Sustainable removal of brilliant green dye from aqueous media using a calcium alginate–polydopamine bio-composite: process optimization and adsorption mechanism

Abstract

The contamination of water with toxic dyes poses serious threats to both aquatic environments and human health. The present study demonstrates a bio-polymer composite of calcium alginate with polydopamine (Ca-alginate/PDA), which was prepared and tested as an effective adsorbent material for brilliant green dye (BG) removal from aqueous solution. The successful formation and structure properties of the Ca-alginate/PDA composite were confirmed via different characterization techniques, including XRD, XPS, FTIR, BET, and FESEM, exhibiting a large surface area of 131.59 m² g⁻¹. Batch adsorption experiments identified the optimal operating conditions for dye removal to be pH 5.7, a temperature of 298 K, an adsorbent dosage of 0.1 g, an initial concentration of dye of 7 mg L⁻¹, and a contact time of 75 minutes, under which the maximum removal efficiency of 99.07% was achieved. Isotherm analysis revealed good correlation with the Freundlich isotherm, indicating that the multilayer adsorption occurred on a heterogeneous surface, but the kinetic analysis followed the pseudo-first-order kinetic model. Also, the thermodynamic study indicated the endothermic (positive ΔH) and spontaneous (negative ΔG) nature of the process. The mechanisms of adsorption were identified as π–π interactions, electrostatic interactions and hydrogen bond interactions. Moreover, regeneration tests showed that poly(dopamine)/Ca-alginate retained high efficiency and stability across four reuse cycles. Optimization using Box–Behnken design provided a reliable statistical model to predict the behavior of adsorption under various parameters. Overall, this study demonstrates the potential of poly(dopamine)/Ca-alginate as a durable and highly effective adsorbent for removing dye (BG) from wastewater.