Nanoparticles to Remove Coral Bleaching Oxybenzone

Sunblocks contain a number of different compounds including oxybenzone which acts as a UV filter. The skeletal structural formula of oxybenzone is shown below:

Oxybenzone is soluble in water.
Before you go snorkeling in the Great Barrier Reef to be amazed by the beautiful corals, fascinating fish and other exciting wonders, you smother yourself in sunblock. When you step into the water, the oxybenzone starts to dissolve. Unfortunately, oxybenzone contributes to coral bleaching, the killing off of the tiny, colourful zooxanthellae marine algae that live inside corals. The result is that the coral loses its colour and appears white, as if it has been bleached.

Researchers have found a way to soak up the oxybenzone from the seawater using magnetite nanoparticles.

Magnetite, Fe₃O₄ , is a mineral made up of iron(II) and iron(III) oxides and is one of the main iron ores, that is, magnetite is mined in order to produce iron. Magnetite is ferromagnetic, that is, it is attracted to a magnet. It is the most magnetic naturally occurring mineral on Earth. If you could get the oxybenzone in the seawater to attach to magnetite nanoparticles then you could pull the oxybenzone out of the water using a magnet.

First, the researchers coated the magnetite nanoparticles with sodium oleate. The skeletal structural formula of sodium oleate is shown below:

Next, they oxidised the oleate coating to increase the number of hydroxyl (OH) functional groups:

Since oxybenzone can interact with other molecules via hydrogen bonds, magnetite nanoparticles  covered in a coating rich with hydroxyl functional groups increases the interactions between oxybenzone and the nanoparticles. Once the oxybenzone has hydrogen bonded to the nanoparticle coating, a magnet can be used to extract the particles from water.

Does it work?
One brave researcher applied sunblock, stepped into the ocean, waited 10 minutes, then collected a sample of the surrounding seawater. Back at the lab, chromatography was used to determine the concentration of oxybenzone in the water, 1.3 ppm. This is a disturbing result since it is known that the concentration needed to bleach coral is measured in parts per billion.
Next, the researchers prepared seawater samples. Some had no magnetite nanoparticles added, others had the nanoparticles added. Then they added 30 ppm oxybenzone to  all the samples. The concentration of oxybenzone in the samples with no nanoparticles did not change in an hour. In the samples that contained the nanoparticles, 95% of the  oxybenzone  was removed within the hour.

Reference
American Chemical Society. "Sopping up sunblock from oceans to save coral reefs." ScienceDaily. ScienceDaily, 21 August 2017.

Further Reading
Solutions Concepts
Water as a Solvent
Transition Metals (magnetism)
Fatty Acids
Carboxylic Acids
Nanoparticles and Nanotechnology
Parts per Million (ppm)
Chromatography
Experimental Design
Variables
2-Dimensional Structural Formula
Skeletal Structural Formula
Molecular Formula

Suggested Study Questions

1. For a molecule of oxybenzone:
• draw the 2-dimensional structural formula
• give the molecular formula
2. On the 2-dimensional structural formula of oxybenzone identify and name each functional group present.
3. Use diagrams to explain why oxybenzone is soluble in water.
4. Draw the 2-dimensional structural formula for oleic acid.
5. On your structural formula of oleic acid, identify and name the functional group(s).
6. Suggest a method by which you could change oleic acid into sodium oleate in the laboratory.
7. Suggest a method by which you could oxidise sodium oleate in the laboratory.
8. Explain the term "nanoparticle".
9. Why do you think the researchers chose nanoparticles of magnetite rather than bulk magnetite for this research?
10. Consider the description of the experiment used to determine the effectiveness of the magnetite nanoparticles in removing oxybenzone from seawater:
• What was the hypothesis being tested?
• What was the aim of the experiment?
• What variables need to be considered in this experiment?
• What is the independent variable in the experiment?
• What is the dependent variable in the experiment?
• Which variables are constant variables in the experiment?
• Why did the experimenters add nanoparticles to some samples but not to others?
• Write out a suitable method for this experiment.