This proof-of-concept study focused on developing a quantitative tool to assess the rates of long-term abiotic transformation occurring at sites impacted by chlorinated solvents including tetrachloroethene (PCE) and trichloroethene (TCE). Abiotic dechlorination of PCE and TCE by naturally present or biogeochemically augmented iron minerals (e.g., mackinawite and green rust) is an important process during natural attenuation of chlorinated ethenes. Although abiotic reduction of TCE and PCE offers cleaner transformation pathways with less accumulation of undesirable intermediates, evidence supporting abiotic degradation in field studies has been difficult to establish. Acetylene is a major product of PCE and TCE reduction by reactive minerals and is an indicator molecule of abiotic degradation. However, the potentially slow rate of acetylene generation and its rapid loss due to mineral adsorption or biological assimilation make it hard to detect at sites undergoing natural attenuation.

In this project, the team developed a sensitive and robust device that can be deployed in situ to selectively accumulate acetylene over a field-relevant time scale. The acetylene collected was quantified using standard instrumental analyses after the sampling activity, and the results are expected to inform the rates and extents of abiotic transformation of chlorinated ethenes. 

In this proof-of-concept project, the team explored two independent approaches to detect and quantify acetylene in water. The original (and the first) approach involved the use of gold (Au) and other metal-based electrodes to detect and quantify acetylene via electrochemical measurements. The choice of Au was based on its strong interactions with acetylene and its application in the past for gaseous acetylene quantification. In the second approach, the use of an azide compound as a highly specific molecular probe for aqueous acetylene was proposed. This approach was based on the copper (Cu)-catalyzed alkyne-azide cycloaddition reaction (or click reaction), in which acetylene reacts with an azide chemical to form a triazole, and quantification of the latter informs the amount of acetylene present. Once the concept of using click chemistry to quantify acetylene was established, the team designed and constructed an acetylene passive sampler containing an azide chemical immobilized in polydimethylsiloxane (PDMS) thin films. The performance of the acetylene passive sampler was verified in laboratory microcosm reactors containing TCE and sulfur-amended Iron(0) materials.

Phase I Results

The original approach using an electrochemical method to quantify acetylene was found to pose significant technical and practical challenges including irreversible poisoning of the Au electrode surface by acetylene oxidation products, interference caused by ethene on a platinum electrode, and the practical difficulty associated with deploying an electrochemical sensing platform in a subsurface environment for long-term monitoring. The team pivoted to the second approach of developing a highly specific acetylene sampler based on the Cu-catalyzed acetylene-azide click reaction. The triazole product of the click reaction was stable and easily quantifiable. Further, the reaction was not affected by variations in groundwater chemistry or the presence of organic and inorganic solutes. The design of the sampler was realized in a simple device containing laboratory-prepared azide-impregnated PDMS membrane. Microcosm studies confirmed the ability of this device to capture acetylene generated during abiotic attenuation of TCE. The amounts of triazole formed correlate quantitatively with the extents of TCE degradation, confirming the validity of the azide-based passive sampler as a simple and inexpensive molecular tool to diagnose the occurrence of abiotic transformation of chlorinated solvents at impacted sites.

The proposed acetylene monitoring device responded directly to a critical research need “to improve the understanding and quantification of natural attenuation mechanisms” at chlorinated solvent sites. If successful, the device and method proposed herein are expected to provide an independent line of evidence for the occurrence of abiotic degradation of chlorinated ethenes and to generate quantitative estimates of abiotic degradation rates and the long-term effectiveness of natural attenuation. Compared to direct water or soil gas sampling, the proposed approach is particularly attractive for sites with low but nontrivial abiotic reduction activity (i.e., having small but non-negligible acetylene generation rates) and for characterizing abiotic attenuation in low-permeability regions that are difficult to acquire samples and investigate. The outcomes fill in critical data gaps in measuring the effectiveness of remedial actions, facilitate investigations into the roles of reactive minerals in natural attenuation of chlorinated solvents, and assist in formulating data-supported long-term site management decisions. (Anticipated Phase II Completion - 2025)