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Ind. Eng. Chem. Res. 2003, 42, 1948-1954
SEPARATIONS Ligand-Enhanced Separation of Divalent Heavy Metals from Aqueous Solutions Using a Strong-Acid Ion-Exchange Resin Ruey-Shin Juang* and Yi-Chieh Wang Department of Chemical Engineering, Yuan Ze University, Chung-Li 320, Taiwan
Experimental studies on the cation-exchange separation of three pairs of divalent metal ions (Zn/Cu, Co/Cu, and Ni/Cu) from water in the presence of water-soluble ligands were made at 298 K. Four ligands including ethylenediaminetetraacetic acid, nitrilotriacetic acid, iminodiacetic acid, and citrate were selected. Experiments were performed as a function of the solution pH (1.0-6.0) and the concentration ratio of the ligand to total metals (0-1). It was shown that the exchange selectivity was enhanced when the anionic ligands were present. The extent of enhancement strongly depended on the solution pH but was not completely related to the metalcomplexing affinity of the ligands. A parameter, based on the combinations of the overall formation constants of the ligands with two metals, was thus proposed to correlate the exchange selectivity obtained under various sets of metal pairs and anionic ligands. This correlation allowed selection of a suitable type or amount of the anionic ligands for effective separation of a given pair of heavy metals. Introduction Ion exchange has been widely applied for the recovery and concentration of metal ions from process and waste streams in chemical process industries because of its simplicity of buildup, well-understood operating principles, and considerable performance data. In addition, it is used in water treatment industries such as the softening of drinking water and the production of ultrapure water. However, selective removal of one or more metals from multi-ion mixtures using common organic cation-exchange resins is generally not feasible, particularly for those ions with the same valence.1 Although the ion-exchange resins with specific chelating groups [e.g., the iminodiacetic acid (IDA) group in Chelex-100 resin] can meet such requirements, the high production and regeneration costs of these resins make them rather limited for practical applications. Other strategies to improve the separation ability of the common organic exchange resins are highly desired. For cation-exchange membrane dialysis and electrodialysis, previous studies have shown that selective transport of metal ions could be achieved by adding anionic ligands as masking reagents to the feed phase.2-7 This can differentiate the equilibrium uptake of metal ions on the membrane so as to increase membrane selectivity for metal ions. The ligands commonly used include F-, SCN-, PO43-, ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), citrate, oxalate, and glycine. However, the enhancement of selectivity would be inevitably at the expense of a decrease of the mass-transfer rate due to a reduction of the driving force, the concentration gradient. On the other hand, * To whom correspondence should be addressed. Tel.: +8863-4638800 ext 555. Fax: +886-3-4559373. E-mail: cejuang@ ce.yzu.edu.tw.
Ramkumar et al.8 added EDTA to the receiving phase to separate Cu2+ from a mixture of Cu2+ and Fe3+ through a Nafion 117 membrane. This ligand-enhanced separation concept has been extended to similar systems such as liquid-liquid extraction and liquid membranes.9-11 For example, Azis et al.9 studied the separation abilities of rare-earth metals by solvent extraction from an aqueous phase containing diethylenetriaminepentaacetic acid. Li et al.10 also employed EDTA to complex Ni2+ to allow effective separation of a trace amount of Co2+ from Ni2+ in emulsion liquid membranes containing bis(2-ethylhexyl)phosphoric acid as mobile carriers. To our best knowledge, little attention has been paid to the solid ion-exchange resin systems. Korngold et al.12 removed Cu2+, Ni2+, Co2+, and Cd2+ from tap water containing small amounts of salts of carboxylic acid by cationic resins having an IDA group, Purolite S930. From the column tests, they indicated that the presence of salts with one carboxylic group such as acetate has little effect on the exchange performance. The presence of salts with two or four carboxylic groups such as tartrate and EDTA dramatically decreases the efficiency of metal exchange to even zero. However, no comments on the separation capability were given in their work. In this work, the influence of water-soluble complexing ligands on the selectivity of exchange separation of three equimolar pairs of divalent heavy metals, Zn2+/ Cu2+, Co2+/Cu2+, and Ni2+/Cu2+, was systematically studied. The ligands EDTA, NTA, IDA, and citrate were selected because their 1:1 overall formation constants with the metals are sufficiently high but have certain differences between the pairs.13,14 In fact, these complexing ligands are often discharged from many industrial sources such as the manufacturing of printed circuit boards and metal finishing.15,16 This is also the
10.1021/ie020854n CCC: $25.00 © 2003 American Chemical Society Published on Web 03/26/2003
Ind. Eng. Chem. Res., Vol. 42, No. 9, 2003 1949 Table 1. Physical and Chemical Properties of the Resin Purolite NRW100 properties
Purolite NRW100
matrix functional group ionic form exchange capacity (equiv/kg of dry resin) moisture content (%) particle size (mm) specific gravity pH limit
polystyrene DVB, gel type sulfonic acid -SO3H+ (>99.9%) 4.5 50-55 0.425-1.2 1.20 none
case in the solutions for remediation washing of metalcontaminated soils17-19 and for the chemical cleaning of power plant boilers.20,21 Experiments were performed at different solution pH values (1.0-6.0) and concentration ratios of the ligands to total metals (0-1). A parameter was finally proposed to correlate the results of the systems of three metal pairs and four anionic ligands.
Figure 1. Amount of metal exchanged at different equilibrium pH values in the single and binary systems in the absence of complexing ligands.
Experimental Details Resins and Solutions. Strong-acid ion-exchange resin Purolite NRW100 (with a -SO3H group) was used in this work. The physical and chemical properties of the resin are listed in Table 1. Prior to use, the resin was washed with NaOH (1 mol/dm3), HCl (1 mol/dm3), and n-hexane to remove possible organic and inorganic impurities and was finally washed with deionized water (Millipore Milli-Q) three times. The resins were converted to Na+ form by column flushing with 1 mol/dm3 NaCl for 12 h. They were then washed with deionized water and dried in a vacuum oven at 333 K. Analytical-reagent-grade EDTA, NTA, IDA, and citric acid were purchased from Merck Co. The aqueous phase was prepared by dissolving equimolar metal sulfates (Merck Co.) and different amounts of complexing ligands in deionized water. The total concentration of metals was fixed at 10.5 mol/m3. The initial solution pH was adjusted to be in the range of 1.0-6.0 by adding a small amount of 0.1 mol/dm3 HCl or NaOH. Experimental Procedures. In exchange experiments, an aliquot of dry resins (0.3 g) and 100 cm3 of an aqueous phase were placed in a 125-cm3 glassstoppered flask and shaken at 120 rpm for 6 h using a shaker (Firstek model B603, Taiwan). The temperature of the water bath was controlled at 298 K. Preliminary runs showed that the exchange reaction studied was complete after 3 h. After equilibrium, the aqueous-phase concentrations of metals were analyzed by an atomic absorption spectrophotometer (Varian model 220FS, Grove City, OH). The solution pH was measured with a pH meter (Horiba model F22, Japan). The resin-phase concentration of each metal ion at equilibrium, qe (mol/kg), was calculated by
qe ) (C0 - Ce)V/W
(1)
where C0 and Ce are the initial and equilibrium concentrations of metal ions in the aqueous phase, respectively (mol/m3), and W/V is the dose of dry resin (3 kg/m3). Each experiment was at least duplicated under identical conditions. Reproducibility of the measurements was within 4%. Results and Discussion Exchange Equilibria in the Binary Zn2+/Cu2+ Systems. In the absence of complexing ligands, the
Figure 2. Effect of added EDTA on the amount of metal exchanged in the binary Zn2+/Cu2+ system.
amounts of exchange qe in the single- and binary-metal systems are compared in Figure 1. As expected, the effective separation of heavy-metal ions by common cation-exchange resins is impossible under the equilibrium pH (pHeq) ranges tested. The low qe value obtained at low pHeq is caused by the competitive exchange of H+ and the Na form of the resins with metal ions. In the single systems, the value of qe for Cu2+ is slightly greater than that for Zn2+; e.g., they are 1.93 and 1.91 mol/kg at pHeq > 2.0, respectively. The values of qe for Zn2+ and Cu2+ reverse in the binary-metal systems, probably because of the stronger complexation of sulfate ions with Cu2+ than Zn2+ in the aqueous phase.14 Figures 2-5 show the amounts of metal exchanged in the binary Zn2+/Cu2+ system by adding different amounts of EDTA, NTA, IDA, and citrate, respectively. In these figures, R is defined as the molar concentration ratio of the ligand to total metals. The related overall formation constants Kf at 298 K and zero ionic strength are listed in Table 2,14 where Kf for divalent metals, for
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Ind. Eng. Chem. Res., Vol. 42, No. 9, 2003
Figure 3. Effect of added NTA on the amount of metal exchanged in the binary Zn2+/Cu2+ system.
Figure 4. Effect of added IDA on the amount of metal exchanged in the binary Zn2+/Cu2+ system.
example, is defined as follows:
M2+ + iLx- T MLi2-ix, Kf )
[MLi2-ix] [M2+][Lx-]i
(2)
It is found that the value of qe for the metal with a stronger complexing affinity (i.e., Cu2+ in this case) significantly decreases, to even zero, at high pH values. In this regard, a higher pH favors the separation of these two metals; however, this is not necessarily the case particularly with the stronger ligands such as EDTA and NTA at R > 0.5. The pH-dependent nature of the qe value may be understood from the following pH trends of species distribution. It is known that EDTA, NTA, IDA, and citrate exist in a number of protonated forms in the aqueous solu-
Figure 5. Effect of added citrate on the amount of metal exchanged in the binary Zn2+/Cu2+ system.
tions.14 They readily form stable complexes with most metals in a 1:1 molar ratio. The pH diagrams of species distribution can be calculated from a set of mass-balance equations. In an equimolar solution of Cu2+ and EDTA (H4L), the species CuL2- predominates at pH > 3.2 and the CuHL- at pH < 3.2, and free Cu2+ ions are absent in the pH range tested.22 In the NTA (H3L) system, the anions CuL- predominate absolutely at pH 2.0-11 and the exchangeable Cu2+ at pH < 2.0.23 On the other hand, the dominant species are the Cu2+ ions at pH < 4.0 and CuL- at pH > 6.8 in the citric acid (H3L) system.22 In addition, the neutral species CuHL predominate within pH 4.0 and 6.8. Results of the Cu-IDA system are basically similar to those of the Cu-NTA system except that each dominating pH is somewhat different (because the values of Kf are different, as shown in Table 2). Exchange Equilibria in the Binary Co2+/Cu2+ and Ni2+/Cu2+ Systems. The amounts of metal exchanged in the binary Co2+/Cu2+ and Ni2+/Cu2+ systems at R ) 0.5 are shown in Figures 6-9 in the presence of EDTA, NTA, IDA, and citrate, respectively. It is found that the differences of the qe values between Co2+ and Cu2+ are larger than those between Ni2+ and Cu2+ in all of the cases of added ligands, indicating a higher separation factor for the former binary-metal system. Moreover, the difference of qe values between Co2+ and Cu2+ appears to match the decreasing order of complexing ability of the individual metal ions (i.e., Kf), i.e., EDTA > NTA > IDA > citrate. The difference between Ni2+ and Cu2+ does not reveal such a tendency. It is expected that the difference in the values of log Kf between Ni2+ and Cu2+, 0.1 (EDTA), 1.4 (NTA), 2.4 (IDA), and 0.5 (citrate), may play a certain role in this subject. Exchange Selectivity in the Binary-Metal Systems. The exchange selectivity of a given pair of metal ions, βM1/M2, is commonly defined as
βM1/M2 )
qM1,e/qM2,e CM1,0/CM2,0
(3)
The results of binary Zn2+/Cu2+ systems at different
Ind. Eng. Chem. Res., Vol. 42, No. 9, 2003 1951 Table 2. Overall Formation Constants (log Kf) for the Complexes of Metals and Anionic Ligands at 298 K and Zero Ionic Strengtha ion H+
Cu2+
Zn2+
Co2+ Ni2+
a
L ) OH-
L ) SO42-
L ) citrate3-
L ) IDA3-
HL (14.0)
HL (1.99)
HL (6.40) H2L (11.16) H3L (14.29)
HL (9.73) H2L (12.63) H3L (14.51)
HL (10.33) H2L (13.27) H3L (14.92) H4L (16.02)
CuL (6.3) CuL2 (11.8) CuL4 (16.4) Cu2L2 (17.7) ZnL (5.0) ZnL2 (11.1) ZnL3 (13.6) ZnL4 (14.8) CoL (4.3) CoL2 (9.2) CoL3 (10.5) NiL (4.1) NiL2 (9.0) NiL3 (12.0)
CuL (2.4)
CuL (7.2) CuHL (10.7) CuH2L (13.8) CuOHL (16.4) ZnL (6.1) ZnL2 (6.8) ZnHL (10.3) ZnH2L (13.3) CoL (6.3) CoHL (10.3) CoH2L (12.9) NiL (6.7) NiHL (10.5) NiH2L (12.9)
CuL (11.5) CuL2 (17.6)
CuL (14.2) CuL2 (18.1) CuOHL (18.6)
HL (11.12) H2L (17.80) H3L (21.04) H4L (23.76) H5L (24.76) CuL (20.5) CuHL (23.9) CuOHL (22.6)
ZnL (8.2) ZnL2 (13.5)
ZnL (12.0) ZnL2 (14.9) ZnOHL (15.5)
ZnL (18.3) ZnHL (21.7) ZnOHL (19.9)
CoL (7.9) CoL2 (13.2)
CoL (11.7) CoL2 (15.0) CoOHL (14.5) NiL (12.8) NiL2 (17.0) NiOHL (15.5)
CoL (18.1) CoHL (21.5)
ZnL (2.1) ZnL2 (3.1) CoL (2.4) NiL (2.3)
NiL (9.1) NiL2 (15.7)
L ) NTA3-
L ) EDTA4-
NiL (20.4) NiHL (24.0) NiOHL (21.8)
The concentration units used to calculate Kf are in mol/dm3.
Figure 6. Effect of added EDTA on the amount of metal exchanged in the binary Co2+/Cu2+ and Ni2+/Cu2+ systems.
Figure 8. Effect of added IDA on the amount of metal exchanged in the binary Co2+/Cu2+ and Ni2+/Cu2+ systems.
Figure 7. Effect of added NTA on the amount of metal exchanged in the binary Co2+/Cu2+ and Ni2+/Cu2+ systems.
Figure 9. Effect of added citrate on the amount of metal exchanged in the binary Co2+/Cu2+ and Ni2+/Cu2+ systems.
amounts of anionic ligands are typically shown in Figures 10 and 11. Two types of figures are found. In the presence of strong metal-complexing ligands such as EDTA and NTA (Figure 10), the selectivity is acceptable (>10, for example) only when R > 0.5. A maximum selectivity is obtained at R ) 1 and a pH near 2.0 (106 with EDTA and 157 with NTA). For the weaker ligands such as IDA and citrate (Figure 11), however,
the exchange separation is impossible at pH < 3 regardless of the amounts of ligands added. At pH > 3, the selectivity increases with an increase in both solution pH and R values under the conitions studied. In addition, a maximum selectivity of 187 is obtained at pH 5.5 and R ) 1 using IDA. Compared to the addition of anionic ligands to the feed phase in other separation processes, the selectivity
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Figure 12. Typical correlations between exchange selectivity of Zn2+/Cu2+ and the amounts of different types of complexing ligands at pH 1 and 3.
Figure 10. Effect of added EDTA and NTA on the exchange selectivity of Zn2+/Cu2+ in the binary systems.
Figure 13. Typical correlations between exchange selectivity of Zn2+/Cu2+ and the amounts of different types of complexing ligands at pH 4 and 5.
Figure 11. Effect of added IDA and citrate on the exchange selectivity of Zn2+/Cu2+ in the binary systems.
obtained here is satisfactory. For example, Huang and Wang4 examined the selective dialysis of metal ions through a cation-exchange membrane Selemion CMV (Asahi Glass Co.) at CCu,0 ) CNi,0 ) 1.5 mol/m3 and pH 4. The ratio of transport flux of Cu2+ to Ni2+ changes from 1.1 (no complexing ligand) to 2.94, 0.34, and 0.09 by adding 3 mol/m3 of EDTA, citrate, and oxalate, respectively. The lower flux ratios caused by the addition of citrate and oxalate are attributed to their larger formation constants with Cu2+ than Ni2+. Li et al.10 added EDTA in the feed phase to complex Ni2+ to a separate trace level of Co2+ from Ni2+ in an emulsion liquid membrane containing a bis(2-ethylhexyl)phosphoric acid carrier. Because of the dual effect of EDTA complexation and carrier reaction, a very high selectivity of Co2+/Ni2+ was obtained (up to 200). However, the
repeated emulsification/demulsification procedure makes such processes complicated and rather impractical. As discussed above, not only the absolute Kf values of two metals (especially for the weaker-complexed metal) but also their differences play an important role in the exchange separation. A parameter γ is thus proposed that considers the above two factors, which is used to correlate the results obtained for different metal pairs and using different types and amounts of the ligands.
γ ) log KSf /(log KLf - log KSf )
(4)
where the superscripts S and L refer to the smaller and larger constants, respectively, for a given metal pair and the ligand. For simplicity, the values of Kf taken in this work are based on the formation of the 1:1 complexes, ML, except for the complex MHL in the citrate systems, as indicated above. Figures 12 and 13 show the typical results in the Zn2+/ Cu2+ system. Such a correlation fails at pH 4 and 5, but it appears to be successful at low pH (10) only when R > 0.5. A maximum selectivity was obtained at R ) 1 and pH 2 (106 with EDTA and 157 with NTA). In the presence of weaker ligands such as IDA and citrate, no exchange separation was found at pH < 3 regardless of the amount of the ligand added. At pH > 3, the selectivity increased with an increase in both the pH and R values. A maximum selectivity of 187 was obtained at pH 5.5 and R ) 1 using IDA. The absolute values of the overall formation constants of two metals with the ligand and their difference played an important role in the exchange separation. A parameter γ was used to correlate the results obtained in the case of different metal pairs and different types of anionic ligands. Such a correlation was successful at low pH (
