A simple and sustainable portable triaxial energy dispersive X-ray fluorescence method for in situ multielemental analysis of mining water samples

https://doi.org/10.1016/j.sab.2019.105762Get rights and content

Highlights

  • Simple, sustainable and reliable method for analysis of Acid Mine Drainage waters

  • Fast and cost-effective screening of mining water composition in the field

  • Analysis of water loaded filters using portable X Ray Fluorescence.

  • Accuracy and precision suitable and comparable with ICP and TXRF

Abstract

In-situ monitoring of potentially toxic elements in mining water samples is of special relevance in order to evaluate the environmental impact of abandoned mining activities. However, it is difficult to develop suitable analytical methods for this purpose considering the complexity of mining water samples matrix as well as the limited sensibility of conventional filed portable X-ray fluorescence spectrometry systems (pXRF).

In the present contribution, a simple, sustainable and reliable triaxial pXRF method (3pXRF) is proposed for multielemental analysis of mining water samples. In this method, several microliters of sample are deposited onto a commercial filter paper retainer and after drying, the loaded filter is directly analyzed in the field using a laboratory pXRF with orthogonal triaxial geometry (p3XRF). This geometry reduces the background of the measured spectra improving peak-background ratios in comparison with conventional pXRF systems.

Using the best analytical conditions (sample deposition volume of 200 μL and 600 s as measurement time), limits of detections were in the low mg∙L1 range and therefore suitable considering the expected high metal concentration in mining water samples. Acceptable accuracy and precision of the results was obtained in comparison with laboratory techniques such as inductively coupled plasma optical emission spectrometry and total reflection X-ray fluorescence spectrometry but with the possibility of in-situ measurements in the field. The developed method was applied to the analysis of different types of mine water samples (stream waters, drainage water tanks and mining lake waters) collected in the mining districts of Tinoca (Portugal) and Cartagena-La Unión (Spain).

Introduction

Metal pollution of the environment as a result of abandoned mining activities is an acute problem nowadays. Although mineral resource extraction has been carried out for centuries, until the last several decades relatively little attention has been given to minimize the metal dispersal around these areas coming from the indiscriminately dumped mining wastes. As a consequence, one of the challenges facing society today is the identification, evaluation and remediation of these old disused areas to protect public health and environment quality [1].

One of the consequences of heavy metal pollution by mining activities is caused when metals contained in excavated rock or exposed in an underground mine are leached and carried downstream as water washes over the rock surface. Tailing piles or ponds may also be a source of acid mine drainage. In both cases, the result is water containing high amounts of potentially toxic elements and dissolved salts [2,3]. Therefore, multielemental analysis of this type of complex liquid samples is of relevance to evaluate the environmental impact of abandoned mining activities.

Usually, the presence of metal and metalloids in water samples is determined by multielemental atomic spectrometry techniques including inductively coupled plasma atomic emission spectrometry (ICP-AES), inductively coupled plasma mass spectrometry (ICP-MS) or total reflection X-ray fluorescence spectrometry (TXRF). However, the direct analysis of complex liquid samples such as acid mine drainage waters is not possible using these analytical techniques and dilution and/or preconcentration may be necessary before the analysis [[4], [5], [6]]. Considering the high salt content of most mining water samples, an additional handicap of laboratory analytical techniques is the precipitation of salts and metals during transportation and storage in the laboratory that can lead to problems for a reliable metal determination. For this reason, the development of in-situ analytical methods is of paramount importance.

Recent technological improvements have led to the widespread adoption of field portable energy dispersive X-ray fluorescence (pXRF) by governmental agencies, environmental consultancies and research institutions for multielemental analysis of environmental samples [7,8]. However, XRF techniques are usually employed for the analysis of environmental solid samples (i.e. soils and sediments) and its application for direct analysis of liquid samples is still limited due to the high X-ray scatter background, the poor signal-to-noise ratio and the high uncertainty of the obtained results. To cope with this fact, several preconcentration strategies can be used to improve analytical performance in the analysis of water samples by XRF [9]. However, most of this methods are laborious and time consuming and, therefore, difficult to be used for in-situ measurements.

In view of these premises, the main aim of the present contribution was the development of a simple, sustainable and reliable analytical pXRF method for in-situ analysis of acid mine drainage water samples. The method is based on a preconcentration step using a commercial paper filter retainer where a small amount of liquid sample is deposited on. In a recent review article, we highlighted the benefits of organic thin layers for preconcentration purposes when dealing with the analysis of aqueous samples by conventional XRF [9]. The main reasons for that are the reduction of matrix effects and the low background of the exciting radiation scatter when analyzing this type of materials. This approach has been successfully used for other type of liquid samples analysis (i.e. blood samples, cultural heritage and TCLP extracts) [[10], [11], [12]] but, according to the authors' knowledge, there is not any contribution published dealing with in-situ multielemental analysis of mining water samples.

In the method proposed, after preconcentration, loaded filters are directly measured in the field using a pXRF system with a triaxial assembly between the X-ray tube, a secondary target, the sample and the detector in order to suppress, due to polarization, the Bremsstrahlung radiation from the X-ray tube - 3pXRF13. This way, increased peak-to-background ratio and sensitivity are achieved maintaining portability and possibility of in situ analysis.

In a first stage, several experimental tests including the sample deposition volume, the homogeneity of the sample on the solid retainer as well as the measurement time, were carried out to obtain a good representative response for the elements of interest. Afterwards, analytical capabilities of the 3pXRF including limits of detection, precision and accuracy for the results were evaluated and compared with those obtained using TXRF and ICP-AES analysis. Once demonstrated the potential of 3pXRF method, it was applied to determine metal content in different types of mining water samples collected in the mining districts of Tinoca (Portugal) and Cartagena-La Unión (Spain).

Section snippets

Reagents and materials

The certified reference solution Transition Mix 1 TraceCert (Sigma-Aldrich Co. LLC) was employed to calibrate the 3pXRF system. It consists of a multielemental solution containing a set of transition metals (V, Cr, Mn, Fe, Co, Ni, Cu and Zn) at the level of 100 mg L1. A monoelemental stock solution of Mo of 1000 mg L1 was employed as internal standard solution for TXRF quantification purposes. Commercial filter paper sample retainers were purchased from Rigaku Co (Japan) and used to

Evaluation of analytical conditions for multielemental 3pXRF analysis of aqueous samples

As aforementioned, the proposed method consists of a simple preconcentration step of the aqueous sample on a solid retainer followed by the direct analysis of the preconcentrated sample using a 3pXRF system. In a first stage, several experimental tests including the sample deposition volume, the homogeneity of the sample on the solid retainer as well as the measurement time, were carried out to obtain a good representative response for the elements of interest.

Conclusions

A simple and sustainable method combining the deposition of few microliters of sample in a suitable retainer with portable XRF spectrometry has been developed for in-situ analysis of acid mine drainage water samples. Acceptable accuracy and precision of the results was obtained in comparison with laboratory techniques such as inductively coupled plasma optical emission spectrometry (ICP-AES) and total reflection X-ray fluorescence spectrometry (TXRF). The method can be used for a fast and

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

S. Pessanha and M. L. Carvalho would like to acknowledge the financial support of Portuguese Foundation for Science and Technology in the funding of LIBPhys (project — UID/FIS/04559/2013). This work was also carried out in the framework of the project CGL2016-78783-C2-2-R funded by the Spanish R + D national Programme. I. Queralt also acknowledges a grant for a Sabbatical stay at Universidade Nova de Lisboa (ref. PRX16-00159) from the Spanish Ministry of Education. The authors thank M. Cabañas

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