Coupling adsorption (batch+column) experiments, CFD modeling, and molecular docking for efficient dye remediation using Zinc-activated Phosphogypsum beads

This study proposes a sustainable method for malachite green (MG) removal using zinc-treated phosphogypsum (PG) beads. Phosphogypsum, a phosphate-fertilizer by-product, was activated with ZnCl<sub>2</sub> and thermally treated to enhance porosity, surface acidity, and available binding sites. Characterization through XRD, FTIR, BET, and zeta potential confirmed the structural and surface modifications enabling superior adsorption performance. Batch and column studies revealed that pH, adsorbent dosage, flow rate, and bed height strongly influenced MG removal, achieving over 90% efficiency at 7 cm bed height and 10 mL/min flow rate. Kinetics followed a pseudo-second-order model, with the Temkin isotherm indicating chemisorption. Thomas and Yoon–Nelson models fit well (R² = 0.98), while Clark showed the best breakthrough prediction (R<sup>2</sup>=0.99). Adams–Bohart provided only moderate agreement. CFD simulations confirmed that the 7 cm bed height generated the most uniform flow, longest residence time, and minimal channeling, enabling efficient MG transport to active sites. Stable turbulence and porosity profiles enhanced external mass transfer, supporting the high experimental removal efficiency (>90%). Molecular docking showed strong electrostatic attraction between MG cations and negatively charged Si–O⁻, Al–O⁻, and S–O⁻ groups on ZnCl<sub>2</sub>-treated PG beads.