Ferric chloride

(endorsed 2005)

Ferric chloride is used as a primary coagulant in the treatment of drinking water, particularly when a broad coagulation pH range is required It is used to remove turbidity, natural organic matter (NOM) (including colour), microorganisms and many inorganic chemicals. Removal of NOM reduces the formation of disinfection by-products, because it removes the organic precursors of the by-products.

General description

Ferric chloride, $\text{FeCl}_{3}$ (anhydrous) or $\text{FeCl}_{3}\cdot\text{6H}_{2}\text{O}$ (crystalline), has a brownish-yellow or orange colouration when in crystalline form and is very hygroscopic. In solution, it has the appearance of a dark-brown syrup. Solutions of ferric chloride are acidic and corrosive to most metals. The typical pH range of a 1% solution of ferric chloride is 3–4. The chemical is significantly more soluble in hot water (535.7 g/100 mL at 100°C) than in cold water (74.4 g/100 mL at 0°C), and is very soluble in alcohol, ether and methanol.

Ferric chloride is available as a powder and in solution at 30–42%. A 42% solution of ferric chloride has a specific gravity of 1.45 at 20°C, contains 14.5% iron and has a pH of 1–2.

Ferric chloride is highly corrosive to most metals, including stainless steel; however, it can be stored or transported in fibreglass, rubber-lined carbon steel, polyvinyl chloride, polyethylene or polypropylene.

Polytetrafluoroethylene and polyvinylidene difluoride are also suitable as lining materials.

Chemistry

Ferric chloride is obtained from ores containing iron and titanium oxides. It is also produced through the reaction of chlorine gas with iron, ferrous sulfate or ferrous chloride.

The positively charged Fe species are available to interact with negatively charged colloidal matter in water. Such matter includes inorganic turbidity particles and the high molecular weight fraction of organic compounds present in natural organic matter (NOM). Fe cations interact with the natural alkalinity to form hydroxides that then act in a charge neutralisation fashion similar to that for aluminium. Charge neutralisation destabilises the repulsive forces between the negatively charged particles, allowing them to approach closely, collide and agglomerate. Metal hydroxides precipitate and can enmesh any colloidal particles. Iron floc is generally large and settles rapidly though it may be weaker than alum floc. As for aluminium, sweep coagulation can also occur at higher doses.

The stoichiometry of the precipitation of iron hydroxide is described as follows:

$$\text{FeCl}_{3}\text{(s)}\rightarrow\text{Fe}^{+3}+\text{3Cl}^{-} \newline \text{Fe}^{+3}+\text{3OH}^{-}\rightarrow\text{Fe(OH)}_{3}\text{(s)}$$

Ferric chloride is an effective coagulant at a pH between 4 and 11. When added to water, ferric chloride consumes more alkalinity than does alum.

Typical use in Australian drinking water treatment

Ferric chloride is used as a primary coagulant, especially when a broader coagulation pH range is required.

The amount of ferric chloride added depends on the properties of the raw water, including factors such as turbidity, NOM, temperature and alkalinity.

Typical ferric chloride doses are 2–100 mg/L $\text{FeCl}_{3}\cdot\text{6H}_{2}\text{O}$, although higher doses may be required if water is particularly dirty. At high doses, product water should be tested to ensure that maximum contaminant levels have not been exceeded.

The dose rate for ferric chloride may refer to crystalline or anhydrous ferric chloride, supplied as liquid or as iron. Care should be taken when interpreting dose rates to ensure that comparisons are relevant.

Contaminants

The purity of chemicals used in Australia for the treatment of drinking water varies, depending on the manufacturing process. The following chemical contaminants may be present in this product (KIWA 1994, NRC 1982):

• antimony

• arsenic

• cadmium

• chromium

• cobalt

• copper

• cyanide

• lead

• manganese

• mercury

• nickel

• phosphorus

• selenium

• silver

• titanium

• vanadium

• zinc

Manganese concentrations in ferric chloride may be high enough to affect the treated water.

Residue and by-product formation in drinking water

When employed in drinking water treatment, ferric chloride should be used in such a way that any contaminant or by-product formed by the use of the chemical does not exceed guideline values in the Australian Drinking Water Guidelines.

Conventional water treatment processes, if optimised, remove almost all of the ferric ions produced when ferric chloride is used for coagulation. Residual chloride is usually at low levels, which do not adversely affect drinking water quality.

The presence of any ferrous iron in the product reduces its effectiveness in water treatment and increases the possibility of soluble iron carry over. This could cause post precipitation of ferric hydroxide (red water) in the distribution system.

Status

Ferric chloride was endorsed by the NHMRC for use as a drinking water treatment chemical in 1983. The revision undertaken in 2003 did not change the status of this chemical for the treatment of drinking water.

References

ANSI (American National Standards Institute)/AWWA (American Water and Wastewater Association) Standard no B407-98. AWWA CD-ROM (April 2003). Available at <www.awwa.org>

Clesceri LS, Greenberg AE and Eaton AD (eds) (1998). Standard Methods for the Examination of Water and Wastewater, 20th edition. American Public Health Association, Washington, DC.

KIWA (1994) Guideline quality of materials and chemicals for drinking water supplies. Inspectorate of Public Health and Environmental Planning, Publication 94-01. Rijswijk, The Netherlands.

NRC (National Research Council) (1982). Water Chemicals Codex. Committee on Water Treatment Chemicals, Food and Nutrition Board, Assembly of Life Sciences, NRC, Washington, DC.