Enhanced chromium (VI) removal: A remediation study employing nano zero-valent iron supported by sawdust

The rapid expansion of industry leads to the emission of heavy metals into the environment, posing a significant global concern presently. The management of heavy metals holds particular significance due to their resilience and persistence in the environment. Removal technologies of heavy metals include physical and chemical treatments. Chromium (VI) was utilized as a representative of heavy metals in this research. Sorption experiments were conducted using a composite material consisting of supported nano zero-valent iron on sawdust, designated as NZVI/SD. The NZVI/SD composite was prepared through a two-step process involving the impregnation of sawdust with ferrous sulfate, followed by a chemical reduction using Sodium borohydride (NaBH₄). The adsorbent was characterized with the help of Scanning Electron Microscopy (SEM), Energy Dispersive X-ray analysis (EDX), X-Ray Diffraction analysis (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and chemical test. The adsorption characteristics have been investigated under various conditions, including different pH values, adsorbent dosages, initial metal ion concentrations, contact times, and varying temperatures. The optimal conditions for employing NZVI/SD for the removal of Cr(VI) were observed at a pH of 3.0, an adsorbent dosage of 0.5 g/L, an initial metal ion concentration of 61.45 mg/L, a contact time of 180 min, and a temperature of 50 °C. Sorption of Cr(VI) on NZVI/SD followed Langmuir Isotherm model. The maximum monolayer sorption capacity on NZVI/SD was 39.8 mg/g at 298 K temperature (equilibrium pH of 3.0) estimated from the Langmuir isotherm. Kinetics and thermodynamic studies on the sorption of Cr (VI) on NZVI/SD were done to understand the sorption mechanism. Sorption kinetics of Cr (VI) on NZVI/SD confirmed that pseudo second order reaction kinetics was followed concluding that the rate of adsorption depends on the analyte and the available free site of adsorbent. By employing the Langmuir isotherm model and Van't Hoff equation, the determination of thermodynamic parameters such as Gibbs free energy, enthalpy change, and entropy change of the adsorption process was facilitated. It was found that the sorption of Cr(VI) was spontaneous (∆G° = −10.36 kJ.mol⁻¹) on NZVI/SD and the process was endothermic (∆H = +4.609 kJ/mol). NZVI/SD can be effectively regenerated by elution with 1 M NaOH for 4 h.