Mineralization Surface ions

In phreeqc, reactive transport is simulated by imagining a number of cells in a numbered sequence, containing minerals, surfaces, ion-exchangers, and an initial fluid equilibrated with these objects. The cells may all contain the same or different minerals and fluids. Another fluid is added to the first cell, equilibrated, with dispersion into the adjacent cell if desired, then that fluid is shifted or advected to the second cell, and the process is repeated until any number of shifts is performed. 

Phosphate ion exchange in soils In soils, phosphate and other ion exchange processes occur on the surfaces of the solid phases (clay minerals, organic matter, and mineral surfaces). Ion exchange reactions are important for maintaining an equilibrium between the solid phase and the solution (soil pore water). Thus, it is important that we understand the origin of surface charge of the solid phase. 

Chemical weathering in a natural process that occurs under the prevailing environmental conditions, resulting in the transfer of matter from unstable minerals to more stable minerals and soluble species. Chemical weathering reactions include dissolution, precipitation, adsorption, and desorption of elements adsorbed on the mineral surfaces, ion exchange, oxidation, reduction and hydrolysis reactions. 

This interface is critically important in many applications, as well as in biological systems. For example, the movement of pollutants tln-ough the enviromnent involves a series of chemical reactions of aqueous groundwater solutions with mineral surfaces. Although the liquid-solid interface has been studied for many years, it is only recently that the tools have been developed for interrogating this interface at the atomic level. This interface is particularly complex, as the interactions of ions dissolved in solution with a surface are affected not only by the surface structure, but also by the solution chemistry and by the effects of the electrical double layer [31]. It has been found, for example, that some surface reconstructions present in UHV persist under solution, while others do not. 

Aetivators. These are used to make a mineral surface amenable to collector coating. Copper ion is used, for example, to activate sphalerite (ZnS), rendering the sphalerite surface capable of absorbing a xanthate or dithiophosphate collector. Sodium sulfide is used to coat oxidized copper and lead minerals so that they can be floated by a sulfide mineral collector. 

Secondary minerals. As weathering of primary minerals proceeds, ions are released into solution, and new minerals are formed. These new minerals, called secondary minerals, include layer silicate clay minerals, carbonates, phosphates, sulfates and sulfides, different hydroxides and oxyhydroxides of Al, Fe, Mn, Ti, and Si, and non-crystalline minerals such as allophane and imogolite. Secondary minerals, such as the clay minerals, may have a specific surface area in the range of 20-800 m /g and up to 1000 m /g in the case of imogolite (Wada, 1985). Surface area is very important because most chemical reactions in soil are surface reactions occurring at the interface of solids and the soil solution. Layer-silicate clays, oxides, and carbonates are the most widespread secondary minerals. 

Activators are those reagents which act in a manner converse to the action of depressants, i.e., they render those minerals floatable which either have been temporarily depressed or would not float without their assistance. They are generally soluble salts which ionize in the aqueous medium. The ions then react with the mineral surface, providing a monomolecular coating and thereby making the mineral surface favourably disposed to the collectors. Sphalerite (ZnS) is essentially not floatable with common collectors. The addition of Cu2+ to the solution, however, alters the mineral surface to CuS, which can adsorb collector. This feature is described elaborately in a later section. 

The adsorption of collectors on sulfide mineral occurs by two separate mechanisms chemical and electrochemical. The former results in the presence of chemisorbed metal xanthate (or other thiol collector ion) onto the mineral surface. The latter yields an oxidation product (dixanthogen if collector added is xanthate) that is the hydrophobic species adsorbed onto the mineral surface. The chemisorption mechanism is reported to occur with galena, chalcocite and sphalerite minerals, whereas electrochemical oxidation is reportedly the primary mechanism for pyrite, arsenopyrite, and pyrrhotite minerals. The mineral, chalcopyrite, is an example where both the mechanisms are known to be operative. Besides these mechanisms, the adsorption of collectors can be explained from the point of interfacial energies involved between air, mineral, and solution. 

The products of hydrolysis and dissociation depend on the pH. In an acid medium, hydrogen sulfide, which has no depressing action, evolves. It is, therefore, necessary to use alkaline circuits in which HS, predominates. These sulfide ions are adsorbed on the copper sulfide mineral surface and react with the surface previously coated with cuprous xanthate. The reaction causes desorption of the collector, and as a result of this desorption the copper sulfide minerals generally become hydrophilic. There is, however, no action of the sulfide ions on molybdenite, and so molybdenite retains its hydrophobic character. 

Holm and Andersson have provided an up-to-date survey of simulation experiments on the synthesis under hydrothermal conditions of molecules important for biogenesis (Holm and Andersson, 2005). It is clear that several research groups have been able to show in the meantime, using simulation experiments, that the conditions present at deep sea vents appear suitable for the synthesis of very different groups of substances. However, it remains unclear how these compounds could have been stabilized and protected against rapid decomposition. At present, metal ions (as complexing agents) and mineral surfaces are the subject of discussion and experiment. 

Layer silicate minerals have a high selectivity of trace transition and heavy metals and greater irreversibility of their adsorption. Some chemisorbing sites such as -SiOH or AlOH groups may be at clay edges and form hydroxyl polymers at the mineral surface. Another possible reason for the high selectivity may be hydrolysis of the metal and strong adsorption of the hydrolysis ion species. 

The surface sites and complexes lie in a layer on the mineral surface which, because of the charged complexes, has a net electrical charge that can be either positive or negative. A second layer, the diffuse layer, separates the surface layer from the bulk fluid. The role of the diffuse layer is to achieve local charge balance with the surface hence, its net charge is opposite that of the sorbing surface. Double layer theory, applied to a mixed ionic solution, does not specify which ions make up the diffuse layer. 

Some emphasis is given in the first two chapters to show that complex formation equilibria permit to predict quantitatively the extent of adsorption of H+, OH , of metal ions and ligands as a function of pH, solution variables and of surface characteristics. Although the surface chemistry of hydrous oxides is somewhat similar to that of reversible electrodes the charge development and sorption mechanism for oxides and other mineral surfaces are different. Charge development on hydrous oxides often results from coordinative interactions at the oxide surface. The surface coordinative model describes quantitatively how surface charge develops, and permits to incorporate the central features of the Electric Double Layer theory, above all the Gouy-Chapman diffuse double layer model. 

The central ion of a mineral surface (in this case we take for example the surface of a Fe(lll) oxide and S-OH corresponds to =Fe-OH) acts as Lewis acid and exchanges its stuctural OH against other ligands (ligand exchange). Table 2.1 lists the most important adsorption (= surface complex formation) equilibria. The following criteria are characteristic for all surface complexation models (Dzombak and Morel, 1990.)... 

Sposito, G. (1990), "Molecular Models of Ion Adsorption on Mineral Surfaces", in M. F. Hochella Jr. and A. F. White, Eds., Mineral-Water Interface Geochemistry, Mineralogical Society of America, Washington, D.C., pp. 261 -279. 

Honeyman, B. D., and J. O. Leckie (1986), "Macroscopic Partitioning Coefficient for Metal Ion Adsorption Proton Stoichiometry at Variable pH and Adsorption Density", in J. A. Davis and K. F. Hayes, Eds., Geochemical Processes at Mineral Surfaces, ACS Symposium, Washington, DC. 

Wehrli, B. (1990), "Redox Reactions of Metal Ions at Mineral Surfaces", in W. Stumm, Ed., Aquatic Chemical Kinetics, John Wiley and Sons, New York, 311 -336. 

This book deals only with the chemistry of the mineral-water interface, and so at first glance, the book might appear to have a relatively narrow focus. However, the range of chemical and physical processes considered is actually quite broad, and the general and comprehensive nature of the topics makes this volume unique. The technical papers are organized into physical properties of the mineral-water interface adsorption ion exchange surface spectroscopy dissolution, precipitation, and solid solution formation and transformation reactions at the mineral-water interface. The introductory chapter presents an overview of recent research advances in each of these six areas and discusses important features of each technical paper. Several papers address the complex ways in which some processes are interrelated, for example, the effect of adsorption reactions on the catalysis of electron transfer reactions by mineral surfaces. 

Sol id Sol utions. The aqueous concentrations of trace elements in natural waters are frequently much lower than would be expected on the basis of equilibrium solubility calculations or of supply to the water from various sources. It is often assumed that adsorption of the element on mineral surfaces is the cause for the depleted aqueous concentration of the trace element (97). However, Sposito (Chapter 11) shows that the methods commonly used to distinguish between solubility or adsorption controls are conceptually flawed. One of the important problems illustrated in Chapter 11 is the evaluation of the state of saturation of natural waters with respect to solid phases. Generally, the conclusion that a trace element is undersaturated is based on a comparison of ion activity products with known pure solid phases that contain the trace element. If a solid phase is pure, then its activity is equal to one by thermodynamic convention. However, when a trace cation is coprecipitated with another cation, the activity of the solid phase end member containing the trace cation in the coprecipitate wil 1 be less than one. If the aqueous phase is at equil ibrium with the coprecipitate, then the ion activity product wi 1 1 be 1 ess than the sol ubi 1 ity constant of the pure sol id phase containing the trace element. This condition could then lead to the conclusion that a natural water was undersaturated with respect to the pure solid phase and that the aqueous concentration of the trace cation was controlled by adsorption on mineral surfaces. While this might be true, Sposito points out that the ion activity product comparison with the solubility product does not provide any conclusive evidence as to whether an adsorption or coprecipitation process controls the aqueous concentration. 

Hydrolysis reactions occur by nucleophilic attack at a carbon single bond, involving either the water molecule directly or the hydronium or hydroxyl ion. The most favorable conditions for hydrolysis, e.g. acidic or alkaline solutions, depend on the nature of the bond which is to be cleaved. Mineral surfaces that have Bronsted acidity have been shown to catalyze hydrolysis reactions. Examples of hydrolysis reactions which may be catalyzed by the surfaces of minerals in soils include peptide bond formation by amino acids which are adsorbed on clay mineral surfaces and the degradation of pesticides (see Chapter 22). 

Using a simple amphoteric model for the mineral surface, we have demonstrated the role specific chemical binding reactions of potential determining Ions In determining the electrical properties and thermodynamics of the oxide/solution interfaces. A by-product of our study Is that under appropriate conditions, an amphoteric surface can show marked deviations from ideal Nernstlan behaviour. The graphical method also serves to Illustrate the... 

Free energy variations with temperature can also be used to estimate reaction enthalpies. However, few studies devoted to the temperature dependence of adsorption phenomena have been published. In one such study of potassium octyl hydroxamate adsorption on barite, calcite and bastnaesite, it was observed that adsorption increased markedly with temperature, which suggested the enthalpies were endothermic (26). The resulting large positive entropies were attributed to loosening of ordered water structure, both at the mineral surface and in the solvent surrounding octyl hydroxamate ions during the adsorption process, as well as hydrophobic chain association effects. 

Metal Ion Adsorption in Mixtures of Multiple Solid Phases. One of the arguments put forth for extending the concepts of solution coordination chemistry to heterogeneous systems is the hypothesis that the mineral components of soils or sediments can be considered as ligands which compete for complexation of adsorbates. To this end, it is important to know the relative ability of different mineral surfaces to complex solutes. 

ESR spectroscopy can be used with adsorbed paramagnetic ions to study the liquid associated with mineral surfaces. Cu(II) and Mn(II) have been used in his type of investigation, although difficulties are encountered with observing a resonance from Mn(II) in distorted environments. Measurements of Cu(II) on silica at room temperature and above have shown that adsorbed water behaves in the same manner as bulk water, but at lower temperatures it experiences a decreased mobility (61). On freezing two types of water are found one which is freezable and undergoes crystallization and the other which is unfreezable, in which the ice structure cannot be formed because of the surface interaction. NMR, IR and differential thermal... 

Chemical reactions of adsorbed species are of importance in vast areas of science, the involvement of adsorbed metal ions in catalysis being one example of great economic value. In addition reactions involving adsorbed species can sometimes produce products that may be either difficult or impossible to prepare away from the mineral surface. Therefore, an understanding of the chemical processes that occur in such systems is of potential economic benefit to industrial operations. Such knowledge is also of much wider significance, however, because the movement of ions in most environmental situations is controlled by sorption processes, and aluminosilicate minerals play a major role in many situations. 

Sorption processes are influenced not just by the natures of the absorbate ion(s) and the mineral surface, but also by the solution pH and the concentrations of the various components in the solution. Even apparently simple absorption reactions may involve a series of chemical equilibria, especially in natural systems. Thus in only a comparatively small number of cases has an understanding been achieved of either the precise chemical form(s) of the adsorbed species or of the exact nature of the adsorption sites. The difficulties of such characterization arise from (i) the number of sites for adsorption on the mineral surface that are present because of the isomorphous substitutions and structural defects that commonly occur in aluminosilicate minerals, and (ii) the difference in the chemistry of solutions in contact with a solid surface as compound to bulk solution. Much of our present understanding is derived from experiments using spectroscopic techniques which are able to produce information at the molecular level. Although individual methods may often be applicable to only special situations, significant advances in our knowledge have been made... 

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