1. Ion exchanger Adsorption at the solid/liquid interface Ion exchange process means an exchange of ions between an electrolyte solution and a solid (ionite). In most cases the term is used to denote the processes of purification, separation, and decontamination of aqueous and other ion-containing solutions with solid polymeric or mineral 'ion exchangers'. This process is also called ion exchange adsorption, because it takes place at the solid/liquid interface. Ion exchanger an inorganic or organic solid substance containing ions (ionogenic groups bounded with the exchanger which can dissociate) which can be replaced by the ions from solution whose electric charge is of the same kind. Ion exchangers are either cation exchangers that exchange positively charged ions (cations) or anion exchangers that exchange negatively charged ions (anions). There are also amphoteric exchangers that are able to exchange both cations and anions simultaneously.
Ion exchangers practically do not dissolve in the solution. The amount of exchanged ions must be electrically equivalent to prevent electroneutrality. Typical ion exchangers are ion exchange resins (functionalized porous or gel polymer), zeolites, montmorillonite, clay, and soil humus. Fig. 1.2. Ion exchange resin Fig. 1.1. Ion exchanger [http://en.wikipedia.org/wiki/ion_exchange]
However, the simultaneous exchange of cations and anions can be more efficiently performed in mixed beds that contain a mixture of anion and cation exchange resins, or passing the treated solution through several different ion exchange materials. Ion-exchange capacity measure of the ability of ionite to undergo displacement of ions previously attached and loosely incorporated into its structure by ions present in the surrounding solution per unit mass (g, kg), or unit volume (cm 3, m 3 ) of the exchanger, and also val/kg (val = miliequivalent), mmol/g, mol/n (n the ion valency). The total capacity of an ion exchanger is defined as the total number of chemical equivalents available for exchange per some unit weight or unit volume of resin. The capacity may be expressed in terms of milliequivalents per dry gram of the exchanger.
Operating capacity, also called useful capacity, is the number of ion exchange sites where exchange has really taken place during the loading run. The ion exchange capacity is expressed as eq/l (equivalents per litre of resin). This value is characteristic of a given process and depends on the solution concentration, kind of ions, temperature, rate of the exchange process. The operating capacity is always smaller than the total capacity.
2. Kinds of ion exchangers Adsorption at the solid/liquid interface In respect of the chemical structure of the exchanger: inorganic organic In respect of the exchanger origin: natural semisynthetic synthetic.
Detailed classification: Adsorption at the solid/liquid interface I. Cationic exchangers Inorganic: natural (clays, aluminosilicates) semi-synthetic (treated glauconite) synthetic (synthetic zeolites) Organic: natural (peat, brown coal) semi-synthetic (sulfonated coal) synthetic (phenyl-formaldehyde resins) II. Anionic exchangers Inorganic: natural (diatomite) semi-synthetic (treated glauconite) synthetic (synthetic zeolites) Organic: natural (peat, brown coal) semi-synthetic (sulfonated coal) synthetic (phenyl-formaldehyde resins)
Natural ion exchangers Their application is smaller than that of the synthetic ones because of their worse physicochemical properties in comparison to those of the synthetic ones. They were used to soften water (zeolites - hydrated aluminosilicates of calcium and sodium). The general formula of zeolites: (Me 2+,Me 2+ )O; Al 2 O 3 nsio 2 mh 2 O This group includes such minerals as: analcime (analcite), chabazite, natrolite, skolecite and others. Adsorption at the solid/liquid interface Fig. 2.1.The microporous molecular structure of the zeolite, ZSM-5 [http://en.wikipedia.org/wiki/zeolite]
The basic structural elements of zeolites are tetrahedrons of SiO 4 and AlO 4 which form 4- or 6-element rings. The aluminosilicate skeleton possesses an excess of negative charge which is compensated by Me + or Me 2+ ions. The ions are not built-in the crystal structure. Therefore they can migrate and be exchanged by other ions from solution. This group of natural ion exchangers includes montmorillonite and glauconite as well as some soils. The soils are amphoteric ion exchangers. Semi-synthetic ionic exchanger These are natural exchangers which have been chemically treated, e.g. sulfonated coals obtained by treatment with concentrated sulphuric acid or oleum. They are known commercially as: Zoe-Karb-H, Permutyt, Wofatyt-Z, Eskarbo-H.
Synthetic ion exchangers These are: synthetic aluminosilicates having the general formula: Al 2 O 3 (SiO 2 )x (Na 2 O)x (H 2 O)z, synthetic resins. Synthetic resins are the most commonly used exchangers. They are mechanically resistant substances, insoluble in water and some organic solvents, like alcohols, ethers, hydrocarbons. They can exchange ions because of the presence of active groups in their matrix. The resins are obtained by polimerization, copolimerization or polycondensation of appropriate monomers whose functional groups can dissociate. The gropus can be acidic exchanging cations or basic exchanging anions.
An ion-exchange resin is in the form of small (1 2 mm diameter) beads, usually white or yellowish. The material has a highly developed structure of pores on the surface of which there are sites with easily trapped and released ions. Ion-exchange resins are widely used in different separation, purification, and decontamination processes. The most common examples are water softening and water purification.
The resin ionite general formula can be written: Cationic resin: R A M + Anionic resin: R B + X Where: R the polimer matrix, A the covalently bonded with the matrix anionic group, for example acidic, COO ; M + the ionically bonded cation with A which can dissociate, e.g. H + or metal cation; B + the covalently bonded with the matrix cationic group, e.g. =N 2+, X ionically bonded anion with B which can dissociate, e.g. OH. One polymer molecule can have many functional groups. Hence ionite is a polyelectrolyte whose ions can dissociate.
Characteristic functional groups of the ion exchange resins: Cationic resins Anionic resins ( SO 3 ) H + ( COO) H + ( O) H + ( S) H + sulphonic carboxylic phenolic thiophenolic ( NH 3 ) + OH (=NH 2 ) + OH ( NH) + OH primary amine secondary amine tertiary amine ( N) + OH quaternary ammonium
Examples: Adsorption at the solid/liquid interface SO 3 -H+ -CH-CH 2 - Cationic ion exchanger -CH 2 -CH-CH 2 -CH-CH 2 -CH- SO 3 -H+ copolymer of styrene and divinylobenzene possessing active sulphonate groups, whose proton H + is capable of exchanging with other cations. -CH 2 -CH-CH 2 -CH-
Examples: Adsorption at the solid/liquid interface OH NH 3 +OH- CH 2 Anionic ion exchanger polymer obtained by polycondensation -H 2 C CH 2 CH 2 - of phenol with fromaldehyde. The amine NH 3 +OH- OH group whose OH - ion can be replaced by other anions is active.
3. The ion exchange process Adsorption at the solid/liquid interface The process on a cation ion exchanger: The process on an anion ion exchanger: RM 2 + M 1 X RM 1 + M 2 X RHX 2 + MX 1 RHX 1 + MX 2 Where: MX 1 the electrolyte solution subjected to the process of ion exchange. The exchange reaction is reversible, therefore under static conditions the mass action law can be used: R M 2 + M 1 R M 1 + M 2 However, in practice the ion exchange process is conducted under dynamic conditions. The solution flows through a bed in the column filled with both cationic and anionic ion exchangers, or by two columns with cationic and anionic exchanger.
For example, if NaCl solution passes through the column filled with a cation exchanger whose H + protons can be substituted by Na + cations, three zones can be distinguished: post-exchange zone (A) upper layer substituted with Na +, exchanage zone (B) middle layer, where the process takes place, both Na + and H + are present in the exchanger and solution, but their concentration depends upon the site of the layer and the solution composition depends on the distance from the column top. pre-exchange exchange (C) lower layer not yet reached by NaCl solution. Fig. 3.1. Schematic representation of ion exchange process in the column: A the post-exchange zone; B the exchange zone; C the pre-exchange zone.
The eluate from the column will be free from Na + cations and contains an equivalent number of hydrogen ions until quantity of the solution passed through the bed does not produce any shifting of the exchange zone to the end of the column bed. If it occurs the break-through point is reached and Na + ions start appearing in the eluate. Their amount in the solution usually increases rapidly and then the exchanger is fully saturated (no exchange of H + for Na + ) and the solution passes through the bed unchanged. To the break-through point there corresponds the break-through volume (operating capacity), which is smaller than the total exchange capacity that occurs when the concentration of Na + ions is the same in the eluate as that of the input solution. Graphical representation of the ion exchange process is shown in Fig. 3.2, where c/c o is a function of the eluate volume V, and c is the ion concentration in elate while c o that in the input solution. This curve is called isoplane. The shaded area represents the total exchange capacity of the bed. This capacity equals the abscissa 'b' at c/c = 0.5, while the break-through volume shows abscissa 'a'.
Fig. 3.2. Isoplane of break-through (breakthrough curve); section a the break-through volume under given conditions, section b the total exchange of the bed.
4.. Factors affecting the ion exchange process Adsorption at the solid/liquid interface The ion exchange process is complicated and therefore it is difficult to be described theoretically. It involves adsorption, absorption, chemisorption and even catalytic reactions. Interpretation of the process takes into account: interaction forces in the crystal lattice (inorganic ion exchangers), adsorption equation of Freundlich and/or Langmuir, Donnan's equilibrium, theory of swelling osmotic pressure. Ion exchange process depends on properties of the exchanger and the ion undergoing exchange as well.
Affinity of the ion for a given exchanger first of all depends on: Electric charge of the ion the larger charge the greater is the attracting force by the functional groups and hence larger is its exchangeable capacity and rate of the process. Ion radius the exchange capability is inversely proportional to its radius. The hydrodynamic radii of ions decrease with the increasing atomic weight and hence their exchange energy increases. Degree of the ion hydration the exchange capacity of cations is inversely proportional to the hydrated radius.
Degree of ion hydration depends on: Adsorption at the solid/liquid interface Solution concentration Temperature Contaminations Other factors. The exchange energy of cations and anios can be arranged in series. In the case of the sulphonated phenolic resin exchangers the series are as follows: Cations: Na + < NH + 4 < K + < Sr 2+ < Cs + < Mg 2+ < Ca 2+ < Cd 2+ < Co 2+ < Al 3+ < Fe 3+ In the case of the weakly basic exchangers the series is: Anions: F < Cl < Br < I < CH 3 COO < PO 3 4 < NO 3 < citrate < CrO 2 4 < SO 2 4 < OH
The ions exchangeability depends to a great extent on ph of the solution, degree of dissociation of the exchanger functional groups, relation between H + and/or OH and other ions concentration. H + and OH compete with other ions in the exchange process. The characteristic parameter of the ion exchanger is its total exchange capacity because it does not depend on the particular conditions of the occurring process. For analytical purposes the break-through capacity is the most important.