Turning Organic Waste into Environmental Solutions: Nano-Structured Hydroxyapatite Beads for Industrial Dye Adsorption

Abstract Water pollution driven by synthetic dyes, coupled with rising societal expectations for cleaner industrial practices, remains a persistent challenge and highlights the need for affordable and sustainable treatment strategies. This study investigates hydroxyapatite–alginate (HAP-ALG) beads synthesized from fish-processing waste as adsorbent for removing Congo Red (CR) and Malachite Green (MG). After 24 h at 20 ± 2 °C, removal efficiencies reached 77.40% for CR and 84.00% for MG, with adsorption capacities of 6.0 mg/g and 66 mg/g using dosages of 3.0 g/L (CR) and 0.5 g/L (MG). Both Langmuir and Freundlich models fitted the equilibrium data well (n F > 1, R² MG ≥ 0.985 and R² CR = 0.999), and F-test comparisons indicated statistically consistent fitting. Kinetic experiments reached equilibrium at 900 min, and the Linear Driving Force model accurately described the temporal evolution, indicating intraparticle diffusion as the dominant rate-controlling mechanism. CR removal occurred mainly through hydrogen bonding, Ca 2+ chelation, and ion exchange at the bead surface, whereas MG adsorption was driven by electrostatic attraction followed by diffusion into the bead interior. A simplified cost assessment showed higher economic viability for MG adsorption, with values consistent with low-cost adsorbents. Reuse tests maintained satisfactory removal for up to three cycles, although performance declined progressively. HAP–ALG beads represent a low-cost, sustainable adsorbent derived from agro-industrial waste, offering competitive performance and alignment with circular-economy principles for dye-contaminated wastewater treatment.