Iron

Typical wastewaters

• coal mine drainage (acid or ferruginous) Fe²⁺ in acid mine water, oxidising to Fe(OH)₃ ochre on aeration; 7.0 daily maximum and 3.5 mg/L monthly average total iron
• power plant metal cleaning wastes total iron with copper, 1.0 mg/L daily maximum and monthly average
• chromate bearing effluent reduced with ferrous sulfate Fe²⁺ reductant oxidised to Fe³⁺ and precipitated as Fe(OH)₃ with Cr(OH)₃ at pH 8 to 9
• wastewater treated with ferric salts for phosphorus FePO₄ and hydrous ferric oxide in the sludge, more than the 1:1 molar ratio dosed

1 · Identity

Symbol, number

Fe, 26

Oxidation states in water

+2 (ferrous, Fe²⁺, dissolved in anoxic groundwater and mine water) and +3 (ferric, hydrolysed to Fe(OH)₃ and hydrous ferric oxide in oxic water; the state of coagulant flocs and pipe rust). 0 as the metal of cast iron and steel mains, which corrode to both.

Note

Iron(II) salts are unstable in drinking water supplies and precipitate as iron(III) hydroxide, a rust coloured silt (WHO background document). Everything below follows from that switch.

2 · Occurrence in water

Natural sources

Dissolution of iron bearing minerals under reducing conditions and low pH; anaerobic groundwater carries iron(II) at up to several milligrams per litre without colour or turbidity until it is pumped and aerated. Lowering the water table or nitrate leaching aerates iron bearing soil layers and changes both groundwater and surface water quality (WHO background document).

Anthropogenic sources

Corrosion of cast iron, steel and galvanised iron distribution pipes; carry over of iron coagulants; acid or ferruginous coal mine drainage, regulated in the US at 7.0 mg/L daily maximum; metal cleaning wastes at power plants, regulated at 1.0 mg/L; iron and steel pickling and mine water in the ledger's mining chapter.

3 · Speciation

Below the oxic boundary, and wherever pH is low, iron is dissolved Fe²⁺, with FeHCO₃⁺ and FeSO₄ ion pairs in mineralised water. In oxic water above pH 6 the equilibrium species is iron(III), and iron(III) is almost entirely solid: Fe(OH)₃ and hydrous ferric oxide, with only nanomolar dissolved hydrolysis products (Fe(OH)₂⁺, Fe(OH)₄⁻) in the neutral range. The practical question is therefore never the equilibrium but the rate of the Fe(II) to Fe(III) step, which is fast above pH 7 in aerated water and slow in acid or organic rich water.

Solubility

Iron(III) hydroxide is the controlling solid in oxic water and keeps dissolved iron(III) far below 0.1 mg/L between pH 6 and 9; iron(II) is soluble at mg/L levels until it is oxidised. The sources read give no solubility products, so none are quoted.

Hydrolysis

Fe³⁺ hydrolyses stepwise to Fe(OH)₂⁺, Fe(OH)₃ and Fe(OH)₄⁻; the hydrolysis releases protons, which is why ferric coagulants consume alkalinity and depress pH.

Complexation

Bicarbonate and sulfate ion pairs with Fe(II); natural organic matter binds Fe(III) and slows its oxidation and settling. Constants are not quoted because the sources read do not print them.

Precipitates

Fe(OH)₃ and hydrous ferric oxide (oxic); FeCO₃ siderite and FeS (anoxic, carbonate or sulfide rich); FePO₄ with phosphate; Fe₃O₄ magnetite and Fe₂O₃ hematite as ageing products and pipe tubercles.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Iron(II) in an anaerobic groundwater sample oxidises and precipitates in the bottle on contact with air (WHO: iron(II) salts are unstable in supplies). Filter 0.45 µm in the field and acidify at once for dissolved iron; acidify unfiltered samples for total iron; measure iron(II) on site. Rust from the sampling tap and the mains is a total iron artefact.

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

Essential nutrient; minimum daily requirement about 10 to 50 mg depending on age, sex and bioavailability; JECFA PMTDI 0.8 mg/kg body weight (1983) as a precaution against iron storage; 0.3 mg/L in drinking water contributes about 0.6 mg to daily intake. No health based guideline is proposed (WHO).

Bioaccumulation

Not addressed in the sources read; iron is regulated by uptake in animals and is not a bioaccumulating contaminant in the sense used for metals such as mercury.

Ecotoxicity

US EPA national recommended aquatic life criterion, freshwater chronic 1000 µg/L (1986); no acute freshwater and no saltwater criterion. The harm in rivers is usually physical: ochre precipitates smother the bed.

Flags

• The 0.5 to 10 mg/L groundwater range and the 0.7 mg/L river median are WHO 1990s compilations and vary strongly by aquifer and region.
• The seawater figure (0.002 mg/L) is a single abundance figure from Jefferson Lab via PubChem, not a range.
• The Fe(II) oxygenation rate constant is cited to Stumm and Morgan chapter 11 from memory of the text, not re-read this session.
• The chromate and chlorite reduction stoichiometries are electron balances written here; the sources name the reagent but do not print the equation.
• The EPA oxidant doses per mg Fe come from a 1986 and 1991 compilation quoted by the 1999 EPA manual; the brief's own example gives 0.64 mg Cl₂ per mg Fe, the EPA table 0.62.
• Abu Dhabi values are two different media (marine outfall 2.0 mg/L, sewer 50 mg/L); other GCC states were not read.
• No detection limit was read for EPA 200.7 or ISO 17294-2 this session.

Gaps

• No source read gives iron concentrations in municipal or industrial wastewater; the mining and steel figures are in the ledger's own chapters, not here.
• No solubility products or hydrolysis constants are quoted; Stumm and Morgan has them but was not re-read.
• No removal percentage for iron is printed in the sources read; the EPA doses and the WHO acceptability level stand in for it.
• Lime softening, ion exchange softening, sequestration with polyphosphate and biological iron filtration are not covered because no read source describes them.
• The phosphorus precipitation and chromate reduction equations are cited to a textbook chapter and the CWW BREF technique description, not to a printed equation read this session.
• Other GCC discharge standards (Saudi, Oman, Qatar) were not read.

Sources

Identity

Name and symbol

Iron, Fe

Atomic number

26 protons

Position

group 8 · period 4 · d-block · transition metal

CAS number

7439-89-6

Atomic structure

Atomic mass

55.845 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶
[Ar] 4s²³d⁶

Electrons per shell

2, 8, 14, 2

Valence electrons

8 ns and (n-1)d

Physical properties

State at room temperature

Solid

Melting point

1,811 K (1,537.85 °C)

Boiling point

3,134 K (2,860.85 °C)

Density

7.874 g/cm3

Appearance

lustrous metallic with a grayish tinge

Thermal conductivity

80.4 W/(m·K)

Electrical resistivity

96.1 nΩ·m at 20 °C

Electrical conductivity

10.41 MS/m

Crystal structure

body-centered cubic

Molar heat capacity

25.1 J/(mol·K)

Chemical properties

Oxidation states

+3, +2

Electronegativity

1.83 (Pauling Scale)

Ionisation energy

7.902 eV
1st 762.5, 2nd 1,561.9, 3rd 2,957 kJ/mol

Electron affinity

0.163 eV

Atomic radius

van der Waals 194 pm

Ionic radius

Fe²⁺ 61 low spin; Fe³⁺ 55 low spin; Fe⁴⁺ 59 low spin; Fe⁶⁺ 25 (4-coordinate) low spin; Fe²⁺ 78 high spin; Fe³⁺ 65 high spin pm

Reactivity

A group 8 transition metal ([Ar] 3d6 4s2), by far the most reactive of its group: the pure metal corrodes rapidly in moist air because its oxide is porous and flakes off rather than passivating, it is pyrophoric when finely divided, and its chemistry runs on the +2 and +3 states.

with water

Rusts in water and moist air to brown-to-black hydrated iron(III) oxides; red-hot iron decomposes steam to iron oxide and hydrogen, the reaction Lavoisier used in $\ce{1774: 3 Fe + 4 H2O -> Fe3O4 + 4 H2}$

with oxygen, air

Reacts readily with oxygen, especially in moist air or when hot, to oxides that offer no protection; finely divided iron is pyrophoric: $\ce{4 Fe + 3 O2 -> 2 Fe2O3}$

with acids

Dissolves easily in dilute acids to iron(II) salts and hydrogen, but is passivated by concentrated nitric acid and other oxidising acids: $\ce{Fe + 2 HCl -> FeCl2 + H2}$

with halogens

Reacts with fluorine, chlorine and bromine to the iron(III) halides: $\ce{2 Fe + 3 Cl2 -> 2 FeCl3}$

Typical compounds

• Fe₂O₃ iron(III) oxide hematite, the main ore; rust and ochre pigments
• Fe₃O₄ iron(II,III) oxide magnetite; black magnetic ore
• FeS₂ iron pyrite fool's gold; iron(II) polysulfide
• FeCl₃ iron(III) chloride water and sewage coagulant; circuit-board etchant
• FeSO₄ iron(II) sulfate green vitriol; precursor to other iron compounds
• Fe(CO)₅ iron pentacarbonyl volatile carbonyl; source of carbonyl iron powder

Occurrence, production and use

Crustal abundance

5.63×104 milligrams per kilogram

Oceanic abundance

2×10-3 milligrams per liter

Occurrence and sources

Iron is a relatively abundant element in the universe. It is found in the sun and many types of stars in considerable quantity. Its nuclei are very stable. Iron is a principal component of a meteorite class known as siderites and is a minor constituent of the other two meteorite classes. The core of the earth 2150 miles in radius is thought to be largely composed of iron with about 10 percent occluded hydrogen. The metal is the fourth most abundant element, by weight that makes up the crust of the earth.

The most common ore is hematite, which is frequently seen as black sands along beaches and banks of streams.

• haematite (Fe2O3), magnetite (Fe3O4) and low grade taconite mined in Australia, Brazil, China, India, Russia, Ukraine, South Africa, Canada, Iran and the United States; world resources above 800 billion tonnes of ore with more than 230 billion tonnes of iron
• pyrite (FeS2) sulphide ore roasted for sulphuric acid, leaving iron oxide; one tonne of acid needs 0.5 tonne of pyrite
• dissolved iron(II) and particulate iron(III) anoxic groundwater and mine water; a measured parameter in water from base metal, precious metal, uranium, iron ore and bauxite extraction

Extraction, production

Blast furnace reduction of ore with coke

$\ce{Fe2O3 (s) + 3 CO (g) -> 2 Fe (l) + 3 CO2 (g)}$

Haematite or magnetite is heated with coke and limestone in a blast furnace to pig iron with about 3 percent carbon, then converted to steel. The equation given is the main ore reduction step of the blast furnace, taken from the added secondary source. World pig iron 1.3 billion tonnes in 2024 (estimate); direct reduced iron and scrap feed electric arc furnaces.

Iron oxide pigments by aqueous oxidation of iron(II) salts

The SIC BREF describes the precipitation and Penniman-Zoph processes (iron(II) sulphate oxidised with air over seed crystals to yellow, red and black oxides), the Laux process (nitro compound reduction with metallic iron steered to pigment grade oxide) and dry calcination of iron sulphate to Fe2O3 and SO3. No stoichiometry is stated, so no equation.

Reduction of aromatic nitro compounds with iron

4 Ar-NO₂ $\ce{+ 9 Fe + 4 H2O -> 4}$ Ar-NH₂ + 3 Fe₃O₄

Equation as stated by the OFC BREF for the fine chemical route to aromatic amines: the nitro compound is added to iron, water and acid with 15 to 50 percent excess iron; 97 to 98 percent of the hydrogen comes from the water, the acid only activates the iron. The iron oxide sludge is a waste stream in the pharma chapter.

Uses

Huge amounts of iron are used to make steel, an alloy of iron and carbon. Steel typically contains between 0.3% and 1.5% carbon, depending on the desired characteristics. The addition of other elements can give steel other useful properties. Small amounts of chromium improves durability and prevents rust (stainless steel); nickel increases durability and resistance to heat and acids; manganese increases strength and resistance to wear; molybdenum increases strength and resistance to heat; tungsten retains hardness at high temperatures; and vanadium increases strength and springiness. Steel is used to make paper clips, skyscrapers and everything in between.

In addition to helping build the world around us, iron helps keep plants and animals alive. Iron plays a role in the creation of chlorophyll in plants and is an essential part of hemoglobin, the substance that carries oxygen within red blood cells. Iron sulfate (FeSO4) is used to treat the blood disease anemia.

Iron is a vital constituent of plant and animal life and works as an oxygen carrier in hemoglobin.

Taconite is becoming increasingly important as a commercial ore. The pure metal is not often encountered in commerce, but is usually alloyed with carbon or other metals.

• Steel and construction: carbon steel (0.1 to 2 percent carbon), alloy steels with nickel, chromium, vanadium, tungsten and manganese, stainless steel with at least 10.5 percent chromium; cast iron (3 to 5 percent carbon) for pipes, valves and pumps; reinforced concrete and girders construction took 28 percent of United States steel shipments in 2024, service centres 23 percent, automotive 15 percent (USGS, US figures)
• Chemicals: iron catalyst for ammonia synthesis at 100 to 250 bar and 350 to 550 degrees C; iron oxide and chromium oxide shift catalyst; pyrite roasting for sulphuric acid; iron oxide pigments from scrap iron and iron sulphate
• Pharmaceuticals and fine chemicals: metallic iron as the reducing agent for aromatic nitro compounds, giving amines and an iron oxide sludge
• Food and beverage: ferric chloride and iron based coagulants and flocculants for effluent treatment and sludge conditioning; iron oxides as the colour additive E172
• Mining: iron ore extraction, tailings and heaps are a sector of the MWEI BREF; ferric chloride as a flotation depressant and iron salts as coagulants in mine water treatment; iron cyanide complexes and high density sludge are waste streams

Safety, toxicity

GHS classification, signal word Danger

• H228 Flammable solid Flammable solids
• H251 Self-heating; may catch fire Self-heating substances and mixtures
• H319 Causes serious eye irritation Serious eye damage/eye irritation
• H335 May cause respiratory irritation Specific target organ toxicity, single exposure; Respiratory tract irritation

Discovery and name

Discovered by

not in sources

Discovered

before 5000 BC

First isolated

not in sources

Named by

not in sources

Origin of the name

probably from a PIE root meaning 'blood', for the color of its oxides

The pure metal is very reactive chemically and rapidly corrodes, especially in moist air or at elevated temperatures. It has four allotropic forms or ferrites, known as alpha, beta, gamma, and omega, with transition points at 700, 928, and 1530C. The alpha form is magnetic, but when transformed into the beta form, the magnetism disappears although the lattice remains unchanged. The relations of these forms are peculiar. Pig iron is an alloy containing about 3 percent carbon with varying amounts of sulfur, silicon, manganese, and phosphorus.

Iron is hard, brittle, fairly fusible, and is used to produce other alloys, including steel. Wrought iron contains only a few tenths of a percent of carbon, is tough, malleable, less fusible, and usually has a "fibrous" structure.

Carbon steel is an alloy of iron with small amounts of Mn, S, P, and Si. Alloy steels are carbon steels with other additives such as nickel, chromium, vanadium, etc. Iron is a cheap, abundant, useful, and important metal.