Mechanism of As(III) removal properties of biochar-supported molybdenum-disulfide/iron-oxide system

Abstract

Sulfate (SO₄^•-) and hydroxyl-based (HO^•) radical are considered potential agents for As(III) removal from aquatic environments. We have reported the synergistic role of SO₄^•- and HO^• radicals for As(III) removal via facile synthesis of biochar-supported SO₄^•- species. MoS₂-modified biochar (MoS₂/BC), iron oxide-biochar (FeO_x@BC), and MoS₂-modified iron oxide-biochar (MoS₂/FeO_x@BC) were prepared and systematically characterized to understand the underlying mechanism for arsenic removal. The MoS₂/FeOx@BC displayed much higher As(III) adsorption (27 mg/g) compared to MoS₂/BC (7 mg/g) and FeOx@BC (12 mg/g). Effects of kinetics, As(III) concentration, temperature, and pH were also investigated. The adsorption of As(III) by MoS₂/FeOx@BC followed the Freundlich adsorption isotherm and pseudo-second-order, indicating multilayer adsorption and chemisorption, respectively. The FTIR and XPS analysis confirmed the presence of Fe-O bonds and SO₄ groups in the MoS₂/FeO_x@BC. Electron paramagnetic resonance (EPR) and radical quenching experiments have shown the generation of SO₄^•- radicals as predominant species in the presence of MoS₂ and FeO_x in MoS₂/FeOx@BC via radical transfer from HO^• to SO₄^2-. The HO^• and SO₄^•- radicals synergistically contributed to enhanced As(III) removal. It is envisaged that As(III) initially adsorbed through electrostatic interactions and partially undergoes oxidation, which is finally adsorbed to MoS₂/FeOx@BC after being oxidized to As(V). The MoS₂/FeO_x@BC system could be considered a novel material for effective removal of As(III) from aqueous environments owing to its cost-effective synthesis and easy scalability for actual applications.

Keywords: Adsorption; As(III); Biochar; Mechanism; Sulfate radicals.