Dissolution and Precipitation

FIGURE 9.29 A typical phosphorus adsorption isotherm showing phosphorus adsorption, desorption, and precipitation in relation to phosphorus concentration in soil pore water. 

FIG U RE 9.30 A simple chemical reaction showing equilibrium relationships. 

FIGURE 9.31 Relationship between solid phases in equilibrium with phosphorus concentration in solution 

For example, if the concentration of an ion in this reaction is increased, the reaction will be driven to the left, until a theoretical equilibrium is reached. Precipitation of solid phase will occur only when ion activity product (lAP) exceeds solubility product. What happens when the concentration of an ion in this reaction is decreased The reaction will be driven to the right, resulting in slow dissolution of the solid phase until a theoretical equilibrium is reached. Dissolution of solid phase will occur only when lAP is lower than the solubility product. 

The status of solid phase can be determined by Saturation Index (SI), which is the ratio of IAP and A. p. If SI 1, then the solution is supersaturated with respect to the solid phase and if SI 1, then the solution is undersaturated with respect to the solid phase. At equilibrium, SI = Any precipitate formed at high concentrations can potentially dissolve, when the concentration of the ion decreases. 

---

Between pH values of ca. 6 and 12 aqueous solutions hold very little dissolved beryllium because of the low solubility of Be(OH)2. When the pH is raised above 12, the hydroxide begins to dissolve with the formation of, first, Be(OH)3 and then, at even higher pH values, Be(OH) (52). The presence of these species in strongly alkaline solutions was confirmed by means of solvent extraction experiments (90) and infrared spectroscopy (31). A speciation diagram is shown in Fig. 7, which was constructed using the values of log /33 = 18.8 and log /34 = 18.6 critically selected from Table III. The diagram illustrates clearly the precipitation and dissolution of Be(OH)2. 

In this chapter we consider how to construct reactions paths that account for the effects of simple reactants, a name given to reactants that are added to or removed from a system at constant rates. We take on other types of mass transfer in later chapters. Chapter 14 treats the mass transfer implicit in setting a species activity or gas fugacity over a reaction path. In Chapter 16 we develop reaction models in which the rates of mineral precipitation and dissolution are governed by kinetic rate laws. 

Comparing the development here to the accounting for the kinetics of mineral precipitation and dissolution presented in the previous chapter (Chapter 16), we see the mass transfer coefficients v and so on serve a function parallel to the coefficients v , etc., in Reaction 16.1. The rates of change in the mole number of each basis entry, accounting for the effect of each kinetic redox reaction carried in the simulation, for example,... 

In calculating most of the reaction paths in this book, we have measured reaction progress with respect to the dimensionless variable . We showed in Chapter 16, however, that by incorporating kinetic rate laws into a reaction model, we can trace reaction paths describing mineral precipitation and dissolution using time as the reaction coordinate. 

The radiochemical assays were done as follows At the end of a polymerisation experiment, when the conductivity had become constant, a ten-fold excess of tritiated water was added from a burette (see Figure 1), the cell was warmed rapidly to room temperature, and any polymer which had been precipitated during the polymerisation was allowed to re-dissolve. It was always noted that no hydrolysis occurred until the solutions reached 0 °C. This could be seen from a rapid drop of conductivity to a very low value. The solvent and most of the tritiated water were then distilled out, within about 15 minutes. The polymer was then dissolved in toluene, also run from a burette into the reaction vessel, which was then cut from the vacuum line. The polymer was precipitated in methanol and prepared for the determinations of radioactivity and DP. For the radiochemical assay the polymers were dissolved in toluene, re-precipitated in methanol, dried, weighed, re-dissolved in toluene, and the activity determined. The processes of precipitation and dissolution were repeated until the activity of the polymer became constant, (up to 7 repetitions). It was assumed that when the activity had become constant, all the excess of tritium had been removed. 

It is interesting to note that many crystal poisons not only interfer with nucleation and the growth of crystals but may also retard their dissolution. As we have seen (Chapter 6), precipitation and dissolution of solids proceed by the attachment or detachment of ions most favorably at kink sites of the crystalline surface. Solutes such as organic substances, or phosphates may upon adsorption immobilize kinks and thus retard dissolution. 

Another source of divergence is the use of different models for the aqueous carbonate systems. Precipitation and dissolution experiments can be carried out in closed or open systems and various ways of pH-adjustments (see 8.2). 

Colloids are present in natural waters (i.e., surface and groundwaters). Surface systems receive terrestrial input as runoff, which carries solid-derived materials into streams, rivers, lakes, or estuaries. Groundwater receives leachates from land fills and percolation water and is frequently well connected with surface water bodies. Colloids may also be formed in situ by native processes of precipitation and dissolution, suspension, or biological activity [103,104]. 

A significant amount of seawater is trapped in the open spaces that exist between the particles in marine sediments. This fluid is termed pore water or interstitial water. Marine sediments are the site of many chemical reactions, such as sulfate reduction, as well as mineral precipitation and dissolution. These sedimentary reactions can alter the major ion ratios. As a result, the chemical composition of pore water is usually quite different from that of seawater. The chemistry of marine sediments is the subject of Part 111. 

The Chemical Speciation of Iron Mineral Precipitation and Dissolution... 

Daccord G, Lenormand R (1987) Fractal patterns from chemical dissolution. Nature 325 41 3 Daccord G, Lietard O, Lenormand R (1993) Chemical dissolution of a porous medium by a reactive fluid, 2, Convection vs. reaction behavior diagram. Chem Eng Sci 48 179-186 Darmody RG, Thorn CE, Harder RL, Schlyter JPL, Dixon JC (2000) Weathering implications of water chemistry in an arctic-alpine environment, north Sweden. Geomorphology 34 89-100 Dijk P, Berkowitz B (1998) Precipitation and dissolution of reactive solutes in fractures. Water Resour Res 34 457-470... 

Fructan was harvested by precipitation from the culture broth by addition of ethanol or isopropanol. Acetone and methanol can also be used. The yield and consistency of the product varied depending on the amount of alcohol added. The fructan started to precipitate at the medium/alcohol v/v ratio of 1 1.2, and the yield peaked at about 1 1.5. Further increase in the ratio hardened the fructan and made the product less fluid. Slightly less isopropanol was needed than ethanol to precipitate levan (fructan). Although most of the bacterial cells, unfermented sugars, and other solubles remained in the aqueous alcohol phase, pre-removal of microbial cells by centrifuging was needed to obtain a pure form of fructan. The product was further purified by repeated precipitation and dissolution in water, followed by dialysis or ultrafiltration. The final product was an... 

Bisphosphonates structurally mimic endogenous pyrophosphate, which inhibits precipitation and dissolution of bone minerals. They retard bone resorption by osteoclasts and, in part, also decrease bone mineralizatioa Indications include tumor osteolysis, hypercalcemia, and Paget s disease. Qinical trials with etidronate, administered as an intermittent regimen, have yielded favorable results in osteoporosis. With the newer drugs clodronate, pamidronate, and alendronate, inhibition of osteoclasts predominates a continuous regimen would thus appear to be feasible. 

The equilibrium solubility of an Fe oxide can be approached from two directions -precipitation and dissolution. The first method involves precipitating the oxide from a supersaturated solution of ions with stepwise or continuous addition of base und using potentiometric measurements to monitor pH and calculate Fej- in equilibrium with the solid phase until no further systematic change is detected. Alternatively the oxide is allowed to dissolve in an undersaturated solution, with simultaneous measurement of pH and Fejuntil equilibrium is reached. It is essential that neither a phase transformation nor recrystallization (formation of larger crystals) occurs during the experiment this may happen with ferrihydrite which transforms (at room temperature) to a more condensed, less soluble phase. A discussion of the details of these methods is given by Feitknecht and Schindler (1963) and by Schindler (1963). 

Iron and manganese oxides are characterized by high specific surface areas and high affinity of their surface hydroxyl groups for adsorption of a variety of trace elements. In addition to adsorption processes, oxidation reactions are catalyzed by these surfaces (18-20). The in situ precipitation and dissolution of these oxides are thus significant for the fate of various trace... 

Geochemical modeling is often used to identify the compounds that primarily control the chemistry of arsenic in aqueous solutions. Modeling studies indicate that the arsenic concentrations of Kelly Lake, Ontario, Canada, are controlled by the precipitation and dissolution of Fe(II) arsenates rather than calcium or Fe(III) arsenates (Sadiq et al., 2002). The arsenic in the lake originated from runoff from the nearby Sudbury mining district and airborne particles from local ore smelters (Sadiq et al., 2002). 

Precipitation and Dissolution. A special case of ionization is the dissolution of a neutral solid phase into soluble species. For example, the binary solid metal sulfide, CuS, dissolves in water according to... 

Stumm, W., Furrer, G., and Kunz, B. (1983). The role of surface coordination in precipitation and dissolution of mineral phases. Croat. Chem. Acta 56, 593-611. 

Mechanistic Multiphase Model for Reactions and Transport of Phosphorus Applied to Soils. Mansell et al. (1977a) presented a mechanistic model for describing transformations and transport of applied phosphorus during water flow through soils. Phosphorus transformations were governed by reaction kinetics, whereas the convective-dispersive theory for mass transport was used to describe P transport in soil. Six of the kinetic reactions—adsorption, desorption, mobilization, immobilization, precipitation, and dissolution—were considered to control phosphorus transformations between solution, adsorbed, immobilized (chemisorbed), and precipitated phases. This mechanistic multistep model is shown in Fig. 9.2. 

Abstract In this paper we discuss a pore scale model for crystal precipitation and dissolution in porous media. We consider weak solutions in general domains and dissol-ution/precipitation fronts in thin strips. The latter yields an upscaled transport-reaction model. 

A second equation for C12 results from a description of the precipitation and dissolution processes. Following the detailed discussion in Knabner et al. [3] we have... 

Many models include features allowing calculations involving the speciation of soluble complexes, the precipitation and dissolution of solid mineral phases and the adsorption of ions from solution on to surfaces. In addition, conditions of oxidation-reduction and partial pressures of gas phases can be superimposed on the calculation. Throughout the chapter some simple examples of chemical modelling calculations will be given. 

Recently, workers (2) have been examining the equilibrium and kinetic factors that are important at the oxic-anoxic interface. The kinetic behavior is difficult to characterize completely due to varying rates of oxidation and absomtion above the interface and varying rates of reduction, precipitation and dissolution below the interface (2.51. Bacterial catalysis may also complicate the system (1). Although one can question the importance of abiotic thermodynamic and kinetic processes at this interface, we feel it is useful to use simple inorganic models to approximate the real system. Recently, the thermodynamics and kinetics of the H2S system in natural waters has been reviewed (0. From this review it became apparent that large discrepancies existed in rates of oxidation of H2S and the thermodynamic data was limited to dilute solution. In the last few years we have made a number of thermodynamic (7.81 and kinetic (9 101 measurements on the H2S system in natural waters. In the present paper we will review these recent studies. The results will be summarized by equations valid for most natural waters. 

XRD experiments can be carried out to characterize gas-solid reactions and, with some limitations, fluid-solid reactions more generally, as long as the fluid contributes little to the pathway of sight for the X-rays. Areas of recent investigation are catalytic gas-solid reactions, electrochemical processes, synthesis procedures involving precipitation and dissolution of solids, temperature-programmed reaction studies of crystallization, and oxidation and reduction of solids. This enumeration covers essentially all phases of the life of a catalyst. 

Precipitation and dissolution of metal hydroxides The solubility product principle can also be applied to the formation of metal hydroxide precipitates these are also made use of in qualitative inorganic analysis. Precipitates will be formed only if the concentrations of the metal and hydroxyl ions are momentarily higher than those permitted by the solubility product. As the metal-ion concentration in actual samples does not vary much (10—1 —10 3 mol -1 is the usual range), it is the hydroxyl-ion concentration which has the decisive role in the formation of such precipitates. Because of the fact that in aqueous solutions the product of hydrogen- and hydroxyl-ion concentrations is strictly constant (A = 10 14 at 25°C, cf. Section 1.18), the formation of a metal-hydroxide precipitate depends mainly on the pH of the solution. Using the solubility product principle, it is possible to calculate the (minimum) pH required for the precipitation of a metal hydroxide. 

---