Monitoring of inorganic environmental pollutants is an ongoing challenge for analytical chemists.
To establish the quality of different compartments of our environment (atmosphere, indoor air, water basins, soil, biota) in general a relatively large number of samples must be taken from a given location over the entire duration of sampling.29,30,31
XRF is a rapid, non-destructive multi-elemental analysis technique with sensitivity in the range of 10-8 and it is perfect for environmental research.31,32 Heavy metals from industry can contaminate water courses and when older industries close the derelict site may need to be decontaminated before the land can switch use into housing or retail development. Heavy metal contamination of soil may persist for many years and even then, there is the danger of toxic metals leaching into groundwater resources.
Soil is a specific compartment of the biosphere where it is a geochemical sink for contaminants and acts as a natural buffer in the transport of elements to the atmosphere, hydrosphere and biota.29,30,31 Equally significant is the use of soil in the production of food. The maintenance of the ecological and agricultural functions of soil depends upon the balance of trace elements. Metals persist in soil much longer than other compartments of the biosphere. Contamination by some metals such as Cr and Pb are effectively permanent.
Excessive concentrations of trace elements may have a detrimental effect of the ecology of the soil or water and can also impact crop development and animals (who may ingest soil during grazing).29 The number of contaminated sites in the European Union and the area affected by different kinds of pollution (for which remediation would cost €17.3 billion annually) underlines the extent of the problem in Europe. An estimated 6.24% or 137,000 km2 of agricultural land in the European Union needs local assessment and eventual remediation action.30 Apart from soil contamination which may lead to the degradation of water quality and a series of negative impacts on the environment the propagation of heavy metals throughout the food chain could have serious consequences for human health.30
Soil testing for heavy metals
The use of a field portable XRF (FPXRF) (x-ray fluorescence) instrument provides a rapid method to test for a range of heavy elements in a set area. Soil Analysis via XRF is a widely used and recognized method for site investigation, assessment, and monitoring.
Handheld XRF elemental soil analysis is a fast and reliable method to test the elemental composition of soil or sludge for all 8 Resource Conservation and Recovery Act (RCRA) metals.
(Ag, As, Ba, Cd, Cr, Hg, Pb, Se), 12 out of the 13 Priority pollutants (Ag, As, Cd, Cr, Cu, Hg, Ni, Pb, Sb, Se, Tl, Zn, Be), and also some important lighter elements (Mg, Al, Si, P, S). XRF analyzers are used for environmental soil analysis as they allow fast mapping of contamination boundaries (in combination with GPS as each reading is acquired), they are complaint with US EPA Method 6200 for defining contamination boundaries in the field and the method is non- destructive allowing samples to go to laboratory based XRF for confirmatory testing when necessary using micro-spot analysis or elemental mapping and quantitative measurements against established reference standards.31,32,33
Of course, XRF isn't the only way to test soil. Watch our video on analyzing heated soil using the Heated Golden Gate ATR spectrometer accessory.
Determining the purity of water | XRF
Water is one of the most important constituents of the human environment. The preservation of fresh water resources has become one of the highest environmental priorities in the world.
The composition of river water, ground water, lake and sea water may be reflected in the biosphere and affect the composition of our food and rain water is an excellent indicator for air pollution. Quality control is particularly important in the case of drinking water, where highly selective analytical techniques are needed to measure low concentrations of essential and potentially toxic elements such as mercury, lead and cadmium.
The objective in the field of water quality engineering is the determination of the environmental controls that must be instituted to achieve a specific environmental quality objective.34 The contamination arises principally from the discharge of the residues of human and natural activities that result in a poor use of water. A wide range of human activities contribute to the trace element pollution of the aquatic environment leaving residues or mercury and even radioactive elements such as uranium.
The major polluting activities include mining and ore processing, coal and fuel combustion, industrial processing (chemical, metal alloys, petroleum), agricultural (fertilizers, pesticides and herbicides), domestic and agricultural effluents or sewage, transportation (urban and motorway run-off), and nuclear activities. Water is used for a variety of different uses from drinking water and for recreation (swimming and watersports) to the water used in industry and power production. Depending on how water is used there are different standards of water purity and required.
Several factors are important for selection of analytical methods used for water quality monitoring: rapidness of analysis, reliability, good precision and accuracy (better than 10%), low detection limits for trace elements determination and low cost. The methods for this at present are atomic absorption spectrometry (AAS) and inductively coupled plasma atomic emission spectrometry (ICP-AES).35,30
XRF is, by its nature, is preferentially used for solid samples and is not very suitable for the assessment of dissolved components in aqueous samples. However, XRF is a superior technique to analyze the suspended fraction in aqueous samples.36
Radioactive environment analysis | XRF
XRF can analyze 15–30 elements with atomic numbers ranging from Z = 11 to Z = 41 and also some rare earth elements (REEs).33 X-ray fluorescence (XRF) is based upon the measurement of characteristic X-ray emissions resulting from de-excitation of inner electron shell vacancy produced in a sample by using a suitable source of radiation (high energy x-ray or gamma radiation).
Energy-dispersive XRF analysis (EDXRF) employs detectors that directly measure the energy of the X-rays by collecting the ionization products using a suitable detector. The earth’s crust contains small quantities of uranium, thorium, potassium and also other trace and major elements such as Cs, Cd, Pb, Fe, Mg and Mn.
The concentrations of these elements depend on the local geology of the environment as well as various natural and anthropogenic processes. The average concentration of uranium in the earths crust has been reported to be in the range of 2–3 ppm, while thorium exists in the range of 8–12 ppm.
A study of clay soils in Ekiti state in Nigeria33 in 2013 revealed the utility of XRF in the detection and quantification of heavy elements. The results of this study showed that the mean concentrations of uranium ranged from 2.2 ± 1.0 mg/kg to 3.2 ± 1.1 mg/kg, that of thorium ranged from 4.0 ± 0.5 mg/kg to 5.7 ± 1.7 mg/kg in all of the clay samples examined indicating an area of potential for extracting useful radioactive isotopes.
Sample preparation for environmental XRF experiments
Environmental samples generally require widespread sampling to map an area of interest and XRF is very good for identification and estimated quantification of trace and heavy elements. Generally, soil, clay and earth samples previously mapped by handheld XRF would be easy to prepare in the laboratory.
By freeze drying and then using the dry powder sample obtained (with or without binder) in one of the Specac hydraulic presses such as the Autotouch automatic hydraulic press, useable pellets can be produced and suitable for accurate XRF analyses. For smaller samples, such as those prepared in the field, a Specac Manual Hydraulic Press (pictured above) can be used instead. Specac also provide a range of XRF dies and cups to ensure that samples are homogenous and uniformly compacted.
Alternatively, a thin film slurry technique can be used for fine powders along with a suitable carrier liquid. Powder samples can also be examined by XRF total reflectance methods if required. Analysis of liquid samples by XRF is a little more difficult and is usually performed in a cup with a thin mylar bottom foil.36 The use of liquids in XRF is not easy and can be fraught with problems (low LoD, difficult sample handling and poor sensitivity).