Arsenic incorporation in natural calcite lattice: Evidence from electron spin echo spectroscopy

Quaternary travertines of the Middle Pecora Valley (Tuscany, Italy) contain up to 257 ppm arsenic. Such a content is environmentally relevant, but low enough to make the exact chemical speciation of arsenic difficult by applying conventional investigation techniques. The task was addressed by use of the Electron Spin Echo (ESE) spectroscopy, taking advantage from the modulation by the arsenic nucleus of the decay spectrum of the paramagnetic ion Mn(II), occurring as replacement of Ca in the calcite lattice. Interpretation of the spectra suggests that arsenic occurs in the calcite lattice in the position of C, through the substitution CO2−3 ⇔ AsO3−3. This mechanism of arsenic incorporation by calcite may be an effective limit of arsenic mobility under conditions where immobilization through sorption by iron and/or manganese oxyhydroxides is not operating.