COD and toxicity reduction of wastewater using a hybrid advanced oxidation process of sonication with chitosan-based hydrogel beads

This study reports the treatment of industrial wastewater (WW) with a hybrid advanced oxidation process (AOP) that uses sonication in the presence of floatable chitosan-based hydrogel beads. These hydrogel beads were synthesized with Fe₃O₄ decorated activated charcoal nanoparticles (Fe₃O₄@AC nanocomposites) as filler material. The hydrogel beads (Fe₃O₄@AC@CH) served dual purpose as adsorbents and heterogeneous Fenton reagents. Statistical experimental design was used to optimize the hybrid AOP. At optimal conditions (0.75 M H₂O₂, 1 g/L Fe₃O₄@AC@CH beads, pH 5.12), a COD removal of 96.12 % and TOC removal of 78.14 % was achieved in 1 h treatment. Several control experiments were performed concurrently to identify synergistic interactions in the hybrid AOP. The surface and porous structure of the hydrogels absorbed substantial amounts of pollutants. Fenton reactions occurring on the hydrogel beads' surface produced radicals ${}^{•}OH$ and ${\mathrm{HO}}_2^{•}$ that successfully degraded and mineralized the pollutants. Sonication induced intense micro-mixing in the medium, enhancing mass transfer between bulk medium and surface/pores of hydrogel beads. The toxicity of WW was reduced by ∼ 70 % after treatment. Major contaminants in the WW degraded during treatment were identified using LC−MS analysis. Fe₃O₄@AC@CH hydrogel beads had excellent recyclability till six consecutive treatment cycles.