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Boron removal from water by complexation to polyol compounds
Journal of Membrane Science 286 (2006) 45–51
Boron removal from water by complexation to polyol compounds Nitzan Geffen a , Raphael Semiat b , Moris S. Eisen c , Yael Balazs c , Ilan Katz a , Carlos G. Dosoretz a,∗ a
Civil & Environmental Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel b Chemical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel c Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel Received 28 May 2006; received in revised form 20 August 2006; accepted 6 September 2006 Available online 16 September 2006
Abstract Boron is an important micronutrient for plants, animals and humans, although the range between deficiency and excess is narrow. The use of desalinated water and treated wastewater for irrigation may result in excess boron. In aqueous environments (i.e. neutral pH) boron is mainly present as boric acid, which is mostly undissociated and therefore only partially rejected by desalination membranes. Boric acid/borate reacts with neutral polyolic compounds, generating anionic complexes. This work reports on the complexation of boron with mannitol integrated with membrane desalination. The separation of the complex was studied in a wide range of conditions by nanofiltration (NF), and partially by reverse osmosis (RO) membranes. NMR analysis and chemical equilibrium modeling system (Mineql+) were applied to provide a better understanding of experimentally observed rejection patterns. The addition of mannitol in excess formed mainly a 2,2-di-borate ester and some monoborate esters as function of the pH, improving boron rejection by NF up to 90% at pH 9. Mineql+ calculations indicate that reactants concentration has a strong influence on the ionized boron species and therefore on the rejection of boron. Sea water-RO membranes having a much higher basal rejection for boric acid, rejected almost 97% mannitol-complexated boron at pH of 9. © 2006 Elsevier B.V. All rights reserved. Keywords: Boron removal; Complexation; Desalination; Reverse osmosis; Nanofiltration
1. Introduction Boron is an important micronutrient for plants, animals and humans, although the range between deficiency and excess is narrow (e.g. 0.3–0.5 mg/l in citrus). A minimal boron concentration is required in the irrigation water for metabolic activities of the crops. However, excess boron is harmful to plant growth; deleterious effects include yellowish spots on the leaves and the fruit, accelerated decay, and ultimately plant expiration [1]. In aqueous environments, boron is mainly present as boric acid and partially as borate ions according to the dissociation reaction (Ka = 6 × 10−10 , pKa 9.1) [2] shown in the following equation: B(OH)3 + H2 O B(OH)4 − + H+
∗
Corresponding author. Tel.: +972 4 8294962; fax: +972 4 8228898. E-mail address: [email protected] (C.G. Dosoretz).
0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.09.019
(1)
The concentration of boron in seawater is between 4 and 5 mg/l [1]. While the average boron concentration in drinking water supplied in Israel is currently around 0.2 mg/l, it may increase with the supply of desalinated seawater [3]. The average boron concentration in wastewater effluents in Israel in 2004 was reported around 0.32 mg/l [4], and is attributed mostly to sources such as household appliances (detergents), urine and various industrial wastes [5]. The increasing reclaim of treated effluent for irrigation has become a common practice in Israel in the recent years as a result of water shortage. In addition, desalinated sea water production is underway and due to become part of the national water supply. These new practices pose a new challenge for boron removal. Boron removal by sea water desalination, using reverse osmosis, reduces its concentration to only 0.9–1.8 mg/l in the permeate product (after a single pass) [6,7], while conventional wastewater treatment procedures and chemicals commonly used in the water treatment industry remove little, if any [8].
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Boron can present reproductive hazards and has suspected teratogenetic properties [6]. The World Health Organization (WHO) has set a limit of 0.5 mg/l of No Observed adverse Effect Level (NOEL) for drinking water [9] and there are limit values between 0.3 and 0.5 mg/l in tender documents for medium and large membrane desalination plants [6]. There is no easy method for removing boron from water. So far, the use of ion exchange resins and composite reverse osmosis (RO) membrane systems, as well as their combination with or without pH increase, are the only used technologies considered effective for the removal of boron. The predominance of undissociated boric acid at neutral pH is of major impact in desalination processes applying RO because its rejection in all type of membrane modules (spiral as well as hollow fiber) is about 65% whilst that of the borate ion is 95% [1]. Since boric acid has no ionic charge it diffuses in a similar way to that of carbonic acid or water itself [10]. The possible increase of the pH in the second pass for shifting the equilibrium to the side of the borate and to obtain high rejection (99.0–99.5% at pH 11) [6,11], will cause immediate calcium carbonate scaling [12], not to mention the cost associated with the injection of large amounts of caustic soda required and associated corrosion. The stability of the membrane and the service life in real operating condition has not yet examined [11]. For selective ion exchange resins the problems are high regeneration costs [13], expensive resins and low capacity with scale up problems [1,14,15]. Extensive research is being carried out today aimed at the development of new technologies as well the reduction of costs in existing ones. Boric acid/borate reacts with chemical compounds containing multiple hydroxyls groups (polyols), such as mannitol (Fig. 1), generating anionic complexes at the neutral pH of water. With carbohydrates and most polyols possessing 1,2-diol systems, the borate ion forms anionic mono (1:1) and bis (1:2) diol-monoborate species [16]. The stability of the borate complex formed is strongly dependent on the type of diol, namely 1,2 or 1,3-diols. If the diol involves the –OH groups oriented in such a way that they accurately match the structural parameters required by tetrahedrally coordinated boron, a strong complex will be formed [2]. The borate esters are formed and dissociated spontaneously in a variety of pH dependent equilibria [2]. Due to the release of acidic protons during complexation there is a concomitant
decrease of pH which tends to reverse the reaction and thus, in order to maintain stable complexes there is a need to avoid pH decrease. The amount of acidification produced upon the addition of polyol is proportional to the extent of borate ester formation. Borate, for example, complexes mainly with two molecules of mannitol at a pK value of 4.1 (50 mM B) [17] according to the following equation:
(2) Boron complexation plays an important role in boron uptake and fixation in plants. Natural boron complexation with natural polyhydric antioxidants such as flavonols has been observed in turmeric (curcum) and other flavor and fragrance plants. Boron complexation with malic acid has also been reported in wines [2,18]. Complexation of boron is also the basis of the selective ion exchange [13] and of supported liquid membranes [19] technologies available for boron removal from water. The study presented in this paper aimed at the complexation of boric acid/borate with mannitol integrated with membrane desalination systems. Mannitol was chosen as the model polyol due to its high equilibrium constant and its compatibility with drinking water. The complexation reaction occurs at neutral pH and produces a negatively charged di-borate ester of 375 Da size. The separation of the complex was examined primary by employing a nanofiltration (NF) membrane as a model of pressure driven-desalination technology and finally with a RO membrane. Nanofiltration was chosen in order to enable the examination of rejection patterns as influenced by the separation mechanism, e.g. molecular weight and the electric charge of the molecule, in wide range of working conditions. 2. Experimental All experiments were performed with pure solutions of boron and/or mannitol in distilled water. The pH of the solutions was adjusted using either 0.1 M NaOH stock solution or 0.2 M of dif-
Fig. 1. Schematic drawings of mannitol and negatively charged borate–mannitol complexes [27].
N. Geffen et al. / Journal of Membrane Science 286 (2006) 45–51
ferent buffers (phosphate pH 7.0, glycine pH 9.2, Tris pH 8.7 and imidazole pH 7.0). Glycine was in addition tested at 6.9 and 8.0 in order to check the effect of the nature of the buffer on the complexation, versus phosphate/imidazole and Tris, respectively. All chemicals used were of reagent grade quality. In order to assess the influence of different solution properties on the rejection of boron, mannitol or their complex, membrane separation experiments were performed in a bench scale membrane filtration module unit (SEPA CF, Osmonics) operated at 15 bar pressure at room temperature (25–30 ◦ C). The unit was equipped with either a NF membrane sheet made of polypiperazine amide (NF200, FilmtecTM ) or RO membrane sheet made of polyamide (BW30 or SW30, FilmtecTM , as indicated) of 190 cm2 net filtration area. The solutions were recycled through the membrane from a 2 l feed reservoir and 30 ml permeate samples were collected for analysis as indicated. Solutions of 7 and 32 mg/l average boron concentration were used in the NF experiments and of 7 mg/l in the RO experiments. An average boron:mannitol molar ratio of 1:4.2 and 1:6.5 was applied in NF and of 1:5, 1:10 and 1:15 in RO. Since the complexation reaction is almost instantaneous, the feedwater was first let to react for 10 min and then the permeate was sampled. The influence of the feed pH on the complexation and further on membrane rejection was examined in a pH range of 6.0–9.6. Baseline rejection of the membranes was determined by measurement of boron rejection of a neat boric acid solution at pH 5.7. All experiments were repeated 2–3 times. Boron rejection was calculated from the measured total boron concentration in the permeate (Cp ) and in the feed solution (Cf ) according to the following equation: Rejection = 1 −
Cp Cf
(3)
2.1. Analytical procedures Boron was assayed either by ICP-emission spectrometry (Optima 3000 DV, Perkin-Elmer) at 249.77 nm or spectrophotometrically upon complexation with Azomethine-H [20]. In the presence of organic buffers mannitol concentration was determined by HPLC (Agilent 1100 series, Hewlett Packard) equipped with a Ion Moderated Partition column (Rezex ROAOrg. Acid, Phenomenex) and monitoring with an Evaporative Light Scattering Detector (PLELS 2100, Polymer laboratories) with distilled water as the eluent at a flow rate of 0.5 ml/min. In all other cases mannitol concentration was determined by measuring the total organic carbon (TOC) concentration in a Shimadzu TOC-5000A analyzer. Two to three replicate analyses of each sample were performed. Nuclear magnetic resonance spectroscopy (1 H NMR, 11 B NMR and 13 C NMR) was applied for qualitative analysis and semi-quantitative determination of boron and mannitol speciation. NMR analysis was performed on a Bruker Avance 500 spectrometer, operating at 160.46, 125.76 and 500.13 MHz for boron 11, carbon 13 and hydrogen 1, respectively, at 25 ◦ C (±0.1 ◦ C) and running Xwin-NMR version 3.5.6. The spectrometer was equipped with a 5 mm broadband (bbo) probe. For the 11 B NMR a total of 128 transients were signal averaged
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using a repetition delay greater than 5 × T1 to allow for full relaxation (200 ms except for the solution with borate which was 1.1 s). A single pulse of 24 kHz (γ/2π) B1 field was used with 3k real points and a spectral width of 22,522 Hz. The FID was processed to 16k real points without applying window functions. To remove the broad background signal from the probe itself, difference spectroscopy was employed. A reference spectrum of the empty probe was acquired with 512 shots and, after scaling, subtracted from the experimental spectrum. Standard parameters were used for proton and carbon NMR. Evaluation of peak areas was done both using the integration routine supplied with Xwin-NMR version 3.5.6 and also using the free software program DMFIT [21] (http://crmhteurope.cnrs-orleans.fr/dmfit/help/dmfit.htm December, 2004). Analyses using either the Xwin-NMR or DMfit programs gave very similar results. NMR samples were prepared in 5 mm glass NMR tubes. 0.1 M (1081 mg/l) boron solutions were prepared by dilution with distilled and deionized water and 10% D2 O with different boron:mannitol molar ratios (without mannitol, 1:2, 1:5 and 2:1). The pH of the solutions was adjusted using either 0.1 M NaOH stock solution or 0.5 M of different buffers (phosphate, glycine and imidazole). The molecular structures of the complex were investigated using 2D 1 H COSY (correlated spectroscopy) [22] and 1 H{13 C}HMQC (Heteronuclear Multiple Quantum Coherence) [23] NMR with standard parameters. Diffusion constants using a BPLED sequence [24] (longitudinal eddy current delay with bipolar gradient pulse pair and two spoil gradients). The gradient pulse length (δ) was 4.4 ms, and the delay for diffusion (Δ) was 50 ms. The gradient strength was changed linearly over 16 steps from 5% to 95% of the maximum value (GREAT-10 gradient amplifier). Each measurement was run with eight scans and 32k real time domain points. For the processing, an exponential window function and single zero filling were applied. During the diffusion measurements, the fluctuation of the temperature was less than 0.1 K. Prior to the NMR scans, all samples were equilibrated for atleast 30 min. The chemical equilibrium modeling software Mineql+ (version 4.5) was used for theoretical computation of equilibrium concentrations of boron species. The pK used for the mono borate ester formation and for the di-borate ester were 6.21 and 4.13, respectively [25], normalized by Davies equation to zero ionic strength and the pH was determined by user from the measured pH in the experiments. Ionized boron species is expressed as the total concentration of free and mannitol-complexated ionized boron species relative to the total boron concentration. 3. Results and discussion The effect of pH on the boron rejection across the NF membrane with and without mannitol was studied at two different reactants concentrations: 7 and 32 mg/l boron and boron:mannitol molar ratio of 1:6.5 and 1:4.2, respectively, with and without buffer (Fig. 2). A boron removal efficiency ranging from approximately 26% to 50% was achieved by adjustment of the pH from 6.0 to 9.6, respectively. These results also indicate that the baseline value for boric acid rejection of this NF
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Fig. 2. Relative rejection of free and mannitol-complexated boron species as a function of equilibrium pH: 7 mg/l ( ) and 32 mg/l ( ) boron in the absence of mannitol (controls); 7 mg/l boron with mannitol at a boron:mannitol molar ratio of 1:6.5 ( ); 32 mg/l boron with mannitol at a boron:mannitol molar ratio of 1:4.2 in the presence of 0.2 M buffer [() phosphate pH 7.0; (×) glycine pH 6.9; () imidazole pH 7.0; (*) glycine pH 8.0; (♦) Tris pH 8.7; () glycine pH 9.2]. Rejection was calculated according to Eq. (3).
membrane is approximately 26%. The influence of pH on boron rejection by RO membranes is well known and is attributed to the ionization of the boron [6,26]. Boron concentration has no influence on the rejection as can be seen in the experiments run at 7 and 32 mg/l as well as in the Mineql+ simulations (see below Fig. 5), in line with the 1/1 boron/borate stoichiometry (pH = pKa + log[B(OH)4 − ]/[B(OH)3 ]) shown in Eq. (1). The addition of mannitol to a 7 mg/l boron solution at a molar ratio of 1:6.5 boron:mannitol increased the rejection in the entire pH range tested, up to 72% at a pH of 8.8, indicating the formation of an ionized boron:mannitol complex. Although such an addition was expected to enable the formation of an ionized complex at neutral pH (due to the low pK; see NMR analysis in Fig. 3 below), the rejection was not constant at these pH values and was almost identical to the rejection of mannitol alone (75% in average). This indicates that there was not complete complexation at these pH values. The addition of mannitol and buffer to a 32 mg/l boron solution at a molar ratio of 1:4.2 boron:mannitol
improved boron rejection substantially to approximately 60% at pH 7.5 and 94% at pH 9.0 as can be seen in Fig. 2. 11 B NMR analysis of complexation of boron (0.1 M = 1081 mg/l) with mannitol as a function of the pH shows that at pH ∼7.7 for a boron:mannitol molar ratio of 1:5 and at pH 8.8 for a boron:mannitol molar ratio of 1:2, complexation goes to completion (Fig. 3). Fig. 3 shows a major product of complexation with a chemical shift of −9.6 ppm which corresponds to the di-borate ester (BP2 ) and three minor peaks with lower chemical shifts (δ = −13.5, −13.7 and −14.7 ppm). These three peaks can be assigned to the monoborate ester (boron bounds to three different pair of hydroxyls of mannitol), in line with previous reports [17,27,28]. The speciation of the products of complexation was confirmed by comparative 11 B NMR analysis at increasing boron:mannitol molar ratio, from excess mannitol (1:5 and 1:2) to excess boron (2:1) at pH > pK of the complexation (Fig. 4). From the spectra presented in Fig. 4, the progress of the reaction as a function of the relative amount of mannitol can be clearly seen, which is in agreement with the relative area of the peak corresponding to the di-borate ester (BP2 , δ = −9.6 ppm). The di-ester is the major product at 1:5 ratio (spectrum b) and a minor product at 2:1 ratio in which mannitol is the limiting factor for the complexation (spectrum d). Note that at pH 11.9 free borate is formed, most probably due to the deficiency of mannitol (spectrum d). The other three peaks clearly visible in spectra c and d (δ = −13.5, −13.7 and −14.7), can be two different monoesters products—one molecule of boron and one of mannitol (BP) or two molecules of boron with one molecule of mannitol (B2 P). Again, boron can bind to three different pair of hydroxyls in the molecule of mannitol in each of the two monoesteres. The identity of the peaks in spectrum c seems to be BP in correspondence with the higher BP2 /BP at the higher excess of mannitol (boron:mannitol molar ratio of 1:5 versus 1:2) in Fig. 3 and in line with previous reports [28,27,17]. B2 P formation as a product of complexation of boron and mannitol was reported by Lorand and Edwards [29] and Van Duin et al. [28]. Product formation was confirmed by 13 C NMR analysis of the complexation at a molar ratio boron:mannitol of 1:2 as a
Fig. 3. 11 B NMR spectra of free and mannitol-complexated boron species as function of the equilibrium pH. Left panel, boron:mannitol ratio of 1:2; right panel, boron:mannitol ratio of 1:5. Boron concentration was 0.1 M. BP, boron:mannitol monoester; BP2 , boron:mannitol di-ester. Bottom spectra, represent undissociated boric acid (left panel) and fully ionized borate (right panel) controls in the absence of mannitol.
N. Geffen et al. / Journal of Membrane Science 286 (2006) 45–51
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Table 1 Diffusion constants (m2 /s) of different components of boron:mannitol solutions with different buffers Componentsa
Mannitol Complex Glycine
Buffer type Glycine (pH 9.5)
Phosphate (pH 6.8)
4.74 × 10−10 3.43 × 10−10 7.47 × 10−10
4.05 × 10−10 3.55 × 10−10 –
a Conditions were as follows: 0.1 M boron solution of 1:5 molar ratio of boron:mannitol, with addition of 0.5 M buffer. Diffusion constants were calculated from 1 H NMR spectra.
Fig. 4. 11 B NMR spectra of spectra of free and mannitol-complexated boron species as function of boron:mannitol molar ratio at different equilibrium pH values: (a) neat undissociated boric acid control (pH 5.7); (b) 1:5 boron:mannitol (pH 7.7); (c) 1:2 boron:mannitol (pH 8.8); (d) 2:1 boron:mannitol (pH 11.9); (e) neat fully ionized borate control (pH 11.9).
function of the pH (4.0, 5.0, 5.7 and 8.8) and as a function of the relative reactants concentration and pH (mannitol alone at pH 7, 1:5 ratio at pH 2.9, 1:5 ratio at pH 7.7, 2:1 ratio at pH 11.9 and 1:2 ratio at pH 8.8). Calculations made from 13 C-spectra corroborated the formation of BP2 (δ = 74.8 and 76.8) as the major complexation product at a ratio of boron:mannitol of 1:2 and 1:5. Indeed, the ratio of complexed mannitol to free mannitol of 1:1.76 (1:1.5 theoretical, correspond to the di-ester), a ratio of 1.8 molecules of mannitol per borate ion in the complex and 1 H COSY and 1 H{13 C}HMQC analyses further confirmed that the complexation-main product is BP2 and it occurred via the 2,2 carbons, all results being in agreement with the literature [17,27]. The influence of the buffers on the rejection efficiency was examined also by 1 H, 13 C and 11 B NMR, and almost complete complexation was observed in all cases upon addition of mannitol to buffered boron solutions at a 1:5 molar ratio, even at pH 5.0 (immidazole buffer), with BP2 being the almost solely product (data not shown). In order to discard the possible aggregation of mannitol free or in the complex as a function of the type of buffer and pH value (see Fig. 2 above), the diffusion constants, which indicate the hydrodynamic size of a molecule, of free and complexed mannitol were calculated from 1 H NMR analysis under different reaction conditions (Table 1). An almost similar diffusion constant was found for free mannitol in both buffers, averaging approximately 4.5 × 10−10 m2 /s which is in line with the diffusion constant of 1.5 M mannitol solution in water (4.9 × 10−10 m2 /s). The calculated diffusion constants of complexated mannitol in both buffers were 15–25% smaller than those of free mannitol, which is in agreement with the expected higher rejection of complex than mannitol alone and show no effect of the buffer
type. Thus, it seems there is no mannitol aggregation during complexation. Theoretical calculations (Mineql+ program) of ionized boron species products (complexated boron and borate) as a function of pH were done for a 7 and 32 mg/l boron at a molar ratio of 1:4.2 and 1:6.5 (Fig. 5). These calculations revealed that the reactants concentration have a strong influence on the ionized boron species and therefore on the expected rejection of boron during membrane separation. In contrast, the reactants ratio displayed a slighter effect on the level of ionized boron species. Addition of buffers has almost no effect on ionized boron species (data not shown). Comparing Figs. 2 and 5, it can be concluded that the major factor governing rejection of the complex at each individual reactants concentration in a NF membrane, as is for free borate, is the degree of ionization. Indeed, for 32 mg/l boron solution an almost complete ionization of boron species is expected at a pH value of approximately 9.5 and for 7 mg/l at pH 10 in practical terms. It must be noted that the experiments were performed in distilled water, whereas in practice the complexation process is envisioned to be applied during the second pass of multistage desalinations plants with recycling of concentrate in order to recycle excess polyol. This
Fig. 5. Theoretical ionization of free and mannitol-complexated boron species as function of the equilibrium pH and reactants concentrations. ( ) 7 mg/l boron in the absence of mannitol (control); ( ) 32 mg/l boron in the absence of mannitol (control); ( ) 7 mg/l boron with mannitol at a boron:mannitol molar ratio of 1:6.5; ( ) 32 mg/l boron with mannitol at a boron:mannitol molar ratio of 1:6.5; ( ) 7 mg/l boron with mannitol at a boron:mannitol molar ratio of 1:4.2; ( ) 32 mg/l boron with mannitol at a boron:mannitol molar ratio of 1:4.2. Theoretical ionized boron species was calculated by means of Mineq1+ software.
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tive BWRO membrane seems to follow the level of ionized boron species at the studied pH (∼73% at pH 9.0). For the SWRO, the obtained rejection was higher than the calculated ionized boron species at the respective pH (∼90% at pH 7.5 and ∼97% at pH 9.0), clearly indicating that other factors besides ionization are determinant, such as membrane selectivity. Furthermore, the dependence of the rejection on the boron:mannitol molar ratio above the stoichiometric ratio becomes almost negligible for the SWRO membrane as can be seen from the overlapping data. In studies of boron removal with a BWRO membrane 62% rejection was obtained at pH 9.5 [26] and 93–99% at pH 10 for both SWRO and BWRO which were tested in the article [6]. Boron removal by complexation with PVA followed by separation of ultrafiltration was reported in the order of 30% [30] and by ion exchange was almost complete (<0.1 mg/l) [12]. 4. Conclusion
Fig. 6. Comparison of experimental membrane rejection and theoretical ionized boron species as a function of pH values. (a) 7 mg/l boron with mannitol: ( ) fit of theoretical ionized boron species data and ( ) fit of experimental NF rejection data ( ) of boron–mannitol complexation reactions, both at molar ratio of 1:6.5; SWRO experimental rejection at molar ratio of 1:5 ( ), 1:10 ( ) and 1:15 ( ), all overlapping; BWRO experimental rejection of 7 mg/l boron with mannitol at molar ratio of 1:5 ( ) and 1:10 ( ), both over) fit of theoretical ionized lapping. (b) 32 mg/l boron with mannitol: ( boron species data and (—) fit of experimental NF rejection data (, ×, , *, ♦, ) of boron–mannitol complexation reactions in the presence of 0.2 M buffer as detailed in Fig. 2, all at molar ratio of 1:4.2. Theoretical ionized boron species was calculated by means of Mineq1+ software. Baseline rejection represents the experimental rejection of a solution of undissociated boric acid at pH 5.7 for each type.
would maintain high bulk concentrations of reactants which, as shown above, favors the complexation reaction and therefore the rejection. Finally, rejection tests were performed with RO membranes of intermediate and high selectivity, Brackish Water RO (BWRO) and Sea Water RO (SWRO) and compared with the rejection of the NF membrane of lower selectivity, and with theoretical ionization of boron species as a function of the pH (Fig. 6). There is a baseline rejection value of undissociated boric acid for each membrane, depending on its density and selectivity, indicated by the dotted lines in Fig. 6, as follows: 26% for NF, 60% for BWRO and 78% for SWRO. For the NF membrane the degree of rejection above this baseline seems to depend directly on the level of ionized boron species at both reactants concentrations (Fig. 6a and b). In a similar fashion, the rejection of the less selec-
Boron rejection by desalination membranes was enhanced by addition of mannitol according to the complexation equilibria. Rejection of the ionized complex by NF membranes, as for free borate, increased with pH increase, according to the degree of ionization, however, only partial rejection was observed in the pH range studied (7–9.2), most probably due to the incomplete ionization. At excess mannitol concentration the complexation reaction yields particularly di-ester and slightly amount of monoester without any aggregation, as demonstrated by NMR analysis. Simulation analysis and NMR experiments strongly indicate that the reactants concentrations and their ratio have a strong influence on the ionized boron species, and therefore on the rejection of boron by desalination membranes with low baseline rejection of undissociated boric acid, such as NF. The use of dense RO membranes improved the baseline rejection of undissociated boric acid and therefore a higher boron rejection could be obtained at a lower pH (97% at pH 9.0), regardless of the boron:polyol ratio above the stoichiometric ratio. As today new membranes with higher boron reduction are in development [31] and hence the required pH for this polyol complexation is expected to be lower. It must be noted that results presented here were obtained with pure boron solution in distilled water and desalination experiments were performed at room temperature (∼25 ◦ C) and 15 bar pressure, whereas the rejection at lower temperature and higher ionic strength is expected to be higher, in line with literature reports [11,12]. Acknowledgements This research was supported by grant of the Grand Water Research Institute, Zakin award for excellent students, and fellowship from Sherman foundation. We are grateful to Dr. Ori Lahav for his kind help with the Mineql+ calculations. We thank Hassan Koseoglu for his technical assistance on RO experiments. M.S.E. thanks the NATAF Program Administered by the Ministry of Industry and Commerce.
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[17] R. Belcher, G.W. Tully, G. Svehla, A comparative study of various complexing agents (polyols) used in the titration of boric acid, Anal. Chim. Acta 50 (1970) 261. [18] F. Dannel, H. Pfeffer, V. Romheld, Update on boron in higher plants—uptake, primary translocation and compartmentation, Plant Biol. 4 (2002) 193. [19] N. Bachelier, C. Chappey, D. Langevin, M. Metayer, J.F. Verchere, Facilitated transport of boric acid by 1,3-diols through supported liquid membranes, J. Membr. Sci. 119 (1996) 285. [20] F.T. Bingham, Boron, in: A.L. Page (Ed.), Methods of Soil Analysis—Part 2, American Society of Agronomy, Madison, WI, 1982, pp. 431–445. [21] D. Massiot, F. Fayon, M. Capron, I. King, S. Le Calve, B. Alonso, J.O. Durand, B. Bujoli, Z.H. Gan, G. Hoatson, Modelling one- and twodimensional solid-state NMR spectra, Magn. Reson. Chem. 40 (2002) 70. [22] S. Berger, S. Braun, 200 and More Basic NMR Experiments, Wiley–VCH, Weinheim, 2004. [23] A. Bax, R.H. Griffey, B.L. Hawkins, Correlation of proton and N-15 chemical-shifts by multiple quantum NMR, J. Magn. Reson. 55 (1983) 301. [24] C.S. Johnson, Diffusion ordered nuclear magnetic resonance spectroscopy: principles and applications, Prog. Nucl. Magn. Reson. Spectrosc. 34 (1999) 203. [25] A.E. Martell, R.M. Smith, Critical Stability Constants, Plenum, New York, 1974. [26] D. Prats, M.F. Chillon-Arias, M. Rodriguez-Pastor, Analysis of the influence of pH and pressure on the elimination of boron in reverse osmosis, Desalination 128 (2000) 269. [27] M. Makkee, A.P.G. Kieboom, H. Vanbekkum, Studies on borate esters. 3: Borate esters of d-mannitol, d-glucitol, d-fructose and d-glucose in water, Recl. Trav. Chim. Pay Bas 104 (1985) 230. [28] M. Van Duin, J.A. Peters, A.P.G. Kieboom, H. Van Bekkum, Studies on borate esters. II: Structure and stability of borate esters of polyhydroxycarboxylates and related polyols in aqueous alkaline media as studied by boron-11 NMR, Tetrahedron 41 (1985) 3411. [29] J.P. Lorand, J.O. Edwards, Polyol complexes and structure of the benzeneboronate ion, J. Org. Chem. 24 (1959) 769. [30] C. Dilek, H.O. Ozbelge, N. Bicak, L. Yilmaz, Removal of boron from aqueous solutions by continuous polymer-enhanced ultrafiltration with polyvinyl alcohol, Sep. Sci. Technol. 37 (2002) 1257. [31] M. Taniguchi, Y. Fusaoka, T. Nishikawa, M. Kurihara, Boron removal in RO seawater desalination, Desalination 167 (2004) 419.
Boron removal from water by complexation to polyol compounds Nitzan Geffen a , Raphael Semiat b , Moris S. Eisen c , Yael Balazs c , Ilan Katz a , Carlos G. Dosoretz a,∗ a
Civil & Environmental Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel b Chemical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel c Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel Received 28 May 2006; received in revised form 20 August 2006; accepted 6 September 2006 Available online 16 September 2006
Abstract Boron is an important micronutrient for plants, animals and humans, although the range between deficiency and excess is narrow. The use of desalinated water and treated wastewater for irrigation may result in excess boron. In aqueous environments (i.e. neutral pH) boron is mainly present as boric acid, which is mostly undissociated and therefore only partially rejected by desalination membranes. Boric acid/borate reacts with neutral polyolic compounds, generating anionic complexes. This work reports on the complexation of boron with mannitol integrated with membrane desalination. The separation of the complex was studied in a wide range of conditions by nanofiltration (NF), and partially by reverse osmosis (RO) membranes. NMR analysis and chemical equilibrium modeling system (Mineql+) were applied to provide a better understanding of experimentally observed rejection patterns. The addition of mannitol in excess formed mainly a 2,2-di-borate ester and some monoborate esters as function of the pH, improving boron rejection by NF up to 90% at pH 9. Mineql+ calculations indicate that reactants concentration has a strong influence on the ionized boron species and therefore on the rejection of boron. Sea water-RO membranes having a much higher basal rejection for boric acid, rejected almost 97% mannitol-complexated boron at pH of 9. © 2006 Elsevier B.V. All rights reserved. Keywords: Boron removal; Complexation; Desalination; Reverse osmosis; Nanofiltration
1. Introduction Boron is an important micronutrient for plants, animals and humans, although the range between deficiency and excess is narrow (e.g. 0.3–0.5 mg/l in citrus). A minimal boron concentration is required in the irrigation water for metabolic activities of the crops. However, excess boron is harmful to plant growth; deleterious effects include yellowish spots on the leaves and the fruit, accelerated decay, and ultimately plant expiration [1]. In aqueous environments, boron is mainly present as boric acid and partially as borate ions according to the dissociation reaction (Ka = 6 × 10−10 , pKa 9.1) [2] shown in the following equation: B(OH)3 + H2 O B(OH)4 − + H+
∗
Corresponding author. Tel.: +972 4 8294962; fax: +972 4 8228898. E-mail address: [email protected] (C.G. Dosoretz).
0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.09.019
(1)
The concentration of boron in seawater is between 4 and 5 mg/l [1]. While the average boron concentration in drinking water supplied in Israel is currently around 0.2 mg/l, it may increase with the supply of desalinated seawater [3]. The average boron concentration in wastewater effluents in Israel in 2004 was reported around 0.32 mg/l [4], and is attributed mostly to sources such as household appliances (detergents), urine and various industrial wastes [5]. The increasing reclaim of treated effluent for irrigation has become a common practice in Israel in the recent years as a result of water shortage. In addition, desalinated sea water production is underway and due to become part of the national water supply. These new practices pose a new challenge for boron removal. Boron removal by sea water desalination, using reverse osmosis, reduces its concentration to only 0.9–1.8 mg/l in the permeate product (after a single pass) [6,7], while conventional wastewater treatment procedures and chemicals commonly used in the water treatment industry remove little, if any [8].
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Boron can present reproductive hazards and has suspected teratogenetic properties [6]. The World Health Organization (WHO) has set a limit of 0.5 mg/l of No Observed adverse Effect Level (NOEL) for drinking water [9] and there are limit values between 0.3 and 0.5 mg/l in tender documents for medium and large membrane desalination plants [6]. There is no easy method for removing boron from water. So far, the use of ion exchange resins and composite reverse osmosis (RO) membrane systems, as well as their combination with or without pH increase, are the only used technologies considered effective for the removal of boron. The predominance of undissociated boric acid at neutral pH is of major impact in desalination processes applying RO because its rejection in all type of membrane modules (spiral as well as hollow fiber) is about 65% whilst that of the borate ion is 95% [1]. Since boric acid has no ionic charge it diffuses in a similar way to that of carbonic acid or water itself [10]. The possible increase of the pH in the second pass for shifting the equilibrium to the side of the borate and to obtain high rejection (99.0–99.5% at pH 11) [6,11], will cause immediate calcium carbonate scaling [12], not to mention the cost associated with the injection of large amounts of caustic soda required and associated corrosion. The stability of the membrane and the service life in real operating condition has not yet examined [11]. For selective ion exchange resins the problems are high regeneration costs [13], expensive resins and low capacity with scale up problems [1,14,15]. Extensive research is being carried out today aimed at the development of new technologies as well the reduction of costs in existing ones. Boric acid/borate reacts with chemical compounds containing multiple hydroxyls groups (polyols), such as mannitol (Fig. 1), generating anionic complexes at the neutral pH of water. With carbohydrates and most polyols possessing 1,2-diol systems, the borate ion forms anionic mono (1:1) and bis (1:2) diol-monoborate species [16]. The stability of the borate complex formed is strongly dependent on the type of diol, namely 1,2 or 1,3-diols. If the diol involves the –OH groups oriented in such a way that they accurately match the structural parameters required by tetrahedrally coordinated boron, a strong complex will be formed [2]. The borate esters are formed and dissociated spontaneously in a variety of pH dependent equilibria [2]. Due to the release of acidic protons during complexation there is a concomitant
decrease of pH which tends to reverse the reaction and thus, in order to maintain stable complexes there is a need to avoid pH decrease. The amount of acidification produced upon the addition of polyol is proportional to the extent of borate ester formation. Borate, for example, complexes mainly with two molecules of mannitol at a pK value of 4.1 (50 mM B) [17] according to the following equation:
(2) Boron complexation plays an important role in boron uptake and fixation in plants. Natural boron complexation with natural polyhydric antioxidants such as flavonols has been observed in turmeric (curcum) and other flavor and fragrance plants. Boron complexation with malic acid has also been reported in wines [2,18]. Complexation of boron is also the basis of the selective ion exchange [13] and of supported liquid membranes [19] technologies available for boron removal from water. The study presented in this paper aimed at the complexation of boric acid/borate with mannitol integrated with membrane desalination systems. Mannitol was chosen as the model polyol due to its high equilibrium constant and its compatibility with drinking water. The complexation reaction occurs at neutral pH and produces a negatively charged di-borate ester of 375 Da size. The separation of the complex was examined primary by employing a nanofiltration (NF) membrane as a model of pressure driven-desalination technology and finally with a RO membrane. Nanofiltration was chosen in order to enable the examination of rejection patterns as influenced by the separation mechanism, e.g. molecular weight and the electric charge of the molecule, in wide range of working conditions. 2. Experimental All experiments were performed with pure solutions of boron and/or mannitol in distilled water. The pH of the solutions was adjusted using either 0.1 M NaOH stock solution or 0.2 M of dif-
Fig. 1. Schematic drawings of mannitol and negatively charged borate–mannitol complexes [27].
N. Geffen et al. / Journal of Membrane Science 286 (2006) 45–51
ferent buffers (phosphate pH 7.0, glycine pH 9.2, Tris pH 8.7 and imidazole pH 7.0). Glycine was in addition tested at 6.9 and 8.0 in order to check the effect of the nature of the buffer on the complexation, versus phosphate/imidazole and Tris, respectively. All chemicals used were of reagent grade quality. In order to assess the influence of different solution properties on the rejection of boron, mannitol or their complex, membrane separation experiments were performed in a bench scale membrane filtration module unit (SEPA CF, Osmonics) operated at 15 bar pressure at room temperature (25–30 ◦ C). The unit was equipped with either a NF membrane sheet made of polypiperazine amide (NF200, FilmtecTM ) or RO membrane sheet made of polyamide (BW30 or SW30, FilmtecTM , as indicated) of 190 cm2 net filtration area. The solutions were recycled through the membrane from a 2 l feed reservoir and 30 ml permeate samples were collected for analysis as indicated. Solutions of 7 and 32 mg/l average boron concentration were used in the NF experiments and of 7 mg/l in the RO experiments. An average boron:mannitol molar ratio of 1:4.2 and 1:6.5 was applied in NF and of 1:5, 1:10 and 1:15 in RO. Since the complexation reaction is almost instantaneous, the feedwater was first let to react for 10 min and then the permeate was sampled. The influence of the feed pH on the complexation and further on membrane rejection was examined in a pH range of 6.0–9.6. Baseline rejection of the membranes was determined by measurement of boron rejection of a neat boric acid solution at pH 5.7. All experiments were repeated 2–3 times. Boron rejection was calculated from the measured total boron concentration in the permeate (Cp ) and in the feed solution (Cf ) according to the following equation: Rejection = 1 −
Cp Cf
(3)
2.1. Analytical procedures Boron was assayed either by ICP-emission spectrometry (Optima 3000 DV, Perkin-Elmer) at 249.77 nm or spectrophotometrically upon complexation with Azomethine-H [20]. In the presence of organic buffers mannitol concentration was determined by HPLC (Agilent 1100 series, Hewlett Packard) equipped with a Ion Moderated Partition column (Rezex ROAOrg. Acid, Phenomenex) and monitoring with an Evaporative Light Scattering Detector (PLELS 2100, Polymer laboratories) with distilled water as the eluent at a flow rate of 0.5 ml/min. In all other cases mannitol concentration was determined by measuring the total organic carbon (TOC) concentration in a Shimadzu TOC-5000A analyzer. Two to three replicate analyses of each sample were performed. Nuclear magnetic resonance spectroscopy (1 H NMR, 11 B NMR and 13 C NMR) was applied for qualitative analysis and semi-quantitative determination of boron and mannitol speciation. NMR analysis was performed on a Bruker Avance 500 spectrometer, operating at 160.46, 125.76 and 500.13 MHz for boron 11, carbon 13 and hydrogen 1, respectively, at 25 ◦ C (±0.1 ◦ C) and running Xwin-NMR version 3.5.6. The spectrometer was equipped with a 5 mm broadband (bbo) probe. For the 11 B NMR a total of 128 transients were signal averaged
47
using a repetition delay greater than 5 × T1 to allow for full relaxation (200 ms except for the solution with borate which was 1.1 s). A single pulse of 24 kHz (γ/2π) B1 field was used with 3k real points and a spectral width of 22,522 Hz. The FID was processed to 16k real points without applying window functions. To remove the broad background signal from the probe itself, difference spectroscopy was employed. A reference spectrum of the empty probe was acquired with 512 shots and, after scaling, subtracted from the experimental spectrum. Standard parameters were used for proton and carbon NMR. Evaluation of peak areas was done both using the integration routine supplied with Xwin-NMR version 3.5.6 and also using the free software program DMFIT [21] (http://crmhteurope.cnrs-orleans.fr/dmfit/help/dmfit.htm December, 2004). Analyses using either the Xwin-NMR or DMfit programs gave very similar results. NMR samples were prepared in 5 mm glass NMR tubes. 0.1 M (1081 mg/l) boron solutions were prepared by dilution with distilled and deionized water and 10% D2 O with different boron:mannitol molar ratios (without mannitol, 1:2, 1:5 and 2:1). The pH of the solutions was adjusted using either 0.1 M NaOH stock solution or 0.5 M of different buffers (phosphate, glycine and imidazole). The molecular structures of the complex were investigated using 2D 1 H COSY (correlated spectroscopy) [22] and 1 H{13 C}HMQC (Heteronuclear Multiple Quantum Coherence) [23] NMR with standard parameters. Diffusion constants using a BPLED sequence [24] (longitudinal eddy current delay with bipolar gradient pulse pair and two spoil gradients). The gradient pulse length (δ) was 4.4 ms, and the delay for diffusion (Δ) was 50 ms. The gradient strength was changed linearly over 16 steps from 5% to 95% of the maximum value (GREAT-10 gradient amplifier). Each measurement was run with eight scans and 32k real time domain points. For the processing, an exponential window function and single zero filling were applied. During the diffusion measurements, the fluctuation of the temperature was less than 0.1 K. Prior to the NMR scans, all samples were equilibrated for atleast 30 min. The chemical equilibrium modeling software Mineql+ (version 4.5) was used for theoretical computation of equilibrium concentrations of boron species. The pK used for the mono borate ester formation and for the di-borate ester were 6.21 and 4.13, respectively [25], normalized by Davies equation to zero ionic strength and the pH was determined by user from the measured pH in the experiments. Ionized boron species is expressed as the total concentration of free and mannitol-complexated ionized boron species relative to the total boron concentration. 3. Results and discussion The effect of pH on the boron rejection across the NF membrane with and without mannitol was studied at two different reactants concentrations: 7 and 32 mg/l boron and boron:mannitol molar ratio of 1:6.5 and 1:4.2, respectively, with and without buffer (Fig. 2). A boron removal efficiency ranging from approximately 26% to 50% was achieved by adjustment of the pH from 6.0 to 9.6, respectively. These results also indicate that the baseline value for boric acid rejection of this NF
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N. Geffen et al. / Journal of Membrane Science 286 (2006) 45–51
Fig. 2. Relative rejection of free and mannitol-complexated boron species as a function of equilibrium pH: 7 mg/l ( ) and 32 mg/l ( ) boron in the absence of mannitol (controls); 7 mg/l boron with mannitol at a boron:mannitol molar ratio of 1:6.5 ( ); 32 mg/l boron with mannitol at a boron:mannitol molar ratio of 1:4.2 in the presence of 0.2 M buffer [() phosphate pH 7.0; (×) glycine pH 6.9; () imidazole pH 7.0; (*) glycine pH 8.0; (♦) Tris pH 8.7; () glycine pH 9.2]. Rejection was calculated according to Eq. (3).
membrane is approximately 26%. The influence of pH on boron rejection by RO membranes is well known and is attributed to the ionization of the boron [6,26]. Boron concentration has no influence on the rejection as can be seen in the experiments run at 7 and 32 mg/l as well as in the Mineql+ simulations (see below Fig. 5), in line with the 1/1 boron/borate stoichiometry (pH = pKa + log[B(OH)4 − ]/[B(OH)3 ]) shown in Eq. (1). The addition of mannitol to a 7 mg/l boron solution at a molar ratio of 1:6.5 boron:mannitol increased the rejection in the entire pH range tested, up to 72% at a pH of 8.8, indicating the formation of an ionized boron:mannitol complex. Although such an addition was expected to enable the formation of an ionized complex at neutral pH (due to the low pK; see NMR analysis in Fig. 3 below), the rejection was not constant at these pH values and was almost identical to the rejection of mannitol alone (75% in average). This indicates that there was not complete complexation at these pH values. The addition of mannitol and buffer to a 32 mg/l boron solution at a molar ratio of 1:4.2 boron:mannitol
improved boron rejection substantially to approximately 60% at pH 7.5 and 94% at pH 9.0 as can be seen in Fig. 2. 11 B NMR analysis of complexation of boron (0.1 M = 1081 mg/l) with mannitol as a function of the pH shows that at pH ∼7.7 for a boron:mannitol molar ratio of 1:5 and at pH 8.8 for a boron:mannitol molar ratio of 1:2, complexation goes to completion (Fig. 3). Fig. 3 shows a major product of complexation with a chemical shift of −9.6 ppm which corresponds to the di-borate ester (BP2 ) and three minor peaks with lower chemical shifts (δ = −13.5, −13.7 and −14.7 ppm). These three peaks can be assigned to the monoborate ester (boron bounds to three different pair of hydroxyls of mannitol), in line with previous reports [17,27,28]. The speciation of the products of complexation was confirmed by comparative 11 B NMR analysis at increasing boron:mannitol molar ratio, from excess mannitol (1:5 and 1:2) to excess boron (2:1) at pH > pK of the complexation (Fig. 4). From the spectra presented in Fig. 4, the progress of the reaction as a function of the relative amount of mannitol can be clearly seen, which is in agreement with the relative area of the peak corresponding to the di-borate ester (BP2 , δ = −9.6 ppm). The di-ester is the major product at 1:5 ratio (spectrum b) and a minor product at 2:1 ratio in which mannitol is the limiting factor for the complexation (spectrum d). Note that at pH 11.9 free borate is formed, most probably due to the deficiency of mannitol (spectrum d). The other three peaks clearly visible in spectra c and d (δ = −13.5, −13.7 and −14.7), can be two different monoesters products—one molecule of boron and one of mannitol (BP) or two molecules of boron with one molecule of mannitol (B2 P). Again, boron can bind to three different pair of hydroxyls in the molecule of mannitol in each of the two monoesteres. The identity of the peaks in spectrum c seems to be BP in correspondence with the higher BP2 /BP at the higher excess of mannitol (boron:mannitol molar ratio of 1:5 versus 1:2) in Fig. 3 and in line with previous reports [28,27,17]. B2 P formation as a product of complexation of boron and mannitol was reported by Lorand and Edwards [29] and Van Duin et al. [28]. Product formation was confirmed by 13 C NMR analysis of the complexation at a molar ratio boron:mannitol of 1:2 as a
Fig. 3. 11 B NMR spectra of free and mannitol-complexated boron species as function of the equilibrium pH. Left panel, boron:mannitol ratio of 1:2; right panel, boron:mannitol ratio of 1:5. Boron concentration was 0.1 M. BP, boron:mannitol monoester; BP2 , boron:mannitol di-ester. Bottom spectra, represent undissociated boric acid (left panel) and fully ionized borate (right panel) controls in the absence of mannitol.
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49
Table 1 Diffusion constants (m2 /s) of different components of boron:mannitol solutions with different buffers Componentsa
Mannitol Complex Glycine
Buffer type Glycine (pH 9.5)
Phosphate (pH 6.8)
4.74 × 10−10 3.43 × 10−10 7.47 × 10−10
4.05 × 10−10 3.55 × 10−10 –
a Conditions were as follows: 0.1 M boron solution of 1:5 molar ratio of boron:mannitol, with addition of 0.5 M buffer. Diffusion constants were calculated from 1 H NMR spectra.
Fig. 4. 11 B NMR spectra of spectra of free and mannitol-complexated boron species as function of boron:mannitol molar ratio at different equilibrium pH values: (a) neat undissociated boric acid control (pH 5.7); (b) 1:5 boron:mannitol (pH 7.7); (c) 1:2 boron:mannitol (pH 8.8); (d) 2:1 boron:mannitol (pH 11.9); (e) neat fully ionized borate control (pH 11.9).
function of the pH (4.0, 5.0, 5.7 and 8.8) and as a function of the relative reactants concentration and pH (mannitol alone at pH 7, 1:5 ratio at pH 2.9, 1:5 ratio at pH 7.7, 2:1 ratio at pH 11.9 and 1:2 ratio at pH 8.8). Calculations made from 13 C-spectra corroborated the formation of BP2 (δ = 74.8 and 76.8) as the major complexation product at a ratio of boron:mannitol of 1:2 and 1:5. Indeed, the ratio of complexed mannitol to free mannitol of 1:1.76 (1:1.5 theoretical, correspond to the di-ester), a ratio of 1.8 molecules of mannitol per borate ion in the complex and 1 H COSY and 1 H{13 C}HMQC analyses further confirmed that the complexation-main product is BP2 and it occurred via the 2,2 carbons, all results being in agreement with the literature [17,27]. The influence of the buffers on the rejection efficiency was examined also by 1 H, 13 C and 11 B NMR, and almost complete complexation was observed in all cases upon addition of mannitol to buffered boron solutions at a 1:5 molar ratio, even at pH 5.0 (immidazole buffer), with BP2 being the almost solely product (data not shown). In order to discard the possible aggregation of mannitol free or in the complex as a function of the type of buffer and pH value (see Fig. 2 above), the diffusion constants, which indicate the hydrodynamic size of a molecule, of free and complexed mannitol were calculated from 1 H NMR analysis under different reaction conditions (Table 1). An almost similar diffusion constant was found for free mannitol in both buffers, averaging approximately 4.5 × 10−10 m2 /s which is in line with the diffusion constant of 1.5 M mannitol solution in water (4.9 × 10−10 m2 /s). The calculated diffusion constants of complexated mannitol in both buffers were 15–25% smaller than those of free mannitol, which is in agreement with the expected higher rejection of complex than mannitol alone and show no effect of the buffer
type. Thus, it seems there is no mannitol aggregation during complexation. Theoretical calculations (Mineql+ program) of ionized boron species products (complexated boron and borate) as a function of pH were done for a 7 and 32 mg/l boron at a molar ratio of 1:4.2 and 1:6.5 (Fig. 5). These calculations revealed that the reactants concentration have a strong influence on the ionized boron species and therefore on the expected rejection of boron during membrane separation. In contrast, the reactants ratio displayed a slighter effect on the level of ionized boron species. Addition of buffers has almost no effect on ionized boron species (data not shown). Comparing Figs. 2 and 5, it can be concluded that the major factor governing rejection of the complex at each individual reactants concentration in a NF membrane, as is for free borate, is the degree of ionization. Indeed, for 32 mg/l boron solution an almost complete ionization of boron species is expected at a pH value of approximately 9.5 and for 7 mg/l at pH 10 in practical terms. It must be noted that the experiments were performed in distilled water, whereas in practice the complexation process is envisioned to be applied during the second pass of multistage desalinations plants with recycling of concentrate in order to recycle excess polyol. This
Fig. 5. Theoretical ionization of free and mannitol-complexated boron species as function of the equilibrium pH and reactants concentrations. ( ) 7 mg/l boron in the absence of mannitol (control); ( ) 32 mg/l boron in the absence of mannitol (control); ( ) 7 mg/l boron with mannitol at a boron:mannitol molar ratio of 1:6.5; ( ) 32 mg/l boron with mannitol at a boron:mannitol molar ratio of 1:6.5; ( ) 7 mg/l boron with mannitol at a boron:mannitol molar ratio of 1:4.2; ( ) 32 mg/l boron with mannitol at a boron:mannitol molar ratio of 1:4.2. Theoretical ionized boron species was calculated by means of Mineq1+ software.
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tive BWRO membrane seems to follow the level of ionized boron species at the studied pH (∼73% at pH 9.0). For the SWRO, the obtained rejection was higher than the calculated ionized boron species at the respective pH (∼90% at pH 7.5 and ∼97% at pH 9.0), clearly indicating that other factors besides ionization are determinant, such as membrane selectivity. Furthermore, the dependence of the rejection on the boron:mannitol molar ratio above the stoichiometric ratio becomes almost negligible for the SWRO membrane as can be seen from the overlapping data. In studies of boron removal with a BWRO membrane 62% rejection was obtained at pH 9.5 [26] and 93–99% at pH 10 for both SWRO and BWRO which were tested in the article [6]. Boron removal by complexation with PVA followed by separation of ultrafiltration was reported in the order of 30% [30] and by ion exchange was almost complete (<0.1 mg/l) [12]. 4. Conclusion
Fig. 6. Comparison of experimental membrane rejection and theoretical ionized boron species as a function of pH values. (a) 7 mg/l boron with mannitol: ( ) fit of theoretical ionized boron species data and ( ) fit of experimental NF rejection data ( ) of boron–mannitol complexation reactions, both at molar ratio of 1:6.5; SWRO experimental rejection at molar ratio of 1:5 ( ), 1:10 ( ) and 1:15 ( ), all overlapping; BWRO experimental rejection of 7 mg/l boron with mannitol at molar ratio of 1:5 ( ) and 1:10 ( ), both over) fit of theoretical ionized lapping. (b) 32 mg/l boron with mannitol: ( boron species data and (—) fit of experimental NF rejection data (, ×, , *, ♦, ) of boron–mannitol complexation reactions in the presence of 0.2 M buffer as detailed in Fig. 2, all at molar ratio of 1:4.2. Theoretical ionized boron species was calculated by means of Mineq1+ software. Baseline rejection represents the experimental rejection of a solution of undissociated boric acid at pH 5.7 for each type.
would maintain high bulk concentrations of reactants which, as shown above, favors the complexation reaction and therefore the rejection. Finally, rejection tests were performed with RO membranes of intermediate and high selectivity, Brackish Water RO (BWRO) and Sea Water RO (SWRO) and compared with the rejection of the NF membrane of lower selectivity, and with theoretical ionization of boron species as a function of the pH (Fig. 6). There is a baseline rejection value of undissociated boric acid for each membrane, depending on its density and selectivity, indicated by the dotted lines in Fig. 6, as follows: 26% for NF, 60% for BWRO and 78% for SWRO. For the NF membrane the degree of rejection above this baseline seems to depend directly on the level of ionized boron species at both reactants concentrations (Fig. 6a and b). In a similar fashion, the rejection of the less selec-
Boron rejection by desalination membranes was enhanced by addition of mannitol according to the complexation equilibria. Rejection of the ionized complex by NF membranes, as for free borate, increased with pH increase, according to the degree of ionization, however, only partial rejection was observed in the pH range studied (7–9.2), most probably due to the incomplete ionization. At excess mannitol concentration the complexation reaction yields particularly di-ester and slightly amount of monoester without any aggregation, as demonstrated by NMR analysis. Simulation analysis and NMR experiments strongly indicate that the reactants concentrations and their ratio have a strong influence on the ionized boron species, and therefore on the rejection of boron by desalination membranes with low baseline rejection of undissociated boric acid, such as NF. The use of dense RO membranes improved the baseline rejection of undissociated boric acid and therefore a higher boron rejection could be obtained at a lower pH (97% at pH 9.0), regardless of the boron:polyol ratio above the stoichiometric ratio. As today new membranes with higher boron reduction are in development [31] and hence the required pH for this polyol complexation is expected to be lower. It must be noted that results presented here were obtained with pure boron solution in distilled water and desalination experiments were performed at room temperature (∼25 ◦ C) and 15 bar pressure, whereas the rejection at lower temperature and higher ionic strength is expected to be higher, in line with literature reports [11,12]. Acknowledgements This research was supported by grant of the Grand Water Research Institute, Zakin award for excellent students, and fellowship from Sherman foundation. We are grateful to Dr. Ori Lahav for his kind help with the Mineql+ calculations. We thank Hassan Koseoglu for his technical assistance on RO experiments. M.S.E. thanks the NATAF Program Administered by the Ministry of Industry and Commerce.
N. Geffen et al. / Journal of Membrane Science 286 (2006) 45–51
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