Mercury and arsenic are examples of heavy metals that can be found in ground water across many parts of the world. These metals reach the ground water either as a result of human activities or through natural processes. In a number of regions, the limit values for the concentrations of these metals are exceeded by many times, especially in the case of arsenic found in drinking water. Therefore, water quality needs to be strictly monitored. This article is focused on field determinations of copper, mercury, and arsenic – directly at the sampling site.
Heavy metals in drinking water as a worldwide global problem
The issue of drinking water contaminated with heavy metals is a global one. Although it is far from the only example, a particularly prominent case is arsenic-contaminated well water in Bangladesh. According to WHO, it is estimated that more than 200 million people globally are exposed to drinking water that contains over 10 μg/L of arsenic, which exceeds the WHO recommended limit value . Increased levels of arsenic caused by geological conditions have affected large regions in, for example, the United States, including the Southwestern states such as Nevada, as well as the Upper Midwest and New England . In Switzerland also, drinking water sources are used in scattered regions that have an arsenic level that exceeds WHO’s guideline value .
The effects of chronic arsenic poisoning are as serious as they are varied. Nervous system disorders, pulmonary tuberculosis, heart attacks, skin lesions, skin cancer and other types of cancers are just a few examples.
How arsenic contaminates drinking water
Natural processes largely contribute to the contamination of ground water with arsenic. For instance, arsenic present in weathered rock is released by river waters and deposited in the sediments, preferentially in river deltas. If the sediments make contact with ground water aquifers, they contaminate the ground water . This is what happens at the Ganges Delta in Bangladesh, the world's largest river delta.
However, in addition to natural processes, human activities contribute to increased concentrations of arsenic in the environment. Arsenic can reach ground water through contaminated wastewater and seepage related to waste landfills and mining operations. Even in developed nations such as the US and Germany, there are new examples to be discovered to the present day [5, 6]. This is the reason why wastes, including waste waters, should be monitored. Government agencies must also regularly check the concentrations of arsenic present in natural waters, especially those nearby to such plants.
Figure 1. Distribution of arsenic in the ground water in the US. Shown here is the 75th percentile, in each case within a radius of 50 km. That means that the concentrations in the samples investigated within a range of 50 km were lower than the specified value in 75% of the cases – and higher in 25%. Figure: U.S. Geological Survey.
Figure 2. Two Indian girls carry well water to their homes. While the construction of wells in the Ganges Delta region has reduced diseases caused by pathogen-contaminated surface water, a large number of these wells produce water that is strongly contaminated with arsenic. Chronic and acute arsenic poisoning that affect large parts of the population are the consequence.
Wood preservatives that contain arsenic
In a number of countries, chromated copper arsenate (CCA) is used in wood preservation agents, which can be problematic because CCA-treated vineyard poles, pasture fences and utility poles can release arsenic into the ground. It is the vineyard poles especially, that can lead to significant contamination of the ground water and soil, owing to the high density of poles per acre . As a result, the EU banned the use of CCA and CCA-treated wood in 2004, except in a few cases. However, its use is still widespread and widely accepted in New Zealand, Australia and the US [8, 9].
Different states of arsenic
In water, arsenic is present in two oxidation states: as less toxic arsenic(V) and as highly toxic arsenic(III).
Therefore, it is not only crucial to record the total concentration of arsenic in arsenic determinations; the speciation of arsenic is also of relevance to water quality ratings. If possible, arsenic(III) should be determined on site, directly following sampling because it is unstable and spontaneously oxidizes to form arsenic(V).
Figure 3. The Ganges Delta, as seen from an airplane. With a surface area of around 140 km2, it is the largest river delta in the world. The arsenic-rich sediments of the Ganges Delta are the source of the high arsenic concentrations in Bangladesh's ground water.
Natural and anthropogenic mercury sources
Aside from arsenic, other heavy metals such as mercury find their way into surface waters and drinking water, which is damaging to both humans and the environment. Large quantities of mercury are continuously released by Earth into the atmosphere through the Earth's crust and vegetation. Added to this, human activities are an ever-increasing contributor to the deposition of mercury into the atmosphere: the anthropogenic share of atmospheric mercury in the 20th Century was about 70% – and growing [10, 11]. For instance, mercury can be introduced into the environment through small-scale gold mining and the burning of fossil fuels. Some industrial processes also pose a problem, particularly metal refining, cement manufacture, and chlor-alkali electrolysis after the amalgam process to extract caustic soda and chlorine. In addition, as is the case with arsenic, ground water can be contaminated with mercury via seepage water from waste landfills . In the past, mercury was used as a low-cost agricultural fungicide, which also resulted in cases of humans getting poisoned .
Part of the released mercury enters the waters via precipitation. It collects in the tissue of crustaceans and fish, thereby posing a hazard to humans; mercury is introduced into the human body when contaminated fish is consumed. Given the toxicity of mercury to humans and the environment, the determination of mercury levels is mandatory to an equal degree in both fished and unfished waters.
Figure 4. Mercury leaches out of the waste at landfills and can seep into the ground and into ground water.
Figure 5. Mercury from the water accumulates in the tissue of fish and crustaceans. A diet rich in fish can therefore lead to increased exposure to mercury.
Figure 6. The Portable VA Analyzer enables on-site measurements and thus enables, for example, reliable speciation analyses of arsenic. The measuring instrument and the necessary accessories fit in the accompanying handy transport case.
Determination of arsenic, mercury and copper on site
Using the 946 Portable VA Analyzer, the concentrations of copper, mercury, total arsenic, arsenic(V) and arsenic(III) can be voltammetrically determined on site. This portable instrument has been specifically designed for determining traces of copper, mercury, and arsenic in water and is appropriate for checking compliance with the relevant WHO guideline values (Table 1).
The 946 Portable VA Analyzer, along with the bottles required for the reagents and the accessories needed for measurement, all fit into a single handy case. The only addition needed is a laptop for instrument control. Measurements can be easily performed directly at the sampling site in a car trunk, for example, which means results can be obtained quickly and without samples needing to be transported to a laboratory first. With regard to arsenic, a reliable determination of the oxidation states is also obtainable with this on-site measurement. If a sample had to be taken to a laboratory first, determination would be flawed by oxidation of the highly unstable arsenic(III).
Table 1. WHO guideline values for arsenic, mercury, and copper in drinking water and the detection limits of the three heavy metals with the 946 Portable VA Analyzer.
WHO guideline value
Detection limit Portable VA Analyzer
Heavy metals can be quickly and conveniently determined using the 946 Portable VA Analyzer. Following sampling, the sample and the electrolyte are filled into the measuring cell. The appropriate method is then selected and measurement is initiated on the laptop, making operation of the 946 Portable VA Analyzer particularly straightforward. If required, the user can modify certain measuring parameters such as the sample volume or the number and volume of standard additions. While measurement is carried out, the user is prompted by the software at the exact points to carry out the standard additions, which are pipetted into the measuring cell via the openings intended for this purpose. Finally, the software reviews the result autonomously and PDF reports can be compiled either automatically or manually.
To perform the measurement, the 946 Portable VA Analyzer uses the unique scTRACE Gold sensor, which combines the gold microwire working electrode with the screen-printed reference and auxiliary electrodes. This provides several benefits over other electrodes; virtually no maintenance of the scTRACE Gold is required and no tedious conditioning is needed. It can also simply be replaced at any time, if necessary.
Sample results in minutes
In environmental analyses across the globe, the determination of heavy metals in water is a key topic. The Metrohm 946 Portable VA Analyzer not only enables the determination of total arsenic and arsenic species, but also copper and mercury through voltammetry, directly at the sampling site. Field measurement allows for more rapid data acquisition, as well as preventing results from being influenced by chemical changes that can take place following sampling such as the oxidation of arsenic(III) to arsenic(V). The design of the software and accessories, especially the maintenance-free scTRACE Gold sensor, enables exceptionally convenient handling. The compact system meets all the requirements for on-field measurement.
Figure 7. Current-voltage curve of an arsenic(III) determination in mineral water with the 946 Portable VA Analyzer with two standard additions of an As(III) standard solution.
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