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The Elements in the Second Rank

9 Palladium

9.1 Identity

Table 9.1
CAS No., EINECS No and molecular weight for palladium

9.2 Physico-chemical properties

Palladium is a heavy metal belonging to the platinum group in the periodic table. It has in most cases the valence +2 (Pd(II)), but +3 (Pd(III)) and +4 (Pd(IV)) are also possible [1]. Palladium is attacked by sulphuric acid and nitric acid [1]. The physico-chemical properties of Pd(II)-compounds resembles the platinum (Pt(II)) compounds. However, the Pd(II)-compounds are generally more reactive and have higher solubility in water. Table 9.2 shows selected data on palladium's physico-chemical properties (data from [2, 6]).

Table 9.2
Physico-chemical data for metallic palladium

9.3 Uses and consumption

9.3.1 Uses

The expansion of the electronics industry is supposed to account for the major part of the recent increase in consumption, e.g. from the use in printed circuit boards [8]. Palladium is also used in significant amounts in catalytic converters in automobiles [8].

Sweden recycles palladium from electronic equipment and catalytic converters and a similar trend is expected in Denmark due to requirements in the national legislation on waste handling [3, 8].

A special use of palladium in Sweden and Denmark is in the alloying of gold in dentistry [8].

According to Swedish experience, palladium is used as the pure metal or as PdCl₂ [8].

9.3.2 Consumption

The global consumption has increased significantly throughout the 20th century [8], and the global production was 240 tons in 1997. From the per capita consumption in USA, the Danish consumption is estimated to approximately 2.4 tons Pd/year [8].

Table 9.3
The relative distribution of the global consumption and the Danish use of Palladium in 1997 based on [8]

Since the 1990's the most significant area of consumption has shifted from the electronic industry to catalysts (including catalytic converters) [8].

9.4 Emissions to and occurrence in the environment

The analysis of waste streams conducted in relation to this study is the only available information on palladium concentration in the environment in Denmark. A Swedish study showed that the level of palladium is highest in lake sediment, sewage sludge, and leachate from waste disposals, cinders, and fly ash [8].

The use of palladium in catalytic converters in automobiles leads to emission on the same level as the platinum-emission. In Sweden this emission is expected to be approximately 3 kg Pd/year which corresponds to 1.8 kg in Denmark. 90 mg/L of palladium is found in road runoff water in Sweden.

Palladium in consumer products will primarily end as solid waste. From the high concentrations found at different waste treatment facilities in Sweden, it is expected that combustion and disposal of solid waste can result in palladium release.

Palladium used in printed circuit boards shall be recycled according to a Danish government order [3].

In aqueous and terrestrial environment palladium is found in low concentrations. Sea water has a concentration of palladium of approximately 0.06 µg/L. Typical concentrations of palladium in the environment are shown in Table 9.4.

Table 9.4
Typical background concentration of palladium in the environment. - indicates that the value is unknown. Data from [5, 7, 8, 12]

A limited investigation of the levels of palladium (and the other elements covered by this study) in the major emissions and waste streams in society was conducted in the autumn of 2001, see Table 9.5.

Table 9.5 shows the palladium concentration in selected emissions and waste products in Denmark. The level of palladium is generally a little higher than that of platinum. In landfill leachate, sewage sludge and sediment from road runoff retention basins the highest concentrations were found. In treated waste water the concentration of palladium was 0.7-0.9 µg/L.

Table 9.5
Levels of palladium in selected emissions and waste products from measurements conducted as part of this study in the autumn of 2001. When the average of two measurements had a deviation of more than ± 50% both measurements are reported.

9.5 Danger classification

Palladium or palladium compounds are not listed on the Danish list of dangerous compounds or the list of undesired compounds [9, 10].

9.6 Toxicology

Palladium has not been identified as acute toxic or causing CMR effects. Contact dermatitis has been reported from exposure of metallic palladium and palladium alloys [11]. Individuals suffering from nickel allergy also seem to react on palladium.

9.7 Environmental properties

The use of palladium in electronic equipment and catalytic converters rises concern of the possibility of palladium dispersion due to the relatively high mobility of the Pd(II)-compounds. The data available is not sufficient to enable a thorough analysis of its environmental effects.

9.7.1 Environmental chemistry

Palladium is found as Pd(OH)₂⁰ in fresh water, and Pd(OH)₂⁰ and PdCl₄^2- in sea water. Palladium can also bind to organic matter in aquatic environments [8]. The palladium assimilated by algae and plankton is released during degradation of organic material.

The relatively high solubility of Pd(II)-ion compounds can result in fast dispersion, impeding accumulation in recipients or other environments that receive palladium, e.g. sludge amended farmland.

Palladium can decrease the enzymatic activity in the organisms that assimilate the metal, due to its ability to bind amino acids.

It is not known whether combustion of palladium rich waste results in emission and subsequent atmospheric deposition, or if the metal immobilises in cinder or in the waste gas cleaning residuals.

9.7.2 Environmental toxicology

Only limited information on the environmental toxicity of palladium was obtained from searching in databases. The only environmental toxicity test found was with the organism Tubifex tubifex. The acute toxicity of PdCl₂ measured as EC₅₀ (assessed as immobilisation) had a value of 237 µg/L. Based on the result of this test, PdCl₂ should be classified as very toxic to aquatic organisms.

Table 9.6
Test results for environmental toxicity (mg/L). Data from AQUIRE [4]

9.7.3 Bioaccumulation

No data on bioaccumulation of palladium or palladium compounds were found.

9.8 Conclusions

The consumption of palladium is increasing due to growth in the use of electronic equipment and catalytic converters for automobiles. This is likely to result in increasing emissions too. Only few data on palladium in waste streams and the environment is available from the literature. Measurements conducted as part of this study show that the palladiums concentration are high in sewage sludge, sediment from road runoff retention basins, and in compost. Palladium (II) compounds are expected to have high mobility due to their high water solubility. There is not sufficient data to evaluate the environmental toxicity of palladium.

9.9 References

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