Comparison of the Batch and Breakthrough Properties of Stable and

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Ind. Eng. Chem. Res. 2008, 47, 6742–6752

Comparison of the Batch and Breakthrough Properties of Stable and Plain Alginate Microcapsules with a Chelating Resin and an Ion Exchanger in Ag+ Adsorption Mohammad Outokesh* Department of Mechanical Engineering, Sharif UniVersity of Technology, Azadi AVe., P.O. Box 11365-8639, Tehran, Iran

Yuichi Niibori and Hitoshi Mimura Department of Quantum Science & Energy Engineering, Graduate School of Engineering, Tohoku UniVersity, Sendai, 980-8579, Japan

Seyed Javad Ahmadi Jaber-ebne-Hayyan Laboratory, Nuclear Science and Technology Research Institute, North Karegar AVe., P.O. Box 14399-5113, Tehran, Iran

A comparative study on the uptake properties of alginate microcapsules containing Cyanex 302 extractant, Lewatite TP 214 chelating resin, and Amberlite 200CT strongly acidic ion exchanger has been conducted using Ag+ as the target ion. The study resulted in an analytical formula, as well as three phenomenological breakthrough formulas that are also applicable in other fixed-bed operations. It also demonstrated a close similarity of the uptake properties of alginate microcapsules (MCs) and the chelating resins. Advantages of MCs over chelating resins include ease of preparation, ability to immobilize different extractants, and, most remarkably, an enormous distribution factor (Kd ≈ 106 cm3/g). As for the stabilization of MCs, two different methods of coating and matrix modification were applied; the latter proved to be superior, because of enhancement of both the uptake capacity and the column performance. Another achievement of the current study was the discovery of a new method for estimating the thickness of the coating layer of MCs. 1. Introduction Solvent extraction (SX) is currently one of the most versatile separation processes with numerous applications in oil and petrochemical, drug and pharmaceutical, and, especially, hydrometallurgical industries.1,2 The process is well-suited for large-scale continuous operations, because of its inherent high capacity and fast kinetics;3,4 however, it also suffers from a few drawbacks, namely, the multistage operations2,5 and the environmental concern resulting from the losses of hazardous extractant.3 Ion exchange (IX), which is another major hydrometallurgical process, has an advantage over SX, in that it can be performed in a single-stage operation within a simple packed column.2,6 In addition, it is more effective in complete separation of the trace ions. Unfortunately, conventional ion exchanger resins are not as selective as the extractants. The merits and shortfalls of SX and IX were very soon recognized and became a great stimulus to the separation scientist to bridge the gap between these two mature processes by developing solvent impregnated resins (SIRs)7 and microcapsules that contain extractant (MCEs).8 Following the introduction of SIRs in the 1970s,9 much research has revealed their different uptake properties.9 Comparatively, much lesser efforts have been exerted on the characterization of MCE, mainly because of their shorter history (since 1990).10–12 There are two major advantages of MCEs over SIRs: (i) greater mechanical stability13 and (ii) higher loading of the * To whom correspondence should be addressed. Tel./Fax: +81 22 795 7915. E-mail address: [email protected].

extractant (e.g., 70% of weight of MCs) (or, in other words, higher uptake capacity).7,14 Other than SIRs and MCEs, two other types of highly selective cation adsorbents are the Levextrel resins9 and the chelating resins.15 The former is prepared by tailoring the extractant molecules to the vinyl monomers, followed by the polymerization. Although it has been claimed that a wide variety of extractants can be incorporated into the Levextrel structure, so far, only a few products that contain TBP, TOPO, and DEHPA have been presented to the market.9 Chelating resins have been known for some 60 years, since the invention of dipicrylamine resins by Skogseid.15 Most of the initial chelating resins were weak Lewis acids16 and, therefore, were ineffective in acidic solutions. Later, new resins that contained weakly basic functional groups (e.g., thiourea) were synthesized that were able to preserve their selectivity in the extreme acidic conditions.17–19 Unfortunately, this modification created a new drawback, which was the formidable elution of the saturated resins.19 Another general shortfall of the chelating resins is their slow kinetics,6 which necessitates a large resin inventory and expensive capital and fixed costs. The aforementioned introduction highlights the fact that the study of MCEs is worthy of consideration, especially with respect to the selective separation of minute amounts of the precious or radioactive metal ions. So far, the uptake properties of MCE have been the subject of many studies;3,5,19–23 yet, a comparative and fundamental approach;especially on the characterization of multinuclear MCEs (e.g., alginate MCE);has been rarely undertaken.5,13,24,25 In a recent study, the equilibrium and kinetics of Ag+ uptake by alginate MCEs were investi-

10.1021/ie071200n CCC: $40.75  2008 American Chemical Society Published on Web 08/05/2008

Ind. Eng. Chem. Res., Vol. 47, No. 17, 2008 6743 13

gated. The current study is an effort to complete the previous attempt by comparison of the uptake properties of alginate MCE with those of chelating resins and strongly acidic ion exchangers. In addition, it involves different methods of stabilization of MCEs and the characteristics of the stabilized products. Ultimate goal of these studies is to develop a commercially acceptable MCE with a simple preparation procedure, high endurance, the immobilizing factor of different extractants, and the uptake properties comparable with the chelating resins. 2. Experimental Section 2.1. Materials. Lewatite TP-214 (abbreviated hereafter as Lewatite), which is a chelating resin with thiourea functional groups, was kindly presented by Energy Lanxess AG, and a strongly acidic ion exchanger “Amberlite 200 CT” (abbreviated as 200 CT) with sulfonic functional groups in sodium form was received from Rohm and Haas Co. For the synthesis of microcapsules (MCs), sodium alginate (NaALG, type 300-400 cP or 500-600 cP) was purchased from Wako Pure Chemical Industries (Osaka, Japan), and an extractant of bis(2,4,4trimethylpenthyl) monothiophosphinic acid (Cyanex 302) with 85% assay was obtained from Cytech Canada, Inc. Other chemicals were reagent-grade and supplied by Wako Pure Chemical Industries. 2.2. Preparation of Microcapsules (MCs). For simplicity, the abbreviations “MC” and “MCs” are used hereafter, as abbreviations for “microcapsule” and “microcapsules”, respectively. In addition, HA is used as an alternative for the trade name of Cyanex 302. The uncoated alginate MCs were prepared according to the methods described elsewhere.13 The NaALG solution (100 cm3, 2.4%, type 300-400 cP) was kneaded with 1.2 g of Cyanex 302 (HA) and then was fully dispersed by an unbubbling kneader. The sol was added dropwise into a 0.1 mol dm-3 Ca(NO3)2 solution through a 0.4-mm (outer diameter, OD) medical needle to cause gellification and form MCs. For the hardening, MCs were gently stirred in the aforementioned solution for 6 h. They were then filtrated, washed with pure water, and left first at ambient for apparent drying, then at 40 °C for at least 14 h. The dried MCs were abbreviated as MC-1. MCs that were prepared with 2.4 or 3.6 g of HA instead of 1.2 g were given the name MC-2 or MC-3, respectively. Calcium alginate (CaALG) granules were also prepared by injecting NaALG sol in a Ca(NO3)2 solution. Some MC-1 and CaALG MCs were coated with polyethyleneimine (PEI). The MCs for this purpose, before drying, were transferred to a 5 g dm-3 PEI solution (Mw ) 70000), whose pH was adjusted at 5.6 with the acetate buffer. The MCs in this solution were gently stirred for 4 h and then washed with plenty of water. The next step was stirring in 1 g/L NaALG solution (type 500-600 cP) for 10 min, followed by a 3-fold wash. After the last wash, MCs were dried and were identified as PEI-1. The CaALG granules (without HA content) coated with PEI were abbreviated as PEI-Ca. MCs were also stabilized by matrix modification by cellulose acetate phthalate (CAP) antacid polymer. Matrix modification was simply affected by adding the appropriate amount of 6% CAP solution to the NaALG solution just before adding of HA to the NaALG solution. The obtained NaALG-CAP solution contained 2% and 0.5% of NaALG and CAP, respectively. The 6% CAP solution itself was prepared by dissolving 6 g CAP in 94 cm3 of a 0.7% ammonia solution, followed by filtration through an 8-µm membrane filter. The gelling solution for these

MCs contained 0.3 mol dm-3 Ca(NO3)2 and 0.03 mol dm-3 HNO3. Dried matrix-modified MCs were abbreviated as CAP20. 2.3. Size Classification of the Particles. To set equal conditions for the kinetics and column studies, commercial resin (including Lewatite and 200 CT) were first dried by reamining in a desiccator that contained a saturated NH4Cl solution, and then a fraction of them (those 500-590 µm in size) was separated and used in the experiments. All MCs except highly agglomerated CAP-20 were also classified, and the same size fraction was used in the experiments. 2.4. Estimation of Thickness of Coating. In a previous manuscript,24 the thickness of the coating of PEI-1 MCs was estimated by the electron probe microanalysis (EPMA) of nitrogen in the vicinity of the surface of MCs, as well as an incorrect uptake method (based on comparison of the Ag+ uptake percent of CaALG, to that of PEI-Ca). In the current study, the previous uptake method was corrected by a new formula. More details about this method are given in section 3.3. 2.5. Leakage of Extractant. With regard to the elaboration of the effect of coating and matrix modification on the stability, the following reliable method was used.13,21,24 Approximately 0.75 g of plain or stable alginate MCs was shaken in 75 cm3 of pure water or 0.5 mol dm-3 HNO3 for ∼24 h. The 0.5 mol dm-3 HNO3 was selected because it causes maximum leakage.13,21 After shaking, MCs were separated, washed with water, and dried. Approximately 0.05 g of the dried samples was dissolved in 60% nitric acid at 160 °C for 8 h, according to the “modified Carius” method,13,21 and then was analyzed for phosphorus using inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The analysis result revealed the HA content of the shaken MCs. The extent of leakage was then obtained by comparing theHA content of the original and the shaken MCs. 2.6. Equilibrium Uptake Experiments. Because the equilibrium and stirred kinetic properties of MC-1, MC-2, and CaALG MCs have been already reported,13 for the purpose of comparison, similar batch experiments were conducted on the Lewatite and 200 CT, as well as the new coated and matrix modified MCs. An aqueous solution (7 cm3) of 10-5000 ppm of Ag+ cations was contacted with 70 mg of the foregoing materials at 25 °C in the polypropylene test tube and shaken at 105 strokes/min for ∼24 h. The supernatant was then analyzed for Ag+ cations: atomic absorption spectrometry (AAS) (Jarrel Ash, model AA-890) was used for measurements below 1000 ppm, or a combination of AAS and the Volhard titration method was used for measurements above this value. The following uptake properties were examined: (i) the effect of Ag+ concentration and the adsorption isotherm; (ii) the effect of HNO3 concentration, from 10-4 mol dm-3 to 3 mol dm-3; and (iii) the selectivity in the presence of Na+ ions, up to 3 mol dm-3 Na+. The nitric acid concentration in all samples (except those used to examine the effect of HNO3) was 10-3 mol dm-3. 2.7. Stirred Kinetics Experiments. The apparatus for kinetic study was a cylindrical flask with a inner diameter (ID) of 85 mm with four triangular baffles. A two-paddle Teflon turbine rotating at 400 rpm agitated the solution. The volume of solution was 500 cm3, and the particle mass was 1.0975 g. 2.8. Column Experiments. A 5-mm-ID glass column packed with 0.4-2.0 g of classified Lewatite, 200 CT, and MCs was used for the dynamic studies. The bed height was 5.75-28 cm, the liquid velocity was 1.3-10.0 cm/min, and the Ag+ concentration mainly 300 and 600 ppm (and occasionally 2400

6744 Ind. Eng. Chem. Res., Vol. 47, No. 17, 2008

Figure 1. Typical SEM images of microcapsules (MCs): (a) morphology and (b) cross section.

adsorption of Ag+ ions. As a result, only core of the PEI-Ca granule (with no HA content) has uptake capacity and its uptake percentage is less than that of the calcium alginate CaALG (see Figure 3). Another useful fact for obtaining the coating thickness is that the adsorption isotherm for the uptake of Ag+ by calcium alginate at [Ag+] < 100 ppm is a straight line that passes through the origin (see Figure 3c). Considering both of the aforementioned facts, and assuming that the density of the coating layer is equal to that of the calcium alginate matrix, results in the following equation (see Appendix ) as the corrected form of a previously presented formula:24

( 100R- R ) ( 100R- R )

CaALG

Table 1. Average Size and Extractant Content of Different Dried MCs type

size (µm)

extractant content (%)

CaALG PEI- Ca MC-1 MC-2 MC-3 CAP-20 PEI-1

618 618 605 692 863 555 660

0 0 28.1 43.7 69.4 31 27.9

ppm). Nitric acid concentration was 10-3-10-1 mol dm-3 and the concentrations of calcium ions ([Ca2+]) and barium ions ([Ba2+]) varied from 0 mol dm-3 to 0.5 mol dm-3. Every 6.5-9 cm3 of solution was fractionated and analyzed via AAS. 3. Results and Discussion 3.1. Definition of Parameters. The equilibrium uptake percentage (R) of Ag+ ions, the distribution coefficient (Kd), the equilibrium adsorption (Q), and the fractional attainment to the equilibrium (Y) are defined as follows:

Kd )

C0 - Cf V × Cf m

C0 - Cf Q) ×V m C0 - Ct Y) C0 - Cf

(r - t)3 r3

(5)

PEI-Ca

Figure 2. Surface morphology of different MCs.

C0 - Cf × 100 R (%) ) C0

)

(1)

where r denotes the radius of the MCs, t the thickness of the coating layer, and the subscripted indices indicate the type of MC. The result of this method is consistent with the values obtained using the EPMA method that has been reported elsewhere24 (see Table 2). 3.4. Stability and Leakage of Extractant. The direct measurement of the HA content before and after shaking, as was described in the Experimental Section, presents an appropriate method for the test of stability. Table 3 indicates that both the coated and matrix-modified MCs have good leakage stability, with a slightly better result for PEI-1. Stability tests showed that the imines groups of PEI in the coating layer of PEI-1 were not corroded in acids. 3.5. Comparison of the Uptake Equilibrium. 3.5.1. Uptake Mechanism. The adsorption of Ag+ by Lewatite18 occurs via an irreversible reaction of the thiourea ligand and the Ag+ receptor. On the other hand, the uptake of Ag+ by 200 CT is reversible and 200 CT retains its ionic identity during the reaction. Ag++ R-S(NH2)2 f R-SAg(NH2)2

(6)

Ag++ R-SO3-Na+ f R-SO3-Ag++ Na+

(7)

Figure 4a shows a breakpoint at the curve of equilibrium concentration (Ceq) of Ag+ adsorbed by Lewatite:

{

(2) (3) (4)

where C0, Ct, and Cf represent the concentration of Ag+ initially, at time t, and at equilibrium, respectively. 3.2. Morphology. Figure 1 shows a scanning electron microscopy (SEM) image of the MC-1 MC, along with its crosssectional morphology. The coating effect on PEI-1 and the large pores of CAP-20 are quite obvious in Figure 2, which show the surface morphology. The average size of MCs from the sieve analyses, along with their HA content, have been given in Table 1. 3.3. Estimation of Thickness of Coating. Coating of alginate MCs by PEI proceeds via the diffusion of positively charged PEI polymeric chains into the alginate matrix, followed by their cross-linking with the negatively charged alginate. Such a cross-linking makes the coating layer neutral, in regard to the

Ceq ) 0 C0 < Cb Ceq ) C0 - Cb C0 g Cb

(8)

as a result of the completion of reaction eq 6. Theoretically, above the breakpoint, no further adsorption occurs, so eq 8 holds. The actual behavior of Lewatite differs from that described by eq 8, probably because of the physical sorption of Ag+. In the case of 200 CT, no breakpoint exists and Ceq monotonically rises. In regard to MC-1,13,23 both of HA and the matrix adsorb the ions: Ag+ + HATAgA + H+ +

(9)

2Ag + Ca(ALG)2T2Ag-ALG + Ca

2

(10)

It has been proven that, for alginate MCs, at concentrations under the breakpoint (Cb), the extraction mechanism of eq 9 controls the uptake, whereas above the Cb, ion exchange by matrix, according to eq 10, predominates.13 Figure 4b indicates the negligible effect of coating on the breakpoint and, generally, the equilibrium properties of MCs. This is due to the adsorption neutrality of the coating layer, as

Ind. Eng. Chem. Res., Vol. 47, No. 17, 2008 6745

Figure 3. (a) An illustration of empty MCs of PEI-Ca and hypothetical granule of CaALG*, (b) comparison of maximum uptake percentage of (×) PEI-Ca with that of (]) calcium alginate, and (c) adsorption isotherm for calcium alginate at a low concentration of Ag+ ions. Table 2. Estimation of the Coating Thickness of the PEI-1 and Its Corresponding Empty MCs (PEI- Ca) by Different Methods method of estimation

thickness (µm)

uptake (current) EPMA

29.8 20

Table 3. Estimation of the Extractant Content of MCs after 24 h of Shaking, Using the Destructive Modified Carius Method after 24 h of shaking

amount of extractant remaining (%)

MC-1 (water) MC-1 (0.5 M HNO3) MC-2 (water) PEI-1(water) PEI-1 (0.5 M HNO3) CAP-20 (water) CAP-20 (0.5 M HNO3)

93.4 91.9 91.5 99.8 98.9 99.5 97.6

Figure 5. (a) Langmuir isotherms for the uptake of Ag+ by (]) MC-1, (0) Lewatite, and (2) 200CT. (b) Distribution coefficient for same adsorbents as (]) MC-1, (0) Lewatite, and (2) 200CT. (In all cases, [H+] ) 0.001 mol dm-3.)

Figure 4. (a) Equilibrium concentration versus initial concentration of Ag+ ions adsorbed by (]) MC-1, (0) Lewatite, and (2) 200CT. (b) Same chart for comparison of (4) CaALG, (]) MC-1, (×) PE-1, and (b) Cap-20. (In all cases, [H+] ) 0.001 mol dm-3.)

Figure 6. Effect of concentrations of nitric acid on Ag+ uptake by (]) MC-1, (0) Lewatite, and (2) 200CT.

well as the approximate equality of the HA content of MC-1 and PEI-1 MCs (see Table 1). Expectedly, the breakpoint of MC-2 with the larger HA content was greater than that of MC1. The actual breakpoint of CAP-20 is greater than the value calculated from its HA content (see Table 1), more likely because the matrix modifier (CAP) has a certain uptake capacity. CaALG as a weakly acidic ion-exchanger shows no breakpoint. Figure 5a shows the Langmuir plot for the adsorption of Ag+ via a different adsorbent, in the form of

3.5.2. Selectivity in the Presence of HNO3 and Na+ Ions. The superiority of MCs and Lewatite appears when the selectivity is taken into consideration. In the 1 mol dm-3 HNO3, the uptake percentages (R) of Lewatite, MC-1, and 200 CT were 99.5%, 94.5%, and 69%, respectively (see Figure 6). The same figure was observed with a Na+ coexisting ion. When the Na+ concentration exceeded 0.1 mol dm-3, MCs were swollen. However, the addition of acid to a Na+ solution remarkably diminished this effect, probably because of the formation of the less-swelling alginic acid (HALG):

( )

Ceq 1 1 ) + C Qeq KQmax Qmax eq The plot reveals the following order of the uptake capacity (Qmax): 200 CT > Lewatite > MC-1. Despite having the lowest capacity, MC-1 shows an enormous distribution coefficient (Kd) (see Figure 5b).

2Haq+ + Ca(ALG)2 T 2H-ALG + Caaq2+

(11)

3.5.3. Kinetics in the Batch Systems. Figure 7a shows the following order of the kinetics: 200 CT > Lewatite > MC-1. The fast kinetics of 200 CT results from the fact that ions do not require covalent binding to its active sites. The same

6746 Ind. Eng. Chem. Res., Vol. 47, No. 17, 2008

Y)

3D C0Rδ

∫ C dt t

(liquid film resistance)

0

1 - 3(1 - Y)2⁄3 + 2(1 - Y) ) 6De t C dt (intraparticle resistance) C0R2 0

∫

1 - (1 - Y)1⁄3 )

k′′ FR

∫ C dt t

0

(12)

(13)

(reaction-controlled kinetics) (14)

In a previous study, it was proven that the kinetics of Ag+ uptake by alginate MCs at low concentrations of coexisting ions (e.g., [H+] ≈ 10-3 mol dm-3) is controlled by chemical reaction of the HA microdroplet with Ag+ ions, and, hence, they obey eq 14, whereas a medium or high concentration of coexisting ions (e.g., [H+] > 10-2 mol dm-3) favors eq 13. 3.6. Column Behavior. In the study of column behavior, we should use the dimensionless concentration (X): 13

+

Figure 7. (a) Kinetics of Ag uptake in stirred systems by (]) MC-1, (0) Lewatite, and (2) 200CT, with [Ag+ ] ) 300 ppm and [H+] ) 0.001 mol dm-3. (b) Same chart for (]) MC-1, (×) PE-1, and (b) Cap-20, but values were obtained under shaking conditions.

X)

Figure 8. Different SCM models for Ag+ uptake by (a) Lewatite and (b) 200 CT resins. In both cases, [H+] ) 0.001 mol dm-3.

argument explains the slow kinetic of the Lewatite as the result of the sluggish chelating of the Ag+ ion to its thiourea groups. With regard to comparing the kinetics of stabilized and plain MCs, instead of a stirred reactor, a shaking system of the equilibrium study with the same procedure was used, with a difference that, for any given time t, one sample was shaken. The slightly faster kinetics of CAP-20, in comparison to MC-1 (see Figure 7b) is due to the involvement of at least one of the following mechanisms: (i) the higher uptake capacity of CAP20, (ii) its greater porosity, (iii) the higher diffusivity of Ag+ ions in a CAP-Alginate composite matrix, and, finally, (iv) the slightly smaller size of CAP-20 (see Table 1). Obviously and expectedly, the coating layer suppressed the kinetics of PEI-1, in comparison to MC-1 (see Figure 7b). Figure 8 shows that the kinetics of Ag+ uptake by Lewatite obeys an intraparticle shrinking core model (SCM). The uptake rate of 200 CT can be either expressed by the Hellfrich model27 or the approximate SCMs (see Figure 8b).26,28 The diffusivity of Ag+ ions in matrix 200 CT favors the liquid film SCM.

C C0

(15)

where C0 and C denote the initial (at inlet of the column) and variable concentrations, respectively. In the current study, unless otherwise noted, the breakthrough point (Vb) corresponds to Xb ) 0.05 at the exit of the column. The practical capacity (Qb, given in moles) shows the adsorption capacity of the column up to the breakthrough point, Qb )

∫

Vb

0

(C0 - C) dV

(16)

and the total capacity (Q, also given in moles) is defined as the multiplication product of the specific capacity of adsorbent (qsat, expressed in units of mol/g) and its mass (mads, given in grams), Q ) qsatmads )

∫

∞

0

(C0 - C) dV

(17)

where V is the volume of solution that passes through the column. Another useful quantity is the utilization factor (Uf), which is defined as the ratio of the practical capacity to the total capacity: Qb (18) Q 3.6.1. Effect of Flow Rate. Least effect of flow rate on the breakthrough behaviors, as well as the steepest breakthrough curves in Figure 9;the flow rate;can be observed for 200 CT, as a result of its high kinetics. In contrast, the slow kinetics Uf )

Figure 9. Effect of flow rate ((]) 0.25 cm3/ min, (0) 0.5 cm3/ min, and (4) 1 cm3/min) on the breakthrough behavior of 0.5 g of (a) Lewatite, (b) MC-1, and (c) 200 CT. In all cases, [H+] ) 0.001 mol dm-3.

Ind. Eng. Chem. Res., Vol. 47, No. 17, 2008 6747 Table 4. Effect of Operational Parameters on Performance, Breakthrough Volume, and Utilization Factora adsorbent

concentration (ppm)

MC-3 MC-2 MC-2* MC-2* MC-1 MC-1 MC-1 MC-1 MC-1 Lewatite Lewatite Lewatite Lewatite Lewatite 200 CT 200 CT 200 CT

600 600 300 300 600 600 600 500 300 600 600 600 600 300 600 2300 2300

mass (g)

flow rate (cm3/min)

residence time, τ (min)

breakthrough volume (cm3)

utilization factor (%)

0.5 0.5 0.5 2 0.5 1 2 0.5 0.5 0.5 1 2 0.5 0.5 0.5 0.5 1

1 1 0.5 0.5 1 1 1 0.25 0.25 1 1 1 0.5 0.5 0.25 0.25 0.25

1.7 1.5 1.5 6 1.30 2.6 5.22 5.22 2.6 1.30 2.6 5.22 2.6 2.6 5 5 10

13 35 41 632 45 145 359 100 93 40 130 541 66 99 256 72 132

6 22 19.8 79 31.6 58.5 69.2 70 79 21.4 44.5 80.1 44.9 54.1 84.8 89.9 93.1

a The breakthrough volume and utilization factor were calculated based on 5% breakthrough (Xb ) 0.05). In cases marked by an asterisk (*), [H+] ) 0.1 mol dm-3; in all other cases, [H+] ) 0.001 mol dm-3.

Figure 10. Effect of the concentration of H+ and Ca2+ ions on the breakthrough behaviors of (a) 200CT, (b) Lewatite, and (c) MC-1. Legend: (]) 0.001 mol dm-3 H+, (0) 0.01 mol dm-3 H+, (4) 0.1 mol dm-3 H+, and (×) 0.1 mol dm-3 Ca2+. (In all of these figures, the flow rate was 0.5 cm3/min.)

of Lewatite results in lower Vb values and more sensitivity of its breakthrough behavior to variations in the flow rate (see Figure 9a). Most of the breakthrough properties of MC-1 lie somewhere between the properties of 200 CT and Lewatite (see Figure 9), despite the fact that MC-1 has the least uptake capacity. Apparently, in the following illustrative equation, when the amount of adsorbent is small, the role of “Kinetics” is more pronounced than that of “Capacity”.

(see Figure 7). Conditions for the foregoing stirred test must be well-defined and equal for the different adsorbents. The choice of using t1/2 as a reference time for measuring the kinetics is made because of complexities that occur under the initial condition (t ) 0) which makes most of the theoretical models (e.g., SCM) inapplicable under that conditions. A more-refined form of eq 19-1 can be obtained by expanding the terms “Kinetics” and “Capacity”:

Column Performance ≡ (Kinetics) X (Capacity)

t1⁄2

(19-1) Here, the parameter “Column Performance” can be arbitrarily defined as the utilization factor. The sign X is a symbol that has been introduced to separate the role of “Kinetics” from that of “Capacity”. It may not be considered as a normal multiplication operator, because of the dimensional inconsistency. “Capacity” in the aforementioned equation has been considered to be the total capacity of the bed, so that both the effects of the specific capacity (qsat, expressed in units of mol/g) and the mass of adsorbent (mads, given in grams) can be taken into account, even if the effect of the former seems to be more significant. “Kinetics” in eq 19-1) may be defined, using a stirred reactor, as “the adsorption rate (dC/dt) at t1/2”, where t1/2 denotes the time needed to attain a 50% saturation level in the stirred system

Column performance ≡

( dCdt )

( )[ C0 τ

dC τq F qsat(FbA)h dt t1⁄2 sat b ) C0Ah C02

]

( )

(19-2) where τ denotes the residence time of the solution in the column, Fb represents the apparent bed density, and A and h ultimately represent the cross section and height of the bed, respectively. The drawback of eq 19-2 is having the term C0 with a power of 2 in its denominator; therefore, it is better to have another measure of the “Kinetics” as the ratio of the residence time τ to t1/2. Using this alteration, we can obtain Column Performance ≡

τ qsat(FbA)h τqsatFb ) t1⁄2 C0Ah t1⁄2C0 (19-3)

6748 Ind. Eng. Chem. Res., Vol. 47, No. 17, 2008

3.6.2. Effects of Mass of Adsorbents and Concentration. When the mass of adsorbent is increased, the residence time τ is proportionally increased; this leads to a better utilization of the adsorbent and greater column performance (see eqs 19-2 and 19-3). The effect according to Table 4 is more remarkable for MCs and Lewatite. Table 4 also shows, with increasing mass, that the ranks of MC-1 and Lewatite are reversed from the standpoint of the “Column Performance”. The phenomenon can be interpreted using eq 19-1, because “Capacity”, which increases proportionally with the mass, has a more important role in the long column. For the 200 CT sample with fast kinetics, “Column performance” is enhanced with increasing concentration (see Table 4), because (∂C/ ∂r), as a driving force of the mass transfer, becomes larger at higher surface concentrations and, consequently, “Kinetics” in eq 19-1) (or dC/dt in eq 19-2) will be improved. The enhancement of dC/dt then can largely cancel out the direct effect of factor 1/C02 in eq 19-2. Contrary to 200 CT, adsorbents with slow kinetics are not able to handle solutions with high concentrations, so increasing C0 via factor 1/C0 in eq 19-3 has an adverse effect on the “Column performance” of both Lewatite and MC-1, especially on the former (Table 4). 3.6.3. Selectivity in Column and Asymptotic Behavior. From the standpoint of the selectivity, 200 CT has the worst behavior, because, in the presence of an acid or coexisting ions, its breakthrough curve greatly shifts toward the lower values and both the practical and total capacities decrease (see Figure 10a). In contrast, Lewatite is not only selective, but also shows a greater uptake capacity at higher concentration of coexisting ions (see Figure 10b). This astonishing behavior may be explained by the effect of the ionic strength on the uptake ability of nitrogen-bearing chelating resins.17 At low concentration of coexisting ions, total capacity of MC-1 is given by superposition of the capacities of both HA and alginate. In the presence of a high concentration of H+ or Ca2+ ions, where uptake ability of the alginate is canceled,13 the total capacity then approaches the capacity when HA is the only constituent:

{

/ qsat ) qCaALG + qHA (at low concentrations ofcoexisting ions) qsat ) qHA (at high concentrations of coexisting ions) (20)

where qHA(mmol/g) and qsat show the capacities of HA and total * capacity of MCs, respectively. The term qCaALG is the capacity presented by the alginate matrix of 1 g of MC-1 and is obtained from the capacity of pure calcium alginate, using the expression / qCaALG ) a × qCaALG

(21)

where a is the weight fraction of calcium alginate in the MCs (a ) 0.7 for MC-1). The capacity of pure calcium alginate (qCaALG) was calculated from the breakthrough curve of CaALG via graphical integration of eq 17 (qCaALG ) 0.616 mmol/g). Figure 10c and Table 5 show that all of the breakthrough curves of MC-1 at higher concentrations of coexisting ions are almost identical, and their corresponding qsat values are equal

(asymptotic behavior). Table 5 also verified the first relations that were presented as eq 20. / qsat ) qCaALG + qHA ) aqCaALG + qHA ) (0.7 × 0.616) + 0.72 ) 1.1512 mmol/g Increasing the concentration of coexisting ions reduces the breakthrough capacity of the MCs (i.e., qb, expressed in units of mmol/g) via two different mechanisms (see Table 5): First, the contribution of qb that is provided by the alginate matrix disappears. Second, at the same time, the ability of the alginate to transfer Ag+ ions to the HA droplets deteriorates. 3.6.4. Effect of the HA Content of MCs. To enhance the uptake capacity of MCs at medium or high concentrations of coexisting ions (CIs), one attractive idea is to increase their HA content (see eq 20). Unfortunately, the results of executing this idea were rather discouraging. Table 4 indicates that increasing the HA content from 29% to 69% reduces the breakthrough volume from 47 cm3 to 13 cm3, respectively. At the same time, the total capacity in the acidic media (qsat) of MC-2 was enhanced (see Table 5). Because the “Capacity” was enhanced, according to eq 19-1, the diminishing of the “Column Performance” can be only attributed to a reduction in the “Kinetics”. There are two reasons why the “Kinetics” diminishes at higher HA contents. First, as Table 1 shows, an increase in the HA content leads to a significant enlargement in the size of the MCs. The second reason addresses the slower transport of Ag+ ions via a matrix of MC-2 and MC-3 as being the mechanism that is causing the slowing of the “Kinetics”. Alginate probably provides a better transferring medium for the Ag+ ions. At present, the data are not adequate to prove any of the aforementioned mechanisms; however, from a practical point of view, these details are not worthy of consideration. In fact, even the “Kinetics” term is not inherently important: what is important is the “Column Performance”, and the next paragraph describes how MCs with a higher HA content can possess better column performance. Similar to the Lewatite, on the long column, the effect of the higher capacity of MC-2 and MC-3 may compensate for their slow kinetics, so that the performance of these MCs improves to even a better value than Lewatite. Table 4 shows that, when the amount of MC-2 increased from 0.5 g to 2 g, the breakthrough volume increased more than 15-fold. Nevertheless, the utilization factor (Uf ) 0.78 at 300 ppm Ag+) was still smaller than that for Lewatite (Uf ) 0.81 at higher concentrations (600 ppm)). Apparently, the only way to achieve MCs with better performance than chelating resins is to reduce their sizes while keeping a high HA content. So far, our entire attempt to conduct such modification by decreasing the viscosity of an alginate-HA sol (by heating or reducing the concentration of NaALG solutions) has been unsuccessful. Therefore, to reduce the size of the MCs, more-sophisticated systems, such as electric or pneumatic dripping systems that are able to adjust the size of the NaALG-HA droplets at the time that the MCs are formed, must be used. 3.6.5. Effect of Coating and Matrix Modification. Table 6 indicates a significant reduction in column performance or,

Table 5. Variation of Total and Breakthrough Capacities (Xb ) 0.05) of MC-1 in Different Ionic Mediaa capacity

for [H+] ) 10-3 mol dm-3

for [H+] ) 10-2 mol dm-3

for [H+] ) 10-1 mol dm-3

for [Ca2+] ) 10-1 mol dm-3

for [Ba2+] ) 10-1 mol dm-3

for [H+] ) 10-1 mol dm-3 (MC-2)b

qsat (mmol/g) qb (mmol/g)

1.15 0.72

0.77 0.42

0.71 0.23

0.74 0.22

0.73 0.16

1.06 0.20

a

In all cases, [Ag+] ) 300 ppm and the flow rate is 0.5 cm3/min. b Data for MC-2.

Ind. Eng. Chem. Res., Vol. 47, No. 17, 2008 6749 Table 6. Variation in the Breakthrough Volumes of MCs by Coating and Matrix Modification Breakthrough Volume, Vb (cm3) type

[H+] ) 10-3 M, [H+] ) 0.1 M, [H+] ) 10-3 M, flow ) 0.25 cm3/min flow ) 1 cm3/min flow ) 0.5 cm3/min

MC-1 PEI-1 CAP-20

168 122

95 57

50 74

in other words, uptake “Kinetics”, as a result of coating the MCs. In contrast, matrix modification enhanced both the “Capacity” and “Kinetics”; therefore, the “Column Performance” of CAP-20 was better than that of MC-1 (see Table 6). 3.6.6. Formulation of the Breakthrough Curves. In a recent study,6 an analytical model for the breakthrough behavior of chelating resins was developed based on SCM and constant pattern approximation (CPA). The latter principle assumes that the entire adsorption occurs in a thin layer of the bed (the “adsorption band”).20,26,29 The foregoing analytical model for the adsorbent that obeys intraparticle SCM leads to the following formula:6

( )

V - Vb ) A ln

X + B[F(X) - F(Xb)] Xb

Figure 11. (a) Fitting of the experimental data by eq 22 for (]) MC-1 and (2) MC-2, both with a flow rate of 0.5 cm3/min, 0.5 g, 300 ppm Ag+, and [H+] ) 0.1 mol dm-3. (b) Fitting of the experimental data by eq 23 for MC-1 at lower acid concentration ([H+] ) 0.001 mol dm-3): (]) 1 g, 300 ppm, 0.25 cm3/min; (4) 1 g, 600 ppm, 1 cm3/min; and (0) 2 g, 600 ppm, 1 cm3/min. In all cases, the solid lines are calculated curves.

(22)

where V denotes the volume of solution that should pass through the column until the dimensionless concentration of effluent reaches a given value X, and Vb is the breakthrough volume that corresponds to Xb ) 0.01. F(X) is defined as F(X) )

{ [

]

[

3 1 - (1 - X)1⁄3 2(1 - X)1⁄3 + 1 ln + √3 arctan 2 X √3

]}

Note that, in the applications of the analytical model, in contrast to the previous discussion, a value of Xb ) 0.01 was used. Here, it is worthwhile to note the difference between the “CPA formulae” and the “CPA models. The CPA formulas (such as eq 22) are exactly valid between the breakthrough point and the exhaustion point. They are not inherently valid for the transition region that precedes the breakthrough point (i.e., V < Vb).6 However, there are some techniques to extend the “CPA formulae” to the transition region,6,30 which creates the “CPA models”. In ref,29 a CPA model based on eq 22 was developed that accurately describes the adsorption of Ag+ ions by Lewatite. For alginate MCs, at medium or high concentrations of coexisting ions (CIs), the intraparticle SCM dominates; therefore, the breakthrough behavior at V >Vb obeys eq 22. On the other hand, at low CI concentrations, a reaction-controlled SCM holds, and in that case, Appendix suggests a new expression (eq 23):

( )

V - Vb ) C ln where G(X) is defined as G(X) )

X + D[G(X) - G(Xb)] Xb

[

2(1 - X)1⁄3 + 1 3 ln[1 - (1 - X)1⁄3] - √3 arctan 2 √3

Figure 12. (a) Elution behaviors of different adsorbents with [HNO3] ) 3 mol dm-3: (2) 200 CT, (]) MC-1, and (0) Lewatite. (b) Elution behavior with 5% thiourea and [HNO3] ) 0.1 mol dm-3: (0) MC-1 and (0) Lewatite.

for the MCs. Therefore, currently, the value of eqs 22 and 23 is only that: an appreciable fitting of the experiments by them that demonstrates the similarity of the uptake mechanisms (e.g., SCM) of alginate MCs and the chelating resins. For the purpose of the current study, this seems to be worthy of consideration. 3.6.7. Elution Characteristics. Any adsorbent that is supposed to be applied in the practical separation processes should be desorbed in an inexpensive method, using a commercially available eluent. Conventional eluents include inorganic acids, bases, salts, and organic complexants. Inorganic eluents are generally preferred, because the high stability of organometallic complexes makes the recovery of metal ions from the stripping solution complicated. Figure 12 shows the elution behavior of Lewatite, MCs, and 200 CT. In this figure, XE (expressed as a percentage) is defined as

(23)

]

Figure 11 shows the remarkable fitting of the experimental breakthrough curves of alginate MCs with eqs 22 and 23, which was obtained using computer software (e.g., EXCEL). The curvefitting resulted in values for A, B, C, and D, but unfortunately, contrary to the results with Lewatite,6 our attempts to determine the mathematical functionality of theses parameters, in the form of eqs A-2-16 and A-2-17 in Appendix A-2, were unsuccessful. Thus, there are still no completed “CPA models“

XE (%) )

Ci∆Vi × 100 Q

(24)

where Ci (given in units of mmol/cm3) is the concentration of each fraction and ∆Vi (expressed in units of cm3) is the volume of each fraction. Q is the total amount of adsorbed Ag+ ions (given in units of mmol). In the case of Lewatite, a HNO3 solution, even with a concentration of 3 mol dm-3, hardly elutes the Ag+ ions, and the addition of thiourea is necessary (see Figure 12). The 200 CT ion exchanger, on the other hand, is easily eluted with a moderate acid. The elution behavior of alginate MCs are more sought than Lewatite, because HNO3 with a concentration of 3 mol dm-3 can readily elute MCSs.

6750 Ind. Eng. Chem. Res., Vol. 47, No. 17, 2008

4. Conclusion

Appendix A-1

Microencapsulation of a liquid extractant in a calcium alginate matrix presents a very simple, but effective, method for the synthesis of highly selective adsorbents with many advantages, such as simplicity of their preparation, cost efficiency, and capability for immobilization of different extractants. The uptake characteristics of these adsorbents (microcapsules, MCs) are amid those of strongly acidic ion exchangers and the chelating resins, and they are rather similar to the latter. For instance, chelating resins maintain all of their adsorption capacity under extreme conditions (such as 1 mol dm-3 HNO3), whereas ion exchangers present very low performance under the same conditions. Alginate MCs, in comparison, lose some of their adsorption capacity, because of the vanishing of the capacity of the alginate matrix, yet the main part of the capacity (e.g., 62.5% for MC-1), which is the so-called “asymptotic capacity”, will remain. If the percentage of extractant (HA) in the MC increases to 65%, the asymptotic capacity exceeds the capacity of Lewatite (1.68 mmol/g). Unfortunately, simple microencapsulation techniques (e.g., gelation in the current study) produce MCs with rather slow kinetics, possibly because of the increasing of size of MCs. However, more-sophisticated microencapsulating techniques, using “electric or pneumatic cutting systems”, are able to overcome this difficulty and produce MCs with a high capacity, fast kinetics, or, in other words, high “Column Performance”. The matter now under investigation is the subject of future studies. Besides the asymptotic behavior, alginate MCs provide a very large value of the distribution coefficient (Kd). In the stirred reactors where a long residence time allows the complete attainment of equilibrium, a large distribution coefficient may be exploited for fully decontamination of the industrial effluents from toxic heavy metal ions (e.g., Hg+). The kinetics behavior of alginate MCs is also rather similar to that of chelating resins, with one difference being that MCs, despite having much lower porosity, exhibit faster kinetics. Moreover, chelating resins (e.g., Lewatite) generally favor the intraparticle shrinking core mechanism (SCM), whereas MCs at a low concentration of coexisting ions (CIs) follow a reactioncontrolled SCM and, at higher CI concentrations, obey an intraparticle SCM. The column behavior of alginate MCs are again similar to the chelating resin. The similar sensitivity to variations in the operational parameters and the same mathematical model (eqs 22 and 23) make the basis of their analogy. The plain (uncoated) alginate MCs suffers from the gradual losses of extractants when they endure a combination of mechanical and chemical stresses. Although the rate of leakage is tolerable in the conventional fixed-bed operations,13 certain techniques are available to avoid this shortcoming. The stabilization techniques that have been attempted in this study included coating by water-soluble polymers (e.g., PEI), and modification of the matrix of MCs by antacid polymers (e.g., cellulose acetate phthalate, CAP). Although both methods were efficient in diminishing the losses of extractant (see Table 2), matrix modification was superior, such that CAP-20 (the matrixmodified MC) had higher kinetics, a larger adsorption capacity, and more acidic stability than both the coated and plain MCs. Besides the experimental issues, the current paper has presented four new equations (eq 23 and eqs 19-1–19-3), which are useful in the interpretation of the results of any column experiments or the modeling of adsorbents with SCM kinetics.

The ratio of maximum uptake by PEI-Ca to that of CaALG in Figure 3b can provide a basis for estimating the thickness of coating layers of MCs. Figure 3c shows that the adsorption isotherm of Ag+ uptake by calcium alginate at concentrations of