Liquid membrane multiple emulsion process of separation of copper(II) from waste waters

Colloids and Surfaces A: Physicochemical and Engineering Aspects 166 (2000) 7 – 25 www.elsevier.nl/locate/colsurfa

Liquid membrane multiple emulsion process of separation of copper(II) from waste waters A.K. Chakravarti a, S.B. Chowdhury b, D.C. Mukherjee b,* a

Department of Chemistry, Dum Dum Motijheel College (Calcutta Uni6ersity), 1, Motijheel A6enue, Calcutta 700 074, India b Department of Chemistry, Calcutta Uni6ersity, 92, Acharya Prafulla Chandra Road, Calcutta 700 009, India Received 14 September 1998; accepted 17 August 1999

Abstract A liquid membrane (multiple emulsion, water-in-oil-in-water, WIII/OII/WI) method of separation of copper(II) from simulated waste water has been described. The effect of variation of oil membrane constituents in OII, different concentrations of sulphuric acid as strippant in the internal phase WI, pH and the object ion concentration in the raffinate WIII have been studied in detail with an aim to optimize the process so that the copper(II) content in the waste water be made lower than the toxic limit (1.5 ppm, WHO.) The ion-exchange behaviour of the carriers in the membrane and the coupled counter-transport resembling a ‘Chemical pump’ have been ascertained. Interference, in the process, of some metals (ions) occurring naturally with copper, e.g. Pb(II), Cd(II), Zn(II), Co(II), Ni(II) and Mn(II) through their co-transport have been studied. The relative extents of their accompaniment have been explained from the consideration of the stability of the carrier-metal (ion) complexes and exchange reactions at the interfaces. The application of the reaction site model and involvement of a pseudo first order process have been found to be reasonably justified in the present transport method of separation of copper(II). © 2000 Elsevier Science B.V. All rights reserved. Keywords: Copper(II) separation; Multiple emulsion process; Waste waters

Among the several different methods of separation of undesirable dissolved materials in aqueous solutions the currently developed liquid membrane permeation process (LMP) has been drawing attention of the workers engaged in the fields of environmental science, ecology, water pollution control, and analytical chemistry. In the liquid membrane permeation process a simultaneous extraction and stripping are accomplished through a * Corresponding author.

large surface area of a liquid membrane prepared with as minimum quantity of an extractant as possible. The liquid membranes are pore-free soluble membranes and the materials to be transported through it must dissolve in it on one side, diffuse across it in the dissolved from, and reversal of the dissolution process should take place on the other side. In principle, stable liquid membranes may be backed up between two liquids as: 1. lamellae stabilized by surface active materials stretched over holes in a supporting wall or free standing;

0927-7757/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 9 9 ) 0 0 4 5 2 - 5

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2. membrane liquids soaked in the pores of a solid membrane [1 – 5]; 3. liquid films between two plane parallel films [6], 4. liquid membranes prepared following the method of multiple emulsions. In regard to the theory and application of the liquid membranes several papers have appeared in the literature[7,8]. Among the different liquid membrane systems, the thinly stretched spherical surfaces with large surface areas ( \ 3000 cm2/cm3) as met with the multiple emulsions are perhaps the most effective. Since the formulation of the separation of some particular hydrocarbons from a mixture of those by Chan and Li [9] using a multiple emulsion of the type O/W/O, and subsequently the successful separation of some organic compounds and inorganic ions by Li et al. and others employing multiple emulsion of the type W/O/W [10–15], the process is considered promising. Since chromium (VI) is considered as one of the harmful species in aqueous solution, its separation from simulated waste waters, below the toxic level (0.5 ppm), was studied in detail by multiple emulsion liquid membrane method and described in an earlier publication [15]. Although copper(II) in aqueous solution is viewed less deleterious (toxic limit 1.5 ppm, WHO), lessening of its content in the effluents from electroplating, engraving and photography industries, in the mining and mineral leaching, rinse tanks and filter clean outs is recommended before discharge. Also, copper is recovered from low grade ores by hydrometallurgical process, for which solvent extraction is currently practised [16]. Recovery of copper (II) from aqueous solutions containing iron (III) in synthetic solutions or in low grade ores by means of supported liquid membranes built with porous back-up like polypropylene, porous trifluoroethylene and LIX 64N or Kelex 100 in kerosene [17], and porous microfilm and Celgard 2400 or LIX 64N [4] are known. A mechanism and kinetics of copper permeation through supported liquid membrane containing hydroxyoxime as a mobile carrier have been reported [18]. Moreover, a mechanism of copper permeation through hollow fibre liquid membrane

[19], and selective concentration of copper(II) by reverse osmosis through hydrophobic polystyrene membrane with acetylacetone as the chelating agent are also reported [20]. A few works concerning application of the emulsion liquid membrane method for the separation of copper(II) from acid solution together with a model therefor [13,21– 23], and from zinc electrolyte solution as well are also reported. Being aware of these works we have attempted to make a thorough study on the simultaneous separation and accumulation of copper(II) from a large volume of simulated waste water to a small volume of aqueous strippant phase using a water immiscible organic liquid in the form of a membrane, processed in the way of multiple emulsion method [2,15,17], with the aim to deplete its content in the raffinate below the toxic level by a single operation within a short time. In this present work one new chelating agent LIX 622 (dodecyl salicylaldoxime) is introduced as the carrier, and its efficacy is compared with a similar one, LIX 84, shown in Table 1[2,24–32], together with other chelating agents, used previously in solvent extraction or in liquid membrane process for the separation of copper. Together with this, the effects of the variation in the basic diluent composition of the membrane phase, the diverse concentration of the acid as strippant in the internal phase, variation of pH of the feed phase, contact period between the feed and the emulsion phases, interference of diverse metal ions in the transport of copper and the extent of counter (coupled) transport of proton from the internal to the feed phase, + + 2RH(O) Cu2(F)

= CuR2(O) + 2H+ (F)(WIII/OII interface) CuR2(O) + 2H+ (A) + = 2RH(O) + Cu2(A) (OII/WI interface),

where, F, feed aqueous phase (WIII); O, organic membrane phase (OII); A, aqueous acid solutionstripping phase or internal phase (WI), have been studied. In addition to these, the applicability of the reaction site model [2] for the transport of copper(II) from feed to the internal phase has been tested.

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1. Experimental In the present studies the bench scale batch stage separation process has been applied for the removal of copper from simulated waste water at 2892°C. The materials and the methods of preparation of the emulsion and the subsequent separation operations are similar to those applied earlier for the separation of chromium(VI) [15], except the carriers used in the present system are LIX 622 and LIX 84, the stirrer speed employed for the preparation of the primary emulsion is 4500–5000 rpm, and the internal stripping phase Table 1

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is sulphuric acid. The variation in kind and quantities of the ingredients in the preparation of the feed phase, emulsions and internal phase are concisely described in Table 2. The primary emulsions have been prepared by pouring a thin steam of the acid solutions into a homogeneous solution of the oil membrane whilst stirring the mixture continuously with a high speed stirrer fitted with six pieces of axial blades. For the separation experiments, the emulsions are poured into the feed aqueous phase in a thin stream whilst mixing them continuously at different rates (325–430) rpm by means of a turbine

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Table 2 Description of the materials for the MELM separation of copper(II) Component reagents Feed phase (WIII) Aqueous solution of copper sulphate (750 ml) (a) Without buffer (b) With buffer (i) ClCH2COOH/CH3COONa (ii) HCl/CH3COOONa (iii) CH3COOH/CH3COONa (c) Interfering ions used Zn(II) Pb(II) Mn(II) Cd(II) Co(II) Ni(II) Oil membrane phase (OII) (a) Diluent oil-mixture of Kerosene oil (r = 0.78, h= 11.7×10−4) and light paraffin (r= 0.84; h= 165×10−4) (b) Membrane strengthener LZ 890* (polybutyl succinimide, r= 0.92) (c) Surfactant-span 80 +(sorbitan monochelate, r= 0.92) (d)Carrier (i) LIX-84 + (2-hydroxy5-nonylacetophenone oxime, r%−0.90) (ii) LIX-622 (dodecyl salicylal-doxime: r =0.92, h= 2 poise (30°C)) Internal Phase (WI) Sulphuric acid

Range used

Remarks

Cu2+ 186, 485, 750

Each Cu2+ concentration is used for individual studies

pH : 5.0 i) pH : 2.0 ii) pH : 3.8 iii) pH :5.95 184–575 200–375 200–400 163–372 187–374 200–392

Only 186 PPM Cu2+ in the feed phase is used in each case for studying the interference in the transport of copper(II)

PPM ppm ppm ppm ppm ppm

wt% Ratio kerosene:Liq. paraffin 88.3:0, 79.6:8.6, 70.6:8.6, 66.8:21.5, 56.8:8.6, 45.3:43 0–11.8 wt%

wt%-Percentage of the liquid paraffin component with respect to the total weight of the oil membrane phase. Polybutyl Succinimide acts simultaneously as a surfactant [15].

0–4.7 wt%

4.6–27.6 wt%

0.95–28.2 wt%

2–5(N)

The ratios of the phases used In the emulsion material- oil membrane 0.4–2.0 phase (O): internal aqueous phase (WI or A) (V/V) In the separation experiment — feed 9.25 aqueous phase (WIII or F) emulsion phase (E) [V/V]:

Most of the studies are performed with 2N acid. The emulsion is a water in oil type.

* , The compound was kindly supplied by Lubrizol, India, viscosity, h in Kg/m/s; +, procured from Sigma (St Louis, MO), density, r in gm/ml; ++, the compound was kindly supplied by Henkel (USA), specific gravity, r%.

type bladed stirrer. The extent of contact between these two phases, for a definite period of mixing, depends on the size of the dispersed emulsion globules in the feed phase, which however depends partly on the speed of this stirrer.

Stability of the primary emulsion (before being used in the separation experiments) was studied by transferring it into deionized water (pH : 6), stirring the mixture continuously for a certain period (: 0.5 h) and monitoring from time to

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time any decrease in pH of the aqueous phase. In other experiments the emulsions preloaded with copper (Cu2 + ) and sulphuric acid together (internal phase 40 – 60 ml containing 2000–3400 ppm Cu(II) in 2N H2SO4) were used in the similar experiments, and any ejection of copper in the aqueous phase were tested. In some cases after accomplishment of the separation experiments, the Cu-loaded emulsions were allowed to remain in the reffmate, in a state of continuous stirring for more than 0.5h and any liberation of the entrapped copper was studied. In the separation experiments the depletion of copper and increase in the hydrogen ion concentration in the raffinate was determined by collecting samples at regular intervals from the bottom outlet of the reaction vessel and using atomic absorption spectrophotometric method (Perkin Elmer 380) and pH measurements (Knick, Labor pH meter), respectively.

2. Results and discussions Only the kerosene oil solutions of both the carriers, LIX 84 and LIX 622, acted as a Cu2 + extractant. Liquid-Liquid extraction of copper

Fig. 1. Effectively of the carriers LIX-622 and LIX-84 in the extraction of copper (Cu2 + ) by simple solvent extraction procedure, and stripping of copper from the organic extract with sulphuric acid.

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with these two reagents and subsequent recovery of it from the extract by stripping with different concentrations of sulphuric acid (2–8 N) have been investigated. Twenty-five millilitres of such an oil extractant (24 ml of kerosene oil and 1 ml of LIX 84 or LIX 622, i.e. 4.8–4.9 wt% of the carriers) on shaking for 4 min (50 strokes min − 1) with a 25 ml aqueous solution of Cu2 + as sulphate (5714 ppm of copper), pH :5.5 extract 1935 ppm and 2100 ppm (i.e. :33.9 and 36.8%) of copper respectively (determined in the aqueous phase). The quantities of the extracted copper recovered using sulphuric acid solutions as strippant depend upon the concentration of the acid and the time of shaking. A 25 ml solution of 2 N H2SO4 strips 1910 ppm and 1845 ppm (i.e. 98.7 and 87.9%) of copper from the above mentioned extracted quantities in the organic phase containing LIX 84 and LIX 622, respectively, with 200 strokes in each case. However, 4 N and 8 N sulphuric acid with 200 strokes strip 94.0 and 96% copper, respectively, from the organic phase containing LIX 622. Again a 25 ml solution of 8N H2SO4 strips 92.5, 95 and 96% of copper from Cu-LIX 622 extract with 50, 100 and 200 strokes, respectively. Further increase in shaking (i.e. \ 200 strokes) does not improve extraction or stripping to any perceptible extent. All these variations in extraction and stripping are shown in Fig. 1. However, when the carriers are used in the multiple emulsion liquid membrane (MELM) process more than 99.9% of the total quantity of copper (186 ppm or 140 mg) in 750 ml of the feed phase is stripped into the internal solution (28–60 ml); only the rate of transport from WIII to WI phase are different for differently concentrated strippant acid. So far as the stability of the system is concerned no noticeable breakage of the emulsion, before and after separation experiments was observed. When the other variables are constant it was found that a suitable combination of the surfactant span 80 and the membrane strengthener LZ

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2.1. Viscosites of the oil and emulsions used The details of the variation of density and viscosity of the emulsions used are the same as that described in the case of Cr(VI) separation [15]. In the present study of Cu(II) separation, carriers of different chemical nature only have been used. All these manipulations regarding the quantities of the ingredients in the oil-membrane phase, the volumes of the aqueous strippants encapsulated in the oil-membrane, the ratio of the volumes of the oil-membrane phase to the internal strippant phase, etc. are adopted with an object to prepare the so called high viscous emulsions. Since from the point of view of stability and ideal behaviour of the emulsion droplets, high viscous emulsions are superior to the low viscous ones and considered to be more useful, although the rate of material transport with the latter is rather faster [2,13,15,33]. Fig. 2. Optimization of the wt% of the surfactant (Span 80) and binder (LZ 890) for preparing emulsions with adequate efficiency.

Fig. 3. wt% Of liquid paraffin in the base-oil of emulsion, stability of the emulsion, and separation of copper (Cu2 + ) from feed phase.

890, (2.3:4.7), in the system produce most effective separation Fig. 2; either of these two is not so operative.

2.2. Composition of the diluent oil and the transport of copper(II) from WIII to WI The diluent oil composition, i.e. kerosene with different quantities of light liquid paraffin, has been observed to have an influence on the transport of the object ion. Gradually increasing quantities of liquid paraffin in the mixture increase the viscosity of the emulsion [15]. Now higher viscosities control undesirable quick rupture of the emulsions, but slower rate of transport. In Fig. 3 the influence of the oil composition is shown (kerosene: liquid paraffin: 79.7: 8.6; 66.8: 21.5; 45.3: 43; 88.3: 0.0) on the transport of copper(II) keeping the other components of the primary emulsion to a constant quantity. Emulsion with only kerosene oil although it yields higher transport rate (being the low viscous type), undergoes a quick loss of cohesion after accumulating Cu2 + ion in it; yet the intended depletion of Cu2 + in the raffinates (B1 ppm) is reached approximately in 10.5 min. An optimum quantity of the light liquid paraffin (viz. 8.6 wt%) makes the emulsion stable and maintains almost the same rate of transport of copper(II) as that with kerosene oil alone, higher quantities, however, cause slower transport and delays attainment of depletion of Cu2 + in the raffinate.

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2.3. Quantity of membrane strengthener in the oil membrane and the transport of copper(II) from WIII to WI

2.4. Concentration of the carriers in the oil-membrane and the transport of copper(II) from WIII to WI

An optimum quantity of the membrane strengthener (polybutyl succinimide LZ 890) in the oil-membrane has been found to produce a higher rate of copper(II) transport. In an experiment with 0.0, 4.7, 11.8 wt% imide in the oil membrane, while keeping the other constituents of the primary emulsion fixed, the best result has been obtained with 4.7% (Fig. 4). A higher quantity of the imide together with a definite quantity of the surfactant (span 80) in the oil increases the viscosity of the oil phase which restricts diffusion of the Cu(II)-carrier complex in the membrane from WIII/OII interface to OII/WI interface. On the other hand, the oil containing surfactant but no strengthener helps swelling the primary emulsion (WI/OII) because of osmotic imbibition of water into the internal phase (WI) and causes a rise in viscosity [14,30,34]. This may be the reason for the quick transport of copper(II).

The rate of transport of copper(II) from the feed to the strippant phase through the oil membrane, at all pH of the fixed phase, attains a maximum value as is seen in Fig. 5 (a, b and c). This trend was also observed in the case of Cr(VI) separation [15]. However, much better separation of copper(II) was achieved with both carriers maintaining the external phase (WIII) pH at a constant value : 6 with a buffer (NaAc/Hac) than without buffer (pH : 5). This may be as a result of a favourable downhill coupled-counter-transport of H+ ions from WI to WIII. It is interesting to note that with relatively high viscous emulsion O/A :0.4, regardless of the nature of the external solution, with or without buffer, for an attainment of highest rate of transport of Cu(II), although LIX 622 could not be used in excess of 14.1 wt%, for LIX 84 a quantity 27.6 wt% could be reached. However, for a comparatively low viscous type emulsion O/A :1, the quantity of LIX 622 could be extended to 28.2 wt% similarly with LIX 84 a quantity of 27.6 wt% could be used in the feed solution without buffer (pH : 5) (curve 1 in Fig. 6a and b). Under the same conditions it can be seen that LIX 622 is more effective as a carrier than LIX 84 (Fig. 7), may be as a result of the inherent difference in the moiety of the reagents other than the common functional group, oxime.

2.5. Effect of concentration of the acid in the stripping phase (WI)

Fig. 4. Effect of wt% of imide as binder on the separation of copper (Cu2 + ) from feed phase.

It has been mentioned that in the present studies sulphuric acid was used as the strippant. The concentration of the acid in the internal phase also influences the rate of separation while the other components in the oil and in the feed phase remain almost the same. In general, higher concentrations of sulphuric acid (3.5–5.0N) in the stripping phase favour the separation of copper to almost the same extent for both LIX 622 and LIX

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Fig. 5. (a) Effect of variation of wt% of carrier LIX-622 on the rate of depletion of copper (Cu2 + ) in the feed phase (pH:6, buffer NaAc/HAc). (b) Effect of variation of wt% of carrier LIX-622 on the rate of depletion of copper (Cu2 + ) in the feed phase (pH : 5, without any buffer). (c) Effect of variation of wt% of carrier LIX-84 on the rate of depletion of copper (Cu2 + ) in the feed phase (pH :5, without any buffer).

84, at pH 5.0 (without buffer) of the feed phase solution, and O/A =1, (Fig. 6a and b). However, a much faster rate of separation has been achieved by using a buffered (NaAc/HAc) feed phase pH: 6.0 with the same higher concentrations of the acid and O/A =0.4.

2.6. Concentration of copper in the feed phase and the rate of transport of copper from WIII to WI Besides the investigations of the role of different components of the emulsion membrane sys-

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Fig. 8. Initial concentration of copper (Cu2 + ) in the feed phase (pH : 6, buffer NaAc/HAc) and its depletion in it with time. Fig. 6. Effect of the concentration of strippant acid (H2SO4) on the depletion of copper (Cu2 + ) in the feed phase (pH :5, without any buffer); (a) carrier LIX-622; (b) carrier LIX-84.

tem with definite quantities of copper(II) in the feed phase, viz. 186 ppm, experiments with higher quantities, e.g. 485 ppm and 750 ppm but optimized quantities of other constituents were performed. In the present studies with the so called high viscous emulsions (O/A : 0.4) and buffered feed phase (pH:6, NaAc/HAc) the rate of depletion of copper in the external solution has been found to be proportional to the starting concentration of the metal ion in the solution, (Fig. 8).

2.7. Hydrogen ion concentration in the external solution (WIII) and the depletion of copper — counter-transport of ions from WI to WIII

Fig. 7. Effectiveness of carriers LIX-622 and LIX-84 on the depletion of copper (Cu2 + ) in the feed phase (pH :6, buffer NaAc/HAc) with optimized composition of the emulsion and strippant.

An aqueous solution of copper sulphate without any buffer is not efficacious. This is because of simultaneous counter transport of hydrogen ion from WI to WIII and causes a lowering of raffinate pH value. The coupled counter transport of hydrogen ions has been ascertained to the quantitative extent. In an experiment using CuSO4 solution of pH: 5 (without buffer), carriers LIX 622 or LIX 84 approximately 6 times the quantities in other experiments, and ratio of the oil membrane phase to the strippant phase (V/

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V)=1 (a somewhat low viscous emulsion), a depletion of copper in the raffmate (750 ml) from 186 ppm (i.e.=140 mg or 0.0022 equivalent) to less than 1.0 ppm and simultaneous fall of pH from 5.0 to 2.5 (i.e. an increase in H+ ion concentration of 0.0023 equivalent) has been found to take place, (Fig. 9). In the acidic raffinate the Cu-carrier complex at the WIII/OII interface is somewhat decomposed in a similar way as dislodging of copper at the OII/WI interface.



1 Cu2 + Carrier 2

“



+ H+ (aq)

org

1 Cu2aq+ + (H+ carrier)org 2

Again, external copper sulphate solutions buffered at different pH values 6–2 through suitable combinations of acetic acid, acetate, monochloroacetic acid and inorganic acids show a gradually decreasing transport of copper with decreasing pH values (Fig. 10), counter transport of H+ ions in the buffered solutions cannot change the pH values remarkably. The most effective transport of copper(II) (i.e. drop of copper content in the external phase (WIII) to B 1 ppm within :10 min) has been found to take place when the raffinate is buffered with sodium acetate/acetic acid to a value of : 6.

2.8. Effect of di6erse ratios of the oil phase to the strippant phase in the primary emulsion and transport of copper(II)

Fig. 9. pH drop in the feed phase because of counter transport of H+ ions from the strippant phase together with copper (Cu2 + ) transport in the opposite direction.

A study with the primary emulsions prepared by using a definite volume (81 ml) of the oil membrane phase (O) containing optimised quantities of its constituents and different volumes of acid strippant phase (2NH2SO4) (A) bearing the ratios O/A (v/v), e.g. 2.0, 1.0 and 0.4 was carried out. It was found (Fig. 11) that higher rate of transport of copper (II) is achieved with lower quantities of the stripping solutions in the internal phase, i.e. by using comparatively low viscous emulsions.

2.9. Effect of the time of contact between the raffinate (WIII and the primary emulsion (WI /OII) and the depletion of copper(II) in the raffinate

Fig. 10. Effect of study of the pH of the feed phase (maintained by buffers) on the separation of copper (Cu2 + ) from it.

For the system with the same chemical composition higher rate of contact between the raffmate and primary emulsion by stirring increases the transport of copper (II) from WIII to WI. For a decrease in the rate of stirring from 430 to 325 rpm for a system of chemical composition as described in the curve 1, Fig. 10 the depletion of copper from 186 ppm to less than 1 ppm takes : 8–10 min, Fig. 12. A higher rate of stirring,

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undergo rupture). It has been found that with the stirring rate of 430–380 rpm. the emulsions do not remain stable after 8–10 min. However, with slower rate of stirring 325 rpm no such rupture of emulsion is observed. Thus, in all experiments the rate of mixing of the raffinate phase (WIII) and the primary emulsion (WI/OII) is generally maintained at 325 rpm.

2.10. Pumping effect

Fig. 11. Effect of variation of the ratio of the volumes of the oil membrane phase to internal aqueous strippant phase (O/A) of the emulsions on the depletion of copper (Cu2 + ) in the feed phase (pH : 6, buffer NaAc/HAc).

Fig. 12. Dependency of the transport of copper (Cu2 + ) on the stirring speed (rpm) for dispersion of emulsion in the feed phase.

however, has an adverse effect (the emulsions

The present studies of the transport of copper(II) involves a coupled counter transport process [2,15] as a result of the ‘pumping effect’ or ‘passive transport’. The results of the typical separate experiments with nearly 18 times more copper(II) (: 3322 ppm) in the internal acid solution (:2 N H2SO4) of the primary emulsion (WI/OII) than that in the feed phase (WIII) (:186 ppm) without buffer (pH: 5.0) together with oil membrane phase containing the carriers, LIX 622 (28.2 wt%) or LIX 84 (27.6 wt%) and O/A : 1, (Fig.

Fig. 13. Effect on the separation of copper (Cu2 + ) from feed phase using emulsions preloaded with copper: (a) carrier LIX84, O/A :1, feed phase pH :5 (without buffer); (b) carrier LIX-622, O/A : 1.0, feed phase pH:5 (without buffer); (c) carrier LIX-622, O/A : 0.4, feed phase pH: 6 (with buffer, NaAc/HAc).

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13(a and b) respectively), and with buffer (pH: 6.0, NaAc/HAc), LIX 622 (4.7 wt%) and 0/A = 0.4, Fig. 13(c) are in accord with this view. The transport of the H+ ion from WI to WIII through OII, because of it’s concentration gradient is in this direction and the decomposition of the LIXCu (II) complex at the OII/WI interface on account of large concentration of H+ ion in WI, may presumably be attributed to this pumping effect or passive transport of copper(II) against its concentration gradient.

2.11. Ideal and non-ideal beha6iour of the emulsion drops and applicability of the reaction-site model It has been mentioned that in the surfactant membrane system with multiple emulsions, waterin-oil-in-water WI/OII/WIII, the low viscosity material is advantageous in effecting higher rate of mass transfer. The concentration of the transferred material in the strippant phase 1, in this instance, is a function of time [13,36]. However, it readily breaks down showing non-ideal behaviour, and in practice, is inoperable. In contrast, with higher viscosity emulsions prepared via adjustment of the constituent parameters [2,15] the phase I droplets are assumed to be almost immobile reaction sites at certain distances from the WIII/OII interface, and the concentration of the transferred material (in phase I) is a function of both the time of travel of the loaded carrier and the distance of the unsaturated core from the WIII/OII interface [2]. The non-ideal behaviour in this state may arise because of loss of cohesion among the primary emulsion globules as a consequence of osmosis and swelling of the emulsion [13] or prolonged stirring. In relation to mass transport through high viscosity emulsion system, some plausible assumptions are: 1. the mass transfer is brought about by diffusion of the carrier molecules in the oil membrane phase II; 2. the reactions pertaining to the carriers, unloaded or loaded, at the phase boundaries, are in equilibrium;

for more than 90% mass transfer (Mt /M ]0.9), the carrier at the raffinate/oil-membrane phase boundary remains loaded to the maximum extent and its concentration is C( /2 (C( , the total concentration of the carrier in the phase II) while at the oil-membrane/strippant phase boundary (II/I) the concentration of the loaded carrier is zero. Unlike the behaviour of the film or flat membranes, an increasing concentration of the carrier in the oil of this membrane system does not help proportional augmentation of the mass transfer rate — rather a levelling-off tendency is observed (Fig. 5a,b and c) and a non-linear rate is observed from the beginning (Fig. 15). The film or the flat membrane model, therefore, appears to be inadequate to explain the present results. The reactionsite model introduced earlier [2,13,15] may be at this stage be used. Here, the free diffusing loaded carrier is assumed to exist only in the region where all phase I droplets are saturated. The copper(II)-carrier molecules unload the copper(II) ions as soon as they meet the unsaturated phase I, re-load with counter ions present there (H+ ions from H2SO4 solution), and return to the III/II interface. The re-loaded carrier and the saturated phase-I droplets are not considered further. Thus within the globules there exists: 1. a sharp boundary at a distance x=j from the III/II interface; 2. between the globule-surface and j all phase-I droplets are saturated; 3. beyond j the phase-I is unsaturated. From the III/II interface within the globule the concentration c =C( /2 (mol/l) decreases gradually following ordinary diffusion (Frick’s second law),

    dc dt

=D

I

d 2c dx 2

for 05x5 j

(1)

and becomes zero at a point x= j. Again, at any time this change of c with time will also depend upon the freely diffusing loaded carriers c and unsaturated phase-I droplets m in a certain unit volume, hence

  dc dt

= − Kcm

(2)

II

K is a constant of proportionality, and the change in m is

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Fig. 14. (a) All holes filled-all holes empty model (after Hermans); (b) reaction site model [2].

 

dm = − Kcm dt

(3)

the net change in c is

   

dc d 2c =D −Kcm dt dx 2

(4)

Since both c and m are dependent on x and t, the system is non-linear, and the concentration c of the freely diffusing particles should show a steep gradient, steeper with larger values of K. For simplification of the treatment, a sharp boundary between the completely saturated and completely unsaturated phase-I droplets is assumed. Up to the boundary K is large, and m is practically zero at all points which have been reached by the diffusing loaded carriers. Thus, at any point x either all phase-I droplets are saturated or none, i.e. the boundary line indicates the abrupt change from all saturated to all unsaturated phase-I droplets (Fig. 14a and b). This idea corresponds to Harmans’s [37] all holes filled — all holes empty model of diffusion with immobilization by precipitation. According to the Fig. 14(a and b), at the III/II phase interface, x = 0 and a constant concentra-

tion a (= C/2) of the diffusing species is maintained. Inside the emulsion from this interface the concentration drops gradually following Fick’s second law of diffusion and becomes zero at a point x= j where the above mentioned sharp boundary exists. The slope of the c versus x curve at this point is determined by the condition: −D

  dc dx

x=j

= S8

  dj dt

(5)

The left hand side of this equation indicates the number of particles diffusing through j in unit time and the right hand side determines the quantity needed to saturate the phase-I droplets in the region covered. S is the saturated concentration. Recalling the validity C= f(x, t) and

  dc dx

x=j

 

d j+

  dc dt

d t =0

(6)

x=j

 

and using Eq. (6), Eq. (5) can be transformed to D

dc dx

2 x=j

= S8

dc dt

(7)

x=j

The solution of the diffusion Eq. (1) with the boundary conditions C(j, t)= 0 and that in the Eq. (7) leads to the result [2]:

20

A.K. Chakra6arti et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 166 (2000) 7–25



 

C( j j = 2 P exp S8 2 Dt 2 Dt

&



2

j/2 Dt

exp( − S) d s

(8)

0

This one dimensional solution indicates the boundary line moves into the emulsion droplet according to the simple relation j/ t = constant for t\0. It can be extended to spheres up to j /gd B 0.5 where gd is the mean diameter of the emulsion droplets. The graphical relationship between j 2/4Dt and C( /S points out that with increasing magnitude of the latter ratio the velocity of penetration of the boundary line decreases asymptotically. This has been found with all the results with gradual increase in copper (II) ion in the internal solution with the progress of time, and particularly, with higher concentration of the carriers (Fig. 5a,b and c) where a leveling off tendency in the increase of copper (II) ion in the internal solution is observed. It has been shown earlier [2,15] that the model is apt to explain the mass transport through high viscous emulsion j =f(t). So long the droplets behave ideally the time dependent mass transport fraction is proportional to the square root of time, a plot of Mt/M against t will produce a straight line. Mt and M are the quantities of the

Fig. 15. Enrichment and depletion profiles of copper (Cu2 + ) in the internal phase (I) and in the feed phase (III), respectively. Experimental points from curve 2, Fig. 7.

permeant transported at any time t and at the final state respectively. In Fig. 15 and in similar figures for different experiments [35], not shown here, it is seen that in the plot of Mt/M versus

t, for each, a straight line is obtained after a short time interval ( t :0.5–0.9) and again near the end of the process the direct course changes slowly. The first and the last slow non linear portions may possibly be attributed to: 1. the duration for mechanical dispersion of high viscosity emulsions in the state of uniform globules in the phase III; 2. an automatic fold up of the process, as a natural consequence, because of non-ideal behaviour of the emulsion drops arising from; 2.1. an increasing rate of growth of emulsion volume and viscosity [13]; 2.2. violation of the condition of constant concentration C( /2, Of the carrier-copper(II) compound at the III/II interface. The concentration of copper(II) ion in the phase III decreases gradually. Although for electrical neutrality the cationic deficiency in this phase is compensated by the transport of H+ ions from phase-I, however, this is incapable of upholding the condition of constant concentration of copper(II) ion. The present studies of the transport of copper (II) concerns the facilitated coupled counter transport and comes about as a result of ‘pumping effect’ or ‘passive transport’ or ‘secondary active transport’ [13]. The constant downhill concentration gradient of H+ ion from I/II to II/III interface (the phase III solution is maintained at a pH: 6 by a buffer system), therefor an increasing carrier borne transport of H+ ion along the direction of the gradient causes, for charge balance, a transport of copper(II) ion in the opposite direction (III/II to II/I interface), even against its gradually increasing higher concentration in the phase I (than in the phase III) to almost an undetectable quantity of it in the phase III. Together with this phenomenon the specificity of complex formation between copper (II) and the carriers (LIX 622 and LIX 84) at the steady pH of the phase III solution seems to be equally attributive. Since the external simulated waste-water (phase III) containing copper(II) in different sepa-

A.K. Chakra6arti et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 166 (2000) 7–25

21

ion and the carrier at the III/II interface. Again, with the ideal state of the primary emulsion globules (WI/OII) i.e. with constant surface area, the plentitude of the light carrier molecules in the oil phase apt to react with the metal ions in the raffnate at any time in the process, may be assumed to be more or less constant, the concentration of the metal ion (copper(II)) in the external phase however, varies continuously. Looking at this part of the system it appears that the rate of depletion of copper(II) in the raffinate (up to 2 to 4 ppm in it) follows a pseudo first order process, − dc/dt =Kc, c being the concentration of copper(II) in ppm at any time t and K the rate constant Fig. 16(a and b). However the value of the rate constant depend upon the composition of the primary emulsion and the condition employed. Fig. 16. Pseudo first order process of depletion of copper (Cu2 + ) in the feed phase (III). (a) from Fig. 13a, curve 1, and (a) from Fig. 7, curve 2.

2.12. Interference of chalcophlile ions in the transport of copper(II)

rate experiments when further contaminated with other chalcophile metal ions, like Ni2 + , Co2 + , Mn2 + , Pb2 + , etc., in considerable quantities (shown afterwards), have been found in all cases to be depleted mainly of copper much below its toxic level, although the added metal ions are susceptible to form complexes with the same donor groups arranged in the same way in similar ligands (viz. Salicylaldoxime) [38] but at other pH values. Generally, it is worth while to study the kinetics of the process. Both the complex formation reaction at the III/II interface and the stripping reaction at the II/I interface may be assumed to be kinetically fast. Naturally, the diffusion of the carrier-metal complex through the oil-membrane phase (III/II to II/I) may be assumed to be the rate governing step. In relation to this particular aspect the results have been discussed above through proposition and utilization of the reaction site model which concerns principally the conduct of the loaded and the light carriers in the membrane phase. However, the depletion of copper(II) with time in the raffmate (WIII) is the experimentally observed phenomenon, and that takes place through reaction between the metal

An investigation has been made for the interference, if any, of some chalcophile divalent cations, Pb(II), Cd(II), Zn(II), Co(II), Ni(II) and Mn(II), in the accomplishment of the separation of copper(II). In one set of studies with binary mixtures of Cu(II) and one of these ions in equal quantities (185 to 200 ppm), and in another with one of these ions in doubly high or more quantities (370 to 575 ppm) than that of Cu(II) in the feed phase, at pH: 6 (buffer, NaAc/HAc), O/A : 0.4 and LIX 622 (4.7 wt% in the oil membrane phase), and in two other separate sets of studies using LIX 622 and LIX 84 (28.2 and 28.6 wt% in the oil phase, respectively), O/A : 1.0 and binary mixtures of Cu(II) (186 ppm) and one among the ions, Cd(II) (163–325 ppm), Ni(II) (200–388 ppm) or Zn(II) (240–400 ppm) in the feed phase without buffer (pH:5.0), it has been found that the target depletion of copper(II) ( B 1.0 ppm) in the raffinate is not unattainable. The results of these two sets of experiments are shown in Table 3 and Table 4 together with only two exemplary diagrams (Figs. 17 and 18). Two effects are noticeable: 1. irrespective of the nature of the feed phase, buffered or unbuffered, the presence of these ions, in general, causes slower transport of copper(II) or its delayed depletion in the raffinate to the target limit (B 1.0 ppm);

A.K. Chakra6arti et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 166 (2000) 7–25

22

2. co-transport of these ions in different extents is a common phenomenon. For the binary mixtures buffered at pH: 6, using LIX 622, in the first series of experiments, containing more or less equal quantity of one of these ions to that of copper(II) (186 ppm) the target depletion of Cu(II) is delayed by 3 – 4 min to-

gether with some co-transport of the contaminant ion (4 ppm for Mn(II) to 71 ppm for Co(II) in 14 min); again, containing twice or more quantities of one of these ions (370–575 ppm) the target depletion is delayed by about twice the time required for only copper(II) in solution, and the co-transport of the added ions are increased more

Table 3 Interference of different chalcophile ionsa Foreign ions together with Cu(II) in the feed phase

Legends of ions (ppm) in the feed phase

I

II

Initial quantity t = 0 min

Depletion in 14 min

Initial quantity t = 0 min

Depletion in 20 min

Cu(II) +Co(II) +Cd(II) +Ni(II) +Pb(II) +Zn(II)

186 187 188 200 200 186

:186 71 48 30 28 26

+Mn(II)

200

:186 374 372 376 375 370 575 400

186 138 52 36 71 40 120 34

a

4

[LIX 622(4.7 wt%); O/A: 0.4; feed phase pH 6.0 (NaAc/HAc Buffer)].

Table 4 Interference of Cd(II), Ni(II) and Zn(II) ions in the transport of Cu(II)a Foreign ions together with Cu(II) in the feed phase

Legends of ions (ppm) in the feed phase

LIX 622

1. Cu(II) +Cd(II) 2. Cu(II) +Cd(II) 1. Cu(I1) +Ni(II) 2. Cu(II) +Ni(II) 1. Cu(II) +Zn(II) 2. Cu(II) +Zn(II) a

LIX 84

Initial Quantity t= 0 min

Depletion in 24 min

Initial Quantity t = 0 min

Depletion in 24 min

186 163 186 325 186 200 186 388 186 240 186 440

186 23 184 45 181 4 178 13 184 52 183 28

186 163 186 325 186 200 186 392 186 240 186 440

184 23 183 45 181 16 177 12 184 36 182 40

[Carriers: LIX 622 (28.2 wt%) and LIX 84 (27.6 wt%); O/A :1.0, feed phase pH : 5.0(without buffer), time 24 min].

A.K. Chakra6arti et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 166 (2000) 7–25

Fig. 17. Interference of zinc (Zn2 + ) in different quantities in the transport of copper (Cu2 + ) from feed phase (III) to the internal phase (I).

Fig. 18. Interference of nickel (Ni2 + ) in different quantities in the transport of copper (Cu2 + ) from feed phase (III) to the internal phase (I).

or less proportionately. Regarding the unbuffered feed phase (pH :5), O/A : 1.0, in the second

23

series of experiments, even when the copper(II) in it is not accompanied by the chalcophile ions a slow depletion of it is an observed phenomenon (25 min for B1.0 ppm) for both the carriers; the gradual decrease of pH of the feed phase because of coupled counter transport of H+ ions has been attributed to it (Fig. 9). However, the presence of these ions in the external phase causes a further delay in the diminution of copper to the target limit and included ions are co-transported. It is interesting to note that with the buffered external solutions the co-transported quantities of these ions are more than those with the unbuffered solutions. It may be mentioned that in the present study the complex formation reaction that takes place at the WIII/OII interface is essentially different from the general ways of complex formation where the quantities of the ligands available are much more than the quantities of the metal ions. In the present studies however, the quantities of the ligands (or carriers) are limited but those of the metal ions copper(II) alone or together with other ions are much high. In the case with two ions together in solution, avid for complex formation with the carrier, a competition will arise. This being the situation, the quantities of the complexes formed by either metal ion at the interface WIII/OII, the extent of their diffusion through the oil-membrane phase (OII) and giving up of metal ions at the OII/WI interface (WI = acid solution) will depend upon the pH of the external solution (WIII), conditional formation or stability constant of the complexes and their solubility in the nonaqueous organic phase (OII). Regarding the oxime complexes (aldoxime or salicylaldoxime), in general, copper(II) forms stable complexes at pH 4–6, whereas stable complexes of Co(II), Ni(II), Zn(II), Mn(II), Pb(II) and Cd(II) are formed at pH above 7.0 [38]. Again, the stability constant values of oxime complexes of Cu(II) and Co(II), in most cases, are close to one another. Those of Ni(II), Zn(II), Pb(II), Cd(II) are smaller than the values of Cu(II) or Co(II), and that of Mn(II) is the smallest [39]. Thus, at a pH 5–6 of the external solution (WIII,) albeit all these ions may participate in complex formation with the carrier (at WIII/OII

24

A.K. Chakra6arti et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 166 (2000) 7–25

interface) they will have tendency to undergo an exchange reaction with Cu(II) present in WIII, to form Cu(II)-carrier complex [40]. + + ? R2Cu(org) +M 2(aq) R2M(org) +Cu2(aq)

(M= Co, Ni, Zn, Cd, Pb, Mn) In effect, although belated, copper(II) is transported mainly, and along with it some amount of the added ion, depending upon its quantity as carrier complex in the oil membrane, will also be carried to the OII/WI interface. Among the added ions the maximum diminution of Co(II) in the raffinate may be because of the reasons mentioned above. Further, in the unbuffered external solutions, the gradual lowering of pH, because of coupled counter-transport of H+ ions, will hamper the complex formation more with the chalcophile ions than with Cu(II) (since these ions form complexes above pH 7). As a result, these ions will remain in this solution in higher quantities than in the buffered ones. In presence of chalcophile ions a fraction of the carrier in the oil membrane will always remain involved in carrying those ions and be unavailable to copper(II). This may also be a reason for delayed depletion of copper(II) in the external solution.

Acknowledgements The author AKC express his gratefulness to UGC (New Delhi) for a minor research grant, and to principal Anil Chandra Guha of Dum Dum Motijheel College for his encourgement in the research work. The authors are also indebted to Henkel Corporation, Tuchson, USA, Dr K.L. Mallik of Lubrizol, India, Dr G. Fritzsch, Max Planck Institute fur Biophysik, FrankPrut, Germany for their generous help and encourgement.

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