Understanding adsorption in water systems
Adsorption occurs when molecules adhere or bond to a surface, creating a thin film. The type of bonding depends on the adsorbent molecule and substrate, but it typically occurs due to physical or chemical forces, or electrostatic attraction. Adsorption of organic molecules to surfaces such as membrane-based water purification systems can impair the membranes’ efficiency and encourage contamination with microbes. To limit this in aquatic systems, the EU-funded ADSORPLAYERS (Adsorbed layers of natural organic macromolecules on solid substrates: Structure, interactions, and mechanisms of growth) project aimed to determine how large organic molecules known as polymers adsorb to different surfaces. ADSORPLAYERS first looked at a well-known mechanism called electrostatic attraction, in which charged polymers called polyelectrolytes bond to a surface with an opposite charge. The researchers found this to be the explanation as to how chitosan, a positively-charged sugar-based polymer with many industrial and therapeutic uses, adsorbs to negatively-charged silica substrates. Intriguingly, researchers discovered that polystyrene sulfonate, a negatively-charged polyelectrolyte with several technical or medical uses, also adsorbs to a negatively-charged silica surface in the presence of multivalent counterions. This is therefore an alternative way in which polyelectrolytes can adsorb to like-charged surfaces. A final mechanism probed by ADSORPLAYERS involved adsorption in the absence of electrostatic attraction. In this case, molecules spontaneously adsorbed to surfaces under conditions in which attractive nonelectrostatic forces overcame the electrostatic barrier to adsorption. These conditions can be achieved by adjusting the solution chemistry once these mechanisms are known ADSORPLAYERS used this knowledge to further elucidate how environmentally relevant processes are affected by adsorption. These include interaction of humic-based organic matter with water-purification systems, and iron-based nanoparticles designed to decontaminate aquifers. Research deliverables have been published in several high-profile peer-review journals. Application of the results promises to increase the efficiency of technologies that will deliver safe water.
