Introduction
Sulfur occurs may occur in several oxidative states, these include, Hydrogen Sulfide, elemental sulfur and sulfate.Sulfate is typically the form of sulfur found in the aquifer. Sulfate is the most reduced state of sulfur and the primary reason for sulfate is that bacteria have oxidized the other forms of sulfur to the sulfate state. Sulfate has no odor and is highly soluble in water in various sulfate compounds.
Sulfur reducing bacteria can reduce sulfate converting it into hydrogen sulfide in an environment devoid of oxygen. Since drinking water aquifers contains no oxygen, when a carbon source is introduced to a ground water containing sulfate, sulfide conversion can be produced by biological activity. The higher in the aquifer from which the water is withdrawn, the more prevalent will be the organic and biological sources necessary for this conversion. The chemical equation for sulfide conversion from sulfate is shown below:
SO4-2 + 2C organic + 2H20 + microbial activity → H2S + 2HCO-3
Hydrogen Sulfide can occur in the aqueous phase in two forms, as Hydrogen Sulfide or H2S and the bisulfide ion or HS-. The form that Hydrogen Sulfide will take is dependant on the pH of the liquid.
Hydrogen Sulfide (H2S) and its conjugate base the bisulfide ion (HS-), are referred to as total sulfide and occur together naturally at the pH ranges found in Florida ground water. As can be seen in the figure below, between the pH of 6 to 9, Hydrogen Sulfide can be present as Hydrogen Sulfide (H2S).
Hydrogen Sulfide is a very volatile dissolved gas and readily escapes as a gas into the air causing unpleasant odors. At pH of 7 both forms of Sulfide are present in equal concentrations. Downwardly adjusting pH to around 6 will result in converting all of the bisulfide ion, to the volatile form. Conversely, raising the pH to above 9, will convert the bisulfide ion to the sulfide ion. These relationships are shown in the figure below.
The four possible forms of Hydrogen Sulfide are discussed below.