<p>The increase in water body contamination by cationic dyes, viz., Methylene Blue (MB) and Malachite Green (MG) has become a serious ecological concern due to their toxicity and persistent behaviour. This research reports the synthesis of a smart hydrogel nanocomposite (NC) designed for the removal of MB and MG dyes via both adsorption and catalytic degradation processes. These processes function through a synergistic adsorption and degradation mechanism, where adsorption concentrates dye molecules at active sites, enhancing their subsequent catalytic breakdown. Characterization techniques confirmed successful composite synthesis with the required porosity and functional groups which increase interaction with dye molecules. The adsorption mechanism includes electrostatic attraction between cationic dyes and negatively charged –COO⁻ groups, along with hydrogen bonding, van der Waals forces, and surface complexation with Zn²⁺/Al³⁺ active sites. The smart bimetallic hydrogel nanocomposite typically has 60–85% porosity, with interconnected micro and mesopores that improve dye adsorption and photocatalytic degradation. This porous architecture facilitates mass transfer and close proximity between adsorbed dye molecules and reactive catalytic sites. The hydrogel composite shows high affinity towards MB and MG dyes under optimum conditions. The optimal efficiency of SAN-2@ZnAl₂O₄ NCs was 96.35 (± 2.81) % for MB dye, and 93.88 (± 2.01) % for MG dye. This adsorption follows the Langmuir isotherm and confirms maximum adsorption capacity of 208.73 mg/g and 206.52 mg/g for MB and MG, respectively. Furthermore, the composite helps in MB and MG dyes degradation, resulting in their conversion into intermediate degradation products. This process is driven by reactive oxygen species (•OH, O₂⁻, and H₂O₂) generated under sunlight irradiation, which enable the oxidative breakdown of dye molecules into simpler forms. The reusable nature of the hydrogel composite is also confirmed as it demonstrates reusability over five cycles, with efficiencies of 87.63% and 85.65% for MB and MG in the fifth cycle, showcasing its capability for application in sustainable water purification. The combined effect of adsorption and photocatalytic degradation leads to enhanced removal efficiency, demonstrating a strong synergistic interaction between the two processes. This smart hydrogel nanocomposite provides an effective and eco-friendly method for waste dye removal from industrial discharge, contributing to environmental protection and public health security.</p> Graphical Abstract <p></p>