On the interactions of ions and odor molecules with nanomaterials in water and synthetic urine

Abstract

Towards the quest of finding materials to be used as adsorbents in real systems, it is crucial to understand the underlying adsorption processesfully. In many applications, adsorption occurs in biological fluids containing a vast range of ions, organic molecules, and biomolecules. Therefore, performing adsorption experiments in ultrapure water, as is the custom in most studies, does not accurately represent the adsorbent-adsorbate interactions.

The main aim of thesis work is to investigate what adsorbent properties are relevant for the adsorption of odor molecules in urine. The model system contains the environmentally and economically friendly porous material activated carbon, whose surface can be readily modified with acid and metal alkoxides to provide new material properties. As the adsorbate, p-cresol is used, a common odor molecule in urine. Synthetic urine, containing the most prevalent ions and molecules in urine and with a constant pH of 6.0, is used as the solvent.

Further steps were taken to investigate the behavior of ions and surfaces in the aqueous environment by studying specific ion effects on silica nanoparticles and crystalline nanocellulose through extensive experimental work. In addition, a molecular dynamics study of the adsorption of p-cresol on graphene and graphene oxide sheets in synthetic urine was performed to provide more information on the relevant interactions on an atomistic scale.

Our results clearly show that ions have a profound effect on both the properties of the adsorbent and the adsorption behavior in synthetic urine. Counter-ions adsorb at the surface of the negatively charged materials to a varying degree according to the extent of their hydration layer, with the trend following the direct Hofmeister series of cations. Although this effect is well-known for the silica surface, we also showed the same type of behavior on crystalline nanocellulose. However, at alkaline pH, the trend was reversed and the Li+ adsorbs more strongly to the surface than the less hydrated cations. We also proved that, for the negative silica surface, there is a significant co-ion effect on the surface charge and gelation rate of the nanoparticles. This was attributed to the formation of ion pairs in water, and this effect appears to be enhanced near charged interfaces.

For the adsorption of p-cresol, we used principal component analysis, a type of multivariate statistics, to derive information on the optimal adsorbent properties. The analyses revealed that a large surface area and a hydrophobic surface are beneficial for p-cresol adsorption in both water and synthetic urine. Surprisingly, the adsorption of p-cresol was enhanced in synthetic urine for the most hydrophobic activated carbon, a result of the salting out effect, which decreases the solubility of organic molecules in the presence of strongly hydrated ions ("structure makers"). The synthetic urine contains sodium, magnesium, calcium, and urea, all considered highly hydrated and, therefore, structure making.

Metal-organic frameworks (MOFs) are a group of highly porous materials formed from metal ions and linker molecules forming extensive networks. Their large surface area makes them promising contenders as adsorbents, which is why we chose two of the more economically and environmentally friendly specimens to use for odor capture. The solvothermal synthesis route is the most common way to synthesize MOFs. Two of the most studied examples are MOF-235(Fe) and MIL-101(Fe), both consisting of Fe(III) ions and terephthalic acid linkers. Despite being synthesized in different solvent compositions and temperatures, the formation of both phases in the same product has frequently been observed in the literature. By testing various combinations of synthesis conditions, i.e., reagent ratios, solvent ratios, equilibration times, and vessel types, we discovered that MOF-235 formation is promoted with larger DMF:ethanol solvent ratios. The product with the largest surface area (~ 3300 m2/g), containing MIL-101, was obtained in the so-called ex-situ synthesis using a larger reagent volume (150 mL) in a regular glass bottle. It was believed that the pressure release and cool down during the sampling changed the reagent environment so that no MOF-235 could be formed.