Aquatic Humic Substances - ACS Publications - American Chemical

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Downloaded by PURDUE UNIVERSITY on June 8, 2013 | http://pubs.acs.org Publication Date: December 15, 1988 | doi: 10.1021/ba-1988-0219.ch044

Mechanistic Interactions of Aquatic Organic Substances with Anion-Exchange Resins Paul L . K. Fu Camp Dresser and McKee, Inc., P.O. Box 9626, Fort Lauderdale, F L 33310 James M. Symons Department of Civil Engineering, University of Houston, Houston, TX 77004

Specific interactions of aquatic organic substances isolated from a natural water source on commercial anion-exchange resins were investigated. Focuses were verifications of the mechanisms of organic removal and the influences of resin properties on the removal. The organic compounds used were concentrated by reverse osmosis and then separated by ultrafiltration into four molecular size fractions. Each organic fraction was characterized for organic carbon and carboxyl contents. Specific interactions of the organic fractions on various types of anion-exchange resins were studied by batch equilibrium experiments. Results indicate that about 90% of the dissolved organic carbon recovered from the natural water was removable by anion-exchange resins via an ion-exchange mechanism, and that both skeletal and pore structures were important to the removal, depending on the characteristics of the organic substances.

REMOVAL OF ORGANIC SUBSTANCES FROM DRINKING WATER SUPPLIES has become a major concern since trihalomethanes were found in chlorinated drinking water by Rook (I) and Bellar et al. (2) in 1974. The U . S . Environ mental Protection Agency recommended that water utilities should lower

0065~2393/89/0219-0797$06.00/0 © 1989 American Chemical Society

In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.


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the organic content of their water to the lowest extent possible prior to the use of any oxidant as a disinfectant (3, 4). The anion-exchange process is an attractive method for the removal of aquatic organic substances because the majority of them are acidic (ionic). Several previous studies focused on the removal of organic substances from waters. They were more empirical than fundamental, however, because the complex nature of the natural waters made the verification of mechanistic interactions of organic fractions on anion-exchange resins difficult. As a result, these interactions still remained speculative. The purpose of this research was, therefore, to understand the resin-organic compound interactions via a systematic study using four different molecular size fractions of aquatic organic substances concentrated and isolated from a single natural water source and six anion-exchange resins of different skeletal and pore structures. This chapter focuses on the portion of the study (5) concerned with mech anisms of aquatic organic compound removal by anion-exchange resins and influences of resin properties on the removal.

Removal Mechanisms Abrams (6) suggested that ion exchange is the most significant mechanism for the removal of organic acids from water supplies by strong-base anionexchange resins. H e assumed that some adsorption (other than ion exchange) also occurred on the aromatic structure of the styrenic resin polymer. In their article concerning organic substance removal by anion-exchange resins, Anderson and Maier (7) stated, "The mechanism by which these organic materials are retained by anion-exchange resins has not been une quivocally established. Nevertheless, it appears that anion exchange plays a prominent part." Kunin and Suffet (8) indicated that some humic substances are removed by anion-exchange resins via a simple anion-exchange process, and that some portions of the humic material are removed by "true" (surface) adsorption involving covalent bonding via van der Waals forces, particularly in acid media when the humic substances are not protonated. None of these authors, however, presented substantial evidence or data to verify the removal mech anisms. According to Thurman's book (9), the acid dissociation constant ( p K j of carboxyl groups on aquatic humic substances ranges approximately from 1.5 to 6.0 and that of phenolic hydroxyl groups falls between 8.0 and 12.0. Because natural water is generally neutral (pH 7-8), carboxyl groups on aquatic organic acids are essentially all ionized and phenolic hydroxyl groups are almost completely nonionized. Therefore, the term "carboxyl group" is used in this research to represent the organic functional groups that would contribute to ion exchange.



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Two possible mechanisms responsible for the removal of aquatic organic acids are proposed: (1) A mechanism that is responsible for the attachment of the carboxyl groups of an organic acid to the resin ionogenic groups involving the replacement of counterions on the resin is described as "ion exchange." This term does not define whether the exchange is reversible. (2) A mechanism that is responsible for the attachment of the nonionic portion of an organic acid to the internal surface of the resin without interacting with any ionogenic groups is termed "surface adsorption". An organic acid could be removed by either mechanism 1 alone or mechanism 2 alone, or by a combination of both mechanisms. These three conditions are demonstrated in Figure 1. When only mechanism 1 is involved (shown in Figure 1A), the removal is by ion exchange. When only mechanism

Figure 1. Removal of an organic acid by an anion-exchange resin through various mechanisms.



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2 is involved (shown in Figure IB), the removal mechanism is surface ad sorption, and the removal is called "electrolyte adsorption". A combination of mechanisms 1 and 2 (shown in Figure 1C) occurs when the carboxyl groups of an organic molecule bind with resin amine functional groups (ion exchange) and the nonionic portion of the molecule attaches to the resin inner surface through surface adsorption. After equilibrating an organic fraction with different quantities of various anion-exchange resins at p H 7.5, a value typical of natural surface waters, the variations of total organic carbon (ΔΤΟΟ) and of counterion concentration ( Δ Ο for chloride-form resins) were measured. The plot of A T O C vs. A C l would yield an approximately straight line only if the removal of the organic fraction involved the ion-exchange mechanism, because the aqueous-phase chloride concentration can be increased only by the exchange of the organic anions for the resin-phase chloride ions. The straight line, however, cannot allow judging whether electrolyte adsorption of some organic compounds or a combination of ion exchange and surface adsorption also occurred. To clarify whether or not electrolyte adsorption of the organic matter was occurring, the following procedure was used. The variation of the ionic concentration in milliequivalents per liter of an organic fraction, ΔΑ, was calculated from A T O C by the following equation: ΔΑ = ΔΤΟΟ(Ω)/1000

(1)

where Ω, the concentration (meq/g ofTOC) of carboxyl groups of the organic fraction at a given p H , was obtained from titrations of acidic functional groups, and ΔΤΟΟ is in milligrams per liter. After ΔΑ values were calculated, ΔΑ vs. Δ 0 1 was plotted. A straight line with a slope higher than 1, dem onstrating ΔΑ > A C l , indicated the existence of some electrolyte adsorption of the organic acid molecules. If, however, the plot resulted in a straight line with a slope equal to 1, which represents the diagonal where ΔΑ = Δ Ο , the stoichiometry of ion exchange and the absence of electrolyte ad sorption was demonstrated. If ΔΑ = Δ Ο , the following steps were carried out to clarify whether surface adsorption, in addition to ion exchange, existed on the same mole cule, as was the case shown in Figure 1C. Selected polymeric adsorbents, which possess skeletal and pore structures similar to the ion-exchange resins but contain no ionogenic functional groups, were used in this investigation. Because of the lack of ionogenic functional groups in polymeric adsorbents, ion exchange was not present. Thus, surface adsorption could be studied without the interference of ion exchange. To obtain the degree of removal through surface adsorption, an organic fraction was equilibrated with the polymeric adsorbents. If little organic matter is adsorbed, surface adsorption would be insignificant to the uptake of the organic fraction by the ion-exchange resins, and ion exchange would


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therefore be the dominant removal mechanism. If, however, a significant quantity of the organic matter is adsorbed, the surface adsorption mechanism would, in addition to the ion-exchange mechanism, also be significantly responsible for the uptake of some of the organic matter by the ion-exchange resins. This verification can only be qualitative, however, because no pol ymeric adsorbent possesses exactly the same skeletal and pore structures as an anion-exchange resin. Nevertheless, these data are useful in under standing the mechanisms involved.

Influences of Resin Properties Synthetic organic anion-exchange resins and adsorbents have two major types of skeletons: polystyrene and aery late. Both of these skeletons are commonly cross-linked with divinylbenzene. The main difference between these two skeletons is their hydrophobicity. Because of its higher content of aromatic rings, the styrenic resin sorbs less water (swells less) and is more hydrophobic than the acrylic-type resin. Using high-performance liquid chromatography ( H P L C ) columns, Sinsabaugh et al. (10) separated aquatic organic matter in the Harwood's M i l l Reservoir (in Virginia) water into hydrophobic, mesic, and hydrophilic fractions. The skeleton of anion-exchange resins, which affects resin hydro phobicity, may therefore be an important property for the uptake of aquatic organic materials by anion-exchange resins. Because the molecular size of aquatic organic substances is polydisperse, the pore structure of anion-exchange resins is important to the removal of aquatic organic fractions. Anion-exchange resins can be made gelular or porous. Gelular resins possess no permanent pores, but they do have "mi cropores" of atomic dimension that depend markedly on their swelling prop erties, described by Kunin and Hetherington (11). Macroporous resins, however, have a "true" pore phase in addition to the gel phase and have an internal surface area. Because of their pore phase, macroporous resins should be more accessible to organic compounds of large molecular sizes than gelular resins. Some important properties of the various anion-exchange resins and polymeric adsorbents used in this research are summarized in Table I. In the literature, Macko (12) was the only researcher who studied the removal by anion-exchange resins of aquatic organic substances fractionated by ultrafiltration (UF). She, however, focused on only one anion-exchange resin (Amberlite IRA904) in her work and did not use a single batch of source organic matter, but collected water samples and UF-fractionated them at different periods during her investigation. Her results, therefore, contrib uted little to the understanding of the influence of resin skeleton and pore structure on the removal of aquatic organic fractions. In this research, how ever, six strong-base anion-exchange (SBA) resins with different skeletal and


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Table I. Properties of Anion-Exchange Resins and Polymeric Adsorbents Used in This Research


Resin

Type?

IRA458 IRA904 IRA938 IRA958 XE510 IRA402 XAD-8 XAD-16

NaCl Exchange Ζαραάψ (meq/g) (meq/mL)

Surface Area (m*/g)

Mean Pore Radius (nm)

ACR-GEL-SBA 1.29 0.1 4.37 STY-MAC-SB A 2.67 0.75 60 35 STY-MAC-SBA 3.91 0.58 7 3500 ACR-MAC-SBA 4.14 0.87 100-150

Aquatic Humic Substances - ACS Publications - American Chemical

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