Analysis of manganese in water using voltammetry

Water supplies are obtained from three types of sources, direct from rivers, from reservoirs by pumping waters from rivers when the flows are high enough, and from ground waters.

Manganese is a naturally occurring element that is found in rock, soil, waters and food.

It usually occurs together with iron and its presence can prove to be a nuisance in water supplies.

Water percolating through soil and rock dissolves manganese and iron before the minerals enter ground water supplies.

Surface waters (rivers and reservoirs) do not usually contain high concentrations of manganese and iron because the oxygen rich water enables both minerals to settle out as sediments.

In deep wells and springs where the oxygen content and pH tend to be low, water containing manganese and iron appears colourless.

After exposure to air, the dissolved minerals react with oxygen and are converted to a coloured solid material that settles out of the water.

For manganese, a brown-black residue forms and this is responsible for the staining properties of the manganese-bearing water and may be enough to restrict the flow of water through pipes in severe cases.

Consumer complaints arise when higher levels than normal of manganese are found in drinking water or domestic water, as a result of the brown-black staining of laundry and objectionable taste of beverages that can occur at concentrations greater than 0.1mg per litre.

Health concerns of manganese to humans.

Manganese is an essential trace element and has a daily nutritional requirement of about 50µg/kg of body weight.

Among other functions, manganese plays a role in bone mineralisation, protein and energy metabolism, and metabolic regulation.

The rate at which it is absorbed varies according to actual intake, chemical form, and the presence of other metals in the diet.

In infants and young animals, very high absorption rates have been observed.

At high levels of exposure not expected to be found in drinking waters, manganese can be associated with a Parkinson-like disease and some reproductive effects that include impotence and decreased fertility among men.

Certain individuals may be more susceptible to the adverse affects from exposure to manganese, these may include people with lung disease, individuals with an iron deficiency, or people with liver disease.

According to the World Health Organisation, no convincing evidence of toxicity in humans associated with the consumption of manganese in drinking waters exists - although only limited studies are available.

Manganese is not known to be a problem in water consumed by livestock.

Impact of manganese contamination.

There is no current regulatory standard for the amount of manganese present in United Kingdom drinking water supplies.

The parametric value adhered to is taken from the European Union limit of 50µg/l.

This is the same level as that administered by the US Environment Protection Agency as a non-enforceable secondary standard for contaminants that cause cosmetic or aesthetic effects.

The concentrations of manganese can vary seasonally due to occasional disturbance of accumulated deposits on the bed of reservoirs when the water is drawn down or it circulates.

Manganese can affect the flavour and colour of food and water, and can also react with tannins present in beverages to form a black sludge affecting both the taste and appearance.

Manganese produces a brown-black staining, not removed by detergents, of laundry and crockery in affected waters.

Manganese deposits can build up in pipelines, pressure tanks, water heaters, and water softeners reducing the available quantity and pressure of the water supply.

This can present an economic problem as a result of water heaters or softeners having to be replaced, as well as the associated increase of energy costs from pumping water through constricted pipework.

Another possible problem caused by manganese is the formation of manganese bacteria in water, although these can be controlled to a certain extent by the addition of chlorine to the supply.

The bacteria are not health-threatening and can occur in soils, shallow aquifers, and surface waters, feeding on the manganese present and forming a brown-black slime.

A colony of these bacteria may clog water treatment systems or distribution pipes. Manganese contamination was a problem that has previously affected some of Mid Kent Water's supplies.

Some 350km of mains within its water supply network are currently undergoing renovation over a four-year timetable, after being given an undertaking by the Drinking Water Inspectorate.

Historically, the water treatment processes were not as thorough as they are today, and as a result water leaving the water treatment works contained traces of manganese and iron.

During transportation to the consumer's homes, the metal ions settled out of solution forming harmless deposits that lined the inside of the water supply pipes.

As the water pressure within the supply network changed or the direction of water flow reversed, deposits were washed off the inside of pipes leading to discoloured drinking water being supplied to some consumers.

Removal of manganese from drinking water supplies.

There are a number of different options available to remove manganese and iron (the two are usually present together) from home water supplies depending upon the concentrations present.

Phosphate compound treatment is a relatively inexpensive way to treat low levels of manganese and iron (<3mg/l) but is only suitable for use in a water containing environment of low to moderate temperatures.

Phosphate compounds are those that can surround minerals and keep them in solution, preventing reaction with oxygen.

A water softener is effective for removing low concentrations of dissolved manganese and iron at less than 5mg/l.

This relies on the process of cation exchange to remove minerals that cause hard water, such as calcium and magnesium, and other nuisance constituents such as iron and manganese.

During the process, manganese and iron are exchanged with sodium on a special resin before being flushed from the resin by backwashing.

The sodium-rich water is passed back through the resin into the treated water supply.

It is important that the untreated water is not exposed to air or chlorine, which converts the dissolved manganese into solid particulates that can clog the cation exchange resin.

An oxidising filter is an option to remove moderate levels of dissolved manganese and iron at combined concentrations of up to 15mg/l.

The filter material (manufactured zeolite coated with manganese oxide) adsorbs dissolved manganese and iron softening the water as it removes the metal contaminants.

Higher levels of the two metals up to 25mg/l can be removed by oxidising to a solid form by aeration before filtration.

Water saturated with air enters an aerator vessel, where the air separates from the water before flowing through a filter that removes the oxidised particles of manganese, iron, and possibly some carbonate and sulphate.

It is important that backwashing of the filter is regularly performed, because manganese oxidation is slower than that of iron: greater quantities of oxygen are required, and thus this technique is not considered suitable for water containing organic complexes of the metals that can clog the filter. Chemical oxidation followed by filtration can be used in those environments where the levels of manganese and iron exceed 10mg/l.

The principle is that an oxidising material, for example chlorine or hydrogen peroxide, converts the manganese and iron from a dissolved to a solid form before removing the precipitates using a sand trap filter.

This type of treatment is particularly effective when iron is combined with organic matter or bacteria. The filtration material requires frequent backwashing to eliminate the solid manganese/iron particulates that accumulate. Voltammetry - a brief overview.

Heyrovsky first introduced polarography in 1922.

The term voltammetry is applied to designate the current-voltage measurement obtained at a given electrode.

Polarography is a special case of voltammetry referring to the current-voltage measurement acquired using a dropping mercury electrode with a constant flow of mercury drops. Stripping voltammetry uses the same instrumentation as traditional polarography but a stationary electrode such as the Hanging Mercury Dropping Electrode (HMDE) is used.

The voltammetric measurement is performed on a stationary mercury drop, allowing one to achieve considerable increases in sensitivity.

The jump in sensitivity is possible by electrochemical preconcentration of the metals in question at the surface of the stationary electrode before the current-voltage curve is recorded.

The recorded current is the redissolution (reoxidation) current of the preconcentrated metal traces. Voltammetry today represents a refined, clean, simple technique that offers outstanding limits of detection and is now the fastest growing analytical technique for trace analysis.

With the Metrohm Multi Mode Electrode (MME) the mercury is hermetically sealed in the reservoir and suffices for around 200,000 drops, ensuring low laboratory running costs.

Method for analysis of manganese in drinking waters.

10mls of sample, 0.5ml of ammonia buffer, 0.5ml of borate buffer, and 50µl of zinc solution were added to the reaction vessel in the Metrohm 757 VA Computrace to give a solution pH between 9.5-10.0.

The role of the electrolyte and additional solutions in voltammetry is crucial.

In the case of the zinc solution added, this was to prevent interference of intermetallic compounds.

Many determinations are pH dependent; so the electrolyte can increase the conductivity and selectivity of the solution.

The solution was then degassed with nitrogen for a period of five minutes to remove the electrochemically active oxygen, before the manganese content was determined using two standard additions with the HMDE.

The HMDE is an electrode mode of the MME.

Four mercury drops of a defined size are formed in succession at the MME, and the last drop remains suspended on the end of the capillary.

The entire voltage sweep is performed on the single stationary drop.

The analysis produced a result of approximately 25µg/l of manganese present in the sample of drinking water. Conclusion.

Although manganese in drinking water supplies poses no toxicological risk to humans based upon current information, its presence at higher levels than one would normally expect proves to be a nuisance - as well as providing potential economic problems if the manganese is not properly controlled through regular monitoring.

It is a good idea to incorporate analysis of manganese in drinking waters into a regulated analytical monitoring program, despite there being no regulatory standard, as one can occasionally get high levels of manganese contamination from unexpected sources - for example when manganese from batteries or pesticides leaches into well-water. It is a matter of priority for the water companies to ensure that their water supply to the consumer is not only safe, but is pleasing in appearance, taste, and odour.

A supply of water that is unsatisfactory in this respect will undermine the confidence of consumers, leading to complaints and maybe the use of water from less safe sources.

Once the confidence of the consumer has been eroded it is very difficult for the water companies to fully regain that trust.

It can result in the use of bottled water and home treatment devices, some of which can have adverse effects on water quality.

Voltammetry is an increasingly popular technique that in many instances offers unrivalled detection limits, even when compared to vastly more expensive analytical techniques. Voltammetry requires no specialist laboratory infrastructure such as expensive fume extraction.

All that is required is a sturdy bench top on which to mount the instrument, and a regulated flow of an inert gas.

The running and maintenance costs of voltammetry are minimal, ensuring a cost-effective analytical solution to meet the demands required by those industries with an environmental bias.

The following internet sites were used extensively as references and can be used to obtain further information: www.bae.ncsu.edu, www.europa.eu.int, www.metrohm.ch, www.midkentwater.co.uk, www.who.int, www.wilkes.edu