Binding of 4-Monochlorophenol to Soil - American Chemical Society

4-monochlorophenol (4-MCP) in the presence of oxygen possibly as a result of covalent binding. Sorbed. 4-MCP was subjected to desorption by consecutiv...
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Environ. Sci. Technol. 1996, 30, 2305-2311

Binding of 4-Monochlorophenol to Soil A L O K B H A N D A R I , * ,†,‡ JOHN T. NOVAK,‡ AND DUANE F. BERRY§ Department of Civil Engineering and Department of Crop and Soil Environmental Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061

Halogenated phenolic compounds such as chlorophenols are capable of binding to organic matter during the humification processes in soil. The incorporation of phenols into organic macromolecules, i.e., covalent bond formation, results from biologically or chemically catalyzed reactions. Standard batch sorption experiments performed in our laboratory showed a significant enhancement in soil associated 4-monochlorophenol (4-MCP) in the presence of oxygen possibly as a result of covalent binding. Sorbed 4-MCP was subjected to desorption by consecutive water and solvent extractions. Nearly 15% of the soil-associated 4-MCP remained unextracted. The unextracted [U-ring-14C]-4-MCP consisted of a fraction incorporated into humic and fulvic acids and a second fraction associated with humin or soil mineral surface and recovered as 14CO2 during soil combustion. Autoclaving the soil before 4-MCP addition resulted in a reduction in the amount of the nonextractable 14C. The addition of H O caused a 4.4-fold increase 2 2 in 4-MCP binding. Hydrogen peroxide, which appears to enhance both biological and abiotic coupling processes, can be added to soil as a stimulant of oxidative coupling activity.

Introduction Monochlorophenol has been used as an intermediate in the manufacture of di- and trichlorophenols that find industrial application in the production of wood preservatives, pesticides, and herbicides (1). These xenobiotics have been detected at a number of locations and pose a serious threat to soil, surface water (2), and groundwater quality (3). Soil organic matter (SOM) has been implicated as the fundamental factor controlling the fate and transport of hydrophobic organic contaminants (HOCs) in soil and the * Corresponding author. † Present address: 181 EWRE, Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, MI 481092125. Telephone: (313) 936-3067; fax: (313) 763-2275; e-mail address: [email protected]. ‡ Department of Civil Engineering. § Department of Crop and Soil Environmental Sciences.

S0013-936X(95)00691-2 CCC: $12.00

 1996 American Chemical Society

subsurface environment (4-6). The process of SOM formation, extent of weathering, aromaticity or polarity of SOM, as well as the hydrogen/oxygen and hydrogen/carbon atomic ratios of SOM can result in preferential sorption of certain HOCs over others (7, 8). Karichkoff (9) described other factors such as pH, particle size, soil ion-exchange capacity, and clay mineral content as having secondary importance in sorption processes in soils. Stone (10) however demonstrated that the interaction of phenolic contaminants with soil mineral oxides such as Mn(III/IV) can significantly affect their fate and transport. Solution ionic strength and pH can be important in the case of ionogenic organic contaminants such as phenols (11, 12). Although sorption data are often modeled using linear isotherms, researchers have also used nonlinear Freundlich isotherms to fit experimental data while studying sorption to soil (13-15). The nonlinearity in adsorption/desorption equilibria as applied to soil systems has been explained in terms of a two-compartment model (16), radial pore diffusion models (17, 18), and a distributed reactivity model (19). DiToro and Horzempa (20) and Pavlostathis and Jaglal (21) have demonstrated the existence of desorption hystereses for polychlorinated biphenyls and trichloroethylene. Irreversible sorption of organic contaminants has been noted on soils (22, 23) and activated carbon (24-26). Xenobiotics that have entered the soil enviroment have been seen to participate in oxidative coupling reactions with SOM, a process that is similar to humic formation from naturally occurring phenolic components (27, 28). The persistence of some pesticides and polycyclic aromatic hydrocarbons in soil has been attributed to their chemical attachment to SOM (23, 29). Various researchers have studied the incorporation of chlorophenols into humic polymers in model systems (30-32). Incubation of phenols in the presence of soil enzymes such as peroxidases, lacasses, and tyrosinases has been shown to produce cross-coupled products (33-36). Sjoblad and Bollag (37) suggested that fungal lacasses play the most important role in the synthesis of soil humic materials derived from phenolic monomers. These Cu-containing enzymes utilize molecular oxygen (O2) and phenolic compounds as electron donors to produce covalent linkages between the phenols. Soil peroxidases also catalyze oxidative coupling reactions but have absolute requirement for H2O2 for activity (38).

14C-labeled

Dove and Novak (39) and Ulrich and Stone (40) showed that transition metal oxides on soil surfaces are capable of retaining high levels of the organic contaminant. Stone and co-workers illustrated that manganese (III/IV) oxides may be responsible for catalyzing abiotic coupling of substituted phenols to SOM (10, 40). At high pHs and in the presence of O2, covalent linkages between phenolic monomers can occur spontaneously as a result of autoxidation. In spite of extensive research with model systems, oxidative coupling of chlorophenols by biotic and abiotic mechanisms has not been thoroughly investigated in natural soils. Knowledge of the binding behavior of MCP in soil systems is essential in determining the compound’s fate in surface soils and in selecting the appropriate

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TABLE 1

Properties of High Organic Matter (HIOM) and Low Organic Matter (LOOM) Soils of the Lakeland Series Collected from Surry County, Virginia (from Burgos et al. (41)) HIOM soil textural class particle size distribution (%) sand silt clay organic matter contenta (%) pH surface areab (m2/g) pore volumeb (cm3/g) av pore radiusb (Å) % surface oxide contentc (mol of oxide/mol of surface)

LOOM soil

sand

sand

89.9 8.9 0.7 3.20 3.80 1.11 0.0029 52.6 5.0

86.8 8.6 1.9 0.52 4.80 1.84 0.0064 69.0 5.2

a Determined by loss-on-ignition at 430 °C. b Measured by N BET 2 analysis with 0.05 P/Po increments between P/Po ) 0.10 and 0.30. c Measured with scanning electron microscopy and X-ray spectrometry (42).

TABLE 2

Properties of Organic Matter Extracted from High Organic Matter (HIOM) and Low Organic Matter (LOOM) Soils Collected from Surry County, Virginia type of soil organic matter LOOM soil fulvic acids humic acids HIOM soil fulvic acids humic acids

absorbance-270a

E4/E6b

0.50 1.01

4.20 5.65

1.40 2.23

4.61 6.87

a Absorbance at 270 nm using 1:10 sample dilution; general measure of the amount of extracted organic matter; from Schnitzer (43). b Spectrophotometric ratio used to distinguish between humic and fulvic acids. For fulvic acids, E4/E6 > 7.6; for humic acids, E4/E6 < 5.8. E4/E6 ) ratio of absorbances at 465 and 665 nm; from Schnitzer (43).

remediation technology for a particular site. The purpose of this research was to investigate the role of oxygen and hydrogen peroxide in the biotic and abiotic binding of 4-MCP to high and low organic matter sandy surface soils.

Materials and Methods Soils. The two soils (both thermic coated Typic Quartzipsamment belonging to the Lakeland series) were collected from Surry County, Virginia. These soils were selected after a thorough review of soil survey data classified them as having been derived from the same parent material and having undergone a similar process of soil genesis. One of the soils, the high organic matter (HIOM) soil, was obtained from the A horizon of a beech/oak forested area and had an organic carbon content of 3.20%. The second soil, the low organic matter (LOOM) soil, was collected from the Ap horizon of an adjacent cropland and contained 0.52% SOM. This method of soil selection minimized variations in the mineral composition of the soil and its particle size distribution. The soils were transported and stored in separate air-tight Coleman box-coolers. Some general properties of the soils and extracted organic matter are listed in Tables 1 and 2. Organic matter was extracted from the soil using the method described by Page (44). Each soil was sieved through an ASTM No. 18 (1-mm opening) sieve to remove large pieces of roots and pebbles

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and to obtain a constant maximum grain size. To control microbial and fungal activity, portions of the soils were autoclaved for 30 min at 25 psi and 120 °C four times over a 10-day period. Multiple autoclaving is effective in killing soil microflora and at the same time does not alter the physical and chemical properties of the soil to a great extent (45). Chemicals. The chlorophenol including [U-ring-14C]4-MCP was purchased from the Sigma Chemical Co. (lots 119F3531 and 128F9211). Labeled 4-MCP had a purity of >99% and a specific activity of 7.4 mCi/mmol. Standard Batch Adsorption Experiments. Adsorption experiments were conducted at 25 ( 2 °C in 8-mL culture tubes containing 4 g (dry wt) of either HIOM or LOOM (autoclaved or nonautoclaved) soils. All glassware were previously acid washed and autoclaved. The soil was equilibrated for a contact time of 2 day with solutions having initial 4-MCP concentrations (Ci) of 8, 31, 62, 78, 310, 620, and 780 mM. Known amounts of [U-ring-14C]-4-MCP were added to the nonradioactive 4-MCP solutions. Three replicates were used for each value of Ci. The pH of the solutions was adjusted to 5.0 ( 0.2 using HCl. Sorption experiments were conducted in oxic conditions, i.e., in the presence of atmospheric or dissolved oxygen, and in anoxic conditions, i.e., in a glovebox from which O2 had been purged. Furthermore, we studied the effect of H2O2 addition on the extractability of 4-MCP by introducing small amounts of H2O2 in the culture tubes at the beginning of the sorption study. Oxic Environment. In the oxic sorption experiments, molecular oxygen was available in the tube headspace or as dissolved oxygen in solution. A 6-mL sample of the desired 4-MCP solution was transferred into tubes containing nonautoclaved and autoclaved soils. An equal volume of solution was also transferred into a tube containing no solids, which served as the abiotic control. Three replicates were incorporated within each treatment. The tubes were capped tightly with Teflon-lined screw caps, and their contents were mixed vigorously for 2 min using a vortex mixer. The tubes were then placed on a shaker table for 2 day. A 2-day contact time was chosen after a preliminary study conducted under identical conditions established that the lag phase for MCP biodegradation in nonautoclaved systems was greater than 2 day. Results of the preliminary study had shown that for nonautoclaved soils, approximately 2.5% of the activity was recovered as 14CO within the 2-day contact period. Mineralization was 2 negligible in autoclaved soils. A 2-day contact period also assumed that, within this time, the initial rapid phase sorption to readily available external binding sites was complete (46), while the slow phase diffusion-limited adsorption at intraparticle sites was kept at a minimum. After the 2-day contact time, tubes were removed from the shaker and centrifuged at 1600g. A 1-mL aliquot of the supernatant from each tube was placed in Scintiverse BD scintillation cocktail. The activity in the sample was determined using a LS-3150T Beckman liquid scintillation counter (LSC). Each sample was analyzed three separate times to minimize variations in the scintillation counting procedure. The soil associated contaminant (qs) was calculated by subtracting the activity remaining in solution from the total activity added and using data from control tubes to adjust for abiotic losses. Biotransformation of the contaminant during the 2-day contact period was moni-

tored using thin layer chromatography (TLC). This information was used to correct for biotic losses. Experiments with H2O2 were designed to study the effect of peroxide addition on irreversible binding. All experimental conditions were kept identical to the oxic system with the exception being the addition of 3 mL of a 30% H2O2 solution to each soil tube. In an earlier work performed in our laboratory, Farmer (47) had shown that this concentration of H2O2 was capable of providing the desired oxygen for biological processes without chemically oxidizing the contaminant. Anoxic Environment. The experimental steps for the anoxic study were identical to the oxic study with the exception that the anoxic adsorption experiments were conducted in a controlled environment inside a N2-filled plexiglass glovebox. All solutions and soil were placed in the glovebox, and the box was evacuated with O2-free N2 for 24 h. Before entry into the glovebox, the N2 was allowed to pass through a column packed with an O2-scavenging catalyst (Oxytrap, Alltech Assoc. Inc., Deerfield, IL) followed by an indicator column capable of detecting O2 in the gas exiting the oxytrap. This setup ensured that O2-free N2 entered the glovebox. The inside of the glovebox was maintained at a positive pressure. The soil and solutions were purged with O2-free N2 within the glovebox. A resazurin indicator solution placed inside the glovebox was monitored to ensure the absence of O2. At the end of 2 day, the tubes were removed from the glovebox and centrifuged at 1600g, and the activity in the supernatant was enumerated in the LSC. Desorption Experiments. To evaluate the desorption characteristics of 4-MCP, the soils contacted with initial solute concentrations of 78 and 780 mM were centrifuged and sequentially extracted with synthetic groundwater solution and methylene chloride, CH2Cl2. The synthetic groundwater was made from distilled-deionized water by adjusting the ionic strength to 10 mM using CaCl2. The pH of the solution was maintained at 5.0 ( 0.2. Consecutive desorptions were performed by multiple fill-and-draw extractions. The supernatant was removed, and the tubes were refilled with 5 mL of synthetic groundwater. The soil and water in the tube were mixed vigorously. The contents of the tube were then allowed to equilibrate for periods varying from 5 min for the first extraction to approximately 5 h for the final extraction before being removed and replaced. The tubes were centrifuged, and the activity in the supernatant was counted in the LSC. The soil was subjected to 25 extractions with synthetic groundwater followed by 7 extractions with 3 mL of CH2Cl2. The amount of 4-MCP removed during extraction by water and CH2Cl2 was defined as extractable, qx, while the amount remaining on soil after extraction was considered bound or nonextractable, qnx. The nonextractable contaminant was quantified by combusting the soil for 4 min at 925 °C in a biological material oxidizer (OX-500, R.J. Harvey Instrument Co., Hillsdale, NJ) and capturing the 14CO produced. One set of soils was combusted after 2 organic matter from the soil had been extracted with 0.1 N NaOH, while a second set was combusted without removing SOM. The difference in the 14CO2 collected in the two cases was a measure of the 14C activity associated with the alkali soluble humic and fulvic acids. A mass balance on the total activity described the exact distribution of the contaminant among the different phases.

FIGURE 1. Sorption isotherm for 4-monochlorophenol (4-MCP) on high organic matter (HIOM) soil under oxic conditions.

FIGURE 2. Consecutive desorption data for 4-MCP sorbed on HIOM soil. The two desorption curves shown here are for soils equilibrated with 4-MCP initial aqueous concentrations, Ci, of (A) 78 and (B) 780 µM.

Results Influence of O2 on 4-MCP Binding. The sorption isotherm of 4-MCP on HIOM soil generated under oxic conditions at a constant sorbent dose and a variable sorbate concentration is shown in Figure 1. Sorption appeared to be near linear with Freundlich KF and n values of 20.2 and 0.95, respectively, and an R2 equal to 0.99. Figure 2 shows the desorption curves generated for soils equilibrated with 4-MCP solutions of Ci ) 78 (A) and 780 (B) µM when the soils were extracted with synthetic groundwater. The qs values after a 2-day contact period were 54 ( 3 and 405 ( 8 mmol/kg (mean ( standard deviation, n ) 3) for Ci values

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TABLE 3

Analysis of Variance (ANOVA) Results Show That Both Treatment (Autoclaved vs Nonautoclaved) and Environment (Oxic vs Anoxic) Had Significant Effects on Nonextractable Binding, qnx, of 4-MCP to HIOM Soilsa Ci ) 78 µM

treatment environment environment × treatment

Ci ) 780 µM

F value

Pr > F

F value

Pr > F

209.61 48.56 34.40

0.0001 0.0001 0.0004

52.38 13.10 NAb

0.0001 0.0056 NAb

a The effect was assumed to be significant with 95% certainity if Pr > F was less than 0.05. Ci ) initial 4-MCP concentration. b Data not available.

FIGURE 3. Recovery of sorbed 4-MCP from HIOM soil, (Ci ) 78 µM). Recoveries are expressed as a percentage of the total sorbed 4-MCP (qs). Actual recoveries were >90%. The alkali fraction represents the humic/fulvic associated 14C activity. The combusted fraction represents the alkali insoluble activity associated with humin and the soil mineral surface. N-OX ) nonautoclaved-oxic; A-OX ) autoclaved-oxic; N-AN ) nonautoclaved-anoxic; A-AN ) autoclaved-anoxic.

of 78 and 780 mM, respectively. As opposed to the near linear sorption of 4-MCP on HIOM soil, the desorption was nonlinear, nonsingular, and hysteretic, i.e., open-looped or not 100% complete. We propose that a significant portion of the contaminant retention observed in Figure 2 was due to oxidative coupling reactions catalyzed by atmospheric O2. Nearly one-third of the soil-associated 14C residue in the HIOM soil could not be recovered in the 25 water extractions. Following the water extractions, the amount of soil-associated 4-MCP was 21 ( 3 and 158 ( 8 mmol/kg for Ci ) 78 and 780 mM, respectively. The soils were further extracted with CH2Cl2 to remove any 4-MCP that may have been physically sorbed on SOM or the soil mineral surface but was resistant to extraction by the synthetic groundwater. We contend that a major portion of 4-MCP remaining after 25 water and 7 solvent extractions was covalently bound to the soil. Such nonextractable binding of 4-MCP to soil provided an explanation for the incomplete desorption of 4-MCP. The amounts of 4-MCP that remained bound to soil after solvent extractions were 14 ( 3.1 mmol/kg for Ci ) 78 mM and 109 ( 20 mmol/kg for Ci ) 780 mM. To strengthen our argument that O2 participated in reactions resulting in the covalent binding of 4-MCP to soil, we conducted experiments in both oxic and anoxic environments. Autoclaved and nonautoclaved soils were used to distinguish between coupling processes mediated by biological or abiotic catalysts. Autoclaving the soil was thought to deactivate extracellular enzymes such as soil lacasses and peroxidases that participate in biological oxidative coupling reactions (33, 37, 38). Extraction of 4-MCP from the nonautoclaved and autoclaved HIOM soil was compared in oxic and anoxic systems (Figure 3). Based on a mass balance approach, the total 14C activity recoveries ranged from 96.3% in the nonautoclaved oxic system to 98.7% in the autoclaved anoxic system. The recoveries illustrated in Figure 3,

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however, are expressed as a percentage of the measured soil-associated 14C activity, qs. The stacked bars show the contaminant recovered during sequential water and solvent extractions, alkali extraction, and combustion at 950 °C, all being expressed in percentages of qs. Although water and solvent extractions were able to remove a major portion of the soil-associated 4-MCP, we observed that a considerable amount of 14C activity remained associated with the alkalisoluble humic and fulvic acids. This observation suggested intraorganic matter retention of the contaminant or the existence of strong intermolecular interactions between the humic/fulvic acids and 4-MCP molecules. Recovery of 14CO during combustion of the alkali-extracted soil indi2 cated that significant amounts of 14C residue also remained associated with the alkali-insoluble organic matter or soil mineral surface. In this study, we have defined the total nonextractable 4-MCP, qnx, as the sum of the 14C activity in the alkali-soluble SOM and that recovered as 14CO2 from the alkali-extracted soils. Approximately 15% (shown as the sum of the alkali and combusted fractions in Figure 3) of the soil-associated 14C was nonextractable from the nonautoclaved soil in the oxic system. This resistant portion was reduced to about 7% of qs when no O2 was present (nonautoclaved anoxic system). This reduction in nonextractability in the anoxic vs oxic system appears to be a strong indicator of the role played by O2 in the binding of 4-MCP to soils. Burgos (42) found that a noncouplable aromatic compound such as naphthalene underwent near complete desorption from soil, while its hydroxylated metabolic intermediate, R-naphthol, did not desorb completely during multiple extractions with ethyl acetate. Similar to Burgos’ observations with R-naphthol (42), we found that autoclaving the soil reduced the bound fraction of 4-MCP in the oxic system from 15% of qs to 7% of qs. The analysis of variance (ANOVA) statistical test for the bound 4-MCP indicated that the environment (oxic vs anoxic) as well as the treatment (autoclaved vs nonautoclaved) had statistically significant effects on binding (Table 3). Multiple comparisons using the Tukey test showed significant effects (at R ) 0.5) of the environment and treatment on nonextractable binding of 4-MCP (Table 4). Only in the case of autoclaved soil was there no significant difference in qnx between oxic and anoxic conditions. These results indicate that in the case of HIOM soil, more than half of the nonextractable binding of 4-MCP occurred when oxygen was present and soil enzymes had not been deactivated by autoclaving (nonautoclaved oxic system).

TABLE 4

Significance of Treatment (Autoclaved vs Nonautoclaved) and Environment (Oxic vs Anoxic) and Treatment/Environment Interactions As Determined by the Standard Tukey Test for 4-MCP Binding on HIOM Soilsa

a S ) significant difference in q ; NS ) nonsignificant difference in nx qnx. The table is valid for 4-MCP Ci values of 78 and 780 µM.

TABLE 5

Nonextractable Binding of 4-MCP on Autoclaved and Nonautoclaved Soils, in Anoxic Conditions and in Presence of O2 or H2O2: Comparison between High (HIOM) and Low Organic Matter (LOOM) Sandy Surface Soils nonextractable 4-MCP, qnx (µmol/kg) system configuration autoclaved anoxic nonautoclaved anoxic autoclaved oxic nonautoclaved oxic autoclaved peroxide nonautoclaved peroxide a

Mean with n ) 3.

b

LOOM soil

HIOM soil

( 5.3 ( 0.2 2.8 ( 0.2 9.9 ( 1.1 31.4 ( 2.2 36.0 ( 2.2

1.3 ( 0.1 3.5 ( 0.2 3.8 ( 0.9 7.7 ( 2.3 21.1 ( 0.2 34.1 ( 1.1

2.6a

0.2b

Standard deviation.

Elimination of either of these factors reduced the fraction of 4-MCP retained on the soil from 15% to 7% qs. This pattern indicates that approximately 50% of the binding observed in the HIOM soil was possibly due to biologically mediated oxidative coupling reactions. When both O2 and soil bioactivity were eliminated (autoclaved anoxic system), nearly 97% of the soil-associated 4-MCP could be recovered by combined water and solvent extraction. Significant amounts of contaminant retention observed in the oxic autoclaved and the anoxic nonautoclaved systems (Figure 3), however, could not be explained by this hypothesis. Monochlorophenol has been previously shown to oxidatively couple to humic and fulvic acids (31, 34), and noncouplable organics have been found to desorb nearly completely (∼98%) from soils (42, 48) and granular activated carbon (49). Based on such evidence and the significant differences in nonextractable binding observed between oxic and anoxic environments and autoclaved and nonautoclaved soils, we believe oxidative coupling to be the factor largely responsible for the nonextractable binding of 4-MCP to the soil. When the LOOM soil was contacted with 4-MCP, nearly twice as much MCP was retained by the soil in the presence of O2 (nonautoclaved oxic system) than when O2 had been removed (nonautoclaved anoxic system) (Table 5). The amount of bound 4-MCP was not significantly different in the oxic and anoxic autoclaved systems. This amount, however, was approximately 50% of the 4-MCP bound in the nonautoclaved anoxic system. It is likely that more

coupling occurred in nonautoclaved anoxic soils than in the autoclaved ones because autoclaving may have attenuated some of the catalyzing capability of mineral surfaces by reducing surface metal oxides that participate in contaminant binding in their oxidized states. These oxides can participate in electron/proton transfer reactions in which the phenolic contaminant is oxidized to a phenoxy radical while at the same time the metal oxide is reduced. Autoclaving the soil could have reduced the surface metal oxides such that their participation in surface redox reactions is minimized. However, it is also possible that binding in the nonautoclaved anoxic systems may have occurred if traces of O2 had remained in the culture tubes. Effect of Organic Matter Content. The two soils used in this study were similar in particle size distribution and surface mineral oxide concentration but very different in terms of the SOM content. As expected, qs was greater for the HIOM soil (54 ( 3 mmol/kg) as compared to the LOOM soil (15.7 ( 1.7 mmol/kg) in nonautoclaved oxic systems. The higher qs for HIOM is consistent with observations made by other researchers (4-6), who implicate the organic matter fraction (fom) of soil as the fundamental soil property controlling adsorption behavior of HOCs to soils. Although qs appeared to be a function of the fom of soil, data in Table 5 reveals that qnx was not controlled by the organic matter content of the soil. The nonextractable or bound MCP in LOOM oxic nonautoclaved soil was not significantly different from qnx in the HIOM oxic nonautoclaved soil on a mole of contaminant per unit mass of soil basis. However, if expressed as per unit weight of organic carbon (not listed in Table 5), the LOOM soil clearly appears to bind more contaminant. Based on the arguments of researchers who hold fom of soil as the primary factor influencing adsorption (4-6), we expected the HIOM soil to bind more 4-MCP than the LOOM soil. Although Murphy et al. (50) have stated that relatively low fom values (∼0.1%) can effectively cover soil mineral surfaces and control soil-HOC interactions, the properties of the soil mineral surface become increasingly important as fom decreases. Pavlostathis and Jaglal (21) have suggested that, for soils with low fom, partitioning is not a function of SOM alone; physical adsorption to mineral surfaces can contribute significantly to partitioning of the HOC on to such soils (9). Burgos (42) used scanning electron microscopy and X-ray dispersive techniques to qualitatively study the surfaces of the LOOM and HIOM soils used in this study. Results indicated the presence of oxides of Fe, Ti, and Mn on both soils. It is postulated that these metal oxides could be responsible for the chemisorption (e.g., hydrogen bonding) or oxidative coupling of 4-MCP to the LOOM soils. Comparison with light microscopy showed that in the case of HIOM soils, a large portion of the mineral surface was covered with organic matter, reducing the amount of surface metal oxides exposed and thus limiting their participation in sorption or binding processes. In their studies with surfaces coated with manganese(III/IV) oxides, Ulrich and Stone (40) showed that these oxides were capable of catalyzing the abiotic oxidative coupling of chlorinated phenols to SOM. It is possible, therefore, that the binding of 4-MCP to the LOOM soil may have resulted from a combination of enzymatic oxidative coupling and simple chemisorption on mineral surfaces with subsequent catalysis and formation of covalent bonds between SOM and the phenol.

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Slow desorption of the contaminant from intraparticle sites (18) may be another possible reason for the high contaminant retention observed for the LOOM soil. Larger amounts of intraparticle/intragranular sites are exposed to the aqueous solution in low fom soils when compared to soils with high organic matter content. In such cases, the sorbate is able to diffuse deep into intraparticle sites. During desorption, the contaminant can be physically trapped in these sites or strongly adsorbed to minerals and surface metal oxides in the intragranular pore spaces. For the physically trapped contaminant, desorption from such sites may occur in the order of tens or even hundreds of days (18). In a short time scale desorption study such as this investigation, this trapped contaminant may be nonextractable and appear to be covalently bound. It is therefore possible that, in addition to the SOM content, the binding of 4-MCP on soil can be affected by the physical characteristics of the mineral surface and the metal oxide coverage. The larger retention of 4-MCP on LOOM soil may have occurred because of greater participation of surface metal oxides in oxidative coupling of the phenol to organic matter as well as due to the retention of larger amounts of 4-MCP at intraparticle sites and soil micropores. When nonautoclaved HIOM and LOOM soils were contacted with the 78 mM 4-MCP solution in oxic conditions after SOM had been removed by combustion of the soils at 550 °C for 24 h, the amount of nonextractable 14C on the soils was found to be nearly identical, i.e., 1.43 ( 0.06 and 1.66 ( 0.22 mmol/kg for the HIOM and LOOM soils, respectively. The retention of 4-MCP on SOM-free soils could be attributed to rate-limited diffusion of the sorbate from soil micropores or to physisorption at intraparticle sites. Hydrogen bonding is thought to be the principal mechanism of association between phenolic OH groups and soil minerals (51). The contribution of hydrogen bonding to overall binding can however be neglected because the low electronegativity of mineral surface oxygens is generally not sufficient for sorption with hydrogen bond formation (51). The apparent “binding” of 4-MCP in SOMfree LOOM and HIOM soils can therefore be attributed to the retention of the contaminant in soil micropores as a result of mass transfer rate-limited diffusion. The magnitude of qnx in the SOM-free soils was found to be very similar to qnx in the anoxic autoclaved system (2.6 ( 0.2 mmol/kg), affirming our contention that the contaminant retention observed in the anoxic autoclaved system is mostly due to rate-limited diffusion of the contaminant from soil micropores. The slightly higher value in the anoxic autoclaved system may have been because of intraorganic matter retention of the chlorophenol. Although we expected autoclaved soils to show similar contaminant retention irrespective of the O2 levels, significantly larger levels of nonextractable 4-MCP were observed for the autoclaved HIOM soil in the oxic as compared to the anoxic environment. This difference in retention may have been due to an incomplete deactivation of soil enzymes during autoclaving. Certain soil enzymes such as peroxidases are fairly resistant to deactivation by heat. Berry and Boyd (34) were able to completely deactivate soil peroxidases only after boiling the enzyme in water for 1 h. It is possible, therefore, that the multipleautoclaving sterilization procedure used in this study was not always successful in eliminating enzymatic activity from the soil. Some biological oxidative coupling may have continued to occur in the HIOM autoclaved soil in oxic

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conditions. On the other hand, O2 may have reoxidized some of the reduced metal oxides in oxic autoclaved systems, thereby regenerating the catalyst and resulting in higher contaminant retention. In the case of the LOOM soil, no significant difference in qnx was observed in the oxic autoclaved vs anoxic autoclaved systems. The nonextractability of 4-MCP in these cases resulted from the retention of the contaminant in soil micropores due to mass transfer rate limited diffusion into the bulk solution. Effect of Hydrogen Peroxide. Hydrogen peroxide has been proposed for use as an oxygen source in bioventing and other in situ aquifer remediation systems. A good understanding of the effects of H2O2 on the binding of phenolic contaminants to soil and sediments can be of considerable importance during cleanup and detoxification processes. As a part of this investigation, we conducted sorption experiments that involved the addition of H2O2 to a soil-water system in order to quantify the effects of H2O2 on binding of 4-MCP to soils. Addition of H2O2 enhanced binding of 4-MCP in all systems (Table 5). An increase in contaminant binding in autoclaved systems suggests that H2O2 participates in abiotic coupling reactions. Hydrogen peroxide is capable of providing a greater coupling potential to mineral surfaces by oxidizing surface metal oxides. The addition of H2O2 to autoclaved soils resulted in a qnx of 21.1 ( 0.2 and 31.4 ( 2.2 mmol/kg for HIOM and LOOM soils, respectively. The greater binding in LOOM soil can be explained by the fact that a larger amount of the mineral surface was exposed in this case. Therefore, by oxidizing the exposed transition metal oxides, H2O2 was able to play a greater role in enhancing the catalytic properties of the oxides in LOOM soils. In addition to being a stronger oxidant than O2, H2O2 is also an essential substrate controlling the activity of soil peroxidase enzymes. Although the availability of H2O2 is limited in natural soil environments, this limitation can be eliminated in engineered systems by introducing H2O2 into the soil being treated. In our research, when peroxide was added to soils in contact with aqueous 4-MCP, a 4.4-fold increase in qnx was observed in nonautoclaved HIOM soils. Enzymatic oxidative coupling reactions, in addition to metal oxide-mediated abiotic binding processes, appear to have been responsible for the additional binding seen in the nonautoclaved HIOM and LOOM soils. Results of our study show that the addition of H2O2 resulted in the binding of nearly 63% of the soil-associated 4-MCP as compared to only 15% in the simple oxic system. Environmental Significance. In the case of surface soils contaminated with “couplable” xenobiotics such as substituted phenols or anilines, immobilization and possibly detoxification of the contaminant may occur as a result of covalent binding of the xenobiotic to soil organic matter during in situ bioremediation processes that involve the addition of O2 or H2O2. Results of this study indicate that portions of the physisorbed 4-MCP can become covalently bound to soil during sorption experiments conducted under aerobic conditions. Various factors may be responsible for the binding. Oxygen and soil enzymes can be implicated in the biotic coupling of 4-MCP to soil. Metal oxides on soil mineral surfaces can mediate abiotic coupling reactions that bind phenols to organic matter. Hydrogen peroxide can enhance both biological and mineral surface-catalyzed contaminant binding and may prove a useful agent that can be added to soil to stimulate oxidative coupling activity.

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Received for review September 18, 1995. Revised manuscript received March 8, 1996. Accepted March 8, 1996.X ES950691C X

Abstract published in Advance ACS Abstracts, May 1, 1996.

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