Broadened operating pH range for adsorption/reduction of aqueous Cr(VI) using biochar from directly treated jute (Corchorus capsularis L.) fibers by H3PO4

Biochar has attracted much attention as a green cost-effective adsorbent for heavy metal removals. However, previous studies generally found that biochar was only effective in a narrow low pH range for Cr(VI) removal, and less attention was paid to biochar which can be effectively operated in a wider range of pH. This study showed that the jute fiber-based biochar pretreated H3PO4 can availably remove and reduce Cr(VI) to Cr(III) in wastewater over a broadened operating pH range of 1-6, in which the intrinsic pH of Cr(VI) solution was included. For this, XRD, Raman, SEM, elemental analysis and N-2 adsorption- desorption techniques were utilized to characterize the structure and compositions of the biochars. FTIR, XPS, Boehm titration and zeta potential measurements were performed to determine the surface group and charge distributions. It was revealed that in comparison with the biochar activated by KOH, H3PO4-treated biochar offered sufficient acidic groups such as phosphonic acid and carboxyl on the surface, from which the supplied protons could maintain the pH within a stable range of 1.9-3.4. XPS analysis exhibited that the electron-rich C-O, P-O and C-P bonds acted electron donating moieties to reduce high potential Cr(VI). Cr(VI) uptake experiments showed that Freundlich multilayer chemisorption occurred on the heterogeneous surface. Langmuir-Hinshelwood kinetic model suggested the surface reaction was the rate-controlling step. The examination for simulated samples confirmed that the H(3)PO(4)treated biochar could control Cr(VI) pollution at the inherent pH in the environment.