Adsorptive Mechanism of Chromium Adsorption on Siltstone–Nanomagnetite–Biochar Composite

In this work, the potential of a novel and highly efficient composite of Eleocharis dulcis biochar with magnetite nanoparticles and siltstone was explored for removing chromium from water. Characterization of the prepared biochar composite was carried out using thermal gravimetric analysis (TGA), X-ray photon spectroscopy (XPS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy, point of zero charge and BET. XRD confirmed magnetite and quartz to be the main phases in biochar composite. TGA results showed higher thermal stability of the composite after the addition of siltstone. Batch adsorption mode was employed for studying the adsorption capacity of sample for the decontamination of chromium as a function of concentrations, time, temperatures and pHs. Kinetic modelling confirmed pseudo second order to fit best to the kinetic data for chromium adsorption on the composite biochar. An increase of adsorption was observed with the rise in temperature from 303 to 318 K showing the endothermic nature of the process whereas pH study showed higher removal efficiency of chromium in the acidic pH range. Langmuir model was applicable to the data with higher value of correlation. The thermodynamic parameter ΔH° (40.46 kJ mol−1) and negative but higher values of (ΔG°) shows the endothermic and spontaneous nature of the adsorption process respectively. Higher value of activation energy (15.08 kJ mol−1) confirmed the chemical nature of the process. Post adsorption FTIR and XPS confirmed the adsorption of chromium on the surface of the composite. The adsorption capacity obtained in the present study was found to be higher as compared to many other reported adsorbents used for chromium removal.