Microplastics from ship paints – “an underestimated source?

Before the study, did you have the concrete “suspicion” that components of marine paints could be responsible for a significant proportion of microplastics?

Barbara M. Scholz-Böttcher: There have been related publications whose results indicated that ship paint or paint particles play a conspicuous role in the microplastic spectrum recorded. In particular, studies of lakes and harbour areas with high shipping frequency, should be mentioned here. We, too, have detected particles of alkyd paints in sediments of the Warnow River in a former survey.

Completely surprising for us in our new study was to find unusual and high plastic related signals far out at sea, in very offshore areas. These differed greatly from those of water samples examined reflecting typical packaging plastics such as polyethylene, polypropylene or PET. Polymers that characterise the microplastic composition in close vicinity to the coast.

This unexpected composition of the microplastic fraction in the water samples we studied prompted us to look for potential sources outside of typical plastic waste. In doing so, we came across the plastic-based binders of marine paints as a possible and plausible interpretive approach.

Your study is the first to provide an overview of the mass distribution of microplastics in the southeastern North Sea. Why did you choose this approach?

Scholz-Böttcher: There are currently two common methods to quantify microplastics in environmental samples. One is the particle-based analysis, which uses spectroscopic methods. These data are of particular interest for ecotoxicological considerations. The smaller particles become, the more relevant they are for uptake by organisms.

But particle shapes are highly variable, have different size distributions. The smaller particles become, the more they are. Accordingly, this results in an enormous range of variation in counting numbers in-between similar samples. Comparing data on a particle basis is incredibly difficult. The relative distribution of plastic types based on particle abundances does not reflect their actual mass quantity distribution.

In mass quantitative analysis, it is not of interest how big the microplastic particles are or what they look like, but they are analysed as the total mass of a particular plastic type. This makes it possible to compare their loads and the respective distribution patterns of samples from very different locations on a special and a temporal scale.

The observation of the mass has the advantage of a strong simplification, as it is related to the plastic type only. The resulting distributions observed in samples can then be compared. These distribution patterns were also the reason for the hypothesis suggested in our publication: for certain samples we observed unexpected  plastic patterns dominated e.g. by polyacrylates.

Both analytical approaches mentioned are important in the analysis of microplastics and complement each other fundamentally. We speak here of complementary analytics.

Which measurement methods did you use and what exactly did you find out?

Scholz-Böttcher: The method used is called pyrolysis-gas chromatography-mass spectrometry coupling and is a thermal process. We decompose the sample and its containing plastics in the absence of oxygen at high temperatures. In this process, the plastics are decomposed into characteristic fragments depending on their respective polymer type. We have determined indicator fragments of each polymer in advance, which allow a direct reference. This allows us to assign the individual plastics even in complex samples. The intensities of these signals can then be used for quantification.

Plastics cannot be distinguished in detail

So far, we have included ten relevant types of plastics in our mass quantification, determined as pure plastics, e.g. polystyrene. It is important for me to emphasise that we have no way to distinguish plastic add mixtures or the way the polymers were applied in detail. We cannot say, for example, “this is an acrylate that has been used as a paint, or a polystyrene from insulation material”. We can only speak of the occurrence of a respective “plastic cluster”, e.g. that we have found the polystyrene or polymethyl methacrylate cluster.

This cluster merge plastics that release the same indicator molecules. In this way, quantities can be compared at the end. However, we are talking about microplastics here, which, since the underlying particles are extremely small, can no longer be assigned 1:1 to a source. The resulted signal might relate to paint particles, but it does not necessarily have to. We can only derive and interpret the composition of the plastic load via the totality of the distribution patterns by looking for causal and plausible relationships.

What kind of components are we talking about?

Scholz-Böttcher: In conspicuous proximity to shipping routes, we observed particularly high proportions of PMMA clusters, which can be linked to polyacrylate-containing plastics or binders. High proportions of PVC (polyvinyl chloride) clusters also puzzled us at first. But a lot of chlorinated rubbers are used in protective coatings (in swimming pools, in the construction sector, but also for ships) because a high corrosion protection is achieved. We found the polycarbonate cluster, which includes epoxy resins, in comparatively small quantities, but also consistently in these samples.

However, coatings are far too complex and heterogeneous to be quantified solely based on their potential binder signal, but these polymer binder makes their removal a component of microplastics.

Are the proportion and composition of concern?

Scholz-Böttcher: There has been a long discussion about the pollution of the marine environment by anti-fouling, especially with a focus on toxic metal ions  involved or,  organo-tin compounds. If we now detect coatings as microplastics, this is essentially only the other side of the same coin to highlight this contamination. But there is a functionality associated with marine paints that consists of a combination of different ingredients and active substances. Their respective detection is a direct indicator that there is a corresponding contamination. Here we have a combination statement, a complementary indication that these coatings are entering the environment and this in a relevant, detectable quantity. As far as the marine microplastic mass load is concerned, its possible ship paint related share, which plays a decisive role for the marine environment, has not yet been made so clear.

These particles are small vectors, they are bio-available, they are leachable, they contain the entire range of active substances of the paints, and this is deliberately taken into account.

What conclusions do you draw from your study results and what do you hope to achieve with them?

“Important: to make the extent clear”

Scholz-Böttcher: First of all, it was important for us to clarify the potential extent to which marine paints contribute to the microplastic load in the ocean. The total concentrations are in the ppt to ppb range, which are not large, but easily detectable amounts. Marine coatings are a source of microplastics, analogous to tyre abrasion on land, where the release of particles is deliberately accepted to ensure functionality. Shipping is a form of transport that will certainly increase in the future. In order to reduce or eliminate this source of contamination for microplastics and the pollutants they contain in the long term, alternative protective coatings should be developed promptly.

It is important for us to focus on the fact that not only packaging waste is an obvious source of microplastics in the environment. Far-sighted creativity is needed for coatings and tyre abrasion as well. What kind of alternatives are there, how can paints be modified to reduce abrasion and thus this form of pollution?

The next analytical step for us will be to examine paints even more systematically in order to be able to support our hypothesis even better. There are still many open questions to be answered.

Why did you concentrate on the southeastern North Sea and are you planning or already conducting comparative studies at other locations?

Scholz-Böttcher: We are also active in other projects. One of them, for example, deals with sources, sinks and the transport of microplastics in the course of the river Weser. In another project, diverse sampling of water, air and sediments up to the northern tip of Norway is planned. This will allow us to compare, condense and refine the data and get a more consistent picture on various aspects including the “skid mark of ships”.