Removal of Copper(II) Chelates of EDTA and NTA from Dilute

In each UF experiment, the feed volume was 200 cm3 and the first 10 cm3 of the ... The retention coefficient of Cu(II) chelates is defined in both way...
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Ind. Eng. Chem. Res. 1997, 36, 179-186

179

Removal of Copper(II) Chelates of EDTA and NTA from Dilute Aqueous Solutions by Membrane Filtration Ruey-Shin Juang* and Ming-Nan Chen Department of Chemical Engineering, Yuan-Ze Institute of Technology, Nei-Li, Taoyuan, 320, Taiwan, ROC

The removal of Cu(II) chelates of ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA) from aqueous solutions was studied by batch ultrafiltration (UF) with the help of poly(ethylenimine) (PEI). The Amicon regenerated cellulose YM10 and YM30 were used as ultrafilters. Experiments were performed as a function of aqueous pH, the concentration ratios of chelating agent to Cu(II) ions (R) and of PEI to Cu(II) ions (β), and the applied pressure (∆P). In general, the removal of Cu-EDTA chelates was more effective compared to that of Cu-NTA chelates. Under the conditions tested (pH 4, ∆P ) 2.04 atm, [Cu2+] < 4 mM), the removal efficiency was higher than 97% for Cu-EDTA chelates when R > 0.5 and β > 10 and higher than 96% for Cu-NTA chelates when R > 0.5 and β > 20. The effect of the presence of inorganic salts on the removal was also investigated. Finally, the membrane fouling during UF was analyzed by modified fouling index, and the regeneration of PEI was examined. Introduction For the treatment of metal-bearing industrial effluents, chemical precipitation methods (e.g., hydroxide, sulfide) are the most economical. However, the presence of strongly chelating agents such as citrate, tartarate, ethylenediaminetetraacetic acid (EDTA), and nitrilotriacetic acid (NTA) may make the precipitation process ineffective for the separation and recovery of metal ions from such effluents, even when coupled with high levels of metal ions (Tunay et al., 1994). Also, the high buffer capacity provided by these chelating agents would require excessive amounts of chemicals for alkaline neutralization. These problems and restrictions warrant investigation of other alternatives of the recovery process. The sources of metal-bearing solutions containing chelating agents such as EDTA and NTA include the discharge from electroless copper plating for printed circuit boards (Spearot and Peck, 1984; Haas and Tare, 1984; Sircharoenchaikit, 1989), metal-finishing industries (Reed et al., 1994), and the washing effluents from heavy metal-contaminated soil (Allen and Chen, 1993; Davis and Singh, 1995; Elliot and Brown, 1989). The treatment methods depend greatly on the particular chelating agent and metal ions used as well as their concentrations. They are generally grouped into three categories, chemical, physical, and electrochemical (Spearot and Peck, 1984). The chemical methods include substitution, reduction of the metal ions, oxidation of the chelating agents, and ion exchange (Fries and Chew, 1993; Haas and Tare, 1984; Sircharoenchaikit, 1989). The physical methods are evaporation, reverse osmosis, and adsorption (Reed, et al., 1994). The electrochemical methods are electrolytic plate out, electrowinning, and electrochemical displacement (Allen and Chen, 1993). Ion exchange and reverse osmosis, while effective in producing an effluent low in metal ions, have high operation and maintenance costs and are subject to fouling. Activated carbon adsorption is an established method for the treatment of organic contaminants. Nevertheless, the performance of a granular activated carbon column was reported to be unsatisfactory for effluents containing EDTA (Reed et al., 1994). * To whom all correspondence should be addressed. S0888-5885(96)00311-9 CCC: $14.00

During recent years, ultrafiltration (UF) has been shown to be a prospective way for removing trace metals from industrial effluents, provided that the metal ions were primarily bound to water-soluble polymers (Chaufer and Deratani, 1988; Geckeler et al., 1980, 1988; Geckeler and Volchek, 1996; Legault et al., 1993; Rumeau et al. 1992; Tabatabai et al., 1995; Tuncay et al., 1994; Volchek et al., 1993; Zhou et al., 1994). The polymers and their metal complexes can be retained by an ultrafilter, whereas the unbound metal ion pass through the membrane. This process can be used for various purposes such as the treatment of waste effluents, ground water, sea water, and radionuclides. The advantages of this method are the low-energy requirements involved in UF (Bemberis and Neely, 1986) and the high selectivity of the separation, owing to the use of selective binding. Poly(ethylenimine) (PEI) was widely used as a polymeric ligand and as a versatile source of complexing derivatives for removing metal ions from aqueous solutions by means of binding UF (Chaufer and Deratani, 1988). In addition to binding abilities, PEI exhibits a high content of functional groups, good water solubility, and chemical stability, all of these features rendering it particularly suitable for this purpose (Chaufer and Deratani, 1988; Legault et al., 1993). Owing to the basic nature of PEI, the removal of the negatively charged species such as HAsO42-, HPO42-, SeO32-, and CrO42by UF has been proven to be efficient (Geckeler et al., 1988; Legault et al., 1993; Tuncay et al., 1994). The aim of this paper is to examine the removal of Cu(II) chelates to EDTA and NTA from dilute aqueous solutions by UF with the help of PEI. Experiments were performed as a function of aqueous pH, the concentration ratios of chelating agent to Cu(II) ions and of PEI to Cu(II) ions, and the applied pressure. The effect of inorganic salts including NaCl, CaCl2, and Na2SO4 on the removal was also studied, and membrane fouling is discussed. Experimental Section Apparatus, Membranes, and Reagents. All UF experiments were carried out in a batch stirred cell (Advantec Model UHP 62, Tokyo, Japan). The cell was stirred by a magnetic motor at 300 rpm, which can © 1997 American Chemical Society

180 Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997

prevent the formation of a serious vortex in the cell. The applied pressure was controlled by nitrogen gas, and the operating temperature was fixed at 298 K by an air conditioner. The UF membranes used in this study were the Amicon hydrophilic YM10 and YM30 (regenerated cellulose). They had a normal molecular weight cutoff of 10 000 and 30 000, respectively, and a pure water permeate flux Jw of 90-120 and 480-600 dm3/ (m2‚h) after 5 min of operation at ∆P ) 3.7 atm. The value of Jw was measured before and after UF in each run to verify the absence of fouling phenomena. Only the membranes with a deviation of Jw smaller than 5% were repeatedly used. PEI with an average molecular weight 50000-60000 was used (Janssen Chimica, Belgium) as a 50 wt % aqueous solution. EDTA, NTA, and other inorganic chemicals were supplied by Merck Co. (Darmstadt, Germany) as analytical reagent grade. The aqueous phase was prepared by dissolving CuSO4 and/or chelating agents in deionized water to the required concentration, in which the pH was adjusted by adding a small amount of HCl or NaOH. Prior to the UF, the solutions containing PEI and Cu(II) chelates were mixed and left standing for 5 days to complete binding. Procedure. In each UF experiment, the feed volume was 200 cm3 and the first 10 cm3 of the permeate was discarded. The permeate flux and removal efficiency were determined by analyzing the next 60 cm3 of the permeate. In this range, the permeate composition was found to be nearly invariable during UF. Here, the removal efficiency is in terms of the retention coefficient of Cu(II) chelates, as defined below. The aqueous pH is the feed and permeate were measured with a Radiometer pH meter (Model PHM82, Denmark). It was reported that the concentration of the PEI complexes of metal chelates, rather than that of the total PEI, cannot be accurately analyzed (Juang and Chen, 1996). In this instance, the concentration of Cu(II) ions in the permeate was alternatively measured with an atomic absorption spectrophotometer (GBC, Model 932) at 324.8 nm. In some cases, the total organic carbon contents (TOC) were also determined using a TOC analyzer (O.I. Analytical Co., Model 700, United States) for comparison. The retention coefficient of Cu(II) chelates is defined in both ways

RCu ) 1 - ([Cu2+]p/[Cu2+]0)

(1)

RTOC ) 1 - (TOCp/TOC0)

(2)

where the subscripts p and 0 refer to the permeate and initial feed, respectively. The final concentration of metal ions in the retentate was also determined to check whether the mass balance was fulfilled or not (in general, within 1%). The used membranes were immediately flushed with deionized water after UF and then were regenerated in sequence by rinsing with solutions of 100 mM NaOH, 1.4 mM NaOCl, and 10 mM HCl using ultrasonic cleaner for 20 min each. Results and Discussion UF Flux of Pure PEI and Cu(II) Chelate Solutions. The effect of applied pressure (∆P) on the normalized UF flux, (Jv/Jw), of pure PEI solution is shown in Figure 1. It is found that when ∆P increases (Jv/Jw) sharply decreases for YM30 compared to YM10. However, rather weak dependence of PEI retention on the applied pressure is seen for both YM10 and YM30

Figure 1. UF fluxes of pure PEI solutions at different applied pressures.

(not shown). In general, raising the applied pressure increases the tangential forces acting on the PEI molecules at the pore entrance, together with the liquid flow velocity through the membrane pores. An increase in these forces also enhances the deformation of PEI and its transport through the membrane (Volchek et al., 1993). The serious drop in (Jv/Jw) for YM30 is likely due to the easier adsorption of the deformed PEI molecules at the larger pore entrance of YM30. Moreover, the effect of ∆P on the removal of Cu(II) chelates and its UF flux by YM10 is found to be weak (not shown), in comparison with the UF of pure PEI solutions. Thus, an applied pressure of 2.04 atm was selected here, and in this case Jw for YM10 and YM30 was about 50 and 280 dm3/(m2‚h), respectively. The retention of PEI with YM30 is also acceptable; nevertheless, the membrane fouling due to concentration polarization is more serious compared to that with YM10, which is supported by the pressure dependence of UF flux (Figure 1). In fact, it was found experimentally that the regeneration of the fouled YM10, probably due to surface deposition, by chemical cleaning is easier than that of the fouled YM30, due to pore adsorption. In this respect, YM10 is selected for further use. Figure 2 shows the effect of aqueous pH on the UF flux of Cu-EDTA chelate solutions by YM10. As expected, (Jv/Jw) decreases by raising the concentration of PEI. It is also seen that (Jv/Jw) increases with aqueous pH in the pH range 2-3; however, a further increase in pH results in a reduction of (Jv/Jw). In the absence of metal ions or metal chelates, it is know that the solution pH has a great effect on the shape of polymer chains (Geckeler and Volcheck, 1996; Rumeau et al., 1992; Volchek et al., 1993). The higher the pH, the less the amount of the protonated positively charged groups in the PEI molecules. This gives small repulsion forces between PEI molecules, enhancing the polymer aggregation and thus raising the UF flux (Volchek et al., 1993). Therefore, continued increase in the pH may cause the “more flexible” PEI aggregates, due to the hard protonation of the amino groups in the PEI

Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997 181

Figure 2. UF fluxes of Cu-EDTA chelates at different pH values.

Figure 4. Removal of Cu-NTA chelates at different pH values.

Figure 3. Removal of Cu-EDTA chelates at different pH values.

Figure 5. Removal of Cu(II) chelates at different pH values.

(Molyneux, 1983), to enter (squeeze into) the pores or make the deposited cake more compressible, resulting in lower flux. The conformational change of polymeric ligands with pH can alter both the retention of solute and the mechanism of membrane fouling (Choe et al., 1986). It is believed that this is also the case in the presence of Cu(II) chelates. Effect of pH on the Removal of Cu(II) Chelates. Although the retentate composition does vary during batch UF, the retention coefficient of Cu(II) chelates defined by eq 1 is found to be nearly constant. Hence, an average RCu or RTOC can be determined. Figures 3-5 illustrate the removal of Cu(II) chelates of EDTA and NTA with and without PEI at different aqueous pH values. In the pH range 2-6, it is found that pure Cu(II) chelates of EDTA and NTA can almost pass

through YM30 (RCu < 0.04) and YM10 (RCu < 0.09), respectively. The small but not negligible retentions of pure Cu(II) chelates are likely due to the formation of soluble hydroxy complexes in the aqueous phase, especially at higher pH. In the presence of PEI, the same trends as the UF flux shown in Figure 2 are observed. All existing data in Figures 3 and 4 reveal that the removal efficiency increases with pH but slightly decreases when the pH is higher than around 4-5. An increase in the retention by raising the solution pH is probably a result of the higher binding with the polymeric ligands. PEI, a proton-acceptor polybase, is involved in competitive reactions of protonation and complex binding (Juang and Chen, 1996; Kobayashi et al., 1987; Volchek et al., 1993). It is experimentally found that the permeate pH is slightly higher than the

182 Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997

feed pH as the feed is weakly acidic (pH 6-7); otherwise, the permeate pH is slightly lower than the feed pH when the feed is weakly alkaline (pH 7-8.5). Consequently, when the feed pH is not far from 7 we have

PEI + H S PEI‚H; Kp

(3)

CuL + nPEI‚H S CuL(PEI‚H)n

(4)

where n is the average number of the protonated PEI ligands bound to one Cu(II) chelate and L denotes EDTA4- or NTA3-. The ionic charges are omitted for simplicity. Although the interaction of PEI and phosphate anions was basically of the simple acid-base type (Zhou et al., 1994), the interaction of PEI and Cu(II) chelates could be predominantly of the electrostatic type. This is because the removal is greatly affected by the presence of inorganic salts (Rouzina et al., 1996), especially at pH < 6 as shown below. If it was purely of the acidbase type, then PEI would bind preferentially to the strongest acid form. This is not the case here at lower pH, as discussed in the work of Juang and Chen (1996). In this instance, the following alternative reaction occurs.

CuL + mPEI S CuL(PEI)m; Kb

(5)

Thus, an increase in the pH leads to a decrease in the fraction of the protonated PEI and thus to a shift in the equilibrium toward formation of the PEI complex. The binding equilibria of PEI with metal chelates of EDTA and NTA in sulfate solutions have been recently studied by UF (Juang and Chen, 1996). The chemical model considers an aqueous solution containing single metal chelates and no insoluble hydroxide solids. The reactions include the protonation of PEI (eq 3), the binding of PEI with metal chelates (eq 5), and formation of soluble metal hydroxy complexes. Moreover, the following justified assumptions were made: (1) free metal chelates and hydroxy complexes pass through the membrane; (2) the equilibrium constants do not depend on the pH or the concentration of the species involved in the reactions; (3) the rejection coefficient of the unbound PEI equals that of the PEI complex of metal chelates. It is shown that log Kp varies from 8.2 to 9.3 in the pH range 3.0-3.8 and both m and Kb increase with β, hence raising the retention of metal chelates (Juang and Chen, 1996). Also, m and Kb for metal-EDTA chelates are larger than metal-NTA chelates, which reflects the higher retention of EDTA chelates. In the Cu(II)-NTA system, the breakdown of the PEI complexes is found at pH ) 2. In the Cu(II)-EDTA system, the effect of the formation of soluble hydroxy complexes is remarkable at pH < 3. As shown in Figure 3, the slight drop in RCu at pH > 5 is probably due to the competition reaction of OHions and PEI with such chelates (Juang and Chen, 1996). At pH > 7, actually, the gelatin-like blue aggregates start to form, which results from the binding of PEI with the hydroxy complexes of Cu(II) chelates, thus reducing the permeate flux. When pH < 3, the extremely low RCu may be due to the formation of the species CuHL (Juang and Chen, 1996) and to the possible breakdown of CuL-PEI complexes (Zezin et al., 1977). An optimal pH range of 3-5 is hence recommended.

Figure 6. Removal of Cu(II) chelates at different R values.

It is known that the nonprotonated PEI, having the unshared electron pair on the nitrogen atom, is capable of forming donor bonds with coordination unsaturated metal ions (Nguyen et al., 1981; Zezin et al., 1977). In the presence of EDTA and NTA, as discussed above, the binding of PEI with metal chelates is believed to be caused by electrostatic interaction rather than coordination (Zhou et al., 1994). Thus, the binding ability of PEI with CuHL would be far weaker than the more negatively-charged CuL. Moreover, as shown in Figure 5, the removal of the bicharged Cu-EDTA chelates is more effective than the monocharged Cu-NTA chelates. In general, such differences become significant at lower PEI concentrations. Effect of Concentration Ratios on the Removal of Cu(II) Chelates. Figures 6-8 show the effects of various concentration ratios, [L]/[Cu2+] (R), [PEI]/[Cu2+] (β), and [PEI]/[L] on the removal of Cu(II) chelates. It is evident that the removal efficiency sharply increases with these ratios and then reaches a plateau. As has been discussed above, the retention coefficient of bicharged Cu-EDTA chelates is larger than that of monocharged Cu-NTA chelates. In case of R < 1, it follows from Figure 6 that RCu decreases by raising [Cu2+] when [PEI] and [L] are fixed. This is because the higher the [Cu2+], the greater the amount of unchelated Cu(II) ions with the chelating agents. In this instance, free Cu(II) ions may directly bind with PEI, thus decreasing RCu, because in the presence of PEI the removal of metal ions is less effective than that of their chelates (Juang and Chen, 1996). In addition, RCu increases with [PEI] when [Cu2+] and [L] are fixed, as shown in Figures 7 and 8. This may be due to the greater the amount of Cu(II) chelates bound at higher [PEI]. The close agreement of RCu and RTOC at sufficiently high pH and [PEI] (Figures 3 and 4) means that Cu(II) ions and EDTA or NTA form stable chelates and mostly bind with PEI, since the TOC in the solution is effected by both chelating agent and PEI only. However, a further increase in PEI concentration (β > 10) gives a lower retention of Cu-EDTA chelates, which may result from

Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997 183

Figure 7. Removal of Cu(II) chelates at different β values.

Figure 8. Removal of Cu(II) chelates at different (β/R) values.

an increase in polarization phenomena near the membrane surface or a decrease in size of PEI molecules due to a more serious aggregation (Nguyen et al., 1981; Volchek et al., 1993). It is seen from Figure 4 that RCu decreases by increasing [NTA] when [PEI] and [Cu2+] are fixed. This may be caused by the possible binding of NTA and PEI with Cu(II) chelates, which is supported by the fact that the retention starts to reduce at a lower pH ()4) compared to the cases of lower [NTA]. It is noted that a comparatively large difference between RTOC and RCu is observed at low R ( 0.96) when R > 0.5 and β ) 20. Effect of Inorganic Salts on the Removal of Cu(II) Chelates. It is known that the ionic strength of aqueous effluents would greatly affect the effectiveness of the binding UF process. Increasing the salt concentration and thus the ionic strength of the soluitons leads to compression of the electric double layer, and hence the electrostatic attraction between Cu(II) chelates and PEI is greatly reduced (Tabatabai et al., 1995). As a result, the Cu(II) chelates unbound with PEI in the solution pass through the membrane leading to a lower retention. Figure 9 shows the effect of the concentrations of inorganic salts on the removal of Cu(II) chelates. It is found that RCu decreases with increasing salt concentration, and the magnitude of such reduction decreases in the order Na2SO4 > CaCl2 > NaCl, as also seen in the UF of metal ions using PEI and its derivatives (Geckeler et al., 1988; Legault et al., 1993; Volchek et al., 1993). This is due to the fact that, at the same molar salt concentration, the ionic strength of the solution containing Na2SO4 (equal to three times the molar concentration) is larger than that of a solution containing NaCl (equal to the molar concentration) (Volchek et al., 1993). On the other hand, the behavior of CaCl2 could be explained from the viewpoint of competitive binding (Geckeler et al., 1988; Legault et al., 1993). By reason of the higher electrostatic affinity to the polymers, the bicharged anions (SO42-) must be stronger competitors for the reagent than the monocharged anions (Cl-). It follows from Figure 9 that RCu < 0.1 with the concentation of Na2SO4 added exceeds about 10 mM. In practice, it is experimentally found that gelatin-like white aggregates form in the presence of either Na2SO4 or MgSO4 at this salt concentration (10 mM) and pH 4.

184 Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997 Table 1. Removal of Cu(II)-NTA Chelates at pH 2 for Different Compositionsa [Cu2+] (mM)

R

β

RCu

[Cu2+] (mM)

R

β

RCu

1 0.5 0.5 0.5 1 1 1 2 2 2 2 4 4

1 2 2 2 1 1 1 0.5 0.5 0.5 0.5 0.25 0.25

1 10 20 40 5 10 40 2.5 5 10 20 2.5 5

0.02 0.10 0.26 0.56 0.05 0.20 0.64 0.01 0.02 0.18 0.42 0.01 0.11

4 4 0.25 0.25 2 2 2 2 1 2 2 2 2

0.25 1 4 4 0.25 0.25 0.25 0.25 0.5 1 1 1 1

10 10 20 40 2.5 5 10 20 20 2.5 5 10 20

0.23 0.38 0.30 0.32 0.06 0.12 0.23 0.37 0.40 0.06 0.15 0.51 0.72

a

YM10, ∆P ) 2.04 atm, temperature ) 298 K.

However, this is not the case for CaCl2. The competitive binding of SO42- and PEI to form certain polymeric complexes is hence apparent. The presence of Na2SO4 at low pH ( 1 and β > 10, the total binding capacity of PEI is so large that it can still react with metal chelates or metal ions, thus reducing the regeneration efficiency. In this case, successful regeneration can be achieved by the addition of Na2SO4 (Figure 9). The causes of these phenomena have been discussed above. Membrane Fouling Analysis. The long-term tests were also carried out to check the flux decline during UF. Figure 10 shows the results. Although RCu or RTOC decreases along the process, its deviation locates within

Figure 10. Long-term test for the removal of UF flux of Cu(II) chelates.

only about 5%, the UF of Cu(II) chelate solutions is more or less affected by membrane fouling, especially in the case of Cu-NTA chelates. This fouling is likely to be a combination of initial pore blocking by PEI chains and the acquisition of a fouling, second layer on the surface which modifies the pore size distribution of original membrane (Lipp et al., 1988; Scott et al., 1992). In this work, the aspect of membrane fouling is simply considered by analyzing the flux data in terms of a fouling index (Scott et al., 1992). The analysis of fouling in membrane filtration derives from the classical equation for filtration in filter beds, in which resistance to flow is a summation of the resistance of the membrane, Rm, and that of the deposited cake fouling layer, Rc (Belfort, 1979; Lipp et al., 1988; Scott et al., 1992)

(1/A)(dV/dtF) ) (1/η){∆P/(Rm + Rc)}

(6)

where A is the membrane cross-sectional area and V is the volume of the permeate collected. Assuming that the fouling potential of the feed, F, is proportional to the concentration of fouling particles, the following relation between cake resistance and fouling potential is obtained (Belfort, 1979; Lipp et al., 1988; Scott et al., 1992)

Rc ) (V/A)F

(7)

Combining eqs 6 and 7 and integrating for a constant applied pressure ∆P gives

(tF/V) ) (ηRm/∆PA) + (ηF/2∆PA2)V

(8)

Thus, a plot of (tF/V) vs V gives a straight line with a slope of (ηF/2∆PA2). The term (ηF/2∆PA2) is commonly called the modified fouling index (MFI) (Scott et al., 1992), which has units of h/dm6. Equation 8 is basically valid in the case of dead-end filtration (Lipp et al., 1988). Its relative simplicity makes it convenient in analyzing fouling behavior, as

Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997 185

the lack of initial pore blockage was proven, and the fouled cellulosic membranes could be easily cleaned. The removal of the bicharged Cu-EDTA chelates was more effective than that of the monocharged Cu-NTA chelates, which results from their binding natures with PEI (electrostatic interaction). It was shown that the removal of Cu(II) chelates first increased sharply with aqueous pH, R, and β but then reached a plateau. Under the ranges tested ([Cu2+] < 4 mM, pH 4, ∆P ) 2.04 atm), the removal efficiency was high for CuEDTA chelates (>97%) when R > 0.5 and β > 10 and for Cu-NTA chelates (>96%) when R > 0.5, and β > 20. The removal efficiency decreased by raising the concentration of salts added, and the extent of such reduction decreased in the order Na2SO4 > CaCl2 > NaCl. This is due to different ionic strength of the solution (for Na2SO4) and to competitive binding of the salt anions and PEI with Cu(II) chelates (for CaCl2). The regeneration of PEI was satisfactory at pH 2 when R < 1 and β < 10. When β > 10, successful regeneration could be achieved by the addition of Na2SO4 at pH < 4. Acknowledgment

Figure 11. Membrane fouling analysis for the UF of Cu(II) chelates. Table 2. Membrane Fouling Index for the UF of Cu(II) Chelate Solutionsa [Cu2+] (mM)

chelate

R

β

pH

membrane

MFI (h/dm6)

1 1 1 1 1

EDTA EDTA EDTA NTA NTA

1 1 1 1 1

10 40 40 10 40

4 6 6 4 6

YM10 YM10 YM30 YM10 YM10

0.09 1.43 2.11 0.45 1.48

a

∆P ) 2.04 atm.

was often done in the membrane filtration of emulsions and suspensions (Juang and Jiang, 1994; Scott et al., 1992). For flowing or stirred filtration processes, such an analysis may be applicable only for low-flux membranes, as the case of this work, since in this instance the change in solute concentration is relatively small (Scott et al., 1992). In general, two regions of this type of analysis were observed previously (Juang and Jiang, 1994; Lipp et al., 1988; Scott et al., 1992). The first region, accredited to initial blockage of the pores, occurs at the start of UF and is reflected in a convex-up shape to the plot in which there is a rapid falloff in flux. The second region is linear and is associated with “cake” formation and buildup (Scott et al., 1992). In the presnt work, however, only a single linear region exists for YM10 and YM30 during UF (Figure 11). The lack of initial pore blockage is evident. The values of MFI obtained are listed in Table 2. It is seen that the membrane fouling is comparatively serious for Cu-NTA chelates, for membranes with higher Jw (YM30), and at higher β. Practically, the blue-colored cake layer can be easily removed by flushing with water immediately after UF.

This was work was supported by the ROC National Science Council under Grant No. NSC85-2214-E-155001, which is greatly appreciated. Nomenclature A ) membrane cross-sectional area (m2) EDTA ) ethylenediaminetetraacetic acid F ) fouling potential defined in eq 7 (1/m2) Jv ) permeate flux of actual solution (dm3/m2‚h) Jw ) permeate flux of pure water (dm3/m2‚h) Kb ) binding constant of PEI and Cu(II) chelates defined in eq 5 (M-m) Kp ) protonation constant of PEI defined in eq 3 (M-1) L ) chelating agent (EDTA or NTA) MFI ) modified fouling index (h/dm6) NTA ) nitrilotriacetic acid ∆P ) pressure difference (atm) PEI ) poly(ethylenimine) Rc ) hydrodynamic resistance of cake defined in eq 7 (1/ m) Rm ) hydrodynamic resistance of membrane (1/m) RCu ) retention coefficient based on Cu(II) concentration defined in eq 1 RTOC ) retention coefficient based on TOC measurement defined in eq 2 tF ) filtration time for the UF of the solutions (h) V ) volume of the permeate collected (dm3) [ ] ) molar concentration of the species (M) Greek Letters R ) initial concentration ratio of chelating agent to Cu(II) ions β ) initiation concentration ratio of PEI to Cu(II) ions η ) viscosity of aqueous solutions (kg/m‚h) Subscript 0 ) initial (total)

Conclusions

Literature Cited

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Received for review June 3, 1996 Revised manuscript received October 2, 1996 Accepted October 21, 1996X IE960311B

X Abstract published in Advance ACS Abstracts, December 1, 1996.