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Removal of metals and anions from drinking water by ion exchange
DESALINATION
! ,;II ELSEVIER
Desalination 155 (2003) 157-170
www.elsevier.com/locate/desal
Removal of metals and anions from drinking water by ion exchange Kaisa Vaaramaa*, Jukka Lehto Laborator)/ of Radiochemistry, Department of Chemistry, Universityof Helsinki, PO Box 55, FIN-O0014Finland Tel. +358-9-19150162; Fax +358-9-19150121; email: kaisa,[email protected] Received 22 July 2002; accepted 15 October 2002
Abstract Five organic and two inorganic ion exchangers were evaluated for the removal of metals and anions from water of two drilled wells. Sodium titanate (CoTreat) and a chelating aminophosphonate resin were the most efficient exchangers in removing transition metals from the total of 1800 bed volumes processed. CoTreat was the best for almost all of the transition metals. The breakthrough level of manganese was below 1% with CoTreat even when its concentration in the feed water was high (1 mg/l). The weak acid cation resin took up transition metals relatively efficiently. Somewhat unexpectedly, the cation exchangers also removed arsenic from water. Arsenic may have been sorbed on iron species, which again was adsorbed and filtered by the exchanger beds. Most of the cation exchangers took up calcium and magnesium at low processing capacities (<400 BV), and the strong base anion resin took up nitrate, bromide and sulphate very efficiently below 700 bed volumes. Neither chloride nor fluoride was taken up by the exchangers tested.
Keywords: Ground water; Purification; Ion exchangers; Metals and anions
I. Introduction Our initial interest has been to evaluate the removal o f radioactive elements from ground water by means of ion-exchange processes. In an earlier work, we studied the performance of the ion-exchange materials in the removal of natural radionuelides from drinking water [1]. Appreciable concentrations o f natural uranium and its *Corresponding author.
daughter radionuclides (radium, polonium and lead) may occur in drinking water obtained from drilled wells when the bedrock contains considerable amounts of these nuclides. Ion exchange can be applied to purify water and to remove uranium and radium. It is also important to study the quality of drinking water after such treatment processes: The exchangers should not be overly effective in removing useful elements. It would also be advantageous if they removed other toxic and harmful elements as well as
0011-9164/03/$- See front matter © 2003 Elsevier Science B.V. All rights reserved PII: S 0 0 1 1 - 9 1 6 4 ( 0 3 )00293-5
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radionuclides. In this study, five organic and two inorganic ion exchangers were evaluated for their ability to remove metals and anions from drilled well waters in column experiments. The quality of ground waters used for drinking water may differ widely from one location to another. The ground waters in Finland are mainly soft calcium bicarbonate waters, although NaCI may dominate in the water from some drilled wells in coastal areas [2]. The high concentrations of Fe and Mn are a problem in some ground waters and these elements cause technical problems in water supplies. Also in some cases water hardness cause problems (mainly Ca 2+ and Mg2+). Strong acid cation exchangers are used to remove Fe, Mn, Ca and Mg. In general, all types of cation-exchange resins remove Ca and Mg effectively, but strong acid resins are often preferred in many applications [3]. Zeolites have replaced polyphosphates for removing hardness from washing water. Zeolite A in the sodium form is commonly used for this purpose [4]. In areas of Finland having a special type of granite (rapakivi), high fluoride concentrations may be found in ground waters [2]. In general, high concentrations of arsenic are found in areas where volcanogenic rock contains volatile elements or argillaceous schists [5]. In Finland, arsenic is a problem mostly in drilled bedrock wells. Concentrations in water from springs or dug wells rarely exceed the limits set for drinking water [6]. Arsenic can be removed from water by using anion-exchange resins [7,8]. A sodium titanate ion exchanger (CoTreat), a commercial inorganic ion exchanger developedat the Laboratory of Radiochemistry of the University of Helsinki, has proved to be highly selective for radioactive cobalt and other activation corrosion products (S4Mn, 59Fe, 65Zn, 63Ni) contained in waste waters generated by nuclear power plants [9]. It is therefore of interest to test the performance of this exchanger in removing these elements from ground water containing
"macro" concentrations (> 1 mg/l) of Fe and Mn and "minor" concentrations (> 1 ktg/l) of Zn and Ni. Chelating aminophosphonateresins have been investigated for use in purifying metallurgical process effluents. They have proved to be highly effective in removing zinc from effluents in the metal plating industry [10]. In addition, aminophosphonate resins separate uranium efficiently from phosphoric acid solutions [11] as well as from acidic ground waters [ 12].
2. Experimental 2.1. Ground water samples
The ground water samples for the experiments were taken from two different drilled wells. Water 3, which was also used in an earlier study [13], has high iron and manganese concentrations, while these concentrations in water 6 were much lower. The separation performance of the exchangers in removing natural radionuclides from water 6 was published earlier [1]. Water from the same batch of water 3 was used in all of the ion exchanger tests. In contrast, samples of water 6 had been collected at different times. This explains the slight variations in the chemical compositions of water 6 for each ion exchanger (Table 1). The pH of water 6 ranged from 7.3 to 8.2; it was 7.5 in water 3. The metal composition was determined by inductively coupled plasma mass spectrometry (ICP-MS) or inductively coupled plasma atomic emission spectrometry (ICP-AES) and the anions by ion chromatography. Before the ICP-MS/AES measurements, the samples were pretreated as follows: Samples of water 6 were first filtered with 0.45 lxm filters and then acidified. It was observed that iron sorbed on the filter material and therefore the pretreatment of water 3 samples was done in a reverse order (acidified and then filtered). The total Fe concentrations in water 6, given in Table 1, are mean values from other batches; they
K. Vaaramaa, J. Lehto / Desalination 155 (2003) 157-170
159
Table 1 Chemical compositions of the ground waters Component
Water 3 with the ion exchangers
Water 6 with Zeolite A
Water 6 with CoTreat
Water 6 with the organic ion exchangers
AI, p.g/! As, Ixg/1 Ba, ~tg/l Cd, ktg/1 Cr, ~tg/l Cu, p.g/l Ni, lag/1 Pb, p.g/l Zn, ~tg/l Ca, mg/l Fe, mg/l K, mg/l Mg, mg/l Mn, mg/l Na, mg/l Sr, mg/l Br, mg/l CI-, mg/1 F-, mg/! NO~, mg/l SO2-, mg/l
7.33 8.54 85.6 0.26 0.29 194.76 2.91 4.51 35.47 58.60 2.94 4.12 19.05 1.08 27.50 0.56 ND ND ND ND ND
86.4 1.76 19 0.03 0.29 196 1.95 8.5 105 36 -0.1 a 11.2 5.99 0.046 31.7 ND 0.1 24 0.32 9.84 25.8
12.2 1.52 7.17 <0.02 <0.20 34.4 0.85 0.83 67.4 22.1 -0.1 a 6.51 3.97 0.025 58.2 ND 0.2 49.4 1.0 3.92 21.1
14.6 1.0 9.55 <0.02 <0.20 27.8 0.93 0.76 37.05 30.55 -0.1 a 9.71 5.96 0.033 53 ND 0.15 37.25 0.63 7.27 26.3
aThe concentration of Fe (tot) was around 0.1 mg/l (see the text in the Experimental section). ND, not determined. were determined by atomic absorption spectrophotometry (AAS). Prior to the column tests, the raw water samples were bubbled with nitrogen gas (purity 99.5%) to expel radon (222Rn), because the samples were also used for radioactivity measurements.
2.2. Ion exchangers The organic and inorganic ion exchangers evaluated in the work are given in Table 2. The particle size was 0.3-0.85 mm for the inorganic ion exchangers and 0.3-1.2 mm for the organic
resins. The pretreatment o f the exchangers was performed as described by Vaaramaa et al. [ 1].
2.3. Column experiments The processing capacities of the ion exchangers for the inactive elements (Na, K, Mg, Ca, Cr, Mn, Fe, Ni, Cu, Zn, Cd, AI, Pb, As, Br-, CI-, F-, NO~ and SO 2-) were determined. The breakthrough values given in this paper were calculated by dividing the ion concentration in the effluent by the ion concentration in the feed water and given in percentages.
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Table 2 Ion exchangerstested Type Organic ion exchangers: Aminophosphonate Strong acid cation Weak acid cation Strong base anion Weak base anion Inorganic ion exchangers: Sodium titanate Synthetic zeolite
Name
Chemical composition
Manufacturer
Purolite $ 9 5 0 Purolite C 145H Purolite C 104 Purolite A 5 0 0 Purolite A105
R-CH2NHCH2PO3H 2 R-SO3H R-COOH R-N(CH3)3OH R-N(CH3)2
Purolite Purolite Purolite Purolite Purolite
CoTreat Zeolite A (Si/AI = 1.1)
-Nal2 [(AIO2)12(SiO2)12]• 27H20
Selion Oy Laporteand Bayer
R, styrene-divinylbenzene.
If the calculated breakthrough value indicated is over 100% (e.g., 120%), this means that the exchanger did not take up the cation/anion from the feed water, but that the exchanger released the element which it had taken up at low processing capacities. This is caused by the "chromatography effect": at lower processed water volumes, the exchangers take up several cations, whereas at higher volumes, when the exchanger capacity is exceeded, the exchanger continues to take up the cations it prefers, but at the same time releases the ones which are less preferred. Thus, if the percentage is 120%, the "extra" 20% in the breakthrough value is the relative amount of the element released from the exchanger. A total of 1800 bed volumes (1 BV = 10 ml) of ground water was processed through the exchanger beds. The 18 L processed correspond to 3-4 months of water consumption in a typical Finnish single-family dwelling, assuming a water consumption of 5 m3/week and a 40-L exchanger bed. To investigate the effect of flow rate on element removal, four different elution rates were used for water 6: first, 6 L at 15 BV/h; then 4 L at 40 BV/h; next 4 L at 60 BV/h, and, finally, 4 L
at 80 BV/h. In the experiments with water 3, the flow rate was constant, 20 BV/h. The chemical compositions of the collected fractions were determined with respect to metals by ICP-MS or ICP-AES for both waters, and with respect to anions by ion chromatography for water 6. The pH values of the effluent fractions were determined. 3. Results and discussion 3.1. p H o f the effluents
The raw water pH in water 6 ranged from 7.3 to 8.2 (see Experimental); it was 7.5 in water 3. The pH of the collected fractions from the sodium titanate (CoTreat) column did not differ significantly from those of the feed waters. The pH of the first fractions from the zeolite A column (not analysed by ICP-MS) in water 6 decreased from 9.3 to 8.2, but after 600 processed bed volumes the pH was 7.9, very close to that of the raw water (7.3). This trend was similar to the pH values of the fractions obtained with water 3. The pH values of the fractions collected from the organic resin columns were relatively close to the pH of the feed water (7-8).
K. Vaaramaa, ,I.. Lehto / Desalination 155 (2003) 157-170 3.2. Removal o f calcium and magnesium Tables 3 and 4 show the breakthrough values of calcium and magnesium for water 6 and water 3, respectively.
3.2.1. Calcium The weak acid cation resin (WAC), zeolite A and CoTreat showed high uptake of Ca from both waters. However, the breakthrough values with these exchangers increased at higher bed volumes, for water 3 being almost 100% at
161
1300 BV for the WAC and CoTreat and at 1700 BV for zeolite A (Table 4). The breakthrough values for calcium with the WAC, zeolite A and CoTreat were lower for water 6 than for water 3 (considering the total processed bed volumes). At low processing capacities, the strong acid cation resin (SAC) also took up calcium; the breakthrough value was 76% (440 BV) for water 3 and 7% (670 BV) for water 6. At higher bed volumes, the uptake of calcium decreased with the SAC. The SAC did
Table 3 Chemical compositionsof effluent fractionsand the breakthroughvalues for Mg, Ca, Pb and As in the column experiments with water 6 Sample
SBA'
WBA'
SAC"
WAC"
$950"
Zeo Aa
CoTreaP
Processedbed v o l u m e s
668 1191 1608 671 1197 1629 679 1209 1628 598 1164 1589 667 1185 1622 601 1043 1578 655 1070 1474
Breakthough value, % Mg
Ca
Pb
As
96.1 99.0 98.7 96.6 98.5 98.5 12.08 144b 114b <1.7c 30.9 142b 60.4 104b 98.5 2.50 72.8 95.2 135b 131b 113b
97.5 96.6 98.9 96.2 95.6 95.6 6.97 82.2 94.9 0.75 11.9 73.3 34.7 92.0 92.6 2.36 33.1 86.9 4.39 53.8 79.2
10.5 11.8 13.2 13.2 5.26 6.58 27.6 7.90 6.58 21.1 17.1 <4c 7.90 10.5 10.5 75.3 5.41 30.6 19.3 15.7 18.1
49.0 115b 89.0 94.0 80.0 78.0 82.0 80.0 80.0 78.0 85.0 71.0 68.0 59.0 71.0 85.8 30.7 59.1 <3.3c 11.8 21.7
"Strong base anion resin (SBA), weak base anion resin (WBA), strong acid cation resin (SAC), weak acid cation resin (WAC), aminophosphonate resin ($950), syntheticzeolite (zeolite A) and sodium titanate (CoTreat). ~l'he percentage above 100% indicates the relative amount of the element released from the exchangers. CTheconcentration of effluent was below the detection limit.
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Table 4 Chemical compositions of effluent fractions and breakthrough values for Mg, Ca and Pb in the column experiments with water 3 Sample
Processed bed volumes
Breakthrough value, % Mg
Ca
Pb
SACa
444 873 1326 1785
152b 98.2 99.2 101b
75.6 102b 104b 101b
14.0 11.8 12.6 10.4
WACa
452 892 1353 1818 455 896 1360 1826 439 874 1328 1774 448 874 1317 1751
30.4 124b 106b 101b 104b 100 99.2 99.7 60.4 96.1 99.7 103b 129b 103b 99.2 98.7
6.16 81.4 99.0 102b 89.9 100 101b 104b 23.4 69.8 88.1 94.5 53.2 91.1 96.8 98.6
14.4 9.31 7.76 5.99 16.0 9.76 8.43 9.76 11.8 14.2 7.32 9.09 9.31 7.98 4.21 3.99
$950~
Zeo A~
CoTreaP
aStrong acid cation resin (SAC), weak acid cation resin (WAC), aminophosphonate resin ($950), synthetic zeolite (zeolite A) and sodium titanate (CoTreat). bA percentage above 100% indicates the relative amount of the element released from the exchangers.
not take up Ca from water 3 at all, but from water 6, which had a Ca concentration two-fold lower than that o f water 3, the resin did take up some calcium. In general, if ground water is soft and especially when it is acidic, corrosion problems may occur in water distribution equipment. Considering the recommendations for household water [14] and the concentrations of Ca in the effluents o f water 6, only at low processing capacities (_<600 BV) were the concentrations o f calcium in the effluents below the recommendations (10 mg/l). One exception was the WAC,
for which the concentration of Ca in the effluent did not comply with the recommendation for Ca until 1500 BV had been processed (Tables 1 and 3).
3.2.2. Magnesium Most o f the cation exchangers took up magnesium at 600 BV and below with water 6 (Table 3) and at 400 BV and below with water 3 (Table 4), but then a release o f the cation was observed at higher bed volumes. This was caused by the "chromatography effect" explained above.
K. Vaaramaa, J. Lehto / Desalination 155 (2003) 157-170 3.3. Removal o f the transition metals The Species Distribution Program (SPE) was used to calculate the distributions of the metals species in water 3 [15]. The stability constants of the anion complexes (carbonate and sulphate) and the hydrolysis species and solubility products o f metals were used. The metal species distribution data are only indicative, because each metal and its species were calculated separately. Competition between the metals was thus excluded. The concentrations of the metals shown in Table 1 were used for the computation,
163
while the concentrations of anions in water 3 had been determined elsewhere [13]. The breakthrough values o f the transition metals are given in Tables 5 and 6. The breakthrough curves as a function of the processed bed volumes for the WAC, CoTreat and the aminophosphonate resin in water 3 are shown in Fig. la-c.
3.3.1. lron With water 3, all of the cation exchangers took up iron rather well (Fig. 2); the break-
Table 5 Chemical compositions of effluent fractions and breakthrough values for Mn, Ni, Cu and Zn in the column experiments with water 6 Sample
SBAa
WBAa
SAC~
WACa
$950a
Zeo Aa
CoTreaP
Processed bed volumes
668 1191 1608 671 1197 1629 679 1209 1628 598 1164 1589 667 1185 1622 601 1043 1578 655 1070 1474
Breakthrough value, % Mn
Ni
Cu
Zn
68.6 82.4 62.8 45.0 70.8 47.5 9.95 4.20 6.71 0.43 6.77 3.80 0.55 0.34 0.40 4.72 3.00 11.9 0.52 0.61 0.52
83.9 74.2 66.7 109b 89.2 93.5 32.3 115b 104b 11.8 28.0 44.1 25.8 49.5 48.4 28.7 49.2 67.2 23.5 17.6 16.5
29.2 35.5 42.4 35.3 41.0 41.7 87.8 77.3 68.0 31.1 37.4 31.7 18.8 18.6 20.6 31.6 16.0 30.7 7.67 11.2 13.8
87.4 90.1 89.6 98.8 92.3 82.3 13.0 118 97.4 1.75 3.05 7.53 1.67 1.57 3.64 6.21 3.05 23.05 1.3 ! 1.16 1.68
aStrong base anion resin (SBA), weak base anion resin (WBA), strong acid cation resin (SAC), weak acid cation resin (WAC), aminophosphonate resin ($950), synthetic zeolite (zeolite A) and sodium titanate (CoTreat). bA percentage above 100% indicates the relative amount of the element released from the exchangers.
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K. Vaaramaa, J. Lehto / Desalination 155 (2003) 157-170
Table 6 Chemical compositions of effluent fractions and breakthrough values for Cr, Mn, Ni, Cu and Zn in the column experiments with water 3 Sample
SAC a
WAC a
$950 '
Zeo A~
CoTreat"
Processed bed volumes
444 873 1326 1785 452 892 1353 1818 455 896 1360 1826 439 874 1328 1774 448 874 1317 1751
Breakthrough value, % Cr
Mn
Ni
Cu
Zn
100 96.6 82.8 <69 c <69 c 93.1 96.6 82.8 72.4 89.7 93.1 82.8 <69 c <69c 75.9 72.4 <69 c 75.9 79.3 72.4
156b 98.1 100 102b 1.85 28.7 47.2 60.2 0.93 0.93 2.78 3.70 33.3 76.9 88.0 94.4 <0.093 ~ <0.093 ~ <0.093 ¢ 0.93
129b 97.3 93.5 99.7 42.6 74.2 71.8 78.7 34.7 34.0 35.1 33.0 70.1 96.6 94.2 96.6 39.5 47.8 43.3 46.7
30.9 30.4 34.2 35.3 25.1 19.5 19.0 19.4 22.2 15.8 14.8 16.1 22.8 28.7 23.5 28.8 15.2 13.2 9.89 9.89
120b 64.0 64.6 60.9 20.3 14.2 10.9 10.3 18.7 11.7 10.3 12.0 17.7 35.3 27.4 33.0 10.7 12.6 5.85 4.89
aStrong acid cation resin (SAC), weak acid cation resin (WAC), aminophosphonate resin ($950), synthetic zeolite (zeo A) and sodium titanate (CoTreat). bA percentage above 100% indicates the relative amounts of the element released from the exchangers. CThe concentration of effluent was below the detection limit.
through values were below 45%. CoTreat was the most efficient in removing iron, and the breakthrough level even tended to decrease (from 27% to 10%) with an increase in the bed volume processed. According to the SPE program, iron was present in the water as Fe(OH)3, i.e., colloidal or particulate iron, and the filtering properties probably improved during the column run. This trend was typical for other exchangers as well. The breakthrough values for the SAC and the W A C were slightly higher (Fig. 2): 2 8 38% and 16--41%, respectively. The break-
through values o f Fe for water 6 are not presented here because iron was sorbed on the filter material before the ICP analysis (as noted in the Experimental section). Therefore, the concentration o f iron in the fractions collected from water 6 was below the detection limit (0.03 mg/l). 3.3.2. Manganese If manganese is present in the water as the +2 oxidation state, then according to the SPE program, it is present as the M n 2+cation when the
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K. Vaararnaa, J. Lehto I Desalination 155 (2003) 15 7-170
100 la
80
,o
= e ,-
60
30
40
20
~
20
t-o~
m
0 500
1000
1500
2000
Volume of processed water 3 (BV)
lb
80 60
9 ~" 40
~
rn
0
O~
500
,
0
,
1000
,rl
,
1500
2000
Volume of processed water 3 (BV) ._. 100 e"
o}
o t--
rn
!
i
500
1000
!
1500
|
2000
Volume of processed water 3 (BV)
20 0
v
0
Fig. 2. Breakthrough curves of Fe for the organic ion exchangers (strong acid cation, SAC •; weak acid cation, WAC ,,; chelating aminophosphonate, A) and for the inorganic ion exchangers (synthetic zeolite, zeolite A, +; sodium titanate, CoTreat, [3) as a function of the processed bed volumes with water 3.
100 t-.
10
v
1C
80 60 40
0 0
500
1000
1500
2000
Volume of processed water 3 (BV) Fig. 1a-c. Breakthrough curves of the transition metals Mn (N), Fe (+), Ni (A), Cu (•), Zn (A), and Cd (o) for a: weak acid cation exchanger (WAC, Purolite C104); b: sodium titanate (CoTreat, Selion); c: chelating aminophosphonate resin (Purolite $950) as a function of the processed bed volumes with water 3.
pH is 7-8, and in that case cation exchangers are able to remove it from water. With water 6, however, all o f the exchangers took up manganese to some extent (Table 5): the breakthrough values for the anion exchangers were
50-80%, and typically lower than 10% for the cation exchangers. The most effective manganese removers were CoTreat and the aminophosphonate resin, both o f which took up more than 99 % of the manganese. The uptake of Mn by the SAC, the WAC and zeolite A was lower for water 3 than for water 6. This discrepancy may be explained by the 30-fold higher concentration of manganese in water 3 as compared to water 6. As can be seen in Fig. la, the WAC resin took up Mn rather well from water 3 at the beginning o f the column run, but the breakthrough o f Mn clearly occurred at 500 bed volumes. With the aminophosphonate resin, the uptake values o f Mn were almost the same as for water 6; they were even better with CoTreat (Tables 5 and 6). With water 3, the breakthrough value o f Mn for CoTreat was below 0.093% (the concentration of the effluent was below the ICP-MS detection limit for Mn) up to 1300 BV, and was 0.93% at 1750 BV (Table 6). At low processing capacities, the SAC resin takes up manganese, but then releases it at 400 BV until at about 1000 bed volumes the concentration of Mn in the effluent is the same as in the feed (Table 6). Iron is present in the waters as Fe(OH)3, and thus one could assume that manganese is present
166
K. Vaaramaa, J. Lehto / Desalination 155 (2003) 157-170
as insoluble Mn(IV)oxides in the prevailing conditions as well. In general, when the breakthrough of an ion occurs in a column process, it shows as an upward curve as a function of the processed water volume. Instead, if the metal is present in the water in its particle form, the breakthrough level of this metal may decrease during the column run (i.e., the breakthrough curve is downward), as was seen in the case of iron. The breakthrough curves for the cation exchangers (Fig. l a-c) support the assumption that manganese is present in water 3 mostly as the soluble metal cation. 3.3.3. Zinc
According to the SPE program, zinc was present in these waters mainly as the Zn 2÷cation at pH 7-8. Soluble zinc carbonate complexes and hydrolysis species were also present to a lesser degree. In water 6, the separation performances of the exchangers for zinc were similar to those for manganese. CoTreat and the aminophosphonate resin were again the most effective exchangers, although the breakthrough values were somewhat higher than for Mn. The breakthrough values of Zn were 1.6--3.6% with the aminophosphonate resin and 1.2-1.7% with CoTreat (Table 5). For the WAC, the flow rate most likely had an increasing effect on the breakthrough of Zn. At flow rates of 15, 60 and 80 BV/h, the average breakthrough values were 1.8%, 3% and 7.5%. The separation performance of the exchangers for zinc were much lower with water 3 than with water 6 (Table 6), which again may be explained by the fact that the concentrations of alkaline earth metals and almost all of the transition metals were higher in water 3. Sodium titanate (CoTreat) was the most effective for Zn with both water 3 (Table 6, Fig. lb) and water 6. The next best for Zn were the aminophosphonate and the WAC resins (Table 6, Fig. la and c). The breakthrough values of Zn for the WAC and the aminophosphonate resins were almost equal (10-20%).
3.3.4. Copper
All of the exchangers took up copper from water 6 (Table 5). The species distribution program showed that copper exists as the soluble carbonate species in pH 7-8 and that a significant portion of the Cu also precipitates as Cu(OH)2. Minor amount are also present as the free metal cation. As in the case of Fe, the significant portion of Cu is present as colloidal form in the waters. Sodium titanate took up Cu most efficiently, especially at low processing capacities (at 600 bed volumes). All of the cation exchangers took up copper from water 3 in the order of: CoTreat > aminophosphonate > WAC > zeolite A > SAC (Table 6). 3.3.5. Nickel
Based on the SPE program, the waters contained nickel mainly as the Ni 2+cation and, to some extent, as soluble Ni-carbonate complexes. In general, the uptake of nickel was lower than that of Cu with both waters. In the experiments with water 6 in which the flow rate was increased during the column run, CoTreat was the most efficient for nickel (Table 5). The flow rate clearly affected the breakthrough of Ni with the chelating aminophosphonate resin (Table 5). The average breakthrough value of Ni rose from 26% to 49% when the flow rate was increased from 40 BV/h to 60 BV/h. The kinetics of the metalion exchange on aminophosphonate resin is known to be rather slow. When the flow rate was constant, as in the run with water 3, the breakthrough values of Ni were constant (3335%) for this resin (Fig. 1c, Table 6). In addition, the aminophosphonate resin was better than CoTreat in removing Ni from water 3. 3.3.6. Cadmium
The concentration of cadmium in the water 6 feed was near or below the detection limit (0.02 p,g/l) of the ICP-MS analysis. Although the concentration of Cd was also low in water 3
K. Vaaramaa, d. Lehto / Desalination 155 (2003) 157-170
(0.26 ~tg/l), the separation performances of the exchangers could be determined. The uptake of Cd was the highest with CoTreat (breakthrough 8% or lower, Fig.lb) and the lowest with the SAC (breakthrough values 66-97%). The second best exchanger for Cd was the WAC (Fig. 1a); the aminophosphonate resin was almost as efficient (Fig. 1c). According to the SPE program, Cd was present mainly as the free metal cation in the water; a minor amount was complexed with carbonate.
3.3. 7. Chromium The concentrations of chromium were also below or near the detection limit of the ICP-MS analyses (0.2 ~tg/1). The breakthrough values determined for Cr with water 3 are given in Table 6. Chromium (III) was present as the soluble hydrolysis species in the water (in pH 6-9 as CrOH 2+, Cr(OH)~, Cr(OH)3, Cr(OH)4). The sodium titanate (CoTreat) and the aminophosphonate resin were overall the most efficient exchangers in removing the transition metals from the waters tested. The weak acid cation resin (WAC) was also reasonably good. A similar trend in the separation efficiency of CoTreat and the aminophosphonate resin for the transition metals was observed with water 3 (Fig. 1 b and c). The separation efficiency was the highest for Mn and the lowest for Fe and Ni. With regard to Cd, Zn and Cu, the trend was somewhat similar. The aminophosphonate resin preferred Cd over Zn, as did CoTreat (up to 1300 bed volumes). Above 1300 BV, the preference order with CoTreat was reversed (Fig. 1b and c). The order of the separation efficiency for metals with the WAC was similar to that of CoTreat below 500 bed volumes: Mn > Cd > Zn > Cu > Fe > Ni. With the WAC resin, the breakthrough of Mn clearly occurred at 500 BV (Fig. la). The separation efficiency of CoTreat and the aminophosphonate resin for Mn, Zn, Cu and Ni in water 6 was similar to that of water 3, except
167
that the both exchangers preferred more clearly Zn over Cu than with water 3.
3.4. Removal of potassium and sodium Zeolite A and CoTreat were the exchangers that took up potassium: at 600 BV, the breakthrough values of potassium in water 6 were 10% and 55%, respectively, and at 400 BV the values for water 3 were 21% and 67%. However, these exchangers did not take up potassium at higher bed volumes; zeolite after 1300 BV and CoTreat after 800 BV. Because of the health effects of sodium, the sodium concentration in drinking water should not exceed the limit of 200 mg/l [3,14]. None of the exchangers took up sodium. All of the cation exchangers (which were initially in sodium form) released sodium. The poorest water quality with respect to sodium was detected with the WAC and zeolite A, when the concentrations in the effluents at 400 BV (with water 3) and 600 BV (with water 6) were about 90 mg/l. Although the cation exchangers released sodium into the treated water, the limit set for drinking water was not exceeded.
3.5. Removal of Al, Pb and As 3.5.1. Aluminium With water 6, the breakthrough ofaluminium was only about 20-30% for all except zeolite A (from which AI leached into the water). The excess aluminium found in the treated water was probably extra-framework aluminium from this exchanger [16]. The recommended limit for aluminium in drinking water in Finland is 200 ~tg/l [14]. With zeolite A, this level of A! in the effluent at 600 BV was exceeded 5-fold. At higher bed volumes, the concentration of A1 in the effluents was the same as in feed water 6. The breakthrough values for water 3 were higher (40-80%, with the exception of zeolite) than for water 6. Aluminium leached into the treated water with zeolite A again, but the concentrations of A! did not exceed the limit of 200 ~tg/l.
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3.5.2. L e a d
The uptake of inactive and radioactive lead was studied with water 6. The behaviour of radioactive lead (21°pb) in the water has been studied in an earlier work [ 1]. With the exception of zeolite A, all of the exchangers behaved similarly for the uptake of both inactive and radioactive lead. This indicates that the inactive lead and 21°pb are in the same chemical form. For an unknown reason, the breakthrough values of inactive lead varied widely for zeolite A (Table 3). Both the anion exchange resins and the cation exchangers took up inactive lead from water 6 (Table 3). On the basis of breakthrough curves for radioactive lead and our earlier studies [ I, 17], we have concluded that 210pbwas present in water 6 as particles. It can further be presumed that some of the inactive lead was also present as particles, which were adsorbed and filtered by the exchanger beds. The breakthrough values of lead with water 3 were also rather low (Table 4). 3.5. 3. Arsenic
Although the concentration of arsenic was low in both raw waters - - 1.4 I~g/I (water 6) and 8.5 ~tg/l (water 3) - - some conclusions on the uptake of As can be drawn. Arsenite (As(Ill)) and arsenate (As(V)) are the most important species of arsenic in ground water supplies for drinking [7,8,18]. Arsenite is typically present as a neutral H 3 A s O 3 molecule at pH 6-9, while arsenate occurs primarily as the H2AsO4 and HAsO 2species. Thus, in order to remove trivalent arsenic efficiently from water by adsorption or ion exchange, As(Ill) must be oxidised to arsenic(V) [7]. Strong base anion-exchange resins are used to remove arsenic from water. The performances of the anion exchange resins were tested for the uptake of anionic arsenic species from water 6. The strong base anion exchanger (SBA) took up arsenic at 600 BV and lower, but then released it at 1000 BV (Table 3). It should be remembered
that, other than N 2 bubbling, the raw water had not been pretreated before the column experiments. Therefore, it is not certain that all of the arsenic was present as +5. On the other hand, the concentration of bicarbonate (HCO~) in these ground waters was quite high, -2.5 mmol/l (water 6) and 4-5 mmol/l (water 3); bicarbonate thus competed with the arsenate anions for the ion-exchange sites. In addition, if the arsenic removal process were simply arsenate anion exchange, the anion exchanger would be the most efficient for the uptake of arsenic. However, sodium titanate was the most efficient in these conditions (Table 3). The breakthrough value of As for CoTreat was below 3.3% (655 BV) while it was 49% (668 BV) for the SBA. All the cation exchangers took up As from water 3. The breakthrough values were of the order of 20% for the SAC, the WAC, the aminophosphonate resin and zeolite A, and 7-12% for CoTreat (Fig. 3). The concentrations of Fe, Mn and As in water 3 were higher than those in water 6. For Fe and As, the breakthrough curves of the exchangers were similar with water 3 (Figs. 2 and 3). The correlation between the behaviour of Fe and As may be explained by the fact that iron was present as Fe(III)hydroxide and by assuming that As precipitated with the Fe(OH)3, which was 50 ~..~ 40 oe - ~ 30 20
m
lO o
t
i
500
1000
i
i
1500
i
2000
Volume of processed water 3 (BV)
Fig. 3. Breakthrough curves of As for the organic ion exchangers(strongacid cation, SAC * ; weak acid cation, WAC m; chelating aminophosphonate, J,) and for the inorganic ion exchangers(syntheticzeolite, zeolite A, +; sodium titanate, CoTreat, I"1) as a function of the processed bed volumes with water 3.
K. Vaaramaa, ,1. Lehto / Desalination 155 (2003) 157-170
then sorbed on the exchanger beds. Driehaus et. al. [ 19] studied granular ferric hydroxide in the removal of arsenic (V) from natural water, and found it to be an effective adsorbent for arsenate removal. In general, arsenic is removed with greater efficiency in sorption processes if it is present in the +5 oxidation state rather than the +3 oxidation state [7]. The concentration of Mn was quite high in water 3, 1.08 mg/l. As discussed above, it seems that manganese was present in the water mostly as soluble Mn 2+. It is also possible that manganese(IV)oxides were present, which may have increased the As(III)oxidation from oxidation state +3 to +5. Manganese dioxide has been found to be an effective oxidising agent for As (III) in water treatment [20]. 3. 6. Removal of anions The removal of five anions (Br-, CI-, F-, NO~ and SO 2-) was studied in column experiments with water 6. The concentrations of the anions in the feed waters are shown in Table 1. Changes in concentrations were observed only with the strong base anion resin (SBA, Fig. 4). At low processing capacities, the SBA took up nitrate very efficiently (<700 BV; breakthrough value
._., 200 150 o 100
50 m
_
OI 500
1000
1500
2000
Volume of processed water 6 (BV) Fig. 4. Breakthrough curves of the anions NO; (=), SO 2- (A), F- (+), CI- (#), and Br- (D) for the strong base anion exchanger (SBA, Purolite A500) as a function of the processed bed volumes with water 6.
169
was 6.5%). It also took up bromine and sulphate very efficiently: the breakthrough of Br was less than 0.1% and that of sulphate 7.8% below 700 BV. Extra chloride was observed to elute from the SBA resin; the water quality with respect to its chloride concentration was, however, reasonably good. 4. Conclusions
The aminophosphonate resin and the CoTreat exchanger were several times more efficient than the conventional resins in removing toxic and harmful transition metals (Cu, Zn, Mn, Ni) from drilled well water. The weak acid cation exchanger (WAC) was also rather efficient in the uptake of transition metals. As expected, the effect of the flow rate was clearly seen for the WAC. Weakly acidic resins and inorganic ion exchangers are known to have relatively slow kinetics for metal-ion exchange. The breakthrough level of manganese was very low with CoTreat (<1%) even when the concentration of Mn in the feed was 1 mg/l. The processing capacity of the strong acid cation resin (SAC), which is usually used to remove Mn from water, was significantly lower than of the other exchangers. Iron, which was present as Fe(III)hydroxide, was taken up well by the cation exchangers. Arsenic, which presumably was sorbed on the iron species, was consequently sorbed on the exchangers. In the past years, one of our interests has been the use of ion exchange to remove natural radionuclides from drinking water. Another method useful for removing uranium and its longlived daughters from water is the reverse osmosis technique, but the disadvantage of this method is the quality of the treated water; the effluent is demineralized during the process [7, 21 ]. During the process of ion exchange, the effluents were not demineralized, but the harmful elements were removed. The quality of the treated water was
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reasonably good, although extra sodium was eluted from the cation exchangers and extra chloride from the anion exchanger. However, after treatment with ion exchangers, water rehardening (alkalisation) is recommended in some cases.
Acknowledgements This work was supported by EU research project (FI4PCT960054), the Maj and Tor Nessling Foundation (Finland) and the Finnish Cultural Foundation. KV also thanks Dr. Teresia M611er for helpful discussions.
References [1] K. Vaaramaa, J. Lehto and T. Jaakkola, Radiochim. Acta, 88 (2000) 361-367. [2] P. Lahermo, M. Ilmasti, R. Juntunen and M. Taka, The Geochemical Atlas of Finland Part 1, Geological Survey of Finland, Espoo, 1990. [3] W. H~511and H-C Fiemming, in: K. Dorfner, ed., Ion Exchangers, Walter de Gruyter, Berlin, 1991, pp. 835-844. [4] A. Dyer, An Introduction to Zeolite Molecular Sieves, Wiley, New York, 1988. [5] T. Koljonen, ed., The Geochemical Atlas of Finland Part II, Geological Survey of Finland, Espoo, 1992. [6] H. Kahelin, K. Jarvinen, P. Forseil and M. Valve, Arsenic in Drilled Wells-Comparison of Arsenic Removal Methods, Report of Investigation 141, Geological Survey of Finland, Espoo, 1998.
[7] E.O. Kartinen, Jr,and C.J. Martin, Desalination, 103 (1995) 79--88. [8] L-L. Homg and D. Clifford, Resins. React. Funct. Polym., 35 (1997) 41-54. [9] R. Harjula, J. Lehto, A. Paajanen, L. Brodkin and E. Tusa, Proc. Waste Management '99, Tucson, AZ, 1999, pp. 2079--2091. [10] H. Leinonen, J. Lehto and A. Makel~ React. Polym., 23 (1994) 221-228. [11] S. Gonzalez-Luque and M. Streat, In: D. Naden and M. Streat, eds., Ion Exchange Technology, Ellis Horwood, Chichester, 1984, pp. 679-689. [12] K. Vaaramaa, S. Puili and J. Lehto, Radiochim. Acta, 88 (2000) 845-849. [13] K. Vaaramaa, J. Lehto and H. Ervanne, Radiochim. Acta, in press. [14] Statute on Household Water Application Guide 461/2000, Finnish Water and Waste Water Works Association, The Association of Finnish Local and Regional Authorities, Helsinki, 2000 [in Finnish]. [15] A.E. Martell and R.J. Motekaitis,The Determination and Use of Stability Constants, VCH, New York, 1988. [16] W.J. Mortier and R.A. Schoonheydt, Prog. Solid St. Chem., 16 (1985) 1-125. [17] J. Lehto, P. Kelokaski, K. Vaaramaa and T. Jaakkola, Radiochim. Acta, 85 (1999) 149-155. [18] T. Viraraghavan, K.S. Subramanian and J.A. Aruldoss, War. Sci. Tech., 40(2) (1999) 69-76. [19] W. Driehaus, M. Jekel and U. Hildebrandt, J Wat. SRT-Aqua, 47(1) (1998) 30--35. [20] W. Driehaus, R. Seith and M. Jekel, Wat. Res. 29(1) (1995) 297-305. [21] M. Annam,~lki and T. Turtialnen, eds., Treatment Techniques for Removing Natural Radionuclides from Drinking Water, STUK-A169, Radiation and Nuclear Safety Authority, Helsinki, Finland, 2000.
! ,;II ELSEVIER
Desalination 155 (2003) 157-170
www.elsevier.com/locate/desal
Removal of metals and anions from drinking water by ion exchange Kaisa Vaaramaa*, Jukka Lehto Laborator)/ of Radiochemistry, Department of Chemistry, Universityof Helsinki, PO Box 55, FIN-O0014Finland Tel. +358-9-19150162; Fax +358-9-19150121; email: kaisa,[email protected] Received 22 July 2002; accepted 15 October 2002
Abstract Five organic and two inorganic ion exchangers were evaluated for the removal of metals and anions from water of two drilled wells. Sodium titanate (CoTreat) and a chelating aminophosphonate resin were the most efficient exchangers in removing transition metals from the total of 1800 bed volumes processed. CoTreat was the best for almost all of the transition metals. The breakthrough level of manganese was below 1% with CoTreat even when its concentration in the feed water was high (1 mg/l). The weak acid cation resin took up transition metals relatively efficiently. Somewhat unexpectedly, the cation exchangers also removed arsenic from water. Arsenic may have been sorbed on iron species, which again was adsorbed and filtered by the exchanger beds. Most of the cation exchangers took up calcium and magnesium at low processing capacities (<400 BV), and the strong base anion resin took up nitrate, bromide and sulphate very efficiently below 700 bed volumes. Neither chloride nor fluoride was taken up by the exchangers tested.
Keywords: Ground water; Purification; Ion exchangers; Metals and anions
I. Introduction Our initial interest has been to evaluate the removal o f radioactive elements from ground water by means of ion-exchange processes. In an earlier work, we studied the performance of the ion-exchange materials in the removal of natural radionuelides from drinking water [1]. Appreciable concentrations o f natural uranium and its *Corresponding author.
daughter radionuclides (radium, polonium and lead) may occur in drinking water obtained from drilled wells when the bedrock contains considerable amounts of these nuclides. Ion exchange can be applied to purify water and to remove uranium and radium. It is also important to study the quality of drinking water after such treatment processes: The exchangers should not be overly effective in removing useful elements. It would also be advantageous if they removed other toxic and harmful elements as well as
0011-9164/03/$- See front matter © 2003 Elsevier Science B.V. All rights reserved PII: S 0 0 1 1 - 9 1 6 4 ( 0 3 )00293-5
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radionuclides. In this study, five organic and two inorganic ion exchangers were evaluated for their ability to remove metals and anions from drilled well waters in column experiments. The quality of ground waters used for drinking water may differ widely from one location to another. The ground waters in Finland are mainly soft calcium bicarbonate waters, although NaCI may dominate in the water from some drilled wells in coastal areas [2]. The high concentrations of Fe and Mn are a problem in some ground waters and these elements cause technical problems in water supplies. Also in some cases water hardness cause problems (mainly Ca 2+ and Mg2+). Strong acid cation exchangers are used to remove Fe, Mn, Ca and Mg. In general, all types of cation-exchange resins remove Ca and Mg effectively, but strong acid resins are often preferred in many applications [3]. Zeolites have replaced polyphosphates for removing hardness from washing water. Zeolite A in the sodium form is commonly used for this purpose [4]. In areas of Finland having a special type of granite (rapakivi), high fluoride concentrations may be found in ground waters [2]. In general, high concentrations of arsenic are found in areas where volcanogenic rock contains volatile elements or argillaceous schists [5]. In Finland, arsenic is a problem mostly in drilled bedrock wells. Concentrations in water from springs or dug wells rarely exceed the limits set for drinking water [6]. Arsenic can be removed from water by using anion-exchange resins [7,8]. A sodium titanate ion exchanger (CoTreat), a commercial inorganic ion exchanger developedat the Laboratory of Radiochemistry of the University of Helsinki, has proved to be highly selective for radioactive cobalt and other activation corrosion products (S4Mn, 59Fe, 65Zn, 63Ni) contained in waste waters generated by nuclear power plants [9]. It is therefore of interest to test the performance of this exchanger in removing these elements from ground water containing
"macro" concentrations (> 1 mg/l) of Fe and Mn and "minor" concentrations (> 1 ktg/l) of Zn and Ni. Chelating aminophosphonateresins have been investigated for use in purifying metallurgical process effluents. They have proved to be highly effective in removing zinc from effluents in the metal plating industry [10]. In addition, aminophosphonate resins separate uranium efficiently from phosphoric acid solutions [11] as well as from acidic ground waters [ 12].
2. Experimental 2.1. Ground water samples
The ground water samples for the experiments were taken from two different drilled wells. Water 3, which was also used in an earlier study [13], has high iron and manganese concentrations, while these concentrations in water 6 were much lower. The separation performance of the exchangers in removing natural radionuclides from water 6 was published earlier [1]. Water from the same batch of water 3 was used in all of the ion exchanger tests. In contrast, samples of water 6 had been collected at different times. This explains the slight variations in the chemical compositions of water 6 for each ion exchanger (Table 1). The pH of water 6 ranged from 7.3 to 8.2; it was 7.5 in water 3. The metal composition was determined by inductively coupled plasma mass spectrometry (ICP-MS) or inductively coupled plasma atomic emission spectrometry (ICP-AES) and the anions by ion chromatography. Before the ICP-MS/AES measurements, the samples were pretreated as follows: Samples of water 6 were first filtered with 0.45 lxm filters and then acidified. It was observed that iron sorbed on the filter material and therefore the pretreatment of water 3 samples was done in a reverse order (acidified and then filtered). The total Fe concentrations in water 6, given in Table 1, are mean values from other batches; they
K. Vaaramaa, J. Lehto / Desalination 155 (2003) 157-170
159
Table 1 Chemical compositions of the ground waters Component
Water 3 with the ion exchangers
Water 6 with Zeolite A
Water 6 with CoTreat
Water 6 with the organic ion exchangers
AI, p.g/! As, Ixg/1 Ba, ~tg/l Cd, ktg/1 Cr, ~tg/l Cu, p.g/l Ni, lag/1 Pb, p.g/l Zn, ~tg/l Ca, mg/l Fe, mg/l K, mg/l Mg, mg/l Mn, mg/l Na, mg/l Sr, mg/l Br, mg/l CI-, mg/1 F-, mg/! NO~, mg/l SO2-, mg/l
7.33 8.54 85.6 0.26 0.29 194.76 2.91 4.51 35.47 58.60 2.94 4.12 19.05 1.08 27.50 0.56 ND ND ND ND ND
86.4 1.76 19 0.03 0.29 196 1.95 8.5 105 36 -0.1 a 11.2 5.99 0.046 31.7 ND 0.1 24 0.32 9.84 25.8
12.2 1.52 7.17 <0.02 <0.20 34.4 0.85 0.83 67.4 22.1 -0.1 a 6.51 3.97 0.025 58.2 ND 0.2 49.4 1.0 3.92 21.1
14.6 1.0 9.55 <0.02 <0.20 27.8 0.93 0.76 37.05 30.55 -0.1 a 9.71 5.96 0.033 53 ND 0.15 37.25 0.63 7.27 26.3
aThe concentration of Fe (tot) was around 0.1 mg/l (see the text in the Experimental section). ND, not determined. were determined by atomic absorption spectrophotometry (AAS). Prior to the column tests, the raw water samples were bubbled with nitrogen gas (purity 99.5%) to expel radon (222Rn), because the samples were also used for radioactivity measurements.
2.2. Ion exchangers The organic and inorganic ion exchangers evaluated in the work are given in Table 2. The particle size was 0.3-0.85 mm for the inorganic ion exchangers and 0.3-1.2 mm for the organic
resins. The pretreatment o f the exchangers was performed as described by Vaaramaa et al. [ 1].
2.3. Column experiments The processing capacities of the ion exchangers for the inactive elements (Na, K, Mg, Ca, Cr, Mn, Fe, Ni, Cu, Zn, Cd, AI, Pb, As, Br-, CI-, F-, NO~ and SO 2-) were determined. The breakthrough values given in this paper were calculated by dividing the ion concentration in the effluent by the ion concentration in the feed water and given in percentages.
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Table 2 Ion exchangerstested Type Organic ion exchangers: Aminophosphonate Strong acid cation Weak acid cation Strong base anion Weak base anion Inorganic ion exchangers: Sodium titanate Synthetic zeolite
Name
Chemical composition
Manufacturer
Purolite $ 9 5 0 Purolite C 145H Purolite C 104 Purolite A 5 0 0 Purolite A105
R-CH2NHCH2PO3H 2 R-SO3H R-COOH R-N(CH3)3OH R-N(CH3)2
Purolite Purolite Purolite Purolite Purolite
CoTreat Zeolite A (Si/AI = 1.1)
-Nal2 [(AIO2)12(SiO2)12]• 27H20
Selion Oy Laporteand Bayer
R, styrene-divinylbenzene.
If the calculated breakthrough value indicated is over 100% (e.g., 120%), this means that the exchanger did not take up the cation/anion from the feed water, but that the exchanger released the element which it had taken up at low processing capacities. This is caused by the "chromatography effect": at lower processed water volumes, the exchangers take up several cations, whereas at higher volumes, when the exchanger capacity is exceeded, the exchanger continues to take up the cations it prefers, but at the same time releases the ones which are less preferred. Thus, if the percentage is 120%, the "extra" 20% in the breakthrough value is the relative amount of the element released from the exchanger. A total of 1800 bed volumes (1 BV = 10 ml) of ground water was processed through the exchanger beds. The 18 L processed correspond to 3-4 months of water consumption in a typical Finnish single-family dwelling, assuming a water consumption of 5 m3/week and a 40-L exchanger bed. To investigate the effect of flow rate on element removal, four different elution rates were used for water 6: first, 6 L at 15 BV/h; then 4 L at 40 BV/h; next 4 L at 60 BV/h, and, finally, 4 L
at 80 BV/h. In the experiments with water 3, the flow rate was constant, 20 BV/h. The chemical compositions of the collected fractions were determined with respect to metals by ICP-MS or ICP-AES for both waters, and with respect to anions by ion chromatography for water 6. The pH values of the effluent fractions were determined. 3. Results and discussion 3.1. p H o f the effluents
The raw water pH in water 6 ranged from 7.3 to 8.2 (see Experimental); it was 7.5 in water 3. The pH of the collected fractions from the sodium titanate (CoTreat) column did not differ significantly from those of the feed waters. The pH of the first fractions from the zeolite A column (not analysed by ICP-MS) in water 6 decreased from 9.3 to 8.2, but after 600 processed bed volumes the pH was 7.9, very close to that of the raw water (7.3). This trend was similar to the pH values of the fractions obtained with water 3. The pH values of the fractions collected from the organic resin columns were relatively close to the pH of the feed water (7-8).
K. Vaaramaa, ,I.. Lehto / Desalination 155 (2003) 157-170 3.2. Removal o f calcium and magnesium Tables 3 and 4 show the breakthrough values of calcium and magnesium for water 6 and water 3, respectively.
3.2.1. Calcium The weak acid cation resin (WAC), zeolite A and CoTreat showed high uptake of Ca from both waters. However, the breakthrough values with these exchangers increased at higher bed volumes, for water 3 being almost 100% at
161
1300 BV for the WAC and CoTreat and at 1700 BV for zeolite A (Table 4). The breakthrough values for calcium with the WAC, zeolite A and CoTreat were lower for water 6 than for water 3 (considering the total processed bed volumes). At low processing capacities, the strong acid cation resin (SAC) also took up calcium; the breakthrough value was 76% (440 BV) for water 3 and 7% (670 BV) for water 6. At higher bed volumes, the uptake of calcium decreased with the SAC. The SAC did
Table 3 Chemical compositionsof effluent fractionsand the breakthroughvalues for Mg, Ca, Pb and As in the column experiments with water 6 Sample
SBA'
WBA'
SAC"
WAC"
$950"
Zeo Aa
CoTreaP
Processedbed v o l u m e s
668 1191 1608 671 1197 1629 679 1209 1628 598 1164 1589 667 1185 1622 601 1043 1578 655 1070 1474
Breakthough value, % Mg
Ca
Pb
As
96.1 99.0 98.7 96.6 98.5 98.5 12.08 144b 114b <1.7c 30.9 142b 60.4 104b 98.5 2.50 72.8 95.2 135b 131b 113b
97.5 96.6 98.9 96.2 95.6 95.6 6.97 82.2 94.9 0.75 11.9 73.3 34.7 92.0 92.6 2.36 33.1 86.9 4.39 53.8 79.2
10.5 11.8 13.2 13.2 5.26 6.58 27.6 7.90 6.58 21.1 17.1 <4c 7.90 10.5 10.5 75.3 5.41 30.6 19.3 15.7 18.1
49.0 115b 89.0 94.0 80.0 78.0 82.0 80.0 80.0 78.0 85.0 71.0 68.0 59.0 71.0 85.8 30.7 59.1 <3.3c 11.8 21.7
"Strong base anion resin (SBA), weak base anion resin (WBA), strong acid cation resin (SAC), weak acid cation resin (WAC), aminophosphonate resin ($950), syntheticzeolite (zeolite A) and sodium titanate (CoTreat). ~l'he percentage above 100% indicates the relative amount of the element released from the exchangers. CTheconcentration of effluent was below the detection limit.
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Table 4 Chemical compositions of effluent fractions and breakthrough values for Mg, Ca and Pb in the column experiments with water 3 Sample
Processed bed volumes
Breakthrough value, % Mg
Ca
Pb
SACa
444 873 1326 1785
152b 98.2 99.2 101b
75.6 102b 104b 101b
14.0 11.8 12.6 10.4
WACa
452 892 1353 1818 455 896 1360 1826 439 874 1328 1774 448 874 1317 1751
30.4 124b 106b 101b 104b 100 99.2 99.7 60.4 96.1 99.7 103b 129b 103b 99.2 98.7
6.16 81.4 99.0 102b 89.9 100 101b 104b 23.4 69.8 88.1 94.5 53.2 91.1 96.8 98.6
14.4 9.31 7.76 5.99 16.0 9.76 8.43 9.76 11.8 14.2 7.32 9.09 9.31 7.98 4.21 3.99
$950~
Zeo A~
CoTreaP
aStrong acid cation resin (SAC), weak acid cation resin (WAC), aminophosphonate resin ($950), synthetic zeolite (zeolite A) and sodium titanate (CoTreat). bA percentage above 100% indicates the relative amount of the element released from the exchangers.
not take up Ca from water 3 at all, but from water 6, which had a Ca concentration two-fold lower than that o f water 3, the resin did take up some calcium. In general, if ground water is soft and especially when it is acidic, corrosion problems may occur in water distribution equipment. Considering the recommendations for household water [14] and the concentrations of Ca in the effluents o f water 6, only at low processing capacities (_<600 BV) were the concentrations o f calcium in the effluents below the recommendations (10 mg/l). One exception was the WAC,
for which the concentration of Ca in the effluent did not comply with the recommendation for Ca until 1500 BV had been processed (Tables 1 and 3).
3.2.2. Magnesium Most o f the cation exchangers took up magnesium at 600 BV and below with water 6 (Table 3) and at 400 BV and below with water 3 (Table 4), but then a release o f the cation was observed at higher bed volumes. This was caused by the "chromatography effect" explained above.
K. Vaaramaa, J. Lehto / Desalination 155 (2003) 157-170 3.3. Removal o f the transition metals The Species Distribution Program (SPE) was used to calculate the distributions of the metals species in water 3 [15]. The stability constants of the anion complexes (carbonate and sulphate) and the hydrolysis species and solubility products o f metals were used. The metal species distribution data are only indicative, because each metal and its species were calculated separately. Competition between the metals was thus excluded. The concentrations of the metals shown in Table 1 were used for the computation,
163
while the concentrations of anions in water 3 had been determined elsewhere [13]. The breakthrough values o f the transition metals are given in Tables 5 and 6. The breakthrough curves as a function of the processed bed volumes for the WAC, CoTreat and the aminophosphonate resin in water 3 are shown in Fig. la-c.
3.3.1. lron With water 3, all of the cation exchangers took up iron rather well (Fig. 2); the break-
Table 5 Chemical compositions of effluent fractions and breakthrough values for Mn, Ni, Cu and Zn in the column experiments with water 6 Sample
SBAa
WBAa
SAC~
WACa
$950a
Zeo Aa
CoTreaP
Processed bed volumes
668 1191 1608 671 1197 1629 679 1209 1628 598 1164 1589 667 1185 1622 601 1043 1578 655 1070 1474
Breakthrough value, % Mn
Ni
Cu
Zn
68.6 82.4 62.8 45.0 70.8 47.5 9.95 4.20 6.71 0.43 6.77 3.80 0.55 0.34 0.40 4.72 3.00 11.9 0.52 0.61 0.52
83.9 74.2 66.7 109b 89.2 93.5 32.3 115b 104b 11.8 28.0 44.1 25.8 49.5 48.4 28.7 49.2 67.2 23.5 17.6 16.5
29.2 35.5 42.4 35.3 41.0 41.7 87.8 77.3 68.0 31.1 37.4 31.7 18.8 18.6 20.6 31.6 16.0 30.7 7.67 11.2 13.8
87.4 90.1 89.6 98.8 92.3 82.3 13.0 118 97.4 1.75 3.05 7.53 1.67 1.57 3.64 6.21 3.05 23.05 1.3 ! 1.16 1.68
aStrong base anion resin (SBA), weak base anion resin (WBA), strong acid cation resin (SAC), weak acid cation resin (WAC), aminophosphonate resin ($950), synthetic zeolite (zeolite A) and sodium titanate (CoTreat). bA percentage above 100% indicates the relative amount of the element released from the exchangers.
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Table 6 Chemical compositions of effluent fractions and breakthrough values for Cr, Mn, Ni, Cu and Zn in the column experiments with water 3 Sample
SAC a
WAC a
$950 '
Zeo A~
CoTreat"
Processed bed volumes
444 873 1326 1785 452 892 1353 1818 455 896 1360 1826 439 874 1328 1774 448 874 1317 1751
Breakthrough value, % Cr
Mn
Ni
Cu
Zn
100 96.6 82.8 <69 c <69 c 93.1 96.6 82.8 72.4 89.7 93.1 82.8 <69 c <69c 75.9 72.4 <69 c 75.9 79.3 72.4
156b 98.1 100 102b 1.85 28.7 47.2 60.2 0.93 0.93 2.78 3.70 33.3 76.9 88.0 94.4 <0.093 ~ <0.093 ~ <0.093 ¢ 0.93
129b 97.3 93.5 99.7 42.6 74.2 71.8 78.7 34.7 34.0 35.1 33.0 70.1 96.6 94.2 96.6 39.5 47.8 43.3 46.7
30.9 30.4 34.2 35.3 25.1 19.5 19.0 19.4 22.2 15.8 14.8 16.1 22.8 28.7 23.5 28.8 15.2 13.2 9.89 9.89
120b 64.0 64.6 60.9 20.3 14.2 10.9 10.3 18.7 11.7 10.3 12.0 17.7 35.3 27.4 33.0 10.7 12.6 5.85 4.89
aStrong acid cation resin (SAC), weak acid cation resin (WAC), aminophosphonate resin ($950), synthetic zeolite (zeo A) and sodium titanate (CoTreat). bA percentage above 100% indicates the relative amounts of the element released from the exchangers. CThe concentration of effluent was below the detection limit.
through values were below 45%. CoTreat was the most efficient in removing iron, and the breakthrough level even tended to decrease (from 27% to 10%) with an increase in the bed volume processed. According to the SPE program, iron was present in the water as Fe(OH)3, i.e., colloidal or particulate iron, and the filtering properties probably improved during the column run. This trend was typical for other exchangers as well. The breakthrough values for the SAC and the W A C were slightly higher (Fig. 2): 2 8 38% and 16--41%, respectively. The break-
through values o f Fe for water 6 are not presented here because iron was sorbed on the filter material before the ICP analysis (as noted in the Experimental section). Therefore, the concentration o f iron in the fractions collected from water 6 was below the detection limit (0.03 mg/l). 3.3.2. Manganese If manganese is present in the water as the +2 oxidation state, then according to the SPE program, it is present as the M n 2+cation when the
165
K. Vaararnaa, J. Lehto I Desalination 155 (2003) 15 7-170
100 la
80
,o
= e ,-
60
30
40
20
~
20
t-o~
m
0 500
1000
1500
2000
Volume of processed water 3 (BV)
lb
80 60
9 ~" 40
~
rn
0
O~
500
,
0
,
1000
,rl
,
1500
2000
Volume of processed water 3 (BV) ._. 100 e"
o}
o t--
rn
!
i
500
1000
!
1500
|
2000
Volume of processed water 3 (BV)
20 0
v
0
Fig. 2. Breakthrough curves of Fe for the organic ion exchangers (strong acid cation, SAC •; weak acid cation, WAC ,,; chelating aminophosphonate, A) and for the inorganic ion exchangers (synthetic zeolite, zeolite A, +; sodium titanate, CoTreat, [3) as a function of the processed bed volumes with water 3.
100 t-.
10
v
1C
80 60 40
0 0
500
1000
1500
2000
Volume of processed water 3 (BV) Fig. 1a-c. Breakthrough curves of the transition metals Mn (N), Fe (+), Ni (A), Cu (•), Zn (A), and Cd (o) for a: weak acid cation exchanger (WAC, Purolite C104); b: sodium titanate (CoTreat, Selion); c: chelating aminophosphonate resin (Purolite $950) as a function of the processed bed volumes with water 3.
pH is 7-8, and in that case cation exchangers are able to remove it from water. With water 6, however, all o f the exchangers took up manganese to some extent (Table 5): the breakthrough values for the anion exchangers were
50-80%, and typically lower than 10% for the cation exchangers. The most effective manganese removers were CoTreat and the aminophosphonate resin, both o f which took up more than 99 % of the manganese. The uptake of Mn by the SAC, the WAC and zeolite A was lower for water 3 than for water 6. This discrepancy may be explained by the 30-fold higher concentration of manganese in water 3 as compared to water 6. As can be seen in Fig. la, the WAC resin took up Mn rather well from water 3 at the beginning o f the column run, but the breakthrough o f Mn clearly occurred at 500 bed volumes. With the aminophosphonate resin, the uptake values o f Mn were almost the same as for water 6; they were even better with CoTreat (Tables 5 and 6). With water 3, the breakthrough value o f Mn for CoTreat was below 0.093% (the concentration of the effluent was below the ICP-MS detection limit for Mn) up to 1300 BV, and was 0.93% at 1750 BV (Table 6). At low processing capacities, the SAC resin takes up manganese, but then releases it at 400 BV until at about 1000 bed volumes the concentration of Mn in the effluent is the same as in the feed (Table 6). Iron is present in the waters as Fe(OH)3, and thus one could assume that manganese is present
166
K. Vaaramaa, J. Lehto / Desalination 155 (2003) 157-170
as insoluble Mn(IV)oxides in the prevailing conditions as well. In general, when the breakthrough of an ion occurs in a column process, it shows as an upward curve as a function of the processed water volume. Instead, if the metal is present in the water in its particle form, the breakthrough level of this metal may decrease during the column run (i.e., the breakthrough curve is downward), as was seen in the case of iron. The breakthrough curves for the cation exchangers (Fig. l a-c) support the assumption that manganese is present in water 3 mostly as the soluble metal cation. 3.3.3. Zinc
According to the SPE program, zinc was present in these waters mainly as the Zn 2÷cation at pH 7-8. Soluble zinc carbonate complexes and hydrolysis species were also present to a lesser degree. In water 6, the separation performances of the exchangers for zinc were similar to those for manganese. CoTreat and the aminophosphonate resin were again the most effective exchangers, although the breakthrough values were somewhat higher than for Mn. The breakthrough values of Zn were 1.6--3.6% with the aminophosphonate resin and 1.2-1.7% with CoTreat (Table 5). For the WAC, the flow rate most likely had an increasing effect on the breakthrough of Zn. At flow rates of 15, 60 and 80 BV/h, the average breakthrough values were 1.8%, 3% and 7.5%. The separation performance of the exchangers for zinc were much lower with water 3 than with water 6 (Table 6), which again may be explained by the fact that the concentrations of alkaline earth metals and almost all of the transition metals were higher in water 3. Sodium titanate (CoTreat) was the most effective for Zn with both water 3 (Table 6, Fig. lb) and water 6. The next best for Zn were the aminophosphonate and the WAC resins (Table 6, Fig. la and c). The breakthrough values of Zn for the WAC and the aminophosphonate resins were almost equal (10-20%).
3.3.4. Copper
All of the exchangers took up copper from water 6 (Table 5). The species distribution program showed that copper exists as the soluble carbonate species in pH 7-8 and that a significant portion of the Cu also precipitates as Cu(OH)2. Minor amount are also present as the free metal cation. As in the case of Fe, the significant portion of Cu is present as colloidal form in the waters. Sodium titanate took up Cu most efficiently, especially at low processing capacities (at 600 bed volumes). All of the cation exchangers took up copper from water 3 in the order of: CoTreat > aminophosphonate > WAC > zeolite A > SAC (Table 6). 3.3.5. Nickel
Based on the SPE program, the waters contained nickel mainly as the Ni 2+cation and, to some extent, as soluble Ni-carbonate complexes. In general, the uptake of nickel was lower than that of Cu with both waters. In the experiments with water 6 in which the flow rate was increased during the column run, CoTreat was the most efficient for nickel (Table 5). The flow rate clearly affected the breakthrough of Ni with the chelating aminophosphonate resin (Table 5). The average breakthrough value of Ni rose from 26% to 49% when the flow rate was increased from 40 BV/h to 60 BV/h. The kinetics of the metalion exchange on aminophosphonate resin is known to be rather slow. When the flow rate was constant, as in the run with water 3, the breakthrough values of Ni were constant (3335%) for this resin (Fig. 1c, Table 6). In addition, the aminophosphonate resin was better than CoTreat in removing Ni from water 3. 3.3.6. Cadmium
The concentration of cadmium in the water 6 feed was near or below the detection limit (0.02 p,g/l) of the ICP-MS analysis. Although the concentration of Cd was also low in water 3
K. Vaaramaa, d. Lehto / Desalination 155 (2003) 157-170
(0.26 ~tg/l), the separation performances of the exchangers could be determined. The uptake of Cd was the highest with CoTreat (breakthrough 8% or lower, Fig.lb) and the lowest with the SAC (breakthrough values 66-97%). The second best exchanger for Cd was the WAC (Fig. 1a); the aminophosphonate resin was almost as efficient (Fig. 1c). According to the SPE program, Cd was present mainly as the free metal cation in the water; a minor amount was complexed with carbonate.
3.3. 7. Chromium The concentrations of chromium were also below or near the detection limit of the ICP-MS analyses (0.2 ~tg/1). The breakthrough values determined for Cr with water 3 are given in Table 6. Chromium (III) was present as the soluble hydrolysis species in the water (in pH 6-9 as CrOH 2+, Cr(OH)~, Cr(OH)3, Cr(OH)4). The sodium titanate (CoTreat) and the aminophosphonate resin were overall the most efficient exchangers in removing the transition metals from the waters tested. The weak acid cation resin (WAC) was also reasonably good. A similar trend in the separation efficiency of CoTreat and the aminophosphonate resin for the transition metals was observed with water 3 (Fig. 1 b and c). The separation efficiency was the highest for Mn and the lowest for Fe and Ni. With regard to Cd, Zn and Cu, the trend was somewhat similar. The aminophosphonate resin preferred Cd over Zn, as did CoTreat (up to 1300 bed volumes). Above 1300 BV, the preference order with CoTreat was reversed (Fig. 1b and c). The order of the separation efficiency for metals with the WAC was similar to that of CoTreat below 500 bed volumes: Mn > Cd > Zn > Cu > Fe > Ni. With the WAC resin, the breakthrough of Mn clearly occurred at 500 BV (Fig. la). The separation efficiency of CoTreat and the aminophosphonate resin for Mn, Zn, Cu and Ni in water 6 was similar to that of water 3, except
167
that the both exchangers preferred more clearly Zn over Cu than with water 3.
3.4. Removal of potassium and sodium Zeolite A and CoTreat were the exchangers that took up potassium: at 600 BV, the breakthrough values of potassium in water 6 were 10% and 55%, respectively, and at 400 BV the values for water 3 were 21% and 67%. However, these exchangers did not take up potassium at higher bed volumes; zeolite after 1300 BV and CoTreat after 800 BV. Because of the health effects of sodium, the sodium concentration in drinking water should not exceed the limit of 200 mg/l [3,14]. None of the exchangers took up sodium. All of the cation exchangers (which were initially in sodium form) released sodium. The poorest water quality with respect to sodium was detected with the WAC and zeolite A, when the concentrations in the effluents at 400 BV (with water 3) and 600 BV (with water 6) were about 90 mg/l. Although the cation exchangers released sodium into the treated water, the limit set for drinking water was not exceeded.
3.5. Removal of Al, Pb and As 3.5.1. Aluminium With water 6, the breakthrough ofaluminium was only about 20-30% for all except zeolite A (from which AI leached into the water). The excess aluminium found in the treated water was probably extra-framework aluminium from this exchanger [16]. The recommended limit for aluminium in drinking water in Finland is 200 ~tg/l [14]. With zeolite A, this level of A! in the effluent at 600 BV was exceeded 5-fold. At higher bed volumes, the concentration of A1 in the effluents was the same as in feed water 6. The breakthrough values for water 3 were higher (40-80%, with the exception of zeolite) than for water 6. Aluminium leached into the treated water with zeolite A again, but the concentrations of A! did not exceed the limit of 200 ~tg/l.
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K. Vaaramaa, J. Lehto / Desalination 155 (2003) 157-170
3.5.2. L e a d
The uptake of inactive and radioactive lead was studied with water 6. The behaviour of radioactive lead (21°pb) in the water has been studied in an earlier work [ 1]. With the exception of zeolite A, all of the exchangers behaved similarly for the uptake of both inactive and radioactive lead. This indicates that the inactive lead and 21°pb are in the same chemical form. For an unknown reason, the breakthrough values of inactive lead varied widely for zeolite A (Table 3). Both the anion exchange resins and the cation exchangers took up inactive lead from water 6 (Table 3). On the basis of breakthrough curves for radioactive lead and our earlier studies [ I, 17], we have concluded that 210pbwas present in water 6 as particles. It can further be presumed that some of the inactive lead was also present as particles, which were adsorbed and filtered by the exchanger beds. The breakthrough values of lead with water 3 were also rather low (Table 4). 3.5. 3. Arsenic
Although the concentration of arsenic was low in both raw waters - - 1.4 I~g/I (water 6) and 8.5 ~tg/l (water 3) - - some conclusions on the uptake of As can be drawn. Arsenite (As(Ill)) and arsenate (As(V)) are the most important species of arsenic in ground water supplies for drinking [7,8,18]. Arsenite is typically present as a neutral H 3 A s O 3 molecule at pH 6-9, while arsenate occurs primarily as the H2AsO4 and HAsO 2species. Thus, in order to remove trivalent arsenic efficiently from water by adsorption or ion exchange, As(Ill) must be oxidised to arsenic(V) [7]. Strong base anion-exchange resins are used to remove arsenic from water. The performances of the anion exchange resins were tested for the uptake of anionic arsenic species from water 6. The strong base anion exchanger (SBA) took up arsenic at 600 BV and lower, but then released it at 1000 BV (Table 3). It should be remembered
that, other than N 2 bubbling, the raw water had not been pretreated before the column experiments. Therefore, it is not certain that all of the arsenic was present as +5. On the other hand, the concentration of bicarbonate (HCO~) in these ground waters was quite high, -2.5 mmol/l (water 6) and 4-5 mmol/l (water 3); bicarbonate thus competed with the arsenate anions for the ion-exchange sites. In addition, if the arsenic removal process were simply arsenate anion exchange, the anion exchanger would be the most efficient for the uptake of arsenic. However, sodium titanate was the most efficient in these conditions (Table 3). The breakthrough value of As for CoTreat was below 3.3% (655 BV) while it was 49% (668 BV) for the SBA. All the cation exchangers took up As from water 3. The breakthrough values were of the order of 20% for the SAC, the WAC, the aminophosphonate resin and zeolite A, and 7-12% for CoTreat (Fig. 3). The concentrations of Fe, Mn and As in water 3 were higher than those in water 6. For Fe and As, the breakthrough curves of the exchangers were similar with water 3 (Figs. 2 and 3). The correlation between the behaviour of Fe and As may be explained by the fact that iron was present as Fe(III)hydroxide and by assuming that As precipitated with the Fe(OH)3, which was 50 ~..~ 40 oe - ~ 30 20
m
lO o
t
i
500
1000
i
i
1500
i
2000
Volume of processed water 3 (BV)
Fig. 3. Breakthrough curves of As for the organic ion exchangers(strongacid cation, SAC * ; weak acid cation, WAC m; chelating aminophosphonate, J,) and for the inorganic ion exchangers(syntheticzeolite, zeolite A, +; sodium titanate, CoTreat, I"1) as a function of the processed bed volumes with water 3.
K. Vaaramaa, ,1. Lehto / Desalination 155 (2003) 157-170
then sorbed on the exchanger beds. Driehaus et. al. [ 19] studied granular ferric hydroxide in the removal of arsenic (V) from natural water, and found it to be an effective adsorbent for arsenate removal. In general, arsenic is removed with greater efficiency in sorption processes if it is present in the +5 oxidation state rather than the +3 oxidation state [7]. The concentration of Mn was quite high in water 3, 1.08 mg/l. As discussed above, it seems that manganese was present in the water mostly as soluble Mn 2+. It is also possible that manganese(IV)oxides were present, which may have increased the As(III)oxidation from oxidation state +3 to +5. Manganese dioxide has been found to be an effective oxidising agent for As (III) in water treatment [20]. 3. 6. Removal of anions The removal of five anions (Br-, CI-, F-, NO~ and SO 2-) was studied in column experiments with water 6. The concentrations of the anions in the feed waters are shown in Table 1. Changes in concentrations were observed only with the strong base anion resin (SBA, Fig. 4). At low processing capacities, the SBA took up nitrate very efficiently (<700 BV; breakthrough value
._., 200 150 o 100
50 m
_
OI 500
1000
1500
2000
Volume of processed water 6 (BV) Fig. 4. Breakthrough curves of the anions NO; (=), SO 2- (A), F- (+), CI- (#), and Br- (D) for the strong base anion exchanger (SBA, Purolite A500) as a function of the processed bed volumes with water 6.
169
was 6.5%). It also took up bromine and sulphate very efficiently: the breakthrough of Br was less than 0.1% and that of sulphate 7.8% below 700 BV. Extra chloride was observed to elute from the SBA resin; the water quality with respect to its chloride concentration was, however, reasonably good. 4. Conclusions
The aminophosphonate resin and the CoTreat exchanger were several times more efficient than the conventional resins in removing toxic and harmful transition metals (Cu, Zn, Mn, Ni) from drilled well water. The weak acid cation exchanger (WAC) was also rather efficient in the uptake of transition metals. As expected, the effect of the flow rate was clearly seen for the WAC. Weakly acidic resins and inorganic ion exchangers are known to have relatively slow kinetics for metal-ion exchange. The breakthrough level of manganese was very low with CoTreat (<1%) even when the concentration of Mn in the feed was 1 mg/l. The processing capacity of the strong acid cation resin (SAC), which is usually used to remove Mn from water, was significantly lower than of the other exchangers. Iron, which was present as Fe(III)hydroxide, was taken up well by the cation exchangers. Arsenic, which presumably was sorbed on the iron species, was consequently sorbed on the exchangers. In the past years, one of our interests has been the use of ion exchange to remove natural radionuclides from drinking water. Another method useful for removing uranium and its longlived daughters from water is the reverse osmosis technique, but the disadvantage of this method is the quality of the treated water; the effluent is demineralized during the process [7, 21 ]. During the process of ion exchange, the effluents were not demineralized, but the harmful elements were removed. The quality of the treated water was
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K. Vaaramaa, J. Lehto / Desalination 155 (2003) 157-170
reasonably good, although extra sodium was eluted from the cation exchangers and extra chloride from the anion exchanger. However, after treatment with ion exchangers, water rehardening (alkalisation) is recommended in some cases.
Acknowledgements This work was supported by EU research project (FI4PCT960054), the Maj and Tor Nessling Foundation (Finland) and the Finnish Cultural Foundation. KV also thanks Dr. Teresia M611er for helpful discussions.
References [1] K. Vaaramaa, J. Lehto and T. Jaakkola, Radiochim. Acta, 88 (2000) 361-367. [2] P. Lahermo, M. Ilmasti, R. Juntunen and M. Taka, The Geochemical Atlas of Finland Part 1, Geological Survey of Finland, Espoo, 1990. [3] W. H~511and H-C Fiemming, in: K. Dorfner, ed., Ion Exchangers, Walter de Gruyter, Berlin, 1991, pp. 835-844. [4] A. Dyer, An Introduction to Zeolite Molecular Sieves, Wiley, New York, 1988. [5] T. Koljonen, ed., The Geochemical Atlas of Finland Part II, Geological Survey of Finland, Espoo, 1992. [6] H. Kahelin, K. Jarvinen, P. Forseil and M. Valve, Arsenic in Drilled Wells-Comparison of Arsenic Removal Methods, Report of Investigation 141, Geological Survey of Finland, Espoo, 1998.
[7] E.O. Kartinen, Jr,and C.J. Martin, Desalination, 103 (1995) 79--88. [8] L-L. Homg and D. Clifford, Resins. React. Funct. Polym., 35 (1997) 41-54. [9] R. Harjula, J. Lehto, A. Paajanen, L. Brodkin and E. Tusa, Proc. Waste Management '99, Tucson, AZ, 1999, pp. 2079--2091. [10] H. Leinonen, J. Lehto and A. Makel~ React. Polym., 23 (1994) 221-228. [11] S. Gonzalez-Luque and M. Streat, In: D. Naden and M. Streat, eds., Ion Exchange Technology, Ellis Horwood, Chichester, 1984, pp. 679-689. [12] K. Vaaramaa, S. Puili and J. Lehto, Radiochim. Acta, 88 (2000) 845-849. [13] K. Vaaramaa, J. Lehto and H. Ervanne, Radiochim. Acta, in press. [14] Statute on Household Water Application Guide 461/2000, Finnish Water and Waste Water Works Association, The Association of Finnish Local and Regional Authorities, Helsinki, 2000 [in Finnish]. [15] A.E. Martell and R.J. Motekaitis,The Determination and Use of Stability Constants, VCH, New York, 1988. [16] W.J. Mortier and R.A. Schoonheydt, Prog. Solid St. Chem., 16 (1985) 1-125. [17] J. Lehto, P. Kelokaski, K. Vaaramaa and T. Jaakkola, Radiochim. Acta, 85 (1999) 149-155. [18] T. Viraraghavan, K.S. Subramanian and J.A. Aruldoss, War. Sci. Tech., 40(2) (1999) 69-76. [19] W. Driehaus, M. Jekel and U. Hildebrandt, J Wat. SRT-Aqua, 47(1) (1998) 30--35. [20] W. Driehaus, R. Seith and M. Jekel, Wat. Res. 29(1) (1995) 297-305. [21] M. Annam,~lki and T. Turtialnen, eds., Treatment Techniques for Removing Natural Radionuclides from Drinking Water, STUK-A169, Radiation and Nuclear Safety Authority, Helsinki, Finland, 2000.