Valorization of ductile cast iron solid waste as a high-performance adsorbent for crystal violet removal: characterization, optimization, and mechanistic insights

Abstract

Industrial textile wastewater containing synthetic dyes cause serious environmental and health risk, whereas ductile cast iron (DCI) foundries generate over 500 000 tons of waste annually. This study utilizes DCI solid waste as an adsorbent to remove the crystal violet (CV) dye from wastewater. Techniques (XRF, XRD, BET, SEM-EDX, FTIR, TGA-DTG, zeta potential) proved that the waste contains 88.0 wt% periclase (MgO) with nanoscale, high surface area, and abundant surface hydroxyl groups. Response surface methodology showed that the most significant parameters were the adsorbent dose and time contact. The optimal conditions give 93.7% removal efficiency at initial concentration: 38.7 mg L⁻¹, adsorbent dose: 6.2 g L⁻¹, shaking rate: 150 rpm, and contact time: 30 min. The isotherm model was the Freundlich model suggesting surface heterogeneity with dispersed binding energies; multilayer coverage supports this, but the Freundlich fit by itself cannot establish it. The maximum physisorption capacity was 116.85 mg g⁻¹, and the mean free energy, E = 3.34 kJ mol⁻¹. Kinetic study demonstrated that the reaction follows pseudo-first-order kinetics (k₁ = 0.1654 min⁻¹) and showed three diffusion phases: the external film diffusion (0–30 min), the intraparticle diffusion (30–120 min), and the equilibration phase (>120 min). The thermodynamic investigation showed that the adsorption is an endothermic process (ΔH° = +22.15 kJ mol⁻¹), accompanied by a positive entropy change (ΔS° = +85.3 J mol⁻¹ K⁻¹) and a negative Gibbs free energy change (ΔG° = −2.94 to −5.68 kJ mol⁻¹), which means spontaneous, entropy-driven physisorption. Post-adsorption XRD showed that MgO was hydroxylated to Mg(OH)₂. The pH optimization revealed maximum removal at pH 7–9. The regeneration technique employing acid and thermal methods yielded a desorption efficiency rate of 95.4%, a cumulative adsorption capacity recovery rate of 78.5% following 15 cycles, and magnesium release lower than all permissible standards (USEPA, WHO, Egyptian Law 4/1994). The initial techno-economic analysis yields a unit treatment cost of approximately $1.09 m⁻³ for a hypothetical 1000 m³ d⁻¹ plant; nevertheless, further confirmation based on scale up and continuous flow operation is essential prior to actual commercialization. This study proves that DCI solid waste is not only economically feasible but also environmentally adsorbent in the context of a circular economy.