1. TRACE-ION
TRACE ION METHOD FOR THE ADVANCED
CHARACTERIZATION OF NF MEMBRANES
N
Laia Llenas1, Xavier Martínez Lladó1, Miquel Rovira1, Joan DePablo1,3, Andriy Yaroshchuk2,3
Martínez-Lladó
1 Environmental Technology Area, CTM Centre Tecnològi Av Bases de Manresa 1 08242 Manresa Spain
Area ic,
ic Av. 1, Manresa,
2 ICREA; 3Department of Chemical Engineering, Polytech
Engineering hnic University of Catalonia Av Diagonal 647 08028 Barcelona, Spain
Catalonia, Av. 647, Barcelona
INTRODUCTION AND OBJECTIVES
Real waters are usually complex mixtures of predominantly ionic solutes Modelling mem
solutes. mbrane performance in electrolyte mixtures requires the knowledge of membrane transport
properties with respect to single ions In the properties obtainable from the measurements of rejection of single salts the membrane transport properties with respect to cations and
ions. s salts,
anions are “ t
i “entangled” d t th spontaneously arising electric fi ld
l d” due to the t l i i l t i fields.
The underlying observation of the approach of trace ions is that the effect of these fields on the trans membrane transfer of traces is much more directly visible than in the rejection of
n trans-membrane
single salts. O th other h d when th t
i l lt On the th hand, h the trace-ion concentrations are sufficiently l
i t ti ffi i tl low, th m b
the membrane properties are unaffected b th presence of t
ti ff t d by the f traces. Th f
Therefore, th l tt can b
the latter be
considered a kind of non-interfering probes for the studies of mechanisms of transfer of dom
gp minant ions [1].
[ ]
In [2] it was shown that the dependence of rejection of various single salts on the trans membrane volume flow can be quite well described by the solution diffusion film model
[2], s-membrane
s solution-diffusion-film model.
Moreover, it was shown that this model can easily explain the experimentally observed high rejections of sulfates from concentrated electrolyte solutions (seawater, brines) whereas the
y p p y g j y ( )
commonly used “nano-porous” model cannot Within the scope of solution-diffusion model, the reflection coefficients of all solutes are considered to be equal to one (no convective
nano porous cannot. solution diffusion
coupling between the volume and solute transfers) This makes possible a very simple description of trans membrane solute transfer and the development of efficient procedures of
transfers). trans-membrane
obtaining membrane t
bt i i b transport properties f
t ti from experimental d t
i t l data.
The objective of this work is to generalize the solution-diffusion-model to include trace ion A very simple analytical formula for the intrinsic rejection of traces vs trans-membrane
ns.
ns vs.
volume fl
l flow i obtained. A procedure i also presented t relate th i t i i rejections t the observable ones b using i f
is bt i d d is l t d to l t the intrinsic j ti to the b bl by i information on th extent of concentration polarization obtainable
ti the t t f t ti l i ti bt i bl
from the treatment of rejection of the dominant salt.
j
DATA TREATMENT PROTOCOL EQUATIONS
Q
csm
 Treat the rejection of dominant salt by means of solution-diffusion-film model: obtain the
ea e ejec o o do a sa ea s o so u o d us o ode ob a e  1 Rsobs expP s  1
1 Pe
c
membrane permeability to the salt and the thickness of unstirred layer. s
Zt  
m
 
    
1
 expPet   1 Rs expPes  1 1 1  Rtobs
ct obs dy
d

 R 
 B using th estimated value of unstirred l
By i the ti t d l f ti d layer thi k
thickness and lit t
d literature d t on th
data the
1   y
Zt
diffusion coefficients of i
diff i ffi i t f ions calculate th concentrations of th d i
l l t the t ti f the dominant salt and th
t lt d the ct 
 
exp  Pet s
obs 
1 

trace at th membrane surface and, th
t t the b f d thus, th i i t i i rejections.
their intrinsic j ti
D D JV
D
Z  Z  DD  
Dt
    Pe,t  δ - thickness of unstirred layer
ZD  ZD
s
ZD  ZD
s
 Present the two-parameter equation for the intrinsic rejection of trace ions in special Ds,t Ds
coordinates where the plot has to be linear; fit one parameter to obtain a linear
dependence and determine the other parameter from the slope
slope.   1  Rs   Z  Z P P P  P P 
Ps
 1       Pt Ps   Zt 1 P      1  Z
  1 R  
 s
Z P  Z P Z P  Z P
 t 
 Estimate the membrane permeabilities to single ions by using thermodynamic
p g y g y ln
1     ln1 Rs 
relationships.
e a o s ps  
  Pt – permeability to the trace P± – permeabilities to the ions of dominant salt
 
RESULTS AND DISCUSSION
NF270, 4750 ppm CaCl2 + 13 ppm NaCl
NF270,
NF270 5000 ppm NaCl + 50 ppm CaCl2
1
0.9
09
0.9
09
0.8
08
P 1636m s
Na . 0.7
Na,
Na intrinsic P  2.0m s
Ca
Na, observable
P 12.6m s
0.5 Na, int., model
, ,
P 13.8m s
0.7
07 Na, obs., model
ion rejec tion (-)
i on r ejec tion (-)
Na, observable
0.6
06 Cl 0.3
03 Cl
c n
Na, intrinsic
Ca, intrinsic
P  78.7 m s P  38.8m s
0.5
05 Na., t
N iint., modell
d
c
0.1 Ca, int., model
0.4 Ca,
Ca observable Na, obs.,outlyers
Ca,
Ca intrinsic
Ca -0.1 Na
0.3 Na, t
N iint., outlyers
tl
0.2
Ca, int.,
Ca int model -0.3
03 Ca,
Ca observable
0.1 -0.5
05
0 -0.7
0 20 40 60 80 100 120 140 1.6
0 20 40 60 80 100 120 0
trans-membrane volume flow (m/s) 1.4 2 trans-membrane volume flow (m/s)
y = 3E-07x + 0 7149x - 0.0013
0.7149x 0 0013 0 0.5 1 1.5 2 2.5 3 3.5
1.2
-1
1 -2
2
0.8
08
-3
P Cl  23.4m s   3.37
NaCl
N
0.6
06
0.4
P Cl  4.7m s
CaCl
C 2   8.26 -4
P 1082m s   0.715
 .
0.2
0
P  72m s

  1.66 -5 2
y 5E 07x 1.6583x
y= 5E-07x - 1.6583x+ 0.009
-6
0 0.5 1 1.5 2
CONCLUSIONS
 In pressure-driven membrane processes, spontaneously arising electric fields cause co s de ab e coupling between the
p essu e d e e b a e p ocesses, spo ta eous y a s g e ect c e ds o
onsiderable coup g bet ee t e
trans membrane
trans-membrane flows of various ions. Permeability Dominant Dominant
 These coupling phenomena can be mastered only if one knows the membrane transport properties with respect to single
p gp y p tp p p g (m/s) NaCl CaCl2
ions (and not only to salts).
( y )
PNa 163.6
163 6 38.8
38 8
 Th effects of spontaneously arising electric fi ld are especially visible i electrolyte mi t
The ff t f t l i i l t i fields i ll i ibl in l t l t mixtures consisting of a d i
i ti f dominant
t PC 78.7
78 7 2.0
20
salt and t
lt d trace i ( )
ion(s). Ca
PCl 12.6
12 6 13.8
13 8
 This can be exploited for the determination of membrane permeabilities with respect to sin
ngle ions
ions.
REFERENCES ACKNOWLEDGEMENTS
[1]. A.Yaroshchuk, V.Ribitsch, J.Membr.Sci.
[1] A Yaroshchuk V Ribitsch J Membr Sci 201 (2002) 85 94
85-94 This study was financially supported by Sociedad General de Aguas de
Barcelona (AGBAR) within th scope of CENIT project “D
B l ithi the f j t “Desarrollos
ll
[ ]
[2]. A.Yaroshchuk, X.Martínez-Lladó, L.Llenas, M.Rovira, J.de Pablo, J.Flores, P.Rubio,
a os c u , a e adó, e as, o a, J de ab o, J o es, ub o, tecnológicos hacia un ciclo del agua urbano auto-sostenible
g g
Desalination and Water Treatment accepted for publication on 20/04/2009 .
Treatment, (SOSTAQUA)
(SOSTAQUA)”.