Using metal stable isotopes to investigate chemical weathering of anthropogenic deposits

Overview

Society must actively move towards more circular economies. Alkaline minerals consitute some of the largest waste streams, yet many are difficult to recycle. This project investigates the UK’s rich history of steel production; the remnants of such industrial activity poses both challenges and opportunities. For example, residual ‘slag’ heaps are a clear point source for release of potentially harmful elements, but also might be re-worked to recover the trace metals of increasing value. Slag heaps might also be an effective location for carbon sequestration to help counterbalance increasing CO2 emissions. To evaluate both the positive and negative impacts of such deposits, it is critical to understand how slag is chemically weathered.

This project focusses on the behaviour, speciation and stable isotope systematics of two trace metals that exist in high concentrations in slag heaps: vanadium (V) and rhenium (Re). Vanadium is used in both existing and new technologies. However, it can have a toxicity similar to arsenic when it exists as vanadate; leading to its increasing recognition as a serious environmental hazard [1]. Rhenium (atomic weight 186) is strongly enriched in coal and mine tailings and highly mobile in oxic environnements, making it an ideal tracer of weathering of anthropogenic deposits such a slag heaps. These novel methods of monitoring legacy deposits will be essential in helping steel makers manage current and future production of slag.

Field Location
Consett Steelworks in County Durham was operational from the 1840s to 1980 (Figure 1). The site is underlain by Carboniferous Lower Peninne coal measures. The residual slag heaps are completely buried, but have been drilled to examine the potential of the site for carbon sequestration [2]. As the slag heaps are weathered, they release alkaline, metal-rich leachates into soils and streams which eventually drain into the River Derwent. The pH of the system is elevated (>10.5). Spectacular carbonate “tuffa” precipitate as a result of these unusual river water chemistries (Figure 2). The hydrology, mineralogy and river water trace metal concentrations have been studied [e.g., 2, 3] thus providing necessary context for examining isotopic compositions.

This project will aim to:
• Determine the retention and release pathways for V and Re from slag heaps
• Determine the speciation of V and Re in proximal soils and river water
• Provide the first combined Re-V isotopic dataset and evaluate the relationship between speciation and isotopic fractionation

Click on an image to expand

Image Captions

Figure 1: Consett Steelworks circa 1950
Figure 2: A “Tuffa” made of calcium carbonate in the river Derwent circa 2017

Methodology

The student will sample river water, bedload sediments, soil profile(s) and tuffa in addition to existing slag borehole material. Consett is only 45 minutes from Durham, allowing a program of water, soil and sediment sampling to evaluate seasonal affects on Re and V cycling.

Durham is the only institute in the world with the expertise and capacity to measure both Re and V isotopic compositions [e.g., 4, 5]. The student will use a combination of cutting-edge techniques and the unique sample set to achieve the research objectives.

Specifically, s/he will use methods including:
• Sample purification by column chemistry in the Earth Science department
• Stable isotope analysis by MC-ICP-MS in the Earth Science department
• Trace metal concentration analysis by ICP-MS in teh Geography Department

Project Timeline

Year 1

Year 1: Literature review and compilation of published river trace element datasets; training in column chemistry, clean laboratory procedures and solid and water sample preparation; training on isotope analysis by MC-ICP-MS; field sampling in Consett; write/ defend Research Proposal;

Year 2

Year 2: Sample and data processing, main geochemical approaches (ICP-MS, MC-ICP-MS); further develop writing skills and manuscript preparation for publication.

Year 3

Year 3: Synthesise field and modelling datasets; attend international conference(s); publication and thesis writing

Year 3.5

Complete and submit thesis; finalise manuscripts for publication.

Training
& Skills

This project would suit a student with a degree in Earth or Environmental Sciences (or a related field) and strong interests in environmental, trace metal and isotope geochemistry.

Excellent time management skills coupled with strong numerical, verbal and written communication are important. Previous analytical experience would be an advantage but is not essential. Training will cover field sampling and cutting-edge geochemical methods, including: i) major ion analysis by ion chromatography; ii) measurement trace metal concentration; iii) Re and V stable isotope measurements by MC-ICP-MS.

The student will join the vibrant Durham Isotope Group, which includes research students and postdocs from the Earth Science, Geography and Archaeology departments. S/he will attend national and international conferences, networking events and outreach activities, developing an important network for feedback and future employment.

References & further reading

[1] Watt et al. 2018. Vanadium: a re-emerging environmental hazard. Environmental Science and Technology, 52, 11973-11975.[X] Miller et al. (2011). Re-assessing the surface cycling of molybdenum and rhenium. Geochimica et Cosmochimica Acta, 75, 7146-7179.[2] Pullin et al. 2019. Atomspheric carbon capture performance of legacy iron and steel waste. Environmental Science and Technology, in press.[3] Hobson et al. Behaviour and fate of vanadium during the aerobic nuetralisation of hyperalkaline slag leachate. Science of the Total Environment, 632, 1191-1199.[4] Dellinger et al. 2019. Measurements of rhenium isotopic composition in low-abundance samples. Journal of Atomic Adsorption Spectroscopy, in press.[5] Prytulak et al. 2011. Determination of precise and accurate 51V/50V isotope ratios by MC-ICP-MS, Part 2: Isotopic composition of six reference materials plus the Allende chondrite and verification tests. Geostandards and Geoanalytical Research 35, 307-318.

Further Information

For further information and informal enquiries, please contact Julie Prytulak, Durham University (julie.prytulak@durham.ac.uk)

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