Efficient nitrate removal from water using an integrated photocatalyst-adsorbent based on chitosan-titanium dioxide nanocomposite
Authors: Sreek Smitha Venu , Parola Athulya , Thaimani Vimala , Pillai Harikumar Puthenveedu Sadasivan , Ramakrishnan Resmi Thoppil 
Published: September 2022

An increased discharge of nitrates to the natural water resources was observed across the globe due to various anthropogenic activities resulting in environmental pollution and associated harmful effects. In the present work, sol-gel-derived functional nanocomposites based on silver (Ag) doped titanium dioxide (TiO2) coated chitosan nanocomposites were successfully synthesized in the form of beads and their application for the removal of nitrates from the water was studied. The synthesized nanocomposite beads were further characterized for their structural, textural, and morphological features using X-ray Diffraction Analysis, Fourier Transform Infrared Spectroscopy, UV-Visible Spectroscopy, BET Surface Area analysis, Scanning Electron Microscopy, and Transmission Electron Microscopy. A uniform coating of doped titania species on the chitosan porous structure was achieved through electrostatic interaction. Adsorption/ photocatalytic reduction of nitrates was further monitored by measuring the concentration of nitrate ions in the model contaminated water in the presence of functional nanocomposite beads when subjected to an adsorption study under dark conditions and photocatalytic study under UV/sunlight for a definite time. Drying conditions of the nanocomposite beads were found to have a significant effect on the adsorption cum photocatalysis efficiencies of the nanocomposite. The freeze-dried chitosan-titania nanocomposite beads containing 0.5 mol% Ag exhibited an adsorption efficiency of ~ 43.5% (under dark for 30 min) and photocatalytic reduction capability of ~ 95% (under sunlight for 2 hours), whereas the adsorption and photocatalytic efficiencies were 40% (under dark for 30 min) and 70% (under UV light for 2 hours) respectively, in the case of oven-dried nanocomposite beads, towards the removal of nitrate ions in an aqueous solution. Continuous flow adsorption cum photocatalytic study using the oven-dried nanocomposite beads was carried out further with the help of an experimental setup fabricated in-house and under varying experimental conditions such as flow rate, bed height, and concentration of feed solution. A nitrate removal efficiency of 87.6% and an adsorption capacity of 7.9 mgg− 1were obtained for the nanocomposite beads in the continuous flow adsorption cum photocatalysis experiment for up to 8 hours when using an inlet concentration of 100 ppm, bed height 12 cm and flow rate 5.0 mlmin− 1. A representative fixed-bed column adsorption experiment using a real groundwater sample collected from the Palakkad District of Kerala was also performed using the oven-dried functional nanocomposite beads that show promising results for nitrate removal (85.9% efficiency) along with a significant removal rate for the other anions as well. Thus, the adsorption cum photocatalytic nitrate reduction ability of the synthesized functional material makes them suitable for the efficient removal of nitrates from water/wastewater through an integrated nanocomposite approach.