Nanofiltration of brackish groundwater by using a polypiperazine membrane

Desalination 286 (2012) 277–284

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Nanofiltration of brackish groundwater by using a polypiperazine membrane Charis M. Galanakis a, 1, Georgios Fountoulis a, Vassilis Gekas b,⁎ a b

Department of Environmental Engineering, Technical University of Crete, Politechnioupolis, GR-73100 Chania, Greece Department of Agricultural Sciences, Biotechnology and Food Science, Cyprus University of Technology, P.O. Box 50329, CY 3603 Lemesos, Cyprus

a r t i c l e

i n f o

Article history: Received 4 August 2011 Received in revised form 27 September 2011 Accepted 12 November 2011 Available online 12 December 2011 Keywords: Desalination Well Drinking water Semi-aromatic polyamide Fouling resistance Hardness removal

a b s t r a c t The purpose of the current study is to investigate the nanofiltration of brackish groundwater by using a polypiperazine membrane. The latter is a polymeric three-layer thin film with an active layer of semi-aromatic/ aliphatic polyamide. Particularly, samples of different hardness and salinity values (up to 762 mg CaCO3 and 1803 mg NaCl/L, respectively) were collected and treated under low transmembrane pressures (6–10 bar), in a cross-flow nanofiltration module. Desalination monitoring was performed by determining performance parameters, total hardness and salinity retention coefficients of brackish samples during experiments. According to the results, the tested membrane was able to provide high hardness retention coefficients (70–76%), satisfactory permeate fluxes (15–47 L·m− 2·h− 1) and high mineral fouling resistance (flux recovery values between 93 and 98%). A disadvantage of the process was the relatively low removal of salinity (44–66% for brackish groundwaters) that restricts the application in samples possessing salinity not much higher than ~ 1100 mg NaCl/L. Finally, the retention of divalent and monovalent ions was governed by size effect and ion valance, probably due to the high ionic conditions of brackish samples and the polyampholytic nature of the membrane. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Membrane operations such as reverse osmosis (RO) and nanofiltration (NF) are now the dominant technologies to treat several surface, well, brackish and sea water in order to produce drinking water of several characteristics [1,2]. RO is a pressure-driven process whereby a semi-permeable membrane rejects dissolved constituents (i.e. salts) present in feed water [3]. The success of RO technology is originated to the easy engineering of the process. For example, it is modular, flexible, automatically operated and easy going during control or scale-up [1]. Nevertheless, RO requires a high operating pressure and energy cost, while it is sensitive to mineral fouling. The latter is caused by pore clogging, adsorption or heterogenous crystallization of sparingly soluble minerals on the membrane surface [2,4,5]. NF offers often a valuable alternative membrane process to RO, as it provides more open pores, higher flux, low operating pressure as well as relatively low investment, operation and maintenance costs. With regard to the rejection of dissolved salts, NF is a very

⁎ Corresponding author at: Department of Agricultural Sciences, Biotechnology and Food Science, Cyprus University of Technology, P.O. Box 50329, Lemesos, CY 3603, Cyprus; Tel.: +357 25002301. E-mail addresses: [email protected], [email protected] (C.M. Galanakis), [email protected] (V. Gekas). 1 Permanent address: Chemical Analytical Laboratories “Galanakis”, Skalidi 34, GR-73131, Chania, Greece. Tel.: + 30 28210 93056; fax + 30 28210 88981. 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.11.035

effective process for hardness removal, as it exhibits higher selective retention of divalent compared to monovalent ions [4,6–8]. In ionic solutions, the interactions between different solutes and these between solutes and membrane are dependent on the concentration, composition and pH value of the feed solution [9]. The separation characteristics of NF are the consequences of a complication between sieving effect, electrostatic and steric interactions (associated with charge shielding), Donnan exclusion and ion hydration [10]. In brackish water conditions (i.e. salinity ~ 4000 mg NaCl/L, hardness ~570 mg CaCO3/L and conductivity ~6700 μS/cm), the shielding of membrane charge is higher than in fresh water, which lies to a weaker Donnan exclusion and a greater influence of size effect [11]. As far as NF of brackish groundwater samples is concerned, researchers have assayed different membrane materials in order to find the balance between divalent and monovalent ion removal, hardness and salinity reduction as well as avoid fouling and operate at lower transmembrane pressures (TMP:s). For example, Haddad et al. applied cellulose acetate based membranes for the desalination of brackish water in Tunisia [6], while Ferjani et al. prepared an additional hydrophobic top layer (made of polymethyl-hydrosiloxane) in order to increase salt rejection [12]. Walha et al. reported utilization of a polyamide-polyethersulphone membrane (0.2 kDa) for the NF of two brackish groundwater samples in Tunisia [7]. According to their experiments, NF was more effective in the rejection of calcium and magnesium cations that led to the reduction of water hardness. On the other hand, salinity was not significantly reduced presumably due to the existing Donnan effect and the extremely low chlorine ion

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permeation, as anions were repulsed from the negatively charged membrane. Monovalent ion removal has also been enhanced by modifying polyamide with polyelectrolyte deposition [8]. Besides, Reddy et al. modified polyamide thin film composite membranes by in situ redox polymerization of acrylate monomers in order to apply NF of brackish groundwater at lower transmembrane pressure (TMP) [13]. Other materials such as the polyether- and polyamide-urea barrier have been suggested for the reduction of fouling phenomena in RO desalination [3]. In a preliminary study of our scientific group, polypiperazine membrane has also shown to reduce mineral fouling phenomena during NF process and resulted in a more gentle removal of ions compared to RO applications [14]. This membrane material is a polymeric three-layer thin film membrane with an active layer of semiaromatic/aliphatic polyamide. The latter is formed by interfacial polymerization and possesses nano-scale heterogeneity, as its outer surface is negatively charged by the un-polymerized carboxylic residual groups and the side next to the support layer is positively charged due to amine groups [15,16].The main difference between aromatic and semi-aromatic polyamide is the degree of sequential aromatic polyamide rings. In particular, completely aromatic networks are more rigid and regularly packed than the semi-aromatic structure. Polypiperazine membranes decline Donnan exclusion effect, retain usually divalent ions at low pressures and provide relatively high permeate fluxes [17]. Despite these advantages, the studies reporting NF of brackish groundwater using polypiperazine materials are limited. Thus, the objective of the current work was to investigate further the NF of brackish groundwater by using a commercial polypiperazine membrane. For this purpose, samples of different hardness and salinity were collected and treated under low TMP:s (6–10 bar), in a cross-flow NF module. Desalination monitoring was performed by determining performance parameters, total hardness and salinity retention coefficients of brackish groundwater samples during experiments. 2. Experimental 2.1. Materials Reagents were of analytical grade. Tap water sample (A) was collected from the local water supply system (Chania, Crete, Greece). Brackish groundwater samples (B, C and D) were collected from three different wells in Milatos (Neapolis, Crete, Greece) by side of a hotel and near the coast. The samples were vacuum filtered (10 mm) before their application in desalination nanofiltration experiments. The commercial polypiperazine membrane NF99 (Alfa Laval, Nakskov, Denmark) was utilized for all the experiments. According to the manufacturer, the specifications of the membrane include a pH range between 2 and 10, a temperature range between 0 and 50 °C, a pressure range between 1 and 55 bar as well as an absolute retention (≥99%) of 2000 mg MgSO4/L solution (by applying 9 bar at 25 °C). 2.2. Experimental set-up and operation control Desalination experiments were performed in a cross-flow membrane filtration module (DSS Labstak M20, Alfa Laval, Nakskov, Denmark), where membranes, filter sheets, support and spacer plates were stacked and compressed in a vertical frame (Fig. 1). Plates were compressed with a hydraulic hand pump at 320 bar (model P392, Enerpac, USA). The membrane specific area was equal to 3.6 · 10 − 2 m 2 (2 sheets × 1.8 · 10 − 2 m 2), while the feed volume was equal to 3 L. Feed circulation (38 mL·s − 1) was carried out in parallel through the compressed membrane plate with an auxiliary pump (hydra cell industrial model G13XDSGHHEMA, Wanner Engineering Inc, USA) equipped with a motor (Varmeca-10, model

013XDSGHHEMA, Leroy Somer, USA). The feed inlet and the outlet temperature were monitored by sensors (Pt100 class A, Pentronic AB, Sweden) and kept constant at room temperature (25 ± 0.5 °C) by placing the feed tank into a water or ice bath. The TMP of the feed liquid was adjusted to the appropriate level with a retentate adjusting valve. The inlet and the outlet TMP:s (6–10 bar) were measured with a module self-contained manometer. The permeate flux was determined gravimetrically as the change of permeate weight versus time by using a laboratory scale balance (XT120A, Precisa Instruments Ltd, Switzerland) and a digital timer (Oregon Scientific, US). Permeate flux was expressed in L·h − 1·m − 2. 2.3. Experimental procedure 2.3.1. Membrane pretreatment NF membrane was placed into Labstak M20 module, packed and pretreated with de-ionized water as feed sample (3 L) in order to minimize membrane compaction during desalination experiments. The membrane was pressurized at 6, 7, 8, 9 and 10 bar in two sequential rounds (10 min duration) using de-ionised water. Permeate was continuously discharged. Fresh de-ionized water was added continuously with the purpose of retaining a 3 L-feed liquid volume and washing out glycerine, which is used as membrane preservative [18]. 2.3.2. Desalination experiments Each water sample (3 L) was processed in membrane module and pressurized at 6, 7, 8, 9 and 10 bar in two sequential rounds (10 min duration). Permeate flux was determined during the process and the relative flux (RF) of the brackish water samples was calculated in percentage according to the following equation: RF ¼

Jv ⋅100ð% Þ; J w0

ð1Þ

where Jv is the permeate flux at steady state (after a 10 min-adaption time in pressure variation) and Jw0 is the de-ionized water flux. The permeability of de-ionized water (Lw) or sample (Lv) was quantified by the following equations [19]: Lw ¼

J w0 TMP

ð2Þ

Jv : TMP

ð3Þ

and Lv ¼

Samples of feed and permeate streams were kept in the freezer (−20 °C) until analysis. After the completion of each desalination experiment, brackish water samples were replaced with deionized water and processed again at the same TMP:s in two sequential rounds to clean the membrane. Permeate flux was again determined at each TMP with fresh de-ionised water and the flux recovery (FR) was calculated in percentage according to the following equation: FR ¼

J wf ⋅100ð% Þ; J w0

ð4Þ

where Jwf and Jw0 are the pure water flux after and prior the desalination experiment, respectively [20]. 2.4. Chemical analysis Conductivity and pH values of water samples were determined as described previously [20]. Chlorine ion (Cl −) concentration of water

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279

Fig. 1. Schematic diagram of the experimental cross-flow nanofiltration module (DSS LabstakM20).

samples was analyzed by titration with a standard silver nitrate solution in the presence of chromic ions (indicator) following the protocol described previously [21]. Hardness, calcium and magnesium determinations were conducted by using the EDTA—titrimetric Method (314B, 311 C and 318 C respectively). Sodium and potassium contents were analyzed by using the Flame Photometric Method (325B and 322B, respectively). Carbonate and biocarbonate ions were determined and calculated according to the HCl — titrimetric Method

(403) [22]. Ammonium (NH4+), Sulfate (SO42 −), nitrite (NO2−) and nitrate (NO3−) ion analysis were performed with commercially available kits and corresponding methods developed by HACH Company (Method 8155, 8051, 8507 and 8171, respectively) [23]. Salinity was estimated by the sum (calcium, magnesium, sodium and potassium) of cation equivalents after their modification to mg NaCl/L. The above determinations were conducted for the characterization of the tap water and brackish groundwater samples (Table 1).

Table 1 Characteristics of the assayed tap water and brackish groundwater samples. Values represent mean ± standard deviation (n = 3). Parameter

Unit

Water sample Moderate harda

Almost harda

Harda

Extremely harda

A

B

C

D

pH Conductivity Hardness Salinity Hardness/salinity

— μS/cm mg CaCO3/L mg NaCl/L mg CaCO3/mg NaCl

7.80 ± 0.03 333 ± 17 161 ± 5 230 ± 12 0.70 ± 0.02

7.84 ± 0.02 633 ± 28 286 ± 13 482 ± 35 0.59 ± 0.02

7.90 ± 0.02 1281 ± 39 531 ± 8 1085 ± 50 0.49 ± 0.02

7.93 ± 0.03 2131 ± 53 762 ± 30 1803 ± 96 0.42 ± 0.01

Cations Ca2 + Mg2 + Na+ K+ NH4+

mg/L mg/L mg/L mg/L mg/L

43 ± 6 13 ± 3 16 ± 3 1±1 n.d.b

85 ± 9 18 ± 2 57 ± 8 2±1 n.d.b

170 ± 7 26 ± 5 181 ± 21 4±1 n.d.b

247 ± 25 35 ± 9 353 ± 25 11 ± 1 n.d.b

Anions Cl− HCO3− CO32 − SO42 − NO3− NO2−

mg/L mg/L mg/L mg/L mg/L mg/L

23 ± 3 156 ± 13 n.d.b 30 ± 4 6±1 n.d.b

82 ± 9 244 ± 21 n.d.b 90 ± 5 1±1 n.d.b

280 ± 15 439 ± 20 n.d.b 167 ± 8 1±1 n.d.b

482 ± 16 716 ± 28 n.d.b 263 ± 19 1±1 n.d.b

a Classification according to hardness values in Klut climax: “extremely soft” for 0–70, “soft” for 70–140, “moderate hard” for 140–220, “almost hard” for 220–320, “hard” for 320–540 and “extremely hard” for > 540 mg CaCO3/L. b “n.d.” for “not detected”.

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2.5. Determination of retention coefficients and permeate characteristics Permeate samples (10 mL) were collected during desalination experiments at the highest TMP (10 bar). Salinity retention coefficients (Rsalinity%) of water samples were calculated by the following equation: Rsalinity% ¼

Afeed −Aperm ⋅100; Afeed

ð5Þ

where Afeed and Aperm were the conductivity values of feed and permeate stream, respectively. Permeate salinity was calculated by the following equation:   Rsalinity% ; permeate salinity ¼ f eed salinity ⋅ 1− 100

ð6Þ

where feed salinity was determined as above (Section 2.4). Hardness retention coefficient (Rhardness%) of water samples was calculated by the following equation: Rhardness% ¼ 100 ⋅

H feed −Hperm ; H feed

ð7Þ

where Hfeed and Hperm were the hardness values of feed and permeate stream, respectively, as determined by the “Total Hardness” titration kit of Merck Aquamerck®. Permeate hardness was calculated by the following equation:   R permeate hardness ¼ f eed hardness ⋅ 1− hardness% ; 100

ð8Þ

where feed hardness was determined as above (Section 2.4). 2.6. Statistical analysis All desalination experiments were performed in triplicates and the results are presented as means ± standard deviations. Data were statistically analyzed using students t-test (pair wise comparisons, Office Excel 2007). Significant differences between samples were observed when the acceptable level of probability was 5% (P ≤ 0.05) for all the comparisons. 3. Results The chemical characterization of the tested water samples is presented in Table 1. All samples possessed a weakly alkalic nature (pH ~ 7.8–7.9). The latter pH-values were not adjusted in order to produce the less treated potable waters as possible in the permeate

stream. Conductivity, hardness and salinity values as well as cations and anions contents were increased sequentially for the three brackish groundwater samples revealing three different types of water according to Klut climax [24]: “almost hard” (B), “hard” (C) and “extremely hard” (D). On the other hand, hardness per salinity ratio decreased as the hardness of samples was increased potentially through the enrichment of the assayed samples with additional seawater. The two brackish water samples C and D possessed hardness and chloride values above World Health Organization (WHO) Guidelines (500 mg CaCO3/L and 250 mg Cl −/L, respectively) [25,26], while the hardest sample (D) possessed sodium and sulfate contents above the corresponding threshold values (200 and 250 mg/L, respectively) [27]. Besides, salinity values of the two hardest water samples were more than 5-fold higher compared to the “moderate hard” water (A). Ammonium cations were not detected at all, indicating the absence of amino acids and proteins from sink effluents in the groundwater samples. Table 2 shows the performance parameters (permeability, RF and FR values) of the assayed samples. Water permeability (Lw0) increased with increasing TMP, although the values were not always significantly different. The same tendency was observed for the samples A, B and C. Nevertheless, the permeability of the hardest brackish water (D) remained constant (1.1–1.4 L·h − 1·m − 2) for all the assayed TMP:s, while values were 2.5- to 5-fold lower compared to these obtained for the other samples. The RF values of the samples A, B and C were very high and remained constant by increasing TMP, while they decreased with increasing hardness (97–98, 92–94 and 82–88%, respectively). On the other hand, the “extremely hard” sample (D) provided much lower RF values, which were decreased with increasing TMP (from 36 to 24%), indicating the presence of concentration polarization phenomena. Finally, the FR values were almost absolute (93–98%) for all the assayed samples, suggesting the lack of fouling problems. The latter observation is more obvious in Figs. 2 and 3, where the feed fluxes of the hardest groundwater samples (C and D, respectively) were plotted as a function of TMP. In both cases, pure water flux prior and after desalination experiment almost coincided for every tested TMP. Fig. 4 illustrates the hardness retention coefficients of the assayed samples and corresponding permeates' harnesses as a function of the initial feed hardness. Retention coefficients were rather high and decreased from 79 to 76, 74 and 70% with increasing feed hardness. Due to the rather low deviation of retention per hardness scalable increase (i.e. ~ 3% per ~200 mg CaCO3/L), the permeate hardness was not affected importantly. Thereby, an increase in feed hardness from 161 to 762 mg CaCO3/L did not result in sharp increase of permeate hardness (i.e. from 34 to 236 mg CaCO3/L, respectively). Besides, even the permeate of the hardest brackish water sample possessed two-fold

Table 2 Performance parameters obtained from the desalination experiments of brackish groundwater samples. Values represent mean ± standard deviation (n = 3). TMPa

De-ionised water

Water sample

b Lw0

Lvc

RFd

FRe

Lvc

RFd

FRe

Lvc

RFd

FRe

Lvc

RFd

FRe

bar

L·h− 1·m− 2· bar− 1

L·h− 1·m− 2· bar− 1

%

%

L·h− 1·m− 2· bar− 1

%

%

L·h− 1·m− 2· bar− 1

%

%

L·h− 1·m− 2· bar− 1

%

%

6 7 8 9 10

3.1 ± 0.3f 3.9 ± 0.9f, g 4.5 ± 0.8g, h 5.1 ± 0.9g, h 5.7 ± 0.5h

3.0 ± 0.2f 3.8 ± 0.7f, g 4.4 ± 0.7g, h 5.0 ± 0.8g, h 5.5 ± 0.5h

98 ± 3f 97 ± 3f 98 ± 2f 98 ± 2f 97 ± 1f

96 ± 3f 94 ± 3f 96 ± 2f 96 ± 5f 98 ± 1f

2.9 ± 0.4f 3.7 ± 0.9f, g 4.2 ± 0.7g, h 4.8 ± 0.9g, h 5.3 ± 0.6h

94 ± 6f 94 ± 3f 93 ± 1f 94 ± 2f 92 ± 2f

98 ± 3f 97 ± 2f 97 ± 1f 98 ± 2f 97 ± 1f

2.6 ± 0.3f 3.4 ± 0.8f, g 3.7 ± 0.6g 4.2 ± 0.7g 4.7 ± 0.6g

83 ± 6f 88 ± 4f 82 ± 3f 83 ± 2f 82 ± 5f

98 ± 3f 97 ± 3f 97 ± 3f 98 ± 1f 98 ± 2f

1.1 ± 0.2f 1.4 ± 0.2f 1.3 ± 0.4f 1.4 ± 0.4f 1.4 ± 0.3f

36 ± 2f 36 ± 2f 28 ± 4g 26 ± 4g 24 ± 4g

96 ± 3f 96 ± 1f 93 ± 3f 96 ± 4f 97 ± 3f

a

A (medium soft)

B (fairly hard)

C (hard)

D (very hard)

“TMP” for “transmembrane pressure”. “Lw0” for the “water permeability” at steady state. Lv” for the “feed permeability” at steady state. d “ RF” for the “relative flux” of the water samples. e “ FR” for the “flux recovery”. f–h Values possessing the same superscripted letter (at least one) within a column are not significantly different (P ≤ 0.05). b

c “

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Fig. 2. Feed fluxes as a function of the applied transmembrane pressure during desalination experiment of the “hard” brackish water sample (C): sample permeate (▲), pure water prior (■) and after (□) desalination experiment.

Fig. 3. Feed fluxes as a function of the applied transmembrane pressure during desalination experiment of the “very hard” brackish water sample (D): sample permeate (▲), pure water prior (■) and after (□) desalination experiment.

281

Fig. 5. Retention coefficient of salinity (%, ■) and permeate salinity values (mg NaCl/L, □) as a function of feed salinity (mg NaCl/L). Values represent mean ± standard deviation (n = 3). The logarithmic (—) equation is: 15.6∙lin(x) + 159.82, with R2 = 0.97. The linear (− −) equation is: y = 0.6173x + 105.78, with R2 = 0.998.

that the relation between retention coefficient and feed hardness as well as this between permeate and feed hardness was linear and almost ideal (R 2 = 0.98 and 0.99, respectively). The salinity retention coefficients of the assayed samples and corresponding permeates' salinity values as a function of the initial feed salinity are shown in Fig. 5. Retention coefficients decreased importantly with increasing feed salinity from 74 to 44%, while permeate salinity increased correspondingly from 60 to 1011 mg NaCl/L. That is to say, permeate of the “extremely hard” sample (D) possessed salinity in the range of “hard” sample (C). The results were subjected to data integration analysis, but this time the relation between salinity retention coefficient and feed salinity value was logarithmic (R 2 = 0.97). The technical expression between permeate and feed salinity was again found to be linear with an ideal regression (R 2 = 0.998). Finally, Fig. 6 presents the data of permeate versus feed hardness per salinity ratio and the obtained linear regression (R 2 = 0.97). Permeate of the two hardest waters (C and D) possessed two-fold lower ratios compared to the initial samples (0.25 and 0.22 compared to 0.49 and 0.42, respectively). 4. Discussion

lower hardness compared to the WHO Guidelines. Moreover, permeate of the “hard” and “extremely hard” samples was found to be a “moderate” and “almost hard” water, respectively (less than ~ 220 and 320 mg CaCO3/L). As it can be concluded from the plots and the implementation of the minimum square approach, it seems clear

As it has been referred above, charge exclusion and sieving effect are two of the primary factors affecting separation in NF applications. Charge exclusion of ions depends on three parameters: (a) membrane charge, (b) ionic strength and (c) ion valance [28]. Subsequently, membrane charge is dependent on the pH of the tested sample.

Fig. 4. Retention coefficient of hardness (%, ■) and permeate hardness value (mg CaCO3/L, □) as a function of feed hardness (mg CaCO3/L). Values represent mean ± standard deviation (n = 3). The linear equation (—) of retention coefficient is: y = − 0.0131x + 80.387, with R2 = 0.98. The linear equation (− −) of permeate hardness is: y = 0.3178x − 21.101, with R2 = 0.99.

Fig. 6. Permeate versus feed hardness/salinity ratio (mg CaCO3/mg NaCl). Values represent mean ± standard deviation (n = 3). The linear equation is: y = 1.3232x − 0.3625, with R2 = 0.97.

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The active semi-aromatic/aliphatic polyamide layer of the membrane possesses a poly-ampholytic nature due to the positive (amine cations) and negative (carboxylate anions) charges. Thereby, the charge net is governed by the isoelectric point of the membrane, which is dependent to the pH value of the electrolyte solution. The isoelectric point of polypiperazine membranes has been reported to be above 3 [29,30], while membrane “NFt50” that is the former brand of “NF99” has been referred to possess an isoelectric point equal to 4 [30]. As the tested water samples showed a pH value of 7.8–7.9 (Table 1), membrane charge was expected to be negative. On the other hand, previous studies with polypiperazine membranes suggested that the charge on the active polyamide layer is fairly low, uneven and thus membrane is behaving more like a hydrogen-bonding hydrophilic polymer [15,16]. The latter hypothesis might be enhanced by the high ionic conditions (conductivity values up to 2131 μS/cm) of brackish waters that shield membrane charge [10,11]. If this is the case, size effect and ion valance would be the main factors affecting separation process. In particular, the ions possessing the higher concentrations in the tested waters (Table 1: Ca 2 +, Na +, Mg 2 +, HCO3−, Cl − and SO42 −) play the major role during desalination experiments. Cations and anions cannot permeate independently, but pass through the membrane while maintaining electro-neutrality [7]. According to the manufacturer, the applied membrane removes quantitatively 2000 mg MgSO4/L. This means that salts possessing a molecular weight (MW) around 120 would be more or less retained. Among the 9 dominant salt combinations occurring in the assayed brackish samples, four of them (Mg(HCO3)2, Ca(HCO3)2, CaSO4, Na2SO4 and MgSO4) possess a MW above or around 120, three (CaCl2, MgCl2, NaHCO3) possess a bit lower MW (~85–95) and only NaCl have a much lower MW (58.5). Thereby, the bigger salts are expected to be retained, while smaller (i.e. the Cl − containing salts) could pass partially or totally through the membrane. Salt ions could step on negative (outer surface) and positive (inner side) residual groups in order to permeate across the pores. Particularly, cations shield negative surface charge and allow anions to pass inside the membrane pores, while anions shield the inner amino residues and allow cations (Ca 2 +, Mg 2 + or Na +) to permeate first. The excess of cations on the permeate stream generates an electrostatic force which increases anion transfer, particularly of Cl −, because SO42 − anions are more hydrated and cannot cross the membrane [7]. Finally, divalent cations (Ca 2 +, Mg 2 +) would pass more difficultly in comparison to monovalent Na + on account of the increased repulsing forces inside membrane pores. The results of the current study confirm the above considerations, as the hardness retention was higher compared to salinity retention (Figs. 4 and 5). The first was exclusively charged to divalent ions, while the second was charged to both divalent and monovalent ions. Moreover, hardness and salinity rejection decreased with increasing salts' concentration of brackish water. Despite the increased concentration polarization of the hardest samples, permeation of ions was higher due to the enhanced ionic conditions of the more concentrated solutions, the lower zeta potential of the membrane [31] and the reduced Donnan exclusion effect as described above. Another possible explanation could be that the effective area of the membrane pore is growing because of the reduced thickness of the electrical double layer [4]. Concentration polarization was shown to occur on account of the decreased RF values and the stable permeability (despite the increasing TMP) of the hardest sample D (i.e. Table 2: 24–36% and ~1.4 L·h − 1·m − 2·bar − 1), respectively. The advanced role of ionic strength in the permeation of ions was also confirmed by the fact that the reduction of hardness and salinity retention coefficients showed a different behavior (linear and logarithmic, respectively). In the second case (Fig. 5), the increase of feed salinity (from 517 to 1141 mg NaCl/L) was followed by a rapid reduction of rejection (from 66 to 48%) suggesting that the electrostatic force between feed and permeate stream was increased enough to allow

ions of smaller valance pass quickly across the membrane pores. Paugam et al. reported a similar behavior during NF of sodium salts and nitrates with a polyamide membrane. Specifically, when the concentration of Na + was increased, the electrostatic interactions became weaker and subsequently the retention was governed by the size effect [11]. With regard to the overall yield of the NF process, results showed that polypiperazine membrane was able to remove successfully hardness of the assayed samples below WHO limit. The linear equations described in Fig. 5 could be utilized in order to predict accurately the hardness retention and the corresponding value of the permeate stream as a function of the initial sample hardness (from 161 to 762 mg CaCO3/L). The ideal linear regression suggests that the prediction could be extended to higher hardness values, too. On the other hand, salinity was satisfactory reduced for the three softer samples (A, B and C). The rather low retention coefficient of salinity for sample D (Fig. 5: 48%) indicates that the permeate stream did not contain Na +, Cl − and SO42 − concentrations lower than the half of these obtained for the initial sample (i.e. higher than ~180, 240 and 130 mg/L, respectively). Moreover, as Na + and Cl − ions pass easier compared to SO42 − through the membrane pores, the corresponding concentrations in the permeate stream are expected to be higher than the limits of WHO (200 and 250 mg/L for Na + and Cl −, respectively). Thereby, the process could be applied for the treatment of samples containing a salinity value not much higher than ~ 1100 mg NaCl/L. Again, the linear equation described in Fig. 5 could be applied for the estimation of permeate as a function of feed salinity, even above 1800 mg NaCl/L. At the latter case, permeate needs further treatment (i.e. with RO) in order to be consumed as potable water and linear equation of Fig. 6 could be applied for the estimation of permeate hardness per salinity ratio (a valuable magnitude for RO applications). A comparison of different membrane materials and modules applied for the desalination of brackish waters in the literature is presented in Table 3. The main advantage of polypiperazine as described in the current study was the low applied TMP (6–10 bar), while the main disadvantage was the moderate salinity retention (44–66%) that restricts application in samples containing a low concentration of monovalent ions. Nevertheless, results were generally in accordance with previous studies concerning salinity retention. For example, Reddy et al. investigated NF of salts like MgSO4, NaCl and CaCl2 contained in standard solutions. According to their experiments, the corresponding retentions were less than 30% [11]. Hassan et al. referred a rejection percentage above 75% for Ca 2 +, Mg 2 +, SO42 − and HCO3−, which was dropped to 40% for monovalent ion (Na +, K +) removal. However, this process was applied with a very high TMP (22 bar) [32]. Kelewou et al. used two commercial NF membranes (polypiperazine and polyamide) for the desalination of salt containing synthetic solutions in a cross-flow unit and reported very high retention of divalent anions (i.e. >90% for SO42 −), but relatively low removal of monovalent ions (i.e. b50% for Cl −) [33]. On the other hand, Hilal et al. reported a very high and medium NaCl rejection (up to 95% and 40%) by treating standard solutions of 5000 and 25,000 mg NaCl/L, respectively, with a polyamide membrane at low TMP (up to 9 bars) [34]. However, this process was tested only with standard solutions and not with real brackish groundwater samples. Another interesting characteristic of the process described herein was the rather advanced performance parameters obtained during desalination experiments. As it can be observed from Fig. 2, permeate flux values during treatment of “hard” sample C were in a satisfactory range (from 15 to 47 L·h − 1·m − 2 by increasing TMP from 6 to 10 bar) and almost coincided with these obtained for pure water flux. For example, Reddy et al. referred similar flux values (10–50 L·h − 1·m − 2) at an operating TMP of ~ 4 bar [13]. Permeate flux versus TMP profiles of the two softer samples (A and B) were respective to sample C (data are not shown). Nevertheless, treatment of the hardest sample D

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provided 2 to 3 fold lower flux values for each TMP (Fig. 3), suggesting that the presence of concentration polarization phenomena is restricting NF application up to the hardness and salinity concentration of the latter sample. This consideration is enhanced by the fact that the permeability of the hardest sample did not increase with increasing TMP (Table 2). In any case, the very high FR values (93–98%) for all the tested samples indicate that polypiperazine membrane possesses a fouling resistance nature to divalent and monovalent ions. This property is important to state, as fouling is the most commonly encountered phenomenon in similar applications. Besides, it is difficult to reverse fouling except by applying extensive and often sever cleansing methods [13]. Although membranes are susceptible to biofouling in alkalic environment, this phenomenon did not seem to be the case neither, probably due to the absence of proteins (accelerate biofouling [35]) and the fact that groundwaters usually possess much lower microbial load compared to wastes (i.e. domestic). 5. Conclusion Polypiperazine membrane and the applied NF processes could be utilized for the treatment of “hard” and “extremely hard” brackish samples and their modification to drinking water, as they provided: (a) high percentage of hardness removal, (b) satisfactory permeate fluxes at rather low TMP:s (6–10 bar) and (c) high mineral fouling resistance. A disadvantage of the process was the relatively low retention coefficient of salinity (44–66% for brackish groundwaters) that restricts the application in samples possessing salinity not much higher than ~1100 mg NaCl/L. Besides, permeate flux was rapidly reduced by increasing sample salinity from 1080 to 1800 mg NaCl/L. As a general rule, the retention of divalent and monovalent ions was governed by size effect and ion valance. A possible explanation could be the high ionic conditions of brackish samples as well as the poly-ampholytic nature of the membrane material that reduced Donnan exclusion effect and generated an electrostatic transfer of anions (and subsequently cations) across membrane pores. Conclusively, the results of the current study contribute to the development of NF applications for the production of low cost drinking water in touristic regions (i.e. islands, coasts and hotel areas), where water deficiency is mainly a seasonal but growing problem.

d

c

b

“MWCO” for “molecular weight cut off”. “TMP” for “transmembrane pressure”. Retention of each parameter described in “Feed Salinity” column. “n.r.” for “not referred”.

References

a

Current study [5] [32] [33] [34] [11] [12] [6] Limited application to 1100 mg NaCl/L Low retention of Na+, Cl− Relatively low retention of Na+, K+, Cl− (~40%) Low retention of Cl− Tested only with standard solutions Low retention of Na+, Cl− High TMP2 High TMP2 High mineral fouling resistance, low TMP2 Retention of sulfate hardness High retention of divalent ions (> 90%) High retention of divalent ions High retention of salinity, low TMP2 Long term fouling resistance High salinity retention High salinity retention 44–66 28 63 b50 95, 41 10–26 78–93 ≤79 120 200 n.r.d 230–280 n.r.d b 1000 n.r.d n.r.d Polypiperazine Polyamide Polyamide Polypiperazine/Polyamide Polyamide Modified polyamide Poly-methyl-hydrosiloxane Cellulose acetate

Cross-flow Tubular Dead-end Cross-flow Cross-flow Dead-end Dead-end Dead-end

6–10 20 22 4–14 2–9 25 16 16

Total salinity ≤1800 mg NaCl/L Total dissolved salts of 2600–5300 mg/L Total dissolved solids ~ 4400 mg/L ~ 5800 5000, 25,000 mg NaCl/L 2000 mg NaCl Total dissolved salts of 4100 mg/L Total salinity 3500–4000 mg NaCl/L

Reference Disadvantages Advantages Retentionc % Feed salinity TMPb bar Module MWCOa Da Membrane active layer

Table 3 Comparison of brackish groundwater desalination by different membrane materials referred in the literature.

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