Kinetics of solvent extraction of metal ions with HDEHP—II Kinetics and mechanism of solvent extraction of V(IV) from acidic aqueous solutions with bis-(2-ethyl hexyl) phosphoric acid in benzene

J. inorg, nucl. Chem. Vol. 42, pp. 421--429 Pergamon Press Ltd., 1980. Printed in Great Britain

KINETICS OF SOLVENT EXTRACTION OF METAL IONS WITH HDEHP~II KINETICS AND MECHANISM OF SOLVENT EXTRACTION OF V(IV) FROM ACIDIC AQUEOUS SOLUTIONS WITH BIS-(2-ETHYL HEXYL) PHOSPHORIC ACID IN BENZENE. F. ISLAM and R. K. BISWAS* Department of Applied Chemistry, University of Rajshahi, Rajshahi, Bangladesh (Received I December 1978; received[or publication 18 July 1979)

Abstract--The rate of extraction of V(IV) from aqueous sulphuric acid solutions (0.5 tool dm-3 sulphate and 0.25 tool dm-3 acetate buffer) with HDEHP [HDEHP = H2A2= bis-(2-ethyl hexyl) phosphoric acid] in benzene and the back extraction of V(IV) from V(IV)-DEHP chelate in the benzene phase to aqueous sulphuric acid phase (0.5 tool dm-3 sulphate and 0.25 tool dm-3 sulphate and 0.25 tool dm-3 acetate) have been studied under various conditions. The rate expression for forward extraction can be written at (30+-I)°C as:-dCvAIdt= 10- 2 ~ [H2A2]Oo~[H+]- i ( 1+ 2.09 [Ac-]) ( 1+ 1.175 [SO2-])-i (i + 1.44 [NO3-]), and the backward extraction rate expression can be written as: - dC°ldt = 10 -o.6~ (Cv) o [H2A2]~o) -o.5 [W] (1 + [NOi]) -I (1 + 0.5 [Ac-])-I. In absence of the sulphate, acetate and nitrate ions, the formation of the l: I complex (VO~++ A- ~ VOA+) is the rate determining step in the forward extraction, whereas, the liberation of a DEHP- (A-) anion from the monomeric complex (VOA2--}VOA+ + A-) is the rate determining step in the backward extraction. The rate data is comparable with the equilibrium data.

INTRODUCTION

There are several reports on the rate of solvent extraction of Be(II), Fe(III), Ga(III), etc. by thenoyltrifluoroacetone (TI'A) and several B-diketones[l-5]. The kinetics of solvent extraction of Fe(II) with HDEHP and other alkyl phosphoric acids were also studied by several workers[6]. In the previous paper of this series[7], the rate of solvent extraction of Ti(IV) from sulphufic acid solutions with HDEHP in benzene and the back extraction of Ti(IV) as TiO-DEHP chelate in benzene into aqueous sulphuric acid solutions were discussed. In all these systems, the equilibria were established very slowly. But recently fast reaction kinetics in the system Co(II)-HYl'A-pyridine base have been reported[8]. The extraction of V(IV) resembles the extraction of Co(II). There is also a paper on the kinetics of extraction of V(V) from HCI solutions with TBP[9]. This paper discusses the kinetics and mechanism of extraction of V(IV) with HDEHP and compares the data with other results and that obtained in equilibria studies. EXPERIMENTAL Reagents. Reagent grade chemicals were used: vanadyl sulphate dihydrate (Fischer Scientific Company), benzene (purified b.p. 79-81~,M. C. and Bell), acetic acid (99.4%, E. Merck), sodium acetate (>99%, M. C. and Bell), sodium sulphate (extrapure, E. Merck), sodium nitrate (extrapure, E. Merck), sulphuric acid (sp. gr. 1.84, BDH), nitric acid (sp. gr. 1.43, BDH), hydrogen peroxide (3t~o, E. Merck), bis-(2-ethyl hexyl) phosphuric acid (Eastman Kodak). All the chemicals including HDEHP were used without further purifications. Recovered benzene was also used. Procedure. The stock solution of V(IV) was prepared by dissolving a weighted amount of VOSO4. 2H20 in dilute sulphuric acid. Its vanadium content was then estimated by a spectronic-

*Author for correspondence. 421

20. Working solutions containing 0.50 mol dm-3 sulphate and 0.25 tool dm-3 acetate, and 460 nagdm-3 ( ~ 9 x 10-3 tool dm-3) V(IV) were prepared from the above solution. The stock solution of V(IV)-DEHP was prepared by dissolving the solid V(IV)DEHP complex in benzene and the vanadium concentration was determined by complete back-extraction with sulphuric acid solution (2 tool dm-3) followed by estimation of the V(IV) in aqueous phase. The details of the procedure are essentially the same as those described in the previous paper[7], except that the acidity of the solutions were measured by a Coming-SA model pH meter. The organic phases were benzene solutions of HDEHP in the forward extraction study and of V(IV)-DEHP complex (containing 450mgdm -3 of V(IV)) in HDEHP for the backward extractions. In the backward extractions the aqueous phases were aqueous sulphuric acid solutions containing 0.5 tool dm-9 sulphate and 0.25 tool dm-3 acetate. The aqueous pH was adjusted by addition of anhydrous sodium carbonate if necessary. The initial volume of each phase was always 10 hal. Two phases were mixed in a 100 ml. stoppered bottle and agitated at 150 strokes/rain, at (30_+I)*C. The shaking speed was chosen to be high enough that no acceleration of the rate of extraction was observed upon further increase, thus any effect due to diffusion to or across the phase interface could be eliminated as has been described in the previous paper[7]. After the desired time, the two phases were separated. No emulsification was encountered. A certain portion of the aqueous phase was pipetted out as soon as possible and the content of V(IV) was estimated spectrophotometfically[10] using a spectronic-20 and measuring the absorbance at 450 m,a. The organic phase concentration of vanadium was then calculated by the method of difference. The solid V(IV)-DEHP complex for the backward extraction rate study was prepared as in[ll], except that in this case, the organic phase was i.0 tool dm-3 HDEHP in benzene, and the aqueous phase contained about 15 g dm-3 vanadium at pH 1.6 The product was a bluish green solid. The molecular weight of the complex was measured in benzene (1975), cyclobexane (2138) and carbontetrachloride (1913) by the cryoscopic method using a Beckmann apparatus. The phosphorous-vanadium atom ratio was determined to be 2:1 as described in the previous paper[7].

422

F. ISLAM and R. K. BISWAS

NOTATION c~, Conc. of V(IV) in the aq. phase, c ° Conc. of V(IV) in the org. phase, Extraction equilibrium constant, Do Partition coefficient of HzAz in between the aq. and ore. phase, gal First acid dissociation constant of H2A2, Second acid dissociation constant of HzA2, Forward and backward rate constants respectively at 0.5 moi dm -3 sulphate and 0.25 moi dm -3 acetate media, ktO,rib *~ o * Forward and backward rate constant respectively in absence of sulphate and acetate, k. k* X V - I / 2 v I d l / 2 v f,xl/2 kYx K ~12x K o t -I] 2 xg~2-1/2 , Forward and backward rate constants respectively at 0.25 mol dm -3 acetate and no sulphate, O* O* kf. , kb" Forward and backward rate constant respectively at 0.5 moi dm -3 sulphate and no acetate.

k?,',k~

where, o~ is equal to k ~ . (ha) b • (h)~; ha and h are the constant concentrations of the extractant and the acid in a series of experiments. The value of " a " was determined by the fractional life-period and the inital rate method[7, 12] and was verified by trial[7]. When the reaction order w.r.t. V(IV) is 1 (as found), the following eqn is obtained by the integration of the eqn (9): log CO = q~ x t + C

where, C is a constant and q~ = - qs/2.303. At constant extraction concentration ha, the quantity qh~+ may be defined:

k*

Treatment of data (a) Extraction equilibrium. Defining the distribution ratio, D by: D~

C vo / C vA

(1)

log q/H+ = log k~ x (ha)b _ cpH

log q/u2A2= log k~ x (h)C + b log [H2A2](o)

where, K¢~ is the extraction equilibrium constant, Z is the number of HzA2 involved in the reaction and Y is the number of H + liberated due to reaction. The suffix (o) represents the organic species. (b) Rate of forward extraction. The rate under our conditions is dependent only on the temperature and the concentrations of the materials in the system. Therefore at constant temperature, the extraction rate may generally be described as[l-5, 7]:

(12)

The rate constant, k~ may be calculated from the eqns (11, 12) and the experimental data, after obtaining the values of " b " and ~6C''"

The effect of sulphate, acetate and nitrate ions on the rate of forward extraction can be represented by the following empirical expression: log qfL = log k~ x (ha) ~ x (h) c + log(l + k/(L)(L])g

(2)

(1 I)

Similarly, at constant acidity (h), the quantity qtH2A2 may be defined:

the extraction equilibrium can be written as: log D = log K,,~ + Z log [H2A2](o)+ Y pH

(10)

(13)

where, g = 1 for L = Ac- or NO3 when k~ = k~.* or k~ respectively and g = - 1 for L = SO42- when k':= k~,', owing to the fact that the reaction order w.r.t, acetate and nitrate ions varies between zero to one and that of sulphate between zero to minus one. So that: L!

[L] "-)o log OIL = log k~ x (ha)b x (h)~

and LI

dCvA = k ~ x (CvA) a ×

dt

[L] --) ® log aaL = log k~ x (ha)b x (h)C + g log (ks(L)[L]). [H2A2]~o) x [H+] ~

( 3)

at constant 0.50moldm -3 sulphate and 0.25 moldm -3 acetate medium. Here, the activity of water is omitted. The distribution coefficient (Ko) and dissociation constant (Ko, and K ~ of the dimeric chelating acid, H2A2 may be described as: KD -- [H2A2](°) [H2A2]

(4)

go, = [HA~'] [H +] [H2A2]

(5)

[H+J[A-] 2 K"2= [HA/]

(6)

[H2A2](o)= KD X K~,I X K;~ x [A-] 2 x [H+]:

(7)

When b = 1/2c the eqn (3) can also be written: dCv A - dt = kl x (CvA)° x [A-] -c

(8)

where, vl/2 v V-I/2 aXa I ~ ZXa2

dCv A dt kb* x (Cv°) a x [H2A2]~o)x [H+]/

dt = o.i x (COY'

(9)

(14)

for 0.5 mul dm -3 sulphate and 0.25 tool dm -3 acetate media. As above, if the extractant concentration is in large excess relative to V(IV)-DEHP complex and the aqueous phase is buffered, one gets:

q~(Cv°)~

(15)

where, qb ffi k *b x (hay x (hi. The value of "d" may be obtained by the differential method applied to single runs, when log (dCvAldt)vs Iog Cv ° plots should be straight lines with slope equal to "d": If the reaction order w.r.t. V(IV)-DEHP is' unity the following equations is obtained by the integrations of eqn (15): log C v ° = q 'b " t + C' ,

As the sulphate and acetate (buffer) concentrations are kept constant and the concentrations of HDEHP is in a large excess relative to vanadium, the rate eqn (3) can be written as[3]:

dC~

dCv ° dt

dCv ° dCv A dt "- d i =

From the eqns (4-6), one gets:

k f --- b. x/all/2 v ~f ~O ~

The values of k~(L)may be obtained from the above plots by trial. (c) Rate of backward extraction. In the like manner, at a constant temperature, the rate of backward extraction can be expressed as [7]:

(16)

where, C' is a constant and q~, =-qd2.303. If the extractant concentration is kept at a certain value, (ha) a quantity qb, ~ may be defined as in eqn (11) by: log q~+ = log kb* x (ha)" - / p H

(17)

Kinetics of solvent extraction of metal ions with HDEHP--II

2.0

and, at [H+] = h, the quantity qb.,A;is defined as in eqn(12) by: log qb,2A~= log kh* X(h)/ + e log[HzA2]lo~

Q

(18)

Using eqns (17-18), the rate constant k~* can be evaluated after finding "e" and "/". It is found that the rate of backward extraction is independent of sulphate concentration over the sulphate concentration ranges studied and is independent of acetate and nitrate concentrations in the lower concentration ranges. But as the rate is inversely first order dependent on acetate and nitrate at higher concentration, the following general empirical expression can be written:

IogqbL=logk'~ .(ha)~ • (h)+- log(l + kblL~[L])

/'

1.0

/ /

0.0

/

0 J

/ ¢,

(19) -1.O

-2.8 B

RESULTS (a) Extraction equilibria The Figs. 1 and 2 represent the variation of the distribution ratio with aqueous pH and the HDEHP concentrations in the organic phase respectively. The Fig. 1 shows that the value of Y in eqn (2) is 2.5 and that of Z is 1.2-1.8. Therefore, the eqn (2) takes the form: (20)

with the average value of Z = 1.5. The extraction equilibrium constant K¢~ has been evaluated and is tabulated in Table 1. (b) Rate of forward extraction The data used for analysis were only those obtained from the experiments in the early stage of extraction when the effect of back extraction reaction may be considered negligible. (i) Reaction order w.r.t. V(IV). The Fig. 3(a) shows the log (-dCvA/dt)vs ]ogCv A plots. The slope of the plots is approximately unity. Moreover, the slope of log tmovs log Cv A is zero (Fig. 3(b)). Hence, the reaction order w.r.t. V(IV) is unity. (ii) Reaction order w.r.t. H +. The log tbH+ vs pH plots at constant HDEHP concentrations of 0.05,0.10 and 0.20 mol dm -3 are shown in the Fig. 4. The lines have a slope of approximately 1. Hence, the reaction order w.r.t. H ÷ is c = - 1 i.e. the rate of extraction is inversely

®

/

where, k'b=k °*, K°.* and k~ for L=SO~-, Ac- and NOr respectively and kb(L)----0 for L = SO~-. The values of the constants kb(L)may be obtained from the log qbLVSlog[L]plots by the method of trial.

log D = log K¢~ + 1.5 [H2A2](o ~+ 2.5 pH

423

I

]

1.2

1.6 pH

.....

J-

Fig. 1. Dependence of the distribution ratio on the aqueous pH. SO42- = 0.5 tool dm-3, Ac- ---0.25 tool dm-3. Slopes (S) and intercepts (I) are calculated by the least squares method. (~), HDEHP = 0.050 tool dm-~ S = 2.47, I = - 3.75, r* = 0,9989; (ID), HDEHP = 0.075 tool dm-3 S = 2.45, I = - 3.55, r = 0.9981; (~), HDEHP=0.100moldm -J S=2.49, I=-3.38, r=0.9905; (@), HDEHP=0.150moldm -3 S=2.50, I=-3.00, r=0.9983; (®), HDEHP=0.300moldm -3 S=2.48, I=-2.48, r=0.9915; (O), HDEHP=0A00moldm -3 S=2.48, I=-2.50, r=0.9903; (@), HDEHP = 0.200 tool dm-3, S = 2.50, I = - 2.70, r = 0.9942. *r = Correlation coefficients. proportional to the hydrogen ion concentrations in the aqueous phase. (iii) Reaction order w.r.t. HDEHP. In Fig. 5, the plots of log tbn A VS 1og[H2A2](o~ at constant pH of 1.70, 1.45 2 2 • . and 1.20 give straight hnes with slope equal to 0.50 approximately. So, the order w.r.t, the extractant is b = 0.50 i.e. the rate is directly proportional to the square root of the HDEHP concentration in the organic phase. (iv) Reaction order w.r.t. SO~-. The Fi~. 6 represents the variation of log qlso,2- with log [SO4~-] at constant acetate and HDEHP concentrations of 0.25 moldm -3 and 0.2 mol dm -3 respectively at pH 2.08. The rate was found to decrease with increasing sulphate concen-

Table l. Evaluation of the extraction equilibrium constant, K~x(eq) Fig. No.

[H2A2(o

0.050 0.075 0.100 0.150 0.200 0.300 0.400

av. eq. pH

w

Intercept, 1 - 3.75 - 3.55 - 3.38 - 3.00 - 2.70 - 2.48 - 2.30

log K~x = I-1.5 log [H2A2](o~ -

log K~x = I-2.5 pH

Mean log K,x

w

0.45 1.00 1.30

1.60 2.01 2.24 2.36

St.dev. 8

1.799 1.863 1.880 1.764 1.651 1.696 1.703 -

0.900 1.075 1.235 1.385 1.540 1.700 1.750

1.88

1.800 - i.650 - 1.790 1.860 1.860 - 2.028 -2.020 -

1.812

0.33

424

F. ISLAM and R. K. BISWAS -1.O

2.0

-1.5

[

1.0

-2.O t% o~ 0 d

0.0

..J -2.5

- 1.0

-3.C

l -1.0

-2. - 1.5

I - 0.5 - 0 2 5

Logt"2A2 OFig. 2. Dependence of the distribution ratio on the HDEHP concentrations. SO~- = 0.5 mol dm -J, Ac- = 0.25 tool dm -3. Slopes and intercepts are calculated by the least squares method. (~), pH =0.900, S = 1.85, I =0.45, r =0.9983; (~), pH = 1.075, S = 1.80, I = 1.00, r = 0.9973; (~), pH = 1.235, S = 1.62, I = 1.30, r =0.9980; (O), pH = 1.385, S = 1.50, I = 1.60, r = 0.9992; (~), pH=1.540, S= 1.40, I=2.01, r=0.9921; (~), pH= 1.700, S= 1.25, I = 2.24, r =0.9931; (O), pH= 1.750, S= 1.20, I = 2.36, r= 0.9902. trations. The experimental points do not fall on a straight line, but a curve is obtained which approaches a negative slope of unity at higher sulphate concentration ranges and zero slope with low sulphate range. The constant kf(so=2-) in eqn (13) was determined by trial and is listed in Table 3. The rate constant, k~," was also evaluated from the asymptote in the lower sulphate range of the curve and is shown in Table 3. (v) Reaction order w.r.r Ac-. The rate was determined as a function of the acetate ion concentration at

t2

0.4

0,4

•

0

~"

Q,O j~"

I 0.5

0

I 1 .O

I t .5

pH

)

I 2 .O

2.5

Fig. 4. Determination of the order w.r.t. H + in the forward extraction . The slopes and intercepts of the lines have been calculated by the least squares methol. (A), HDEHP= 0.5 tool dm -3, S = 0.9272, I = - 2.5494, r = 0.9943. (I-7), HDEHP = 0.2 tool dm -3, S = 0.9786, I = 2.7855, r = 0.9997. (O), HDEHP = 0.1 tool dm ~3, S = 0.9668, I = 3.0360, r = 0.9989.

an H D E H P concentration of 0.2 mol dm -3 and pH of 1,5 with sulphate concentration 0.5 tool dm -3. Figure 7 shows the results, where, log qfAc- is plotted against log[Ac-]. The rate is found to be accelerated by the acetate ion and becomes almost proportional to the acetate concentration in the higher concentration range. The values of the constants k1~Ac-)and k~-" were obtained as above and are quoted in Table 3. (vi) Reaction order w.r.t. N O r . It is found that the rate of extraction of V(IV) is greatly increased by the presence of nitrate in the aqueous phase. Figure 8 shows the variation of log qlso3- with log[NO3-] for the system containing 0.5 tool dm -3 sulphate in the aqueous phase at pH 1.2 and 0.2 tool dm -3 HDEHP. The log qtNo,- values increase with increasing log[NO3-], at higher[NO3-] ranges approaching unit slope, but in the lower nitrate ranges, the rate is independent of the nitrate ion concentration. The values of the constants kf~No3-) and k~ are given in Table 3. (vii) Evaluation o[ rate constants. The value of the rate constant k* for 0.Smoldm -3 sulphate and

]

~-I.0

o _J

J 0"0.9

2.1

2.3

25

2

8"

_J

-1,5

Log C~v~ Fig. 3(a) and 3(b). Determination of the order w.r.t. V(IV) in the forward extraction. (a) log (-dCvAIdt) vs log CvA plots. The slopes and intercepts of the lines have been calculated by the least squares method. (O), dt = (5-2.5) sees., S = 1.2849, I = 2.2646, r = 0.9864; (©), dt = (5 - 0) sees., S = 0.8755, I = 1.3279, r=0.9723; (A), dt =(7.5-5) sees., S=0.9381, I=1.6818, r= 0.9928; ([]), dr=(10-5) sees., S=0.9703, I=-1.8335, r= 0.9932. (b)log tmoVS IogCva plots. (A), pH =2.00, HDEHP = 0.30 tool dm -3, S = 0. (~7), pH = 1.78, HDEHP = 0.40 tool dm -3, S=0. -

- 2 .O <2

-o.e

-o,4

o

Log [H2A2](o-T---~ Fig. 5. Determination of the order w.r.t. HDEHP in the forward extraction. ([2]), pH = 1.20, S =0.6318, I = - i.2344, r =0.8810; (A), pH= 1.45, S=0.6068, I = - i.0218, r =0.9627; (O), pH = 1.70, S = 0.4949, I = -0.6429, r = 0.9951.

Kinetics of solvent extraction of metal ions with HDEHP--II

425

1.O

/ /

~ , ~ O.O

/ _I

-~ - 2 . 0

- IoO

\

\

\ /

---'~.o~-~"

L

l"

-,.o

I

~o

,.o

Locj [SO41

-2.~ O

2.0

I

I

- 1.O

,

Fig. 6. Effect of sulphate on the rate of forward extraction: log q/so~ vs log[SO42-] plots. The solid curve is log q/so 2-= l o g k ~ ' (ha) °'5 (h) - I - l o g (l+k/so2_×(SO42-]), where, ~a = 0.2 tool dm -3, - log h = 2.08 and k/(so(~-,is found to be 1.175). The broken lines are two asymptotes: log q/so2-= log k~.' (0.2)05+ 2.08 = I and log q/so2 = I - log (k/,so42-[SO44' 2 -]). The dotted and broken line represents log q/so :- value at 0.5 mol dm -3 sulphate concentration, k~ ° is calcdlated to be 10-2-'~, Ac = 0,25 mol dm -3.

1.O

LogINO3 ]

)

1.7

l.S

¢

A .,)

/

//////

2.O

Fig, 8. Effect of nitrate on the rate of forward extraction. The solid curve is logq/so3 = l + l o g ( l + k / ( N o : ) [NO£]), where, I = logk~x(ha)°Sx(h) -''°, (ha)=0.2moldm 3, and logh=l.20. The value of k;(No:) is found to be 1.44. The broken lines are two asymptotes: log qfsoc=l = -1.66 and log q:~o, = - 1.66 + log (k/(No:) [NO3 ~] x SO42- = 0.5 tool dm -~, A c 0.25 tool dm -3.

1.O

I o.o

I

O.O

%>

O

O

1.3

0 -1

2'

g, ~ -t.O / / I - 2,~

- 1.O

I

I

Q.O

I.O

Loq[Ac-]

o,,

2.O

.)

Fig. 7. Effect of acetate on the rate of forward extraction. The solid curve is log ql,o- = log k~'. (ha)O5 (h)-~° + log (1 + k/(Ac ) [Ac ]), where, ha =0.2moldm -3, - l o g h = 1.5 and k/,A¢ ) is found to be 2.04. The broken lines are two asymptotes: log qI,,,- = log k~,* x (ha) °'5 x (h)-L° ---I = - 1.41 and log q/^o- = I + log (k/(^c-)[Ac-]). The dotted and broken line represents the log q/~- value at 0.25 tool dm 3 acetate, k~' is calculated to be 10 ~.56 SO4~- = 0.5 tool dm -3. 0.25 mol dm -3 acetate media was evaluated from the intercepts of the lines in Figs 4 and 5, with the help of eqns (11, 12). The values are given in Table 2. The rate constant, k ~ ' , in the absence of sulphate and acetate is evaluated from eqn (3) which is for 0.5 tool dm -3 sulphate and 0.25 mol dm -3 acetate media and the following relation:

2.2

2A

4

5

(ha)

pH= -log(h)

0.10 0.20 0.50

----

- 3.0360 - 2.7855 - 2.5494

1.20 1.45 1.70

- 1,2544 - 1.0218 - 0.6429

~

2.8

Log~ Fig. 9. Determination of the order w.r.t. V(IV)-DEHP complex in the backward extraction. The slopes and intercepts are calculated by the least squares method. (O), HDEHP = 0.09 tool dm -3, pH = 0.50, S= 1.2833, I=-1.6093, r=0.985 (A), HDEHP= 0.09 tool dm -3, pH = 1.88, S = 0.9884, I ---- 1.2096, r = 0.987 ([]), HDEHP = 0.20 tool dm -3, pH = 0.45, S = 1.0793, 1 = - 1.2131, r = 0.9842.

dCv A dt = k ~ " x C v A x [H2A2]°o~[H÷] - '

x (1 + 1.175 [SO~-])-' (1 + 2.04[Ac-])(1 + 1.44[NO3-]) and is shown in Table 3(a).

Table 2. Evaluation of the forward rate constant, k~s Fig. No.

I 2.6

Intercept, k~= Mean I 10cl-pH) k~ = 10(I 0.51ogha) k~ ---10-2"454a 10 -:'4718 10 -2"4429

10-~4~s° 10-2.5555 10 z~994

426

F. ISLAM and R. K. BISWAS O.O~

-0.6

-0.5

1+3:-1.0

o-

1 81 -'-"1.00

-1.5

I -0.75

I -0.50

I -0.25

OD

L°9 [ H2 A2 ](o)

-2"00

0.5

10 pH

1.5

2.0

,

Fig. 10. Determination of the order w.r.t. H÷ in backward extraction. The slopes and intercepts of the lines have been estimated by the least squares method. (A), HDEHP= 0.50moldm-3, S=-0.8832, I=-0.5535, r=-0.9931; (O), HDEHP = 0.25tool dm-3, S = - 1.0495, I = -0.2691, r = -0.9949; ([~), HDEHP=0.10moldm-3, S=-0.7132, I=-0.3358, r= -0.9904.

Fig. 11. Determination of the order w.r.t. HDEHP in the backward extraction. Ths slopes and intercepts of the lines have been estimated by the least squares method. (@), pH =0.55, S = -0.4873, I = - 1.2095, r=-0.9815; (rT), pH =0.85, S =-0.5665, I = - 1.4673,r = -0.9909; (©), pH = 1.15,S = - 0.5650, I= -1.8611, r = -0.9833.

0.0 \ T,, T ~ -1.0

(c) Rate of backward extraction The initial organic phase V(IV) complex concentration was always 450 rag dm -3. (i) Reaction order w.r.t. V(IV)-DEHP chelate. The Fig. 9 represents the plot of log (dCvA/dt)vs log Cv° for three different systems having pH, H2A2(o), acetate and sulphate concentrations as parameters. The plots show a straight line relationship with slope equal to approximately unity. This indicates that the order w.r.t. V(IV)-DEHP complex in the backward extraction is I. (ii) Reaction order w.r.t. H +. It was observed that the rate of backward extraction was enhanced by increase of acidity. The log qbu+ VS pH plots are shown in Fig. 10 for the extractant concentrations of 0.50, 0.25 and 0.10moidm -3. The slopes of the line obtained were approximately-f = - 1 (cf. eqn 17), indicating that the rate is directly proportional to the H + concentrations. (~1) Reaction order w.r.t. HDEHP. The reaction order w.r.t. HDEHP in the backward extraction was determined by plotting log q~2^2 against log [H2A2](Fig. l l) with the help of eqn 08) and was found to be approximately -0.5 (iv) Reaction order w.r.t. SO42-. The Fig. 12 (log qso42- vs log [SO42-] plot) indicates that the hackward rate is not effected by the sulphate concentration in the aqueous phase. In eqn (19), the rate constant k °* will thus be identical with k b*. (V) Reaction order w.r.t. Ac-. Unlike sulphate, the presence of acetate retarded the rate of backward extraction. Figure 12 shows the log qbAo- VSlog [Ac-] plot.

Y) \ - 3.~

-'.0

I

-1.0

I

0.0 Lo9 [At:-] LoglSOz:]

The value of 1ogqbAc- decreases with the increasing acetate concentration. The decrease is less pronounced in lower acetate ranged, but, at high acetate concentrations, the rate is inversely proportional to the acetate concentrations. The value of the constant kb<^c-) in eqn (19), was evaluated by trial. The value of the rate constant, kb°,* was obtained from the asymptote with lower acetate concentration range. The value of k °" together with kb
(a) Constants for the eqn: dCv A dt

•

=/¢!

o*. (CvA) [H2A2]}~o~in+]-, (l + ky(so,-)[SO2-])-1 (1 + kf(N03-)[NO3,])(1 + k1(Ac->(Ac-]).

k~ *= 10-2't84; kf(so42-)= 1,175; k/~o~-)-- 1.44; kytA~-) = 2.04 (b) Constants for the eqn.: d C v ° = k ° * . (Cv ° ) [n2A2]~-°~5 x

2.0

).

Fig. 12. Effect of sulphate and acetate on the rate of backward extraction, (@),logq~so.2- vs log [SO42-) plot, Ac- = 0.25 tool dm-3, HDEHP --"0.2 mol dm-3, pH = 1.0; (©), log ql,,vs log [Ac-] plot. The solid curve is drawn by log qbA=-=I - log (l+kb~Ac-, [Ac-]), where I=log kq*x(ha)-°'5, ha= 0.2 tool dm-s, -log h = 0.8 and kb~^c-~is found to be 0.50. The broken lines are two asymptotes: IOgqbAc-=I=0.97 and log qb,~- = 0.97 -- log k~A-~- log [Ac-]. The broken and dotted line represents the log qb~- value at 0.25mol dm-3 at 0.25tool dm-3 acetate, k °* is calculated to be 10-°ss and SO42-= 0.5 tool dm-3.

Table 3. Summary of rate and other constants

-

I

1.0

[H+](I + kb(NO3-)[NO3-]Xl+ kb(Ac-)[Ac-])" dt k °*= 10-°'63a7; kb(NO3-)= 1.0; kbtAc-) = 0.5. (C) Other constants: k~.=10-2.2se; k ro. = 10-2560 ' ;kb,o* = 10-°'63;k°-*= 10-0.ss

427

Kinetics of solvent extraction of metal ions with HDEHP--II

•

power dependence of V(IV)-DEHP, HDEHP and H + concentrations on the backward rate, lead one to write the following rate expressions (cf. eqns 3, 14): -1 o0

~

~

--~-

d Cv A

dt = k~ × CvA x I_ rH2A21(o)t lo.s rH+l-1 J

(20)

and

I -I.o

I

I ~.o

ao Log [ N O -3 ]

dCv ° -~=k*xCv dt

\

1-o.s trH+ll ° X trH2A2J
(21)

2.0

Thus K,~tr,t.)=k~'/k*

".

= 10 -1"7653,

but from Table 1.

K,x<.q) = 10-1"812.

Fig. 13. Effect of nitrate on the rate of backward extraction. The solid line is drawn by log qbNo:= log k o*. (ha)-°s (h)-log (1 +. kbtso3~)[NOf])=I-log (l+/q, tNo:~ [NO3-]; where ha= 0.2 mol dm-3, -log h = 0.8 and kbtNo:) is found to be 1.0. The broken lines are two asymptotes: Iogq~o-=-l.04 and log qbNof= -- 1.04-IogkbtNOf)- log [NO3-]).SO4="-= 0.5 mol dm-3, Ac- = 0.25 mol dm-3. (vi) Reaction order w.r.r NOi. The rate was determined as a function of nitrate concentration in the aqueous phase when the HDEHP concentration and pH were 0.20 tool dm-s and 0.80 respectively. Figure 13 shows the log qb~o3- VS log [NOf] plot; the rate is almost inversely proportional to the nitrate concentration in the higher concentration range. The constant kbtNO:) in eqn (19) was evaluated as above and is shown in the Table 3. (vii) Evaluation of backward rate constants. With the help of eqns (17, 18), the value of backward rate constant kb*, for the 0.50moldm -3 sulphate and 0.25moldm -3 acetate media, was determined from the intercepts in Figs. 10 and 11. The values are in Table 4. The rate constant in absence of sulphate and acetate kb°" was evaluated from eqn(14) which is valid at [SO~-] = 0.5 mol dm-3, and [Ac-] = 0.25 mol dm-3 and the following relation:

dC°v * k ° ' x C v - ~ = dt

° x H2A2(o) -0.5 [H + ]

× (1 + (NO~-)-' (1 +0.5 Ac-)-' The value is quoted in Table 3(b). DISCUSSION

At constant sulphate and acetate concentrations of 0.50moldm -3 and 0.25 moldm -3 respectively with no nitrate, the first, one-half and inverse first power dependence of V(IV), HDEHP and H + concentrations on the forward rate; and the first, inverse one-half and first

The above two equations show that the equilibrium constant obtained from the equilibriumstudy agrees with that obtained from the rate study. T. Sekine et al. have reported the kineticsof solvent extraction of Be(II)[I],Fe(llI)[2--4]and Ga(III)[5] with thenoyltrifluoroacetone(TTA). Freiser et al. have studied the kinetics of the solvent extraction of divalent metal ions with dithizone and its derivatives[13-15].S. M. Karpacheva and L. V. llozheva[16], I. Saburo et a/.[17], A. Ekstrom and D. A. Johnson[18] and H. Akaiwn et aL[8] have reported the kinetics of extractions of Fe(III) with HDEHP in synthine, Ni(II) with 6-(2-thiozolylazo) naphthol (TAN) in chloroform, U(VI) with 4-(12-pyridylazo) resorcinol (PAR); and Ni(II) and Co(II) with TTA-pyridine base systems respectively. All these workers have independently established that the rate controlling step of the solvent extraction in these systems is the formation of the 1:1 chelate in the aqueous phase (with the exception of Zn-extraction with di-a-napthyldithiocarbazone [15] and titanium(IV) extraction with HDEHP[7]), if the two phases are vigorously shaken so that the effect of the diffusion of materials and transfer across the interface is minimized. The acceleration extraction in the presence of other ligands (anions) in the aqueous phase has been discussed. T. Sekine et al. [1, 3] have found that the rate of extraction of Be(II) and Fe(III) with "ITA in MIBK is increased by the addition of sodium perchlorate to the aqueous phase. Finston et al.[19-21] have also reported that the rate of forward extraction of Fe(III) and Zr(IV) with TTA in benzene is enhanced by the addition of ammonium thiocyanate to the aqueous perchlorate medium and that the increase is larger when MIBK is added to the organic phase. Both authors suggested that the acceleration of the extraction caused by thiocyanate or perchlorate is due to a change in the extraction mechanism, the 0 0 4 - or SCN- complex of metal ions formed in the aqueous phase is first extracted into the organic phase and then the 0 0 4 - are replaced by TI'A in the organic phase. The acceleration due to these anions

Table 4. E v a l u a t i o n of the backward rate constant, k ~ Fig. No.

10

(ha)

pH = - log (h)

Intercept, I

k~ = 10(l+pH)

0.I0 0.25 0.50

-~ --

-- 0.3358 -0.2691 - 0,5535

----

I0 -°'ssSs I0 -°'5691 10 -0.7035

---

0.55 0.85

- 1.2095 - 1.4673

I0 -°'6s9° 10 -0"6173

---

--

1.15

- 1.8611

10 -0"7111

--

kb* = i0
Mean k b*

10-0.6907

428

F. ISLAM and R. K. BISWAS

probably arises because the above two processes are faster than the T r A chelate formation in the aqueous phase, which is the rate determining step in the absence of SCN- or CIO4-. They also noted that the synergic enhancement of metal chelate extraction can be explained in terms of this type of kinetic effect. In this connection, H. Akaiwn et al.[8] reported that the formation of the first T r A complexes with Ni(II) and Co(II) are accelerated by adding pyridine bases. Moreover, the addition of acetate ions has been reported to enhance the rate of zinc dithizonate extraction, (ZnOAc- reacts 25 times as fast as Zn 2+) whereas the formation of NiOAc+ does not enhance the nickel dithizonate extraction[22]. The extraction of Cr(III) with TTA is accelerated by the addition of F- to the aqueous phase [23], O. M. Petrukhin et al. explained this in terms of the formation of an intermediate fluoride complexes, which destroyed the hydrated shell of Cr(III) but did not prevent the formation of extractable Cr(III)-TrA complexes. Based on these reports, one can discuss the results which was obtained in this case in the following way: From the results obtained in Figs. 3-5, the rate expression at constant sulphate and acetate concentrations of 0.5 tool dm -3 and 0.25 mol dm -3 respectively may be written (cf. eqn (3-8)):

dCvA =/:i x Cv A x [A-] dt and this states that the formation of 1 : 1 chelate (VOA-) in the aqueous phase is the rate determining process. Thus, the reaction VO2+ + A- ~ VOA+ appears to be rate determining. The acceleration on increase in acetate and nitrate concentrations may be explained as follows. In presence of acetate, the rate expression may be written as (0.5 tool dm -s sulphate media):

(where, k : = k~,* × ~DI?"2/2 Av"txalIE-I/2 /~ ~a2Itr-l/2"t,/ This is compatible with the reaction

VOSO~

+ A-

~to~ > V O A ÷ +

S042-

A- [ fast fast ~ VOA2~o)

VOA2

at high sulphate concentration. The results obtained in the Figs. 9--11 indicate that the backward extraction rate may be expressed as follows (in presence of 0.Smoldm -3 sulphate and 0.25 tool dm -3 acetate, cf. eqns (4-7) and (14)): d Cv ° dt = kb x Cv ° x [A-]-~

(25)

The above equation indicates that the replacement of the first anion of the ligand in the complex VOA2 is the rate determining step. The tool. wt. and P-V atom ratio indicates the complex is in the trimeric form in the organic phase, but only the reaction VOA2--->VOA+ + A- appears to be rate determining. In the presence of acetate and nitrate, the rate was found to be decreased with increasing concentrations in the aqueous phase. Another slow step must be involved which is slower than the above. Among the following two steps: VOA÷ + L VOAL

VOAL

(26a)

> VOL ÷ + A-

(26b)

)

(22)

where, L = Ac- or NO3-; the step represented by the eqn (26a) is probably fast and that the step given by eqn (26b) is slower than VOA2--->VOA+ + A-. Although, the sulphate ion appreciably decreases the rate of forward extraction, it has little effect on the backward extraction. It is therefore suggestable that the substitution of A- in VOA- by SO42- is probably very fast and has no influence on the rate.

where, k~ = k~," x K ~ 2 x K~l/2 x K~]/2 and k~' = 2.09 k~. The eqn (22) suggests that there are two rate determining steps in presence of acetate VO2++A--~VOA + and

Acknowledgement--The authors are grateful to the University Grants Commission(U.G.C.), Dacca, Bangladeshfor their financial support.

dCvA dt = k~ x CvA x [A-] + k}' x C ~ x [A-] [Ac-]

slow

VO2++Ac-A-

)VOAAc

Afast

fast

~Ac-+VOA2-

)

VOA2to). A similar suggestion may be put forward for the effect of nitrate on the system. In view of the following rate equations: dCvA dt = k~" x Cv A x [A-] + k f f x Cv A x [A-] [NO3-] (23) where, k~" = k~' x KD~/2x K'~/2 x K~]/2 and kff = 1.44 k?"; the following reaction: VO2+ + NO3- + slow

A-

~ VONOsA, is also involved as a rate determining

step. But the explanation of the behaviour with sulphate is different. The rate expression can be treated as follows: dCv a dt = kF × CvA x [A-] (1 + 1.75 [SO42-1)-t

(24)

REFERENCES 1. T. Sekine,Y. Koikeand Y. Komatsu, Bull. Chem. Soc. Japan, . 44, 1903 (1971). 2. T. Sekine, J. Yumikura and Y. Komatsu, Bull. Chem. Soc. Japan, 46, 2356 (1973). 3. T. Sekine and Y. Komatsu, J. lnorg. Nucl. Chem. 37, 185 (1975). 4. Y. Komatsu, H. Honda and T. Sekine,J. lnorg. Nucl. Chem. 38, 1861 (1976). 5. T. Seldne, Y. Komatsu and J. Yumikura, J. Inorg. Nucl. Chem. 35, 3891 (1973). 6. C. F. Colemannand J. W. Boddy, Solvent Extraction Review, (Edited by Y. Marcus), Vol. I, p. 63. Marcel Dekker, New York (1971) 7. F. Islam and R. K. Biswas, J. lnorg. Nucl. Chem. 39, 559 (1978). 8. H. Akaiwn,H. Kownmato and T. Ishii, J. Inorg. Nucl. Chem. 36, 2077 (1974). 9. I. V. Vinaroceand N. P. Kirichenko,Zh. Phy. Khim. 50, 2834 (1976); Chem. Abstr. 86, 79468r (1976). 10. A. I. Vogel,A Text Book of Quantitative Inorganic Analysis: 3rd Edn, p. 790. Longmans,Green, London (1961).

Kinetics of solvent extraction of metal ions with HDEHP--II 11. F. Islam, Bangladesh Journal o[ Scientific and Industrial Research, p. 106 Vol. IX, Nos. 3--4,July-October (1974). 12. A. A. Frost and R. G. Pearson, Kinetics and Mechanism, 2nd FAn, pp. 43, 45. John Wiley New York (1%1). 13. J. S. Oh and H. Freiser, Anal. Chem. 39, 295 (1967). 14. C. B. Honker and H. Freiser, J. Phy. Chem. 66, 127 (1%2). 15. B. E. McClellan and H. Freiser, Anal. Chem. 36, 2262 (1964). 16. S. M. Karpacheva and L. V. Ilozheva, Chem. Abstr. 72, 7016.1 (1969). 17. I. Saburo et al., Chem. Abstr. 79, 35488J (1973). 18. A. Ekstrom and D. A. Johnson, J. lnorg. Nucl. Chem. 36, 2549 (1974).

JINC Vol. 42, No. 3.--H

429

19. H. L. Finston and Y. lnoue, J. lnorg. Nucl. Chem. 29, 199 (i%7). 20. H. L. Finston and Y. Inoue, J. Inorg. Nucl. Chem. 29, 2431 (1%7). 21. H. L. Finston and E. Gnizi, Solvent Extraction Research (Edited by A. S. Kertes and Y. Marcus), p. 333. Wileylnterscience, New York (1969). 22. H. Freiser, Anal. Chem. 40, 522R (1968). 23. O. M. Petrukhin et. al., Chem. Abstr. 65, 57668 (1967).