Application of homogeneous and heterogeneous cation-exchange membranes in coagulant recovery from water treatment plant residuals using Donnan membrane process

Journal of Membrane Science 237 (2004) 131–144

Application of homogeneous and heterogeneous cation-exchange membranes in coagulant recovery from water treatment plant residuals using Donnan membrane process Prakhar Prakash, David Hoskins, Arup K. SenGupta∗ Department of Civil and Environmental Engineering, 13 E. Packer Avenue, Lehigh University, Bethlehem, PA 18015, USA Received 18 July 2003; received in revised form 30 January 2004; accepted 12 March 2004

Abstract The Donnan membrane process (DMP) was applied for recovery of alum from water treatment plant residuals or sludge. Alum is widely used as a coagulant in drinking water treatment plants for efficient removals of suspended solids and colloidal particles. The application was studied for two different types of membrane viz. the homogeneous Nafion 117 membrane and the heterogeneous Ionac 3470 membrane. Homogeneous membranes are coherent ion exchanger gels while heterogeneous membranes consist of colloidal ion exchanger particles embedded in an inert binder. In the process, the recovered Al3+ could be concentrated to a high value of over 4500 mg/L (80% recovery) with Nafion 117, but the recovery was relatively low (25% recovery) with Ionac 3470. Since the Donnan membrane process is driven by the electrochemical potential gradient, the presence of high turbidity and natural organic matters in the sludge did not influence alum recovery and no noticeable membrane fouling was observed even after multiple runs for long hours of operation. The Al3+ recovery profile led to identification of three zones of mass transport viz. kinetically driven linear zone, equilibrium driven saturation zone and osmosis driven dilution zone. All the three zones were observed in Nafion 117 during a 24 h experimental run. For Ionac 3470, only the linear zone was observed during the experimental period. Osmosis effect was not noted for Ionac 3470. The interdiffusion coefficient value of DAl–H was found to be one order of magnitude greater for Nafion 117. Within an ion exchange membrane, a diffusing ion hops from one charged site to the next and that constitutes the primary intramembrane ion transport mechanism. Scanning electron microphotograph and x-ray fluorescence showed clusters of non-conducting inert phases within the Ionac 3470 membrane containing no ionogenic groups. The lower interdiffusion coefficient for Ionac 3470 was attributed to its larger fraction of the non-conducting phase compared to Nafion 117. © 2004 Elsevier B.V. All rights reserved. Keywords: Donnan membrane process; Ion exchange membrane; Homogeneous and heterogeneous membrane; Alum recovery; Coupled transport of ions; Coagulant; Water treatment residuals

1. Introduction Ion-exchange membranes have been used industrially for separation processes such as in recycling and in water and wastewater treatment processes employing electrodialysis [1–3]. They have also been applied for recovery of spent acids by diffusion dialysis [4,5]. Several researchers have also utilized Donnan membrane process (DMP) using ion-exchange membranes as a pretreatment technique in wastewater and drinking water treatment, and in hydrometallurgical operations [6–9]. In most of the DMP applications ∗

Corresponding author. Fax: +1-610-758-6405. E-mail address: [email protected] (A.K. SenGupta).

0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.03.016

discussed in literature, ionic contaminants (heavy metal ions and anions) have been concentrated from a low concentration feed solution to a high concentration sweep solution, for further handling. Although information on applications of DMP in pure solutions is available in the open literature, no work is reported on the use of DMP to treat a sludge or slurry with high concentration of suspended solids or large organic molecules. Recent findings strongly suggest that a single-step Donnan membrane process can selectively recover alum, a widely used coagulant, from water treatment plant sludge, henceforth referred to as water treatment plant residuals (WTR) [10]. There are over a thousand (1000) drinking water treatment plants in USA which use alum, Al2 (SO4 )3 ·14H2 O, as

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a coagulant for efficient removal of particulate solids and colloids from surface water supply. More than 2 million tonnes of WTR are generated from these utilities every day [11,12]. Alum is finally converted during the coagulation process into insoluble aluminum hydroxide, a major component (50–75%) of the solids in WTR, along with suspended inorganic particles, natural organic matters (NOM) and trace amounts of heavy metal precipitates [13]. Due to regulatory changes in the recent past, WTR now have to be disposed of into landfills or through land application [13,14]. In addition, the toxicity of free and complexed aluminum species toward various aquatic life and benthic organisms has been the focus of several studies [15,16]. Some researchers have linked aluminum’s contributory influence to occurrence of Alzheimer’s disease [17]. In this regard, the prospect of alum recovery from WTR and its reuse are worthy of scientific investigation. Several efforts were made in the past to recover alum from WTR. Acid digestion process is the most commonly tried process at laboratory, pilot-scale, and plant level [18–20]. In this process, WTR are sufficiently acidified with sulfuric acid, dissolving insoluble aluminum hydroxide in the form of alum up to aluminum concentration levels of 360–3700 mg/L. However, the process is non-selective; with the dissolution of aluminum hydroxide, NOM like humates and fulvates get dissolved too and the resulting dissolved organic carbon (DOC) concentration ranges from 326 to 1800 mg/L [21]. This recovered alum, if reused as a coagulant, may impart a high trihalomethane formation potential (THMFP) during the chlorination stage of water treatment. The trihalomethanes are suspected carcinogens, regulated by the USEPA [22–24]. As an alternative to

acid digestion process, the amphoteric nature of aluminum oxide also permits alum recovery from the WTR under alkaline conditions. However, the alkali digestion process suffers from the same limitation as the acid digestion process i.e., NOM concentration is very high in the recovered solution. Fig. 1 shows both DOC and aluminum concentrations of the Allentown water treatment plant (AWTP) in WTR at different pH levels. It must be noted that DOC tends to increase with dissolved Al(III) under both acidic and alkaline conditions, confirming a non-selective nature of these processes. Ultrafiltration [25] application, another technology used in alum recovery, is associated with problems of fouling. The two-step composite membrane process (TCMP) [26], though selective, does not concentrate aluminum to high concentrations. In addition, composite exchangers are not available commercially in large quantities. The liquid ion exchange [27] process does not concentrate aluminum to a high level and there is always some solvent carryover that requires further treatment. The Donnan membrane process, as presented later, is uniquely capable of recovering alum from WTR in a single-step process using sulfuric acid and cation-exchange membrane. 1.1. Role of membrane structure: homogeneous versus heterogeneous An investigation into the transport of exchanging ions in the DMP application for alum recovery should consider the structure of the membrane, as the transport of ions is related to the resistance offered in the membrane phase. Based on the structure and method of preparation, two different types

10000

DOC, Al(III) Concentration (mg/L)

Al(III)

1000

100

DOC

10

1 0

2

4

6

8

10

12

14

Equilibrium pH

Fig. 1. Variation of dissolved organic carbon and aluminum concentration with pH for water treatment residuals from Allentown water treatment plant.

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finert

f gel

(a)

133

: Inert phase containing non charged polymer matrix and/or inert binding

: Gel phase containing ionogenic groups

Homogeneous membrane

Heterogeneous membrane

(b)

Fig. 2. Conceptualized distribution of ionogenic groups within cation-exchange membranes for (a) homogeneous membrane and (b) heterogeneous membrane.

of ion-exchange membranes have been identified. Homogeneous membranes are coherent ion-exchanger gels. Heterogeneous membranes consist of colloidal ion-exchanger particles embedded in an inert binder. Methods of preparing these membranes are different and available in open literature [28]. Several researchers have studied the influence of non-uniformity of ion-exchange membranes on various transport properties of membranes such as electrical conductivity, transport numbers and diffusion permeability [29–31]. A membrane may be considered to be a system of two or more phases. Timashev [32] proposed a two-phase model with hydrophobic backbone and hydrophilic ion-exchange group, including sorbed water. Zabolotsky and Nikonenko [33] suggested that an ion exchange membrane could be represented as a combination of a conducting phase that included a gel phase with a dense distribution of ionogenic groups and a non-conducting inert phase with more hydrophobic matrix and the binder. Fig. 2a depicts the distribution of the ionogenic groups within a homogeneous membrane, while Fig. 2b represents the same inside a heterogeneous membrane. It may be noted that the non-conducting inert fraction (finert ) is significantly greater within the heterogeneous membrane. Under non-equilibrium conditions, a diffusing cation hops from one charged site to the next within the membrane and that constitutes the primary ion transport mechanism for homogeneous membranes as illustrated in Fig. 3a. For heterogeneous membranes, the non-conducting or inert fraction offers resistance to ion transport in the following two ways: first, tortuosity or the path length of ion transfer through the membrane is increased; and second, the stagnant inert phase with minimal or no ionogenic groups is always less conducive to transport of ions. For counter-transport of aluminum and hydrogen ions in a heterogeneous membrane, Fig. 3b illustrates the foregoing effects of the inert phase. In this study, a homogeneous cation-exchange membrane, namely, Nafion 117 from DuPont, Delaware and heterogeneous membrane, namely, Ionac 3470 from Sybron Chemicals, NJ. were used. An SEM-XRF analysis was done to

identify the distribution of charged sites within Nafion 117 and Ionac 3470. The results of SEM analysis are shown in Fig. 4a and b. It is apparent from the microphotographs that there are inert regions with no ionogenic groups in Ionac 3470 while such discontinuities are absent in Nafion 117 membrane. A “dot-map” to identify regions containing Na+ is shown in Fig. 5a and b and the finding confirms this point. Sodium ions are uniformly distributed in homogeneous Nafion 117 membrane while they are unevenly distributed i.e., in clusters in Ionac 3470. A series of laboratory experiments were carried out to investigate alum recovery from water treatment residuals using these two membranes. The general goal of this study is to: first, present the experimental results under varying conditions to confirm the potential of Donnan membrane process for selective alum recovery from WTR; and second, discuss the underlying scientific reasons to validate the differences in the performance of homogeneous and heterogeneous membranes.

3+

Feed Side

Sweep Side

Al

H

+

(a) Homogeneous membrane

Feed Side

3+

Al

Sweep Side +

H

(b) Heterogeneous membrane Movement of ions through ionogenic groups Movement of ions through inert phase

Fig. 3. Counter-transport of Al3+ and H+ through homogeneous and heterogeneous membranes.

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Fig. 4. SEM picture of (a) Nafion 117 cross-section and (b) Ionac 3470 (magnification: 200×).

2. Theoretical Let us consider aluminum sulfate and sulfuric acid solutions in a Donnan membrane cell divided into two chambers (called left-hand side and right-hand side for the sake of discussion) by a cation-exchange membrane that allows only cations to migrate from one side to the other but rejects any passage of anions according to Donnan’s co-ion exclusion principle [34]. At equilibrium, the electrochemical potential of aluminum ion Al3+ ion (µ) ¯ in the electrolyte solution on the left-hand side (LHS) of the membrane will be the same as that in the electrolyte solution on the right-hand side (RHS) i.e.:

Fig. 5. SEM-XRF “dot-map” of Na+ in a cross-section of (a) Nafion 117 and (b) Ionac 3470 (magnification: 200×).

electrical potential, respectively. z refers to the charge of the diffusing ion, which is 3 for trivalent aluminum ion Al3+ and 1 for H+ ions. Eq. (2) gives the following equality for aluminum ions on two sides of the membrane: 
R 1/3 aAl F(φL − φR ) (3) = ln L RT aAl In a similar way, it can be shown for hydrogen ions that:
 aR F(φL − φR ) = ln H (4) L RT aH

(2)

Assuming non-ideality effects are about the same on both sides of the membrane, activities can be replaced by molar concentrations. Eqs. (3) and (4) then yield the following:

 R R 3 CAl CH (5) = L L CAl CH

where superscripts ‘0’, ‘L’, and ‘R’ refer to standard state, LHS and RHS; and µ, ¯ µ, a, F and φ the electrochemical potential, chemical potential, activity, Faraday constant and

R /C L is 10, it means C R is 1000 times greater If the ratio CH H Al L than CAl . Thus, by maintaining high hydrogen ion concentration on the right-hand side of the membrane, aluminum ions

µ ¯ LAl = µ ¯R Al

(1)

or L R µ0Al + RT ln aAl + zFφL = µ0Al + RT ln aAl + zFφR

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135

Fig. 6. A schematic of Donnan membrane process illustrating selective alum recovery from water treatment residuals.

can be driven from the LHS to the RHS even against a positive concentration gradient i.e., from a lower concentration region to a higher concentration one. In the conceptualized selective alum recovery from WTR of Fig. 6, the following can be highlighted: (i) aluminum hydroxide precipitates can be dissolved and then concentrated in the right-hand side; (ii) negatively charged NOM, sulfate and chloride cannot permeate through the membrane; and (iii) the transmembrane pressure does not influence the aluminum transfer flux. The distribution of Al3+ and H+ ions in the membrane during this coupled transport is represented in Fig. 7a and b. Fig. 7a shows the membrane condition at the start of the experimental run and Fig. 7b shows the concentration distribution when Donnan equilibrium is achieved. To explain the Donnan membrane process, the fundamental conditions for

coupled ion transport have been discussed in several papers [9,35,36]. These are as follows for Al3+ –H+ system. (1) The Nernst–Planck equation:   ∂qAl F ∂φ ¯ Al JAl = −D + zAl qAl ∂L RT ∂L   ∂qH F ∂φ ¯ + z H qH JH = −DH ∂L RT ∂L

(6b)

where J is the ion flux (mol/m2 s) and q (mol/m3 ) the molar concentration (also used interchangeably with equivalent concentration, meq/L) of ions in the ion-exchange membrane. L is the distance (m) in the ¯ the self-diffusion coefficient (m2 /s) membrane and D

Membrane

qAl,I

(6a)

Membrane

qH,II

CAl,II

CAl,I CH,I

CH,II Feed Side (I)

qAl

Feed Side (I)

Sweep Side (II)

Sweep Side (II)

qH, CAl,I CH,I

qH,I qAl,II

(a)

CH,II

CAl,II (b)

Fig. 7. An illustration of the instantaneous Al3+ and H+ ion concentration in the liquid phase and within membrane under intramembrane transport limited condition at (a) start of experiment and at (b) Donnan equilibrium. Note that the diffusion boundary layer (DBL) has not been considered owing to the high concentration in the feed and sweep solutions.

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of the diffusing ions in the membrane phase. Subscripts Al and H refer to Al3+ and H+ ions. (2) The principle of electroneutrality, by which the concentration of all counter ions is equal to those of fixed ions viz. the exchange capacity (Q): zH qH + zAl qAl = Q

(7)

Q is the exchange capacity of the membrane and has the same units as q. (3) Because of the condition of zero current, the sum of the flux of feed ions JAl and that of the sweep ions JH is zero: zH JH + zAl JAl = 0

(8)

Under these conditions, the flux for Al3+ transfer can be described by ¯ Al–H JAl = −D

∂qAl ∂L

where




¯ Al–H = D ¯ HD ¯ Al D

(9)

z2H qH + z2Al qAl ¯ H + z2 qAl D ¯ Al z2 q H D H

(10)

Al

(4) The selectivity coefficient K for Al3+ with respect to H+ in a membrane can be described by the following equation: (1 − xAl )3 CT2 yAl xAl (1 − yAl )3 Q2

3.1. Feed and sweep solutions The feed solution in some experiments consisted of WTR collected from the Allentown Water Treatment Plant (Allentown, PA), which uses alum as a coagulant. Table 1 provides a typical composition of WTR based on samples obtained from the Allentown plant. In other experiments, synthetic feed solutions were prepared using aluminum sulfate (Fisher Scientific) of Al3+ concentration of 2000 mg/L. The sweep side solution consisted of 1–2 M sulfuric acid purchased from EM Science. In order to investigate the effect of turbidity on solute flux and membrane fouling, fine (<200 mesh size) inert glass powder (Potters Industries, Product number: 6000) was used as the source of turbidity. NaCl for Osmosis experiments was bought from Fisher Scientific. 3.2. Donnan membrane cell



refers to the interdiffusion coefficient in the membrane phase. Replacing the charges with 3 and 1 for aluminum and hydrogen, respectively, the resulting equation is given as   qH + 9qAl ¯ ¯ ¯ DAl–H = DH DAl (11) ¯ H + 9qAl D ¯ Al qH D

Al KH =

3. Materials and methods

The Donnan membrane cell was made of Plexiglas, partitioned into two chambers, where the sweep solution side was separated from the feed solution side by a cation-exchange membrane, as shown in Fig. 8. Fixed volumes of feed and sweep solutions were used for every run; the ratio of feed volume to sweep volume could be adjusted independently in the test cell. The area available for exchange could also be adjusted to a maximum of 600 cm2 . Both the solutions were agitated during the entire course of the experiment using instrument quality air using air spargers connected to an instrument air line. 3.3. The exchange membranes

(12)

y refers to the equivalent ionic fraction in the membrane and x to the equivalent ionic fraction in solution. CT refers to the total concentration in the electrolyte (mol/m3 or meq/L). In applying the above conditions, the following fundamental assumptions have been made for the Donnan membrane process: (a) The diffusional resistance under experimental conditions lies entirely in the membrane phase i.e. intermembrane solute transport is the primary rate-limiting step [37]. (b) The amount of ions in the membrane is negligible compared to the amount in the solution. (c) The amount of co-ions (anions) in the membrane is negligibly small compared to its ion-exchange capacity i.e., Donnan co-ion exclusion is valid. (d) Local equilibrium is assumed at the membrane–water interface.

The cation-exchange membranes used for experiments were homogeneous Nafion 117 from Dupont, USA and heterogeneous membrane Ionac 3470 from Sybron chemicals. Pertinent details are provided in Table 2. Their exchange capacities were determined in the laboratory following standard experimental protocols [38,39].

Table 1 Typical composition of water treatment residuals Substance

Amount (mg/L)

Total suspended solids Total aluminum Total iron Total calcium Total copper Total zinc pH Arsenic Total dissolved organic carbon

100000–150000 1000–3000 100–300 <30 < 10 <20 7.0–7.2 <0.3 150–600

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137

Fig. 8. Laboratory set-up for a variable volume Donnan membrane cell. Note that the dimensions are not to scale and are in ‘cm’, unless mentioned separately.

3.4. Equilibrium isotherm experiment

3.5. SEM-XRF analysis

The membrane was converted to H+ form and dipped in an Al3+ –H+ electrolyte solution of known strength CT and the solution was stirred for 24 h to attain near equilibrium conditions. Aluminum uptake by the membrane was determined by mass balance. Several sets of such equilibrium experiments were carried out using different masses/areas of both Nafion 117 and Ionac 3470 at 50 meq/L electrolyte concentration. Subsequently, isotherms were constructed by plotting xAl (equivalent aluminum fraction in water) versus yAl (equivalent aluminum fraction in membrane).

A 1/2 in. × 1/2 in. square piece of the two membranes was taken and converted to Na+ form. Since the Sybron membranes are available in Na+ form, they were treated “as received”. For Nafion 117, the membrane was converted to Na+ form. For this purpose, the membrane was dipped in a 2% NaOH solution (for 30 min, stir) followed by a DI water wash (or soak) and rinsed. The process was repeated three times with “fresh” NaOH. Two hundred milliliters quantity of 2% NaOH was used for each treatment step. Once the membrane was converted to Na+ form, it was dried, a cross-section of the sample taken and mounted for SEM. A “dot-map” of the Na+ was performed using XRF. The sample was taken from the sample’s center to eliminate edge effect. The Sybron membrane was also soaked in DI water and after hydration, the membrane was air dried before preparing a cross-section sample for SEM-XRF analysis.

Table 2 Properties of Ionac 3470 and Nafion 117 membranes

Type Ionic form as shipped Exchange capacity (wet basis) Membrane thickness (wet basis) Reinforcement

Ionac 3470

Nafion 117

Heterogeneousa Na+a 1896 eq/m3

Homogeneousa H+a 1372 eq/m3

0.466 mma

0.200 mma

Inert bindinga

No reinforcementa

a Information obtained from http://www.dupont.com and http://www. sybronchemicals.com.

3.6. Experimental procedure and analytical techniques The feed solution was allowed to exchange Al3+ ions with H+ ions from the recovery solution for a period of 9–24 h, depending on the objectives of the experiment. Samples were collected at regular intervals from both feed and the sweep side. Aluminum was analyzed using UV-Vis

P. Prakash et al. / Journal of Membrane Science 237 (2004) 131–144

Spectrophotometer. This analysis involved the Eriochrome Cyanine R Method described in Standard Methods [40]. Other metals such as iron, copper, calcium, arsenic and zinc were analyzed with Atomic Absorption Spectrometer (Perkin Elmer: Model AA100 and Perkin Elmer: Model SIMAA 6000). The DOC was measured using a TOC Analyzer (Dohrman: Model DC-190) and sulfate ions were analyzed for Donnan exclusion using Dionex Ion Chromatograph (Model 4500i). SEM-XRF analysis was carried out with ThermoNORAN Quest which is a combination SEM and XRF.

6000.0

5000.0

81% 68%

4000.0

Al(III), m g/l

138

3000.0 43% 2000.0

24%

24% 13%

1000.0

7% 3%

0.0

3.7. Osmosis experiment A smaller Donnan cell was selected for this experiment in which the feed solution side consisted of 0.5 × 10−3 m3 of DI water and the receiver solution side had 0.2 × 10−3 m3 of 1 M electrolyte solution of NaCl (purchased from Fisher Scientific). The area of contact was 5 × 10−3 m2 . Water transport from feed to receiver solution was measured at different time intervals and water flux Jˆ Osmosis calculated from the obtained data. 4. Results and discussion 4.1. Alum recovery from water treatment residuals In the Donnan membrane cell, the feed side of the membrane contained 3.0 L of the decanted and slightly acidified WTR collected from the AWTP, while the sweep side contained 1.0 L of 1 M sulfuric acid solution. Fig. 9 shows the results of the run for a period of 24 h for both Nafion 117 and Ionac 3470; the percentage aluminum recovery and the concentration of aluminum in the sweep chamber were plotted against time. It can be seen that for Nafion 117, over eighty percent recovery (81%) was attained in 24 h. The noteworthy observation is that the recovered aluminum concentration was over 4700 mg/L as Al and it was significantly greater than the total aluminum concentration (1900 mg/L) present in the parent WTR. For Ionac 3470, alum recovery was low

0.0

5.0

10.0 15.0 Tim e (hrs)

20.0

25.0

Fig. 9. Aluminum recovery from AWTP residuals during Donnan membrane process for Nafion 117 and Ionac 3470: % recovery and increase in Al concentration in sweep solution.

(24%) and the sweep side concentration at 1500 mg/L was lower than the initial feed concentration. Fig. 10a and b provides detailed composition of the recovered alum from the Donnan membrane process for the two membranes. Besides Fe(III), other constituents are present at very low concentrations i.e., nearly two orders of magnitude lower than aluminum. Note that the presence of Fe(III) in the recovered alum is desirable for ferric sulfate is also an efficient coagulant. In a jar test investigation (results not included here), the recovered alum with Nafion membrane performed as efficiently as commercial alum in removing turbidity from Lehigh River water. 4.2. Membrane–solution equilibrium experiments The isotherm plot for Al3+ –H+ equilibrium is shown in Fig. 11. Fig. 11 depicts the experimental points and theoretical isotherm plot for Nafion 117 and Ionac 3470. Theoretical plots were generated in conjunction with Eq. (12). A comparison of the two plots shows that Nafion 117 has a more rectangular isotherm than Ionac 3470, suggesting that the homogeneous membrane has a higher selectivity for Al3+

Fig. 10. Composition of alum recovered from AWTP using Donnan membrane process: (a) Nafion 117 and (b) Ionac 3470.

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139

Equivalent Ionic Fraction of Al3+ in m em brane, yAl

1

K H = 3.21 Al

0.8

K H = 0.11 Al

0.6

CT: 50 meq/L Nafion 117 (Experimental)

0.4 Nafion 117 (Theoretical) Ionac MC 3470 (Experimental)

0.2

Ionac MC 3470 (Theoretical) 0 0

0.2

0.4

0.6

0.8

1

Equivalent Ionic Fraction of Al3+ in solution, xAl

Fig. 11. Al3+ –H+ Isotherm plots for Nafion 117 and Ionac 3470 membranes.

4.3. Osmosis of water When two electrolyte solutions at different concentration are separated by a semi-permeable membrane, solvent diffuses from dilute concentration side to the strongly concentrated side. The solvent flux is proportional to the osmotic pressure difference between the two solutions. Osmotic pressure is directly related to the total electrolyte concentration. A water flux term can be written as Jˆ Osmosis (m3 /m2 h) which is related to the concentration difference by the equation: Jˆ Osmosis = kC

(13)

where C = Csweep − Cfeed , C is the summation of molar concentration of all ionic species in the given solution and k is the proportionality constant or the osmosis parameter. For Nafion 117, the results of osmosis experiment are shown in Fig. 12. The findings showed that water flux was linearly related to the difference in concentration between the sweep and the feed side. The osmosis parameter kOsmosis was found to be 1.13 × 10−7 m4 /mol h. However, for Ionac 3470, no notable diffusion was recorded during the 24 h experimentation. 4.4. Donnan exclusion and fouling One of the primary attributes of the Donnan membrane process is its ability to exclude anions, NOM and particulate matters while recovering aluminum from the WTR without being fouled. Fig. 13a and b shows the dissolved organic carbon and sulfate (SO4 2− ) concentrations in the feed side

and sweep side of Donnan membrane cell employing Nafion 117 and Ionac 3470, for a run that lasted 24 h. DOC is a measure of NOM in the aqueous phase and these figures show that both DOC and sulfate remained nearly constant on both sides. The sulfate concentration values were very high (∼10,000 mg/L on the feed side and ∼100,000 mg/L on the sweep side), yet their exclusion was nearly complete. The DOC concentration in the recovered alum was consistently nearly 20 mg/L for both the membranes. It is possible that due to some sorption of DOC molecules on the membrane surface, some amount of dissolved organic molecules leaked to the sweep side. The feed concentration was about 150 mg/L. Likewise, sulfate concentrations in the feed side and sweep side did not change during 24 h experimental run. This observation confirms that both Ionac 3470 and Nafion 117 disallowed permeation of NOM and sulfate in accordance with Donnan co-ion exclusion principle. A visual comparison of the feed (WTR) and recovered alum solutions in Fig. 14 shows the dark coloration (yellow) of the

0.0005 0.0004 Flux (m 3/m 2/hr)

Al was deions. In fact, the average selectivity coefficient KH termined to be 3.21 for Nafion 117 and 0.11 (over 30 times less) for Ionac 3470.

0.0003 0.0002

k Osmosis

0.0001

-7

4

= 1.13*10 m /moles/hr

0 0

500

1000

1500

2000

2500

3000

3500

4000

Concentration Difference (moles/m 3)

Fig. 12. Osmotic flux values for water transport with concentration difference between sweep and feed solution in Nafion 117 membrane.

140

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DOC Concentration (m g/L)

140 120 100 Nafion 117 Feed

80

Nafion 117 Sw eep Ionac MC 3470 Feed

60

Ionac MC 3470 Sw eep 40 20 0 0

5

10

(a)

15

20

25

30

Tim e (hrs)

Sulfate (SO 4 2-) Conce ntr ation, (m g/L)

1000000

100000

10000

Nafion 117 Feed Ionac MC 3470 Feed Nafion 117 Sw eep Ionac MC 3470 Sw eep

1000 0

5

10

(b)

15

20

25

30

Tim e (hrs )

Fig. 13. (a) Dissolved organic carbon concentration in feed and sweep solutions and (b) sulfate ion concentration in feed solution.

Fig. 14. Visual comparison of feed side (left) and sweep side (right) during alum recovery using Donnan membrane process (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

feed due to high DOC, as opposed to the transparent alum solution in the sweep side due to its low DOC content. Two consecutive experiments were carried out to investigate possible fouling of membranes caused by NOM and particulate matters. Fig. 15a and b shows the results of two successive runs using an identical feed sample collected from AWTP. For Nafion 117, the rate of increase in aluminum concentration with time in the sweep side did not undergo any noticeable change between the two runs, confirming absence of fouling. For Ionac 3470, the second run resulted in slightly higher yield. The reason for an increased aluminum flux during the second run could not be explained but the results confirm the absence of fouling. Since the negatively charged NOM do not adhere to the membrane surface and additionally, since the Donnan membrane process is non-pressure driven, the possibility of membrane fouling due to particulate matter clogging the active charged sites is absent.

P. Prakash et al. / Journal of Membrane Science 237 (2004) 131–144

(a)

141

(b)

Fig. 15. Aluminum recovery in two consecutive runs with WTR feed in (a) Nafion 117 and (b) Ionac 3470.

4.5. Al3+ recovery profile In order to develop an insightful understanding of the effect of the sulfuric acid (sweep solution) concentration on aluminum transport, experiments were conducted at two different initial sulfuric acid concentrations (C0 ), namely, 1 and 2 M. Identical synthesized feed solutions of aluminum sulfate (∼2000 mg/L as Al) were used in all four experiments that lasted for 24 h. Fig. 16a and b provides plots of aluminum recovery versus time for both Nafion 117 and Ionac 3470. The following observations are noteworthy: • For Nafion 117, the aluminum recovery profile approached saturation after 12 h but the same was linear after 24 h for Ionac 3470. • Increasing the initial sulfuric acid concentration, C0 , from 1 to 2 M increased aluminum recovery uptake for both membranes but there was a drop in aluminum concentration at the end for Nafion 117. This observation of drop in aluminum concentration beyond saturation is counter-intuitive and was reconfirmed through a replicate experiment. Characteristically, the aluminum recovery plot for Nafion 117 could be identified as a combination of three zones: Zone A (where the plot is linear), Zone B (where the plot

reaches a plateau) and Zone C (where it tapers downward). The following provides a scientific explanation leading to the existence of the three zones. In the beginning of the run, the driving force was characterized by a high electrochemical potential gradient in the membrane phase. This initial period of aluminum transfer is analogous to the condition in Fig. 7a shown earlier. The driving force remained nearly constant for the first 5 h and consequently, alum recovery rate was almost linear as shown in Zone A of the recovery plot. Zone A can therefore be termed as “kinetically driven linear zone”. As time progressed, the condition of Donnan equilibrium characterized by the situation in Fig. 7b was approached. Sufficient electrochemical potential gradient was no longer available, thus lowering the aluminum transfer flux. The aluminum recovery rate was thus governed by equilibrium conditions during this period and Zone B can be termed the “equilibrium driven saturation zone”. In membranes such as Nafion 117, the transfer kinetics is fast enough to exhibit the two zones clearly during a 24 h experiment, as demonstrated in Fig. 16a. Unlike it, Ionac 3470 is kinetically slower and offers higher resistance to aluminum transport. Therefore, the conditions of equilibrium were delayed and only linear profile of Zone A was observed with Ionac 3470 during the entire period of experimentation, as shown in Fig. 16b.

6000

5000

2500 Feed: 3L Sweep: 1L Specific surface area: 120cm2/Liter

Feed: 3L Sweep: 1L Specific surface area: 120cm2/Liter

2000

4000

Zone C

Al(III), mg/L

1500

Zone B

Al(III), mg/L

3000 1000 2000

1000

500

Nafion 117: Co=1M

Zone A

Ionac MC 3470: Co=2M

0

0 0.0

(a)

Ionac MC3470: Co=1M

Nafion 117: Co=2M

5.0

10.0

15.0

Time (hrs)

20.0

25.0

0.0

5.0

10.0

(b)

Fig. 16. Al3+ recovery profile for (a) Nafion 117 and (b) Ionac 3470.

Time (hrs)

15.0

20.0

25.0

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In Fig. 16a, an interesting and counter-intuitive observation was the drop in aluminum concentration with time, when a 2 M acid sweep solution concentration was used. Note that the osmosis of water is an ongoing phenomenon in Nafion 117 during the Donnan membrane process. It is noteworthy that osmosis took place independent of the aluminum transfer process. Thus when the aluminum recovery rate became negligible, the water transport due to osmosis still continued leading to a lowering or dilution of aluminum concentration in the recovery side. This situation was identified in Zone C which is termed ‘osmosis driven dilution zone’. Such a tapering was not observed in the experiment involving 1 M sweep solution because the osmosis effect was weaker during the period of experimentation. Only Zone A was observed in Ionac 3470, primarily due to low aluminum transport flux and partly due to higher resistance offered to solvent osmosis in comparison to Nafion 117. Increasing the sweep side concentration to 2 M led to greater recovery, as shown in Fig. 16b, because a higher driving force was available in the sweep solution. 4.6. Intramembrane diffusivity An increase in liquid-phase agitation in both the WTR feed and the sweep solution did not increase transmembrane aluminum flux or rate of aluminum recovery during the experimental run. This observation implies that under the hydrodynamic conditions of the experiments, transport from the bulk liquid to the membrane interface was much faster relative to the intramembrane transport of ions i.e., diffusional resistance for intramembrane transport of ions was the rate-limiting step. For this situation, ¯ Al–H ) and aluminum–hydrogen interdiffusion coefficient (D self-diffusion coefficients of Al3+ and H+ were determined from the experimental data for both homogeneous Nafion 117 and heterogeneous Ionac 3470 in accordance with the following procedure: • Experimental data points in Fig. 16 provide aluminum concentration in the sweep side at different times. These concentration values were used to compute the aluminum transport rate or flux (JAl ) at different time intervals. • At any given time, transmembrane aluminum flux is given by q q − qAl,II ¯ Al–H Al = −D ¯ Al–H Al,I JAl = −D (9 ) L L Every JAl value corresponds to known solution compositions in the feed and sweep side. Using selectivity coefficient values determined from equilibrium isotherm data in Fig. 11, both qAl,I and qAl,II values at both sides of the membrane surface were determined. All other pa¯ Al–H values were subsequently rameters being known, D determined from Eq. (9). Table 3 shows the computed in¯ Al–H ) values for the two memterdiffusion coefficient (D branes at different times during the run. As expected, the ¯ Al–H values of Nafion 117 are significantly greater than D

Table 3 Interdiffusion coefficient values Time (s)

¯ Al–H (m2 /s) Nafion 117: D

900 1800 2700 5400 9000 13500 19800 33300 64800

1.04 × 10−10 1.09 9.33 8.77 8.49 4.54 3.88 1.56

× × × × × × ×

¯ Al–H (m2 /s) Ionac 3470: D 1.85 × 10−11

10−10 10−11 10−11 10−11 10−11 10−11 10−11

1.18 1.21 1.47 1.22 1.40 1.19

× × × × × ×

10−11 10−11 10−11 10−11 10−11 10−11

those of Ionac 3470, under identical conditions. However, ¯ Al–H values are not constant and vary as the solution D composition varies at the membrane–water interface with time. ¯ Al–H values are dependent on the • As shown earlier, the D membrane composition i.e., on the relative aluminum and hydrogen ion loading as shown below:   qH + 9qAl ¯ Al–H = D ¯ HD ¯ Al D (11) ¯ H + 9qAl D ¯ Al qH D Rearrangement of Eq. (11) gives the following equality:   ¯ Al − D ¯H 1 1 1 D = + ¯ Al–H ¯ Al ¯ HD ¯ Al 1 + (1/9)(qH /qAl ) D D D (14) Or, in terms of ionic fraction of Al3+ in the membrane phase, the equation can be expressed as ¯H ¯ Al − D 1 1 D = + ¯ ¯ ¯ ¯ DAl–H DAl DH DAl   1 × 1 + (1/9)((1 − yAl )/yAl )

(15)

¯ Al–H are From the experimental data, the values of 1/D ¯ Al plotted against 1/(1 + (1/9)((1 − yAl )/yAl )) and D is determined from the intercept using Eq. (15). Subse¯ H is computed from the slope after substituting quently, D ¯ Al . Table 4 includes the self-diffusion coefficient for D values of aluminum and hydrogen and they are both one order of magnitude greater for homogeneous Nafion 117 in comparison with Ionac 3470 membrane. Another interesting inference that can be made from Tables 3 and 4 is that the interdiffusion coefficient values are close to the self-diffusion coefficient values of H+ ions. The relative distribution of Al3+ and H+ in the membrane governs the value of DAl–H . Both Nafion 117 and Ionac Table 4 Self-diffusion coefficient values

Nafion 117 Ionac 3470

¯ H (m2 /s) D

¯ Al (m2 /s) D

1.1 × 10−10 1.42 × 10−11

1.6 × 10−12 1.9 × 10−13

P. Prakash et al. / Journal of Membrane Science 237 (2004) 131–144

3470 are strong-acid cation-exchange membranes and exhibit much greater affinity for ions with higher valence [37]. Equilibrium isotherm experiments in Fig. 11 reveal that binary separation factor, which is a measure of relative affinity for two competing ions for an ion exchanger, is over 70 for Nafion 117 and about 30 for Ionac 3470 at 50 meq/L solution concentration. Thus, at anytime during the process, qAl is significantly greater than qH within the ion exchange membrane. Considering the extreme case where hydrogen ion is essentially a trace species compared to aluminum, i.e., qAl qH , Eq. (11) degenerates into the following: ¯ Al–H = D ¯H D

(16)

Thus for the alum recovery process, hydrogen ion is a minor species within the membrane. Consequently, the interdiffusion coefficient, DAl–H , approaches that of faster diffusing ¯ H . The composition of the membrane hydrogen ions, D changes with the progress of the process, thus altering the DAl–H values, but they always remain significantly greater than the self-diffusion coefficient of aluminum ions.

5. Conclusions In the current research, a comparison was made between a homogeneous membrane Nafion 117 and a heterogeneous membrane Ionac 3470 for alum recovery from water treatment residuals. The primary findings of the study can be summarized as follows: • Selective alum recovery was possible in the presence of high concentration of suspended solids and natural organic matter. The concentration of aluminum in the recovered alum was significantly greater than that in residuals. • Factors such as high turbidity and high DOC concentration in residuals did not alter the recovery rate and fouling of ion exchange membrane was practically absent after multiple runs. • Alum recovery with homogeneous Nafion 117 after 24 h was over three times greater than that of heterogeneous Ionac 3470 under similar conditions. The quality of the recovered alum in terms of its purity was about the same for both the membranes. The low recovery in Ionac 3470 was attributed to the lower Al3+ –H+ interdiffusion coefficient in Ionac 3470 membrane. • The physical morphology of the heterogeneous ion exchange membrane is quite different from that of the homogeneous membrane as illustrated by the SEM and SEM-XRF pictures. It is postulated that the lower interdiffusion coefficient value is caused due to the presence of much higher non-conducting inert zones within the membrane, which offered greater diffusional resistances. • For both the membranes, H+ ion was a trace species compared to aluminum and, therefore, the interdiffusion ¯ Al–H approached the self-diffusion coefficient value D ¯ coefficient value DH .

143

Acknowledgements Industrial Ecology Fellowship grant from the National Science Foundation and Lucent Technology is gratefully acknowledged. We are also thankful for partial financial support from Pennsylvania Infrastructure and Technology Alliance (PITA). The authors gratefully acknowledge valuable suggestions from Mr. Dennis Curtin of Dupont Chemicals. Nomenclature a Al C CT ¯ D f F H J Jˆ k K L Q, q R t T x y z I, II

activity (mol/m3 ) aluminum concentration of ions in solution (mol/m3 or meq/L) total concentration of ions in solution (eq/m3 or meq/L) diffusion coefficient of ions in membrane (m2 /s) fraction in membrane Faraday constant (96,496 C/mol) hydrogen flux of ions in membrane (mol/m2 s) flux of water (m3 /m2 h) osmotic parameter (m4 /mol s) selectivity coefficient distance in membrane (m) concentration of ions in membrane (mol/m3 or eq/m3 ) gas constant (J/K mol) time (s) temperature (K) ion fraction in solution ion fraction on membrane charge on ion solution–membrane surface interface

Greek letters µ, ¯ µ electrochemical potential, chemical potential (C V/mol; cal/mol [4.1868 J/mol]) φ electrical potential (V)

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