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Literature Review - Coggle Diagram

- - - - A high degree of crosslinkage and the tight network creates a hard, brittle resin, and prevents
        the contacting solution from diffusing rapidly throughout the entire resin matrix. Thus, the theoretical capacity is never reached due to certain tightly enmeshed “pockets”, causing inaccessible to the solvent. The time required for establishment of equilibrium between a resin and the contacting solution varied directly with the degree of crosslinking. Simultaneously, the high degree of crosslinking will prevent extreme swelling or shrinkage of the resin with changing environment. .
    - - Adsorb moisture from air. Amount of adsorption depends on the cross-linking and the type of functional group. Eg: low cross-linking gel resins with functional groups of sulfonic acid or quaternary ammonium has high water adsorption, resulting in swelling. Resin volumes change with conversion to ionic forms of differing degrees of hydration. For a cation exchanger, there is a volume change with the monovalent ion species, Li+ > Na+ > K+ > Cs+ > Ag+. With polyvalent ions, hydration is reduced by the cross-linking action; therefore, Na+ > Ca2+ > Al3+. In more concentrated solutions, less water is taken up owing to greater osmotic pressure. Frequent swelling and contraction will reduce the life of resin.
    - - Available in different size of particle, commonly manufactured in form of polydisperse spherical beds with the size distributed within the range 0.25-1.25mm or in form of uniform particle size (UPS). Smaller particles will improve the kinetics of the ion exchange reaction but increase the water pressure drop and decrease the flow rate.
    - - An ion exchange resin is a crosslinked polymer, high molecular substances, mostly solid, organic or inorganic, which contains either acidic or basic structural units and which can exchange either cations or anions on contact with a solution. Resins are insoluble in all solvents, due to their infinite molecular size which arises from the crosslinks. Ion exchange sites (acidic or basic sites) containing inorganic functional group with mobile ions are present throughout the 3D copolymer matrix where either cations or anions are attached to the polymer network from the surrounding liquid media. It is an insoluble matrix normally in the form of small microbeads with size range from 0.25mm to 0.5mm radius. The beads are fabricated from an organic polymer substrate, naturally porous, providing a large surface area on and inside them. There are multiple types of ion-exchange resin and the most commercial resins are made of polystyrene sulfonate. The ion exchange process is reversible because this process is governed by “Law of Mass Action”. The law of mass action states that the rate of a chemical reaction is proportional to the product of the masses of the reactants. Hence, the resins can be used repeatedly by converted from one ionic form to another and back and reused. The process of converting the ion exchange resins to the desired ionic form at the end of cycle of its intended use is known as ‘regeneration’.
    - - Mechanical or physical stability of ion exchange resins depends on the toughness of the polymer structure, which is depending on the level of cross-linking of the polymer.
        The stability is also depending on the frequency of swelling contraction cycles. Chemical degradation of ion exchange resins may be caused by contaminating or fouling the resin pores by breaking polymer structure, precipitates (e.g., iron hydroxide), loss of ion exchange capacity due to a modification of the functional groups.
    - - Capacity of resins means the number of chemical equivalents of ions that can be taken up by a unit amount of the resin, where the unit amount of resin can be dry weight, wet weight or wet volume. Loss of ion exchange capacity can be cause by a modification of the functional groups. High cross-linking of polymer backbone will decrease the capacity of resin measured on the dry basis as fewer functional groups (ion exchange sites) may be attached to the polymer molecules. However, high cross-linking will decrease hydration of resin, so the capacity measured on the wet basis increases with an increase of the cross-linking level.
  - - - Addition polymerization of olefinic compounds. Contain only carbon-carbon bonds as the links which join the monomers.
        These polymers have the same empirical composition as the monomers. Such polymers are stable to heat and pH changes, and may be used directly as obtained. An advantage of
        addition polymerization over condensation methods in resin preparation is the addition methods can be adapted to the preparation of the polymer in bulk, powdered or bead forms of various sizes. The condensation reactions usually give a bulk product which must be broken to desired sizes.
    - - Accomplished by reacting polyfunctional molecules in such a
        manner that new carbon-carbon, carbon-nitrogen or carbon-oxygen bonds are formed. These act as the joining links which bind the starting molecules with the polymeric network. Condensation reactions are always accompanied by the formation of one small molecule such as water, alcohol or ammonia, for each new carbon-carbon, carbon-nitrogen or carbon-oxygen bond created. Such polymers have a composition which differs from that of the monomers by the elements of the molecule which was split off in the polymerization. Some of the resins prepared in this fashion are not stable over the entire pH range, as carbon-oxygen and carbon-nitrogen links are susceptible to hydrolytic cleavages.
  - - - Work like weak acids, able to neutralize strong bases. They derive functionality from a carboxylic group (-COOH) and available in hydrogen form (R-COOH). They have high bonding ability for hydrogen ions, so they are easily regenerated by stoichiometric amount of acid. Its ion exchange capacity increases with increase in the pH of solution. WAC resins are not working efficiently for the treatment of acidic solutions but working very efficiently for demineralization and dealkalization of water. When operated in the hydrogen form, WAC resins remove cations that are associated with alkalinity, producing carbonic acid: Ca(HCO3)2 + 2RCOO.H ⇆ 2RCOO.Ca + 2H2CO3 When combine with strong resins, it can use for full demineralization. Has higher regeneration efficiency than SAC resins.
    - - Work like strong bases. Neutralizes strong acids and convert neutral salts into their corresponding bases. They derive their functionality from quaternary ammonium functional groups. Two types of quaternary ammonium groups, referred to as Type I and Type II, are used. Type I sites have three methyl groups while in a Type II resin, one of the methyl groups is replaced with an ethanol group. Type II resins provide a greater regeneration efficiency and a greater capacity for the same amount of regenerant chemical used. Generally, SBA resin is found in hydroxide form (R-NH3OH) and it remove all commonly encountered anions. Generally used for softening and demineralization of water. The fully utilized SBA resins may be regenerated by concentrated sodium hydroxide (NaOH) solution.
    - - Work like strong acids. Neutralizes strong bases and convert neutral salts into their corresponding acids. Derive their functionality from sulfonic acid groups (HSO3¯) and generally available in form of hydrogen (R-SO3H) or sodium (R-SO3Na). General cation exchange reaction is 2(R-SO3Na) + CaCl2 ⇆ (R-SO3)2Ca + 2NaCl. Remove nearly all raw water cations, replacing them with hydrogen ions. Cross-linking level is from 8% to 10% and its ion exchange capacity does not depend on the solution pH. Used for water softening and demineralization. Commonly, regeneration in form of hydrogen (acid) is performed by a strong acid (e.g., HCl) while regeneration in form of sodium salt is performed by sodium chloride solution (NaCl).
    - - Work like weak base, able to neutralize strong acids. They derive their functionality from primary (R-NH2), secondary (R-NHR'), or tertiary (R-NR'2) amine groups. They are readily removing sulfuric, nitric, and hydrochloric acids. Its ion exchange capacity is directly dependent on the pH of solution. Ion exchange capacity increases when decrease pH of solution decreases. WBA resins are not used for treatment solutions having pH more than 6 and it is regenerated by small amounts of weak base salt such as ammonia or sodium carbonate. Can be used for dealkalization, partial demineralization, or (in combination with strong resins) full demineralization.
- - - - The theoretical dissociation electric potential and energy consumption for Al(OH)3 were 0.638 V and 61.57 kJ/mol, respectively. Furthermore, the total charge amount of cations and anions from Al(OH)3 dissociation reaction was 3 times as much as that from H2O when the same amount of regenerant was dissociated. This shows that Al(OH)3 was easily dissociated to Al3+ and OH- when applied electric field and more free ions could be used to regenerate resins. The selectivity order of strongly acidic cation exchange resins follows Fe3+ > Al3+ > Ca2+ > Mg2+ > K+ > Na+ > H+. Al3+ had second highest affinity for the resins which was preferentially exchanged by the cation resins. For weakly basic anion exchange ion, the selectivity order was OH-> SO2-4> NO-3 > Cl- > HCO-3 > HSiO-3. According to the selectivity order for the strongly acidic cation exchange resins, Al3+ was easier to exchange with other cations in the spent resins. It also had less ionic mobility, and the migration rate of Al3+ was
        Low, causing long contact time of Al3+ with resins. Thus, the regeneration efficiency of resin with Al(OH)3 regenerant was high.
- - - - P.N.G.N, et al. has used ammonium bicarbonate to regenerate weak acid and weak base ion exchange resin by using chemical-thermal regeneration and the result shows nearly 100% regeneration of exhausted resin without the need of acid and base washing. Ammonium bicarbonate permit low-cost regeneration process. It decomposes in solution above 60°C so the energy required for regeneration is less than for other desalination processes. After decomposition, the gases (NH3 and CO2) can then be collected, either by column distillation or membrane distillation (MD) and used for to regenerate AB solution. Thus, the chemical waste produced from regeneration process can be eliminated.
    - - Traditional regenerants are salts (such as NaCl, KCl, NH4Cl), acids (HCl, H2SO4, HNO3, acetic acid, citric acid), and alkali
        (NaOH, KOH, NH3, Na2CO3). For example, NaCl is normally used to regenerate SAC resins in the softening process. SAC resins need to be regenerated with a strong acid for the purpose of decationization, and SBA resins are always regenerated with NaOH in the demineralization process. The disadvantages of the use of traditional regenerants are the low regeneration efficiency and the discharge of large amount of wastewater, leading to serious environmental pollution and resource waste.