Design and Synthesis of Polymeric Nanocomposite Materials for Efficient Removal of Cr(VI) from Aqueous Solutions

Abstract

Water pollution has become a significant global issue due to the increased presence of heavy metal ions and various pollutants in aquatic systems giving a serious threat to living organisms. Cr(VI) is highly toxic among heavy metal pollutants which affects millions of people worldwide. Even minimal exposure to Cr(VI) above permissible limit can cause a number of diseases which include nausea, diarrhea, respiratory disorders, DNA mutations and genetic information damage, which can lead to malignant tumors and failure of kidney. So, the remediation of Cr(VI) is a main focus for the researchers. This thesis presents a detailed studies on the synthesis of novel polymeric nanocomposite materials designed for the adsorptive sequestration of hexavalent chromium. Four number of novel polymeric nanocomposites mainly polypyrrole zirconium phosphate, polyaniline zirconium tungstophosphate, polyaniline yttrium phosphate and polypyrrole modified layer double hydroxides are synthesized by incorporating pyrrole, aniline as an organic component and zirconium phosphate, zirconium tungstophosphate, yttrium phosphate, layer double hydroxides as inorganic components. The assessment of the structural, morphological, textural functional, and thermal stability of the synthesized materials is conducted using various analytical techniques like XRD, TEM, FESEM, EDX, Raman spectroscopy, TGA-DTA, FTIR, N2 sorption isotherms, Zeta potential measurements, XPS and other instrumental analyses. A polypyrrole modified zirconium phosphate (PPY-ZrPO4) nanocomposite is synthesized by in situ oxidative polymerization process and used to remove hexavalent chromium from synthetic solution. The pseudo-second-order kinetic is the best fitted model for Cr(VI) adsorption with maximum adsorption capacity of 62.5 mg g-1 following the Langmuir isotherm model. The spontaneity, endothermic nature and feasibility of this adsorption process is confirmed from thermodynamic data. The adsorption with partial reduction of Cr(VI) to Cr(III) are the basic mechanism being followed in this adsorption process. The surface functional group of -N= and -NH- are responsible for Cr(VI) reduction. A novel nanocomposite of polyaniline zirconium tungstophosphate (PANI-ZrWPO4) is synthesized following an in situ oxidative polymerization reaction and subsequently used to remove Cr(VI) from aqueous solution. The TEM and FESEM images revealed that polyaniline are decorated on the surface of the ZrWPO4. The experimental data are best fitted to Langmuir isotherm model as compared to other model with a maximum uptake capacity of 71.4 mg g-1. The XPS spectra confirmed the adsorption and partial Cr(VI) reduction to Cr(III) through in situ chemical reduction. The regeneration efficiency of PANI-ZrWPO4 is able to remove around 80% of Cr(VI) even after five cycles. Polyaniline-modified yttrium phosphate (PANI-YPO4) nanocomposites is synthesized following oxidative polymerization process in in-situ condition and used to remove Cr(VI) from both wastewater as well as aqueous solutions. Morphological analysis by FESEM and TEM revealed a flower-like structure of the nanocomposite. This adsorption is governed by the kinetics models of pseudo-second-order and Langmuir isotherm model having an uptake capacity of 91.0 mg g-1. The adsorption mechanism involves the electrostatic attraction among anions of Cr(VI) and the PANI-YPO4 nanocomposite, accompanied by the in situ chemical reduction of hexavalent chromium to less toxic trivalent chromium. The nanocomposite shows successful regeneration and reuse up to five cycles with 85% removal efficiency. Furthermore, the PANI-YPO4 nanocomposite exhibited impressive adsorption efficiency of 99% for real chromium wastewater solutions containing Cr(VI). The NiFe-layer double hydroxide is prepared following hydrothermal process and subsequently synthesized polypyrrole modified NiFe-layer double hydroxide through in situ oxidative polymerization process and used to remove Cr(VI) by batch mode experiment. The experimental data are best fitted to pseudo second order kinetics model as compared to other kinetics model and Langmuir isotherm model having a maximum uptake capacity of 96.7 mg g-1. The spontaneity and endothermic nature are evidenced by the thermodynamic study. The adsorption mechanism is due to adsorption, reduction, ion exchange in the region of interlayer of nanocomposite, hydrogen bonding on the surface and electrostatic interaction. The material shows more than 85% regeneration ability even after five cycles.