Chemical fractionation of arsenic in contaminated soils using a nonspecific sequential extraction method and chemometric data analysis Patrick M. Amaibi*1, Mark Cave2, Jane Entwistle3 and John R. Dean1 1 Department
of Applied Sciences, Northumbria University, Newcastle upon Tyne, NE1 8ST. UK. 2 British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK 3 Department of Geography, Northumbria University, Newcastle upon Tyne, NE1 8ST, UK.
1. Introduction Potentially harmful elements (PHEs) in soil are from various sources (Fig. 1) both natural (e.g. weathering & volcanic eruption) and anthropogenic (e.g. industrial waste, mining & smelting) origins which can pose severe threat to the environment and human health.
3. Method This work is based on the use of a non-specific extracting solution (aqua regia) prepared in an increasing concentration range from 0.01 – 5.0 M (Fig. 3). At each extraction step, the soil sample was extracted twice and the supernatant transferred into a clean sample tube. For each test sample, 14 acid extracts were produced (with the pH of the extractant progressively decreasing from extract 1 to 14), for which 23 major, minor and trace elements were analysed by inductively coupled plasma optical emission spectrometer (ICP-OES) followed by chemometric data analyses . Fig. 3 . Extraction of soil using CISED method
3. Results & Discussion
4. Conclusion
The CISED identified 16 physicochemical components from the soil extracts with elevated levels of As compared to other trace elements present(Fig. 4).
Arsenic is found in association with selected mineral oxides and hydrous metal oxides, either as part of the mineral structure or as sorbed species. Our study suggests that the As which is bound to the Fe and Ca may be sorbed species from an anthropogenic source. Further research on the mineralogy (by x-ray diffraction) and the particle phases that make up the sample (via computer controlled scanning electron microscopy coupled to energy dispersive spectrometry), are currently underway.
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Fig.4. Distribution of trace elements in different components
Concentration (mg/kg)
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Fig. 1 Arsenic cycle
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Arsenic (As), a well known carcinogen, has two common oxidations states (III and V) which are of different toxicity and mobility (Fig. 2). Research has shown that pH and redox potential changes control the release of PHEs in different chemical forms [1].
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The concentration of As in the extract increases alongside the extraction number (1-14). At higher acidic and redox potential conditions, more As is released from the host mineral assemblage (Fig. 5). 45
Fig.5. As extractions in the 3 major soil components
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As mg/kg extracted
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Fe-Si-Al 15
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Fig. 2 Eh-pH diagram for As (Smedley & Kinniurgh, 2002)
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A wide variety of chemicals, including sequential chemical extractions, have been used in an attempt to extract PHEs from particular soil phases. The non-specificity of most of these extractants is also widely reported in the literature, along with the difficulty of identifying a suitable method for a range of soils types and elements [3]. The aim of this research is to use a nonspecific sequential extraction method, the Chemometric Identification of Substrates and Element Distributions (CISED; Cave et al. 2004 [4]) to understand the chemical fractionation of As in contaminated soils.
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There are 8 As – bearing phases (Fig 6). In the Fe-As-Ca phase, As is high and bound to Fe and Ca . 3000
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Fig. 6. Solid phase distribution of As
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1. Molinari, A., Ayora, C., Marcaccio, M., Guadagnini, L., Sanchez-Vila, X., Guadagnini, A. (2014). Geochemical modeling of arsenic release from a deep natural solid matrix under alternated redox conditions. Environmental Science and Pollution Research, 21(3), 1628-1637. 2. Smedley, P. L., & Kinniburgh, D. G. (2002). A review of the source, behaviour and distribution of arsenic in natural waters. Applied Geochemistry, 17(5), 517-568. 3. Bacon, J.R. and Davidson, C.M., 2008. Is there a future for sequential chemical extraction? Analyst, 133, 25–46. 4. Cave, M. R., Milodowski, A. E., & Friel, E. N. (2004). Evaluation of a method for identification of host physico-chemical phases for trace metals and measurement of their solid-phase partitioning in soil samples by nitric acid extraction and chemometric mixture resolution. Geochemistry: Exploration, Environment, Analysis, 4(1), 7186
6. Acknowledgments
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mg/kg extracted
2. Research Aim
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5. References
This research forms part of a PhD research studentship to PA from NDDC, Nigeria.
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Corresponding author: Patrick Amaibi (
[email protected])
