Envlron. Scl. Technol. 1991, 25, 1501-1506
Influence of Sorbate Structure on Nonequilibrium Sorption of Organic Compounds Mark L. Brusseau”
Soil and Water Science Department, 429 Shantz Building, 38,University of Arizona, Tucson, Arizona 85721 P. Suresh C. Rao
Soil Science Department, University of Florida, Gainesville, Florida 326 1 1
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We investigate the relationship between sorbate structure and nonequilibrium sorption. The rate-limited sorption of compounds representing eight classes of organic chemicals, including chlorinated benzenes, unsubstituted and alkyl-substituted aromatics, chlorinated ethenes and ethanes, chlorinated phenols, nitrogen heterocycles, striazines, substituted amides, and substituted ureas, was examined by use of a single sorbent (sandy aquifer material) and the miscible displacement technique. The breakthrough curves were analyzed by using a bicontinuum model wherein sorption is assumed instantaneous for a fraction of the sorbent and rate limited for the remainder. Sorbate structure was shown to exert minimal impact on the nature of rate-limited sorption for nonionic, low-polarity compounds comprising relatively simple structures and for ionogenic compounds in neutral form. In contrast, sorbate structure appeared to have a significant impact for compounds comprising more complex structures (Le., pesticides). First-order reverse rate constants determined for the pesticides were a t least 1 order of magnitude smaller than those of the non-pesticides. This difference was attributed to differences in degree of constraint on diffusion within the polymeric structure of organic matter.
Introduction Several investigations of the rate-limited sorption (both adsorption and desorption) of nonionic, low-polarity “hydrophobic” organic compounds (HOCs) by natural sorbents have recently been reported. Chlorinated aromatics have received the majority of attention (cf. refs 1-7), with polynuclear aromatics (7,8)and chlorinated ethenes (6, 7 , 9 , IO) receiving lesser attention. Knowledge of the role of solute (or sorbate), solvent, and sorbent properties on mediating nonequilibrium sorption is required for a complete understanding of this process. Such knowledge is also required to successfully model and predict the effect of nonequilibrium sorption on the transport and fate of contaminants in soils, sediments, and aquifers. Unfortunately, few systematic investigations of this type have been reported. The nature of the sorbent apparently has only a second-order effect on the nature of nonequilibrium sorption, as evident in the similarity of rate-limited sorption of numerous HOCs for a wide variety of soils, sediments, and aquifer materials (1, 7,11,12). This holds for only natural sorbents, however, as values for sorption rate constants measured for systems comprising natural sorbents were significantly different from those measured for synthetic sorbents (e.g., organic-modified silica) (7). Conversely, the nature of the solvent (e.g., polarity) has been shown to significantly affect the kinetics of sorption of HOCs (13-16). For example, a model that describes a log-linear increase in the reverse rate constant with increasing volume fraction of organic cosolvent was presented by Brusseau et al. (13, 14). Qualitative observations of a relationship between degree of nonequilibrium or values of rate constants and sorbate 0013-936X191/0925-1501$02.50/0
hydrophobicity, which is a reflection of the nature and size of the sorbate molecule, have been reported by several researchers (cf. refs 1,4, and 17). The relationship between sorption kinetics and equilibrium sorption has recently been quantified by Brusseau and Rao (11),who analyzed literature data using the linear free energy relationship approach. They reported the first-order, reverse rate constant to be a log-log-linear inverse function of the equilibrium sorption constant for a number of HOCs and a wide variety of natural sorbents. This functionality has since been substantiated by experiment (6, 7, 12). The size of the sorbate molecule appears to have a major influence on the degree of nonequilibrium sorption experienced by a given sorbate, based on the observations reported above. However, the structure of the sorbate molecule (e.g., nature, size, and reactivity of functional groups) may also significantly affect nonequilibrium sorption ( 7 , I l ) . On the basis of their analysis of literature data, Brusseau and Rao (11) reported the degree of nonequilibrium sorption exhibited by relatively complex sorbates, such as pesticides, to be significantly greater than that exhibited by simpler sorbates such as alkyl- and chlorobenzenes. These observations are tentative, however, considering that the data were gathered from many literature sources, were obtained by several different experimental techniques, and represent many different sorbents. The number of carbons comprising the alkyl functional group was shown to affect the degree of nonequilibrium sorption for a series of substituted benzenes (7). These two appear to be the only published reports concerning a quantitative analysis of the influence of sorbate structure on nonequilibrium sorption of HOCs. The purpose of this work is to specifically investigate the relationship between sorbate structure and nonequilibrium sorption. This was accomplished by examining the rate-limited sorption of compounds representing eight classes of organic chemicals, including chlorinated benzenes, unsubstituted and alkyl-substituted aromatics, chlorinated ethenes and ethanes, chlorinated phenols, nitrogen heterocycles, s-triazines, substituted amides, and substituted ureas, by a single sorbent. In addition, the nature of nonequilibrium sorption of ionogenic chemicals in their neutral form is investigated.
Experimental Section Materials. The following analytical grade chemicals were used for the experiments: benzene, toluene, p-xylene, naphthalene, chlorobenzene, 1,3-dichlorobenzene, 1,2,4trichlorobenzene, 1,2-trans-dichloroethane,l,l,l-trichloroethane, 1,2-trans-dichloroethene,trichloroethene, tetrachloroethene, p-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, 2,3,4,5-tetrachlorophenol, pentachlorophenol, quinoline, atrazine, cyanazine, simazine, trietazine, alachlor, propachlor, monuron, and monolinuron. The structures, and pK, values where appropriate, of all chemicals discussed are provided in Figure 1.
@ 1991 American Chemical Society
Environ. Sci. Technol., Vol. 25, No. 8, 1991 1501
Benzene
Toluene
1 .bDichlorobenzene
1.2.4-Trichlorobenzene
p-Chlorophenol
2.4.6Trichlorophenol ( 7
6,)
is.^
2.4-Dichlorophenol
Atrazine
