Technical uses of platinum and anthropogenic sources
Due to its different physical and chemical properties described previously, Pt serves for several applications related to corrosion resistance even at high temperatures, a high melting point, a considerable mechanical strength, and good ductility. Principal use of Pt, and other PGEs, relates to their unique catalytic properties. Due to all these properties, Pt is used in the glass, petroleum, electric, and electronic industries, and the manufacture of jewelry (Pawlak et al., 2014). This last application is common since more than 2000 years, and most Pt- jewelry manufacturing countries, produce jewels made in a purity of at least 85 % Pt. This sector accounted for ~ 10 % of the European Pt demand in 2017 (Figure 4; Johnson Matthey, 2017). Since the early 1960’s, Pt-based drugs serve for cancer treatment, as Pt has the ability, in certain chemical forms, to inhibit the division of living cells. Since then, various molecules with Pt as the active agent (e.g Cisplatin, Carboplatin, Oxaliplatin) have been developed for anti-cancer drugs to decrease their toxicity and increase their efficiency (Eckstein, 2011). The medical use in Pt-based drugs does not exceed 5 % of the total European Pt demand. Furthermore, due to its chemical inertia (it does not corrode inside the body) and its good electrical conductivity, Pt components serve in pacemakers and other heart treatments or in dental alloys. Platinum may also be used in the chemical processing sector (nitric acid, silicones…), in electronic components, as a sensor in engine control systems to measure oxygen and nitrous oxide levels and in home safety devices to detect carbon monoxide.
A diversity of other Pt applications exist including glass, watches, fuel cells which correspond to devices for generating electrical power, spark plugs, and turbine blade coatings. Platinum catalysts are also used to produce petroleum and petrochemical feedstocks for plastics, synthetic rubber and polyester fibers. The most important application sector, representing more than 70 % of the total European demand for Pt in 2017, are the car catalytic converters, introduced in Europe in the late 1980’s.
A car catalytic converter is a unit that fits into the front part of motor exhaust systems, close to the engine, and which aims at reducing gaseous pollutant emissions, mainly carbon monoxide (CO), nitrogen oxides (NOx), and hydrocarbons (HC). It is usually fitted with a heat shield to limit heat losses (Figure 5). Removal of the pollutant gases from the exhaust occurs either after reduction, or oxidation (Ravindra et al., 2004).
Car catalytic converters convert more than 90 % of CO, HC and NOx into carbon dioxide (CO2), water (H2O), and nitrogen (N2) respectively. Both CO and HC are eliminated together by oxidation reactions while NOx are reduced to harmless N2. The regulation system (lambda-probe and fuel injection management) controls the quantity of O2 that enters into the engine ensuring that O2 and fuel compounds are in optimum stoichiometric proportions to optimize converter efficiency (Ravindra et al., 2004).
Modern catalytic converters consist of a monolithic honeycomb support made of alumina-coated cordierite (a phase of 2MgO.2Al2O3.5SiO2; Mg: magnesium, Si: silicon) housed in a stainless steel box (Figure 5). This activated, high surface alumina layer is called “washcoat” and consists of ~ 90 % -Al2O3 and a mixture of base metal additives, mainly oxides of cerium (Ce), zirconium (Zr), lanthanum (La), Ni, Fe, and some alkaline earth metals improving the catalyst performance (Ravindra et al., 2004). The PGEs (Pt, Rh and Pd) are fixed on the washcoat surface usually by impregnation, or coating from a solution of hexachloroplatinic (IV) acid (H2PtCl6.6H2O), palladium chloride (PdCl2), and rhodium chloride (RhCl3) salts. After evaporation of the solvent, decomposition and reduction of PGE salts are performed to obtain the highly dispersed catalytically “active” metallic forms (Ravindra et al., 2004).
There is a wide range of PGE combinations and concentrations in converters. Accordingly, Pt, Pd/Rh, Pt/Rh, Pt/Pd or Pt/Pd/Rh catalysts exist, yet, with overall percentage regarding the whole converter mass of less than 0.1 %. The combination and amount of Pt, Rh and Pd in catalytic converters have been influenced by technological developments and by changes in clean air legislation. First, from 1976 to 1979, the two-way catalysts, containing Pt and Pd, allowed the oxidation of HC and CO while the second generation (from 1979 to 1986) are the “three-way catalysts” including Pt, Pd, and Rh which serve also to reduce NOx (Ravindra et al., 2004). The third generation of three-way catalytic converters comprises the same composition but is better adapted to high temperature working conditions in relation with new fuel efficient engines. Finally, from 1992 to the mid-1990s, another generation of three way Pd-rich catalysts was extensively applied by car manufacturers both in US and in Europe, in the new models to meet the even stricter emission legislation (Ravindra et al., 2004).
Since the beginning of 1993, all new cars sold within the E.U. must be fitted with catalytic converter (Jarvis et al., 2001). A catalytic converter designed for a family car normally contains about 1.75 g of PGE and a typical ratio of Pt/Rh of 5 (Barefoot, 1997). Catalytic converters generally have a service life of 80,000 – 160,000 km, although several factors may reduce this lifetime. Factor influencing its correct functioning include Pb pollution or short journey use that prevents the converter from reaching the optimal working temperature (around 400 °C, Ravindra et al., 2004). However, the principal cause of converter failures is carbon pollution, leading to a partial, or sometimes, a total blockage of the catalyst, and the internal fracture of the catalyst surface, usually induced by external/internal physical damage.
Although catalytic converters are recycled, typical recovery is only around 20 – 30 %. Therefore over the 5 – year (or 80,000 km) average lifetime of a catalyst, up to 80 % of the Pt used in production will be released into the environment in one form or another (Jarvis et al., 2001).
However Pt emissions are not the only source of anthropogenic Pt dispersion in the environment.
Platinum dispersion and behavior in aquatic environments
The diversity of applications of PGEs in modern technologies is accompanied with anthropogenic dispersion of these elements in the natural environment. Consequently, PGEs are considered as emerging inorganic contaminants in the environment for which it is required to make reasonable quantitative estimates of not only environmental pathways, loads, and concentrations but also the socioeconomic drivers and « upstream » control measures (control, reduction, or elimination of emissions, Cobelo-García et al., 2015; Rodrigues et al., 2009).
Mitra and Sen (2017) established a quantification of natural and anthropogenic Pt fluxes across the different spheres: geo-, atmo-, bio-, hydro-, pedo-, and anthropo-spheres. Human contributions to the total anthrobiogeochemical Pt cycles are 5 %. When the Earth’s surficial processes only are considered, soil erosion is the dominant flow for Pt mobilization, comprising 13 % of the total mobilization on the Earth’s surface. On the other hand, mining activities, fossil fuel burning and automobile emissions are the most important anthropogenic flows and human contributions to the anthrobiogeochemical Pt cycles, considering only surficial processes, are greater than 70 %.
First works on Pt contamination of natural environments considered the exponentially increasing use of Pt in relation with car catalytic converters. Considering thermal and mechanical abrasion from the surface of the device, Pt, and other PGEs, particles are released into the environment and deposited in roadside soils. Consequently, first monitoring studies of Pt pollution in urban areas considered terrestrial environments with the analysis of soils and/or different types of plants (Morton et al., 2001; Schäfer et al., 1998). Anthropogenic PGE signals have been detected in all Earth compartments (Mitra and Sen, 2017). In addition, in urban areas, potential sources include not only automobile catalysts but also industrial processing, fossil fuel combustion and medical centers (Rauch and Peucker-Ehrenbrink, 2015). After emissions, this different Pt inputs may be dispersed in aquatic systems as described in the following section.
Anthropogenic dispersion of platinum in aquatic systems
Platinum Group Elements pollution is monitored in aquatic systems near mining zones in order to prevent potential environmental risks. Accordingly, in a recent study Almécija et al. (2017) evaluated the concentrations of PGE in stream sediments of the Hex River, which drains the mining area of the Bushveld Igneous Complex (South Africa). The highest concentrations were observed closer to the mining area, decreasing with distance and in the < 63 µm fraction, probably derived from atmospheric deposition and surface runoff of PGE-rich particles released from mining activities. Thus, mining activities are causing some disturbance of the surface PGE geochemical cycle, increasing the presence of PGE in the fine fraction of river sediments (Almécija et al., 2017).
Distant from mining zones, Pt and other PGEs anthropogenic dispersion occurs especially in urbanized environments. In such systems anthropogenic emissions have largely been attributed to car catalytic converters. Accordingly, during the release of the exhaust gases from the engine, the surface of the washcoat is chemically and physically stressed by the fast change of oxidative/reductive conditions, high temperature and mechanical abrasion. This produces the emission of Pt, and other PGEs, containing particles into the environment. It is generally believed that mechanical erosion of the catalysts surface is the major cause, although thermal and chemical processes may also contribute to the emission (Rauch and Morrison, 2008). The amount and rate of PGE emissions from the catalytic converter are affected by the speed of the automobile, the type of the engine, the type and age of the catalyst, and the type of fuel additives (Artelt et al., 1999b). Emission can be intensified by unfavorable operating conditions (misfiring, excessive heating), which may even destroy the converter (Schäfer and Puchelt, 1998 and references herein). Measurements under laboratory conditions indicate that emission rates are in the low ng.km-1 range (Artelt et al., 1999b; König et al., 1992). Several studies suggest that emission rates are significantly higher for diesel catalyst than for three-way catalysts used in gasoline engines (Moldovan et al., 2002), and at higher speeds (König et al., 1992). Using the average emission rate of automobile catalytic converters and different deposition rates at different environments, the total deposition of Pt, Pd and Rh in the northern hemisphere is calculated. Total deposition from automobiles lies between 9 – 20 metric tons Pt.year-1, 20 – 50 metric tons Pd.year- 1 and 2 – 4 metric tons Rh.year-1 (Rauch et al., 2005). However, accurate estimation of PGE emissions from car catalytic converters remains uncertain despite nearly 30 years of research (Rauch and Peucker-Ehrenbrink, 2015).
Overall, calculated flows of Pt from automobile emissions represent the majority of total anthropogenic flows (Mitra and Sen, 2017). Following their emission, traffic related heavy metals are either subjected to atmospheric transport or are deposited on the road (Haus et al., 2007). Automobile atmospheric Pt dispersion is expected to have a relatively limited extent because PGEs are bound to fine particles (Rauch and Peucker-Ehrenbrink, 2015). A sharp decrease in PGE concentrations is observed within a few meters from automobile traffic (Jarvis et al., 2001; Schäfer and Puchelt, 1998). Therefore, when deposited on the road, Pt is washed into the drainage system during a later precipitation event, from where it may enter freshwater systems or sewage treatment systems (Haus et al., 2007).
Studies report that most of the PGEs released from catalytic converters are in particulate form (Ravindra et al., 2004). These elements might be emitted in the form of abraded washcoat particles onto which the PGEs are bound, and Ce is a major component of the washcoat. Platinum Group Elements are closely associated with Ce in road sediments, suggesting that they remain bound to autocatalyst particles and have a limited mobility (Rauch et al., 2000). Except for the association Pt-Rh, PGEs show a low degree of association with each other suggesting that they are emitted separately. The association of Pt and Rh might be explained by the formation of a Pt-Rh alloy in the autocatalyst (Rauch et al., 2000). Emitted particle sizes range from the sub-micron to > 63 µm in automobile exhaust and in the urban environment (Rauch and Morrison, 2008). Difference in particle sizes is related to the fact that the morphology of PGEs in the converter is subject to physical and chemical alteration during vehicle operation (Rauch et al., 2001). Thus, while fresh catalysts present a smooth dispersion of noble metal particles in the range of 1 – 10 nm, the size of Pt in the aged catalyst can range from 20 nm to several microns due to sintering (Rauch et al., 2001). As aforementioned, typical Pt/Rh ratio for car catalyst are of 5 (Barefoot, 1997), but ageing of these catalyst through vehicle operation may lead to increasing ratio from 5.5 to 12 (Rauch et al., 2001).
Previous publications state that examination of these particles showed that around 99 % of Pt is in the metallic state Pt(0) sorbed to small particles of alumina matrix that is considered to be extremely inert (Artelt et al., 1999b). If at all, soluble platinum is emitted in only very small quantities i.e. 1 %. However, another study confirmed the particulate form of catalyst-derived Pt but showed that soluble Pt amount in exhaust fume samples of fresh gasoline and diesel catalysts is important even though it represents less than 10 % (Hill and Mayer, 1977; Moldovan et al., 2002). Because soluble fraction is defined as filter passing (< 0.45 µm), it may also include PGE nanoparticles (Rauch and Morrison, 2008). Accordingly, recent research performing road dust sample characterization through single particle Inductively Coupled Plasma-Mass Spectrometry (spICP-MS) revealed that Pt in the extracted leachate (0.2 to 18 % of the road dust material) is entirely present as nanoparticles of sizes between 9 and 21 nm. Although representing only a minor fraction of the total content in road dust, the nanoparticulate Pt leachate is most susceptible to biological uptake and hence most relevant in terms of bioavailability (Folens et al., 2018).
Furthermore, although weathering by oxidation of PGE in natural system is low, moderately strong oxidizing solutions containing complexing agents such as chlorides or humic matter could lead to PGE dissolution processes (Jarvis et al., 2001). Accordingly, PGEs emitted by the catalytic converters, may not be in metallic form in exhaust fumes, or at least they could be rapidly altered, once they are deposited in the environment (Jarvis et al., 2001). Rauch et al. (2000) confirm that transformation of PGEs into a more mobile form might occur in the roadside environment, during transport through the storm water system or in urban rivers. Rainwater usually provides an important medium for the transportation of PGE particles until deposition. In run-off water up to 1 µg.L-1 Pt was found (Laschka and Nachtwey, 2000). Thus, measurements conducted in municipal wastewater treatment plants show an increase in average Pt loads in periods of rainy weather as compared to dry weather conditions, attributed to car traffic that causes increased Pt inputs into sewage treatment plants after rainfall (Laschka and Nachtwey, 2000).
Recently, Rauch and Peucker-Ehrenbrink (2015) addressed in a review the question: Are automobiles the main source of PGE in urban areas? While many studies support this source, other present discrepancies such as relatively small differences reported between cities with contrasting population sizes, vehicle numbers and catalyst introduction dates. Other divergences include traffic patterns and intensities at specific sampling locations. Discrepancies between expected and observed PGE concentrations or abundance ratios (used as a tracer of catalytic converter pollution) suggest that a number of sources contribute to PGE fluxes in urban areas. Non-automobile sources may contribute to both local and global PGE cycles (Figure 6).
Industrial sources emitting Pt into the atmosphere are likely but difficult to quantify according to Helmers and Kümmerer (1999), because few data are available for industrial PGE emissions. Moreover, Pt pollution by hospitals is also to consider in urbanized areas. As aforementioned, for the past 25 years Pt compounds have been used to treat numerous types of tumors (e.g. testicular, ovarian, bladder). Given the number of cancer cases, another important source of environmental pollution with Pt are hospitals, particularly those with chemotherapy departments (Pawlak et al., 2014). After treatment, chemotherapeutic agents are excreted by patients. For instance, carboplatin is mainly excreted via patient urine in its intact form. Hence, it can be deduced that aquatic systems most of the emitted compound is present as parent drug, while oxaliplatin degradation in aqueous media depends on chloride concentration (chloro-aqua-complexes, (Hann et al., 2005). Since effluents from hospitals are not treated in any special way, these compounds are released directly to municipal wastewater systems (Pawlak et al., 2014). The concentration of Pt in the sewage of various hospitals was determined and the enriched Pt concentrations were detected with diurnal variations (higher excretion during daytime, Kümmerer et al., 1999). Variability of Pt discharges was also observed over a three-week period of time monitoring effluents from a major UK hospital (Vyas et al., 2014). About 70 % of the Pt, administered in the form of either cisplatin, or carboplatin, is rapidly excreted, and, therefore, would end up in hospital effluents (Ravindra et al., 2004). Such excretion also continues outside the hospital, as more and more patients leave for home directly after the treatment (Vyas et al., 2014). Therefore, the lack of appropriate treatment methods for the purification of such active compounds contributes to Pt contamination of the aquatic environment. Although predicted concentrations are below European Medicines Agency (EMA) guidelines warranting further risk assessment, the presence of substances in surface waters that are potentially carcinogenic, mutagenic and teratogenic and whose environmental effects are not understood is cause for concern (Vyas et al., 2014). Platinum emitted by hospitals represent between 3 and 12 % of the amount of Pt emitted by car catalytic converters (Reith et al., 2014). Elevated PGE concentrations reported in urban sewage and waste originating from various aforementioned sources, may potentially become secondary PGE sources. They may lead to PGE emissions during sewage treatment, reuse, or disposal (Rauch and Peucker-Ehrenbrink, 2015). Thus, depending on characteristics of the sewage network, sewage discharges as secondary sources can result in the release of PGE into both the agricultural and the aquatic environments.
Today PGE are used in a wide range of applications and emissions might occur during PGE production, manufacture of PGE-containing products and use and disposal of these products. Although emissions from PGE production and manufacture are expected to be limited or relevant to specific sites, the use and disposal of PGE-containing items are of concern because of the potential leaching of PGE (Rauch and Morrison, 2008). Same authors also reported that the contribution of natural sources, including erosion and volcanic emissions, and the potential impact of human activities on some natural sources also need to be investigated since increased erosion resulting from agriculture or deforestation may for instance contribute to elevated concentrations at remote sites where no direct anthropogenic sources are present. Mitra and Sen (2017) have addressed such Pt flows revealing that mining activities, fossil fuel burning and automobile emissions are the most important anthropogenic flows (Figure 6). In addition, PGEs are of great relevance in the development of emerging key technologies and are in this context considered as Technology-Critical Elements (TCEs). Associated environmental impacts of such elements (from mining to end-of-life waste products) is not restricted to a national level but covers most likely a global scale (Cobelo-García et al., 2015).
Figure 6: Stocks and flows of the platinum cycle. Natural flows are marked by black lines, whereas anthropogenic flows are marked by red lines. Atmospheric stock of Pt is calculated by adding the natural and anthropogenic flows that are moving elements from different spheres to the atmosphere. Authors have assumed that the atmosphere is not a proper repository and PGEs have a short residence time, like other industrial metals (Mitra and Sen, 2017).
Overall, most of the PGEs were thought to behave in an inert manner, and to be immobile (Zereini et al., 1997). However, other studies proving their solubility and occurrence in various environmental compartments proved the importance of studying how these metals may become chemically/biochemically active, and mobile in interactions with different environmental matrices and especially in aquatic systems. Despite uncertainties in emission and release estimates, available data are consistent with the anthropogenic dispersion of PGE and more particularly Pt into the natural aquatic environment. According to the source of emission and to the recipient compartment, Pt present different speciation and chemical behavior.
Platinum geochemical behavior in aquatic systems: river, estuary, coastal systems, open oceans
Aquatic ecosystems can be considered as an important sink of PGEs. As aforementioned, different sources, like road runoff or industrial effluents are directly discharged into these ecosystems (Ruchter et al., 2015).
Table of contents :
Chapter 1: Scientific context
1.1. The platinum element
1.1.1. Physical and chemical properties of platinum
1.1.2. Platinum mineralogy and deposits: natural sources
1.1.3. Technical uses of platinum and anthropogenic sources
1.2 Platinum dispersion and behavior in aquatic environments
1.2.1. Anthropogenic dispersion of platinum in aquatic systems
1.2.2. Platinum geochemical behavior in aquatic systems: river, estuary, coastal systems open oceans
1.3. Platinum bioavailability and bioconcentration in aquatic organisms
1.3.1. Field studies on aquatic organisms
1.3.2. Laboratory studies on Pt uptake and biological response in aquatic organisms
Chapter 2: Material and methods
2.2. Study area and sampling strategy
2.3.1. Sediment cores from the Lot River
2.3.2. Biomonitoring samples and field campaign in the Gironde Estuary
2.3.3. Field campaigns in the Arcachon Bay
2.3.4. Biomonitoring samples from the northwestern Mediterranean Coast
2.3.5. Sampling from two highly anthropogenically impacted coastal sites: the Genoa Harbor and the Toulon Bay
2.3.6. Laboratory exposure experiment – experimental design
2.4. Analytical procedures for platinum determination in environmental and experimental samples
2.4.1. Analytical techniques
2.4.2. Determination of dissolved platinum in seawater
2.4.3. Determination of particulate platinum in sediment and in suspended particulate matter
18.104.22.168. Sediment cores – AdCSV
22.214.171.124. Suspended Particulate Matter – ICP-MS
2.4.4. Determination of platinum in wild living biota
2.4.5. Application of isotopically-labelled platinum in exposure experiments
Chapter 3: Platinum accumulation and physiological response in oyster Crassostrea gigas
3.2. Tracing platinum accumulation kinetics in oyster Crassostrea gigas, a sentinel species in coastal marine environments
3.3. Organotropism and biomarker response in oyster Crassostrea gigas exposed to platinum in seawater
Chapter 4: Temporal and spatial aspects of biogeochemical platinum cycles in coastal environments
4.2. Past and present platinum contamination of a major European fluvial–estuarine system: Insights from river sediments and estuarine oysters
4.3. Platinum in sediments and mussels from the northwestern Mediterranean coast: temporal and spatial aspects
4.4. Spatial variability and sources of platinum in the Genoa Harbor – from geochemical background to urban levels
4.5. Short-term variations in platinum partitioning in contrasting European coastal environments: the role of primary production and local point sources
Chapter 5: General conclusions and perspectives