Environ. Sci. Technol. 1989, 23, 421-428
S., Ed.; Air Pollution Control Association: Pittsburgh,PA,
Russell, A. G.; McRae, G. J.; Cass, G. R. Atmos. Environ. 1983, 17, 949-964.
1987; pp 564-575.
Russell, A. G.; Cass, G. R. Atmos. Environ. 1986, 20,
Hegg, D. A.; Hobbs, P. V.; Lyons, J. H. Atmos. Enuiron.
2011-2025.
1985,19, 1147-1167.
Russell, A. G.; McCue, K. F.; Cass, G. R. Environ. Sei.
Richards, L. W.; Anderson,J. A.; Blumenthal,D. L.; Brandt, A. A.; McDonald, J. A.; Waters, N.; Macias, E. S.; Bhardwaja, P. S. Atmos. Environ. 1981, 15, 2111-2134. Hegg, D. A. University of Washington, personal communication, 1985. Richards, L. W.; Anderson, J. A.; Blumenthal, D. A.; McDonald, J. A.; Macias, E. S.; Hering, S. V.; Wilson, W. E., Jr. Atmos. Enuiron. 1985, 19, 1685-1704. Richards, L. W.; Anderson, J. A.; Blumenthal, D. L.; McDonald, J. A. Annual Meeting of the Air Pollution Control Association, New Orleans, LA, June 1982, Air Pollution Control Association, Pittsburgh,PA, 1982; paper
Technol. 1988,22, 263-271.
Pilinis, C.; Seinfeld, J. H.; Seigneur, C. Atmos. Enuiron. 1987,21, 943-955.
Pilinis, C.; Seinfeld, J. H. Atmos. Environ. 1987, 21, 2453-2466.
Godden,D.; Lurmann, F. Development of the PLMSTAR Model and Its Application to Ozone Episode Conditions in the South Coast Air Basin. Southern California Edison Co., Rosemead, CA, 1983. Killus, J. P.; Whitten, G. Z. Technical Discussion Relating to the Use of the Carbon-Bond Mechanism in OZIPM/ EKMA. U S . Environmental Protection Agency, Research Triangle Park, NC, EPA-450/4-84-009; 1984. Seinfeld, J. H. Atmospheric Chemistry and Physics of Air Pollution; Wiley: New York, 1986. Friedlander, S. K. Smoke, Dust and Haze-Fundamentals of Aerosol Behavior; Wiley: New York, 1977. Sehmel, G. A.; Hodgson, W. J. AIChE Symp. Ser. 1978,76, 218-230.
Sears, F. W.; Zemansky, M. W.; Young, H. D. University Physics; Addison-Wesley, Reading, MA, 1977; p 245. Saxena, P.; Seigneur, C. Atmos. Enuiron. 1987,21,807-812. Seigneur,C.; Hudischewskyj,A. B.; Seinfeld, J. H.; Whitby, K. T.; Whitby, E. R.; Brock, J. R.; Barnes, H. M. Aerosol Sei. Technol. 1986,5, 205-222. McMurry, P. H. J. Colloid Interface Sci. 1983,95, 72-80. Saxena, P.; Hudischewskyj, A. B.; Seigneur, C.; Seinfeld, J. H. Atmos. Environ. 1986, 20, 1471-1483. Mie, G. Ann. Phys. 1908, 25, 377. Hudischewskyj, A. B.; Saxena, P.; Seigneur, C. Visibility Protection: Research and Policy Aspects; Bhardwaja, P.
82-24.6.
US.-Canadian Memorandum of Intent, Workgroup 3B. Emissions, Costs and Engineering Assessment, 1982. Seigneur, C.; Bergstrom, R. W.; Johnson, C. D.; Richards, L. W. Atmos. Environ. 1984, 18 2231-2244. Received for review May 16,1988. Revised manuscript received September 9,1988. Accepted October 4,1988. The aerosol model was developed under contract with the U.S. EPA Atmospheric Sciences Research Laboratory, Aerosol Research Branch, by Systems Applications, Inc. We would like to thank the project officer, H.M. Barnes, for suggesting development of this model and for his support. Although the research described in this article has been funded in part by the US.EPA, it does not necessarily reflect the views of the agency and no official endorsement should be inferred. The incorporationof the aerosol module into a plume model and the evaluation of the model with experimental data was sponsored by Southern CaliforniaEdison (SCE).
Oxidation of Chlorophenols Adsorbed to Manganese Oxide Surfaces Hans-Jakob Ulrlch and Alan T. Stone" Department of Geography and Environmental Engineering, The Johns Hopkins University, Baltimore, Maryland 2 1218
Adsorption and oxidation of nine chloro-substituted phenols by manganese(III/IV) oxide particles in aqueous suspension are examined. Partition coefficients for adsorption to manganese oxides increase as the number of chloro substituents is increased, reflecting increased hydrophobicity. Partitioning and overall reaction rate (measured by Mn2+ release) increase as the pH is decreased, reaching a maximum level below the pK, of each chlorophenol. Electron-transfer rates depend quite strongly upon the position of chloro substituents on the aromatic ring; placing a chloro substituent at an ortho or para position instead of a meta position causes a 6-10-fold increase in rate of Mn2+ release per mole of adsorbed phenol. A mechanism is described involving specific adsorption of chlorophenols onto manganese oxide surface sites, followed by electron transfer and product release. Results indicate that manganese oxides may be important oxidants of chloro-substituted phenols in aquatic environments. Introduction
Minerals containing iron(III), manganese(III), manganese(IV), and other transition-metal oxidants are potential oxidizing agents for both natural and xenobiotic organic compounds. Because the solubilities of these higher valent states of iron and manganese are exceedingly low, redox 0013-936X189/0923-0421$01.50/0
reactions must take place at the mineral/water interface ( I ) . For this reason, overall rates of organic compound oxidation depend upon rates and extent of adsorption, as well as upon rates of electron transfer and subsequent reactions (2). As organic compounds are oxidized by the surface chemical reaction, surface metal centers are reduced and released into overlying solution. In an earlier work, we examined how substituent effects influence rates of Mn2+ release from manganese(III/IV) oxides by a series of monosubstituted phenols (3). Rates of reductive dissolution generally decrease as Hammett constants of ring substituents become more positive, reflecting trends in the basicity, nucleophilicity, and half-wave potential of substituted phenols. The present study extends the examination of structure-reactivity relationships to include mono-, di-, tri-, tetra-, and pentachlorophenols. The extent of phenol adsorption onto the oxide surface and oxidative loss during reaction have been measured by a reversed-phase HPLC technique and used to calculate reaction stoichiometry and rates of surface chemical reaction. Wide ranges in hydrophobicity and acid/base speciation exhibited by chlorophenols provide important insights into the reaction mechanism, and into factors that influence their relative susceptibility toward oxidation by manganese(III/IV) oxides.
0 1989 American Chemical Society
Environ. Sci. Technol., Vol. 23, No. 4, 1989 421
Table I. Chemical Properties of Phenol and Chlorophenols" abbrev
compound
PK
logK?w
P 2-MCP 3-MCP 4-MCP 2,4-DCP 3,4-DCP 3,5-DPC TCP TeCP PCP
phenol 2-chlorophenol 3-chlorophenol 4-chlorophenol 2,4-dichlorophenol 3,4-dichlorophenol 3,5-dichlorophenol 2,4,64richlorophenol 2,3,5,6-tetrachlorophenol pentachlorophenol
9.98 8.53 9.13 9.43 7.85 8.63 8.18 6.15 5.16 4.75
1.47 2.17 2.48 2.41 3.23 3.38 3.56 3.72 4.31 5.24
apK, values are from ref 10-13 and log KO, values are from ref 13-16.
Environmental Chemistry of Chlorophenols. Chlorophenols are highly toxic and widely distributed pollutants in the environment, arising from their use as insecticides, herbicides, and fungicides (such as 2,4-T, 2,4,5-T, and Silvex), and as intermediates in the synthesis of dyes and other important industrial chemicals (4). They have been widely found in surface waters (5) and in waste waters, such as pulp mill effluent (6, 7). Although chlorophenols have been degraded by bacteria (8) and by plants (9) in some instances, their observed persistence in the environment suggests that conditions are often unfavorable for these biotic degradation processes to occur. As the number of chloro substituents added to phenol is increased, chlorophenolsbecome more hydrophobic and their pK, values are lowered. Octanol-water partition coefficients and pK, values are listed in Table I. Thus, chlorophenols are hydrophobic ionizable compounds (17), capable of partitioning into the natural organic matter component and adsorbing to the mineral surface component of solids and aquifer sediments (13). Reaction Mechanism. A mechanism for the reaction of chlorophenols with manganese(III/IV) oxide surfaces is presented below, in order to facilitate discussion of the effects of chloro substituents on reaction rate (3). surface complex formation: >Mn"-OH
k + ArOH -1 ,>MnlI1-OAr + HzO k-1
(1)
k
& (>Mn",'OAr) k-2
(2)
release of phenoxy radical: k
+ HzO -k, >Mn"-OHZ + 'OAr k-s
(>Mn",'OAr)
(3)
release of reduced Mn(I1): >Mn1LOHZ
k
Mn2+ (+ free underlying site) (4)
coupling and further oxidation:
-
ArO' quinones, dimers, and polymeric oxidation products (5) The symbol ">" denotes bonds between surface metal centers and the oxide lattice. The successor complex to electron transfer (given in parentheses in reactions 2 and 3) may be either inner-sphere or outer-sphere. Manganese(1V) oxide surface sites may participate in reactions analogous to reactions 1-4. Phenol (ArOH), phenolate 422
Environ. Scl. Technol., Vol. 23,No. 4, 1989
Experimental Section Unless otherwise stated, all solutions were prepared from reagent-grade chemicals and 18 pfl cm resistivity water (DDW, Millipore Corp.). Substituted phenols listed in Table I were purchased from Aldrich Chemical Co. (purity 97-99%) and used without further purification. All glassware was soaked in 5 N HN03 and thoroughly rinsed with DDW prior to use. Preparation of the Manganese Oxide. Manganese oxide particles were prepared by oxidation of Mn(I1) solutions by sodium hypochlorite. First, a 1.1 x M NaOH solution was heated to boiling and purged with argon for 1h; 10 mL of 1.009 M Mn(C10& and 75 mL of Chlorox (0.705 M NaOC1) were then added, creating the manganese oxide particles. At this point, the reaction M NaOH, 1.98 X 10" M solution contained 1.03 X M NaOC1. Argon purging was MII(C~O~)~, m d 1.04 X continued for an additional 35 min. The particles were centrifuged, resuspended in DDW, and sonicated three times in order to remove electrolyte ions. A final resuspension was performed in 500 mL of DDW. A dilute suspension was made by adding 2 mL of 2.77 M NaOH and 125 mL of this concentrated slurry to 5000 mL of DDW. High-resolution transmission electron micrographs (HRTEM) revealed that the oxide is a mixture of platelets and acicular crystals. Crystallographic d spacings determined by electron diffraction were 2.49,2.16,1.64, and 1.43
A.
electron transfer: >Mn"'-OAr
(ArO-), and phenoxy radical (ArO')species are represented in this scheme. A complex mixture of oxidation products is formed, with a composition that depends upon the nature of the phenolic substrate, speciation of the oxide surface, and medium composition (3, 18). Details concerning this scheme have been presented elsewhere (2,3, 19). This mechanism postulates that electron transfer takes place via the precursor complex >Mn"'-OAr. The inner-sphere or outer-sphere nature, stoichiometry, and protonation level of the precursor complex are not known with certainty, but generalizations can be made concerning substituent effects on surface chemical reaction rates (3, 19). In this work, chemical properties of various chlorophenols will be used as a basis for understanding their adsorption and surface chemical reaction behavior.
Total manganese in particle suspensions was measured by atomic absorption spectrophotometry (AAS) and oxidizing titer measured by back-titration of excess oxalate with permanganate (20). The total manganese concentration in the dilute suspensions was found to be 6.05 X lov4M, with an average manganese oxidation state of +3.93. Thus, the stoichiometry of the oxide particles was Mn01.97. Design of Kinetic Experiments. Stock solutions of the phenols listed in Table I were prepared in DDW and used within 24 h. In the case of the tri-, tetra-, and pentachlorophenols, saturated solutions were made by mixing an excess of the phenols overnight, followed by filtration (0.2-pm Nucleopore filter). The ionic strength was maintained at 5.00 x M with NaCl and the temperature held constant at 20 "C by using double-walled Pyrex beakers and a circulating constant-temperature bath. Manganese oxide suspensions were equilibrated in the reaction solution under argon purging for at least 1 h to keep dissolved oxygen to a minimum. Parallel experiments performed under normal laboratory (fluorescent) lighting and protected from light by aluminum foil yielded the same results, All subsequent experimenb were performed under normal laboratory lighting.
Constant pH was maintained with either acetate or constant-P(C02)buffers. In previous experiments, it was shown that acetate has a negligible effect on the reaction rate (3). Constant-P(C02)buffer was used for reactions above pH 6. Alkalinity was set by addition of NaOH, and suspensions were purged with 1% C02 (balance nitrogen) for at least 2 h before addition of the chlorophenols. Negligible differences were observed between the reactions performed with acetate buffer and reactions performed at the same pH with constant-P(C02)buffer. Dissolved manganese concentration measurements provided one method for monitoring the progress of the reaction. Dissolved and particulate manganese were distinguished by filtering samples of the reaction suspension with 0.2-pm Nucleopore membrane filters. Concentrations of dissolved manganese in supernatant solution were determined by AAS. In the absence of reductant, dissolution of manganese oxide was below the detection limit. Chlorophenol Adsorption Measurements. Adsorption of chlorophenols during the reductive dissolution reaction was measured by the following procedure: (i) collection of oxide particles by filtering 50 mL of reaction suspension through 0.2-pm polycarbonate (hydrophilic) Nucleopore filters, (ii) fast reduction of the oxide particles M ascorbic acid, reby addition of 10 mL of 1.0 X leasing the adsorbed chlorophenols, and (iii) chlorophenol analysis by reverse-phase high-performance liquid chromatography (HPLC) using a Waters 490 Programmable Multiwavelength Detector. [ArOH],& is the moles of adsorbed chlorophenols per liter of reaction suspension, determined by this extraction and HPLC analysis technique. [ArOH]bullris the dissolved concentration of chlorophenols measured in filtered aliquots of reaction suspension. In order to test for adsorption of chlorophenols to the filter membranes, parallel experiments were performed where chlorophenol solutions with and without manganese oxide particles were filtered and extracted following the same procedure. At pH 4.1, where adsorption onto filter membranes is highest, the following results were obtained [ArOH],dl, mol/L of sumension filter 1.6 x 10-4 M filter only MnToT 1.2 x 10-7 3.2 X 3.5 x 10-8 1.4 x 10-7
+
2,4,6-TCP PCP
[ArOHIbulk,M 1.0 x 10-4 1.5 X
Oxidation of adsorbed chlorophenols by manganese oxide also occurs to some extent during extraction, which lowers the measured value of [ArOH]ad,. This loss is highest at low pH, where reaction rate is highest. For the examples presented above, oxidative losses of approximately 3.5 X M 2,4,6-TCP and 3.0 X lo4 M PCP from [ArOH]ad, can be estimated. Thus, in this worst case, adsorption onto membrane filters increases [ArOH],& approximately 30% above its true value, while oxidation during extraction lowers [ArOHIad,approximately 30% below its true value. Based upon this assessment, the percent uncertainty in [ArOH],d, is high when the amount adsorbed is low (1.0 X lo-' M). Most reported values of [ArOH],d,, however, are greater than 1.0 X lo-' M, and the percent uncertainty is proportionately smaller. As a rule, two [ArOH],& values must differ from one another by at least a factor of 2.0 in order for the difference to be significant.
Results and Discussion The production of dissolved Mn2+as a function of time in suspensions containing 2,4,6-TCP is shown in Figure 1. This behavior is typical of the dissolution experiments
25i
1
DH4.1
-
100 I50 200 Time (minutes) Figure 1. Reductive dissolution of manganese oxide particles by varying concentratlons of 2,4,6-trlchlorophenoI at different pH values. M total manganese (added as oxide Reaction conditions: 3.0 X M acetate buffer, 5.00 X lo-* M NaCI. particles), 2.50 X
50
0
ln
+- a, "c
z
-Lo
0
T i m e (minutes) Figure 2. Bulk concentration and extent of adsorption of 2,4,6-trichlorophenol during the reaction with manganese oxide. Reaction conditions at the onset of reaction: 1.02 X lo3 M 2,4,6-TCP, 1.6 X lo4 M total manganese, pH 5.42 (2.50 X lo3 M acetate buffer), and 5.00 X lo-' M NaCI.
performed with the various chlorophenols listed in Table I under different concentration and pH conditions; production of dissolved Mn2+is nearly constant at the start of the dissolution experiment, but diminishes as the reaction progresses. Dissolution rates (d[Mn2+]/dt) reported in this work are initial values, calculated by linear regression of [Mn2+]measurements against time up to the point where 10% of the added manganese oxide is dissolved. [ArOHIbulk,[ArOH],ds, and [Mn2+] are shown as a function of time in Figure 2 during dissolution of 1.6 X lo4 mol/L manganese oxide by 1.00 X lo4 M 2,4,6-TCP at pH 5.42. Because 2,4,6-TCP is present in large excess, changes in [ArOH]bulkare small in this experiment. A small increase in [ArOHIad,during the course of the reaction is, however, observed. Results from experiments performed in 1.41 X M PCP are shown in Figure 3. Higher concentrations of PCP cannot be used, because of solubility limitations. Consumption of PCP by the surface chemical reaction leads to a decrease in [ArOH]bulkover the course of the reaction. Decreases in [ArOH]b&, in turn, lead to decreases in [ArOH],& to below the detection limit as the reaction progresses. In order to make comparisons among chlorophenols of varying solubilities, a partition coefficient Qadsis defined as the ratio of [ArOH],d, to [ArOH]bulk. Qads represents the affinity of chlorophenols for the manganese oxide Environ. Sci. Technol., Vol. 23, No. 4, 1989
423
-
r
1
T i m e (minutes) Flgure 3. Bulk concentration and extent of adsorption of pentachlorophenol during the reaction with manganese oxide. Reaction conditions at the onset of reaction: 1.41 X M PCP, 1.6 X M total manganese, pH 4.08(2.50 X lo3 M acetate buffer), and 5.00 X lo-* M NaCI.
complex formation is the rate-limiting step, or when surface complex formation reactions (forward and reverse) are fast relative to subsequent steps. Rates of manganese oxide dissolution (d[M$+] /dt) are directly proportional to the concentration of 2,4,6-TCP (in the concentration range 5 X 10-6-1 X M) within the pH range of study (4.15 < pH < 6.50). This is indirect evidence that the fraction of surface sites occupied by 2,4,6-TCP is low, since dissolution rates do not increase in direct proportion to bulk reductant concentration once a significant fraction of sites are occupied by reductant molecules (2). Adsorption of Chlorophenols to Oxide Surfaces. According to the scheme outlined in reactions 1-5, the degree of partitioning of chlorophenols onto manganese oxide particles is determined by relative rates of three processes (2): adsorption: Rads = k,[ArOH]b~k[>MnlllOH] desorption: Rdes = k_,[>Mn"'-OAr] electron transfer: Re, = k,[>Mn"'-OAr]
-0-.3
0
20
40 60 80 100 120 Time (minutes)
Flgure 4. Partition coefficient Qads([ArOH]ads/[ArOH]bulk) showing increases in some cases as the reaction progresses. Reaction conditions: 1.6 X lo-' M total manganese, 2.50 X lo3 M acetate buffer, 5.00 X lo-* M NaCi.
surface, which is a function of pH, medium composition, nature of the oxide surface, and available surface area. As shown in Figure 4, Qadsmay change substantially over the course of a dissolution experiment, most notably for PCP. Changes in Qada are typically greater than changes in [ArOHlad,,perhaps reflecting slow desorption kinetics as [ArOH]bulkdecreases. In most of the discussions that follow, values of [ArOH]d and Qah measured after 5.0 min of reaction are reported. Although both may change during the course of a reaction, values measured a t 5.0 min are useful reference points for comparing reactions performed with different chlorophenols and under different medium conditions. Effect of Chlorophenol Concentration. The relationship between [ArOHIbdkand overall rates of reaction has been outlined and discussed earlier (3). According to reactions 1-5, rates of Mn2+production are proportional to [ArOHIbulkas long as [ArOH],d, is a small fraction of ST,the total moles of available manganese oxide sites per liter of suspension. This conclusion is valid when surface 424
Environ. Sci. Technoi., Vol. 23, No. 4, 1989
(6)
- k-z[>Mnll,'OAr]
(7)
(8)
In the special case where adsorption and desorption reactions are fast relative to electron transfer, reaction 1can be treated as a pseudoequilibrium step, and an equilibrium constant can be defined that describes the partitioning behavior ( K , = k 1 / k l ) . Changes in phenolic structure, medium composition, or oxide speciation that increase the magnitude of kl relative to k-, and kz increase the extent of partitioning, regardless of the identity of the rate-limiting step. Three forces contribute to the partitioning of organic compounds onto mineral surfaces: (i) inner- or outer-sphere complex formation with surface metal centers, (ii) electrostatic interaction with charged surfaces, and (iii) hydrophobic exclusion of organic compounds from bulk solution. We will now examine the effects of chloro substitution on the magnitude of these three forces, and their combined influence in determining partition coefficients (Qada). Table I lists pKa and log KO, (octanol-water partition coefficient) values for the chlorophenols included in the study. pKa values decrease as the number of chloro substituents is increased, from 9.98 for phenol to 4.75 for pentachlorophenol, The basicity of the phenolate group decreases with each successive chloro substituent, shifting the aqueous speciation from neutral phenol molecules to phenolate anions. These changes should influence the stability of inner- or outer-sphere complexes with oxide surface sites (19). As shown in Table I, octanol-water partition coefficients increase by almost 3 orders of magnitude in going from 4-MCP to PCP. Increased exclusion from bulk solution (13) and increased driving force for partitioning onto mineral surfaces should accompany changes in hydrophobicity of this magnitude. In general, Qads values increase as the number of chloro substituents and log KO, values of chlorophenols increase (see Table I1 and Figure 5). Values of [ArOHIad,and Qadsdetermined from experiments with mono- and dichlorophenols are presented in Table 111. Since the pKa values of this group of chlorophenols are all above 7.5, electrostatic interactions have a negligible effect on adsorption (see reaction 1). Among monochlorophenols, [ArOH],d, and Qads decrease in the
Table 11. Influence of the Number of Chloro Substituents on Partitioning and Reactivity at pH 4.2'
compound 1.5 X 1.5 X 1.5 X 1.5 X 2.7 X 1.4 x 1.5 X 1.1 x
PH 4.20 4.21 4.20 4.20 4.21 4.20 4.20 4.21
10"' M phenol 10"' M 2-MCP lo4 M 4-MCP M 3,4-DCP lo" M 3,4-DCP 10-4 M TCP M TeCP M PCP
d[Mn2+]/dt 3.1 x 10-7 1.7 X 10-7 3.5 x 10-1 4.2 X loT8 1.1 x 10-6 2.8 x 10-7 2.7 X 5.2 X
[ArOHl~ds
&ads
kX
3.9 x 104 2.4 X 10-7
1.5 x 10-3 3.4 x 10-3
0.28 1.17
1.1 x 10-7
1.0 x 10-2
0.47
M NaCl. Units: M acetate buffer, 5.00 X 'Reaction conditions: 1.6 X 10"' M total manganese (added as oxide particles), 2.50 X d[Mn2+]/dt [moles/(liter)(minute)]; [ArOH],* (moles of phenol adsorbed/liter of suspension at t = 5.0 min); &ads (moles of adsorbed phenol/moles of dissolved phenol); k x (l/minute). Table 111. Differences in the Partitioning and Reactivity of Various Isomers of Monochlorophenol and Dichlorophenol at pH 4.8'
compound
PH
d[Mn2+]/dt
1.5 X 10"' M 2-MCP 1.5 X 10"' M 3-MCP 1.5 X 10"' M 4-MCP 1.1 X 10"' M 2,4-DCP 1.2 X lo4 M 3,4-DCP 1.2 X 10"' M 3,5-DCP
4.84 4.84 4.84 4.84 4.84 4.84
1.7 X 4.5 x 10-9 6.9 X 8.7 x 10-8
t
2,3,5,6 TeCP
,$3,5-DCP
-
0
1 /
/
g 2 , 4 DC P n3,4-DCP
-2.8
1
- 3.0 I .o
2-MCPo /I 2.0
&ads
1.2 x 1.7 x 2.4 x 2.1 x 1.9 x 2.5 x
10-3 10-3 10-3 10-3 10-3 10-3
M total manganese (added as oxide particles), 2.50 X 10" M acetate buffer, 5.00 X
"Reaction conditions: 1.6 X are the same as in Table 11.
-2.4
1.1 x 10-8 1.3 x 10-9
[ArOHlads 1.6 x 10-7 2.5 x 10-7 3.7 x 10-7 2.2 x 10-1 2.4 X lo-' 2.8 x 10-7
1
3.0
1 4.0
I 5.0
6.0
Log Kow Flgure 5. Partition coefficient Qedsat pH 4.1 showing increases as a function of the octanoi-water partition coefficient (K,,) of chlorophenols. Reaction conditions: 1.6 X io-' M total manganese, 2.50 X lom3M acetate buffer, 5.00 X lo-* M NaCi.
order 4-MCP > 3-MCP > 2-MCP. 2-MCP is the most acidic (pK, = 8.53) and least hydrophobic (log Kow= 2.17) of the monochlorophenols. Both effects tend to diminish the extent of adsorption (3,191.In addition, chloro substituents are bulkier than hydrogen substituents, creating steric hindrance to inner-sphere surface complex formation at the ortho position that is far greater than that observed for chloro substitution at the meta and para positions. Differences in [ArOH],d, and Qads for the dichlorophenols in the study are smaller than for the monochlorophenols. This may arise because hydrophobic exclusion from bulk solution has become a bigger contributor to adsorption relative to surface complex formation. Steric interactions inhibiting surface complex formation of 2,4DCP are comparable to those of 2-MCP, but the enhanced importance of hydrophobic forces lessens the effect. Differences in Kowvalues among dichlorophenolsare small relative to differences among monochlorophenols. With tri-, tetra-, and pentachlorophenols, the contribution of hydrophobic forces to adsorption becomes even more dominant. At pH 4.1, where comparisons are made
k, 0.11 0.018 0.19 0.40 0.045 0.0046 M NaCl. Units
in Figure 5, the concentration of the protonated, neutral form of all the chlorophenols included in the study is greater than the concentration of the phenolate anion in overlying solution. Despite dramatic decreases in basicity (the pKa value of pentachlorophenolis 5.23 pH units below that of phenol), Qadacontinues to increase as chloro substituents are added, reflecting increases in log KOw. When hydrophobic effects are sufficiently strong, nonspecific partitioning may completely overshadow surface complex formation. This behavior has been observed in the adsorption of amphipathic molecules onto metal oxides. Ulrich et al. (21) observed that complex formation is the dominant contributor to the overall free energy of adsorption for short-chain aliphatic acids (less than eight carbon atoms long), while nonspecific hydrophobic exclusion from bulk solution is the dominant contributor for intermediate- and long-chain fatty acids (such as caprylic, capric, and lauric acids). The fitted line in Figure 5 represents the degree of partitioning arising from nonspecific hydrophobic effects. Thus, only 3-MCP and 4-MCP exhibit contributions from surface complex formation large enough to make an impact on their partitioning behavior. Figure 4 is important because it illustrates that, in some instances, Qadaincreases as the reaction progresses. The oxidation of phenols and chlorophenols takes place via formation of phenoxy radicals, which are subsequently consumed by further oxidation and oxidative coupling reactions (18). Dimers, trimers, and higher coupling products are much more hydrophobic than reactant phenol molecules and therefore likely to adsorb to oxide surfaces. Buildup of products on the oxide surface may have important implications for the adsorption of chlorophenols and for subsequent surface chemical reactions. Increases in Qads as the reaction progresses may be driven by favorable hydrophobic interactions of chlorophenolreactant molecules with adsorbed coupled product molecules. Electron transfer and subsequent dissolution reactions may be inhibited by adsorbed product molecules, since oxide surface sites are blocked. The effect of pH on the partitioning of TCP, TeCP, and PCP onto manganese oxide surfaces is shown in Figure 6. Since the oxidation state of the manganese oxide employed Environ. Sci. Technol., Vol. 23, No. 4, 1989 425
-3.0 4
5
6
7
0
I
5
6
7
I 0
I
4
6 - 7 0 PH Flgure 6. pH dependence of the adsorption coefficient Qado for 2,4,6-TCP, 2,3,5,6-TeCP, and PCP. The dashed lines show the pK, values of the correspondlng chlorophenols. Reaction condltlons: 1.6 X lo4 M total manganese, 2.50 X lo3 M acetate buffer, 5.00 X lo-* M NaCI. 4
5
in these experiments is nearly +4, the pH,,, is most likely low (1.4 < pH,,, < 4.5) (22, 23), and the oxide surface carries a neutral or negative surface charge. An electrostatic repulsive force away from the oxide surface, which is not experienced by neutral chlorophenol species, is experienced by phenolate anions and becomes important at pH values above the pK, of each chlorophenol. In addition, phenolate anions are considerably less hydrophobic than corresponding phenols, lessening the force for exclusion from bulk solution. Figure 6 shows that Qadsdecreases substantially as the pH is increased in the vicinity of the pK,, consistent with our expectations. Since pKa values of chlorophenols decrease strongly with increasing chloro substitution, the maximum of the partition coefficient Qadsshifts toward lower pH values; for TCP, the maximum is observed near pH 6, while for PCP, the maximum occurs below pH 4. The driving force for inner-sphere complex formation with oxide surface sites is also affected by pH. Recent work (24-26) has demonstrated that changes in the extent of inner- and outer-sphere complex formation arising from changes in adsorbate concentration, pH, and medium composition can be reliably modeled once the proper stoichiometry of predominant surface complexes has been found. If the proton stoichiometry for the surface complex given in reaction 1 is correct, a maximum in the extent of adsorption by chlorophenols should occur somewhere near the isoelectric point of the manganese oxide surface (3). This explanation is also consistent with the results presented in Figure 6. Chloro Substitution and Isomeric Effects on Mn2+ Production. According to the mechanism outlined in eq 1-5, overall rates of Mn2+production are proportional to the extent of chlorophenol adsorption: d[Mn2+]/dt = k,[ArOH],d, (9)
k, (in units of I/min) is an empirical rate constant found by dividing d[Mn2+]/dt by [AroH],d,. Simultaneous determination of [AroHlad,and d[Mn2+]/dt and calculation of k, allow the effects of chloro substitution on adsorption/desorption to be distinguished from effects on subsequent electron-transfer and dissolution reactions; k, can be seen as a reaction rate normalized per unit of chlorophenol surface concentration. Isomeric effects on k, are effectively demonstrated in Table 111. For monochlorophenols, k, for the meta-substituted isomer is 6 times lower than for the ortho-sub426
Environ. Scl. Technol., Vol. 23, No. 4, 1989
stituted isomer and 10 times lower than for the parasubstituted isomer. A similar inhibitory effect of chloro substituents in the meta position is observed with dichlorophenols; k, for 3,5-DCP (two meta) is 10 times lower than for 3,4-DCP (one meta, one para) and 90 times lower than for 2,4-DCP (one ortho, one para). Adding chloro substituents at ortho and para positions actually increases rates of electron transfer and dissolution. k, for 2,4-DCP is higher than for both 2-MCP and 4-MCP, and k, for 3,4-DCP is higher than k, for 3-MCP. In contrast, k, for 3,5-DCP is lower than for 3-MCP. This trend is observed with all the chlorophenols included in the study. Chlorophenols where numbers of oor p-chloro substituents are high relative to numbers of rn-chloro substituents (4-MCP, 2,4-DCP, and 2,4,6-TCP) have k, values substantially higher than those with a low ratio (3-MCP, 3,5-DCP, and 2,3,5,6-TeCP). The electron-transfer step (reaction 2) is the most likely source of this isomeric effect. One-equivalent oxidation of phenol or phenolate anions generates phenoxy radicals, which are highly reactive and quickly consumed by subsequent radical reactions (see ref 3). Ring substituents that stabilize phenoxy radicals accelerate the reaction, since the transition-state structure bears some resemblance to the product. All chloro substituents regardless of position are a-electron acceptors, which lower the stability of the phenoxy radical by lowering electron density in the aromatic ring. Chloro substituents in ortho and para positions are also n-electron donors (27), which provide electron density and stabilize the phenoxy radical by resonance interaction. Resonance structures are shown below:
9-
9-
CI
+
This r-resonance interaction that promotes reaction by stabilizing the phenoxy radical is apparently strong enough to overcome the a-electron-acceptor nature of o- and pchloro substituents. rn-Chloro substituents are unable to participate in ?r-resonanceinteractions, and their effect on reactivity is dominated by their c-electron-acceptor nature. Thus, isomeric effects on k, can be fully accounted for by considering substituent effects on phenoxy radical stability. pH Dependence and Reaction Stoichiometry. The effects of pH on rates of reaction between substituted phenols and manganese oxide surfaces have been outlined and extensively discussed previously (3). Rates of TCP reaction with manganese oxide are shown as a function of pH in Figure 7, using different TCP concentrations. The apparent reaction order with respect to [H+]is defined as n (3). Overall reaction rates in Figure 7 increase as the pH is decreased, but not linearly; n has a value close to 2.0 at pH 7.0, but drops to a value of 0.3 at pH 4.0. This behavior is quite similar to the pH dependence of p methylphenol and other monosubstituted phenols described earlier (3). This curvature is observed with other chlorophenols, but the effect is less dramatic; 3,4-DCP has a value of n near 0.6 at pH 4.0,4-MCP and 2,3,5,6-TeCP have values of n near 1.0, and PCP has a value of n near 1.5. The shift in speciation from neutral phenol molecules to phenolate anions with increasing numbers of chloro substituents can be expected to have an important role in determining values of n. As shown in Figure 6, Qah values for TCP, TeCP, and PCP increase as the pH drops below
0
20
40
60
00
120
100
140
Time (minutes) Phenol
:
(I)
PH Flgure 7. Initial rates of reductive dissolution of manaanese oxide particles as a function of pH In 5.0 X lom5M ( O ) , 2.5 2 M (A), and 1.00 X M (0)2,4,6-trichlorophenoi solutions. Reaction M total manganese, 2.50 X M acetate conditlons: 3.02 X buffer, 5.00 X lo-* M NaCI.
the pKa of each chlorophenol. This increase in partitioning and surface coverage undoubtably causes higher rates of Mn2+release (d[Mn2+]/dt).The observed pH dependence may also depend upon changes in acidlbase surface speciation that affect rates of electron transfer and Mn2+ release from the oxide lattice (2, 3). Reaction stoichiometry (SIX,) is defined as the change in chlorophenol supernatant concentration ([ArOH],,,) divided by the change in dissolved manganese concentration ([Mn2+]). The value of S,,, depends upon several factors, which may change during the course of a reaction. Early in the reaction, SI,,may be large because the adsorptive removal of chlorophenol from solution is not yet matched by electron transfer and release of Mn2+. Eventually, as [ArOHIad,and [Mn(II)],d, (the amount of adsorbed Mn2+)reach steady-state values, s,,, should approach a constant value. This final value of SI,,should reflect the overall stoichiometry of the reaction: the moles of manganese oxides per mole of chlorophenol reactant consumed. SI, values are shown in Figure 8 as a function of time during the reaction of manganese oxides with phenol and six chlorophenols. In every case, S, decreases during the first 20-70 min and then levels out at a constant value. Initial values of SI, above 3.0 were observed for 2-MCP, 3,4-DCP, 2,4,6-TCP,2,3,5,6-TeCP,and PCP. The highest SI, value observed was nearly 8. The characteristic time for reaching a constant value of S, may be useful in future work to examine the kinetics of reductant adsorption and desorption relative to subsequent surface chemical reactions. It is interesting to note that final values of S, near 1.0 were observed in reactions with phenol, 4-MCP, and 2,4,6-TCP; these three phenols provide the highest yield of dissolved Mn2+per mole of consumed phenol. Final values of 2-MCP, 2,3,5,6-TeCP, and PCP were between 2.0 and 3.0, which represent low yields of dissolved Mn2+per mole of consumed phenol. Oxidative coupling and phenoxenium ion formation (with subsequent benzoquinone formation) represent competitive oxidative degradation pathways for phenolic compounds, which may influence final values of S,,,. Oxidative coupling is strongly ortho- and para-position directed (18). Chloro substituents block coupling at these positions, altering the distribution of oxidation products and eventual reaction stoichiometry. Although no clear pattern is discernable in results from the present work, additional studies may find a relationship between chlo-
I
c:[ 20
0
40
60
80
Time (minutes)
2,4,6-TCP
= 2
m I
0
20
40
60
80
100
120
0 4 3
20
40
60
80
100
120
(I)
PCP
x '1 I
40 60 80 100 120 T i m e (minutes) Figure 8. Reaction stoichiometry S,,, showing decreases as the reaction with various chlorophenolsprogresses. Reaction conditions: pH 4.1, 1.6 X lo4 M total manganese, 2.50 X M acetate buffer, 5.00 X lo-' M NaCI.
0
20
rophenol structure and reaction stoichiometry. Significance. The importance of manganese oxides in the degradation of chlorophenols depends upon the oxide surface area available for reaction. Although the overall abundance of manganese oxides in many environments (such as soils and aquifer sediments) is low, enrichment can occur in the vicinity of oxic/anoxic interfaces. Accumulation of manganese oxides occurs when Mn2+(aq) generated by reduction reactions under anoxic conditions diffuses into oxic environments and becomes reoxidized and precipitated within a restricted area. In Lake Zurich, for example, there is a manganese-enriched particulate layer above the sedimentlwater interface that contains, on average, approximately7 pM manganese oxide (28). By using the experimental results reported in Figure 7 (at pH 7) and assuming that Lake Zurich manganese oxides have the same surface area as the synthetic oxides used in the experiments, a half-life for TCP oxidation can be estimated as from 0.9 to 1.8 years. This rate is probably slow relative to competitive biological and chemical degradative processes. Within the surface layer of oxic sediment, however, an even greater manganese oxide enrichment exists. In Lake Charlotte, Nova Scotia, for example, manganese oxides comprise as much as 2.5% of the dry weight of sediment (29). Under these conditions, available manganese oxide surface areas can easily be several orders of magnitude higher than those found anywhere within the water column, dramatically increasing rates of chlorophenol degradation. Adsorption of chlorophenols onto settling particles provides a mechanism for transport into Environ. Sci. Technol., Vol. 23,No. 4, 1989 427
sediments. Downward transport is probably most efficient for the more strongly hydrophobic chlorophenols. This combined transport and reaction pathway may be an important contributor to losses of chlorophenols from the water column. Conclusions
Several conclusions can be drawn concerning the susceptibility of chloro-substituted phenols toward oxidation by manganese oxides in aqueous environments. Partition coefficients for adsorption to manganese oxides (QadJ increase as the number of chloro substituents is increased, reflecting increased hydrophobicity, and an increased contribution of hydrophobic interactions toward the overall driving force for adsorption. The extent of partitioning (Figure 6) and overall rate of reaction (Figure 7) increase as the pH is decreased, reaching a maximum level below the pKa of the chlorophenol. Since the pKa values decrease substantially as the number of chloro substituents is increased, this maximum level of adsorption and reaction shifts progressively to lower pH values. Rates of electron transfer and overall rates of reaction depend quite strongly upon the position of chloro substituents on the aromatic ring; placing a chloro substituent in an ortho or para position instead of a meta position causes as much as an order of magnitude increase in rates of Mn2+release per mole of adsorbed phenol. All chloro substituents exert a a-electron-withdrawingeffect. For chloro substituents in the meta position, this effect dominates, increasing the energy required for phenoxy radical formation and lowering rates of reaction. Chloro substituents in ortho and para position also participate in s-donor resonance interactions in the phenoxy radical. This resonance interaction may stabilize the phenoxy radical and accelerate the reaction rate, overcoming the unfavorable a-electronwithdrawing effect. The entire class of chloro-substituted phenols is surprisingly susceptible toward oxidation by manganese(III/IV) oxides within the pH range of natural waters. Increased resistance toward oxidation arising from the a-electron-withdrawing nature of chloro substituents is offset by lessened resistance arising from s-electron-donating resonance interactions of chloro substituents at ortho and para positions and by increases in hydrophobicity that increase the extent of partitioning onto manganese oxide surfaces. Acknowledgments
Ken Livi and David Veblen assisted with HRTEM and electron diffraction analysis. Registry No. 2-MCP, 95-57-8; 3-MCP, 108-43-0; 4-MCP, 106-48-9;2,4-DCP, 120-83-2;3,4-DCP, 95-77-2; 3,5-DCP, 591-35-5; TCP, 88-06-2; TeCP, 935-95-5; PCP, 87-86-5; P, 108-95-2; Mn, 7439-96-5; manganese oxide, 11129-60-5.
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(2) Stone, A. T.; Morgan, J. J. In Aquatic Surface Chemistry; Stumm, W., Ed.; Wiley: New York, 1987; Chapter 9. (3) Stone, A. T. Environ. Sci. Technol. 1987,21, 979. (4) Freiter, E.R. Chlorophenols. In Kirk-Othmer Encyclopedia of Chemical Technology;Wiley-Interscience: New York, 1979; Vol. 5. ( 5 ) Wegman, R. C. C.; Hofstee, A. W. M. Water Res. 1979,13, 651. (6) Giger, W.; Schaffner, C. In Advances in the Identification and Analysis of Organic Pollutants in Water;Keith, L. H., Ed.; Ann Arbor Science: Ann Arbor, MI, 1981; Vol. I. (7) Leuenberger, C.; Coney, R.; Graydon, J. W.; Molnar-Kubica; Giger, W. Chimia 1983,37, 345. (8) Baker, M. D.; Mayfield, C. I.; Inniss, W. E. WaterRes. 1980, 14, 1765. (9) Weiss, U. M.; Moza, P.; Scheunert, I.; Haque, A.; Korte, F. J . Agric. Food Chem. 1982, 30, 1186. (10) Martell, A. E.; Smith, R. M. Critical Stability Constants; Plenum: New York, 1977; Vol. 3. (11) Martell, A. E.; Smith, R. M. Critical Stability Constants; Plenum: New York, 1982; Vol. 5. (12) Serjeant, E. P.; Dempsey, B. Ionization Constants of Organic Acids in Aqueous Solution;Pergamon: Oxford, 1979. (13) Schellenberg, K.; Leuenberger, C.; Schwarzenbach, R. P. Environ. Sci. Technol. 1984, 18, 652. (14) Leo, A.; Hansch, C.; Elkins, D. Chem. Rev. 1971, 71, 525. (15) Callahan, M. A.; Slimak, M. W.; Gabel, N. W.; May, I. P.; Fowler, C.; Freed, J. R.; Jennings, P.; Durfee, R. L.; Whitmore, F. C.; Maestri, B.; Mabey, W. R.; Holt, B. R.; Gould, C. Water Related Environmental Fate of 129 Priority Pollutants; EPA-440/4-79-029; 1979. (16) Xie, T. M.; Dyrssen, D. Anal. Chim. Acta 1984, 160, 21. (17) Westall, J. C.; Leuenberger, C.; Schwarzenbach, R. P. Enuiron. Sci. Technol. 1985, 19, 193. (18) Musso, H. In Oxidative Coupling of Phenol;Taylor, W. I., Battersby, A. R., Ed.; Dekker: New York, 1967. (19) Stone, A. T. In GeochemicalProcesses at Mineral Surfaces; Davis, J. A,, Hayes, K. F., Eds.; ACS Symposium Series 323; American Chemical Society: Washington, DC, 1986; Chapter 21. (20) Skoog, D. A.; West, D. M. Fundamentals of Analytical Chemistry;Holt, Rinehart, and Winston: New York, 1976. (21) Ulrich, H.-J.; Stumm, W.; Cosovic, B. Enuiron. Sci. Technol. 1988, 22, 37. (22) Parks, G. A. Chem. Rev. 1965, 65, 177. (23) Balistrieri, L. S.; Murray, J. W. Geochim. Cosmochim. Acta 1982,46,1041. (24) Kummert, R.; Stumm, W. J. Colloid Interface Sci. 1980, 75, 373. (25) Sigg, L.; Stumm, W. Colloids Surf. 1981, 2, 101. (26) Balistrieri, L. S.; Murray, J. W. Geochim. Cosmochim.Acta 1987,51, 1151. (27) March, J. Advanced Organic Chemistry, 3rd ed.; McGraw-Hi11: New York, 1985. (28) Sigg, L.; Sturm, M.; Kistler, D. Limnol. Oceanogr. 1987, 32, 112. (29) Kepkay, P. Limnol. Oceanogr. 1985,30, 713.
Received for review March 21, 2988. Accepted November 8,1988. This work was supported by the Environmental Engineering Program of the National Science Foundation under Grant ECE-8519793. Donation of the high-performance liquid chromatograph by MilliporelWaters Corp. of Milford, MA, is gratefully acknowledged.
