Experimental and DFT investigation on N-functionalized biochars for enhanced removal of Cr(VI)

In this study, N-functionalized biochars with varied structural characteristics were designed by loading poplar leaf with different amounts of urea at 1:1 and 1:3 ratios through pyrolysis method. The addition of urea significantly increased the N content of biochar and facilitated the formation of amine (-NH-, -NH₂), imine (-HCNH), benzimidazole (-C₇H₅N₂), imidazole (-C₃H₃N₂), and pyrimidine (-C₄H₃N₂) groups due to substitution reaction and Maillard reaction. The effect of pH on Cr(VI) removal suggested that decrease in solution pH favored the formation of electrostatic attraction between the protonated functional groups and HCrO₄⁻. And, experimental and density functional theory study were used to probe adsorption behaviors and adsorption mechanism which N-functionalized biochars interacted with Cr(VI). The protonation energy calculations indicated that N atoms in newly formed N-containing groups were better proton acceptors. Adsorption kinetics and isotherm experiments exhibited that N-functionalized biochars had greater removal rate and removal capacity for Cr(VI). The removal rate of Cr(VI) on N-functionalized biochar was 10.5–15.5 times that of untreated biochar. Meanwhile, N-functionalized biochar of NB3 with the largest number of adsorption sites for -C₇H₅N₂, -NH₂, -OH, -C₃H₃N₂, and phthalic acid (-C₈H₅O₄) exhibited the supreme adsorption capacity for Cr(VI) through H bonds and the highest adsorption energy was −5.01 kcal/mol. These mechanistic findings on the protonation and adsorption capacity are useful for better understanding the functions of N-functionalized biochars, thereby providing a guide for their use in various environmental applications.