Nanofiltration: Role of the Electrolyte and pH on Desal DK

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Nanofiltration: Role of the Electrolyte and pH on Desal DK Performances† Carolina Mazzoni, Luigi Bruni, and Serena Bandini* Dipartimento di Ingegneria Chimica, Mineraria e delle Tecnologie Ambientali, UniVersity of Bologna, Viale Risorgimento 2, I-40136, Bologna, Italy

Nanofiltration through Desal DK polyamide membranes is considered. The membrane separation efficiency is determined through permeation experiments with NaCl-water solutions as well as with CaCl2-water solutions. Membrane characterization is completed by introducing a wide study on proton rejection properties. Salt rejections and proton rejections are measured in a wide range of salt concentrations and pH values in the feed side, in the pressure range from 3 to 30 bar, at room temperature. The role of the electrolyte type as well as the influence of the feed pH on membrane separation properties is assessed in a general way: the common aspects and the main differences are put in evidence. In the concentration range investigated, NaCl rejection always decreases as the salt concentration increases; in contrast, CaCl2 rejection goes through a maximum value as the salt concentration increases. For both solutions, salt rejection is greatly affected by the feed pH, and proton rejection remarkably depends on operative conditions (feed pH and pressure). Salt and proton rejections are dependent upon the membrane charge (value and sign) existing at the corresponding conditions. On the other hand, the membrane charge is greatly affected by the electrolyte type and by the feed pH. All the behaviors observed can be simply explained case by case, taking into account the membrane charge behavior. In the case of CaCl2-water solutions, the electrolyte concentration is the key parameter in determining the amphoteric behavior of the membrane. Introduction Nanofiltration (NF) is a well-defined pressure-driven membrane technology, intermediate between ultrafiltration and reverse osmosis, which can be used as an alternative to conventional water treatment methods. NF membranes are highly selective with simple sugars and multivalent electrolytes, whereas lower rejections are obtained with univalent electrolytes. Hardness removal in water softening for industrial purposes or for drinking water production is one of the simplest low-energy requirement applications of this process. NF is also particularly favorable in all cases in which selective removal of electrolytes is required. The separation efficiency of the process depends on the membrane material, on the chemical nature of the feed solutions, and on the acidity conditions kept in the feed side. Polymeric membranes (polyamide, polysulfone, polyethersulfone, cellulose acetate, etc.) as well as inorganic membranes (alumina, titania, etc.) show different behaviors depending on the type of the electrolyte they are put in contact with. In the permeation of aqueous solutions containing strong electrolytes, two cases can be considered. In the case of single symmetric salts, such as NaCl and KCl, salt rejection generally decreases as the concentration increases at constant pH values, whereas rejection goes through a minimum value as feed pH increases.1-13 In the case of nonsymmetric electrolytes, on the contrary, chemical interactions of the salt with the membrane can be relevant; trends are often reversed with respect to the NaCltype behavior.2-6,8,11-16 The most interesting behaviors are obtained with polyamide membranes in aqueous solutions * To whom correspondence should be addressed. Tel.: +39 0512093138. Fax: +39 051581200. E-mail: serena.bandini@ mail.ing.unibo.it or [email protected]. † Paper presented at the ECI meeting on “Advanced Membrane Technology III: Membrane Engineering for Process Intensification” (June 11-16, 2006, Cetraro (ITALY)).

containing calcium or magnesium ions. As an example, MgCl2 rejection increases as the concentration increases8,14 and a maximum rejection value is obtained as pH increases.3 In the case of CaCl2-water solutions,16 the role of the electrolyte concentration is remarkable: CaCl2 rejection increases and goes through a maximum value as the salt concentration increases, at constant pH values in the feed. Such reversed behaviors cannot be qualitatively explained by only taking into account electrostatic partitioning effects (Donnan equilibrium and dielectric exclusion, mainly), which arise as a consequence of a net charge existing on the membrane surface.14,15,17,18 Nowadays, it is recognized that the surface of a NF membrane is endowed with charges which are strictly dependent on feed pH, as well as on type and concentration of electrolytes.19,20 Electrokinetic measurements of streaming potential21-27 show that physical-chemical interactions of multivalent ions with the membrane are stronger than the ones typically existing with univalent electrolytes. In particular, at fixed pH values, calcium binding on ionized sites of the membrane can be so remarkable that zeta-potentials switch from negative to positive values and the points of zero charge are greatly affected by salt concentration.21 The aim of this work is to put in evidence, in a synthetic and complete way, which are the relevant parameters determining the performances of Desal-type polyamide membranes. The effect of salt concentration and feed pH is reported both on salt rejection and on proton rejection, for the two representative solutions NaCl-water and CaCl2-water. In both cases, the common aspects as well as the main differences are focused on. The paper represents a synthesis of the research activity partially published by the same authors in refs 10 and 16; the new aspect is the study on the proton rejection which completes the membrane characterization for those specific cases. The paper is concluded through the calculation of the volumetric membrane charge values corresponding to each operative condition investigated by the Donnan steric pore model

10.1021/ie060974l CCC: $37.00 © 2007 American Chemical Society Published on Web 01/23/2007

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Figure 1. Scheme of the experimental apparatus: (a) experimental setup; (b) semicell and flow pattern.

and dielectric exclusion.15 The results obtained allow us to understand which are the main phenomena at the basis of the different behaviors observed. Materials and Methods An experimental investigation was performed testing NF Desal-5 DK membranes with NaCl-water and CaCl2-water solutions in a wide range of compositions and pH values kept in the feed side. pH and electrolyte concentration effects were studied both on salt rejection and on proton rejection. Desal-5 DK membranes are manufactured by Desal Inc. (Vista, CA); DK membranes are polymeric flat thin film composite membranes, in which a polyamide selective layer is supported on a polysulfone layer; nominal characterization given by the manufacturer is 98% rejection, measured at 1000 ppm MgSO4 at 25 °C and 6.9 bar. Various samples were used taken from two different batches and preserved in 0.5 wt % sodium bisulfite-water solutions; the samples taken from the batches delivered in 1999 and in 2002 are abbreviated as DK99 and DK02, respectively. The membranes were located in a radial flow circular cell in which the feed enters the center of the cell perpendicular to the membrane and flows outward in the radial direction (Figure 1b): the depth of the feed side chamber was close to 1 mm and the useful membrane area was 39 cm2. The membrane apparatus was arranged in a typical benchscale plant, operating in total recirculation mode; the flow rate was kept at 700 dm3 h-1. A flow sheet of the plant is reported in Figure 1a. The feed solution was continuously recirculated from a reservoir sufficiently large to keep the salt concentration constant during the experiments; the reservoir temperature was controlled by a thermostatic system. Feed and retentate pressures were measured at two points close to the inlet and the outlet sections of the module, respectively; the driving force (∆P) was calculated as the arithmetic mean of those pressure values. A buret on the retentate loop was used to measure the trasmembrane volume flux. Experiments were performed at 25 °C, with NaCl-water and CaCl2-water solutions in the range from 1 to 50 mol/m3 (pH 3-6.5) and from 1 to 500 mol/m3 (pH 5-6.5), respectively.

The applied pressure in the feed side was varied from 3 to 30 bar. Solutions were prepared from reagent-grade chemicals in deionized water, obtained through ionic exchange resins followed by a reverse osmosis step. The pH was adjusted by HCl or NaOH addition. NaCl and CaCl2 concentrations in the feed and permeate sides were measured by a conductimeter in the experiments at pH higher than 4.5; in the experiments at pH lower than 4.5, sodium concentration was determined through high performance liquid chromatography (HPLC), with a cationic column followed by an electric conductivity detector. For each operative condition three permeate samples were taken and analyzed. The retention datum is reported as the arithmetic mean of the corresponding rejection values; the maximum discrepancy obtained was less than 3%. pH measurements were performed in the feed side and in the permeate side and proton rejections were calculated; the instrument precision was (0.06. Experimental Results Standard procedures were followed for membrane characterization with pure water: the transmembrane flux was measured vs the applied pressure kept in the 3-30 bar range at 25 °C; the average water permeability was calculated as 6.41 × 10-8 m h-1 Pa-1 and as 7.07 × 10-8 m h-1 Pa-1 for DK99 and DK02 membranes, respectively. The role of salt concentration and feed pH in membrane performances both on salt rejection and on proton rejection is studied for two typical solutions of NaCl-water and CaCl2water. The effect of osmotic pressure on salt rejection is also reported. Salt Rejection. The effect of salt concentration and pH in the feed side on the salt rejection is summarized in Figure 2, in which Na+ and Ca2+ rejections are plotted versus the corresponding salt concentration, at various feed pH values. Data are reported with reference to the case of 25 bar pressure difference in the inlet section; however, the behavior was reproduced in the whole pressure range investigated.10,16 First of all, we can observe that, in the case of NaCl-water solutions, DK99 performs in the same way as DK02 (Figure 2a,b). In the concentration range investigated, Na+ rejection decreases as the salt concentration increases, and correspond-

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Figure 3. NF of aqueous solutions containing NaCl through DK99, at 25 °C. Rejection vs (a) applied pressure and (b) effective pressure difference across the membrane, at various salt concentrations in the feed.

Figure 2. NF of aqueous solutions containing NaCl or CaCl2 through DK99 and DK02 membranes, at 25 °C. Effect of salt concentration and pH in the feed on salt rejection, at ∆P ) 25 bar applied at the inlet section. Arrows indicate the rejection behavior with increasing values of pH.

ingly goes through a minimum value as the feed acidity increases (Figure 2a,b); with DK99 and DK02 the lowest rejections are close to pH 4.5 and pH 5, respectively. At feed pH values in the range from 5.3 to 6.2, DK02 is less pH sensitive than DK99, although, at pH 5.8, NaCl rejections are very close for both membranes (in this case, permeate concentrations measured with DK99 are 15-20% lower than DK02’s).

Figure 4. NF of aqueous solutions containing CaCl2 through DK02, at 25 °C. Rejection vs (a) applied pressure and (b) effective pressure difference across the membrane, at various salt concentrations in the feed.

Second, the relevance of the electrolyte type on membrane performances is self-evident in Figure 2c, in which CaCl2

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Figure 5. NF of aqueous solutions containing NaCl through DK99 membranes, at 25 °C. Effect of salt concentration and pH in the feed on H+ rejection. ∆PIN ) applied pressure at the inlet section. Arrows indicate the rejection behavior with increasing values of pH.

rejection is reported as a function of the salt concentration and pH. In this case, calcium rejection goes through a maximum value as the salt concentration increases; the same behavior is observed in the whole pressure range investigated.16 At low salt concentrations, the rejection dependence upon feed composition and pH is exactly reversed with respect to the NaCl behavior. Since the experimental conditions are kept the same in all the cases, that result is suggestive of different physical-chemical interactions of the dissolved ions with the membrane material. Typically polyamides show chemical affinity for alkaline-earth metals of the second group such as calcium and magnesium. On the other hand, also streaming potential measurements clearly remark the relevant role of the calcium ion in determining the membrane charge.21,23 The effect of osmotic pressure on salt rejection is reported in Figures 3 and 4, in the cases of NaCl-water and CaCl2water solutions, respectively. Rejection data are plotted versus the effective pressure, ∆P-∆π, in which ∆π is the osmotic pressure difference calculated at the corresponding bulk conditions existing in the feed side and in the permeate side, with reference to the case of pH 5.8. Apparently, in the case of NaCl-water solutions, the effect is quite negligible (Figure 3). In the case of CaCl2-water solutions, on the contrary, the effective driving force is greatly decreased by the osmotic contribution, above all at CaCl2 concentrations higher than 50

mol/m3 (Figure 4). The same behavior is obtained in the whole range of pH values investigated, for all the membranes tested. Proton Rejection. The effect of salt concentration and pH in the feed side on the proton rejection is reported in Figures 5-7. For both mixtures investigated, H+ rejections are remarkably dependent on the feed pH; rejections are obtained in a wide range of values, from very low negative values to positive values. It must be pointed out that the proton concentration in the permeate side is affected by an error related to pH measurements which amplifies the statistical error on H+ rejection, thus giving a more scattered behavior with respect to salt rejection data. However, proton rejection data considered on the whole allow us to draw a general trend. In the case of NaCl-water solutions with DK99 membranes (Figure 5), proton rejection is negative at pH 3, switches to positive values at pH in the range from 4 to 5, and then decreases again to negative values as pH increases. The same behavior is obtained also with DK02 membranes (Figure 6); however, in this case the dependence on pH is much more remarkable. As an example, in the pH range from 4.5 to 6, at 5 mol/m3 NaCl, at ∆P ) 25 bar applied at the inlet section, H+ rejection is obtained in the range from 84% to -140% and from 79% to -216% for DK99 and DK02, respectively. Generally speaking, with NaCl-water solutions, the effects of pH on sodium and proton rejections are exactly opposite:

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Figure 6. NF of aqueous solutions containing NaCl through DK02 membranes, at 25 °C. The effect of salt concentration and pH in the feed on H+ rejection. ∆PIN ) applied pressure at the inlet section. Arrows indicate the rejection behavior with increasing values of pH.

Na+ rejection goes through a minimum value located in the pH range from 4 to 5 (Figure 2a,b), and correspondingly, H+ rejection goes through a maximum value (Figures 5b and 6b). The feed pH at which proton rejection assumes its maximum value seems to be independent of the salt concentration (Figures 5b and 6b). Proton negative rejections are not a surprise:7,28-30 depending on feed pH, the permeate pH can be alternatively lower or higher than the corresponding value existing in the feed. The trend observed is qualitatively in agreement with what was measured also by other authors7,30 for the same kind of membranes, in the case of NaNO3-water solutions. Also in the case of CaCl2-water solutions, proton rejection achieves a maximum value as feed pH increases (Figure 7a). However, the pH at which H+ rejection approaches a maximum value remarkably depends on salt concentration. As an example, at 1 mol/m3 CaCl2, H+ rejection approaches a maximum at pH 6.5. Finally, it is worth noting that the influence of the electrolyte type and concentration on membrane performance is relevant also in determining the rejection of a trace ion such as proton; it is synthetically reported in Figures 5c, 6c, and 7b. For DK02 membranes, with NaCl-water solutions, proton rejection increases as the concentration increases (Figure 6c); with CaCl2 solutions, on the contrary, proton rejection goes through a minimum value as the salt concentration increases. The trend

observed parallels in a reverse way the trend obtained in parts b and c, respectively, of Figure 2 with regard to sodium and calcium rejection. All the behaviors observed, scattered and complicated at first glance, can be easily explained by taking into account a different behavior of the membrane charge with the salt concentration. Membrane Charge. All the NF experimental data available for DK membranes, partially reported in refs 10 and 16 and in this paper, were used to evaluate the adjustable membrane parameters. The membrane has been characterized through the use of three adjustable parameters, such as the average pore radius (rp), the effective membrane thickness (δ), and the volume membrane charge (X), which were calculated through the Donnan steric pore model and dielectric exclusion (DSPM&DE), introduced and widely discussed by the same authors in ref 15. The model takes into account the dielectric exclusion phenomenon as a partitioning mechanism at the membrane/external phase interfaces in addition to steric hindrance and to Donnan equilibrium. The contribution of the image forces is considered dominant with respect to Born dielectric partitioning; dielectric properties of the solution inside the membrane pores are also assumed equal to those existing in the external feed solution which are finally approximated as the pure water values at room temperature.15 As a consequence of those assumptions, no further adjustable parameters are introduced to characterize the membrane in addition to the typical parameters of the Donnan steric pore model, originally developed by Bowen et al.17,18 All the partitioning effects are located at the feed/membrane as well

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Figure 7. NF of aqueous solutions containing CaCl2 through DK02 membranes, at 25 °C. Effect of salt concentration and pH in the feed on H+ rejection. ∆PIN ) applied pressure at the inlet section. Arrows indicate the rejection behavior with increasing values of pH.

as at the membrane/permeate interfaces; the membrane charge is considered constant over all the membrane volume and it is representative of the overall effects of the interactions between the dissolved ions and the membrane material. The calculation procedure to obtain the membrane charge values has been described in detail by the authors in refs 10 and 16 for the NaCl-water and the CaCl2-water cases, respectively. In particular, since the role of concentration polarization in the feed side was relevant in the case of CaCl2-

water solutions, the membrane charge values were calculated at the corresponding salt concentrations existing at the feed/ membrane interface. In this paper, for the sake of clarity, only the final results are reported in Figure 8, in that they are strictly necessary to understand the experimental trends discussed in the previous section. With regard to the NaCl-water solutions, both for DK99 and for DK02 membranes (Figure 8a,b), the absolute value of the membrane charge increases as the feed salt concentration

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In the case of CaCl2-water solutions (Figure 8c), the membrane charge does not show a monotone behavior with the salt concentration: at low CaCl2 concentrations the membrane charge is negative, it switches to positive values as the concentration increases, and it finally decreases to negative values at higher concentrations. Maximum values are observed in the concentration range from 100 to 150 mol/m3 CaCl2, very close to the concentration values corresponding to the maximum rejections experimentally measured (Figure 2c). The trend is reproduced at each pH value investigated; membrane charge is an increasing function with feed pH. The points of zero charge also depend on feed pH, although their dependence on salt concentration is remarkable. Basing on those calculations, we can easily put in evidence that what is important in determining the membrane performance is the role of the electrolyte in the mechanism of membrane charge formation. In the case of NaCl-water solutions the behavior obtained for the membrane charge can be easily explained by supposing a competitive adsorption of Na+ and Cl- ions on the membrane surface.19,20,32 The trend observed with CaCl2 is rather different from the behavior obtained in the case of NaCl-water solutions: it is suggestive of a preferential calcium specific adsorption on the membrane, which is probably due to prevailing site-binding effects on the negatively charged sites of the membrane.32 At a constant pH value, as CaCl2 concentration increases, when all the ionized membrane sites are saturated by site-binding effects, chloride adsorption might become dominant so that the membrane charge decreases from positive to negative values.32 It must be stressed that the calculated membrane charges are average values which include the overall effects of the interactions between the dissolved ions and the membrane material. According to the basic idea introduced in the DSPM&DE model, they can be directly correlated to the ionic concentration existing at the feed/membrane interface. However, they are fitting values and their absolute values depend on the model used as well as on the values of all the other adjustable parameters. Membrane charge calculations were performed keeping the pore radius and the membrane thickness as constant values, without any variation of the dielectric constants. We can use these data with confidence to get information about general trends, since we only need to know the relative values for a certainty. Discussion and Conclusions

Figure 8. Volume membrane charge values vs salt concentration calculated at the feed/membrane interface, at 25 °C. (a) NaCl-water solutions through DK99 membranes (rP ) 0.57 nm, δ ) 19.3 µm); (b) NaCl-water solutions through DK02 membranes (rP ) 0.59 nm, δ ) 23.7 µm); (c) CaCl2-water solutions through DK02 membranes (rP ) 0.59 nm, δ ) 23.7 µm).

increases and, correspondingly, increases as the feed pH increases. A sign change is obtained at pH values close to 4-4.5, which approximately corresponds to the point of zero charge obtained by Hagmeyer and Gimbel2 in streaming-potential measurements with KCl-water solutions. Obviously, the trend obtained reproduces in a qualitative way the behavior observed in the case in which surface membrane charges were calculated based on electrokinetics measurements for membranes of the same kind.25

A very different behavior has been observed in salt rejection as well as in proton rejection for NaCl-water and CaCl2-water solutions, as a function of operative conditions. Membrane performances with NaCl or CaCl2 as dominant salts are not only affected by a different role of the dielectric exclusion and Donnan equilibrium on ionic partitioning at the membrane/ external solutions interfaces.15,31 Remarkably, the role of the electrolyte type is more important in determining value and sign of the membrane charge as a function of the salt concentration. With regard to NaCl-water solutions, by considering the membrane charge behavior observed in Figure 8,b, we can draw the following conclusions. At a constant salt concentration, at pH >4-4.5, since the membrane charge is negative and its absolute value increases as feed pH increases, we can expect that the chloride (the coion) rejection increases; correspondingly, at pH 4-4.5 the membrane charge is negative and the proton (the counterion with the higher mobility) is greatly attracted by the membrane toward the permeate and its rejection decreases to very low negative values. At pH