Distinct chromium removal mechanisms by iron-modified biochar under varying pH: Role of iron and chromium speciation.

Iron-modified biochar (Fe-biochar) has been widely developed to attenuate Cr(VI) pollution in both acid and alkaline environments. However, there are few comprehensive studies on how the iron speciation in Fe-biochar and chromium speciation in solution influencing the removal of Cr(VI) and Cr(III) under varying pH. Here, multiple Fe-biochar containing Fe 3 O 4 or Fe(0) were prepared and applied to remove aqueous Cr(VI). Kinetics and isotherms suggested that all Fe-biochar could efficiently remove Cr(VI) and Cr(III) via adsorption-reduction-adsorption. The Fe 3 O 4 -biochar immobilized Cr(III) by forming FeCr 2 O 4 , while amorphous Fe–Cr coprecipitate and Cr(OH) 3 was formed when using Fe(0)-biochar. Density functional theory (DFT) analysis further indicated that pH increase caused more negative adsorption energies between Fe(0)-biochar and the pH-dependent Cr(VI)/Cr(III) species. Consequently, the adsorption and immobilization of Cr(VI) and Cr(III) species by Fe(0)-biochar was more favored at higher pH. In comparison, Fe 3 O 4 -biochar exhibited weaker adsorption abilities for Cr(VI) and Cr(III), which were in consistent with their less negative adsorption energies. Nonetheless, Fe(0)-biochar merely reduced ∼70% of adsorbed Cr(VI), while ∼90% of adsorbed Cr(VI) was reduced by Fe 3 O 4 -biochar. These results unveiled the importance of iron and chromium speciation for chromium removal under varying pH, and might guide the application-oriented design of multifunctional Fe-biochar for broad environmental remediation. [Display omitted] • Fe-biochar efficiently removed aqueous Cr via adsorption-reduction-adsorption. • Fe speciation altered the contribution of adsorption and reduction to Cr removal. • The role of Fe speciation in Cr removal process was dependent on pH. • The interaction between Cr species and Fe-biochar at varying pH was analyzed by DFT. [ABSTRACT FROM AUTHOR]