Multi-valent ions

Rejection is a property of the specific feed water component and the membrane of interest. Table 3.2 lists the general rejection ability of the most common polyamide composite RO membranes. Note that ionic charge of the component of interest plays a role its rejection by an RO membrane the rejection of multi-valent ions is generally greater than for mono-valent ions. 

Multi-valent ions interact with organics, inorganic colloids and membranes. Real systems are very complex. An effort to simplify natural systems without losing important factors has been made and the systems were characterised thoroughly. 

Additional questions are raised when the moving ion is immersed in water instead of a simple dipolar liquid discussed in Sec. 5.4.The hydration phenomena observed by many different experiments show a remarkable variety in its dependence on size and sign of ions, which may not be characterized by a simple chemical model such as ion-water complex formation, or a solventberg model. Such a model seems to apply to very small monovalent ions like Li+ and F and multi-valent ions which make a stable hydration shell with substantial lifetime. However, water molecules in the first hydration shell around ions with greater size are more mobile and disordered than those in bulk water. The behavior which has been referred to as structure breaking by Frank and Wen [85] and as negative hydration by Samoilov [86] is due to the competition between two forces acting on water molecules in the shell, the... 

The released metal ions are covered by a hydration shell, the size of which depends on the valency of the ion and the size of the nucleus. The hydrate complexes [M(H20) ] of multi-valent ions can deprotonate (i.e. they act as acids), which affects the chemical equilibrium of the reaction (3.21). As a result, the apparent solubility is a function of the pH-value. Deprotonation leads to a transformation of the hydrate complexes into hydroxide complexes [M(OH) ] ". The neutral hydroxide complexes are hardly soluble. The total amount of dissolved material and its composition can be calculated from the acid-base-equilibria of the complex ions and the pH. In general, the lower the main group of the metal is, the higher the oxide solubility. 

Impact of Multi-valent Ions on the Dissolution of Chddes... 

Table 7.6 cites a few sample values of y as a function of concentration. Note how multi-valent anions and cations cause y to vary more greatly than do mono-valent ions. The implications are vast if an indium electrode were to be immersed in a solution of In2(S04)3 of concentration 0.1 mol dm-3, for example, then a value of y = 0.035 means that the activity (the perceived concentration) would be about 30 times smaller ... 

Transition metal ions have an incompletely filled d-shell, i.e. their electron configuration is d". The optically active electrons are thus bound to central potential as well as experiencing crystal field potential, and are not shielded by outer electrons. Most transition metal ions are multi-valent. Mainly the number of 3d electrons and the crystal field determine their optical properties. Thus the groups below have similar optical behavior ... 

That concept had led to the synthesis of so-called "halato-telechelic polymers" (which means a "salt" or "neutralized" telechelic polymer, acidic or basic). Although that is a very general denomination covering all the chains formed by any type of ion-pair coupling in any way, a particularly handy and representative class of such structures can be obtained from the complete neutralization of a,o)-dicarboxylato-polymers (PX), by a di (or multi-) valent metal derivative, (19), according to the general equation ... 

Gelation of polyanionic or polycationic polymers by the addition of multi-valent counter-ions is a simple and common method of entrapment of enzymes and whole cells. In one common embodiment, whole cells or enzymes are entrapped by the drop-wise addition of an aqueous solution of sodium alginate and the biocatalyst to a concentrated solution of a Ca2+ salt. The cation acts as a cross-linking agent towards the alginate biopolymer and the droplets precipitate as beads with the biocatalysts entrapped within the network. Although the beads are relatively soft and unstable, this method has been one of the preferred methods for entrapment of whole cells. A... 

Two reactions can convert alkali silicate glass into a solid mass with which contaminated sludges can be bound (1) by adding acid to form silica gel, whereby the evaporation of water does not occur (alternative to vaporization ) and (2) reaction with multi-valent metal ions (e.g., calcium chloride) while forming aqueous metal silica gel, where heavy metals are precipitated and are mechanically bonded into the gel structure. The CHEM-FIX-Process is primarily used in the United States for inorganic contaminants the Belgian SOLIROC-Process contains additives to make it usable for the solidification of organic wastes. 

Cationic surfactants and polymers adsorb readily on silica due to electrostatic interaction since pH of most practical systems is above 2 and silica is negatively charged under these conditions. Anionic surfactants or polymers do not adsorb on silica at neutral pH due to the presence of similar charge on the solid and the adsorbate. However, anionic surfactants can adsorb on silica in the presence of multi-valent metal cations in the pH range in which metal ions hydrolyze to their first hydroxyl complex. This has been attributed to the chemisorption of the first hydroxyl species of metal ions on silica and modification of the quartz surface [3]. 

Manipulation of the temperature or pH, or addition of multi-valent metal ions to dilute polymer solutions are common techniques for creating hydrogels, but these fall outside of conventional superabsorbent technology. Gels of gelatin, certain natural gums, and sodium silicate are not the same as superabsorbent polymer gels. The key difference Is spontaneity. The former require assistance to gel while the latter form gel merely upon exposure to water. 

In contrast, multivalent ions are reduced down to the zero-valent state according to a multi-step reaction mechanism involving intermediate valencies by disproportionation. These redox reactions, which are often diffusion-controlled, have extensively been studied by pulse radiolysis in the case of several free or complexed metal ions. 

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