Environ. Sci. Technol. 2000, 34, 5193-5200
Intraparticle Diffusion during Selective Sorption of Trace Contaminants: The Effect of Gel versus Macroporous Morphology PING LI AND ARUP K. SENGUPTA* Department of Civil and Environmental Engineering, Lehigh University, 13 East Packer Avenue, Bethlehem, Pennsylvania 18015
Ion exchanger beads are commercially available in two different physical morphologies, namely, gel and macroporous. While gel-type (or microporous) exchanger particles are essentially homogeneous solid phases with closely spaced functional groups, a single macroporous ion exchanger bead may be viewed as an ensemble of tiny microgels with an interconnected network of pores. Macroporous exchangers are gaining acceptance in environmental applications due to their durability, resistance to oxidation, and less susceptibility to fouling by natural organic matters. Many environmentally significant synthetic aromatic compounds, namely, chlorophenols, naphthalene and benzene sulfonic acids, nitrophenols, and quaternary benzylammonium compounds exist as ions in the aqueous phase over a wide range of pH and are very selectively removed by polymeric exchangers with hydrophobic matrixes. Anionic pentachlorophenol (PCP-), a widely used pesticide and wood preservative, was used in the study as a representative aromatic anion. This paper presents significant differences in intraparticle diffusion behaviors during sorption of trace PCP- onto gel and macroporous anion exchangers. While an increase in competing chloride ion concentration had no appreciable effect on the effective intraparticle diffusivity for gel exchangers, the same resulted in a significant enhancement of intraparticle diffusivity of macroporous exchangers. For exchangers with gel morphology, solid-phase diffusion is the primary intraparticle solute transport mechanism. For macroporous exchangers, on the contrary, the slower pore diffusion between the microgels of a single spherical bead is the rate-limiting step. The experimental observations were substantiated using an intraparticle parallel solute transport model. From a generic viewpoint, the macroscopic observation of the effect of competing solute (chloride) concentration on the effective intraparticle diffusivity during selective sorption may be used to gather microscopic knowledge about the morphology of the sorbent material. Geosorbents, although not spherical and quite complex in composition, are characteristically similar to macroporous exchangers where clusters of sorption sites are separated by pores filled with liquid. It is postulated that an enhanced understanding of the sorption behaviors of geosorbents can be obtained through the use of surrogate macroporous polymeric materials. 10.1021/es001299o CCC: $19.00 Published on Web 11/10/2000
2000 American Chemical Society
Introduction Many environmentally significant synthetic aromatic compounds, namely, chloropheols, naphthalene and benzene sulfonic acids, nitrophenols, and quaternary benzylammonium compounds exist as ions in the aqueous phase over a wide range of pH and are often referred to as hydrophobic ionizable organic compounds (HIOCs) (1, 2). Contrary to nonionized organic solutes, HIOCs are poorly partitioned into the soil phase and hence are more mobile. While conventional adsorbents, namely, activated carbon, alumina, and synthetic sorbents, are practically ineffective in removing HIOCs in engineered processes, polymeric ion exchangers with hydrophobic matrixes show high sorption affinity toward these ionic solutes (3-5). An ion exchange type sorption is however a coupled transport process where electroneutrality is maintained in both aqueous and exchanger phase independently. Previous studies (5, 6) have shown that the uptake of anionic pentachlorophenol (PCP-), a widely used pesticide and wood preservative, is accompanied by the desorption of an equivalent amount of competing ion (say chloride) in the aqueous phase:
(R+)Cl- + PCP-(aq) h (R+)PCP- + Cl-(aq)
(1)
where the overbar represents the exchanger phase and (R+) denotes the functional group with fixed positive charges. Assuming ideality in both aqueous phase and ion exchanger phase, the pseudo-equilibrium constant or separation factor (R) for the exchange reaction in eq 1 is given by
R or RPCP/Cl ≡
qPCPCCl qClCPCP
(2)
where qi and Ci are the concentrations of solute i in the exchanger phase (mequiv/g) and in the aqueous phase (mequiv/L), respectively. The total exchange capacity of the exchanger, Q, and the total aqueous-phase concentration, CT, however, remain unchanged during the sorption process, i.e., Q ) qPCP + qCl and CT ) CCl + CPCP. After these equalities were applied and this was considered to be a selective exchange in favor of trace concentration of PCP-, i.e., R . 1, deductions in Appendix A show that the partitioning of PCP- between the exchanger phase and the solution phase is given as
qPCP ) KCPCP
(3)
where
K)
RQ CT
(4)
Polymeric anion exchangers with polystyrene matrix and quaternary ammonium functional groups exhibit high affinity toward PCP- (5). Two types of such polymeric anion exchangers are available commercially, namely, gel and macroporous. Both of them are essentially perfect spherical beads with identical chemical composition; the difference lies only in the presence of a network of pores within the beads as elaborated later in this paper. Macroporous exchangers, although more expensive as compared to their gel counterparts, are gaining acceptance in many environmental applications because of their durability, resistance * Corresponding author phone: (610)758-3534; fax: (610)758-6405; e-mail:
[email protected]. VOL. 34, NO. 24, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Transmission electron microphotographs (TEM) of gel and macroporous ion exchangers (×50 000 magnification) and representative schematics. to oxidation, and less susceptibility to fouling by natural organic matter. The primary objective of this study is to present experimental evidence pertaining to the distinct difference in the nature of intraparticle diffusion between macroporous and gel-type polymeric resins for selective exchange processes. Specifically, the effective intraparticle diffusivity for trace contaminant sorption is influenced differently by changes in competing ion concentrations for exchangers with different morphologies. Attempts are also made to elucidate the underlying sorption and solute transport mechanism responsible for such a difference in kinetic behavior between these two widely used exchangers, under otherwise identical conditions. Pentachlorophenol is used in this study as a representative target contaminant because of its environmental significance and widespread occurrence in water bodies primarily as an anion due to its relatively low pKa value as shown:
Four different polymeric ion exchangers from three different manufacturers were used in the study to substantiate the general premise of the investigation. Natural geosorbents, as explained later, are characteristically similar to macropor5194
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ous materials. It is postulated that certain findings of the study can be extended toward enhanced understanding of sorption behaviors of geosorbents with surrogate polymeric materials.
Gel versus Macroporous Ion Exchangers: Intraparticle Solute Transport Although both of them are spherical, the physical morphology of a gel particle is significantly different from that of a macroporous one. A single macroporous ion exchanger particle may be viewed as an ensemble of tiny microgels with an interconnected network of pores. These macroporous ion exchangers are manufactured using the process of suspension polymerization; the resulting microgels are essentially perfect microspheres (7, 8). While the sizes of the macroporous bead vary from 0.2 to 1.0 mm, the sizes of microgels (
