assessing of phosphorus removal by polymeric anion exchangers
TRANSCRIPT
Desalination 281 (2011) 111–117
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Desalination
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Assessing of phosphorus removal by polymeric anion exchangers
Md. Rabiul Awual ⁎, Akinori Jyo ⁎⁎
Department of Applied Chemistry and Biochemistry, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555, Japan
⁎ Correspondence to: R. Awual, National Institute forMaterials Research Laboratory for Energy and EnvironIbaraki-ken 305-0047, Japan. Tel.: +81 96 342 3871; fa⁎⁎ Corresponding author. Tel.: +81 96 342 3871; fax
E-mail addresses:[email protected], [email protected] (A. Jyo).
0011-9164/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.desal.2011.07.047
a b s t r a c t
a r t i c l e i n f oArticle history:Received 28 June 2011Received in revised form 21 July 2011Accepted 22 July 2011Available online 17 August 2011
Keywords:Anion-exchange resinsPhosphorus removalEutrophicationCompeting ionsRapid removal
Assessing of phosphorous removal by the polymeric anion exchange resins was studied in this paper, aimingat the determination of effects of pH, competing ions, concentration and flow rates. Batch study was carriedout of both weak-base and strong-base resins to determine the selectivity to phosphorus in presence of Cl–,NO3
– and SO42–. Batch studies clarified that weak-base resin preferred phosphorus to competing anions and
equilibrium adsorption capacity was from 4.95 to 1.39 mmol/g. Strong-base resin was strongly affected bycompeting anions and equilibrium adsorption capacity as low as 0.39 mmol/g. Column studies also performedof fixed-bed experiments using both resins packed column. The breakthrough capacities of weak-base resinswere not so strongly affected by flow rates from 50 to 150 h−1 and the breakthrough capacities were from0.41 to 0.20 mmol/g indicating high kinetic performances. Trace level of phosphorus was removed by weak-base resins from feed containing of 0.026 mM phosphorus at high flow rate of 350 h−1. Adsorbed phosphoruswas quantitatively desorbed with 1 M HCl acid and regenerated into hydrochloride form simultaneously fornext adsorption operation. Therefore, weak-base resin is to be an effective means to treat wastewater toprevent eutrophication in the receiving water bodies.
Materials Science, Exploratoryment, 1-2-1 Sengen, Tsukuba,x: +81 96 342 3679.: +81 96 342 [email protected] (M.R. Awual),
l rights reserved.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Phosphorus is a macronutrient constituent of most biologicaltissues and basic materials of agriculture, fertilizer, chemical andmetal-plating industries but one of the major concerns in environ-mental chemistry [1–3]. The municipal wastewater in many countriesis treated biologically and the secondary effluents still content most ofthe nutrients such as nitrate, ammonium and phosphate [4]. However,natural aquatic environment and water quality are affected due to thepresence of high concentration of phosphorus. It is well known thattrace amount of phosphorus (P) in lakes, bays, coastal areas, andinland seas are also liable for eutrophication problem [5]. Then thephosphate concentration in water is regulated and the maximumpermissible limit is to below 10 μg/L of P to escape from eutrophica-tion problems [6–8].
Purification and separation processes using an ion exchange resinsare enormously used in industry, especially in desalination of seawater,deionization of water, and production of pure water for the electronicand microelectronic industries. Recently the usage of different types ofion exchange adsorbents are expanding as a possible process to reducetoxic anions such as arsenic and phosphate concentration in water and
wastewater based on operational simplicity and adaptability tochanging water and wastewater feed flow rates and compositions[4,6,9–12] There are various physical methods available for removal ofphosphate from polluted wastewaters and these are mainly carried outeither by biologically or chemically [13–17]. Therefore, phosphateremoval from water and wastewater has been achieved by severalmethods such as coagulation, chemical precipitation, biological treat-ment and adsorption by selective adsorbents including metals loadedligand exchange adsorbents based on cation exchange and chelatingresins [11,18–25]. Granular and ligand exchange adsorbents such asactivated alumina, zirconium oxide and iron oxide particles are wellknown inorganic adsorbents to take up phosphate [20,22,23,26].However, these are lack of mechanical strength, unable to take upphosphate efficiently in column adsorption more than flow rate of50 h−1 in space velocity, unable to retaining functionality and uptakecapability in terms of adsorption activity after multiple regenerationscycles of adsorbents from the stand point of long-termuses [6,15,12,26].
Recently, aliphatic crosslinked polymer matrices having primaryamino group resins and fibers have been reported for selectiveremoval of arsenic and phosphate at high kinetic performances[9,11,12,15]. In this work, we have studied the phosphate selectivityand adsorption capacity by weak-base Diaon WA20 and strong-baseDiaion SA10A resins through batch and column approaches. Most ofthe studies have been focused on various individual aspects on the ionexchange performances. Ion exchange operations are often carriedout in column method. Therefore, breakthrough curves experimentsfor the adsorption of phosphate on weak-base and strong-base resinsin packed bed column were investigated to evaluate an efficient
112 M.R. Awual, A. Jyo / Desalination 281 (2011) 111–117
phosphate removal from various phosphate containing aqueoussolutions. Moreover, column adsorption also studied in differentparameters such as feed flow rate, competing anions compositions,initial phosphate concentration and trace influent concentration tomaximize the performance and efficiency of the column for selectivephosphate removal by these ion exchange resins.
2. Materials and methods
2.1. Materials
All materials and chemicals were of analytical grade and used aspurchased without further purification. Two types of resins were usedin this study. One was weak-base anion exchange resins namedDiaion WA20 and the other was strong-base anion exchange resinsnamed Diaion SA10A. The properties of these resins are shown inTable 1. Both resins have same matrix of styrene-divinylbenzene.Diaion WA20 and Diaion SA10A were produced by MitsubishiChemical Corporation, Tokyo, Japan. Phosphoric acid (85 wt%),NaH2PO4 · 2H2O and Na2HPO4 · 12H2O were guaranteed grade andwere obtained fromWako Pure Chemical Industries Ltd., Osaka, Japan.The phosphorus standard solutions were also purchased from WakoPure Chemical Industries Ltd., Osaka, Japan. Ultra-pure waterprepared by a Milli-Q Academic-A10 (Nippon Millipore Co., Tokyo,Japan) was used in all experimental work.
2.2. Batch studies
The weak-base anion exchange resin (Diaion WA20) was used asfree amine form. Diaion WA20 was treated with 1 M sodiumhydroxide to convert into free amine form and washed with waterin column method. After air-drying, the resin was dried in vacuum at55 °C. Therefore, Diaion WA20 was ready to use for batch study as infree amine form. Strong-base anion exchange resin (Diaion SA10A)was conditioned by a conventional method, and it was used aschloride form. A series of phosphorus (P) test solutions with differentpH values were prepared by mixing of 0.010 M H3PO4, 0.010 MNaH2PO4, and 0.010 M Na2HPO4. The initial analytical concentrationof P in the test solutions was 0.010 M. Each test solution (50 mL) wastaken into a series of Erlenmeyer flasks (100 mL) and then DiaionWA20 in free amine form or Diaion SA10A in chloride form (0.050 g ofeach) was added to each flask. All flasks were shaken in atemperature-controlled water bath with a mechanical shaker at30 °C for 24 h at a constant agitation speed of 95 rpm to achieveequilibrium phosphate adsorption capacities of Diaion WA20 andDiaion SA10A. The amount of equilibrium phosphate adsorption by
Table 1Properties of polymeric anion exchange resins.
Resins Diaion WA20
Characteristic Weak-base anion exchang
Structure CH CH
CH
CH2
CH2NH(CH2C
Functional group Primary and secondary amManufacturer Mitsubishi Chemical CorpNitrogen content (mmol/g) 8.34 (in free amine form)Capacity (meq/g) (in dry basis) 6.4
Diaion WA20 and Diaion SA10A was calculated as follows:
Q e = Ci−Cf
� �V =m ð1Þ
where Qe is the equilibrium phosphate adsorption (mmol/g), V is thesolution volume (L), and m is the mass of anion exchange adsorbentsof Diaion WA20 or Diaion SA10A (g), Ci and Cf are the initial andequilibrium phosphate concentrations in the solution, respectively.The pH and phosphate concentration of each supernatant at theequilibrium were measured. The P concentrations were determinedby inductively coupled plasma atomic emission spectrometry (ICP-AES, IRIS, Nippon Jarrell Ash Co. Ltd., Kyoto, Japan).
2.3. Preparation of ion exchange resins for column studies
The free amine form of Diaion WA20 (0.85 g) was placed in apolyethylene column (inner diameter 1.3 cm) and then 1 M HCl(100 mL) was fed into the column at a flow rate of 4 mL/h to convertthe resin into hydrochloride form. The volume of the Diaion WA20resin bed was 2.0 mL following equilibration with HCl, which wasused as the reference volume to convert the flow rate in mL/h into thespace velocity (SV) in h−1 and volumes of supplied solutions or waterto the column in mL into bed volumes (BV) in mL/mL-resin. In thiswork, all flow rates were measured in space velocity. Then, thecolumnwas washedwith water (100 mL) at a flow rate 5 h−1 in SV toremove free hydrochloric acid and the adsorption operation wasstarted. Similarly, Diaion SA10A (0.95 g) was also placed in polyeth-ylene column and converted into chloride form by supplying 2 M HClat flow rate of 3 h−1. However, the volume of the Diaion SA10A resinbed was also 2.0 mL, which was used as reference volume fordetermination of other experimental conditions in column study.Flow rates of all solutions were expressed by SV, which is designatedby the ratio F/Vbed in h−1. Here F is the flow rate of a solution or waterin mL/h and Vbed is the reference volume of the resin bed in mL. Allsolutions volumes supplied to the columns were expressed by BV,which is designated by the ratio, Vsupplied/Vbed; here Vsupplied is thevolume of a feed supplied to the column in mL.
2.4. Column study
Feed solutions containing phosphate were prepared by dissolvingNaH2PO4 · 2H2O in water and their final pH was adjusted with diluteHCl or NaOH as required. Feeds containing phosphate were alsosupplied to the column at a given flow rate. Prior to desorption andregeneration operations, the column was washed with water(100 mL) at a flow rate of 5 h−1. The adsorbed phosphate on Diaion
Diaion SA10A
er Strong-base anion exchanger
H2NH)nH
CH CH2 CH CH2
CH
H2C N
CH3
CH3
CH3 Cl
n
ines (polyamines) Quaternary ammoniumoration Mitsubishi Chemical Corporation
3.65 (in OH− form)3.4
0
1
2
3
4
5
2 3 4 5 6 7 8
Control
Chloride
Nitrate
Sulfate
Pho
spha
te u
ptak
e (m
mol
/g)
Equilibrium pH
0
0.5
1
1.5
2
2 3 4 5 6 7 8 9
Control
Chloride
Nitrate
Sulfate
Pho
spha
te u
ptak
e (m
mol
/g)
Equilibrium pH
(a)
(b)
Fig. 1. Equilibrium phosphate adsorption as a function of equilibrium pH in thepresence or absence of competing anions. Exchangers: (a) Diaion WA20 and (b) DiaionSA10A (50 mg in dry state); solutions: 0.010 M phosphate solution of various pH(50 mL) without and with a competing anion; equilibration: 30 °C for 24 h; competinganions: 0.010 M of Cl−, NO3
− and SO42−.
113M.R. Awual, A. Jyo / Desalination 281 (2011) 111–117
WA20 and Diaion SA10A resin column was desorbed with 1 M HCl(100 mL) and 2 M HCl (100 mL) at a flow rate of 5 h−1, respectively.During desorption operation with HCl acid, the column wassimultaneously regenerated into hydrochloride form and used fornext phosphate adsorption operation after rinsing with water. Thebreakthrough point (beyond the feed concentration of 1.0 mg/L of P)was designated as the feed volume supplied to the column up toC/C0=0.01. Here, C0 and C represent concentrations of phosphate infeeds and column effluents, respectively. In the case of feed solutionconcentration below 1.0 mg/L of P, the breakthrough point wasdefined by the feed volume up to C of P=0.01. The total phosphateadsorption (TPA) in the adsorption operations was calculated fromthe following equation:
TPA mmol=gð Þ = CdVd− ∑n
j=1CjVj
!=m ð2Þ
where Cd, Vd andm are the concentrations of P in the feed, the volumeof the supplied feed, and amount of loaded resins, respectively; also Cj,Vj, and n are the concentration of P and the volume of the j-th fraction,and the number of the last fraction in the adsorption operation.Breakthrough capacity (BC) of exchangers for P was calculated fromthe following relation:
BCðmmol=gÞ = CfVp− ∑n1
i=1CiVi
!=m ð3Þ
Here, Vp is the volume of the feed up to the breakthrough point andn1 is number of the last fraction up to the breakthrough point. Thedesorbed amount of phosphate (EAP) was calculated from Eq. (3)
EAPðmmol=gÞ = 1=mð Þ ∑n2
q=1CqVqÞ ð4Þ
Here, Cq, Vq and n2 are the concentration of P and volume of the q-thfraction, and number of the last fraction in the desorptionoperations. Allcolumn effluents were collected on a fraction collector and Pconcentration in each fraction was determined by ICP-AES.
3. Results and discussion
3.1. Batch studies
3.1.1. Effect of pHThe acid dissociation constants in aqueous solution of phosphoric
acid are pKa1=2.16, pKa2=7.21, pKa3=12.32 [27]. In batch adsorp-tion, free amine form of Diaion WA20 was contacted with phosphatesolution atpH from2.25 to 6.94. Total equilibriumphosphate adsorptioncapacities decreased with an increase in pH as judged from Fig. 1(a).Adsorption of phosphate byweak-base anion exchanger decreaseswithincrease in solution pH due to the number of protonated sites decreaseswith increase in pH [12,15]. Hydrogen ions transfer was possible fromparent acid (H3PO4) and monovalent ions (H2PO4
−) to R-NH2 for takingup dominant species (H2PO4
− and HPO42−) according to following
reaction. Here, the symbol R-NH2 does not representmonoamines but itrepresents polyamine (primary and secondary amine).
R �NH2 þ H3PO4 ⇌ R �NHþ3 ·H2PO
�4 ð5Þ
R � NH2þ Naþþ H2PO�4 ⇌ R �NHþ
3 ·HPO2�4 ·Naþ ð6Þ
The equilibriumpHof test solutions always increases from the initialpHof test solutions because of proton transfer to resin phase. The Eq. (5)will occurwhen the equilibriumpHwas below4.7 (=(pKa1+pKa2)/2)as judged from dissociation constant values. Similarly, the reaction (6)
will be dominant when the equilibrium pH above 4.7. Fig. 1(b) alsoshows the adsorption of phosphate by Diaion SA10A in the chlorideform as a function of equilibrium pH of test solutions. The phosphateadsorption was slightly increased with increase in pH up to ca. 7.0.However, further increase in pH, the total adsorption was decreased alittle. Therefore, phosphate adsorption by Diaion SA10A was notmarkedly dependent on pH of test solutions because anion exchangecapacity of strong-base anion exchange resins is essentially indepen-dent of test solutions pH. In addition, equilibrium pH did not somarkedly differ from initial ones, since no acid–base reaction occurs inphosphate adsorption by the strong-base resin in chloride form.Moreover, the total phosphate adsorption capacities by Diaion WA20are much higher than Diaion SA10A. Phosphate adsorption is favored inlow pH area. Adsorption capacity is higher at low pH region than at highpH area by iron oxide adsorbents and ligand exchange adsorbents[13,15,18,23,25]. In addition, we have also determined the phosphateadsorption of DiaionWA20 at three different temperatures, namely 10,20 and 30 °C under identical experimental conditions. However,phosphate adsorption was not affected significantly. Similar behavioris observed inhybrid ion exchange adsorbent for phosphate removal [6].
0
0.2
0.4
0.6
0.8
1
0 80 160 240 320 400 480 560 640
50 h-1
100 h-1
200 h-1
C/C
0
0
0.2
0.4
0.6
0.8
1
C/C
0
Feed volume (BV)
0 80 160 240 320 400 480 560 640
10 h-1
50 h-1
Feed volume (BV)
(a)
(b)
Fig. 2. Phosphate adsorption under different feed flow rates by (a) Diaion WA20 and(b) Diaion SA10A. The numerical results and conditions refer to entry nos. 1–5 inTable 2.
114 M.R. Awual, A. Jyo / Desalination 281 (2011) 111–117
3.1.2. Effect of competing anionsThemain common anions dissolved in freshwater and river waters
are chloride, nitrate and sulfate. Phosphate adsorption may competein the presence of these anions. Moreover, sulfate possesses higherionic charges and would offer greater competition through enhancedelectrostatic interaction in phosphate adsorption by polymeric ionexchangers. Then, phosphate adsorption was clarified in the presenceof these anions by Diaion WA20 and Diaion SA10A. Fig. 1(a) and (b)shows the phosphate adsorption capacity as a function of equilibriumpH in the presence of these competing anions by Diaion WA20 andDiaion SA10A, respectively. The phosphate adsorption by DiaionWA20 was not strongly affected (Fig. 1(a)) in the presence ofmonovalent competing anions of chloride and nitrate when theconcentration was in equimolar level. At pH 6.55, phosphateadsorption capacity in absence of interfering anions was 1.98 mmol/g.On the other hand, in the presence of chloride and nitrate, thephosphate adsorption at pH 6.56 was 1.96 and 1.51 mmol/g,respectively. Conventional anion exchange resins prefer less hydratedanions to highly hydrated ones and cannot take up highly hydratedanions in the presence of less hydrated anions and exhibitedHofmeister anion series [28,29]. Here, Diaion WA20 prefers phosphateto these univalent anions, indicating the non-Hofmeister anionselectivity sequence. On other the hand, phosphate adsorption byDiaion SA10A was markedly affected by chloride and nitrate and itwas ca. 0.32 mmol/g in acidic pH region below at 3.0. Phosphateadsorption by both resins was strongly affected in the presence ofsulfate according to Helfferich's electroselectivity concept [30].Adsorption capacity by Diaion WA20 was markedly depressed inthe presence of equimolar sulfate below pH 5.0 where the monovalentphosphate species (H2PO4
−) was dominant. Above pH 5.0, thedominant species of divalent phosphate species (HPO4
2−) increasesand phosphate adsorption also increase. In the presence of sulfate,phosphate adsorption by Diaion WA20 was 0.903 mmol/g at pH 6.60.This is half of the total phosphate adsorption compared with controlphosphate adsorption by Diaion WA20 as judged from Fig. 1(a). Inaddition, the maximum phosphate adsorption by Diaion WA20 wasobserved between pH 5.0 and 7.5 in the presence of sulfate. On thecontrary, the maximum phosphate adsorption by Diaion SA10A wasonly 0.39 mmol/g. The anion selectivity of weak-base resin iscompletely different from that of strong-base resin [12] becausedivalent phosphate (HPO4
2) is able to form a hydrogen bond with theprotonated site (R-NH3
+) of weak-base Diaion WA20 resin asindicated from the studies on “anion coordination chemistry” [31].It is demonstrated that the polymeric ligand exchanger are moreeffective for phosphate removal than the strong-base anion exchangersuch as IRA 900 and 958 [4,8]. Similarly, metal loaded ligand exchangeadsorbents including hybrid anion exchange adsorbent has nopractical effect in the presence of high concentration of competinganions [6,23] but their kinetic performances are not so high.
3.2. Column studies
3.2.1. Effect of feed flow rateIn column studies, both resins were used hydrochloride form in
phosphate adsorption operations. Most lake and river waters pH arearound 7.0. Then phosphate adsorption in column approach wascarried out at neutral pH area. In kinetic performance of phosphateadsorption by Diaion WA20 and Diaion SA10A, flow rate effect wasinvestigated at different feed flow rates. Breakthrough curves ofphosphate by Diaion WA20 are shown in Fig. 2(a) for three differentfeed flow rates. The numerical data and experimental conditions areshown in Table 2 (entry nos. 1–3). Breakthrough capacities ofphosphate gradually decreased with an increase in feed flow rate.Increasing flow rate from 50 to 150 h−1, the breakthrough capacitiesdecreased from 0.411 to 0.203 mmol/g. In other words, increasing theflow rate 3 times, breakthrough capacities decreased more than 50%.
However, the flow rate of 150 h−1 is much higher than metal loadedligand adsorbents [26] and Diaion WA20 is able to take up phosphateefficiently. On the other hand, feed flow rate effect was alsodetermined by Diaion SA10A packed column. Fig. 2(b) shows thebreakthrough profiles of phosphate adsorption by Diaion SA10A anddata are summarized in Table 2 (entry nos. 4 and 5). Phosphateadsorption was strongly affected by Diaion SA10A when the flow rateincreased from 10 to 50 h−1, indicating low kinetic performances.Thus, weak-base resin can effectively take up phosphate in wide flowrate ranges from 50 to 150 h−1 different from strong-base resins andligand exchange type adsorbents [6,11].
3.2.2. Effect of concentrationThe breakthrough point is not only dependent of feed flow rates
but also initial concentration of a target ion. Therefore, breakthroughcapacity was determined for weak-base resin of Diaion WA20 usingfeed solutions containing phosphate from 0.087 to 0.366 mM at pH ca.7.0. Here, the flow rate of feeds was at 100 h−1. The numerical dataand results are summarized in Table 2 (entry nos. 2, 6 and 7). Fig. 3shows breakthrough capacities and points as a function of feedsolution concentration (C0). Decreasing in feed solution concentra-tion, the breakthrough point also increased but the breakthroughcapacity decreased slightly. Higher initial phosphate concentrationscaused a faster breakthrough points as expected. Similar tendency
Table 2Phosphate adsorption by weak-base and strong-base resins under different feed flow rates and concentrations.
Entryno.
Feed Breakthroughpoint (BV)
Breakthroughcapacity(mmol/g)
Phosphateadsorbed(mmol/g)
Phosphatedesorbed(mmol/g)
Recovery(%)
Flow rate (h−1 in SV) C0 of phosphate (mM) Volume (BV)
1 50 0.365 600 479 0.411 0.539 0.547 1012 100 0.366 600 328 0.283 0.485 0.479 98.73 150 0.363 600 238 0.203 0.361 0.368 1024 10 0.369 600 329 0.231 0.429 0.437 1025 50 0.368 600 57 0.044 0.156 0.149 95.56 100 0.185 1220 582 0.253 0.447 0451 1017 100 0.087 2450 1065 0.218 0.397 0.388 97.78 50 0.026 5000 3080 0.187 0.398 0.393 98.79 350 0.026 5000 3020 0.183 0.376 0.381 101
Wet resins bed: 2.0 mL, feed solution pH 7.00±0.10.
115M.R. Awual, A. Jyo / Desalination 281 (2011) 111–117
was observed in the effect of initial concentration for phosphateadsorption by a weak anion exchange fiber having primary aminogroup [12,32]. However, 1% breakthrough capacities for phosphateadsorption at feed concentrations of 0.366 and 0.087 mM were 0.283and 0.218 mmol/g, respectively.
3.2.3. Effect of competing anionsIn batch studies, monovalent chloride and nitrate was not strongly
interfered with phosphate adsorption by Diaion WA20. Therefore,phosphate adsorption by weak-base resin was clarified in thepresence of chloride and sulfate as competing anions at equimolarin column method. Breakthrough curves of phosphate show in Fig. 4and numerical data and experimental conditions are summarized inTable 3. Phosphate adsorption was not strongly affected in thepresence of chloride and sulfate; breakthrough capacities forphosphate in the presence of equimolar chloride and sulfate were0.256 and 0.181 mmol/g, respectively. However, the total phosphateadsorptions by these anions were 0.457 and 0.402 mmol/g in thepresence of chloride and sulfate, respectively (entry nos. 10 and 11 inTable 3) and these values were much close to phosphate adsorptionin the absence of competing anions (0.485 mmol/g, entry no. 2 inTable 3). Therefore, Diaion WA20 prefers phosphate to chloride orsulfate, different from phosphate adsorption in the presence ofcompeting anions by strong-base anion exchange resins [6,12,15].
3.2.4. Rapid removal of phosphateThe behaviour of DiaionWA20 in a column was investigated using
feeds containing trace concentration level of phosphate at relatively
0.1 0.2 0.3 0.4
400
600
800
1000
1200
BP
BC
Phosphate concentration (mM)
BP
(m
L/m
L-r
esin
)
0.16
0.24
0.32
0.40
BC
(mm
ol/g)
Fig. 3. Phosphate adsorption by Diaion WA20 under different initial phosphate feedconcentration. For detailed conditions and results refer to entry nos. 2, 6 and 7 inTable 2. BC and BP mean breakthrough capacity and breakthrough point, respectively.
high feed flow rate. It is desire the high feed flow rate to purify hugevolume of low concentration level phosphate from water below at1.0 mg/L of P. Then phosphate removal was examined using a solutioncontaining P as low as 0.81 mg/L. Breakthrough curves of phosphateadsorption at high feed flow rate (350 h−1) in neutral pH region areshown in Fig. 5(a). The results and conditions are summarized inTable 2 (entry nos. 8 and 9). In this adsorption operation, the observedbreakthrough points and capacities are 3050 BV and 0.185 mmol/g,respectively. The data revealed that 6.1 L of water containingphosphorus at 0.81 mg/L was purified to less than 10 μg/L by 0.85 gpacked weak-base Diaion WA20 resin, although the test solutioncontained phosphate only.
3.2.5. Desorption and regenerationAdsorbed phosphate on Diaion WA20 and Disaon SA10A was
quantitatively desorbed with 1 M and 2 M HCl, respectively at a flowrate of 5 h−1. Fig. 5(b) shows an example of desorption of adsorbedphosphate on Diaion WA20. During desorption operation, the weak-base resin was simultaneously regenerated into hydrochloride formfor the next adsorption operation after rinsing with water to removefree hydrochloric acid. Therefore, Diaion WA20 was able to use forphosphate removal from aqueous solutions repeatedly. These resultssuggest that the phosphate-loaded material can be easily desorbedusing hydrochloric acid and has the potential to be re-used as a goodadsorbent. All results in the column study were obtained using thesame columnwithout replacement of resins packed column. Thus, thiswork suggests that weak-base Diaion WA20 resin is a promisingadsorbent for phosphate removal from water.
0.1
0.3
0.5
0.7
0.9
0 80 160 240 320 400 480 560 640
Control
0.4 mM Cl-
0.4 mM SO42-
C/C
0
Feed volume (BV)
Fig. 4. Phosphate adsorption in the presence of Cl− and SO42− by Diaion WA20. The
numerical results refer to entry nos. 2, 10 and 11 in Table 3.
Table 3Phosphate adsorption by weak-base resin in presence of competing anions.
Entryno.
Feed Breakthroughpoint (BV)
Breakthroughcapacity(mmol/g)
Phosphateadsorbed(mmol/g)
Phosphatedesorbed(mmol/g)
Recovery(%)
C0 of phosphate (mM) NaCl (mM) Na2SO4 (mM)
2 0.366 328 0.283 0.485 0.479 98.710 0.365 0.4 298 0.256 0.457 0.461 10111 0.369 0.4 209 0.181 0.402 0.410 102
Wet resin bed: 2.0 mL, flow rate of feed: 100 h−1, feed solution pH 7.00±0.07, volume of feed: 600 BV.
116 M.R. Awual, A. Jyo / Desalination 281 (2011) 111–117
4. Conclusions
Selectively phosphate removal was studied by both weak-base(Diaion WA20) and strong-base (Diaon SA10A) resins. The specificanion selectivity was investigated in batch studies by both resins.Batch study clarified that Diaion WA20 prefers phosphate in thepresence of competing anions such as chloride, nitrate and sulfate.However, Diaion SA10A prefers sulfate, nitrate and chloride tophosphate. The solution pH played an important role in phosphateadsorption operation. Higher pH can cause lower phosphate adsorp-tion by Diaion WA20 and the equilibrium capacity also decreased
0
0.05
0.1
0.15
0.2
0.25
0.3
0 1000 2000 3000 4000 5000
Run 1
Run 2
C/C
0
Feed volume (BV)
0
5
10
15
20
25
30
35
0 10 20 30 40 50
P c
once
ntra
tion
(m
M)
Eluent volume (BV)
(a)
(b)
Fig. 5. (a) Phosphate adsorption by Diaion WA20 from feeds containing trace levels ofphosphate at a feed flow rate of 350 h−1. The numerical data refers to entry nos. 8 and 9in Table 2. (b) Desorption of adsorbed phosphate on Diaion WA20 with 1 M HCl at aflow rate of 5 h−1. For detailed adsorption condition refer to entry no. 9 in Table 2.
from 4.95 to 1.39 mmol/g when pH was increased from 2.25 to 7.6,respectively. Moreover, equilibrium capacity of Diaion WA20 forphosphate is much higher than that of the strong-base resin of DiaionSA10A. Fixed-bed experiments were performed and the performancewas dependent on the feed flow rate. The weak-base resin was able totake up phosphate at high feed flow rate (150 h−1) but strong-baseresin was strongly affected even the flow rate at 50 h−1. Therefore,Diaion WA20 showed high kinetic performances opposed to DiaionSA10A. In addition, increasing the initial phosphorus concentrationbreakthrough capacities also increased. Trace level of phosphoruswas also removed by Diaion WA20 resins from feed containingof 0.026 mM phosphorus at high flow rate of 350 h−1 and thebreakthrough points and capacities were 3050 BV and 0.185 mmol/g,respectively. The adsorbed phosphate on Diaion WA20 was quanti-tatively desorbedwith 1 MHCl and the DiaionWA20was regeneratedinto hydrochloride form simultaneously for the next adsorptionoperation after rinsing with water. Therefore, weak-base anionexchange resin is able to use repeatedly for phosphate removalfrom water for long time.
Acknowledgments
The authors wish to thank the anonymous reviewers and theeditor for their helpful suggestions and enlightening comments toimprove the paper.
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