Abstract
Composite materials of BEA zeolite and polyaniline (PANI) were prepared by the chemical oxidative polymerization of aniline in the presence of zeolite in water (without added acid) and in an aqueous solution of sulfuric acid, using ammonium peroxydisulfate as an oxidant. Protonated (as-synthesized) and deprotonated forms of the composites and pristine PANIs were characterized by scanning electron microscopy, conductivity and zeta potential measurements, FTIR, Raman and XPS spectroscopies, and thermogravimetric analysis. Adsorption properties of synthesized materials for removal of nicosulfuron pesticide from aqueous solutions were studied, using HPLC technique. The obtained adsorption isotherms were analyzed using Freundlich and Langmuir-Freundlich equations. Protonated PANI/BEA composites showed excellent adsorption capacity (18.4–25.4 mg g−1), that was higher than the adsorption capacity of pristine BEA zeolite (18.2 mg g−1) but slightly less than neat PANI. Among PANIs, the highest adsorption capacity of 29.8 mg g−1 of adsorbent was found for protonated PANI prepared in sulfuric acid solution. Analysis of adsorption isotherms revealed high degree of surface homogeneity for all prepared composite materials and PANIs. Proposed mechanism for enhanced adsorption of nicosulfuron on protonated composites is based on hydrogen bonding of nicosulfuron O- and N-containing groups with bridging hydroxyls of BEA zeolite and –NH/ = NH+/−NH•+ groups in protonated emeraldine salt form of PANI chains (PANI-ES), accompanied with electrostatic attractive interaction between anionic nicosulfuron species and positive = NH+/−NH•+ groups in bipolaron/polaron containing structures of PANI-ES. Presence of protons in bridging hydroxyls in BEA zeolite and in protonated PANI-ES chains is essential for excellent adsorption of nicosulfuron via hydrogen bonding on all protonated composite samples. In support of this interpretation, deprotonated PANI/BEA composites and deprotonated PANIs showed significantly lower adsorption capacities (in the range 5.5–13.0 mg g−1) compared to those of their protonated counterparts.
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