Adsorption research: Where are we headed?

What do cigarette butts, used coffee grounds, and banana peels have in common? According to the scientific literature, all of them could make great adsorbents for water treatment. Adsorption as a research topic has been booming for the last 20 years: the number of scientific publications annually is increasing exponentially.
Porous material on a petri dish

However, a conventional adsorption research article can be very formulaic at its worst. You take material X, which can sometimes be rather trivial (like the mentioned cigarette butts), and apply it to separate component Y from water. Frequently, the conditions in these experiments are highly idealized, which decreases their practical relevance. There is also criticism expressed against some commonly used mathematical models in the adsorption studies (see an example here). Consequently, many water-focused scientific journals have started to almost categorically refuse articles about adsorption. Has this research area become a bubble?

In our everyday life, we are frequently, and often without knowing, relying on adsorption technology. In the natural aqueous environment, adsorption on mineral surfaces is an important phenomenon affecting the material cycles of nutrients, for instance. In drinking water treatment, for example in Oulu, surface water is commonly treated by pumping it through a bed of activated carbon to adsorb organic compounds affecting taste or color. This is very much proven technology as it has been in use for over 100 years and has improved our quality of life significantly. However, at the same time, activated carbon is considered expensive due to its regeneration (i.e., restoring the adsorption capacity after the material has been used), which is conducted with a thermal process.

In the research of adsorption materials, the Holy Grail would be to develop a material that is at the same time highly selective, effective, and inexpensive. Sounds simple? In fact, many scientific articles claim that the developed adsorbents are “green” or “low-cost”, but such terms are often thrown loosely without conducting any sustainability or economic analysis. Due to the immense publication volumes, there are also some errors, for example in the used equations, that propagate from one article to another via citation. As a result, a new genre of scientific articles has emerged, which focuses on criticizing the mistakes of other adsorption studies (see examples here and here).

All that being said, the need for efficient and affordable adsorbents really exists. There are more than 100 000 synthetic compounds registered in the EU of which many can be detected in various water resources thanks to the improvements in analytical chemistry. Micro- and nanoplastics are ubiquitously present in the environment. Heavy metals (a.k.a., potentially toxic elements) can pollute water, in many cases via the dissolution of naturally occurring minerals in ground water. There is also a need to recover valuable components from wastewaters, such as nutrients. In all these examples, adsorption technology may play an important role and other types of adsorbents than just conventional activated carbon are needed.

Interestingly, there is an obvious discrepancy between the volume of adsorption articles and the number of materials that end up being commercially available. In fact, only a handful of adsorbents are widely used on an industrial scale. This question, the challenges of productizing a new adsorbent, is being addressed in a collaboration between the Fibre and Particle Engineering and Industrial Engineering and Management research units at the University of Oulu. In the development of new adsorbents, the circular economy also plays an instrumental role. Activated carbon, too, is often prepared of industrial side streams, such as coconut shell. Within the Instreams community, geopolymers and alkali-activated materials are being actively studied as adsorbents. They can utilize several aluminum- and silicon-rich industrial side streams as their raw materials.

To conclude, more focus should be given to avoid the pitfalls of common mistakes, consider a bit more practical feasibility, and to actually understand the theoretical underpinning of each mathematical model used to describe the adsorption phenomena. Let´s make adsorption research plausible again!


Tero Luukkonen
Associate Professor
Fibre and Particle Engineering
University of Oulu

Tero Luukkonen is an Assistant Professor at the Fibre and Particle Engineering Research Unit.