Arsenic species as an indicator of redox conditions in groundwater

Abstract

Although the thermodynamically based concept of oxidation-reduction potential has for many decades been an accepted tool for interpretation of the chemistry of hydrochemical systems, attempts at measurement of actual redox levels in natural waters have been fraught with difficulty. Existing methods of measurement involve use of potential-sensing inert metal electrodes or analytical determination of redox-indicator species such as dissolved O 2 or Fe 2+ or redox couples such as SO 2− 4-HS − and HCO − 3-CH 4. As a result of recent advances in analytical methods, it is now possible to determine the concentrations of both As(III) and As(V) at sufficiently low levels so that the apparent redox condition, as pE or Eh, can be computed from measured concentrations of As(III) and As(V) species. The arsenic pE or Eh domain obtained using published thermodynamic data for As species and the assumption of redox equilibrium, provides a basis for obtaining an indication of redox levels within the central portion of the redox field for natural waters. The redox domain for the As couple is largest at high total dissolved As concentrations, but even at concentrations as low as 1–10 μg/l the domain has significant extent. Oxidation and reduction of As(III) and As(V) in laboratory trials with redox agents common to natural waters, such as O 2, H 2S and Fe, suggests that oxidation or reduction of As species in natural waters occurs at rates sufficiently slow to enable water samples to be collected, transported and analysed before excessive change in species distribution takes place, but rapid enough for As species to adjust to the dominant redox condition of the water if periods of years or longer are available for equilibration. Because of the long equilibration time and the position of the pE-pH domain for the As couple, groundwater is best suited for use of As as a redox indicator.