Elsevier

Analytica Chimica Acta

Volume 575, Issue 1, 4 August 2006, Pages 114-119
Analytica Chimica Acta

Microwave-assisted UV-digestion procedure for the accurate determination of Pd in natural waters

https://doi.org/10.1016/j.aca.2006.05.062Get rights and content

Abstract

A procedure was developed for the selective determination of Pd in environmental aquatic solutions. The method is based on a preliminary microwave-assisted UV-digestion for the degradation of dissolved organic material, and the subsequent determination of Pd using a recently presented enrichment procedure with ETAAS detection. Due to the simultaneous use of microwave and UV irradiation only extremely small quantities of hydrogen peroxide were necessary to yield a quantitative degradation of interfering organic ligands. Thus the on-line pre-concentration of Pd in the digested samples using the complexing agent N,N-diethyl-N′-benzoylthiourea was possible without any further sample pre-treatment. Using a sample volume of 1.57 ml for FI-ETAAS analysis a limit of detection of 15 ng l−1 was obtained for the combined procedure, with a relative standard deviation being not more than 4.9%. The method was applied to quantify the water-soluble fraction of Pd in urban road dust. Extractions with a rain sample of pH 5.9 revealed that an average of 2.2% (n = 8) of the total Pd present in urban road dust was water soluble.

Introduction

Platinum group elements (PGE) can be naturally found only at very low concentration in the earth crust, a situation which is changing with the introduction of PGE based catalytic converters in the automobile industry. Platinum (Pt), rhodium (Rh), and palladium (Pd) are used in motor vehicle exhaust systems to catalyse the conversion of hydrocarbons, carbon monoxide, and nitrogen oxides into the less harmful substances carbon dioxide, nitrogen, and water. While improvement in air quality is an important benefit of catalytic converters, the use of these devices has led to widespread release of PGE in the environment [1], [2]. Recently it has been shown that the emitted metals undergo environmental transformations into soluble and thus more reactive species [3], [4], [5]. From a toxicological point of view, these soluble PGE species are more bioavailable and have the potential to directly affect organisms and plants, and therefore they present a greater environmental risk. To date, very little research has been carried out in this field. For Pt, the element that has received the most attention among the PGE studies, the water soluble fraction was determined to be in the order of 0.01–0.5% of the total platinum contents of the investigated materials [6]. Since for Pd an enhanced degree of accumulation in the aquatic biosphere has been reported [4], [7], it is important to study how this noble metal may become mobile and chemically or biochemically active.

The accurate determination of Pd in various parts of the water ecosystem, i.e. ice, snow, rain-, river- or seawater is a very challenging task. Extremely low concentration levels (in the lower ng g−1 range) combined with numerous interferences caused by the matrix are considered to be the major difficulties. For overcoming this analytical challenge several methods for sample preparation, matrix separation and/or pre-concentration have been developed, with nickel sulphide fire assay, co-precipitation and ion-chromatographic separation techniques being most frequently used [8], [9]. As demonstrated by a growing number of published studies an enrichment of Pd with simultaneous matrix separation can be achieved with fully automated flow injection (FI) systems [10], [11], [12], [13]. Sorption of metal complexes of low polarity from the aqueous phase by a reversed-phase mechanism using non-polar sorbents such as C18-bonded silica are considered to be the most efficient on-line sample pre-treatment approaches [14], [15]. The applicability of such FI-procedures for the analysis of Pd in completely mineralised airborne particulate matter and road dust samples has been demonstrated recently [16], [17].

Since natural aquatic samples contain numerous dissolved organic substances (mainly alcohols, aldehydes, carboxylic acids and macromolecular compounds with several functional groups like humic and fulvic acids), which could affect complexation of the analyte or displace the retained metal-complex from the stationary phase, it is necessary to decompose these competitive ligands prior to the application of such FI-techniques. For the destruction of dissolved organic material (DOM) in aqueous solutions wet digestion with chemical oxidants such as nitric acid, sulphuric acid, perchloric acid or hydrogen peroxide, sometimes in the presence of ultra-violet (UV) radiation has been used [18], [19]. A serious drawback of this approach is the addition of large concentrations of oxidants to the sample, which must be decreased after digestion (via evaporation or sample dilution) to met the conditions required for analysis.

The present work describes a microwave-assisted UV-digestion procedure, which avoids the drawbacks of common sample pre-treatment techniques, and allows in combination with a recently presented flow injection procedure with ETAAS detection an accurate determination of Pd in aqueous samples. The method has been applied for the determination of the water-soluble Pd content in urban road dust.

Section snippets

Instrumentation

Digestion of liquid and solid samples was performed with a Multiwave 3000 digestion unit (Anton Paar, Austria) equipped with an IR-sensor for monitoring the temperature of the individual reaction vessels. For the decomposition of DOM in aqueous sample solutions an 8XQ80 Rotor fitted with four 80 ml quartz vessels has been used. Each reaction vessel included an electrodeless Cd discharge lamp, which has been directly immersed in the investigated sample solution. The generated UV radiation has

Development of the digestion procedure

In a first set of experiments the necessity of DOM removal prior to the Pd determination in natural waters with the FI-ETAAS procedure was investigated. For this purpose aliquots of a natural rain sample with a DOC content of 11.49 mg L−1 have been spiked with Pd at concentration levels ranging from 200 to 1000 ng l−1 and analysed using the described FI-ETAAS procedure. The results derived for the individual samples underestimated the spiked Pd contents significantly. This observation could be

Conclusions

The applicability of a recently presented FI-procedure with ETAAS detection for the selective determination of Pd in environmental samples was found to be limited to completely mineralised samples, the presence of organic matrix interfered the FI-ETAAS analysis of natural aquatic samples. For the degradation of DOM in aqueous samples a microwave-assisted UV-digestion procedure was developed. Due to the simultaneous use of microwave and UV irradiation only extremely low quantities of chemical

Acknowledgements

The author would like to thank M. Handler for his assistance with sample collection and sample pre-treatment. Anton Paar (Graz, Austria) is gratefully acknowledged for continuous support and providing the equipment required for multiwave-assisted UV-digestions.

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