Facet-dependent photo-Fenton degradation of p-arsanilic acid and arsenic redistribution on hematite

Abstract

Iron minerals play a critical role in regulating arsenic fate through adsorption and photocatalysis. Organoarsenic compounds such as p-arsanilic acid (p-ASA) can be transformed into toxic inorganic arsenic, posing serious environmental risks. Although the surface reactivity of hematite is known to be facet-dependent, the facet-specific photo-degradation mechanisms of organoarsenics, especially the speciation transformation and fate of arsenic group remain unclear. In this study, the photo-Fenton catalytic degradation of p-ASA by H₂O₂ was systematically investigated using hematite nanoplatelets (HNPs, dominated by {001} facets) and nanocubes (HNCs, dominated by {012} facets) as catalyst. HNCs exhibited significantly faster and more complete degradation than HNPs, with a higher pseudo-first-order rate constant ((5.29 ± 0.79) × 10⁻¹vs. (7.62 ± 0.53) × 10⁻²). Notably, HNCs also promoted greater dearsenification, releasing more inorganic arsenic into solution (27.3% vs. 6.9%), indicating a higher potential environmental risk. In situ shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) revealed that molecular p-ASA persisted on HNP after reaction, whereas HNC surfaces were dominated by degradation products. Mechanistic studies identified hydroxyl radicals as the dominant reactive species, with higher concentrations generated in the HNC system. Its superior activity stemmed from a higher concentration of surface-labile Fe species and stronger co-adsorption of H₂O₂ (2 mmol L⁻¹) and p-ASA, which enhanced radical generation. This work demonstrates that the photocatalytic function of iron minerals is intrinsically facet-dependent, governing both pollutant transformation and arsenic speciation. These findings highlight the importance of facet analysis in understanding the environmental fate of organoarsenicals and suggest that facet engineering offers a strategic pathway for designing targeted remediation technologies.