Environ. Sci. Techno/. 1994, 28, 1859-1866
Interaction of Metals and Protons with Algae. 4. Ion Exchange vs Adsorption Models and a Reassessment of Scatchard Plots; Ion-Exchange Rates and Equilibria Compared with Calcium Alginate? Ray H. Crist, J. Robert Martin, Donald Carr, J. R. Watson, and Heather J. Clarke
Messiah College, Grantham, Pennsylvania 17027 DeLanson R. Crist'
Department of Chemistry, Georgetown University, Washington, D.C. 20057 Sorption is frequently described in terms of adsorption isotherms (Langmuir, Freundlich), but metal sorption on algae has been recognizedto be an exchange process where sorption is accompanied by the release of ions. An experimental ion-exchange constant for Zn displacing Ca from Rhizoclonium was used to calculate concentrations over a wide enough range to assess interpretations given to Langmuir and Scatchard plots. While such plots may be convenient to describe maximum sorption and systematize data, misinterpretations can occur at low metal concentrations in an ion-exchange system. Values of Ke, for seven metals displacing Ca from Vaucheria correlated with formation constants of the metal acetates and with K,, of the metals on calcium alginate, a model of cell wall components, indicating the bonding of metals to carboxylate groups of algal cell walls. Rates of metal desorption from Vaucheria by EDTA-Li are inversely related to binding strengths, and rates of Cd desorption from calcium alginate justify assumptions in the K,, expression. Removal of Cd with a calcium alginate column was investigated for possible application to water treatment.
+
KL
Zn2+ 2 ~ - (ZnX,) [Zn2'l/(ZnX2) = [Zn2+l/(ZnX,),,
(1)
+ 1/KL(ZnX2),, (2)
Ion Exchange.
Introduction The role of metals in environmental issues has become increasingly more prominent in recent years as ecological awareness has achieved global proportions. An important development is the use of algae and other biomass to scavenge toxic and precious metals as discussed in recent reviews (1-3). The basis of these applications, however, is the nature of the sorption process itself. Sorption of metals on algal cell walls has been treated in terms of two models: (1)adsorption, in which the metal becomes bonded to one of several unoccupied sites with no additional changes at that site, and (2) ion exchange, in which a metal displaces another ion in the sorption process. An example of an adsorption model is the Langmuir adsorption isotherm represented by equilibrium 1taking Zn2+as a specific example and X- as a univalent anionic site on the solid surface. The equilibrium constant, KL, is a measure of binding strength. This model gives eq 2, where [Zn2+lis the concentration in solution, (ZnX2) is the amount adsorbed, and ( Z ~ X Z ) is~ = the maximum amount adsorbed. This model, derived for gas-solid systems with weak van der Waals-type attractions, and the related Freundlich adsorption isotherm are useful for systematizing data. t Presented in part at the Federation of European Microbiological Societies Symposium Metals-Microorganisms Relationships and Applications, Metz, France, May 1993. 0013-936X/94/0928-1859$04.50/0
The ion-exchange model can be illustrated by equilibrium 3, where quantities in parentheses represent species and reverse rates sorbed on a solid phase. Forward (Rf) (R,) are given by eq 4, where f M factors are the fraction of sites on a solid phase occupied by metal M. Setting forward and reverse rates equal gives the equilibrium expression eq 6 with a dimensionless Kev. In our earlier work, an effort was made to relate the sorption of metals on algae to electronic structure using alkali, alkaline earth, and transition metals ( 4 , 5 ) . Data were represented by Langmuir adsorption isotherms. This method was satisfactory since, as will be shown in the present work, ion concentrations were sufficiently high, and it was also used for the sorption of amines, diamines, and amino acids by Vaucheria (6). Reaction rates aided in understanding proton-metal interactions (4, 5, 7). Langmuir Adsorption.
0 1994 American Chemical Society
R, = k,[zn2+1fcs R, = k,[Ca2+1fzn where
C, = (CaX,) + (ZnX,) at a fixed pH = ion exchange capacity
The occurrence of ion exchange was demonstrated in the displacement of H, Ca, and Mg when Vaucheria (5) and marine algae (8)were treated with various metals. The stoichiometryof ion exchangewas used to demonstrate an ionic process for Cd desorption from Vaucheria by Environ. Scl. Technol., Vol. 28, No. 11, 1994 1859
sulfide ions (accompanied by partial retention of CdS) (7) and for Cd sorption from a Cd(OH)2 suspension at pH 10 (7, 8). Preliminary ion-exchange constants were first reported for the displacement of Ca from Vaucheria by several metals, as determined from both forward and reverse directions (7). For a new sorption system, it should be determined experimentially whether the fundamental process is one primarily of adsorption or ion exchange if one wants to make conclusions about structural effects on sorption. Since most researchers do not do this, an important question is what conclusions can be drawn from data plotted by the more convenient adsorption model when the actual process is one of ion exchange. We now report results that have a bearing on this question as well as the underlying rate assumptions (eq 4) in the ion-exchange model. We also report new K,, constants and their pH behavior for a variety of metals with Ca-Vaucheria and with calcium alginate, considered a model for components of algal cell walls. Rates of metal desorption from Vaucheria using EDTA were also investigated as well as a potential application, the use of calcium alginate (CaG2) to remove Cd from aqueous solution. Experimental Section
Materials. Algae were obtained from local limestone spring waters and stored at 0 "C. Samples were prepared as described previously (4, 6 ) . The material was filtered through 40-60 mesh stainless steel. After several washings it was dried with absorbent paper and stored at 0 "C. Individual samples were 0.015-0.03 g of dry weight. Calcium alginate (CaG2)powder was prepared by adding 50 mL of acetone to 10 g of the Na salt of alginic acid (Sigma). To this was added an equivalent amount of Ca(NO& as a saturated solution with stirring over 30-60 min in order to avoid gel formation. Another equivalent of Ca(N03)~ was added slowly. The mixture was transferred to an open pan, and acetone was allowed to evaporate giving a powder. This sample was washed with water and then dried at 80 "C. The metal content of a sample was determined by treatment with 10 mL of 3 M HN03 for 10 min followed by atomic absorption analysis of the nitrate with a PerkinElmer instrument Model 2380 A (triplicate runs). All data are presented per gram of dry weight of the alga as determined by heating a sample in an oven at 80 "C to constant weight. Ion Exchange vs Adsorption. Experimental data for this purpose were taken from the sorption of Zn at pH 7 on Rhizoclonium, which had been pretreated with Ca to remove all native Mg. For each added Zn amount, aqueous concentrations of [Zn2+land [Ca2+lwere determined as well as sorbed (ZnXz). The total of sorbed metals (measured after the maximum amount of Zn had been added) gave the ion-exchange capacity C, = (ZnX2) + (CaX2) = 590 pmol g-l. The value of K,, for each EZn2+l was calculated from eq 6 (see Results for data). Verification of Ion-Exchange Reaction Rate Assumptions. (a) Rate Proportional to Ca Concentration. A 0.05-g sample of CaG2was put into 50 mL of 1.0 mM Cd(NO& for 50 s, vacuum filtered, and washed. The sample was then suspended in 50 mL of HzO from which five, 10-mL portions, all with the same f c d = (CdGd/ [(CaGz) + (CdG2)], were taken with a pipet. Each was 1860
Environ. Sci. Technol., Vol. 28, No. 11, 1994
mixed with 10 mL of Ca at the concentrations 1,2,4,and 10mM and filtered after 20 s, and the filtrate was analyzed for Cd released. This amount released per 20 s gave the rate of exchange for samples with fixed fCd at varying [Ca2+l. (b) Rate Proportional to fed. Calcium alginate samples with different values of fCd were prepared by adding 0.02-g CaG2 samples to solutions 1,2,4,7, and 10 mM in Cd for 50 s. Each sample was filtered, washed, and suspended in 10 mL of Hz0. To each was added 10 mL of Ca at 1.0 mM. After 20 s, the suspension was filtered, and (CdX2)was determined, thereby giving fCd. The Cd released to the filtrates in these 20 s gave the rate of Ca exchange for varying fCd at constant [Ca?. This short time was chosen to reduce the importance of the reverse reaction. Ion Exchange for Vaucheria. A sample of 0.07-0.100 g (dry weight) was washed with 100 mL of deionized water and then suspended in 500-600 mL of water. This suspension was adjusted to the desired pH and held there until stable when the proton-metal equilibrium was established. From this suspension,five moist samples were prepared by filtration;one for blank analysis and the others for mixing with 100 mL of the sorbing metal at concentrations of 0.03,0.05,0.07, and 0.12 mM. For La and Al, the concentrations were somewhat lower, while for Na and K, they were 0.05-10 mM. A metal solution was brought to the desired pH, and the moist alga was added. If the pH decreased due to proton release, it was brought back with 0.01 M KOH, thus maintaining the pH and also giving a measure of protons released. On filtration, the solution and sample were analyzed for Ca and Mg. Ion Exchange for Calcium Alginate (CaG2). A 0.050-g sample of CaGz powder was suspended in 50 mL of HzO, and 10 mL was pipetted into each of the five beakers. This 10 mL was then added to 90 mL of metal solution and held for 2 h for equilibrium. This long time was necessary since the rate was found to be much slower than for algae. The suspension was vacuum filtered, and the filtrate and sample were analyzed for metals. For CaG2, the exchange capacity C, was 2000 vmol g-' (for M2+). Sorption was also considerably higher than for algae, making it difficult to find a concentration range suitable for reliable results. Thus, at low concentration the metal would be used up leaving too little for precise measurement, while at higher concentrations f~ is too close to C, leading to unreliability. As a consequence,the important and very reactive trivalent metals Al, Er, and La could not be used nor could univalent alkali metals which tend to form gels. Rates of Removal of Sorbed Metal from Vaucheria by EDTA. Rates of desorption of Mg, Cd, Cu, and P b were measured in the presence of EDTA to complex desorbed metal thereby minimizing its resorption. The procedure for each metal was the same as that described for Pb. A 0.05-g alga sample was treated for 10 min with 5 mM P b at pH 6.00 to avoid precipitation of the hydroxide. After filtration, the Pb-sorbed sample was washed twice with deionized water. The sample was then added with stirring to 100 mL of 1mM EDTA-Li (acid EDTA made basic withLiOH) at pH 9.00. Five milliliters of the mixture was removed at 15,30,60,90,120,200,400,600,800,1200, 1400,and 1600 s and filtered immediately with wire mesh. The liquid was analyzed for Pb, Ca, and Mg, as was the residual sample after washing. The original P b content was obtained from that in the residue together with that
~~
Table 1. Determination of K,, for Zn on Ca-Rhizocloniu& 1
exDeriment 2
3
4
[Zn2+],M X 105 1.05 2.32 4.04 6.66 1.58 2.42 3.60 4.20 [Ca2+],M X lo5 (ZnXz),pmol g1 82.7 141 168 194 (CaXZ),pmol g-1 507 449 422 396 Kex 0.245 0.328 0.356 0.309 0.310 f 0.032 av Kex Data on Rhizoloniurn treated with Ca2+ to remove Mg2+;all experiments at pH 7. Calculated from (CaXz) = C, - (ZnXz) = 590 - (ZnXz).
displaced by the EDTA-Li. The counter ion (Li) flow was obtained from Li analysis of the sample. Rate data were analyzed by use of the first-order rate equation, In C/Co = -kt, where C is taken as the P b content of the sample at time t and COis that a t t = 0. The latter is obtained by adding the final amount left in the sample to that released in the desorption process. A plot of In C vs t gives a straight line with slope 4 . Column Separation of Cd by CaG2. A 0.5-g sample was put into a plastic tube giving a column 3 cm long x 0.5 cm in diameter. A 1.0 mM Cd solution was eluted under pressure at ca. l.OmL min-l. Samples were analyzed over several hours to determine the breakthrough point. The CaG2 powder was removed in four equal sections, and each was analyzed for Cd. Results Comparison of Adsorption and Ion-Exchange Models. In order to compare the two models properly, the key parameters Keg and C, were determined experimentally for an actual system, namely, sorption of Zn on Rhizoclonium pretreated so as to have only one exchangeable site (CaX2). The value of K,, was determined by adding increasing amounts of Zn2+ to this Rhizoclonium and measuring equilibrium concentrations of [Zn2+land [Ca2+l as well as sorbed (ZnX2). These data are given in the first three rows of Table 1. The value of (CaX2)for each (ZnX2) was gotten by the difference from C, = (CaX2) + (ZnX2) = 590 pmol g-l. Values of Kex calculated from eq 6 with these data are given in Table 1 along with the average value of 0.310 f 0.032. Comparison of the two models requires a wide concentration range, one that cannot be covered adequately for various experimental reasons. For the Zn-Rhizoclonium system, a wide range of concentrations of the various species was therefore calculated using the appropriate ion-exchange model. These calculated data, considered to represent what would be found experimentally, were then plotted according to the Langmuir method to see what conclusions might be drawn if one had assumed an adsorption model. Thus, using the ion-exchange model, [Zn2+lvalues were calculated for selected (ZnX2) from eq 6 using an experimental K,, = 0.300, C , = 590 pmol g-’, and the fact that E a 2 ] in solution equals (ZnX2) sorbed. To illustrate the procedure for the first entry of Table 2, a value of 530 pmol g-l was chosen for (ZnX2). This determines (CaX2) as 60 pmol g-l from C,. Also, the amount of ea2+released into solution equals the amount of Zn sorbed or 530 pmol. If one assumes 1.0 L of solution, [Ca2+lis therefore 530 pmol L-l or 0.53 mM. Use of these values in eq 6 gives [Zn2+l= 15.6 mM. Also shown in Table 2 is the Langmuir
Table 2. Comparison of Adsorption ( K L )and Ion-Exchange (Kex)Constants from Calculated Data for Sorption of Zn on Rhizocloniuma
(ZnXdb 530 510
490 450 400 350 300 250 200
[Zn2+lC [Zn2+1/(ZnXz)
KL
[Ca2+lK~ = K.,
15.6 10.82 8.00 4.82 2.81 1.70 1.03 0.613
566
0.299 0.302 0.299 0.300 0.300 0.300 0.301 0.300 0.300 0.300
29.4 21.2 16.3 10.7
7.02 4.66 3.34 2.45 1.70 0.068
0.341
0.068
100
591 612 667 750 858 1004 1200
1503 3001
From experimental data (Table l),K,, was found to be 0.310 and [(LnXz) + (CaXz)]= 590pmolg1. Values in this tablewere calculated using K,, = 0.300 and [(ZnXz)+ (CaXz)] = 590 pmol g-l for arbitrary b In pmol g-1. values of (ZnXz) from K,, = ([Ca2+1/[Zn2+l)(f~n/j~a). Concentration in mM. a
30
’ /
/
i 20
1
t
t
2
4
6
8
IO
le
14
16
[Zn’+], mM
Flgure 1. Ion-exchange data (Table 2) plottedaccordingto the Langmuir l/KL(ZnXz),, adsorptionequation [Zn2+]/(ZnX2) = [Zn2+]/(ZnXz), where (ZnX,) is the amount sorbed and KLis the equilibrium constant. For linear plots, (ZnX2)maxis obtained from the slope and KL is from the intercept.
+
ratio [Zn2+1/(ZnX2)and the Langmuir constant K L of 566 calculated from eq 2 using ( Z I I X ~ ) , = ~ total number of exchangeable sites = C,. The last column shows that, as = Kex expected by comparison of eqs 2 and 6, [Ca2+lK~ within roundoff error. The rest of the values in Table 2 were obtained in a similar way. These [Zn2+land (ZnX2)data, calculated from K,,, were plotted according to the Langmuir model as [Zn2+l/(ZnX2) vs [Zn2+lin Figure 1. Although there is curvature a t low [Zn2+l,the ion exchange data fit a Langmuir isotherm provided the metal concentration in solution is sufficiently high. The value of (ZnX2),=, the reciprocal of the slope of a Langmuir plot by eq 2, is 588 pmol g-1 from the linear part of Figure 1a t high [Zn2+l,as expected from the value of 590 pmol g-1 taken for C,, which also represents (Znx~),~,. At high [Zn2+l,Langmuir “constants” calculated from ion-exchange concentrations of Table 2 approach a constant value (Figure 2A), and the typical saturation effect observed in Langmuir systems is also apparent (Figure 2B). If a Scatchard plot is made from ion-exchange dataof Table 2, there are two limiting regions corresponding to low and high amounts of sorbed material (see Figure 3). Verification of Ion-Exchange Rate Assumptions. The amount of Cd released from a CdG2 sample in 20 s, Environ. Scl. Technol., Vol. 28,
No. 11, 1994 1861
30
4 2
4
B
6
12
10
14
fa EO
/ i
I
1 2
E
6
4
12
10
14
16
[Zn''] , m M
r
1
Flgure 2. Ion-exchange data (Table 2) interpreted according to the Langmuir adsorption equation. (A) KL "constants" calculated from eq 2 for increasingaqueous [Zn2+] concentrations. (B) Typical saturation effect on sorption with increasing [Zn2+].
2
3
4
5
6
7
B
9
I
IO
[Ca2+l , m M Flgure 4. Verification of rate assumptions made in the ion-exchange model (eq 5). Amount of desorption of Cd in 20 s from cadmium alginate by Ca. (A) For fixed [Ca2+], the rate is linearly dependent on f, = (CdG2)/[(CdG2) (CaG2)].(6)For fixed fa,the rate is linearly dependent on [Ca2+].
+
-
i 2
12
l.ol
:
O
o
0.44
I
\
O
.
I
I
0 24 00 100
200
300
400
500
600
(ZnXa, pmol g-' Flgure 3. Scatchard plot of data calculated from an ion-exchange model (Table 2) having one type of site (one Kex)and one (sorbed (=Cp). amount),
hence a rate, is shown in Figure 4 for different conditions. When [Ca2+]is constant at 1.0mM, this rate is proportional to fCd for samples prepared with different initial amounts of CdGz (Figure 4A). Also, when samples with the same fCd value are placed. in solutions of varying [Ca2+] concentrations, there is a linear relationship between rate of Cd release and [Ca+21(Figure 4B). Relative Amounts of Ca, Mg, and H on Vaucheria Cell Walls. When suspensions of Vaucheria, Spirogyra, Tribonema, and Oedogonium are acidified, anionic sites become protonated and Ca and Mg are released to the solution, as shown in Figure 5. This means that wall composition varies with pH, as shown for these Vaucheria data as plotted in Figure 6. It is important to note that 1882 Environ. Scl. Technol., Vol. 28,
No. 11, 1994
PH Flgure 5. Displacement of Ca and Mg by protons over a pH range of 7.0-1.0 for the freshwater algae Vaucheria (0); Tribonema (0); Spirogyra (A);and Oedogonium (0).
while pH affects metal-exchange capacity via the amount of (HX), it should not affect K,, since the denominators
7.0
6.0
1.0
4.0
3.0
2.0
1.0
PH
Figure 6. Amounts of sorbed species on Vaucheria as a function of PH.
Table 3. Ion-Exchange Constants K,. of Metals for Cae
Vaucheria
PH metal
K Ag Mg Ba Zn Cd Pb cu La A1 metal Li Mg Zn Sr
4.00
5.00
0.47
0.42
0.92 0.82 1.90 2.75
1.06 0.93 2.25 2.35
6.00 0.11 0.45 0.48 0.87 1.25 1.76 2.78
7.00
av
0.003
0.003 0.11 0.43 f 0.02 0.48 0.92 f 0.06 1.04 f 0.06 2.09 f 0.26 2.62 f 0.19 2.35 2.84
0.39 0.85 1.17 2.45 2.35
2.84
Calcium Alginate (CaGd, pH 7 K B X metal 0.017 0.098 f 0.006 0.54 f 0.02 1.32 f 0.08
Cd La
Pb
K,. 1.97 f 0.12 11.71 f 3.57 16.20 f 1.36
Calculated from K,. = ([Ca2+1/[M2+1)~~/fca).
of eqs 5 cancel in giving eq 6. This K,, independence of pH was demonstrated experimentally (see below). Desorption by protons is fast and complete at low pH. In this work, metal content was therefore obtained by putting for 10 min followed by analysis the sample in 3 M "03 of the solution for various metals. Ion-Exchange Constants. Values of Kegfor various metals displacing Ca from Vuucheria are given in Table 3 for several pH values. Because of hydroxide precipitation, experiments with P b must be done at pH < 7, Cu < 6, and A1 < 4. Reliable results for La could be gotten only at pH 7. This is because at higher pH, the hydroxide precipitates, and at lower pH, the reduced amount of (CaX2) and (MgX2) present (see Figure 5) combined with the very high reactivity of La does not allow the determination of K,, values over a reasonable range of metal concentrations. For the same reason, no data could be obtained for Er. With K only, pH 7.0 was used because at lower pH levels the stronger binding of protons prevented sufficient sorption of K. For Mg, Zn, Pb, Cu, and Cd, the constancy of Kexvalues over the pH 4-7 range shows that pH in itself is not a factor. Hence, the primary effect of lowering pH is to reduce the number of sites available for metal exchange
but not their intrinsic ability t o exchange. This result suggests that the binding sites act independently. Not shown is proton release which is about the same for Cu, Pb, A1 and Er, while for trivalent La it was surprisingly low and not observable for Mg. Ion-exchange constants for CaG2 are also given in Table 3. This surface presents only hydroxy and carboxylate groups as possible ligands. Desorption Rates of Metals from Vaucheria by EDTA. A typical first-order kinetic plot is shown in Figure 7 for the desorption of Cu from Vaucheria by EDTA. The curves show a three-step desorption process. The first 10-15 s accounts for about 50 7% of the total desorbed, and the 20-400-9 interval accounts for an additional 20 93,with the remainder desorbed in the slowest phase. Rate constants for these steps are taken as the slope of these regions. For Cu, these are 524,20, and 1.00 X lo4 s-l for k&, respectively. That for the first step is an estimate, since only two points at 0 (not shown) and 15 s were available, the rate being too fast to obtain data at shorter times. First-order rate constants for Ag, Mg, Cd, Pb, and Cu are given in Table 4 as averages for triplicate runs. The last column, Li content of a sample after desorption, shows that the reaction is 2Li+ (MX2) 2(LiX) M. Since Li is present in large excess (3 mM), these are pseudofirst-order rate constants. Desorption of Metals from CaGz by EDTA. Preliminary experiments desorbing P b from CaG2 using EDTA-Li showed two slow processes. Significantly, there was no initial fast process. Further quantitative work was not possible, however, since the counterflow of Li from EDTA-Li causes gel formation with sample disintegration. Cd Removal by CaG2. Passing a 1.0 mM solution of Cd through a CaG2 column (see Experimental Section) gave the following results:
-
+
total Cd added in pmol g1 [Cd2+]in effluent, pmol L-l
+
1
2
3
4
580
600 2
610 10
620 20
1
Entry 1represented a breakthrough point. When 580 pmol of Cd had been sorbed (per g of CaGZ),the eluant contained the measurable amount of Cd of 1.0 pmol L-1 (0.112 ppm). When more Cd was added, increasing amounts of Cd appeared in the effluent. The distribution of sorbed Cd at various column lengths was measured by determining Cd in each of four equal sections. After a total of 620 pmol g-' had been added (entry 4 above), analysis showed that the amount of Cd was greatest in the input section and least in the output section, with values of 28,90,168, and 264 pmol g-l per section for a total of 550 pg sorbed. Discussion
Adsorption vs Ion Exchange. The widespread use of the Langmuir isotherm in sorption studies is largely due to longstanding applications and its convenience in determining maximum sorption capacity. It is also a way of systematizing data at high concentrations. The problem with this model, however, is that it frequently does not really apply to the actual chemical process involved. In fact, we have shown that for algae, sorption of metal ions is accompanied by displacement of other cations. An ionexchange model is therefore more consistent with the chemical system. Unfortunately, more experimental work is necessary to describe the system by the ion-exchange Environ. Sci. Technol., Vol. 28, No. 11, 1994
1863
2.8
kl (0-15
S) =
342 x lo4 s . ~ 2.6
1.4'
200
400
800
800
1000
1200
1400
1800
Time, s Flgure 7. Rate of desorption of Cu from Vaucherla by EDTA as plotted according to the first-order rate equation In C = -kt where slopes of linear regions give rate constants k. The value of klwas calculated from two points (In C = 3.17 at t = 0, not shown, and In C = 2.65 at t = since
ZZ/r, pm" Figure 8. Ion-exchange constants for metals with Ca-Vaucheria plotted against a charge density factor P / r where Z is the ion charge a d r is its radius. For reference, a line is drawn for the points K, Ea, and AI, which do not have transitlon metal bonding characteristics.
mechanism (7,8),since the stoichiometry for sorption and desorption must be demonstrated and four concentrations determined rather than just the two values of "adsorption", namely, solute concentration and sorbed amount. However, the resulting K,, values, which are dimensionless, give a better perspective in comparing results from one algae specieswith another and in ultimately understanding the effect of cell wall chemistry. If one were to plot data from an ion-exchange system in the Langmuir manner, a linear plot results, but only at sufficiently high sorbent concentrations. The Langmuir constant calculated from this region is related to the actual K,, by the equation K,, = KLEmetal displaced]. 1864
Environ. Sci. Technol., Vol. 28, No. 11, 1994
It is clear from inspection of Figure 1that the Langmuir treatment is not appropriate for experimental data at low solute concentrations. The slope and intercept change with concentration in this region because the reverse reaction with Ca+2 is not considered in the Langmuir model. Our earlier work that was described in terms of the adsorption model (5, 6, 8) utilized high metal concentrations in the linear region. Another frequently used treatment of sorption data is derived by from the rearrangement of the Langmuir equation. For simple adsorption or the mathematically equivalent binding of a substrate to a macromolecule,the resulting equation suggests a Scatchard plot of (sorbed
0.6
Table 4. First-Order Rate Constants (Three-Step Process) for Desorption of Metals from Vaucberia by EDTA (Li) at pH 9.00.
reaction stepb 2
1
0.4-
Li/M equiv
3
A€! 530f 5 63f 20 Mg 515 f 32 82 f 20 8.6 f 2.0 1.11 f 0.05 Cd 6382~90 21 f 8 9.4 f 3.0 1.09 f 0.05 Pb 172f24 15 1 2.6 f 0.3 0.94 f 0.03 Cu 524f92 20 f 7 4.7 f 1.2 1.09 f 0.12 a For the reaction MX2 + 2HY3- + 2Li+ MY2- + 2LiX 2H+. MY2- is determined from the solution, and LiX is determined from the sample. Values shown are rate constants X lo4 and are in s-1 units.
*
~~~~
~~
~
-
0.2-
0.0-
+
~
~
amount)/(solutionconcentration) vs (sorbed amount). This plot gives a straight line with slope of -KL and intercept A curved plot which appears of (KL)(sorbedamount),,. linear at low and high sorbate concentration would normally be interpreted as adsorption on a material with two types of sites having different KL and (sorbed amount),,, because of the different slopes and intercepts. However,a Scatchard plot of the calculated ion-exchange data of Table 2 is also nonlinear (Figure 3). These data were generated assuming one type of site with one K,, and one (sorbed amount),ax (=C,). A curved Scatchard plot observed in biosorption studies should therefore be viewed with caution, since it may indicate that the chemical process is one of ion exchange, not simple adsorption. Ion-ExchangeConstants. The ion-exchangeapproach has therefore been justified for several reasons: stoichiometry, constant values for over a 6-fold range of concentrations for Zn on Vaucheria (Table 11, and verification of rate assumptions for calcium alginate, a model for the cell wall polymer. Two important trends can be noted for K,, values for various metals displacing Ca from Vaucheria (Table 3). First, K,, plotted against Z 2 / r (where Z is the charge of an ion and r is its radius) increases with Z 2 / r through the series K, Ba, and A1 suggestingthat the bonding is primarily of an electrostatic character (see Figure 8). P b and Cu are distinctly off the curve, which means that some additional factor is involved, perhaps more covalent bond character in bonding to carboxylate groups or chelation. This result therefore suggests another comparison, that of Vaucheria K,, with formation constants of metal acetates (9) for the same metals. A linear free energy relationship (LFER) (10)of these constants was reasonably linear (see Figure 9A). Since log K = -AGo/RT, this means that the free energy change for ion exchange on Vaucheria is related to the free energy change for formation of the corresponding metal acetates. A similar result was found for CaG2 (Figure 9B). As expected, there is a LFER for Kex for Vaucheria with Ke, for CaG2 (Figure 10). These correlations suggest the importance of complexation with carboxylate groups in Vaucheria,since CaG2 contains only carboxylate and hydroxy functional groups. It might be pointed out that CaG2 may not provide a precise model for the sorption character of the native material. Nevertheless, the present results do lead to a further understanding of the essential chemical aspects of sorption to algae. Rates of Desorption by EDTA. Various factors are probably involved in determining rates of metal desorption, such as the extent of hydration of metal ions and cell wall
0.4
0.6
0.8
1.0
1.2
1.4
3.6
1.8
2.0
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
B 1.0-
0.0.-
-1.5 0.4
Log Kf for metal acetates Figure 9. Free energy relationships between ion-exchange constants of metals and formation constants of the correspondingmetal acetates. (A) Displacement of Ca from Ca- Vaucheria. (B) Displacement of Ca from calcium alginate.
"'I
- I
0.2
;.
-g 9 L.
8
0.0-
d -0.2-
-i.5
-1,o
-0.5
0.0
0.5
1.0
5
Log K,, for Calcium alginate
Figure 10. Free energy reiationshlp between ion-exchange constants for metals with Ca- Vaucheria and with calcium alginate.
microstructure. However, an important factor appears to be binding strength, as indicated by the LFER found Environ. Sci. Technol., Vol. 28, No. 11, 1994 1865
2.07
A
1
so that even lower effluent concentrations should be possible. Removal of P b should be more efficient, since its K,, is 16.2 compared to 1.97 for Cd. Biosorption of Cd by flow-through columns containing the marine alga Ascophyllum has recently been reported (11). Literature Cited (1) Volesky, B., Ed. Biosorption of Heavy Metals; CRC Press, Inc.: Boca Raton, FL, 1990.
1A i
-0.4
.
-0.3
-0.2
i
-0.1
0.0
0.i
0.2
0.3
0.4
0.5
Log Ke, for Vaucheria Figure 11. Free energy relationships between rate constants for metal desorption from Ca- Vaucheria and ion-exchange constants of the metals. Desorption rates decrease as binding strengths increase.
between rate constants kz and k3 vs Kex for the various metals (Figure 11). Metals with higher K,,, and hence stronger binding relative to Ca, are desorbed more slowly. Column Sorption of Cd by CaG2. In principle, the present K,, values could be used to describe the extent of sorption throughout a column, assuming equilibrium a t each microscopic layer. This is under current investigation. However, as estimated by extrapolation of the present data, 1lb of CaGz would reduce Cd in 790 gal from 10 to 0.112 ppm. The distribution of sorbed Cd in four successive column sections appeared to decrease linearly
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(2) Darnall, D. W.; Greene, B.; Hosea, M.; McPherson, R.; Henzl, M.; Alexander, M. D. In Trace Metal Recovery from Aqueous Solutions; Thompson, R. T.; Ed.; Burlington House: London, U.K., 1986; pp 1-24. (3) Lembi, C. A.,Waaland, J. R., Eds. Algae and Human Affairs; Cambridge University Press: New York, 1988. (4) Crist, R. H.; Oberholser, K.; Shank, N.; Nguyen, M. Environ. Sci. Technol. 1981, 15, 1212-1217. (5) Crist, R. H.; Oberholser, K.; Schwartz,D.; Marzoff, J.; Ryder, D.; Crist, D. R. Environ. Sci. Technol. 1988, 22, 755-760. (6) Crist, R. H.; Oberholser, K.; Wong, B.; Crist, D. R. Environ. Sci. Technol. 1992,26, 1523-1526. (7) Crist, R. H.; Martin, J. R.; Crist, D. R. In Mineral Bioprocessing;Smith, R. W., Misra, M., Eds.; The Minerals, Metals, and Materials Society: Warrendale, PA, 1991; pp 275-287. (8) Crist, R. H.; Oberholser, K.; McGarrity, J.; Crist, D. R.; Johnson, J. K.; Brittsan, J. M. Environ. Sci. Technol. 1992, 26,496-502. (9) Ringbom, A. Complexationin Analytical Chemistry;Interscience: New York, 1963; p 320. (10) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in Organic Chemistry, 3rd ed.; Harper and Row: New York, 1987; pp 143-158. (11) Volesky, B.; Prasetyo, I. Biotechnol.Bioeng. 1994,43,10101015.
Received f o r review December 27, 1993. Revised manuscript received May 23, 1994. Accepted June 15, 1994.@ @
Abstract published in Advance ACS Abstracts, July 15, 1994.
