School Email Exchange
What is the significance of selenium found in the plastics? Where is it coming from?
Question submitted by The Packer Collegiate Institute
Hi, Kristen Mitchell here to answer your selenium question. First let me thank you for your excellent questions. I will do my best to answer them.
Selenium was discovered by two Swedish scientists, J.J. Berzelius and J.G. Gahn, in 1817. The two researchers analyzed a residual slime that was formed during the oxidation of sulfur dioxide from copper pyrites. The residue was initially thought to be tellurium, named from the Latin, tellus, meaning Earth. Tellurium had been discovered approximately 35 years before. However, upon further investigation it proved to be a unique element, and was named after the Greek lunar deity, Selene. Selenium is one of the rarest trace elements in the Earth’s crust with an average abundance of 0.05 ppm (parts per million). It is also unevenly distributed on the surface of the Earth.
It is true that selenium is used in electronics. It is a good semi-conductor and is also light-sensitive, and has been used in everything from photocopiers to solar panels. It is also true that selenium is an essential micronutrient. It is toxic, so it can also be consumed in excess and the difference between not enough and too much is quite narrow. Selenium deficiency in animals becomes apparent at levels between 0.05 and 0.10 mg/kg and toxicity effects occur at levels exceeding 5 to 15 mg/kg. It turns out that nearly every form of life needs selenium, from bacteria all the way up to humans. Selenium is a main part of the 21st amino acid, selenocysteine which was first discovered in 1976. This was also the first amino acid that was found to use a stop codon, UGA, to make selenocysteine. This was the first time that a codon was found to have two functions. Selenocysteine is essential for the formation of over 20 selenoproteins. Selenoproteins generally function as antioxidants.
My research focuses generally on the marine selenium cycle. So I study the inputs to the oceans, the outputs from the oceans and how selenium moves through different ‘compartments’ or reservoirs in the oceans including the atmosphere, surface oceans, deep oceans and sediments. Selenium is found naturally in the oceans, with inputs from rivers, volcanoes, some hydrothermal inputs and some atmospheric inputs. I have found in my research that phytoplankton play a key role in moving selenium through the compartments of the ocean.
Selenium in the surface ocean occurs at extremely low concentrations (<1 nM or <0.08 ppb) and can be variable depending on where you are. Phytoplankton take up selenium and concentrate it to higher levels usually around 5 ppb. When they die the phytoplankton sink through the water column, where some of the selenium is released back to the water column and the rest is transported to the sediments as organic selenium. Once in the sediments selenium accumulates to possibly even higher concentrations depending on the amount of phytoplankton and other materials buried. Over millions of years these rocks are tectonically shifted and can end up on continents, and they become areas of relatively high selenium concentration on land, which is generally as low and variable as it is in the oceans. This is where my interest in plastics began.
Plastics tend to be colonized by biofilms which could also concentrate selenium from the seawater, thus making it less available for phytoplankton to take up. This could seriously disrupt the natural selenium cycle depending on the amount of plastic in the oceans. I am currently only sampling what we are calling macroplastics, larger pieces of plastic, because I need quite a bit of biofilm/algae sample for the selenium concentration measurements I will be doing once I get back to my lab. This is because while the selenium is more concentrated it is still quite a small amount to measure. It is likely that there are biofilms on some of the small pieces of plastic that could also be concentrating selenium but this will have to be done in future research. There is another interesting aspect to selenium and plastics. Elemental selenium is red and is sometimes used as a colorant in red and orange plastics. So as these red and orange plastics breakdown they could actually become a source of selenium. This is also a possibility for future research.
Kristen Mitchell scrapes biofilm off a buoy that was just brought aboard. She will test the sample for selenium back in her lab in Canada.
My PhD research focused on using selenium isotopes as a proxy for biogeochemical environments in geologic history. We found that most of the rocks that we analyzed had the isotopic signature of phytoplankton, no matter how old they were, and we tested rocks as old as 3.5 billion years old. To my knowledge selenium concentration and selenium isotopes have not been used to trace food up the food chain but it is definitely a possibility.
As for running out of selenium, that will likely not happen. One of the reasons this may have been said is that selenium does not occur in ores that are easily mined, which makes it hard to collect for use in electronics. It is generally collected in its pure form, as elemental selenium, as a byproduct of other industrial activities, like copper smelting, coal burning and sulfur mining.