Towards understanding the dissolution mechanism of clay minerals
BORIS DOI
Subtitle
an ab initio simulations study
Abstract
Clay minerals can be abundantly found in sediments, soils and argillaceous rock formations. Together with their sorption capability, pH-buffering effects and other properties they dominate the geochemistry of aforementioned soils and subsurface formations. Many applications of clay minerals rely on their specific reactivity or ability to retain transition and heavy metals through sorption, surface complexation or even incorporation. The dissolution of such structures would lead to the release of potentially hazardous elements back into the environment, which is why the knowledge of the dissolution mechanism is of utter importance to make predictions of the retention time of detained elements.
Batch experiments to determine dissolution rates in different temperature and pH conditions revealed an initial incongruent dissolution while long term observations show stoichiometric dissolution for different types of clay minerals. Quantum mechanical simulations offer the possibility to investigate phenomena at the atomic scale to gain deeper insight into the reaction mechanism.
This study utilizes ab initio simulations to investigate the atomistic scale dissolution mechanism of pyrophyllite in pure water at neutral pH, where the pyrophyllite is considered as a proxy for 2:1 clay minerals. The dissolution has been studied starting from the two most stable edge faces (110) and (010) with different water pressure configurations. The Metadynamics approach was used to sample the free energy surface of the system with three different coordination numbers of the detaching species as collective variables that span the reaction space.
The results show a clear preference for octahedral Al over tetrahedral Si detachment in either configuration. The dissolution reaction itself is a multistep process with several reaction intermediates in which the connectivity of the detaching complex to the edge surface changes step-by-step. The overall mechanism in all setups can be considered as a sequence of concurrent, reversible elementary reactions:
1) H2O molecules or OH groups perform a nucleophilic attack on the dissolving surface site.
2) One or more ligand exchange reactions in the first coordination shell of the reacting sites lead to changes of its conformation and denticity at the mineral edge surface.
3) Collective proton transfer reactions between acidic and basic oxygen sites mediated via a chain of the hydrogen bonded molecules in the first and second coordination shell of the reacting site binds otherwise dangling bonds of the exchanged ligand.
Some specific phenomena connected to the pressure settings of the bulk water molecules are otherwise rarely observed octahedral coordinated Si and tetrahedral coordinated Al sites as intermediate complexes during the respective detachment reaction.
Batch experiments to determine dissolution rates in different temperature and pH conditions revealed an initial incongruent dissolution while long term observations show stoichiometric dissolution for different types of clay minerals. Quantum mechanical simulations offer the possibility to investigate phenomena at the atomic scale to gain deeper insight into the reaction mechanism.
This study utilizes ab initio simulations to investigate the atomistic scale dissolution mechanism of pyrophyllite in pure water at neutral pH, where the pyrophyllite is considered as a proxy for 2:1 clay minerals. The dissolution has been studied starting from the two most stable edge faces (110) and (010) with different water pressure configurations. The Metadynamics approach was used to sample the free energy surface of the system with three different coordination numbers of the detaching species as collective variables that span the reaction space.
The results show a clear preference for octahedral Al over tetrahedral Si detachment in either configuration. The dissolution reaction itself is a multistep process with several reaction intermediates in which the connectivity of the detaching complex to the edge surface changes step-by-step. The overall mechanism in all setups can be considered as a sequence of concurrent, reversible elementary reactions:
1) H2O molecules or OH groups perform a nucleophilic attack on the dissolving surface site.
2) One or more ligand exchange reactions in the first coordination shell of the reacting sites lead to changes of its conformation and denticity at the mineral edge surface.
3) Collective proton transfer reactions between acidic and basic oxygen sites mediated via a chain of the hydrogen bonded molecules in the first and second coordination shell of the reacting site binds otherwise dangling bonds of the exchanged ligand.
Some specific phenomena connected to the pressure settings of the bulk water molecules are otherwise rarely observed octahedral coordinated Si and tetrahedral coordinated Al sites as intermediate complexes during the respective detachment reaction.
Date of Publication
2022
Year of graduation
2022
Theses Type
dissertation
Subject(s)
Language(s)
en
Author(s)
Schliemann, René Jens |
Faculty/Graduate School
Institute
Access(Rights)
open.access
Primary OA Publication
true