45.8 in 2023 and projected 51.8 by 2028
• Food and animal feed: animal feed supplements from phosphoric acid; purified phosphoric acid for food uses; monocalcium phosphate in baking powder; bone ash in fine china
• Industrial phosphorus chemicals: glyphosate herbicide, the main outlet for elemental phosphorus; phosphorus trichloride and phosphoryl chloride; detergent phosphates, now being phased out; trisodium phosphate cleaner and water softener; matches, flares and incendiaries
• Batteries: lithium-iron-phosphate cathodes; more than 90 percent of LFP batteries were made in China in 2024, and purified phosphoric acid plants are being built in Quebec for them
• Steel, glass and ceramics: phosphor bronze; special glasses for sodium lamps

Safety, toxicity

Phosphorus is very poisonous, 50 mg constituting an approximate fatal dose. Exposure to white phosphorus should not exceed 0.1 mg/m3 (8-hour time-weighted average per 40-hour work week). White phosphorus should be kept under water (as it is dangerously reactive in air) and should be handled with forceps, as contact with the skin may cause severe burns.

When exposed to sunlight or when heated in its own vapor to 250°C, it is converted to the red variety, which does not phosphoresce in air as does the white variety. This form does not ignite spontaneously and is not as dangerous as white phosphorus. It should, however, be handled with care as it does convert to the white form at some temperatures and it emits highly toxic fumes of the oxides of phosphorus when heated. The red modification is fairly stable, sublimes with a vapor pressure of 1 atm at 17C, and is used in the manufacture of safety matches, pyrotechnics, pesticides, incendiary shells, smoke bombs, tracer bullets, etc.

GHS classification, signal word Danger

• H228 Flammable solid Flammable solids
• H412 Harmful to aquatic life with long lasting effects to the aquatic environment, long-term hazard
• H250 Catches fire spontaneously if exposed to air Pyrophoric liquids
• H300 Fatal if swallowed Acute toxicity, oral
• H314 Causes severe skin burns and eye damage Skin corrosion/irritation
• H330 Fatal if inhaled Acute toxicity, inhalation
• H400 Very toxic to aquatic life Hazardous to the aquatic environment, acute hazard

Discovery and name

Discovered by

Hennig Brand

Discovered

1669

First isolated

not in sources

Named by

not in sources

Origin of the name

from the greek Φωσφόρος, meaning 'light-bearer'

Phosphorus exists in four or more allotropic forms: white (or yellow), red, and black (or violet). Ordinary phosphorus is a waxy white solid; when pure it is colorless and transparent. White phosphorus has two modifications: alpha and beta with a transition temperature at -3.8°C.

It is insoluble in water, but soluble in carbon disulfide. It takes fire spontaneously in air, burning to the pentoxide.