- Email: [email protected]
Removal of Ni(II) and Zn(II) from an aqueous solutionby reverse osmosis
DESALINATION
ELSEVIER
Desalination 174 (2005) 161-169
www.elsevier.corrdlocate/de sal
Removal of Ni(II) and Zn(II) from an aqueous solution by reverse osmosis Ubeyde Ipek Department of Environmental Engineering, Faculty of Engineering, Firat University, 23119 Elazig, Turkey Fax: +90 (424) 241-5526; email: [email protected] Received 30 March 2004; accepted 24 September 2004
Abstract
The removal ofNi +~and Zn÷2from an aqueous solution by reverse osmosis (RO) at different pH, conductivity and EDTA concentrations was investigated. In addition, the removal in the pretreatment units (PU) with filtration (F) and granular activated carbon (GAC) at the same conditions was also determined. It was observed that Zn+2rejections did not change much with pH, usually less than 0.88 mg/1, and Ni +~rejection was below the detection limits of AAS in the range ofpH 4-8. While Ni +2and Zn÷2were removed 23-25% and 25-45%, respectively, by PU, the rejection of Ni ÷2 and Zn÷2was, respectively, determined to be higher than 99.2% and 98.8% by PU+RO at experiments related to the determination of the effect of initial metal concentration on rejection. It was found that the influent conductivity affected metal rejection at an unimportant level, but the increase of conductivity affected the effluent conductivity. The addition of EDTA into the aqueous solution increased Ni ÷2and Zn÷2rejection from 99.3% to 99.7% and from 98.9% to 99.6% at an EDTA concentration of 240 ppm, respectively.
Keywords: Reverse osmosis; Zinc (II); Nickel (II); EDTA
I. Introduction
The world is facing pollution problems due to toxic elements in the environment. Large quantities o f metals are being released into the environment (i.e., air, water and soil) from mining, industrial agricultural and other anthropogenic waste due to inadequacies o f technology in the processing o f metals or through other routes. The higher concentrations o f heavy metals in soils, water and plants, their chemicals forms and the
mobility and availability to the food chain provide the basis for a range o f problems related to the crop, animal and human health [ 1]. Nickel and zinc are toxic heavy metals that are widely used in industry, including silver refineries, electroplating, zinc base casting, storage battery industries and (more recently) landfill leachates [2,3]. The chronic toxicity o f nickel to humans and the environment has been well documented. For example, high concentrations o f metals cause
0011-9164/05/$- See front matter © 2005 Elsevier B.V. All rights reserved doi: 10.1016/j.desa1.2004.09.009
162
U. Ipek / Desalination 174 (2005) 161-169
cancer of lungs, nose and bone. It is essential to remove metals from industrial wastewater before being discharged. For this reason, advanced treatment processes are generally used such as chemical reduction, ion exchange, reverse osmosis (RO), electrodialysis, and activated carbon adsorption [4]. Granular activated carbon removes the heavy metals from waste solutions by physical adsorption where the properties of the surface area has an important role. Membranes with very low-pressure requirements for operation are promising in removing heavy metals from wastewater [5]. The number of researchers who investigated the efficiencies of low-pressure membranes to remove heavy metals has recently been increasing. Low-pressure RO membranes with pressures below 690 kPa for Zn +2and Cu +2removal have been used [6] as well as low-pressure RO in which operating pressures were varied in the range of 300 kPa to 1800 kPa [7]. The main aim of the present work was to investigate Zn +2 and Ni +2 removal from wastewater by a RO unit by changing some parameters such as the feed pH and metal concentration. Furthermore, EDTA as an aid to the membrane separation process was used to remove heavy metals from wastewater; its effect on the removal of heavy metals was also investigated. Manometers
J Feed
Pump
I
ter'
Concentrate
GAC
Fig. I. Experimental set-up.
High pressure pump
BMan~
2. Materials and methods
2.1. Experimental set-up The experimental work was carried out by using an apparatus at pilot scale. The experimental set-up is illustrated in Fig. 1. The filter used in the experimental work, manufactured by Osmonics, has extended blown microfiber technology to meet the requirements for a pure polypropylene deep filter with an exceptional dirt holding capability. This translates to longer life on fewer changes than existing string-wound or resin-bonded filters. The Desal-11, a thin-film R e membrane, was used as a membrane throughout the work. This membrane (AG4021FF) manufactured by Desal (Osmonics) is characterized by high flux and excellent sodium chloride rejection. The properties and operating parameters of the membrane are given in Table 1. Granular activated carbon (AquaSorb 1000) was manufactured by steam activation from selected grades of bituminous coal. The perfect balance between adsorption and transport pores provides optimum performance in a wide range of water treatment applications. The product is a high-density adsorbent and provides maximum volume activity. The total pore volume and apparent density of the adsorbent are 0.88 cm3/g and 500 kg/m3, respectively. Its surface area is 950 m2/g. The bed height
U. lpek / Desalination 174 (2005) 161-169 Table 1 Properties and operatingparametersof the membrane Model Cross reference GPD, m/d NaCI rejection,% (avg/min) Active area, m2
AG4021FF BW30-4021 1.050 (3.97) 99.4/99.0 3.72
Membrane Typical operatingpressure, kPa Maximumpressure, kPa Maximumtemperature, o RecommendedpH: optimumrejection operating range cleaning range
Thin-film (TFM) 1.379 2.758 50 6.5-7.5 4.0-11.0 2.0-11.5
and inner diameter of the pretreatment units are 50 and 6.5 cm, respectively. The bed dimensions of the membrane are 63.5 cm in height and 12.5 cm in out diameter, and the general dimensions of the entire system are 45x45x80 cm (wxlxh).
2.2. Operating conditions The water present in the RO system was discharged during 20 s at the beginning of experimental studies. Then, the flow was regulated and the concentrate stream was recycled to the feed tank. The system pressure was 1100 kPa. Samples of the pretreatment effluents were taken from sample point on the pretreatment unit. The permeates from the RO were collected in another tank and the flux measured. After each experiment the RO membrane was washed with distilled water. The total volume of samples used in each experiment was 15 L. The pH and conductivities of all samples were measured with a pH meter (WTW pH 330) and conductivity meter (WTW LF 330), respectively. The pH of each sample was adjusted by adding NaOH or H2SO4 stock solution under vigorous stirring. The synthetic wastewater was prepared by dissolving sodium chloride (NaC1), zinc chloride (ZiC12), nickel chloride (NiC12.
163
6H20 ) and ETDA (disodium salt) in distilled water. After the samples were collected, water samples were acidified with HNO3. The concentrations of zinc and nickel in the samples were analyzed by using an atomic absorption spectrophotometer (ATI Unicam 929 and Perkin Elmer 370), respectively.
3. Results and discussion 3.1. Effect o f p H The pH of the feed water can alter the nature of the membrane surface charge and also influence the characteristics of the solutes in the feed water and, thus, the membrane separation performance on the solutes [8]. Figs. 2 and 3 indicate the concentrations ofNi +zand Zn +2in the influent and effluent at the different pH and water matrices. The samples were taken both in the influent and effluent of RO (PU+RO) and the pretreatment units (PU) equipped with a RO system to eliminate the suspended solids in the influent. It was shown that the variations in Ni +2 rejections with pHs were not important (Fig. 2). While Ni +2 concentrations in influent vary between 8.22 and 10.29 mg/1, its concentrations in effluent of PU decreased to degrees between 4.07 and 6.56 mg/1. However, Ni +2 concentrations of PU+RO were below the detection limits of AAS. Although the influent has high Ni +2concentrations at pHs in the range of 5.5-7, Ni ÷2 indicated much more decreasing according to the other pHs in effluent of PU. For zinc metal, the same situation was also shown at pH = 6 (Fig. 3). Zn ÷2concentrations of influent ranged between 10.74 and 13.72 mg/1, and its concentrations of effluent of PU decreased to degrees between 7.14 and 9.56 rag/1. Zn+2concentrations in permeate did not change much with pH, generally less than 0.88 mg/l. The times of run varied between 1462.5-1530 s and 14851545 s for Ni +2and Zn+~, respectively. Figs. 4 and 5 indicate the levels of conductivity in different water matrixes at the experiments
164
U. Ipek / Desalination 174 (2005) 161-169 o
~
Influont
..-m-..
PU+RO
---•---
o
PU
12
15
1o
12
Influent
...ta... PU+RO
---*-.- PU
O
=
6
• ~1.. ° . ,
oo
u •"
4
U
.o$ ....
-°'$-
9
.°.~....oL
.
.
.
.
.
.
.
.
.
.
.
.
° " ~11% .
..
•'"
.•..
.•..
,s .,
,.
o
6
. . . . . . . . .
3
"-" 2 Z
NI ...--Q...
-r
-r
-r
5
6 pH
7
0 8
Fig. 2. Ni ÷2 concentrations o f influent and effluent at different pHs.
o
Influent
...t,... PU+RO
4
5
6 pH
7
8
Fig. 3. Zn +2 concentrations ofinfluent and effluent at different pHs.
o
---*--. PU
Influent
..-m... PU+RO
.--*--- PU
100
80
~
~60 OO
::k
80
• 60
i~ 40
.....
$ .....
~ .....
$ .....
•..
_.'~'"
..O, °.•'''--O""
..-•'-,.o•°.~'°
....
• .....
• .....
$ .....
O""
m.....
O .....
G .....
"
•
40 ~ .....
4
0 .....
Q .....
Q .....
Q .....
Q .....
g .....
I
t
I
5
6 pH
7
20
Q .....
- - -..Q
8
4
.....
5
6
G .....
Q .....
7
D .....
i
8
pH
Fig. 4. Variation o f conductivity with pH in the experiments o f N i ÷z removal•
Fig. 5. Variation of conductivity with pH in the experiments o f Zn ÷2 removal•
performed to determine the removal capacities of Ni +2and Zn+2at the different pHs, respectively. It was shown that the conductivity also decreased independently from pHs at two experiment series. While the conductivity of influent for the Ni +2 experiment series ranged between 46.4 and 75.5 mS/cm, it ranged between 34.4 and 44.1 mS/cm at the effluent of PU. Further, the conductivity was less relatively than 3.8 mS/cm in the final effluent. The conductivity of influent at the Zn+2experiment series ranged between 60
and 85 #S/cm, and it decreased to the values between 43 and 61 #S/cm after PU. Although the conductivity increased in the effluent of PU at higher pH values than pH 6.5, permeate generally has conductivity of 4.0 #mS/cm.
3.2 Effect of initial metal concentrations Ni +2 concentrations in influent were varied in the range of 44-169 mg/l in order to determine the effect of metal concentration of feed on
U. lpek / Desalination 174 (2005) 161-169 --
PU+RO
-.-~--- PU
100
* 26
165
Influent
•
PU÷RO
--- ~,--- PU
700 600 E
99,5
~500 ,0
,
'~"
d
99
24 '\
© 98,5
.
.
.
.
~,
,~400
.~t..
°
"~ 300 200
.
A"
100 98
I 44
I 86
I 129
22 0
169
50 1O0 150 Initial Ni(II) concentration, mg/l
Ni(II) concentration,rrg/1 Fig. 6. Variation o f rejection vs. different Ni ÷2 concentrations. •
PU+RO
• 50
98
45
-"
PU+RO
... A.-. PU
E 600
96
O .,~
35 ~ ~
Influent
800
40 ~=-
0
"g
Fig. 7. Variation o f conductivity with different Ni ÷2 concentrations.
---~--PU
100
200
94
:~ 400
i.q
30 ©
°
92 90
I
64
-o,
25
I
93
e~
200
°
139
t
20
I
170
Zn(ll) concentration, rag/1
0
•
i
•
!
50 1O0 150 Initial Zn(II) concentration, rng/I
n
200
Fig 8. Variation o f rejection vs. different Zn ÷~ concentrafions.
Fig. 9. Variation o f conductivity with different Zn ÷2 concentrations.
removal by using RO. The pH of feeds was adjusted to 4. The rejection capacity ofpre-units varied in the range of 23-25%, but the rejection capacity increased up to 100% by using RO, generally higher than 99.2% (Fig. 6). The effect of increasing Ni +2content in feed could be negligible, but this situation increased the conductivity dramatically in either the feed or PU effluent. For example, the conductivity in feed and PU effluent
increased from 197 to 650 #S/cm and from 134 to 456 #S/cm, respectively. The conductivity of permeate (PU+RO) increased from 12 to 24 #S/ cm with an increase in the feed metal concentration (Fig. 7). Although the rejection capacity decreased from nearly 45% to 25% by PU with increasing Zn ÷2concentration in feed from 64 to 170 mg/1, it varied in the range of 98.8-99.6% with PU+RO
U. Ipek / Desalination 174 (2005) 161-169
166
units (Fig. 8). The increase in feed Zn +2 concentrations rinsed the feed conductivity from 238 to 606/zS/cm. This varied between 199 and 442 #S/ cm after PU and between 16-26 #S/cm in permeates after RO (Fig. 9).
3.3. Effect of salt ions In order to investigate salt ions on removal of heavy metal by PU and PU+RO configurations, sodium chloride was applied to feed water containing various heavy metal concentrations, and thus, the conductivity was adjusted to determine levels in the range of 200 and 1200 #S/cm. The pH of the influent was adjusted to 6. The times of runs varied between 1545-1590 s and 15451605 s for Ni +2and Zn +2, respectively, while Ni +2 rejection after PU combination decreased slowly with an increase in the influent conductivity, it did not vary in permeates with this increase and was completely less than the detection limit of Ni +2 by AAS (Fig. 10). However, the conductivities after PU were measured in the range of 150 and 900 #S/cm. As can be seen in Fig. 11, the rejection capacity of PU combination was so low about 25%. The final conductivity also increased with an increase in the influent conductivity after PU+RO combination, but this was low, varying in the range of 5.1 to 31.7 #S/cm.
While Zn ÷2 rejection decreased with an increase in the influent salt ions by using the PU combination, it did not considerably vary with this increase by using the PU+RO combination, and was usually less than 0.48 mg/1 (Fig. 12). However, the conductivity in the PU combination effluent increased with an increase in the influent conductivity in the range of 148 and 1036 #S/cm. That is, the rejection capacity was less than 25%. When the overall system is taken into consideration, the conductivity in permeate changed from 6 to 36 #S/cm (Fig. 13). It was shown that an increase in the influent conductivity did not significantly affect the rejection capacity by using the overall system (PU+RO).
3.4. Effect of EDTA The application of complexation in combination with membrane separation processes is based on the reaction of various types of ligands with endogenous cations in aqueous metallic constituents to form a metal-containing complex (chelate) and the removal of these metals by RO. The opposite charges of the ionised ligand and the metal attract each other and form a stable complex [6]. Thereby, the chelating agents aid the removal of metals. In this study, the effects of o
o
lnfluent
.--~--
PU+RO
- - - * - - - PU
12
---g---PU ,I
~I0 0 •~
PU+RO
1000 800
. . . . .
~m °" .o
°~ o, •
°°.°~"
'~ 600
8
il o ° .o
"O
oo
o ¢d,
o"
°•
400 °Q-"
84 "-~ 2 Z 0
ra
°°-
u~ o
200
o
0
0
o
400 600 800 1000 1200 Conduetivity,gS~m Fig. 10. Variation of Ni+2 concentrationsofinfluent and effluent at different conductivities.
200
Q..
0
9 200
°,°°
"i ~ ? :i 400 600 800 1000 1200 Influentconductivity,ixS/cm
Fig. 11. Variation of effluent conductivitywith influent conductivityin the experimentsof Ni+2removal.
U. lpek / Desalination 174 (2005) 161-169 e
Influent
...m-..PU+RO
.--#...PU
167 o
15
PU+RO
...g--*PU
1200 .4~,..,.+.-°4~
~1000
°
# ! +# ~
9
t.: "°'°°--~
!
' ~ 800
k,, o-
•~
.w""
600
°o °,
0 ¢J
.°
400
.-, 3 e~ N 0
ra."
.°
200 L. . . . . . . . .
200
I J........
400
.~ . . . . . . . .
.[ .........
II ........
Q.•
°-
i
.I
0 600 800 Conductiv~y, gS/cm
1000
I
1200
Fig. 12. Variation o f Zn÷2 concentrations of influent and effluent at different conductivities.
complexation on the behaviour o f metal ion removal by RO membranes can also be evaluated from Figs. 14-- 17. During this experimental work, the pH of feeds was adjusted to 4. Ni +2 concentrations in permeate decreased with an increase in the EDTA, as the chelating agent. Without adding EDTA, the feed solutions of Ni +2 show lower degree of metal ions removal. Ni +2 values in permeate were also plotted logarithmically in order to see the metal rejection in detail (Fig. 14). At the experiments with and without adding EDTA, the initial Ni +2 concentrations were in the range of 152.2 and 181 rag/1. Thus, the conductivity was high in feed solution; usually about 680 #S/cm of conductivity because o f high Ni +2 concentrations prepared from NiCI 2. The conductivity increased the addition of EDTA to the feed solution, but it decreased to 34.8 #S/cm in the permeate (Fig. 15). The same situation was also applied for Zn +2metal ions (Figs. 16 and 17), and Zn +2removal increased as the concentrations of EDTA increased. The conductivity in permeate increased from 12.8 to 167.7 #S/cm for ZnEDTA (Fig. 17).
3.5. Evaluation offluxes The flux decreased to nearly 2.85 and 3.75% in the experiments o f N i ÷2and Zn ÷2, respectively,
200
'f
i
400 600 800 1000 lrrfluentconductivity, IxS/cm
1200
Fig. 13. Variation o f effluent conductivity with influent conductivity in the experiments o f Zn +2 removal. e
~
lnfluent
...m... PU~RO
...e--. PU
200 150 -. .........
O.°.
o
--""
.-O.
"° -..
.o..~1,......°
-o
.4,
100 O
~
50 0
1000
.=o
100 ........... i .......... i .......... J .......... "$
10 8
1
.........
l -
--r'-
~ .....
[
"'-G
i ........
..o
Z
0,1 0
50
1O0
150
200
250
~ T A , n',~/I Fig. 14. Variation of Ni +2 concentrations with EDTA.
as the conductivity was increased from 200 to 1200 #S/cm (Fig. 18). This situation can be due to an accumulation o f salt ions on the membrane surface. The experiments to determine the effect
U. Ipek / Desalination 174 (2005) 161-169
168 *
Influent
-..m..- PU+RO
...o--- PU
*
800
lnfluent
...m..-
PU+RO
---o---
PU
1000 i _ . . . _ . . . . . O - - -
~OO6 0 0
o
.-n
. . . . . . . . .
..O..
-'~ ..........
-o
~O .
.
.
.
.
-O
.~ 400
H
800 600
.=> o
400
~, 200
200 .........
0
.•
..........
•
..........
•
..........
i
i
i
i
50
1O0
150
200
..--"
• i.....
0 250
.t,
........... i •
o
50
EDTA, rag0
.I--" i
~
n
]oo
15o
200
Fig. 15. Variation of conductivity with EDTA in the experiments o f N i ÷2 removal.
Fig. 17. Variation o f conductivity with EDTA in the experiments o f Z n +: removal. ---o---Zn(II)
¢
~
200
•~
is0
Intluent ...t,... PU+RO
---~-- PU
*
Ni(II)
9.4 9.3
. . . . . . . . . " ~. .. . . . . . . .
¢.
4' ....
" ......
4..
-o..
~
o 100 ~ M
250
EDTA, rr~/'l
50
9.2
9.1 l.n
_
.
~.
r~
9
1000
l
i
i
i
i
200
............
~100
.~ . . . . . . . . : . ~ . . . . . . ~ . . . ~
400 600 800 1000 lnfluent Conductivity, ixS/cm Fig. 18. Variation o f fluxes with conductivities.
1o
---~--- Zn(II) . .
10
* .
.
1200
Ni(II)
.
9.8 0,1 0
50
100 150 EDTA, m8/l
200
250
9.6 "O,.
Fig. 16. Variation o f Zn +2 concentrations with EDTA. 9.4
of pH on metal separation by RO were performed, and it was shown that the flux decreased to be nearly 4.40 and 3.90% for Ni *2 and Zn +2, respectively, because of an increase of OH- ions on the membrane (Fig. 19).
"O ....
9.2 4
5
6 pH
Fig. 19. Variation o f fluxes with pH.
U. Ipek / Desalination 174 (2005) 161-169
169
4. Conclusions
References
A very high-quality feed is required for efficient operation of RO units. Membrane elements in the RO unit can be fouled by colloidal matter and constituents in the feed stream. Thus, cartridge filters with a pore size of 5 to 10/~m and activated carbon have also been used to reduce residual suspended solids and other toxic matter. These units used before RO can contribute to better quality effluent for the product. It was shown in this study that the combination of RO and its PU sufficiently removed nickel and zinc ions from aqueous solutions. The metal rejections seem not to be greatly affected by different conductivity and pH. EDTA increased Zn +2and Ni +2removal, but the effluent conductivity also increased, especially in Zn +2 removal. The removal of nickel and zinc from metals that are toxic to humans and the environment by RO is extremely important to control these metals.
[I] S.D. Sujatha, S. Sathyanarayanan, P.N. Satish and D. Nagaraju, Env. Jeo.,40 (2001) 1209. [2] K. Kadirvelu, K. Thamaraiselvi and C. Narnasivayam, Sep. Pur. Teclmol., 24 (2001) 497. [3] H. Hasar, Y. Cuci, E. Obek and M.F. Dilekoglu, Adsorp. Sci.Technol., 21 (2003) 799. [4] H. Hasar, J. Haz. Mat., 97 (2003) 49. [5] H. Ozald, K, Sharrna and W. Saktaywin, Desalination, 144 (2002) 287. [6] Z. Ujang and G.K. Anderson, Water Sci. Tech., 34 (1996) 247. [7] D. Bhattacharyya, R. Adams and M. Williams, Biological and synthetic membranes. Liss, New York, 1989, p. 153. [8] J.J. Qin, M.H. Oo, M.N. Wai and F.S. Wong, J. Membr. Sci., 217 (2003) 261.
ELSEVIER
Desalination 174 (2005) 161-169
www.elsevier.corrdlocate/de sal
Removal of Ni(II) and Zn(II) from an aqueous solution by reverse osmosis Ubeyde Ipek Department of Environmental Engineering, Faculty of Engineering, Firat University, 23119 Elazig, Turkey Fax: +90 (424) 241-5526; email: [email protected] Received 30 March 2004; accepted 24 September 2004
Abstract
The removal ofNi +~and Zn÷2from an aqueous solution by reverse osmosis (RO) at different pH, conductivity and EDTA concentrations was investigated. In addition, the removal in the pretreatment units (PU) with filtration (F) and granular activated carbon (GAC) at the same conditions was also determined. It was observed that Zn+2rejections did not change much with pH, usually less than 0.88 mg/1, and Ni +~rejection was below the detection limits of AAS in the range ofpH 4-8. While Ni +2and Zn÷2were removed 23-25% and 25-45%, respectively, by PU, the rejection of Ni ÷2 and Zn÷2was, respectively, determined to be higher than 99.2% and 98.8% by PU+RO at experiments related to the determination of the effect of initial metal concentration on rejection. It was found that the influent conductivity affected metal rejection at an unimportant level, but the increase of conductivity affected the effluent conductivity. The addition of EDTA into the aqueous solution increased Ni ÷2and Zn÷2rejection from 99.3% to 99.7% and from 98.9% to 99.6% at an EDTA concentration of 240 ppm, respectively.
Keywords: Reverse osmosis; Zinc (II); Nickel (II); EDTA
I. Introduction
The world is facing pollution problems due to toxic elements in the environment. Large quantities o f metals are being released into the environment (i.e., air, water and soil) from mining, industrial agricultural and other anthropogenic waste due to inadequacies o f technology in the processing o f metals or through other routes. The higher concentrations o f heavy metals in soils, water and plants, their chemicals forms and the
mobility and availability to the food chain provide the basis for a range o f problems related to the crop, animal and human health [ 1]. Nickel and zinc are toxic heavy metals that are widely used in industry, including silver refineries, electroplating, zinc base casting, storage battery industries and (more recently) landfill leachates [2,3]. The chronic toxicity o f nickel to humans and the environment has been well documented. For example, high concentrations o f metals cause
0011-9164/05/$- See front matter © 2005 Elsevier B.V. All rights reserved doi: 10.1016/j.desa1.2004.09.009
162
U. Ipek / Desalination 174 (2005) 161-169
cancer of lungs, nose and bone. It is essential to remove metals from industrial wastewater before being discharged. For this reason, advanced treatment processes are generally used such as chemical reduction, ion exchange, reverse osmosis (RO), electrodialysis, and activated carbon adsorption [4]. Granular activated carbon removes the heavy metals from waste solutions by physical adsorption where the properties of the surface area has an important role. Membranes with very low-pressure requirements for operation are promising in removing heavy metals from wastewater [5]. The number of researchers who investigated the efficiencies of low-pressure membranes to remove heavy metals has recently been increasing. Low-pressure RO membranes with pressures below 690 kPa for Zn +2and Cu +2removal have been used [6] as well as low-pressure RO in which operating pressures were varied in the range of 300 kPa to 1800 kPa [7]. The main aim of the present work was to investigate Zn +2 and Ni +2 removal from wastewater by a RO unit by changing some parameters such as the feed pH and metal concentration. Furthermore, EDTA as an aid to the membrane separation process was used to remove heavy metals from wastewater; its effect on the removal of heavy metals was also investigated. Manometers
J Feed
Pump
I
ter'
Concentrate
GAC
Fig. I. Experimental set-up.
High pressure pump
BMan~
2. Materials and methods
2.1. Experimental set-up The experimental work was carried out by using an apparatus at pilot scale. The experimental set-up is illustrated in Fig. 1. The filter used in the experimental work, manufactured by Osmonics, has extended blown microfiber technology to meet the requirements for a pure polypropylene deep filter with an exceptional dirt holding capability. This translates to longer life on fewer changes than existing string-wound or resin-bonded filters. The Desal-11, a thin-film R e membrane, was used as a membrane throughout the work. This membrane (AG4021FF) manufactured by Desal (Osmonics) is characterized by high flux and excellent sodium chloride rejection. The properties and operating parameters of the membrane are given in Table 1. Granular activated carbon (AquaSorb 1000) was manufactured by steam activation from selected grades of bituminous coal. The perfect balance between adsorption and transport pores provides optimum performance in a wide range of water treatment applications. The product is a high-density adsorbent and provides maximum volume activity. The total pore volume and apparent density of the adsorbent are 0.88 cm3/g and 500 kg/m3, respectively. Its surface area is 950 m2/g. The bed height
U. lpek / Desalination 174 (2005) 161-169 Table 1 Properties and operatingparametersof the membrane Model Cross reference GPD, m/d NaCI rejection,% (avg/min) Active area, m2
AG4021FF BW30-4021 1.050 (3.97) 99.4/99.0 3.72
Membrane Typical operatingpressure, kPa Maximumpressure, kPa Maximumtemperature, o RecommendedpH: optimumrejection operating range cleaning range
Thin-film (TFM) 1.379 2.758 50 6.5-7.5 4.0-11.0 2.0-11.5
and inner diameter of the pretreatment units are 50 and 6.5 cm, respectively. The bed dimensions of the membrane are 63.5 cm in height and 12.5 cm in out diameter, and the general dimensions of the entire system are 45x45x80 cm (wxlxh).
2.2. Operating conditions The water present in the RO system was discharged during 20 s at the beginning of experimental studies. Then, the flow was regulated and the concentrate stream was recycled to the feed tank. The system pressure was 1100 kPa. Samples of the pretreatment effluents were taken from sample point on the pretreatment unit. The permeates from the RO were collected in another tank and the flux measured. After each experiment the RO membrane was washed with distilled water. The total volume of samples used in each experiment was 15 L. The pH and conductivities of all samples were measured with a pH meter (WTW pH 330) and conductivity meter (WTW LF 330), respectively. The pH of each sample was adjusted by adding NaOH or H2SO4 stock solution under vigorous stirring. The synthetic wastewater was prepared by dissolving sodium chloride (NaC1), zinc chloride (ZiC12), nickel chloride (NiC12.
163
6H20 ) and ETDA (disodium salt) in distilled water. After the samples were collected, water samples were acidified with HNO3. The concentrations of zinc and nickel in the samples were analyzed by using an atomic absorption spectrophotometer (ATI Unicam 929 and Perkin Elmer 370), respectively.
3. Results and discussion 3.1. Effect o f p H The pH of the feed water can alter the nature of the membrane surface charge and also influence the characteristics of the solutes in the feed water and, thus, the membrane separation performance on the solutes [8]. Figs. 2 and 3 indicate the concentrations ofNi +zand Zn +2in the influent and effluent at the different pH and water matrices. The samples were taken both in the influent and effluent of RO (PU+RO) and the pretreatment units (PU) equipped with a RO system to eliminate the suspended solids in the influent. It was shown that the variations in Ni +2 rejections with pHs were not important (Fig. 2). While Ni +2 concentrations in influent vary between 8.22 and 10.29 mg/1, its concentrations in effluent of PU decreased to degrees between 4.07 and 6.56 mg/1. However, Ni +2 concentrations of PU+RO were below the detection limits of AAS. Although the influent has high Ni +2concentrations at pHs in the range of 5.5-7, Ni ÷2 indicated much more decreasing according to the other pHs in effluent of PU. For zinc metal, the same situation was also shown at pH = 6 (Fig. 3). Zn ÷2concentrations of influent ranged between 10.74 and 13.72 mg/1, and its concentrations of effluent of PU decreased to degrees between 7.14 and 9.56 rag/1. Zn+2concentrations in permeate did not change much with pH, generally less than 0.88 mg/l. The times of run varied between 1462.5-1530 s and 14851545 s for Ni +2and Zn+~, respectively. Figs. 4 and 5 indicate the levels of conductivity in different water matrixes at the experiments
164
U. Ipek / Desalination 174 (2005) 161-169 o
~
Influont
..-m-..
PU+RO
---•---
o
PU
12
15
1o
12
Influent
...ta... PU+RO
---*-.- PU
O
=
6
• ~1.. ° . ,
oo
u •"
4
U
.o$ ....
-°'$-
9
.°.~....oL
.
.
.
.
.
.
.
.
.
.
.
.
° " ~11% .
..
•'"
.•..
.•..
,s .,
,.
o
6
. . . . . . . . .
3
"-" 2 Z
NI ...--Q...
-r
-r
-r
5
6 pH
7
0 8
Fig. 2. Ni ÷2 concentrations o f influent and effluent at different pHs.
o
Influent
...t,... PU+RO
4
5
6 pH
7
8
Fig. 3. Zn +2 concentrations ofinfluent and effluent at different pHs.
o
---*--. PU
Influent
..-m... PU+RO
.--*--- PU
100
80
~
~60 OO
::k
80
• 60
i~ 40
.....
$ .....
~ .....
$ .....
•..
_.'~'"
..O, °.•'''--O""
..-•'-,.o•°.~'°
....
• .....
• .....
$ .....
O""
m.....
O .....
G .....
"
•
40 ~ .....
4
0 .....
Q .....
Q .....
Q .....
Q .....
g .....
I
t
I
5
6 pH
7
20
Q .....
- - -..Q
8
4
.....
5
6
G .....
Q .....
7
D .....
i
8
pH
Fig. 4. Variation o f conductivity with pH in the experiments o f N i ÷z removal•
Fig. 5. Variation of conductivity with pH in the experiments o f Zn ÷2 removal•
performed to determine the removal capacities of Ni +2and Zn+2at the different pHs, respectively. It was shown that the conductivity also decreased independently from pHs at two experiment series. While the conductivity of influent for the Ni +2 experiment series ranged between 46.4 and 75.5 mS/cm, it ranged between 34.4 and 44.1 mS/cm at the effluent of PU. Further, the conductivity was less relatively than 3.8 mS/cm in the final effluent. The conductivity of influent at the Zn+2experiment series ranged between 60
and 85 #S/cm, and it decreased to the values between 43 and 61 #S/cm after PU. Although the conductivity increased in the effluent of PU at higher pH values than pH 6.5, permeate generally has conductivity of 4.0 #mS/cm.
3.2 Effect of initial metal concentrations Ni +2 concentrations in influent were varied in the range of 44-169 mg/l in order to determine the effect of metal concentration of feed on
U. lpek / Desalination 174 (2005) 161-169 --
PU+RO
-.-~--- PU
100
* 26
165
Influent
•
PU÷RO
--- ~,--- PU
700 600 E
99,5
~500 ,0
,
'~"
d
99
24 '\
© 98,5
.
.
.
.
~,
,~400
.~t..
°
"~ 300 200
.
A"
100 98
I 44
I 86
I 129
22 0
169
50 1O0 150 Initial Ni(II) concentration, mg/l
Ni(II) concentration,rrg/1 Fig. 6. Variation o f rejection vs. different Ni ÷2 concentrations. •
PU+RO
• 50
98
45
-"
PU+RO
... A.-. PU
E 600
96
O .,~
35 ~ ~
Influent
800
40 ~=-
0
"g
Fig. 7. Variation o f conductivity with different Ni ÷2 concentrations.
---~--PU
100
200
94
:~ 400
i.q
30 ©
°
92 90
I
64
-o,
25
I
93
e~
200
°
139
t
20
I
170
Zn(ll) concentration, rag/1
0
•
i
•
!
50 1O0 150 Initial Zn(II) concentration, rng/I
n
200
Fig 8. Variation o f rejection vs. different Zn ÷~ concentrafions.
Fig. 9. Variation o f conductivity with different Zn ÷2 concentrations.
removal by using RO. The pH of feeds was adjusted to 4. The rejection capacity ofpre-units varied in the range of 23-25%, but the rejection capacity increased up to 100% by using RO, generally higher than 99.2% (Fig. 6). The effect of increasing Ni +2content in feed could be negligible, but this situation increased the conductivity dramatically in either the feed or PU effluent. For example, the conductivity in feed and PU effluent
increased from 197 to 650 #S/cm and from 134 to 456 #S/cm, respectively. The conductivity of permeate (PU+RO) increased from 12 to 24 #S/ cm with an increase in the feed metal concentration (Fig. 7). Although the rejection capacity decreased from nearly 45% to 25% by PU with increasing Zn ÷2concentration in feed from 64 to 170 mg/1, it varied in the range of 98.8-99.6% with PU+RO
U. Ipek / Desalination 174 (2005) 161-169
166
units (Fig. 8). The increase in feed Zn +2 concentrations rinsed the feed conductivity from 238 to 606/zS/cm. This varied between 199 and 442 #S/ cm after PU and between 16-26 #S/cm in permeates after RO (Fig. 9).
3.3. Effect of salt ions In order to investigate salt ions on removal of heavy metal by PU and PU+RO configurations, sodium chloride was applied to feed water containing various heavy metal concentrations, and thus, the conductivity was adjusted to determine levels in the range of 200 and 1200 #S/cm. The pH of the influent was adjusted to 6. The times of runs varied between 1545-1590 s and 15451605 s for Ni +2and Zn +2, respectively, while Ni +2 rejection after PU combination decreased slowly with an increase in the influent conductivity, it did not vary in permeates with this increase and was completely less than the detection limit of Ni +2 by AAS (Fig. 10). However, the conductivities after PU were measured in the range of 150 and 900 #S/cm. As can be seen in Fig. 11, the rejection capacity of PU combination was so low about 25%. The final conductivity also increased with an increase in the influent conductivity after PU+RO combination, but this was low, varying in the range of 5.1 to 31.7 #S/cm.
While Zn ÷2 rejection decreased with an increase in the influent salt ions by using the PU combination, it did not considerably vary with this increase by using the PU+RO combination, and was usually less than 0.48 mg/1 (Fig. 12). However, the conductivity in the PU combination effluent increased with an increase in the influent conductivity in the range of 148 and 1036 #S/cm. That is, the rejection capacity was less than 25%. When the overall system is taken into consideration, the conductivity in permeate changed from 6 to 36 #S/cm (Fig. 13). It was shown that an increase in the influent conductivity did not significantly affect the rejection capacity by using the overall system (PU+RO).
3.4. Effect of EDTA The application of complexation in combination with membrane separation processes is based on the reaction of various types of ligands with endogenous cations in aqueous metallic constituents to form a metal-containing complex (chelate) and the removal of these metals by RO. The opposite charges of the ionised ligand and the metal attract each other and form a stable complex [6]. Thereby, the chelating agents aid the removal of metals. In this study, the effects of o
o
lnfluent
.--~--
PU+RO
- - - * - - - PU
12
---g---PU ,I
~I0 0 •~
PU+RO
1000 800
. . . . .
~m °" .o
°~ o, •
°°.°~"
'~ 600
8
il o ° .o
"O
oo
o ¢d,
o"
°•
400 °Q-"
84 "-~ 2 Z 0
ra
°°-
u~ o
200
o
0
0
o
400 600 800 1000 1200 Conduetivity,gS~m Fig. 10. Variation of Ni+2 concentrationsofinfluent and effluent at different conductivities.
200
Q..
0
9 200
°,°°
"i ~ ? :i 400 600 800 1000 1200 Influentconductivity,ixS/cm
Fig. 11. Variation of effluent conductivitywith influent conductivityin the experimentsof Ni+2removal.
U. lpek / Desalination 174 (2005) 161-169 e
Influent
...m-..PU+RO
.--#...PU
167 o
15
PU+RO
...g--*PU
1200 .4~,..,.+.-°4~
~1000
°
# ! +# ~
9
t.: "°'°°--~
!
' ~ 800
k,, o-
•~
.w""
600
°o °,
0 ¢J
.°
400
.-, 3 e~ N 0
ra."
.°
200 L. . . . . . . . .
200
I J........
400
.~ . . . . . . . .
.[ .........
II ........
Q.•
°-
i
.I
0 600 800 Conductiv~y, gS/cm
1000
I
1200
Fig. 12. Variation o f Zn÷2 concentrations of influent and effluent at different conductivities.
complexation on the behaviour o f metal ion removal by RO membranes can also be evaluated from Figs. 14-- 17. During this experimental work, the pH of feeds was adjusted to 4. Ni +2 concentrations in permeate decreased with an increase in the EDTA, as the chelating agent. Without adding EDTA, the feed solutions of Ni +2 show lower degree of metal ions removal. Ni +2 values in permeate were also plotted logarithmically in order to see the metal rejection in detail (Fig. 14). At the experiments with and without adding EDTA, the initial Ni +2 concentrations were in the range of 152.2 and 181 rag/1. Thus, the conductivity was high in feed solution; usually about 680 #S/cm of conductivity because o f high Ni +2 concentrations prepared from NiCI 2. The conductivity increased the addition of EDTA to the feed solution, but it decreased to 34.8 #S/cm in the permeate (Fig. 15). The same situation was also applied for Zn +2metal ions (Figs. 16 and 17), and Zn +2removal increased as the concentrations of EDTA increased. The conductivity in permeate increased from 12.8 to 167.7 #S/cm for ZnEDTA (Fig. 17).
3.5. Evaluation offluxes The flux decreased to nearly 2.85 and 3.75% in the experiments o f N i ÷2and Zn ÷2, respectively,
200
'f
i
400 600 800 1000 lrrfluentconductivity, IxS/cm
1200
Fig. 13. Variation o f effluent conductivity with influent conductivity in the experiments o f Zn +2 removal. e
~
lnfluent
...m... PU~RO
...e--. PU
200 150 -. .........
O.°.
o
--""
.-O.
"° -..
.o..~1,......°
-o
.4,
100 O
~
50 0
1000
.=o
100 ........... i .......... i .......... J .......... "$
10 8
1
.........
l -
--r'-
~ .....
[
"'-G
i ........
..o
Z
0,1 0
50
1O0
150
200
250
~ T A , n',~/I Fig. 14. Variation of Ni +2 concentrations with EDTA.
as the conductivity was increased from 200 to 1200 #S/cm (Fig. 18). This situation can be due to an accumulation o f salt ions on the membrane surface. The experiments to determine the effect
U. Ipek / Desalination 174 (2005) 161-169
168 *
Influent
-..m..- PU+RO
...o--- PU
*
800
lnfluent
...m..-
PU+RO
---o---
PU
1000 i _ . . . _ . . . . . O - - -
~OO6 0 0
o
.-n
. . . . . . . . .
..O..
-'~ ..........
-o
~O .
.
.
.
.
-O
.~ 400
H
800 600
.=> o
400
~, 200
200 .........
0
.•
..........
•
..........
•
..........
i
i
i
i
50
1O0
150
200
..--"
• i.....
0 250
.t,
........... i •
o
50
EDTA, rag0
.I--" i
~
n
]oo
15o
200
Fig. 15. Variation of conductivity with EDTA in the experiments o f N i ÷2 removal.
Fig. 17. Variation o f conductivity with EDTA in the experiments o f Z n +: removal. ---o---Zn(II)
¢
~
200
•~
is0
Intluent ...t,... PU+RO
---~-- PU
*
Ni(II)
9.4 9.3
. . . . . . . . . " ~. .. . . . . . . .
¢.
4' ....
" ......
4..
-o..
~
o 100 ~ M
250
EDTA, rr~/'l
50
9.2
9.1 l.n
_
.
~.
r~
9
1000
l
i
i
i
i
200
............
~100
.~ . . . . . . . . : . ~ . . . . . . ~ . . . ~
400 600 800 1000 lnfluent Conductivity, ixS/cm Fig. 18. Variation o f fluxes with conductivities.
1o
---~--- Zn(II) . .
10
* .
.
1200
Ni(II)
.
9.8 0,1 0
50
100 150 EDTA, m8/l
200
250
9.6 "O,.
Fig. 16. Variation o f Zn +2 concentrations with EDTA. 9.4
of pH on metal separation by RO were performed, and it was shown that the flux decreased to be nearly 4.40 and 3.90% for Ni *2 and Zn +2, respectively, because of an increase of OH- ions on the membrane (Fig. 19).
"O ....
9.2 4
5
6 pH
Fig. 19. Variation o f fluxes with pH.
U. Ipek / Desalination 174 (2005) 161-169
169
4. Conclusions
References
A very high-quality feed is required for efficient operation of RO units. Membrane elements in the RO unit can be fouled by colloidal matter and constituents in the feed stream. Thus, cartridge filters with a pore size of 5 to 10/~m and activated carbon have also been used to reduce residual suspended solids and other toxic matter. These units used before RO can contribute to better quality effluent for the product. It was shown in this study that the combination of RO and its PU sufficiently removed nickel and zinc ions from aqueous solutions. The metal rejections seem not to be greatly affected by different conductivity and pH. EDTA increased Zn +2and Ni +2removal, but the effluent conductivity also increased, especially in Zn +2 removal. The removal of nickel and zinc from metals that are toxic to humans and the environment by RO is extremely important to control these metals.
[I] S.D. Sujatha, S. Sathyanarayanan, P.N. Satish and D. Nagaraju, Env. Jeo.,40 (2001) 1209. [2] K. Kadirvelu, K. Thamaraiselvi and C. Narnasivayam, Sep. Pur. Teclmol., 24 (2001) 497. [3] H. Hasar, Y. Cuci, E. Obek and M.F. Dilekoglu, Adsorp. Sci.Technol., 21 (2003) 799. [4] H. Hasar, J. Haz. Mat., 97 (2003) 49. [5] H. Ozald, K, Sharrna and W. Saktaywin, Desalination, 144 (2002) 287. [6] Z. Ujang and G.K. Anderson, Water Sci. Tech., 34 (1996) 247. [7] D. Bhattacharyya, R. Adams and M. Williams, Biological and synthetic membranes. Liss, New York, 1989, p. 153. [8] J.J. Qin, M.H. Oo, M.N. Wai and F.S. Wong, J. Membr. Sci., 217 (2003) 261.