Enhanced removal of phenolic compounds by ferrate(VI): Unveiling the Bi(III)-Bi(V) valence cycle with in situ formed bismuth hydroxide as catalyst

Abstract

The use of 2-hydroxybenzophenone (2-HBP) in personal care products is of great concern due to its potential negative effects on the ecosystem and public health. This paper presents the degradation of 2-HBP by bismuth(III) (Bi³⁺)-ferrate(VI) (Fe^VIO₄^2-, Fe(VI)) (Bi³⁺-Fe(VI) system). Experimental studies at different pH and dosages of Bi³⁺ and Fe(VI) showed that the Bi³⁺-Fe(VI) system increased the degradation rate and removal efficiency of 2-HBP compared to Fe(VI) alone. The in situ formed flake-like white flocculent precipitate of Bi(OH)₃ showed catalytic performance through the Bi(III)-Bi(V)-Bi(III) valence cycle which was demonstrated through spectroscopic measurements. The hydrogen transfer-mediated reactions between Fe(VI) and Bi(OH)₃ as well as subsequent formation of Bi(V) were supported by performing density functional theoretical (DFT) calculations. Seventeen identified transformation products of 2-HBP by Fe(VI) with and without Bi³⁺ revealed hydroxylation, bond breaking, carboxylation, and polymerization reaction pathways. Significantly, Bi³⁺ facilitated the polymerization reaction and the dioxygen transfer-mediated hydroxylation reaction pathways. The ions (anions and cations) and humic acids (HA) present in the Bi³⁺-Fe(VI) system had minimal influence on the removal efficiency of 2-HBP. Reusability tests and use of real water samples as well as toxicity assessments of transformation products unveiled the practical application aspect of the Bi³⁺-Fe(VI) system. Finally, the results showed that the system exhibits good removal efficiency for all 12 phenolic compounds, indicating theuniversality. The Bi³⁺-Fe(VI) system may be an easy-to-implement cost-effective method for the catalytic degradation of benzophenones by Fe(VI).

Keywords: 2-Hydroxybenzophenone; Bismuth hydroxide; Catalytic mechanism; Ferrate(VI); Theoretical calculations.