Hydrothermal Conversion of Electrolytic Manganese Residue into Silicate-Based Adsorbent for Removal of Cu(II) from Wastewater

A novel silicate-based adsorbent (M-EMR) was synthesized via a hydrothermal process using electrolytic manganese residue (EMR) and calcium carbide slag (CS) in a NaOH medium. The adsorption behavior of M-EMR for Cu(II) ions from aqueous solution was systematically investigated under varying conditions. The adsorption kinetics followed a pseudo-second-order model with a high correlation coefficient (<i>R</i>² = 0.9999), suggesting chemisorption as the rate-limiting mechanism. Equilibrium data were best fitted by the Langmuir isotherm (<i>R</i>² > 0.999), with maximum adsorption capacities of 174.22, 185.19, and 232.56 mg/g at 288.15, 298.15, and 308.15 K, respectively. Thermodynamic analysis revealed a spontaneous and endothermic process, characterized by negative Δ<i>G</i>° values and positive Δ<i>H</i>° (41.25 kJ/mol) and Δ<i>S</i>° (203.88 J/mol·K). Competitive adsorption studies showed that Pb(II) significantly interfered with Cu(II) uptake, while Co(II) and Ni(II) exhibited negligible effects. M-EMR demonstrated high selectivity for Cu(II) and strong resistance to interference from coexisting cations (K⁺, Ca²⁺, Na⁺, Mg²⁺). Mechanistic analysis confirmed that ion exchange, surface precipitation, and complexation were the primary removal pathways, further supported by X-ray diffraction (XRD), Fourier-transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS) results. Cu(II) could be efficiently desorbed using 30 mmol/L EDTA, achieving a recovery efficiency of 96.62%. This study provides a cost-effective and environmentally sustainable approach for the valorization of industrial waste and the treatment of Cu(II)-contaminated wastewater.