Aquifer ph and enhanced reductive dechlorination

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Source: REGENESIS

The recent Battelle Memorial Institute symposium on bioremediation1 in Baltimore, MD featured a focused session of platform presentations on the timely topic of Bioremediation in Low-pH Aquifers. Throughout the session we heard a variety of case studies and viewpoints, with the primary focus of the session being biological reductive dechlorination of chlorinated ethenes (PCE, TCE, DCE and VC) in low pH environments. The presentations and discussions included cases of inhibited dechlorination at low pH, some observations of successful dechlorination at low pH (<5), as well as some attempts (successful and unsuccessful) to raise the pH of naturally acidic aquifers.

As a leading manufacturer of controlled-release electron-donor substrates, Regenesis takes a strong interest in the influences of chemical and geochemical parameters on biological reductive dechlorination. With respect to pH, the current concerns include: 'Does reductive dechlorination occur in low pH aquifers?' and 'Can naturally acidic aquifers be buffered with injectable chemicals?'

Based on over ten years of experience and groundwater data pertaining to HRC® applications, we conclude that most aquifers have significant buffering capacity, and that bulk groundwater rarely exhibits a significant drop in pH due to fermentation of injected substrates.  That observation aside, the topic of dechlorination at low pH is a complex one.  Most commercial dehalogenating cultures, including Regenesis’ BDI-Plus®, are not active at a pH of less than 5.2 or above a pH of about 8.5.  In addition, sites with low pH often show low activity for microbially-mediated dechlorination.   On the other hand, there are numerous anecdotal accounts of bio-dechlorination activity in low-pH aquifers (pH range 3.5 to 5). This may be explained by heterogeneity of aquifers and/or the ability for microbial communities in certain sites to dechlorinate under acidic conditions.  This issue is a matter of ongoing study in laboratories and field sites around the world, and is of interest to those who provide in-situ remediation technologies for low-pH aquifers.

Speaking to the second question, pH adjustment of naturally acidic aquifers is a formidable challenge.  Some remediation practitioners are attempting to accomplish this by injecting bases (like carbonates or hydroxides) into aquifers.  However, due to the naturally high buffering capacities of soils, it proves to be physically and financially impractical to adjust the pH of many aquifers through the addition of chemical additives.  As more of these projects are completed, the limitations of the process will become clearer.  To date, such methods are producing mixed results in terms of pH adjustment and subsequent dechlorination progress.

While the issues of dechlorination at low pH and aquifer pH adjustment are complex and perhaps different at every site, Regenesis recommends that you consider the following when deploying one or more of our controlled-release electron donors:

  1. The controlled-release electron donors HRC® and 3-D Microemulsion (3DMe)® are effective at doses far below 1% of the mass of aquifer soils within a treatment volume.  Therefore, the buffering capacity of the soil typically outpaces acid quantities generated by fermentation and dechlorination.
  2. Organic acids, the building-blocks of our controlled-release substrates, are excellent fermentable substrates for enhanced reductive dechlorination.  After their application into wells or direct-push points, we observe a sustained lowering of pH in the immediate column of injection, due to high concentrations of active ingredients (organic acids).  This pH is rarely communicated away from the concentrated electron donor to the bulk of the aquifer, and will not inhibit bioremediation of chlorinated solvents.
  3. In-situ adjustment of pH adds cost to any bioremediation project.  Unless it is warranted by an inherently low buffering capacity aquifer, it may be prudent to avoid this unnecessary chemical cost.
  4. Aquifers with extremely low buffering capacities may be subject to a lowering of pH during the course of fermentation of an injected electron donor. In such cases, Regenesis offers a formulation of 3-D Microemulsion (3DMe)® that can be injected at a pH of approximately 6.5.
  5. Lastly, a clarification of chemical terms.  The word “buffer” is used to describe a mixture of a weak acid and base that maintains a certain pH in solution, despite addition of other acids and bases to the system.  Soil functions in this manner as a solid buffer.  Injectable chemicals, however, are typically simple bases or acids that attempt to adjust the equilibrium pH of the groundwater/soil system.

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