Ind. Eng. Chem. Res. 1996,34, 2676-2604
2676
Trace Contaminant Sorption through Polymeric Ligand Exchange Dongye Zhao, Arup K. SenGupta,* and Yuewei Zhu Environmental Engineering Program, Fritz Engineering Laboratory, Lehigh University, Bethlehem, Pennsylvania 18015
Various industrially and environmentally important trace contaminants in water and wastewater bodies are anionic ligands, namely, phosphate, selenite, arsenate, sulfide, phthalate, oxalate, ethylenediaminetetraacetate, cyanide, and a host of organic derivatives. Traditional sorbents such as activated carbons, ion exchangers, iron oxyhydroxides, etc. are not effective for selective sorption of the foregoing contaminants. This study reports the results and underlying sorption mechanisms of a tailored polymeric ligand exchanger (PLE) which shows very high affinities toward phosphate, selenite, and oxalate. The polymeric ligand exchanger used in the study is essentially a copper(I1)-loaded specialty chelating polymer, and its preference toward anionic ligands is attributed to concurrent electrostatic and Lewis acid-base type interactions. PLE is also amenable t o efficient regeneration with brine, and more than 90% phosphate was recovered in less than 10 bed volumes.
Introduction It is well recognized that the sorption affinity toward target contaminants can be greatly enhanced by modifylng and tailoring the interfacial chemistry of polymeric ion exchangers. Helfferich (1961,1962) was the first to conceive use of Cu(I1)- or "1)-loaded weak-acid polymeric cation-exchange resins for ligand sorption through fairly strong Lewis acid-base interactions. In such a ligand-exchange process, the water molecules (weak ligands) present at the coordination spheres of immobilized Cu(I1) or Ni(I1) in the cation-exchange resins are replaced by relatively strong ligands, such as ammonia or ethylenediamine. The following provides a typical ligand-exchange reaction with ammonia where "M" represents a divalent metal ion like Ni(I1) or Cu(11)with strong Lewis acid properties and the overbar denotes the exchanger phase: (RCOO-),M2+(H20),
nNH,
+
+
(RCOO-)2M2+(NH3), nH20 (1)
Helfferich (1962)also provided a quantitative approach twoard determining the ligand-exchange capacity of metal-loaded cation exchangers. The investigations in ligand exchange, however, remained confined to only non-ionized (uncharged) ligands, namely, various amines and ammonia derivatives (Dobbs et al., 1975;Hernandez and Walton, 1972; Groves and White, 1984). In reality, most of the inorganic and organic ligands of industrial and environmental importance are anionic, such as cyanide, selenite, sulfide, acetate, phosphate, oxalate, phthalate, phenolate, and a host of other organic derivatives. Ligand-exchange processes using polymeric cation exchangers as metal hosts, as depicted below, are unable to sorb any anionic ligand, i.e.,
-
(RC00-)2M2f(H20),+
no reaction
[CH,COOThe metal-loaded weak-acid cation-exchange resins are electrically neutral and do not have any anion-exchange
capacity, and also the negatively charged fixed co-ions (carboxylates in this case) of the polymer will not allow uptake of any anions due to the Donnan co-ion exclusion effect. Thus, in spite of being strong ligands, the anions in eq 2 cannot displace water molecules (much weaker ligands) from the coordination spheres of the metal ions (Lewis acids). It is recognized that, in order to sorb anionic ligands selectively, the polymeric substrate (metal host) upon metal loading must possess fixed positive charges, Le., it should act as an anion exchanger. Obviously, such functional polymers should have high preference toward metal ions so that the metals do not bleed or bleed only negligibly during the ligand-exchange process. Characterization of the Polymeric Ligand Exchanger (PLE) and Key Objectives. Our primary goal is to selectively separate and concentrate trace or low concentrations @gIL to mg/L) of anionic ligands (namely, oxalate, phosphate, phthalate, ethylenediaminetetraacetate (EDTA), arsenates, nitrilotriacetate (NTA), selenites, cyanides) onto a suitable sorbent from the background of high concentrations of competing anions (namely, chloride and sulfate) and, then, to regenerate the sorbent (effkiently) so that it can be used for multiple number of cycles. Conceptually, transition-metal cations, say copper(II),if held firmly onto a solid phase at high concentrations, may act as anion-exchange sites with relatively high affinities toward aqueous-phase anions with strong ligand characteristics. Thus, in a generic way, the polymeric ligand-exchange process can be viewed as the formation of a ternary complex in the polymer phase as shown below: (3)
where RL is the electrically neutral polymer-phase ligand, Ma+ is the immobilized transition metal cation, and Ln- is 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) immobilized onto the polymeric substrate, and three overbars represent the covalently attached functional groups (Lewis bases) with no fixed charges. Coordination requirements of the metal ion
0008-5005/95/2634-2676$09.QOIQ 0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34,No. 8, 1995 2677 Table 1. Background Information on Ion Exchangers ~
composition of the functional groupa
Q
a
~_____ ~~
matrix, porosity
manufacturer and trade name
high metal-ion affinity
characteristic
polystyrene, macroporous
DOW Chemical , Midland, MI; DOW 2N or XFS 43084
strong-base anion exchanger
polystyrene, macroporous
Rohm and Haas Co., Philadelphia, PA, IRA-900
strong-base anion exchanger
polyacrylic, macroporous
Rohm and Haas Co., Philadelphia, PA, IRA-958
high metal-ion affinity
polystyrene, macroporous
Rohm and Haas Co., Philadelphia, PA, IRC 718
“ R inside the circle denotes the repeating unit of the polymer matrix.
Table 2. Properties of Target and Competing Anions ligand characteristic
parent acid
Se032- (selenite)
anion
bidentate ligand with two oxygen donor atoms
H2Se03
HP042- (phosphate)
bidentate ligand with two oxygen donor atoms
C ~ 0 4 (oxalate) ~-
bidentate ligand with two oxygen donor atoms
HzC204
sop
very poor ligand with ability to form ion vairs with metal cations weak monodentate ligand
HCl
c1a
pKa valuesa pK1 = 2.6
iK3 = 12.4 pK1 = 1.3 pK2 = 4.3 pK1= negative VKZ= 1.9 negative
PK=
Source: Lunge’s Handbook of Chemistry; Dean, J. A., Ed.; McGraw-Hill Inc.: New York, 1979.
(shown by arrows) are satisfied by both RL and Ln(Lewis bases), and at the same time electrostatic interactions (or ion-pair formations) are also operative between Mn+ and Ln- (shown by dashed lines). Chanda et al. (1988) extensively studied ligand sorption onto Fe(II1)-anchored chelating exchangers, especially XFS 4195 from DOW Chemical, USA. Iron is a nontoxic, innocuous metal, and hence, it is more desirable over other relatively toxic transition-metal(I1) cations such as copper and nickel. However, Fe3+is a hard cation (or hard Lewis acid) and has a poor affinity toward XFS 4195 with only nitrogen donor atoms. Also, they observed strong competing effects of sulfate and chloride ions on As(V) sorption onto Fe(II1)-loadedXFS 4195 chelating exchanger. 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 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. 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(111, 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. The polymeric ligand exchanger, thus prepared, will be referred to as PLE or DOW 2N-Cu in this paper for typographical convenience. Also, for comparison, two strong-base anion exchangers (SBA) with quaternary ammonium functional groups (namely, IRA-900
and IRA-958) and one chelating exchanger with iminodiacetate h c t i o n a l group (IRC 718), all from Rohm and Haas Co., Philadelphia, PA, were also included in the study. Table 1 provides the pertinent background information of these polymeric sorbents. Three environmentally significant anionic ligands, namely, phosphate, selenite, and oxalate, were included in this study as target trace contaminants while sulfate and chloride were used as the competing anions. Ligand characteristics and other related properties of these solutes are provided in Table 2. Phosphate at trace concentration is responsible for eutrophication (algal bloom), selenium is included in USEPA’s list of priority pollutants, and oxalate is a model compound for various hydrophilic organic anions. Several previous studies (Maneval et al., 1985; Clifford, 1990; Liberti et al., 1976; Brauch, 1984) have confirmed the need to identify and characterize appropriate sorbents which would exhibit high affinities toward selenite, phosphate, and low molecular weight organic anions in the presence of other electrolytes. Primary objectives of this paper are 3-fold: (1)t o provide experimental evidence pertaining to selective sorption of phosphate, selenite, and oxalate onto the PLE; (2) t o present regeneration behaviors of the PLE following exhaustion; and (3) to discuss the anion uptake mechanism and present guidelines for modeling ligand exchange equilibria.
Experiments: Materials and Procedure This study includes experimental results for four different synthetic sorbents, namely, DOW 2N (or XFS 430841, IRA-958, IRC 718, and IRA-900. Names of their manufacturers, chemical compositions of their functional groups, and other salient properties are provided
2678 Ind. Eng. Chem. Res., Vol. 34, No. 8,1995
in Table 1. These resins were obtained in spherical bead forms with sizes varying from 0.3 to 0.8 mm. They were conditioned following the standard procedure of cyclic exhaustion with 1N hydrochloric acid and 1N sodium hydroxide. Finally, DOW 2N and IRC-718 were converted into copper-loaded forms by passing 500 mg/L Cu(I1) solution at pH = 4.5 through these resins in separate columns until saturation. The solubility product of copper(I1)hydroxide was not exceeded at this pH. Analytical grade CuSO&H20 and CuC12-2H20 (Fisher Chemical) were used; the resins loaded with cupric sulfate were used for binary equilibrium experiments involving sulfate as the competing ions. The resins loaded with cupric chloride were used for column runs and also for the equilibrium experiments where chloride was the competing ion. Following copper loading, the resins were rinsed with distilled deionized water and air-dried for 72 h. MinicolumnExperiments and Sorption Equilibria. With chelating exchangers, it is extremely difficult to maintain a constant aqueous-phase pH during batch equilibrium tests. Therefore, all the equilibrium data with DOW 2N-Cu in this study were generated by minicolumns where aqueous solutions of fxed 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 phosphate, oxalate, or selenium concentrations with those of the influent. The resin in minicolumn was considered to have attained equilibrium when the effluent solute concentration was at least 95% of the influent concentration. Following a short rinse (10 min) with deionized water, the minicolumn resin was regenerated with 2% ammonia. The evidence of very high regeneration efficiency of DOW 2N-Cu with ammonia and the stoichiometries of regeneration have been provided elsewhere (Sengupta et al., 1991). Solute concentrations (P, oxalate, Se, sulfate, chloride) in the spent regenerant were analyzed and the resin’s uptakes at equilibrium were determined by mass-balance calculations. Chemical Analyses. Selenium(IV) samples were analyzed using an atomic absorption spectrophotometer (Perkin Elmer Model 2380) with graphite furnace accessory and electrodeless discharge lamp (EDL). All sample injections were 20 pL of nickel nitrate (1000 mgL as Ni) matrix modifier. Oxalate, sulfate, and chloride analyses of samples were carried out after necessary dilutions using a Dionex ion chromatograph (Model 4500i) with conductivity detectors using bicarbonatekarbonate as the eluent. Phosphate was analyzed by stannous chloride colorimetric procedure (Standard Methods, 1992). Column Runs. Fixed-bed column runs were carried out using Plexiglas columns (11mm diameter), constantflow stainless steel pumps, and an ISCO fraction collector. The ratio of column diameter to exchanger bead diameter was approximately 20.0; our earlier work (Sengupta and Lim, 1988) showed that over 24 000 bed volumes of chromate free effluent could be obtained before chromate breakthrough occurred, suggesting that channeling due to wall effects was practically absent under such conditions. The superficial liquid velocity (SLV) and the empty bed contact time (EBCT) were recorded for each column run.
INFLUENT IN
1
f7 *
11”
1.5 cm
I
It--GLASS WOOL
1
6IRC 718 -I+
( VIRGIN )
EFFLUENT OUT
Figure 1. Experimental details of the fxed-bed column using DOW 2N-CU.
m
0.80
> 1, (K - 1)CL >> C", and K - 1 = K, from eq 15, qLeQ
(16)
In essence, when the target ligand is a trace species (i.e., CLis very low), the sorption capacity, q L , is linearly dependent on CL.For much higher concentrations of CL,however, q L approaches the total capacity Q. Characteristically, the behavior conforms to the Langmuir isotherm. Equation 15 can be further refined to an approximate Langmuir isotherm by expressing aqueousphase solute concentration in normalized equivalent fraction which is XL
= CJC"
Equation 15 now becomes
(17)
3.00
Ind. Eng. Chem. Res., Vol. 34,No. 8, 1995 2683
, a K
--
I 1.29 mmq./g 21.01
I
Sorbont: DOW 2N-CU
1/XL Figure 13. Fitting the phosphate isotherm data into a linearized Langmuir-type model.
Considering K to be much greater than unity, eq 18, upon linearization, takes the following form:
small due to the high preference of DOW 2N toward dissolved copper over other alkali-metal and alkalineearth-metal cations such as Na+, K+, and Ca2+. The copper bleeding was completely eliminated by providing a small amount of a virgin chelating exchanger at the exit of the column. 2. Upon exhaustion of the column, phosphate could be regenerated very efficiently with concentrated brine (6% NaCl a t pH = 4.3). Over 90% phosphate was recovered in less than 10 bed volumes. High oxalate regeneration efficiencies were reported earlier by Zhu (1991) using different regenerants, namely, sodium chloride, sodium carbonate, and ammonia. 3. Anion exchange accompanied by Lewis acid-base interaction is the underlying reason for PLE's unusually high affinity toward phosphate, selenite, and oxalate. The PLE, however, conforms t o ion-exchange stoichiometry; i.e., total desorption capacity in equivalent unit remains constant. Upon rearrangement, the sorption behavior of PLE can be represented mathematically by Langmuir-type isotherms and the experimental data fitted well into the same.
Acknowledgment (19) Phosphate sorption data for the phosphate-sulfate isotherm with DOW 2N-Cu, when plotted in accordance with eq 19 (i.e., l l q L vs 1/xL),provided good agreement (see Figure 13)with the following values of parameters K and Q:
K = 21.08 (dimensionless) Q = 1.29 mequivlg correlation coeficient, r2 = 0.99 For quantitative modelling, polymeric ligand exchange for preferred trace solutes can mathematically be presented through a Langmuir-type isotherm.
Conclusions One of the growing areas of separation science and technology involves selective sorption of trace concentrations (milligrams per liter to micrograms per liter) of target contaminants in the presence of much higher concentrations of other competing species (Worthy, 1991). Many such trace contaminants are anionic ligands which can be sorbed selectively onto tailored polymeric ligand exchangers through electrostatic as well as Lewis acid-base interactions. Three anionic ligands, namely, phosphate, selenite, and oxalate were included in the study while a copper(I1)-loaded chelating polymer containing only nitrogen donor atoms was used as the polymeric ligand exchanger. Other polymeric sorbents, namely, two strong-base anion exchangers and one chelating cation exchanger, were also used for comparison. Primary conclusions of the study can be summarized as follows: 1. The PLE showed much higher affinities toward phosphate, selenite, and oxalate compared to other sorbents used in the study. The column run experiments with various sorbents also confirmed that DOW 2N-Cu offered the highest phosphate and oxalate removal capacities. Bleeding of copper(I1)from the PLE during the column run and regeneration was extremely
This study received partial financial support from the Environmental Protection Agency through Grant R-819228-01-0.
Nomenclature Ci = concentration of species i in aqueous phase (mmoVL, mequiv/L, or mg/L) c" = total electrolyte concentration in aqueous phase (mequiv/L) Go = Gibbs free energy at standard state (kcallmol or kJ1 mol) K = equilibrium constant Q = total capacity of polymer phase (mmol/g or mequivlg) qi = concentration of species i in polymer phase (mmoYg, mequivlg, or mglg) xi = equivalent fraction of i in aqueous phase (dimensionless) yi = equivalent fraction of i in polymer phase (dimensionless) ay = separation factor of i with respect t o j (dimensionless) Literature Cited Brauch, H. J. Adsorption of Natural Aqueous Organic Compounds on Activated Carbon. Ph.D. Dissertation (in German), University of Karlsruhe, Germany, 1984. Chanda, M.; O'Driscoll, K. F.; Rempel, G. L. Ligand Exchange Sorption of Arsenate and Arsenite Anions by Chelating Resins in a Ferric Ion Form. React. Polym. 1988,9, 277. Clifford, D. Ion Exchange and Inorganic Adsorption. In Water Quality and Treatment; Pontius, F. W., Ed.; McGraw-Hill Inc.: New York, NY, 1990; pp 561-640. Dean, J. A., Ed. Lunge's Handbook of Chemistry; McGraw-Hill Inc.: New York, NY, 1979. Dobbs, R. A.; Uchida, S.; Smith, L. M.; Cohen, J. M. Ammonia Removal from Wastewater by Ligand Exchange. MChE Symp. Ser. 1975,71 (1521, 157-163. Groves, F. R.; White, T. Mathematical Modeling of Ligand Exchange Process. MChE J . 1984,30(3), 494-496. Helfferich, F. Ligand Exchange: A Novel Separation Technique. Nature 1961,189 (4769), 1001-1002. Helfferich, F. Ligand Exchange I: Equilibria. J . Am. Chem. SOC. 1962,84(171, 3237-3242. Hernandez, C. M.; Walton, H. F. Ligand Exchange Chromatography of Amphetamine Drugs. Anal. Chem. 1972,46,890. Liberti, L.; Boari, G.; Passino, R. Selective Renovation of Eutrophic Wastes: PhosphateISulfate Exchange. Water Res. 1976,11, 5 17-523.
2684 Ind. Eng. Chem. Res., Vol. 34,No. 8, 1995 Maneval, J. E.; Klein, G.; Sinkovic, J. Selenium Removal fmm Drinking Water by Ion Exchange; EPA Project Summary, Report No. EPA/600/S2-85/074, Water Eng. Res. Lab., Environmental Protection Agency, Office of Research and Development: Cincinnati, OH, 1985. Matejka, Z.; Weber, R. Ligand Exchange Sorption of Carboxylic and Aminocarboxylic Anions by Chelating Resins Loaded with Heavy Metal Cations. React. Polym. 1990, 13,299. Ramana, A. A New Class of Sorbents for Selective Removal of As(V) and Se(IV) oxy-anions. M.S. Thesis, Lehigh University, Bethlehem, PA, 1990. SenGupta, A. K.; Lim, L. Modeling Chromate Ion-exchange Processes. AIChE J. 1988,34 (12), 2019-2029. SenGupta, A. K.; Zhu, Y. Selective and Reversible Ligands Sorption through a Novel Regeneration Scheme. Ind. Eng. Chem. Res. 1994,33,382-386. SenGupta, A. K.; Zhu, Y.; Hauze, D. Metal(I1) Ion Binding onto Chelating Exchangers with Nitrogen Donor Atoms. Environ. Sci. Technol. 1991,25(3), 481-488.
Standard Methods for the Examination of Water and Wastewater, 18th ed.; Am. Public Health Assoc., Am. Water Works Assoc., Water Environ. Fed.: Washington, DC, 1992. Worthy, W. Separations Technology, Long Overlooked, Is a Growing Field. Chem. Eng. News. 1991, Dee 2, 27. Zhu, Y. Chelating Polymers with Nitrogen Donor Atoms: Their Unique Properties in Relation to Heavy Metals Sorption and Ligand Exchange. Ph.D. Dissertation, Civil Engineering Department, Lehigh University, Bethlehem, PA, 1992. Received for review November 3, 1994 Revised manuscript received February 22, 1995 Accepted March 13, 1995@ IE940643L
@
Abstract published in Advance ACS Abstracts, July 1,
1995.
