Synthesis and characterization of NiZnAl-LDH/PVA and NiS-ZnS/LDH/PVA nanocomposites for environmental applications

Abstract

Nickel–zinc–aluminum-based layered double hydroxide (NiZnAl-LDH) has been synthesized in the presence of polyvinyl alcohol (PVA) via the in situ composite formation (co-formation) method. The obtained materials were characterized by scanning electron microscopy, X-ray diffractometer (XRD), UV–visible and Fourier-transform infrared spectrometer. NiS-ZnS nanoparticles were synthesized based on the obtained NiZnAl-LDH/PVA nanocomposite by the successive ionic layer adsorption and reaction (SILAR) method. The effects of SILAR cycles, reaction parameters, and the metal ion ratio on particle size, crystal formation and structure have been extensively investigated. Some binary sulfide phases were observed in the XRD pattern of NiS–ZnS/LDH/PVA nanocomposite. The peak broadening of the metal sulfide phase is attributed to structural defects in the main lattice. The maximum absorbance band of the NiS–ZnS/LDH/PVA (Ni:Zn = 1:1) nanocomposite increased significantly in the UV–Vis spectrum because of the enhanced light-harvesting ability. It also shows incredibly low Urbach energy (E_U = 0.37 eV), which is a characteristic of disordered semiconductors. The sorption and photocatalytic degradation performance of the synthesized NiZnAl-LDH/PVA and NiS-ZnS-doped LDH/PVA nanocomposites were studied using various model pollutants under UV-C, visible light (150 W tungsten lamp), and solar irradiation. Rhodamine 6G was selected as a cationic, Ponceau 4R as an anionic, Patent Blue V as an anionic–zwitterionic dye, and 2,4-dichlorophenoxyacetic acid as a phenoxy herbicide. The sorption efficiency of NiZnAl-LDH/PVA nanocomposite is significantly high with anionic dye (97.9%) and incredibly low with cationic dyes. The sorption of cationic dyes increased significantly after sulfidation of LDH, which can be explained by three facts: the changes in surface activity, an increase in available sorption sites, and surface charge. Langmuir constant value (K_L = 0.01 L/mg) of the sorption of 2.4 D onto NiZnAl-LDH/PVA is close to the Langmuir–Hinshelwood constant (K_L–H = 0.009), which combines adsorption equilibrium with surface reaction rate. The cumulative drug release activity of the obtained nanocomposites on the 2,4D herbicide increased from 5.3% to 16.73% during the period of 0.5–60 h.