Environ. Sci. Technol. 1991, 25, 134-142
Nonequilibrium Sorption of Organic Chemicals: Elucidation of Rate-Limiting Processes Mark L. Brusseau* Department of Soil and Water Science, 429 Shantz Building 38, University of Arizona, Tucson, Arizona 85721
Ron E. Jessup and P. Suresh C. Rao Soil Science Department, University of Florida, Gainesville, Florida 326 11
The results of experiments designed to identify the process(es) responsible for nonequilibrium sorption of hydrophobic organic chemicals (HOCs) by natural sorbents are reported. The results of experiments performed with natural sorbents were compared to rate data obtained from systems wherein rate-limited sorption was caused by specific sorbate-sorbent interactions. This comparison showed that chemical nonequilibrium associated with specific sorbate-sorbent interactions does not significantly contribute to the rate-limited sorption of HOCs by natural sorbents. Transport-related nonequilibrium was also shown to not be a factor for the systems investigated. Hence, attempts were made to interpret the data in terms of two, sorption-related, diffusive mass-transfer conceptual models: retarded intraparticle diffusion and intraorganic matter diffusion. The analyses provide strong evidence that intraorganic matter diffusion was responsible for the nonequilibrium sorption exhibited by the systems investigated in this paper. W
Introduction Rate-limited or, nonequilibrium, sorption of organic chemicals by natural sorbents &e., soils, sediments, and aquifer materials) has been a topic of interest for quite some time. Perusal of the literature reveals that uncertainty exists in regard to the effects and relative importance of nonequilibrium sorption and its impact on the transport and fate of organic chemicals in surface (sediment/water) and subsurface (soil, aquifer) systems. Much of the confusion, we believe, can be attributed either directly or indirectly to inadequate knowledge of the mechanisms responsible for rate-limited sorption. Several different processes have been proposed as causing nonequilibrium sorption. However, past analyses have been based upon supposition or the fitting of models to data. The purpose of this paper is to report the results of experiments specifically designed to identify the mechanism(s) responsible for nonequilibrium sorption of hydrophobic organic chemicals (HOCs) by natural sorbents. Conceptual Framework Rate-LimitingProcesses. Several processes have been proposed as being responsible for nonequilibrium sorption. To enhance forthcoming analyses and discussion, as well as to clarify terminology, these processes will be briefly reviewed. Rate-limiting processes have been grouped into two general classes: transport related and sorption related (1,2). Transport-related nonequilibrium, often referred to as physical nonequilibrium, results from the existence of a heterogeneous flow domain. The influence of macroscopic heterogeneities (e.g., aggregates, macropores, stratified media) on solute transport has been well documented (see refs 1 and 3 for recent reviews). It should be noted that transport-related nonequilibrium affects both sorbing and nonsorbing solutes. Sorption-related nonequilibrium may result from chemical nonequilibrium or from rate-limited diffusive mass 134
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transfer. Chemical nonequilibrium (e.g., chemisorption) is caused by rate-limited interactions between the sorbate and sorbent. Specific sorbate-sorbent interactions may be relatively unimportant for the sorption of HOCs since their sorption is generally thought to be driven by a partitioning between the solution and organic matter components of the sorbent (4-7). Thus, chemical nonequilibrium may be ruled out as a probable nonequilibrium mechanism for HOCs (1, 8). This conclusion will be evaluated in light of experimental results in a forthcoming section. It must be stressed that, while chemical nonequilibrium may be unimportant for nonpolar organic chemicals, it may well be important for other organic chemicals such as pesticides (8,9), which often have one or more polar functional groups. Three different processes involving diffusive mass transfer can cause sorption-related nonequilibrium: film diffusion, retarded intraparticle diffusion, and intrasorbent diffusion. Film diffusion will not be considered further, as many researchers have shown that this mechanism is generally insignificant in comparison to other mechanisms (see ref 1 and references cited therein). Because much of the forthcoming results will be analyzed in terms of retarded intraparticle diffusion and intraorganic matter diffusion, these two are discussed in some detail in the following three sections. Retarded Intraparticle Diffusion. Retarded intraparticle diffusion involves aqueous-phase diffusion of solute within pores of microporous particles (e.g., sand grains) mediated by retardation resulting from instantaneous sorption to pore walls. Such a mechanism was proposed for the rate-limited uptake/release of HOCs by sediments (10) and by aquifer materials (11). This model is based on the radial-diffusion models that have been developed in chemical engineering. An important assumption associated with this model is that most, if not all, sorption occurs inside the particles. For HOCs, whose sorption is generally controlled by organic matter, this means that most, if not all, organic matter must reside inside the particles. Hindered diffusion is assumed to not be a factor in the usual conceptualization employed for the retarded intraparticle diffusion model. However, it has been suggested (1, 3, 12), based upon analyses of applications of the retarded intraparticle diffusion model to experimental observations, that this model cannot describe all aspects of the data without calling upon the concept of hindered diffusion. Hindered diffusion in fixed-pore systems, such as catalyst beads and zeolites, has received a great deal of attention. Equations of the following type log (Do/D,) = -0.5 - 1.98X (1) where Do is the aqueous diffusion coefficient ( L 2 / T )D, , is the pore diffusion coefficient, and X is the ratio of solute molecular diameter to pore diameter, have been reported by, among others, Satterfield et al. (13)and Chantong and Massoth (14). The values used for the regression coeffi-
0013-936X/91/0925-0134$02.50/0
0 1990 American Chemical Society
cients in eq 1 are averages of those reported by the cited researchers. By use of eq 1 and a representative value of
