Sulfide exists in ground water due to decomposition of sulfate by the naturally-occurring sulfate reducing bacteria. In drinking water, sulfide can exist in two main forms: (1) unionized dissolved hydrogen sulfide gas; and (2) ionized form, either as HS- or S2-. The distribution of total sulfide between the dissolved hydrogen sulfide gas and the ionized form depends on temperature and pH.
At low pH, most of the total dissolved sulfide exists as unionized hydrogen sulfide, while at alkaline pH, most of the total dissolved sulfide exists as ionized sulfide, mainly HS-. The reason why this distinction is important is because degasification can only remove the unionized dissolved hydrogen sulfide and cannot remove any of the ionized sulfide. Further, this limitation is not due to the method of degasification, but is valid for all degasification methods.
It is also important to recognize that pH is also impacted by the amount of dissolved carbon dioxide, since it forms carbonic acid. Dissolved carbon dioxide is also present in well water naturally along with hydrogen sulfide. During degasification, hydrogen sulfide and carbon dioxide are both removed from the water, thereby resulting in an increase of pH, which results in converting more of the dissolved sulfide into ionized sulfide and less of dissolved unionized hydrogen sulfide.