Recent Progress in Ion Exchange

In recent years extensive research has been carried out on new crystalline inorganic and inorgano-organic layered compounds which possess ion exchange properties. Each layer in their structure can be considered as a planar macromolecule, while the substance as a whole is assumed to be a molecular crystal formed by these planar macromolecules. A reversible process of intercalation between the layers occurs due to interactions of guest species with active sites on the surface of the layer (lamella). However, the layers are unable to move spontaneously in a direction perpendicular to the plane. This is due to a certain rigidity of layers that plays an important role in intercalation reaction mechanism and energetics. Like other ion exchange materials, the charged layered solids may be strong, medium or weak cationic (or anionic).

The exchange of protons of a-ZrP phase for Li+, Na+ and Ca2+ occurs rapidly in acidic solutions, while H# exchange for larger or strongly hydrated cations like NH#, Rb +, Cs+, Ba2 +, Mg2 +, Cu2 + and Cr3 + is quite slow at room temperature due to the high activation energies of interlayer expansion. Exchange can be facilitated in materials with large interlayer distances like a-Zr(HPO4)(NaPO4). 5H2O (¿=11.8 A) or in intercalation compounds with ethanol or alkylamines. The compounds that can be protonated are preferably used as guest species. For example, an amino derivative of cyclodextrin has been used for intercalation, increasing the interplanar distance in a-ZrP up to d = 35.6 A. These distances for other guest species are; 14.2 A for ethanol, 20.4 A with benzimidazole, 22.8 A with 1-hexylamine and 23.1 A with lysine. The layered compounds under discussion can swell upon introduction of water or other solvents into the interlayer space. Sometimes the process leads to delamination, i.e. destruction of the crystal into separate lamellae. Withdrawal of the solvent results in reaggregation of the lamellae in thin films or membranes. Inorgano-organic derivatives, phosphonates of layered a-structure, can be obtained by introducing the corresponding acid H2O3PR (where R =-CH,, -C6H5, -O(CH2)nCH, etc.) into the reaction, instead of H3PO4. It is also possible to synthesize those compounds by substitution of existing OH groups in the a-ZrP structure by R or OR. Another interesting group of compounds is covalently pillared zirconium diphosphonates of general formula MIV(O3P-R-PO3). If the R group is small, then a low degree of interlayer microporosity is observed, while for pillared compounds containing fragments of 3,3(5,5)-tetramethylbiphenyldiphos-phonic acid the value of interlayer porosity is raised to 375m2g_1 (an average pore size of 5 A). Inor-gano-organic derivatives have also been obtained for y-ZrP by substitution of the interlayer O2P(OH)2 groups for O2PRR'. Pillared phases of y-ZrP with, for example, biphenylphosphonate groups have a volume of micropores of 320 m2 g_1 and an average size of 5.8 A.

A limited number of inorganic anion exchangers is known. Layered double hydroxides (or hydrotalcite-like anionic clays) can exchange a large number of inorganic and organic anions, while layered ZrPO4Cl can selectively replace chloride anions with other monodentate anionic ligands.

Currently, ion exchange is of extreme importance for processing of irradiated nuclear fuel and treatment of spent fuel elements of nuclear power stations, where it is often combined with other techniques, e.g. extraction. The processes of sorption play an important role in deactivation of nuclear industry wastes and in purification of cooling water from nuclear reactors. Different kinds of ion exchangers are widely used for the clean-up of the world's worst nuclear accident at Chernobyl. For example, Strelko et al. are carrying out both research and application of highly selective inorganic granulated ion exchangers for elimination of radioactive isotopes from drinking water, milk, etc. Ion exchange is extensively used in medicine for haemosorption (or haemoperfusion) - the method of blood purification from toxic compounds by direct contact of the sorbent with the patient's blood. This method was applied for the first time by Muirhead and Reid in 1948; they directed the blood flow through a mixture of cation and anion exchangers taken in a ratio of 9:1. Haemosorption can be applied alone or in combinations with haemodialysis (when the toxins are distributed between two liquid phases, separated by a semipermeable membrane). The ion exchangers are used to regenerate dialysate from the artificial kidney apparatus. Further improvement of the haemosorption method is connected with the necessity to resolve problems of selective blood purification, as well as the problem of better sorbent compatibility with biological fluids. Another possible medical application of ion exchangers consists of the creation of drugs and pharmaceuticals with prolonged activity, offering the possibility to release an active component inside the patient's body over time and maintaining its necessary concentration.

A nontraditional application of ion exchangers in nonpolar organic media is the ultra-purification of organometallic compounds used as precursors in chemical vapour deposition. These precursors are widely utilized for synthesizing materials possessing valuable properties for micro-, opto- and acousto-electronics, and protective and optical coatings. The organometallics in question react readily with atmospheric oxygen and moisture, while at the same the requirements on their purity are quite rigorous (less then 1 x 10~3% of the sum of contaminants). The above requirements can be met by treatment with a sorbent composition containing inorganic ion exchangers based on titanium and zirconium phosphates, thus replacing energy-intensive and expensive traditional methods (sublimation or distillation).

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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