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Pollution

These Volatile Pollutants Can’t Fly, but Can They Hop?

Article: Josep Sanchís, Ana Cabrerizo, Cristóbal Galbán-Malagón, Damià Barceló, Marinella Farré, and Jordi Dachs. Unexpected Occurrence of Volatile Dimethylsiloxanes in Antarctic Soils, Vegetation, Phytoplankton, and Krill. Environ. Sci. Technol. Article ASAP. DOI: 10.1021/es503697t

 

Introduction

Decamethyl cyclopentasiloxane, known as D5, is normally added into personal care products (e.g. shampoo, lotion and cream) to help soften, smooth and moisten. D5 belongs to the group of cyclic volatile methyl siloxanes (cVMS), which in combination with linear volatile methyl siloxanes (lVMS), have wide applications in the manufacture of industrial and consumer products.

During manufacturing processes as well as normal use of siloxane-contaning products, these compounds can easily evaporate into the atmosphere. They were believed to be “fliers”, which means they have strong tendency to stay in the atmosphere and not to deposit onto the ground. Scientists believed that transport through the atmosphere was the only efficient way for these compounds to reach remote places such as Antarctica. Because once they arrive at Antarctic they cannot reach the ground, scientists did not expect these compounds to be detected in the Antarctic ecosystem.

Although volatile methyl siloxanes are inert, meaning they hardly react with anything, recently the European Commission claimed that octamethylcyclopentasiloxane (D4) could interfere with human hormone function. Also, this group of compounds has the potential to bioaccumulate and biomagnify. If they do manage to deposit on terrestrial or marine surfaces, they could accumulate in organisms.

 

Methods

In order to test that volatile methyl siloxanes would not accumulate in remote ecosystems, scientists collected soil, vegetation, phytoplankton and krill samples in the southern ocean near Antarctica in 2009. Sampling locations are shown in Figure 1. All sampling operations were performed using silicone-free materials to avoid any possible contamination. Back in the lab, samples were extracted by ultrasound-assisted procedures with hexane. Samples were analyzed by gas chromatograph coupled to tandem mass spectrometry (GC-MS/MS). Blanks were used to assess the contamination during the whole analytical process.

Figure 1.  Location and cyclic volatile methyl siloxanes concentrations of phytoplankton (above) and krill (below) samples.

Figure 1. Location and cyclic volatile methyl siloxanes concentrations of phytoplankton (above) and krill (below) samples.

 

Results

In general, among all the samples collected, cyclic volatile methyl siloxanes were wider distributed and at higher concentrations than linear volatile methyl siloxanes. D5 and dodecamethylcyclohexasiloxane (D6) are the two dominant cyclic volatile methyl siloxanes detected on land as shown in soil, lichens, mosses and grass samples; while hexamethylcyclotrisiloxane (D3) and D4 showed the highest concentrations in the marine environment, as shown in phytoplankton and krill samples (Figure 1).

An interesting finding is that cyclic volatile methyl siloxanes concentrations in phytoplankton samples were inversely correlated with sea surface salinity (SSS). This means that concentrations of these compounds were higher in waters that received greater inputs of freshwater from seasonal ice and snow melting. It was hypothesized that they are depositing to the Antarctic and Southern Ocean mainly during winter and are released from snowmelt in the austral summer.

This hypothesis can be used to explain the different compositions of volatile methyl siloxanes in different compartments. For example, because D3 is the least reactive with radicals in the atmosphere, it has the largest ability to travel by air. Phytoplankton samples proved this by reflecting the predominance of D3. In soils and vegetation, the relative contribution of D3 was lower than in phytoplankton. This is because D3 volatilizes faster than the other group members.

Figure 2. Snow melting may be the major input of volatile methyl siloxanes to the Antarctic ecosystem.

Figure 2. Snow melting may be the major input of volatile methyl siloxanes to the Antarctic ecosystem.

 

Significance

This study is the first to evaluate the potential of volatile methyl siloxanes to affect surface environments in Antarctic. The results bring into question the belief that these compounds are not able to reach remote terrestrial and marine surfaces. It was found that snow scavenging is the major input of volatile methyl siloxanes to Antarctic surfaces. Due to their ability to undergo atmospheric deposition, they should not be considered as “flyers” that stay put in the atmosphere, but rather as “hoppers”  that can travel between gas, liquid and particles.

The concentrations of volatile methyl siloxanes detected were relatively high, raising some concern; these compounds in the environment may be expected to continuing increasing, considering the large global consumption of these compounds. It remains to be investigated whether restrictions on the usage of volatile methyl siloxanes are necessary. Studies on potential replacement of these compounds are also underway, seeking less toxic and more eco-friendly alternatives.

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