Environ. Sei. Technol. 1993, 27, 2381-2386
Catalysis of Picolinate Ester Hydrolysis at the Oxide/Water Interface: Inhibition by Adsorbed Natural Organic Matter Alba Torrentst and Alan T. Stone'
Department of Geography and Environmental Engineering, G. W. C. Whiting School of Engineering, The Johns Hopkins University, Baltimore, Maryland 21218 Natural organic matter (NOM) has been found to inhibit TiOz-catalyzed hydrolysis of methyl picolinate (MEP), which is used to represent a broader class of hydrolyzable pollutants. NOM must adsorb at the oxide/water interface in order for inhibition to take place. At equal NOM surface coverage (in mg C/m2),a nearly 60%variation is observed among NOM samples from five different sources. As NOM-laden water passes through soils and sediments, surface-active NOM molecules are removed by adsorption. This fractionation process has been simulated in the laboratory by pretreating a NOM sample with TiOz, FeOOH, and Alz03. FeOOH is extremely efficient at removing the NOM subfraction responsible for the inhibitory effect, while A1203 is the least efficient of the three oxides. I t is concluded that ecosystem-to-ecosystem NOM variations and prior NOM contact with subsurface solids can substantially alter the ability of NOM to inhibit surface-catalyzed hydrolysis reactions. ~~~~~~~~
Introduction In order to estimate rates of pollutant hydrolysis in the environment, it is necessary to account for the effects of other dissolved and particulate-bound chemical constituents. The first paper in this series (1)reported that the representative ester phenyl picolinate is subject to surfacecatalyzed hydrolysis in the presence of FeOOH and Ti02 surfaces; Alz03 and Si02 surfaces were found to possess no catalytic activity. More recently, we reported that the adsorption of inorganic ions (calcium and phosphate) and low molecular weight organic acids (benzoate, picolinate, and salicylate) inhibited surface-catalyzed hydrolysis of phenyl picolinate (PHP) and a related ester methyl picolinate (MEP) (2). Although inhibition arises from the occupation of surface sites, equal adsorption densities by various coadsorbate species yield markedly different degreesof inhibition. These differences cannot be ascribed to differences in the area occupied by each coadsorbate molecule alone; structural-chemical differences among surface sites must be postulated to explain inhibition phenomena (2). Natural organic matter (NOM) is present in virtually all aquatic environments (3).The objective of this study is to investigate the ability of raw-water NOM samples and NOM isolates to inhibit TiO2-catalyzedhydrolysis of the representative ester MEP. The multiple-component nature of NOM is a key concern here. As NOM-laden water moves through the subsurface, surface-active NOM molecules are preferentially removed by adsorption onto soil solids and aquifer sediments (4). We will simulate this natural fractionation process by pretreating NOM samples with various oxide surfaces. By removing subfractions of NOM material in this way, the magnitude of the inhibitory effect may be altered.
* Author to whom correspondence should be addressed. + Present address: Department of Civil Engineering, College of Engineering, University of Maryland, College Park, MD 20742. 0013-936X/93/0927-2361$04.00/0
0 1993 American Chemical Society
N O M Adsorption at Oxide/Water Interface. NOM must adsorb at the oxide/water interface in order for inhibition of surface-catalyzed reactions to occur. Extensive research into the adsorption of NOM onto oxide surfaces has emphasized the importance of (i) the physical and chemical properties of the NOM sample and available surface, (ii) the NOM to surface area ratio, and (iii) the major ion composition and pH of the supporting aqueous medium (5-9). NOM adsorption onto oxide surfaces increases as the pH is decreased, reaches a maximum value around pH 4-5, and then decreases (7). A pH dependence of this kind is also observed for the adsorption of simple aliphatic and aromatic carboxylic acids at the oxide/water interface (10). NOM adsorption is quantified by performing a carbon balance: adsorption is expressed in units of mg of C/L lost from solution or in terms of surface density (mg of C/m2 of surface). NOM adsorption onto oxide surfaces is governed by the law of mass action. At fixed pH, the mass of organic carbon adsorbed per liter of suspension increases as the concentration of NOM in solution is increased and as the oxide loading is increased. In order for a high percentage of NOM to be adsorbed, the surface area loading must be high relative to the NOM concentration. Under these conditions, the density of NOM molecules on the surface (expressed in units of mg of C/m2)is expected to be low. In order for the density of NOM molecules on the oxide surface to be high, the NOM concentration must be high relative to the oxide loading (5). Expressing NOM adsorption on a weight-carbon basis does not account for chemical differences which arise between dissolved and adsorbed fractions. Observations that weakly acidic groups are removed in greater proportion than strongly acidic groups (41, and that high molecular weight molecules are removed in greater proportion than low molecular weight molecules (9) confirm that sorption onto oxides causes chemical fractionation. In a recent study, McKnight et al. (11) examined NOM sorption and accompanying changes in NOM chemical compositionupon mixing with an iron oxide and aluminum oxide-laden tributary. Aromatic moieties, carboxylicacid groups, N- and S-containing groups, and amino acid residues were preferentially removed by sorption onto hydrous oxides. The nature of the fractionation process is more consistent with a ligand exchange-surface complexation mechanism than on a mechanism relying on hydrophobic interactions alone (11). Recent studies also indicate that fractionation alters the chemical composition of NOM as it migrates through the subsurface. Dunnivant et al. (12) observed that laboratory columns containing aquifer material retained NOM sub-fractions in the following order: hydrophobic neutral > hydrophobic acidic > hydrophilic. McCarthy et al. (13) injected NOM-rich surface water into a sandy aquifer. During the early stages of this experiment, larger (>3000 molecular weight) and more hydrophobic NOM Environ. Sci. Technol., Vol. 27, No. 12, 1993 2381
Table I. Physical and Chemical Properties of Oxides Used in This Study
oxide
surface area (mz/g)
Ti02 FeOOH
55.0
pH,,,
6.5
site density (site/nm2) 3
50.1
7.9
6-7
A1203
92.8
8.9
2-4
subfractions migrated more slowly than small and more hydrophilic fractions. Differences in the migration rates of various subfractions diminished during the 2-week injection period, indicating approach toward steady-state adsorption (13). NOM adsorption brings about physical and chemical changes to the oxide surface (8).Baccini et al. (14)observed that the adsorption of NOM decreased the extent of copper, zinc, and phosphate adsorption onto aluminum oxide and postulated that competition for available surface sites was responsible. Because NOM molecules are anionic, adsorption causes the oxide surface charge to become more negative (15). As a consequence, the nature of particleparticle (16), solute-particle (17), and solvent-particle interactions are changed. Mineral surfaces, in turn, may alter the orientation and structure of adsorbed NOM and, thus, affect NOM-organic pollutant interactions ( 4 ) . Two NOM samples yielding the same oxide surface coverage on a weight-carbon basis may occupy a different fraction of the total number of surface sites and bring about different physical and chemical changes to the oxide surface. When significant structural-chemical differences exist among surface sites (2),the pattern of coverage by two NOM samples may differ. Carboxylate and other ligand-donor groups represent potential points of attachment between NOM molecules and individual surface sites ( 4 ) . Thus, the number of surface sites occupied at a fixed weight-carbon surface coverage should increase as the O/C ratio (and potentially the N/C and SIC ratios) increases. Carboxylate-rich NOM would also impart more negative charge onto the oxide surface than a comparable coverage by carboxylate-poor material, In a similar fashion, differences in relative amounts of aliphatic and aromatic carbons may affect the nature of hydrophobic interactions at the oxide/water interface.
Materials and Methods All solutions and suspensions were prepared from 18 MOScm resistivity water (DDW, Millipore Corp.). All glassware were soaked in 5 N HN03 and rinsed several times with DDW prior to use. Inorganic reagents were analytical grade (Fisher Chemicals), and the organic reagents were used as received (from Aldrich and American Tokyo Kasei). Titanium dioxide (TiOz, primary anatase, type P25), aluminum oxide (A1203,type C), and amorphous silica (SiOz,Aerosil200) were obtained from Degussa Corp. and used without purification. Synthesis of iron oxide (FeOOH, type 2) was described in an earlier paper (1). Physical and chemical properties listed in Table I were measured following procedures outlined previously (I). Two raw samples and three International Humic Substances Society (IHSS) NOM isolates were used in this study. Great Dismal Swamp Water (DSW) was collected from Jericho Ditch near Suffolk, VA, on October 1990. The sample contained 99 mg of C/L NOM and had a pH of 3.7. Georgetown Swamp Water (GSW) was collected in March 1991 from Gator Pond at the Baruch Forest 2382 Envlron. Scl. Technol., Vol. 27, No. 12, 1993
Science Institute in Georgetown,SC (operated by Clemson University). GSW contained 65 mg/L NOM and had a pH of 4.2. DSW and GSW have not been subject to any isolation or preconcentration process. Black and Christman (18) determined that fulvic acids constitute most of the NOM (up to 80 % ) in freshwater swamps of this kind along the southeast coast of the United States. Gu et al. (19) determined that GSW adsorption onto iron oxides varies little between 2 and 18 h of contact. Collection of IHSS NOM isolates begins with column extraction (XAD-8) of a “hydrophobic” fraction from natural samples (201, followed by desalination and lypholization (21). This procedure removes metals and inorganic anions. The recovery of low molecular weight NOM by this procedure may be poor; additional low molecular weight, “hydrophilic”material can be recovered using XAD-4 resin (20). Collected material is further divided into fulvic acids (soluble at all pHs) and humic acids (insoluble at acidic pH but soluble a t alkaline pH). Fulvic acids typically possess a lower molecular weight distribution than humic acids and possess higher oxygen but lower carbon contents (22). For fulvic acids, these properties translate into more functional groups per mg of C and less aromaticity than humic acids. The same holds true for aquatic NOM in comparison to soil-derived NOM (22). Some chemical alteration accompanies all extraction and fractionation procedures. The LEON IHSS sample was extracted from an oxidized form of lignite coal with high humic acid content (23). PEAT was isolated from Pohokee peat (Ocachobee, FL) and is comprised mostly of humic acids with 56.82% C and a aromaticity of 47 9% (4). SRH was isolated from the Suwanee River, which drains the Okefenokee Swamp in southern Georgia. SRH has a carbon content lower than typical soil humic acid (54.3%) and a lower degree of aromaticity (37%) (22). Adsorption and hydrolysis experiments were conducted in 30-mL amber-glass vials sealed with Teflon/silicon rubber septa. Suspensions were continuously stirred in a constant temperature bath using Teflon-coated magnetic stir bars. The ionic strength was kept constant using 0.01 M NaC1, and the pH was adjusted to 5.0. Stock MEP solutions were prepared by mixing an excess of ester in DDW, followed by filtration (0.2-pm Nuclepore Corp. filter), and the solutions were then used immediately. FeOOH was removed by filtration (0.2-pm Nuclepore Corp. filter). TiO2, A1203,and Si02 were removed from suspension by centrifugation (15 000 rpm for 7 min). Dissolved organic carbon (DOC) measurements were carried out using a Dohrman DC-80 total organic carbon analyzer. NOM adsorption experiments were conducted by adding a known amount of oxide to NOM solutions at fixed pH and ionic strength and then equilibrating them at 25 “C overnight (I10 h). Mass balance and DOC measurements of the supernatant recovered by centrifugation were used to calculate NOM surface coverage (in mg of C/m2). After NOM adsorption and pH were measured, 1.0 mM acetate buffer was added. The hydrolysis experiment was begun by adding a known amount of methyl picolinate (MEP). Reaction progress was monitored with a Waters HPLC system using a p-Bondapac-Cis column (Waters Corp.). During the hydrolysis reaction, the extent of MEP adsorption was below the detection limit of our analytical technique. First-order kinetics were observed (-d[esterl/
I* E
v
B 8
0
0-0
DSW
m-*
GSW
A-A
LEON
A-A
PEAT
0-0
SRH
0.05
Ln
0.00
B NOM source
n
A-A
DSW GSW LEON
c
A-A
PEAT
0-0
0
*-•
v
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X
0.4
--
0.2
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n Y
0.0 0.00
0.05
0.10
0.15
0.20
surface Coverage (rng~/rn2)
Figure 1. (A) Adsorption of NOM from five different sources onto 10.0
g/L Ti02,and (6) the effect of NOM adsorption on TI02-catalyzed MEP hydrolysis. Reaction conditions: 1.7 X acetate).
M MEP, pH 4.8 (1.0 mM
dt = kobs[esterl). Hydrolysisrate constants in the presence of NOM (kobs(X)) and in the absence of NOM (kobe(0)) were determined from the slope of logLester1versus t. The effect of NOM on surface-catalyzed hydrolysis will be reported as the ratio kobs(X)/kobs(O).The smaller this ratio, the greater the inhibitory effect. The loss of catalytic activity can also be defined as 1 - kobs(X)/kobs(0). Results
Hydrolysis in Particle-Free Solutions. A series of experiments was conducted to assess the influence of NOM on the hydrolysis of MEP in homogeneous solution. Different reaction vessels were prepared with an initial MEP concentration of 1.7 X M and increasing concentrations of DSW up to 100 mg of C/L at pHs 5.0 and 7.0. DSW in particle-free solution was found to have no effect on the rates of MEP hydrolysis. Effect of Untreated NOM Samples on TiO2-Catalyzed Hydrolysis. A series of experiments was conducted to assess the influence of NOM from different sources and at different concentrations on TiOz-catalyzed MEP hydrolysis. Suspensions were maintained at pH 4.8, where the extent of NOM adsorption and rates of surfacecatalyzed MEP hydrolysis are near to their maximum measured values (24). The extent of DOC removed by adsorption varied substantially from source to source (values range from 64% to 97%). These variations are reflected in plots of NOM surface coverage (expressed in units of mg of C/m2)as a function of the DOC concentration remaining in solution (Figure 1A). As shown in Figure lB, the observed inhibitory effect (kobs(X)/kobs(O)) is proportional to the NOM surface coverage. (Comparable plots of observed inhibitory effect versus [DOC],, are less
strongly correlated.) At a surface coverage of 0.08 mg of C/m2, there is nearly 60% variation in the magnitude of the inhibitory effect among the five samples. The inhibitory effect is greatest for the two raw water sources (DSW and GSW), followed by the aquatic IHSS sample (SRH). The two soil-derived IHSS samples (LEON and PEAT) yield the smallest inhibitory effect. (Inspection of Figure 1A,B indicates that differences in the inhibitory effect arising from the five NOM sources do not correlate well with DOC concentration remaining in solution.) An experiment was conducted using DSW surface coverages below 0.015 mg of C/m2; no inhibitory effect on surface catalysis was observed. Effect of Prior Fractionation. As discussed earlier, prior contact of NOM-containing water with oxide solids brings about loss of subfractions by adsorption. We have attempted to simulate this process in the laboratory by pretreating 100 mg of C/L of LEON with 1.0 g/L TiO2, FeOOH, and A1203. The pH was adjusted to 5.0 and 7.0 by adding strong acid (HC1) or base (NaOH) at constant ionic strength (0.01 M NaC1). After 24-h equilibration, the supernatant was collected by centrifugation, and its DOC content was analyzed. Each NOM-containing supernatant was then diluted to 40 mg of C/L and sufficient Ti02 was added to yield a 10.0g/L suspension. The NOM/ Ti02 suspensions were then adjusted to pH 4.8 and allowed to equilibrate for 24 h. A 3 mL aliquot was removed at this time, centrifuged, and analyzed using the DOC analyzer. In all cases, less than 2.0 mg of C/L (5 % ) NOM remained in solution. The MEP hydrolysis experiment was begun by adding 1.0 mM acetate buffer and MEP stock solution. The first set of experiments involves NOM pretreatment at pH 5.0. As shown in Figure 2A, 60% of added NOM was adsorbed by 1.0 g/L A1203,in comparison to only 35 % adsorbed by 1.0 g/L Ti02 and FeOOH. Higher removal by A1203 is caused by its high surface area, which is 1.7 times the area of Ti02 and 1.9 times the area of FeOOH. In fact, the NOM surface coverage on a weight-carbon basis is comparable for all three oxides: 0.63-0.70 mg of C/m2. Figure 2B shows the loss in Ti02 catalytic activity arising from the addition of 40 mg of C/L pretreated NOM. Raw LEON caused a 38 % loss in catalytic activity. Despite equal pH (pH = 4.8) and equal NOM surface coverage (0.27-0.28 mg of C/m2),pretreated NOM samples caused muchless loss in catalytic activity: 10 % loss for the A1203treated sample, 12 % loss for the TiO2-treated sample, and only 1% loss for the FeOOH-treated sample. Results from experiments involving NOM pretreatment at pH 7.0 are shown in Figure 3. In agreement with earlier studies of NOM adsorption onto FeOOH (8) and A1203 (9),the extent of NOM adsorption during pretreatment decreases as the pH is increased: 25% of the added NOM was adsorbed by 1.0 g/L A1203,16% by TiO2, and 13% by FeOOH. Equal pH (pH = 4.8) and equal surface coverage (0.27-0.28 mg of C/m2)were once again maintained in all the TiOz-catalyzed MEP hydrolysis experiments. The same trends are observed as in the pH 5.0 experiments, but the results are less pronounced. There is a 22 % loss in catalytic activity for the A12Os-treated sample, a 22% loss for the TiO2-treated sample, and a 16% loss for the FeOOH-treated sample. Environ. Sci. Technol., Vol. 27, No. 12, 1993
2383
1.0
A
-
1.0
I
IA
-
I .
m
0
N
v
c,
c,
m
O I4
30 -
30 cl R
R
:
5
$2
Y
N
v
a
20
5
-
8
N
2
2 0 al
E
m
0
2
a al Y
d
3
Figure 2. LEON pretreatmentexperimentsat pH 5.0. (A) 1.O g/L Ti02, FeOOH, and Ai203 remove different amounts of LEON from solution at pH 5.0. (B) The three treated samples (40 mg of C/L) cause different amounts of TiOpcatalytic activityto be lost (1 k*(X)/k&O)). Condltions M MEP, 10 g/L Ti02, for the MEP hydrolysis experiment: 1.7 X pH 4.8 (1.0 mM acetate).
-
Discussion Rates of MEP hydrolysis may be affected by interaction of ester with dissolved NOM molecules, by interaction of ester with adsorbed NOM molecules, and by NOM interference in the interaction of ester with oxide surface sites. In this section, we will evaluate the experimental findings in light of these possible interactions. Carboxylate and other functional groups within the NOM structure may act as general acid or base catalysts of ester hydrolysis. Perdue and Wolfe (25) provided a "buffer catalysis factor" for dealing with these effects by considering the concentrations of contributing functional groups and their relative nucleophilicities. Their conclusion that NOM should have a negligible effect on neutral and base-catalyzedorganic ester hydrolysis is in agreement with our own observations: NOM addition did not affect MEP hydrolysis in homogeneoussolution. Small amounts of dissolved metals present in raw-water NOM samples (DSW and GSW) also had a negligible effect on MEP hydrolysis NOM addition to oxide suspensionsmay lower the extent of ester adsorption through favorable ester-NOM interaction in solution. Measurements of the extent of MEP adsorption in the presence or absence of NOM could prove or disprove this hypothesis. Unfortunately, MEP adsorption under the experimental conditions employed is
.
2384
Envlron. Sci. Technol., Vol. 27, No. 12, 1993
Figure 3. LEON pretreatmentexperimentsat pH 7.0. (A) NOM removal by 1.0 g/L TiO2, FeOOH, and A1203. (B) The three treated samples (40 mg of C/L) cause different amounts of Ti02 catalytic activity to be lost. Conditions for the MEP hydrolysis experiment:1.7 X M
below the detection limit of our analytical method ( I , 2). Significant inhibitory effects have, however, been observed in the absence of appreciable dissolved NOM concentrations. More importantly, the magnitude of the inhibitory effect increases in proportion to the NOM surface coverage for all five NOM samples, as shown in Figure 1B. We can therefore conclude that adsorbed NOM, not dissolved NOM, is responsible for the inhibitory effects observed in our experiments. Our earlier work ( 1 , 2 )indicated that surface site-ester chelate formation is necessary for surface-catalyzed hydrolysis to take place. We can therefore speculate that inhibition arises from (i) association between adsorbed ester and adsorbed NOM that impedes chelate formation; (ii) occupation of oxide surface sites by NOM functional groups; or (iii) steric or electrostatic effects that impede the approach of ester and nucleophile toward catalytic surface sites. The three untreated IHSS samples (LEON, PEAT, SRH) contain a high humic acid content. Raw surfacewater samples (i.e.,DSW,GSW) containamixtureoffdvic acids and hydrophilic organic acids (20) with lesser amounts of humic acids. These samples are therefore likely to possess more carboxylate and other functional groups and less aromaticity on a weight-carbon basis (22). Increased numbers of functional groups should occupy more surface sites through a ligand exchange-surface complexation mechanism (11). The effects of a lower degree of aromaticity are more difficult to evaluate. Such
changes may affect the compactness of NOM molecules and the oxide surface area occupied per molecule and may alter the magnitude of hydrophiliclhydrophobic interactions at the oxide/water interface. Whatever underlying phenomena are responsible, the raw-water samples exhibit a greater inhibitory effect on a weight-carbon basis than the more humic acid-rich samples (Figure 1B). In the pretreatment experiments, net NOM removal is primarily controlled by differences in available surface area: NOM surface coverage in mg of C/m2 is approximately the same on all three oxides. Various pretreated NOM samples exhibit substantial differences in their effect on TiO2-catalyzed MEP hydrolysis. Although A1203 removes 1.7 times the amount of NOM removed by Ti02 at pH 5.0,the effect on inhibition is nearly equal. Although FeOOH removes the same amount of NOM as TiO2, it nearly eliminates the inhibitory effect. We can conclude that FeOOH is extremely efficient at removing the NOM subfraction responsible for the inhibitory effect. Ti02 is moderately efficient, and A1203 is the least efficient of the three oxides. Differences in surface structure or hydrophilicity may be partially responsible for variations in selectivity among the three oxides. Ti02 is a more acidic surface than FeOOH and A1203,affecting the protonation behavior of the surface and determining the magnitude of solute-surface electrostatic interactions. The greatest source of variation, however, probably arises from differences in the surface complexation behavior of A P , Ti1”, and FelI1. All three are classified as “hard” surface-bound metals, denoting a strong ionic contribution toward metal-ligand bond formation. Transition metals experience an additional covalent contribution to bond formation, which explains why complex formation constants for FeIn typically surpass those of A P f o r carboxylate, phenolate, and other oxygendonor ligands (see ref 26) and for nitrogen-donor ligands (27). We reported earlier (2) that adsorption of low molecular weight organic acids possessing metal coordination properties most similar to the hydrolyzable ester had the greatest inhibitory effect on TiO2-catalyzed hydrolysis. Consider the properties listed below: ligand
MEP picolinate salicylate
donor groups N-donor (pyridyl) 0-donor (carbonyl) N-donor (pyridyl) 0-donor (carboxylate) 0-donor (phenolate) 0-donor (carboxylate)
chelate ring size 5-atom ring 5-atom ring
teristic. In order to confirm this theory, extensive functional group information concerning inhibitory and noninhibitory subfractions would be required; the ability of each of the three oxides to remove particular functional groups would need to be evaluated. Inhibition of Surface-Catalyzed Hydrolysis Reactions in the Environment. The composition and abundance of NOM depend upon inputs of aromatic compounds (e.g., lignins and terpenoids from vascular plants) and aliphatic compounds (e.g., carbohydrates), upon the chemical and biological reworking of this material, and upon rates of mineralization to CO2. The five NOM samples included in this study exhibit significant differences in their adsorption behavior onto oxide surfaces. At fixed surface coverageexpressed on a weight-carbon basis, the inhibitory effect varies by nearly 60%. Based upon this preliminary survey, ecosystem-to-ecosystemvariations in the inhibitory effect arising from NOM adsorption are expected to be quite significant. As mentioned earlier, selective adsorption of particular NOM subfractions has been observed in both hydrous oxide-laden streams (11) and aquifers (13). Evidence indicates that transport of dissolved NOM through soils and aquifer sediments should preferentially remove NOM subfractions that adsorb strongly to mineral surfaces. The pretreatment experiments reported here demonstrate that a portion of the adsorbable NOM has a particularly high potential for inhibiting surface-catalyzed hydrolysis reactions. This most inhibitory subfraction would be removed during the passage of NOM-laden waters through FeOOH-rich subsurface material. Passage through A1203rich subsurface material, in contrast, would remove less of this inhibitory NOM subfraction. Fractionation processescause the properties of dissolved and adsorbed NOM to be substantially different from one another. In addition, many aquifers contain detrital organic matter laid down with the original sediments. In order to evaluate the importance of surface-catalyzed hydrolysis in soils and aquifers, we must therefore collect and evaluate the inhibitory effect of both dissolved and particulate-bound NOM. Acknowledgments This work was supported by the Water Resources Research Program of the U.S.Geological Survey (Grant 14-08-0001-G1647) and by the Office of Exploratory Research of the U S . Environmental Protection Agency (Grant R-818894-01-0). The “Ministerio de Educacion y Ciencia” of Spain provided fellowship support for A.T.
6-atom ring
At equal surface coverage (0.18 pmol/m2),picolinate caused a 79 % loss in Ti02 catalytic activity while salicylate caused only a 51 % loss in catalytic activity. Ti02 and other oxide surfaces are believed to contain structurally and chemically distinct sites which exhibit different catalytic activity (2). In comparison to salicylate, picolinate apparently adsorbs more strongly to the subfraction of surface sites most responsible for the catalysis of MEP hydrolysis. We can theorize that pretreatment with Ti02 and FeOOH selectively removes NOM molecules possessing the greatest capacity for binding catalytic sites on the Ti02 surface. A1203 is not catalytic and, therefore, does not contain surface sites exhibiting this selectivity charac-
Literature Cited Torrents, A.; Stone, A. T. Environ. Sci. Technol. 1991,25, 143. Torrents, A.; Stone, A. T. Enuiron. Sci. Technol. 1993,27, 1060. Thurman, E. M. Organic Geochemistry of Natural Waters; Nijhoff/Junk Publishers: Boston, 1985. Murphy, E. M.; Zachara, J. M.; Smith, S. C. Environ. Sci. Technol. 1990, 24, 1507. Davis, J. A. Geochim. Cosmochim. Acta 1982, 46, 2381. Hunter, K. A. Limnol. Oceanogr. 1980,25, 807. Davis, J. A. In Contaminants and Sediments; Baker, R. A. Ed.; Ann Arbor Science: Ann Arbor, MI, 1980; Chapter 15, pp 279-304.. Tipping, E. Geochim. Cosmochim. Acta, 1981, 45, 191. Davis, J. A.; Gloor, R. Environ. Sci. Technol. 1981,15,1223. Environ. Sci. Technol., Voi. 27, No. 12, 19Q3 2385
(10) Kummert, R.; Stumm, W. J . Colloid Interface Sci. 1980, 75, 373. (11) McKnight, D. M.; Bencala, K. E.; Zellweger, G. W.; Aiken, G. R.; Feder, G. L.; Thorn, K. A. Environ. Sci. Technol. 1992, 26, 1388. (12) Dunnivant, F.M.; Jardine, P. M.; Taylor, D. L.; McCarthy, J. F. Soil Sci. SOC.Am. J. 1992,56, 437. (13) McCarthy, J. F.; Williams, T. M.; Liang, L.; Jardine, P. M.; Jolley, L. W.; Taylor, D. L.; Palumbo, A. V.; Cooper, L. W. Environ. Sci. Technol. 1993, 27, 667. (14) Baccini, P.;Grieder, F.; Stieri, R.; Goldberg, S. Schweiz. 2. Hydrol. 1982, 44, 99. (15) Gibbs, R. J. Environ. Sci. Technol. 1983, 17, 237. (16) Liang, L.; Morgan, J. J. In Chemical Modeling of Aqueous Systems II; Melchior, D. C., Bassett, R. L., Eds.; ACS Symposium Series 416; American Chemical Society: Washington, DC, 1990; Chapter 23. (17) Stone, A. T. J. Colloid Interface Sci. 1989, 132, 81. (18) Black, A. P.; Christman, R. F. Am. Water Works Assoc 1963, 55, 753. (19) Gu, B.; Schmitt, J.; Chen, Z.; Liang, L.; McCarthy, J. F. Environ. Sci. Technol., submitted for publication. (20) Aiken, G. R.;McKnight, D. M.; Thorn, K. A.; Thurman, E. M. Org. Geochem. 1992, 18, 567. (21) Malcolm, R. L.; Aiken, G. R.; Bowles, E. C.; Malcolm, J. D. In Humic Substances in the Suwannee River, Georgia:
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Interactions, properties, and proposed structures; Averett, R. C., Leenheer, J. A,, McKnight, D. M., Thorn, K. A., Eds.; Open-File Report 87-557;US.Geological Survey: Denver, 1989;Chapter B. (22) Stevenson, F. J. In Humic Substances in Soil, Sediment, and Water: Geochemistry, Isolation, and Characterization; Aiken, G. R., McKnight, D. M., Wershaw,R. L., MacCarthy, P., Eds.; John Wiley & Sons: New York, 1985;Chapter 2, pp 13-52. (23) Hernandez, T.; Moreno, J. 1.; Costa, F. Geoderma 1989,45, 83. (24) Torrents, A. P b D . Thesis, Johns Hopkine University, Baltimore, MD, 1992. (25) Perdue, E. M.; Wolfe, N. L. Environ. Sci. Technol. 1983,17, 635. (26) Morel, F. M. M.; Hering, J. G. Principles and Applications of Aquatic Chemistry;Wiley-Interscience: New York, 1993; Chapter 6, pp 338-342. (27) Evers, A.; Hancock, R. D.; Martell, A. E.; Motekaitis, R. J. Inorg. Chem. 1989,28, 2189-2195. Received for review November 30, 1992. Revised manuscript received June 28, 1993. Accepted July 15, 1993.' Abstract published in Advance ACS Abstracts, September 15, 1993. @
