Synthesis and characterization of multifunctional porous nanocomposites for the removal of pollutants from water

Abstract

The emerging problem of water pollution (a persistent challenge worldwide) is leading researchers to find a convenient cheap, and efficient solution for water purification. The presence of toxic metal ions, organic pollutants, and bacteria in water leads to damage to the ecosystem. The industrial revolution and advances in technology have led to many new developments, but this also means that water pollution is on the rise from industrial waste. 1 For example, industrial battery waste does not get recycled and contaminates water reservoirs via landfill waste, which plays an instrumental role in water pollution. Battery waste is primarily the origin of heavy and toxic metal ions, such as Cd+2, Ni+2, Pb+2, Hg+2, Co+2, Cu+2, and Li+ , and others, which contribute to the problem of water pollution. Ideas have been proposed to build novel hierarchically ordered porous nanocomposites with distinctive properties and applications. 2 However, such nanocomposites have not been utilized due to their low surface area, poor ion-exchange properties, and low hydrothermal stability. In this study, crystalline zeolite ZSM-5 (particle sizes 100-150 nm) nanoparticles were used as building blocks to develop hierarchically ordered porous magnetic nanocomposites. These nanocomposites were characterized with a high surface area (327 m2 /g), a crystalline (~83%) wall structure, high ion-exchange efficiency, and superparamagnetism [Ms = 33 emu/g, mr/ms (squareness) = 0.02]. These nanocomposites were used for the removal of toxic metal ions in an easy, one-step magnetic separation technique for re-usability of the nanocomposites. The magnetic nanocomposites were further employed to incorporate titanium dioxide and silver nanoparticles to introduce two additional properties, i.e. photocatalytic and antimicrobial properties, for the removal of organic pollutants and the inhibition of bacterial infections in water, respectively. Similarly, polymer-based commercial resins (cationic: Amberlite and iv Amberjet; anionic: Ambersep) were used to develop organic-inorganic hybrid nanocomposites by introducing iron oxide (superparamagnetic properties) and titanium dioxide (photocatalytic properties) nanoparticles in order to develop multifunctional applications for the nanocomposites. The ion-exchange capacity of hybrid nanocomposites was tested alongside novel hierarchically ordered porous nanocomposites for the selective removal of specific toxic metal ions from water. The reduction of As (V) and As (III) ions was evaluated as around 100% and 95%, respectively, from a concentration of 1000 ppm in water. Hierarchically ordered porous alumina-silicate (HOPAS) nanocomposites exhibited higher efficiency in comparison to commercial resins. Hierarchically ordered porous alumina-titano-silicate (HOPATS) nanocomposites were also developed for the first time as novel nanocomposites with similar characteristics (macropore sizes 400-500 nm with interconnecting windows, crystalline wall, BET surface area value of around 218 m2 /g,) to HOPAS. HOPATS nanocomposites were further used for the selective oxidation of cyclooctene to epoxide (cyclooctene oxides, around 19% selectivity), using hydrogen peroxide as an oxidizing agent. The ion-exchange efficiency of these nanocomposites was extensively studied for removing Cd+2, Pb+2, and Ni+2 ions from water. In addition, the other cations, such as Cu+2, Co+2, Zn+2, Hg+2, and Li+ , were also tested. The removal efficiency of these ions using the HOPAS nanocomposite was found to be around 100%, which was slightly higher than hybrid inorganic-organic nanocomposites (HIONC) and nanocrystalline pure ZSM-5. Titanium incorporated materials such as Ti-HOPAS, magnetic Ti-HOPAS, and modified hybrid resins (Ambersep and Amberjet) were tested for the degradation of methylene blue, which resulted in a reduction of the initial concentration of 3.5 μM to 0.45, 0.41, 0.34, and 0 μM, respectively. Thus, HOPAS and other modified hybrid forms of resins were successfully applied to multiple applications, hence defining these nanocomposites as ‘multifunctional’.