In situ speciation analysis and kinetic study of arsenic adsorption on ferrihydrite with surface-enhanced Raman spectroscopy

Arsenic pollution poses a serious threat to human health, and is one of the most concerning environmental problems worldwide. The adsorption, fixation, and dissolution behaviors of arsenic on the surface of iron-(hydr-) oxides influence the environmental routes of arsenic cycle geochemistry. Both inner-sphere and outer-sphere adsorption configurations of arsenic on iron oxides have been proposed based on X-ray adsorption spectra. However, there is no systematic study on the in situ speciation analysis and adsorption kinetics of these species at such interfaces, because of the lack of an efficient monitoring strategy. The correlation of surface speciation and environmental stability is still unknown. Here, a shell-isolated SiO₂@Ag@Au-based surface-enhanced Raman spectroscopy (SERS) platform was developed for speciation analysis of the adsorbed arsenic species by eliminating the chemical interaction between arsenic and silver. Using ferrihydrite as a typical iron oxide, the intrinsic Raman spectra of the inner-sphere (~ 830 cm⁻¹) and outer-sphere (~ 660 cm⁻¹) complexes at the adsorption interface were identified. For the first time, the in situ kinetic monitoring of the formation and transformation of these species was realized. By correlating the speciation to the sequential extraction results, the environmental stability of arsenic on ferrihydrite was shown to be closely related to the adsorption configuration. It was shown that stability can be significantly promoted by transforming loosely bonded species (outer-sphere complexes) into inner-sphere structures. Our work demonstrated the applicability of SERS with shell-isolated plasmonic particles for arsenic geochemical cycle monitoring and mechanism studies. It also provided a convenient tool for developing effective strategies for arsenic pollutant control and abatement.