Environ. Sci. Technol. 1884, 18, 652-657
Weschler, C. J.; Kelty, S. P.; Lingousky, J. E. J. Air Pollut. Control Assoc. 1983,33, 624-629. Stevens, R. K.; Dzubay, T. G. U.S. Environ. Prot. Agency 1978, EPA 60012-78-112. Gagosian, R. B.; Peltzer, E. T.; Zafiriou, 0. C. Nature (London) 1981,291, 312-314. Van Vaeck, L.; Broddin, G.; Van Cauwenberghe, K. Environ. Sci. Technol. 1979, 13, 1494-1502.
(14) Storck, W. J. Chem. Eng. News 1981, 59 (29), 9. (15) Junge, C.E. In "Fate of Pollutants in the Air and Water Environments"; Suffet, I. H., Ed.; Wiley: New York, 1977; Part 1, pp 7-25.
Received for review July 11,1983. Revised manuscript received March 1, 1984. Accepted March 27, 1984.
Sorption of Chlorinated Phenols by Natural Sediments and Aquifer Materials Kurl Schellenberg,+ Chrlstlan Leuenberger, and Rend P. Schwarzenbach"
Swiss Federal Institute for Water Resources and Water Pollution Control (EAWAG), CH-8600 Dubendorf, Switzerland Laboratory experiments have been conducted to study the sorption of chlorinated phenols by sediments and aquifer materials. It is shown that sorption not only of the nondissociated phenols but also of their conjugate bases (phenolates) can occur. At typical ambient concentrations, sorption equilibrium can be described by the equation S = DC, where S = concentration in the solid phase, D = overall distribution ratio, and C = concentration in the liquid phase. In natural waters of low ionic M) and of pH not exceeding the strength (i.e., I 6 pK, of the phenol by more than one log unit, phenolate sorption can be neglected and the overall distribution coefficient may be expressed by D = KpQ, where K = partition coefficient of the nonionized phenol and = degree of protonation. K p may be estimated from the octanol/water partition coefficient of the compound and from the organic carbon content of the sorbent. In the case of tetra- and pentachlorophenol,phenolate sorption usually has to be considered. It is strongly influenced by the organic carbon content of the sorbent and by the ionic strength of the aqueous medium. N
4
Assessing the transport, fate, and the potential biological effects of xenobiotic chemicals in the aquatic environment requires knowledge of the sorption behavior of the chemicals, i.e., of their distribution between the solid and aqueous phases. Recently, much effort has been directed toward understanding the sorption of a variety of hydrophobic organic pollutants by sediments, soils, and aquifer materials (1-5). The vast majority of the chemicals investigated were compounds exhibiting no ionizable functional group(s). For such neutral compounds, including polycyclic aromatic hydrocarbons ( 2 , 3 ) ,halogenated hydrocarbons (1,5),and certain pesticides (4),approximately linear sorption isotherms have been found at concentrations typically encountered in natural waters: S = KpC (1)
S is the concentration of the compound in the solid phase, C is the concentration in the liquid phase, and K p is the equilibrium partition coefficient of the compound between the sorbent and water. The partition coefficient K has been shown to be primarily dependent on the lipophificity of the compound, as expressed by its octanol/water partition coefficient KO, and on the organic carbon content f, of the sorbent (1-5). Mathematical relationships between Kp,f,, and Kowhave been derived for various sets of compounds and natural sorbents (2-5): Kp
= f O 3 O C = fOCb(K0W)"
'Present address: Sandoz AG, 4002 Basel, Switzerland. 852
Environ. Sci. Technol., Vol. 18, No. 9, 1984
(2)
where KO,is the partition coefficient of the compound between water and a hypothetical natural sorbent of 100% organic carbon representing the organic material present in the sorbents investigated and a and b are constants. Values reported for a and b include a = 1.00 and b = 0.48 for polycyclic aromatic hydrocarbons, a = 0.52 and b = 4.4 for a certain group of pesticides ( 4 ) , and a = 0.72 and b = 3.2 for alkylated and chlorinated benzenes (5). From the presently available data, it can be concluded that the values of a and b are primarily determined by the type of compounds (i.e., compound class(es), range of lipophilicity) on which the relationship is established, and only to a smaller degree by the type of natural sorbents used. Thus, the reported relationships are very useful for predicting partition coefficients of neutral hydrophobic organic compounds between water and natural sorbents of very different origins. It should be noted that eq 2 is valid only for sorbents containing more than about 0.1% organic 2 0.001). For organic-poor sorbents, interaccarbon tions of the chemical with the inorganic matrix of the sorbent may become important ( 5 , 6 ) . The simple model used to describe the sorption of neutral hydrophobic organic chemicals by natural sorbents (eq 1and 2) is applicable only to a limited degree to compounds which are fully or partially ionized at natural pH values. Such compounds include amines, carboxylic acids, and phenols. The sorption of benzidine, e.g., was found to be largely controlled by the pH of the aqueous phase, and nonlinear sorption isotherms were obtained which were interpreted to be the result of the superposition of several different sorption processes (7). Also, a significant enhancement of the sorption above that expected based on simple partitioning, Le., predicted from eq 2 (derived for polycyclic aromatic hydrocarbons), was observed for two polycyclic aromatic amines (8). For anthracene-9carboxylic acid which is predominantly present as anion at the pH of natural waters, the same authors found no significant differences between predicted (eq 2) and experimentally determined "partition coefficients". In any case, when dealing with the sorption of hydrophobic compounds containing functional groups which may ionize or which may strongly interact with the various organic and inorganic constituents of natural sorbents, processes such as ion exchange, ligand exchange (9),formation of ion pairs or ion complexes (that may be transferred into the organic phase), etc., have to be considered in addition to simple partitioning. In the laboratory study reported here, the sorption of a series of chlorinated phenols by natural sediments and aquifer materials has been investigated. Chlorinated phenols are of growing concern as environmental pollutants (10-12). They have been widely found in surface waters
uOc
0013-936X/84/0918-0652$01.50/0
0 1984 Amerlcan Chemical Soclety
and in wastewaters, in particular, in pulp mill effluents (13-16). At typical ambient pH values, especially, the highly chlorinated phenols are present in the water predominantly as phenolate anions. The goal of this study was to establish quantitative relationships for describing the overall sorption of chlorinated phenols by sediments and aquifer materials, and to evaluate the contribution of the processes responsible for the sorption of the phenolate species (referred to as "phenolate sorption") to the overall sorptive process.
Table I. Characterization of the Sorbents Investigated
Experimental Section Chemicals. The chlorinated phenols used in this study (the names are given in Table 11) were purchased from Fluka AG, Buchs, Switzerland, or Chem Service, West Chester, PA, and used without further purification. The water for high-performance liquid chromatography (HPLC) mobile phases was doubly distilled in quartz, and the methanol was of spectral quality (SpectrAR, Mallinckrodt, S. Louis, MO). All other chemicals, acids, and salts were of the highest available purity (Fluka, purissimum and purum, or Merck p.a.). Acidity Constants. Acidity constants were determined by titration of 5 X 10-5-5 X 10"' M solutions of the chlorinated phenols in M NaN03 solution using M NaOH in thermostated vessels (20 f 0.5 "C) under a nitrogen atmosphere. Proton activity was determined with a glass pH electrode (Metrohm AG, Herisau, Switzerland) which was calibrated before each titration. Dissociation constants were calculated with eq 3 on the basis of at least
"g, = grams of solid; g, = grams of organic carbon. bSurface sediment (air-dried) from Greifensee, Switzerland. Sample from field site in the lower Glatt Valley, Switzerland (see ref 5); prepared by dry sieving; size fraction 4 < 63 pm.
six measurements at protonation degrees in the range of 0.2-0.8. [H+], = proton concentration, log [H+], = (E Eo)/k (Nernst equation), CB = amount of added base per total volume solution (Vo VC),K, = mol2 LP2,and At = initial concentration of chlorinated phenol. M solutions of five In a second approach, (2-5) X chlorinated phenols in 5 X M phosphate buffers were prepared, and the concentration ratio of deprotonated to neutral phenols [A-]/[AH] at different pH was determined by UV spectroscopy. Absorbances were measured at 11 wavelengths between 280 and 340 nm, and a nonlinear least-squares calibration and data reduction technique was used to allow determination of concentrations of both neutral and anionic species (which exhibit absorption maxima a t different wavelengths). The dissociation constant K, was then determined with a linear regression by using eq 4:
+
log ([A-lw/[AHlw) PH - PKa (4) Octanol/Water Partition Coefficients. Octanol/ water partition coefficients were determined according to OECD guidelines (17). Since it was found that the presence of more than one chlorinated phenol did not influence the distribution of the individual compounds, the di- and trichlorophenols and, as a second group, the tetrachlorophenols and pentachlorophenol were investigated simultaneously. All extraction experiments were carried out in at least four replicates. The two phases, 1-octanoland 0.01 M aqueous HCl, were saturated with each other before use; 45, 40,and 35 mL of the aqueous phase containing the phenols were added to 5,10, and 15 mL, respectively, of 1-octanol in a 100-mL ground-glass-stoppered bottle. The initial concentration of each phenol in the organic phase was ca. lo4 M for the di- and trichlorophenols and ca. M for the tetrachlorophenols and pentachlorophenol. The
no. 1 2
3 4 5
sorbent description lake sedimentb river sedimentc aquifer materialC aquifer materialc y-Al203
organic specific c&bon surface area (BET), content cf,), m2 g;la g.x 0.094 3.8 0.026 4.8 0.0084 6.4 0.0003 5.7
