The molecular structure of the mineral beudantite PbFe3(AsO4,SO4)2(OH)6 - Implications for arsenic accumulation and removal

Observed bands are assigned to the stretching and bending vibrations of (SO4)2- and (AsO4)3- units, stretching and bending, stretching and bending vibrations of hydrogen bonded (OH)− ions and Fe3+-(O,OH) units. The approximate range of O-H⋯O hydrogen bond lengths is inferred from the Raman spectra. Raman spectra of crystalline beudantite and of the amorphous gel differ in that the beudantite spectrum is more complex, where bands are sharp and the degenerate bands of (SO4)2- and (AsO4)3- are split and more intense. Lower wavenumbers of δ H2O bending vibration in the spectrum of the amorphous gel may indicate the presence of weaker hydrogen bonds compared with those in beudantite. The formation of beudantite offers a mechanism for the removal of arsenic from the environment.