Competitive Adsorption of 4-Hydroxybenzaldehyde and Toluene onto High Silica Zeolites

Abstract To evaluate the ability of zeolites to remove natural low molecular organic substances, p-hydroxybenzaldehyde (p-HBA), a phenolic compound derived from lignin, was chosen as a representative of naturally occurring dissolved organic substances. Two distinct high-silica zeolite materials, namely ZSM-5 and Y, were utilised for the study, and the adsorption process was investigated under a wide range of conditions. It has been observed that p-HBA is adsorbed by both zeolites, and the pH significantly impacts the adsorption of p-HBA, particularly within the low concentration range, while exerting minimal influence on the saturation capacity. For both zeolites, various isotherm models were assessed to accurately describe the adsorption data obtained from aqueous solutions of p-HBA. In addition, to comprehend the selectivity of the adsorbents towards natural organic substances and xenobiotics, the competitive adsorption of mixtures of p-HBA and toluene (TOL) was investigated. The zeolite’s saturation capacity for p-HBA diminishes with increasing contaminant concentration. Conversely, the adsorption of toluene remains minimally affected by p-HBA, and it has been demonstrated that toluene can displace adsorbed p-HBA from the zeolites’ sites. This finding has been confirmed by diffractometric study that indicates that TOL and p-HBA occupy “the same” adsorption sites. Furthermore, Rietveld refinements reveal the formation of p-HBA complexes interacting with the framework and stabilising the guest structures within the zeolite porosity. The results obtained are important for the selection of proper adsorbent for the removal of hydrocarbons in environmental application (natural waters).