Adsorption mechanism of aqueous Cr(<sc>vi</sc>) by Vietnamese corncob biochar: a spectroscopic study

Cr(vi) is highly toxic and carcinogenic, posing significant threats to health and ecosystems. This study utilizes solid waste from corncobs to synthesize biochar (CCBC) for the removal of Cr(vi) from water. The most effective Cr(vi) removal was achieved at pH 2.0, with a maximum adsorption capacity (Qm, Langmuir, mg g-1) of 38.1, higher than that of activated carbon (25.69), composite (35.84), and magnetic biochar (25.94) derived from corncobs. Brunauer-Emmett-Teller (BET) results indicated that Cr(vi) was adsorbed on the internal surface instead of external surface. Scanning electron microscope (SEM-mapping) images combined with the pHPZC value (7.6) demonstrated that Cr(vi) interacts with the material surface via electrostatic mechanisms. Energy-dispersive X-ray (EDX) spectra combined with Fourier-transform infrared (FTIR) spectra demonstrate that two key adsorption mechanisms in this study are surface adsorption (Cr(vi)-biochar) followed by the reduction of Cr(vi) to Cr(iii), allowing ion exchange adsorption to occur. X-ray diffraction (XRD) patterns indicate no precipitation on the surface, and the material remains stable after four reuse cycles. These results suggest that CCBC can be used as an efficient, cost-effective, and environmentally friendly adsorbent for Cr(vi) removal from water. This is the first study to combine spectroscopic methods and theoretical models to gain deeper insights into the Cr(vi) adsorption mechanisms onto CCBC.