thermal power plants. This example will cover details about the application of heavy metal determination in boiler feed water.
Cd of heavy metals in purified water with different sensors and with corresponding R
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All thermal power plants use water for cooling and steam generation. Only extremely pure water guar- antees efficient and trouble-free operation of these processes. Monitoring the water quality is therefore crucially important in these situations. Heavy metals such as copper or iron act as important corrosion indi- cators and can reflect potential safety issues. To facil- itate trouble-free operation of power plants, various guidelines for boiler feed water have been established [3]. The guideline values for both copper and iron are in the low μg/L range (between 10 μg/L and 50 μg/L).
Zn, Cr (VI), Ni, Co
Pb, Se(IV), Pb Co, Ni, Sb(III) Bi
A − SEA WATER
Stripping voltammetry can be employed for the sensi- tive determination of very low concentrations of heavy metals in boiler feed water. Concentrations down to approximately 0.3 μg/L can be determined for both copper and iron using the mercury-free scTRACE Gold sensor. In Figure 2, examples for the copper and iron determination in deionized water are presented.
Figure 2. Determination of copper (A) and iron (B) in deionised water with the 884 Professional VA and 30 s deposition time. B A
sensi- heavy wn to both Gold
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Sodium and chloride, along with magnesium, sulfate, calcium, potassium, bromide, and carbonate, represent more than 99% of the ions dissolved in sea water. Apart from these ions, there are more than 50 other naturally occurring elements in sea water. In many cases, they are present in trace concentrations. However, their low concentrations do not correlate well with their importance to the ecosystem. A good example of this concept is iron. The oceanic concen- tration of iron is extremely low due to its poor solubility and its biological uptake. It is one of the bioactive elements and is essential for marine organisms. The iron concentration in sea water significantly affects the ecosystem and controls the growth of microorganisms, as it is a micronutrient. Due to its trace concentrations, the determination of total dissolved iron in sea water requires a highly sensitive analytical approach.
Figure 2. Determination of copper (A) and iron (B) in deionized water with the 884 Professional VA and 30 s deposition time.
The concentration of total iron in sea water can be determined using adsorptive stripping voltammetry (AdSV) with 2,3-dihydroxynaphthalene (DHN) as a complexing agent. Iron can be determined down to approximately 0.3 μg/L with this method using the mercury-free scTRACE Gold sensor. An example showing the iron determination in an artificial sea water sample is presented in Figure 4. In addition to iron, also nickel, cobalt, chromium (VI), cadmium, lead, antimony (III), thallium, and zinc can be determined with mercury-free sensors in sea water samples.
Figure 4. Example for (A) iron determination in an artifi cial sea water sample β(Fe) = 23.3 μg/L and for (B) a calibration curve in a low μg/L range. B
Protection Agency (EPA) have established limit values for heavy metals in drinking water. However, in some countries there are no limit values for drinking water set by the local authorities. To assist in such situations, the World Health Organization (WHO) has defi ned certain guideline values. In the WHO’s ‘Guidelines for Drinking-water Quality’, limits for organic, inorganic, radiological, microbiological, and additional parameters are set. For instance, the guideline concentration value for cadmium is set at 3 μg/L, and 10 μg/L for lead [4]. To monitor the limit and guideline values, sensitive analytical methods are required.
Metrohm provides anodic stripping voltammetry-based (ASV) methods that are suitable to monitor down to the WHO guideline values for multiple heavy metals in drinking water. One of the available methods employs the Bi drop electrode to simultaneously determine concentrations as low as 0.5 μg/L for lead and 0.1 μg/L for cadmium. An example of a tap water sample containing 2 μg/L cadmium and lead is shown in Figure 3.
Table 2. Results for the analysis of tap water sample containing 2.0 μg/L Cd and 2.0 μg/L Pb. Sample
The AdSV method is simple to perform, specific, and free of interferences. It is a viable, less sophisticated alternative to atomic absorption spectroscopy (AAS) or inductively coupled plasma (ICP), requiring only a moderate investment in hardware and low running costs. However, the main advantage of this analytical method lies in the innovative and cost-effective sensing platform. The scTRACE Gold is a combined sensor consisting of a small gold wire working electrode, Ag/AgCl reference, and carbon auxiliary electrode on a ceramic substrate. The semi-disposable sensor does not require maintenance such as mechan- ical polishing or mechanical cleaning. It can be used conventionally in the laboratory with the 884 Profes- sional VA, or alternatively in the field with the 946 Portable VA Analyzer. This method is best suited for manual and semiautomated systems.
Tap Water
Cd (μg/L) 2.0
Pb (μg/L) 2.0
4 3 B Co, Co
B A
more than 99% of the ions dissolved in sea water. Apart from these ions, there are more than 50 other naturally occurring elements in sea water. In many cases, they are present in trace concentrations. However, their low concentrations do not correlate well with their importance to the ecosystem. A good example of this concept is iron. The oceanic concen- tration of iron is extremely low due to its poor solubility and its biological uptake. It is one of the bioactive elements and is essential for marine organisms. The iron concentration in sea water significantly affects the ecosystem and controls the growth of microorganisms, as it is a micronutrient. Due to its trace concentrations, the determination of total dissolved iron in sea water requires a highly sensitive analytical approach.
Sample
Sample size β(Cu) β(Fe)
− DRINKING WATER
Figure 2. Determination of copper (A) and iron (B) in deionized water with the 884 Professional VA and 30 s deposition time.
The concentration of total iron in sea water can be determined using adsorptive stripping voltammetry (AdSV) with 2,3-dihydroxynaphthalene (DHN) as a complexing agent. Iron can be determined down to approximately 0.3 μg/L with this method using the mercury-free scTRACE Gold sensor. An example showing the iron determination in an artificial sea water sample is presented in Figure 4. In addition to iron, also nickel, cobalt, chromium (VI), cadmium, lead, antimony (III), thallium, and zinc can be determined with mercury-free sensors in sea water samples.
The AdSV method is simple to perform, specific, and free of interferences. It is a viable, less sophisticated alternative to atomic absorption spectroscopy (AAS) or inductively coupled plasma (ICP), requiring only a moderate investment in hardware and low running costs. However, the main advantage of this analytical method lies in the innovative and cost-effective sensing platform. The scTRACE Gold is a combined sensor consisting of a small gold wire working electrode, Ag/AgCl reference, and carbon auxiliary electrode on a ceramic substrate. The semi-disposable sensor does not require maintenance such as mechan- ical polishing or mechanical cleaning. It can be used conventionally in the laboratory with the 884 Profes- sional VA, or alternatively in the field with the 946 Portable VA Analyzer. This method is best suited for manual and semiautomated systems.
3
Drinking water is defined as water that is used for drinking purposes or food preparation, including both tap water and mineral water. Safe drinking water must not present any significant risks to human health over a lifetime of its consumption. The term «safe» is defined by legal entities and includes limit values for physical and chemical parameters like the concentration of heavy metals, organic compounds, total suspended solids, and more. Due to the detrimental impact of heavy metals on human health, legal entities including the European Union (EU) and the U. S. Environmental Protection Agency (EPA) have established limit values for heavy metals in drinking water. However, in some countries there are no limit values for drinking water set by the local authorities. To assist in such situations, the World Health Organization (WHO) has defined certain guideline values. In the WHO’s «Guidelines for Drinking-water Quality», limits for organic, inorganic, radiological, microbiological, and additional parame- ters are set. For instance, the guideline concentration value for cadmium is set at 3 μg/L, and 10 μg/L for lead [4]. To monitor the limit and guideline values, sensitive analytical methods are required.
Metrohm provides anodic stripping voltammetry-based (ASV) methods that are suitable to monitor down to the WHO guideline values for multiple heavy metals in drinking water. One of the available methods employs the Bi drop electrode to simultaneously determine
Table 1. Results for the analysis of deionized water sample containing 1.0 μg/L Cu and 2.0 μg/L Fe, respectively.
Deionized water (spiked)Environmental Analysis & Electrochemistry 10.0 mL 1.0 μg/L 2.0 μg/L
B
concentrations as low as 0.5 μg/L for lead and 0.1 μg/L for cadmium. An example of a tap water sample containing 2 µg/L cadmium and lead is shown in Figure 3.
21
Figure 3. Example for determination of cadmium and lead in tap water sample containing 2 μg/L of each element.
Figure 3. Example for determination of cadmium and lead in tap water sample containing 2 µg/L of each element.
Table 2. Results for the analysis of tap water sample containing 2.0 μg/L Cd and 2.0 μg/L Pb.
Sample Tap water
Cd (µg/L) 2.0
Pb (µg/L) 2.0
The main advantage of using this ASV method is the innovative and cost-efficient sensor: the Bi drop elec- trode. With this mercury-free sensor, concentrations in the low µg/L and even ng/L range can be reliably measured. The Bi drop electrode benefits users with its high stability and long lifetime. Its optimal performance is easily reached in a fully automated system when used for sample series in the laboratory environment.
Figure 4. Example for (A) iron determination in an artificial sea water sample β(Fe) = 23.3 µg/L and for (B) a calibration curve in a low µg/L range.
Iron can be determined down to approximately 0.3 μg/L with this method using the mercury-free scTRACE Gold sensor. An example showing the iron determination in an artifi cial sea water sample is presented in Figure 4. In addition to iron, also nickel, cobalt, chromium (VI), cadmium, lead, antimony (III), thallium, and zinc can be determined with mercury-free sensors in sea water samples.
5
The AdSV method is simple to perform, specifi c, and free of interferences. It is a viable, less sophisticated alternative to atomic absorption spectroscopy (AAS) or inductively coupled plasma (ICP), requiring only a moderate investment in hardware and low running costs. However, the main advantage of this analytical method lies in the innovative and cost-effective sensing platform. The scTRACE Gold is a combined sensor consisting of a small gold wire working electrode, Ag/AgCl reference, and carbon auxiliary electrode on a ceramic substrate. The semi-disposable sensor does not require maintenance such as mechanical polishing or mechanical cleaning. It can be used conventionally in the laboratory with the 884 Professional VA, or alternatively in the fi eld with the 946 Portable VA Analyzer. This method is best suited for manual and semiautomated systems.
Figure 4. Example for (A) iron determination in an artificial sea water sample β(Fe) = 23.3 µg/L and for (B) a calibration curve in a low µg/L range.
Summary
The main advantage of using this ASV method is the innovative and cost-effi cient sensor: the Bi drop electrode. With this mercury-free sensor, concentrations in the low μg/L and even ng/L range can be reliably measured. The Bi drop electrode benefi ts users with its high stability and long lifetime. Its optimal performance is easily reached in a fully automated system when used for sample series in the laboratory environment.
Sea water
Sodium and chloride, along with magnesium, sulphate, calcium, potassium, bromide, and carbonate, represent more than 99% of the ions dissolved in sea water. Apart from these ions, there are more than 50 other naturally occurring elements in sea water. In many cases, they are present in trace concentrations. However, their low concentrations do not correlate well with their importance to the ecosystem. A good example of this concept is iron. The oceanic concentration of iron is extremely low due to its poor solubility and its biological uptake. It is one of the bioactive elements and is essential for marine organisms. The iron concentration in sea water signifi cantly affects the ecosystem and controls the growth of microorganisms, as it is a micronutrient. Due to its trace concentrations, the determination of total dissolved iron in sea water requires a highly sensitive analytical approach.
Figure 4. Example for (A) iron determination in an artificial sea water sample β(Fe) = 23.3 µg/L and for (B) a calibration curve in a low µg/L range.
The concentration of total iron in sea water can be determined using adsorptive stripping voltammetry (AdSV) with 2,3-dihydroxynaphthalene (DHN) as a complexing agent.
5
This article has presented an overview of several mercury-free methods for the voltammetric determination of heavy metals in aqueous samples along with some specifi c application examples. Due to the accumulation step applied during the voltammetric determination, the sensitivity increases signifi cantly. As a result, the limit values for heavy metal concentrations in different types of water samples defi ned by legal entities or guideline values recommended by WHO can be easily monitored. Problems resulting from the use of metallic mercury no longer play a role as Hg-free methods for the determination of 16 heavy metals are now available. Four different ‘green’ sensors, such as the scTRACE Gold, screen-printed electrodes, the glassy carbon electrode, and the Bi drop electrode can be used to determine low concentrations of heavy metals in different sample matrices, such as boiler feed water, drinking water, and sea water.
5 References
1. Wang, J.; Lu, J.; Hocevar, S. B.; et al. Bismuth-Coated Carbon Electrodes for Anodic Stripping Voltammetry. Anal. Chem. 2000, 72 (14), 3218–3222. DOI:10.1021/ac000108x 2. Švancara, I.; Mikysek, T.; Sýs, M. Polarography with Non-Mercury Electrodes: A Review. Electrochemical Science Advances 2022, e2100205. DOI:10.1002/elsa.202100205 3. Engineering ToolBox. Boiler Blowdown
https://www.engineeringtoolbox.com/
boiler-blowdown-d_908.html (accessed 2022-12-24).
4. World Health Organization. Guidelines for Drinking-Water Quality, 4th Edition, Incorporating the 1st Addendum, 2017.
https://www.who.int/publications/i/item/9789241549950.
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