Efficient removal of Cr(VI) using low-temperature biochar derived from stevia residues via wet-process phosphoric...

To develop a low-temperature biochar for chromium (Cr) removal from wastewater, stevia residue was employed as the raw material. It was pretreated with wet-process phosphoric acid (WPPA) to lower the pyrolysis temperature during biowaste conversion. The morphology and physicochemical properties of adsorbents pyrolyzed at different temperatures (200-400 °C) are characterized by scanning electron microscopy (SEM), surface area analysis, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Phosphorylated biochar showed higher Cr(VI) removal efficiency compared to original biochar. The maximum adsorption capacity of the biochar prepared at 250 °C (HBC250) for Cr(VI) reached 191.1 mg/g. Low temperature retains the rich oxygen-containing adsorption sites of biochar. The predominant mechanism of Cr(VI) adsorption by HBC250 can be proposed involving electrostatic interactions, pore filling, chemical reduction and complexation. Combined with the phosphorylation via low-cost WPPA, the multilayer anchoring of Cr(VI) is significantly boosted. This work provides an economic approach for biochar synthesis, offering an engineered strategy for the efficient utilization of biowastes and the removal of Cr(VI) from aqueous solutions.