Optimisation of preparation conditions and removal mechanism for trivalent antimony by biochar-supported nano zero-valent iron

In this study, the preparation conditions of nano zero-valent iron biochar (nZVI-BC)​ was optimised with the aim of removing Sb(III) from aqueous solutions. Among all the prepared nZVI-BC, 5-1nZVI-BC (98.25 mg/g) and 7-4nZVI-BC (91.26 mg/g) were considered to be more efficient adsorbents for Sb(III) adsorption, and their adsorption capacity increased by 457% and 157%, respectively, than that of pristine BC. Both the Langmuir and Freundlich models were well fitted (R2 ≥ 0.92), indicating that the adsorption process occurred through the synergistic effects of monolayer adsorption and multilayer adsorption. With the increase in the ratio of biochar to iron, the second-order kinetics could better describe the adsorption behaviour, which was controlled by chemisorption. Investigation of the adsorption mechanism using X-ray photoelectron spectroscopy (XPS) and kinetic modelling showed that there were two adsorption behaviours for Sb(III): (i) Sb(V) from the oxidisation of Sb(III) or Sb(III) combined with the functional groups on the surface of nZVI-BC to form antimonite/antimonate, which was attached to biochar surface to remove Sb(III). (ii) Negatively charged Sb(III)/Sb(V) combined with Fe(II)/Fe(III) to form precipitates, such as FeSb2O4 and FeSbO4, that were attached to the biochar surface, resulting in removing of Sb(III). Moreover, the reusability and recyclability of the screened nZVI-BC samples were tested in this study. The removal rate for Sb(III) remained approximately 75% after three reuses and 50%–80% through adsorption/desorption after two cycles. These results show that the screened nZVI-BC has a promising prospect as an economical and efficient adsorbent for Sb(III) removal from wastewater.