DESCRIPTION
Inspiration
Chemistry Lessons
From chemistry lessons, we learnt that palladium is a transition metal with atomic number 46. It is a rare metal discovered in 1803 by William Hyde Wollaston1. Palladium, platinum, rhodium, ruthenium, iridium, and osmium form a group of metals known as the platinum group metals2. They have similar properties though palladium is the least dense and has the lowest melting point.
Catalytic Converters
Palladium is used in catalytic converters, devices used to control automobile exhaust emission. Palladium catalyses the oxidation of carbon monoxide, nitric oxides, and unburnt hydrocarbons, turning them into harmless gases with the following reactions:
- 2CO + O2 —> 2CO2,
- CxHy + (x+y)O2 —> xCO2 + (y/2)H2O, and
- 2NO → N2+ O2 .
Catalytic converters can filter up to 90%3 of the aforementioned toxic pollutants. Palladium allows adsorption of the gases and lowers the activation energy of the oxidation reaction. A lower activation energy allows more particles to obtain sufficient energy to react, thus a higher rate of reaction.
Humans are highly dependent on the diminishing fossil fuel supply. A more sustainable and green alternative to fossil fuels would be hydrogen, since the combustion of hydrogen only produces water. There are already ways to obtain hydrogen, such as electrolysis, though these methods are costly and are not environmentally friendly. However, due to palladium’s strong affinity to hydrogen, it has been proven useful in hydrogen storage4.
Pharmaceutical Industry
In the pharmaceutical industry, palladium is commonly used as a catalyst in cross coupling reactions in which two organic fragments are joined together. An example would be the production of the drug (PF-03052334-02) which results in less musculoskeletal side effects during the treatment of coronary diseases. To produce this medicine, a Suzuki reaction is used between triflate and styrenyl boronic acid. After that, the product is submitted to ozonolysis, providing aldehyde in 76%. The side chain is then installed by Wittig reaction, then chemo- and regioselective reductions and deprotection make PF-03052334-026. This reaction would not be possible without a palladium catalyst.
Jewelry Business
Due to palladium’s high malleability and its high resistance to tearing and tarnishing, it is commonly used in the jewelry business. Palladium is used instead of platinum since palladium is harder than platinum. Therefore palladium jewelry has higher durability. Jewelry makers also add palladium to the production of white gold, which is a type of reinforced gold and can cost more than normal gold in most conditions7.
Scarcity and Shortage of Palladium
Palladium is a precious platinum metal which has an abundance of 0.015 parts per million in the Earth’s crust with a wide range of uses9. However, the supply of palladium cannot satisfy the demands so the price for palladium is fuelled up.
In 2019, there was a total primary supply of 200,400 kg of palladium (Table 1)8. Compared to 2018, the number increased by 1,800 kg8. However, the total net demand for palladium in 2019 was around 227,300 kg8. In which, the gross demand for autocatalysts was 273,100 kg, while some of the gross demand was satisfied by recycled palladium. Compared to 2018, the net demand increased by 22,800 kg8. Observing the data below, the rate of increase of the total net demand for palladium was larger than the rate of increase of the total primary supply of palladium. From 2018 to 2019, the palladium market shortage increased from around 5,900 kg to 26,900 kg8.
The effects of shortage can be reflected through price. In April 2018, the price of palladium was about USD$35,000 per kg. In April 2019, the price of palladium increased by 51.4% to about USD$53,000 per kg. In January 2020, the price of palladium further increased by 49.15% to USD$79,000 per kg. (Fig. 2)10. The main reason for such a bullish market is that the supply of palladium is highly inelastic, meaning the quantity supplied cannot be increased drastically with respect to price changes11.
To alleviate both the problems of scarcity and high prices, an easily scalable method is needed to recycle palladium. In this project, we aim to develop a low cost method to recycle used palladium by peptide reduction. This can increase the supply of palladium to curb the deficit of the palladium market, thus ensuring that palladium can be used in daily life products in the long term at a low cost.
| Supply (Kilograms) | 2017 | 2018 | 2019 |
|---|---|---|---|
| South Africa | 72,206 | 72,093 | 74,446 |
| Russia | 69,513 | 84,368 | 84,680 |
| Others | 41,164 | 42,156 | 41,276 |
| TOtal primary supply | 182,883 | 198,617 | 200,402 |
| Gross demand (Kilograms) | 2017 | 2018 | 2019 |
|---|---|---|---|
| Autocatalyst | 239,979 | 250,326 | 273,148 |
| Jewelry | 4,734 | 4,196 | 3,827 |
| Industrial | 51,596 | 54,601 | 50,009 |
| Investment | -10,943 | -16,273 | -2,466 |
| Total gross demand | 285,366 | 292,850 | 324,518 |
| Recycling | -81,108 | -88,337 | -97,182 |
| Total net demand | 204,258 | 204,513 | 227,336 |
| Movement in stocks | -21,375 | -5896 | -26.932 |
Table 1, Palladum supply and demand
Figure 1, Demand and supply of palladium
Figure 2, Palladium Price Trend by Month, 2012 to 2020
Hazards
97 tons of palladium were recycled in 20198. However, most of the palladium ended up in landfills12. If the palladium leaks, it will affect both animals and plants as well as agricultural yield.
Palladium is found to be very toxic to aquatic animals. It has been shown that palladium compounds can cause cardiac malformation in zebrafish embryos13, as bioaccumulation of palladium decreases the embryonic heart rate and affects the expression of mRNA of cardiac-related genes.
Palladium is also harmful to terrestrial plants. Exposure to palladium can alter cell structure, and hence decrease agricultural yield14. Research has shown that palladium nanoparticles reduce the growth rate of lettuce seeds, as measured by shoot to root ratio15. Palladium nanoparticles also destroy the cell membrane of kiwi pollen fruits, and depletes its endogenous calcium, causing an emergence of an impaired pollen tube16.
Animals might also be exposed to palladium through contaminated soil and water sources. Studies have shown acute toxicity of palladium salts in mammals such as rats and rabbits17. Signs of toxicity included death, cardiovascular effects, biochemical changes such as changes in activity of hepatic enzymes and changes in blood parameters indicative of proteinuria or ketonuria17.
Palladium does not only affect the environment, but humans can also be affected as well. Even though palladium and its compounds have low acute oral toxicity, contact with palladium can result in symptoms of allergy including skin disorders18 and asthma14.
Recycling and Reduction of Pd
Many technologies for recycling palladium and other precious metals have been developed and implemented commercially. Pyrometallurgy processes and smelting are recycling methods that have the advantage of being able to accept scraps in various forms19. However, additional measures and costs are required to enhance metal refinement and reduce toxic emissions20. Therefore, a common alternative way is hydrometallurgy which is a mature technology that involves chemical leaching and separation to recover precious metals from solids21. Hydrometallurgy is commonly used in commercial size recycling of precious metals and is preferred over pyrometallurgy for its low emission of dust, low energy consumption and high recovery rate. However, the chemicals used in the process have raised environmental and safety concerns due to hazardous gas emissions and waste emission22.
In commercial hydrometallurgy, crushed metal wastes such as spent catalysts are leached in liquid mediums used to extract metals called lixiviants. Commonly used lixiviants include cyanide, chlorine and aqua regia. Leaching of metal waste results in a solution of different metal ions in the leaching solvent. More steps of precipitation, solvent extraction or ion exchange are needed to further separate the metals from aqueous solutions23. The metals are then separated and extracted are in compounds such as chloropalladate salts rather than in elemental state24. Therefore further reduction of the extracted Pd ions is required to obtain recycled elemental palladium.
While technologies of Pd leaching are well-developed, technologies of reduction of Pd and other metal ions are still in initial development stages and have only been applied on a laboratory scale. Studies focusing on using biotechnology such as cultures to reduce Pd(II) to Pd(0) have emerged25,26 as environmental friendly alternatives to traditional metal reduction technologies, which involve the use of strong reductants e.g. NaBH4 that require stabilizers and carrier materials to prevent aggregation of particles27 and is performed at high temperatures28. However, hydrogen is required in the reduction process using bacteria, e.g. Desulfovibrio fructosivorans and Shewanella oneidensis27, which is not very worthwhile since hydrogen should be used for greater applications. Pure cultures are highly susceptible to even small environmental changes and are difficult to maintain long term while mixed cultures have not yet been reported to reduce Pd30. Methods that can provide a continuous flow and cost-effective route for reducing Pd ions, e.g. upflow anaerobic granular sludge bed reactors33, are still being explored and evaluated. Being able to synthesize Pd nanoparticles through reduction is ideal as Pd nanoparticles are more active and require less catalyst compared to bulk materials of Pd31.
Our Aim
Witnessing such a situation, our team aims to design a peptide which has a high binding and reducing efficiency to palladium (II) ions. We believe that having a biological substitute to reduce palladium will provide more options for industries and companies to further process the leached palladium (II) compounds, thus increasing the amount of palladium ready for reuse.
References
[1] Wisniak, J., 2018. William Hyde Wollaston. The platinum group metals and other discoveries. Educación Química, 17(2), p.130.
[2] Platinum group metals (PGM); occurrence, use and recent trends in their determination
[3] Catalytic Converter (n.d.). Retrieved August 30, 2020, from http://www.chemistry.wustl.edu/~edudev/LabTutorials/CourseTutorials/Tutorials/AirQuality/CatalyticConverter.html
[4] Konda, S., & Chen, A. (2016). Palladium based nanomaterials for enhanced hydrogen spillover and storage. Materials Today, 19(2), 100-108. doi: 10.1016/j.mattod.2015.08.002
[5] Matthey, J. (n.d.). Platinum and Palladium in the Pharmaceutical Industry. Retrieved August 30, 2020, from https://www.technology.matthey.com/article/2/3/86-89/
[6] Biajoli, A. F. et. al,(2014). Recent Progress in the Use of Pd-Catalyzed C-C Cross-Coupling Reactions in the Synthesis of Pharmaceutical Compounds. Journal of the Brazilian Chemical Society. doi:10.5935/0103-5053.20140255
[7] Guide to Precious Metals. (n.d.). Retrieved August 29, 2020, from https://www.gemvara.com/Precious-Metal-Guide/pages/v/education/metals/palladium/
[8] Johnson Matthey (2020). Pgm Market Report: May 2020. Retrieved October 15, 2020, from https://matthey.com/en/news/2020/pgm-market-report-may-2020
[9] Palladium Uses, Properties, & Facts | Britannica
[10] Britannica (2019). Palladium. Retrieved October 15, 2020, from https://www.britannica.com/science/palladium-chemical-element
[11] Rising Price of Palladium Could Impact High-Reliability MLCC Markets in 2020
[12] Passive Components (2020, February 11). Rising Price of Palladium Could Impact High-Reliability MLCC Markets in 2020. Retrieved October 15, 2020, from https://passive-components.eu/rising-price-of-palladium-could-impact-high-reliability-mlcc-markets-in-2020/
[13] Yao, W., & Patterson, W. (2020, February 13). Precious metals' golden year to continue. Retrieved October 15, 2020, from https://think.ing.com/articles/co-precious-still-outshine/
[14] WHO (2002). Palladium. Retrieved October 15, 2020, from https://apps.who.int/iris/bitstream/handle/10665/42401/WHO_EHC_226.pdf?sequence=1&isAllowed=y
[15] Chen, M., Chen, S., Du, M., Tang, S., Chen, M., Wang, W., Yang, H., Chen, Q., & Chen, J. (2015). Toxic effect of palladium on embryonic development of zebrafish. Aquatic toxicology (Amsterdam, Netherlands), 159, 208–216. https://doi.org/10.1016/j.aquatox.2014.12.015
[16] Leso V, Iavicoli I. Palladium Nanoparticles: Toxicological Effects and Potential Implications for Occupational Risk Assessment. Int J Mol Sci. 2018;19(2):503. Published 2018 Feb 7. doi:10.3390/ijms19020503
[17] Shah, V.; Belozerova, I. Influence of metal nanoparticles on the soil microbial community and germination of lettuce seeds. Water Air Soil Pollut. 2009, 197, 143–148.
[18] Speranza, A.; Leopold, K.; Maier, M.; Taddei, A.R.; Scoccianti, V. Pd-nanoparticles cause increased toxicity to kiwifruit pollen compared to soluble Pd(II). Environ. Pollut. 2010, 158, 873–882.
[19] Melber C., Mangelsdorf I. (2006) Palladium Toxicity in Animals and in vitro Test Systems — An Overview. In: Zereini F., Alt F. (eds) Palladium Emissions in the Environment. Springer, Berlin, Heidelberg. https://doi.org/10.1007/3-540-29220-9_39
[20] Orion E., Wolf R. (2006) Contact Dermatitis to Palladium. In: Zereini F., Alt F. (eds) Palladium Emissions in the Environment. Springer, Berlin, Heidelberg. https://doi.org/10.1007/3-540-29220-9_36
[21] Ghosh B, Ghosh MK, Parhi P, Mukherjee PS, Mishra BK, Waste Printed Circuit Boards Recycling: An Extensive Assessment of Current Status, Journal of Cleaner Production (2015), doi: 10.1016/j.jclepro.2015.02.024.
[22] J. Cui, L. Zhang / Journal of Hazardous Materials 158 (2008) 228–256 Metallurgical recovery of metals from electronic waste: A review https://doi.org/10.1016/j.jhazmat.2008.02.001
[23] Akcil, A., et al. Precious metal recovery from waste printed circuit boards using cyanide and non-cyanide lixiviants – A review. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.01.017
[24] Recovery of precious metals from electronic waste and spent catalysts: A reviewhttps://doi.org/10.1016/j.resconrec.2018.10.041
[25] M.K. Jha et al. / Hydrometallurgy 142 (2014) 60–69 https://doi.org/10.1016/j.hydromet.2013.11.009
[26] A review of methods of separation of the platinum-group metals through their chloro-complexes https://doi.org/10.1016/j.reactfunctpolym.2005.05.011
[27] Bioreductive deposition of palladium (0) nanoparticles on Shewanella oneidensis with catalytic activity towards reductive dechlorination of polychlorinated biphenyls https://doi.org/10.1111/j.1462-2920.2005.00696.x
[28] Palladium recovery as nanoparticles by an anaerobic bacterial communityhttps://doi.org/10.1002/jctb.4064
[29] Bio-palladium: from metal recovery to catalytic applications Simon De Corte, Tom Hennebel, Bart De Gusseme, Willy Verstraete and Nico Boon* doi:10.1111/j.1751-7915.2011.00265.x
[30] Bioreductive deposition of platinum nanoparticles on the bacterium Shewanella algae Yasuhiro Konishi a,∗, Kaori Ohno a, Norizoh Saitoh a, Toshiyuki Nomura a, Shinsuke Nagamine a, Hajime Hishida b,Yoshio Takahashi c, Tomoya Uruga doi:10.1016/j.jbiotec.2006.11.014
[31] Corte, S. D., Hennebel, T., Gusseme, B. D., Verstraete, W., & Boon, N. (2011). Bio-palladium: From metal recovery to catalytic applications. Microbial Biotechnology, 5(1), 5-17. doi:10.1111/j.1751-7915.2011.00265.x
[32] Martins, M., Assunção, A., Martins, H., Matos, A. P., & Costa, M. C. (2013). Palladium recovery as nanoparticles by an anaerobic bacterial community. Journal of Chemical Technology & Biotechnology. doi:10.1002/jctb.4064
[33] Removal and recovery of palladium in a UASB reactor DOI 10.1002/jctb.4708