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Molecular scale understanding of competitive cation adsorption on swelling clay minerals

English title Molecular scale understanding of competitive cation adsorption on swelling clay minerals
Applicant Marques Fernandes Maria do Sameiro
Number 192219
Funding scheme Project funding
Research institution Paul Scherrer Institut
Institution of higher education Paul Scherrer Institute - PSI
Main discipline Other disciplines of Environmental Sciences
Start/End 01.05.2021 - 30.04.2025
Approved amount 624'480.00
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Keywords (6)

Adsorption; Competitiveness; Clay minerals; Molecular scale understanding; X-ray absorption spectroscopy; Atomistic simulations

Lay Summary (German)

Lead
Quellfähige Tone sind natürliche Minerale, die in Böden, Sedimenten und Sedimentgesteinen vorkommen, und wegen ihrer großen Oberfläche und pH-unabhängigen negativen strukturellen Ladung wesentlich dazu beitragen, dass einerseits Wasser und gelöste Nährstoffe pflanzenverfügbar gepuffert, zum anderen aber auch Schadstoffe effizient zurückgehalten werden. Daher werden sie geotechnisch häufig als Barrieren für Schadstoffquellen eingesetzt. Obwohl die Sorptionseigenschaften von quellfähigen Tonen seit Jahrzehnten systematisch untersucht werden, gibt es noch wesentliche Lücken hinsichtlich der Retentionsmechanismen von Elementen (z.B. Schwermetalle, Radionuklide), die - als Schad- oder Spurenelemente - nur bei niedrigsten Konzentrationen vorkommen und daher im Sorptionsprozess mit anderen gelösten Elementen eventuell konkurrieren können (z.B. Fe, Mn).
Lay summary

Inhalt und Ziel

Unser übergeordnetes Ziel ist, zu einem verbesserten Verständnis der Prozesse beizutragen, die die Rückhaltung von Metallionen an der Tonmineral-Wasser-Grenzfläche regeln, mit besonderem Fokus auf den kompetitiven Effekten bei niedrigsten Konzentrationen. Wir erwarten hierbei, die unterschiedlichen Retentionsmechanismen die mitwirken (z.B. Einbau ins Kristallgitter, Chemiesorption an den amphoteren Oberflächengruppen oder Kationenaustausch), zu identifizieren und zukünftig besser vorhersagen/modellieren zu können. Diese Prozesse werden mit konventionellen chemischen und state-of-the-art spektroskopischen Methoden untersucht, parallel dazu aber auch mit atomistischen Simulationen; wir erhoffen uns dadurch auch, die Untersuchung dieser Prozesse mit quantenchemischen Methoden wesentlich vorantreiben zu können.

Wissenschaftlicher und gesellschaftlicher Kontext

Unsere Arbeit wird neue und wichtige Informationen über das Rückhaltevermögen von Tonen bezüglich von Elementen im Spurenbereich generieren. Dadurch werden Risiken bei der Ausbreitung von kationischen Schadstoffen, die zum Beispiel bei industriellen Prozessen oder in zukünftigen radioaktiven Tiefenlagern entstehen, besser abschätzbar. Die Ergebnisse werden daher ein besseres Umweltmonitoring ermöglichen sowie es erlauben, bessere Massnahmen zu ergreifen, um den Schutz der Bevölkerung vor Umweltrisiken sicherzustellen.

Direct link to Lay Summary Last update: 03.04.2020

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Abstract

Clay minerals are fundamental components of soils, sediments (rivers, lakes) and sedimentary rocks, and contribute substantially to their chemical reactivity. Because of their small particle size, high surface area and structural charge, clay minerals act as chemical sink that retains efficiently water, dissolved micronutrients and pollutants. In spite of decades of research on metal adsorption onto clay minerals, there are still substantial knowledge gaps in understanding the mechanism of ion uptake on specific adsorption sites. Previous experimental and spectroscopic studies strongly suggest the existence of high and low-affinity adsorption sites responsible for the metal uptake. The exact molecular nature of these adsorption sites, however, has not been fully explained yet. The aim of the present project is twofold. On the one hand, the project will improve the molecular scale understanding of the retention of common di- and trivalent metal ions onto clay minerals. On the other hand, the project will foster advanced atomistic simulation approaches as well as spectroscopic techniques for the quantitative description of interface processes on mineral/fluid interfaces. With this challenging goal in mind, we will investigate the single and competitive adsorption behaviour of Ni and Lu on a synthetic saponite and on its natural counterpart, vermiculite. The adsorption phenomena in this carefully selected model system will be characterised by a highly complementary combination of wet chemistry, surface complexation modelling, EXAFS spectroscopy and atomistic calculations. Saponite, a trioctahedral mineral of the smectite group, is environmentally relevant and - in contrast to many other clay minerals - can be synthesized relatively easily in the laboratory, allowing the production of pure clays with well-tuned compositions (reference clay). The project proposal comprises two parts. In Part 1, metal-clay systems will be set up, which are well defined and well constrained in terms of mineralogy and surface speciation, and well characterised by a variety of complementary analytical tools (EXAFS, AFM, XRD and FTIR). To this end, different steps are necessary. First, we will synthesize pure and Lu/Ni-doped saponites (reference samples). Second, we will perform acid-base titrations and Ni/Lu (competitive) adsorption experiments on the pure saponites and pointwise on the natural vermiculite. A thermodynamic adsorption model will then be developed from the macroscopic data for both elements. With the aid of the thermodynamic model, we will prepare Ni/Lu clay adsorption samples optimized for specific surface composition, and investigate them by Lu-L3 and Ni-K edge EXAFS spectroscopy along with selected reference compounds (aqueous Lu/Ni and Ni/Lu-doped saponites). Part 2 is dedicated to the atomistic simulations of the clay systems set up in Part 1. First, different approximations and DFT models will be tested to ensure full consistency of the modelled and measured structural parameters for the reference compounds. Next, ab initio molecular dynamics trajectories will be obtained and used to calculate the reference EXAFS spectra for the Ni/Lu saponite system. These ab initio spectra and the experimental spectroscopic data (obtained in Part 1) will be used for quantitative interpretation of the Ni/Lu adsorption on saponite over a wide range of experimental conditions.Particular strength of this project is the use of dedicated experimental data for an independent testing and calibration of the spectroscopic measurements and atomistic modelling techniques. The gained knowledge will contribute to a better fundamental understanding of the processes controlling the speciation and competitive behavior of metals in natural environment at the atomic and molecular scale as well as to a further advancement of the atomistic simulations to describe reliably reactions occurring at the water-mineral interface. The methodological development of simulation approaches and spectroscopic techniques undertaken in this project goes far beyond clay mineral geochemistry and migration of hazardous metals in the environment: The cross benchmarking of spectroscopic techniques and ab initio modelling methods against dedicated experimental data as foreseen in this project will serve as reference for the application of these methods to many other systems, including not only geological and geo-engineered environments in general, but also for instance in materials sciences, biotechnology and catalysis.
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