Chitosan sponges modified with BiOCl and TiO₂ for enhanced Congo red capture and light-responsive dye removal

The practical application of particulate adsorbents for wastewater is often limited by difficult recovery, particle loss, and restricted reusability after adsorption-site saturation. In this study, three-dimensional macroporous chitosan (CS) sponges were developed as recoverable monolithic adsorbents for Congo red (CR) removal, with bismuth oxychloride (BiOCl) and titanium dioxide (TiO₂) incorporated to provide additional light-responsive functionality. A chitosan concentration of 20 mg/mL (20CS) produced an aligned open-cell structure with 85.69% porosity and a hydraulic permeability of 42,960 millidarcy. CR adsorption on 20CS followed the pseudo-second-order kinetic model and the Langmuir isotherm, with a theoretical maximum adsorption capacity (q_max) of 407.8 mg/g. Incorporation of BiOCl and TiO₂ enhanced CR removal, and the optimized 30BiOCl-30TiO₂/20CS composite removed more than 80% of CR during the initial dark adsorption stage for 30 min and achieved 98.0% total removal after a further 90 min of illumination. Bulk CR removal remained adsorption-dominated, whereas the visibly lighter used composite suggested partial light-assisted decolorization of surface-bound CR. Despite the reduction in stiffness caused by inorganic loading, the composite retained its mechanical integrity. It also maintained approximately 90% CR removal in the second cycle and showed no measurable increase in Ti release or detectable Bi leaching under the tested conditions. These results demonstrate that the BiOCl-TiO₂/chitosan monolithic sponge combines rapid dye capture, localized light-responsive surface decolorization, convenient solid recovery, and short-term reuse within a single treatment.