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The 5th AUN/SEED-Net Regional Conference on Global Environment November 21st-22nd, 2012

Application of Electrodeionization (EDI) for Humic Acid Removal Khoiruddin, I. S. Yunus, J. Sucipto, I G. Wenten* Department of Chemical Engineering, ITB, Jl. Ganesha 10, Bandung 40132, Indonesia *Email: [email protected]

Abstract. In this study, an electrodeionization (EDI) system is applied for removal of humic acid from peat water. The experiment is conducted using both diluted and natural peat water solution which previously prepared using Ultrafiltration membrane (UF). The EDI stack consist of one diluate compartment, two concentrate compartments, one anode compartment and one cathode compartment. Commercially available cation exchange membrane (MC3470) and anion exchange membrane (MA-3475) are used as ionic selective barriers of the stack. Both diluate and concentrate compartments are filled with mixed ion exchange resins. Results show that applied voltage has significant impact upon ionic removal percentage and current density. Feed conductivity significantly affect all responses except for current density. While the interaction of both factors only has significant effect on current density. Experiment using pretreated natural peat water shows that ionic removal percentage could be acquired more than 90 % in batch operation mode. The reduction of absorption value are about 99 %, 60 % and 70 % in electrode, diluate, and concentrate stream respectively. Moreover, A dimensionless coefficient Kt is used to distinguish the “enhanced transfer” and “electoregeneration” mechanisms. It is found that the EDI system operates under electroregeneration regime. However, it is observed that absorption of humic acid is the control mechanism during process and it is leading to a reduction in EDI performance. Therefore, it should be emphasized that fouling formation on the anion-exchange membrane surface and anion-exchange resins need to be controlled to achieve a better performance. Keywords: humic acid, peat water, electrodeionization, ion-exchange membrane, ionexchange resin.

1

Introduction

Peat water is one of water source that widely spread in sumatera and borneo, Indonesia. However, peat water usage as clean water source is inhibited by its high dissolve organic matter especially humic acids and humic substances. Humic substances constitute a major fraction of soil organic matter are generally divided into three classes of materials on the basis of operational definitions. Fulvic acids are materials soluble in water under all pH conditions,

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The 5th AUN/SEED-Net Regional Conference on Global Environment November 21st-22nd, 2012 humic acids are soluble at pH > 2 and humins are insoluble at all pH [1]. Humic substances contribute to odor, color, taste as well as acidity problems [2, 3]. Removal of humic substances, mainly humic acid, is a critical issue in the production of clean or drinking water from surface water. Because humic acids can initiate the formation of disinfectan by-products (Trihalomethanes) [4] as potentially carcinogenic organic compounds when water react with chlorine [5, 6]. Thus, humic acids removal from water becomes the main issue for water purification researchs and development. Recently, membrane processes have been used in humic acids removal. However, low water flux and serious fouling during humic acid removal were found when using Nano Filtration (NF) and Reverse Osmosis (RO)[7, 8]. In addition, RO is operated in high pressure operation condition which lead to high production cost [9]. The concept of modified Electrodialysis (well known as electrodeionization, EDI) process has been extensively recognized since the mid-1950s. After the first commercial unit introduction in 1987 [10], EDI has continued to be an attractive deionization process with various advantageous than conventional ion exchange deionizaton (IX) in production of Ultrapure Water (UPW) from technological and economic standpoint [11, 12]. The main reason for EDI commercial success is its capability on eliminating regeneration process and associated hazardous chemical. As a chemical free operation or environmentfriendly technology, this process has dominant the choice of alternative technology for large scale UPW production [13 – 21]. EDI is mainly applied to water and wastewater treatment, but it has also shown great potential to be applied in a number of different applications. The use of this technique in biotechnology and organic acid removal is also challenging [22 – 30]. Since dilute solution has relatively high electrical resistance, using electrically active media (ion exchange resin filler) as the bridge over current is a strategy to decrease the resistance and the energy consumption of the ED stack [31]. By adopting this strategy, low concentration solution can be treated by using EDI process. Therefore, in this study, we investigated the use of EDI process as an alternative technology for removal of humic acid from natural peat water.

2

Experimental

Peat water was taken from Sungai Masjid Dumai, Riau, Indonesia. Experiment is conducted using model solution prepared by diluting peat water into ultrapure water to get aqueous peat water. Schematic diagram of experimental set up used

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The 5th AUN/SEED-Net Regional Conference on Global Environment November 21st-22nd, 2012 in this study is shown in Fig. 1.The process for humic acid removal from natural humic water consisted of pretreatment filtration using ultrafiltration and EDI system. The pretreatment filtration unit provided suspended solid-free filtrate. Its permeate was directly fed into the EDI stage.

Figure 1 Experimental Set Up of electrodeionization System The EDI stack consisted of one diluate compartments, two concentrated compartment, one anode compartment, and one cathode compartment. Commercially available cationexchange membrane (MC-3470) and anion-exchange membrane (MA-3475) – both from Sybron Chemical Inc., USA, were used as ionic selective barriers of the EDI stack. The specification of membrane used in this experiment are shown in Table 1. The effective surface area of each membrane was about 140 cm2. All diluate and concentrate compartments were filled with mixed-bed ion-exchange resins. The ratio of anionexchange resin to cation-exchange resin was 2 : 1 (Amberlite strong acid cationexchange, IR-120NA and strong base anion resins, IRA-402CL, see Table 2). Platinum and stainless steel were used for anode and cathode, respectively. The internal spacer for each compartment was 3 mm. An adjustable power supply (Lab scale, made by GPD Filter) was used to produce direct current. It could supply voltage and direct current in the range of 0 – 200 V and 0 – 1.0 A, respectively. Conductivity was analyzed using HI98303 (Hanna Instrument) in the range of 1999 µS/cm (Resolution: 1 µS/cm, accuracy: ±2% F.S.).

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The 5th AUN/SEED-Net Regional Conference on Global Environment November 21st-22nd, 2012 Table 1

Membrane Specification

Membrane

Type

Category

Thickness (mm)

MC-3470 MA-3475

Cation Anion

Hetero Hetero

0.38 0.41

Table 2

Burst Strength (bar) 10 10

IEC (meq/g) 1.5 0.9

Gel rm α (%) water (Ω m2) m 35 31

10-25 25-50

Ion-Exchange Resin Specification

Properties

Cation-Exchange Resin

Anion-Exchange Resin

Functional Group Ionic form as Shipped Total Exchange Capacity Moisture Holding Capacity Harmonic Mean Size

IR-120Na Styrene Divinylbenzene Copolymer Sulfonate Na+ ≥ 2.00 eq/L 45 – 50 % 0.6 – 0.8 mm

IRA-402Cl Styrene Divinylbenzene Copolymer Trimethyl ammonium Chloride ≥ 1.2 eq/L 49 – 60 % 0.6 – 0.75 mm

Type Matrix

3 3.1 3.1.1

96 99

Results and Discussions Diluted peat water Analysis of main factors and interaction

Operating parameters such as applied voltage and feed water conductivity were evaluated. Response used to determine EDI performance are percentage of ionic removal, operation time to obtain desired product conductivity, and current density. The relative importance of the main effects and their interactions was observed on the Pareto chart (Fig. 2 – 4). A student's t-test was performed to determine whether the calculated effects were significantly different from zero, which shown in the Pareto chart by horizontal columns. As shown in Figures, the values that positioned around references line are not significant factor. While the values that exceed a refence line which corresponding to 95 % confidence interval in this study, are significant values. According to Fig. 2 – 4, the main factors that extend beyond the reference line was significant at the level of 0.05. Applied voltage (the main factor A) represented significant effect on % removal and current density, but had not significant level on operation time to obtaining desired product conductivity. In other hand, feed conductivity (the main factor B) had significant effect on all responses except on current density. While the interaction factor (AB) had only significant effect on current density.

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Standardized Effect

3,5

Factor(A: Voltage; B: Feed Conductivity) Ref. Line

3,0 2,5 2,0 1,5 1,0 0,5 0,0

A

Term

B

AB

Figure 2 Pareto chart of standardized effect, response is % removal 8,0 Factor (A: Voltage; B: Feed Conductivity)

Standardized Effect

7,0 6,0

Ref. Line

5,0 4,0

3,0 2,0 1,0 0,0

B

AB

A

Term

Figure 3 Pareto chart of standardized effect, response is operation time

Standardized Effect

20,0

Factor (A: Voltage; B: Feed Conductivity) Ref. Line

16,0 12,0 8,0

4,0 0,0 A

AB

B

Term

Figure 4 Pareto chart of standardized effect, response is current density

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3.1.2

Effect of Applied Voltage

Cd/Co

Ratio of conductivity in diluate to feed stream at different time and different applied voltage is illustrated in Fig. 5. It was observed that a higher migration rate of ions in diluate stream could be achieved at higher voltage (15 volt). The ions amount transferred from diluate to concentrate was increased gradually and approached to constant after diluate conductivity reached 1 μS/cm (a: Cd/Co = 0.07 and b: Cd/Co = 0.02). When the applied voltage is too low, it will be difficult to maintain a desired transfer rate of ions due to a lower driving force. Increased in applied voltage may improved the diluate product stream and decreased operation time to achieved desired product conductivity. However, when the applied voltage is too large, intensive water splitting will occur and resulting in decrease current efficiency [32, 33]. 1,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0

a

0

5

V = 5 Volt V = 15 Volt

10

15

20

25

Cd/Co

Time (minutes) 1,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0

b V = 5 Volt V = 15 Volt

0

10

20

30

40

50

60

Time (minutes)

Figure 5 Variation of Cd/Co with time at different applied voltage (a: Co = 15 μS/cm;b: Co = 42 μS/cm ).

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a

100 80 % Removal

60


40 20

0 0

10 20 Time (minutes)

30

b

100 % Removal

80 60


40 20 0 0

20

40

60

Time (minutes) Figure 6 Variation of % Removal with time at different applied voltage ( a: Co = 15 μS/cm; b: Co = 42 μS/cm )

Ratio of conductivity in diluate to feed stream at different operation time and different applied voltage is illustrated in Fig. 6. It shows that EDI was able to reach ions removal in diluted peat water at more than 93 %. When the applied voltage is increased to 15 V, a higher removal rate is acquired within 90 % removal ( Co = 15 μS/cm ) and 80 % removal ( Co = 42 μS/cm ). Meanwhile, using 5 V applied voltage resulting in relatively constant removal rate.

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Effect of Feed Conductivity

Cd/Co (Diluate Conductivity / feed conductivity)

Cd/Co (Diluate Conductivity / feed conductivity)

3.1.3

a

1,20 1,00

Co = 15 MicroS/cm Co = 42 microS/cm

0,80 0,60 0,40

0,20 0,00 0


60

b

1,20 1,00

Co = 15 microS/cm Co = 42 microS/cm

0,80 0,60 0,40

0,20 0,00 0


60

Figure 7 Variation of Cd/Co with time at different Feed conductivity (a: V = 5 volt ; b: V = 15 volt). The effect of feed conductivity on Cd/Co and % removal are shown in Fig. 7 and Fig. 8 respecitvely. The feed conductivity was inversely proportional to operation time since longer operation time were needed to achieved desired product conductivity (1 μS/cm) at feed conductivity 42 μS/cm than at 15 μS/cm. Ion exchange resins have high preferences under low concentration of feed solution which induces sharp exchange zone boundary in EDI process. Meanwhile at a high feed concentration those zone becomes dispersed which induces easy breakthrough. Therefore, a high feed conductivity directly leads to reduction of EDI performance and high operation time [34].

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% Removal


a

100 90 80 70 60 50 40 30 20 10 0

Co = 15 microS/cm Co = 42 microS/cm

0

20

40

60

Time (minutes)

b

100 % Removal

80 60 Co = 15 microS/cm Co = 42 microS/cm

40 20 0 0

10

20

30

40

50

Time (minutes) Figure 8 Variation of % removal with time at different Feed conductivity (a: V = 5 volt ; b: V = 15 volt).

3.1.4

Current Density and Resistance of Membrane Stack

Variation of current density and membrane stack resistance over time at different applied voltage are shown in Fig. 9 and Fig. 10, respectively. It was observed that Fig. 9 and Fig. 10 reveal an obvious tendency of increasing current density and decreasing resistance at increasing applied voltage as well as feed conductivity. It was also observed that current density decreased with time. In other hand, the resistance of membrane stack is increased over time, because conductivity of diluate and electrode become depleted as ion transferred to concentrate compartment.

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Current Density (A/m2)

Therefore, the current density is decreased and total resistance of membrane stack is increased. 2,00 1,80 1,60 1,40 1,20 1,00 0,80 0,60 0,40 0,20 0,00

a

0

5

10

V = 5 Volt V = 15 volt

15

20

25

Time (minutes)

Current Density (A/m2)

3,00

b

2,50

V = 5 Volt

2,00

V = 15 volt

1,50 1,00 0,50 0,00 0

20

40

60

Time (minutes) Figure 9 Variation of current density with time at different applied voltage ( a: Co = 15 μS/cm; b: Co = 42 μS/cm ).

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Resistance of membrane Stack (ohm)


a

1800 1600 1400 1200 1000 800 600 400 200

V = 5 Volt V = 15 volt

0

10

20

30

Time (minutes)

b

Resistance of membrane Stack (ohm)

1400 1200 1000 800


600 400 200 0

20

40

60

Time (minutes)

Figure 10 Variation of membrane stack resistance with time at different applied voltage ( a: Co = 15 μS/cm; b: Co = 42 μS/cm ).

3.1.5

Ionic Removal Mechanism

Mechanism of ionic removal in EDI process is explained in two different regimes of operation, enhanced transfer and electroregeneration regime [34]. In the first operation regime, resin plays as a conducting media which reducing the stack resistance as well as increasing current efficiency. It is different from other regime, where water dissociation occurs in diluate compartments and the ion exchange resins are continuously regenerated by those phenomena.

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The 5th AUN/SEED-Net Regional Conference on Global Environment November 21st-22nd, 2012 In order to distinguish the two regimes in this process, a dimensionless coefficient Kt can be introduced in mass balance analysis [36] which is defined as the following equation:

Kt value is generally around unit (Kt =1) while a steady state of process achieved. Enhanced transfer regime is occur when Kt ≤ 1, while electroregeneration regime is occur when Kt > 1.

Kt

It can be observed at Fig. 11, using model solution at V = 5 volt that EDI process is operated under enhanced transfer regime with Kt value around 1. However, when the applied voltage rose to 15 V at Co = 15 μS/cm, the value of Kt is found to be larger than 1 along the operation, which indicated that the process is operated under electroregeneration regimes. This operation regime is also occured when the process operated using Co = 42 μS/cm (V = 15 Volt) within 20 minutes. 1,20 1,15 1,10 1,05 1,00 0,95 0,90 0,85 0,80

a

0

10

20

Steady State V = 5 volt V = 15 volt

30

Time (minutes)

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b

1,20

Steady State V = 5 volt V = 15 volt

1,15 1,10

Kt

1,05 1,00 0,95 0,90 0,85 0,80 0

20

40

60

Time (minutes) Figure 11 Variation of Kt value with time at different applied voltage ( a: Co = 15 μS/cm; b: Co = 42 μS/cm ).

3.2

Prefiltered Natural Peat Water

The performance of EDI process during humic acid removal from natural peat water is illustrated in Fig. 12 and Fig. 13. The process is conducted for 8 hours with prefiltered natural peat water. The final conductivity of diluate, concentrate and electrode stream are 6 μS/cm, 432 μS/cm, 1 μS/cm respectively (Fig. 12). It is observed that the high ion transfer is obtained within first 50 minutes and significantly decreased over time. During 20 minutes operation, the rate of ion transfer in diluate stream is higher than in electrode, while on the next period of operation, solution conductivity in electrode compartment was depleted faster and resulting in lower conductivity value than in diluate compartment. In the other hand, the reduction of absorption value are occur in all compartment. The highest reduction is obtained in electrode stream, while the lowest reduction is achieved in the diluate stream (Fig. 13).

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Conductivity (µs)

500

a

400 300

Cc

200

Ce

100 0 0

50

100

150

200

250

300

Time (Minutes)

Conductivity (µs)

60

b

50

Ce Cd

40 30 20 10 0 0

50

100

150

200

250

300

Times (Minutes) Figure 12 Variation conductivity with time. (Applied voltage = 15 Volt; Co = 55 μS/cm; a: Conductivity of diluate, concentrate and electrode compartment; b: comparison of diluate and electrode compartment).

In EDI system, ion-exchange resin acts as conducting medium, which enhances the transport of ionic components from bulk solution toward the ion-exchange membrans under the force of a direct current. Those ability is much more important than the ion exchange capacity of the resin [11]. Meanwhile, anionexchage resin has ability to adsorb negatively charge species, in this case the humic substances [37]. These substances can adsorbed within the anion beads, reducing their exchange capacity and leading to an increasing in total resistance of EDI stack (Fig. 14). As well as in anion-exchange resin, anion-exchange membrane has high fouling potential with humate substances [38]. Negatively charged organic matter move toward anion-exchange membranes under an electric field and deposit on the surface of membranes due to electrical

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The 5th AUN/SEED-Net Regional Conference on Global Environment November 21st-22nd, 2012 interactions between the membrane surface and humic substances (Fig. 15). Thus, those phenomena could increased the total electric resistance of membranes stack and definitely decreased the ionic diffusion accros the membranes which causing deterioration in the membrane performance. Reduction of absorption Value (%)

100 80 60 40 20 0 Diluate Figure 13

Figure 14

Concentrate

Electrode

Reduction of absorption value

Fouled Anion-Exchange Resins

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The 5th AUN/SEED-Net Regional Conference on Global Environment November 21st-22nd, 2012 Figure 15

Fouled Anion-Exchange Membranes

Mechanism of ionic removal was investigated as well. Kt value plot was presented in Fig. 16. It can be seen that the value increased drastically during initial 50 minutes and gradually approached to relatively constant during the following operation time. When prefiltered natural peat water was used as feed solution, the Kt value exceeded unit all along the operation which suggested that the EDI process was operated under electroregeneration regime. 2,6 2,4 2,2 2,0

Kt

1,8

Steady State

1,6

Peat Water

1,4 1,2

1,0 0,8 0

50

100

150

200

250

300

Time (minutes) Figure 16 Variation of Kt value with time ( V: 15 volt; Feed: natural peat water, Co = 55 μS/cm ).

4

Conclusion

In this paper, a lab scale EDI system was studied for humic acid removal from peat water. The main factors, including applied voltage, feed water conductivity and interaction of both factors were investigated. An equilibrium coefficient Kt was also introduced to estimate the operation regime of EDI process. Applied voltage was found to have significant impact upon ionic removal percentage and current density. In other hand, feed conductivity had significant effect on all of the responses except on current density. Meanwhile, the interaction factor only had significant effect on current density. Experiment on prefiltered natural peat water showed that the EDI system operated under electroregeneration regimes. Ionic removal percentage more than 90 % could be obtain at batch operation mode. In addition, reduction of absorption value was about 99 %, 60 % and 70 % in electrode, diluate, and concentrate stream respectively.

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The 5th AUN/SEED-Net Regional Conference on Global Environment November 21st-22nd, 2012 Moreover, it is observed that absorbed humic substances within anion-exchange resins and anion-exchange membranes surface leading to a reduction in EDI performance. Therefore, it should be emphasized that fouling formation on the anion-exchange membrane surface and anion-exchange resins need to be controlled to achieve a better performance.

5 [1] [2]

[3]

[4] [5]

[6]

[7]

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The 5th AUN/SEED-Net Regional Conference on Global Environment November 21st-22nd, 2012 [36] Fu, L., Wang, J., Su, Y., 2009. Removal of low concentrations of hardness ions from aqueous solutions using electrodeionization process. Separation and Purification Technology 68, 390–396. [37] Brattebo, H., Odegaard, H., Halle, O., 1987. Ion Exchange for The Removal of Humic Acids In Water Treatment. Wat. Res. Vol. 21, No. 9, pp. 1045-105 [38] Lee, H. J., Choi, J. H., Cho, J., Moon, S. H., 2002. Characterization of anion exchange membranes fouled with humate during electrodialysis. Journal of Membrane Science 203, 115–126.

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