Assessing the interactions of chlorinated solvents at clay mineral surfaces

Biogeochemical Cycles

IAP2-19-148

Overview

Once used widely in industry, chlorinated solvents have been banned for use in Europe and North America since decades because of their carcinogenic and detrimental environmental effects. Yet, their threat to human and environmental health persists even today, as chlorinated solvents are only slowly degraded by micro-organisms and abiotic processes and several microbial degradation products are even more recalcitrant and toxic than the parent compounds. [1] Consequently, most research has focussed on identifying the conditions favouring abiotic degradation processes and has linked reductive degradation of chlorinated solvents to the presence of iron-rich minerals in subsurface sediments. [1] Recent findings on several types of iron-bearing minerals, including clay minerals, suggests, however, that chlorinated solvents cannot be degraded even over extended periods of time although the reaction is thermodynamically feasible. [2]

Interestingly, clay minerals are known to take up significant amounts of chlorinated solvents and subsequently function as a long-term source of chlorinated solvents to the aqueous phase. [3] While these field-based observations suggest interactions between chlorinated solvents and clay mineral surfaces, laboratory experiments showed that chlorinated solvent uptake was significant for clay minerals containing oxidised iron (ferric iron: Fe(III)) but negligible when the clay mineral iron had been reduced to ferrous iron (Fe(II)). [2] At present, it is unclear why and how this change in iron oxidation state resulted in this dramatic alteration in partitioning behaviour of chlorinated solvents. In this project, we will address this knowledge gap and assess the processes governing the interactions between chlorinated solvents and clay mineral surfaces. To improve our understanding of chlorinated solvent transport and degradation in the environment, the project will address the following research questions:
1) Which clay mineral properties affect chlorinated solvent sorption and how?
2) How does solution chemistry impact sorption of chlorinated solvents to clay minerals?
3) Can these interactions observed at the macroscopic scale be related to processes at the micro- to nano-scale, such as sorption at specific surface charge sites?

Methodology

The student will be based in the School of Engineering at Newcastle University (supervised by A. Neumann), where laboratory-based experiments will be carried out to answer questions 1 and 2. The student will visit Durham University to learn and apply specific experiments, as well as computational chemistry tools (molecular dynamics: supervised by C. Greenwell; density functional theory (DFT): supervised by S. Clark), to complement experimental observations with a fundamental understanding of the governing molecular level processes and mechanisms.

Laboratory experiments (Question 1) will be carried out with well-characterised clay minerals differing in total Fe content, extent and location of excess charge, and including kaolinite-type and smectite-type minerals. Additionally, Fe-containing clay minerals will be redox-altered, i.e. Fe(III)-bearing clay minerals will be reduced and Fe(II)-bearing clay minerals will be oxidised to study the effect of redox changes on chlorinated solvent sorption. Sorption kinetics, swelling capacity and extent will be quantified for a large range of relevant contaminants, including hexachloroethane (HCA), tetrachloroethene (PCE), trichloroethene (TCE), isomers of dichloroethene (DCE), and vinylchloride (VC). In a second step and to answer Question 2, a subset of the same clay mineral will be assessed for the effect of interlayer cations (Na+ vs K+ vs Ca2+), anions (Cl- vs SO42- vs CO32-), ionic strength, and pH value on the chlorinated solvent sorption. Bulk sorption experiments will be combined with measurements of mineral charge (zeta-potential) and, where needed, with studies of surface wettability and bulk swelling. Question 3 will be addressed by implementing classical atomistic molecular dynamics simulations and DFT calculations for selected reaction conditions tested in laboratory experiments, where significant effects are noted.

Key equipment and methods to be used in the Environmental Engineering laboratories at Newcastle University include an anaerobic glovebox, enabling experiments under controlled environmental conditions; analytical instruments for organic contaminant analysis (GC-MS, GC-ECD) and ion quantification (ICP-OES for cations, IC for anions); and techniques for mineral characterization (XRD, FT-IR, Mössbauer spectroscopy, zeta-potential measurements). Equipment for studying surface wettability (including contact angle apparatus/goniometer, chemical force microscopy and fluorescence microscopy) is available in the Greenwell Group chemistry labs at Durham University, as are bespoke instruments for measuring compacted bulk clay mineral swelling and in situ X-ray diffraction of crystalline swelling, (supervisor: C. Greenwell). Durham University also hosts the equipment/access to software needed for the molecular dynamics (GROMACS or LAMMPS) and DFT calculations (with Clark a developer of the CASTEP code) as well as the Hamilton high performance computing service.

Project Timeline

Year 1

Detailed literature review; training in laboratory techniques and computational chemistry tools; commencement of experiments pertaining to question 1.

Year 2

Continuation with and conclusion of experiments related to question 1; molecular dynamics and DFT calculations for selected clay minerals (1st part of question 3); preparation of 1st journal article and conference presentation towards end of year.

Year 3

Experiments related to question 2; molecular dynamics and DFT calculations for selected solution chemistry and clay minerals (2nd part of question 3); laboratory and modelling work to be concluded by end of third quarter; writing up of thesis and preparation of journal article(s).

Year 3.5

Completing writing up of thesis and preparation of journal article(s).

Training
& Skills

The student will be trained in all laboratory skills, analytical techniques, and all aspects of computational chemistry as required for the project. Training in all aspects of laboratory work, from planning over implementing to critically assessing, will be provided at Newcastle University. Here, the student will also receive training in experimenting and working with samples under the exclusion of oxygen, be trained in FT-IR and Mössbauer spectroscopy, become familiar with the routine analysis methods for chlorinated solvents (GC-MS, GC-ECD) as well as for cations (ICP-OES) and anions (IC). Specialized training in clay mineralogy and techniques and approaches for their characterization will be delivered through an established 1-week course at the James Hutton Institute in Aberdeen (https://hutton.ac.uk/events/clay-mineralogy-and-its-application-oil-industry). Similarly, the student will receive training at Durham University for carrying out wettability measurements (if required) and in the theory and application of molecular dynamics and DFT calculations. Durham is usually involved in the CCP programmes and previous students have also attended specific CECAM workshops.
Further training needs will be assessed during the first three months of the PhD, involving the PhD student and the entire supervisory team, and a detailed training plan will be developed. This training plan might also include relevant taught MSc modules at Newcastle University or Durham University. Furthermore, the PhD student will be encouraged to make use of the broad suite of training opportunities in transferable skills provided at Newcastle University. The supervisory team will build on these skills trainings and consolidate and deepen the student’s critical analysis and writing skills, their proficiency in preparing manuscripts for publication in peer-reviewed journals, and their competency at delivering conference presentations.

References & further reading

[1] He, Y.T., Wilson, J.T., Su, C. and Wilkin, R.T. (2015) Review of Abiotic Degradation of Chlorinated Solvents by Reactive Iron Minerals in Aquifers. Ground Water Monit Remediat 35, 57-75.[2] Entwistle, J., Latta, D. E., Scherer, M. M., Neumann, A. (2019) Abiotic Degradation of Chlorinated Solvents by Clay Minerals and Fe(II): Evidence for Reactive Mineral Intermediates. Environ Sci Technol, accepted[3] Parker, B. L., Chapman, S. W., Guilbeault, M. A. (2008) Plume persistence caused by back diffusion from thin layers in a sand aquifer following TCE source-zone hydraulic isolation. J Cont Hydrol 102, 86-104

Further Information

For more information, please contact Dr Anke Neumann (anke.neumann@ncl.ac.uk).

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