Comparative study of hydrothermal-assisted Schiff base crosslinked and grafted chitosan-microalgae-Kronos composites for the removal of Brilliant Green dye

The constant depletion of water resources on earth has been a growing concern for the continuous existence of humans and other life forms on the planet. Due this the urgency of the current situation around the globe the United Nations has included this area of research in the 17 Sustainable Development Goals (SDGs) pertaining the 17 global goals addressing the most pressing challenges the world is facing and promoting a more sustainable future for all (sixth goal: Ensure availability and sustainable management of water and sanitation for all). Meanwhile, textile industries are one of the major contributors to the pollution of global water resources with the discharge of effluents containing various chemicals and dyes into natural water resources. Majority of dyes used in industries are synthetic and organic dyes due to their lower cost, however the complex chemical structure of these dyes makes them resistant to biodegradation. Hence, in this research the feasibility of the employment and efficiency of dye removal (brilliant green (BG) dye) by four different chemically crosslinked (crosslinking agents used: glyoxal and benzil) or grafted (grafting agents used: salicylaldehyde and benzaldehyde) chitosan-microalgae-kronos (TiO2). The physicochemical properties of the four synthesised adsorbents have been investigated using various analytical techniques (FT-IR analysis, XRD analysis, FE-SEM imaging with EDX analysis, pHpzc determination analysis, and elemental (CHNS/O) analysis) and the impact of the main adsorption parameters (adsorbent dose, pH and time) on the dye removal of each adsorbent was determined. Box-Behnken Design from Response Surface Methodology was used for design of experiments and parameter optimisation and the optimised parameter values were used to investigate the adsorption equilibrium behaviour. The pseudo-first order and pseudo-second order kinetic models’ parameters for each adsorbent (for the removal of BG dye) were calculated to determine the rate determining factor in the adsorption systems. Furthermore, the adaptability of the adsorption equilibrium data with Langmuir, Freundlich and Temkin isotherm models was investigated to better understand the adsorption process of BG removal by the four synthesised adsorbents. Moreover, the feasibility and spontaneity of the adsorption process was investigated by calculating the thermodynamic parameters for the BG removal process by the adsorbents. Additionally, based on the obtained data from all analyses a plausible adsorption mechanism has been proposed for each adsorbent. Finally, the results of the analyses for all adsorbents have been compared to determine the better approach (crosslinking or grafting) to achieve a more efficient adsorption system for the removal of BG dye. The obtained results revealed that amongst the four modified adsorbents in this study, the salicylaldehyde-grafted adsorbent has the highest efficiency (qmax=957mg/g) towards the removal of the target pollutant (BG dye), while the glyoxal-crosslinked adsorbent, despite yielding the lowest dye removal capacity (qmax=88.3mg/g), exhibits efficient removal in acidic pH values (highest removal achieved at pH=4) which can prove efficient for the removal of pollutants in acidic environments without the need for pH adjustments.