Thursday, May 19, 2016

Nano-zinc oxide and the Environment

Increasingly, we are making use of nanoparticles because of their unique properties compared to the same substance in bulk material. Many cosmetics, including sunscreens and sunblocks, now contain nanoparticles. When you go swimming or wash, these nanoparticles are washed off. Depending on where the nanoparticles are washed off, the waste water may directly enter a natural water system such as a river or ocean, it may end up in sewerage sludge, and it may eventually end up on land. What scientists do not know is just how many nanoparticles are entering the earth, air and water.

It is estimated that carbon nanotubes, which form part of a composite material in objects such as bicycle frames and tennis rackets, can take 10 years to breakdown and be released into the environment. On the other hand, about half of the cosmetic nanoparticles enter our waste water within one year.

Europe currently produces about 39,000 tons of nano-titanium dioxide per year, and it is estimated that the concentration of these nanoparticles in effected areas is now 61 micrograms per kilogram of ground. For humans, the maximum "safe" levels for exposure to these nanoparticles is set at:

  • 2,500 mg/kg/day for oral exposure
  • 2.4 mg/m3 for inhalation
While small amounts of zinc oxide are beneficial to plant growth, larger amounts can impair seed germination. Plants take up the free zinc ions in aqueous solution rather than the zinc oxide particles. This zinc becomes incorporated into the plants we eat. Zinc is an essential element in the human diet. The recommended dietary allowance of zinc for men is 11 mg/day, and for women is 8 mg/day. There are concerns that the increasing level of zinc in  plants may lead to accumulation of zinc in humans which will be detrimental to our health. Ingesting more than about 100 mg of zinc per day may lead to chronic toxicity.

Research into the environmental impact of nanoparticles, and their impact on plant and animal health, will continue for a long time.


Further reading
Graphene and Fullerenes:
Solutions Concepts:
Weight percent (w/w):
Parts per MIllion (ppm):

Suggested Study Questions:

  1. What is meant by the term "nanoparticle"?
  2. If a nanoparticle of zinc oxide has a diameter of 20 nm, what is its diameter in:
    • metres
    • centimetres
    • millimetres
    • micrometres
  3. Give an example of one property of bulk zinc oxide that is different to nanoparticles of zinc oxide.
  4. Explain why zinc oxide nanoparticles are used in sunscreens.
  5. What is a carbon nanotube?
  6. Why are carbon nanotubes used in the production of bicycle frames?
  7. Why are concentrations of titanium dioxide nanoparticles in soil given in units of micorgrams per kilogram of soil rather than in moles per litre?
  8. Convert the following concentrations into parts per million (ppm)
    • 2,500 mg kg-1
    • 2.4 mg m-3
  9. Using the recommended dietary allowance figures in the article, determine the mass in grams of zinc allowed for a:
    • 58 kg woman each day
    • 79 kg man each day
  10. A typical vitamin pill contains 25 mg of zinc. By consuming 1 tablet per day, will the man or woman above exceed the recommended daily allowance of zinc?
  11. 6 raw oysters contain 32 mg of zinc. How many oysters can the man and woman above eat before exceeding the recommended dietary allowance of zinc?
  12. 85 g of cooked beef contains 7 mg of zinc. What mass of beef can the man and woman above ingest before exceeding the recommended dietary allowance of zinc.
  13. 28 g of dry roasted cashews contain 1.6 mg of zinc. What mass of zinc, in grams, is present in 750 g bag of cashews?
  14. 1/2 cup of cooked red kidney beans contain 0.9 mg of zinc. How many cups of red kidney beans would our man and woman above need to consume in order to achieve their recomended dietary allowance of zinc?
  15. Do you think you should take a daily vitamin pill containing zinc? Justify your answer.

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