pH Buffering – Overview

Acidity Testing – Avoid pH complications before they happen

Every organism has a pH range within which growth is possible and typically a well-defined optimum pH for maximum growth rates and activity.  Although organisms that thrive in low pH (acidophiles) and high pH (alkaliphiles) environments have been isolated, most microorganisms are classified as neutrophiles with pH optimums between pH 6 and 8.  Many important bacterial groups responsible for biodegradation of chlorinated solvents belong to this neutrophile category. For example, the pH for optimal growth of Dehalococcoides ethenogenes strain 195, the only known bacterium capable of complete reductive dechlorination of PCE to ethene, is between pH 6.8 and 7.5. Likewise, optimal pH ranges for several Desulfitobacterium species capable of reductive dechlorination of chlorinated ethenes and ethanes are near neutral. Thus maintenance of a circum neutral pH in subsurface environments can be an important factor in promoting reductive dechlorination.

In pristine aquifer systems, low groundwater pH is relatively uncommon although oxidation of sulfides can lead to pH values as low as 4 to 5 under natural conditions. More often, pH excursions are a direct result of site activities including:

  • co-contamination with strong acids used in a wide variety of industrial processes,
  • source area treatment using Fenton’s-based in situ chemical oxidation, and
  • organic acid production following subsurface injection of an electron donor (e.g. HRC, EOS, molasses, etc.) designed to stimulate biological reductive dechlorination.

Aquifer pH can be increased by the subsurface circulation or injection of a dissolved base or alkaline material. The added alkalinity, however, will be consumed by groundwater acidity and acidic mineral surfaces in the aquifer matrix. Consequently, determination of groundwater and soil acidity is a key component of remediation system design. The acidity and buffering analysis provides the equivalents of base needed to overcome aquifer acidity and maintain a near neutral pH required for optimum biological activity.

Acidity-button

References

Gerritse et al. 1999. Influence of different electron donors and acceptors on dehalorespiration of tetrachloroethene by Desulfitobacterium frappieri TCE1. Appl. Environ. Microbiol. 65(12): 5212-5221.

Maymo-Gatell, X. 1997. Dehalococcoides ethenogenes Strain 195, A novel eubacterium that reductively dechlorinates tetrachloroethene (PCE) to ethene. Report No. AL/EQ-TR-1997-0029.

Suyama et al. 2001. Isolation and characterization of Desulfitobacterium sp. strain Y51 capable of efficient dehalogenation of tetrachlorethene and polychloroethanes. Biosci. Biotechnol. Biochem. 65(7): 1474-1481.