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Uphill permeation of chromium (VI) using Cyanex 921 as ionophore across an immobilized liquid membrane
Hydrometallurgy 61 Ž2001. 13–19 www.elsevier.nlrlocaterhydromet
Uphill permeation of chromium žVI/ using Cyanex 921 as ionophore across an immobilized liquid membrane F.J. Alguacil a,) , A.G. Coedo a , M.T. Dorado a , A.M. Sastre b a
(CSIC), AÕda. Gregorio del Amo 8, Ciudad UniÕersitaria, 28040 Madrid, Spain Centro Nacional de InÕestigaciones Metalurgicas ´ Department of Chemical Engineering, ETSEIB, UniÕersitat Politecnica de Catalunya, Diagonal 647, E-08028 Barcelona, Spain `
b
Received 20 September 2000; received in revised form 10 January 2001; accepted 11 January 2001
Abstract The transport of chromium ŽVI. through an immobilized liquid membrane ŽILM. containing Cyanex 921 Žphosphine oxide. as a carrier has been studied. The permeation of the metal is investigated as a function of various experimental variables: hydrodynamic conditions, concentrations of chromium ŽVI. and HCl in the source phase, carrier concentration and diluent in the membrane, strippant in the receiving phase and support characteristics. The mass transfer coefficient and the thickness of the aqueous boundary layer were calculated from the experimental data. Furthermore, the selectivity of Cyanex 921-based ILM towards different metal ions and the behaviour of the system against other carriers are presented. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Chromium ŽVI. transport; Immobilized liquid membrane; Cyanex 921
1. Introduction Despite its high toxicity, chromium ŽVI. has been used widely in different industrial processes Ži.e. electroplating., its recovery from the corresponding wastewaters being a primary target before their discharge to natural waters. Several techniques have been developed to remove andror recover chromium ŽVI. from these industrial processes w1x. Among them, supported liquid membrane ŽSLM. technology could be competitive when the metal is present at low concentrations in the aqueous solution. In SLMs,
)
Corresponding author. E-mail addresses: [email protected] ŽF.J. Alguacil., [email protected] ŽA.M. Sastre..
both the advantages of solvent extraction and membrane separation processes are combined, giving simultaneous separation, concentration and purification of metallic species from solutions. An SLM is expected to be one of the most efficient membranes for separation processes, since facilitated transport is more effective than passive transport w2x. Among carriers used for chromium ŽVI. transport, quaternary ammonium salts have been most extensively studied w1,3–10x, and only recently has the use of a phosphine oxide been reported w11x. In the present work, results obtained for the transport of chromium ŽVI. using the phosphine oxide, Cyanex 921 as carrier, are presented. Several variables, which could affect the permeation process: stirring speed of the source and receiving phases, metal and carrier concentrations, diluent of the or-
0304-386Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X Ž 0 1 . 0 0 1 4 7 - 5
F.J. Alguacil et al.r Hydrometallurgy 61 (2001) 13–19
14
ganic phase, etc., are studied. The selectivity of the system against other metallic species and the behaviour of Cyanex 921 vs. other carriers are also reported.
concentrations of chromium in the source phase at elapsed time t and time zero, respectively. For the calculations of the chromium metal-accompanying permeabilities, the same Eq. Ž1. was used. 2.2. Membrane support
2. Experimental The phosphine oxide Cyanex 921 was obtained from Cytec ŽCanada. and was used without further purification, the active substance of the extractant being tri-n-octylphosphine oxide Žabout 99%. w12x. Extractants, TBP ŽFluka., DBBP ŽAlbright and Wilson., Cyanex 923 and Cyanex 471X ŽCytec., and Alamine 304 ŽCognis., were also used without further purification. The chloride salt of the tertiary amine, Alamine 304, was synthesized according to the procedure described in the literature w13x. The analytical grade diluents xylene, toluene and cumene ŽFluka. were used as such. Stock CrŽVI. solutions were prepared by dissolving K 2 Cr2 O 7 ŽMerck. in distilled water. All other chemicals were of AR grade. 2.1. ILM preparation and measurements The characteristics of the cell used in the present investigation were similar to those described in a previous work w11x. The source and receiving phases were mechanically stirred at 1600 and 1500 miny1 , respectively, at 25 " 18C to avoid concentration polarization conditions at the membrane interfaces and in the bulk of the solutions. Membrane permeabilities were determined by monitoring concentration by AAS in the source phase as a function of time, except in the case of the mixture containing CrŽVI.r CrŽIII. which was analysed by ICP-MS, previously labelling chromium ŽIII. with 58 Cr isotope. The chromium concentration in the various phases was found to be reproducible within "3%. The permeation coefficient Ž P . was computed using: ln
w Cr x t A s y Pt w Crx 0 V
Ž 1.
where A is the effective membrane area, V is the volume of the source phase, wCrx t and wCrx0 are the
The organic membrane phase was prepared by dissolving Cyanex 921 in the organic diluent to obtain carrier solution of various concentrations. The support was impregnated with the corresponding organic solutions by immersion for 24 h, then left to drip for a few seconds before being placed in the cell. The physical characteristics of the supports used in the present work can be obtained elsewhere w11x.
3. Results and discussion 3.1. Influence of the stirring speed of the source solution Previous experiments were carried out to establish adequate hydrodynamic conditions. The permeability of the membrane was studied as a function of the stirring speed on the source solution side. The agitation of the receiving solution was kept constant at 1500 miny1 . Constant permeability for stirring speeds higher than 1400 miny1 was obtained ŽTable 1.. Consequently, the thickness of the aqueous diffusion layer and the aqueous resistance to mass transfer were minimized and the diffusion contribution of the aqueous species to mass transfer process is assumed to be constant.
Table 1 Influence of the stirring speed in chromium ŽVI. permeation Speed Žminy1 .
P Žmmrs.
600 800 1000 1200 1400 1600
33.5 34.9 37.7 41.6 45.3 45.7
Source phase: 1.92P10y1 mM CrŽVI., 0.5 M HCl. Membrane phase: 30% wrv Cyanex 921 in xylene on Durapore GVHP 4700 support. Receiving phase: 0.5 M NaCl.
F.J. Alguacil et al.r Hydrometallurgy 61 (2001) 13–19
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3.2. Influence of the stirring speed and stripping agent of the receiÕing solution
Table 3 Influence of HCl concentration and ionic strength on CrŽVI. permeation
The effect of varying the stirring speed in the receiving solution was also investigated using the same phases as described in Table 1, and maintaining a 1600 miny1 constant speed in the source solution. Results obtained show that the variation of the stirring speed Ž500–1500 miny1 . in the receiving phase does not affect chromium permeability Ž Paverage s 45.4 " 0.7 mmrs.. Different aqueous solutions were studied as stripping agents. The results are given in Table 2, the higher permeability coefficient is obtained when a 0.1 M NaCl solution is used as strippant, whereas NaCl solution seems to be more effective than HCl solution to strip chromium ŽVI.. As a result of the previous experiments, 0.1 M NaCl was used as the stripping agent.
HCl
P Žmmrs.
y3
10 M 10y2 M 10y1 M 0.5 M 1M 10y3 M a
9.5 67.4 224.0 45.7 6.12 9.5
Source phase: 1.92P10y1 mM Cr ŽVI. at different HCl concentrations. Membrane phase: 30% wrv Cyanex 921 in xylene on Durapore GVHP 4700 support. a With 0.1 M NaCl.
tration in the source phase was fixed in the subsequent experiments. 3.4. Influence of the diluent
3.3. Influence of HCl concentration and ionic strength in the source phase In order to study the significance of the role of the HCl concentration in the source phase solution during the permeation of chromium, HCl variation concentration studies in the range 10y3 –1 M were carried out, the receiving phase being 0.1 M NaCl. As seen in Table 3, permeability of chromium ŽVI. increases with increasing HCl concentration up to 0.1 M, and then decreases with increasing aqueous acidity. At a fixed HCl concentration, the increase of the ionic strength Žby NaCl addition. has no effect on chromium transport. Thus, a 0.1 M HCl concen-
As seen from Fig. 1, the permeability of chromium ŽVI. depends strongly on the nature of the organic diluent. Removal of CrŽVI. from the source side is more effective when xylene is used as the diluent for Cyanex 921. The low solubility of Cyanex 921 in aliphatic diluents Ži.e. kerosene type. prevented its use in the present study. 3.5. Influence of the support characteristics on the chromium flux Two solid supports, with different characteristics w11x, were used to study their effect on chromium transport. The flux values: J s Cr Ž VI .
Table 2 Influence of the composition of the receiving phase on CrŽVI. transport Receiving phase
P Žmmrs.
0.01 M NaCl 0.1 M NaCl 0.5 M NaCl 0.1 M HCl
37.6 57.7 45.7 34.6 y1
Source phase: 1.92P10 mM CrŽVI., 0.5 M HCl. Membrane phase: 30% wrv Cyanex 921 in xylene on Durapore GVHP 4700 support.
TOT P
Ž 2.
are given in Table 4, and the best flux value was obtained when Durapore GVHP 4700 was used as the support because of its higher performance expressed as: ´ Ž 3. do t However, it is possible to correct the influence of the different physical properties of the supports studied by the use of an expression w14,15x in which the flux values Ž J N . were normalized to the physical charac-
F.J. Alguacil et al.r Hydrometallurgy 61 (2001) 13–19
16
Fig. 1. Effect of various diluents on permeability of CrŽVI.. Source phase: 0.192 mM CrŽVI., 0.1 M HCl. Membrane phase: 15% wrv Cyanex 921 in each diluent on Durapore GVHP 4700 support. Receiving phase: 0.1 M NaCl.
teristics of the Durapore support, and nearly the same flux values were obtained for both supports ŽTable 4.. 3.6. Influence of initial metal concentration Fig. 2 shows a plot of the initial chromium ŽVI. flux Ž J . vs. the concentration of chromium in the source phase. At low metal concentrations, the initial flux is a function of the initial concentration in the source phase. Thus, the permeation process is con-
Table 4 Influence of the support used in the membrane phase on the chromium transport Support
J Žmolrm2 rs.
J N Žmolrm2 rs.
Durapore GVHP 4700 Fluoropore FGLP 4700
4.3P10y5 2.4P10y5
4.3P10y5 4.0P10y5
Source phase: 1.92P10y1 mM CrŽVI., 0.1 M HCl. Membrane phase: 30% wrv Cyanex 921 in xylene on each support. Receiving phase: 0.1 M NaCl.
trolled by diffusion of metal species in the lower range of chromium concentrations; however, beyond a certain limiting concentration, the flux approaches a maximum and becomes independent of metal concentration, one probable reason for this may be the rate-determining step for the transport process. 3.7. EÕaluation of limiting permeability (Pl i m ) The influence of different Cyanex 921 concentrations on permeability was studied in the range 5–30% wrv. Results obtained are shown in Fig. 3; it can be seen that at higher carrier concentrations, P is independent of Cyanex 921 concentration and this region is representative of an aqueous diffusion film-controlled permeation process. This constant permeability value Plim , known as limiting permeability, can be expressed as: P lim s
1 s D aq
Daq d aq
Ž 4.
F.J. Alguacil et al.r Hydrometallurgy 61 (2001) 13–19
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Fig. 2. The influence of initial concentration of CrŽVI. on the permeability flux Ž J . of the metal. Source phase: various CrŽVI. concentrations, 0.1 M HCl. Membrane phase: 30% wrv Cyanex 921 in xylene on Durapore GVHP 4700 support. Receiving phase: 0.1 M NaCl.
Assuming the value of Daq as 10y9 m2rs w11x and P lim as 224 mmrs, the value of the thickness of the aqueous boundary layer is estimated to be 4.5 mmrs. This value is the minimum thickness of the stagnant aqueous diffusion layer in the present experimental conditions. 3.8. SelectiÕe transport of Cr(VI) Õs. Mn(VII), Fe(III), Cr(III), Cu(II), Zn(II) Because chromium is widely used in a number of industries, the corresponding wastewaters may contain a wide range of concentrations of chromium ŽVI. and ŽIII. and other heavy metals, i.e. coating plants ŽCrŽVI. 0.005–5.0 mgrL, iron 0.41–170 mgrL., chemical milling and etching ŽCrŽVI. 0.005–335 mgrL, copper 0.21–270 mgrL, zinc 0.112–200 mgrL, iron 0.0075–260 mgrL., printed board industry ŽCrŽVI. 0.004–3.54 mgrL, copper 1.6–540 mgrL, nickel 0.027–9 mgrL, lead 0.044– 10 mgrL., anodizing plants Žtotal chromium 0.27–80 mgrL, CrŽVI. 0.005–5.0 mgrL..
Table 5 shows results obtained in the transport of chromium ŽVI. when the source phase also contained other accompanying metals. A highly selective transport of chromium ŽVI. is obtained within the experiments that allow recovery of high purity CrŽVI. from the receiving solution. However, there is an influence on chromium ŽVI. transport when these metals are present in the source solution; thus, chromium ŽVI. permeation is negatively influenced within the apparent order: CuŽII., ZnŽII. ) NiŽII., FeŽIII., CrŽIII. ) MnŽVII., despite that some of the metals were not detected in the receiving phase Ži.e. zinc ŽII., copper ŽII., nickel ŽII. and iron ŽIII.. or are slightly transported Žchromium ŽIII., manganese ŽVII... Such behaviour may be explained in various ways: a negative salting-out effect, formation of metal-chlorocomplexes which also decreases the initial acidity of the source phase, or to the main metal transport which needs an induction period and decreases the nominal carrier concentration in the membrane phase Ži.e. iron ŽIII. is transported through an ILM impregnated with Cyanex 921 w16x..
F.J. Alguacil et al.r Hydrometallurgy 61 (2001) 13–19
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Fig. 3. The influence of Cyanex 921 concentration on permeability of CrŽVI.. Source phase: 0.192 mM CrŽVI., 0.1 M HCl. Membrane phase: Cyanex 921 in xylene on Durapore GVHP 4700 support. Receiving phase: 0.1 M NaCl.
3.9. BehaÕiour of the Cyanex 921 carrier system compared to other potential chromium (VI) carriers The transport of chromium ŽVI. was also studied using other carriers to compare with results obtained using Cyanex 921. The carriers investigated were: TBP Žphosphoric ester., DBBP Žphosphonic ester.,
Table 5 Chromium ŽVI. transport in presence of different metallic ions Žmmrs.
System
PCr
CrŽVI. CrŽVI. –MnŽVII. CrŽVI. –CrŽIII. CrŽVI. –FeŽIII. CrŽVI. –NiŽII. CrŽVI. –CuŽII. CrŽVI. –ZnŽII.
224.0 169.2 136.0 146.1 122.0 104.3 93.5
Remark PMn -6 mmrs PCr - 5 mmrs No Fe transport No Ni transport No Cu transport No Zn transport
Source phase: 0.01 grL CrŽVI. and 0.01 grL of each metal, 0.1 M HCl. Membrane phase: 30% wrv Cyanex 921 in xylene on Durapore GVHP 4700 support. Receiving phase: 0.1 M NaCl.
Cyanex 471X Žphosphine sulphide., Cyanex 923 Žmixture of phosphine oxides. and the chloride salt of Alamine 304 Žquaternary ammonium salt.. Results are summarized in Table 6, showing the best and comparable chromium ŽVI. permeabilities when phosphine oxides are used as carriers. Only slight metal transport is obtained when TBP, DBBP
Table 6 Chromium ŽVI. transport using different carriers Extractant
P Žmmrs.
Cyanex 921 Cyanex 923 Alamine 304 salt TBP Cyanex 471X DBBP
224.0 180.3 120.2 17.5 13.6 7.1
Source phase: 1.92P10y1 mM CrŽVI., 0.1 M HCl. Membrane phase: 0.77 M of each extractant in xylene on Durapore GVHP 4700 support. Receiving phase: 0.1 M NaCl.
F.J. Alguacil et al.r Hydrometallurgy 61 (2001) 13–19
and Cyanex 471X are used, which can be interpreted in terms of the lower donor properties of the active substance of the reagents if compared with that of the phosphine oxides. In the case of the amine salt, and despite the permeability coefficient shown, it should be pointed out that this value was obtained up to 30 min of reaction, since at longer times, chromium transport became constant, this being attributable to membrane saturation within the present experimental conditions.
4. Conclusions The transport of chromium ŽVI. can be effectively performed using Cyanex 921 in xylene as carrier across an ILM. Metal transport is influenced by a number of variables of the source phase Žstirring speed, HCl concentration, metal concentration., the membrane phase Žcharacteristics of the support, diluent, carrier concentration. and the receiving phase Žcomposition of the solution.. Chromium ŽVI. permeation is independent of carrier concentration when higher concentrations are used, and thus, the transport process is controlled by the diffusion in the aqueous stagnant film. The values of the mass transfer coefficient of the aqueous film and the thickness of the aqueous boundary layer are 224 mmrs and 4.5 mm, respectively. Chromium ŽVI. can be separated from other metals present in the source phase, however, chromium ŽVI. transport is negatively affected. The advantage of using Cyanex 921 in xylene, as carrier for chromium ŽVI. transport, against other reagents is demonstrated, a comparable permeation coefficient value is obtained when using Cyanex 923. Nomenclature J Permeability flux ´ Membrane porosity Thickness of the membrane do t Tortuosity of the membrane Transport resistance due to diffusion by the D aq aqueous source boundary layer
Daq d aq
19
Average aqueous diffusion coefficient of the metal-containing species Thickness of the aqueous source boundary layer
Acknowledgements To the Comunidad de Madrid ŽSpain. for project 07Mr0053r1998. Also to Mr. Bascones and Mr. Lopez for technical assistance. ´ References w1x J. Ruffo, A. Miret, J.L. Cortina, A. Sastre, in: N. Piccinni, R. Delorenzo ŽEds.., Proceedings of the European Meeting of Chemical Industry and Environment II, Cerdena, ˜ 1996, p. 479. w2x R. Molinari, M.G. Buonomenna, E. Drioli, in: P. Massacci ŽEd.., Proceedings of the XXI International Mineral Processing Congress vol. A, Elsevier, Amsterdam, 2000, pp. A6– A95. w3x E. Salazar, M. Ortiz, A. Urtiaga, A. Irabien, Ind. Eng. Chem. Res. 31 Ž1992. 1516. w4x A. Alonso, A. Urtiaga, A. Irabien, I. Ortiz, Chem. Eng. Sci. 49 Ž1994. 901. w5x A. Zouhri, M. Burgard, D. Lakkis, Hydrometallurgy 38 Ž1995. 299. w6x A.I. Alonso, C.C. Pentelides, J. Membr. Sci. 110 Ž1996. 151. w7x I. Ortiz, B. Galan, ´ A. Irabien, J. Membr. Sci. 118 Ž1996. 151. w8x K. Scott, Handbook of Industrial Membranes, Elsevier, Kidlington, 1997. w9x A.M. Sastre, A. Kumar, J.P. Shukla, R.K. Singh, Sep. Purif. Methods 27 Ž1998. 213. w10x I. Ortiz, B. Galan, ´ F. San Roman, ´ A.M. Urtiaga, in: I. Gaballah, J. Hager, R. Solozabal ŽEds.., Proceedings of the Global Symposium on Recycling, Waste Treatment and Clean Technology ŽRewas ’99. vol. III, TMS-Inasmet, Warrendale, 1999, p. 2173. w11x F.J. Alguacil, A.G. Coedo, M.T. Dorado, Hydrometallurgy 57 Ž2000. 51. w12x E. Dziwinski, J. Szymanowski, Solvent Extr. Ion Exch. 16 Ž1998. 1515. w13x A. Lopez-Delgado, F.A. Lopez, F.J. Alguacil, Rev. Metal. ´ ´ ŽMadrid. 36 Ž2000. 165. w14x T.B. Stolwijk, E.J.R. Sudholter, D.N. Reinhout, J. Am. Chem. ¨ Soc. 109 Ž1987. 7042. w15x A. Sastre, A. Madi, J.L. Cortina, N. Miralles, J. Membr. Sci. 139 Ž1998. 57. w16x F.J. Alguacil, M. Alonso, Hydrometallurgy 58 Ž2000. 81.
Uphill permeation of chromium žVI/ using Cyanex 921 as ionophore across an immobilized liquid membrane F.J. Alguacil a,) , A.G. Coedo a , M.T. Dorado a , A.M. Sastre b a
(CSIC), AÕda. Gregorio del Amo 8, Ciudad UniÕersitaria, 28040 Madrid, Spain Centro Nacional de InÕestigaciones Metalurgicas ´ Department of Chemical Engineering, ETSEIB, UniÕersitat Politecnica de Catalunya, Diagonal 647, E-08028 Barcelona, Spain `
b
Received 20 September 2000; received in revised form 10 January 2001; accepted 11 January 2001
Abstract The transport of chromium ŽVI. through an immobilized liquid membrane ŽILM. containing Cyanex 921 Žphosphine oxide. as a carrier has been studied. The permeation of the metal is investigated as a function of various experimental variables: hydrodynamic conditions, concentrations of chromium ŽVI. and HCl in the source phase, carrier concentration and diluent in the membrane, strippant in the receiving phase and support characteristics. The mass transfer coefficient and the thickness of the aqueous boundary layer were calculated from the experimental data. Furthermore, the selectivity of Cyanex 921-based ILM towards different metal ions and the behaviour of the system against other carriers are presented. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Chromium ŽVI. transport; Immobilized liquid membrane; Cyanex 921
1. Introduction Despite its high toxicity, chromium ŽVI. has been used widely in different industrial processes Ži.e. electroplating., its recovery from the corresponding wastewaters being a primary target before their discharge to natural waters. Several techniques have been developed to remove andror recover chromium ŽVI. from these industrial processes w1x. Among them, supported liquid membrane ŽSLM. technology could be competitive when the metal is present at low concentrations in the aqueous solution. In SLMs,
)
Corresponding author. E-mail addresses: [email protected] ŽF.J. Alguacil., [email protected] ŽA.M. Sastre..
both the advantages of solvent extraction and membrane separation processes are combined, giving simultaneous separation, concentration and purification of metallic species from solutions. An SLM is expected to be one of the most efficient membranes for separation processes, since facilitated transport is more effective than passive transport w2x. Among carriers used for chromium ŽVI. transport, quaternary ammonium salts have been most extensively studied w1,3–10x, and only recently has the use of a phosphine oxide been reported w11x. In the present work, results obtained for the transport of chromium ŽVI. using the phosphine oxide, Cyanex 921 as carrier, are presented. Several variables, which could affect the permeation process: stirring speed of the source and receiving phases, metal and carrier concentrations, diluent of the or-
0304-386Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X Ž 0 1 . 0 0 1 4 7 - 5
F.J. Alguacil et al.r Hydrometallurgy 61 (2001) 13–19
14
ganic phase, etc., are studied. The selectivity of the system against other metallic species and the behaviour of Cyanex 921 vs. other carriers are also reported.
concentrations of chromium in the source phase at elapsed time t and time zero, respectively. For the calculations of the chromium metal-accompanying permeabilities, the same Eq. Ž1. was used. 2.2. Membrane support
2. Experimental The phosphine oxide Cyanex 921 was obtained from Cytec ŽCanada. and was used without further purification, the active substance of the extractant being tri-n-octylphosphine oxide Žabout 99%. w12x. Extractants, TBP ŽFluka., DBBP ŽAlbright and Wilson., Cyanex 923 and Cyanex 471X ŽCytec., and Alamine 304 ŽCognis., were also used without further purification. The chloride salt of the tertiary amine, Alamine 304, was synthesized according to the procedure described in the literature w13x. The analytical grade diluents xylene, toluene and cumene ŽFluka. were used as such. Stock CrŽVI. solutions were prepared by dissolving K 2 Cr2 O 7 ŽMerck. in distilled water. All other chemicals were of AR grade. 2.1. ILM preparation and measurements The characteristics of the cell used in the present investigation were similar to those described in a previous work w11x. The source and receiving phases were mechanically stirred at 1600 and 1500 miny1 , respectively, at 25 " 18C to avoid concentration polarization conditions at the membrane interfaces and in the bulk of the solutions. Membrane permeabilities were determined by monitoring concentration by AAS in the source phase as a function of time, except in the case of the mixture containing CrŽVI.r CrŽIII. which was analysed by ICP-MS, previously labelling chromium ŽIII. with 58 Cr isotope. The chromium concentration in the various phases was found to be reproducible within "3%. The permeation coefficient Ž P . was computed using: ln
w Cr x t A s y Pt w Crx 0 V
Ž 1.
where A is the effective membrane area, V is the volume of the source phase, wCrx t and wCrx0 are the
The organic membrane phase was prepared by dissolving Cyanex 921 in the organic diluent to obtain carrier solution of various concentrations. The support was impregnated with the corresponding organic solutions by immersion for 24 h, then left to drip for a few seconds before being placed in the cell. The physical characteristics of the supports used in the present work can be obtained elsewhere w11x.
3. Results and discussion 3.1. Influence of the stirring speed of the source solution Previous experiments were carried out to establish adequate hydrodynamic conditions. The permeability of the membrane was studied as a function of the stirring speed on the source solution side. The agitation of the receiving solution was kept constant at 1500 miny1 . Constant permeability for stirring speeds higher than 1400 miny1 was obtained ŽTable 1.. Consequently, the thickness of the aqueous diffusion layer and the aqueous resistance to mass transfer were minimized and the diffusion contribution of the aqueous species to mass transfer process is assumed to be constant.
Table 1 Influence of the stirring speed in chromium ŽVI. permeation Speed Žminy1 .
P Žmmrs.
600 800 1000 1200 1400 1600
33.5 34.9 37.7 41.6 45.3 45.7
Source phase: 1.92P10y1 mM CrŽVI., 0.5 M HCl. Membrane phase: 30% wrv Cyanex 921 in xylene on Durapore GVHP 4700 support. Receiving phase: 0.5 M NaCl.
F.J. Alguacil et al.r Hydrometallurgy 61 (2001) 13–19
15
3.2. Influence of the stirring speed and stripping agent of the receiÕing solution
Table 3 Influence of HCl concentration and ionic strength on CrŽVI. permeation
The effect of varying the stirring speed in the receiving solution was also investigated using the same phases as described in Table 1, and maintaining a 1600 miny1 constant speed in the source solution. Results obtained show that the variation of the stirring speed Ž500–1500 miny1 . in the receiving phase does not affect chromium permeability Ž Paverage s 45.4 " 0.7 mmrs.. Different aqueous solutions were studied as stripping agents. The results are given in Table 2, the higher permeability coefficient is obtained when a 0.1 M NaCl solution is used as strippant, whereas NaCl solution seems to be more effective than HCl solution to strip chromium ŽVI.. As a result of the previous experiments, 0.1 M NaCl was used as the stripping agent.
HCl
P Žmmrs.
y3
10 M 10y2 M 10y1 M 0.5 M 1M 10y3 M a
9.5 67.4 224.0 45.7 6.12 9.5
Source phase: 1.92P10y1 mM Cr ŽVI. at different HCl concentrations. Membrane phase: 30% wrv Cyanex 921 in xylene on Durapore GVHP 4700 support. a With 0.1 M NaCl.
tration in the source phase was fixed in the subsequent experiments. 3.4. Influence of the diluent
3.3. Influence of HCl concentration and ionic strength in the source phase In order to study the significance of the role of the HCl concentration in the source phase solution during the permeation of chromium, HCl variation concentration studies in the range 10y3 –1 M were carried out, the receiving phase being 0.1 M NaCl. As seen in Table 3, permeability of chromium ŽVI. increases with increasing HCl concentration up to 0.1 M, and then decreases with increasing aqueous acidity. At a fixed HCl concentration, the increase of the ionic strength Žby NaCl addition. has no effect on chromium transport. Thus, a 0.1 M HCl concen-
As seen from Fig. 1, the permeability of chromium ŽVI. depends strongly on the nature of the organic diluent. Removal of CrŽVI. from the source side is more effective when xylene is used as the diluent for Cyanex 921. The low solubility of Cyanex 921 in aliphatic diluents Ži.e. kerosene type. prevented its use in the present study. 3.5. Influence of the support characteristics on the chromium flux Two solid supports, with different characteristics w11x, were used to study their effect on chromium transport. The flux values: J s Cr Ž VI .
Table 2 Influence of the composition of the receiving phase on CrŽVI. transport Receiving phase
P Žmmrs.
0.01 M NaCl 0.1 M NaCl 0.5 M NaCl 0.1 M HCl
37.6 57.7 45.7 34.6 y1
Source phase: 1.92P10 mM CrŽVI., 0.5 M HCl. Membrane phase: 30% wrv Cyanex 921 in xylene on Durapore GVHP 4700 support.
TOT P
Ž 2.
are given in Table 4, and the best flux value was obtained when Durapore GVHP 4700 was used as the support because of its higher performance expressed as: ´ Ž 3. do t However, it is possible to correct the influence of the different physical properties of the supports studied by the use of an expression w14,15x in which the flux values Ž J N . were normalized to the physical charac-
F.J. Alguacil et al.r Hydrometallurgy 61 (2001) 13–19
16
Fig. 1. Effect of various diluents on permeability of CrŽVI.. Source phase: 0.192 mM CrŽVI., 0.1 M HCl. Membrane phase: 15% wrv Cyanex 921 in each diluent on Durapore GVHP 4700 support. Receiving phase: 0.1 M NaCl.
teristics of the Durapore support, and nearly the same flux values were obtained for both supports ŽTable 4.. 3.6. Influence of initial metal concentration Fig. 2 shows a plot of the initial chromium ŽVI. flux Ž J . vs. the concentration of chromium in the source phase. At low metal concentrations, the initial flux is a function of the initial concentration in the source phase. Thus, the permeation process is con-
Table 4 Influence of the support used in the membrane phase on the chromium transport Support
J Žmolrm2 rs.
J N Žmolrm2 rs.
Durapore GVHP 4700 Fluoropore FGLP 4700
4.3P10y5 2.4P10y5
4.3P10y5 4.0P10y5
Source phase: 1.92P10y1 mM CrŽVI., 0.1 M HCl. Membrane phase: 30% wrv Cyanex 921 in xylene on each support. Receiving phase: 0.1 M NaCl.
trolled by diffusion of metal species in the lower range of chromium concentrations; however, beyond a certain limiting concentration, the flux approaches a maximum and becomes independent of metal concentration, one probable reason for this may be the rate-determining step for the transport process. 3.7. EÕaluation of limiting permeability (Pl i m ) The influence of different Cyanex 921 concentrations on permeability was studied in the range 5–30% wrv. Results obtained are shown in Fig. 3; it can be seen that at higher carrier concentrations, P is independent of Cyanex 921 concentration and this region is representative of an aqueous diffusion film-controlled permeation process. This constant permeability value Plim , known as limiting permeability, can be expressed as: P lim s
1 s D aq
Daq d aq
Ž 4.
F.J. Alguacil et al.r Hydrometallurgy 61 (2001) 13–19
17
Fig. 2. The influence of initial concentration of CrŽVI. on the permeability flux Ž J . of the metal. Source phase: various CrŽVI. concentrations, 0.1 M HCl. Membrane phase: 30% wrv Cyanex 921 in xylene on Durapore GVHP 4700 support. Receiving phase: 0.1 M NaCl.
Assuming the value of Daq as 10y9 m2rs w11x and P lim as 224 mmrs, the value of the thickness of the aqueous boundary layer is estimated to be 4.5 mmrs. This value is the minimum thickness of the stagnant aqueous diffusion layer in the present experimental conditions. 3.8. SelectiÕe transport of Cr(VI) Õs. Mn(VII), Fe(III), Cr(III), Cu(II), Zn(II) Because chromium is widely used in a number of industries, the corresponding wastewaters may contain a wide range of concentrations of chromium ŽVI. and ŽIII. and other heavy metals, i.e. coating plants ŽCrŽVI. 0.005–5.0 mgrL, iron 0.41–170 mgrL., chemical milling and etching ŽCrŽVI. 0.005–335 mgrL, copper 0.21–270 mgrL, zinc 0.112–200 mgrL, iron 0.0075–260 mgrL., printed board industry ŽCrŽVI. 0.004–3.54 mgrL, copper 1.6–540 mgrL, nickel 0.027–9 mgrL, lead 0.044– 10 mgrL., anodizing plants Žtotal chromium 0.27–80 mgrL, CrŽVI. 0.005–5.0 mgrL..
Table 5 shows results obtained in the transport of chromium ŽVI. when the source phase also contained other accompanying metals. A highly selective transport of chromium ŽVI. is obtained within the experiments that allow recovery of high purity CrŽVI. from the receiving solution. However, there is an influence on chromium ŽVI. transport when these metals are present in the source solution; thus, chromium ŽVI. permeation is negatively influenced within the apparent order: CuŽII., ZnŽII. ) NiŽII., FeŽIII., CrŽIII. ) MnŽVII., despite that some of the metals were not detected in the receiving phase Ži.e. zinc ŽII., copper ŽII., nickel ŽII. and iron ŽIII.. or are slightly transported Žchromium ŽIII., manganese ŽVII... Such behaviour may be explained in various ways: a negative salting-out effect, formation of metal-chlorocomplexes which also decreases the initial acidity of the source phase, or to the main metal transport which needs an induction period and decreases the nominal carrier concentration in the membrane phase Ži.e. iron ŽIII. is transported through an ILM impregnated with Cyanex 921 w16x..
F.J. Alguacil et al.r Hydrometallurgy 61 (2001) 13–19
18
Fig. 3. The influence of Cyanex 921 concentration on permeability of CrŽVI.. Source phase: 0.192 mM CrŽVI., 0.1 M HCl. Membrane phase: Cyanex 921 in xylene on Durapore GVHP 4700 support. Receiving phase: 0.1 M NaCl.
3.9. BehaÕiour of the Cyanex 921 carrier system compared to other potential chromium (VI) carriers The transport of chromium ŽVI. was also studied using other carriers to compare with results obtained using Cyanex 921. The carriers investigated were: TBP Žphosphoric ester., DBBP Žphosphonic ester.,
Table 5 Chromium ŽVI. transport in presence of different metallic ions Žmmrs.
System
PCr
CrŽVI. CrŽVI. –MnŽVII. CrŽVI. –CrŽIII. CrŽVI. –FeŽIII. CrŽVI. –NiŽII. CrŽVI. –CuŽII. CrŽVI. –ZnŽII.
224.0 169.2 136.0 146.1 122.0 104.3 93.5
Remark PMn -6 mmrs PCr - 5 mmrs No Fe transport No Ni transport No Cu transport No Zn transport
Source phase: 0.01 grL CrŽVI. and 0.01 grL of each metal, 0.1 M HCl. Membrane phase: 30% wrv Cyanex 921 in xylene on Durapore GVHP 4700 support. Receiving phase: 0.1 M NaCl.
Cyanex 471X Žphosphine sulphide., Cyanex 923 Žmixture of phosphine oxides. and the chloride salt of Alamine 304 Žquaternary ammonium salt.. Results are summarized in Table 6, showing the best and comparable chromium ŽVI. permeabilities when phosphine oxides are used as carriers. Only slight metal transport is obtained when TBP, DBBP
Table 6 Chromium ŽVI. transport using different carriers Extractant
P Žmmrs.
Cyanex 921 Cyanex 923 Alamine 304 salt TBP Cyanex 471X DBBP
224.0 180.3 120.2 17.5 13.6 7.1
Source phase: 1.92P10y1 mM CrŽVI., 0.1 M HCl. Membrane phase: 0.77 M of each extractant in xylene on Durapore GVHP 4700 support. Receiving phase: 0.1 M NaCl.
F.J. Alguacil et al.r Hydrometallurgy 61 (2001) 13–19
and Cyanex 471X are used, which can be interpreted in terms of the lower donor properties of the active substance of the reagents if compared with that of the phosphine oxides. In the case of the amine salt, and despite the permeability coefficient shown, it should be pointed out that this value was obtained up to 30 min of reaction, since at longer times, chromium transport became constant, this being attributable to membrane saturation within the present experimental conditions.
4. Conclusions The transport of chromium ŽVI. can be effectively performed using Cyanex 921 in xylene as carrier across an ILM. Metal transport is influenced by a number of variables of the source phase Žstirring speed, HCl concentration, metal concentration., the membrane phase Žcharacteristics of the support, diluent, carrier concentration. and the receiving phase Žcomposition of the solution.. Chromium ŽVI. permeation is independent of carrier concentration when higher concentrations are used, and thus, the transport process is controlled by the diffusion in the aqueous stagnant film. The values of the mass transfer coefficient of the aqueous film and the thickness of the aqueous boundary layer are 224 mmrs and 4.5 mm, respectively. Chromium ŽVI. can be separated from other metals present in the source phase, however, chromium ŽVI. transport is negatively affected. The advantage of using Cyanex 921 in xylene, as carrier for chromium ŽVI. transport, against other reagents is demonstrated, a comparable permeation coefficient value is obtained when using Cyanex 923. Nomenclature J Permeability flux ´ Membrane porosity Thickness of the membrane do t Tortuosity of the membrane Transport resistance due to diffusion by the D aq aqueous source boundary layer
Daq d aq
19
Average aqueous diffusion coefficient of the metal-containing species Thickness of the aqueous source boundary layer
Acknowledgements To the Comunidad de Madrid ŽSpain. for project 07Mr0053r1998. Also to Mr. Bascones and Mr. Lopez for technical assistance. ´ References w1x J. Ruffo, A. Miret, J.L. Cortina, A. Sastre, in: N. Piccinni, R. Delorenzo ŽEds.., Proceedings of the European Meeting of Chemical Industry and Environment II, Cerdena, ˜ 1996, p. 479. w2x R. Molinari, M.G. Buonomenna, E. Drioli, in: P. Massacci ŽEd.., Proceedings of the XXI International Mineral Processing Congress vol. A, Elsevier, Amsterdam, 2000, pp. A6– A95. w3x E. Salazar, M. Ortiz, A. Urtiaga, A. Irabien, Ind. Eng. Chem. Res. 31 Ž1992. 1516. w4x A. Alonso, A. Urtiaga, A. Irabien, I. Ortiz, Chem. Eng. Sci. 49 Ž1994. 901. w5x A. Zouhri, M. Burgard, D. Lakkis, Hydrometallurgy 38 Ž1995. 299. w6x A.I. Alonso, C.C. Pentelides, J. Membr. Sci. 110 Ž1996. 151. w7x I. Ortiz, B. Galan, ´ A. Irabien, J. Membr. Sci. 118 Ž1996. 151. w8x K. Scott, Handbook of Industrial Membranes, Elsevier, Kidlington, 1997. w9x A.M. Sastre, A. Kumar, J.P. Shukla, R.K. Singh, Sep. Purif. Methods 27 Ž1998. 213. w10x I. Ortiz, B. Galan, ´ F. San Roman, ´ A.M. Urtiaga, in: I. Gaballah, J. Hager, R. Solozabal ŽEds.., Proceedings of the Global Symposium on Recycling, Waste Treatment and Clean Technology ŽRewas ’99. vol. III, TMS-Inasmet, Warrendale, 1999, p. 2173. w11x F.J. Alguacil, A.G. Coedo, M.T. Dorado, Hydrometallurgy 57 Ž2000. 51. w12x E. Dziwinski, J. Szymanowski, Solvent Extr. Ion Exch. 16 Ž1998. 1515. w13x A. Lopez-Delgado, F.A. Lopez, F.J. Alguacil, Rev. Metal. ´ ´ ŽMadrid. 36 Ž2000. 165. w14x T.B. Stolwijk, E.J.R. Sudholter, D.N. Reinhout, J. Am. Chem. ¨ Soc. 109 Ž1987. 7042. w15x A. Sastre, A. Madi, J.L. Cortina, N. Miralles, J. Membr. Sci. 139 Ž1998. 57. w16x F.J. Alguacil, M. Alonso, Hydrometallurgy 58 Ž2000. 81.