Langmuir 1993,9, 3635-3639
3636
Adsorption Behavior of Cadmium and Zinc Ions on Oxide/ Water Interfaces J. Liu, S. M. Howard, and K. N. Han' Department of Metallurgical Engineering, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701 Received July 12, 1993. In Final Form: September 27,199P The adsorption behavior of cadmium and zinc metal ions on silica and alumina particles has been studied as a function of concentrationof these ions at 25 O C and pH 6-6.5. The range of the concentration of these ions studied was 109-106M. Amodified Stern-Grahame equationhas been developedto delineate the adsorption mechanism of the systems considered in this study. The surface activity coefficient values of these ions at the solid substrates were evaluated and the results were discussed.
Introduction Effective removal of metal ions from effluent streams has been an industrial problem for a long time. The society demands a clean and safe environment and government agencies comply with these demands by imposing strigent regulations and reinforcement. The environmental issue has become a daily chore in U.S.industry since the 1970s. The Environmental Protection Agency (EPA) has put forward guidelines for almost every effluent stream. For example, the amount of cadmium and zinc in effluent streams should not be allowed to be more than 0.1 and 1.0 ppm, respectively.112 It has been reported3 that the cost to U.S.industry in order to meet these guidelines amounts to about $80 billion per year. One simple but effective way of removing metal ions from water is by adsorption of these metal ions on to metal oxides. There have been numerous investigations to delineate the mechanisms of the metal ion adsorption on the water/oxide interfaces.b10 In many of these studies, the Stern-Grahame equation has been used to describe the adsorption behavior of metal ions. One of the shortcomings of this equation is that activity coefficients of the adsorbed species are usually assumed to be unity. In this study, this aspect has been rectified and a modified Stern-Grahame equation has been developed to describe the adsorption behavior of cadmium and zinc ions on silica and alumina.
ni = n? e-*iIRT (1) where ni is the moles of species i adsorbed on the surface, nio is the moles of species i in the bulk solution, Wi is the work required to bring 1 mol of species i from the bulk solution to the surface, R is the gas constant, and Tis the absolute temperature. This equation is based on statistical thermodynamics and is the starting point for the analysis of most adsorption processes. However, the equation is based on the assumption of ideal behavior which is rarely observed in practice. For a real behavior, the equation must be modified by replacing the molar amounts by activities; however, the definition of the adsorbed species' standard state is not always clear when this is done. The authors prefer a more formal way to arrive at the desired equation based on classical thermodynamics. The following approach leaves no doubt as to the definition of the standard states. Equilibrium between the species i in the bulk solution, ib, and on the surface, is,is represented by the equilibrium reaction ib + i, (2) At equilibrium the chemical potential of each species must be the same = (3) Since activity is defined by pi = pio RT In ai, eq 3 becomes pb
Theory The Stern-Grahame adsorption equation is based on the statistical thermodynamics and can be presented as @Abstractpublished in Advance ACS Abstracts, November 15, 1993. (1) Ruggiero, D. D.; Kohlmann, H. J. Proceedings of the 15th Annual AIME Mineral Economics Symposium; SME-AIME:New York, 1989; P 29. (2) Ritcey, G. M. Tailings Management; Elsevier: Amsterdam, 1989; 13 r
970.
(3) Hahn, R. W. Proceedings of the 15th Annual AZME Mineral Economics Symposium; SME-AIME, 1989; p 1. (4) James, R. 0.; Healy, T. W. J.Colloid Interface Sci. 1972,40,42, 53, 65. (5) Fueretenau, D. W. Pure Appl. Chem. 1970,24, 135. (6) Benjamin, M. M.; Leckie, J. 0. J. Colloid Interface Sci. 1981, 79, 209. (7) Hachiya, K.; Saaaki, M.; Saruta, Y.; Mikami, N.; Yasunaga, T. J. Am. Chem. SOC.1984,88, 23. (8) Kinniburgh, D. G.; Sridhaf, K.; Jackson, M. L. h o c . Fifteenth Hanford Life Science Symposium; Technical Information Center: Springfield, VA, 1977; p 231. (9) Huang, C. P.; Rhoads, E. A.; Hao, 0.J. Water Res. 1988,22,1001. (10) Fokkink, L. G. J.;De Keizer,A.; Lyklema, J. J. J.Colloid Interface Sci. 1987, 118, 454.
+ pb0 + RT In ab = pa* + RT In a,
(4)
where ab is the activity of species i in the bulk solution, a, is the activity of species i on the surface, pbo is the chemical potential of the species i in ita standard state in the bulk solution, and pa* is the chemical potential of adsorbed species i in ita standard state on the surface. Standard statesare chosen to make the conversion from activity to concentration or vice versa convenient. The standard state for the species in the bulk solution throughout this investigation is the one molar solution behaving as the infinitely dilute solution. Whatever standard state is chosen for the adsorbed species, the standard Gibbs energy of adsorption from the bulk solution is the difference between the standard (11) Kruyt,H. R. Colloid Science; Elsevier: New York, 1962; p 389. (12) Murray, D. J.; Healy, T. W.; Fueretenau, D. W. Adu. Chem. Ser. 1968, No.79, 74. (13) Loganathau, P.; Burau, R. G. Geochim. Cosmochim. Acta 1978, 37, 1277.
0743-7463/93/2409-3635$04.oo/o0 1993 American Chemical Society
3636 Langmuir, Vol. 9,No. 12, 1993
Liu et al.
Surface Concentration ' I
Figure 1. Schematic illustration of the Raoultian and Henrian standard states for the adsorbed species.
where YH is the activity coefficient for adsorbed species i when the Henrian standard state is chosen, Ii' is the adsorption density, moles/area of solid, and d is the diameter of the adsorbed species. The ratio ( r i d )is called the surface concentration of the adsorbed species. Figure 1 illustrates the relationship of the Henrian standard state to the Raoultian standard state. The tangent line to the adsorbed species activity at infinite dilution represents the ideal solution along which YH = 1. The Henrian standard state lies where the extension of this tangent line reaches a surface concentration of unity as shown in the figure. Substitution of eq 9 for a8 into eq 5 gives
chemical potentials of the species in the bulk solution and the adsorbed state (5) Two different standard states for the adsorbed species are common-the Raoultian and the Henrian. Both are presented here. Raoultian Standard State. The Raoultian standard state for the adsorbed species is defined as the species i adsorbed on the surface at maximum coverage. The activity of adsorbed species is then described as a R = YRXR
(6)
where YR is the activity coefficient of the adsorbed species and XR is the fractional adsorption. The fractional adsorption XRis the ratio of the adsorption density, ri, to the maximum adsorption density, rmar
X, = rimmar
(7)
The maximum adsorption density is the density a t the plateau when plotting adsorption density versus bulk concentration. This is sometimes loosely referred to as the condition of monolayer coverage. Figure 1illustrates the relationship of the Raoultian standard state to the overall solution behavior. From eq 5
-1
AGOa&. = -RT In[ YRXR YbCb
where the activity for the species in the bulk solution has been replaced with the usual product of the activity coefficient, Yb, and the molar concentration of the species in the bulk solution, c b . The values for AGoads;~ depend on the determination of the bulk concentration at maximum surface layer coverage. Uncertainties in these concentrations result in uncertainand corresponding uncertainties in YR ties in A G o a a ; ~ values. However, these uncertainties do not propagate into equilibrium calculations based on these values since the equilibrium condition is independent from the choice of standard state. That is, any uncertainties exactly compensate. Henrian Standard State. The Henrian standard state for the adsorbed species is the hypothetical adsorbed species behaving as the infinitely dilute adsorbed species, but at an adsorption concentration of unity. In such a case, the activity of the adsorbed species is then described as
One could determine AGoads;~directly from experimental data of (I'Jd)versus c b is the corresponding bulk concentrations were sufficiently low to assure that the ideal adsorption range had been reached. That is, where both Yb and YH are unity. However, since one cannot simply assume those conditions have been met L'Hopital's rule should be used. In the limit of infinite dilution, both Yb and Y Hare unity by definition. Therefore, the quotient in eq 10 becomes YH(ri/d)
(ri/d)
2%[TI = 2%[ T I (11) which equals the slope of the plot of (I'i/d)versus
cb.
Experimental Section
Batch adsorption experiments were conducted. Silica and alumina powders were used as adsorbenta for cadmium and zinc ions. High-purity silica (99%)was obtained from Custer, SD, and alumina powders were obtained from Mager Scientific,Inc. The size of the oxide particles was measured using a Coulter counter. The average sizes of these particles were measured to be 2.2 Nm and 1 pm respectively for silica and alumina. Surface area of these oxide particles was measured using a Quantasorb (Model QS-15) manufactured by Quantachrome Corp. The specificsurface areas of these oxides were measured to be 5.4 and 6.0 m2/grespectively for silica and alumina. The points of zero charge of these oxides were measured using a Zeta Meter to be pH 2 and 9 respectively for silica and alumina. Chemicals used in this study were of reagent grade. The concentration of metal ions was measured using a Perkin-Elmer atomic absorption spectrophotometer, Model ICP/5500. Batch adsorption testa were conducted in a vessel which was placed in a constant temperature bath. The slurry suspension was continuously stirred during the adsorption process. When equilibrium was reached, the solution was subjected to centrifugation and the pregnant solution was then subjectedto chemical analysis. Metal distribution diagramsof cadmiumand zinc as a function of pH were constructed using thermodynamic data provided in the literature.14 These are shown in Figurea 2-5. In these figures, precipitated metal hydroxides are not shown. The total moles of all the species,either ionized or precipitated,were kept constant over the entire pH range. Baaed on these diagrams, a few observations could be made. First, when pH of the solution is less than 6, the predominant metal species is either Zn2+or Cd*+. (14)Butler, J. N. Ionic Equilibrium: A Mathematical Approac)r; Addison-Wesley: Reading, MA, 1964; p 547.
Adsorption Behavior of Cadmium and Zinc Ions I
I
v
I
I
Langmuir, Vol. 9, No. 12, 1993 3637
1
----I
10-f Cd*
cd(II) 25OC TOT. m.= M I0-f
5
i
g
0
10-7
V W
8 I0-8
0 I 0-9
0
I
1
2
4
A
I1
6
8
1
0
1
2
1
4
PH
Figure 2. Distribution of cadmium species aa a function of pH. Total concentration of cadmium = 106 M. I
I
I
2
4
6
8
1
I1
0
1
2
1
4
PH
Figure 4. Diatributionof zinc species aa a functionof pH. Total concentration of zinc = 1.5 X 106 M. I
I
I
I
6
8
1
1
I6 3
I
I
I0-3 Cd(ll)
IO-^
TOT. CONC. = 10-314 Iom4 a!
I
8
z
z 10-5
0
E
10-5
8
z W z V
8 I o= Id6
-
I0-7
IO-'
0
I
1
2
4
I IO
1 2 1 4
PH
0
2
4
6
8
IO
1 2 1 4
PH
Figure 3. Distribution of cadmium species aa a functionof pH. Total concentration of cadmium = 10-9 M.
Furthermore, when pH of the solution ia greater than 7 or 8, metalhydroxidea will start precipitating. Therefore,in thia study, pH of the solution waa maintained below 7. Results and Discussion Figure 6 presents the typical adsorption density versus concentration for zinc and cadmium on a log-log graph. Plota of log adsorption density of zinc and cadmium on alumina versus log molar concentration of these ions in
Figure 5. Diatributionof zinc species aa a functionof pH. Total concentration of zinc = 10-9 M.
the bulk solution were made. A similar observation was also made for silica powders. It can be noted that the adsorption density of zinc is more than that of cadmium. This may be attributed to the fact that the size of the hydrated zinc ion (6 A) is greater than that of cadmium ion (5 A1.15 In accordance with eq 8, the standard Gibbs energy for the adsorption to the Raoultian standard state, A G ' ~ ; R , (16) Garrela, R.M.;Christ, C.L.Solutions, Minerab, and Equilibria; H W r and Row: New York, 196s; p 450.
3638 Langmuir, Vol. 9, No.12,1993
Liu et al.
t
TEMPERATURE = 25OC
0 vj
4
0.0
W
s
0.2
-I I
0.4 (I
0.6
0.8
I .o
XR)'
Figure 7. Darken plot for the adsorption of cadmium and zinc on silica and alumina.
1
1
I
I
1
J
-5
-4
-3
-2
-I
0
where
LOG CONC.
Figure 6. log (adsorption density)versus log (concentration)of cadmium of zinc on alumina. Table I. Comparison of S l o w at Infinite Dilution for system Cd2+ on AlzOa Zn2+ on Al2Os Cd2+ on Si02 Zn2+ on Si02
(wa)vs ob second demee fiv 14.7 17.5 41.8 51.9
third demee fitb
9.88 12.6 29.4 37.1 a Origin plus next two data points. Origin plus next three data
*
points.
is evaluated from the species concentration in the bulk solution at I?- and the corresponding Yb. The values of Yb are obtained from the Debye-Huckel the01y.l~ Values of YR at concentrations other than the standard state are determined using the so obtained value of AGoads;~and experimental data using eq 8. Once the standard Gibbs energy for adsorption to the Henrian standard state, AGoa,~, is determined, values of YH are determined using experimental data in accordance with eq 10. However, a problem arises in this procedure because measurement uncertainty at the lowest concentrations make uncertain the slope of the plot (I'ild) versus c b near infinite dilution. For example, the slopes at infinite dilution computed from a least-squares second and third degree fit of the present work are compared in Table I. A more precise method of determining AGO,&L;His based on the concept of changing standard states. In this case the standard state is to be changed from Raoultian to Henrian states as defined above. The chemical potentials of species i adsorbed on the surface is independant on the choice of standard state and therefore at equilibrium py
+ RTIn aR= p? + R T h aH
(12)
where piR is the chemical potential of the adsorbed species i in the Raoultian standard state and p,H is the chemical potential of the adsorbedspecies i in the Henrian standard state on the surface. Were one to determine the difference in standard chemical potentials, one could use the difference to determine AGo,,~ from AGO,,& That is
The value of AGOH-R is determinable from any set of activities a~ and QR corresponding to the same adsorption concentration. The easiest set corresponds to the hypothetical solution behaving as the infinitely dilute solution at a fractional surface coverage, XR,of unity. Point A on Figure 1 shows the location of this condition. The value of O R at that condition is aR e YoR
The value of
CZHat
(14)
that condition is
since the activity coefficient YH is unity by definition for the standard state and surface concentration at (IR = 1 is ( I ' d d ) . Therefore, eq 13 becomes
Once AG',&L;H is determined, values of YH can be determined from the experimental results in accordance with eq 10. The best way of determining TOR is to make a so-called Darken16plot as shown in Figure 7. The activity coefficient data for adsorption onto silica may be summarized as
h YR = a(1 - XR)2 wherea for Cd2+is -2.75 and for Zn2+-2.77,as determined from the slopes of the lines in Figure 7. This linear behavior exhibited for adsorption of Cd and Zn onto silica is characteristic of regular behavior as f i s t described by Hildebrand." His treatment applies to solutions and is based on ideal mixing entropy. The comparable condition for adsorption would require ideal adsorption entropy. That is, entropy changes for adsorption would result from configurational changes only between the adsorbing species and the adsorption sites. A consequence of regular behavior is the constancy of In y d ( 1 - X R )over ~ the range 0 IXR I1. It should be (16)Darken, L. S. Trans. TMS-AIME 1967,239,8.
(17)Darken, L. 9.; Gurry, R. W. Physical Chemistry of Metale; McGraw-Hill Book Co.: New York, 1953; p 266.
Adsorption Behavior of Cadmium and Zinc Ions
Langmuir, Vol. 9, No. 12,1993 3639
Table 11. Values of TOE As Determined from the Darken Plot and Calculated AGO~
Cd2+ on A l a 0 3
_
_
_
_
0.111 0.121 0.0657 0.0644
Zn2+on A1208 Cd2+on Si02 Znz+ on Si02
_
-6219 -6654 -9894 -10016
Table 111. Results for Raoultian and Henrian Activity Coefficients (Wd)
(mol/ (mol/L) cm9 cb
4.663-3 2.293-3 8.713-4 3.1904 6.703-5 1.733-5 1.013-5 4.493-3 2.193-3 8.223-4 3.013-4 6.013-6 1.4105 8.203-6 6.50E-4 3.863-4 2.093-4 5.153-5 1.153-5 5.403-6 7.813-4 6.133-4 3.533-4 1.853-4 4.073-6 7.403-6 4.003-6
ri (mol/ cm2)
XR Yb YR Cd'J+on A l a 0 3 3.783-3 1.893-10 LOO0 0.593 1.OOO 2.383-3 1.193-10 0.630 0.679 0.893 1.433-3 7.17E-11 0.379 0.777 0.646 9.023-4 4.513-11 0.239 0.854 0.413 3.6604 1.833-11 0.097 0.928 0.232 1.4104 7.033-12 0.037 0.962 0.162 1.1OE-4 5.523-12 0.029 0.971 0.122 Zn2+ on Ala03 4.7703 2.863-10 LOO0 0.598 1.OOO 2.883-3 1.733-10 0.605 0.684 0.922 1.653-3 9.893-11 0.346 0.782 0.693 9.153-4 5.453-11 0.192 0.857 0.501 3.703-4 2.223-11 0.078 0.932 0.269 1.483-4 8.853-12 0.031 0.966 0.164 1.103-4 6.583-12 0.023 0.974 0.129 Cd2+ on Si02 1.8603 9.283-11 LOO0 0.802 1.OO0 1413-3 7073-11 0.762 0.841 0.817 1.123-3 5.593-11 0.602 0.879 0.585 5.983-4 2.993-11 0.322 0.936 0.287 2.783-4 1.393-11 0.150 0.969 0.143 1.80E-4 9.023-12 0.097 0.979 0.104 ZnZ+ on Si02 2.253-3 1.353-10 1.OO0 0.787 1.OO0 1.933-3 1.163-10 0.859 0.807 0.937 1.523-3 9.103-11 0.674 0.847 0.722 1.193-3 7.13011 0.528 0.885 0.505 6.083-4 3.653-11 0.270 0.943 0.231 2.323-4 1.393-11 0.103 0.975 0.114 1.643-4 9.853-12 0.073 0.981 0.088
UR
YH
3.783-03 1.50E-03 5.44E-04 2.153-04 3.54005 5.23006 3.22006
8.989 8.030 5.804 3.713 2.089 1.456 1.092
4.77E-03 1.74E-03 5.701.763-04 2.87E-05 4.56006 2.523-06
8.248 7.609 5.715 4.132 2.217 1.353 1.062
1.86003 1.08E-03 6.733-04 1.93E-04 4.163-05 1.753-05
15.211 12.433 8.894 4.366 2.170 1.580
2.253-03 1.66M3 1.02E-03 6.28E-04 1.64E-04 2.39E-05 1.20E-05
15.534 14.553 11.216 7.838 3.589 1.774 1.374
noted that alumina is net positively charged, while the silica surface exhibits a net negative under the conditions studied here. This means that the adsorption of these cations on silica is strictly based on the Coulombic attraction between the positive cations and the negative surface. On the other hand, the adsorption of these metal ions on the alumina surface could be attributed to surface
modification with the aid of anions present in the system.18Jg As a result, the absence of regular solution behavior of metal ions and alumina surface is not unreasonable. The values of AGO,&& are in fact the Gibbs energy change for the adsorption species going from the aqueous solution standard state to the Raoultian adsorption standard state. If one treats the adsorption process involving adsorbingspecies and adsorption sites, then the excess partial molar Gibbs energy for the adsorption species onto silica is A G L B a & . S= RTa(1- XR)' where the values of a for Cd2+and Zn2+are identical to those given above the adsorbing species. The standard state for the species would be the species at a fractional coverage of unity. At this condition the surface vacancy fraction would be 0 and X R = 1. The values of YRO obtained from a second-order polynomial fit of the data are as shown in Table 11. The Gibbs standard free energies of adsorption of zinc are smaller than those of cadmium as evidenced in the experimental data shown in Figure 6. The values Of YR and for YH each data point investigated are shown in Table 111.
Conclusions A thermodynamic approach to the adsorption behavior of zinc and cadmium on silica and alumina has been developed. The Gibbs standard free energy values of the adsorption of zinc and cadmium were evaluated based on Raoultian standard state and Henrian standard state. It was concluded that the Gibbs free energies of the adsorption of zinc are in general greater than those of cadmium due to more favorable adsorption of zinc than that of cadmium. This may be attributed to the fact that the hydrated zinc ion is larger than that of cadmium. It was also observed that the adsorption of cadmium and zinc on silica follows more ideal behavior due to simple Coulombic adsorption, while that on alumina behaves less regular due to a more complicated adsorption mechanism. Acknowledgment. This work has been supported in part by a Mining and Mineral Resources Research Institute (MMRRI) grant, Grant Number G1104146. (18) Hohl, H.;Stumm, W. J. Colloid Interface Sei. 1976,65, 281. (19) Riemadijik, W. H.V.; DeWit, J. C. M.; Koopal, L. K.;Bolt, G. H. J. Colloid Interface Sci. 1987, 116, 511.
