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1 Introduction

The elevated content of Pt and Pd in the urban and roadside environment, which significantly exceeds the geochemical background (1 ng g−1), is mainly connected with emission of these metals from industrial and transport sources. The biological processes occurring in various compartments of environment can lead to the transformation of platinum group elements (PGE) into forms available to plants and other living organisms. In the last decade the studies related to emission, occurrence, toxicity, transformation and determination of Pt and Pd in the environment were undertaken (Kalavrouziotis and Koukoulakis 2009; Spada et al. 2012; Zereini et al. 2012). In the near future regular monitoring of these metals in the environment will be probably compulsory.

The determination of Pt and Pd in environmental samples is still a challenging task as a content of these analytes is at a level of ng g−1. The application of sensitive analytical technique, such as inductively coupled plasma mass spectrometry (ICP-MS) (Gomez et al. 2003; Shinotsuka and Suzuki 2007), inductively coupled plasma optical emission spectrometry (ICP-OES) (Herincs et al. 2013), electrothermal atomic absorption spectrometry (ETAAS) (Schuster and Schwarzer 1996; Boch et al. 2002), chemiluminescence (CL) (Malejko et al. 2012) or adsorptive stripping voltammetry (AdVS) (Zimmermann et al. 2003), and careful optimization of sample analytical procedure are necessary to obtain reliable results of analysis. Generally, for elimination of interference arising from complex matrix of samples the application of separation/preconcentration procedure is essential. For this purpose classical methods of separation, such as precipitation and liquid-liquid extraction have been developed. Co-precipitation of Pt with tellurium (Gomez et al. 2003) and Pd with mercury (Zimmermann et al. 2003) was successfully applied for separation of analytes from environmental samples. A variety of dispersive liquid-liquid microextraction (Liang and Zhao 2011) and cloud point microextraction methods (Meeravali et al. 2008) have been used as well. Among other methods, the solid phase extraction (SPE) is most widely recommended (Godlewska-Żyłkiewicz 2004; Myasoedova et al. 2007). The retention mechanism of analytes on solid sorbents include adsorption (Schuster and Schwarzer 1996; Ebrahimzadeh et al. 2010; Rastegarzadeh et al. 2010; Leśniewska et al. 2005; Bagheri et al. 2012), chelation (Rojas et al. 2006, 2009), ion-exchange (Shinotsuka and Suzuki 2007; Nakajima et al. 2009; Krishna et al. 2009; Herincs et al. 2013) and ion-pair formation. Biosorbents, including free and immobilized biomass such as algae Chlorella vulgaris (Dziwulska et al. 2004), yeast Saccharomyces cerevisiae (Godlewska-Żyłkiewicz and Kozłowska 2005) and fungi Aspergillus sp. (Woińska and Godlewska-Żylkiewicz 2011; Malejko et al. 2012), have proved to be an attractive alternative to traditional, commercially available sorbents. The biosorption process has been successfully applied for separation of PGE from environmental samples and their determination by ETAAS.

Even if it is widely recognized that the sample pre-treatment procedure is a main source of errors in the analytical process, certain factors are still underestimated by some analysts. The most critical aspects of the sample pre-treatment procedure, that affect the accuracy of the results, such as sample digestion and separation of analytes from interfering matrix before determination of Pt and Pd in road dust samples by ETAAS technique, will be discussed in detail in this chapter. As an example, the procedure based on separation of Pt and Pd on immobilized biomass of fungi Aspergillus sp. (Woińska and Godlewska-Żyłkiewicz 2011), has been chosen. The analysed samples of road dust were collected from the main intersection located in the centre of Bialystok (Poland) in the period from 2000 to 2011. The fraction of size <0.075 mm was taken for analysis.

2 Digestion Procedure for Road Dust Samples

Inhomogeneous distribution of PGE in road dust and their resistance to various chemicals, complex matrix of such samples often induce problems with complete transfer of the analytes into solution (Balcerzak 2002; Godlewska-Żyłkiewicz 2004). The selected digestion procedure must provide quantitative dissolution of analytes and their transformation into chemical forms suitable for a further pre-treatment step.

Pt and Pd of anthropogenic origin are mainly emitted to the environment in elemental (nano-metallic) and oxide forms (Merget and Rosner 2001). Wet chemical digestions utilizing various mineral acids (HCl, HNO3, HClO4, HF) in open (Chwastowska et al. 2004) or closed systems heated in convection or microwave ovens are commonly proposed for decomposition of road/tunnel dust or soil (Table 1) (Boch et al. 2002; Limbeck et al. 2003; Matusiewicz and Lesiński 2001; Tsogas et al. 2008). The necessity of using HF for digestion of samples containing silicate matrix was discussed in the literature (Balcerzak 2002; Boch et al. 2002; Leśniewska et al. 2005). According to some authors (Boch et al. 2002; Tsogas et al. 2008) Pd is strongly bound to silicate matrix of dust and ceramic particles emitted to environment from automotive catalysts, therefore its quantitative releasing into solution arises after total digestion of the sample matrix. However, the addition of concentrated HF to the sample induces some analytical problems. The larger amount of dissolved metal ions from silicates or aluminosilicates, e.g. Al(III), Fe(III), Pb(II), Ni(II), Co(II), Cu(II) in the final solution may cause spectral interference in the detection step (Brzezicka and Szmyd 1999; Brzezicka and Baranowska 2001; Godlewska-Żyłkiewicz and Leśniewska 2006, 2010) or problems with efficient separation of matrix.

Table 1 Digestion procedures for the determination of Pt and Pd by ETAAS in environmental samples
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The removal of these heavy metals from the digested solutions is difficult, because in the presence of HF negatively charged fluoride complexes are formed, which cannot be separated from anionic chlorocomplexes of Pt and Pd, and often are simultaneously enriched on anion-exchange columns (Brzezicka and Baranowska 2001; Godlewska-Żyłkiewicz and Leśniewska 2006). The excess of HF in the analysed solutions might also cause serious damage to the column packing used for the separation of the matrix, e.g. silica gel (Boch et al. 2002). To avoid these adverse effects the evaporation of excess of HF (Limbeck et al. 2003) or masking of fluoride ions with H3BO3 (Boch et al. 2002; Tsogas et al. 2008) have been proposed. Some examples of successful application of alone aqua reqia for digestion of road dust (Rauch et al. 2000; Müller and Heumann 2000) and BCR 723 (Leśniewska et al. 2005; Godlewska-Żyłkiewicz and Leśniewska 2006; Woińska and Godlewska-Żyłkiewicz 2011) for determination of PGE show that addition of HF to such samples is not essential.

It must be also considered that during storage and pre-treatment of samples, the specific problems connected with chemical reactions and physical processes taking place in solutions or at the phase boundary, e.g. adsorption of analytes on the vessel’s surface or changes in the oxidation state may occur. The type of vessels used for preparation and storage of standard and sample solutions (Godlewska-Żyłkiewicz 2002; Cobelo-Garcia et al. 2007), and the roughness of their surface have also an effect on the accuracy of analysis. The memory effect cannot be neglected as well (Kowalska et al. 2005). The storage of Pt solutions in quartz vessels and Pt solutions in glass vessels is recommended, because in this way the adsorption effect is diminished. Efficient cleaning of laboratory ware is possible with solutions containing chemical agents complexing Pt and Pd, such as acidic solutions of thiourea (Godlewska-Żyłkiewicz 2002; Cobelo-Garcia et al. 2007). The crucibles of unknown history should be cleaned before use in concentrated acids, e.g. in a mixture of HCl, HNO3 and HClO4, HCl or aqua regia.

Due to the variety of chemical forms of Pt and Pd in solutions obtained after digestion of samples and various stability of the species, the special treatment of samples should be performed in order to transform the analytes into the form suitable for separation step. A high amounts of chloride and nitrate(V) ions are often added during sample preparation procedure, e.g. during leaching of PGE from road dust with aqua regia. It was found that the presence of hydrochloric acid (0.5–4.0 mol L−1) and nitric(V) acid (0.5–2.0 mol L−1) cause increase of the analytical signal of Pt by 20–50 % and the signal of Pd by 30–60 % measured by ETAAS (Godlewska-Żyłkiewicz and Leśniewska 2010). The possibility of formation of stable nitrosyl and/or hydroxynitrate compounds of some PGE (Pt, Ru, Pd) must be carefully considered during sample preparation procedure. The excess of acids should be removed from a digested sample while different species of analytes should be transferred into stable chlorocomplexes. The repeated evaporations of the solution in the presence of concentrated HCl is recommended to that end. It must be stressed that this stage may seriously influence the accuracy of results. On the one hand, the complete evaporation of the solution can lead to the irreversible adsorption of the analyte on the surface of the vessel. On the other hand, often the non-quantitative conversion of analytes takes place as the efficiency of the conversion reaction depends on the number of evaporation steps and the volume of HCl used. Therefore, this step should be optimized experimentally, depending on the type and amount of the digested sample matrix.

In this work the effect of sample pre-treatment step on the accuracy of determination of Pt by ETAAS in road dust was studied in detail. The recovery of Pt from BCR-723 and road dust samples spiked with 20–30 ng of analyte was controlled after variable number of evaporation steps performed with addition of different volumes of concentrated HCl. Such pretreated samples were next loaded on the columns filled with immobilized fungi Aspergillus sp. The proper evaporation procedure was chosen on the basis of recovery data and the shape of absorbance signals of the analyte registered from column eluate (Fig. 1). Initially, the digested sample was evaporated two times, almost to dryness, in cleaned quartz crucibles with addition of 1 mL of HCl. However, the repeatability of Pt determination was low (>15 %). Furthermore, a high signal of background was also registered. When road dust sample was evaporated four times with addition of 1 mL of HCl, the recovery of Pt decreased to 80 %. When samples were evaporated three times with 2 mL of HCl the recovery of Pt from road dust reached 105 %, and from BCR-723 was equal to 107 %. It was confirmed that this procedure provides acceptable recovery of the analyte as well as the efficient elimination of the background signal. For comparison, the atomization signal of Pt obtained from directly injected digested sample is also shown in Fig. 1a.

Fig. 1
figure 1

Atomization signals of Pt from BCR-723 (road dust) (taken from Woińska and Godlewska Żylkiewicz 2011, with Elsevier permission); a — Pt in digested sample; −−− sample background; Pt in eluent (sample evaporated 2 times with 1 mL HCl); eluent background; b — Pt in eluent (sample evaporated 4 x with 1 mL HCl); −−− eluent background; Pt in eluent (sample evaporated 3 times with 2 mL HCl); eluent background

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3 Separation and Determination of PGE

For separation of trace amounts of Pt and Pd from road dust the SPE technique based on biosorption process on various microorganisms was developed (see chap. “Appraisal of Biosorption for Recovery, Separation and Determination of Platinum, Palladium and Rhodium in Environmental Samples”). It was found that fungi Aspergillus sp. immobilized on Cellex-T resin were very efficient in terms of selectivity and reproducibility (Woińska and Godlewska-Żyłkiewicz 2011). The analytes were efficiently retained from standard solutions on this biosorbent at pH range from 1 to 4 and quantitatively eluted from the column with thiourea solutions (0.25 mol L−1) prepared in HCl (0.3 mol L−1). However, in order to apply the developed procedure to analysis of road dust samples some further optimization of the procedure had to be done.

Generally, the large content of transition metal ions in the solution obtained after evaporation of digested road dust sample may influence the recovery of the analyte from biosorbent. The retention of the analyte may be diminished due to the competition of the analyte and interfering ions for active sites of sorbent or overloading sorption capacity of the used sorbent. The signals of the analytes registered from eluates are decreased due to interferences from foreign ions co-eluted together with the analytes, and this effect influences their recoveries. Therefore, in many cases the analytical procedure should be completed with additional step of column rinsing before elution of the analyte. The addition of acids, bases and buffering agents used for adjusting of sample pH to the value required to efficient separation as well as reagents used for elution can influence the analytical signal of analytes measured by ETAAS.

The retention of palladium and platinum on the fungi Aspergillus sp. immobilized on Cellex-T was not diminished in the presence of high excess of transition metal ions (10 mg L−1-10 g L−1 of Al(III), Cr(III), Co(II), Fe(III), Ni(II), and Pb(II)) (Woińska and Godlewska-Żyłkiewicz 2011). However, their presence considerably influenced the recovery of analytes. The strongest decrease of platinum and palladium recoveries (to 46.5 and 56.2 %, respectively) was observed in the presence of Co(II) ions (10 mg L−1). The significant background signals registered from eluate solutions indicate that transition metal ions nonspecifically retained on the low mass of sorbent were co-eluted together with the analytes. Removing of matrix ions retained on the sorbent was performed by rinsing the column before the elution step with water or nitric acid of different concentrations. The example below shows the process of optimization of rinsing procedure, which had been used for the elimination of negative effect of Co(II) ions (10 mg L−1) on the recovery of Pt (50 µg L−1) from the column (Fig. 2). As mentioned above the recovery of Pt from such solution was only 46.5 %. Rinsing the column with 3, 5 or 20 mL of water resulted in significant reduction of the background signal and increase of Pt recovery to 55, 64 and 78 %, respectively (Fig. 2a). Because the recovery of Pt was not quantitative, solutions of nitric acid of various concentrations (0.001–0.1 mol L−1) were tested for removal of Co(II) ions from the column. As can be seen in Fig. 2b, the use of 3 and 5 mL of 0.001 mol L−1 HNO3 resulted in 51 and 83 % recovery of Pt. The recovery equals 79 % was obtained using for this purpose of 3 mL of 0.05 mol L−1 HNO3. Accurate results of the analysis (recovery: 93.2 ± 2.0 %, n = 3) and the total elimination of the background signal was obtained when column was rinsed with 5 mL of 0.05 mol L−1 HNO3. The small losses of Pt (recovery: 86 %) with rinsing solution were observed when higher concentration of nitric acid (>0.1 mol L−1) was used. The effectiveness of the developed procedure was proved by analysis of standard Pt solutions (50 µg L−1) containing a high concentrations of foreign ions, namely a mixture of Al(III), Cr(III) and Co(II) ions of total concentration from 30 mg L−1 to 2,608 mg L−1. The recovery of Pt from such solutions was in the range from 101.7 ± 2.3 % to 106.8 ± 5.2 % (n = 4). The recovery of platinum decreased to 60.3 ± 0.4 % (n = 3), when the solution contained as much as 10 g L−1 of Al(III), 400 mg L−1 of Cr(III) and 30 mg L−1 of Co(II). In the same way the procedure of column rinsing was optimized for Pd determination. The recovery of Pd (50 µg L−1) from the column in the presence of other ions (2.5 g L−1 of Fe(III), 500 mg L−1 of Ni(II), 9 mg L−1 of Co(II) and 6 mg L−1 of Pb(II)) amounted to 100.3 ± 0.4 % (n = 4). Finally, it was found that using of 5 mL of 0.05 mol L−1 HNO3 for column rinsing in case of Pt determination, or 5 mL of 0.1 mol L−1 HNO3 in the case of Pd determination, allows for quantitative recovery of analytes and significant elimination of interferences even in the presence of 50,000–60,000-fold excess of other ions.

Fig. 2
figure 2

Atomization signals of Pt from solutions containing 50 µg L−1 Pt + 10 mg L−1 Co after separation on fungi Aspergillus sp immobilized on Cellex-T: a Pt signal in sample solution; sample background; Pt signal in eluent, column rinsed with: 3 mL of H2O, 20 mL of H2O; eluent background, column rinsed with: 3 mL of H2O, 20 mL of H2O; b Pt signal in eluent, column rinsed with: 3 mL 0.001 mol L−1 HNO3, 5 mL 0.001 mol L−1 HNO3, 5 mL 0.05 mol L−1 HNO3; eluent background, column rinsed with: 3 mL 0.001 mol L−1 HNO3, 5 mL 0.001 mol L−1 HNO3, 5 mL 0.05 mol L−1 HNO3

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4 Analysis of Road Dust Samples

The content of Pt and Pd in road dust samples collected in the center of Białystok (300,000 inhabitants, main intersection of urban roads, sampling at the distance of 2 meters from a footpath with traffic lights) was determined using a developed procedure consisting of digestion of samples in aqua regia, evaporation of samples with concentrated HCl, separation of analytes on fungi Aspergillus sp. immobilized on Cellex-T and their determination by ETAAS. The obtained results are presented in Fig. 3. The content of Pt in road dust increased from 93 µg kg−1 in 2000 to 263 µg kg−1 in 2011, while the content of Pd increased from 43 µg kg−1 in 2000 to 328 µg kg−1 in 2011. These values are significantly higher than background values, which suggest the input of PGE to the environment from automobile catalyst systems. The rapid increase of content of PGE in road dust is probably an effect of 140 % increasing of the number of cars registered in Białystok within this period (according to the Department of Inhabitants Service, City Office, Białystok from 58,000 in 2001 to 138,407 in 2011). The increase of Pd content in road dust was more rapid than the increase of Pt, which can be related to the changes in composition of catalytic converters introduced at the beginning of 2000, when Pt was replaced by Pd due to its high price. Since 2003 almost constant ratio of content of Pt to Pd in collected road dust has been observed (Ptcontent/Pdcontent. = 0.746 ± 0.055).

Fig. 3
figure 3

Pt and Pd content in road dust collected in years 2000-2011 from main intersection in Bialystok

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The obtained values of Pt and Pd content in road dust in Białystok are similar to the results obtained in the UK (Prichard et al. 2009), Spain (Simitchiev et al. 2008), the USA (Spada et al. 2012) or Greece (Tsogas et al. 2008) (Fig. 4) in road dust collected from the places with high traffic. The traffic intensity, the speed of cars and driving conditions influence the emission rate of PGE. The emission is also higher during continued stop/start cycles as well as during engine’s ignition (Tsogas et al. 2008), what often happens at traffic lights. The content of PGE in samples collected in Białystok is quite high, which can be explained by the sampling place situated close to traffic lights. The content of Pt and Pd in samples collected in 2009 from ring road of the city was much lower, namely 48 and 97 µg kg−1. The content of PGE in tunnel dust (the USA) is among highest values.

Fig. 4
figure 4

Ranges of Pt (a) and Pd (b) content in road dust in different countries (Pan et al. 2013; Spada et al. 2012; Lee et al. 2012; Tsogas et al. 2008; Smitchiev et al. 2008; Prichard et al. 2009, this work)

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The PGE content in road dust obtained in Hong Kong and Macao (China) is also comparable to our results, as in those cities catalytic converters were applied to vehicles as early as in European countries (Pan et al. 2013). In Beijing and Shanghai (China) the PGE content in road dust samples is relatively lower, what was explained by considerable dilution of dust due to engineering (road) construction in the sampling time of 2007. The lower content of PGE in road dust in Kolkata (India), Qingdao and Wuhan (China), and Seoul (Korea) (Lee et al. 2012) can be connected with shorter time period when vehicles have been equipped with catalytic converters.

5 Conclusions

It is of key importance that the reliable determination of Pt and Pd in road dust by spectrometric techniques (e.g. ETAAS) requires careful optimization of sample pre-treatment procedure. The steps, which seems to be less important, as evaporation of acids from digested samples or rinsing of column after separation of analytes, have the significant effect on the final result, what was described in this chapter. Therefore, all steps of procedure should be optimized experimentally using samples of similar matrices or certified reference materials.