Analysis of mercury in waters using voltammetry

Mercury is a naturally occurring element and there are many potential sources, often it is found in the environment as the ore cinnabar also known as mercury sulphide.

Mercury can be present in numerous chemical forms and both the elemental and inorganic forms can be transformed into methylated organic forms by biological systems.

Methylated forms of mercury are especially toxic in addition to being bioaccumulative when they pass through the aquatic food chain.

Natural sources of mercury include volcanoes, geological deposits of mercury and volatilisation from the ocean.

Once in the atmosphere, mercury can travel long distances and be deposited into streams and rivers through atmospheric fall out, often making it very difficult to pinpoint sources of contamination.

A major release of mercury into water and air is through certain industrial processes, fuel combustion, waste incineration and improper disposal of mercury containing products.

Once in streams and rivers mercury generally does not remain suspended in the water column but is deposited in the sediment.

A tiny amount is incorporated into the food chain and undergoes biomagnification - a natural process whereby contaminants like mercury are found at progressively higher concentrations as they pass from prey to predator up the food chain.

The biomagnification of mercury can lead to concentrations in fish that exceed the safe levels recommended for human consumption.

The need to monitor and control water sources is paramount even still today, despite mercury releases being considerably reduced over recent years, to ensure the continued wellbeing of humans and the environment in which they exist.

Voltammetry - a brief overview.

Jaroslav Heyrovsky first introduced the concept of polarography circa 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.

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.

For stripping analysis in the region below 1mg/l, Metrohm also offers a Rotating Disk Electrode (RDE) with exchangeable tips of glassy carbon for mercury film techniques and gold for determinations of mercury.

RDE are often employed as an alternative to the traditional mercury-working electrode and offer well defined, reproducible conditions for voltammetric analysis.

The surface of the solid state electrode is constant throughout the entire analysis allowing low limits of detection.

The jump in sensitivity is made 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.

Applications of mercury.

Mercury is perhaps best known as the silver liquid present in thermometers but also has over 3000 industrial uses.

Mercury is used in a variety of different industries and is commonly used in laboratory work for making barometers, diffusion pumps and in analytical instruments such as polarography systems using the classic mercury-dropping electrode.

A gaseous form of mercury is used in mercury vapour lamps and for advertising signs.

It is also used for mercury switches and other electrical apparatus.

Historically, mercury has been used in the manufacture of certain pesticides, anti fouling paints as well as the production of chlorine and caustic soda although these types of applications have declined over recent decades due to the environmental consequences of their use.

Other uses include catalysts for synthesis and the production of batteries or mercury cells.

Mercury will dissolve numerous metals to form amalgams and has been used in the extraction of gold dust from rocks, as well as with tin, silver and gold in dental fillings.

World-wide dental and medical institutions have previously caused considerable mercury contamination to local water supplies but better control procedures now require safe and ecologically sound disposal of the unwanted waste.

Mercury and the environment.

Mercury is an element existing in elemental, inorganic and organic compounds meaning it will always be present in the world around us.

It is possible to remove contaminated solid waste such as soil and sludge and dispose of it elsewhere, such as in a hazardous waste landfill site but the mercury will still be present.

Mercury is easily vaporised so the air around chlorine-alkali plants, smelters, waste incineration plants and sewage treatment plants may contain increased levels of mercury.

The adoption of pollution prevention and recycling schemes for mercury bearing waste such as thermometers, fluorescent lights and switches has led to significant reductions in the amount of mercury in solid waste streams.

Mercury is being phased out of certain products such as batteries and industrial permits are becoming ever more stringent in addition to better control technology being developed and adopted for the likes of industrial or agricultural waste.

As a consequence, mercury discharges into streams and rivers over recent decades have been greatly reduced.

Historically, there have been few permissible consent levels with regard to contamination by mercury in different water sources.

Today, the Water Supply (Water Quality) Regulations 2000 state that at the point of the consumers' tap then the maximum concentration of mercury permitted is 1 mg/l.

Methylation is a product of complex processes that move and transform mercury.

Atmospheric deposition contains the three principal forms of mercury but once in surface water, mercury enters a complex cycle in which one form can be converted to another.

Mercury that is attached to particles can settle onto sediments where it can diffuse into the water column and be suspended, buried or methylated.

Methyl mercury can enter the food chain or it can be released back to the atmosphere by volatilisation.

The concentration of dissolved organic carbon and pH play a strong role in determining the fate of mercury in a particular ecosystem.

Studies have shown that for a water system with increased acidity or dissolved organic carbon content, then the levels of mercury observed are increased due to the enhanced mobility of the mercury under these conditions.

The exact mechanism by which mercury enters the food chain is largely unknown and may vary among ecosystems.

It is known that certain bacteria process sulphate in the environment and take mercury up in its inorganic form before converting it to methyl mercury through metabolic processes.

Toxicity of mercury.

It has been known for hundreds of years that mercury is toxic in large quantities however recent evidence demonstrates that even low levels of mercury may be hazardous.

Exposure to mercury can occur through inhalation or ingestion and the form of mercury determines the amount of mercury taken in by the body.

Only 0.01% of elemental mercury is absorbed but almost 100% of methyl mercury is absorbed from the gastrointestinal tract.

The biological half-life is some 60 days so that even when the exposure has been reduced the mercury burden will remain active in the body for some months.

In the human body mercury accumulates in the liver, kidney, brain and blood and can cause chronic or acute health effects.

Acute exposure is not as common as it once was due to greater precautions and decreased handling although severe acute effects may include gastrointestinal damage, cardiovascular collapse or kidney failure all of which can be fatal.

Chronic effects include pronounced damage to the central nervous system as a result of elemental mercury dissolved in the blood where it may be transported across the blood/brain barrier before being oxidised and retained in brain tissue.

Other symptoms include possible kidney failure and birth defects in unborn children as a result of neurological damage from methyl mercury.

Symptoms of chronic mercury exposure on the nervous system include increased excitability, mental instability, a tendency to weep, trembling of the hands and feet and personality changes.

The expression "mad as a hatter" is actually derived from the symptoms of mercury exposure in workers who manufactured felt hats using a mercury containing process.

Extreme mercury pollution - a case study.

Widespread poisoning of Japanese fisherman and their families occurred in Minamata, Japan as a result of consumption of fish contaminated by methyl mercury.

The Chisso Corporation initially produced fertilisers but evolved over a number of years to manufacture petrochemicals and plastics using acetaldehyde that was produced using mercury as a compound.

The company was considered a success because it was one industry that maintained economic development despite Japan's suffering during and right after World War II.

During this time, Chisso was the only manufacturer of a plasticiser called diotyl phthalate that allowed the organisation to expand rapidly between the years 1952-1960.

Throughout the years 1932-1968 it is estimated that the company dumped an estimated 27tonnes of mercury compounds into Minamata Bay.

It took until the middle of the 1950s before people began to notice the effects of a strange disease.

Throughout this period, the local people and animals in the surrounding areas of the bay whose normal diet included fish developed peculiar symptoms not previously seen before.

The major symptoms of Minamata disease included involuntary trembling, sensory disorders of the limbs, muscular co-ordination failure, speech and language disorders, reduced fields of vision and a loss of equilibrium.

Curious phenomenon such as cats committing suicide and birds falling from the sky were observed.

It took until 1968 for Chisso to stop polluting the waters of Minamata Bay primarily because the method of mercury production had become so outdated.

The years of neglect and environmental destruction were allowed to propagate for such a long period of time because it is alleged that the company paid off certain victims and intimidated prominent outspoken campaigners into silence.

Over 3000 victims have been recognised as having Minamata disease and it has taken many years for some to receive compensation for the event.

As late as 1993, the Japanese courts were still resolving suitable compensation for certain victims.

In addition to those who were recognised as having the condition, many thousands of others were classified as being non-certified victims and in 1995 the government awarded payment to 10,000 people including those since deceased.

It took until 1997 for the governor to declare that the mercury levels present in fish and shellfish in Minamata Bay were safe for human consumption.

This declaration coincided with the complete removal of the net that for the previous 23 years had prevented mercury-polluted fish in the bay from entering the sea in an effort to curb Minamata disease.

Method for analysis of mercury in waters.

Samples of waters can often contain sufficient quantities of organic material requiring some sample pre-treatment before the analysis can be undertaken.

The Metrohm 705 UV digester is eminently suited to this task and can eliminate moderate to high (after dilution) amounts of dissolved organic matter that ordinarily would disturb the voltammetric analysis of mercury.

5mls of water sample and 5mls of deionised water were added to the quartz cuvettes in the UV digester.

The pH of the solution was reduced to 2.0 with 10ml of concentrated hydrochloric acid before the addition of 100ml of hydrogen peroxide to aid the photolytic generation of OH radicals.

The samples were then digested at a temperature of 90C for a period of 120 minutes.

The digestion of organic matter is achieved through photolytic generation of OH radicals that in turn react with the organic compounds and decompose them.

Hydrogen peroxide serves as an initiator of the radical reaction and a mercury lamp provides radiant energy that is converted to heat accelerating the digestion process.

The advantage of UV photolysis over other sample digestion techniques is that only a little hydrogen peroxide has to be added ensuring that blank values can be kept low.

This is crucially important for analysis of water samples as the low content of mercury coupled with the extreme sensitivity of the technique means that the samples are prone to contamination if extreme care is not taken.

10ml of deionised water, 1ml of UV digested water sample, 1ml of sodium chloride solution, 400ml of EDTA solution and 300ml of concentrated perchloric acid were added to the reaction vessel in the Metrohm 797 VA Computrace.

The role of the electrolyte and additional solutions in voltammetry is crucial as many determinations are pH dependent and the electrolyte can increase the conductivity and selectivity of the solution.

The solution was then degassed with nitrogen for a period of 60 seconds to remove the electrochemically active oxygen, before the mercury content was determined with two standard additions using the gold RDE.

Automation with voltammetry.

For a large numbers of samples then it is possible to automate the process of voltammetry through the use of an autosampler, pump units and liquid dosing units known as dosimats.

The Metrohm 813 Compact Autosampler allows fully automatic precise, reproducible analysis of multiple samples of a similar nature and can accommodate a maximum of 18 samples that are transferred using a peristaltic pump to the measuring vessel of the Computrace.

The analyte concentration is determined by means of automatic standard additions carried out by Metrohm 765 Dosimats.

After each sample the measurement vessel is emptied and rinsed by two Metrohm 772 Pump Units.

The sample data is entered into a sample queue that is automatically processed by the 797 software.

Conclusion of analysis of mercury in waters.

Mercury is introduced into different sources of waters from industry as waste products from a variety of processes and its release must be monitored and carefully controlled where appropriate.

Even today despite stringent control of emissions we continue to be exposed to low levels of mercury in our diets primarily from fish and shellfish.

Over recent years, voltammetry has undergone a tremendous surge in popularity and today represents a refined, clean simple technique that offers outstanding limits of detection and is now the fastest growing analytical technique for trace analysis and is eminently suited to mercury analysis of waters.

Voltammetry is an increasingly popular technique that in many instances offers unrivalled detection limits even when compared to vastly more expensive analytical techniques.

For samples of water, then voltammetry often requires little or no sample preparation and the result determined by standard addition obtained in less than ten minutes.

The advantage of using standard addition as a means of calibration and quantification is that matrix effects present in the sample are taken into account.

Voltammetry requires no specialist laboratory infrastructure like 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 surpass the demands required by those organisations interested in quantifying mercury in sources of water.