Ion Exchange

Ion exchange is a process in which ions of a particular species in solution are replaced by ions with a similar charge but of different species attached to an insoluble resin. In essence, ion exchange is a sorption process and can also be considered a reversible chemical reaction. The common appli cations of ion exchange are water softening (removal of "hardness" ions such as CA2+ and Mg2+) and nitrate removal in advanced wastewater treatment operations. These ion exchange resins are either naturally occurring inorganic zeolites or synthetically produced organic resins. The synthetic organic resins are the predominant type used today because their characteristics can be tailored to specific applications.

An organic ion exchange resin consists of an organic or inorganic network structure with attached functional groups that can exchange their mobile ions for ions of similar charge from the surrounding medium. Each resin has a distinct number of mobile ion sites that set the maximum quantity of exchanges per unit of resin. Ion exchange resins are called cationic if they exchange positive ions and anionic if they exchange negative ions. Cation exchange resins have acidic functional groups such as sulfonic, whereas anion exchange resins are often classified by the nature of the functional group as strong acid, weak acid, strong base, and weak base. The strength of the acidic or basic character depends on the degree of ion-ization of the functional groups, similar to the situation with soluble acids or bases. Accordingly, a resin with sulfonic acid groups would act as a strong cation exchange resin.

Ion exchange reactions are stoichiometric and reversible, and in that way they are similar to other solution phase reactions—for example (Equation 3.38):

In this reaction, the magnesium ions of the magnesium sulfate (MgSO4) are exchanged for the calcium ions of the calcium hydroxide Ca(OH)2 molecule. Similarly, a resin with hydrogen ions available for exchange will exchange those ions for magnesium ions from solution. The reaction can be written as follows (Equation 3.39):

R indicates the organic portion of the resin and SO3 is the immobile portion of the ion active group. Two resin sites are needed for magnesium ions with a plus 2 valence (Mg+2).

As stated previously, the ion exchange reaction is reversible. The degree the reaction proceeds to the right will depend on the resins preference or selectivity for magnesium ions compared with its preference for hydrogen ions. The selectivity of a resin for a given ion is measured by the selectivity coefficient, K, which in its simplest form for the following reaction (Equation 3.40):

is expressed as: K = (concentration of B+ in resin/concentration of A+ in resin) X (concentration of A+ in solution/concentration of B+ in solution).

The selectivity coefficient expresses the relative distribution of the ions when a resin in the A+ form is placed in a solution containing B+ ions. Table 3.1 shows the selectivities of strong acid and strong base ion exchange resins for various ionic compounds. It should be pointed out that the selectivity coefficient is not constant but varies with changes in solution conditions. It does provide a means of determining what to expect when various ions are involved. As indicated in Table 3.1, strong acid resins have a preference for magnesium over hydrogen. Despite this preference, the resin can be converted back to the hydrogen form by contact with a concentrated solution of sulfuric acid (H2SO4) (Equation 3.41):

This step is known as regeneration. In general terms, the higher the preference a resin exhibits for a particular ion, the greater the exchange efficiency in terms of resin capacity for removal of that ion from solution.

Table 3.1. Selectivity of ion exchange resins for some ions in order of decreasing preference (source: Weber, 1972).

Strong Acid Cation Exchanger

Strong Base Anion Exchanger

Barium

Iodide

Calcium

Nitrate

Copper

Bisulfite

Zinc

Chloride

Magnesium

Cyanide

Potassium

Bicarbonate

Ammonia

Hydroxide

Sodium

Fluoride

Hydrogen

Sulfate

Greater preference for a particular ion, however, will result in increased consumption of chemicals for regeneration.

Ion exchange resins are classified as cation exchangers that have positively charged mobile ions available for exchange, and anion exchangers, whose exchangeable ions are negatively charged. Both anion and cation resins are produced from the same basic organic polymers. They differ in the ionizable group attached to the hydrocarbon network. It is this functional group that determines the chemical behavior of the resin. Resins can be broadly classified as strong or weak acid cation exchangers or strong or weak base anion exchangers.

Strong acid cation resins

Strong acid resins are so named because their chemical behavior is similar to that of a strong acid. The resins are highly ionized in both the acid (R-SO3H) and salt (R-SO3Na) form. They can convert a metal salt to the corresponding acid by the reaction (Equation 3.42):

The hydrogen and sodium forms of strong acid resins are highly dissociated and the exchangeable Na+ and H+ are readily available for exchange over the entire pH range. Consequently, the exchange capacity of strong acid resins is independent of solution pH. These resins would be used in the hydrogen form for complete deionization; they are used in the sodium form for water softening (calcium and magnesium removal). After exhaustion, the resin is converted back to the hydrogen form (regenerated) by contact with a strong acid solution, or the resin can be converted to the sodium form with a sodium chloride solution. Hydrochloric acid (Hcl) regeneration would result in a concentrated magnesium chloride (Mgcl2) solution.

Weak acid cation resins

In a weak acid resin, the ionizable group is a carboxylic acid (COOH) as opposed to the sulfonic acid group (SO3H) used in strong acid resins. These resins behave similarly to weak organic acids that are weakly dissociated.

Weak acid resins exhibit a much higher affinity for hydrogen ions than do strong acid resins. This characteristic allows for regeneration to the hydrogen form with significantly less acid than is required for strong acid resins. Almost complete regeneration can be accomplished with stoichio-metric amounts of acid. The degree of dissociation of a weak acid resin is strongly influenced by the solution pH. Consequently, resin capacity depends in part on solution pH.

Strong base anion resins

Like strong acid resins, strong base resins are highly ionized and can be used over the entire pH range. These resins are used in the hydroxide (OH) form for water deionization. They will react with anions in solution and can convert an acid solution to pure water (Equation 3.43):

Regeneration with concentrated sodium hydroxide (NaOH) converts the exhausted resin to the hydroxide form.

Weak base anion resins

Weak base resins are like weak acid resins in that the degree of ionization is strongly influenced by pH. Consequently, weak base resins exhibit minimum exchange capacity above a pH of 7.0. These resins merely sorb strong acids: they cannot split salts.

Evaluation of resins

Resin vendors usually provide detailed information regarding the properties of resins they sell, as is shown in a typical ion exchange resin property sheet from DOW Chemical (Table 3.2). However it is still sensible to evaluate the resins in service for any change in capacity of the resins. The potential loss of active ion exchange sites, due to reduction of cross-linking and other deleterious effects of long-term services, is of particular concern. The common properties of resins for uses in wastewater treatment undergoing evaluations are

• Dry weight capacity

• Wet weight capacity

• Wet volume capacity

• Percentage of moisture content

Table 3.2. A commercial ion exchange resin property sheet (courtesy of DOW Chemical Company).

Commercial Name

DOWEX®* 1 X . . .

DOWEX® 50 WX . . .

Strongly Basic

Strongly Acid

Type

Anion Exchanger

Cation Exchanger

Functional group

Trimethyl ammonium

Sulfonic acid

Cross linkage (% DVB)

2 or 8

2, 4, or 8

Ionic form as shipped

Cl-

Na+ (analytical grade)

H- (practical grade)

Shipping density (kg/l)

0.7

0.8

Volume change (%)

Cl- b OH- ~ + 20 %

Na+ b H+ ~ + 8 %

Effective working

0 - 14

0 - 14

range (pH)

Selectivity for ions

I- > NO3- > Br- > Cl-

Ag+ > Cs+ > Rb+ > K+

> acetate- > OH-

> NH4+> Na+ > Li+

> F-

> Ba2+ > Sn2+ > Ca2+

> Mg2+ > Be2+

Total exchange

1.3

1.9

capacity (eq/l)

OH- form max. 50°C

Na+ form max. 120°C

(122°F)

(248°F)

Thermal stability

Cl- form max.150°C

H+ form max. 80°C

(302°F)

(176°F)

Moisture (%)

39 - 80

40 - 82

*DOWEX® is a registered trademark of Dow Chemical Company.

*DOWEX® is a registered trademark of Dow Chemical Company.

Ion exchange systems

In order to design an ion exchange system for removing ions from complex food and agricultural wastewater, several runs of a laboratory scale ion exchange column are necessary to develop system design criteria. Eckenfelder (1989) suggested an experimental procedure for conducting experiments on a lab-scale ion exchange column:

1. Rinse the column for 10 minutes with deionized water at a rate of 50 ml/min.

2. Switch to a waste-containing solution passing through the column at the same flow rate as deionized water.

3. Measure the initial volume of solution to be treated.

4. Start the treatment cycle and develop the breakthrough curve until the ion concentration reaches the maximum effluent limit.

5. Backwash to 25% bed expansion for 5-10 minutes with distilled water.

6. Regenerate at a flow rate of 6 ml/min using the concentration and volume recommended for the resin by the vendor and collect the spent regenerant and measure the recovered ions.

7. Rinse the column with distilled water.

After several runs of the experiment, it is possible to select optimal operating conditions in terms of resin utilization and regenerant efficiency.

Most practical applications of ion exchange use fixed-bed column systems, the basic component of which is the resin column. Complete de-mineralization operations generally involve the wastewater passing first through a bed of strong acid resin to replace metal ions with hydrogen ions (thus lowering the pH) followed by a weakly basic anion exchanger as shown in Fig. 3.18 of a schematic diagram of this two-stage type of arrangement. Weak base resins are preferred over strong base resins be

Figure 3.18. A schematic diagram of a two-stage ion exchange system.

Effluent

Figure 3.18. A schematic diagram of a two-stage ion exchange system.

cause they require less regenerant chemical. A reaction between the resin in the freebase form and HCl would proceed as follows (Equation 3.44):

The weak-base resin does not have a hydroxide ion form, as does the strong-base resin. Consequently, regeneration needs only to neutralize the absorbed acid; it need not provide hydroxide ions. Less expensive weakly basic reagents such as ammonia (NH3) or sodium carbonate can be employed.

Ion exchanger systems used for wastewater treatment have been based on a process called DESAL (Downing et al, 1968), which utilizes a three-step operation:

1. A weak base anion resin in the bicarbonate form, R-(NH)HCO3

2. A weak acid cation in the hydrogen form, R-COOH

3. A weak base anion resin in the free base form

A schematic diagram of the DESAL process is shown in Fig. 3.19.

Ma* HCtfj HiO+HiCOj

Figure 3.19. A schematic diagram of the DESAL ion exchange system.

Ma* HCtfj HiO+HiCOj

Figure 3.19. A schematic diagram of the DESAL ion exchange system.

110 Food and Agricultural Wastewater Utilization and Treatment Chapter Remarks

For organic-rich food and agricultural wastewater, biological treatment has its unrivaled advantages. However, physicochemical processes are still important in treating this type of wastewater stream. First, physico-chemical treatment plants have small footprints; this is important for densely populated areas. Second, physicochemical processes can be easily expanded as and when required—for example, if subsequent treatment using biological methods is planned. Third, the processes are often fast compared to biological treatment; they may be included as integral parts of an overall wastewater management strategy if the influent streams are mixed with municipal or other industrial wastewater. Finally, certain pollutants in wastewater are not biodegradable, thus requiring physicochem-ical processes to remove them.

The disadvantages of physicochemical processes are well known: high operating and capital costs, relatively modest treatment performance, and larger sludge volume. It is not accidental that physicochemical processes in practice are often interspersed with biological treatment processes to achieve optimal results. The ultimate choices of physicochemical processes for a given treatment task are largely dependent upon the deliberate consideration of technological and economical facts within the constraints of treatment requirements and regulatory compliance.

One of the most challenging aspects of treatment process design is the analysis and selection of the treatment processes capable of meeting the permit or recycling requirements. The methodology of process analysis that leads to process selection includes several evaluation steps. These evaluations vary greatly with the project and characteristics of wastewater. Nevertheless, any process analysis needs to consider several important factors: process applicability, applicable flow range and variation, reaction kinetics and reactor selection, performance, treatment residuals and odor, sludge treatment, chemicals/polymers requirements, and energy requirements. Once process analysis is done, process selection or design commences; several methods of process design or selection may be considered—process selection based on empirical relationship from experience or literature and process design based on kinetic analysis or modeling. Chapter 1 provides the basic tools to assist the selection process.

Further Reading

Weber, Walter J., Jr. 1972. Physicochemical Process: For Water Quality Control. New York: John Wiley & Sons.

Visvanathan, C. and Ben Aim, Roger. 1989. Water, Wastewater, and Sludge Filtration. Boca Raton, FL: CRC Press.

Metcalf and Eddy, Inc. (Tchobanoglous, G. and Burton, F.L.). 1991. Wastewater Engineering, Treatment, Disposal, and Reuse, 3rd edition. New York: McGraw-Hill, Inc.

Droste, Ronald L. 1996. Theory and Practice of Water and Wastewater Treatment. New York: John Wiley & Sons.

Eckenfelder, W. Wesley, Bowers, Alan R., and Roth, John A. 1996. Chemical Oxidation: Technology for the Nineties, Volume IV, Boca Raton, FL: CRC Press.

Cooney, David O. 1998. Adsorption Design for Wastewater Treatment. Boca Raton, FL: CRC Press.

Qusim, Syed R. 1998. Wastewater Treatment Plants: Planning, Design, and Operation, Second Edition. Boca Raton, FL: CRC Press.

Drinan, Joanne E. 2000. Water and Wastewater Treatment: A Guide for the Nonengineering Professionals. Boca Raton, FL: CRC Press.

Lin, Shun Dar and C.C. Lee. 2001. Water and Wastewater Calculations Manual. New York: McGraw-Hill Professional.

Hahn, H., Hoffman, E., and Odegaard, H. 2002. Chemical Water and Wastewater Treatment VII: (Gothenburg Symposia). London: IWA Publishing.

Tang, Walter Z. 2003. Physicochemical Treatment of Hazardous Wastes. Boca Raton, FL: Lewis Publishers (CRC Press).

Parsons, Simon. 2005. Advanced Oxidation Processes for Water and Wastewater Treatment. London: IWA Publishing.

References

Canale, R.P. and Borchardt, J.A. 1972. "Sedimentation." In Walter J. Weber, Jr., Physi-cochemical Process: For Water Quality Control. New York: John Wiley & Sons.

Cheryan, M. 1986. Ultrafiltration Handbook. Lancaster, PA: Technomic Publishing Co.

Davis, T.A. 1990. "Electrodialysis." In Handbook of Industrial Membrane Technology. Porter, M.C., ed. Park Ridge, NJ: Noyes Publications.

Downing, D.G., Kunin, R., and Pollio, F.X. 1968. "DESAL process—Economic ion exchanger system for treating brackish and acid mine drainage waters and sewage waste effluents." Water—1968. Chemical Engineering Progress Symposium Series, 64, 90. American Institute of Chemical Engineers, New York.

Eckenfelder, W. Wesley. 1989. Industrial Water Pollution Control, 2nd edition. New York: McGraw-Hill, Inc.

Glasgow, L.A. and Liu, S.X. 1995. "Effects of Macromolecular Conformation upon SolidLiquid Separation and Water Treatment Plant Residuals," Environmental Technology 16: 915-927.

Liu, S.X. 1995. The Essential Aspects of Floc Structure and Breakage. Chemical Engineering Department, Kansas State University, Manhattan, KS.

Lopez-Leiva, M. 1988. "The use of electrodialysis in food processing, II. Review of practical applications." Lebens-mittel Wissemschaft und Technologie 21: 177-182.

Metcalf and Eddy, Inc. (Tchobanoglous, G. and Burton, F.L.). 1991. Wastewater Engineering, Treatment, Disposal, and Reuse. 3rd edition, New York: McGraw-Hill, Inc.

Moulin, P., Manno, P., Rouch, J.C., Serra, C., Clifton, M.J., and Aptel, P. 1999. "Flux improvement by dean vortices: Ultrafiltration of colloidal suspensions and macromolecu-lar solutions." Journal of Membrane Science 156: 109-130.

Mulder, M. 1991. Basic Principles of Membrane Technology; Dordrecht, Germany: Kluwer Academic Publishers.

Rautenbach, R. and Albreht, R. 1989. Membrane Processes. Chichester, Britain: John Wiley & Sons.

Rosenberg, M. 1995. "Current and future applications for membrane processes in the dairy industry." Trends in Food Science and Technology 6: 12-19.

Weber, J.W., Jr. 1972. Physicochemical Processes for Water Quality Control. 640pp. New York: John Wiley & Sons.

Food and Agricultural Waste Water Utilization and Treatment

Sean X. Liu

Copyright © 2007 by Blackwell Publishing

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