Final Report Summary - RASMIM (Reactivity of Aluminium Sulphate Minerals In Mine wastes (RASMIM))
Final Technical Report for the RASMIM Project
(Reactivity of Aluminium Sulphate Minerals in Mine Wastes)
Marie Curie IEF Fellowship Program
Researcher: Dr. Patricia Acero; patri.acero@gmail.com
Scientist in Charge: Dr. Karen Hudson-Edwards; k.hudson-edwards@bbk.ac.uk
Dpt. of Earth and Planetary Sciences
University of London, Birkbeck (London, United Kingdom)
1. The relevance of aluminium in natural systems
Aluminium is one of the main elements present in many waters affected by acid drainage, in poorly-buffered lakes receiving acid rain and in the pore-water of acid sulphate soils. In these environments, Al plays a key role on the mobility of other elements potentially pollutant and it may also have severe effects on ecosystems (e.g. as a gill toxicant on fish or causing growth inhibition of plants) and even on human health (e.g. Alzheimer’s disease). Thus, the knowledge of the factors controlling aluminium mobility in these environments is of paramount importance in order to mitigate these potential problems.
2. The RASMIM project: understanding the dissolution of key aluminium sulphate minerals
The RASMIM (Reactivity of Aluminium Sulphate Minerals in Mine wastes) project is focused on understanding the behaviour of two of the main sulphate minerals controlling the mobility of aluminium: alunite (KAl3(SO4)2(OH)6) and felsobanyaite (Al4SO4(OH)10·4-5H2O). Some of the questions addressed in the project are the following:
• How stable are these minerals?
• How fast do they dissolve and release aluminium to the environment?
• How important are the temperature and acidity (pH) of the dissolving environment for their dissolution and dissolution rate?
• What happens with the composition of solid alunite and felsobanyaite when they dissolve?
• How (on a molecular scale) does dissolution take place?
3. Laboratory dissolution experiments to understand alunite and felsobanyaite dissolution
To shed light on these questions, the dissolution of both alunite and felsobanyaite was studied under laboratory conditions mimicking the ones commonly found in natural systems. In the dissolution experiments, a small amount of pure, synthetic and powdered mineral was stirred for 1 to 300 hours in contact with different solutions representing a wide range of pH (between 2.5 and 8) and temperature (between 6 and 40oC) conditions. The evolution of both mineral and solution composition throughout the experiments was assessed by different techniques (Scanning Electron Microscopy; SEM, X-ray Photoelectron Spectroscopy; XPS, Inductively Coupled Plasma Atomic Emission Spectroscopy; ICP-AES, Gas Adsorption Isoterms, etc) in order to characterize the dissolution process. Moreover, the obtained results were interpreted with the assistance of two types of numerical modelling; geochemical modelling and atomistic computer simulations.
4. Project results and conclusions
The results obtained during the experiments show that alunite dissolution rates are between 10-10 and 10-11 mol·m-2 s-1 whereas the dissolution rates for felsobanyaite are up to 100 times faster under similar pH and temperature conditions. When dissolved in acidic sulphate solutions (pH ≤ 4.5) both minerals tend to dissolve faster when the solution acidity and temperature are increased. For alunite, the pH increase above pH around 4.6 also seems to promote a faster mineral dissolution.
When dissolved in contact with acidic (pH ≤ 4.5) sulphate solutions, alunite tends to release Al, K and sulphate in similar proportions to the ones present in the pure mineral (i.e. congruent dissolution). On the contrary, for less acidic or basic solutions (pH between 4.5 and 8), the experimental results suggest the precipitation of other aluminium minerals on the dissolving grains. This leads to the release of lower proportions of Al than the ones in the dissolving alunite (i.e. incongruent dissolution) but do not seem to hinder alunite dissolution.
For felsobanyaite, dissolution in sulphate solutions seems to be incongruent even under acidic conditions (pH ≤ 4) due to the possible precipitation of other aluminium minerals or to the preferential release of sulphate over Al.
Atomistic computer simulations for alunite suggest that most mineral surfaces expose K atoms and/or OH- groups. Thus, these components should be the easiest to detach from dissolving alunite, whereas Al and SO4 are much less accessible to the solution and should be the ones limiting alunite dissolution.
5. Relevance of the obtained results and future research lines
The obtained rates for alunite and felsobanyaite can be incorporated into mathematical expressions that allow quantifying their dissolution and, particularly, the associated release of aluminium. Such expressions are, in turn, key for the assessment and modelling of the Al behaviour under a wide range of pH and temperatures typical from most natural waters rich in sulphate. The experimental results suggest, for instance, that pH values around 4.5 and low temperatures would diminish alunite and felsobanyaite dissolution, favouring their preservation and decreasing the release of aluminium to the environment.
Some interesting questions related to the dissolution of both minerals remain open and should be the topic for future research lines, such as the exact mineralogical nature of secondary precipitates, the structure of precipitated surface coatings or the influence of other types of solutions (e.g. more concentrated) on dissolution rates and mechanisms.
(Reactivity of Aluminium Sulphate Minerals in Mine Wastes)
Marie Curie IEF Fellowship Program
Researcher: Dr. Patricia Acero; patri.acero@gmail.com
Scientist in Charge: Dr. Karen Hudson-Edwards; k.hudson-edwards@bbk.ac.uk
Dpt. of Earth and Planetary Sciences
University of London, Birkbeck (London, United Kingdom)
1. The relevance of aluminium in natural systems
Aluminium is one of the main elements present in many waters affected by acid drainage, in poorly-buffered lakes receiving acid rain and in the pore-water of acid sulphate soils. In these environments, Al plays a key role on the mobility of other elements potentially pollutant and it may also have severe effects on ecosystems (e.g. as a gill toxicant on fish or causing growth inhibition of plants) and even on human health (e.g. Alzheimer’s disease). Thus, the knowledge of the factors controlling aluminium mobility in these environments is of paramount importance in order to mitigate these potential problems.
2. The RASMIM project: understanding the dissolution of key aluminium sulphate minerals
The RASMIM (Reactivity of Aluminium Sulphate Minerals in Mine wastes) project is focused on understanding the behaviour of two of the main sulphate minerals controlling the mobility of aluminium: alunite (KAl3(SO4)2(OH)6) and felsobanyaite (Al4SO4(OH)10·4-5H2O). Some of the questions addressed in the project are the following:
• How stable are these minerals?
• How fast do they dissolve and release aluminium to the environment?
• How important are the temperature and acidity (pH) of the dissolving environment for their dissolution and dissolution rate?
• What happens with the composition of solid alunite and felsobanyaite when they dissolve?
• How (on a molecular scale) does dissolution take place?
3. Laboratory dissolution experiments to understand alunite and felsobanyaite dissolution
To shed light on these questions, the dissolution of both alunite and felsobanyaite was studied under laboratory conditions mimicking the ones commonly found in natural systems. In the dissolution experiments, a small amount of pure, synthetic and powdered mineral was stirred for 1 to 300 hours in contact with different solutions representing a wide range of pH (between 2.5 and 8) and temperature (between 6 and 40oC) conditions. The evolution of both mineral and solution composition throughout the experiments was assessed by different techniques (Scanning Electron Microscopy; SEM, X-ray Photoelectron Spectroscopy; XPS, Inductively Coupled Plasma Atomic Emission Spectroscopy; ICP-AES, Gas Adsorption Isoterms, etc) in order to characterize the dissolution process. Moreover, the obtained results were interpreted with the assistance of two types of numerical modelling; geochemical modelling and atomistic computer simulations.
4. Project results and conclusions
The results obtained during the experiments show that alunite dissolution rates are between 10-10 and 10-11 mol·m-2 s-1 whereas the dissolution rates for felsobanyaite are up to 100 times faster under similar pH and temperature conditions. When dissolved in acidic sulphate solutions (pH ≤ 4.5) both minerals tend to dissolve faster when the solution acidity and temperature are increased. For alunite, the pH increase above pH around 4.6 also seems to promote a faster mineral dissolution.
When dissolved in contact with acidic (pH ≤ 4.5) sulphate solutions, alunite tends to release Al, K and sulphate in similar proportions to the ones present in the pure mineral (i.e. congruent dissolution). On the contrary, for less acidic or basic solutions (pH between 4.5 and 8), the experimental results suggest the precipitation of other aluminium minerals on the dissolving grains. This leads to the release of lower proportions of Al than the ones in the dissolving alunite (i.e. incongruent dissolution) but do not seem to hinder alunite dissolution.
For felsobanyaite, dissolution in sulphate solutions seems to be incongruent even under acidic conditions (pH ≤ 4) due to the possible precipitation of other aluminium minerals or to the preferential release of sulphate over Al.
Atomistic computer simulations for alunite suggest that most mineral surfaces expose K atoms and/or OH- groups. Thus, these components should be the easiest to detach from dissolving alunite, whereas Al and SO4 are much less accessible to the solution and should be the ones limiting alunite dissolution.
5. Relevance of the obtained results and future research lines
The obtained rates for alunite and felsobanyaite can be incorporated into mathematical expressions that allow quantifying their dissolution and, particularly, the associated release of aluminium. Such expressions are, in turn, key for the assessment and modelling of the Al behaviour under a wide range of pH and temperatures typical from most natural waters rich in sulphate. The experimental results suggest, for instance, that pH values around 4.5 and low temperatures would diminish alunite and felsobanyaite dissolution, favouring their preservation and decreasing the release of aluminium to the environment.
Some interesting questions related to the dissolution of both minerals remain open and should be the topic for future research lines, such as the exact mineralogical nature of secondary precipitates, the structure of precipitated surface coatings or the influence of other types of solutions (e.g. more concentrated) on dissolution rates and mechanisms.
