Environmental risk
The mining and smelting operations for PGEs may affect the workers and local population, creating environmental contamination. In addition, traffic emissions and their impact on urban air quality, health, and atmospheric processes has been the subject of increasing interest in recent years [Ely et al. 2001; Dahlheimer et al. 2007]. The increased use of these noble metals in automobile catalyst converters, has led to their release into the environment and biological accumulation on roadsides, since 1974 in USA and 1993 in European countries [Zereini et al.1998; Palacios et al. 2000; Ely et al. 2001; Jarvis et al. 2001; Riga-Karandinos et al. 2006; Croy et al. 2008]. Catalytic converters are used to treat automobile emissions: reduction of nitrogen oxides to nitrogen and oxygen (2NOx → xO2 + N2), oxidation of carbon monoxide (toxic) to harmless carbon dioxide (2CO + O2 → 2CO2) and oxidation of unburnt carcinogenic hydrocarbons (HC) to carbon dioxide and water (2Cx Hy + (2x+y/2)O2 → 2xCO2 + yH2 O). However, Pt, Pd and Rh may be released from catalytic converters and their amount and rate is affected by the speed of the automobile, type of engine, type and age of catalyst, and type of fuel additives [Ely et al. 2001; Whiteley and Murray 2003]. The investigation of samples from road dust, soil and grass indicated that greater proportion of PGE emissions is from automobile catalysts, in the form of nanometer-sized catalyst particles. In soil, PGE can be transformed into more mobile species through complexation with organic matter. There are indications that environmentally formed Pd species are more soluble and hence more mobile in the environment than Rh and Pt [Dahlheimer et al. 2007].
Representative dust samples from Katehaki, Messoghion, Iera odos roads and the National high-way road Athens – Thessaloniki of Greece demonstrated that catalytic converter attrition is responsible for elevated abundances and that concentrations of PGEs increased with traffic density, reaching values up to 2070 ppb Pt and 1980 ppb Pd in dust. Gold, ranging between 14 and 990 ppb (average 310) in dust samples and from 27 and 160 ppb Au (average 95) in soils [Economou-Eliopoulos and Sfendoni 2010]. The PGE level along the highways may be washed by rain and concentrated into local water systems. Also, the determination of the PGE content of different plants grown on contaminated soil close to highways showed a transfer of the PGE from the contaminated soil to the plants. The enrichment of Pd and Pt contents in grasses and tree-leaves (average 5.6 ppb and 3.8 ppb, respectively) along roadsides confirm the aspect that under appropriate pH and redox potential conditions (humic or fulvic acids) they are more mobile [Dahleimer et al. 2007]. The good positive relationship between both Pd and Pt contents in plants with their corresponding contents in soils [Economou-Eliopoulos and Sfendoni 2010] is consistent with their solubility in soils [Wood 1990, 2005; Wood et al. 1994; Rauch and Morrison 2000].