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Chemical and Biological Remediation Tetrachloroethene – Case Study

Tetrachloroethene is the systematic name for tetrachloroethylene, or perchloroethylene (“perc” or “PERC”), and many other names.  It is a manufactured chemical that is widely used in the dry-cleaning of fabrics, including clothes. It is also used for degreasing metal parts and in manufacturing other chemicals. Tetrachloroethene is found in consumer products, including some paint and spot removers, water repellents, brake and wood cleaners, glues, and suede protectors.

Tetrachloroethene is a common soil contaminant. With a specific gravity greater than 1, tetrachloroethylene will be present as a dense nonaqueous phase liquid(DNAPL) if sufficient quantities are released. Because of its mobility in groundwater, its toxicity at low levels, and its density (which causes it to sink below the water table), cleanup activities are more difficult than for oil spills (which has a specific gravity less than 1).

In the case study, researchers from Manchester Geomicro, a geo-microbiology and molecular environmental science research group affiliated with the University of Manchester, used combined chemical and microbiological contaminant degradation processes to remediate tetrachloroethene at a contaminated site in Germany.

In the study, the researchers used Carbo-Iron®, an applied composite material consisting of colloidal activated carbon and embedded nanoscale zero valent iron (ZVI). In a recent long term study of a field site in Germany, it was injected into an aquifer contaminated with tetrachloroethene (PCE). Carbo-Iron® particles accumulated the pollutants and promoted their reductive dechlorination via a combination of chemical and microbial degradation processes.

Schematic illustrating Carbo-Iron® particle structure and key chemical and microbial dechlorination pathways

The presence of the dominant degradation products ethene and ethane in monitoring wells over the duration of the study indicates the extended life-time of ZVI’s chemical activity in the composite particles. However, the identification of the partial dechlorination product cis-dichlorethene (cis-DCE) at depths between 12.5m and 25m below ground level one year into the study, suggested additional microbially mediated degradation processes were also involved.

Hydrogen produced by the aqueous corrosion of ZVI contributed to a decrease in the redox potential of the groundwater up to 190 days promoting organo-halide reducing conditions that lasted for months after. The long lasting reducing effect of Carbo-Iron® is crucial to efficiently supporting microbial dehalogenation, because growth and activity of these microbes occurs relatively slowly under environmental conditions. Detection of increased levels of cis-DCE in the presence of various organohalide reducing bacteria supported the hypothesis that Carbo-Iron® was able to support microbial dechlorination pathways. Despite the emergence of cis-DCE, it did not accumulate, pointing to the presence of an additional microbial degradation step.

The results of state-of-the-art compound specific isotope analysis in combination with pyrosequencing suggested the oxidative degradation of cis-DCE by microorganism related to Polaromonas sp. Strain JS666. Consequently, the formation of carcinogenic degradation intermediate vinyl chloride was avoided due to the sequential reduction and oxidation processes. Overall, the moderate and slow change of environmental conditions mediated by Carbo-Iron® not only supported organohalide-respiring bacteria, but also created the basis for a subsequent microbial oxidation step.

This study, published in Science of the Total Environment (Vogel et al. 2018, vol. 628-629, 1027-1036) illustrates how microbes and nanomaterials can work in combination for targeted remediation. The work was led by collaborators (Katrin Mackenzie and Maria Vogel) at the Helmholtz Centre for Environmental Research in Leipzig, Germany, and adds to a growing portfolio of research highlighting the potential of Carbo-Iron® as an in situ treatment for contaminated groundwater.

 

Remediation of Trichoroethane (TCE) – contaminated groundwater by persulfate oxidation

Researchers in Taiwan performed field trials on the ability of persulfate to remediate trichloroethane (TCE) contaminated groundwater.  The purpose of the field trial was to (1) evaluate the efficacy of TCE treatment using persulfate with different injection strategies; (2) determine the persistence of persulfate in the aquifer; (3) determine the persulfate radius of influence and transport distance; and (4) determine the impact of persulfate on indigenous microorganisms during remediation.

The researchers discovered that persulfate removed up to 100% TCE under specific conditions.  Overall, they found a single, higher does of persulfate was more effective at destroying TCE than two separate, smaller doses.

Results show that sequential injections of a large amount of persulfate are suggested to maintain good long-term performance for TCE treatment. This paper is available at http://pubs.rsc.org/en/content/articlehtml/2018/ra/c7ra10860e.

Arsenic found to control uranium contamination

As reported by World Nuclear News, an international team led by the University of Sheffield has discovered that the toxic element arsenic prevents uranium from an abandoned mine in the UK migrating into rivers and groundwater.  The discovery could help in the remediation of former uranium mines and other radioactively contaminated areas around the world, the scientists believe.

The team of scientists – led by the Department of Materials Science and Engineering at the University of Sheffield – studied the uranium and arsenic in the topsoil at the abandoned South Terras uranium mine in Cornwall, England.

The researchers used some of the world’s brightest synchrotron X-ray microscopes – the Swiss Light Source and the USA’s National Synchrotron Light Source – to unearth what is believed to be the first example of arsenic controlling uranium migration in the environment.  These microscopes use intense X-ray beams to focus on a spot just one-millionth of a metre in diameter.

“We use synchrotron X-rays to identify and isolate the microscopic uranium particles within the soils and determine their chemical composition and mineral species,” said co-author of the study, Neil Hyatt.  “It’s like being able to find tiny uranium needles in a soil haystack with a very sensitive metal detector.”

Source: © Claire Corkhill
The abandoned South Terras mine in Cornwall where uranium was mined until 1930

According to the study – published on 14 December in Nature Materials Degradation – ore extraction processes and natural weathering of rock at the South Terras mine has led to the proliferation of other elements during degradation, particularly arsenic and beryllium, which were found in significant concentrations.  The arsenic and uranium were found to have formed the highly insoluble secondary mineral metazeunerite.

“Significantly, our data indicate that metazeunerite and metatorbernite were found to occur in solid solution, which has not been previously observed at other uranium-contaminated sites where uranyl-micas are present,” the study says.

Claire Corkhill, lead author of the study, said: “Locking up the uranium in this mineral structure means that it cannot migrate in the environment.”

The researchers concluded that this process at South Terras – which operated between 1873 and 1930, producing a total of 736 tonnes of uranium – is the result of a set of “rather unique” geological conditions.  “To identify this remediation mechanism at other sites, where arsenic and uranium are key co-contaminants, further detailed mineralogical assessments are required,” they said.  “These should be considered as an essential input to understand the ultimate environmental fate of degraded uranium ore.”

“The study has far-reaching implications, from the remediation of abandoned uranium mines across the world, to the environmental clean-up of nuclear accidents and historic nuclear weapons test sites,” according to the scientists.  “It also shows the importance of local geology on uranium behavior, which can be applied to develop efficient clean-up strategies.”

Performance Assessment of Pump and Treat Systems

Researchers at the U.S. Department of Energy’s Pacific Northwest National Library recently released a paper on the Performance Assessment of Pump-and-Treat Systems.

The pump-and-treat (P&T) remediation technology is comprised of three main aspects:  groundwater extraction for hydraulic control and contaminant removal, above-ground treatment, and groundwater monitoring to assess performance.

Pump-and-treat (P&T) is a widely applied remedy for groundwater remediation at many types of sites for multiple types of contaminants. Decisions regarding major changes in the remediation approach are an important element of environmental remediation management for a site using P&T. Performance assessment during P&T remedy implementation may be needed because of diminishing returns, the complex nature of the site and contamination, or other factors.

While existing guidance documents for the performance assessment of pump-and-treat systems provide information on design, operation, and optimization for P&T systems, these documents do not provide specific technical guidance to support remedy decisions regarding when to transition to a new remedy or to initiate closure of the P&T remedy.

In the paper, the researchers describe a structured approach for P&T performance assessment that was developed  using analysis of three example P&T systems. These examples highlight key aspects of the performance assessment decision logic and represent assessment outcomes associated with optimizing the P&T system, transitioning from P&T to natural attenuation, and supplementing P&T with another technology to hasten transition to natural attenuation.

Decision elements for the P&T performance assessment include:

  • Contaminant concentrations and trends
  • Contaminant mass discharge from source areas or at selected plume locations
  • The attenuation capacity of the aquifer
  • Estimated future plume behavior and time to reach remedial action objectives for the site
  • P&T system design, operational, and cost information

Categories of decision outcomes for the P&T assessment include:

  • Initiate P&T remedy closure
  • Continue with existing or optimized P&T
  • Transition to Monitored Natural Attenuation
  • Supplement P&T with other treatment technologies
  • Transition to a new remedy approach