Heavy metals are commonly defined as a set of metals/metalloids that possess densities at least 5 times higher than that of water. Heavy metals are natural constituents of the earth's crust, and natural phenomena such as weathering and volcanic activity contribute significantly to heavy metal pollution. Anthropogenic activities such as mining, smelting, industrial, and agricultural use increase the abundance of heavy metals in the environment by either releasing or concentrating them into the environment. Heavy metals are widely used in industrial, agricultural, medical, and technological applications. They can be toxic to living organisms at very low levels of exposure. Excessive usage of heavy metals has raised concerns for human health over time, and their impacts on the overall environment are being studied extensively. Based on their direct threats to human health and the environment, four of the top 10 chemicals of major public concerns reviewed by the World Health Organization (WHO) are heavy metals (arsenic, lead, mercury, and cadmium). In addition, chromium is also considered as a very toxic heavy metal.

Table 1 US EPA Regulatory Guidelines for Heavy Metals in Soil

• Arsenic (As)

Generally, soils contain about 5.0 ppm of arsenic, but soils with known historical or arsenic applications contain an average of about 165 ppm. In some places, such as Buns, Switzerland and the Wiatapu Valley, New Zealand, the arsenic level in the soil may reach 104 ppm; a substantial portion of arsenic in soil and soil-like material (sediment clays, sand, etc.) is expected to be found in soluble form and probably can be dislodged easily by the action of water moving through the soil. Soluble forms of arsenic are relatively more mobile in the environment and pose a greater potential of containing both groundwater and surface water. Soluble forms of arsenic from soil and soil-like material are likely to enter the bioconversion chain through their initial uptake by vegetation.

For the detection of arsenic, the commonly used methods are spectrophotometry, atomic absorption spectrometry, inductively coupled plasma atomic-emission spectrometry, inductively coupled plasma mass spectrometry and neutron activation analysis and photon activation analysis.

Merry and Zarcinas have described a silver diethyldithiocarbamate method for the determination of arsenic and antimony in soil. The method involved the addition of sodium tetrahydroborate to an acid-digested sample that has been treated with hydroxylammonium chloride to prevent the formation of insoluble antimony compounds. The generated arsine and stibine react with a solution of silver diethyldithiocarbamate in pyridine in a gas washtub. Absorbance is measured twice at wavelengths of 600 and 504 nm.

• Lead (Pb)

Most of the lead in soil exits in a slightly soluble form. When 2784 ppm of lead nitrite was added to the soil, it was found that the soluble lead content was only 17 ppm after three days. It is to be expected that all ions in nature will accumulate in the form of less soluble compounds , such as oxides, carbonates, silicates, and sulfates, the relative proportions of which depend on the nature and solubility of the soil.

For the detection of lead, methods usually used are spectrophotometry, atomic absorption spectrometry, atomic fluorescence spectroscopy, anodic stripping voltammetry, polarography and X-ray fluorescence spectroscopy.

• Mercury (Hg)

As for mercury, spectrophotometric method, atomic absorption spectrometry and anodic stripping voltammetry are commonly used.

Kimura and Miller have shown that at room temperature, reduction with tin(II) and aeration is suitable for quantitative separation of microgram quantities of mercury(II) from sulfuric and nitric acid extracts of soil over wide ranges of concentrations. Mercury is concentrated during the separation and is determined by a direct photometric dithizone procedure. The standard deviation of 0.05 μg of mercury in a single measurement ranged from 0 to 0.5 μg.

Kamburova reported a spectrophotometric method based on the formation of the mercury-triphenyltetrazolium chloride complex for the determination of mercury in soils.

• Cadmium (Cd)

Cadmium is readily taken up by most plants. The occurrence of cadmium in motor oils, car tires, phosphorus fertilizers, and impurities in zinc compounds all indicate its accumulation in the soil; the cadmium contents of soils in nonpolluted areas are below 1 ppm, but values as high as 50 ppm can be found.

Atomic absorption spectrometry and inductively coupled plasma atomic-emission spectrometry and inductively coupled plasma spectrometry are usually used in the detection of cadmium.

The determination of cadmium by graphite-furnace atomic-absorption spectrometry is especially difficult because cadmium is a volatile element, and matrix constituents cannot be removed by charring without a loss of cadmium. The use of selective volatilization often makes it possible to obtain a cadmium peak before the background rises to such a high value that it interferes with cadmium measurements.

Inductively coupled plasma atomic emission spectrometry has proved to be an excellent technique for the direct analysis of soil extracts because it is precise, accurate, and not time-consuming, with level of matrix interference being very low. Of course, the graphite-furnace technique yields lower detection limits than the inductively coupled plasma procedure.

• Chromium (Cr)

Spectrophotometric methods and atomic absorbance spectrometry are widely used in chromium detection.

Qi and Zhu studied a highly sensitive method for the determination of chromium in soils. In this method, chromium VI is reacted with o-nitrophenyl-fluorone in the presence of cetyltrimethyl ammonium bromide to form a purplish-red complex at pH 4.7 to 6.6 by heating at 50 °C for 10 min. The wavelength of maximal absorbance was 582 nm, and the molar absorptive was 111,000 L mol^?1?cm^?1. Interference due to copper(II), iron(III), and aluminum(III) was eliminated by the addition of a masking reagent containing potassium fluoride trans-1,2-diaminocyclohexanetetra-acetic acid and potassium sodium tartrate. This method was more sensitive than the diphenyl-carbazone method.

Smith and Lloyd determined the chromium VI in the soil by a method based on complexation with sodium diethyl-dithiocarbamate in pH 4 buffered medium followed by extraction of the complex with methyl-isobutyl ketone and analysis of the extract by atomic absorption spectrometry. Levels of chromium V between 90 and 176 mg?L^?1 were found in pastureland on which numerous cattle fatalities occurred.

References

• Arbab-Zavar, M. H., Chamsaz, M., & Heidari, T. (2010). ‘Speciation and analysis of arsenic(iii) and arsenic(v) by electrochemical hydride generation spectrophotometric method.’ Analytical Sciences, 26(1): 107-110.

• Zhong, X. L., Zhou, S. L., & Huang, M. L. (2009). ‘Chemical form distribution characteristic of soil heavy metals and its influencing factors.’ Ecology & Environmental Sciences.

• Rui, Y. K., Kong, X. B., & Qin, J. (2007). 'application of icp-ms to detection of heavy metals in soil from different cropping systems.’ Spectroscopy & Spectral Analysis, 27(6): 1201-1203.

• Sun, J., Ma, L., Yang, Z., Lee, H., & Wang, L. (2015). ‘Speciation and determination of bioavailable arsenic species in soil samples by one‐step solvent extraction and high‐performance liquid chromatography with inductively coupled plasma mass spectrometry.’ Journal of Separation Science, 38(6): 943-950.

• Kara, D., Fisher, A., & Hill, S. J. (2009). ‘Determination of trace heavy metals in soil and sediments by atomic spectrometry following preconcentration with schiff bases on amberlite xad-4.’ Journal of Hazardous Materials, 165(1): 1165-1169.

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