Modified biopolymers for removal of organics dyes from aqueous solution

Abstract

An extensive search for a highly efficient, reusable, and non-toxic adsorbent materials for the removal of organic dyes from wastewater continues to be of great importance to the world. Activated carbon is the most widely used adsorbent material for treating dye contaminants from water owing to its high removal capacity and large surface area. However, activated carbon is expensive and not easy to regenerate. Hence, the use of biodegradable, non-toxic, and cost-effective biopolymer-based hydrogel adsorbents has attracted great attention. These adsorbents have high swelling capacity and number of adsorptive functional groups to allow adsorption of methylene blue dye. Hence in this work, we present carboxymethyl cellulose crosslinked with poly (acrylic acid) incorporated with magnetic cloisite 30B clay (CMC-cl-pAA/Fe3O4-C30B) and sodium alginate crosslinked with poly (acrylic acid) incorporated with zinc oxide (SA-cl pAA/ZnO) hydrogel nanocomposites (HNCs) for the removal of methylene blue from aqueous solution. The hydrogel nanocomposites were synthesised through in situ free radical polymerisation. The structural properties of the prepared materials were studied using Fourier transform infrared (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and dynamic mechanical analysis (DMA). The FTIR and XRD confirmed the successful synthesis of the CMC-cl-pAA and SA-cl-pAA hydrogels, Fe3O4-C30B and ZnO nanoparticles (NPs) and their hydrogel nanocomposites. Furthermore, the co-existence of the metal oxide nanoparticles in the CMC-cl-pAA and SA-cl-pAA hydrogel matrices was confirmed by XRD. The SEM revealed that upon the incorporation of the Fe3O4- C30B NPs onto CMC-cl-pAA, the resulting material showed spherical particles of the magnetite nanoparticles on the irregular shaped hydrogel structure. As well as on the SA-cl-pAA after modification by ZnO nanoparticles, the spherical ZnO particles were embedded on the hydrogel surface. The successful modification with metal oxide nanoparticles was also confirmed by the presence of characteristic elements of the incorporated materials on the EDS. The TEM coupled with selected area electron diffraction (SAED) confirmed the presence of Fe3O4-C30B on the hydrogel structure, in which circular bright dotted lines were observed corresponding to light diffracted by the lattice planes of different energies on the Fe3O4 structure. The thermogravimetric analysis was conducted to study the thermal stability of the materials, the results showed that the incorporation of Fe3O4-C30B and ZnO nanoparticles on CMC-cl-pAA and SA-cl-pAA hydrogels, respectively improved their thermal stability. Furthermore, DMA was used to study the mechanical stability of the prepared hydrogels and their composites. In the case of CMC-cl-pAA hydrogel, the storage modulus of CMC-cl pAA/Fe3O4-C30B nanocomposite was higher than of the hydrogel, indicating improved mechanical stability, and on SA-cl-pAA hydrogel the storage modulus decreased, indicating a decrease in mechanical stability on the SA-cl-pAA/ZnO HNC. Consequently, the swelling studies revealed that the SA/AA/ZnO HNC was highly efficient for water uptake in comparison to SA/AA hydrogel. Whereas, CMC-cl pAA/Fe3O4-C30B had lower swelling capacity than CMC-cl-pAA hydrogel. Various factors influencing the adsorption of adsorbents were systematically investigated. The kinetics, isotherms, and thermodynamics of adsorption were examined, and results showed that equilibrium data fitted the Langmuir isotherm model, and the adsorption kinetics of MB followed pseudo-second-order model in both the CMC-based HNC and SA-based HNC. Maximum adsorption capacities of 1129 and 1529.6 mg/g were achieved for SA/AA hydrogel and SA/AA/ZnO HNC, respectively, in 0.25 g/L MB solution at pH 6.0 within 40 min. Whereas maximum capacities of 1165 mg/g (pH 5) and 806.51 mg/g (pH 7) for CMC-cl-pAA hydrogel and CMC-cl-pAA/Fe3O4- C30B HNC, respectively. Thermodynamic parameters for SA/AA and CMC-cl-pAA hydrogels exhibited exothermic adsorption processes and their nanocomposites SA/AA/ZnO and CMC-cl-pAA/Fe3O4-C30B exhibited endothermic nature of the adsorption processes, respectively. Moreover, the CMC-cl-pAA/Fe3O4-C30B NCH showed improved mechanical and thermal properties as compared to CMC-cl-pAA hydrogel. In contrast, the SA/AA/ZnO HNC presented outstanding reusability with relatively better adsorption efficiencies than SA/AA hydrogel.