Using Iron to Control Sulfides?

Many areas of the United States still use ferric or ferrous chloride for H2S control. Whether you are using ferric or ferrous, choride or sulfate, the usual discussion of sulfide removal emphasizes only the iron chemistry and glosses over other reaction products resulting from the chloride and sulfate that is released, when the iron combines with sulfur to form iron sulfide, a black precipitate.

What happens when iron sulfate is used for odor control in wastewater? 

The first step in the reaction produces iron sulfide as a precipitate and sulfuric acid. The use of ferrous choride is just as problematic, producing hydrochloric acid and creating significant challenges downstream for water re-use.

If this were all that happens in wastewater, iron would be fed at a stoichiometric rate of 4.5 lbs. of ferrous sulfate to remove 1 pound of H2S. 

However, in real world applications, 2 to 3 times that amount is necessary for sulfide reduction, because there are competing reactions in wastewater that will consume iron. Phosphorus, chlorides, sulfates, hydroxide, carbonate, oxygen and other common compounds compete for the iron - especially in wastewaters high in saltwater infiltration.

The next question to ask is this... what is likely to happen to the H2SO4 that is produced as a reaction product.

First, it is highly corrosive and works to lower the pH in the wastewater attacking both concrete or iron infrastructure. Second, sulfuric acid rapidly dissociates and will allow SO4 to be used as an oxygen source by the same Sulfate Reducing Bacteria (SRB) that produced the problematic H2S in the first place, ultimately producing even more H2S.

In practice, iron sulfate could produce as much sulfide as it removes stoichiometricly, which then must be removed with additional iron. Furthermore, the lowered wastewater pH will cause more H2S to be released as gas, in accordance with both the Law of Equilibrium acid-base chemistry and Henry’s Law of partial pressure.

THIOGUARD® as a Total System Treatment (TST)

In contrast THIOGUARD® technical grade magnesium hydroxide does not lower the pH of the wastewater, produces no reaction products that can lead to additional sulfide production, and produces no sludge to settle out in the system or to be dewatered at the WWTP. 

The chemical reaction is as follows:

The Magnesium binds the sulfide in a similar manner to iron and it will not be released unless the pH drops to 5.5, which does not occur under normal wastewater conditions.  More importantly, THIOGUARD® is preventative and controls the formation of sulfide by maintaining, or possibly increasing pH and retarding Sulfate Reducing Bacteria activity.