Nucleation and growth of iron sulfides: linking theory and experiment

Iron sulfides are widespread in the environment, where they regulate and control the global geochemical iron and sulfur cycles. However, despite their application as indicators for seawater anoxia and recorders of early-life isotopic and paleomagnetic data, iron sulfide minerals are still largely unexplored compared to, for example, iron oxide minerals or the silicates or carbonates.

Numerous iron sulfide phases are known, but many are highly unstable or only partially stable for a short time in the environment. Even the least reactive iron sulfide, pyrite, is no longer stable once exposed to air at the Earth's surface. Its dissolution leads to the problem of acid mine drainage, where sulfuric acid and any trapped toxic metals are released with devastating effects on the environment near the mine. However, iron sulfides also have beneficial effects on the environment, as they easily incorporate metals within their structure, and thus could be sinks for toxic metals or radioactive elements.

An intriguing aspect of iron sulfides is the crucial role they may have played in the Origin of Life. Thin layers of iron-nickel sulfide are believed to have formed in the warm, alkaline springs on the bottom of the oceans on Early Earth. They are increasingly considered to have been the early catalysts for a series of chemical reactions leading to the emergence of life. The oxygen-free production of various organic compounds, including amino acids and nucleic acid bases - the building blocks of DNA - is thought to have been catalyzed by small iron-nickel-sulfur clusters, which are structurally similar to the highly reactive present day iron sulfide minerals greigite and mackinawite, yet we know little about how they form.

In view of the likely role of such reactive minerals in the conversion of pre-biotic CO2 on Early Earth, we may well be able to harness iron sulfides as present-day catalysts for the same process, thereby potentially aiding the slowing down of climate change by converting the CO2 we produce into useful chemicals. In today's world, the importance of such iron-nickel-sulfide clusters as catalysts has been confirmed, as several life-essential iron-sulfur proteins help transform volatiles such as H2, CO and CO2 into other useful and less harmful chemicals.

In all of the above examples, it is important to understand that the reactions that lead to the formation of all these minerals which are necessary for any of the geologically stable minerals to exist (i.e., pyrite) all rely on our understanding of the nucleation and growth of unstable precursors or of the reaction transforming one phase to another. Furthermore, the structure and reactivity of each of these phase determines its role and potential application in the environment. A few research groups in the UK and abroad have carried out high quality investigations of the properties of a number of iron sulfide minerals, but it is particularly difficult to investigate events early on in the nucleation process, even though they set the scene for all subsequent transformations.

In this project we propose to employ a robust combination of state-of-the-art computation and experiment to unravel the nucleation of iron sulfide mineral phases. We aim to follow the reactions from the emergence of the first building block in solution, through agglomeration into larger clusters, their aggregation into nano-particles and the eventual transformation into the final crystal. We anticipate that this project, investigating short-lived processes which are only now accessible to study through the development of high temporal and spatial resolution in-situ characterization techniques and High Performance Computing platforms, will lead to in-depth step-by-step quantitative insight into the iron sulfide formation pathways and enhance our fundamental understanding of how a mineral nucleates in solution.

The beneficiaries of this project range from (i) the general public, (ii) environmental protection agencies in the UK and abroad, and (iii) the Government and public sector, to potentially (iv) a variety of industries, as well as (v) academic colleagues, as outlined in the Academic Beneficiaries section.

(i) General Public
It is likely that iron sulfides have played a major role as primordial catalysts in the synthesis of the earliest organic molecules on Earth, which will be of great interest to the general public. As a consequence of their catalytic role in the Origin of Life, iron sulfides also show promise as potential present-day catalysts for the conversion of CO2 at ambient temperatures and pressures, and again, the general public will be clear beneficiaries. We are all under threat from the potentially disastrous effects of climate change, such as rising sea levels and extreme weather conditions, and methods for the reduction of CO2 levels will therefore benefit all humans as well as the natural environment generally.
In addition, any progress made in understanding immobilisation of heavy metals or re-generation of acidified soil after mining/ground operations also benefits us all, whether by providing new routes to regenerate contaminated land to make it suitable for housing or for leisure/recreational purposes.

(ii) Environmental protection agencies
In addition to carbon conversion, this project will impact on other areas of environmental concern: acid mine drainage and immobilisation of heavy metals in the soil. Greater insight into when and how heavy metals become incorporated into the sulfide minerals during their formation may provide routes to more efficient clean-up procedures, whereas fundamental understanding of the processes involved in the pathway from pre-cursor phases through meta-stable intermediates to the final stable minerals may assist in designing conditions during mining and other groundwork operations that will either prevent or remediate this acidification process of the surrounding soil.

(iii) Commercial Sectors
A number of commercial sectors may be interested in the findings from this project. Catalyst developers/manufacturers are clearly one sector, which would benefit greatly from the knowledge of the formation of iron sulfide nano-particles with catalytic centres for the conversion of CO2 into useful chemicals. We already work closely with Johnson Matthey on a number of projects and they, as well as catalyst industries generally and chemicals companies will benefit from the project (letter attached).
Moreover, in addition to the interest from Government environment agencies, commercial providers of environmental monitoring and clean-up of soil contamination and other land managers will benefit from the research outcomes.

(iv) Government/Public Sector
With ever more stringent legislation put in place to guarantee a cascade of international agreements to reduce CO2 and other greenhouse gases to acceptable levels, viable routes to carbon reduction are clearly of prime importance to policy makers and legislators. New catalysts for CO2 conversion, that can be produced and operated under ambient conditions are therefore of great interest.
Similarly, DEFRA will be interested in the fundamental research into the minerals and compounds causing - but potentially also remedying - acid mine drainage, and their potential for pollutant immobilisation.

(i) Third Sector
More speculative beneficiaries of this research are voluntary organisations. With climate change leading to more frequent weather-induced disasters, such as hurricanes, floods and droughts, the call on voluntary aid organisations is increasing rapidly and climate stabilisation would alleviate this burden to sustainable levels.
Local conservation charities will benefit from any insight into clean-up processes this project may help to develop.