Experimental and computational study of basic blue 41 adsorption onto Ceratophyllum demersum-grafted Poly(Acrylic Acid-co-Acrylamide) nanocomposite hydrogel: Kinetics, thermodynamics, DFT, and MD simulations

This study reports the grafting of poly(acrylic acid- co -acrylamide) onto dried biomass of Iraqi submerged aquatic macrophyte Ceratophyllum demersum (CD), and validates its dye-remediation performance through a coupled experimental-DFT-molecular dynamics (MD) framework rarely applied to biomass-grafted hydrogels. The nanocomposite, designated CD-g-P(AA-AM), was synthesised via free-radical copolymerisation with N,N ′-methylenebisacrylamide as crosslinker and potassium persulfate as initiator, and characterised by FTIR, XRD, FE-SEM, EDX and TGA/DTG. Characterisation confirmed successful incorporation of -COOH and -CONH 2 functionalities onto the biomass backbone, with a porous sponge-like morphology and nanoscale particles (∼40–66 nm). The composite achieved 93.7 % removal of Basic Blue 41 (BB41) cationic dye versus 71.8 % for raw CD under optimum conditions (0.05 g adsorbent, pH 10, 120 min, 20 °C). Crucially, grafting shifted the adsorption regime from heterogeneous (Freundlich, n = 1.87) to homogeneous (Langmuir, q max = 71.3 mg g⁻¹). This transition was confirmed by Sips-isotherm fitting (n S changed from 0.65 for CD to 1.00 for the composite) and site-energy-distribution (SED) analysis (37 % narrower distribution after grafting). Thermodynamic analysis yielded spontaneous, endothermic adsorption (ΔH° = 44.0 kJ mol⁻¹, ΔS° = 169.2 J mol⁻¹ K⁻¹). DFT calculations with explicit Na + counterions and PCM solvation preserved the mechanism ranking (bidentate -COO⁻ > H -bond > n -π) with an effective binding energy of −76.6 kJ mol⁻¹. MD simulations (100 ns, single- and multi-dye) quantified a primary PMF minimum of −34.2 kJ mol⁻¹ and confirmed negligible lateral dye-dye interactions (+0.46 kJ mol⁻¹ per pair) consistent with Langmuir behaviour. Regeneration with 0.1 M HCl/ethanol (1:1) achieved 89.3 % desorption, with 78.4 % capacity retention after six cycles. On real Iraqi textile wastewater, the composite reached 87.5 % dye removal and 49.1 % COD reduction at pH 10. This work establishes a transferable multi-scale framework linking grafted-hydrogel microstructure to macroscopic adsorption behaviour, and demonstrates that C. demersum can be upcycled into a competitive, cost-effective biosorbent for industrial dye remediation.