Adsorptive performance of sustainable biosorbent from <i>Macadamia integrifoli</i> shell powder for toxic methylene blue dye removal: desirability functions and dye uptake mechanism

Herein, the potential of Macadamia integrifolia nutshell powder (MSP) as a sustainable, renewable, and cost-effective biosorbent for methylene blue (MB) dye was evaluated. The physicochemical properties of MSP were characterized <i>via</i> XRD, FTIR, FESEM-EDX, and pH_pzc analysis. The biosorption process was optimized using the Box-Behnken design (RSM-BBD), to evaluate the influence of MSP dose (0.02-0.1 g/100 mL), contact time (20-300 min), and solution pH (4-10). The desirability function further refined and validated the BBD results, demonstrating that maximum MB removal (98.7%) was achieved at an MSP dose of 0.09 g/100 mL, contact time of 276.1 min, and solution pH of 8.7. Kinetic modeling indicated that MB biosorption onto MSP conformed to the pseudo-second order (PSO) model. The intraparticle diffusion (IPD) model supports a multi-step adsorption process, consisting of surface adsorption, gradual diffusion, and equilibrium stages. The adsorption equilibrium data were well described by the Langmuir and Freundlich isotherm models, confirming a combination of monolayer and multilayer adsorption profiles. The maximum adsorption capacity (<i>q_max</i>) of MSP was estimated to be 128.3 mg/g. The biosorption mechanism was attributed to hydrogen bonding, π-π interactions, electrostatic forces, and pore filling, as evidenced by spectroscopy and bioadsorbent morphology results. The reusability study demonstrated that MSP retained significant adsorption capacity over multiple cycles, highlighting its moderate recyclability. These findings establish MSP as a highly efficient, scalable, and environmentally sustainable bioadsorbent for MB dye removal, offering a practical solution for wastewater treatment applications and options for sustainable water management.