Design tool for estimation of buffer requirement for enhanced reductive dechlorination of chlorinated solvents in groundwater

To assist in the design of enhanced reductive dechlorination systems for in situ remediation of chlorinated solvent source zones, the software BUCHLORAC (BUffering of deCHLORination ACidity) was developed to predict the amount of buffer required to maintain the groundwater pH in a DNAPL treatment zone within the optimal range for dechlorinating bacteria. Reductive dechlorination is an acid-forming process with hydrochloric acid and organic acids typically building up in the treatment zone. Remediation of source zones is associated with such extensive localized dechlorination that it may be common for the soils’ natural buffering capacity to be exceeded. As groundwater acidification may inhibit the activity of dechlorinating microorganisms and thus slow or stall the remediation process, sufficient alkalinity must be present to maintain a near-neutral pH. BUCHLORAC is a Windows Graphical Interface based on an abiotic geochemical model that is implemented through the program PHREEQC. BUCHLORAC allows users to estimate the buffer requirements for their specific operating and design conditions including, for example, site water chemistry, mineralogy, amount of chlorinated solvent to be degraded, type of organic substrate and buffering additive and design inhibition pH.

pH control for enhanced reductive bioremediation of chlorinated solvent source zones

David Andrew Barry, Clare Robinson

Enhanced reductive dehalogenation is an attractive 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 acidification thereby inhibiting the activity of dehalogenating microorganisms. Where the soils’ natural 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 insight into the processes influencing the groundwater acidity as dehalogenation proceeds, and to predict the amount of bicarbonate required to maintain the pH at a suitable level for dehalogenating bacteria (i.e., > 6.5). The model accounts for the amount of chlorinated solvent degraded, site water chemistry, electron donor, alternative terminal electron- accepting processes, gas release and soil mineralogy. While calcite and iron oxides were shown to be the key minerals influencing the soil’s buffering capacity, for the extensive dehalogenation likely to occur in a DNAPL source zone, significant bicarbonate addition may be necessary even in soils that are naturally well buffered. Results indicated 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 required to control the pH for user- specified operating conditions.

Elsevier2009

Insights and Modelling Tools for Designing and Improving Chlorinated Solvent Bioremediation Applications

David Andrew Barry, Alessandro Brovelli, Clare Robinson

The chlorinated solvents tetrachloroethene (PCE) and trichloroethene (TCE) have been used extensively in industry and are now amongst the most common and hazardous groundwater contaminants. These solvents are typically present as dense, non-aqueous phase liquids (DNAPLs) and represent long-term source zones that produce persistent contamination plumes in aquifers. Under anaerobic conditions, chlorinated ethenes may be biodegraded via reductive dechlorination (the biologically mediated, step-wise removal of chlorine) to form ethene, a relatively innocuous end-product. The rate of reductive dechlorination can be enhanced by stimulating the activity of dechlorinating bacteria by injection of an electron donor (typically an organic substrate that generates hydrogen upon fermentation), nutrients and, in some cases, microbial communities known to dechlorinate effectively to ethene (i.e., bioaugmentation). Reductive dechlorination has been shown to be a viable technology for in situ treatment of dissolved chlorinated solvent plumes, and recent laboratory studies have suggested that this strategy may also be effective for chlorinated solvent DNAPL. Here, the source zone is targeted directly, with the aim of reducing its lifespan by enhancing dissolution from the DNAPL and sorbed phases and coupling this with effective and sustained dechlorination near the DNAPL-water interface and within the plume. This bulletin focuses on modelling of enhanced dechlorination processes in groundwater, including the modelling tools developed in the SABRE project (under which this report was written, http://www.claire.co.uk/index.php?option=com_content&task=view&id=53&Itemid=47&showall=1), insights gained from the models concerning factors controlling the rates and extent of enhanced source zone DNAPL bioremediation, and how the modelling tools can be used to assist future applications of this technology.

CL:AIRE (Contaminated Land: Applications in Real Environments)2010

Buffering agent for pH control during in situ bioremediation of chlorinated solvent source zones

David Andrew Barry, Christof Holliger, Elsa Marina Lacroix

Chloroethenes such as tetrachloroethene (PCE) and trichloroethene (TCE) are among the most prevalent contaminants in groundwater. Because of their low aqueous solubility, PCE and TCE are often present as dense non aqueous phase liquid (DNAPL) that can act as a long term source of groundwater contamination. In situ enhanced bioremediation is an attractive technology for removal of PCE and TCE. It relies on a process called dehalorespiration in which dehalogenating bacteria reduce PCE to ethene, which is a harmless compound for the environment. Enhanced bioremediation is achieved by stimulating the activity of these specialized bacteria through the addition of an electron donor. This method has been widely used for remediation of chlorinated ethene plumes for more than thirty years and recent studies have indicated that it is a promising technology for bioremediation of source zone. However, application of source zone bioremediation is still a significant technical challenge. One of the main issues is groundwater acidification due to dechlorination and fermentation processes thereby inhibiting activity of dehalogenating micro-organisms. Objective of this project is to develop an efficient buffering strategy for source zone remediation. Utilisation of silicate mineral as buffering agent has been investigated through geochemical modeling and batch experiment. A methodology to select appropriate silicate minerals for pH control based on their kinetics properties and field characteristics has also been developed. A geochemical model, done with the chemical speciation code Phreeqc, including transport phenomena has been implemented to assess the influence of silicate dissolution on acidity. Preliminary results have demonstrated that certain minerals belonging to aluminosilicate (nepheline) and magnesium iron silicate (olivine) present kinetics fast enough to neutralize acidity typically produced during bioremediation of DNAPL source zone. Experimental studies including mineral dissolution and reductive dechlorination by bacterial culture are currently performed to validate the results of this model.