Distribution of Hydrophobic Ionogenic Organic Compounds between Octanol and Water: Organic Acids Chad T. Jafvert" Environmental Research Laboratory, U.S. Environmental Protection Agency, Athens, Georgia 3061 3
John C. Westall Department of Chemistry, Oregon State University, Corvallis, Oregon 9733 1
Erwin Grieder and Ren6 P. Schwarzenbach Swiss Federal Institute for Water Resources and Water Pollution Control (EAWAG), CH-6047 Kastanienbaum, Switzerland
The octanol-water distributions of 10 environmentally significant organic acid compounds were determined as a function of aqueous-phase salt concentration (0.05-0.2 M LiC1, NaCl, KC1, CaCl,, or MgC1,) and pH. The compounds were pentachlorophenol, 2,3,4,5-tetrachlorophenol, (2,4,5-trichlorophenoxy)aceticacid, 4-chloro-a-(4-chloropheny1)benzeneacetic acid, 2-methyl-4,6-dinitrophenol, (2,4-dichlorophenoxy)aceticacid, 4-(2,4-dichlorophen0xy)butanoic acid, 3,6-dichloro-2-methoxybenzoicacid, 2,3,6-trichlorobenzeneaceticacid, and 2-(2,4,5-trichlorophen0xy)propionic acid. The experimental results were interpreted quantitatively with an equilibrium model that accounts for acid dissociation in the aqueous phase and partitioning into the octanol phase by the neutral organic species, free inorganic and organic ions, and ion pairs. The partition constants for the neutral ion pairs correlate well with the partition constants of the neutral acids. Two experiments address the applicability of these octanolwater distribution data to the distribution of ionogenic compounds in the environment: the distribution of 2methyl-4,6-dinitrophenolon a natural sorbent as a function of salt concentration (NaCl and CaC1,) and pH, and competitive adsorption of pentachlorophenol and 2,3,4,5tetrachlorophenol on an environmental sorbent.
Introduction Partitioning between water and natural sorbents is an important factor influencing the fate and toxicity of organic chemicals released into the aquatic environment. Thus, extensive research has focused on the partitioning of neutral hydrophobic compounds to natural sorbents, including soils, sediments, and biological lipids and tissues (I, 2). In most instances, sorption of these compounds is dominated by hydrophobic interactions. As a result, the affinity of a sorbent for a hydrophobic solute in most cases can be reliably correlated with its organic carbon or lipid content. All other factors, such as clay mineral content and composition, ion-exchange capacity, pH, and particle size, have been labeled as factors of "second-order" importance ( 3 ) . 0013-936X/90/0924-1795$02.50/0
Although the kinetics of adsorption and desorption is important in many instances, sorption to soils and sediments has been generally treated as a rapid process, approaching equilibrium within a few hours or days. The equilibrium partition constants, normalized to the organic carbon content of the sorbent (K,,J, have been shown to correlate to octanol-water partition constants, water solubility (corrected for crystal energy), reverse-phase HPLC retention, and topological parameters of the chemical involved. Lyman et al. (4) compiled a list of some of the most widely used equations that correlate these parameters. Although these relationships are empirical, they provide an excellent means of estimating pollutant sorption behavior from the properties of pollutants and sorbents. Reasonable estimates may be developed for the adsorption of virtually any simple hydrophobic compound to any sediment or soil having an organic carbon content greater than 0.1%. For ionogenic compounds, such as organic acids and bases, the complexity and variety of possible interactions between these compounds and natural heterogeneous sorbents have made development of similar estimation techniques more difficult. Indeed, for ionogenic compounds, factors such as pH are no longer of second-order importance, but are major factors that influence their distribution between water and natural sorbents. For example, the sorption of organic cations to clays or natural aquifer material may be driven primarily by ion-exchange reactions rather than by hydrophobic interactions (5-7). On the other hand, the partitioning of organic anions to natural sorbents is believed to be influenced significantly by both hydrophobic and electrostatic interactions. For example, Hand and Williams (B), studying the sorption of linear alkylbenzenesulfonates (LAS) to four sediments, found that sorption increased with alkyl chain length, indicating that hydrophobicity contributes to the total energy of sorption. However, sorption to the four sediments did not correlate well with the organic carbon content of the sediments, as would be expected from the simplest of hydrophobic mechanisms. In another study,
0 1990 American Chemical Society
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Table I. Reactions Involving HA with either MOH and MCl, or MCl, in Octanol-Water Systemsa expression
(A) Reactions of HA HA=Ht+AKa H A = m Kow (B) Reactions in KC1, NaC1, and LiCl Salt Systems 3. Mt C1- = M+ Kix 4. M' + Cl- = MCI Kipx 5. Mt+H-=W+F Ki 6. Mt+A-=m Kip 7. M ' + O H - =W+m 8. M' + OH- = MOH 1. 2.
+
+
-
( C ) Reactions in MgC1, and CaCl, Salt Systemsb MZt + 2C1- = -+ Kdix MZt + 2C1- = MC1, Kdipr 11. MZt + 2A- = MA+ + E Kdi MZt A- C1- = MA+ 8 12. Kdim 9. 10.
+ +
13.
Mzt
+
+ 2A- = MA,
kdip
"Adapted from Westall e t al. (10). HA represents a neutral organic molecule, A- represents the corresponding monovalent organic anion, Mt represents a monovalent inorganic cation (Lit, Kt, or Nat), Mzt a divalent inorganic cation (CaZt or MgZt). The overbar indicates species in the octanol phase. bAn equilibrium expression involving the species and A- can be derived from reactions 10-12, with the corresponding equilibrium constant equal to (KdixKdi)/Kdirn.
Ogram and co-workers (9) indicated that the sorption of (2,4-dichlorophenoxy)aceticacid (2,4-D),which was primarily in the anionic form at the pH of the experiment, correlated with the organic carbon content of four selected soils or clays. Repulsion of 2,4-D from negatively charged montmorillonite was also observed, however, indicating that electrostatic properties such as surface charge, pH, and ionic medium may be significant factors influencing the sorption of organic anions. Mechanism of Sorption. A first step in estimating or predicting the sorption behavior of ionogenic organic compounds is a survey of the major mechanisms that may account for sorption. Westall and co-workers (10) considered four types of interactions of ionogenic organic compounds with nonaqueous phases. Several of these interactions can be represented by the reactions in Table I. As it is our purpose to address factors that influence the partitioning of acidic compounds (phenols and carboxylic acids), these four mechanisms will be described as they relate to this class of compounds. The first mechanism is transfer of a neutral species between an aqueous phase and a nonaqueous phase. This mechanism may be represented by reaction 2 of Table I and is the basis for linear free energy relationships relating K, and KO,for sorption of nonpolar organic compounds (see refs 1,4, and 11). For any ionogenic compound, this mechanism is important when a significant fraction of the total compound exists in the neutral form. The second mechanism is the transfer of ionic species as free ions or ion pairs from one phase to another. This transfer is represented by reactions 5 and 6 or reactions 11-13 of Table I. This mechanism has been shown to be significant in the distribution of phenate species between octanol and water (10) and is believed to be responsible for the sorption of phenate species on natural sorbents (12). The results of these two studies are reviewed briefly below. Westall and co-workers (10) used a set of reactions similar to that given in Table I to describe the distribution of pentachlorophenol (PCP) and 2,3,4,5-tetrachlorophenol 1796
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(TeCP) between octanol and an aqueous solution of KC1 at pH 12. Partitioning of K+ and C1- could be described by reactions 3 and 4. Partitioning of the organic species was described by reactions 5 and 6. Under the conditions of the experiment (pH value of approximately 12), the transfer of H+ and OH- was found to be negligible. Also at this pH, transfer of the neutral organic species (reaction 2) was shown to be negligible, as the majority of the organic species in the aqueous phase exists as an anion (reaction 1). Schellenberg and co-workers (12) studied the sorption of chlorinated phenols by sediments and aquifer materials. For solution pH values a t which the phenols were not strongly ionized (pH - pK, < l),the distribution between solution and the natural sorbents could be described with reactions 1 and 2 of Table I. Using mass action and material balance equations associated with these reactions, they derived an equation for the distribution ratio as a function of pH, KO,,and K, for the chlorinated phenols at pH - pK, < 1: D = 1.05f~0,0~s2[1/(1 + K,/[H+])] (1) where D is the experimentally determined overall distribution ratio, f,, is the fraction of organic carbon in the sorbent, and K, is the acid dissociation constant of the phenol. At solution pH values where the phenols were strongly ionized, it was necessary to invoke reactions 5 and 6 to describe the data. This finding agrees with the work of Mdler and co-workers (13),who showed that adsorption of p-nitrophenol onto activated carbon occurs by both the neutral and dissociated species and is highly pH dependent. The third mechanism involves transfer of organic ions from solution to a two-dimensional lipophilic organic surface. It is difficult to distinguish between this mechanism and the second mechanism with data from conventional distribution experiments (as it is difficult to distinguish between "solvation" and "adsorption" for neutral hydrophobic compounds). Indeed, a continuum between these mechanisms is certain to exist in natural systems. The fourth mechanism involves surface complexation between specific ionic functional groups of the organic compounds and those of the sorbent surfaces. Considerable work has been published regarding surface complexation of organic compounds by pristine oxide surfaces. The importance of this mechanism for anionic compounds in natural systems has yet to be addressed in detail. Scope of This Study. The emphasis of this study is to extend our understanding of the reactions given in Table I for the distribution of selected ionogenic species between octanol and water, a system akin to natural sorbent-water or lipid-water. Since we are dealing with octanol-water distributions, the term "octanol phase" refers to watersaturated octanol, and "water phase" refers to octanolsaturated water. We report on the octanol-water distribution of 10 phenolic or carboxylic acid compounds as a function of aqueous-phase salt concentration (for KC1, NaC1, LiC1, MgC12, and CaC1,) and aqueous-phase pH. From these experiments, equilibrium partition coefficients of the neutral and charged forms, as well as pK, values for several of these compounds have been calculated. Of particular interest are the effect of divalent versus monovalent cations on the distribution of anionic organic compounds, the existence of free ions versus ion pairs in the octanol phase, and the degree of correlation of equilibrium constants with other properties of the hydrophobic ionogenic organic compounds (HIOCs).
Table 11. Equilibrium Constants Involving Ionogenic Organic Compounds in Octanol-Water" common name
compound
PCP
pentachlorophenol
TeCP
2,3,4,5-tetrachlorophenol
2,4,5-T
acid (2,4,5-trichlorophenoxy)acetic
DDA
4-chloro-cu-(4-chlorophenyl)benzeneacetic acid
DNOC 2,4-D 2,4-DB dicamba fenac silvex
2-methyl-4,6-dinitrophenol (2,4-dichlorophenoxy)aceticacid 4-(2,4-dichlorophenoxy)butanoicacid 3,6-dichloro-2-methoxybenzoic acid 2,3,6-trichlorobenzeneaceticacid 2-(2,4,5-trichlorophenoxy)propionicacid
aqueous soln
log Ki
log Kip
log KO,
KCl NaCl LiCl MgClz CaCl, KC1 NaCl KC1 NaCl LiCl KC1 NaCl LiCl KC1 KC1 KCl KCl KCl KCl
-1.95 -1.97 -1.62 5.Olc 4.72' -2.03 -2.05 -3.69 -3.71 -3.36 -3.58 -3.53 -3.18 -3.88 -4.16 -3.81 -5.95 -5.66 -3.75
2.64 2.68 3.10 -0.254d
5.09,5.24'
4.83,4.15'
1.81 2.09 0.604 0.515 1.25 1.83 1.83 2.31 0.016 -0.042 0.781 -0.654 0.140 0.916
4.87'
6.35'
3.31,3.38'
2.83,2.80 f 0.0d
4.64,4.48e
3.66
2.14,2.16e 2.83e 3.53e 2.4ge 3.20e 3.80e
4.46,4.3Y 2.90 f 0.04' 4.95 f 0.ov 1.90 f 0.05h 3.80 f 0.04' 3.07 f 0.04'
-log K,
"Ki and Kipwere determined simultaneously in experiments a t high pH; Kowand K,, unless otherwise noted, were determined simultaneously in experiments by varying aqueous-phase pH. bDeterminated by Schellenberg et al. (12). 'Values reported are for log Kdi. dValue reported is for log Kdim. eIndependently determined by using aqueous phases of either 0.1 or 0.5 M HCl. /Determined potentiometrically a t 25 "C, ionic strength
