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July 17, 2013

Biogeochemical cycling of platinum

About Platinum
Paula Johnson in her book about Platinum introduces platinum discussing its properties, occurrence, extraction and purification. Platinum is one of the many chemical elements that exist with 78 as its atomic number and Pt as its chemical symbol. Some of its properties include high density, malleability, ductility, and gray-white transition. It is also a precious metal. Platinum is among the rarest elements to be found on the earth’s crust and has an average abundance of five μg/kg. South Africa accounts for 80% of the world’s production of platinum.Platinum has a unique property of being non-reactive. This places it in 2 categories: the platinum group of elements and group 10 in the periodic table. It is known as a noble metal due to its ability to resist corrosion even under high temperatures. Consequently, it is normally found as a native element, uncombined with other chemicals.

Platinum finds its use in several appliances, such as thermometers, dentistry equipment, catalytic converters, electrical contacts and electrodes, laboratory equipment and jewelry. It is considered a highly precious and valuable metallic commodity due to its scarcity in production. Only few hundreds of tones are produced annually. However, this metal has a disadvantage. Just like other heavy metals, it causes health hazards upon its exposure to it. Luckily, it is not toxic since it does not corrode. In fact, certain compounds containing platinum, for example cisplatin, are applied during chemotherapy against various kinds of cancers.

In the platinum group of elements are six elements: Palladium, Rhodium, Platinum, ruthenium, osmium and iridium. Their symbols are Pd, Rh, Pt, Ru, Os and Ir respectively. All the six elements are rare in occurrence and hence precious. However, the elements that have commercial use are platinum, pallidum, and rhodium. These elements tend to occur as metal in their state or bonded with sulfur or any group Va or Via ligands. Their occurrence normally is as trace minerals in rocks. These elements existed in the early stages of the evolution of the solar system, planets formation and its differentiation as well as the biogeochemical cycling. Their purification was initially fulfilled in the late 1700s. At this time, some of their properties were realized, for example, their high melting point, unreactiveness, and ability to catalyze reactions. This led to their heavy industrial use in fuel production and engine emission control. The PGE has become a highly valued commodity.

Platinum Placers
As discussed earlier in this paper, platinum rarely forms compounds but occurs as native platinum in rocks where magnetic forces and processes have held it. During the early stages of magnetic processes, platinum is seen to occur as rare disseminations in deposits. When erosion takes place in such deposits, platinum placers are formed. Examples of these placers are found in Ural Mountains, Russia, Alaska, and Colombia. In terms of its scarcity, platinum placers surpass gold by almost 90% of its annual production. The platinum placers occur in different sizes ranging from small grains to large ones of about 20 pounds in weight. They find their use in jewellery and other equipments.

In 1926, platinum placers were discovered in an area south of Goodnews Bay, Southwestern Alaska. They were worked on for seven years beginning in 1927. The methods used were small-scale mining. Later, dragline excavators and dredge were put into use. Being the only commercial source of platinum in the United States as well as being of high grade, these deposits are of great importance. Recently, the district has had interest in its revival following another discovery and commercial exploitation of the platinum placers. According to a geological survey of 1937 into the area showed that the principal placers lie in two streaks, the valley floor of Salmon River and in the ancient stream channel east of this valley.

Platinum occurs as an extremely rare metal at a very low concentration of only 0.005 ppm within the earth’s crust. It is sometimes confused with silver (Ag). Since platinum is generally not reactive, it is mostly found uncombined with chemical elements but as native platinum. It is normally alloyed with iridium to obtain platiridium. More often than not, platinum is found in alluvial deposits together with other platinum group of elements. Since the time of the pre-Colombian people in the Choco department in Colombia up to date, the alluvial deposits are used as a source of platinum group of metals. Another huge alluvial deposit is found in the Ural Mountains, Russia (CRC contributors 2007–2008).

Platinum occurs mainly in South Africa and Canada as well as other former USSR countries. In Canada, extraction of deposits of a mixture of ores associated to volcanic rocks is carried out. This contains copper –nickel sulfides. Platinum and palladium occurs in equal portions in the same while gold and silver are residual. In South Africa, the deposits are located in Meresky, which is in the northwest of Johannesburg. The platinum here occurs in rocks as pyroxene in the order of 4 to 10ppm. It is always associated with copper, iron or nickel sulfides. In Norilsk, Siberia, Russia, deposits of this metal occur in minerals like peridotite. In smaller amounts, platinum can also be extracted in Colombia and in Alaska. Its abundance in the earth’s crust is about 0.01 gram per ton.

Platinum group metals occur as sulfides (PtS, pdS), tellurides (PtBiTe), arsenides (PtAs2) and antimonides (PdSb) as well as alloys with copper or nickel. Another major source of platinum in nickel ores in theSudbury Basin deposit in Ontario, Canada is the Platinum arsenide, sperrylite (PtAs2). Alaska was another source of platinum but ceased operation in 1990. About 545,000 troy ounces had been produced between 1927 and 1975.

Biogeochemical cycling of Platinum
Associated with the existence of platinum ore are certain microorganisms. These microorganisms, examples of bacteria and archae are concerned with the biogeochemical cycling of platinum. Platinum exhibits negative oxidation at surfaces trimmed down electrochemically (Ghiliane et al. 2007). They are involved in the formation of a secondary mineral, dissolution, precipitation, and mobility of platinum. While in the biosphere, various biogeochemical reactions take place that cause transformation of platinum. Under the influence of microorganisms, weathering occurs leading to the mobility and the dissolution of platinum entangled in minerals (Helmut & Sigel 2005: 304).

Following the destabilization of platinum by microorganisms, together with its precipitation and mineralization leads to formation of secondary platinum. These occur as microcrystalline as well as nano-particles. Their existence on surfaces of platinum grains indicates biogeochemical solubility as well as re- precipitation of platinum, which in turn results in its mobility in surface environments. This has been reported to be witnessed in Brazil and Australia. The process is reported to have contributed majorly to the formation of secondary platinum deposits.
Studies show that microorganisms cause various processes to take place within the platinum ore. The first being solubility through excretion of metabolites such as cyanides, Aminoacids and thiosulphates. The second event is the precipitation of platinum forming its precipitate internally and extra cellularly hence forming a secondary mineral. Thirdly, biochemical responses have been developed to deal with toxic pt complexes.

Comparison of biogeochemical cycling between gold and platinum in the surface environment

The biogeochemical cycling of gold and platinum has been discovered to be a major contributor in the formation of deposits of secondary platinum (pt) as well as gold (Au). To compare the cycling 0p of the two elements, we will refer to an experiment that was carried out in New South Wales, Australia. This was done to establish a comparison in mobility of Pt and Au as one process in the biogeochemical cycling of the two. The materials included collected soils, ground water, and nuggets containing platinum and gold. They were then analyzed using x-ray tomography, synchrontron-XRF, as well as thermodynamic modeling.

The formation of micro crystals and the presence of nano-particles of platinum or gold on the surfaces of gold or platinum grains indicate biogeochemical dissolution and precipitation within the cycling process. This promotes the mobility of Pt and Au within the surface environment. These were also supported by an X-ray tomography of embedded Pt and Au grains on deep lead materials such Fe-oxides, silicates and clays. Synchrotron XRF indicated differences in Au and Pt mobility. Data from groundwater and thermodynamic modeling showed a lower reactivity of pt compared to au, which translated to lower mobility in surface environments (Thomas & Canuel 2011).

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