Some chemical relationships among sulfur species and dissolved ferrous iron

Abstract

Sulfur species most likely to occur in natural water are HS-1, H2 S, HSO.T1 , and SO4~2 ; and free sulfur (S°) may be a stable form. The stability fields for these five species are shown on an Eh-pH diagram. A second Eh-pH diagram shows the range of conditions under which iron activity could amount to 10~7 molal or more in the presence of solid-phase ferrous sulfide and a total dissolved sulfur content 10~5 molal, in the absence of bicarbonate. Reactions involving oxidation or reduction of sulfur are slow but may be speeded by biochemical influences. If equilibrium is established, natural water containing a few hundred ppm (parts per million) of S04~2 and HCOs1 could contain over 10 ppm of iron only if pH is less than about 6.1 over a rather wide Eh range. Some ground-water samples are not at equilibrium, however. Wells may obtain water from several poorly interconnected zones in which Eh, iron, and sulfur content differ. Oxidation of pyrite evidently can be an important source of iron in ground water. The changes in Eh with depth below the water table and the fluctuation of the water table may result in stratification of iron content in ground water and in erratic changes in iron content of water from wells; they may cause iron deposits to form in aquifers and wells. Such deposits could reduce the capacity of wells to yield water and might interfere with ground-water movement.