Hydroxylated ferric sulfate

(endorsed 2005)

Hydroxylated ferric sulfate is used as a coagulant for the treatment of drinking water. It is effective over a broad pH range and generally produces a stronger floc than other ferric salts.

General description

Hydroxylated ferric sulfate (HFS), $\text{Fe}_{x}\text{(SO}_{4}\text{)}_{y}\text{(OH)}$, also known as polymerised ferric sulfate, is one of several hydroxylated iron coagulants produced from ferrous sulfate. It is a translucent, dark reddish liquid, with no odour. It is available in various ferric iron and basicity concentrations, but typically contains 12.5 % Fe.

HFS has a pH of less than 2 and a specific gravity of 1.45–1.6 at 25°C. It is corrosive, but can be stored in fibreglass, rubber-lined steel, stainless steel, polyethylene, polyvinyl chloride or polytetrafluoroethylene.

For commercial coagulant solutions, the basicity varies from about 5 to 15% for prehydrolysed iron salts. As the basicity exceeds 15%, it becomes increasingly difficult to keep the metal hydroxide precipitate from forming in the product solution during shipping and extended storage. The typical basicity for HFS is 10%.

Chemistry

HFS is produced by oxidising ferrous sulfate. It can also be produced by dissolving ferrous oxide in sulfuric acid under controlled conditions. The chemical is similar to ferric sulfate, but its small polymeric chains provide additional coagulation properties, so it may be preferable to ferric sulfate for water that is difficult to treat.

In water, 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. The stoichiometry of the precipitation of iron hydroxide is described as follows:

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

Typical use in Australian drinking water treatment

HFS coagulates relatively quickly over a wide pH range (pH 4–11). It forms a dense floc and does not cause significant variation in pH. The floc produced is usually similar in characteristics to alum floc, but has less impact on pH and alkalinity. HFS generally produces a more robust floc than other iron salts. It is often preferred over conventional coagulants for low alkalinity waters containing colour because it does not consume as much alkalinity as other coagulants; thus, the need to add alkali in addition to coagulant is reduced.

The dose of HFS used depends on the properties of the raw water, including factors such as turbidity, natural organic matter, temperature and alkalinity. As with other coagulants, higher doses are required as turbidity and colour increase, and colder temperatures slow down reaction times.

Typical HFS doses (expressed as supplied HFS in mg/L) are 5–100 mg/L, although higher doses may be required if the 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 of HFS may be expressed as ferric sulfate, supplied HFS liquid or iron. Care should be taken when interpreting dose rates to ensure that relevant comparisons are made. Because of its reactivity, HFS should be used neat if possible, or not pre-diluted such that is hydrolyses before contact with the water to be treated.

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 HFS:

• antimony

• arsenic

• cadmium

• chromium

• cobalt

• copper

• cyanide

• lead

• manganese

• mercury

• nickel

• phosphorus

• selenium

• silver

• titanium

• vanadium

• zinc

In some products, manganese concentrations may be significant.

Residual and by-product formation in drinking water

When employed in drinking water treatment, HFS 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 ferric ions used for coagulation are removed by conventional water treatment processes. Residual sulfate is usually at low levels that do not adversely affect drinking water quality.

Status

HFS was endorsed by the NHMRC for use as a drinking water treatment chemical in 2005.

References

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.