Surfactant modified natural zeolites as adsorbents for contaminants of emerging concern: Non-Steroidal Anti-Inflammatory drugs

Abstract

In this study, composites of two zeolite-rich rocks were modified with cationic surfactants cetylpyridinium chloride (containing one hydrophobic tail) and Arquad® 2HT-75 (having two hydrophobic tails) and tested for the adsorption of contaminants of emerging concern – diclofenac, ibuprofen, naproxen, and ketoprofen. Modifying a clinoptilolite- and phillipsite-rich tuffs with different amounts of selected surfactants resulted in the preparation of composites with monolayer or bilayer of each surfactant at zeolitic surfaces. Starting materials and prepared composites were characterized by ATR–FTIR and STA coupled with EGA. The point of the zero charge was determined for starting materials and composites, providing the information of materials surface properties and their buffering capacities. Zeta potential measurements confirmed that the negative charge of the natural zeolites was compensated and close to zero when surfactants formed monolayers at the zeolitic surface or reversed to positive when bilayers of surfactants are present at the zeolitic surface. Extensive washing of the composites and repeated zeta potential measurements have shown that surfactants were unstable at the zeolite surface, especially in bilayer formation. However, composites with bilayers prepared using the two-tailed surfactant Arquad® 2HT-75 had excellent stability. These results suggested that SMNZs could be tailored for safe applications for water treatment purposes. The capabilities of the prepared sorbents to adsorb four drugs were estimated by the determination of adsorption isotherms and kinetic runs. Maximum adsorption capacities, obtained from the Langmuir model, were in the range from 1-35 mg/g, where the best results were achieved with composites containing bilayers of surfactants at the zeolitic surfaces. Physicochemical properties (pKa and log Kow) of investigated drugs helped explain the mechanism of their adsorption by SMNZs. Between the two used zeolite-rich tuffs, composites prepared using clinoptilolite-rich tuff had the better performance. As far as the two surfactants are considered, composites with cetylpyridinium chloride gave better results. Using the best performing adsorbent, drug adsorption isotherms were conducted in the natural water collected from Grindstone creek (Columbia, Missouri, USA) to have a more realistic system. Consequently, a slight decrease in diclofenac and ibuprofen adsorption was observed, while adsorption of NAP and KET was quite similar. Kinetic runs were performed in distilled water as well as in the presence of inorganic ions (sulfates and bicarbonates), and these interfering agents have caused an adsorption decrease for bilayer composites. For comparison reasons, kinetic runs for each drug were performed using activated carbon under similar experimental conditions. ATR-FTIR analysis and zeta potential measurements were performed on composites after adsorption of drugs, confirming the presence of drugs at zeolitic surfaces without any change of structures of composites.