Understanding the role of clay mineral redox reactions for Zn sequestration and release

Biogeochemical Cycles

IAP2-19-150

Overview

Anthropogenic activities have in the past and continue today to pollute water and terrestrial ecosystems with metal contaminants. Discharges from abandoned base metal mines are by far the single biggest source of toxic metals such as zinc and cadmium to the aquatic environment of England and Wales [1], and mining is a globally important source of aquatic pollution. Once released into the environment, metal contaminants are particularly difficult to remediate because, in contrast to organic contaminants, they cannot be transformed into benign compounds.

Current remediation strategies often rely on reactions involving minerals, which sequester metals from the aqueous phase, rendering the metal contaminants unavailable to living organisms. For example, sorption to mineral surfaces of iron oxides or formation of new minerals such as sulfides have been employed in active and passive treatment systems to remove Zn from minewater effluents [2]. These same processes also occur in natural environments and may strongly affect the environmental fate of metal contaminants but a class of ubiquitously present minerals in sediments and soils, clay minerals, have largely been overlooked when assessing the fate of metals in the environment. Recently, however, it was shown that interactions of clay minerals with dissolved iron (Fe(II)) lead to the redox-activation of clay mineral iron and the formation of new iron mineral phase(s) [3]. Currently, the role of these processes and the resulting mineral assemblage for metal contaminant fate is unknown.

This project will investigate how interactions of dissolved and clay mineral iron affect the environmental fate of metal contaminants. Here, Zn will be used as a representative metal contaminant because (1) it is a highly relevant contaminant in surface water in the UK and around the world, (2) as a cation it can interact with the negatively charged surfaces of the clay mineral, and (3) it is known to sorb to and incorporate into iron oxide/hydroxides [4]. To understand the contributions and interplay of different processes during the interaction between clay minerals and Zn, the project will address the following research questions:
(1) How do the formed iron precipitate(s) bind Zn (sorption vs incorporation)?
(2) How do environmental conditions (e.g. clay mineral identity, pH, dissolved iron concentration, organic material) affect the type of precipitate(s) formed?
(3) Do the precipitates transform over time and what happens to the metal contaminant?

Methodology

The student will be based in the School of Engineering at Newcastle University (supervised by A. Neumann), with visits to the University of St Andrews to learn and apply specific elemental and isotopic analysis techniques (supervised by P. Savage). This project will be mainly laboratory-based, with the opportunity to ground-truth laboratory observations with field measurements towards the end of the project. This fieldwork (supervised by A. Jarvis) will be carried out in the Coledale, Cumbria, since Newcastle University holds a substantial body of historic water quality for the river (Coledale Beck) in this mining-impacted catchment.

Laboratory experiments with well-characterized clay minerals will be carried out and Zn sequestration will be studied for clay minerals alone, after and during reaction with aqueous Fe(II). To distinguish Zn sorption to the minerals from incorporation into newly formed solids, a selective, sequential extraction method will be developed and verified. Solid and aqueous samples will be analysed for their Zn stable isotope composition to determine whether this tool can distinguish between sorbed and incorporated Zn and be useful for determining the sequestration pathway in natural sediments. In a second step, Zn sequestration will be compared for different clay minerals (different Fe content, excess charge, and charge distribution), pH values, aqueous Fe(II) concentrations, and organic material, to observe trends of sorption vs incorporation of Zn. Again, Zn stable isotope analysis will be used for selected samples to delineate different Zn removal pathways. Finally, the evolution of Zn distribution over the different binding environments (sorbed, incorporated) will be studied over time to determine whether Zn can be irreversibly sequestered and under which environmental conditions. If irreversible Zn sequestration is observed, natural sediments from the Coledale Beck will also be analysed for their mineral composition and Zn binding distribution.

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 metal analysis (ICP-OES, ICP-MS); and techniques for mineral characterization (XRD, FT-IR, Mössbauer spectroscopy). Stable isotope analysis of Zn will be carried out in St Andrews Isotope Geochemistry (STAiG) laboratories, which houses all equipment and clean room facilities necessary for sample preparation and purification as well as multiple collector inductively coupled plasma mass spectrometers (MC-ICP-MS) required for reliably measuring individual isotopes of heavy elements such as Zn.

Project Timeline

Year 1

Detailed literature review; training in laboratory techniques and development of specific, sequential extraction method for Zn in different binding environments; commencement of experiments pertaining to question 1.

Year 2

Continuation with and conclusion of experiments related to question 1; Commencement of experiments pertaining to question 2; preparation of 1st journal article and conference presentation towards end of year.

Year 3

Continuation with and conclusion of experiments related to question 2; experiments pertaining to question 3; laboratory 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 field work 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 (NCL). 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, as well as become familiar with the routine analysis methods of ICP-OES/MS for Zn. 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 the University of St Andrews (UStA) in state-of-the-art MC-ICP-MS analysis of Zn isotopes and all techniques and skills required for sample preparation. 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 NCL or UStA. Furthermore, the PhD student will be encouraged to make use of the broad suite of training opportunities in transferable skills provided at NCL. 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] Mayes, W.M., Potter, H.A.B. and Jarvis, A.P. (2013) Riverine flux of metals from historically mine orefields of England and Wales. Water Air & Soil Pollution, 224:1425.[2] Environment Agency (2014) Mitigation of pollution from abandoned metal mines: Investigation of passive compost bioreactor systems for treatment of abandoned metal mine discharges. Environment Agency Science Report SC090024/1. Environment Agency, Bristol, UK.[3] Schaefer, M.V., Gorski, C.A. and Scherer, M.M. (2011) Spectroscopic Evidence for Interfacial Fe(II)-Fe(III) Electron Transfer in a Clay Mineral. Environmental Science & Technology, 45: 540.[4] Frierdich, A.J. and Catalano, J.G. (2011) Controls on Fe(II)-Activated Trace Element Release from Goethite and Hematite. Environmental Science & Technology, 46:1519.

Further Information

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

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