Determination of Cd, Cu, Pb and Zn in environmental samples: microwave-assisted total digestion versus aqua regia and nitric acid extraction

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Abstract

We validated the determination of the content of Cd, Cu, Pb and Zn using two digestion protocols: a microwave-assisted total digestion and an aqua regia extraction procedure based on the International Organization for Standardisation (ISO) 11466 method. Our goal in validating these two protocols, along with a nitric acid digestion, is to propose a rapid, cheap and easily automated digestion method for monitoring heavy metal content in environmental samples. We applied the digestion protocols to samples with a wide range of organic matter such as sediments, soils, sludges and plant material.

For samples with a low carbonate or organic matter contents, such as sediments and agricultural soils, aqua regia digestion in an aluminium block was revealed to be an optimum estimator for the total metal content. For samples with a high organic matter content, such as organic horizons of forest floor layers, plant material and organic soils, nitric acid digestion could substitute microwave (MW)-hydrofluoric acid (HF) digestion. Except in a few samples with high organic matter or low heavy metal content, the RSD values obtained after the application of the proposed digestion procedures were lower than 5%. Based on the obtained results, a decision flow chart for choosing the fit-for-purpose digestion procedure is suggested.

Introduction

Trace elements, especially heavy metals, are considered to be one of the main sources of pollution in the environment, since they have a significant effect on its ecological quality. Human activities often mobilise and redistribute natural substances in the environment so much so that they can cause adverse effects [1]. Otherwise, high levels of heavy metals in sediments, sludges and soils, and through transfer processes, also in groundwater and plants, may have a negative effect on animals and human health [2].

As a result, the number of environmental samples submitted to analyses in the frame of routine monitoring or risk and sustainability assessment studies is continuously growing. The first step of a monitoring action is to determine whether the total metal content is within the range of background levels or over the concentration limits according to the national legislation. In the latter case, a further step is to assess the mobility rate of the heavy metals in the soil-plant system for a better evaluation of the environmental impact of a contamination event [3].

The most common methods used nowadays for the determination of heavy metals in environmental samples involve highly sensitive spectroscopic techniques, such as atomic absorption spectroscopy (FAAS, ETAAS) and inductively coupled plasma-optical emission and -mass spectrometry (ICP-AES and ICP-MS). The drawback of these techniques is that they first require the solid sample to be transformed into solution where the metal content is determined [4].

Sample digestion is mainly carried out by a fusion or a wet procedure based on an acid digestion with a heated mixture of mineral acids. There are different heating systems that can be used for digestion such as, sand-bath [5], heating plate [6], [7], pressure digestion bombs [8], [9] and aluminium blocks [10], [11]. The introduction of microwaves, with both open and closed pressurised systems, has allowed a considerable reduction in the total time of analyses as well as in the risk of sample contamination [12], [13], [14].

Whereas the use of closed digestion systems is mandatory for total determination of volatile elements to avoid losses (eg. As, Se, Hg, Cr), the use of opened systems allows a higher sample intake and facilitates the acid evaporation to dryness, reducing in that way following analytical problems associated with high acid concentration [13], [15].

Considering the matrix of most environmental samples (soils, sediments, sludges), a total digestion scheme must include the use of hydrofluoric acid (HF) to completely release the trace elements included in the aluminosilicate phase [9], [15], [16], [17], [18]. However, the use of HF leads to long, dangerous, and cumbersome schemes and its use is not recommended for routine analyses [11].

Thus, in environmental analytical chemistry, acid leaching has become a common procedure as an alternative to total digestion. Since the existence of the ISO 11466 method [19], aqua regia extraction is one of the acid leaching methods more widely used both, refluxing [10], [20], [21] or using microwave-assisted techniques [22], [23], [24]. There are already a few studies aimed at establishing a recovery yield for this extraction with respect to the total metal amounts. It is believed that aqua regia extraction leads to the maximum soluble acid amount of metals [21] with recoveries from 89 to 110% for some metals in soils and sediments [24]. However, in many cases, the aqua regia protocol is not harmonised or it has not been previously optimised, thus leading to an inefficient extraction of the metal [25].

Since the Environmental Protection Agency (EPA) has recommended the microwave-assisted method with nitric acid [26], this leaching procedure of metals has been widely applied in sediments, soils and sludges. Nitric acid is strong enough to solubilize metals from fly ashes [25], from soils with an organic carbon content up to 38% [27] and from plant materials for environmental monitoring [28]. However, the strong dependence of element recovery on the applied leaching procedure makes it mandatory in each case to do a previous comparison with a total digestion approach, including the use of HF.

In this paper, we report the comparison of a microwave-assisted total digestion with an aqua regia extraction for materials with low carbonates or organic matter content. For those materials with a high carbonate or organic matter content which show foaming when adding acid, we also compare the protocols with a nitric acid leaching. The heavy metals determined were some of those considered being of great environmental concern such as, Cd, Cu, Pb and Zn. Our first step was to validate the methods using a high number of certified reference materials (CRMs) that covered a wide range of environmental matrices, including sediments, soils, sludges, and plants. The second step was to apply the methods within a set of samples (sediments, forest horizons, contaminated agricultural soils, peat soils, and plants) with a wide range of mineral and organic matter content.

Section snippets

Reagents and material

All solutions were prepared using doubly de-ionised water (USF PureLaB Plus, 18.2  cm−1) in a class 100 laboratory dotted with a workbench with vertical airflow, in accordance with the Fed-Std-209 E norm.

All the concentrated acids used for the experiments were Instra-Analyzed (J.T Baker) quality. Cd, Cu, Pb and Zn standard solutions were prepared from wires of purity higher than 99.998% from Alfa Johnson Matthey Co. (JMC).

All the material was soaked in HNO3 10% for at least 16 h, and then rinsed

Optimisation of microwave-assisted total digestion

The use of an experimental design for the optimisation of the programme of temperatures and times in the focused microwave oven was rejected because the design conditions could not be experimentally followed. This was due to operational problems such as overheating and burning of the residue in the evaporation-to-dryness step, and also due to the formation of a thick foam.

Thus, some experiments not reported here were undertaken using samples with a different matrix composition. Table 1 shows

Conclusions

In the analyses of the heavy metal content in environmental samples, the digestion of the solid sample is one of the more time-consuming steps. The use of HF ensures a total digestion of the aluminosilicate matrix, and its use with microwave has shortened the time devoted to this step. However, other approaches such as an aqua regia extraction in reflux conditions in an aluminium block, also allow digesting a high number of samples simultaneously and the extractable metal amounts are similar to

Acknowledgements

This work was funded by the AMB99-0430 project of the Ministerio de Ciencia y Tecnologı́a. The authors would like to thank the Serveis Cientı́fico-Técnics (Universitat de Barcelona) for the helps in XRF, ICP-AES, and ICP-MS measurements.

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