pH Buffering

The importance of pH buffering in biological reductive dechlorination of volatile chlorinated hydrocarbons (CVOCs)

pH buffering plays a crucial role in biological reductive dechlorination of CVOCs, such as perchloroethylene (PCE) and trichloroethylene (TCE), in contaminated soils and groundwater. During this process, chlorinated compounds are reduced step by step to less harmful substances, such as ethylene. This is done by microorganisms that use electron donors to replace the chlorides.

One of the byproducts of this process is hydrogen ions (H⁺), which can lower the pH in the environment. A decreasing pH has a negative effect on the efficiency of the biological process because many microorganisms involved are sensitive to acidic conditions. Too low a pH can lead to reduced enzyme activity and growth rate of the microbes, slowing or even stopping the reduction process. In particular, the Dehalococcoides mccartyi strains, those responsible for the complete conversion of PCE and TCE to ethylene, are very sensitive. At pH < 6, their activity drops to zero.

Applying pH buffering, such as carbonate buffers or the addition of alkaline substances such as sodium bicarbonate or magnesium hydroxide , prevents the pH from dropping too much. This maintains a stable, neutral to slightly alkaline environment in which the microorganisms involved can function optimally. This makes reductive dechlorination more efficient and allows the remediation process to proceed successfully.

In short, pH buffering is essential to support microbial activity and make the remediation process of CVOCs effective and sustainable.

Figure 1. Relative dechlorination rates (RpH /Rmax) versus pH for consortia SL2-PCEa, SL2-PCEb, AQ-1 and PM (adapted from Lacroix et al. 2014)[32]. Source: Enviro.wiki

pH Buffers

QM Environmental International supplies various buffers of different strengths based on calcium carbonate and sodium bicarbonate and magnesium hydroxide. For groundwater with very low pHs, CoBupHMg is an ideal buffer with a very long-lasting effect.

CoBupHMg

Long-term pH control

CoBupHMg is a safe, easy-to-use, long-term colloidal buffer that can be mixed with the EOS® electron donors if needed.

The colloidal suspension of a natural mineral has an average particle size of 1 µm (micron) making it easy to inject even in very dense soils. The dissociation of hydroxide ions (OH–) from the mineral is controlled by the acidity (pH) of the medium. In groundwater with a low pH, more OH– ions will be released, raising the pH. As pH rises, the dissociation of OH– ions decreases and remineralization occurs.

This allows for long-term action where the mineral releases its hydroxide ions at low pHs and absorbs at higher pHs.

Advantages

  • Long-term pH adjustment
  • Minimal risk of pH overshoot
  • Can be combined with electron donors
  • Small packs for easy dosing
  • Easy to dilute with water on site

pH development with CoBupHMg for 1 year

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