Analysis of manganese in water using voltammetry
Metrohm (UK)contact supplier
Jonathan Bruce, applications manager for Metrohm, provides this backgrounder on the effects of manganese in drinking waters, its removal, and the use of voltammetry for analysis
Water supplies are obtained from three types of sources, directfrom rivers, from reservoirs by pumping waters from rivers whenthe flows are high enough, and from ground waters.
Manganese is anaturally occurring element that is found in rock, soil, watersand food.
It usually occurs together with iron and its presencecan prove to be a nuisance in water supplies.
Water percolatingthrough soil and rock dissolves manganese and iron before theminerals enter ground water supplies.
Surface waters (rivers andreservoirs) do not usually contain high concentrations ofmanganese and iron because the oxygen rich water enables bothminerals to settle out as sediments.
In deep wells and springswhere the oxygen content and pH tend to be low, water containingmanganese and iron appears colourless.
After exposure to air, thedissolved minerals react with oxygen and are converted to acoloured solid material that settles out of the water.
Formanganese, a brown-black residue forms and this is responsiblefor the staining properties of the manganese-bearing water andmay be enough to restrict the flow of water through pipes insevere cases.
Consumer complaints arise when higher levels thannormal of manganese are found in drinking water or domesticwater, as a result of the brown-black staining of laundry andobjectionable taste of beverages that can occur at concentrationsgreater than 0.1mg per litre.
Health concerns of manganese tohumans.
Manganese is an essential trace element and has a dailynutritional requirement of about 50µg/kg of body weight.
Amongother functions, manganese plays a role in bone mineralisation,protein and energy metabolism, and metabolic regulation.
The rateat which it is absorbed varies according to actual intake,chemical form, and the presence of other metals in the diet.
Ininfants and young animals, very high absorption rates have beenobserved.
At high levels of exposure not expected to be found indrinking waters, manganese can be associated with aParkinson-like disease and some reproductive effects that includeimpotence and decreased fertility among men.
Certain individualsmay be more susceptible to the adverse affects from exposure tomanganese, these may include people with lung disease,individuals with an iron deficiency, or people with liverdisease.
According to the World Health Organisation, noconvincing evidence of toxicity in humans associated with theconsumption of manganese in drinking waters exists - althoughonly limited studies are available.
Manganese is not known to bea problem in water consumed by livestock.
Impact of manganesecontamination.
There is no current regulatory standard for theamount of manganese present in United Kingdom drinking watersupplies.
The parametric value adhered to is taken from theEuropean Union limit of 50µg/l.
This is the same level as thatadministered by the US Environment Protection Agency as anon-enforceable secondary standard for contaminants that causecosmetic or aesthetic effects.
The concentrations of manganesecan vary seasonally due to occasional disturbance of accumulateddeposits on the bed of reservoirs when the water is drawn down orit circulates.
Manganese can affect the flavour and colour offood and water, and can also react with tannins present inbeverages to form a black sludge affecting both the taste andappearance.
Manganese produces a brown-black staining, notremoved by detergents, of laundry and crockery in affectedwaters.
Manganese deposits can build up in pipelines, pressuretanks, water heaters, and water softeners reducing the availablequantity and pressure of the water supply.
This can present aneconomic problem as a result of water heaters or softeners havingto be replaced, as well as the associated increase of energycosts from pumping water through constricted pipework.
Anotherpossible problem caused by manganese is the formation ofmanganese bacteria in water, although these can be controlled toa certain extent by the addition of chlorine to the supply.
Thebacteria are not health-threatening and can occur in soils,shallow aquifers, and surface waters, feeding on the manganesepresent and forming a brown-black slime.
A colony of thesebacteria may clog water treatment systems or distribution pipes.Manganese contamination was a problem that has previouslyaffected some of Mid Kent Water's supplies.
Some 350km ofmains within its water supply network are currently undergoingrenovation over a four-year timetable, after being given anundertaking by the Drinking Water Inspectorate.
Historically, thewater treatment processes were not as thorough as they are today,and as a result water leaving the water treatment works containedtraces of manganese and iron.
During transportation to theconsumer's homes, the metal ions settled out of solutionforming harmless deposits that lined the inside of the watersupply pipes.
As the water pressure within the supply networkchanged or the direction of water flow reversed, deposits werewashed off the inside of pipes leading to discoloured drinkingwater being supplied to some consumers.
Removal of manganese fromdrinking water supplies.
There are a number of different optionsavailable to remove manganese and iron (the two are usuallypresent together) from home water supplies depending upon theconcentrations present.
Phosphate compound treatment is arelatively inexpensive way to treat low levels of manganese andiron (<3mg/l) but is only suitable for use in a watercontaining environment of low to moderate temperatures.
Phosphatecompounds are those that can surround minerals and keep them insolution, preventing reaction with oxygen.
A water softener iseffective for removing low concentrations of dissolved manganeseand iron at less than 5mg/l.
This relies on the process of cationexchange to remove minerals that cause hard water, such ascalcium and magnesium, and other nuisance constituents such asiron and manganese.
During the process, manganese and iron areexchanged with sodium on a special resin before being flushedfrom the resin by backwashing.
The sodium-rich water is passedback through the resin into the treated water supply.
It isimportant that the untreated water is not exposed to air orchlorine, which converts the dissolved manganese into solidparticulates that can clog the cation exchange resin.
Anoxidising filter is an option to remove moderate levels ofdissolved manganese and iron at combined concentrations of up to15mg/l.
The filter material (manufactured zeolite coated withmanganese oxide) adsorbs dissolved manganese and iron softeningthe water as it removes the metal contaminants.
Higher levels ofthe two metals up to 25mg/l can be removed by oxidising to asolid form by aeration before filtration.
Water saturated withair enters an aerator vessel, where the air separates from thewater before flowing through a filter that removes the oxidisedparticles of manganese, iron, and possibly some carbonate andsulphate.
It is important that backwashing of the filter isregularly performed, because manganese oxidation is slower thanthat of iron: greater quantities of oxygen are required, and thusthis technique is not considered suitable for water containingorganic complexes of the metals that can clog the filter.Chemical oxidation followed by filtration can be used in thoseenvironments where the levels of manganese and iron exceed10mg/l.
The principle is that an oxidising material, for examplechlorine or hydrogen peroxide, converts the manganese and ironfrom a dissolved to a solid form before removing the precipitatesusing a sand trap filter.
This type of treatment is particularlyeffective when iron is combined with organic matter or bacteria.The filtration material requires frequent backwashing toeliminate the solid manganese/iron particulates that accumulate.Voltammetry - a brief overview.
Heyrovsky first introducedpolarography in 1922.
The term voltammetry is applied todesignate the current-voltage measurement obtained at a givenelectrode.
Polarography is a special case of voltammetryreferring to the current-voltage measurement acquired using adropping mercury electrode with a constant flow of mercury drops.Stripping voltammetry uses the same instrumentation astraditional polarography but a stationary electrode such as theHanging Mercury Dropping Electrode (HMDE) is used.
Thevoltammetric measurement is performed on a stationary mercurydrop, allowing one to achieve considerable increases insensitivity.
The jump in sensitivity is possible byelectrochemical preconcentration of the metals in question at thesurface of the stationary electrode before the current-voltagecurve is recorded.
The recorded current is the redissolution(reoxidation) current of the preconcentrated metal traces.Voltammetry today represents a refined, clean, simple techniquethat offers outstanding limits of detection and is now thefastest growing analytical technique for trace analysis.
With theMetrohm Multi Mode Electrode (MME) the mercury is hermeticallysealed in the reservoir and suffices for around 200,000 drops,ensuring low laboratory running costs.
Method for analysis ofmanganese in drinking waters.
10mls of sample, 0.5ml of ammoniabuffer, 0.5ml of borate buffer, and 50µl of zinc solution wereadded to the reaction vessel in the Metrohm 757 VA Computrace togive a solution pH between 9.5-10.0.
The role of the electrolyteand additional solutions in voltammetry is crucial.
In the caseof the zinc solution added, this was to prevent interference ofintermetallic compounds.
Many determinations are pH dependent; sothe electrolyte can increase the conductivity and selectivity ofthe solution.
The solution was then degassed with nitrogen for aperiod of five minutes to remove the electrochemically activeoxygen, before the manganese content was determined using twostandard additions with the HMDE.
The HMDE is an electrode modeof the MME.
Four mercury drops of a defined size are formed insuccession at the MME, and the last drop remains suspended on theend of the capillary.
The entire voltage sweep is performed onthe single stationary drop.
The analysis produced a result ofapproximately 25µg/l of manganese present in the sample ofdrinking water.Conclusion.
Although manganese in drinking water supplies posesno toxicological risk to humans based upon current information,its presence at higher levels than one would normally expectproves to be a nuisance - as well as providing potential economicproblems if the manganese is not properly controlled throughregular monitoring.
It is a good idea to incorporate analysis ofmanganese in drinking waters into a regulated analyticalmonitoring program, despite there being no regulatory standard,as one can occasionally get high levels of manganesecontamination from unexpected sources - for example whenmanganese from batteries or pesticides leaches into well-water.It is a matter of priority for the water companies to ensure thattheir water supply to the consumer is not only safe, but ispleasing in appearance, taste, and odour.
A supply of water thatis unsatisfactory in this respect will undermine the confidenceof consumers, leading to complaints and maybe the use of waterfrom less safe sources.
Once the confidence of the consumer hasbeen eroded it is very difficult for the water companies to fullyregain that trust.
It can result in the use of bottled water andhome treatment devices, some of which can have adverse effects onwater quality.
Voltammetry is an increasingly popular techniquethat in many instances offers unrivalled detection limits, evenwhen compared to vastly more expensive analytical techniques.Voltammetry requires no specialist laboratory infrastructure suchas expensive fume extraction.
All that is required is a sturdybench top on which to mount the instrument, and a regulated flowof an inert gas.
The running and maintenance costs of voltammetryare minimal, ensuring a cost-effective analytical solution tomeet the demands required by those industries with anenvironmental bias.
The following internet sites were usedextensively as references and can be used to obtain furtherinformation: www.bae.ncsu.edu, www.europa.eu.int, www.metrohm.ch,www.midkentwater.co.uk, www.who.int, www.wilkes.edu
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