Sustainable sludge derived activated carbon polyaniline composite removes lead and cadmium from industrial wastewater

This study presents the fabrication of a novel, sustainable, and cost-effective hybrid composite (SAC-PANI) derived from sewage sludge calcined at 600 °C, activated carbon, and polyaniline through in situ oxidative polymerization. Comprehensive Characterization using FTIR, XRD, SEM, BET, and TGA confirmed the successful formation of the composite and its enhanced physicochemical properties. BET analysis revealed a substantial increase in surface area (90 m²/g) compared to raw calcined sludge (58 m²/g), indicating improved porosity. Process optimization was performed using Response Surface Methodology (RSM) based on a Central Composite Design (CCD). The developed quadratic model (R² = 0.984) adequately described the system and revealed significant interactions among pH, adsorbent dosage, and initial metal concentration, with optimum removal achieved at pH 6.0. Adsorption equilibrium data were best described by the Langmuir isotherm (R² = 0.995 and 0.992 for Pb(II) and Cd(II), respectively), yielding maximum adsorption capacities of 128.5 ± 2.3 mg/g for Pb(II) and 115.7 ± 2.1 mg/g for Cd(II). Kinetic studies were well fitted by the pseudo-second-order model (R² = 0.998 for Pb(II) and 0.997 for Cd(II)), with rate constants of k₂ = 0.035 ± 0.001 g/mg·min for Pb(II) and k₂ = 0.029 ± 0.001 g/mg·min for Cd(II), indicating that chemisorption is the dominant adsorption mechanism owing to interactions between the metal ions and the nitrogen-containing functional groups of PANI. Thermodynamic analysis demonstrated that the adsorption process is spontaneous (ΔG° = -19.8 kJ/mol for Pb(II) and - 17.6 kJ/mol for Cd(II) at 298 K), endothermic (ΔH° = +33.5 kJ/mol for Pb(II) and + 25.3 kJ/mol for Cd(II)), and characterized by increased entropy (ΔS° = +132 J/mol·K for Pb(II) and + 98 J/mol·K for Cd(II)), indicating chemisorption-driven metal uptake. The composite also exhibited good stability, retaining more than 82% of its removal efficiency after five adsorption-desorption cycles for both Pb(II) and Cd(II).). Environmental safety assessment via the TCLP leaching test demonstrated that the spent adsorbent is non-hazardous, with Pb and Cd leachate concentrations (0.58 and 0.18 mg/L, respectively) well below EPA regulatory limits (5.0 mg/L for Pb and 1.0 mg/L for Cd). Importantly, the SAC-PANI composite was successfully tested on real electroplating wastewater, achieving Pb(II) and Cd(II) removal efficiencies of 75.0 ± 1.5% and 72.6 ± 1.4%, respectively, in the presence of competing ions and complex matrix effects. Overall, SAC-PANI demonstrates considerable potential as an efficient, sustainable, and low-cost adsorbent for the removal of heavy metals from wastewater.