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Nickel(II) removal by mixtures of Acorga M5640 and DP8R in
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pseudo-emulsion based hollow fiber with strip dispersion
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technology
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R.Gonzaleza,b, A.Cerpab, F.J.Alguacila,*
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a
Centro Nacional de Investigaciones Metalurgicas (CSIC Agency), Avda. Gregorio del
Amo 8, 28040 Madrid, Spain. *E-mail corresponding author: fjalgua@cenim.csic.es
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b
Universidad Europea de Madrid, Dpto. de Electromecanica y Materiales, Edificio C,
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Campus Universitario-c/Tajo s/n, Urbanización El Bosque. Villaviciosa de Odon,
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28670-Madrid, Spain
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Abstract
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This work presents the investigation of
Ni(II) extraction from aqueous solution
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through pseudo-emulsion based hollow fiber strip dispersion (PSEHFSD) containing the
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mixture Acorga M5640+DP8R/Exxsol D100 as extractant in the form of a pseudo-
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emulsion with H2SO4. The permeation of Ni(II) is investigated as a function of various
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experimental variables: hydrodynamic conditions, feed pH, extractants mixture
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concentration in the pseudo-emulsion, initial Ni(II) concentration in feed phase, H2SO4
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concentration in the pseudo-emulsion as strippant. In PEHFSD, pseudo-emulsion is an
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emulsion that is formed temporarily between organic and stripping solutions. The
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organic and aqueous strip phases are separated when the stirring device is stopped. In
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this investigation, feed was circulated through the lumen side in counter-current mode.
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The selectivity of Acorga M5640+DP8R/Exxsol D100 based PEHFSD toward different
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metals was also examined.
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Keywords: Nickel (II); Pseudo-emulsion hollow fiber strip dispersion; Acorga M5640;
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DP8R; Metal transport; Wastewater treatment
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1. Introduction
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Over the past decades, membrane technology has led to important innovations either
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in the processing and in products specially when it was refereeing to sustainable
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industrial growth.
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Membrane technology and its inherent accompanying science has been demonstrate in
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a wide range of applications including: metal ion extraction, wastewater treatment,
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fermentation, pharmaceuticals, etc. (Pabby et al., 2008).
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Including in this science and technology, liquid membranes (LMs) have offered their
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potentials and among them, supported liquid membranes (SLMs), in whatever its
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configuration, are constantly under theoretical and practical consideration. Thus, the
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removal of metals from dilute or even concentrate solutions, using this technology, has
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received a considerable attention for the recovery of valuable metals or decontamination
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of liquid effluents.
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Though LMs and SLMs variations offered their attraction, some of the problems
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encountered in their use, specially their apparent lack of stability, had led to a less
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frequent scaling-up of such membrane processing. Thus, modifications and
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improvements are needed continuously, specially when refereed to find a more stable
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technology. In this context, non-dispersive solvent extraction (NDSX) (Kumar et al,
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2002), supported liquid membrane with strip dispersion (SLMSD) (Ho, 2003), and
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hollow fiber renewal liquid membrane (HFRLM) (Ren et al., 2008) had been developed
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in order to find a suitable technology for practical use. All the three above can be
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operated using a hollow fiber contactor which gives a high surface area to volume ratio,
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and whereas stripping is performed in NDSX in a second module, both SLMSD and
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HFRLM perform the stripping in a single hollow fiber contactor.
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Being nickel a metal widely used in the industry and currently reaching elevated
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prices, it is also one of the metals which appeared in liquid streams. In surface waters,
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this element arises from runoff from soil and tailing pipes, from atmospheric deposition
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and from landfill leachates. Industrial and municipal wastewater is another important
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source of nickel in surface waters. Leaching of nickel from soil into groundwater
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accounts for much of the element found in these waters being this process probably
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accelerated in regions in which acid precipitation occurs. Nickel absorption for humans
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is higher when the element is present in drinking water as opposed to food, and though
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there is not an apparent EPA-mandated legal limit of amount of nickel in drinking
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water, the recommendation of the agency for the maximum contaminated level (MCL)
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is 0.1 mg Ni per liter of drinking water. The toxicological character of nickel to humans
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is that its ingestion is consistently associated, among others, with lung and
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nasopharyngeal cancer (Sutherland and Costa, 2002).
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Various technologies were used to remove nickel from aqueous solutions, i.e. solvent
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extraction, LMs, and ion exchange resins (Algarra et al., 2005; Alguacil et al., 2006;
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Amini et al., 2009; Argun, 2008; Bukhari, et al., 2006; Cerpa and Alguacil, 2004;
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Cheng, 2006; Dabrowski et al., 2004; Dimitrov et al., 2008; Egorov et al., 2010; Jung et
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al., 2008; Kandah and Meunier, 2007; Molinari et al., 2008; Ortiz et al, 2001; Reddy
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and Priya, 2006; Tanaka et al, 2008; Van der Voorde et al., 2004; Wilson et al., 2006;
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Yang and Cussler, 2000).
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The prime objective of this investigation is to study Ni(II)/Acorga M5640+DP8R in
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relation to several experimental variables: hydrodynamic conditions, pH of feed,
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extractants Acorga M5640 and DP8R concentrations in Exxsol D100, Ni(II)
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concentration in feed solution, etc., by pseudo-emulsion based hollow fiber strip
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dispersion technology using a single module for extraction as well as stripping. In
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addition mass transfer parameters were estimated and the selectivity of Ni(II) against
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metals such as Co(II), Fe(III) and Cr(III) was also evaluated.
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2. Experimental
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2.1. Materials
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The active substance of DP8R is di(2-ethylhexyl) phosphoric acid, whereas the active
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substance of Acorga M5640 is 2-hydroxy-5-nonylbenzaldehyde oxime to which a fatty
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ester is added as a modifier.
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DP8R and Acorga M5640 extractants were supplied by Daihachi and Avecia,
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respectively. Both reagents were used as supplied by the manufacturers as also did with
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Exssol D100 diluent (ExxonMobil) containing >99% aliphatics.
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All other chemicals used in the present study were of AR grade.
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2.2. Hollow fiber apparatus used for PEHFSD, preparation and methods
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The hollow fiber device (Liqui-cel 8x28 cm 5PCG-259 LLE contactor and 5PCS-
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1002 Liqui-Cel LLE unit) used for experimentation was a commercially available unit
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from Celgard (now Membrana). The module details and hollow fiber membrane
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characteristics were given elsewhere (Kumar et al., 2005).
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The hollow fiber strip dispersion process comprises one membrane module for
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extraction and stripping, one stirred tank for preparing a pseudo-emulsion of Acorga
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M5640+DP-8R/Exxsol D100 and H2SO4, and a supplementary second tank which
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contained the feed solution homogenized by gentle stir. The experimental set-up for the
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separation of the metal consists of two gear pumps of varying flows for both phases.
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The organic phase wet the porous wall of the fiber because of its hydrophobic nature.
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The interface was maintained at the pore by applying a higher pressure to the feed phase
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than to the pseudo-emulsion solution, thus, in the feed phase the pressure was
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maintained 0.2 bar higher than in the pseudo-emulsion phase. The differential pressure
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was always kept below the breakthrough pressure.
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The view of PEHFSD using a single hollow fiber contactor in the recirculation mode
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is shown in Figure 1. The PEHFSD operation was carried out by passing the feed
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solution containing nickel (II) through the tube side and the pseudo-emulsion through
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the shell side in counter-current mode. The pseudo-emulsion was prepared by mixing in
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the corresponding tank, the organic and strip solutions, being the latter dispersed into
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the organic phase. The stirring rate in this tank was kept below 500 rpm, this stirring
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speed was fixed as best suited for this experimentation. The character of the pseudo-
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emulsion may be such that it should have clear and fast phase separation (organic and
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strippant phases) when the mixing is stopped. The recovery of nickel from pseudo-
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emulsion can be accomplished (pseudo-emulsion breaks down in about 30 seconds after
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the mixing of phases stopped) and strip and organic phases separated automatically.
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The volume of pseudo-emulsion phase is 800 cm3 (400 cm3 of Acorga
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M5640+DP8R/Exxsol D100 and 400 cm3 of H2SO4 solution); 3000 cm3 of feed solution
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of the desired Ni(II) concentration was prepared by taking a suitable aliquot of the
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nickel sulphate stock solution. Further, the desired feed pH was adjusted and controlled
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by adding small aliquots of H2SO4 or NaOH solution.
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At predetermined time, aliquots of the aqueous phases were taken and analyzed for
metal concentration by AAS.
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3. Results and discussion
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For extraction of Ni(II) by PEHFSD containing DP8R (HL) and Acorga M5640 (HR)
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as mobile carrier, the concentration profile across the hollow fiber membrane pores in
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the contactor is schematically shown in Figure 2. The nickel (II) ions in the aqueous
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solution complex with the extractants dissolved in the suitable diluent.
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The extraction equilibrium is expressed as (Alguacil, 2002):
2

Ni aq
 pH 2 L 2 org  qHR ) org  NiL 2 p R q H 2 p  q  2 org  2H aq
(1)
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which is related to a cation exchange reaction, and where p can be 2 or 4 depending of
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the extractant concentration and q is 2. The extracted complex then diffuses through the
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pore of the hollow fiber toward the membrane pseudo-emulsion interface, where nickel
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is stripped in the pseudo-emulsion phase after coming into contact with H2SO4. This
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reaction, in which the above equilibrium is shifted to the left, is fast and instantaneous.
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At the same time, the carrier is regenerated and diffuses back through the hollow fiber,
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after which the process is repeated. Such a process is called counter-transport, where the
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difference in H+ concentration between the feed and the strip solutions is used as the
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driving force for nickel transport.
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In the case of PEHFSD, calculation of the overall permeability coefficient of the
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experimental system is based on a first-order mass transfer equation with instantaneous
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chemical reaction in the stripping side when a recycling mode is operated.
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The transport of nickel across the liquid membrane can be described by considering
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only diffusional parameters. The interfacial transport of Ni(II) due to the chemical
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reaction has been neglected as the chemical reactions seem to take place at the feed
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solution-membrane and membrane-pseudo-emulsion phase interfaces, and it had been
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previously suggested that chemical reactions occur simultaneously relative to the
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diffusion process (Zuo et al, 1996; Alguacil and Alonso, 2003; Juang and Huang, 2003).
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The transport of Ni(II) in a hollow fiber membrane system operating in the recycling
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mode can be explain by consideration of equations which described i) the change of
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metal concentration in the feed and stripping streams when are circulating through the
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hollow fiber module and ii) the change of metal concentration in the feed and stripping
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tanks when the aqueous solutions are continuously recirculated. Also, linear
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concentration gradient and absence of back-mixing are assumed.
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Accordingly (Kumar et al, 2000), experimental results can thus be fitted to a firstorder kinetic law:
 VFEED ln
Nit
Ni0
 S t
(2)
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where S is a factor dependent on the geometry of the fibers and the module, the linear
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velocity of the fluids, and the overall permeability of the system. This last value, PNi,
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can be obtained by the following relation:
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PNi 
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S  ri  v FEED
2  r  L  N  v FEED  S  L  
(3)
2
i
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where N, L, ri, ε (membrane porosity) and vFEED are known values dependent of the
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module characteristics (N, L, ri, ε), and vFEED is the experimental feed linear flow
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velocity.
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The separation-concentration of nickel using the hollow fiber module and overall
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permeation coefficient center on three mass transfer resistances. One occurs in the
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liquid flowing through the hollow fiber lumen. The second correspond to the nickel-
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complex diffusion across the liquid membrane immobilized in the porous wall of the
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fiber. The third resistance is due to the aqueous interface created on the outside of the
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fiber.
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The reciprocal of the overall permeability coefficient is given by:
r 1 ri 1
1
1
  i 
 
PNi k i rlm Pm ro k o
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(4)
The membrane permeability is related to the partition coefficient of nickel (DNi) with
the mixture Acorga M5640+DP8R via eq. (1):
   H L   HR 
Pm  D Ni  k m  K ext  H 
2
aq
2
p
2 org
q
org
 km
(5)
where DNi is defined as:
D Ni 
NiL
2p

R q H 2 p  q 2  org
Ni 
2
(6)
aq
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Inserting eq.(5) in eq.(4), and if the reaction is instantaneous on the stripping side, the
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contribution of the outer phase is removed from eq. (4) and:
 
2
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H  aq
ri
1
1



p
q
PNi k i rlm k m  K ext  H 2 L 2 org
 HR org
(7)
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3.1. Influence of flow rate on nickel transport
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Table 1 showed the effect of different feed flows for testing 0.01 g L-1 concentrations
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of Ni(II) using 10% v/v DP8R+10% v/v Acorga M5640/Exxsol D100. PNi values
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increased with increasing the flow of the feed phase up to 280 cm3 min-1 and further
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decreased at a flow rate of 360 cm3 min-1. As expected, PNi first increased with flow and
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then decreased. The increase of PNi with the flow is caused by a decrease of the
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thickness of the aqueous boundary layer when the flow in the fiber lumen increased,
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whereas the decrease in PNi value could be lower residence time at higher flow rate,
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which provides insufficient time to complex Ni(II) with the carriers. This resulted in
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incomplete loading of the extractants with nickel, which finally contributed to the lower
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value of PNi. In this experimental study, it was necessary to maintain the interface in the
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pore of the fiber mouth. To avoid the problem of the contamination of feed phase by the
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pseudo-emulsion at the highest flow rate, the 280 cm3 min-1 flow rate is selected for
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further investigations. The importance of pressure control across the tube sides of the
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contactor is described elsewhere (Sonawane et al., 2008).
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3.2. Influence of H2SO4 concentration in the strip phase on metal transport
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To study the influence of the sulphuric acid concentration in the stripping solution on
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the transport of nickel, a series of experiments were conducted using feed phase of 0.01
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g L-1 Ni(II) at pH 4.0±0.1, an organic solution of 10% v/v DP-8R+10% v/v Acorga
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M5640/Exxsol D100 and strip phases containing 0.5-2 M H2SO4. Results obtained
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indicated that the variation in the H2SO4 concentration in the strip phase has no
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influence on metal permeation, with PNi values very close to 4.4x10-4 cm s-1. On the
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other hand, the variation of the H2SO4 concentration in the strip phase has also a minor
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effect on the percentage of nickel recovered in this phase.
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3.3. Influence of initial metal concentration on Ni(II) transport
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Studying the effect of the initial concentration of nickel (II) (0.01-3.6 g L-1) in the feed
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phase, when the stripping solution contains no nickel, it was revealed that the metal
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permeation continuously decreased as the initial metal concentration increased in the
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feed phase. The results of nickel permeation through the fibers as a function of the
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initial metal concentration are shown in Figure 3, indicating that the concentration limit
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for the proper function of the technology is below 0.4 g L-1 Ni(II) in the feed phase..
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This behaviour can be related to that within the present range of nickel concentrations
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used, the transport process is controlled by diffusion of nickel species. The interfacial
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transport of Ni(II) seem to take place at the feed solution/fiber and fiber/pseudo-
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emulsion interfaces. As it was mentioned above, chemical reactions occur at the same
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time to the diffusion process. Thus, the nickel(II) transport rate is determined by the
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diffusion rate of nickel that contains species across the aqueous feed diffusion layer and
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by the diffusion rate of Ni(II)/Acorga M5640+DP8R complexes through the fibers. The
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decrease in transport at higher nickel concentrations in the feed solution could be further
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enhanced either by increasing the surface area of the contactor or running the operation
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for longer time.
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3.4. Effect of feed phase pH on the transport of nickel (II)
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In the present PEHFSD system, the pH gradient between the feed and the stripping
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solutions is the driving force for the transport of metal ion. To asses the role of this
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variable, pH variation studies in the range 2.0-5-0±0.1 were carried out. The stripping
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solution consisted of 1 M H2SO4, whereas the concentration of the organic solution was
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10% v/v DP8R+10% v/v Acorga M5640/Exxsol D100. It is evident from Table 2 that
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the metal permeability increases with an increase in pH from 2.0 to 4.0, though at
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higher pH it remained unaffected. Thus, it is considered that the pH value of 4.0 may be
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optimum for nickel permeation. Moreover, at low pH values the diffusion of the carrier
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mixture through the fibers become dominant. Two limiting cases can be considered:
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i) at low pH:
2
PNi 
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if p=q=2 in eq. (1),
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ii) at high pH:
 
rlm  k m  K ext  H 2 L 2 org  HR org  H 
2
2
aq
(8)
ri
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PNi  k i
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and the permeability is independent of pH. In this case, PNi = ki = 4.4x10-4 cm s-1.
(9)
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3.5. Influence of the extractant mixture concentrations on nickel (II) transport
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The influence of the organic phase composition on nickel transport was studied using
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different extractants concentrations in the organic solution of Exxsol D100. Table 3
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shows metal permeabilities values for the transport of nickel through the hollow fiber
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module with solutions 1+1 to 20+20 % v/v DP8R+Acorga M5640 in the diluent. In the
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absence of extractants in the organic phase, no transport of nickel occurred. As can be
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seen, the permeability increased with the initial extractants mixture concentrations, thus,
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it can be considered that in the transport process, permeability is controlled by
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membrane diffusion. At extractants concentrations in the 5+5 % v/v range a maximum
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in permeability is obtained. This maximum or limiting permeability is explained by
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assuming that diffusion in the fibers is negligible and the permeation process is then
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controlled by diffusion in the stagnant film of the feed phase. In this condition, it can be
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assumed:
Plim 
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D aq
d aq
(10)
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where Daq is the aqueous diffusion coefficient of the nickel-containing species (in the
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10-5 cm2 s-1 order) and Plim = 5.7x10-4 cm s-1, then the overall thickness of the aqueous
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film (daq) estimated from the above equation is 1.8x10-2 cm. This value can be
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considered as the minimum thickness of the feed diffusion layer under the present
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experimental conditions. At higher extractant concentrations the decrease of
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permeability can be explained in terms of the increase in the organic solution viscosity
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that increases membrane (fibers) resistance.
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3.6. Selectivity
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A series of experiments were conducted to investigate the selectivity of nickel in order
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to examine the effect of several metal ions generally accompanying Ni(II), and their
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interferences with the overall permeation of this element. Two types of synthetic
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solutions were used in this study; in one case (Solution I) the metal accompanying to
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nickel was cobalt (II), being this case representative of the cobalt-nickel mining
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operation. In the second case (Solution II), the metals accompanying to nickel were iron
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(III) and chromium (III), and this case being representative of rinse waters from
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stainless steel piclinkg.
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Table 4 summarizes the results obtained in the first case and in sulphate medium (Cerpa
280
and Alguacil, 2004), it can be seen that the two metals were extracted, and even cobalt
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transport is a little better than that of nickel.
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In the second case, nitrate-fluoride media (Lobo-Recio et al., 2004), the same Table 4
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showed the results obtained from this investigation; here, the transport order
284
Fe(III)>Cr(III)>Ni(II) was encountered.
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From the results above and the calculations of the separation factors as:
SF 
286
PNi
PMetal
(11)
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and given in the same Table 4, it is concluded that the present system presented a poor
288
selectivity against the presence of other accompanying metals in the feed solution,
289
though still is useful to remove nickel from the above solutions.
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3.7. Continuous cycles in PEHFSD
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This experimentation was carried out using 18 L of 0.01 g L-1 Ni(II) at pH 4.0±0.1 as
293
a feed (linear flow velocity of 1.03 cm s-1) and a pseudo-emulsion formed by 1 M
294
H2SO4 and 10+10 % v/v DP8R+Acorga M5640/Exxsol D100 to evaluate concentration
295
factor, defined as the ratio of the final concentration of Ni(II) in the stripping phase to
296
the initial concentration in the feed solution under similar experimental conditions. In
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this investigation, feed was continuously replaced with fresh feed without changing
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pseudo-emulsion. Thus, metal was allowed to concentrate in pseudo-emulsion in
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recirculation mode (linear flow velocity 0.05 cm s-1). After passing 18 l, mixing unit
300
was stopped and pseudo-emulsion was allowed to settle for few minutes. Finally, strip
301
solution was checked for nickel concentration. This was around 25 times the initial
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metal concentration in the feed. The total time taken for this experiment was 6 hours.
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The feasibility of recovering Ni(II) with PEHFSD using DP8R+Acorga M5640 mixture
304
in Exxsol D100 as a liquid membrane was thus proven.
305
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3.8. Estimation of the mass transfer coefficients
307
From eq. (7) it can be seen that the overall permeation coefficient is a function of the
308
individual transfer coefficients ki and km. The tube mass transfer coefficient (ki) is
309
known to depend on the flow conditions in the fiber tube. For this coefficient the
310
following correlation was given (Kumar and Sastre, 2008):
1
311
D aq  d i2  v FEED  3


k i  1.64
d i  L  D aq 
(12)
312
Thus, being Daq= 10-5 cm2 s-1, di= 24x10-3 cm, vFEED= 1.03 cm s-1, and L= 15 cm, the
313
value of ki for the present system is estimated as 1.1x10-3 cm s-1. The variation in the
314
value of ki estimated from eqs. (11) and (9) and (10), in this two last cases the value of
315
ki is very close, may be due to that eq. (11) overestimates experimentally determined
316
mass transfer coefficients at low flows; this can be attributed to non-uniform flow
317
caused by polydispersity in hollow fiber diameter (Wickramasinghe et al., 1992). Also,
318
tube side flow distribution not always is uniform (Park and Chang, 1986).
319
320
321
The membrane mass transfer coefficient can be determined from the following
expression (Kumar et al., 2004):
km 
Dm  
  d o  d i / 2
(13)
322
For the system considered here, the membrane tortuosity (τ)= 3, ε= 0.3, do and di=
323
30x10-3 and 24x10-3 cm, respectively, and the diffusion coefficient of the nickel
15
324
complex (es) in the membrane (Dm) in the 10-6 cm2 s-1 order. Thus, km was about 3.3x10-
325
5
326
327
cm s-1.
On the other hand, the effective diffusion coefficient (Deff) of nickel complex(es)
through the organic membrane phase is defined as follows:
D eff  k m  d org  
328
329
(14)
where dorg is the membrane thickness of 3x10-3 cm. Then, Deff is about 3x10-7 cm2 s-1.
330
331
4. Conclusions
332
Pseudo-emulsion based hollow fiber strip dispersion inverstigations were carried out
333
with a unique module for simultaneous extraction and stripping in counter-current
334
operation. From the experimental study it is concluded that using the initial conditions
335
of i.e. pseudo-emulsion formed by mixing 5 % v/v + 5 % v/v DP8R+Acorga M5640 in
336
Exsol D100 and 1 M H2SO4 and maintaining a linear flow velocity in the feed phase of
337
1.03 cm s-1, are suitable for the efficient extraction and concentration of nickel (II)
338
under optimum conditions, i.e. initial Ni(II) concentration in the feed solution below 0.4
339
g L-1. For concentration of the extractants of 5% v/v + 5 % v/v DP8R+Acorga M5640 a
340
limiting value of 5.7x10-4 cm s-1 for permeability is obtained, and the transport process
341
is controlled by the diffusion of the feed film, as occurred at high pH values in the feed
342
phase. However, it is also apparent that the role of membrane diffusion becomes
343
dominant under the conditions of low feed pH and low carriers mixture concentrations
344
in the organic phase. The stability of PEHFSD was found to be good enough, thus, this
345
technology is a challenge to conventional procedures for the recovery of Ni(II) from
346
liquid effluents, however, Co(II), Fe(III) and Cr(III) are co-transported with Ni(II), if
347
present in the feed solution.
16
348
Acknowledgements
349
To CSIC Agency (Spain) for support.
350
351
Nomenclature (not described in text)
352
DNi
partition coefficient of nickel
353
ki (cm s-1)
interfacial coefficient corresponding to the inner aqueous boundary layer
354
ko (cm s-1)
interfacial coefficient corresponding to the outer aqueous boundary layer
355
km (cm s-1)
membrane mass transfer coefficient
356
Kext
extraction equilibrium constant
357
L (cm)
fiber length
358
N
number of fibers in the contactor
359
[Ni]t/[Ni]0
nickel concentrations in the feed solution at an elapsed time/time zero
360
Pm (cm s-1)
membrane permeability
361
ri (cm)
inner radius of fiber
362
rlm (cm)
hollow fiber log mean radius
363
ro (cm)
outer radius of fiber
364
t (s)
time
365
VFEED (cm3)
volume of feed solution
366
367
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