Competitive adsorption of VOCs and humic monomers from water on high-silica zeolites

We studied the adsorption of caffeic acid (CA) and p-hydroxybenzaldehyde (p-HBA) on hydrophobic synthetic zeolites. Adsorption is contributed by formation of H-bond between adsorbates and adsorbent and it occurs with the inclusion of water molecules. The adsorption experiments were carried out at different pH and they indicate that dissociation greatly affected the sorption due to electrostatic repulsion of anionic form of the adsorbates and the surface of the adsorbent. The effect of CA and p-HBA on the adsorption of toluene (TOL) onto zeolites was also investigated. Experimental results, reveal that both caffeic acid and para-hydroxybenzaldheyde are highly adsorbed on zeolites. The host-guest-interactions were highlighted by combined FTIR and Rietveld analysis, in agreement with Density Functional Theory calculations of the adduct stabilization energies. However, competitive adsorption of mixtures of TOL and CA or p-HBA on zeolites shows that aromatic hydrocarbons are preferentially adsorbed. The findings provide new understanding of the interactions between ionizable organic compounds and zeolites which has implications for fate of ionizable pollutants and in the removal of organic pollutants from water in presence of natural organic matter.