Understanding the molecular mechanisms of trace metal interactions with mineral surfaces in natural systems: an integrated stable isotope, XAS and HRTEM study

Fundamental understanding of the molecular-level processes involved in metal sorption in dynamic natural systems is still lacking. Research questions concern role of the surface structure of nanoscale substrate, the adsorbed layer composition and thickness, the details of isotope fractionation associated with sorption/desorption and the dynamics of metal sorption and release at near- and far- from equilibrium.

The geochemical cycling of trace elements in the environment is largely controlled by inputs from continental weathering and riverine transport and removal from seawater through a variety of processes at the mineral-water interface. In the environment, interfacial reactions such as mineral dissolution, precipitation, adsorption and desorption of chemical species are responsible for the release of trace elements to soils, sediments and groundwaters. In open oceans, such processes are fundamental to understanding the supply and limitation of nutrients and their bioavailabilities to microbial life. Whereas adsorption and precipitation processes lead to reducing toxicity and bioavailability of key toxic elements, desorption and mineral dissolution reactions have an opposite effect. The surface structure and size of a mineral particle, organic coatings on a mineral surface, microbial activities and redox reactions all have profound influence on the fate of trace elements in the environment. While a full understanding of these processes is of fundamental importance, there are many unresolved questions regarding the detailed mechanisms occurring in unperturbed natural environments. Probing the exact nature of interfacial processes using experimental or theoretical approaches is difficult because of the complexity of the natural systems, especially with the large number of interacting biotic and abiotic processes that occur at different rates.

Here we attempt to resolve this problem by combining advanced analytical techniques of both simplified model systems and carefully isolated samples from natural environments. In simplified model systems, variables will be controlled to simulate interfacial processes in complex natural systems. Parallel experiments with natural samples collected from the field will be carried out to help place constraints on the variables and types of interacting processes occurring in laboratory experiments that can control the speciation and transformation of chemical species or affect the pathways of certain surface reactions. Because the toxicity and bioavailability of elements is determined by the speciation or chemical forms of the element, this project will employ advanced surface speciation techniques such as synchrotron X-ray absorption spectroscopy (XAS). The XAS technique determines the coordination environment of molecular entities interacting with solid surfaces, including the type and behaviour of chemical species adsorbed on a mineral surface, and thus providing a molecular-level description of its speciation. Critically, sorption reactions are commonly associated with significant isotope fractionation, and so isotopes can be used in unexplored ways to investigate these processes. This fractionation results from a difference in the coordination environment of adsorbed and dissolved metal ions. The magnitude of this effect observed in the laboratory matches that seen in a number of natural systems. For example, previous work on non-traditional isotope systems such as Zn, Cd, Cr, U, Ba, Ni, Cu, Mo, Fe, Nd, Si have demonstrated the association of isotopic effect with sorption processes, but the exact mechanisms are yet to be discovered.

Another important component of this research relates to investigating the size effects of natural nanomaterials especially as it is becoming well-established that many nanostructures (e.g. iron and manganese oxides) form and persist under a variety of environmental conditions. These nanomaterials have profoundly distinct surface properties primarily due to surface and quantum effects. Nanoparticles have large surface areas and high particle number densities, with the fraction of the atoms present at their surfaces much higher than that found in microscale particles or bulk solids. These factors not only affect the physical properties (e.g. magnetic, electrical, optical or mechanical) of nanomaterials, but also their chemical reactivities, which ultimately govern the speciation, fate and transport of elemental species in the environment.

This ‘game-changing’ project employs a novel approach by combining advance spectroscopic methods including synchrotron X-ray absorption spectroscopy (XAS), trace element chemistry and isotopic measurements to unravel, in a series of experiments, the details of molecular interactions at the water-mineral interface.  The objective of this project is to untangle the contributions of different factors that govern trace metal sorption behaviour on mineral surfaces. These factors include: rates of exchange, stability of sorbates, sorption reversibility and hysteresis, mineral aging, surface precipitation, degree of crystallinity of nanoparticles and the stability and aggregation of nanoparticles in natural systems. The ultimate goal of this project is to provide a molecular-level description of the speciation of trace elements associated with mineral surfaces and mechanistic details of interfacial processes in representative natural environments.

Another innovative aspect of this study is the use of isotopes as a precise experimental tool to track time-dependent exchange and equilibrium, and as a signature for identifying the dynamics and processes determined at the molecular and microscopic scale on reactive mineral surfaces. In turn this work will provide the fundamental understanding for interpreting environmental data on metal isotopes.

Impacts of the research

1. Studies of this type should provide a new paradigm for understanding microscopic processes and macroscopic biogeochemical phenomena in natural systems.

2. A unique set of data on fundamental processes will be derived that will used for developing simulation models, for identifying processes controlling transport and element behaviour in different natural, laboratory, and industrial environments, and in reconstructing paleo-environments from trace metals incorporated into sedimentary rocks.

3. There have been many recent studies documenting the isotope variations of a wide range of elements in the environment. In the absence of clear fundamental understanding of the processes involved, quantitative interpretations of environmental data that are used to explain trace element transport and cycling are necessarily limited. The results of this project are therefore expected to be widely used in many future environmental studies.

4. The approaches developed here, and the data obtained will also apply to many other fields, including materials science, environmental protection, and industrial engineering. An important aspect of the project will be in widely disseminating the results and identifying diverse applications.

Aims of the Project

(1) To provide a molecular-level description of elemental sorption and desorption processes in unperturbed environmental systems and the associated isotope fractionation.

(2) To untangle the complex contributions of physiochemical factors that govern the sorption and release of trace metals at the mineral-water interface.

(3) To provide mechanistic details of trace metal speciation at the mineral-water interface

Methods to be used

Thermal Ionization Mass Spectrometry (TIMS); Multiple Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS); Synchrotron X-ray Absorption Fine Structure Spectroscopy (XAS); X-ray Diffraction (XRD); Fourier-transform Infrared Spectroscopy (FTIR); High-Resolution Scanning and Transmission Electron Microscopy (SEM and HRTEM).

Specialised skills the student will need

The candidate will receive adequate training in all relevant experimental techniques. However, this project will be suitable for a candidate with environmental chemistry, mineralogy, geochemistry, materials chemistry or related background.

If interested please contact Don Porcelli donald.porcelli@earth.ox.ac.uk and Imad Ahmed Imad.ahmed@earth.ox.ac.uk