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CHAPTER ION EXCHANGE AND INORGANIC ADSORPTION Dennis A Clifford, Ph.D., P.E., DEE Professor and Chairman Department of Civil and Environmental Engineering University of Houston Houston, Texas INTRODUCTION AND THEORY OF ION EXCHANGE Contaminant cations such as calcium, magnesium, barium, strontium, and radium, and anions such as fluoride, nitrate, fulvates, humates, arsenate, selenate, chromate, and anionic complexes of uranium can be removed from water by using ion exchange with resins or by adsorption onto hydrous metal oxides such as activated alumina (AAl) granules or coagulated Fe(II), Fe(III), Al(III), and Mn(IV) surfaces This chapter deals only with the theory and practice of ion exchange with resins and adsorption with activated alumina (AAl) The reader interested in cation and anion adsorption onto hydrous metal oxides in general is referred to Schindler’s and Stumm’s publications on the solid-water interface (Schindler, 1981; Stumm, 1992) as a starting point Ion exchange with synthetic resins and adsorption onto activated alumina are water treatment processes in which a presaturant ion on the solid phase, the adsorbent, is exchanged for an unwanted ion in the water In order to accomplish the exchange reaction, a packed bed of ion-exchange resin beads or alumina granules is used Source water is continually passed through the bed in a downflow or upflow mode until the adsorbent is exhausted, as evidenced by the appearance (breakthrough) of the unwanted contaminant at an unacceptable concentration in the effluent The most useful ion-exchange reactions are reversible In the simplest cases, the exhausted bed is regenerated using an excess of the presaturant ion Ideally, no permanent structural change takes place during the exhaustion/regeneration cycle (Resins swell and shrink, however, and alumina is partially dissolved during 9.1 9.2 CHAPTER NINE regeneration.) When the reactions are reversible, the medium can be reused many times before it must be replaced because of irreversible fouling or, in the case of alumina, excessive attrition In a typical water supply application, from 300 to as many as 300,000 bed volumes (BV) of contaminated water may be treated before exhaustion Regeneration typically requires from to bed volumes of regenerant, followed by to 20 bed volumes of rinse water These wastewaters generally amount to less than percent of the product water; nevertheless, their ultimate disposal is a major consideration in modern design practice Disposal of the spent media may also present a problem if it contains a toxic or radioactive substance such as arsenic or radium Uses of Ion Exchange in Water Treatment By far the largest application of ion exchange to drinking water treatment is in the area of softening, that is, the removal of calcium, magnesium, and other polyvalent cations in exchange for sodium The ion-exchange softening process is applicable to both individual home use and municipal treatment It can be applied for wholehouse (point-of-entry or POE) softening or for softening only the water that enters the hot water heater Radium and barium are ions more preferred by the resin than calcium and magnesium; thus the former are also effectively removed during ionexchange softening Resins beds containing chloride-form anion exchange resins can be used for nitrate, arsenate, chromate, selenate, dissolved organic carbon (DOC), and uranium removal, and more applications of these processes will be seen in the future Activated alumina is being used to remove fluoride and arsenate from drinking water, particularly high total dissolved solids (TDS) waters, at point-of-use (POU), (POE), and municipal scales The choice between ion exchange or alumina adsorption (to remove arsenic from water, for example) is largely determined by (a) the background water quality— including TDS level, competing ions, alkalinity, and contaminant concentration— and (b) the resin or alumina affinity for the contaminant ion in comparison with the competing ions The affinity sequence determines the run length, chromatographic peaking (if any), and process costs As previously mentioned, process selection will be affected by spent regenerant and spent medium disposal requirements, and regenerant reuse possibilities, particularly if hazardous materials are involved Each of these requirements is dealt with in some detail in the upcoming design sections for the specific processes summarized in Table 9.1 Past and Future of Ion Exchange Natural zeolites (i.e., crystalline aluminosilicates) were the first ion exchangers used to soften water on a commercial scale Later, zeolites were completely replaced by synthetic resins because of the latters’ faster exchange rates, longer life, and higher capacity Aside from softening, the use of ion exchange for removal of specific contaminants from municipal water supplies has been limited This is primarily because of the expense involved in removing what is perceived as only a minimal health risk resulting from contaminants such as fluoride, nitrate, or chromate.The production of pure and ultrapure water by ion-exchange demineralization (IXDM) is the largest use of ion exchange resins on a commercial scale The complete removal of contaminants, which occurs in demineralization (DM) processes, is not necessary for drinking water treatment, however Furthermore, costs associated with these treatments ION EXCHANGE AND INORGANIC ADSORPTION 9.3 TABLE 9.1 Advantages and Disadvantages of Packed-Bed Inorganic Contaminant Removal Processes Ion exchange Advantages ● Operates on demand ● Relatively insensitive to flow variations, short contact time required ● Relatively insensitive to trace-level contaminant concentration ● Essentially zero level of effluent contaminant possible ● Large variety of specific resins available ● Beneficial selectivity reversal commonly occurs upon regeneration ● In some applications, spent regenerant may be reused without contaminant removal Disadvantages ● Potential for chromatographic effluent peaking when using single beds ● Variable effluent quality with respect to background ions when using single beds ● Usually not feasible at high levels of sulfate or total dissolved solids ● Large volume/mass of regenerant must be used and disposed of Activated alumina adsorption Advantages ● Operates on demand ● Relatively insensitive to total dissolved solids and sulfate levels ● Low effluent contaminant level possible ● Highly selective for fluoride and arsenic Disadvantages ● Both acid and base are required for regeneration ● Relatively sensitive to trace-level contaminant concentration ● Media tend to dissolve, producing fine particles ● Slow adsorption kinetics and relatively long contact time required ● Significant volume/mass of spent regenerant to neutralize and dispose of are high compared with those of the alternative membrane processes (i.e., reverse osmosis and electrodialysis) for desalting water (see Chapter 11) Adherence to governmentally mandated maximum contaminant levels (MCLs) for inorganic contaminants (IOCs) will mean more use of ion exchange and alumina for small-community water treatment operations to remove barium, arsenic, nitrate, fluoride, uranium, and other IOCs An AWWA survey (1985) indicates that 400 communities exceeded the 10 mg/L nitrate-N MCL, 400 exceeded the 4.0 mg/L fluoride MCL (USEPA, 1985), and 200 exceeded the 2.0 mg/L secondary limit on barium Regarding radiological contaminants, an estimated 1,500 communities exceed the proposed 20 µg/L MCL for uranium (USEPA, 1991), and many others may exceed the MCL goal for radon (Rn) contamination when it is established In most of these cases, new contaminant-free sources cannot readily be developed, and a treatment system will eventually be installed ION EXCHANGE MATERIALS AND REACTIONS An ion exchange resin consists of a crosslinked polymer matrix to which charged functional groups are attached by covalent bonding The usual matrix is polystyrene 9.4 CHAPTER NINE crosslinked for structural stability with to percent divinylbenzene The common functional groups fall into four categories: strongly acidic (e.g., sulfonate, SO3−); weakly acidic (e.g., carboxylate, COO−); strongly basic (e.g., quaternary amine, N+(CH3)3); and weakly basic (e.g., tertiary amine—N(CH3)2) A schematic presentation of the resin matrix, crosslinking, and functionality is shown in Figure 9.1.The figure is a schematic three-dimensional bead (sphere) made up of many polystyrene polymer chains held together by divinylbenzene crosslinking The negatively charged ion exchange sites (SO3−) or (COO−) are fixed to the resin backbone or matrix, as it is called Mobile positively charged counterions (positive charges in Figure 9.1) are associated by electrostatic attraction with each negative ion exchange site The resin exchange capacity is measured as the number of fixed charge sites per unit volume or weight of resin Functionality is the term used to identify the chemical composition of the fixed-charge site, for example sulfonate (SO3−) or carboxylate (COO−) Porosity (e.g., microporous, gel, or macroporous) is the resin characterization referring to the degree of openness of the polymer structure An actual resin bead is much tighter than implied by the schematic, which is shown as fairly open for purposes of illustration only The water (a) (b) FIGURE 9.1 (a) Organic cation-exchanger bead comprising polystyrene polymer cross-linked with divinylbenzene with fixed coions (minus charges) of negative charge balanced by mobile positively charged counterions (plus charges) (b) Strongacid cation exchanger (left) in the hydrogen form and strong-base anion exchanger (right) in the chloride form ION EXCHANGE AND INORGANIC ADSORPTION 9.5 (40 to 60 percent by weight) present in a typical resin bead is not shown This resinbound water is an extremely important characteristic of ion exchangers because it strongly influences both the exchange kinetics and thermodynamics Strong- and Weak-Acid Cation Exchangers Strong acid cation (SAC) exchangers operate over a very wide pH range because the sulfonate group, being strongly acidic, is ionized throughout the pH range (1 to 14) Three typical SAC exchange reactions follow In Equation 9.1, the neutral salt CaCl2, representing noncarbonate hardness, is said to be split by the resin, and hydrogen ions are exchanged for calcium, even though the equilibrium liquid phase is acidic because of HCl production Equations 9.2 and 9.3 are the standard ion exchange softening reactions in which sodium ions are exchanged for the hardness ions Ca2+, Mg2+, Fe2+, Ba2+, Sr2+, and/or Mn2+, either as noncarbonate hardness (Equation 9.2) or carbonate hardness (Equation 9.3) In all these reactions, R denotes the resin matrix, and the overbar indicates the solid (resin) phase 2R S O −3H + + CaCl2 ⇔ (R S O 3−)2C a2+ + 2HCl (9.1) 2 RS O 3−N a+ + CaCl2 ⇔ (R S O 3−)2C a2+ + 2NaCl (9.2) 2R S O N a + Ca(HCO3)2 ⇔ (R S O )C a + 2NaHCO3 − + − 2+ (9.3) Regeneration of the spent resin is accomplished using an excess of concentrated (0.5 to 3.0 M) HCl or NaCl, and amounts to the reversal of Equations 9.1 through 9.3 Weak acid cation (WAC) resins can exchange ions only in the neutral to alkaline pH range because the functional group, typically carboxylate (pKa = 4.8), is not ionized at low pH.Thus,WAC resins can be used for carbonate hardness removal (Equation 9.4) but fail to remove noncarbonate hardness, as is evident in Equation 9.5 2 RC O O H  + Ca(HCO3)2 ⇒ (R C O O −)2C a2+ + H2CO3 2R C O O H  + CaCl2 ⇐ (R C O O −)2C a2+ + 2HCl (9.4) (9.5) If Equation 9.5 were to continue to the right, the HCl produced would be so completely ionized that it would protonate (i.e., add a hydrogen ion to the resin’s weakly acidic carboxylate functional group, and prevent exchange of H+ ions for Ca2+ ions) Another way of expressing the fact that Equation 9.5 does not proceed to the right is to say that WAC resins will not split neutral salts (i.e., they cannot remove noncarbonate hardness) This is not the case in Equation 9.4, in which the basic salt, Ca(HCO3)2, is split because a very weak acid, H2CO3 (pK1 = 6.3), is produced In summary, SAC resins split basic and neutral salts (remove carbonate and noncarbonate hardness), whereas WAC resins split only basic salts (remove only carbonate hardness) Nevertheless, WAC resins have some distinct advantages for softening, namely TDS reduction, no increase in sodium, and very efficient regeneration resulting from the carboxylate’s high affinity for the regenerant H+ ion Strong- and Weak-Base Anion Exchangers The use of strong-base anion (SBA) exchange resins for nitrate removal is a fairly recent application of ion exchange for drinking water treatment (Clifford and W J Weber, 1978; Guter, 1981), although they have been used in water demineralization 9.6 CHAPTER NINE for decades In anion exchange reactions with SBA resins, the quaternary amine functional group (N+[CH3]3) is so strongly basic that it is ionized, and therefore useful as an ion exchanger over the pH range of to 13 This is shown in Equations 9.6 and 9.7, in which nitrate is removed from water by using hydroxide or chlorideform SBA resins (Note that R4N+ is another way to write the quaternary exchange site, N+(CH3)3) R R4N 4N +O H − + NaNO3 ⇔  +N O 3− + NaOH (9.6) R R4N 4N +C l− + NaNO3 ⇔  +N O 3− + NaCl (9.7) In Equation 9.6 the caustic (NaOH) produced is completely ionized, but the quaternary ammonium functional group has such a small affinity for OH− ions that the reaction proceeds to the right Equation 9.7 is a simple ion exchange reaction without a pH change Fortunately, all SBA resins have a much higher affinity for nitrate than chloride (Clifford and W J Weber, 1978), and Equation 9.7 proceeds to the right at near-neutral pH values Weak-base anion (WBA) exchange resins are useful only in the acidic pH region where the primary, secondary, or tertiary amine functional groups (Lewis bases) are protonated and thus can act as positively charged exchange sites for anions In Equation 9.8 chloride is, in effect, being adsorbed by the WBA resin as hydrochloric acid, and the TDS level of the solution is being reduced In this case, a positively charged Lewis acid-base adduct (R3NH+) is formed, which can act as an anion exchange site As long as the solution in contact with the resin remains acidic (just how acidic depends on basicity of the R3N:, sometimes pH ≤ is adequate), ion exchange can take place as is indicated in Equation 9.9—the exchange of chloride for nitrate by a WBA resin in acidic solution If the solution is neutral or basic, no adsorption or exchange can take place, as indicated by Equation 9.10 In all these reactions, R represents either the resin matrix or a functional group such as CH3 or C2H5, and overbars represent the resin phase R 3N : + HCl ⇔ R 3N H +C l− (9.8) R3N R3N  H +C l− + HNO3 ⇔  H +N O 3− + HCl (9.9) R 3N : + NaNO3 ⇒ no reaction (9.10) Although no common uses of WBA resins are known for drinking water treatment, useful ones are possible (Clifford and W J Weber, 1978) Furthermore, when activated alumina is used for fluoride and arsenic removal, it acts as if it were a weakbase anion exchanger, and the same general rules regarding pH behavior can be applied Another advantage of weak-base resins in water supply applications is the ease with which they can be regenerated with bases Even weak bases such as lime (Ca[OH]2) can be used, and regardless of the base used, only a small stoichiometric excess (less than 20 percent) is normally required for complete regeneration Activated Alumina Adsorption Packed beds of activated alumina can be used to remove fluoride, arsenic, selenium, silica, and humic materials from water Coagulated Fe(II) and Fe(III) oxides (McNeill and Edwards, 1995; Scott, Green et al., 1995) and iron oxides coated onto sands (Benjamin, Sletten et al., 1996) can also be employed to remove these anions, ION EXCHANGE AND INORGANIC ADSORPTION 9.7 but these processes are not covered in this chapter The mechanism, which is one of exchange of contaminant anions for surface hydroxides on the alumina, is generally called adsorption, although ligand exchange is a more appropriate term for the highly specific surface reactions involved (Stumm, 1992) The typical activated aluminas used in water treatment are 28- × 48-mesh (0.3- to 0.6-mm-diameter) mixtures of amorphous and gamma aluminum oxide (γ-Al2O3) prepared by low-temperature (300 to 600°C) dehydration of precipitated Al(OH)3 These highly porous materials have surface areas of 50 to 300 m2/g Using the model of an hydroxylated alumina surface subject to protonation and deprotonation, the following ligand exchange reaction (Equation 9.11) can be written for fluoride adsorption in acid solution (alumina exhaustion) in which Al represents the alumina surface and an overbar denotes the solid phase  Al − O H + H+ + F− ⇒   A l− F − + HOH (9.11) The equation for fluoride desorption by hydroxide (alumina regeneration) is presented in Equation 9.12  A l− F  + OH− ⇒  A l− O H  + F− (9.12) Another common application for alumina is arsenic removal, and reactions similar to Equations 9.11 and 9.12 apply for exhaustion and regeneration when H2AsO4− is substituted for F− Activated alumina processes are sensitive to pH, and anions are best adsorbed below pH 8.2, a typical zero point of charge (ZPC), below which the alumina surface has a net positive charge, and excess protons are available to fuel Equation 9.11 Above the ZPC, alumina is predominantly a cation exchanger, but its use for cation exchange is relatively rare in water treatment An exception is encountered in the removal of radium by plain and treated activated alumina (Clifford, Vijjeswarapu et al., 1988; Garg and Clifford, 1992) Ligand exchange as indicated in Equations 9.11 and 9.12 occurs chemically at the internal and external surfaces of activated alumina A more useful model for process design, however, is one that assumes that the adsorption of fluoride or arsenic onto alumina at the optimum pH of 5.5 to 6.0 is analogous to weak-base anion exchange For example, the uptake of F− or H2AsO4−, requires the protonation of the alumina surface, and that is accomplished by preacidification with HCl or H2SO4, and reducing the feed water pH into the 5.5 to 6.0 region The positive charge caused by excess surface protons may then be viewed as being balanced by exchanging anions (i.e., ligands such as hydroxide, fluoride, and arsenate) To reverse the adsorption process and remove the adsorbed fluoride or arsenate, an excess of strong base (e.g., NaOH) must be applied.The following series of reactions (9.13–9.17) is presented as a model of the adsorption/regeneration cycle that is useful for design purposes The first step in the cycle is acidification, in which neutral (water-washed) alumina (Alumina⋅HOH) is treated with acid (e.g., HCl), and protonated (acidic) alumina is formed as follows: A lu m in a⋅H O H  + HCl ⇒ A lu m in a⋅H C l + HOH (9.13) When HCl-acidified alumina is contacted with fluoride ions, they strongly displace the chloride ions providing that the alumina surface remains acidic (pH 5.5 to 6.0) This displacement of chloride by fluoride, analogous to ion exchange, is shown as A lu m in a⋅H C l + HF ⇒ A lu m in a⋅H F  + HCl (9.14) 9.8 CHAPTER NINE To regenerate the fluoride-contaminated adsorbent, a dilute solution of 0.25 to 0.5 N NaOH alkali is used Because alumina is both a cation and an anion exchanger, Na+ is exchanged for H+, which immediately combines with OH− to form HOH in the alkaline regenerant solution The regeneration reaction of fluoride-spent alumina is A lu m in a⋅H F  + 2NaOH ⇒ A lu m in a⋅N aO H  + NaF + HOH (9.15) Recent experiments have suggested that Equation 9.15 can be carried out using fresh or recycled NaOH from a previous regeneration This suggestion is based on the field studies of Clifford and Ghurye (1998) in which arsenic-spent alumina was regenerated with equally good results using fresh or once-used 1.0 M NaOH The spent regenerant, fortified with NaOH to maintain its hydroxide concentration at 1.0 M, probably could have been used many times, but the optimum number of spent-regenerant reuse cycles was not determined in the field study To restore the fluoride removal capacity, the basic alumina is acidified by contacting it with an excess of dilute acid, typically 0.5 N HCl or H2SO4: A lu m in a⋅N aO H  + 2HCl ⇒ A lu m in a⋅H C l + NaCl + HOH (9.16) The acidic alumina, alumina⋅HCl, is now ready for another fluoride (or arsenate or selenite) ligand-exchange cycle as summarized by Equation 9.14 Alternatively, the feed water may be acidified prior to contact with the basic alumina, thereby combining acidification and adsorption into one step as summarized by Equation 9.17: A lu m in a⋅N aO H  + NaF + 2HCl ⇒ A lu m in a⋅H F  + 2NaCl + HOH (9.17) The modeling of the alumina adsorption-regeneration cycle as being analogous to weak-base anion exchange fails in regard to regeneration efficiency, which is excellent for weak-base resins but quite poor on alumina This is caused by the need for excess acid and base to partially overcome the poor kinetics of the semicrystalline alumina, which exhibits very low solid-phase diffusion coefficients compared with resins that are well-hydrated, flexible gels offering little resistance to the movement of hydrated ions A further reason for poor regeneration efficiency on alumina is that alumina is amphoteric and reacts with (consumes) excess acid and base to produce soluble forms (Al(H2O)6 3+, Al(H2O)2(OH)−4) of aluminum Resins are totally inert in this regard (i.e., they are not dissolved by regenerants) Special-Purpose Resins Resins are practically without limit in their variety because polymer matrices, functional groups, and capacity and porosity are controllable during manufacture Thus, numerous special-purpose resins have been made for water-treatment applications For example, bacterial growth can be a major problem with anion resins in some water supply applications because the positively charged resins tend to “adsorb” the negatively-charged bacteria that metabolize the adsorbed organic material—negatively charged humate and fulvate anions.To correct this problem special resins have been invented, which contain bacteriostatic long-chain quaternary amine functional groups (“quats”) on the resin surface These immobilized quats kill bacteria on contact with the resin surface (Janauer, Gerba et al., 1981) The strong attraction of polyvalent humate and fulvate anions (natural organic matter, [NOM]) for anion resins has been used as the basis for removal of these total organic carbon (TOC) compounds from water by using special highly porous resins Both weak- and strong-base macroporous anion exchangers have been manufactured ION EXCHANGE AND INORGANIC ADSORPTION 9.9 to remove these large anions from water The extremely porous resins originally thought to be necessary for adsorption of the large organic anions tended to be structurally weak and break down easily More recently, however, it has been shown that both gel and standard macroporous resins, which are highly crosslinked and physically very strong, can be used to remove NOM (Fu and Symons, 1990) Regeneration of resins used to remove NOM is often a problem because of the strong attraction of the aromatic portion of the anions for the aromatic resin matrix This problem has at least been partially solved using acrylic-matrix SBA resins More details on the use of ion exchange resins to remove NOM appears later in this chapter Resins with chelating functional groups such as imino-diacetate (Calmon, 1979), amino-phosphonate, and ethyleneamine (Matejka and Zirkova, 1997) have been manufactured that have extremely high affinities for hardness ions and troublesome metals such as Cu2+, Zn2+, Cr3+, Pb2+, and Ni2+ These resins are used in special applications such as trace-metal removal and metals-recovery operations (Brooks, Brooks et al., 1991) The simplified structures of these resins are shown in Figure 9.2 Table 9.2 summarizes the features of some of the special ion exchangers available commercially from a variety of sources (Purolite, 1995) FIGURE 9.2 Structure of highly selective cation exchangers for metals removal ION EXCHANGE EQUILIBRIUM Selectivity Coefficients and Separation Factors Ion exchange resins not prefer all ions equally This variability in preference is often expressed semiquantitatively as a position in a selectivity sequence or, quantitatively, as a separation factor, αij, or a selectivity coefficient, Kij, for binary exchange The selectivity, in turn, determines the run length to breakthrough for the contaminant ion; the higher the selectivity, the longer the run length Consider, for example, Equation 9.18, the simple exchange of Cl− for NO3− on an anion exchanger 9.10 CHAPTER NINE TABLE 9.2 Special Ion Exchangers—Commercially Available Type of resin Functional group Typical application Chelating Thio-uronium Selective removal of metals, especially mercury Chelating Imino-diacetic Selective removal of polyvalent ions, especially transition metals Chelating Amino-phosphonic Decalcification of brine and removal of metals from wastewaters Silver impregnated, SAC Sulfonic Softening resin with bacteriostatic properties NSS, Nitrate-over-sulfate selective (sulfate rejecting), hydrophobic, SBA Triethyl and tripropyl quaternary amines Nitrate removal in high sulfate waters Iodine releasing Quaternary amine SBA in triiodide form, R4N+I3− Disinfection by iodine release into product water Source: Purolite, 1995 whose equilibrium constant is expressed numerically in Equation 9.19 and graphically in Figure 9.3a: Cl− + NO3− ⇒ N  O 3− + Cl− − (9.18) − {N O  } {Cl } K =  {C l−} {NO3−} (9.19) In Equations 9.18 to 9.20, overbars denote the resin phase, and the matrix designation R has been removed for simplicity; K is the thermodynamic equilibrium constant, and braces denote ionic activity Concentrations are used in practice because they are measured more easily than activities In this case, Equation 9.20 based on concentration, the selectivity coefficient KN/Cl describes the exchange Note that KN/Cl includes activity coefficient terms that are functions of ionic strength and, thus, is not a true constant (i.e., it varies somewhat with different ionic strengths) [N O 3−] [Cl−] qN CCl = KN/Cl =  [C l−] [NO3−] qCl CN (9.20) where [ ] = concentration, mol/L qN = resin phase equivalent concentration (normality) of nitrate, eq/L CN = aqueous phase equivalent concentration (normality), eq/L The binary separation factor αN/Cl, used throughout the literature on separation practice, is a most useful description of the exchange equilibria because of its simplicity and intuitive nature: distribution of ion i between phases yi / xi αij =  =  distribution of ion j between phases yj / xj (yN/xN) yN xCl (qN/q) (CCl/C) αN/Cl =  =  =  (yCl/xCl) xN yCl (CN/C) (qCl/q) (9.21) (9.22) ... activated aluminas used in water treatment are 2 8- × 48-mesh (0. 3- to 0.6-mm-diameter) mixtures of amorphous and gamma aluminum oxide (γ-Al2O3) prepared by low-temperature (300 to 600°C) dehydration of... resins is a rapid process at near-ambient temperature For example, the halftime to equilibrium for adsorption of arsenate onto granular 2 8- × 48-mesh (0.2 9- to 59-mm-dia) activated alumina was found... 9.11 Two-column roughing-polishing system operated in a merry-go-round fashion overruns During normal operation of After exhaustion of column 1, it will be taken out multiple-parallel-column