Heavy metals are naturally occurring elements that accumulate in the soil due to anthropogenic activities such as mining, smelting, corrosion, and agricultural activities. These elements can be toxic to humans and plants if ingested in high quantities. Due to their high degree of toxicity, lead, arsenic, cadmium, chromium, and mercury are among the detrimental metals even at a minor level of exposure. Soil testing, as part of the environmental characterization process, for heavy metal contaminants is a significant procedure in determining their concentration prior to soil remediation .
The methods for soil testing for lead, arsenic and other heavy metals may be classified into two: analytical techniques for solid samples and atomic spectrometry for extracted elements. Below are some of these existing methods.
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Analytical techniques for solid samples in environmental soil characterization
These are analytical techniques that directly apply to solid samples .
Neutron activation analysis (NAA)
The Neutron activation analysis technique employs irradiation of a given sample with a neutron beam. The energies and intensities of the radioisotopes’ gamma-ray emissions are calculated using a sensitive semiconductor detector. Results are compared with standards irradiated simultaneously with the unknown substance to specify the elements and their quantities .
Laser-induced breakdown spectrometry (LIBS)
Laser-induced breakdown spectrometry is a rapid chemical analysis method that utilizes a short laser pulse to create a micro-plasma on the sample surface, thus allowing its chemical identification. It has adaptable sampling protocols that include fast raster of the sample surface and depth profiling, and thin-sample analysis without the concern of the substrate interference .
X-ray fluorescence spectrometry (XRF)
X-ray fluorescence spectrometry bombards an atom and excites electrons orbiting it using high energy photons. Some of these photons expel an electron that is bound to the atom’s nucleus. Consequently, an electron from a higher energy orbital will be transferred to the lower energy orbital. During this transition, the atom emits photons which becomes the characteristic of that particular element. Since the energy difference between two specific orbitals is always the same, the photon’s energy is constant. By measuring the energy of the photons, the element that is emitting the photons and its concentration in the soil is known .
Atomic spectrometry methods that require digestion processes like total or pseudo total digestion and single or sequential extraction to break down the complex matrix of a sample and extract the heavy metals .
Atomic absorption spectrometry (AAS)
Atomic absorption spectrometry uses a wavelength that is unique to the sample element. It is based on the absorption of discrete wavelengths of light by ground-state, gas-phase atoms. Atoms of the elements absorb characteristic wavelengths from a hollow cathode lamp with a specific frequency that is proportional to the quantity of the element in the sample. Using a standard curve, the concentration is determined .
Inductively coupled plasma spectrometry (ICP)
Inductively Coupled Plasma spectrometry is a method used to detect and measure elements based on its ionization by an extremely hot plasma, usually created from argon gas. ICP spectrometry is classified into two.
Inductively coupled plasma mass spectrometry (ICP-MS)
In inductively coupled plasma mass spectrometry, ions are taken out through several cones and passed into a mass spectrometer. They are separated according to their mass-to-charge ratio, and a detector acquires ion signals relative to the concentration of different elements. The concentration can be determined by ICP spectroscopy units based on verified reference standards.
Inductively coupled plasma atomic emission spectrometry (ICP-AES)
Inductively coupled plasma atomic emission spectrometry is based on the exciting ions releasing electromagnetic radiation at wavelengths distinct to a specific element. ICP-AES uses shear gas to eliminate plasma at a particular location. The light radiated by ions is directed by one or two lenses on a diffraction grating breaking it into its component wavelengths before examining by an optical spectrometer .