Sludge Biochar Loaded with Polyethylene Glycol-Stabilized Nanoscale Zero-Valent Iron for the Enhanced Removal of Cd(II) from Aqueous Solution

In this study, polyethylene glycol-stabilized nanoscale zero-valent iron (PEG-nFe0) was loaded on sludge biochar (SBC) via a liquid-phase reduction method for the removal of Cd(II) from aqueous solutions, where nFe0 is the Fe0-FeO/FeOOH mixture. The morphology, functional groups, and crystal structure of the resulting PEG-nFe0@SBC composites were characterized using scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) techniques. The incorporation of PEG introduced numerous –OH functional groups, which facilitated the dispersion of nFe0 as smaller particle size on the SBC surface. Kinetic studies revealed that the Cd(II) removal process by PEG-nFe0@SBC adhered to a pseudo-second-order kinetic model. The Freundlich isotherm model provided the best fit for describing the adsorption process. Thermodynamic analysis indicated that the removal of Cd(II) by PEG-nFe0@SBC was a spontaneous endothermic reaction. Under the initial conditions of 200 mg/L Cd(II), 1 g/L of PEG-nFe0@SBC, pH 5.0, and a temperature of 298 K, more than 85.8% of Cd(II) was removed within 24 h. The presence of PEG significantly enhanced the oxidation resistance of nFe0 in air. The adsorption capacity of PEG-nFe0@SBC for Cd(II) remained stable at 123.60 mg/g even after 90 days of air exposure, compared to 73.87 mg/g for nFe0@SBC. Typical macroscopic composition of natural water (Ca(II), Mg(II), humic acid (HA)) had minimal impact on Cd(II) removal by PEG-nFe0@SBC. Mechanistic analysis suggested that the primary removal mechanisms for Cd(II) by PEG-nFe0@SBC involved precipitation/co-precipitation and complex formation.