Removal mechanisms of Cd(II) and Pb(II) from aqueous solutions using straw biochar: batch study, Raman and X-ray photoelectron spectroscopy techniques

Abstract

Recent interests in aqueous cadmium and lead removal has been driven by the rapid progress of the adsorption on biochar, which is an environmentally friendly adsorbent with high effectiveness. The biochar was prepared from pyrolysis of straw under 400°C and characterized by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, Brunauer–Emmett–Teller, Fourier-transform infrared spectroscopy and Raman spectra analysis. The batch experiments were performed to investigate the capacity and mechanism of the adsorption. The kinetic of either Cd(II) or Pb(II) adsorption fitted in both pseudo-first-order kinetic model and pseudo-second-order kinetic model. The adsorption capacity increased rapidly in the initial 30 min and then slowed down until the plateau after 80 min. Both the capacity of the Cd(II) and Pb(II) removal increased with pH value elevation at the relatively low pH range while the Pb(II) adsorption capacity decreased with pH value elevation in the range of 8.5–11.0. Nevertheless, these adsorptions were independent of the ionic strength. Either Cd(II) or Pb(II) adsorption process fitted the Langmuir model and Freundlich model well. The thermodynamic data demonstrated the cadmium adsorption on biochar as the endothermic and spontaneous reaction with the ΔH° = 21.31, ΔS° = 136.77 and ΔG° < 0 at 293, 313 and 333 K. The lead adsorption exhibited similar natures with ΔH° = 17.20, ΔS° = 127.92 and ΔG° < 0. Thus, biochar would be a green material as the potential adsorbent for the cadmium and lead removal in aqueous solutions.