Environmentally persistent free radicals mediated removal of Cr(VI) from highly saline water by corn straw biochars

[Display omitted] •Biochars show highly selective adsorption of Cr(VI) in salt solution.•Cr(VI) on biochars has been reduced to Cr(III) in neutral solution.•Persistent free radicals play the role in reducing Cr(VI). Heavy metal ions coexisting with salts in the contaminant water are difficult to remove due to the interference of salts. Herein, biochars were pyrolyzed by corn straw at different temperatures, aiming to remove Cr(VI) in the presence of salts. Results show that biochars had surprisingly selective adsorption of Cr(VI). X-ray photoelectron and X-ray absorption near edge spectra revealed that Cr(VI) was reduced to Cr(III). All the adsorption was conducted at pH ∼ 7, which differed from the previous studies that Cr(VI) could only be reduced at pH 2–4. Environmental persistent free radicals (EPFRs) on biochars were found to play the role in reducing Cr(VI) in neutral solutions, which was confirmed by electron spin resonance and free radical quenching. The biochar with EPFRs reveals a highly selective removal of Cr(VI), which has implications for the remediation of contaminated water. This work provides a new insight into biochar’s properties and potential environmental applications.