Reduction of Chromium Using Polyaniline Based Composite Membrane In A Photocatalytic Membrane Reactor

Abstract

The presence of heavy metals in the aquatic environment has become a severe threat to the ecosystem. This work focuses on the remediation of heavy metal Chromium present in the aqueous solution using a visible-light-driven photocatalytic membrane reactor via simultaneous photoreduction and separation. The hydrothermal method was used to synthesize TiO2-based photocatalysts such as TiO2-WO3 and TiO2-SnO2. The photocatalytic membranes were prepared either by blending the photocatalyst nanoparticle before casting the composite membranes or by coating the photocatalytic nanoparticles on the membrane by physical method to obtain hierarchical membranes. Polyaniline was selected as the membrane material due to its interesting properties such as permselectivity, better electrical tunability, higher conductivity, economic viability, environmental stability, excellent tunable properties, and redox behaviour. Polyaniline (PANI) is also a very good adsorbent of Cr(VI) in acidic pH, and we could observe that the adsorption of Cr(VI) on the PANI membrane follows Freundlich and Jovanovic’s multilayer adsorption isotherm, which indicates that the adsorption occurs by both physisorption and chemisorption. PANI/TiO2 nanocomposite offered better adsorption and reduction of Cr(VI) to Cr(III) in acidic pH under visible light. The addition of nanoparticles in the membrane can impart synergistic effects such as improved hydrophilicity and permeability. The maximum performance of 98.50 % was observed in the nanofiltration membrane separation process operated for 60 min using 5 wt% TiO2-WO3 nanoparticle loading in the PANI membrane. The hierarchical membrane allows the immobilization of nanoparticles to facilitate catalyst recovery. TiO2-based nanosheets, nanotubes, and nano-thorns have shown higher surface adhering capacity to allow a stable film formation. In this work, we could successfully synthesize TiO2-WO3 nanocubes-coated hierarchical PANI membranes. The membrane ‘M1’ (50 mg loaded) showed better photocatalytic efficiency (removal/separation) of around 79.30 % in a custom-made photocatalytic membrane reactor. The hierarchical assembly also offered self-cleaning ability, and the coating on the membrane prevented the PANI emeraldine salt membrane from over-oxidation.