Persulfate Chemical Oxidation and Sulfates in Groundwater
I read with interest a recent article (Peroxygen Talk, June 2010) by FMC's Technical Manager, Phil Block, concerning the fate of sulfate in groundwater when employing FMC's patented Klozur® chemical oxidation process. The article discussed the formation of sulfate and sulfides resulting from the application of persulfate when employing the Klozur® process. Sulfate reducing bacteria can (and do) reduce residual sulfate in the groundwater, the end product from the chemical reaction of persulfate by sulfate reducing bacteria. One concern about the application of persulfate is that sulfate by itself is a pollutant and there are secondary drinking water criteria for sulfates. This is an obvious concern when applying persulfates in drinking water aquifer settings.
Another concern, not addressed in the article, is the formation of sulfuric acid from sulfides which are formed by sulfate reducing bacteria. This occurs commonly in sewers where sulfate reducing bacteria and organic matter are present according to the following reactions:
SO4-- + organic matter ---> S-- + H20 + C02
S-- + 2 H+ ---> H2S
In the presence of oxygen and bacteria in sewers, the bacteria will then oxidize the hydrogen sulfide to sulfuric acid:
H2S + 2O2 ---> H2SO4
The formation of sulfuric acid will cause corrosion of concrete sewers over time. This phenomenon is known as "crown corrosion" as the acid gas rises and attacks the top or "crown" of the pipe. Since sewer systems are often subject infiltration from groundwater, this can be a problem with persulfate oxidation projects.
Interestingly, a remedy for this is simply to apply hydrogen peroxide for conversion of the acid to elemental sulfur:
H2O2 + H2S ---> 2H2O +S
In conclusion, when utilizing persulfate oxidation processes are employed for groundwater contamination cleanup, the formation of sulfate and hydrogen sulfide should be anticipated and appropriate control measures should be considered as part of the application process to avoid violation of secondary drinking water standards for sulfate and potential creation of crown corrosion problems where sewer systems are present in the treatment area.
Norfolk Ram Group has successfully employed the Klozur® process for numerous groundwater treatment projects. Call Brian Moran at (508) 478-1276 x12 should you wish to discuss any chemical oxidation project needs you may have.