Research Topics

New proxies to study metals in the environment

Non traditional stable isotopes have a unprecedented promise to trace trace metal cycles and geochemical processes in the environment. The aim of our work is to explore applications of these novel proxies, to calibrate them carefully and to understand underlying fractionation mechanisms. 

Recently completed work studying aerosols in selected cities around the work (London, Barcelona, Sao Paolo) suggests that Cu and Zn isotope ratios enable us to differentiate if these air pollutants are derived from break and tire wear or from high combustion emission sources.  This presents a major breakthrough in tackling urban pollution and identifying human sources of Zn and Cu.  Present work focusses on field studies in specific cities around the world (Beijing, Seoul, Delhi, London, Philadelphia) to test this hypothesis.  The field studies are associated with laboratory and computational experiments which determine the extent and direction of isotopic fractionation during important industrial processes including waste combustion, smelting, galvanization and more.  To this end, we have just recently successfully established a model of isotope fractionation during coal composition for Zn.  Furthermore, we have proposed the first conceptual model accounting for observed isotope fractionation of Cu and Zn during uptake in higher plants.  This has already become an important reference point in the literature for the interpretation of observed isotope signatures during numerous environmental studies.  Our next aim is to comprehensively test this model.  To this end, we are investigating isotope fractionation during complexation with phyto-siderophores and other organic ligands.  Recent laboratory experiments, we have confirmed the predicted extent of isotope fractionation of Zn during complexation with phytosiderophores.  We likewise have identified the structural characteristics that determine isotopic fractionation and we have carried out preliminary theoretical computations using DFT theory to model equilibrium fractionation mechanisms during complex formation.  These are first important steps towards the goal to develop a predictive modelling tool for isotope fractionation.  Future work is aiming to improve the theoretical computations and to test the proposed fractionation model for other key elements in the plant – microorganisms – soil – rock system such as Mo, Fe, and Co.

for recent findings, see for example Weiss et al., Chem Geol, 2021, where we present a complete model for isotope fractionation of Zn in the rhizosphere of rice.  

 

Air pollution and its environmental impact 

How does air pollution affect atmospheric trace metal cycles? Understanding this process is critical to develop and implement environmental policies and to assess possible effects for the environmental. 

Of particular interest is the study of the impact of emerging economies and megacities on atmospheric trace element cycles. Emerging economies have increasing industrial activities leading to significant atmospheric emission but the less stringent emission controls typical for these countries have the danger to inversely affect regional and global environmental and ecosystem health. We study different processes including long range transport and atmospheric processing. We quantify the contribution of anthropogenic and natural sources in urban areas as well in remote regions.  Recent focus has been on South America and South East Asia as these regions have among the greatest economic growths. We have recently conducted a field campaign using passive samplers in the Amazon Basin, we collected a suit of surface and deep-water samples from the South Atlantic Ocean and we sampled lichens and peat bogs in the Falkland Islands.  A key part of this work is also to understand the natural atmospheric element cycles. To this end, we use the inorganic, organic and isotope geochemistry of peat bogs and aerosols to identify changes in sources and in deposition fluxes of mineral dust and trace elements.

for recent findings see for example Resongles et al, PNAS, 2021 demonstrating the persistence of gasoline lead in the urban atmosphere

 

Towards sustainable plant production in a changing world

We try to understand how plants and other organisms acquire micronutrients in soils. What mechanisms do they use and how do changes in the environment (climate, pollution) affect these processes? Answering these fundamental questions is critical to our ability to develop effective and sustainable management practises when cultivating these plants. 

Recent work showed strong evidence for the possible role of siderophores and the subsequent uptake of metal-ligand (M-PS) complex into the rice plant and present works tries to improve our knowledge of the reactivity and structures between trace elements and the mugineic acid family of phytosiderophores. We have only recently achieved the successful synthesis of mugineic acid and its derivatives in the laboratory in collaboration with the Vilar group at Imperial and the Suzuki group in Achii Steel and started now thermodynamic and kinetic studies of the complexation reaction. Experimental work on reaction and structures are supported with DFT calculations. Once the bioinorganic chemistry of the M-PS complex is better understood, we will move on and study the uptake of the metal-ligand complexes using stable and radioactive isotope tracers, apply novel imaging techniques using chromophores and develop further our predictive solubilisation model. Furthermore, we try to understand how increasing salinization due to rising sea surface levels affects biogeochemical processes in soils and aquifers. We are studying changes in chemical speciation (in particular the formation of metal ligand complexes) and the effect on adsorption and dissolution. This is achieved using field experiments in collaboration with the International Rice Research Institute and applying an array of different analytical techniques (i.e., isotope dilution mass spectrometry, voltammetry, XAS, sequential extraction, micro beam analysis and microscopy, potentiometric titration). Soil chemical processes are tested in well-constrained laboratory experiments, including the assessment of changes in the labile soil pool and the dynamics of uptake and translocation using stable isotopes and high precision isotope ratio measurements. Surface complexation modelling and reactive transport modelling is used to test our understanding of the system and to develop predictive models. 

for recent findings, see Northover et al, Sci Reports, 2021, showing the impact of salination of the complexation of siderophores with zinc and the impact on micronutrient availability.

 

Waste management in a zero carbon economy  

Nuclear energy and carbon capture play a central role in our efforts to create a zero carbon economy. However, concepts for the safe storage of waste associated with these technologies is needed. 

Our group is at present studying the actinide chemistry in alkaline and saline solutions. The release of uranium from disintegrating cement containers is the critical controlling process during leakage of radioactive nuclides from nuclear waste repositories but the chemistry in these ‘non-ideal’ and alkaline solutions is little understood. In particular, we are interested in the effect of natural organic molecules, including bacterially generated siderophores, and of colloid formation on An mobility. Our most recent work demonstrated for the first time that the formation of colloids does not necessarily lead to increased mobility as previously thought. We use density functional theory calculations to understand whether the organic molecules, acetate, oxalate and desferrioxamine B can complex with uranium in solutions between pH 4 to 6, and 10 to 12, representative of conditions uranium is mobilized in quartz sand. Preliminary results indicate that the organic molecules can complex with uranium, and potentially enhance mobility in acidic conditions (pH 4 to 6), but not in alkaline conditions (pH 10 to 12). Furthermore, we explore the reaction mechanisms leading to the formation of uranium-oxalate complexes in acidic conditions.

for recent findings, see for example Kirby et al, Colloids and Surfaces, 2020, where we show the that colloid formation in alkaline solutions prevents mobility in quartz columns even if pore sizes are much larger than colloid diameters.

 

Effective and sustainable water treatment  

Effective and sustainable water treatment processes are critical in our efforts to meet the UN sustainable development goals.  To this end, we are involved in the development and study of new resins that remove toxic elements from contaminated waters, with a special focus on Arsenic. 

Most of our recent work has focussed on adsorption and photo oxidation mechanisms of arsenite on complex multi-phase systems.  We are developing accurate surface complexation models using a wide range of surface characterization techniques to predict the mechanism and amount of arsenite adsorbed on multi mineral systems.  We are also trying to understand the effect of mixed minerals on oxidation rates and photo catalysis mechanisms, with a special focus on understanding the role of changing band gaps. The work is driven by our involvement in the development of arsenic treatment plants in rural communities in low income countries that are using iron oxide and titanium dioxide combined in a bi-functional composite material. These materials integrate the adsorption performance of the iron phase and photo-catalytic activity of the titanium phase.  Whilst the work has direct application to water treatment, the importance of understanding and predicting photo oxidation and adsorption reactions on multi mineral systems is of fundamental significance to earth and environmental science.

for recent findings, see for example Heiba et al, J Photochem  Photobiol A: Chemistry, 2021 where we develop a novel technique to enable accurate and precise determination of oxidation rates of arsenic over photocatalysts.