Swiss National Science Foundation
January 2009 – March 2013
Prof. Jason Gerhard, University of Western Ontario, Canada
The main focus of this project is enhanced remediation of chlorinated solvent source zones. Enhanced remediation typically involves the introduction of a suitable microbial consortium, and an electron donor (organic substrate that undergoes fermentation) or other source of molecular hydrogen, which is subsequently used in microbial anaerobic degradation of aqueous-phase chlorinated solvents. However, the means to achieving successful remediation of such zones in practice have yet to be resolved. Issues that confound remediation schemes include source location and extent, location and timing of electron donor addition to the subsurface, inhibitory aquifer acidification arising from rapid dechlorination as well as microbially induced dissolved-phase carbon dioxide, and addition of buffering agents to counter the aforementioned acidification.
To address these issues, this project will carry out small and medium scale laboratory experiments designed to validate a comprehensive numerical model applicable to enhanced chlorinated solvent degradation processes in aquifers. Laboratory experiments will include
- small scale experiments to quantify models of biologically enhanced dissolution,
- 1D small scale column experiments to evaluate buffer addition, and
- 2D medium scale (approximately 2 m × 1 m) experiments to investigate mixing of injected electron donor and buffer with contaminant source and degradation products in a heterogeneous aquifer.
The model will be used to investigate the following aspects of source-zone remediation schemes:
- develop further understanding of the process interactions involved in chlorinated solvent bioremediation,
- identify the means by which mixing of added substances and resident groundwater can be enhanced,
- evaluate controlling parameters affecting remediation scheme efficacy, and (iv) evaluate remediation design optimization.
To develop a novel method for long-term control of groundwater pH, it was decided to test ground silicate minerals as pH-buffering agent. Silicate minerals may act as a long-term source of alkalinity release as i) they dissolved slowly compared to carbonates and ii) their dissolution rate and solubility is pH dependent and increase with acidic pH. In addition, they are easily available at affordable cost as raw material or by product of industrial processes.
Silicate minerals are the most common rock forming mineral and constitute a very diverse group with highly variable dissolution rates, solubilities and compositions. Only a restricted numbers of these minerals present appropriate characteristics to act as buffering agents. A screening methodology, based on numerical simulations, thermodynamic and kinetic considerations, was developed to select potential candidates for pH control. A geochemical model including the main microbial processes driving groundwater acidification and silicate mineral dissolution was also developed. This model provides a useful design tool to estimate the mineral requirement in the perspective of field applications. The results of numerical simulations showed that a dozen of silicate minerals have the potential to act as buffering agent.
Abiotic batch experiments were conducted with five silicate minerals (nepheline, fayalite, forsterite, diopside and andradite) to validate and improved the geochemical model. Abiotic experiments confirmed the buffering potential of these minerals and revealed the importance of secondary precipitation, a process not included in the original formulation of the model. Precipitation of secondary phases can decrease the reactivity of silicates, reduce the aquifer porosity and precipitate nutrients. Therefore, prediction of secondary precipitations was included in the model in order to predict and avoid this type of reaction.
The influence of silicate mineral dissolution on OHRB and fermentative bacteria was investigated in batch cultures. As expected, the five silicate minerals (except nepheline) were able to maintain the pH in the tolerance range for the three microbial consortia tested. However, transformation of cis-DCE to ethene was completely inhibited in most of the experiments in presence of minerals. These results showed that compatibility of silicate minerals with the bacterial community involved in in situ bioremediation has to be carefully evaluated prior to their use for pH control at a specific site.
Subsequently, the long-term buffering potential of the most promising buffering agents (diopside, fayalite, forsterite) was tested in continuous-flow column studies simulating chloroethene source zone conditions for six month and a half. In contrast to batch experiments, transformation of cis-DCE to ethene was not inhibited by mineral dissolution in continuous flow systems. Olivine minerals (such as fayalite and forsterite) appeared as suitable pH buffering agents. They successfully maintained the pH in the neutral range (7.5 for forsterite and 6.5 for fayalite) and sustained the activity of OHRB bacteria. In contrast, the buffering potential of diopside rapidly decreased due to the formation of a less-reactive cation depleted leached layer at the mineral surface.
This project has demonstrated the potential of silicate minerals to act as a long-term source of alkalinity release for groundwater pH control. A global strategy for the selection of appropriate buffering agents based on site characteristics was developed. This methodology was applied to the particular case of chlorinated solvent ISB but can be extended to any groundwater remediation technology requiring close to neutral pH conditions.