Novel Iron-based Hybrid Nanoadsorbents for Removal of Arsenic from Aqueous Stream

Abstract

Contamination of water by highly noxious arsenic (As) species is a severe environmental threat that threatens human life negatively by causing adverse health complications. Nowadays, various advanced techniques such as adsorption, membrane filtration, capacitive deionization, ion exchange, etc. are developed by different researchers to obtain As-free drinking water. Adsorption of As species by iron-based nano-adsorbents is regarded as the most promising one. The high affinity of iron-based nano-materials toward As species is mainly due to their specific properties such as improved reactivity, biocompatibility, high stability, greater charge density, multi-functionality, and dispersibility nature. However, pristine iron-based nano-materials possess certain limitations. This includes the irreversible accumulation of individual nanoparticles (NPs), which may hinder their effectiveness in sorption. Therefore, the fabrication of iron-based functional nanohybrids is highly essential, which improves their stability, dispersibility, and performance. Other distinctive properties of well-dispersed functional nanohybrids are that they can be directly implemented for the in-situ elimination of As from contaminated water. In light of the foregoing concerns, the key objective of this doctoral dissertation is to investigate the synthesis, physicochemical characteristics, and applicability of various iron-based hybrid nanostructures for As adsorption. Here, we have successfully synthesized various iron-based hybrid and functional architectures such as α-Fe2O3 decorated hydroxyl functionalized porous graphitic carbon nitride (P-gCN-OH/α-Fe2O3) binary nanohybrid, L-Cysteine functionalized mesoporous magnetite (Fe3O4@Cy) nanosphere, maghemite & graphene oxide (GO) embedded polyacrylonitrile (PAN) electrospun nanofibers matrix (PAN/GO/γ-Fe2O3), and magnetite & amine-functionalized gCN embedded PAN nanofibers matrix (PAN/gCN-NH2/Fe3O4) by suitable synthetic approach. All the synthesized materials have been characterized by various instrumental characterization techniques such as X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), Vibrating-sample magnetometry (VSM), Raman, Brunauer-Emmett-Teller (BET), Thermogravimetric analysis (TGA), Universal testing instrument (UTI), Contact angle (CA), Zeta potential (η), and X-ray photoelectron spectroscopy (XPS), etc. to investigate their physicochemical characteristics. XRD and other characterization results indicate the presence of α-Fe2O3, Fe3O4, γ-Fe2O3, and Fe3O4, phases of the iron-oxide in P-gCN-OH/α-Fe2O3, Fe3O4@Cy, PAN/GO/γ-Fe2O3, and PAN/gCN-NH2/Fe3O4, respectively. FESEM and TEM results signify the formation of ultrafine spherical α-Fe2O3, γ-Fe2O3, and Fe3O4 NPs in the respective adsorbent system. UTI provides the enhanced mechanical behavior of the PAN/GO/γ-Fe2O3 and PAN/gCN-NH2/Fe3O4 nanofibers matrix with greater elastic modulus and tensile strength. From the Batch adsorption experiment, it is clear that pH is an important variable for the adsorption of As. Therefore, adsorption of As(III) occurs nearly at neutral conditions as it exists as non-ionic species under pH 2-9, while As(V) adsorption occurs at lower pH as it exists in anionic form, which facilitates the strong electrostatic attraction between positively charged adsorbent surface and negatively charged As(V). Also, the kinetics, isotherm, and thermodynamics investigation provides insight into the adsorption mechanism. The maximum adsorption capacity of P-gCN-OH/α-Fe2O3, Fe3O4@Cy, and PAN/gCN-NH2/Fe3O4 towards As(III) adsorption were found to be 22.22, 20.0, and 32.26 mg/g respectively from the Langmuir isotherm plot. Also, the maximum adsorption capacity of P-gCN-OH/α-Fe2O3, Fe3O4@Cy, PAN/GO/γ-Fe2O3, and PAN/gCN-NH2/Fe3O4 towards As(V) were found to be 27.8, 34.0, 36.1, and 33.22 mg/g respectively. From the effect of the co-ion study, it is clear that in all the cases phosphate (PO43-) is a good competitor for As adsorption, as P and As are present in the same group of the periodic table. All the adsorbents possess good regeneration ability with retention of their applicability. The proposed mechanism by XPS analysis revealed that the adsorption of As on the adsorbent surface follows a physio-chemical process. Based on the overall investigation it is demonstrated that the addition of various supporting materials such as gCN, GO, PAN, or subsequent functionalization of iron-based materials improves their stability and adsorption efficiency as compared to the pristine one. In the future study, we will apply these nanostructures to analyze real water samples and design an engineering prototype for industrial and domestic water filtration.