Environ. Sci. Technol. ISS3. 27, 2133-2140
Characterizing a New Class of Sorptive/Desorptive Ion Exchange Membranes for Decontamination of Heavy-Metal-Laden Sludges Sukalyan Sengupla and Arup
K. Sengupta'
Fritz Engineering Lab 13, Civil Engineering Department, Lehigh University, Bethlehem, Pennsylvania 18015
Selective removal of small amounts ( Ni Environ. Sci. Technol., Vol. 27, No. 10, 1993 2155
!I
Table 111. Cu2+/Ca2+Separation Factors for Composite IDA Membrane8
c
aqueous-phase Cu2+uptake Ca2+uptake Cu2+concn, by membrane, by membrane, acu/ca= Ccu (mg/L) 4cu (mequiv/g) PCs (mequiv/g) mJCcdqcaCcu
0'
A 0
00000 Pure Solution A A A A A 10s Sand (-200
A 0 A
A 0 A
A
u
3.2 5.5 10.4 23.0 44.0 90.0
0.682 1.091 1.462 1.949 2.436 2.923
0.98 0.619 0.464 0.310 0.155 0.077
69.09 101.81 96.25 86.84 113.48 134.00
I
c
50
1
0
pH = 3.0 Background CJ" Conc. = 200 mgn
100
c
A
L
00
E 4.0
200
300
:
i
400
500
Figure 7. Comparison of copper(I1) uptake rate of the composite membrane for pure solution and 10 % slurry under identicalconditions.
-2 E "3.0
35
0
E
;I 'b '
E
i
Composite IDA Membrane Competing Caa' Conc. ZOO mgil
m \
,35
:
30
-
-
Oxalate = 4000 mgll 5% Sand + CaCO, Sluny pH = 9.0
E
30
00000 Ni
oonooCu
*****Cu
E
Li
\
12.5 L
1.0
0.0
i1
0
A
Time (Minutes)
cn
E
&
(3 100
pH = 3.0. Aqueous-phase Ca2+ concentration = 200 mg/L.
I
O A
mesh) Slurry
E
at pH = 4.0 at pH = 3.0
a t pH = 5.0 W m m m m P b at pH = 3.0
I
;Ad
I
I
;dd
i
3bd
I
' ibd
1
56d ,
1 I
'600
Aqueous Phase Concentration (mg/I)
Figure 5. Nickel(II), copper(II), and iead(I1) sorption isotherms for composite IDA membrane in the presence of competing calcium ions.
>E
225
e 5
c1
-
ooow Cu. at Ni. at Pb. at Cu. at
v
c 200
Number
4k \
175-
pH pH pH pH
= 5.0 = 3.0 = 3.0 = 3.0
C
0
;150-
0
c
a 1250
2
r?
a
100
10
-
100
1
200
300
400
500
Time (Minutes) Figure 6. Plot of aqueous-phase metal concentration versus time during the batch kinetic study of the composlte IDA membrane.
> Pb; and fourth, as expected, the membrane has a much higher affinity toward heavy metals over competing Ca, and that is why Ca uptake by the composite membranes was rather low (see Table 111). Batch Kinetic Studies for Pure Solution and Slurry. Figure 6 shows the results of individual sorption rate studies with composite membrane for Pb, Cu, and Ni solutions free of suspended solids, and the change in aqueous-phase metals concentration is plotted against time. It may be noted that the uptake rate of the composite membrane is about the same for these dissolved heavy metals. Figure 7 shows Cu uptake rates for two sets of batch kinetic tests which were carried out under identical conditions except that one batch consisted of pure solution while the other was a 10% w/v slurry containing inert 2136
Envlron. Scl. Technol., Vol. 27, No. IO, lQ93
Cycles
Figure 8. Cumulative copper, calcium, and oxalate recovery with increasing number of cycles at alkaline pH.
Composite IDA Membrane
0
Of
fine-sand (-200 mesh) particles. Note that the copper uptake kinetics remained almost unaffected in the presence of a high concentration of suspended solids. Heavy Metals Uptake in the Presence of a Strong Organic Ligand and a Solid Buffer. Sludges/slurries may also contain a separate solid phase with high buffer capacity (e.g., calcite) and organic substances capable of forming strong complexes with heavy metals. Due to the high buffer capacity of the solid phase, the pH of the sludge may be above neutral and will require an excessive amount of acid dosing to lower the pH. Laboratory experiments were simulated to confirm whether the proposed cyclic process using composite membrane may selectivelyremove heavy metals from such a sludge at above-neutral pH. The solid phase of the sludge (5% w/v) was prepared by mixing fine sand (70%), Na2C204 (21%), calcite (7.8%), and CuO (0.6%). The sludge pH was maintained at 9.0, and CaCz04 and Cu(0H)z were the controlling solid phases under this condition. Oxalate was used as a model organic ligand because first, it forms fairly strong complexes with most of the heavy metals; second, calcium oxalate is sparingly soluble and likely to be present in the solid phase; and third, most natural organic matter contains carboxylate functional groups like oxalate. The cyclic two-step recovery process was carried out for 10 cycles using HzSO4 as the regenerant. Figure 8 shows the recovery of Cu, Ca, and CzO4 into the regenerant solution. Although Cu is present primarily as Cu-C204 complex in the sludge phase, Cu recovery was significant and increased steadily with every cycle. Ca recovery was
1.0
1
1000 j
,
1
3
Initial Cu. Conc. = 200 mg/l
Copper ooooo Calcium
00000
pH
Oxalata = 4000 mg/l 5% Sand + CsCO, Slurry pH s 9.0
E
v
=
3.0 0
h0.6 3
G
.o- 0.2
I
'
' ' ' Of Cycles
'
' ' ' 4 ' ' ' ' 6 ' ' ' ' 8 ' ' ' ' 10 ' ' ' ' 12
Number
Flgure 9. Dlssolved copper and calcium concentrations at alkaline pH during the recovery process. ~
~~~~
Table IV. Stability Constants for Metal(I1)-N-Benzyl Iminodiacetate Complex'
a
metal(I1)
log K (1:l)
Ni2+ Pb2+ cu2+ Ca2+
7.92 7.39 10.62 3.17
All values taken from ref 11.
much lower and tended to approach an asymptotic concentration in the regenerant with an increase in the number of cycles. Dissolved sludge-phase concentrations of Cu and Ca remained fairly constant with the number of cycles (Figure 91, suggesting that they are controlled by the solubility products of the solid phases.
Discussion Membrane Characterization and Heavy Metals Uptake. Comparison of acid titration curves (Figure 3) suggests that the protonation behavior of the parent chelating microbeads is unaffected in the composite membrane environment, although sharp end points were not observed in any titration curves. Identifiable inflection points of polymeric weak acid and weak base ion exchangers were also absent during titration in several previous studies (8,9).This observation (lack of inflection points) is an equilibrium phenomenon caused by the close proximity of the functional groups in a rigid polymeric background where weak acid and weak base moieties often lend themselves to special interactions through hydrogenbonding and/or Coulombic effects (8, 10). Monomeric benzyl iminodiacetate can be viewed as a model compound for the polymeric chelating microbeads, and its first stability constants with Cu(II), Ni(II), and Pb(I1) at 25 "C are provided in Table IV (11). Note that Ni(I1)and Pb(I1) have virtually the same stability constant values, while that of Cu(I1) is much greater. In general, the affinity sequence of these metal ions toward the composite membrane, as may be seen from Figure 5, follows the same trend (i-e., Cu2+ > Ni2+ = Pb2+ >> Ca2+, which suggests that Lewis acid-base interaction is the predominant mechanism for selective sorption of heavy metals. (Cu(I1)is the strongest Lewis acid, while Ca is the weakest.) Thus, the proposed decontamination process would be very selective even for a sludge containing high concentrations of competing alkali and alkaline earth metal cations (e.g., Na+, K+, Ca2+,and Mg2+)in its aqueous phase.
0
O
00000 Composite IDA Membrane
e
m o o 0 Parent Chelatin (50-100
LL
'$7- 1
00
0
0
10
Resin
mesh?
100
Time (Minutes)
1[
Figure 10. Copper uptake rates for the composlte membrane versus the parent chelating exchanger under otherwise identical conditions.
Sorption Kinetics and a Viable Model. Figure 7 provides evidence that the presence of a high concentration of fine suspended solids does not influence the metals uptake rate by the composite membrane, thus confirming its suitability for treating sludges or slurries. Figure 6 shows almost identical uptake rates for Cu(II), Ni(II), and Pb(I1) by the composite membrane, although they are not equally labile from a chemical reaction viewpoint. It is, therefore, likely that the chemical reaction kinetics would not be the rate-limiting step. Previous investigations in this regard with spherical chelating exchangers have demonstrated intraparticle diffusion to be the most probable rate-controlling step (12,13). Also, since the PTFE fibers do not sorb any metal ions, solute transport by surface diffusion is practically absent. However,a significantly different physical configuration of the composite membrane may introduce additional diffusional resistances, retarding the sorption kinetics. The electron microphotograph and the schematic (Figure 1) suggest the presence of the following two possible contributors toward increased diffusional resistances within the composite membrane. (1)Fairly stagnant pore liquid is present in the channels of the composite membrane between individual microbeads, and the solutes need to be transported through this pore liquid for sorption or desorption. (Note that the above-mentioned pore diffusion is quite different from the more widely used "pore diffusion" existing within the voids of a single particle.) (2) Since the microbeads are interlaced within PTFE fibers, peripheral surface area of the microbeads (Le., 4 d for a bead of radius r) is only partially accessible for solute transport. Figure 10 shows the results of a batch kinetic study (fractional metal uptake versus time) comparing the parent chelating exchanger with composite membrane under otherwise identical conditions. Fractional metal uptake, F ( t ) ,is dimensionless and defined as the ratio of the metal uptake q ( t ) after time t and the metal uptake at equilibrium, q o , i.e., F ( t ) = q ( t ) / q o . Although the parent chelating exchanger microbeads were much bigger (50100 mesh) than the microbeads within the composite membrane, the copper uptake rate, as speculated, was slower with the composite membrane. In order to overcome the complexity arising due to the heterogeneity of the composite membrane (chelating microbeads randomly distributed in nonadsorbing PTFE fibers), we proposed a model where the thin-sheet-like composite membrane may be viewed as a flat plate Envlron. Sci. Technol., Vol. 27, No. 10, 1993 2137
. Entrapped< sorbent microbeads
Solute Transpon Equahon
lnlbal CondiUon q = 0. t = 0.0 < x < w Boundary Condibons $=o.==o,
x-0
v
~
%,,*,-,
t > o
I = 1.
t t 0
Flguro 11. Schematic model showing sorption through a flat plate with constant diffusivity.
containing a pseudohomogeneous sorbent phase, as shown in Figure 11. Consideringan apparent metal ion diffusivity of D within the membrane phase, the metal uptake rate through the plane sheet (the thickness of the membrane) from a solution of limited volume is given by
200 -b
(4) where x = the axial space coordinate in the direction of membrane thickness and q = heavy metal loading of the composite membrane. Under the experimental conditions, it may he assumed that (i) the surface of the membrane is in equilibrium with the bulk of the liquid phase; (ii) the volume of the membrane is insignificant compared to the total liquid volume, and (iii) the solute (heavy metal) has high affinity toward the thin-sheet sorbent material. The fractional uptakeparameter,F(t),whichis aratioofthe totalamount of heavy metal, qt, in the composite membrane after time t and the corresponding quantity after infinite time (i.e., at equilibrium), qo. is given by the following relationship (14) for a membrane of thickness 2w:
Again, at any time t , q ( t ) is related to the aqueous-phase metal concentration C ( t ) by the following equation:
VICo- C(t)l = mq(t) (6) in which V, CO,and m are the volume of the solution, initial heavy metal concentration, and mass of the composite membrane, respectively. Figure 12 shows the results of three independent kinetic studies in which aqueous-phase copper concentrations are plotted against time. Two sets of studies had different initial Cu concentrations (200 mg/L versus 100 mg/L), while the third one was a 10% fine-sand slurry, all other conditions remaining identical. Solid lines in Figure 12 indicate model predictions computed from eqs 5 and 6 for aconstantcopper ion diffusivity into the membrane equal to 8.0 X cm2/s. Note that although the exact morphology, i.e., distribution of voids and particles within the PTFE fibers,is not known, the suggested pseudohomogeneous plane sheet model showed good agreement for 2138
Environ. Sci. TeChnOI., VoI. 27. No. io. 1993
0
-Imrnr,immn 0
100
-7 --
200
300
400
0
Time (Minutes) Figure 12. Comparison of kinetic model predictions (solM ilnes) wkh three independent experimental data sets.
the three sets of independent kinetic data produced with different initial concentrations and suspended solids content. Copper Recovery from the Buffered Sludge i n t h e Presence of a Strong Ligand. Figure 8 showed convincing evidence that even a t a slightly alkaline pH of 9.0 andin thepresenceofafairlyhighconcentrationofoxalate, Cu could he selectively separated from a highly buffered solid phase. However, in order to extend and apply this methodology to similar other systems, the following two seemingly anomalous observations need to he looked into and comprehended. (1)Under the experimental conditions of Figure 9, free Cu ions, Cu2+,are practically absent, and most of the dissolved Cu exists primarily as anionic or neutral Cu-C204 complexes. The computed percentage distribution of important Cu(I1) species based on stability constants data in the open literature (11) is as follows: = 90.0, [ C ~ ( C z 0 a )= ~ l9.26, [Cu2+1= 1.44 [C~(CzO4)2~-1 X lo4, [Cu(OH)+l = 0.014, [Cu(OH)z01 = 0.69, and [Cu(OH),-l = 0.014%. Significant Cu uptake by the membrane, as observed even under such unfavorable conditions, demands scientific explanation. (2) Oxalate exists as the divalent anion CzOaZ a t pH = 9.0 and, according to the Donnan exclusion principle, should be rejected by the cationic chelating exchanger with the iminodiacetate functional group. Instead, oxalate is carried over to the regenerant solution fairly steadily.
2.5
I
I
/-
>LO
E
Oxalate = 4000 ppm pH = 9.0 Composite IDA Membrane
-
-2.0
s 2
Table V. Stability Constants for Cua+-LigandComplex.
E E
m C o p er OxaPate
v
2.5
:1.5
Y
0
ligand
metakligand
log K
oxalate oxalate N-B-IDA N-B-IDA
1:l 1:2 1:l 1:2
6.23 10.27 10.62 15.64
Y
1: -1.0
a
Q
8 0.5 0.0 , 0.b
a
3
0.5
,
I
C
2.k
I
I
Z
I
5.b
I
I
I
I
(
I
7.15
I
I
1
lO',O
1
1
'
1
1
1
12.5
1
1
1
1
1
0.0
15.0
Aqueous Phase Copper Concentration (rng/l)
Flgure 13. Equilibrium uptakes of copper(I1) and oxalate by the
composite membrane at pH = 9.0. 0xSl.te
C
C
?
0
I
I
I
-0
Cu2'
a
All values taken from ref 11.
ligand interaction) is believed to be the primary pathway for sorption of oxalate onto the membrane and subsequent carry-over into the regenerant as exhibited in Figure 8. Although free Cu ions (Cu2+) were practically absent under the experimental conditions of Figures 8 and 13, Cu uptake was quite significant. Besides sorption of [Cu(C204)IO as explained before, it is very likely that ligand substitution was another major mechanism by which Cu(11) was sorbed onto the chelating microbeads of the composite membranes. Table V shows 1:l and 1:2 metalligand stability constants of Cu(I1)with oxalate and benzyl iminodiacetate (IDA),and the much higher ligand strength of IDA is quite apparent. Also, Cu(I1) complexes are known to be labile (fast kinetics). Thus, the ligand substitution reaction, where oxalate in the aqueous phase is replaced by IDA in the exchanger phase, is favorable and results in an increased Cu(I1) uptake as shown below: [Cu(C204)J2-+ RN(CH,COO-Na+), e
Flgure 14. Sorption of neutral copper-oxalate complex onto chelating
mlcrobeads through formation of the ternary oxalate-copper-IDA complex.
It was speculated that the suspended particles of the insoluble calcium oxalate and copper oxide were probably trapped in the large pores of the composite membrane and subsequently carried over to the regenerant phase, thus exhibiting significant copper and oxalate recovery. In order to eliminate such a possibility, a sorption isotherm experiment was carried out in the absence of suspended solids at pH = 9.0 and total aqueous-phase oxalate concentration of 4000 mg/L but at varying dissolvedcopper concentrations. Figure 13 shows Cu and C2Oa uptakes under these experimental conditions free of suspended solids. Note that the ( 2 2 0 4 uptake by the composite membrane is significant and remains very much the same, while Cu uptake increases with an increased Cu concentration. These observations strongly suggest that Cu and C2O4 are removed from the solution (or sludge) phase primarily through sorption processes. The following are identified as plausible binding mechanisms for C204 and Cu(I1) onto the composite IDA membrane. As already indicated, the neutral copper-oxalate complex [Cu(C204)10was significantly present in the aqueous phase under experimental conditions, and only two of the four primary coordination numbers of Cu(I1) are satisfied in this complex. Since these complexes are electrically neutral, they can permeate readily to the sorption sites containing nitrogen donor atoms, which can favorably satisfy the remaining coordination requirements of Cu(11). Figure 14 shows how electrically neutral 1:l copperoxalate complexes can be bound to the neighboring nitrogen donor atoms of the iminodiacetate moieties through formation of ternary complexes. This mode of sorption (Le.,formation of ternary complex through metal-
RN(CH2COO-)2Cu2++ 2Na+ + 2C2042- (7) From an application viewpoint, the foregoing experimental observations are particularly important, for they strongly suggest that heavy-metal-laden sludges with high buffer capacity can be decontaminated even at alkaline pH in the presence of fairly strong ligands, and pH reduction through addition of large amounts of acids is not necessary.
Conclusions Conventional fixed-bed and membrane processes are unable to treat heavy-metal-laden sludges with high suspended solids contents. An extensive study was undertaken to explore a new type of composite chelating membranes which are not susceptible to fouling by particulate matter. Primary conclusions of the study can be summarized as follows: (1) the acid-base characteristics and the toxic-metal-removal properties of the composite membranes are almost identical to those of the parent chelating exchangers; (2) a kinetic model, where the composite membrane may be viewed as a pseudohomogeneous flat plate with intraparticle diffusion being the rate-limiting step, may well predict the heavy metals uptake rate under varying conditions; and (3) heavy metals removal, accordingto this process, is not necessarilylimited to acidic pH conditions. Cu(I1) was efficiently removed from a synthesized sludge with high buffer capacity at pH of 9.0 and in the presence of organic ligand (oxalate). Scientific analyses of the data suggest that the significant Cu uptake by the membrane, under these seemingly unfavorable conditions, was due to (i) the formation of ternary Cu(I1) complexes in the membrane phase and (ii) ligand substitution reactions. Acknowledgments This study received partial financial support from the United States Environmental Protection Agency through Environ. Scl. Technol., Vol. 27, No. 10. 1993 2139
Grant R-817438-01-0; Dr. Louis Swaby is the program director. Literature Cited (1) Waitz, W. H. Ion Exchange in Heavy Metals Removal and Recovery;Amber-Hi-Lites 162; Rohm and Haas Co.: Philadelphia, PA, 1979. (2) Bolto, B. A,; Pawlowski, L. Wastewater Treatment by Ion Exchange; E & F. N. Spon: London, 1987. (3) Benjamin, M. M.; Bailey, R. P.; Bennett, T. Trace Metal Removal with Oxide-Coated Sand. Polymeric Complexing
Agents in Environmental Separations;Abstracts ofPapers, 203rd National Meeting of the American Chemical Society, San Francisco, CA, Spring 1992; American Chemical Society: Washington, DC, 1992. (4) Theis, T. L.; Iyer, R.; Ellis, S. K. J. Am. Water WorksAssoc. 1992, 84, 101. (5) Shao, X.; Hu, S.; Govind, R. Ind. Eng. Chem.Res. 1991,30, 1231.
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(6) Danesi, P. R. Sep. Sci. Technol. 1985, 19, 857. (7) Sengupta, A. K.; Shi, B. J. Am. Water Works Assoc. 1992, 84(1), 96. (8) Garcia, C . A.; King, C. J. Ind.Eng. Chem.Res. 1989,28,204. (9) Clifford, D.; Weber, W. J. Nitrate Removal from Water Supplies by Ion Exchange; EPA-60012-78-052;U.S.Environmental Protection Agency, U.S.Government Printing Office: Washington, DC, 1978. (10) Roy, T. K. M.S. Th-sis, Lehigh University, Bethlehem, PA, 1989. (11) Smith, R. M.; Martell, A. E. Critical Stability Constants 2; Plenum Press: New York, 1974; p 135. (12) Hoell, W. React. Polym. 1984, 2, 10, 103. (13) Helfferich, F. J. Phys. Chem. 1965, 69, 1178. (14) Crank, J. The Mathematics of Diffusion;Oxford University Press: London, 1975.
Received for review January 11, 1993. Revised manuscript received May 28, 1993. Accepted June 3, 1993.
