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Adsorption of lead ions on immobilized humic acid
Adsorption of Lead Ions on Immobilized Humic Acid HIDESHI SEKI, 1 AKIRA SUZUKI, AND ISAMU KASHIKI Department of Chemistry, Faculty of Fisheries, Hokkaido University, Hakodate 041, Japan Received December 20, 1988; accepted March 22, 1989 A fundamental study of the recovery of lead from dilute solution was carried out. Humic acid (HA) was immobilized by calcium alginate gel containing activated carbon powder. To understand the adsorption mechanism of lead on the adsorbent (HA-gel), a model for complexation between divalent metal ions and carboxyl groups on organic polymers was proposed. A general model could be proposed from this work. The results show that the complexation constant of the lead-humic acid system and the number of available sites for complexation of humic acid was not influenced by immobilization. However, the alginate gel used as an immobilizing agent affected the rate of adsorption to a remarkable extent. © 1990 Academic Press, Inc.
In the present study we used two carrier materials, that is, calcium alginate gel and activated carbon powder, to immobilize humic acid. Based on a simple model for the complexation of divalent metal ions on organic polymers with acidic groups, the adsorption mechanism and the ability of this newly developed adsorbent will be discussed.
INTRODUCTION
For the recovery of valuable elements or the removal of harmful heavy metals from aqueous environments, adsorption is a most promising technique. Humic substances (such as humic acid and fulvic acid) are widely present in natural waters and they have a highcomplexing ability with various heavy metal ions. Moreover, they are ecologically acceptable matter. Most previous studies dealt with the structure and the nature of humic substances or their complexes (e.g., 1, 2). As a few examples of their practical applications, Ho and Miller (3) and Heitkamp and Wagener (4) tried to use humic acid for the recovery of uranium. However, it is difficult to use humic acid as the sole adsorbent because of its high solubility in water. To immobilize it, Ho and Miller, and Heitkamp and Wagener employed hematite particles and anion exchange resins as the carriers, respectively. Since the carriers are almost inert materials in adsorption, the ratio of humic acid to the carrier is very important. In the studies mentioned above, the percentage of humic acid in the whole adsorbent was only 3.5% (3) or 25% (4) on a dryweight basis.
ORGANO-METALLIC COMPLEXATION MODEL
Schnitzer and his associates (5) accounted for the acidic properties o f h u m i c compounds in terms of a range of aromatic carboxyl groups and to a lesser extent phenolic hydroxyl groups. Wilson and Kinney (6) showed that more than 90% of the COOH sites on humic acid were dissociated in seawater in pH 8, and virtually all the phenolic hydroxyl sites remained protonated. They also showed that only minor proportions (8-38%) of the acidic sites on dissolved organic matter ( D O M ) are available for metal ion binding, because most of the sites are not in close proximity and do not have the polydentate characteristic necessary for chelation of metal ions. Taking into account these facts, we built a simple model for the complexation of divalent heavy metal ions and an organic polymer such as humic acid in an acidic media. The model is based on three assumptions:
i To whom all correspondence should be addressed. 59
0021-9797/90 $3.00 Journal of Colloid and Interface Science, Vol. 134, No. 1, January 1990
Copyright © 1990 by Academic Press, Inc. All rights of reproduction in any form reserved.
60
SEKI, SUZUKI, A N D K A S H I K I
(a) A divalent heavy metal ion binds with two sites (carboxyl groups) on an organic polymer.
parameters of N and K can be obtained from the slope and the intercept of the line, respectively.
(b) The polymer has a definite number of available sites for complexation. The unavailable sites are not taken into account. (c) In acidic media o f p H < 4.3, the available sites are either protonated to - C O O H or complexed with metal ions. From the above assumptions, the complexation of metal ions and an organic polymer in acidic media can be expressed by the cation exchange reaction, 2 ( - C O O H ) + M 2+ ~
(COO)2M
+
2H +,
MATERIALS
Chemicals Lead nitrate and sodium salt of alginic acid (AA) were obtained from Wako Pure Chemical Industries (Japan), and sodium salt of humic acid from Aldrich Chemical Co. They were used as received. Activated carbon powder (ACP) was purchased from Wako Pure Chemical Industries. It was screened by a 100mesh (0.149-mm) sieve and the undersize fraction was used.
[11 where M 2+ represents the divalent heavy metal ion. The complexation constant ofEq. [ 1], K, is defined as
K-
OMC2 - O~CM OMC~ - (1 - OM)2CM '
[21
where On and 0M represent the fractions of the sites on the polymers covered by H + and M 2+ at the equilibrium state, respectively, and CI~ and CM are the equilibrium concentrations of H + and M 2+, respectively. Defining the amount of divalent metal ions complexed with (or adsorbed on ) the polymer as X mole. g-l, and the number of available sites for complexation as N mole- g-l, Eq. [2] is modified to
(2X/N) C2 K = {1 - ( 2 X / N ) } z c M
Calcium alginate gel containing HA and ACP was used as the adsorbent. An aqueous solution containing HA (0-15%), sodium alginate ( 1% ), and the HA's weight in ACP was well mixed. It was trickled down into a 0.1 M calcium chloride solution in a stirred vessel and was converted to the gel particles. After 30 min the formed adsorbent (HA-gel) was removed from the calcium solution and it was washed repeatedly with distilled water. The detailed preparation method of HA-gel is given elsewhere (9). The shapes of the particles were nearly spherical and they had a diameter of 2-4 mm, as shown in a photograph of Fig. 1.
0
[3]
0
or
2 ( X / C H ) ( C M / X ) 1/2 = N(CM/X)I/2/CH
Adsorbent
-
( 2 N / K ) 1/2.
[4]
Equation [4] represents the adsorption isotherm of divalent metal ions on the organic polymer. The plot of 2 ( X/Cr~ ) ( C M / X ) 1/2 vs ( C M / X ) 1/2/CH gives a straight line, and the Journal of Colloid and Interface Science, Vol. 134,No. 1, January 1990
• 0
•
0
0 Q Q
• Q
FIG. 1. Photograph of HA-gel's particles.
ADSORPTION ON IMMOBILIZED HUMIC ACID
61
ACP was added for suppressing the discharge of H A from the gel's interior to the water phase. The excess addition of ACP inhibited the formation of HA-gel. Separate experiments by the authors (9) showed that HA-gel without ACP released 10-20% of HA, while the gel containing the same weight of ACP as H A discharged only 1% of HA.
the thermal equilibrium at 30°C, a certain a m o u n t of HA-gel (ca. 1 g) was added to the solution. After reaching the adsorption equilibrium, the concentrations of lead and p H in the bulk phase were measured.
EXPERIMENTAL METHODS
The structure and the properties of humic acid, in general, greatly depend on the H A ' s origin. To examine the validity of the complexation model represented by Eq. [4] and to know the adsorption characteristics of the H A present, complexation experiments were carried out for the P b - H A system. Figure 2 shows a typical result of the kinetic experiments. The reaction proceeded very rapidly and a few seconds or so were enough to attain the equilibrium. From the results, we determined the contact time for the equilibrium experiments (shown in Fig. 3 ) as 30 min. The ordinate of Fig. 3, XH, represents the equilibrium a m o u n t of Pb complexed with 1 g of HA. The figure shows that the p H increase of the adsorption system resulted in more for-
Complexation between Lead and Humic or Alginic Acid Kinetic experiments for the P b - H A system were conducted to determine the time for attaining equilibrium. The reaction was carried out in a 10-ram spectrofluorometer cell (JASCO FP-550A). The extent of complexation was determined from the measurement of the intensity of fluorescence (7). The maxi m u m emission intensity for the H A present occurred at 450 n m upon excitation at 365 nm. Equilibrium experiments were carried out in the following manner. A 0.285-dm -3 solution containing a certain a m o u n t of lead nitrate was prepared. The p H was adjusted to a desired value by nitric acid. A 0.015-dm -3 polymer solution containing 0.015 g of H A or 0.01 g of AA was prepared. After reaching the thermal equilibrium at 30°C, both solutions were mixed and the complexation reaction was initiated. The solution was stirred for a necessary time to attain the complexation equilibrium. Then, it was filtered through a m e m brane filter (pore size = 0.05 # m ) , and the p H and lead concentration of the filtrate were measured by a p H meter (Orion Research 501 ) and by atomic absorption spectrophotometer (Hitachi A- 1800), respectively. The a m o u n t of lead complexed with the polymer was determined from the difference between the lead concentrations in the initial and the equilibrium states.
Adsorption of Lead on HA-Gel A 1-dm -3 solution containing a necessary a m o u n t of lead nitrate was prepared and its p H was adjusted by nitric acid. After reaching
RESULTS AND DISCUSSION
Organo-Metallic Complexation
100
7 v
O
50~-
g
'-0--0--0
0--
I
I"
50
100
Contact
150
Time ( s e c )
FIG. 2. Time dependence of lead-humic acid complexation at 30°C. Initial concentrationsof lead ion and humic acid are 5.0 × 10-5 mole.dm 3 and 0.02 g.dm 3 (A), and 1.0 × 10-3 mole.dm -3 and 0.16 g-dm -3 (©), respectively. Journal of Colloid and Interface Science, VoL 134, No. 1, January 1990
62
SEKI, SUZUKI, AND KASHIKI 1.5
2.0
7, ~
re
~j 8j
0.5
o/
08/0/0/
%
1.0
f
/
.0..0 qb '~
1.0
0/O
7/ I
0.0
3
5
4 pH
FIG. 3. Effect of equilibrium pH on lead-humic acid complexation at 30°C. Initial concentrations of lead ions and humic acid are 1.0 × 10 -4 mole.dm -3 and 0.05 g • dm-3, respectively. The solid line represents the theoretical curve calculated from Eq. [6].
marion o f P b - H A complexes. The comparison o f the data o f Fig. 3 with the complexation model expressed by Eq. [4] is shown in Fig. 4. The data agreed well with the model (solid straight line), and the number o f available complexing sites on 1 g o f HA, NH, and the complexation constant o f the P b - H A system, KH, were derived from the slope and intercept o f the line as NH = 1.84 × 10 -3 m o l e . g - I and KH = 1.53 × 10 -3 m o l e . d m - 3 , respectively. Alginic acid, used here as the immobilizing agent, also has a complexing ability with di-
I
I
3.5
4.0
0
3.
4.5
pH
FIG. 5. Effect of equilibrium pH on lead-alginic acid complexation at 30°C. Initial concentration of lead ions and alginic acid are 1.0 × 10 4 mole.dm-3 and 0.033 g. dm-3, respectively. The solid line represents the theoretical curve calculated from Eq. [7 ].
valent metal ions. Figure 5 shows that the experimental results o f the P b - A A system are similar to those in Fig. 3. Since the complexation o f metal ions with A A in a liquid state is very rapid (8), we adopted the same contact time as that in the previous P b - H A system. The comparison o f the data o f Fig. 5 with Eq. [ 4 ] is presented in Fig. 6. Similarly to HA, the complexation o f Pb with A A was also consistent with the model. The number o f available sites on AA, NA, and the complexation con-
15 10 A
I0
o/
Slope 3.44x10 -3 Intercept -0.72
"i
re
(~)J
jo
v 5 jO
(13
ca
tO
CD 0
0
i
0.5
(CM/XH)½/CHx l 0
1.5 -3
FIG. 4. Fitting of the data of Fig. 3 to Eq. [4]. Journal of Colloid and Interface Science, Vol. 134, No. 1, January 1990
,,i.o°'~
I 1.0
I 2.0
(CM/XA}~/CH x l 0
-3
FIG. 6. Fitting of the data of Fig. 5 to Eq. [4].
3.0
ADSORPTION
ON IMMOBILIZED HUMIC
stant, KA, were obtained as NA = 3.44 × 10 -3 mole- g-i and KA = 1.32 × 10-2 mole" dm-3, respectively. Comparing the complexing abilities of AA and HA with Pb ions, the former is obviously superior to the latter in both terms of capacity and strength. However, AA is not suitable as an adsorbent material because it is poorly soluble in water and a solution containing more than 10% of AA cannot be prepared. Activated carbon powder (ACP), used as another immobilizing agent in HA-gel, is also a well-known adsorbent for various kinds of molecules, but it cannot be expected to have a complexing ability with ions. By the adsorption experiments for the Pb-ACP system, it was confirmed that ACP adsorbed a negligible amount of lead, compared with HA or AA. Adsorption of Lead on HA-Gel Prior to the equilibrium experiments, kinetic experiments for the lead-HA-gel system were conducted. The HA-gels containing 5, 10, and 15% of HA on a wet-weight basis (45.5, 47.6, and 48.4% on a dry-weight basis) were used for the experiments. Typical examples of the results are presented in Fig. 7. X denotes the adsorption amount of lead on 1 g of HA-gel. As 50 h were enough to attain the equilibrium for all the gels, we adopted the contact time of 50 h for the equilibrium experiments. From Figs. 2 and 7 it is seen that the present method of immobilization apparently had a serious effect on kinetic behavior of organo-metallic complexation. The remarkable decrease in the adsorption (or complexation) rate is caused by the increase in diffusion resistance of lead ions due to the gel network. Therefore, the preparation of a small or thinner HA-gel will be required for actual operation. The influence of pH on the equilibrium amounts of lead adsorbed, X, on several HAgels is shown in Fig. 8. Triangle symbols refer to a particular HA-gel which contained no humic acid. The adsorbed amount increased with the increase of the HA content. The same
63
ACID
2.0
% -;
mf
m f
/ 1.0
m
'~o
f
0
~ 0 - -
~ •/...-.-.-.----- •
• l
0.0
0
s0 C o n t a c t Time ( h r )
100
FIG. 7. Time dependence of lead adsorption on HAgels containing 5% (O), 10%(O), and 15%(ll) ofhumic acid at 30°C. All the HA-gelscontain 1%alginieacid and humic acid's weight in activated carbon powder. Initial concentration of lead ion is 1.0 × 10 4 mole-dm-L
data are fitted to Eq. [ 4 ] in Fig. 9. The plotted symbols correspond to those in Fig. 8. The adsorption mechanism of lead on HA-gel was also consistent with the foregoing organo-metallic complexation model. The slopes of the lines express the numbers of available sites on 1 g of HA-gel. Estimation of the Amount of Lead Adsorbed on HA-Gel From the fact that the adsorption of lead on HA-gel (i.e., immobilized HA and AA) agreed with the complexation model, the adsorbed amount of lead on 1 g of HA-gel, X, can be expressed as X = XH.PH + XA.PA,
[5]
where XH and PH represent the amount of lead adsorbed by the HA component and the weight fraction of HA in the gel, respectively. Similarly, XA and PA denote the quantities with respect to AA. Applying the adsorption isotherm of Eq. [ 4 ] to the components in the gel, XH and XA can be evaluated as XH = (NH/4KH)[(2KH + C) - {C(4Kn + C)} '/21 [61 Journal of Colloid and Interface Science, Vol. 134, No. 1, January 1990
64
SEKI, SUZUKI, A N D K A S H I K I 0.2
2.0
-= ) A i
j
~0 •
0.1
/II
i
ss/II1.0
/0
1
v
?o
,/"
/
/
o,-" a -,-"-'A--A 0 I
a0
I
3
4
I 2.0
1.0
5
Xcalc
(mol.g-I) xlO4
pH
FIG• 8. The p H dependence of lead adsorption on HAgels containing 0% (A), 5% (O), 10% (©), and 15% (11) of h u m i c acid at 30°C. Initial concentration of lead ions is 2.0 × 10 4 mole. dm-3.
strates the agreement of the calculated adsorption amounts on HA-gels with the experimental.
XA = (NA/4KA)[(2KA + C)
- {C(4KA + C)}1/21,
[7]
where C =-- C ~ / C M . T h e solid lines in Figs• 3 and 5 represent the theoretical curves calculated from Eqs. [6] and [7], respectively• PH and PA are the values determined from the composition of HA-gel• Since NH, NA, Kn, and KA were already obtained, we can calculate X by using Eqs. [ 5 ] - [ 7 ]. Figure 10 demon-
8.0
Slope
2.84xi0-4
/
4.0 "
1.94x10
°//f,0I
/0//"
I I/~o" 0
FIG. 10. Comparison between the experimental amounts of lead adsorbed on several kinds of HA-gels and calculated from Eqs. [ 5 ] - [ 7 ]. The symbols are the same as those in Fig. 8.
-4
1.23xl 0 -4
SI°pe 3"47x10-5
/ A -
2.0
4.0
(cMIXl½/Cnx l o -4 FIG. 9. Fitting of the data of Fig. 8 to Eq. [4].
Journal of Colloid and Interface Science, Vol. 134,No. 1, January 1990
CONCLUSION
Humic acid, which has an excellent complexing ability with heavy metal ions, was used as an adsorbent material for the recovery of lead. It was immobilized by a combination of alginate gel and activated carbon powder• By the present immobilizing method, we obtained the adsorbent (HA-gel) comprising as much as about 50% of HA on a dry-weight basis. A general complexation model for divalent heavy metal ions and organic matter was proposed. The model was confirmed by two kinds of experiments in which humic and alginic acids in liquid state were used as adsorbents. The complexation constants and the number of available sites were given as KH = 1.53 × 10 -3 mole. dm -3 and NH = 1.84 × 10 -3 mole- g-1 for humic acid, and KA = 1.33 × 10 .2 and NA = 3.44 × 10 .3 for alginic acid, respectively. The model was applied to the adsorption of lead on HA-gels. The comparison of the model with the experimental data showed that the model also explained the adsorption on HAgets.
ADSORPTION ON IMMOBILIZED HUMIC ACID ACKNOWLEDGMENT The authors thank Y. Yasuda for taking the experilental data.
REFERENCES 1, Ramamoo~hy, S.,andKushner, D.J.,Nature(London) 256, 31 (1975). 2. Buflle, J.,Greter, F., and Haerdi, W., Anal Chem. 49, 2 (1977).
65
3. Ho, C. H., and Miller, N. H., J. Colloid Interface Sci. 106, 281 (1985). 4. Heitkamp, D., and Wagener, K., Ind. Eng. Chem. Process Des. Dev. 21, 781 (1982). 5. Schnitzer, M., and Khan, S. U., "Humic Substances in the Environment." Dekker, New York, 1972. 6. Wilson, D. E., and Kinney, P., Limnol. Oceanogr. 22, 281 (1977). 7. Saar, R. A., and Weber, J. H., Anal, Chem. 52, 2095 (1980). 8. Smiderod, O., and Hang, A., Acta Chem. Scand. 19, 329 ( 1965 ). 9. Seki, H., Suzuki, A., and Kashiki, I., Bull. Fac., Fish. Hokkaido Univ. 39, 304 ( 1988 ). [In Japanese]
Journal of Colloid and Inte(ace Science, Vol. 134, No. 1, January 1990
In the present study we used two carrier materials, that is, calcium alginate gel and activated carbon powder, to immobilize humic acid. Based on a simple model for the complexation of divalent metal ions on organic polymers with acidic groups, the adsorption mechanism and the ability of this newly developed adsorbent will be discussed.
INTRODUCTION
For the recovery of valuable elements or the removal of harmful heavy metals from aqueous environments, adsorption is a most promising technique. Humic substances (such as humic acid and fulvic acid) are widely present in natural waters and they have a highcomplexing ability with various heavy metal ions. Moreover, they are ecologically acceptable matter. Most previous studies dealt with the structure and the nature of humic substances or their complexes (e.g., 1, 2). As a few examples of their practical applications, Ho and Miller (3) and Heitkamp and Wagener (4) tried to use humic acid for the recovery of uranium. However, it is difficult to use humic acid as the sole adsorbent because of its high solubility in water. To immobilize it, Ho and Miller, and Heitkamp and Wagener employed hematite particles and anion exchange resins as the carriers, respectively. Since the carriers are almost inert materials in adsorption, the ratio of humic acid to the carrier is very important. In the studies mentioned above, the percentage of humic acid in the whole adsorbent was only 3.5% (3) or 25% (4) on a dryweight basis.
ORGANO-METALLIC COMPLEXATION MODEL
Schnitzer and his associates (5) accounted for the acidic properties o f h u m i c compounds in terms of a range of aromatic carboxyl groups and to a lesser extent phenolic hydroxyl groups. Wilson and Kinney (6) showed that more than 90% of the COOH sites on humic acid were dissociated in seawater in pH 8, and virtually all the phenolic hydroxyl sites remained protonated. They also showed that only minor proportions (8-38%) of the acidic sites on dissolved organic matter ( D O M ) are available for metal ion binding, because most of the sites are not in close proximity and do not have the polydentate characteristic necessary for chelation of metal ions. Taking into account these facts, we built a simple model for the complexation of divalent heavy metal ions and an organic polymer such as humic acid in an acidic media. The model is based on three assumptions:
i To whom all correspondence should be addressed. 59
0021-9797/90 $3.00 Journal of Colloid and Interface Science, Vol. 134, No. 1, January 1990
Copyright © 1990 by Academic Press, Inc. All rights of reproduction in any form reserved.
60
SEKI, SUZUKI, A N D K A S H I K I
(a) A divalent heavy metal ion binds with two sites (carboxyl groups) on an organic polymer.
parameters of N and K can be obtained from the slope and the intercept of the line, respectively.
(b) The polymer has a definite number of available sites for complexation. The unavailable sites are not taken into account. (c) In acidic media o f p H < 4.3, the available sites are either protonated to - C O O H or complexed with metal ions. From the above assumptions, the complexation of metal ions and an organic polymer in acidic media can be expressed by the cation exchange reaction, 2 ( - C O O H ) + M 2+ ~
(COO)2M
+
2H +,
MATERIALS
Chemicals Lead nitrate and sodium salt of alginic acid (AA) were obtained from Wako Pure Chemical Industries (Japan), and sodium salt of humic acid from Aldrich Chemical Co. They were used as received. Activated carbon powder (ACP) was purchased from Wako Pure Chemical Industries. It was screened by a 100mesh (0.149-mm) sieve and the undersize fraction was used.
[11 where M 2+ represents the divalent heavy metal ion. The complexation constant ofEq. [ 1], K, is defined as
K-
OMC2 - O~CM OMC~ - (1 - OM)2CM '
[21
where On and 0M represent the fractions of the sites on the polymers covered by H + and M 2+ at the equilibrium state, respectively, and CI~ and CM are the equilibrium concentrations of H + and M 2+, respectively. Defining the amount of divalent metal ions complexed with (or adsorbed on ) the polymer as X mole. g-l, and the number of available sites for complexation as N mole- g-l, Eq. [2] is modified to
(2X/N) C2 K = {1 - ( 2 X / N ) } z c M
Calcium alginate gel containing HA and ACP was used as the adsorbent. An aqueous solution containing HA (0-15%), sodium alginate ( 1% ), and the HA's weight in ACP was well mixed. It was trickled down into a 0.1 M calcium chloride solution in a stirred vessel and was converted to the gel particles. After 30 min the formed adsorbent (HA-gel) was removed from the calcium solution and it was washed repeatedly with distilled water. The detailed preparation method of HA-gel is given elsewhere (9). The shapes of the particles were nearly spherical and they had a diameter of 2-4 mm, as shown in a photograph of Fig. 1.
0
[3]
0
or
2 ( X / C H ) ( C M / X ) 1/2 = N(CM/X)I/2/CH
Adsorbent
-
( 2 N / K ) 1/2.
[4]
Equation [4] represents the adsorption isotherm of divalent metal ions on the organic polymer. The plot of 2 ( X/Cr~ ) ( C M / X ) 1/2 vs ( C M / X ) 1/2/CH gives a straight line, and the Journal of Colloid and Interface Science, Vol. 134,No. 1, January 1990
• 0
•
0
0 Q Q
• Q
FIG. 1. Photograph of HA-gel's particles.
ADSORPTION ON IMMOBILIZED HUMIC ACID
61
ACP was added for suppressing the discharge of H A from the gel's interior to the water phase. The excess addition of ACP inhibited the formation of HA-gel. Separate experiments by the authors (9) showed that HA-gel without ACP released 10-20% of HA, while the gel containing the same weight of ACP as H A discharged only 1% of HA.
the thermal equilibrium at 30°C, a certain a m o u n t of HA-gel (ca. 1 g) was added to the solution. After reaching the adsorption equilibrium, the concentrations of lead and p H in the bulk phase were measured.
EXPERIMENTAL METHODS
The structure and the properties of humic acid, in general, greatly depend on the H A ' s origin. To examine the validity of the complexation model represented by Eq. [4] and to know the adsorption characteristics of the H A present, complexation experiments were carried out for the P b - H A system. Figure 2 shows a typical result of the kinetic experiments. The reaction proceeded very rapidly and a few seconds or so were enough to attain the equilibrium. From the results, we determined the contact time for the equilibrium experiments (shown in Fig. 3 ) as 30 min. The ordinate of Fig. 3, XH, represents the equilibrium a m o u n t of Pb complexed with 1 g of HA. The figure shows that the p H increase of the adsorption system resulted in more for-
Complexation between Lead and Humic or Alginic Acid Kinetic experiments for the P b - H A system were conducted to determine the time for attaining equilibrium. The reaction was carried out in a 10-ram spectrofluorometer cell (JASCO FP-550A). The extent of complexation was determined from the measurement of the intensity of fluorescence (7). The maxi m u m emission intensity for the H A present occurred at 450 n m upon excitation at 365 nm. Equilibrium experiments were carried out in the following manner. A 0.285-dm -3 solution containing a certain a m o u n t of lead nitrate was prepared. The p H was adjusted to a desired value by nitric acid. A 0.015-dm -3 polymer solution containing 0.015 g of H A or 0.01 g of AA was prepared. After reaching the thermal equilibrium at 30°C, both solutions were mixed and the complexation reaction was initiated. The solution was stirred for a necessary time to attain the complexation equilibrium. Then, it was filtered through a m e m brane filter (pore size = 0.05 # m ) , and the p H and lead concentration of the filtrate were measured by a p H meter (Orion Research 501 ) and by atomic absorption spectrophotometer (Hitachi A- 1800), respectively. The a m o u n t of lead complexed with the polymer was determined from the difference between the lead concentrations in the initial and the equilibrium states.
Adsorption of Lead on HA-Gel A 1-dm -3 solution containing a necessary a m o u n t of lead nitrate was prepared and its p H was adjusted by nitric acid. After reaching
RESULTS AND DISCUSSION
Organo-Metallic Complexation
100
7 v
O
50~-
g
'-0--0--0
0--
I
I"
50
100
Contact
150
Time ( s e c )
FIG. 2. Time dependence of lead-humic acid complexation at 30°C. Initial concentrationsof lead ion and humic acid are 5.0 × 10-5 mole.dm 3 and 0.02 g.dm 3 (A), and 1.0 × 10-3 mole.dm -3 and 0.16 g-dm -3 (©), respectively. Journal of Colloid and Interface Science, VoL 134, No. 1, January 1990
62
SEKI, SUZUKI, AND KASHIKI 1.5
2.0
7, ~
re
~j 8j
0.5
o/
08/0/0/
%
1.0
f
/
.0..0 qb '~
1.0
0/O
7/ I
0.0
3
5
4 pH
FIG. 3. Effect of equilibrium pH on lead-humic acid complexation at 30°C. Initial concentrations of lead ions and humic acid are 1.0 × 10 -4 mole.dm -3 and 0.05 g • dm-3, respectively. The solid line represents the theoretical curve calculated from Eq. [6].
marion o f P b - H A complexes. The comparison o f the data o f Fig. 3 with the complexation model expressed by Eq. [4] is shown in Fig. 4. The data agreed well with the model (solid straight line), and the number o f available complexing sites on 1 g o f HA, NH, and the complexation constant o f the P b - H A system, KH, were derived from the slope and intercept o f the line as NH = 1.84 × 10 -3 m o l e . g - I and KH = 1.53 × 10 -3 m o l e . d m - 3 , respectively. Alginic acid, used here as the immobilizing agent, also has a complexing ability with di-
I
I
3.5
4.0
0
3.
4.5
pH
FIG. 5. Effect of equilibrium pH on lead-alginic acid complexation at 30°C. Initial concentration of lead ions and alginic acid are 1.0 × 10 4 mole.dm-3 and 0.033 g. dm-3, respectively. The solid line represents the theoretical curve calculated from Eq. [7 ].
valent metal ions. Figure 5 shows that the experimental results o f the P b - A A system are similar to those in Fig. 3. Since the complexation o f metal ions with A A in a liquid state is very rapid (8), we adopted the same contact time as that in the previous P b - H A system. The comparison o f the data o f Fig. 5 with Eq. [ 4 ] is presented in Fig. 6. Similarly to HA, the complexation o f Pb with A A was also consistent with the model. The number o f available sites on AA, NA, and the complexation con-
15 10 A
I0
o/
Slope 3.44x10 -3 Intercept -0.72
"i
re
(~)J
jo
v 5 jO
(13
ca
tO
CD 0
0
i
0.5
(CM/XH)½/CHx l 0
1.5 -3
FIG. 4. Fitting of the data of Fig. 3 to Eq. [4]. Journal of Colloid and Interface Science, Vol. 134, No. 1, January 1990
,,i.o°'~
I 1.0
I 2.0
(CM/XA}~/CH x l 0
-3
FIG. 6. Fitting of the data of Fig. 5 to Eq. [4].
3.0
ADSORPTION
ON IMMOBILIZED HUMIC
stant, KA, were obtained as NA = 3.44 × 10 -3 mole- g-i and KA = 1.32 × 10-2 mole" dm-3, respectively. Comparing the complexing abilities of AA and HA with Pb ions, the former is obviously superior to the latter in both terms of capacity and strength. However, AA is not suitable as an adsorbent material because it is poorly soluble in water and a solution containing more than 10% of AA cannot be prepared. Activated carbon powder (ACP), used as another immobilizing agent in HA-gel, is also a well-known adsorbent for various kinds of molecules, but it cannot be expected to have a complexing ability with ions. By the adsorption experiments for the Pb-ACP system, it was confirmed that ACP adsorbed a negligible amount of lead, compared with HA or AA. Adsorption of Lead on HA-Gel Prior to the equilibrium experiments, kinetic experiments for the lead-HA-gel system were conducted. The HA-gels containing 5, 10, and 15% of HA on a wet-weight basis (45.5, 47.6, and 48.4% on a dry-weight basis) were used for the experiments. Typical examples of the results are presented in Fig. 7. X denotes the adsorption amount of lead on 1 g of HA-gel. As 50 h were enough to attain the equilibrium for all the gels, we adopted the contact time of 50 h for the equilibrium experiments. From Figs. 2 and 7 it is seen that the present method of immobilization apparently had a serious effect on kinetic behavior of organo-metallic complexation. The remarkable decrease in the adsorption (or complexation) rate is caused by the increase in diffusion resistance of lead ions due to the gel network. Therefore, the preparation of a small or thinner HA-gel will be required for actual operation. The influence of pH on the equilibrium amounts of lead adsorbed, X, on several HAgels is shown in Fig. 8. Triangle symbols refer to a particular HA-gel which contained no humic acid. The adsorbed amount increased with the increase of the HA content. The same
63
ACID
2.0
% -;
mf
m f
/ 1.0
m
'~o
f
0
~ 0 - -
~ •/...-.-.-.----- •
• l
0.0
0
s0 C o n t a c t Time ( h r )
100
FIG. 7. Time dependence of lead adsorption on HAgels containing 5% (O), 10%(O), and 15%(ll) ofhumic acid at 30°C. All the HA-gelscontain 1%alginieacid and humic acid's weight in activated carbon powder. Initial concentration of lead ion is 1.0 × 10 4 mole-dm-L
data are fitted to Eq. [ 4 ] in Fig. 9. The plotted symbols correspond to those in Fig. 8. The adsorption mechanism of lead on HA-gel was also consistent with the foregoing organo-metallic complexation model. The slopes of the lines express the numbers of available sites on 1 g of HA-gel. Estimation of the Amount of Lead Adsorbed on HA-Gel From the fact that the adsorption of lead on HA-gel (i.e., immobilized HA and AA) agreed with the complexation model, the adsorbed amount of lead on 1 g of HA-gel, X, can be expressed as X = XH.PH + XA.PA,
[5]
where XH and PH represent the amount of lead adsorbed by the HA component and the weight fraction of HA in the gel, respectively. Similarly, XA and PA denote the quantities with respect to AA. Applying the adsorption isotherm of Eq. [ 4 ] to the components in the gel, XH and XA can be evaluated as XH = (NH/4KH)[(2KH + C) - {C(4Kn + C)} '/21 [61 Journal of Colloid and Interface Science, Vol. 134, No. 1, January 1990
64
SEKI, SUZUKI, A N D K A S H I K I 0.2
2.0
-= ) A i
j
~0 •
0.1
/II
i
ss/II1.0
/0
1
v
?o
,/"
/
/
o,-" a -,-"-'A--A 0 I
a0
I
3
4
I 2.0
1.0
5
Xcalc
(mol.g-I) xlO4
pH
FIG• 8. The p H dependence of lead adsorption on HAgels containing 0% (A), 5% (O), 10% (©), and 15% (11) of h u m i c acid at 30°C. Initial concentration of lead ions is 2.0 × 10 4 mole. dm-3.
strates the agreement of the calculated adsorption amounts on HA-gels with the experimental.
XA = (NA/4KA)[(2KA + C)
- {C(4KA + C)}1/21,
[7]
where C =-- C ~ / C M . T h e solid lines in Figs• 3 and 5 represent the theoretical curves calculated from Eqs. [6] and [7], respectively• PH and PA are the values determined from the composition of HA-gel• Since NH, NA, Kn, and KA were already obtained, we can calculate X by using Eqs. [ 5 ] - [ 7 ]. Figure 10 demon-
8.0
Slope
2.84xi0-4
/
4.0 "
1.94x10
°//f,0I
/0//"
I I/~o" 0
FIG. 10. Comparison between the experimental amounts of lead adsorbed on several kinds of HA-gels and calculated from Eqs. [ 5 ] - [ 7 ]. The symbols are the same as those in Fig. 8.
-4
1.23xl 0 -4
SI°pe 3"47x10-5
/ A -
2.0
4.0
(cMIXl½/Cnx l o -4 FIG. 9. Fitting of the data of Fig. 8 to Eq. [4].
Journal of Colloid and Interface Science, Vol. 134,No. 1, January 1990
CONCLUSION
Humic acid, which has an excellent complexing ability with heavy metal ions, was used as an adsorbent material for the recovery of lead. It was immobilized by a combination of alginate gel and activated carbon powder• By the present immobilizing method, we obtained the adsorbent (HA-gel) comprising as much as about 50% of HA on a dry-weight basis. A general complexation model for divalent heavy metal ions and organic matter was proposed. The model was confirmed by two kinds of experiments in which humic and alginic acids in liquid state were used as adsorbents. The complexation constants and the number of available sites were given as KH = 1.53 × 10 -3 mole. dm -3 and NH = 1.84 × 10 -3 mole- g-1 for humic acid, and KA = 1.33 × 10 .2 and NA = 3.44 × 10 .3 for alginic acid, respectively. The model was applied to the adsorption of lead on HA-gels. The comparison of the model with the experimental data showed that the model also explained the adsorption on HAgets.
ADSORPTION ON IMMOBILIZED HUMIC ACID ACKNOWLEDGMENT The authors thank Y. Yasuda for taking the experilental data.
REFERENCES 1, Ramamoo~hy, S.,andKushner, D.J.,Nature(London) 256, 31 (1975). 2. Buflle, J.,Greter, F., and Haerdi, W., Anal Chem. 49, 2 (1977).
65
3. Ho, C. H., and Miller, N. H., J. Colloid Interface Sci. 106, 281 (1985). 4. Heitkamp, D., and Wagener, K., Ind. Eng. Chem. Process Des. Dev. 21, 781 (1982). 5. Schnitzer, M., and Khan, S. U., "Humic Substances in the Environment." Dekker, New York, 1972. 6. Wilson, D. E., and Kinney, P., Limnol. Oceanogr. 22, 281 (1977). 7. Saar, R. A., and Weber, J. H., Anal, Chem. 52, 2095 (1980). 8. Smiderod, O., and Hang, A., Acta Chem. Scand. 19, 329 ( 1965 ). 9. Seki, H., Suzuki, A., and Kashiki, I., Bull. Fac., Fish. Hokkaido Univ. 39, 304 ( 1988 ). [In Japanese]
Journal of Colloid and Inte(ace Science, Vol. 134, No. 1, January 1990