Novel Fe-Mn binary oxide-biochar as an adsorbent for removing Cd(II) from aqueous solutions

In this study, a pristine biochar (BC) and Fe-Mn binary oxide-biochar (FMBC) were prepared using Pennisetum sp. straw as the feedstock for Cd(II) removal from aqueous solutions. Scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and specific surface area (SSA) analyses revealed the physico-chemical characteristics of the pristine and designer adsorbents, suggesting that an ultrasonic treatment during synthesis enhanced the SSA and pore volume of the BC, and assisted successful loading of Fe-Mn binary oxide particles on the BC surface. The Cd(II) adsorption data of the adsorbents were fitted to the Langmuir isothermal and pseudo-second-order kinetic models. At a system temperature of 25 °C and pH 5, the maximum Cd(II) adsorption capacities of BC (30.58 mg/g) and FMBC (95.23 mg/g) were obtained. Multiple Cd(II) adsorption mechanisms by FMBC were identified, including precipitation with minerals, complexation with surface functional groups, Cd(II)-π interactions, and cation exchange. As the most dominant adsorption mechanism, Cd-O bonds were formed on the FMBC surfaces precipitating Cd(OH)2 (63.9 wt%) and CdO (36.1 wt%). The FMBC thus could be potentially used as an effective adsorbent for Cd(II) removal from aqueous solutions.