The Control of Solution Composition on Ligand-Promoted Dissolution

Sep 28, 2009 - Dissolution kinetics of barite is faster in an aqueous solution of a ... Lihong Qin , Wenjun Zhang , Jianwei Lu , Andrew G. Stack , and...
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DOI: 10.1021/cg9007894

The Control of Solution Composition on Ligand-Promoted Dissolution: DTPA-Barite Interactions

2009, Vol. 9 5266–5272

Magdalena Kowacz,* Christine V. Putnis, and Andrew Putnis Institut f€ ur Mineralogie, Universit€ at M€ unster, Correnstrasse 24, 48149 M€ unster, Germany Received July 10, 2009; Revised Manuscript Received September 11, 2009

ABSTRACT: The mechanism and kinetics of barite (BaSO4) dissolution in the presence of diethylenetriaminepentaacetic acid (DTPA) has been investigated as a function of solution composition. The dependence of the reaction rate on the background electrolyte present in solution (NaCl or KCl) and on the concentration of the chelating agent (DTPA) is explained by considering chemical speciation and conformational changes of DTPA in the aqueous phase. A mechanism for the promotion of the dissolution reaction by dissociated ionic species is proposed for an organic polyelectrolyte with a strong affinity to Ba2þ ions (DTPA) and for simple inorganic electrolytes. The mobilization of ions from the crystal structure is suggested to be induced mainly by water molecules and not by specific additive-surface interactions. Recognition of the correlation between solution composition, ion-water interactions (hydration phenomena), and the dissolution process enables an explanation of the faster dissolution kinetics of barite in the aqueous solution of a simple inorganic salt (NaCl) compared to in the solution of a strong chelating agent (DTPA). Our findings imply that because the mechanisms of complexation in solution and mobilization of ions from the solid surface are different, the sequestering capacity of the ligand toward dissolved ions cannot be used to predict the dissolution rate.

Introduction Interactions of minerals with aqueous solutions containing organic species are of interest for understanding biomineralization or natural weathering processes and for the proper design of industrial applications. The reaction rates are essential for an estimation of the environmental impact of the processes and for engineering implementation. No quantitative treatment is possible without an understanding of the kinetics of interactions of fluids with solid surfaces. In order to get insight into the mechanism of such interactions, experiments are preferentially carried out in simplified systems. Nevertheless, the weakness in the application of experimental results to natural or industrially relevant multicomponent systems is understanding the often profound influence of “foreign” species in solution composition. Because of their high sequestering capacity toward most of the metal ions, polyaminocarboxylic ligands such as ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), or trietylenetetraaminehexaacetic acid (TTHA) find a broad range of applications in research and are among the most widely used chelating agents in industry.1,2 Two examples where mineral-chelator interactions are of particular importance are in agriculture, where enhanced mobilization of metal ions from soil is used to oppose the effect of trace element deficiency, and in oilfields where dissolution of scale-forming minerals (e.g., barite, BaSO4) increases the efficiency of oil production. Because of their low biodegradability, synthetic organic ligands are also pervasively present in the natural environment.3 Moreover, the chemical structure of these polyaminocarboxylic compounds makes them useful models in studies of naturally occurring ligands such as humic acids in aquatic environment or metalchelators secreted by bacteria.4 Because of the ubiquitous *To whom correspondence should be addressed. E-mail: magdakowacz@ uni-muenster.de. Phone: þ492518333487. pubs.acs.org/crystal

Published on Web 09/28/2009

applications and environmental relevance, several studies have been dedicated to understanding the mechanism of organic ligand-mineral interactions at the molecular level and to the establishment of a theoretical framework allowing an estimation of the reaction rate constants.5-11 Owing to the high affinity of chelators to metal ions, it is commonly assumed that these organic molecules attach to the ions at the crystal surface and inhibit growth sites or actively remove ions from the crystal structure and promote dissolution. As a result, several studies explain reaction control and morphology modification by structural fit or steric hindrance of access of the organic molecules to the respective sites at the crystal surface.12,13 However, it has been shown that modification of reaction kinetics and morphological features in the presence of additives can in some cases be explained without considering any specific interactions between the species present in solution and the crystal surface but are instead controlled by water mediated processes.14 Furthermore, for ligands forming weak complexes with lattice ions, a dissolution mechanism was also proposed15 in which water mobilizes ions that are subsequently sequestered from solution by the respective ligand. The aim of this study is to give some insight into the mechanism underlying the dissolution of barite in the presence of the strong chelating agent, DTPA. We address reaction kinetics and nanoscale phenomena at the mineral surfacefluid interface for barite interactions with aqueous solutions of DTPA as a function of solution composition. There is a need to broaden our knowledge of mineral dissolution as a comprehensive theory of a surface dissolution mechanism is currently lacking.16 Dissolution of barite in particular is of special interest for the offshore oil industry where this mineral represents one of the most common and problematic scales. The most cost-effective way to remove this scale is by chemical treatment with a solution of DTPA.17 Nevertheless, according to the Convention for the Protection of the Marine Environment (OSPAR)18 DTPA should be r 2009 American Chemical Society

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Table 1. Composition of Solutions Used in the Respective Experiments DTPA = [M]

other components

pH

experiment type

0.72 M NaCl 0.65 M NaCl 0.65 M NaCl 0.65 M NaCl 1 M NaCl 0.9 M KCl artificial seawater22 artificial seawater22

11-12 11-12 7 and 9 11-12 11-12 11-12 11-12 11-12 9 8

AFM AFM, closed (i)a, flowb AFM, closed (i), flow AFM, closed (ii)c AFM, closed (ii) AFM AFM AFM closed (ii) closed (ii)

0. 5 0.05 0.05 0.175 0.35 0.05 0.05 0.05 a

Closed reactor system, type (i). b Flow through system. c Closed reactor system, type (ii).

on the substitution list for offshore chemicals as it is not biodegradable in the OECD 306 test.19 Experimental Methods Solution Preparation. DTPA solutions were made following the recipe used by the Mobil Research and Development Corporation20 and following successful trials as a barite solvent in the oil and gas fields of the North Sea. DTPA (Aldrich 99% M.W. 393.4) was dissolved in a 2 M solution of KOH to make a standard 0.5 M DTPA solution. This solution was used to make respective working solutions. The pH was kept in the range of 11-12 by further addition of KOH when necessary. Such a pH range ensures full deprotonation of DTPA and a high stability constant of Ba-ligand complexes (log K = 8.6).21 The composition of solutions used in experiments is given in Table 1. Artificial seawater used in the experiments was prepared according to the recipe given by Kester et al. (1967).22 AFM in Situ Experiments. Experiments were performed at room temperature in a fluid cell of a Digital Instruments (Veeco Instruments, GmbH) Multimode AFM, working in contact mode. Optically clear barite single crystals were cleaved immediately before each experiment to expose a (001) surface. The working solution (Table 1) was injected at intervals of about 2 min before each AFM scan, which gives an effective flow rate of approximately 50 mL/h. Images were continuously taken and the time was automatically recorded. Bulk Experiments: Flow through Reactor System. Experiments were performed in a Teflon flow through cell. 0.3 g of freshly cleaved barite crystals (average grain volume 2 mm3) were placed in the cell and working solutions were pumped through the cell. A constant rate fluid flow of 2.33 mL/min was ensured by a Gilson Minipuls Evolution peristaltic pump. During the experiment, the cell contents were gently agitated by a magnetic stirrer. The outlet solution was sampled at given time intervals and subsequently analyzed for barium concentrations using inductively coupled plasma optical emission spectroscopy (ICP-OES). Bulk Experiments: Closed Reactor System. Experiments were carried out in closed Teflon flasks at room temperature. Two sets of experiments were performed: (i) 0.2 g of freshly cleaved barite crystals (average grain volume 2 mm3) were placed in 200 mL of the respective working solution (Table 1) and gently agitated by a magnetic stirrer. The solution was sampled at given time intervals and analyzed with ICP-OES for the barium concentration. (ii) 1 g of freshly cleaved Barite crystals (average grain volume 25 mm3) was placed in 250 mL of the respective working solution (Table 1) and left to react for 5 or 7 days under gentle stirring. The barium concentration in solution was then analyzed with atomic absorption spectroscopy (AAS). Selected mineral grains were withdrawn, rinsed with Milli-Q water, and dried and their surface morphology was analyzed by atomic force microscopy (AFM) imaging in air.

Results Dissolution Kinetics in NaCl vs DTPA Solution. AFM investigations indicate that the kinetics of barium sulfate

Figure 1. AFM deflection images showing the morphology of a barite (001) surface (5  5 μm) after 5 min and 25 min of dissolution in 0.72 M NaCl (a) and (b) and 0.05 M DTPA (c) and (d). Maximum depth of the dissolution etch-pits is comparable in both solutions. The density and lateral dimensions of deep etch-pits is considerably higher in a NaCl solution. In a DTPA solution, mainly shallow (half barite unit cell) etch-pits are formed.

Figure 2. The concentration of barium released into the solution of 0.72 M NaCl and 0.05 M DTPA, respectively, during barite dissolution in a closed reactor system (left axis) and the corresponding ratio of [Ba2þ] in 0.72 M NaCl to [Ba2þ] in 0.05 M DTPA (right axis). In the first 5 h of reaction, almost twice as much barium is released into the NaCl solution, but as the activity of Ba2þ increases, the system approaches equilibrium. In DTPA solutions, barium ions are sequestered by the ligand; therefore, undersaturation is maintained and more barium can be mobilized into solution.

dissolution is faster in 0.72 M NaCl than in a solution of 0.05 M DTPA (Figure 1). Although the molalities were different, the concentration of charges (as expressed by ionic strength) was comparable in both solutions, because each DTPA molecule can dissociate into an anion bearing five negatively charged groups counterbalanced by five positively charged cations. The nanoscale observations are confirmed by the results of bulk experiments that show that the rate of barite dissolution is indeed faster in solutions of simple inorganic electrolytes than in the presence of the organic ligand DTPA (Figures 2 and 3).

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Figure 3. Rate of dissolution of barite in 0.72 M NaCl and 0.05 M DTPA, respectively, expressed by the concentration of barium released into solution in a flow through system.

Figure 4. AFM deflection images showing the morphology of a barite (001) surface after 2 h of reaction with (a) 0.05 M DTPA solution and (b) 0.5 M DTPA solution. In 0.05 M DTPA, deep etchpits develop on the surface while in 0.5 M DTPA the crystal surface remains intact in the experimental time scale and coverage of the surface by a thin layer of “soft” precipitate can be observed (similar surface phenomena were previously documented by Putnis et al.23).

Figure 5. Graph presenting inverse relationship between concentration of barium released into the DTPA-0.65 M NaCl solution and the concentration of DTPA. Closed reactor system, reaction time: 7 days.

Dissolution Kinetics As a Function of Ligand Concentration. Results of AFM experiments on Barite dissolution in aqueous solutions of DTPA are in agreement with previous findings of Putnis et al.23 and imply that the kinetics of Barite dissolution decreases with increasing ligand concentration (Figure 4). The lower efficiency of the dissolution of barite at higher ligand concentrations has also been established for mineral dissolution in mixed DTPA-NaCl solution in bulk experiments (Figure 5). The AFM investigations of the nanoscale mechanism of barite dissolution in DTPA-NaCl solutions

Figure 6. AFM deflection images of a barite (001) surface (5  5 μm) reacting with a 0.05 M DTPA-0.65 M NaCl solution: (a) simultaneous dissolution (formation of etch-pits of half a barite unit cell in depth, 3.5 A˚) and precipitate deposition (indicated by arrows) on the barite surface just after injection of the working solution and (b) propagation of deep etch-pits on the surface already fully covered by precipitate; (c-e) gradual deposition of a precipitate that repeats the morphology of the underlying surface.

confirm an inverse relationship between the ligand concentration and the barite dissolution rate. At the lowest concentration of DTPA, dissolution of barite proceeds by etchpit spreading (Figure 6a). With increasing concentration of DTPA, the process is inhibited and virtually no further dissolution features developed on the surface (Figure 7). Surface Phenomena. Concurrently with the dissolution process the surface of the barite crystal is gradually covered with a layer of a precipitate (Figures 6 and 7). The precipitate is clearly controlled by the crystallography of the mineral and repeats the morphology of the underlying barite surface. The precipitate always develops with a characteristic thickness of 35 or 70 A˚ corresponding to the height of 5 and 10 barite unit cells, respectively. The precipitate layer could be mechanically removed, for example, by forcing the fluid flow, but immediately it again covered the surface. The precipitate deposition was faster at lower DTPA concentrations. Because of the inverse relationship between barite dissolution kinetics and ligand concentration, it can be concluded that the rate of surface precipitation is controlled by the kinetics of the dissolution of barite. The precipitation phenomenon is evidently coupled with the dissolution reaction. Such a conclusion is also supported by the experiments on barite dissolution in seawater with or

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Figure 9. AFM deflection images (in air) showing the morphology of a barite surface after 5 days of interaction in a closed reactor system with: (a) artificial seawater and (b) 0.05 M DTPA in artificial seawater medium. Surface etching can be observed in the presence of DTPA. Correspondingly 0.04 and 1.44 μM of barium was released into the artificial seawater and DTPA-seawater solution, respectively. Scale bars: 2.5 μm.

Figure 7. AFM deflection images of a barite (001) surface (10  10 μm) reacting with 0.175 M DTPA-0.65 M NaCl for (a) 35 min and (b) 65 min (surface fully covered with precipitate) and (c) 0.35 M DTPA-0.65 M NaCl for 75 min. The crystal surface is gradually covered with a precipitate and no dissolution features can be observed.

Figure 10. AFM deflection images (5  5 μm) showing: (a) immediate inhibition of the barite dissolution process and evolution of the surface precipitate after injection of 0.05 M DTPA-1 M NaCl solution (before the addition of the DTPA-NaCl solution the surface was dissolving in the presence of 0.72 M NaCl) and (b) dissolution of a barite surface in 0.05 M DTPA-0.9 M KCl (the reaction proceeded mainly by spreading of shallow etch-pits).

Figure 8. AFM deflection image (10  10 μm) showing precipitate deposition on a barite surface in contact with artificial seawater containing 0.05 M DTPA after more than 2 h of interaction.

without DTPA present in solution. In the time frame of the AFM experiments, no dissolution features (etch-pits formation or step retreat) could be observed on the barite surface in contact with seawater or with DTPA-seawater media, but in the latter case a surface precipitate gradually developed (Figure 8). AFM observation of the surface morphology of the barite crystals immersed in the respective solutions for 5 days and corresponding analysis of the concentration of barium released into the solution reveal that the precipitate deposition on the barite surface in DTPA-seawater media was accompanied by a dissolution process (Figure 9). Dissolution kinetics of barite in a seawater medium containing

0.4 M NaCl is incomparably slower than in the aqueous solution of 0.72 M NaCl. This can be expected because seawater contains a high concentration of sulfate (∼28 mM) and as soon as barium ions are released into the solution the system becomes close to equilibrium. However, DTPA present in solution can sequester and immobilize Ba2þ in solution, so that undersaturation is maintained. Dependence of the Dissolution-Precipitation Process on the Type of Background Electrolyte. Inhibition of the etch-pit spreading on a barite surface in DTPA-NaCl solutions was observed with increasing ligand concentration (Figures 6 and 7) or with increasing NaCl concentration at a constant concentration of DTPA (Figure 10a). At the same time, the presence of 0.9 M KCl as a background electrolyte did not impede the dissolution process and did not induce the formation of the surface precipitate (Figure 10b). Discussion Dissolution Kinetics in Aqueous Solution. Our results show that the kinetics of barium sulfate dissolution is faster in an aqueous solution of the inorganic electrolyte (NaCl) than in a solution of a strong chelating agent (DTPA). This rather striking observation implies that the dissolution kinetics of barite is not determined by the affinity of dissolved species with ions making up the crystal nor by their ability to bind to and actively remove ions from the crystal surface. Therefore, we suggest that predominately water molecules actually

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mobilize ions from the barite crystal structure and then dissolved ions can be sequestered by species present in solution. The kinetics of such a water-promoted dissolution reaction will be assisted by increased affinity of solvent molecules toward lattice ions and by the enhanced mobility of water (diminished mutual attraction of solvent molecules).14 In salt solutions, ion-water interactions are stabilized in comparison to pure water24 as a result of the electrostatic attraction between water dipoles oriented in an ion solvation shell and the counterions present in solution. This attraction lowers the potential energy of respective water molecules.25,26 Mobility of water molecules depends on the hydration characteristics of the dissolved species.27 According to such an approach, enhancement of the dissolution rate of barite by background electrolytes (ionic strength effect) was attributed to the stabilization of water of solvation by ionic charges.14 Salt specific effects at constant ionic strength were shown to depend on the ion distribution in solution, which determines the screening and propagation of the electric fields, and on the influence of ions on water mobility.14 Mechanism of DTPA - Promoted Dissolution of Barite. Our findings suggest that the mechanism underlying the dissolution of barite in aqueous solutions of organic ligands can be the same as in the case of a simple inorganic electrolyte. Dissociated DTPA can then assist dissolution of barite as a result of the stabilizing effect of its charge on water of solvation of barite building units (Ba2þ and SO42-). Therefore, a solution of a simple electrolyte of high ionic strength can be more effective (in terms of kinetics) in the dissolution of a barite mineral than a solution containing a chelating agent. We do not intend to say that there is no direct interaction between DTPA and the mineral surface, but that ion-ligand bonding is not a prerequisite to accelerate the dissolution process. Dependence of the kinetics of waterpromoted dissolution of barite on the charge of DTPA is corroborated by the rather counterintuitive experimental observation that the efficiency of dissolution of barite decreases with increasing concentration of DTPA.23 At higher concentration of DTPA, the chelating capacity of the ligand-containing solution toward dissolved barium ions increases23 but the degree of ionization of DTPA diminishes.28,29 Therefore, reaction kinetics slow down even though more barium can be immobilized in solution. Furthermore, the dissolution rate of barite in the presence of DTPA is enhanced at higher temperatures,23 which is accompanied by the increase in the ionization constant of DTPA28,29 and increased mobility of water molecules due to breaking of hydrogen bonds. Both the enhanced degree of dissociation of DTPA and an increased mobility of solvent molecules assist the dissolution kinetics of barite, although the stability of the ligand-metal complex diminishes with temperature.29 In our experiments, we observed the dependence of the dissolution rate of barite on the type and concentration of background salt accompanying DTPA, and this can be also compared with the effect of the ligand concentration and of temperature (i.e., degree of dissociation and water structure dynamics). Effect of Background Ions on DTPA-Promoted Dissolution of Barite. Changes in the degree of dissociation of DTPA on ligand concentration can originate from the association effects of DTPA in a polar aqueous solvent. Such effects are characteristic for the species containing hydrophobic (e.g., CH2) and hydrophilic (e.g., COO-) constituents in their structure. With increasing concentration, amphiliphilic

Kowacz et al.

solutes tend to aggregate and therefore reduce the exposure of the hydrophobic components to the aqueous solvent and consequently can separate from the water phase (e.g., methanol,30 t-buthanol,31 glycine32). Assumption of the tendency of DTPA to associate in aqueous solutions with increasing concentration is supported by the accompanying reduction in heat capacity of DTPA28,29 that depends on solventexposed surface area.33 With increasing temperature, trends are opposite;the ionization constant and heat capacity of DTPA increases.28,29 The addition of specific ionic salts can promote similar association effects as an increase in organic molecule concentration.31,32 This phenomenon is known from organic chemistry and gives rise to the so-called Hofmeister series, ranking salts according to their effect on water-miscibility of amphiliphiles, nonpolar solutes, or hydrophilic ionic liquids.34-36 The exact mechanism responsible for these effects is still not well understood. Nevertheless, the sequence of ions in the Hofmeister series follows their specific hydration properties. Potassium and sodium can be characterized as negatively (chaotropic) and positively (kosmotropic) hydrated ions, respectively, and exert opposite effects on conformation of amphiliphilic species in polar aqueous solvents. Kosmotropes (Naþ) promote aggregation and separation from the water phase while chaotropes (Kþ), on the contrary, support miscibility with water.35 Therefore, an inhibition of the dissolution process with increasing NaCl concentration and the higher effectiveness of the potassium form of DTPA in the dissolution of barite (as observed in our experiments and reported by the producers of commercially available scale dissolvers37) correlates well with a different tendency of background electrolytes to promote/ hinder association of DTPA in aqueous solution. The lower stability of the K-DTPA in comparison to Na-DTPA complexes4 can also partially account for higher dissolution rates of barite in the presence of Kþ as a background cation, but additionally, specific bonding of counterions diminishes the hydrogen-bond donating ability of anionic groups, thus reducing hydrophilicity of the organic molecule and the screening of repulsive forces that prevent aggregation.23 Apart from the effect of differently hydrated background ions on association effects and the degree of dissociation of the DTPA, ionic hydration can also be characterized by changes induced by ions in “local water structural temperature”.27 Negatively hydrated ions (Kþ) enhance water mobility in their vicinity and increase water activity, while positively hydrated ions (Naþ) reduce water mobility in their environment and reduce water activity which can be compared to an increase or decrease in water temperature, respectively.27,38,39 Therefore, higher efficiency of K-DTPA in comparison to Na-DTPA in the dissolution of barite can result from both a higher degree of dissociation of the ligand (and enhanced charge effect) and increased mobility of water molecules in the presence of potassium as a background cation. Surface Phenomena: Hydration of DTPA and Solution Composition. Despite the similarity between “concentration and salt effects” on the conformation of amphiliphiles in aqueous solution, solute-solute contact upon association can be different in nature. Bowron and Finney31 showed that in a salt-free system, amphiliphiles aggregate through nonpolar surface contact, while in the presence of a kosmotropic salt (NaCl), through salt bridging between their hydrophilic

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components (polar-polar mode of solute-solute interaction). This implies that if DTPA undergoes analogous changes, then in salt-free solutions or in solutions of a chaotropic salt (KCl) hydrophilic functional groups (COO-) would remain exposed to the solvent, while under the influence of NaCl they would be mutually associated. As a consequence in the former case, functional groups that could bind to ions at the surface remain available, but due to association effects the affinity of DTPA to the water phase (hydration) decreases. This can lead to passivation of the surface by the layer of weakly adsorbed DTPA molecules and would be consistent with AFM observations of a “soft” precipitate layer covering the barite crystal surface at higher DTPA concentrations.23 However, in NaCl-DTPA systems, polar functional groups that could bind to the surface would be inaccessible while hydrophobic components would remain exposed. This can explain the origin of the other interesting phenomenon, namely, a precipitate formation on a barite surface dissolving in NaClDTPA and DTPA-seawater solutions. The presence of an additive that reduces the dynamics of water structure (e.g., methanol by hydrogen-bonding through hydrophilic (OH) and/or water cage formation around hydrophobic groups (CH2)) can facilitate cation dehydration40-43 and assist in its incorporation into a solid phase14 and can therefore induce barite nucleation.44-46 It is also recognized that weakly hydrated ions are preferentially excluded from the bulk highly structured solvent.47-50 Thus, the following scenario is proposed: when functional groups of DTPA are hindered, due to a polar-polar mode of association of the ligand (in NaCl) or complexation with dissolved ions such as Ca, Mg, or Sr (in seawater) toward which DTPA has higher affinity than toward Ba, dissolved Barite lattice ions are released by water and can then reprecipitate in the form of some solid solution with other ions that are excluded from the structured fluid phase (H-bonded with hydrophilic and forming cages around hydrophobic components of DTPA molecules). In the presence of KCl as a background electrolyte, no precipitation occurred, supporting the hypothesis that this surface phenomenon results from the effect of the hydration characteristics of DTPA on the hydration of other ions present in solution and not from the interaction of DTPA with the mineral surface (e.g., adsorption). We could not identify the chemical character of the precipitate. Nevertheless, a precipitate formation by reprecipitation of the released lattice ions is corroborated by the coupling of dissolution and precipitation reactions and by specific characteristics of the precipitate. Preservation of the morphology of the underlying crystal surface by the layer of the precipitate that can grow only to the “critical thickness” is described in the literature as an indication of foreign ion incorporation.51,52 Another possibility is that because of the high pH, DTPAcontaining solutions can absorb carbon dioxide from the atmosphere, so that the foreign ions incorporating into the barite structure are carbonate anions or that the precipitate is composed mainly of barium carbonate. This is supported by preliminary experiments on barite dissolution in a Na2CO3 solution, in which a very similar precipitate layer with the same characteristic height also formed. Precipitate formation on a barite surface dissolving in the presence of DTPA in a seawater medium, which also contains carbonate ions, also corroborates this hypothesis.

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Conclusions Our results imply that dissolution of barite in the presence of NaCl or DTPA proceeds mainly by a water-mediated reaction mechanism. The dissolution rate of barite is increased by dissociated ionic species due to the charge effect on stabilization of water molecules in solvation shells of barite building units. Therefore, the high chelating ability of an organic ligand toward ions in solution does not ensure fast reaction kinetics of removal of respective ions from the crystal surface. Simple inorganic electrolytes can be more efficient, in terms of dissolution kinetics, than a complexing ligand. Furthermore, the application of a lower concentration of DTPA to promote dissolution of barite is more reasonable because association effects at higher concentration, which hinder the dissolution process, are avoided. Effectiveness of DTPA to act as a mineral dissolver cannot be derived on the basis of its capacity to sequester metal ions from solution but requires consideration of specific mineral surface-fluid interactions that are affected by aqueous speciation and the presence of ligand molecules in the fluid phase. Acknowledgment. This work was carried out within the EU Early Stage Training network MIR (Mineral-fluid Interface Reactivity) Contract No. MEST-CT-2005-021120. Experimental facilities were supported by the Deutsche Forschungsgemeinschaft (DFG).

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