Rapid assessment of soil pollution using a field portable x-ray fluorescence spectrometer.
Oyourou, Jean-Nazaire
Oyourou, Jean-Nazaire
Citations
Altmetric:
Abstract
The large number of abandoned and unattended mines in South Africa, combined with a long history of application of copper-based fungicides in horticulture, has raised concerns regarding metal pollution of the environment. The need for monitoring environmental pollution through the implementation of regular soil surveys and risk assessments necessitates the use of a rapid analytical technique, such as portable-X-ray fluorescence (pXRF) spectroscopy, in combination with conventional inductively coupled plasma-optical emission spectroscopy (ICP-OES). In this study, pXRF spectroscopy was applied to screen soils from an abandoned mine contaminated with lead and zinc, and from mango orchards thought to be polluted with copper. Prior to field investigations, the reliability of the pXRF spectrometer for the in situ determination of the metals in soils was verified. After calibration of the spectrometer using appropriate certified reference materials, soil samples were analysed to determine their metal content, and the results were compared to those obtained from the analysis of the same samples using ICP-OES and wavelength dispersive X-ray fluorescence (WDXRF) spectroscopy. The pXRF results correlated well with those from ICP-OES, as reflected by the coefficients of determination (r2) of 0.8250; 0.9743 and 0.9899 for copper, zinc and lead, respectively. Quantitative data could be classified as quality level Q2 according to the USEPA for copper, zinc and lead, with reference to values obtained for the r2, relative standard deviations (RSDs) and by applying inferential statistics. The metal concentrations determined using the pXRF spectrometer were also confirmed by an independent analysis of the samples using WD- XRF spectrometry. Once the reliability of the pXRF spectrometer for field determination of lead, zinc and copper had been established, large scale screening was conducted to determine the spatial and depth distribution of metals in soils at the abandoned Edendale Mine (Mamelodi, Pretoria) and in mango and citrus orchards near Hoedspruit (Limpopo). Contamination indices were applied to establish the levels of pollution of the soils. As part of risk assessment, the mobility of the contaminants in soils was determined from the sequential extraction of representative soil samples. Risk assessment indices were applied to determine the ecological risk associated with the contamination. The screening of soils at the old Edendale Mine using the pXRF revealed a maximum concentration of 7300 mg/kg for zinc and 21 000 mg/kg for lead. Of the 167 points sampled, 46% of the samples for zinc and 77% for lead contained more of these metals than the baseline concentrations (185 mg/kg for Zinc and 56 mg/kg for lead) set for South African agricultural soils (Department of National Health and Population Development). The geo-accumulation index indicated that 2% and 15% of the sampling points at the Edendale Mine were extremely polluted with zinc and lead, respectively. The high concentrations of lead present in the exchangeable, reducible, and oxidisable fractions of the soils following sequential extraction, indicated that the metal occurs in mobile forms. It was deduced that zinc was present in moderately mobile forms, since 61% of the total concentration was determined in the residual fraction. The risk assessment indices consistently indicated that lead in the soils poses a high ecological risk, but that zinc is associated only with a moderate risk. A maximum copper concentration of 2600 mg/kg was measured in mango orchard soils using the pXRF spectrometer. Copper generally accumulated in the surface soil, while the concentration decreased with an increase of depth. This trend was ascribed to a high content of organic matter in the surface soil that confined the copper to the surface through binding mechanisms. Most of the soil samples analysed (64%) could be described as significantly to extremely contaminated with copper, resulting from the fungicides applied to the trees through spraying. Sequential extraction of selected orchard soil samples indicated that copper resided mainly in the oxidisable (32%) and reducible (28%) fractions. Only 14% of total copper was confined to the exchangeable fraction, reflecting limited mobility and therefore a moderate environmental risk for copper. A phytoremediation trial was implemented in a contaminated mango orchard using two aromatic plant species i.e. Rosmarinus officinalis and Ocimum labiatum. After 15 months, plant samples were harvested. High copper concentrations were determined in the leaves of R. officinalis (73.6 to 236 mg/kg) and O. labiatum (333 to 675 mg/kg). Although the bioaccumulation and translocation factors for plant leaves were above 1, the plants did not qualify as phytoaccumulators. If these two aromatic species were to be interspersed between the mango trees, they could play an important role in mitigating the copper pollution. Such an intercropping practice could possibly benefit the orchard by attracting pollinators, yielding a commercial crop in the form of an essential oil, and perhaps even contributing to a decline in insect parasites, due to the repellent effect of emitted volatile organic compounds. In conclusion, portable spectroscopy was successfully applied in situ, in combination with ICP-OES spectroscopy, to establish the levels of lead and copper pollution of soils that were found to be concomitant with extreme and moderate ecological risk at the abandoned Edendale Mine (Mamelodi), and mango orchards (Hoedspruit), respectively. The high levels of soil pollution are extremely concerning and remediating interventions should be initiated by local authority and stakeholders.
Description
Submitted in partial fulfilment of the requirements for the degree, Doctorate Technologiae: Chemistry in the Department of Chemistry, Faculty of Science at the Tshwane University of Technology.
Date
2016-11-01
Journal Title
Journal ISSN
Volume Title
Publisher
Tshwane University of Technology
Collections
Research Projects
Organizational Units
Journal Issue
Keywords
Copper, Horticulture, Pollution, Spectroscopy, South Africa