Effect of ion-exchange resin structure on nitrate selectivity

Reactive Polymers, 12 (1990) 277-290 Elsevier Science Publishers B.V., Amsterdam

277

EFFECT OF I O N - E X C H A N G E RESIN S T R U C T U R E ON NITRATE SELECTIVITY M.B. JACKSON and B.A. BOLTO

CSIRO, Division of Chemicals and Polymers, Private Bag 10, Clayton, Victoria 3168 (Australia) (Received July 31, 1989; accepted in revised form January 9, 1990)

The nitrate/sulfate selectivity of a range of anion-exchange resins with backbones consisting of crosslinked polystyrene, polyepichlorohydrin or polydiallylamine derivatives and containing different nitrogen substituents is discussed. It is shown that the preference of an anion-exchange resin for nitrate over sulfate increases as the resin becomes more hydrophobic (less polar). It appears not to matter whether the increase in the hydrophobic character is caused by the nature of the polymer backbone or the nature of the nitrogen substituents. The most nitrate-selective resins found were those with a polystyrene backbone and large trialkyl substituents on the amino-nitrogen atom. Nitrate-selective resins are more difficult to regenerate with sodium chloride solution than sulfate-selective resins. Experiments designed to overcome the difficulty of regeneration of nitrate-selective resins by regeneration at higher temperatures and also regeneration of weak-base resins using stoichiometric amounts of sodium hydroxide solution are described.

INTRODUCTION

The consequences of high nitrate levels in drinking water have been discussed widely [1-7]. Ion exchange is considered the most feasible method for the full-scale treatment of drinking water [8]. However, when the work reported in this paper was commenced all commercially available ion-exchange resins which could be used to adsorb nitrate had a high preference for sulfate over nitrate so that if the water contains sulfate, as it usually does, sulfate is preferentially adsorbed. A considerable amount of work has been done 0923-1137/90/$03.50

with the aim of developing ion-exchange resins which prefer nitrate over sulfate [9-19]. Recently several nitrate-selective resins have become available: Imac HP555 [20], Purolite A-520E [21], Wofatit SN 35L and Wofatit SN 36L [22]. The resin Imac HP555 is being evaluated by several different groups [23-26]. The continuation of our earlier work [27] on the relationship between the structure of ion-exchange resins and their anion selectivity is described in this paper with the aim of developing nitrate-selective resins. It is shown that a variety of nitrate-selective resins can be prepared.

© 1990 Elsevier Science Publishers B.V.

279 from the UV absorption at 220 nm using an LKB Biochrom Ultrospec 4050 instrument. Sulfate was determined by either a titration procedure [36] or a turbidimetric method [37]. Chloride, nitrate and sulfate were also determined using an ion chromatograph consisting of a Bio-rad HPLC column heater, a Bio-rad HPLC pump, model 1330, and a Biorad conductivity monitor. A Hamilton PRPX100 (100 × 4.1 mm) column at 35°C was eluted with potassium hydrogen phthalate, 2 × 10 3 M, pH 4.3, at 2.0 ml/min.

Selectivity measurements (K values) Ksy values were determined by shaking a known amount of the air-dried resin in the nitrate form (0.6 to 0.7 meq) with 0.050 N sodium sulfate (50 ml) in a stoppered bottle for 2 to 4 days. After settling--by standing or centrifuging--an aliquot was diluted and analysed for nitrate and sulfate. K ~ values were determined by equilibrating 0.500 meq (in these cases the capacity of the resins was determined by eluting the nitrate form of the resin with 1 M NaC1 rather than 1 M Na2SO4) of air-dried resin in the nitrate form in each of 7 bottles with various volumes of 0.010 N NaNO 3 and 0.010 N NaCI with the total volume being 100 ml so that the chloride composition at equilibrium covered the range from Xcl = 0.1 to Xcl = 0.9. The solutions were shaken for 2 days, the resin allowed to settle and the solutions analysed for nitrate and chloride. Equilibrations at 80 °C were carried out by tumbling the bottles in an oven at 8 0 ° C overnight, allowing the resin to settle while the bottles were still at 80 ° C in the oven and then removing aliquots for analysis with a hypodermic syringe through the rubber caps.

the resin was transferred to a small sintered glass column of 1.0 cm diameter. A solution containing nitrate and sulfate was then fed to the resin using an LKB Microperpex 2132 peristaltic pump at the rate of 30 m l / h . The feed contained nitrate between 70 and 98 mg/1 and sulfate between 170 and 190 mg/1 so that S O 2 - / N O 3 > 2. Effluent from the column was collected as 15 ml fractions using an LKB Redirac 2112 fraction collector (30 min per fraction). The fractions were weighed and analysed for chloride, nitrate and sulfate using an ion chromatograph. Nitrate concentrations were also determined by UV spectroscopy. After the adsorption was complete, deionized water was passed through the resin at 4 m l / h for 5 hours and then the regenerant (5% NaC1) passed through at 4 m l / h with 2 ml fractions being collected, weighed and analysed. Regenerations at 70 ° C were achieved by surrounding the lower half of the column with a continuous flow of water from a water bath heated at 70 ° C. Regeneration of resins completely in nitrate form was carried out similarly, except that the resins were converted to their nitrate forms in separate columns and then transferred to the small column for regeneration. The ease of regeneration of the weak-base resins with N a O H was determined by measuring the capacity of the resin in chloride form. A sample of resin (1.0 ml) in nitrate form in a small column was then eluted with a given number of milliequivalents of 0.3 N NaOH at 4 B V / h and washed successively with water (1 ml), 0.5 M HC1 (6 ml) at 4 B V / h and pH 3 HC1 (4 ml). The capacity was then remeasured.

RESULTS AND DISCUSSION

Column behaviour--Adsorption and regeneration

Definition of selectivity

A sample of air-dried resin in the chloride form (1.0 ml) in sufficient water to just cover

The selectivity of an ion-exchange resin for one ion over another may be expressed in

280

several different ways [38]. For the system (1)

equation (4),

B+A~A+B

NSS -- log KsN - log C + 1

(1)

the separation factor, a A, is given by eqn. (2), and the molar selectivity coefficient, K A, is given by eqn. (3),

(x~' =

(2)

~A

K A = CAz"CBzA

(3)

? ZAcAZ.

D

where the overbar signifies the resin phase, X and X indicate equivalent ionic fractions in the resin and in the solution, respectively, of the counterions A and B, CA and CB refer to molar concentrations, and Z A and Z B to the charges on the ions. Guter [39] has defined a nitrate-selective resin as one which in column operation with c o m m o n ground waters retains nitrate as the last ion to break through when exchanging ions at anionic concentrations of 10 m e q / L . A quantitative measure of the nitrate/sulfate selectivity (NSS) can then be defined by

-('CH2CH 0 ")if--{" CH2 CHO I I ® CH2 CH2 NR 3 1

X

CIG

where C is the resin capacity in m e q / L . This definition takes into account the use of low solution concentrations which are most relevant for water purification processes and the resin capacity. If NSS > 0, the resin is nitrate selective. Since the preference of an ion exchange resin for the ion of higher valence increases with dilution of the solution [40], there will be some solution concentration below which any resin will prefer sulfate over nitrate. An alternative method of characterizing the nitrate/sulfate selectivity of resins is by the value of the solution concentration at which the cross-over occurs.

Effect of resin structure on Ksu values Figure 1 shows the idealized structures of the ion-exchange resins for which the KsN values are listed in Table 1. Table 1 shows that the PECH-based resins are all sulfateselective since the NSS values are all negative.

m ~ C Cl® Cl® + Me Me - I ' n N-Me

I

CH2 I

-'{ CH2CHO )y

( CH2CNO ~,n I

®

CH2NR 3

• POly (QHEXA/DADMAC)

CI® Resin

based on crosslinked PECH

~CI®

Cle

-{- cu2 CH -)~---{-CH2Ca -Fn

-.(.CH2CH .~x

PolyTAA.HOl (R= H)

n~

cI®

Me Me Poly O.HEXA

CH2NR3

Cte

Clo +

(4)

Resin based on crosslinkedd

polystyrene

Jr n

Fig. 1. Idealized structures of ion-exchange resins.

281 TABLE 1 N i t r a t e / s u l f a t e selectivity of a n i o n exchange resins Resin Resin structure 1 2 3 4 5

PECH PECH PECH PECH PECH

Me 2 n-Bu 2 n-Bu 3 -(CH2) 5-(CH2) 6-

Capacity K TM (meq/ml)

NSS

1.5 0.36 0.36 1.0 1.1

4.2 11 2.0 2.0 4.0

- 1.6 -0.5 -1.3 - 1.7 - 1.4

0.94 0.54 0.46 0.42 0.30

9.5 71 160 530 470

- 1.0 0.1 0.5 1.1 1.2

6 7 8 9 10

PTAA PMeTAA PDoMeTAA(22%) PDoMeTAA(57%) PDoMeTAA(83%)

11

0.39

12

- 0.5

13 14 15

DADMAC/ 20% Q H E X A DADMAC/ 30% Q H E X A Q H E X A (30% solids) Q H E X A (48% solids) Q H E X A (65% solids)

0.49 0.55 0.87 0.92

34 39 165 450

- 0.2 -0.2 0.3 0.7

16 17 18

PSty IRA93 M e 2 PSty I R A 4 0 0 M e 3 PSty I R A 9 0 0 M e 3

1.2 1.3 0.9

20 87 90

-0.8 - 0.2 0.0

19 20 21 22 23 24

Imac HP555 a Et 3 n-Pr 3 n-Bu 3 n-Bu 2 n-Am 3

1.0 0.66 0.62 0.42 1.1 0.32

3000 400 1700 3000 370 1600

1.5 0.8 1.4 1.9 0.5 1.7

12

a

Believed to c o n t a i n Et 3 substituents.

A comparison of the selectivity of PTAA and quaternized PTAA containing increasing amounts of the hydrophobic (non-polar) dodecyl group shows that the strong-base resin 7 has a higher preference for nitrate than the corresponding weak-base resin 6, and as the dodecyl content increases the preference for nitrate increases. Table 1 shows that the homopolymers of 1,6-bis( N, N, Ndiallylmethylammonium)hexane dichloride (QHEXA) are more selective for nitrate than the copolymers of QHEXA and diallyldimethylammonium chloride (DADMAC) and the selectivity increases with increasing de-

grees of crosslinking, as indicated by the increasing capacity per milliliter. The designation 65% solids for resin 15 means that the concentration of monomers before the polymerization was more than twice that used to make resin 13 and a more concentrated solution would be expected to give a more highly crosslinked resin. The copolymers (resins 11 and 12) have lower preferences for nitrate, with the preference being greater for the resin containing the greater amount of the crosslinking monomer, QHEXA. However, resins 11 and 12 are sulfate selective (NSS values negative) and therefore, if the usual statement that the preference for the preferred ion increases with increasing degrees of crosslinking is true, the values of K ~ for resins 11 and 12 would be expected to decrease with increased crosslinking [41,42]. In fact, the selectivity towards nitrate increases with increased crosslinking even if the resin actually prefers sulfate. Increasing the crosslinking would reduce the flexibility of the resin and thus reduce its ability to reorient to enable two nitrogen sites to satisfy the divalent sulfate ions [12]. Accordingly, the preference for nitrate relative to sulfate increases with an increase in the degree of crosslinking. Table 1 shows that for resins with a polystyrene backbone a strong-base resin has a higher preference for nitrate than the corresponding weak-base resin and that the preference for nitrate for nitrate increases as the nitrogen substituents increase in size from methyl to butyl. Figure 2 summarizes the relationship between the resin structure and the separation factor. NSS values are positive for all resins above the line and negative for all resins below the line. It shows that weak-base resins are generally less selective for nitrate than the corresponding strong-base resins and the selectivity for both types of resins toward nitrate increases as the resin becomes more hydrophobic (less polar). It seems not to matter whether the increase in the hydro-

282 Sty/strong bose Sty/weak bose PTAA (dodecyl 83%.Me) OHEXA PTAA (Me quot)

~]

I N0~ selective

3

$ SOCselective

Sty/N Me3 Sty/N Me2 DADMAC/OHEXA PECH/amines PTAA PECH/ Nn Bu] 0

' 10

2'0

30'

Fig. 2. Relationship between resin structure and nitrate/sulfate selectivity.

phobic character is the result of the nature of the polymer backbone or the nature of the nitrogen substituents.

haviour. The adsorption behaviour has been quantified using quantities A s and A N as defined in the footnotes to Table 2. When A S = AN, as is the case for IRA 900, nitrate and sulfate break through together. When A N > A s, as is the case for all other resins in this table, sulfate breaks through before nitrate. The regeneration efficiencies in equivalents of nitrate removed from the resin per equivalent of regenerent refers to regeneration of the resin in fully loaded nitrate form. The results show that all the nitrate-selective resins are more difficult to regenerate than sulfate-selective resins. In addition, regeneration with sodium sulfate is much less efficient than regeneration with sodium chloride.

10000 -~ SO~ -

Adsorption and regeneration behaviour of resins with a polystyrene backbone

2000 -~

2000 1000

500

Since the experiments under equilibrium conditions showed that resins with polystyrene backbones have the greatest preference for nitrate, the adsorption and regeneration behaviour in columns of such resins was examined. For the resin Amberlite IRA900 nitrate and sulfate break through together whereas for resin 22 and all other nitrateselective resins sulfate breaks through before nitrate. As noted previously [23], with Amberlite IRA900, there is a region where the product has nitrate levels greater than the feed. A comparison of Figs. 3 and 4 shows that it is more difficult to regenerate a resin with sodium chloride if it prefers nitrate over sulfate. Table 2 summarizes the adsorption and regeneration behaviour of some polystyrene-based resins. These results are for shallow beds intended for equilibrium study and are not representative of deep-bed be-

500

200"

100

T

100

~ so

L 2O

20

10

10

NO~ i

i

i

20 30 BVof 5"0% NOCI 10

BVof 5 ' 0 % NoC~

Fig. 3. Regeneration curves for Amberlite IRA900 with 5% NaC1 after being loaded using a flow rate of 30 BV/h and a feed containing 71 mg NO~-/L and 180 mg S O 2 - / L . Flow rate = 4 BV/h. Fig. 4. Regeneration curves for resin 22 with 5% NaC1 after being loaded using a flow rate of 30 BV/h and a feed containing 71 mg NO~-/L and 180 nag SO42-/L. Flow rate = 4 BV/h.

283 TABLE 2 Adsorption and regeneration behaviour under column conditions of polystyrene-based resins Resin

IRA900 HP555 20 21 22 23 25 26

a AS =

Adsorption a Regeneration Efficiency (from a solution (of resin in NO 3- form) containing N O 3- + SO42- )(eq NO 3 - / e q regenerant)

N-substituent

Me 3 b Et 3 nPr 3 nBu 3 nBu z isoBu 2 isoAm 3

AS

AN

with 5% NaCI

with 10% Na2SO 4 •lOH20

190 135 200 170 130 170 160 130

190 370 410 400 550 450 440 400

0.098 0.060 0.075 0.056 0.027 0.060 n n

0.048 0.014 0.025 0.014 0.009

number of bed volumes passed before output o f 5 0 4 2 capacity of resin in m e q / m L

=

n n n

input of NO 3

number of bed volumes passed before output of NO 3- = input of NO 3AN capacity of resin in m e q / m L b Believed to contain Et 3 substituents. n = not measured.

Possible solutions to the problem of regeneration Although a number of different resins can be made which prefer nitrate over sulfate, they are all difficult to regenerate. The use of large amounts of sodium chloride for regeneration poses serious disposal problems. Such problems have recently been discussed [2326,43,44]. The cost of regeneration is a major

factor in determining the running cost of a plant because not only is there a cost of providing chemicals but often of more significance is the cost of disposal [24]. Van der Hoek et al. [26,43] have made similar comments and proposed the use of biological denitrification for regeneration of the nitratel o a d e d resin. In the c o m b i n a t i o n of ion exchange and biological denitrification, N a H C O 3, a poor chemical regenerant, is also

TABLE 3 Effect of temperature on nitrate selectivity of anion-exchange resins with a polystyrene backbone Resin

IRA900 HP555 20 21 22

N-substituent

Me 3 Et 3 n-Pr 3 n-Bu 3

n = n o t measured.

K~

Kffl

20°C

80°C

20°C

80°C

Regeneration efficiency (of resin in NO 3- form) with 5% NaCI (eq NO 3 - / e q NaCI) 20°C

70°C

90 3000 400 1700 3000

20 300 n n 80

2.8 6.6 3.9 7.3 14.4

2.4 6.0 3.7 7.4 9.3

0.098 0.060 0.075 0.056 0.027

0.099 0.061 0.077 0.057 0.041

284

used [43]. Croll and Hayes [25] report that there are plans to evaluate the resin Imac HP555 at Anglian Water in pilot plant trials in the next two years. He says that the most efficient regenerant is sodium chloride and that further research is warranted because attempts to find alternatives which would allow liquor to be disposed to local agricultural land have not been successful. Ambrus et al. [23] found that sodium chloride was the most efficient regenerant for Imac HP555. They also used combination regeneration, brine followed by bicarbonate, or brine followed by bicarbonate and then sulfate. Such a regeneration scheme reduces the wide swings in the product composition. Table 3 shows that the KsN values for the sulfate-selective IRA900 resin and also for the nitrate-selective resins HP555 and resin 22 are much lower at 80 ° C than at 20 ° C. Kffl values show a similar although less marked trend, which suggests that it should be possible to regenerate the resins more efficiently at higher temperatures. However, from the regeneration efficiencies (Table 3) it can be seen that only resin 22 is more easily regenerated at 70 ° C. The differences in the KsN and Kc~ values for the latter resin at 2 0 ° C and 8 0 ° C are greater than for the other resins and therefore the temperature effect is apparent only for this resin during regeneration conditions; K ~ values relate to equilibrium conditions whereas in column regeneration equilibrium would not be estabfished. In a more realistic situation the resin would be loaded with a mixture of nitrate and sulfate and, since sodium sulfate is much less effective than sodium chloride for regeneration, regeneration would need to be with sodium chloride. Figure 5 shows the effect of regenerating resin 22 with 5% sodium chloride at 70 ° C after loading it under the conditions indicated. A comparison of Figs. 4 and 5 shows that resin 22 is more easily regenerated at 70 ° C than at 20 ° C. Since hydrophobic interactions increase

10000 5000

2000 1000 500

2OO 100

T_.,

so] 20

~No;

lO

,'0

2'o t'0

,'0

BVof 5 0 % NoCI

Fig. 5. Regeneration curves for resin 22 with 5% NaC1 at 7 0 ° C after being loaded using a flow rate of 30 B V / h and a feel containing 71 mg NO3-/L and 180 mg S O 2 - / L . Flow rate = 4 BV/h.

with increasing temperature [45], and since the nitrate selectivity of the anion-exchange resins increases with an increase in the hydrophobic nature of the resin, it might have been expected that the nitrate selectivity would increase with temperature. Hydrophobic interactions within the polymer would favour nitrate selectivity by contributing to the rigidity of the network, thus making it more difficult to bring two ion-exchange sites close to one sulfate anion. In addition, the interaction between the hydrophobic charged site and the nitrate ion would be expected to increase with temperature because it has been found that the solubility in water of benzyl-tri-n-butyla m m o n i u m nitrate decreases with an increase in temperature [46]. A possible explanation for the selectivity for nitrate decreasing with an increase in temperature may be that the greater thermal motion of the polymer chains at higher temperatures, which allows the resin

285 TABLE 4 Nitrate/sulfate selectivity of weak-base anion exchange resins with a polystyrene backbone Resin

27 28 23 29

N-substituent

Et 2 n-Pr 2 n-Bu 2 iso-Am 2

Capacity (meq/mL)

1.02 0.80 1.1 0.92

Ksy (20 ° C)

NSS

82 148 370 750

- 0.10 0.27 0.50 0.91

Adsorption a (from a solution containing NO 3-- + 5042- ) As

AN

175 240 170 130

200 500 450 400

a See footnote to Table 2.

TABLE 5 Regeneration of weak-base resins with NaOH Resin

27

28

23

29

N-substituent

Et 2

n-Pr 2

n-Bu 2

iso-Am 2

%

NaOH treatment ( eq NaOH 1 eq resin ]

Capacity (CI- form, after regeneration)

0 0.9 1.8

1.02 0.76 0.84

75 82

0 1.0 2.1

0.92 0.61 0.63

66 68

0 1.0 2.4

0.92 0.44 0.56

48 61

0 1.2 2.8

0.92 0.53 0.62

58 67

to bring two ion-exchange sites close to one sulfate ion, outweighs the hydrophobic effects. The finding that the preference for nitrate relative to sulfate decreases with an increase in temperature (Table 3) is contrary to that reported by Sabadell and D'Amico [47]. Although weak-base resins have less preference for nitrate than the corresponding strong-base resins, Table 4 shows that weak-

Regeneration

(meq/mL)

TABLE 6 Strong-base (SB) and weak-base (WB) capacities of nominal weak-base resins Resin

2"/ 28 23 29

N-substituent

Et 2 n-Pr 2 n-Bu 2 iso-Am 2

Capacities (meq/g of CI- form) total

SB

WB

%WB

3.62 3.43 3.28 3.27

0.85 0.86 1.97 1.31

2.77 2.57 1.30 1.96

77 75 40 60

286 base resins at room temperature with dibutyl, diamyl or dipropyl substituents but not diethyl ones are still nitrate selective (NSS values positive) and, accordingly, difficult to regenerate. With the weak-base resins, however, the possibility exists of regenerating with near-stoichiometric amounts of N a O H and then converting the resin to the chloride form for further nitrate adsorption. Table 5, which summarizes the results of such a procedure, shows that the butyl and amyl resins were only about 50% regenerated with 1 eq of N a O H (or less, since the experimental procedure overestimates the extent of regeneration). More than doubling the quantity of N a O H only increased the degree of regeneration by a small amount. The failure to achieve complete regeneration of the resins with N a O H is easily explained by the high amount of strong-base groups in the resins (Table 6). These high amounts of strong-base groups would not be expected to be as readily regenerated with NaOH. Nevertheless, as expected, the weak-base groups in the resins can be regenerated with stoichiometric amounts of sodium hydroxide and hydrochloric acid. In contrast, complete regeneration of nitrateselective strong-base resins such as HP555, with a regeneration efficiency of 0.06 eq N O f / e q C1- (Table 2), requires up to 16 eq of sodium chloride per equivalent of resin for regeneration. Thus considerable savings in the chemical costs could be achieved using nitrate-selective weak-base resins and regeneration with stoichiometric amounts of sodium hydroxide/hydrochloric acid. For this approach to be successful it is obviously necessary to have resins with high weak-base capacities and only low strong-base capacities. On current chemical costs in Australia the cost of regenerating weak-base resins with the latter would be one fifth of the cost of sodium chloride needed to regenerate the weak-base resin (and also the strong-base resin). It should be pointed out that the use of 16 eq of sodium chloride per equivalent of

resin for regeneration as used in the above calculations represents an extreme case. For example, it has been found that partial regeneration, using 1 eq of chloride per equivalent of resin, of only a 40 to 50% nitrate-loaded resin is satisfactory for a sulfate-selective resin [44,48]. So far, all our preparations of weak-base resins have strong-base capacities which are too high (Table 6). These high strong-base capacities presumably result in the resins having greater preference for nitrate than would the completely weak-base forms of the resins. A di-n-butyl resin similar to resin 23 has recently been described [49,50].

Mode of adsorption The ratio of sulfate plus nitrate adsorbed to chloride displaced provides some insight into the origins of nitrate selectivity. The loadings on the resins after adsorption and the amount of chloride eluted were determined by the areas under the adsorption curves. The resins were then washed with water and regenerated with 5% sodium chloride. The loadings were then recalculated from the areas under the regeneration curves (e.g., Figs. 3 and 4). The calculated values for four resins are listed in Table 7. Both the sulfateselective IRA900 resin and the weakly nitrate-selective resin 28 have about 25% of their sites loaded with nitrate under the adsorption conditions used (last column). On the other hand the nitrate-selective resins HP555 and resin 22 have 50 to 60% of their sites loaded with nitrate. However, with the latter resins, the loadings of sulfate plus nitrate calculated from adsorption curves greatly exceed the amount of chloride eluted. This can be explained if a divalent sulfate ion in the vicinity of monovalent fixed group, and far from a second fixed group, produces a local net negative charge which the resin attempts to neutralize by taking up a sodium co-ion from the external solution. Such an explanation is consistent with the fact that

287 TABLE 7 Sulfate and nitrate loadings of resins by integration of adsorption or regeneration curves Loadings calculated from adsorption (meq/ml) Resin

N-substituent

SO42-

NO 3-

total 5042 - q_NO 3-

CI- eluted

Percent NO 3 on the resin relative to C1- eluted

IRA900 28 HP555 22

Me 3 nPr 2 a nBu 3

0.68 0.83 0.84 0.45

0.24 0.25 0.37 0.18

0.92 1.08 1.21 0.53

0.88 1.02 0.78 0.35

27 25 47 51

Loadings calculated from regeneration (meq/ml) Resin

Nsubstituent

SO42-

NO 3-

total SO42- + NO 3-

Percent NO 3on the resin relative to total capacity

IRA900 28 HP555 22

Me 3 n-Pr 2 a n-Bu 3

0.73 0.80 0.43 0.16

0.20 0.30 0.59 0.25

0.93 1.10 1.02 0.41

22 27 58 61

a

Believed to contain Et 3 substituents

the l o a d i n g of sulfate, a f t e r w a s h i n g w i t h w a t e r b u t b e f o r e r e g e n e r a t i n g , is a b o u t h a l v e d f o r b o t h H P 5 5 5 a n d resin 22. D u r i n g the w a s h i n g w i t h w a t e r the f o l l o w i n g p r o c e s s t a k e s place: --N

l

--N

+.-.SO2--.-Na

T h e r e m a y b e a r e l a t i o n s h i p b e t w e e n this effect a n d the c h a r g e s e p a r a t i o n c o n c e p t of S u b r a m o n i a n a n d C l i f f o r d [51].

+ H20) ACKNOWLEDGEMENTS

+'"SO

2-'''Na

+

I

T h e skilled a s s i s t a n c e of M r L.J. Vickers a n d M r P.J. P a s i c w i t h the e x p e r i m e n t a l w o r k is g r a t e f u l l y a c k n o w l e d g e d .

I --i --N+

+ ..'"';SO2- + Na2SO4

288

LIST OF S Y M B O L S AND ABBREVIATIONS

a measure of the number of bed volumes to sulfate break through (as defined in Table 2) separation factor bed volumes BV the capacity of a resin (meq/L) DADMAC diallyldimethylammonium chloride divinylbenzene DVB HEXA 1,6-bis( N, N-diallylamino)hexane molar selectivity coefficient milliequivalents meq N nitrate NSS nitrate/sulfate selectivity value (eqn. 4) PECH polyepichlorohydrin PTAA polytriallylamine QHEXA 1,6-bis( N, N, N-diallylmethylammonium)hexane dichloride sulfate S polystyrene Sty TAEC/SB total anion-exchange capacity/strong-base capacity equivalent ionic fraction of sulfate in solution Xso4 As

REFERENCES 1 J.W.H. Adam, Health aspects of nitrate in drinking-water and possible means of denitrification (literature review), Water S.A., 6 (1980) 79-84. 2 Committee report, An AWWA survey of inorganic contaminants in water supplies, J. Amer. Water Works Assoc., 77 (5) (1985) 67-72. 3 C.R. Lawrence, Nitrate-rich groundwaters of Australia, Australian Water Resources Council, Technical Paper No. 79, 1983. 4 F. Pearce, The hills are alive with nitrates, New Sci., (10 December 1987) 22. 5 G. Solt, Nitrate removal: A compromise solution, Water Qual. Int., (1) (1987) 29-30. 6 G. Solt, Removing nitrate from potable water, Chem. Eng., (May 1987) 33-36. 7 WQI Review, Nitrates: A question of time?, Water Qual. Int., (1) (1987) 24-28. 8 J.M. Montgomery, Water Treatment, Principles and Design, John Wiley and Sons, New York, NY, 1985, pp. 327-330. 9 R.W. Beulow, J.W. Knopp and J.M. Symons, Nitrate removal by anion-exchange resins, J. Amer. Water Works Assoc., 67 (Sept. 1975) 528-534.

10 D.A. Clifford and W.J. Weber, Nitrate removal from water supplies by ion exchange, executive summary, report 600/8-77-015, U.S. Environmental Protection Agency, Cincinnati, OH, 1977. 11 D.A. Clifford and W.J. Weber, Nitrate removal from water supplies by ion exchange, Report 600/2-78052, U.S. Environmental Protection Agency, Cincinnati, OH, 1978. 12 D.A. Clifford and W.J. Weber, The determinants of divalent/monovalent selectivity in anion exchangers, Reactive Polym., 1 (1983) 77-89. 13 G.L. Dalton, The removal by ion exchange of nitrates from borehole waters at Aroab SWA, Report National Institute for Water Research, Pretoria, 1978. 14 R.B. Gauntlett, Nitrate removal from water by ion exchange, Water Treat. Examin., 24 (1975) 172-193. 15 R.R. Grinstead and K.C. Jones, Nitrate removal from wastewaters by ion exchange, Report 17010 FSJ01/71, U.S. Environmental Protection Agency, Cincinnati, OH, January 1971. 16 G.A. Guter, Removal of nitrate from water supplies using a tributyl amine strong base anion exchange resin, U.S. Patent 4,479,877, October 30, 1984. 17 C.E. Meloan, The selective removal of nitrate and nitrite from polluted water, Prepared for Office of Water Resources Research, NTIS No. PB-231471,

289

18 19

20

21 22 23

24

25

26

27

28

29

30

31

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