Chemical tweak makes seawater-soluble plastic potentially more biodegradable
Wageningen researcher Julian Engelhardt has further developed and chemically refined a salt-responsive plastic during his PhD research. The material, which dissolves in seawater, now contains a built-in chemical structure designed to enhance its biodegradability. This could help prevent discarded plastic from accumulating in the oceans and harming marine life.
Conventional plastics are strong, cheap and ubiquitous. When they end up in the environment, they can remain there for tens to hundreds of years. During his PhD, Engelhardt worked on previously developed saloplastics: plastics that break down into individual polymer chains when exposed to salt water. By using alternative building blocks, he succeeded in incorporating oxygen atoms into the molecular backbone of the material. This chemical architecture is designed to ensure that, after breaking down, the plastic biodegrades. The aim is explicitly not to dispose more plastic in the sea, but to provide a safety net for material that ends up there despite recycling efforts.
Ester groups make biodegradation more likely
Engelhardt captured the highlight of his PhD on film. "I placed two beakers side by side. One contained plain water, the other artificial seawater: table salt dissolved in water. I submerged a piece of my plastic in each beaker. After ten minutes, the plastic in the seawater had already partially dissolved, while the piece in ordinary water remained unchanged. After half an hour, there was no trace of the plastic left in the salt water." For the young researcher, it was the first proof that his salopolymers performed exactly as intended: dissolving in seawater. "I celebrated with a large coffee," he says with a grin. "It was a huge relief."
The fact that salt acts as the trigger is no coincidence. A large proportion of the world’s plastic waste ends up in the oceans: around 11 million tonnes each year. Partly because of their traditional carbon-based backbone, these plastics degrade either very slowly or not at all. This should be different for the new salopolymers. Even so, it is too early to describe the material as fully biodegradable. The fact that Engelhardt’s salopolymers dissolve in seawater has been clearly demonstrated. The PhD researcher also confirmed the presence of the chemical bonds containing oxygen atoms he introduced into the material, known as ester groups. These molecular bonds are known for their biodegradability by water and micro-organisms. Similar ester groups, for example, make polylactide (PLA) biodegradable. Engelhardt therefore considers further degradation likely. “But we still need to prove it,” he says.
From coral reef restoration to agricultural films
For now, the most promising applications for salopolymers do not lie in shopping bags or packaging. The material remains too sensitive to humidity for such uses. Instead, Engelhardt sees greater potential in agricultural films or even niche applications where contact with water is desirable. One example is coral reef restoration. "We could embed young corals in a salopolymer matrix. The structure would initially provide support in the ocean and then gradually disappear. In the end, only the coral would remain, without any residual adhesives or plastic waste."
The technology is still at an early stage. During his PhD, Engelhardt produced only a few dozen grams of material. He believes scaling up should be technically feasible, as the required feedstocks can come from existing biomass streams. The biggest challenge is likely to be cost. "Giving plastic building blocks an electrical charge and making them responsive to salt requires additional chemical processing," he explains. Each step requires solvents, purification processes and specialised reagents. As a result, the material must compete with a petrochemical industry that has been producing plastics on a massive scale for decades. Despite these challenges, Engelhardt sees considerable potential in developing plastics that are not only useful during their lifetime, but also know when their job is done.
Julian Engelhardt performed his PhD at Organic Chemistry, Physical Chemistry and Soft matter, and WFBR at Wageningen University & Research. He defended his thesis on 22 May 2026.
