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Study on lithium separation from salt lake brines by nanofiltration (NF)and low pressure reverse osmosis (LPRO)

A. Somrani a,b, A.H. Hamzaoui a, M. Pontie b,⁎a LVMU, Technopôle de Borj Cédria, Centre National de Recherche en Sciences des Materiaux, Tunisiab L'UNAM University, Angers University, GEPEA UMR CNRS 6144, 2 Bd. Lavoisier 49045, Angers Cedex 01, France

H I G H L I G H T S

• Separation of Mg2+/Li+ by nanofiltration vs low pressure reverse osmosis membranes• Nanofiltration fouling by salt lake brines diluted 10 times• Dialysis with nanofiltration for a total separation Na+/Li+

a b s t r a c ta r t i c l e i n f o

Article history:Received 22 December 2012Received in revised form 5 March 2013Accepted 6 March 2013Available online 9 April 2013

Keywords:LithiumExtractionSalt lake brineNanofiltrationLow pressure reverse osmosisDialysis

The aim of the present work is to study the separation of lithium from salt lake brines by NF and LPRO. NF90membrane compared to the XLE a LPRO membrane appeared more efficient for Li+ extraction due to its higherhydraulic permeability to pure water and 0.1 M NaCl solution, its lower critical pressure (Pc = 0), its higher se-lectivity between monovalent ions (40%) obtained at low operating transmembrane pressure (below 15 bar)and its lower average roughness (105 ± 10 nm) decreasing the propensity to be fouled. NF90 exhibited 100%rejection of magnesium in the first step separation from brine diluted ten times as 15% for Li+, with a final sep-aration of 85% between Mg2+/Li+. The permeability to the diluted brine is 0.7 L.h−1.m−2.bar−1 usable to sizefull scale experiments, but the fouling mechanism has to be discovered in the future work. In a second step wehave not succeeded to separate totally Li+ and Na+ in the permeate obtained before (15% of separation only be-tween Li+ and Na+). To solve this problem,we did dialysis.We obtained a total separation between Li+ andNa+

with a diffusion flux (4.42 10−7 mol.s−1.m−2 at 20 °C) for NaCl 0.1 M 5 times higher for NF90 vs XLE.© 2013 Elsevier B.V. All rights reserved.

1. Introduction

The use of membrane technology has gained momentum in recenttimes and is preferred over conventional techniques like chemicalprecipitation or solvent extraction [1–12]. Chemical precipitationtechnology has inherent disadvantages of using large amounts ofchemicals and producing a lot of sludge.

Lithium is one of the rare elements on earth. It has important ap-plications as batteries, refrigerants, ceramics and medicines and itsdemand is expected to expand [1–12]. A huge amount of lithium re-sources is widely distributed in brine or seawater, whose quantitiesvary between 10 to 350 mg/L [12–14]. Actually membrane processesconcerned this refrigeration agent, specially using the operation ofnanofiltration (NF) [13,15,16]. They based their work on the first ap-plication of NF membrane as softening membranes [1,13–17]. Veryrecently NF works reported on its use to separate Mg2+ from Li+

and other monovalent ions in seawater or brines [11,14,18–21].

The main interest herein is to estimate the best separation betweenMg2+/Li+ from Tunisian salt lake brine solution by nanofiltration (NF)vs low pressure reverse osmosis (LPRO) membranes. To achieve thisaimwe determinedmembrane permeabilities to water and salted solu-tions, Li+ retention vs Mg2+, membranes roughness from AFM experi-ments and diffusion flux of NaCl and LiCl electrolyte solutions bydialysis experiments.

2. Experiments

2.1. SEM measurements

The surfaces were characterized by scanning electron microscopewith a field emission gun (FEG-SEM) using a JSM-6301F from JEOL(SCIAM, Angers University, France). Apparatus at a 3 KeV accelerationvoltage allowing both to minimize the irradiation damages and to ob-tain a resolution of few nanometers (nm) with a working distance of13 mmwas used. Observations were conducted until ×5000 magnifi-cation. To increase the image quality it is necessary to cover the ana-lyzed surfaces by an ultra-thin layer of carbon (2–5 nm thickness)

Desalination 317 (2013) 184–192

⁎ Corresponding author. Tel.: +33 2 41 73 52 07; fax: +33 2 41 73 54 21.E-mail address: mpontie@aol.com (M. Pontie).

0011-9164/$ – see front matter © 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.desal.2013.03.009

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deposited by evaporation under a vacuum (BAL-TEC MED 020 BalzersLiechtenstein apparatus).

2.2. AFM measurements

The AFM equipments used are conducted with a Thermo micro-scope autoprobe CP-research device from Brucker (France). Themembrane morphologies are imaged in contact mode in air with ascan rate of 1 Hz and 400 × 400-pixel resolution. The cantileversused for such imaging are from Brucker, with specified spring con-stant of 0.2 nm−1 and a resonant frequency of 13 kHz and the tipsin Si.

The mean roughness (denoted as Ra) is the mean value of surfacerelative to the center plane. The plane for which the volume is enclosedby the image above and below this plane is equal and is calculated as

Ra ¼ 1LxLy

∫Ly0 ∫Lx

0 Z x; yð Þj jdx dy ð1Þ

where z(x,y) is the surface relative to the center plane and Lx and Ly arethe dimensions of the surface analyzed. The same cantilever is used forall AFM images and all the AFMs were treated in this way. The mem-brane samples are dried at room temperature in a desiccator. The sam-ples are then attached to steel discs with double-sided scotch tape. Theimages are obtained over the area 50 μm × 50 μm. Image analysis iscarried out by means of (SPMLab602) software from Brucker (France).Focussing only on virgin membrane in Ra values in the present studyis resulting from two field analyses on each sample using contact anal-ysis mode. The Ra values reported are the average values of the twofields analyzed (size 50 μm × 50 μm).

2.3. Membranes properties

The studied membranes are thin-film membranes composed of twolayers: A thin polyamide film as active layer and a large mesoporouspolysulfone as the support layer, as described elsewhere [22]. The twostudied membranes are NF, denoted as NF90, and LPRO, denoted asXLE. All membranes are purchased from Filmtec (DOW, USA). The spec-ifications of the studied membranes are given in Table 1. A free mem-brane sample used in the case of XLE was purchased as received in theform of flat sheet samples as in the case of NF90. The samples obtainedfrom the module NF90-2540 (n°F6749226) are dismantled.

Before using the membranes, they should be rinsed with ultra-purewater (denoted asUP) (ElgaUV-UF system, France) until the conductiv-ity of the permeate remained belowUPwater conductivity (b1 μS/cm).The effective membrane area is 140 cm2.

The salts used (NaCl, LiCl) are of analytical grade from Aldrich(France) and used as received. All solutions are prepared from a MilliQwater (Millipore system, France) with a purity water presenting a con-ductivity of 0.4 μS/cmandpH = 6.7. The salt analyses are carried out bya conductimeter (Cond. 315i, order number 2C10-0011FB, from WTW,France) after standardization for each salt and concentrations is de-duced for single salt's solutions.

2.4. Bench scale pilot plant

The NF/LPRO pilot plant is supplied by Dow, France (moduleOsmonics GE), and consisted of a feed tank, a pump Wanner (WannerGP, US) and a planar module, as detailed in Fig. 1. All studies are doneusing a low flow rate ratio (5%) and a tangential rate around 0.2 m.s−1

for a Reynolds number of 400. The applied transmembrane pressuresare in the range of 0–25 bar. The temperature maintained at 21 °C.

Permeated solutions are recycled during the runs expected forsamples withdrawn for the calculation of observed retention denotedas Robs according to:

Robs ¼ 1−Cp

Coð2Þ

where Cp and Co are the concentrations in the permeate and feedsolutions, respectively. But within hydraulic conditions, considerRobs = Rreal onto low flow rate ratio.

Pure water flux through a membrane can be described by:

Jv ¼ LpΔP ð3Þ

Table 1NF and LPRO membrane properties from the supplier.

XLE NF90

Supplier Filmtec (DOW) Filmtec (DOW)

Max.temp, °C 45 35Pressure range, bar 0–41 0–41pH range 3–10 3–9NaCl rejection (%) (ROSA™ simulatorresults with 0.1 M NaCl, ΔP = 15 bars)

98.6 96.6

Material active layer Polyamide PolyamideMWCOa (Da) __ 250 (supplier)

213 [13]

a MWCO = molecular weight cut-off.

Fig. 1. Design of the elements of the bench scale laboratory pilot.

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with Lp as the hydraulic permeability to ultrapure water (Lp' the hy-draulic permeability to salted waters), Jv the permeate flow, and ΔPthe difference of hydrostatic pressure between each sides of themembrane.

The volume reduction factor (denoted as VRF) is calculated by

VRF ¼ V0

V0�Vp tð Þ ¼V0

VR tð Þ ¼ 1:33 ð4Þ

where V0 is the initial feed volume (8 L), VR(t) the retentate volume attime t and Vp(t) the permeate volumes at time t (2 L) and t = 6 h. Inthe experiments conducted the NF90 with diluted brine worked witha VRF of 1.33.

2.5. Salt diffusion experiments

For any givenmembrane, the mass transfer is the result of the cou-pling between the phenomena of diffusion, convection and migration.This transfer of material may also be due to a thermodynamic dis-equilibrium as is shown by Kedem and Katchalsky [23]. Knowledgeof the volume (V) of diluted compartment and the surface of themembrane (S) in contact with the solutions is used to calculate theflow of electrolyte (denoted as Js) transferred across Hittorf's cells:

Js ¼ VΔC2=SΔt ¼ VΔχ=SKΔt ð5Þ

with 1/K = 1,365,079 S−1.cm2.mol (K: calibration constantlinking the conductivity to the concentration), V = 50.10−6 m3,S = 113,097.10−6 m2, Δχ = conductivity variation (μS/cm) andΔt = time variation (s).

The principle of the method for measuring Js with LiCl and NaClelectrolytes is to follow the diffusion of salts separately through thestudied membranes, in order to determine the solute permeability, asreported elsewhere [24]. Thismembrane separates a concentrated solu-tion and UP water (Millipore, France) with conductivity of 0.4 μS/cm.The temperature is maintained at 22.0 ± 0.1 °C. The determinationof concentrations from conductivity measurements requires preciseknowledge of the diluted compartment volume (50 mL). The twocompartments are separated by the membrane, the first (C1) is filledinitially with a solution of NaCl (or LiCl) 0.1 mol/L and the second(C2) is filled with UP water. The conductivity of solution compart-ments C2 is measured by a conductimeter (Cond. 315i, order number2C10-0011FB, from WTW, France). The measurements are madeduring 90 min.

2.6. Analytical tools

Emission apparatus type Sherwood 410 (US) allows to determinethe Na+ and Li+ concentrations after calibration between (0–30)g.L−1 for Na+ and (0–30) mg.L−1 for Li+. And, Mg2+ is determinedusing Vario-6 atomic absorption spectrometer (US) after calibrationbetween 0 and 10 g.L−1.

2.7. The Chott Djerid (Tunisia) brine geographical localization and chemicalcomposition

Chott Djerid is the largest salt lake located in the south of Tunisia,as shown in Fig. 2. The characteristics of this brine are given inTable 2.

The water brine salinity is 5 to 10 times higher than usual sea waterwhich is situated between 35 and 45 g.L−1. Chott Djerid brine water ischaracterized with high concentration of magnesium and sodium ionsare more concentrated than sea water (1.29 g.L−1 and 10.8 g.L−1

Fig. 2. Geographical position of Chott Djerid (Tunisia) (geographical coordinates: 33°42′ 00″ north).

Table 2The composition of Chott Djerid brine water.

Ions analyzed Cl−

(g/L)Ca2+

(g/L)Mg2+

(g/L)SO4

2−

(g/L)Na+

(g/L)K+

(g/L)Li+

(mg/L)

Chott Djerid (±0.1) 144.1 1.6 3.4 6.7 80.0 5.6 59.9

Fig. 3. Effect of operating pressure on the permeate flux of NaCl solution for the studiedmembranes, NF90 and XLE (T = 21 °C, NaCl 10−1 M).

Table 3Pure water (Lp) and saline solution (Lp') (NaCl 10−1 M) permeabilities and criticalpressures (Pc) of the NF90 and XLE membranes.

Membrane Lp (±0.1)(L.h−1.m−2.bar−1)

Lp' (±0.1)(L.h−1.m−2.bar−1)

aPc (bar)(±0.1)

XLE 8.3 4.4 2.6NF90 15.0 7.5 0.0

a Critical pressure, denoted as Pc, is the pressure of starting filtration.

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respectively). Also the main interest in brine is its more concentratedlithium ion since seawater contains only 0.18 mg.L−1 [25]. The feedtested in this work is obtained by diluting ten times the brine solutionwith UP water because at this high concentration of ions the risk ofsalt precipitation (e.g. NaCl, Na2SO4 and MgSO4) is avoided. The sameapproach using dilution of brine has also recently reported [14]. Duringthe first stage separation using NF90, 2 L of permeate solution (denotedas permeate 1) from 8 L of feed is prepared. During the second stage thetreatment of nanofiltration is completed by filtrating 2 L of permeate 1with the NF90.

Simulation: NF simulation is conducted by ROSA™ 6.1 in order topredict Li+, Na+ and Mg2+ selectivity in NF90 with brine composi-tion diluted 10 times in comparison to the experiments.

3. Results and discussions

3.1. Characterizations of NF and LPRO membranes

3.1.1. Saline aqueous solution permeabilitiesThe interest in knowing the permeability of a membrane for a

salty solution is to predict the fluxes which could be obtained forreal water, without fouling. This parameter is not usually given byall the suppliers. In Fig. 3, the flow rate is reported as a function ofthe transmembrane pressure (ΔP) for an NaCl solution at a concen-tration of 10−1 mol.L−1 (6000 ppm and a theoretical osmotic pres-sure of 4.8 bars) in order to compare the NF90 with the XLE. Thelinearity observed suggests that this salty solution follows theKedem–Katchalsky model (i.e. Spiegler–Kedem model, with pressureand osmotic linear gradients) [23]: the hydraulic permeabilities withpure water Lp, with the salt solution Lp' and the critical pressure (Pc)obtained are reported in the Table 3.

The Pc values show that the flux through the LPROmembrane startsat 2.6 bars, under the theoretical osmosis pressure (Πth.) of 4.8 bar (seeFig. 3), suggesting that this LPRO membrane has a different behaviorthan a usual RO membrane, as reported elsewhere [13,26]. Then be-tween NF and LPRO, NF is less limited by theoretical osmosis pressuredue to its microporosity as the LPRO which is a dense membrane. SoNF90 membrane shows the higher hydraulic permeability to 0.1 MNaCl solution with Lp' = 7.5 L.h−1.m−2.bar−1 and a Pc of zero.

3.1.2. SEM/AFM images of NF90 and XLE membranesIn Fig. 4, SEM and 3D-AFM images of NF90 and XLE membranes

are reported. According to the SEM measurement results Fig. 4a, b,it is observed that the membrane surfaces of both studiedmembraneshave the same morphology. In addition AFM characterizations havebeen performed to determine the roughness of the studied mem-branes. The results obtained for the one scan of 50 × 50 μm2 of the

b

a

b

a

2 µm Ra = 105±10 nm

Ra = 133±10 nm

2 µm

Fig. 4. Images of studied membranes NF90, XLE by SEM (magnification ×5000) (a) (b) and AFM (a') (b'), respectively.

Fig. 5. LiCl observed retention versus transmembrane pressure for NF90 and XLE mem-branes (T = 21 °C, LiCl = 30 ppm).

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two membranes are reported in Fig. 4a', b'. It may be seen that the av-erage roughness (denoted as Ra) is higher for the XLE membrane.Then XLE can be easily fouled, clogged, plugged than NF90 mem-brane, as reported elsewhere [13,26,27]. In the case of the NF90 pre-vious results observed a higher Ra with 390 ± 20 nm [13] and also anoutstanding high roughness [28,29]. In the present study the samplesof NF90 used present a large decrease in its roughness as previouslyreported for other samples from other batch [13]. So supposing achange in the NF90 process of elaboration is obtained to correct thisprevious high roughness.

3.1.3. Determination of Li+ retention betweenNF90 andXLE andmembraneselectivity

Usually, to compare the performances of different membranes, thegraph of the observed retention, denoted as, Robs, as a function of thetransmembrane pressure applied (ΔP) is used (see Fig. 5) [13,26]. Thestudied membranes rejected Li+ ion following the order: XLE > NF90.As attempted, the highest rejections are obtained for the LPRO mem-brane. For the latter, the membrane has no pores and the mechanismof exclusion is only chemical based on thewell-known solution–diffusionmodel [30,31]. In NF, the selectivity is not only based on chemical phe-nomena, the pore size effect, the charge effect, the pH effect, the ionicstrength effect, the solute type, the temperature effect and the hydrationeffect [30–40] can influence salt permeation, depending on the operatingconditions and on the kind of NF membrane. As reported elsewhere[13,16,18,19,35–41], ROmembrane cannot be used for partial and for se-lective demineralization but NF membrane is more suitable.

In Fig. 6 the retention observed for Na+ and Li+ using NF90 andXLE membranes versus transmembrane pressure, is reported. It is ob-vious that the rejection of all ions increases with increasing pressure.

The sequence of rejection of monovalent anions for the NF90 mem-brane can be written as R (Li+) > R (Na+) from 0 to 20 bars. The se-quence of rejection of monovalent anions for the XLE membrane issimilar to the NF90 but starts at higher pressure due to the osmoticpressure of the feed, from10 to 18 bars. After 18 bars an inversion is ob-served for the retention in Fig. 6b. This modification can be attributed to

the inversion of predominant mass transfer which occurs at low pres-sure vs high pressure, as reported elsewhere [31,42].

Under 20 bars the observed retention order of the two ions and forboth studied membranes is similar to the hydration energy order forthe monovalent ions (Table 4) where Li+ has a higher hydration energyand then is better retained than Na+. The main difference between bothmembranes is the higher selectivity observed for the NF90 in the rangeof 0–15 bars with a maximum of selectivity observed at 8 bars with adifference in Li+ and Na+ rejection of 40%. A well-known two masstransfer mechanisms occur in NF: solution–diffusion and convection.As recently reported [42] the Spiegler–Kedem–Katchalsky model helpsto observe that an old ROmembrane acquires a convectivemass transfer,as usually dedicated to porousmembranes. The Peclet number (denotedas Pe′) is used to distinguish between diffusional and convective masstransfers with a Pe′ > 1 for predominant convection mass transport. Inseawater desalination the old used membrane becomes an NF mem-brane with Pe′ between 1.27 and 3.95 in a transmembrane pressurerange of 10 to 50 bars. In this case, the inversion in ion rejection ordercan be translated in a change in the predominant mass transfer. Before20 bars the mass transfer is diffusional with the order of rejections inthe same order as the Hofmeister series (see Table 4) RLi+ > RNa+, asfor a pressure higher than 20 bars the convection becomes predominantand inversion of rejection is observed.

To conclude, the selectivity is always higher in NF as reported else-where [13,31] and also it is higher at low operating pressure wherethe solution–diffusion mass transfer is often predominant. As it isseen previously the same mechanism with the problematic watercontaining chloride ions and fluoride in excess was compared to theregulation. Different NF/LPRO membranes are investigated and thebest one obtained is an NF, because of its properties which presenta predominant mass transfer by solution–diffusion, under a low pres-sure [13,43]. F− and Li+ have somewhat the same behavior in NF be-cause there are both ions with lower radius in the column of theMendeleyev classification, but they have the highest hydration ener-gy in their group. As a result, F− and Li+ ions are the higher rejectedmonovalent ions in NF/RO, especially in NF membrane presenting asolution-diffusion predominant mass transfer, as well as the case forthe NF90 under low transmembrane pressure.

The results show that NF90 membrane is more efficient for Li+

extraction due to its higher hydraulic permeability to pure water(Lp = 15 L.h−1.m−2.bar−1), its higher hydraulic permeability to0.1 M NaCl solution (Lp' = 7.5 L.h−1.m−2.bar−1), its lower Pc(Pc = 0), its higher selectivity between monovalent ions (40%)obtained at lower operating pressure (below 15 bar), (Fig. 6a) andalso its lower roughness decreasing the propensity to be fouled.

3.2. Application of NF90 for Li+ extraction from Tunisian brines

3.2.1. First step: separation of Li+ from Mg2+/(Li+, Na+) mixturesIn Fig. 7, it is reported that the rejections of Mg2+, Na+ and Li+

were obtained with Chott Djerid brine diluted 10 times versus

Fig. 6. Rejections of Na+ and Li+ cations versus transmembrane pressure for (a) NF90 and (b) XLE membranes for separate electrolytes LiCl and NaCl at 30 ppm of Li+ and 8 g/L of Na+.

Table 4Bulk diffusion coefficient (Ds), Stokes radii (rs), and hydration energy of Li+, Na+,Mg2+, Ca2+, Cl− , F− and SO4

2−.

IONS Ds (×109)(m2.s−1)

rs(nm)

Absolute values ofhydration energy(kJ.mol−1) [30]

Li+ 1.03 0.238 636Na+ 1.33 0.184 454Mg2+ 0.71 0.348 2555Ca2+ 0.792 0.347 1615Cl− 2.03 0.121 325F− – 0.117 442SO4

2− 1,065 0.229 1047

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transmembrane pressure with the NF90 membrane (Fig. 7a) in com-parison to ROSA™ simulation (Fig. 7b).

It can be seen that NF90 exhibits an excellent retention ofmagnesiumions (100%) but poor retentionof lithiumand sodium ions (10 to 30%), asattempted [16,18,35–39]. Similar experiments have been conducted re-cently by Yang and co-authors [18] with the DK NF membrane wherethey validated the separation between Mg2+ and Li+ with a differenceof rejection of 31%with large selectivity under the influence of thework-ingpressure. In this case, at a pressure of 15 bars a larger difference in re-jection betweenMg2+ and Li+ vs Yang and co-authors is obtained, with80%. The main reason behind this higher selectivity is the choice of theNF90 membrane. In this case, the molecular weight cut-off of the NF90is around 200 Da as for the DK NF the MWCO is around 300 Da. The dif-ference in MWCO is sufficient to explain the difference of selectivity ob-served. The solution–diffusion mass transfer must be predominant tomaintain a high selectivity between divalent and monovalent ions.

Previous results illustrating the selectivity observed in NF70 mem-branes between the ions Mg2+, Ca2+ and Na+ are reported for seawa-ter [40]. Monovalent/divalent ions selectivity is obtained with the NF70membrane [40]. The rejection order followed very well the Hofmeisterseries of the hydration energy, i.e. Mg2+ > Ca2+ > Na+. It meansthat NF70 predominant mass transfer is a solution–diffusion, like RO.To compare selectivity between divalent and monovalent ions the roleof hydration has to be evocated. As reported by Pontalier andco-author [37], hydration can be considered as a force which is neces-sary to extract the solute from the solvent to push it into the pores. In

this way, it would require more energy to extract Mg2+ and to push itinto the pores in comparison with Li+ or Na+. Today, it is knownmore clearly that the state of water inside those membranes differsfrom the bulk state in accordance to recent studies where the authorstried to obtain a water dielectric constant inside the membrane withdifferent values simulated between 25 and 60 [38,39] in comparisonto the bulk value of 78. Free water has its proper behavior concerningions or polar molecules. Furthermore, the microporosity of the mem-brane in combination with the hydration effect could explain verywell the selectivities observed between ions and the effects of the oper-ating conditions. As illustrated in Fig. 6, the ion rejection is mainly de-pendent under its hydration energy in the solution upstream and itwill be more retained if it has higher hydration energy than the others.

The same result is obtained by simulating the NF operation withROSA™. The ions rejection is following also the Hofmeister series:RMg2+ > RLi+ > RNa+. At once, a dramatic difference between exper-iments and simulation is observed. Those differences are attributed to thefact that ROSA™ simulator is essentially based on a solution–diffusionmodel of mass transfer, but this mechanism is not the only one in NFwhere the active layer presents microporosities and where convectioncan play a none negligible role, as reported recently [44]. In the presentresults, (see Fig. 7) the difference between the simulation and the exper-iments increases with the transmembrane pressure.

Another phenomenon can sometimes overlap and permit the ob-servation of negative retention of monovalent ions; it is based onthe Donnan effect. The authors have also developed more complex

Fig. 7. Rejections of Mg2+, Na+ and Li+ cations in Chott Djerid brine diluted 10 times versus transmembrane pressure for NF90 (a) experiment (b) simulation ROSA™.

Fig. 8. Evolution of water flow rate in the preparation of 2 L of permeate from the diluted brine (VRF = 1.33).

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model, putting into consideration, all the transport engaged in NF i.e.DSPM (Donnan steric pore model) or PPTM (pore and polarizationtransport model) models that are well described in [14,16,45,46].

3.2.2. Membrane fouling after 1st step NF90To separate Mg2+ from Li+/Na+ mixture in the diluted brine, the

transmembrane pressure should be fixed at 14 ± 2 bar because atthis pressure the best separation Mg2+/Li+ is obtained (see Fig. 6a).During this first step of filtration conducted with the NF90 mem-brane, the flow (denoted as Q) vs time during the preparation of 2 Lpermeate from 8 L of diluted brine is determined.

It may be seen in Fig. 8a that Q decreased dramatically versustime. At the end, after 6 h of filtration, a loss of 50% of the initialflow rate is observed. The membrane surface is analyzed in order todetermine the presence of particles explaining this lost of productiv-ity. As illustrated in Fig. 8b, AFM image is very similar to Fig. 4a', witha higher average roughness value of 126 ± 10 nm. So finally, themain fouling mechanism is supposed to be internal. Further experi-ments should be conducted to better explain the real mechanism offouling.

Furthermore, the flow rate is stabilized after 350 min. A constantvalue of 0.2 L/h is obtained with a geometrical area of 140 cm2 andfor 14 bars applied, then the membrane permeability to the dilutedbrine is 0.7 L.h−1.m−2.bar−1 in comparison to the UP water perme-ability obtained before at 15 L.h−1.m−2.bar−1. With this result, fullscale experiments now can be prepared by considering this problemof fouling during time.

3.2.3. Second step: separation of Li+/Na+ mixture with NF90 membraneTaking permeate 1 as described in Table 5, the separation of the

monovalent ions Li+ and Na+ was attempted. Fig. 9 shows the reten-tion observed with the NF90 membrane for Na+ and Li+ in both thepermeate obtained from the previous experiment and for a syntheticsolution made up to the same concentration (3000 ppm and 2 ppm,respectively).

The following retention order is expected: RLi+ > RNa+ in the casewhere the hydration energy is the major mechanism of selectivity. Theorder in Fig. 9a iswell observed only under very lowpressure (b5 bars).But after 5 bars, the opposite order is obtained: RLi+ b RNa+ with avery low constant difference of rejection (15%).

Sometimes the same kind of inversion is reported [30], it is due tothe increase in the convective part of themass transportwith increasingtransmembrane pressure. In Fig. 9b, on the contrary of the Fig. 9a, allover the range of pressure study the retention order remains theattempted following order: RLi+ > RNa+ with a higher retention dif-ference of 30% observed at 5 bars. The previous behavior of the NF90membrane can be attributed to the effect of other ions present in thepermeate ([K+] = 0.5 g.L−1 + [Cl−] = 14 g.L−1) and then the differ-ences between Fig. 9a and b is attributed to the ionic strength differenceplaying a decrease in the Debye length increasing pore size and chang-ing the predominant membrane mass transfer.

To conclude, the results indicated that in the case of monovalentLi+/Na+ mixture, NF90 is not suitable for the selective recovery ofLi+. To try to improve this separation, it is important to test otherprocesses.

In comparison, liquid membrane technology is considered as anefficient technology for the selective separation and concentrationof different chemical species, especially in low concentrations. Theimportance of supported liquid membrane (SLM) process has beenrealized in the last three decades only [46]. Therefore, new sourcesof lithium are being explored and methods are being developed forits extraction from secondary sources. The concentration of lithiumin geothermal water, available abundantly in New Zealand, is around20 ppm and its recovery is possible with the use of SLM technique[2–10,12,47].

As a prospective work and based on previous work conducted byelectrodialysis and/or dialysis [41,48] and considering the good selec-tivity observed in the present work under low pressure, the testdialysis experiments at pressure zero are decided in order to compareboth studied membranes with the aim to totally extract Li+ frommonovalent ions mixture.

3.3. Diffusion flow measurements: preliminary results

The variations of the conductivity of electrolyte solutions at a con-centration of 0.1 mol/L as function of time are measured for the stud-ied membranes NF90 and XLE, as reported in Fig. 10.

The conductivity increases linearly with time for the XLE as for theNF90 the conductivity stays constant to the conductivity of UP water.The diffusion flows are calculated from the slopes of these curves,according to the Eq. (5). Diffusional flow values Js, for LiCl and NaClelectrolytes and for both tested membranes, are reported in Table 6.

Table 5Conductivity data, Li+, Na+ and Mg2+ concentrations for the brine solution, the brinediluted 10 times, 1st and 2nd step permeates of NF90.

Brine Brinediluted 10 times

1st step NF90permeate 1

2nd step NF90permeate 2

Conductivity 21 °C(mS/cm) ± 0.02

34.00 7.79 nd 2.96

Li+(mg/L) ± 0.1 59.9 6.0 5.0 2.2Na+(g/L) ± 0.1 80.0 8.0 6.4 3.1Mg2+(g/L) ± 0.01 3.40 0.30 0 0

nd = Not determined.

Fig. 9. Monovalent observed retention versus transmembrane pressure for (a) permeate solution after 1st step NF90 and (b) synthetic solution ([Na+] = 3000 ppm + [Li+] = 2 ppm).

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Based on the data presented in Table 6 it is observed that LiCl elec-trolyte has no diffusion flux with a slope of zero for both studiedmembranes. However, for NaCl electrolyte NF90 membrane diffusedwith a flow of 4.42 10−7 mol.s−1.m−2 is 5 times more than XLEmembrane. According to those results the use of NF90 membrane indialysis is the most rational way of having the best selectivity be-tween Na+ and Li+. In this way a total selectivity is obtained betweenNa+ and Li+, and the best membrane is the NF90 vs XLE.

4. Conclusion

The results of the previous experiments comparing NF90 and XLEmembranes show that NF90 membrane is more efficient for Li+ extrac-tion froma diluted brinedue to its higher hydraulic permeability to purewater (Lp = 15 L.h−1.m−2.bar−1), its higher hydraulic permeability to0.1 M NaCl solution (Lp' = 7.5 L.h−1.m−2.bar−1), its lower criticalpressure Pc (Pc = 0), its higher selectivity between monovalent ions(40%) obtained at lower operating pressure (below 15 bar) and alsoits lower roughness decreasing the propensity to be fouled. Further-more, NF90 was exhibited in the first step of the NF of Tunisian saltlake brine diluted ten times, a 100% rejection of magnesium and only15% for Li+, with a separation of 85% under low pressure (b15 bars).The diluted brine nanofiltration operation has shown after 6 h of filtra-tion a permeability of 0.7 L.h−1.m−2.bar−1with a loss of 50% of produc-tivity due tomembrane fouling. This value will help in the future to sizefull scale experiments.

In the second step nanofiltration conducted with the permeate 1,NF90 is not suitable to obtain a sufficient selective separation of Li+

in presence of Na+ (b15%).To try to improve this separation, dialysis at pressure zero is test-

ed. The dialysis results show a total separation between Li+ andNa+ with no diffusion of LiCl and a diffusion flux of NaCl of 4.4210−7 mol.s−1.m−2 at 20 °C.

Further experiments should be conducted to better explain thereal mechanism of fouling observed and also to validate the prelimi-nary results obtained in dialysis experiments.

Acknowledgments

The authors would like to thank Adel M'NIF (laboratory managerin Tunisia) for the funds supporting the work conducted in Angers(France) by A. Somrani. Lots of thanks also to the microscopy depart-ment of Angers University (SCIAM, France) and a special thanks toRomain Mallet. Thank you for the Li+ analytical apparatus from thechemical engineering dept of Angers University.

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Table 6Results of diffusion flow Js for the membranes NF90 and XLE with NaCl and LiCl electro-lytes (electrolyte concentrations in the most concentrated compartment are 0.1 mol/L).

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