pHcontrol for enhanced reductive bioremediation of chlorinated solvent source zones



Enhanced reductive dehalogenation is an attr active treatment technology for in situ remediation of chlorinated solvent DNAPL source areas. Reductive dehalogenation is an acid-forming process with hydrochloric acid and also organic acids from fermentation of the electron donors typically building up in the source zone during remediation. This can lead to groundwater acidifica tion thereby inhibiting the activity of dehalogenating microorganisms. Where the soils’ na tural buffering capacity is likely to be exceeded, the addition of an external source of alkalinity is needed to ensure sustained dehalogenation. To assist in the design of bioremediation systems, an abiotic geochemical model was developed to provide in sight into the processes influencing the groundwater acidity as dehalogenation proceed s, and to predict the amount of bicarbonate required to maintain the pH at a suit able level for dehaloge nating bacteria (i.e., > 6.5). The model accounts for the amount of chlorinated solvent degraded, site water chemistry, electron donor, alternative termin al electron-accepting processes, gas release and soil mineralogy. While calcite an d iron oxides were s hown to be the key minerals influencing the soil’s buffering capacity, for the exte nsive dehalogenation likely to occur in a DNAPL source zone, sign ificant bicarbonate addition may be necessary even in soils that are naturally we ll buffered. Results indi cated that the bicarbonate requirement strongly depends on the electron donor used and availability of competing electron acceptors (e.g., sulfate, iron(III)). Based on understanding gained from this model, a simplified model was developed for calculating a preliminary design estimate of the bicarbonate addition requ ired to control the pH for user-specified operating conditions.

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