Iron (Fe) and water

Iron in water: occurrence, chemistry, health and treatment

Iron is common in rocks, soils and groundwater. Its concentration and behaviour depend on oxygen, pH, organic matter and corrosion. These conditions determine whether iron remains dissolved or forms particles that can colour water and create deposits.

How does iron occur in water?

Iron enters water through the weathering of iron-bearing minerals and, in some settings, through corrosion of iron or steel pipes and equipment. Dissolved iron is often elevated in oxygen-poor groundwater, where ferrous iron (Fe(II)) can remain soluble. In oxygenated water, Fe(II) is commonly oxidised to ferric iron (Fe(III)); depending on pH and other water-chemistry conditions, it can hydrolyse and form poorly soluble iron (oxyhydr)oxide particles. Natural organic matter can bind iron and keep some of it in dissolved or colloidal forms.

Iron in water may therefore be dissolved, organically complexed, attached to particles or present in corrosion products. A total-iron result alone does not identify the form or source. Metallic iron exposed to water and oxygen corrodes; the resulting rust is a mixture of iron oxides and oxyhydroxides, not a single compound.

Sources and environmental behaviour

Natural sources include iron-rich minerals and sediments. Reducing conditions in aquifers, wetlands and lake sediments can release dissolved Fe(II). Iron can also enter water from corrosion in distribution systems, from industrial discharges, or as residuals from treatment processes that use iron salts.

Iron is an essential micronutrient for organisms, but its availability in water is controlled by chemical form. In oxygenated water, ferric iron often binds to organic matter or particles and may settle as a precipitate; under reducing conditions, more iron may remain dissolved. These changes influence iron transport and biological availability.

Red, orange or brown water and deposits can occur when dissolved iron oxidises or when accumulated pipe corrosion products are disturbed. A sudden discoloration event should be investigated; iron may be present alongside other substances released from deposits, depending on the source and distribution system.

Health and drinking-water values

Iron is an essential nutrient, and food is usually the main source of dietary iron. WHO has not established a health-based guideline value for iron in drinking-water at concentrations commonly encountered; iron is mainly a concern for acceptability because it can affect colour, taste and staining. This does not mean that every discoloured supply is safe or that iron results replace other water-quality tests.

Reference values differ by jurisdiction and purpose. The U.S. EPA lists 0.3 mg/L as a non-enforceable federal secondary standard aimed at aesthetic effects. The EU Drinking Water Directive sets an indicator parametric value of 0.2 mg/L. Health Canada’s 2024 technical document sets an aesthetic objective of 0.1 mg/L for total iron. These are not interchangeable health-based limits; consult the applicable local requirements.

Which water-treatment technologies can remove iron?

Treatment selection depends on the iron form and concentration, pH, dissolved oxygen, alkalinity, organic matter, manganese and the required treated-water quality. Analyse representative samples for total and dissolved iron; where needed, distinguish Fe(II) from Fe(III) and assess the distribution system as well as the source water.

Oxidation and filtration: aeration or a suitable oxidant can convert dissolved Fe(II) into particles, which are then removed by filtration. Performance depends on pH, contact time, oxidant demand and solids separation.
Coagulation and filtration: coagulation may help remove fine particles or iron associated with natural organic matter. The process and dose should be selected through water analysis and testing.
Ion exchange: this may be considered for low iron concentrations and suitable water chemistry. Iron precipitates can foul resin, so pretreatment and operating conditions matter.
Membrane or other advanced treatment: membranes and adsorptive processes may be appropriate in specific applications, but performance depends on iron speciation, fouling control and the wider water matrix.

Iron is also used as a coagulant in wastewater treatment, for example to help remove phosphorus. This process use is different from removing naturally occurring iron from a water supply. Lenntech provides further information on drinking-water preparation, sand filtration, ion exchange and coagulation and flocculation.