Ferric sulfate

(endorsed 2005)

Ferric sulfate 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 sulfate, $\text{Fe}_{2}\text{(SO}_{4}\text{)}_{3}$, is a yellow crystal or greyish-white powder that is soluble in water. In water treatment, it is usually supplied as an aqueous solution of 39–45% w/w ferric sulfate (11–12.5% Fe). The liquid solution has a specific gravity of 1.5–1.6 and is red-brown in colour. A 1% solution of ferric sulfate is acidic (pH 3–4).

Ferric sulfate is also available in granular form with an iron content of 18.5–20% and a pH of less than 1. It is not as corrosive as ferric chloride. Ferric sulfate can be stored or transported in stainless steels, lead, fibreglass, rubber-lined carbon steel, polyvinyl chloride, polyethylene or polypropylene.

Ferric sulfate can be used in a system built for alum dosing, whereas ferric chloride cannot.

Chemistry

Ferric sulfate is produced by the oxidation of ferrous sulfate or by dissolving ferric oxide in sulfuric acid. In water, the ferric (iron III) ion hydrolyses and precipitates, to an extent that depends on pH and dosage. Iron precipitates formed are goethite, $\text{HFeO}_{2}$, and iron hydroxide, $\text{Fe(OH)}_{3}$, which are less soluble than aluminium precipitates. At equilibrium, the concentration of the soluble species is very low. 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 hydroxides is described as follows:

$$\text{Fe}_{2}\text{(SO}_{4}\text{)}_{3}\rightarrow\text{2Fe}^{+3}+\text{3SO}_{4}^{2-} \newline \text{Fe}^{+3}+\text{3OH}^{-}\rightarrow\text{Fe(OH)}_{3}\text{(s)}$$

Ferric sulfate is an effective coagulant at pH values between 4 and 11.

Typical use in drinking water treatment

Ferric sulfate is used as a primary coagulant in the treatment of drinking water, particularly when a broad coagulation pH range is required.

The dose of ferric sulfate used depends on the properties of the raw water, including factors such as turbidity, natural organic matter (NOM), temperature and alkalinity.

Typical ferric sulfate doses, expressed as mg/L $\text{Fe}_{2}\text{(SO}_{4}\text{)}_{3}$, range from 2 mg/L to 100 mg/L although higher doses may be required if the raw water is excessively dirty. At high doses, product water should be tested to ensure that maximum contaminant levels have not been exceeded.

The dose rate of ferric sulfate may be expressed as crystalline ferric sulfate, as supplied liquid or as iron. Care should be taken when interpreting dose rates to ensure that any comparisons made are relevant.

Contaminants

The purity of chemicals used in Australia for the treatment of drinking water varies depending on the manufacturing process. Chemical contaminants that may be present in ferric sulfate are (JECFA, KIWA 1994, NRC 1982):

• antimony

• arsenic

• cadmium

• chromium

• cobalt

• copper

• cyanide

• lead

• manganese

• mercury

• nickel

• phosphorus

• selenium

• silver

• titanium

• vanadium

• zinc

In some products, manganese concentrations may be high enough to affect the treated water.

Residual and by-product formation in drinking water

When employed in drinking water treatment, ferric sulfate 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.

Almost all of the ferric ions used for coagulation are removed by optimised conventional water treatment processes. Residual sulfate 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. Iron residuals after filtration can cause floc to form in the distribution system, which can give rise to customer complaints. To minimise residual levels of iron, pH and dosage conditions should exceed the solubility of iron. Poor dosage selection or inadequate mixing also leads to elevated iron residuals.

Status

Ferric sulfate 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

LS, Greenberg AE and Eaton AD (eds) (1998). Standard Methods for the Examination of Water and ANSI (American National Standards Institute)/AWWA (American Water and Wastewater Association) Standard no B406-97. AWWA CD-ROM (April 2003). Details 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.

JECFA (Food and Agriculture Organization of the United Nations (FAO)/World Health Organization (WHO) Joint Expert Committee on Food Additives). Compendium of Food Additive Specifications. FAO Food and Nutrition Papers 52 (two volumes). Available at https://www.who.int/foodsafety/publications/jecfa/en/

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.