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Study of iron-bearing dolomite dissolution at various temperatures: Evidence for the formation of secondary nanocrystalline iron-rich phases on the dolomite surface Mathieu Debure, Pascal Andreazza, Aurélien Canizarès, Sylvain Grangeon, Catherine Lerouge, Paul Mack, Benoit Madé, Patrick Simon, Emmanuel Véron, Fabienne Warmont, and Marylene Vayer ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.7b00073 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 12, 2017
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Study of iron-bearing dolomite dissolution at various
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temperatures: Evidence for the formation of secondary
3
nanocrystalline iron-rich phases on the dolomite surface
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Mathieu Debure(1,*), Pascal Andreazza(2), Aurélien Canizarès(3), Sylvain Grangeon(1),
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Catherine Lerouge(1), Paul Mack(4), Benoît Madé(5), Patrick Simon(3), Emmanuel
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Veron(3), Fabienne Warmont(2), Marylène Vayer(2)
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(1) BRGM – French Geological Survey, Water, Environment and Ecotechnologies Division, Storage
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and Deep Geological Settings Unit; 3, avenue Claude Guillemin - BP 36009, 45060 Orléans Cedex 2 -
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France.
10
(2) ICMN - UMR 7374 CNRS - Université d’Orléans 1b rue de la Férollerie CS 40059, 45071 Orléans
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cedex 2, France.
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(3) CEMHTI, UPR 3079 CNRS, - Université d’Orléans, CS90055, 45071 Orléans, France.
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(4) Thermo Fisher Scientific, Imberhorne Lane, East Grinstead, West Sussex, RH19 1UB UK
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(5) Andra, 1 – 7 rue Jean Monnet, 92298 Châtenay-Malabry, France
15
Abstract
16
We investigated the dissolution of a natural Fe-containing dolomite [Ca1.003Mg0.972Fe0.024Mn0.002(CO3)2]
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under acidic conditions (pH 3-5.5) with atomic force microscopy (AFM) at 20 °C, and with batch
18
dissolution experiments at 80 °C. Dolomite dissolution proceeded by two identified mechanisms:
19
removal of dolomite layers through spreading and coalescence of etch pits nucleated at defect points,
20
and stepped retreat from surface edges. The dolomite dissolution rate increased when pH decreased
21
(from 0.410 nm s-1 at pH 3 to 0.035 nm s-1 at pH 5). Rates calculated from edge retreat (vedges) and
22
from etch-pit spreading rates (vsum) were consistent; the etch-pit digging rate was almost 10 times
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slower than its spreading rate. Nanocrystalline secondary phases precipitated in the course of
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dolomite dissolution at pH 3 and 80 °C were identified as (nano)hematite, ferrihydrite and an ankerite
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like mineral using X-ray diffraction, transmission electron microscopy, MicroRaman and X-ray
26
photoelectron spectrometry. In addition, Mg enrichment of the surface layer was observed at 80 °C.
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The characterizations performed at a nanocrystalline scale highlighted the role played by impurities in
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the dolomite dissolution/precipitation scheme and evidenced that the preponderant mechanism
29
explaining the incongruent dolomite dissolution is secondary phase precipitation from major and minor
30
elements initially present in the pristine dolomite.
*
Corresponding author: BRGM – French Geological Survey, Water, Environment and Ecotechnologies Division, Storage and Deep Geological Settings Unit, 3, avenue Claude Guillemin - BP 36009, 45060 Orléans Cedex 2 France. Tel: +33 238 643 177; Fax: +33 238 644 797; E-mail:
[email protected] ACS Paragon Plus Environment
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Keywords: Dolomite, nanocrystalline secondary phases, Fe oxyhydroxide, Ca-Mg-Fe carbonates, in
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situ AFM experiments.
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Graphical abstract for TOC
34
Fe
Mg Ca
Fe oxyhydroxides
Fe2++ O2
Fe Ca
Ca
Mg Mg Ca
Fe Fe3+ Edge retreat Mg Mg Mg 2+ Ca Fe2+ Ca Fe Mg Mg Edge pit Ca Fe2+ Ca Fe2+ spreading
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Ca-Fe-Mg carbonate
Fe3+
Fe2+ Mg Mg Mg Ca Fe2+ Ca Fe2+ Mg Mg Ca Fe2+ Ca Fe2+
Edge pit digging
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1. Introduction
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Carbonate minerals play an important pH-buffering role1 in both natural and engineered systems.
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Understanding and quantifying calcite (CaCO3) and dolomite (CaMg(CO3)2) dissolution and
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precipitation mechanisms as a function of environmental conditions is necessary if we are to make
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predictions about the evolution of natural systems containing carbonates such as aquifers in karst ,
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marine water chemistry, and the CO2 global cycle , and engineered systems such as in CO2
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sequestration or radioactive waste geological disposal. In the oil and geothermal industries,
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hydrochloric acid injections in wells promote carbonate dissolution in reservoirs and improve
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performance related to porosity and permeability6-8. In CO2 sequestration projects, the high amount of
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injected CO2 reduces the pH and dissolves carbonate minerals in carbonate reservoirs9. Conversely, a
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CO2 injection in basaltic rock leads to the precipitation of Ca-Mg-Fe carbonates and Fe hydroxides . In
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the mining industry, oxidation of sulfur minerals due to natural conditions and/or anthropogenic
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disruption (excavation, pumping) cause acid mine drainage and metal release (Pb, As, Hg, etc.) into
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the environment, acidify the soils and consequently induce carbonate dissolution if those are present
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in the soil10. The behavior of carbonates in acidic fields is also studied for desalination and water
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delivery purposes. Inherently, desalination treatment leads to demineralization. So carbonate
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dissolution is investigated as a way of remineralizing water to protect the distribution system from
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corrosion damage11. In nuclear waste management, transitional acidic events are expected to occur
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because of the oxidation of pyrite in the clay host-rocks , and carbonate mineral dissolution is
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expected to buffer the pH in the host-rock pore water. In these systems, dolomite is often present in
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significant amounts, and participates in controlling pore water composition and pH through dissolution
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and precipitation processes.
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The dolomite dissolution rate depends on several parameters: saturation state, pH, temperature, ionic
60
strength, ion concentrations, and hydrodynamic conditions
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extensively investigated over the years in environmental fields3. The stoichiometry of dolomite
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dissolution as a function of conditions is still under debate as well as the mechanisms involved in fluid
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rock interaction
. Dolomite incongruent dissolution has been explained by the lower hydration
64
energy of Ca
2+
compared to Mg2+, which leads to a lower stability of Ca2+ at the dolomite/water
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interface and to a Ca-depleted layer at the dolomite surface
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reported the formation of a MgCO3 phase at the dolomite surface involving a dissolution/precipitation
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scheme as evidenced for wollastonite21 and for other minerals20,
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investigated dolomite surface reactivity exposed to a continuous flow of supersaturated solutions. The
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precipitation of dolomite layers without any other secondary phase precipitation occurred only from a
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fluid highly oversaturated with respect to magnesite, calcite and dolomite. Indeed, one might expect
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that other Mg or Ca-bearing phases may have formed but they were not observed. Such contradictory
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results underline the complexity of dolomite dissolution and the complex reactions occurring at the
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dolomite surface. Moreover, there is a lack in the understanding of trace-element behavior during
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natural dolomite dissolution, and in particular of Fe behavior, although this is a common trace-element
75
incorporated in the dolomite lattice.
2
3-5
5
12
13-15
. Most of these parameters have been
14-23
13, 14, 19, 22
. More recently, Urosevic, et al.
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. Berninger, et al.
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lately
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Overall, there has been to our knowledge no systematic characterization of the secondary phases
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formed in batch reaction experiments15, 16, 26. Studies focusing on nanoscale resolution have provided
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new insights into the kinetics and mechanisms of dolomite dissolution and precipitation on almost pure
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dolomite without discussing the role of trace elements. Those works found evidence to explain
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dolomite dissolution by either preferential release of Ca over Mg or secondary phase precipitation but
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did not establish the role of impurities inside the structure, or a combination of several mechanisms. It
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is usually admitted in literature that a concentration gradient appears at the mineral surface during its
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dissolution that explains the secondary phase precipitation. However, other even minor mechanisms
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such as preferential release of an ion can occur. Thus, nanoscale characterizations are required to
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clearly identify the products of dolomite dissolution to infer the reaction mechanism.
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This study examined the dissolution of a Fe-bearing dolomite under various pH (3 to 5.5) and
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temperature (20 and 80 °C) conditions, and with reaction time ranging from 30 minutes to 21 days. We
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studied the secondary phase precipitation that occurred during the experiments. The objective was to
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better understand the congruence or incongruence observed during dolomite dissolution, and to
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determine quantitatively the crystal-dissolution rates, the reaction path, how Fe influences the phases
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formed in situ and how these impact the dolomite dissolution and precipitation scheme.
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2. Materials and methods
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2.1. Natural dolomite
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Our dolomite sample came from the French Geological Survey (BRGM) collection (sampled in Ariège,
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southern France). Chemical compositions, determined with a tri-acid attack (HCl, HNO3, HF) on bulk
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powder and by twenty spot analyses of the crystal surface using an electron microprobe, are in good
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agreement (Table 1). The trace element composition is shown in Table S1 of the supporting
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information. The dolomite was cleaved along the (104) surface, with this atomic plane being indexed
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relative to the model proposed by Anthony, et al.
27
. A smooth, bright plane was obtained.
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Table 1. Chemical compositions of dolomite determined by tri-acid attack (HCl, HNO3, HF) on bulk powder
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and by spot analyses using an electron microprobe. The data is given in atoms per formula unit Ca (Fe,
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Mg, Mn)(CO3)2).
Analysis technique
Ca
Fe
Mg
Mn
Tri-acid
0.972 ± 0.049 0.032 ± 0.002 0.984 ± 0.049 0.002 ± 0.002
Microprobe
1.002 ± 0.023 0.024 ± 0.003 0.972 ± 0.024 0.002 ± 0.002
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2.2. Experiments
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Dolomite dissolution was investigated using two types of experiment (Table 2). Atomic force
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microscopy (AFM) experiments were used to follow the dissolution mechanism in situ. These
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experiments were performed at ambient temperature, at three pHs, and were short. Batch experiments
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were performed at 80 °C, for longer contact times (30 min, 1 day and 21 days), and at pH80°C ~3, for
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which the highest dolomite surface reactivity was observed. The temperature of 80 °C was chosen
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based on previous experimental studies on dolomite dissolution (e.g. Gautelier, et al. al.
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and Berninger, et al.
15
, Pokrovsky, et
25
) and temperatures attained in an oil reservoir or CO2 injection well29, 30.
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Table 2. Experimental conditions of the dolomite dissolution tests under various environmental
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conditions (the name of the experiment includes type of experiment, pH and time expressed in hours).
Time
Experiment
pHsolution
(days:hours:min)
Surface 2
(mm )
Volume of solution
Temperature (°C)
(mL)
Environment
AFM-pH3-2.4
00:02:25
3.0 ± 0.1
5.0 ± 0.04
0.7 ± 0.01
20 ± 1
HCl
AFM-pH5-3.1
00:03:07
4.9 ± 0.1
3.9 ± 0.04
0.7 ± 0.01
20 ± 1
HCl
AFM-pH5-20
00:20:00
5.5 ± 0.1
5.6 ± 0.04
0.7 ± 0.01
20 ± 1
Pure water
Batch-pH3-0.5
00:00:30
2.8 ± 0.1
120.5 ± 0.04 44.8 ± 0.06
80 ± 1
HCl
Batch-pH3-24.3
01:00:20
2.8 ± 0.1
36.2 ± 0.04
45.0 ± 0.06
80 ± 1
HCl
Batch-pH3-504
21:00:00
2.9 ± 0.1
23.2 ± 0.04
45.0 ± 0.06
80 ± 1
HCl
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2.2.1.
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Atomic force microscopy experiments were carried out in Teflon cells with an inner volume of 0.7 mL
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at room temperature (20 °C). Each solution consisted of ultrapure water (resistivity = 18.2 mΩ cm)
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prepared immediately before the experiment to avoid equilibration with the ambient atmosphere. The
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amount of carbonate and bicarbonate ions in solution can thus be considered as negligible initially and
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as only coming from dolomite dissolution together with Ca, Mg and Fe. The solution pH was adjusted
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with HCl. The (104) dolomite surface was put in contact with the solution for different time periods
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depending on the pH. The surface exposed to the solution was around 5 mm (Table 2).
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Laboratory observations and measurements used an AFM equipped with a Molecular Imaging Pico+
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fluid cell, working in contact mode under ambient conditions (T = 20 °C). The AFM images were
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collected using Si3N4 contact tips and analyzed with the Gwyddion software (Version 2.44).
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Measurements of step-retreat velocity (or etch-pit spreading rate, vsum) were made from sequential
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images scanned in the same direction. One type of step has an acute angle of 78° (-) because the
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step-edge atoms of the upper terrace overhang those below the step edge31. The other type of step is
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exposed opposite and has an obtuse angle of 102° (+). In all etch pits, the equivalent steps are
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adjacent, whereas nonequivalent steps are parallel. The retreat velocity was given by vsum = (v+ + v-),
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where v+ and v- represent the retreat velocities of + and - steps, respectively17, 32, 33. These two types
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of steps retreat to opposite directions with different velocities as the mineral surface dissolves, and
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hence, the dissolution of dolomite is anisotropic. The vsum values were calculated by measuring the
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length increase per unit time between opposite parallel steps in sequential images. Overall dissolution
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rates, RAFM (in mol cm s ), were calculated according to equation 1 given by Shiraki, et al. :
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=
Atomic Force Microscopy
2
-2
-1
31
∆
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-1
135
where Vdol is the molar volume of dolomite (64.34 cm mol ), Δh is the difference in height along the
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step orientation in sequential images after their time duration, and L is the length between the center
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of the etch pit and its edge.
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The macroscopic dolomite dissolution rates (RMac) were calculated when possible with equation 2: =
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.
(2)
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where [Ca] is the final Ca concentration in mol L-1, V the volume of the solution in L, A the surface area
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exposed to the solution in m , and t the total duration time of the experiment.
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2.2.2.
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Batch tests were performed in 50 mL PTFE reactors at 80 °C. The solutions were prepared following
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the same protocol as AFM experiments. The amounts of solution and surfaces of dolomite in contact
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with the solutions are given in table 2. At the end of the experiments, the reactors were opened, the
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solutions were filtered through 0.1 µm PVDF filters, and then pH and alkalinity were measured if the
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pH during the experiment got higher than 4.5. The monoliths were removed from the reactor, carefully
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rinsed with deionized water and dried for one day at room temperature before characterization.
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2.3. Solution analyses
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The pH measurement of batch solutions was carried out with a Mettler Toledo Seven multi pH meter
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using NIST 1.7, 4, 7 and 9 buffers. Cations (Ca2+, Mg2+ and total Fe) were analyzed in solution by
152
inductively coupled plasma atomic emission spectroscopy (ICP-AES, Jobin Yvon), or by mass
153
spectroscopy (ICP-MS, Thermo Fisher Scientific). Chloride anions were analyzed by ionic
154
chromatography (HPLC, Dionex). Element concentrations in solution were determined with an
155
uncertainty of 3%. Alkalinity was measured using a Titrando 905 and a Dosino 800 equipped with a
156
5 mL syringe (Metrohm) to inject the HCl solution (10-3 mol L-1) into the sample. The alkalinity was then
157
calculated with the Gran method described by Neal 34.
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2.4. Solid characterizations
159
Scanning electron microscopy (SEM) observations were done with a Low Vacuum – Field Emission
160
Scanning Electron Microscope (LV-FE-SEM) TESCAN Mira3XMU (TESCAN, Brno, Czech Republic)
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coupled with an EDAX TEAM system equipped with a SDD detector APOLLO XPP. Detectors used for
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SEM observations were Dual BSE/CL Tescan detectors for backscattered electrons (BSE).
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Observations were performed at HV = 25 kV.
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Transmission electron microscopy (TEM) observations were done with a Philips CM20 operated at
165
200 kV. Prior to observation, samples were scratched with a PTFE spatula and then dry-deposited on
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a carbon-coated copper grid.
2
Batch experiments
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Spot analyses of initial dolomite were performed on monoliths covered with a carbon coating, using a
168
CAMEBAX SX Five electron microprobe with an accelerating voltage of 15 kV, a beam current of
169
12 nA, and a 1 µm beam diameter. Peak and background counting times were 10 s for Ca, Mg, Mn
170
and Fe. Detection limits were 20 ppm for Ca, 8 ppm for Mg, 44 ppm for Mn and 40 ppm for Fe.
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Standards used included both well-defined natural minerals and synthetic oxides. Matrix corrections
172
were done with a ZAF software program .
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The Raman spectra were recorded at room temperature using a Renishaw InVia Reflex Raman
174
microscope to characterize dolomite and the secondary phases formed after reaction. A 633 nm He–
175
Ne laser was initially employed and Raman spectra were acquired with an acquisition time of 1 to 30 s
176
(depending on the phase fluorescence) for a relative wavenumber ranging from 100 cm-1 to 1400 cm-1.
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Measurements were done with a 20 mW laser power using the Renishaw StreamLine
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In this imaging mode, the laser focuses as a line on the sample via a cylindrical lens, decreasing the
179
power density of laser to around 200 µW on each analyzed point on the sample and avoiding phase
180
transformation by laser heating as observed by El Mendili et al. on maghemite
35
TM
36, 37
38
configuration.
and potentially
181
observed by de Faria, et al.
182
were used.
183
X-ray diffraction was performed at fixed incidence of 5° to enhance the signal arising from the surface
184
of the sample. Using a D8 ADVANCE diffractometer with Cu Kα radiation (8041 eV) at 40 mA and
185
40 kV, we selected step sizes of 0.04° with time-by-step fixed at 40 s over a 2θ range of 13° to 60°.
186
The small angle X-ray scattering (SAXS) experiments were performed on the “Xeuss” Xenocs X-ray
187
scattering set-up with a monochromatic X-ray beam at 8041 eV . X-ray scattering measurements
188
were performed versus the components of the wave-vector transfer (transferred momentum) q = ki - kf,
189
defined by the incident ki and the scattered kf wave vectors. To enhance the scattering signal coming
190
from the dolomite surface, an X-ray grazing incidence angle of 0.22° was selected. The two-
191
dimensional SAXS pattern was recorded using a high sensitivity 2D detection (Pilatus 300K hybrid
192
pixel detector) placed at a distance of 2.52 m from the sample, perpendicular to the edge. The
193
scattered beam intensities were collected in a range between [0; 3.5 nm ], by averaging ten frames to
194
improve the counting statistics. To extract morphological information, we analyzed two sections of 2D
195
patterns, or fully integrated the scattering signal. From these 1D data, the contribution of the form
196
factor could be extracted with a size evaluation
197
X-ray photoelectron spectra (XPS) were recorded on a Thermo Fisher Scientific ESCALAB250
198
Surface Science Instrument spectrometer, at a vacuum of 2×10
199
radiation (1486.6 eV) was employed, which was obtained by bombarding the Al anode with an
200
electron gun operating at a beam current of 10 mA and an accelerating voltage of 15 kV. The
201
spectrometer energy-scale calibration and the charge correction considered that the C 1 s signal of the
202
contaminating carbon (C−C or C−H bonds) was centered at 284.8 eV. The detection limit for each
203
element was evaluated at about 0.1 At% and the accuracy for high concentrations about at 1 At%.
on wüstite. A 100x high numerical aperture and a 600 lines/mm grating
39
-1
40, 41
.
−9
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2.5. Saturation index calculations
205
Saturation indices were calculated with PHREEQC v342,
206
Thermodynamic constants for the primary phases and potential secondary phases are given in Table
207
S2 (supporting information). Only potential phases are shown, but no phases in the databases were
208
discarded.
209
3. Results
210
3.1. Solution analyses and saturation index calculation
211
Batch experiments performed at 80 °C with three reaction times and for an initial solution pH80°C ~ 3
212
showed that the pH remained acidic after 24 hours of contact, but was near neutral after 504 hours of
213
contact (Table 3). Ca and Mg concentrations increased in solutions congruently (within uncertainties)
214
according to dolomite dissolution. In the meantime, Fe concentration increased between 30 min and 1
215
day, and then was below the detection limit once a steady state was reached (Batch-pH3-504). In the
216
AFM experiments, the congruence between Ca and Mg was also observed at pH 3 but not at pH 5;
217
AFM-pH5-3.1 displayed a higher Ca concentration in solution than Mg.
43
using the Thermoddem44 database.
-1
218
Table 3. Solution composition at the end of the experiments in mol L (QL: quantification limit,
219
QLFe = 4.8 10 mol L for AFM-pH3-2.4 and AFM-pH5-3.1, 1.8 10 mol L for Batch-pH3-24.3 and Batch-
-6
-1
-6
-7
220
-1
-1
pH3-504 and 3.6 10 mol L for Batch-pH3-0.5).
Experiment
Final pH
Alkalinity
Ca
Mg -5
Fe -5
(1.70±0.07) 10-6
Batch-pH3-0.5
2.82
n.m.
(8.48±0.34) 10
Batch-pH3-24.3
3.00
n.m.
(3.60±0.14) 10-4 (3.44±0.14) 10-4 (9.13±0.37) 10-6
Batch-pH3-504
7.85
(3.2±0.13) 10
AFM-pH3-2.4
3.25
n.m.
-4
-4
(7.24±0.29) 10
-4
(1.58±0.06) 10
-5
(7.82±0.31) 10
-4
