Ind. Eng. Chem. Res. 1994,33, 382-386
382
GENERAL RESEARCH Selective and Reversible Ligands Sorption through a Novel Regeneration Scheme Arup K. Sengupta' and Yuewei Zhu Environmental Engineering Program, Fritz Engineering Laboratory 13, Lehigh University, Bethlehem, Pennsylvania 18015
A new class of tailored chelating polymers show high sorption affinity toward target anionic ligands of environment importance, namely, oxalate, phthalate, selenite, and arsenate. This study presents a new two-bed regeneration scheme achieving very high desorption efficiency. Ligand recovery from dilute solutions may now be quite viable.
Introduction There remains a general need to identify and synthesize sorbents for selective sorption of low concentrations of target solutes in the areas of product purification, environmental pollution control, and chemicals recovery (Worthy, 1991). Ideally, suchaprocess should be reversible so that the target solutes can be desorbed and recovered at high concentrations, thus leading to energy-efficient separation processes. From an equilibrium viewpoint, however, high sorption affinities tend to diminish desorption or regeneration efficiencies, thus offsetting the advantages of selective sorption. In this regard, we characterized a new class of tailored chelating polymers for selective sorption of anionic ligands of industrial and environmental importance, such as oxalate, phthalate, selenite, arsenate, cyanide, ethylenediaminetetraacetate (EDTA), and others. The new polymeric sorbents, referred to as polymeric ligand exchangers (PLE), are essentially copper(I1)-loaded chelating polymers with nitrogen donor atoms, and exhibit high affinities toward anion ligands through electrostatic as well as Lewis acid-base interactions. However, conventional regeneration processes using sodium chloride/ hydroxidelcarbonate are inefficient in desorbing the sorbed anions in concentrated forms for recovery or disposal. In a generic way, the polymeric ligand-exchange process can be viewed as the formation of a ternary complex in the polymer phase: where RL is the electrically neutral polymer-phase ligand,
Mn+is the immobilized transition metal cation, and Lnis the target anionic ligand. The overbars denote the polymer phase and of them one overbar indicates exchangeable anions participating in the ligand-exchange reactions, two overbars indicate the metal ions (Lewis acids) immobilizedonto the polymeric substrate, and three overbars represent the covalently attached functional groups (Lewisbases) with no fixed charges. Coordination requirements of the metal ion (shown by arrows) are satisfied by both RL and L" (Lewis bases), and at the same time electrostatic interactions (or ion-pair formations) are also operative between Mn+and L" (shown by the dashed line). All other conditions remaining identical, selective separation of a specific ligand, Ln-, would be OSSS-5S85/94/2633-0382$04.50/0
influenced by the ligand strength or Lewis base properties of the anion. However,once sorbed into the polymer phase, it is rather difficult to desorb them through chemical regeneration. The poor reversibility during regeneration is a major limitation of otherwise attractive polymer ligand exchange process (Zhu and Sengupta, 1992). Matejka and Weber (1990) used copper(I1)- and chromium(II1)-loaded chelating polymers containing oligoethyleneamine functional groups for removal of anionic ligands, namely, citrate, EDTA and nitrilotriacetate (NTA). Competing effects of chloride and sulfate anions on ligand uptake were quite significant, and magnesium sulfate was used as the regenerant for desorption of ligand anions. Chanda et al. (1988)used Fe(II1)-loadedchelating polymers for selective removals of arsenate and arsenite anions. Since Fe3+ is held very weakly by chelating polymers with nitrogen donor atoms, the arsenate removal capacity in the presence of competing sulfate and chloride was rather low. The present study uses spherical beads of a chelating polymer with two nitrogen donor atoms as the polymeric substrate (DOW 2N or XFS 43084) obtained from Dow Chemical Co., Midland, MI. Upon loading with copper(II),the chelating polymer is essentially converted into an anion exchanger capable of interacting with solutes (anions) through both electrostatic (i.e., ion-pair formation) and Lewis acid-base (i.e., metal-ligand) interactions. For comparison, a strong-base anion exchanger (SBA) with quaternary ammonium functional group (IRA-900;Rohm and Haas Co., Philadelphia, PA) is also included in the study. Table 1provides the chemical compositions of the repeating functional groups of DOW 2N and IRA-900along with other pertinent information. The polymeric ligand exchanger will be referred to as PLE or DOW 2N-Cu while the strong-base anion exchanger will be abbreviated as SBA or IRA-900 in this paper. The key objectives of this paper are to present experimental evidence in support of high sorption affinities of anionic ligands toward the PLE and, more importantly, very efficient desorption of the target solutes from the PLE through a novel regeneration scheme.
Experiments: Materials and Procedure The chelating exchanger DOW 2N (or XFS 43084) was obtained in spherical bead forms with sizes ranging from 0.3 to 0.8 mm. The resin beads were conditioned following 0 1994 American Chemical Society
Ind. Eng. Cham. Res., Vol. 33, No. 2, 1994 383 Table 1. Background Information on Ion Exchangers composn of functional groupa
e ..
(ii>
characteristic high metal-ion affinity
matrix, porosity polystyrene, macroporous
Dow Chemical, DOW 2N or XFS 430M
manufacturer and trade name
strong-base anion exchanger
polystyrene, macroporous
Rohm and Haae Co., IRA-900
T
CH2-!-CH2CHOHCH3
6
CHJ-NLCH,
I
cH3
a
'R" inside the circle denotes the repeating unit of the polymer matrix.
the standard procedure of cyclic exhaustion with 1 N hydrochloric acid and 1 N sodium hydroxide. Finally, DOW 2N was converted into copper-loaded forms by passing 500 mg/L Cu(I1) solution a t pH = 4.5 through a fEed-bed DOW 2N column until saturation. The solubility product of Cu(I1) was not exceeded at this pH. Analytical grade CuSO&H20 and CuCLH20 (Fisher Chemical) were used; the resins loaded with cupric sulfate were used for binary equilibrium experiments involving sulfate as competing anions, and the resins loaded with cupric chloride were used for column runs. The copper loading capacity of DOW 2N was 1.2 mmol/g of resin. Sorption Equilibria, Column R u n s a n d in-Situ Regeneration. The equilibrium sorption data with DOW 2N-Cu in this study were generated by minicolumn experiments where aqueous solutions of fixed composition and pH (predetermined) were passed through short glass columns containing approximately 0.2-0.5 g of resin in question at room temperature (24 f 2 "C). The volume of solution fed was much in excess of the stoichiometric requirement to ensure attainment of equilibrium, which, however, was confirmed separately by comparing exiting concentrations of solutes with those of the influent. Following a short rinse (5 min) with deionized water, the minicolumn resin was regenerated with 200 mL of 4% ammonia. Solute concentrations (oxalate, sulfate) in the spent regenerant were analyzed, and resin uptakes at equilibrium were determined by mass-balancecalculations. The fixed-bed column runs were carried out using Plexiglas columns (10-mm diameter), constant-flow stainless steel pumps, and an ISCO fraction collector. The ratio of column diameter to exchanger bead diameter was approximately 20.0. All column runs were carried out essentially under the same hydrodynamic conditions; the superficial liquid velocity (SLV) and the empty bed contact time (EBCT) were identical and equal to 0.94 m/h and 7.7 min, respectively. Samples were collected from the exit of the column every 30 min; the run length with DOW 2N-Cu for oxalate removal lasted over 4 weeks. In order to avoid any possible bleeding of copper from DOW 2N-Cu into the exit of the column, a small amount of a virgin chelating exchanger (non copper-loaded) in sodium form was kept at the bottom of the column occupying less than 10% of the total bed height. Further details about the experimental procedure are provided by Zhu (1992). Regeneration experiments with sodium chloride, sodium carbonate, and ammonia were also carried out in the Plexiglas columns. However, for ammonia regeneration, a second column containing a weak-acid cation exchanger (IRC 50; Rohm and Haas) was placed in series with the PLE as discussed later in the paper. Results Equilibrium Isotherms and Column Run. Figure 1 provides binary oxalate-sulfate isotherms at 23 f 2 "C for
1.00
0.80
Sep. Factor = 70.0
Av.
Aq. Phase Anion Conc.= 3.5 meq/L pH = 6.5
0.40
0.20
j
IRA Av.
-
900 900
Sep. Factor = 1.0 Sew
0.00 0.00
0.10
0.05
0.15
0. 15
0.20
Figure 1. Binary oxalate-eulfate isotherms for the polymeric ligand exchanger (PLE)and the strong-baee anion exchanger (SBA)under identical conditions.
the strong-base anion exchanger (IRA-900) and the polymeric ligand exchanger (DOW 2N-Cu) at pH = 6.5. Both the aqueous-phase and exchanger-phase oxalate concentrations of the binary system are expressed in dimensionless equivalent fractions which are defined as follows:
Qox
--
- 4,
=
Qox
(3)
+ Qs
where Ci and qj are the equivalent concentrations of solute "i" in the water (mequi v/L) and polymer phase (mequiv/ g), respectively. Both oxalate and sulfate exist as divalent anions at pH 6.5, and for binary isotherms, xox x , = 1.0 and yox + ys = 1.0, where subscripts "OX" and "s" represent oxalate and sulfate, respectively. Note that the oxalate isotherm for DOW 2N-Cu is well above that of IRA-900, confirming much higher oxalate affinity toward DOW 2N-Cu. The binary separation factor is a quantitative measure of relative selectivity between two solutes toward a specific sorbent and is defined in this case as follows:
+
aoxJs
=
-QOXCS
YOPS
QSCOX
YSXOX
(4)
The average oxalate-sulfate separation factor, aOxI),as determined from the binary isotherm data for DOd2NCu is approximately 70.0 while that for IRA-900 is 1.0. Figure 2 shows effluent histories of oxalate during separate column runs using two different sorbents, namely,
384 Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 Hours 0
100
200
300
400
500
600
Influent: pH 2-65 SO4 = 100 mg/L CI- = 200 mg/L Oxalate = 10 mg/L SLV = 0.94 M/Hr EBCT = 7.7 MIN
AAAAAIRA
-DW
900
2N-c~ 0
~ , , , . ~ ~ . . . i . . ~ ~ . . . . . i n ~ n ~
1000
2000
3000
4000
5000
Bed Volume Figure 2. Oxalate effluent histories during separate column runs with PLE and SBA under otherwise identical conditions: SLV = superficial liquid-phase velocity; EBCT = empty bed contact time. Regeneration Scheme
obtained with sodium carbonate and brine regenerations. Equally important, no copper was lost from the system. In order to validate the general applicability of the new regeneration scheme, we loaded both DOW 2N-Cu and IRA-900 separately with 10 mg/L concentration of phthalate, a more hydrophobic (aromatic) anionic ligand. Table 2 provides some key properties of oxalate and phthalate for ready reference. Both DOW 2N-Cu and IRA-900 were subsequently regenerated under identical hydrodynamic conditions: DOW 2N-Cu with ammonia in a two-bed system as described before and IRA-900with alkaline brine. (Alkaline brine is a widely used regenerant for desorption of hydrophobic anions from strong-base anion exchangers (Kunin, 1982)). Figure 5 clearly dem~ ~ ~ ~ ~ l ~ ~ ~ ~ ~ . . ~ . onstrates that ammonia regeneration is highly reversible and requires lesser number of bed volumes for complete desorption of phthalate. The reversibility of the two-bed regeneration system was also observed for a PLE loaded with selenite (SeO$), a toxic inorganic ligand found in many groundwater supplies (see Figure 6). More information about selenite sorption onto and desorption from the PLE is available elsewhere (Ramana, 1990).
Discussion Sorbcnl:
Suklrale ZN-Cu
Figure 3. Two-bed, two-step regeneration scheme (ammonia followed by acid) for efficient desorption of oxalate.
the polymeric ligand exchanger (DOW 2N-Cu) and the strong-base anion exchanger (IRA-900). Influent composition, pH, and the operating hydrodynamic conditions, namely, empty bed contact time (EBCT) and superficial liquid velocity (SLV), were identical for the two column runs and are provided in Figure 2. Note that oxalate breakthrough for DOW 2N-Cu occurred after 3000 bed volumes although competing sulfate and chloride were present in the influent at much greater concentrations. In contrast, the strong-base anion exchanger (IRA-900)was completely exhausted in less than 250 bed volumes. Regeneration of the Exhausted Sorbent. Upon completion of the fixed-bed column run with the PLE in Figure 2, three equal portions of the exhausted sorbent (1.6 mL each) were regenerated separately with the following regenerants: (i) 2 5% NaCl + 1'36 NaOH; (ii) 2 '36 Na2C03; (iii) 2 5% "3. While the regenerations with alkaline sodium chloride and sodium carbonate were carried out in single columns, ammonia regeneration used a weak-acid cation-exchange column in series as shown in Figure 3. Figure 4 shows the desorption profiles of oxalate concentrations during the three regenerations. Both sodium carbonate and sodium chloride were capable of desorbing/eluting oxalate from the PLE, but a long tail is observed in both cases due to prolonged desorption from the exchanger. By contrast, ammonia regeneration was far more efficient and required lesser bed volumes of regenerant for complete desorption. Oxalate concentration in the eluent was also much greater than those
Sorption Affinity. Figure 7 provides a schematic presentation of the underlying mechanisms which govern the binding of oxalate (a bidentate divalent anion) onto the PLE and the SBA. At around neutral to slightly alkaline pH, two of the four primary coordination numbers of each Cu(I1)are satisfied by the two nitrogen donor atoms on the chelating polymer, and as a result, copper ions are held firmly onto these sites. The remaining two coordination numbers and the two positive charges of every immobilizedCu(I1)ion are satisfied by the divalent oxalate ion with two oxygen donor atoms. Thus, both ion-pair formation (i.e., electrostatic interaction) and relatively strong Lewis acid-base (LAB) interaction are operative for binding of oxalate onto the PLE. For the SBA, on the contrary, only ion-pair formation is involved between the positively charged quaternary ammonium functional groups (RdN+) and the oxalate anion. As a result, the oxalate uptake by the SBA is significantly reduced by the competing sulfate ions. Chemistry of PLE Regeneration. The underlying chemistry leading to high regeneration efficiency of the PLE with ammonia for an anionic ligand (say, oxalate) in a dual-bed system can be explained with the following stepwise reactions: i. Ammonia desorbs the metal-sorption center, Cu(II), of the PLE irreversibly thus eluting the oxalate,
ii. When the eluting solution subsequently passes through a weak-acid cation-exchange column in sodium form, positively charged copper(I1)-ammonia complexes get sorbed releasing sodium ions. 2RCOO-Na
+ [Cu(NH3),12++ Ox2-+ (RCOO-)2[Cu(NH3),12++ 2Na++ Ox2- (6)
Oxalate anions are, however, completely rejected by the
Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 385 zoo0
P Regenerant: 2% NH40H
l
a
n d \
p 1200
v
$
I
Regenerant: 2% NaCl 1% NaOH
"Y m x l ' . I .
m
Regenerant: 2% Na2C03
Figure 4. Oxalate desorption profiles from the PLE during regeneration with three different regenerants.
--
5000
cation exchanger due to Donnan's co-ion exclusion effect and appear at the exit rapidly at relatively high concentrations. iii. Following oxalate recovery, the weak-acid cationexchange column can be regenerated efficiently with dilute mineral acid and the copper(I1) from the eluting solution can be reloaded onto DOW 2N very efficiently because DOW 2N has high copper(I1) affinity even under highly acidic conditions (Sengupta and Zhu, 1991).
m
IRA 900 3% NaCl 1%NaOH
(RCOO-),[CU(NH,),]~~+ ( n + 2) HCl *
.n,
+
2RCOOH + Cu2+ nNH4++ ( n + 2) C1- (7)
* N
N
2c
'
N
N
DOW 2N-Cu 2% Ammonia
(8)
10
Icu-4,WI' 1
The above regeneration scheme, although unconventional, is very efficient and is ideally suited for recovering anionic ligands at high concentrations. Equally important, this regeneration scheme does not produce large amount of regenerant waste if disposal is necessary. Ammonium chloride is the major constituent in the waste stream which may also be treated in specialty electrodialysis membrane processes to recover ammonia and hydrochloric acid (Chang, 1979). By contrast, the regeneration of the exhausted PLE with brine and sodium carbonate are strictly anion-exchange processes where the anionic ligand, L2-, is replaced by C1or carbonate:
3 5.0
0.0
10.0
15.0
20.0
Bed Volume Figure 5. Comparison of phthalate desorption from IRA-900 and DOW 2N-CU.
51
4000.0
P
A
NO. OF BE0 VOLUMES
Figure 6. Selenite desorption profile from DOW 2N-Cu during two-bed regeneration.
Since oxalate and other anionic ligands are greatly preferred by the PLE over chloride and carbonate, the regeneration process is unfavorable from an equilibrium viewpoint and the ligand elution is very gradual or nonsharpening. That is why more bed volumes of these
regenerants are required and the concentration of ligands recovered are relatively low. Conclusions
Polymeric ligand exchange was conceptualized and formally introduced by Helfferich (1961, 1962) over 30
386 Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 Table 2. Important Properties of Oxalate and Phthalate. first stability Darent acid chem formula pK1 pK2 const with Cu(I1) COOH 1.27 4.2 108.8 oxalic I COOH
o-phthalic
@;:a);
2.9
5.4
103.46
Data from Lunge’s Handbook of Chemistry; Dean, J. A., Ed.; McGraw-Hill: New York, 1979.
+ IRA-900
regeneration scheme is also explained after taking into consideration appropriate metal-ligand and acid-base reactions. From an application viewpoint, the polymeric ligand exchangers are now particularly attractive because they can selectively remove many target ligands of industrial and environmental importance and can also be regenerated very efficiently.
Acknowledgment An anonymous reviewer’s insightful comments helped improve the manuscript. The study received partial financial support from the U.S.Environmental Protection Agency through Grant Nos. R-817438 and R-819228.
MI. Qhrin
OH
c-c
+O
O*
c-c
Literature Cited
+O
Ion Palrlng
Ion Palrlng + LAB
Ion Palrlng -.-. Lewis ACld-BaSe lnteractlon (LAB) __..
Figure 7. Illustration of underlying mechanisms of oxalate binding onto DOW 2N-Cu and IRA-900.
years ago. Subsequent investigations in this area have, however, been confined primarily to non-ionized ligands, namely, various amines and ammonia derivatives. This study identifies a new chelating polymer with nitrogen donor atoms which, when loaded with Cu(I1) ions, exhibit very high affinity toward a hydrophilic organic anion, oxalate. Earlier works have shown the effectiveness of these polymer ligands exchangers also for selective removals of toxic inorganic ligands, namely, arsenates and selenites in the presence of competing sulfate and chloride anions (Ramana, 1990). The most significant finding of this paper is the highly reversible two-bed ammonia regeneration of the polymeric ligand exchangers. The target ligands, namely, oxalate, phthalate, and selenite, were desorbed from the bed favorably at high concentrations without any loss of copper from the system. The underlying chemistry of this novel
Chanda, M.; O’Driscoll,K. F.;Rempel, G. L. Ligand exchangesorption of arsenate and arsenite anions by chelating resins in a ferric ion form. React. Polym. 1988,9, 277. Chang, Y. Conversion of ethylenediamine dihydrochloride into ethylenediamine by electrodialytic water-splitting. J. Appl. Electrochem. 1979, 9, 731. , Helfferich,F. Ligand exchange: anovelseparation technique. Nature 1961,189, 1001. Helfferich, F. Ligand Exchange. I. Equilibria. J. Am. Chem. SOC. 1962,843237.
Kunin, R. The role of organics in water treatment, Part B. Amberhi-lites No. 169; Rohm and Haas Co.: Philadelphia, 1982. Matejka, Z.; Weber, R. Ligand exchange sorption of carboxylic and aminocarboxylicanions by chelatingresins loaded with heavy metal cations. React. Polym. 1990, 13, 299. Ramana, A. A new class of sorbents for selective removal of arsenic(V) and selenium(1V)oxyanions. M.S. Thesis, Fritz Engineering Laboratory, Lehigh University, Bethlehem, PA, 1990. Sengupta, A. K.; Zhu, Y. Metals sorption onto chelating polymers: a unique role of ionic strength. AZChE J. 1992, 38 (l),153. Worthy, W. Separations technology, long overlooked, is a growing field. Chem. Eng. News 1991, Dec 2,27. Zhu, Y. Chelating polymers with nitrogen donor atoms. Ph.D. Dissertation, Fritz Engineering Laboratory, Lehigh University, Bethlehem, PA, 1992. Zhu, Y.; Sengupta, A. K. Sorption enhancement of some hydrophilic organic solutes through polymeric ligand exchange. Environ. Sci. Technol. 1992,26, 1990. Received for review May 28, 1993 Revised manuscript received October 5, 1993 Accepted October 18, 1993” Abstract published in Advance ACS Abstracts, December
15, 1993.
