Phosphate mineral dissolution

U-bearing minerals and adsorption processes (Salah et al. 2000 Perez del Villar et al. 2000). The vertical and lateral flow of groundwater is responsible for the oxidation and dissolution of primary sulphides, leading to acidic solutions that facilitated the oxidation and dissolution of uraninite. The resulting uranyl cations migrated and precipitated as uranyl minerals, mainly phosphates, silicates, silico-phosphates. In certain local conditions, reduction of these uranyl cations allowed precipitation of coffinite with a high content of P and LREE. Adsorption of uranium, together with P, mainly occurs on Fe-oxyhydroxides, but this kind of uranium retention seems less efficient than the precipitation, at least in the close vicinity to the... 

A thermodynamic model of dissolution is presented in this chapter, which relates the solubility product constant to the thermodynamic potentials and measurable parameters, such as temperature and pressure of the solution. The resulting relations allow us to develop conditions in which CBPCs are likely to form by reactions of various oxides (or minerals) with phosphate solutions. Thus, the model predicts formation of CBPCs. 

Carbonates are one plausible sohd source for metal ions which display similar dissolution/complexation barriers for various ions (that is, different COj salts, e.g. of Fe(II), Mg, Cu, Zn or Mn(II), are similarly soluble in water (pKs 11 Mizerski 1997)) and form mixed crystals among each other. In addition, the products of chemical evolution, with HCOOH, many higher carboxylic acids with or without amino groups being prominent in yields (Lemmon 1970), are usually acidic and thus would dissolve carbonates readily. Contrary to carbonates, other kinds of minerals like phosphates. 

The rates of reactions between minerals and groundwater are also difficult to predict because of their dependence on the surface characteristics of mineral grains, adsorbed trace substances on mineral surfaces, and often on the activities of organisms (Berner 1978). Laboratory rates of mineral dissolution may be orders of magnitude faster than observed in nature because of enhanced reactivities of the laboratory-prepared mineral grains and the adsorption of dissolution-inhibiting trace species such as phosphate in the natural system. Other laboratory rates, such as that of Fe(II) oxidation, are much slower than naturally observed oxidation rates when the latter are catalyzed by microorganisms. 

Equation 4.73 suggests that, as soils are made very acid (pH < 5), the increase in AP" " activity from clay mineral dissolution and the decrease in OH activity from the pH change could result in either a lower or higher solubility of phosphate. Because it is commonly assumed that AP" activity is controlled in acid soils by A1(0H)3 precipitation, that is, by the solubility product... 

I.I.3.2. Phosphate minerals Dissolution equilibria for aluminum phosphate can be written as follows ... 

In the laboratory, phosphate adsorption by layer silicates is rapid for a few hours and then continues more slowly for weeks. The initial rapid reaction can be envisioned as a combination of nonspecific adsorption and ligand exchange on mineral edges. The slower reaction probably consists of a complex combination of mineral dissolution and precipitation of added phosphate with exchangeable cations or cations within the lattices. 

Solubility of phosphates in anatectic melts can be fruitfully analyzed by use of the concept of saturation index (SI) (see also Watt and Harley 1993). The saturation index is defined as the log of the ratio of the ion activity product (lAP) to the solubility constant (K ), SI = log (lAP/Kgp), and is a measure of the degree of over saturation (positive SI) or unclersaturation (negative SI) of a mineral. Dissolution of the phosphates apatite, monazite and xenotime into a silicate liquid may be described by the reactions... 

The Ca(Il) coaceatratioa ia blood is closely coatroUed aormal values He betweea 2.1 and 2.6 mmol/L (8.5—10.4 mg/dL) of semm (21). The free calcium ion concentration is near 1.2 mmol/L the rest is chelated with blood proteias or, to a lesser extent, with citrate. It is the free Ca(Il) ia the semm that determines the calcium balance with the tissues. The mineral phase of bone is essentially ia chemical equiUbrium with calcium and phosphate ions present ia blood semm, and bone cells can easily promote either the deposition or dissolution of the mineral phase by localized changes ia pH or chelating... 

Inorganic reactions in the soil interstitial waters also influence dissolved P concentrations. These reactions include the dissolution or precipitation of P-containing minerals or the adsorption and desorption of P onto and from mineral surfaces. As discussed above, the inorganic reactivity of phosphate is strongly dependent on pH. In alkaline systems, apatite solubility should limit groundwater phosphate whereas in acidic soils, aluminum phosphates should dominate. Adsorption of phosphate onto mineral surfaces, such as iron or aluminum oxyhydroxides and clays, is favored by low solution pH and may influence soil interstitial water concentrations. Phosphorus will be exchanged between organic materials, soil inter-... 

P. Hinsinger and R. J. Gilkes, Dissolution of phosphate rock in the rhizosphere of five plant species grown in an acid, P-fixing mineral substrate. Geoderma 75 231 (1997). 

During the lifetime of a root, considerable depletion of the available mineral nutrients (MN) in the rhizosphere is to be expected. This, in turn, will affect the equilibrium between available and unavailable forms of MN. For example, dissolution of insoluble calcium or iron phosphates may occur, clay-fixed ammonium or potassium may be released, and nonlabile forms of P associated with clay and sesquioxide surfaces may enter soil solution (10). Any or all of these conversions to available forms will act to buffer the soil solution concentrations and reduce the intensity of the depletion curves around the root. However, because they occur relatively slowly (e.g., over hours, days, or weeks), they cannot be accounted for in the buffer capacity term and have to be included as separate source (dCldl) terms in Eq. (8). Such source terms are likely to be highly soil specific and difficult to measure (11). Many rhizosphere modelers have chosen to ignore them altogether, either by dealing with soils in which they are of limited importance or by growing plants for relatively short periods of time, where their contribution is small. Where such terms have been included, it is common to find first-order kinetic equations being used to describe the rate of interconversion (12). 

Apatite is used to remediate Pb contaminated soils because apatite dissolution releases phosphate, which combines with Pb to form highly insoluble Pb-phosphate minerals. Apatites follow linear (zero-order) dissolution kinetics (Manecki et al., 2000) with rates of Pb uptake by the apatites decreasing in the same order as the apparent dissolution rate... 

In soils the organic matter (range 2 in Fig. 9.12) is a significant pH and pe buffer because it represents a reservoir of bound H+ and e When organic matter is mineralized alkalinity and [NO3], [SO ] increase and Fe(II) and Mn(II) become mobilized. Phosphate, incipiently bound to Fe(III)(hydr)oxides, is released as a consequence of the partial reductive dissolution of the Fe(III) solid phases. At lower pe values (range 3 in Fig. 9.12) the concentration of Fe(II) and Mn(II) further... 

Phosphate is remineralized during the oxidation of organic matter and dissolution of hard parts, such as bones and teeth, that are composed of the minerals hydroxyapatite and fluoroapatite. Unlike the other products of remineralization, pore-water phosphate concentrations are regulated only by mineral solubility, such as through vivianite (iron phosphate) and francolite (carbonate fluoroapatite). Redox reactions are not significant because phosphorus exists nearly entirely in the h-5 oxidation state. 

Secondary U and REE minerals include autunite, Ce-phosphate, and Ld-Nd phosphates. The geochemical behaviour can be explained through pyrite oxidation that increases acidicity and releases sulphate and Fe(III), that would allow oxidative dissolution of the U ore, possibly precipitating uranopilite. When the pH increased at sites more distant from pyrite dissolution, U(VI) was hydrolysed and eventually co-precipitated with Fe3+-oxyhydroxides. 

The study of the basaltic dykes in evaporites demonstrates that dissolution and precipitation of phosphate minerals is a key process for the control of REE mobility and REE fractionation. In the present case, all REE found in secondary apatite in the basalt and in the salt are derived from the dissolution of primary magmatic apatite during basalt corrosion. This loss of REE from the basalt to the salt was not sufficient to lower significantly the REE concentrations of the basalt and it could only be detected by the analysis of the salt. The absolute quantity of REE transferred from the basalt into the salt, however, cannot be quantified because we have no three-dimensional control on the REE concentrations around the basalt apophy sis. 

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