Precipitation mechanisms

There are two important precipitation mechanisms including volume diffusion and surface reaction. Volume diffusion is diffusion of ions in aqueous solution towards the surface of minerals. Surface reaction means the precipitation reaction at crystal surface. 

Surface reaction mechanisms include adsorption, desorption, surface nucleation, polynucleation, mononucleation and ion exchange reaction. The dependencies of amounts of precipitate and solution composition on time are different for each mechanism. For example, linear, exponential and logarithmic rate equations are established for volume diffusion, polynuclear growth and spiral growth, respectively. 

Precipitation kinetics of salts such as barite (BaS04) has been studied to prevent the formation of scale (BaS04, CaS04 2H20) in the pipes for the transportation of oil and geothermal water. 

Nielsen and Toft (1984) discussed the precipitation mechanism using PA (= —logXA) — PB(—logXiB) diagram (Fig. 3.5). It is shown in this diagram that precipitation rate and mechanism depends on total cation and anion concentrations. 

Calcite is the most well studied mineral with regard to precipitation kinetics (e.g., Inskeep and Bloom 1985 Shikazono and Shiraki 1994). 

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Central pore being filled by the dissolution precipitation mechanism... 

Sludge Precipitated mechanically or biologically separated solid matter produced during water and or sewage treatment or industrial processes. Such solids may be amenable to biological control. 

This boundary between two different precipitation mechanisms for barite determined by the experiments at 150°C (Shikazono, 1994) roughly coincides with that between dendritic crystals and well-formed crystals which has been experimentally determined at temperatures lower than 100°C. 

Shikazono, N. (1994) Precipitation mechanisms of barite in back-arc basins. Geochim. Cosmochim. Acta, 58, 2203-2213. 

Measurements of the chemical composition of an aqueous solution phase are interpreted commonly to provide experimental evidence for either adsorption or surface precipitation mechanisms in sorption processes. The conceptual aspects of these measurements vis-a-vis their usefulness in distinguishing adsorption from precipitation phenomena are reviewed critically. It is concluded that the inherently macroscopic, indirect nature of the data produced by such measurements limit their applicability to determine sorption mechanisms in a fundamental way. Surface spectroscopy (optical or magnetic resonance), although not a fully developed experimental technique for aqueous colloidal systems, appears to offer the best hope for a truly molecular-level probe of the interfacial region that can discriminate among the structures that arise there from diverse chemical conditions. 

The retarding effect of pores on solvent relative to the polymer described in Section 6.1 has now Iseen proposed as the reason for the good separations obtained by precipitation mechanisms ( -10, 13) ... 

Oxidation/reduction of Pb electrode has been studied using in situ spectroscopic techniques - Raman [114, 130-132], fourier transform infrared (FTIR) [133-135], Auger [136], and photocurrent spectroscopy [131, 137-141]. El-Kpsometric studies underlined nonuniform PbS04 film growth a dissolution-precipitation mechanism with nucleation and three-dimensional growth has been proposed as a result of large oversaturation of Pb(II) ionic species [142]... 

Ion removal by solids could involve more phenomena, as for example in inorganic natural materials where ion uptake is attributed to ion exchange and adsorption processes or even to internal precipitation mechanisms (Inglezakis et al., 2004). 

As will be shown later, the former three mechanisms mentioned above are applicable to TLC separation of polymers. From the standpoint of TLC applied to polymer separation, the partition mechanism may be better expressed by phase-separation or precipitation mechanism, as will be explained in Section II.3. It should be noted that all these mechanisms are generally present during a chromatographic separation. Therefore, one mechanism should be made to be predominant for a given separation aim. This can, in principle, be done by properly selecting the developer and adsorbent. However, such a selection is the major problem in application of TLC, especially, to polymer separation, and the following three sections will be devoted to describing the rules that have been established to solve this problem. 

SFC is complementary to other classical techniques of liquid or gas chromatography. The migration of the analyte is explained by a dissolution-precipitation mechanism that depends on the solvation power of the mobile phase. Thus, it is governed by the pressure that determines the density of the supercritical phase. Resistance to mass transfer between the stationary and mobile phases is less than that found in HPLC because diffusion is faster. The C factor in Van Deemter s equation is smaller so the velocity of the mobile phase can be increased (see Fig. 6.3). Moreover, because the viscosity of the mobile phase is similar to that of a gas, it is possible to use capillary columns like those used in capillary GC. However, the... 

The precipitation mechanism proposed by Inagaki offers a good qualitative explanation for the fractionation of polymers by TLC. The quantitative agreement between TLC precipitation and bulk precipitation is lacking at the present time. Further experiments are needed to provide exact definition of the role of the adsorbent. 

This model has been recently quoted1 in some discussions 120 122). To avoid misunderstandings, it might be stressed that a precipitation mechanism was proposed I14> on the base of... 

The precipitation mechanism is governed by the solubility of the foulant, while the adsorption is affected by electrostatic and hydrophobic interactions between the foulant and the membrane surface. The molecular size and... 

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