Although a relatively simple parameter, the effect of pH on enhanced reductive dechlorination (ERD) can be complex. When growing bacteria in the laboratory, it has been clearly established that maintaining a pH close to 7.0 is desirable for most bacteria involved in ERD. However, in trying to achieve a significant rate of reduction, it has been observed that maintaining this pH may not be the best approach.
Recent data has surfaced from a study indicating the effects of adding yeast extract and lactate to Bio-Dechlor Inoculum (BDI™), a culture of dechlorinating microbes including Dehalococcoides ethenogenes. The initial growth rate (microbial production per day) is higher at a pH close to 7, but the dechlorination rate is near zero as the bacteria multiply. Inversely, as the lactate is metabolized, the pH decreases (pH ~4.0), causing a low rate of microbial growth and a higher degree of reductive dechlorination. In the field, the effect of pH becomes more complicated. The pH measured in groundwater may represent the pH of the environment of suspended bacteria but may not be representative for colonies sorbed to soil particles.
For most bacteria, the optimum pH for growth lies between 6.5 and 7.5.1 There is an optimum pH for each bacterial activity, but these values are often different from those necessary for growth. Numerous references have insisted a stall in dechlorination will occur at a pH below 6.0.2-9 This is of particular interest as, during ERD, pH tends to be reduced either by the acidic properties of the added electron donor or by the production of acids during the fermentation of the added electron donor. In a recent study by Lu, Wilson, and Kampbell10, pH was measured in comparison to microbial activity. A total of 10 locations were studied, comprising 15 contaminated plumes. Examination of 81 wells revealed a range in pH of 4.7 to 7.4 and no correlation between viable reductive dechlorinating microbes and pH.
Certainly pH is an important parameter in site characterization; however, shifts in pH are more important than the absolute pH within the subsurface. In our experience, at over 1,500 projects employing Regenesis’ HRC products, we have seen that the buffering capacity of the aquifer matrix serves to dampen any short-term negative effects of pH shifts. This is a direct benefit of the Regenesis Controlled-Release Technology (CRT™) – allowing for a slow and easily buffered release of lactate, as opposed to flooding the subsurface with soluble and rapidly fermenting sugars or organic acids.
If you would like to discuss this in more detail or would like a copy of the references cited, please feel free to contact Regenesis’ R&D department.