Development of adsorbents based on phosphate-containing hyper-cross-linked polymers for selective removal of tetracycline from water : unveiling the role of phosphate groups in adsorption

The design of adsorbents capable of selectively removing antibiotic pollutants from environmentally relevant water matrices is one of the most emerging environmental issues. This study fits this research trend and presents a series of phosphate-containing hyper-cross-linked polymers (P-HCPs), synthesized by crosslinking of two aromatic building blocks: 4,4′-bis(chloromethyl)-1,1′-biphenyl (BCMB) and diphenyl phosphate (DPP), as promising adsorbents for the elimination of tetracycline (TC) from water. The primary goal of the study was to unravel the role of BCMB and DPP molar ratio selected during the polymer synthesis in controlling the physicochemical properties and adsorption capacity ($q_{e}$) of the resulting materials. A significant part of the study also covers assessing the adsorption mechanism and kinetics. It was found that the most efficient P-HCP exhibited ca. 1.5 times higher adsorption capacity than the polymer without phosphate species ($q_{e}$ = 330 vs. 216 mg/g, respectively), despite its remarkably lower BET surface area. This optimal phosphate-containing polymer was also highly selective in the elimination of tetracycline from complex water matrices, even at trace concentration of the TC antibiotic (50 µg/L) and in the presence of significantly higher concentrations of competing cations and naturally occuring organic matter in water samples. Analysis of the adsorption mechanism revealed that the enhanced adsorption efficiency of best-performing material resulted from the optimal compromise between its surface area and loading of phosphate species, which were recognized as the main adsorption sites owing to ionic interactions and/or hydrogen bonding with the antibiotic.