[Trifluoroacetic acid-modulated synthesis of defective UiO-66 for highly efficient removal of sodium p-perfluorous nonenoxybenzenesulfonate].

Abstract

Per- and polyfluoroalkyl substances （PFAS）， widely known as "forever chemicals"， present a severe and escalating global threat to ecological systems and human health. Their extreme persistence originates from remarkably strong carbon-fluorine bonds， while their amphiphilic nature facilitates widespread environmental dispersal and bioaccumulation. Among them， sodium p-perfluorous nonenoxybenzenesulfonate （OBS） has been extensively used as a commercial alternative to perfluorooctanesulfonic acid （PFOS）， which is under strict global regulation. However， mounting evidence has revealed that OBS exhibits higher bioaccumulation， higher environmental persistence， and significant cytotoxicity， raising urgent environmental concerns and creating an urgent demand for efficient remediation technologies. To tackle this challenge， this study proposes a defect-engineering strategy for the design of advanced adsorbent materials. Specifically， a series of trifluoroacetic acid （TFA）-modulated UiO-66-type metal-organic frameworks （MOFs）， denoted as U-TFA-10 M， UN-TFA-10 M etc.， were successfully synthesized via a solvothermal method. The intentional introduction of structural defects was designed to increase the density of active sites and tailor surface properties， thereby achieving enhanced pollutant capture performance. Comprehensive characterization confirmed the successful material synthesis and modification. Scanning electron microscopy （SEM） and transmission electron microscopy （TEM） images revealed that U-TFA-10 M and UN-TFA-10 M particles exhibited irregular， fragmented morphologies， a stark contrast to the regular octahedral shape of pristine UiO-66， visually indicating abundant structural defects. X-ray diffraction （XRD） patterns displayed a new broad peak in the 2θ range of 3°-5°， further confirming defect formation. Fourier-transform infrared spectroscopy （FT-IR） identified characteristic vibrations： peaks at 556 cm^-1 and 482 cm^-1 corresponded to Zr-O coordination， while a distinct peak at 1 212 cm^-1 confirmed the presence of -CF₃ groups. X-ray photoelectron spectroscopy （XPS） provided definitive evidence， with a clear F 1s signal verifying the successful incorporation of trifluoromethyl moieties into the MOF framework-a critical modification for enabling specific interactions with the target pollutant. Systematic adsorption evaluation and optimization were conducted. The optimal synthesis condition was identified as using 10 mol/L TFA as the modulator. The pH of the solution profoundly influenced adsorption， with maximum OBS removal achieved at pH 3， where the protonated adsorbent surface exerted strong electrostatic attraction towards anionic OBS molecules. Notably， adsorption efficiency remained stable across a range of ionic strengths （Cl^- mass concentration 0-100 mg/L）， indicating strong resistance to interference from common salts， a vital advantage for practical application. Adsorption equilibrium data fitted the Langmuir isotherm model （R² of 0.999 9 for U-TFA-10 M and 0.999 5 for UN-TFA-10 M）， suggesting monolayer adsorption on homogeneous active sites. The maximum adsorption capacities were 684.9 mg/g for U-TFA-10 M and 602.4 mg/g for UN-TFA-10 M. Kinetic analysis followed the pseudo-second-order model （R² of 0.999 9 for U-TFA-10 M and 0.998 8 for UN-TFA-10 M）， implying chemisorption as the rate-limiting step. The materials also exhibited excellent regenerability and reusability， maintaining high efficiency after repeated adsorption-desorption cycles. Mechanistic studies revealed that the superior performance stems from a synergistic combination of multiple interactions： （1） pore-size matching for physical entrapment； （2） electrostatic attraction between the protonated surface and the OBS sulfonate group； （3） acid-base interaction between unsaturated metal sites （Lewis acids） and the sulfonate group； （4） specific fluorine-fluorine （F-F） interaction between the -CF₃ groups and the OBS perfluorinated chain， enhancing selectivity； and （5） hydrophobic interaction promoting partitioning from the aqueous phase. Compared with other reported OBS adsorbents， U-TFA-10 M and UN-TFA-10 M combined ultrahigh capacity and fast kinetics， with a simple and scalable synthesis route， showing great practical potential. In summary， this work successfully developed U-TFA-10 M and UN-TFA-10 M as efficient， stable， and easily prepared adsorbents for OBS removal. It also validated defect engineering coupled with -CF₃ functionalization as a powerful material design strategy. The mechanistic insights provide a valuable theoretical foundation for developing next-generation materials for the separation and remediation of persistent aquatic pollutants.