RSM and ANN-driven optimization of Reactive Blue 221 biosorption by inactive Aspergillus niger: Mass transfer mechanisms, pareto analysis, and a circular economy approach

Synthetic reactive dyes are persistent pollutants often found in textile wastewater due to their complex aromatic and sulfonated structures. This study evaluated inactive Aspergillus niger biomass as a biosorbent for removing Reactive Blue 221 (RB 221) from aqueous solutions. Batch experiments indicated that the Langmuir isotherm best described the equilibrium data, with a maximum monolayer capacity of 32.89 mg/g at 30 °C. Kinetic data fit well with the pseudo-second-order and Elovich models, pointing to physicochemical interactions on a heterogeneous surface. Kinetic evaluation via Weber-Morris and Boyd models indicated that the mass transfer rate was co-limited by both external film and intraparticle diffusion. Meanwhile, SEM, EDS, and FTIR analyses confirmed the localized physicochemical binding mechanisms, revealing post-biosorption morphological changes, dye-specific elemental signals, and the participation of protonated amines in binding sulfonate-rich RB 221. Thermodynamic parameters (ΔG° < 0 and ΔH° > 0) confirmed a spontaneous, endothermic process. Response Surface Methodology (RSM) with a Central Composite Design optimized the process, where conditions of pH 5.25, 7.95 g/L dosage, 30 °C, and 87.50 mg/L dye concentration yielded a maximum predicted removal efficiency of 89.17%, which was experimentally validated at 89.25%. Contribution analysis highlighted biosorbent dosage as the key operational parameter, accounting for ∼90% of the total process variance. An Artificial Neural Network (ANN) with a 4-20-10-1 topology yielded superior predictive accuracy compared to the quadratic RSM model (R 2 = 0.9941 vs. 0.968). Combined with a preliminary cost assessment and a proposed waste-to-energy disposal route, these findings demonstrate that inactive A. niger is a practical, low-cost, and sustainable biosorbent for textile wastewater treatment.