I n d . Eng. C h e m . Res. 1992,31, 1516-1522
1516
Equilibrium and Kinetics of Cr(V1) Extraction with Aliquat 336 Ernest0 Salazar, M. Inmaculada Ortiz, and Ane M. Urtiaga Departamento de Ingenieria Quimica, Facultad de Ciencias, Universidad del Pais Vasco, Apartado 644, 48080 Bilbao, Spain
J. Angel Irabien* Departamento de Quimica, Escuela Polit6cnica Superior de Ingenieria, Universidad de Cantabria, Calle Sevilla, 6, 39001 Santander, Spain
Solvent extraction has been reported as a useful technology for the recovery of &(VI) from industrial effluents. The equilibrium and kinetics of the solvent extraction process based on a complex organic phase (0.5% Aliquat 336,5% n-decanol, and kerosene as diluent) have been studied. The influence of the chromium(VI) concentration on the distribution coefficient has been fitted by a semiempirical model, which allows evaluation of the equilibrium conditions in the mass-transfer kinetic model. A mass-transfer coefficient in the organic phase k, = 1.64 X s-l, according to the film theory, or an effective diffusion parameter Deff/R2= 5.03 X s-l, according to a radial diffusion model, describes the mass-transfer kinetics of the process.
Introduction In recent years there has been considerable interest in treating polluted water generated by industrial processes. Different technologies have been developed and are available to remove toxic materials from wastewater for many industrial processes, thereby preventing the pollution of rivers, streams, and sewer systems. Chromium(VI) coming from sulfuric baths employed in electroplating processes as chromic acid is one of the most toxic elements to be discharged to the environment. Standard technologies for the treatment of these effluents (chemical reduction of Cr(V1) to Cr(II1) and chemical precipitation) reduce the chromium concentration in the wastewater to a low level, but it is not possible to recycle the metallic element to the industrial process. The development in the early 1960s of the high-molecular-weight amine reagents allowed the application of solvent extraction procedures for the removal of several anions from aqueous solutions. Some works utilizing amines (Alamine 336, Aliquat 336) for the removal of common anions have been reported: C1- and Br-, Ivanov and Zaitsev (1988) and Kholkin et al. (1988); SO:-, Kholkin et al. (1988); I-, Rakhman’ko et al. (1988). The removal of metal ions in anionic form such as zinc (Nakashio et al., 1986), iron (Kholkin et al., 1988; Harada et al., 1984),copper and nickel (McDonald and Bajwa, 1977), uranium (Ochkin, 1988), tungsten (Marr et al., 1983; Ivanov and Zaitsev, 1988), vanadium (Marr et al., 1983), rhenium (Strzelbicki et al., 1984), molybdenum (Sato et al., 1986), and rare earths (Sedov et al., 1989) has been extensively investigated. Some of these processes have been recommended for utilization in several industries; a comprehensive and quite detailed account of solvent extraction processes covering a range of metals is given in the literature (Ritcey and Ashbrook, 1979). Amines of high molecular weight have been extensively investigated for the removal of Cr(V1) (Hochhauser and Cussler, 1975; Strzelbicki et al., 1984; Weiss and Castaiieda, 1988; Mori et al., 1990; Salazar et al., 1990a,b, 1992a,b; Fuller and Li, 1984), and several economic studies have shown the viability of the Cr(V1) recovery from washing operations of final metallic products (Clevenger and Novak, 1983; Inuetagoyena, 1987),but until not there has not been any reference of the practical application of solvent extraction processes to chromium recovery.
* Author to whom correspondence should be addressed.
Hexavalent chromium, &(VI), may exist in the aqueous phase in different forms; the total amount of chromium and the pH are the main variables of the chromium species in an aqueous phase. If there is no other chemical species in the aqueous solution, the Cr(V1) equilibrium reactions are: (Sengupta et al., 1988) H2Cr04+ H+ + HCr04(log K = -0.8) (1) HCr04- + H+ + Cr0422HCr04- + Cr2072-+ H 2 0
(log K = -6.5)
(2)
(log K = 1.52) (3)
+
HCr207-+ H+ Cr2072- (log K = 0.07) (4) Higher forms of chromate polymers have also been reported in acidic solutions at high concentration (Sengupta et al., 1988): Cr2072-+ H+ + HCr04- + Cr30102-+ H 2 0 (5) Cr3Olo2-+ H+ + HCr04- + Cr40132-+ H20 (6) It has been noted that higher polymers of chromate do exist at concentrations higher than 0.1 M (Kirk and 0thmer, 1965). In addition to the reactions previously mentioned, high concentrations of bisulfate (HS04-) and chloride (C1-) may form mononuclear complexes (Haight et al., 1964; Tong and King, 1953; Tong and Johnson, 1966): HCr04- + HS04- + Cr03S042-+ H 2 0 (7) HCr04- + H+ + C1- t CrO3C1- + H 2 0
(8)
Similar complexes were not found when nitrate (NO,-) was present. Since the distribution of chromate species is dependent on both the pH and the total Cr(V1) concentration, a predominance diagram Sengupta et al. (1988) may be drawn with both pH and total Cr(V1) concentration as variables. It is well-known that quaternary ammonium salts exhibit low selectivity in ion-exchange reactions, since simultaneous reaction with several anionic species present in the aqueous feed is possible (Fuller and Li, 1984; Mori et al., 1990). The pH value may have an important effect in the distribution of the anionic species in the aqueous phase, and it may be the main variable in the extraction reagent affinity for chromium. The complexity of the extraction process between Cr(V1) in aqueous solution and the extractant Aliquat 336 together with the lack of references
0888-5885/92/2631-1516$03.00/00 1992 American Chemical Society
Ind. Eng. Chem. Res., vel. 31, No. 6, 1992 1517 concerning the modeling of the system makes necessary a systematic study of the equilibrium and kinetics of the process with a view to the industrial application of the extraction technology using quaternary amines to reduce the Cr(V1) concentration in water effluents. In the present work the equilibrium and kinetics of the extraction system Cr(V1)-Aliquat 336 have been studied. Taking into account the species of Cr(V1) in aqueous solution, a careful experimental design has been planned in order to evaluate their effect on the distribution coefficient and kinetic behavior.
Table I. Influence of the Composition of the Aqueous Phase on the Extraction Equilibrium (tc,60 rnin; V J V , = 20; Organic Phase: Aliquat 336 = 0.5% V,, n-Decanol = 5% V.,) aqueous phase exDt NaOH, N Kr(V1)L mn/L I 5-200 I1 10-5 5-200 I11 10-3 5-200 IV 5 x 10-3 5-200 V 0.01 7-100 VI 0.1 1-100 ~~
Experimental Section AnalaR grade reagents Cr03 (Merck) and NaOH (EKA Nobel) were used to prepare aqueous solutions. The components of the organic phase were solvent, kerosene (PETRONOR S.A.); extractant, Aliquat 336 (Henkel Co.); and stabilizer, n-decanol (Fluka A.G.). In the equilibrium experiments the aqueous and organic phases were put into contact in a rotatory SBS stirrer (5-140 rpm). For the kinetic studies a glass stirred tank provided with four stainless steel baffles and a plastic turbine type impeller connected to a stirrer Heidolph RZR-2000 speed controller (0-700 rpm) was used. A constant stirring rate, 300 rpm, allowed the homogeneous dispersion of the organic phase in the aqueous phase, and it was taken as a constant parameter in the kinetic study. Samples were taken from the tank, and after separation of the two phases, the aqueous phase was prepared for chemical analysis. The quaternary ammonium salt Aliquat 336 is commercially available in the chloride form. In this work previous to the extraction reaction, the Aliquat 336 was converted to the hydroxide (OH-) form according to the following procedure: (1) dilution of Aliquat 336 to the desired concentration with kerosene and n-decanol, (2) contact of the organic solution containing Aliquat 336 with 1 M NaOH (equal volumes) in a glass tank stirred with a magnetic stirrer for 10 min, and (3) settling of the aqueous phase. This procedure was repeated four times. A pH meter CRISON Digital MicropH 2002 was used to measure the pH of the aqueous phase. The concentration of chromate was determined with a Perkin-Elmer 1100 B absorption spectrophotometer and with a Dionex 2000i ion-exchange chromatograph.
Results and Interpretation Extraction Equilibrium of the System Cr(V1)-AIiquat 336. According to the literature the extraction of Cr(V1) oxyanions by Aliquat 336) is only slightly dependent on the equilibrium pH (Henkel Co., 1961; Fuller and Li, 1984, Strzelbicki et al., 1984). However, the Cr(V1) oxyanion predominance depends on the pH value, and therefore it may have an influence on the distribution coefficient of Cr(V1) between the aqueous and organic phases. It has been reported that quaternary ammonium salts exhibit reasonable extraction abilities for Cr(V1) oxyanions, higher for the monovalent oxyanions than for the divalent oxyanions (Strzelbicki et al., 1984). The extraction reactions for HCr04- and with Aliquat 336 are as follows: HCr04-
Cr04’-
+
+
RsNOH
R,NHCrO,
CH3
CH3
I
2R3NOH
I
CH3
=
I
[R,N],CrO,
I
+ +
OH-
20H-
(9)
(10)
CH3
Taking into account the extraction possibilities shown by eqs 9 and 10, an experimental design has been planned
”\
-,-
-1
0
2
1
3
log C, (mg/l)
Figure 1. Distribution coefficient in acidic medium. Experiments I (0)and I1 (X) of Table I.
23 3’0
5
0 0
-m
\
+*\
-0,s
0,O
0,s
1,O
1,s
2,O
2,s
3,O
log C, (mgW Figure 2. Distribution coefficient in basic medium. Experiments I11 ( O ) , IV ( O ) , and V (+) of Table I.
to study the influence of the initial Cr(V1) concentration and the pH of the aqueous phase on the extraction equilibrium (Table I). Figure 1shows the distribution coefficient CDvs the final concentration of Cr(V1) in the aqueous phase in a logarithmic plot for the experiments of Table I in acidic medium. As can be observed in Figure 1, there is a region with a constant distribution coefficient when the concentration of Cr(V1) in the aqueous phase is lower than 1 mg/L. For higher concentrations the distribution coefficient is linearly related to the chromium concentration. When the initial concentration of NaOH in the aqueous M (Figure 2), several facts should phase is higher than be mentioned: the pH of the solution decreases with the concentration of &(VI); starting from a basic solution it
1518 Ind. Eng. Chem. Res., Vol. 31, No. 6, 1992 I
35 I
0
0
m
-
-,-
-1
2
1
0
log Ca
3
(mgll)
Figure 3. Influence of the volumetric ratio between the aqueous and organic phases: (+) 40, (0)20, and ( X ) 4.
Figure 4. Influence of the stabilizer (+) without n-decanol and (0) with 5% (v/v) n-decanol.
becomes neutral and at high Cr(V1) concentration the pH decreases to an acidic value. As a result, the representation of CDvs C, shows three well-defined regions. For a basic medium there is a linear relationship between CD and C, with a value of the slope similar to that encountered in acidic medium but with lower values of the distribution coefficient. In this region the predominant oxyanion would be Cr0,2-. For an acidic medium, corresponding to higher concentrations of chromium, and to the presence of HCr0,-, a similar linear relationship is observed, and finally for neutral values of pH, when both species Cr042and HCr04- are present in the aqueous solution, an inflection region with a very high slope and joining the two linear zones was found. In the analysis of the influence of the solute counteranion in the extraction equilibrium, experiments have been performed at different volumetric ratios between the aqueous and organic phases. Figure 3 shows a representation of the distribution coefficient as a function of the aqueous concentration of hexavalent chromium in a double logarithmic scale for three different volumetric ratios Val V, = 40,20, and 4. Two different zones can be observed: at high chromium concentration the results obtained for the different volumetric ratios are similar,but in the region of constant value of the distribution coefficient, it becomes related to the volumetric ratio, being higher as the V,/ V,, ratio increases. The influence of the stabilizer on the extraction equilibrium has been also studied. Several authors (Henkel, 1961; Strzelbicki et al., 1984; Fuller and Li, 1984; Mori et al., 1990) have recommended the addition of different stabilizers to the organic phase to avoid the formation of a third phase on the interphase between the aqueous and organic phases that could inhibit the extraction reaction. Figure 4 shows a representation of the distribution coefficient as a function of the hexavalent chromium concentration for the experiments without and with 5% (v/v) n-decanol in the organic phase. I t can be observed that the experimental results obtained without stabilizer show a large dispersion and a lower value of the distribution coefficient in the flat region. Accounting for the findings previously mentioned, for the description of the extraction equilibrium in the region where the value of CD is linearly related to the concentration of Cr(V1) in the aqueous phase, an empirical relationship can be deduced:
Table 11. Eauilibrium Parameters (CM= 4.5 X mol/Lb NaOH concn, M parameter 10-3 5 x 10-3 10-2 a 22.31 14.72 11.16 b, L/mol 4957 3270 2480 K , L/mol 1.11 x lo5 0.48 X lo5 0.277 x lo5
CD
= ACLB
(11)
In acidic medium, after linear regression of the experimental results (Figure l),the obtained equation has been C D = 741.3C,-0.96
(12)
r2 = 0.99 where C, is expressed as mg/L. The parameter B , close to 1.0, shows that the influence of the concentration of Cr(VI) in the aqueous phase on the chromium concentration in the organic phase is very small, the latter remaining almost constant, so that the organic phase will extract Cr(V1) from the aqueous phase until saturation is reached. When the aqueous-phase concentration of Cr(V1) is too low to saturate the organic phase, this variable will decrease until a maximum value of the distribution coefficient is obtained depending on the activity of the chemical species and with the competition between the Cr(V1) and the counteranion for the Aliquat 336 becoming important. At lower levels of Cr(V1) the distribution coefficient will remain constant. In the fitting of the experimental results of basic medium (NaOH = 5X and loT2M) the obtained equation has been CD
= 124.4Ca-0.s9
(13)
r2 = 0.99 where C, is expressed as mg/L. The lower distribution coefficient at infinite dilution obtained in basic medium is related to the predominance in the aqueous phase of the oxyanion Cr042-. The application of the mass action law or an adsorption model to the chromate exchange reaction leads to the equation CD1I2 = a
- bC,
(14)
where a = (CMK)1/2and b = (K/CM)1/2. The representation of CD1l2vs C,, Figure 5, shows that both variables are linearly related; a linear regression of the experimental results leads to the values of the parameters a and b shown in Table 11.
Ind. Eng. Chem. Res., Vol. 31, No. 6, 1992 1519 SO
60 50 70
.
A
20
\ l,o
1,5
2,o
Co10
2,5 3
3,o
4,o
3,5
150 0
0
5
.
1
I
I
15
20
.
10
(mol/t)
.
25
t h e (mln)
M
Figure 5. Linear fitting of the equilibrium model: (0) M NaOH. NaOH, ( 0 )5 X 10" M NaOH, and (A) 300
1
n
Figure 7. Extraction kinetics of chromium at different initial concentrations: (0)10, (0)7.5, and (A)5 mg/L.
I
I1 -I
-
" 0,O
0,2
0.4
0,6
0.8
1.0
1.2
0
l o 4 (mot/l) Figure 6. Comparison between the experimental values (0, M NaOH) and model prediction (-) of the distribution coefficient. C,
From the results, the large value of the extraction equilibrium constant and the influence of the sodium hydroxide concentration on the equilibrium parameters can be observed. Figure 6 shows the agreement between the experimental and calculated distribution coefficient as a function of Cr(VI) concentration in the aqueous p k , according to eq 14. Kinetic Modeling. The kinetics of the extraction of Cr(VI) with Aliquat 336 in a basic medium containing M NaOH have been studied. Under these conditions, in the aqueous phase the oxyanion CrOd2-will prevail, neglecting the simultaneous reaction between HCrOL and Aliquat 336. For the study of the kinetics of extraction of Cr(V1) with Aliquat 336 in basic media, several kinetic experiments with a concentration of 1-10 mg/L Cr(V1) and M NaOH in the aqueous phase, with an organic phase containing 0.5% (v/v) Aliquat 336 and 5% (v/v) decanol, and with a volumetric ratio V,/V, = 25 were carried out in the experimental setup previously described. The obtained results of the extraction yield vs experimental time are given in Figures 7 and 8. It can be observed that the extraction rate corresponding to an initial concentration of 1 mg/L is lower than could be expected; this lower extraction rate is due to the value of the distribution
5
10
15
20
25
time (mln)
Figure 8. Extraction kinetics of chromium at different initial concentrations: (0) 5, ( 0 )2.5, and (A) 1 mg/L. Table 111. Parameters of the Extraction Kinetics no. of param a b, L/mol k,, std dev 1.64 X 1.70 X lo4 equil 1 equil 2.30 X 0.60 X lo-' 2 36.22 9740
coefficient, which a t this concentration of solute reaches a constant value, lower than the one corresponding to a linear relationship with the aqueous concentration of chromium. Taking into account that ionic reactions usually show a very fast rate, it has been assumed that the main contribution to the kinetic model is the maas transfer. According to the film theory, the extraction rate would be expressed as N = k,(C,* - C,) (15) and the equilibrium equation CD1/' = u - bC,* leading to C , = ( 1 / 2 u b ) [ u 2+ b2(C, + N / ~ z ,-) ~ (C, + N / k , ) / C , ] - N / k o (16) Nonlinear regression of the experimental data C,, C,, and N using the Marquardt optimization subroutine
1520 Ind. Eng. Chem. Res., Vol. 31, No. 6, 1992
r =R (C,/Ca)1/2= a - bCo (22) 3. There is a mass-transfer control of the external aqueous phase:
where
0,m
0
3
6
9
12
15
is the overall mass-transfer resistance. Defining the dimensionless variables
time (min)
Figure 9. Comparison between the theoretical (--) and calculated chromium concentration in the aqueous phase for the experiments: (0)10, ( 0 )7.5, and (A) 5 mg/L.
(Marquardt, 1963) led to the mass-transfer parameters of Table 111. The values of the parameters of Table I11 allow the conclusion that the main resistance to the mass transfer is located in the organic phase, the contribution of the aqueous phase to the overall resistance being negligible. Although the standard deviation decreases slightly when the parameters a and b are optimized together with the mass-transfer parameters of the organic phase, the values obtained for a and b do not agree well with the values calculated in the analysis of the extraction equilibrium, so that the value of k , is taken as k , = 1.64 X s-l. Figure 9 shows the comparison between experimental and simulated data with the latter value of k,; the good agreement c o n f i i s the location of the main resistance to the mass transfer in the organic phase. Due to the properties of surface activity of the quaternary ammonium salts, it is possible to assume that the organic droplets behave as rigid spheres of high stability. Following this consideration, the evolution of the concentration of Cr(VI) in the organic and aqueous phases will be represented by the following equations: aqueous phase
Equations 17-23 take the following form: aqueous phase
organic phase
g=o
7=0
(%).o
q=o
1.
g = l
q = l
where Cmnx
= km
where organic phase
Cmax = 3. q = l
with the boundary conditions
r=O
(a)=o
To describe the boundary condition of the system in r = R, three possibilities have been taken into consideration: 1. The concentration of Cr(V1) in the organic phase at r = R is a constant: r=R C,=km (21) 2. The concentration of Cr(V1) in the organic phase at r = R is in equilibrium with the concentration of Cr(V1) in the aqueous phase:
(1 + 2abCn,J - (1
+ 4~bC,o)l/~
2b2C,o
(g)
=
h=l
gcmax a2 + b2Cmm2g2 - 2abgCrn,
)]
(30)
where the Biot number
Bi
=
KoR Detfcrnax
The resolution of the system of differential equations allows the simulation of the concentration of Cr(V1) with time in the organic and aqueous phases. The second-order, nonlinear differential equation was solved with the program DGEAR available from the ISML library. The program finds approximations to the resolution of a system of first-order ordinary differential equations with initial
Ind. Eng. Chem. Res., Vol. 31, No. 6,1992 1521
time (min)
Figure 10. Linear fitting of the calculated dimensionless time and experimental time, h = 0.5, according to the boundary condition of eq 21 (0) and the boundary condition of eq 22 (0).
conditions. The basic methods used by DGW are of the implicit linear multistep type. Prior to solving the second-order differential equation, it was transformed to a first-order ordinary differential equation by orthogonal collocation with the boundary condition dC:
N+l
"," 0
5
irl
The resolution of eq 34 in order to introduce the value of the CN+, parameter in eq 31 has been made by the bisection method to find the real roots of nonlinear functions (Ruckdeschel, 1981). Ekpations 31-34 were solved to a tolerance of 1.0 X lo4 for the concentration of Cr(V1) in the organic phase. For the discrimination between the different hypotheses the values of the calculated dimensionless time, 7,according to the boundary conditions of eqs 21 and 22 have been compared with the real time, t , corresponding both to a constant dimensionless concentration, h = 0.5, as shown in Figure 10. The linear plot of the experimental results shows that the correlation obtained with the hypothesis that the solute concentration in the boundary of the organic phase is in equilibrium with the solute concentration in the aqueous phase, eq 22, leads to the best fitting of the experimental results. The influence of the Biot number (mass-transfer resistance in the aqueous phase) can be neglected, taking into account that the best fitting of the simulated and experimental results has been obtained for Bi = as shown in Figure 11. From the slope of the representation 7 vs t the params-l. eter DefrfR2was obtained Deff/R2= 5.03 X
15
Conclusions As a result of the study of the extraction reaction between chromium(V1) and Aliquat 336, the main parameters of the equilibrium and kinetic behavior have been evaluated. Hexavalent chromium may have in the aqueous phase different ionic forms depending on the concentration and pH. In acidic media, the distribution coefficient can be described, by the equations C D = 741.3Ca4.96 C, > 1 mg/L CD = 521.7
N+1
10
tlme (mln) Figure 11. Linear fitting of the calculated dimensionless time and experimental time, h = 0.5, according to the boundary condition of eq 23 (+) Bi = m, (0) Bi = 25000, and (A) Bi = 1OOOO.
C,
< 1 mg/L
In basic media, a semiempiripl model has been proposed: CD1/2= u - bC, where a = (CMlW2and b = (K/CM)1/2show different values depending on the NaOH concentration. The surface-active behavior of quaternary ammonium salts leads to the dispersion of the organic phase in the bulk of the aqueous phase as rigid spheres of high stability. According to the film theory, the extraction rate will be N = k,(C,* - C,) s-l. where k, = 1.64 X The description of diffusional transport of Cr(VI) in the organic phase by means of a radial diffusion model gives a value of the quotient between the effective diffusion coefficient of solute and drop radius: D,fr/R2 = 5.03 X lo4 s-'
These results allow the mathematical modeling of the equilibrium and kinetics of the extraction of chromium(VI) with Aliquat 336.
Nomenclature A = empirical equilibrium parameter, eq 11 = symmetrical orthogonal collocation parameter a = (CMK)ll2 = equilibrium parameter B = empirical equilibrium parameter, eq 11 B,c = symmetrical orthogonal collocation parameter b = (K/CM)1/2= equilibrium parameter, L/mol C = solute concentration, mol/L C, = solute concentration in the aqueous phase, mol/L
1522 Ind. Eng. Chem. Res., Vol. 31, No. 6, 1992
C, = initial solute concentration in the aqueous phase, mol/L CD = distribution coefficient, C,/C, CM = maximum Cr(V1) concentration in the organic phase under saturation conditions, eq 14,mol/L = maximum Cr(VI) concentration in the organic phase, eq 28, mol/L C, = solute concentration in the organic phase, mol/L C,* = solute concentration at the W/O interphase, mol/L Cd = initial solute concentration in the organic phase, mol/L Deff= effective diffusivity, mz/s g = C,/C, = dimensionless Cr(V1) concentration in the organic phase h = Ca/CsO= dimensionless Cr(V1) concentration in the aqueous phase K = extraction equilibrium constant in basic media k = mass-transfer coefficient rate at the interphase, l / s k, = mass-transfer coefficient in the aqueous phase, l / s kG = overall mass-transfer coefficient, l / s k , = mass-transfer coefficient in the organic phase, l / s N = extraction rate, mol/(L 8 ) R = drop radius, m r = radial position, m V, = volume of aqueous phase, L V, = volume of organic phase, L v = V,/ V , = volumetric ratio between aqueous and organic phases Greek Letters q = r/R = dimensionless radial position T = Def&/R2 = dimensionless time
,C
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