Rare earth elements (REEs) make possible the high-tech world we live in today—from the miniaturization of electronics, to the enabling of green energy and medical technologies, to supporting a myriad of essential telecommunication and defense systems. These elements have become irreplaceable in the world of technology, owing to their unique magnetic, phosphorescent, and catalytic properties [1
]. They have also gained enormous attention due to their spectroscopic characteristics for advanced new materials. The main environmental risk posed by these elements are tailings, which are a mixture of small-sized particles, wastewater, and floatation chemicals used in the processing stages [2
]. Most rare earth elements also consist of radioactive materials, which impose the risk of radioactive dust and water emissions [3
]. The abundance of electrical gadgets around us, and agricultural and industrial use of rare earth elements also pose environmental hazards if not properly monitored [4
]. Innovations in crop quality after the 1970s were a result of the use of rare earth micro fertilizers. This led to largescale application of the fertilizers for crops such as wheat, rice, maize and mungbean [5
], resulting in a large amount of REEs entering the environment. Thus, more attention is now being paid to understanding their environmental and ecological effects. Determining the REE concentrations in different soil profiles and different plant parts is of great value to environmental ecology [6
Excessive amounts of REEs in the environment can have devastating effects on humans, aquatic life, vegetation, and micro fauna. The increased use of REEs in agriculture in some countries has led to their scattering and bioaccumulation in the environment [8
]. Soil fauna, which is an important component of the terrestrial ecosystem, plays an important role. While precautionary measures are being put in place to reduce the threats to human health and the environment from radionuclides, little attention is being paid to REEs as another source of radionuclides. The toxicology of REEs to humans, plants, aquatic and other terrestrial organisms is also not well understood [7
]. Mining and exploration of REEs are a major source of their scattering in the environment. Studies carried out in China and Spain show that areas near mining activities are heavily polluted with REEs [6
]. The geology and chemistry of rare earth elements make their processing after mining a huge task. Their separation and purifying requires several industrial processes and the use of dangerous chemicals such as sulphuric acid and hydrofluoric acid [15
]. Huge amounts of wastewater and industrial waste carry radioactive elements, including REEs. In a study, Jinxial et al. [16
] revealed that the production of REEs in the Baotau region in China has caused the surface and ground water to be affected by radioactive substances and light rare earth elements (LREEs).
The distribution of REEs in fresh water and sediments differ from place to place, depending on contaminant sources. In normal circumstances, unpolluted fresh water systems should contain minimum traces of REEs. Wood and Shannon [17
] carried out a study analyzing REEs to picogram levels in natural waters. They established that REEs can be found in very low concentrations or may need pre-concentration steps to be determined. Several researches on REE determination in sediments and natural waters have found that each source of environmental matrice is different [14
]. The sources of contamination—industrial, mining or agriculture—have different impacts.
Electrochemical techniques, which use sensors to combine selectivity, sensitivity, simplicity and rapidity, offer quick monitoring. However, the chemical complexity of REEs make it difficult for researchers to analyze some RREs using these techniques. The modification of carbon electrodes with metals such as antimony (Sb) [20
], bismuth (Bi) [22
] and organic materials [24
] have helped to move away from mercury, which is poisonous and an environmental hazard. Technologies used in modification of sensors, conducting polymers, and nanotechnology can be explored to bridge the gap between the problem and the solution.
On the other hand, spectroscopic techniques, such as inductive coupled plasma mass spectrometry (ICP-MS), inductive coupled plasma-optical emission spectrometry (ICP-OES), instrumental neutron activation analysis (INAA), and x-ray fluorescence (XRF) have been applied for REE determination [25
]. Although they offer good detection capabilities, analysis and capital costs are high. Thus, in order to ensure better research, these techniques should complement each other. According to a review by Zawisa et al. [25
], the concentrations of REEs in natural waters normally range from ppb to ppt levels. Two general approaches to analysis of REE in water are employed: separation or enrichment prior to quantification, and direct sample analysis without removing matrix (with or without dilution) [25
]. The latter approach was used in this study to evaluate both voltammetry and spectroscopic techniques.
The aim of the present study is to quantify trace amounts of cerium, lanthanum and praseodymium, using a glassy carbon antimony film electrode (GC/SbFE) and ICP-OES analysis of fresh and surface water samples, obtained from an area rich with REE deposits in Northern Cape Province, South Africa. In order to compare our results with the established spectroscopic method, we used Spectro Arcos, a high resolution ICP-OES spectrometer for quantification of the rare earth elements.
Electroanalytical techniques, such as differential pulse adsorptive striping voltammetry, offer relative simplicity, low-equipment cost, sensitivity, and low-detection limits in environmental matrices. This has been explored in this study, which aimed at monitoring traces of rare earth elements. The chemistry of the rare earth elements and their group’s natural occurrence posed a challenge to selectivity. Although the selectivity of the method has shown limited but satisfactory results, huge progress has been made to determine La(III) and Pr(III) on carbon electrodes, a step that was unsuccessful in previous studies. The ICP-OES spectrometry analysis was able to separate and analyze all the three REEs simultaneously, compared to stripping voltammetric determination by a GCE/SbF sensor. Further optimization or employing of pre-concentration and separation techniques of elements before the stripping analysis may be employed to enhance sensitivity and selectivity in the future. The results obtained by the GCE/SbF sensor platform were in agreement with the results achieved by the established ICP-OES method. The results obtained by both techniques were good, despite the limitations encountered for Pr(III) analysis at low concentrations. The LOD obtained for the stripping analysis was 0.06, 0.42 and 0.71 µg/L for Ce(III), La(III) and Pr(III), respectively. For ICP-OES, the detection limit obtained was 2.45, 3.12 and 3.90 µg/L for Ce(III), La(III) and Pr(III), respectively. The method was found to be accurate and fast and can be an alternative low-cost technique to determine REEs in environmental samples. It can also be used to complement spectroscopy analysis.