Insights into simultaneous adsorption and oxidation of antimonite[Sb(III)]by crawfish shell-derived biochar:spectroscopic investigation and theoretical calculations

Removal of antimonite[Sb(Ⅲ)]from the aquatic environment and reducing its biotoxicity is urgently needed to safeguard environmental and human health.Herein,crawfish shell-derived biochars(CSB),pyrolyzed at 350,500,and 650℃,were used to remediate Sb(Ⅲ)in aqueous solutions.The adsorption data best fitted to the pseudo-second-order kinetic and Langmuir isotherm models.Biochar produced at 350℃(CSB350)showed the highest adsorption capacity(27.7 mg g^(−1)),and the maximum 78%oxidative conversion of Sb(Ⅲ)to Sb(V).The adsorption results complemented with infrared(FTIR),X-ray photoelectron(XPS),and near-edge X-ray absorption fine structure(NEXAFS)spectroscopy analyses indicated that the adsorption of Sb(Ⅲ)on CSB involved electrostatic interaction,surface complexation with oxygen-containing functional groups(C=O,O=C-O),π-πcoordination with aromatic C=C and C-H groups,and H-bonding with-OH group.Density functional theory calculations verified that surface complexation was the most dominant adsorption mechanism,whilstπ-πcoordination and H-bonding played a secondary role.Furthermore,electron spin resonance(ESR)and mediated electrochemical reduction/oxidation(MER/MEO)analyses confirmed that Sb(Ⅲ)oxidation at the biochar surface was governed by persistent free radicals(PFRs)(•O_(2)^(−)and•OH)and the electron donating/accepting capacity(EDC/EAC)of biochar.The abundance of preferable surface functional groups,high concentration of PFRs,and high EDC conferred CSB350 the property of an optimal adsorbent/oxidant for Sb(Ⅲ)removal from water.The encouraging results of this study call for future trials to apply suitable biochar for removing Sb(Ⅲ)from wastewater at pilot scale and optimize the process.