[Advances in Chemistry] Aquatic Humic Substances Volume 219 (Influence on Fate and Treatment of Pollutants) || Influences of Natural Organic Matter on the Abiotic Hydrolysis of Organic Contaminants in Aqueous Systems

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  • 21 Influences of Natural Organic Matter on the Abiotic Hydrolysis of Organic Contaminants in Aqueous Systems

    Donald L . Macalady

    Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, CO 80401

    Paul G. Tratnyek and N . Lee Wolfe

    Environmental Research Laboratory, U.S. Environmental Protection Agency, College Station Road, Athens, GA 30613

    This chapter reviews investigations that attempt to provide a system-atic understanding of the role of natural organic matter in aqueous hydrolytic reactions of anthropogenic organic chemicals. In partic-ular, the suggestion that humic substances exert a catalytic effect on a wide variety of hydrolytic reactions is evaluated in the light of experimental evidence. With the possible exception of the hydrolytic dechlorination of the chloro-1,3,5-triazine herbicides, available data do not support the idea of a general catalytic effect. On the contrary, more evidence exists for an inhibitory role than for one of enhance-ment. The (limited) experimental evidence that may lead to an un-derstanding of the possible role of natural organic matter in mediating abiotic hydrolytic reactions is discussed. Several models that imply a general effect of natural organic matter on the kinetics of hydrolysis are outlined.

    HYDROLYTIC DEGRADATION OF CONTAMINANT ORGANIC CHEMICALS is one

    of the most important and widely investigated pollutant transformation proc-

    0065-2393/89/0219-0323$06.00/0 1989 American Chemical Society

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

  • 324 AQUATIC HUMIC SUBSTANCES

    esses in aqueous systems. Microorganisms play an important role in the hydrolysis of organic chemicals, either directly (J, 2) or indirectly through their molecular components, such as soil enzymes (3). For many contaminants, however, abiotic hydrolytic reactions are more important under conditions found in most natural waters. Therefore, abiotic hydrolyses of anthropogenic organic chemicals, especially pesticides, have been extensively investigated (4-8). In particular, there has been considerable focus on the factors that enhance or retard the rates of these abiotic hydrolytic reactions.

    This review wil l deal only with hydrolysis, as distinguished from other reactions between organic chemicals and water, such as acid-base, addition, elimination, and hydration. In the hydrolytic process, an organic molecule, RX, reacts with water to form a new carbon-oxygen bond with the water molecule and cleave a C - X bond in the original molecule. The net reaction is commonly a direct displacement of X by O H (9). Important classes of organic molecules that undergo hydrolytic reactions include aliphatic halides; esters of carboxylic, phosphoric, phosphonic, sulfonic, and sulfuric acids; carbamates; epoxides; amides; and nitriles. Hydrolytic half-lives for abiotic reactions at p H 7 and 25 C vary from millions of years to about 1 s (8).

    Under pH-buffered reaction conditions, many hydrolytic reactions can be represented by a pseudo-first-order rate law -d[P]/dt = fc[P], where [P] represents the concentration (activity) of the pollutant molecule and k is the pseudo-first-order rate constant, valid for a fixed temperature and p H only. The rate constant, k, is system-specific. It may be composed not only of contributions from specific acidic or basic and neutral hydrolyses, but also from catalytic processes such as general acid-base catalysis. A profile of the p H dependence of the hydrolytic rate constant for the methyl ester of (2,4-dichlorophenoxy)acetic acid (2,4-DME), typical of many carboxylic acid esters, is represented in Figure 1. This figure does not include generalized acid or base catalysis, and either specific acid or base catalysis may be the rate-controlling process at p H values commonly encountered in natural waters (8, 9). A few metal ions also catalyze specialized hydrolytic reactions (10, 11). Under most reaction conditions that exist in natural waters, however, neither general acid-base nor metal-ion catalysis is expected to contribute significantly to the overall rate of hydrolysis (12).

    This review addresses the effects of natural organic matter in aquatic systems (humic substances) on the rates of hydrolytic reactions. In a recent review of the interactions of humic substances with environmental chemicals, Choudhry (13) stated that humic substances can play an important role in hydrolytic transformations of a wide variety of organic chemicals in the aqueous environment. We wil l reevaluate the evidence for this generalization, take into account studies published since 1983, and offer alternate suggestions that may lead to a more general understanding of the influences of natural organic matter on hydrolytic processes. Except in a few cases

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  • 21. MACALADY ET AL. Abiotic Hydrolysis of Organic Contaminants 325

    "T" 2

    ~T~ 4 6

    p H 10

    Figure 1. The pH-rate profile for the hydrolysis of the methyl ester of 2,4-D in buffered distilled water at 30 C. Data are from ref 28. The dashed lines

    represent extrapolations of the exprimental data.

    included for comparison, this discussion refers only to abiotic reactions. The means by which sterility is ensured in the various experimental systems wil l not be discussed. Though considerations of these methods may be important, the present state of the literature does not warrant further discussion at this time (14).

    Triazines The catalytic effects of humic acids (HAs) on the dechlorohydroxylation of the chloro-1,3,5-triazine herbicides in soils and aqueous solutions are well documented. Harris (15) first reported that the decomposition of simazine, atrazine, and propazine in soils proceeds predominantly by way of the acid-catalyzed hydrolysis at the 2-position of the triazine ring to produce the hydroxy derivatives, as shown in Reaction I.

    Evidence for the catalytic effect on this reaction of solution-phase HAs and soil organic matter has been presented by (among others) Armstrong and Chesters (16), L i and Felbeck (17) and Khan (18). Nearpass (19) reported similar evidence for propazine. Although these catalytic effects are well

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  • 326 AQUATIC HUMIC SUBSTANCES

    Cl OH

    ( E t ) H N ^ N ^ N H ( i . p r ) H20

    cat.

    (Et)HN N NH(iPr)

    H* CI"

    Reaction I. Hydrolysis of atrazine.

    documented, the mechanism is poorly understood. Attempts to establish a pathway for the observed enhancement have produced some contradictory evidence. The effect of HAs on the activation energy for this hydrolysis, for example, has been reported to be substantial by L i and Felbeck (17) and negligible by Kahn (18). Indeed, Metwally and Wolfe (20) failed to observe rate enhancement of acid-catalyzed hydrolysis of atrazine in sediment-water versus buffered distilled water systems.

    Attempts to explain these observations in terms of general acid catalysis are not convincing, and specific interaction between HAs and the triazines is more likely. Hydrogen bonding between H A surface acid groups and the ring nitrogens of the triazines has been suggested as a mechanism for reduction of the activation energy barrier for hydrolytic cleavage of the C - C l bond (13).

    Interpretation of the effects of HAs on triazine hydrolyses is complicated by the complexity of the reaction pathways for these reactions at lower p H values. Plust et al. (21) demonstrated that the complex p H dependence of the atrazine rate of hydrolysis (Figure 2) is due to the protonation of atrazine at lower p H values. They also verified the absence of a pH-independent hydrolytic pathway for atrazine.

    Under certain reaction conditions, the surface acidity of clays wi l l facilitate the hydrolysis of adsorbed atrazine (22, 23). This activity indicates that the catalytic effects observed in soils may not be entirely due to natural organic matter. Where clean surfaces are available, they may contribute a catalytic effect confounded with that from other sources (24).

    Solution-Phase Reactions

    For convenience, one can divide the role of natural organic matter in hydrolytic reactions into solution-phase and sediment-phase interactions. Fu l vic and humic acids in water are known to interact with many (generally hydrophobic) contaminants (25). Investigations of the effect, if any, of such associations on rates of hydrolysis are few. Struif et al. (26) considered the role of fulvic acids in the hydrolyses of a series of n-alkyl esters of 2,4-D and concluded that the presence of this form of organic matter accelerated

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  • 21. MACALADY ET AL. Abiotic Hydrolysis of Organic Contaminants 327

    CO

    )

    PH

    Figure 2. The pH-rate profile for the hydrolysis of atrazine. The dashed lines represent the rates that would result if there were no contribution from di-protonated atrazine. {Reproduced from ref. 21. Copyright 1981 American

    Chemical Society.)

    the reactions. However, Perdue and Wolfe (27) later demonstrated that, at least for base-catalyzed hydrolyses, the opposite is true. Solution-phase humic substances (those that pass through a 0.2- filter) were shown to retard the rate of the base-catalyzed hydrolysis of the n-octyl ester of 2,4-D (Figure 3). Perdue and Wolfe attributed the rate enhancements observed by Struif et al. to microbially catalyzed processes. No reports of similar investigations of the effect of solution-phase organic matter on acid-catalyzed or pH-independent hydrolytic reactions have been published.

    Heterogeneous Systems In sediment-water or soil-water systems, associations of hydrophobic contaminant molecules with the organic phase of the solids ("sorption") can also

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  • 328 AQUATIC HUMIC SUBSTANCES

    f 1 1 1 I 0 1 0 0 2 0 0 3 0 0

    time (m inutes )

    Figure 3. Effect of partitioning on the hydrolysis of 2,4-DOE at pH 10 with varying amounts of "dissolved humic substances". (Reproduced from ref 27.

    Copyright 1982 American Chemical Society.)

    be considered in terms of effects on hydrolytic reactions. Studies of rates of hydrolysis in the presence of soils and sediments are more numerous than solution-phase studies, and considerable progress has been made toward a general understanding of the effects of sorption on abiotic hydrolyses. In particular, a large and growing body of evidence supports the idea that sorption has little or no effect on the rates of pH-independent (i.e., non-acid/base catalyzed) hydrolyses. For a wide variety of organic molecular structures and a concomitant variety of hydrolytic mechanisms, sediment organic matter does not have a measurable effect on pH-independent rates of hydrolysis.

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  • 21. MACALADY ET AL. Abiotic Hydrolysis of Organic Contaminants 329

    Macalady and Wolfe (28, 29) demonstrated that the hydrolyses of chlor-pyrifos, diazinon, and ronnel (Chart I), all organophosphorothioate esters, proceed at rates that are independent of sediment sorption. These reactions presumably proceed via an S N 2 pathway involving attack by water at the ester carbon atoms (30). The S N 1 hydrolysis of benzyl chloride (Chart I) is

    S

    Chart I. Molecular structures of compounds mentioned in the text. A, chlor-pynfos; B, diazinon; C, ronnel; D, benzyl chloride; E, hexachlorocyclopen-tadiene; F, 4-(p-chlorophenoxy)butyl bromide; and G, chlorostUbene oxide.

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  • 330 AQUATIC HUMIC SUBSTANCES

    also unaffected by the presence of sediments (28). The S E 2 hydrolysis of hexachlorocyclopentadiene (31, Chart I) and the nucleophilic substitution reaction of 4-(p-chlorophenoxy)butyl bromide (32, Chart I) are also not measurably affected by sorption of the substrate to sediment organic matter (28). Recently, El-Ammy and M i l l (33) have demonstrated a similar lack of influence of aquifer materials on the disappearance kinetics of halogenated al-kenes, which probably involves hydrolytic reactions. Metwally and Wolfe (20) have recently demonstrated that the hydrolysis of chlorostilbine oxide (Chart I) at near-neutral p H values proceeds at experimentally equivalent rates in the presence and absence of sediments.

    A similarly convincing array of evidence exists that base-catalyzed hydrolytic reactions are retarded by natural organic matter. The homogeneous retardation of 2 ,4-DOE hydrolysis by dissolved HAs is matched by influences of solid-phase organic matter (34). The alkaline hydrolysis of chlor-pyrifos is characterized by pseudo-first-order kinetics with rate constants that, in the presence of sediments, are lower by factors of 0.01 to 0.001 than those in buffered distilled water at the same p H (29).

    For acid-catalyzed hydrolyses, the picture is considerably less resolved. An attractive working hypothesis is that acid-catalyzed hydrolyses should be enhanced in the presence of natural organic matter. This hypothesis is consistent with the rationalization of rate retardation observed for the alkaline case. The retardation of base-catalyzed hydrolysis can be envisioned in terms of the changes in solution che...

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