Elemental Metals Analysis in Water

Water is essential to life; therefore, providing access to safe and clean water is a major concern for health development at the local, national, and regional levels. Enhancing water quality by improving sanitation systems needs a profound understanding of water chemistry, especially for the element analysis in water. Every organic compound contains the elemental carbon together with one or more of the other elements in the periodic table, in definite proportions. Elemental analysis serves to measure the proportion of each element present in the compound. The following figure lists common organic/inorganic pollutants in water. Generally, contaminated water contains toxic metallic elements such as As, Cd, Co, Cr, Cu, Hg, Ni, Pb, Sn, and Zn.

Fig.1 Organic/Inorganic pollutants in water (Brandt, M. J., etc, 2017)

Metal ions in aquatic media originate from both anthropogenic activities and natural processes. Mining and industrial discharges are major sources for the introduction of significant quantities of metal ions into water. Besides, the emergence of large amounts of e-waste is another major source of toxic metal ions. Metal ions are highly important pollutants due to their physiological role in human and other living organisms. Recently, Metallomics has emphasized the importance of metal ions and their interactions in ecosystems.

Metals are categorized into three sets: toxic metals (As, Cd, Co, Cr, Cu, Hg, Ni, Pb, Sn, Zn, etc.), precious metals (Ag, Au, Pd, Pt, Ru, etc.), and radionuclides (Am, Ra, Th, U, etc.). These metals, commonly referred to as heavy metals, are described as having high density metal elements and are fatal even at trace concentrations. As, Cd, Hg, and Pb are highly toxic and are on the top 10 of Agency for Toxic Substance and Disease Registry (ATSDR) and WHO’s toxic substance lists. Arsenic is a metalloid but is universally referred to as a heavy metal. All metals accumulate in human’s internal organs, and excessive concentrations result in adverse reactions and diseases. Thus, DW tainted with metal ions even at trace levels leads to severe health disorders, and the maximum allowable limits of metal ions in drinking water are summarized as follows.

Table 1 The maximum permissible limits of metal ions in drinking waters

Metal ion	ATSDR (2015 ranks)	EU (μg/L)	USEPA (μg/L)	WHO (μg/L)
Arsenic (As)	01	10	10	10
Cadmium (Cd)	07	5.0	5.0	3.0
Chromium (Cr)	78	50	100	50
Copper (Cu)	118	2000	1300	2000
Lead (Pb)	02	10	15	10
Mercury (Hg)	03	1.0	2.0	6
Nickel (Ni)	57	20	–	70
Selenium (Se)	145	10	50	40
Thallium (Tl)	275	–	2.0	–
Uranium (U)	235a	–	30	30

Some cations are essential to human health and to the physiological functions of other organisms. However, high concentrations of exposure can adversely affect the health of the organism and cause damage to the ecosystem. Ions such as Hg²⁺, Cr⁶⁺, Pb²⁺, or Cd²⁺ are toxic as they are not essential for biological functions and they accumulate in the organism. Household and industrial effluents are common sources of those pollutants, which directly affect the structure and productivity of ecosystems. The importance of controlling the level of toxic metals in natural and potable water has led to the development of novel sensitive analytical methods including methods for speciation analysis.

For the determination of toxic metals at low levels, atomic spectrometry is commonly employed, which includes atomic absorption spectrometry (AAS) and the techniques based on inductively coupled plasma (ICP) as an ion reservoir/source, such as optical emission spectrometry (ICP OES) and mass spectrometry (ICP-MS). These techniques can be coupled with powerful separation techniques such as high-performance liquid chromatography (HPLC) and capillary electrophoresis (CE). Other methods can be used, such as potentiometry using ion selective electrodes, ion chromatography (IC) and redox titration, depending on the substance being analyzed and its level.

Fig.2 Environmental analytical chemistry¹

• Spectrometric methods

AAS and ICP OES are based on the electronic transitions that occur in free atom clouds in reservoirs at high temperatures. In AAS the measured absorbance (which is a logarithmic function of the transmittance) is a result of the absorption by atoms and photons from an electromagnetic radiation source. In optical emission, ground state atoms are transformed to excited state ions due to thermal and collisional processes that occur in the Ar+ rich ICP. The return of ions to the ground state emits characteristic photons that can be related to the nature and quantity of the target species. Both techniques allow determinations at the μg·L^?1 (parts per billion) levels and present established protocols to determine more than 70 metals and metalloids in different types of water samples using approaches that vary upon the employed atomization reservoirs (atomizer) and ways to introduce the analyte into atomizers. ICP is the standard source for OES, and for AAS, different atomizers are available and used according to the nature of the analytes, their levels in samples, and expected interferences of the sample matrices. In Flame Atomic Absorption Spectrophotometry (FAAS), the aqueous sample is aspirated into a flame where the process of atomization takes place. The air/acetylene flame is usually employed to determine elements such as Al, Co, Fe, Zn, and others, while hot nitrous oxide/acetylene is utilized for the determination of Cu, Ag, Au, Ga, In, Ti, Si, As, Sb, Se, V, and others. Analyte preconcentration on solid phases, such as silica gel chemically modified with N-(1-carboxy-6-hydroxy) benzylidenepropylamine, enables the determination even at sub μg·L^?1 levels for analytes such as Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb in natural water samples.

ICP-MS has become the most powerful analytical tool for elemental determination in water samples. It covers almost all elements of the periodic table and enables isotopic analysis. Modern analytical systems allow a very efficient collection of the ions formed in the ICP. Reaction and collisional cells have been employed to minimize isobaric interferences that impair the detection of the most abundant isotopes of many elements. The spectral (mass-to-charge) resolution has been improved over the years making quadrupole analyzers the best cost-benefit analyzer for routine analysis. Sector field mass spectrometers allow very high resolution to minimize isobaric interference. Nowadays, the coupling of ICP-MS with either HPLC or CE has become a routine for speciation analysis and can even be pre-concentrated prior to introduction of ICP. The use of isotope dilution for quantification, extremely low detection limits and the robustness of the system obtained over time makes ICP-MS essential for any environmental control laboratory. Applying these two techniques, and after optimizing the experimental conditions, Li, Be, B, Al, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Rb, Sr, Ag, Cd, Sb, Cs, Ba, Hg, Tl, Pb, and Bi have been determined in natural water samples in the μg·L^?1 range.

Fig.3 Atomic spectrometric methods (http://slideplayer.com/slide/4617174/)

• Ion chromatography

Ion chromatography (IC) is applied for separation and analysis of both anions and cations in environmental samples. The separation of analytes on a column leads to the same identical analytical results as in other separation techniques. The differences in ion-exchange affinities of different species of ions result in their separation, and the most common detection is through conductivity detectors. The most widely used approach is the ion (cation) exchange with reversible complexation. However, for complex samples, anion exchange with irreversible complexation is also employed. But nowadays the most effective one is the bifunctional ion exchange column that is mostly utilized to separate heavy metals and transition metals. The application of of the chelation IC, and the online sample pretreatment quantification of lead, copper, cadmium, and other transition metals, has been carried out in water samples. The analysis of drinking water resulted in quantities of transition metals and heavy metals in the ng·mL^?1 range such as Pb (2.0), Cu (0.2), Cd (0.6), Co (0.2), and Ni (0.2).

Fig.4 Ion chromatography (https://en.wikipedia.org/wiki/Ion_chromatography)

References

• Brandt, M. J., Johnson, K. M., Elphinston, A. J., & Ratnayaka, D. D. (2017). ‘Chapter 7 – chemistry, microbiology and biology of water.’ Water Supply, 235-321.

• Desireddy H K R . (2017) ‘Encyclopedia of Sustainable Technologies || Water Pollution Control Technologies’ Encyclopedia of Sustainable Technologies, 3-22.

• Mahmoodi, R., Shahriari, M., & Jafari, J. (2009) ‘Finite Element Analysis for Water Hammer Problems in Pipeline Systems.’ Third International Conference on Mechanical Engineering & Mechanics, pp.23.

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