Development of Chitosan/Graphene oxide/nano-silver Hydrogel for Photocatalytic Dye Degradation and Antibacterial Action

Abstract

Water pollution, due to human activities has emerged as one of the biggest challenge in 21st century creating an inevitable hazard for environment and human health. Majority of water contamination occurs due to the discharge of industrial waste from textile, leather tannery, paper and pulp containing a copious amount of colored dye which is released directly into the water resources creating an ecological imbalance. Being complex in nature, the dye molecules are difficult to degrade and therefore persist over a long duration in water systems resulting in severe health hazards. It is unfortunate that the existing treatments (physical, chemical, and biological) or technologies have few limitations for being universally employed in water treatment purposes, which in turn restrict their applicability for complete removal of dye pollutants from water sources. Therefore, there is an utmost need to develop some effective, cheap and eco-friendly methods for dye removal and minimizing the ecotoxicological implications. In recent past, nano-driven technologies have provided a few promising solutions for using nanomaterials in water purification applications and enhancing the environmental sustainability. In the present study, a biogenically-derived polymeric nanocomposite of nano-silver/chitosan/graphene oxide hydrogel was synthesized in order to evaluate its catalytic performance to degrade methylene blue (MB) as a model dye under sunlight illumination. A porous hydrogel nanocomposite with semi-interpenetrating network of chitosan (CS) and graphene oxide (GO) was fabricated, which acted as a nanoreactor for in-situ synthesis of silver nanoparticles (AgNPs) and their subsequent immobilization. The hydrogel nanocomposites were characterized using electron microscopy which indicated a stable formation of nanocomposite with immobilized AgNPs onto polymeric networks. The synthesized hydrogels showed remarkable swelling capacities with swelling ratio of 83.17% and 80.23% for GO-CS and GO-CS-Ag respectively. Experiment conducted under sunlight (natural source) with GO-CS hydrogel showed a 99.7% degradation of MB (initial concentration, 40 ppm) within 5 hrs. The introduction of silver into gel although delayed its degradation efficiency, a dye degradation of 98.7% was still achieved after 7 hrs of its exposure to sunlight. Modelling experiments indicated that dye removal follows second order model kinetics using these nanocomposites. The CS/GO hydrogels (pristine and Ag-loaded) were also examined for its disinfection property against Bacillus subtilis (Gram- positive) and E. coli (Gram-negative). The antibacterial studies done under liquid culture and solid media indicated that the pure GO-CS hydrogels could not demonstrate any significant antibacterial effect. On the contrary, nano-silver immobilized GO-CS hydrogel showed an unprecedented biocidal action where a complete disinfection (intial bacterial count, 103-105 CFU/ml) was achieved even after 60 minutes and 180 minutes against Bacillus subtilis and E.coli strains respectively. This study therefore provides a novel tool to use hydrogel based system for minimizing water contamination with dual performance attributed its photocatalytic dye removal activity and antibacterial action.