Water-soluble (liquid, dissolved) Polymers
Many studies on polymers in the environment focus on microplastics, which include solid plastic particles and fibers. So far, water-soluble polymers and their effects on the environment and organisms have received little attention.
Since the water solubility of an organic chemical compound, such as polymers, enables long-range transport in aquatic ecosystems, water-soluble polymers are increasingly found in water and soil samples. Just like microplastics, they act as anthropogenic stressors, are only broken down very slowly in the environment and affect organisms and ecosystems worldwide.
How can the multitude of different water-soluble polymers be grouped
Synthetic water-soluble polymers can be divided into four groups. This includes:
- Non-ionic homopolymers and
- Hydrophobically associating polymers.
Polyelectrolytes are polymers that have charged groups. They are subdivided into polycations (positively charged), polyanions (negatively charged) or amphoters (zwitterions, positively and negatively charged). The polyelectrolytes include proteins, but also numerous synthetically produced additives for thickening cosmetics. These include, for example, polyacrylate (Cabomer), polyquaternium 6 and polyvinylpyrrolidone (PVP).
The group of water-soluble nonionic homopolymers include, e.g., polyethylene oxides, poly-N-vinylpyrrolidones, polyvinyl alcohols and polyacrylamides.
Synthetic water-soluble polymers are now an indispensable part of many everyday products.
They are added to many cosmetics, hairsprays, and lotions (collectively as personal care and cosmetic products; PCCPs). In the medical field, they serve both as film material for sustained-release capsules and as binders for medical materials and as active ingredient components.
In papermaking, polymers are added as auxiliaries so that the white pigments are finely and evenly distributed on the cellulose fibers. Another application of soluble polymers is their use to improve the wearability of textile fibers in the finishing process.
Polymers are added to detergents to prevent limescale build-up on textiles and heating rods in washing machines. The paint and lacquer industries use water-soluble polymers as pigment dispersants and rheology modifiers.
Water-soluble polymers are also used as flocculants or flocculation aids in water treatment and wastewater treatment. In agriculture, they help to increase the effectiveness of pesticides and fertilizers, which reduces the consumption of resources.
Another application of the water-soluble polymers is in the construction industry, e.g., as an aid for concrete to achieve the highest possible degree of compaction.
How do soluble polymers work?
Polyvinyl alcohol (PVAL or PVOH) is one of the most widely used water-soluble polymers. It is used because of its layer-forming, emulsifying and adhesive properties. It also has high tensile strength and flexibility. Due to its hydrophilic properties, PVAL can absorb water, which acts as a plasticizer. As the water content increases, PVAL loses its tensile strength and thereby gains elasticity.
Polyacrylamide (PAM) is another widely used water-soluble polymer. In low concentrations it can increase the viscosity of aqueous solutions, in high concentrations it forms solid gels.
How do water-soluble polymers get into the environment?
Just like insoluble polymers - microplastics - water-soluble polymers are also released directly into the environment: via plastic waste (e.g., shampoo bottles that have not been completely emptied), peelings and additives in cosmetics and household cleaners or detergents that are introduced into the water cycle when swimming or washing, for example, and are transported within it. In the wastetwater treatment plant, water-soluble polymers are partially extracted from the wastewater, the remainder is released into the environment with the treated wastewater.
Indirect entry paths can be caused by rain events that wash the polymer particles from coatings and other products. In addition, the soluble polymers removed from the wastewater in the sewage treatment plant end up in the sewage sludge and on agricultural fields when this is used there as fertilizer.
As with microplastics, the wastewater treatment plant can be seen as an important entry path for soluble polymers into the water cycle and the environment.
In contrast to microplastics, there are no coherent data on the distribution of soluble polymers in the various environmental compartments (water - air - soil); their fate is largely unknown. Since they are dissolved in water, easily attach to other substances in water and are mostly not biodegradable, it can be assumed that soluble polymers also represent a global environmental problem with high health risks.
Water-soluble polymers and the analytical problem
As in the field of microplastic detection, the analysis of soluble polymers in environmental samples is in its infancy. There is currently no generally accepted method for the detection and characterization of water-soluble polymers. However, some methods of detecting polyethylene glycols (PEG), a polymer used in various industries, can be found.
What about the data situation on the ecotoxicological relevance of soluble polymers?
There is hardly any information about the (eco) toxicological effects of water-soluble polymers, as they depend on the monomers of which a polymer is made, as well as on environmental parameters, concentration, and sensitivity of the exposed organism. PEG has been examined for toxicity to Chlorella sp. and L. aequinoctialis.
Neither species showed any significant effects even when exposed to high concentrations (≤ 12,000 mg L-1) of the water-soluble polymer. In contrast, the use of PEG resulted in the death of the descendants of M. macleayi.
When used as a coating for active ingredients or in combination with other chemicals (detergents, waxes, personal care, and cosmetic products), soluble polymers that get into the environment can contain particles of active ingredients that adhere chemically to them. With water, the polymers and toxins that are bound to them enter the body of aquatic and terrestrial organisms with unsafe consequences.
There is currently little transparent and comparable scientifically usable data available on soluble polymers and microplastics. In the future, it is important to harmonize the data. Standardized analysis is essential for this. It is also essential to advance investigations into the risk potential of synthetic polymers, on the one hand to determine the role of the polymers as transport vector and matrix and on the other hand to quantify the risk potential of the polymer itself.