Electrophoretic mobility of solvent extraction oil drops: Effects of mixed extractants and metal ions

Colloids and Surfaces, 20 (1986) 109-119 Elsevier Science Publishers B.V., Amsterdam

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in The Netherlands

Electrophoretic Mobility of Solvent Extraction Oil Drops: Effects of Mixed Extractants and Metal Ions K.L. LIN and K. OSSEO-ASARE Department of Material; Science and Engineering, University Park, PA 16802 (U.S.A.)

The Pennsylvania State University,

(Received 28 October 1985; accepted in final form 10 April 1986)

ABSTRACT The electrophoretic mobility of hexane droplets dispersed in aqueous solution has been investigated in the presence of mixed extractant systems consisting of anti-5,8-diethyl-7-hydroxy-6dodecanone oxime (the active extractant in LIX63) + dinonyhmphthalenesulfonic acid (HDNNS), di(2-ethylhexyl)phosphoric acid (HDEHP) + HDNNS, and tricaprylyhnethylammonium chloride (Aliquat 336) + HDNNS. LIX63 was found to lower the electrophoretic mobility of HDNNS-hexane droplets, an additive effect in electrophoretic mobility was seen in HDEHP + HDNNS systems, while the presence of Aliquat 336 in HDNNS solution reversed the interfacial charge from negative to positive. The effects of aqueous metal ions, Ni(II), Co(I1) and Fe(III), on the electrophoretic mobility of LIX63-, HDNNS- and LIX63 + HDNNS-containing hexane droplets were also investigated. It was found that the effects of these metal ions are strongly pH-dependent. Charge reversals (CRs) were generally observed in the pH range 9-11 for Ni(II)- and Co(II)-containing systems, and in a wider pH range (3-11) for Fe(II1). The first CR is attributed to a partial coating of the oil drops by metal hydroxide precipitates, while the second CR reflects the behavior of a completed surface layer of metal hydroxide precipitate.

INTRODUCTION

The importance of interfacial activity in hydrometallurgical solvent extraction processes is receiving increasing recognition [l-6 1.The interfacial effects are particularly interesting when extractant mixtures are used. For example, Osseo-Asare and Keeney [5] have applied phase-transfer catalysis and micellar catalysis concepts to the situation where dramatic enhancements in metal extraction are achieved when a highly surface-active reagent such as dinonylnaphthalenesulfonic acid (HDNNS) is added to a chelating but less surfaceactive extractant. Although the interfacial activity in such systems has been viewed primarily in terms of the effects of interfacial concentration, it is likely that where ionizable interfacial groups are present, electrosttic ‘$ienomena may also be important. Thus in the case of uranium extraction by di(2-ethyl-

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0 1986 Elsevier Science Publishers

B.V.

110 TABLE 1 Structures of the extractant compounds studied in this work Extractant LIX63

Structure 9%

C2%

c.,H2 ~H-t-~H& ,N

Extractant

Structure

HDEHP

CH2 CH3

C&

ip//”

OH (CH (CH 1 CHCH I-O’ 23 3 I 2 CH2CH3

HO

HDNNS

I (CH3(CH2)3CHCH21-0 ‘OH

Aliquat 336

hexyl)phosphoric acid (HDEHP) and tributylphosphate (TBP) mixtures, there are indications that the observed enhancement in extraction rate may be ascribed, in part, to an increase in interfacial electrical potential [2,3]. In a recent study [7], the interfacial electrification of oil drops in the presence of various solvent extraction reagents [LIX63 (anti-5,8-diethyl-7-hydroxy6-dodecanone oxime), HDNNS, HDEHP and Aliquat 336 (tricaprylylmethylammonium chloride)] was examined with the aid of electrophoretic mobility (EM) measurements. The present communication focuses on extractant mixtures involving these reagents. In addition, the effects of metal ions [Ni(II), Co(I1) and Fe(III)] on the EM of oil drops were investigated. EXPERIMENTAL

The chemical structures of the aqueous insoluble extractant compounds used in the experiments are presented in Table 1. The purification procedures used for the extraction reagents have been described in a recent work [7]. Nickel(B) nitrate, cobalt(H) nitrate and iron(II1) nitrate were the sources of metal ions. Aqueous phase pH and ionic strength were controlled with reagent grade KOH, HN03 and KNOB. The EM was measured with a Riddick Zeta-Meter and a Teflon electrophoretie cell as described previously [7]. The organic droplets were prepared by vigorously shaking 0.35 cm3 of a hexane solution containing the appropriate extractant concentrations in 500 cm3 aqueous solution for 1 h. For studying the metal ion effects, 1 cm3 of 0.05 kmol m-3 freshly prepared aqueous metal nitrate solution was added to a 500 cm3 aqueous solution with a pre-determined pH and an ionic strength of 4x 10W3 kmol rnB3 KNO, right before conditioning. All experiments were carried out at room temperature ( w 25’ C).

111 0

-2

;

E ” > A ‘I. 5

-4

-6

I w -8

-10 2

3

4

5

6

7

8

9

IO

II

12 log

PH

([HDNNSI,

Fig. 1. Effect of pH on the electrophoretic mobility of LIX63-HDNNS-hexane strength: 4~ lo-:’ kmol m-” KNO,.

kmoln?)

oil drops; ionic

Fig. 2. Effect of HDNNS concentration on the electrophoretic mobility of LIX63-HDNNS-hexane oil drops; ionic strength: 4 x lo-” kmol rn-” KNO,,. RESULTS

The LIX63-HDNNS-hexanelwater

system

The EM of hexane droplets in the presence of LIX63-HDNNS mixtures is presented in Figs 1 and 2. Itcan be seen in Fig. 1 that in the pH region of 3 to 11, the organic/aqueous interface is negatively charged for all the four systems shown (i.e. pure hexane, hexane-LIX63, hexane-HDNNS, and hexane-LIX63-HDNNS). The magnitudes of the EM for the LIX63-HDNNS mixture are slightly lower than those of the pure HDNNS system, but, are much greater than those of the pure LIX63 system. The concentration effects of HDNNS and LIX63 on the EM of hexane droplets at constant pH (3.0) are shown in Fig. 2. It can be seen from these results that an increase in HDNNS concentration leads to a dramatic increase in the magnitude of the EM. On the other hand, the presence of LIX63 tends to make the EM less negative. The HDNNS-HDEHP-hexanelwater

system

The EM of the mixed HDNNS-HDEHP system shows two different trends with increasing pH, as shown in Fig. 3. The EM becomes increasingly negative as the pH increases until pH -8. After this point, the magnitude of the EM decreases as the pH is further increased. At low pH (14), the EM of the mixed extractant system has a magnitude similar to that of the HDNNS system. However, the mixed system becomes

112

CONSTANT

-^ ‘E 0

[HDNNS1=1.4xl~‘k4kmol

’

1.5 x lo-’

-6 -6 -0 -10 2

3

4

5

6

7

6

9

IO

II

12

t

1

Ll 2

3

I

I

I,

4

5

6

PH

I, 7

6

9

l

l

IO

II

J I2

PH

Fig. 3. Electrophoretic mobility of HDNNS-HDEHP-hexane kmol m-” KNO,,.

oil drops; ionic strength: 4x 1O-3

Fig. 4. Effect of Aliquat 336 on the electrophoretic mobility of HDNNS-hexane oil drops; ionic strength: 4x lo-:’ kmol m-” KNO,.

more negatively charged than the pure HDNNS system in the intermediate pH region (4-8). Eventually, at pH >8, the magnitude of the EM decreases and falls between that of the HDNNS and HDEHP systems. The HDNNS-Aliquat

336-heranelwater

system

The Aliquat 336 concentrations examined were 1.5 x lo-* and 7.0x lop3 kmol rne3. The latter concentration corresponds to the Aliquat 336 concentration which was found to inhibit nickel extraction by 3.7 x lo-’ kmol me3 HDNNS (5 vol. % of the 40% active as-received commercial reagent) in hexane [5]. The presence of 1.5 x 10d4 kmol m-’ Aliquat 336 in the HDNNS-containing organic phase makes the EM of the hexane droplets less negative, as shown in Fig. 4. the EM value remains essentially constant until the pH reaches N 8.8, after which it becomes more negative. In the case of 7.0 x 10e3 kmol rnM3Aliquat 336, the hexane droplets become highly positively charged in the entire pH range (3-11) investigated.

113

4r

I

I

I

[HDNNSI

I =1.4x

I

I

dkmol

I

I

I

d

t

ILIX631 = I.5 x IO-’ [N?+l = 1.0 x lO-4

[HDNNSI

= 1.4 xKi4kmol

CLIX631

= I.5 ~16~

CNi?

= I.0 x l(r4

c coz+l

= I .o x 10-4

m-3

i

LIX63-HDNNS/N

,I

::[*, 2

3

4

5

6

7

6

9

IO

II

12

2

3

4

5

6

7

6

9

IO

II

I2

PH

PH

Fig. 5. Effect of aqueous Ni2+ on the electrophoretic mobility HDNNS-bexane oil drops; ionic strength: 4 x 10e3 kmol m-a KNO,.

of LIX63-hexane

and

Fig. 6. Effect of aqueous Ni2+ and Co’+ on the electrophoretic mobility of LIX63-HDNNS-hexane oil drops; ionic strength: 4 x lo-” kmol m-’ KNO,.

Metal ion effects The EM behavior of hexane droplets in metal-containing aqueous phases with the systems LIX63/Ni(II), HDNNS/Ni(II), was investigated LIX63_HDNNS/Ni(II), LIX63-HDNNS/Co(II), LIX63-HDNNS/Fe(III), and LIX63-HDNNS/Fe(III)-Ni(I1). Individual metal ion concentrations were maintained at 1.0 x 10e4 kmol rne3. It can be seen from Figs 5-7 that the effects of metal ions on the EM of these systems are strongly pH-dependent. A general trend observed in the Ni- and Co-containing systems is that the magnitude of the EM increases with increasing pH up to about pH 5. Then follows a plateau region in the pH range 5-9. An abrupt shift of the EM from negative to positive takes place after pH 9, and a second chargereversal occurs above a pH of w 10.5. The effect of Fe(II1) on the EM of the mixed LIX63-HDNNS system, as shown in Fig. 7, is somewhat different from the behavior observed for Ni(I1) and Co(I1) ions in that a wide range of positive EM occurs between about pH 3.5 and 10. Below pH 10, the behavior of the system containing both Ni(I1) and Fe(II1) differs little from that of the pure Fe(II1) system, thus indicating that Fe(II1) is the predominant species which controls the EM in this pH region. DISCUSSION

The LIX63-HDNNS

system

The negative charge exhibited by pure hexane extractants), Fig. 1, is a result of the preferential

drops (i.e. in the absence of adsorption of hydroxyl ions

-_-_-_-_-_

[Fe=1 = CNi2f=I.0~16*kmo1 -6

-

[LIX

631

CHDNNSI

2

I

,

3

4

IT?

= 1.5 xIO-~ = 1.4 xU4

II 5

6

I 7

I 8

( 9

Ill IO

II

I2

PH

Fig. 7. Effect of aqueous Fe”’ and Fe”+ + Ni2+ on the electrophoretic LIX63-HDNNS-hexane oil drops; ionic strength: 4~10~~ kmol m-3 KNO,.

mobility

of

at the organic/water interface, while the great enhancement in EM as the HDNNS concentration increases, Figs 1 and 2, is a result of the increasing population of the interface by deprotonated HDNNS molecules [ 7 1. Interfacial tension data presented by Osseo-Asare and Keeney [8,9] indicate that a monolayer of HDNNS molecules forms at the interface when the bulk concentration is as low as 10e5 kmol mW3. The fact that the EM of the HDNNS system becomes less negative in the presence of LIX63, is a result of the interaction between LIX63 and HDNNS molecules. This interaction diminishes the surface population density of HDNNS and, consequently, lowers the EM of the HDNNS system. Evidence for such an interaction comes from interfacial tension data [8,9] which indicate a shift in the CMC to higher HDNNS concentrations as LIX63 concentration is increased, infrared spectra [8], and thermodynamic analysis of the interfacial tension data [9] which suggest the presence of hydrogen bonding between the functional groups of LIX63 and HDNNS.

The HDNNS-HDEHP The HDNNS-HDEHP that addition of HDEHP

system system differs from the LIX63-HDNNS system in to HDNNS leads to a more negatively charged inter-

115

face. This difference in the effects of HDEHP and LIX63 is due to the fact that HDEHP is more surface active [8, lo] and deprotonates more readily. Increase in pH enhances the dissociation of both HDNNS and HDEHP. In considering the difference in the pKa values of naphthalenesulfonic acid and HDEHP, 0.57 for the former [ll] and 3.49 for the latter [lo], the lack of additive effect at low pH values is understandable. Higher pH is required for HDEHP to dissociate. According to Fig. 3, a lowering in the magnitude of the EM of the HDNNS-HDEHP mixtures occurs at high pH. Similar results are also observed for the pure HDNNS and HDEHP systems, although less evident there. However, this behavior was not seen in any system not containing HDNNS or HDEHP. Since the ionic strength of the systems studied was kept constant, a possible explanation for this observation through double-layer compression [ 12,131 must be ruled out. A reasonable explanation may, however, be deduced from the idea of electrical interaction. As indicated in Fig. 1, the pure hexane/ aqueous interface becomes increasingly negatively charged as the pH increases. Thus it is likely that the resulting electrostatic repulsion forces the anionic surfactant molecules back into the organic phase, thereby decreasing the net interfacial charge density. The Aliquut 336-HDNNS system The fact that both HDNNS and Aliquat 336 are good ionizable surfactants [8, 141 is the basis for the EM behavior of HDNNS-Aliquat 336 mixtures. Charge neutralization occurring at the interface between the positively charged RIN + groups of Aliquat 336 and the negative charge of the sulfonate group on HDNNS lowers the magnitude of the EM of the HDNNS system, Fig. 4. An increase in Aliquat 336 concentration to 7.0 x low3 kmol me3 results in a positively charged interface. With the aid of interfacial tension data, it was previously shown [5] that at this concentration, the quaternary amine has completely displaced HDNNS from the interface. The effects of metal ions The strong pH-dependence of the EM in the presence of metal ions, Figs 5-7 is similar to the behavior observed with colloidal dispersion of metal oxides [15, 161 and is due to the increasing adsorption and precipitation of metal hydroxo species as the pH is raised. According to the available thermodynamic data [17,18], in 10m4 kmol me3 solutions, the hydroxides of Fe(III), Ni(I1) and Co(I1) are expected to precipitate at pH 2.3,8.4 and 8.6, respectively. It is clear from Figs 5 and 6 that even at pH values below the pH of bulk precipitation, the presence of nickel’& cobalt affects the observed electrophoretic mobility. For example, at pH 7 in the absence of Ni(II), a hexane-LIX63 drop has an

116

EM of - 4.5 pm s-‘/V cm-’ whereas in the presence of Ni(II), the corresponding EM has a value of - 3 pm s-‘/V cm-‘. Below the pH of bulk precipitation, the effect of metal ions is probably the combined result of the adsorption of hydroxo species and surface induced precipitation of metal hydroxides [ 15,161. For all three metals, the first charge reversal (CRl) occurs above the pH of bulk precipitation and reflects a partial coating of the oil surface with an amount of metal hydroxide that is sufficient to neutralize the negative interfacial charge. In the Ni(I1) and Co(B) system, the second charge reversal (CR2), observed in the neighborhood of pH 11, must be attributed to the PZC of a completed surface layer of metal hydroxide [15, 161. In a previous study [16] the PZC values of nickel and cobalt hydroxides were found to occur in this same pH region. A full coverage of these metal hydroxides on the droplet surface drastically changes the surface property of the oil droplets. In the case of the Fe(III)containing system, the low value of the pH of bulk precipitation gives rise to an extended region of positive interfacial charge on the oil droplet, Fig. 7. In the presence of Fe(III), CR2 also occurs between pH 10 and 11. However in this case the CR2 differs somewhat from the PZC which is typically in the neighborhood of pH 8.5 [ 161. Implications for metal extraction In liquid-liquid extraction systems, where the organic phase extractants (such as those investigated in this work) typically have extremely low aqueous solubility, the organic/aqueous interfacial region is the meeting place for the charged metal ions and the extractant molecules. It is necessary, then, that metal ions be able to get close to the interface in order to interact with an extractant. In view of this, for certain systems, a charged interface might be important and critical in the metal extraction reactions. For example, it is reasonable to expect that a negatively charged organic droplet would favor the extraction of a cationic species, whereas a positively charged interface would repel a positively charged species. Based on a first-order rate law for the metal extraction reaction [l], an expression for the ratio of the reaction rate of a charged and uncharged interface, may be derived as [ 19-211,

Rate(v) = exp(-z) Rate(O)

(1)

At pH 3, the zeta potential of the HDNNS-hexane system is equivalent to about - 70 mV, when the Smoluchowski relation [ 221 is applied to the data presented in Fig. 1. Referring to Eqn (l), one can see that the presence of an interfacial potential of - 70 mV would enhance the interfacial reaction rate by about ten times (z = 2 for M2+) when compared with an uncharged interface.

117

Thus, from the extraction kinetics point of view, the existence of an interfacial charge may be extremely important. In the LIX63-HDNNS system, a chelating extractant, LIX63, is used together with a sulfonic acid (HDNNS or HD). As shown by the EM results, the presence of HDNNS imparts a strong negative charge to the organic/water interface. It is likely therefore that the presence of this favorable interfacial charge situation contributes to the reported ability of the HDNNS molecule to act as a phase-transfer catalyst [5]. Thus the following reaction may be regarded as the first step in the extraction process: M?+ 1nt mt + D-1nt = MD.+

(2)

Similarly, the reported observations that the addition of minute amounts of with HDNNS (up to N 1.0~ lo-’ kmol mP3) enhances metal extraction LIX64N and with Kelex 100 [23, 241 may be related in part to the ability of the adsorbed HDNNS molecules to impart a negative charge to the organic/aqueous interface. The inhibition effect of Aliquat 336 on nickel extraction with HDNNS was previously attributed to a decrease in the interfacial HDNNS concentration as a result of the competitive adsorption of Aliquat 336 [5]. However, in view of the results of the present work, it is likely that an additional effect, i.e., electrostatic repulsion, must be considered. By converting the EM results of Fig. 4 to zeta potentials, it can be shown by using Eqn (1) that at pH 3, the ratio of the interfacial reaction rate for the extraction of a divalent cation with Aliquat 336-HDNNS mixtures to the rate with pure HDNNS (c= + 77, - 35 and - 68 mV at pH 3 for 7.0 x 10h3, 1.5 x 1O-4 and 0 kmol rnM3 Aliquat 336, respectively) is given by Rate (Aliquat 336-HDNNS) Rate (HDNNS)

= 0.28 for 1.5 x low4 kmol mP3 Aliquat 336 = 0

for 7.0~ lop3 kmol m-3 Aliquat 336

GM (3b)

These results clearly show the dramatic lowering in the rate of a first-order interfacial reaction by the presence of Aliquat 336 and are consistent with the experimental extraction results [ 51. The effects of surfactants on the kinetics of metal extraction have also been studied by Yagodin et al. [25]. These investigators found in a study of copper extraction by hydroxyoxime reagents that the presence of anionic surfactants (e.g. sodium lauryl sulfate) in the organic phase increased the metal extraction rate. On the other hand, the introduction of a cationic surfactant (cetylmethylammonium bromide) decreased the extraction rate. Similar findings were reported by Miyake et al. [26] who found that copper extraction with 2-hydroxy-

118

5-t-nonylacetophenone oxime (the active reagent in the Shell extractant SME 529) decreased in the presence of a cationic surfactant (C&H,,N(CH&Br) but was enhanced in the presence of an anionic surfactant (C&,H2,0S0,Na). Both groups of investigators attributed their findings to interfacial electrostatic effects induced by the ionogenic surfactants, although they presented no interfacial charge data. However, the EM results provided in the present paper support their interpretation. Besides the effects of the organic phase species discussed above, the presence of aqueous phase metal ions can also modify the interfacial charge, as shown in Figs 5-7. An important implication of this observation is that metal ions which are not extracted (for instance because of steric effects) may, nevertheless, influence the rate of reaction of the extractable species. The interaction of aqueous metal ions with charged organic droplets may also play a role in the formation of the interfacial films which adversely affect mass transfer and phase separation (i.e., crud formation [27-291 and structural mechanical barriers [30, 311. It appears from the present results that the first step in the formation of such films may be the interaction of aqueous metal hydroxo complexes with charged organic droplets; the presence of metal ions in the aqueous phase has been found above to have a strong pH-dependent effect on the EM of organic droplets. It has been reported that interfacial film formation is most severe in systems containing highly hydrolyzable cations, i.e. the polyvalent cations such as AP+, Fe3+, Zr4+ and Mo6+ , etc. [ 27-311. According to Yagodin et al. [31], when the organic phase is given a positive electrostatic charge (via an electrode immersed in the organic phase) or cationic surfactants are added, film formation is enhanced at the organic/aqueous Ta-solution interface. In contrast, the opposite effect is observed with Zr solution. The implication is that in the case of Ta, film formation is due to the adsorption of anionic metal hydroxo complexes whereas cationic metal hydroxo complexes are responsible for film formation in the Zr system. ACKNOWLEDGEMENTS

This work was supported No. CPE 8110756.

by the National

Science Foundation

under Grant

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