Sustainable wastewater treatment via nitrogen/sulfur co-doped graphene composite: Mechanistic insights & process optimization

Abstract

The persistent discharge of dye-laden industrial effluents poses environmental and health risks due to the toxic, mutagenic, and carcinogenic nature of synthetic dyes. This study reports the synthesis of a nitrogen/sulfur co-doped three-dimensional graphene composite (NSGH) via a simple hydrothermal method for crystal violet (CV) dye removal from aqueous media. Characterization techniques (SEM, XRD, FTIR, EDX) confirmed successful heteroatom incorporation and a porous 3D structure with abundant active sites. Batch adsorption experiments were systematically designed using a Box–Behnken design to investigate the effects of pH, contact time, initial dye concentration, and adsorbent dosage on CV adsorption onto NSGH. Adsorption performance was evaluated using three key metrics: removal efficiency (RR), adsorption capacity (q), and effective adsorption capacity (EAC), a dimensionless parameter integrating both removal rate and capacity. Parametric modeling via response surface methodology (RSM) and artificial neural networks (ANN) revealed ANN's superior predictive accuracy (R² = 0.993) over RSM (R² = 0.975). Multi-objective optimization using the desirability function identified optimal conditions (pH: 7.0, 33.9 min, 39.9 mg L^-1, 0.019 g), achieving 90.84 % RR, 31.35 mg g⁻¹ q, and 1.08 EAC. Sensitivity analysis indicated initial dye concentration as the most influential variable across all metrics. Kinetic data were best fitted by the pseudo-first-order model, supporting diffusion-controlled physisorption, while statistical physics and thermodynamic analyses confirmed multilayer, spontaneous, and endothermic adsorption. DFT simulations reinforced experimental outcomes, showing strong π–π and electrostatic interactions on NSGH (−18.82 kcal mol⁻¹). Although a gradual decline in performance was observed over five adsorption–desorption cycles, NSGH demonstrated appreciable reusability, reinforcing its applicability as a high-efficiency adsorbent for dye-laden wastewater. By integrating dual doping, process modeling, and systematic evaluation, this work offers a practical framework for guiding the development of future water treatment materials.