Environ. Sei. Technol. 7904, 18, 43-48
Nonreversible Sorption of Phenolic Compounds by Sediment Fractions: The Role of Sediment Organic Matter Peter J. Isaacson* and Charles R. Frlnk The Connecticut Agricultural Experiment Station., New Haven, Connecticut 06504
rn The sorption and desorption of phenol, 2-chlorophenol, and 2,4-dichlorophenol by a fine and coarse sediment fraction were measured in a continuous flow stirred cell. Sorption was extensive (up to 0.3 mmol/g of sorbent) and was not due solely to hydrophobic interactions. Desorption was slower than sorption, and in some cases up to 90% of the sorbate was irreversibly held. Sorption of the three phenols by the two sediment fractions was also compared with sorption by the same sediment fractions with organic matter partially removed. The penetrability and accessibility of organic matter associated with sediment are proposed to exert a major influence in the sorption of compounds which take part in extensive hydrogen-bond interactions.
Introduction The environmental fate of many organic chemicals will be determined largely by their sorption interactions with soils or sediments (1-6) or with soluble humic compounds (7). The organic matter content of these natural sorbents has been shown to be an important factor in determining their sorption properties. This behavior has been summarized in terms of a mechanism involving solute partitioning between aqueous solution and the sorbent organic matter (2-12), in which the partition coefficient is influenced by the hydrophobicity of solute rather than by the makeup of the organic matter itself. However, the apparent uniformity of the sorbent character of organic matter may be partially the result of “averaging” which is inherent in observations of such complex material. The early formulation of an index for soil sorption equilibria (8)required the postulation of a “corrected”organic matter content, or “active fraction” of organic matter, in order to obtain the best indexing results. Also, different linear relationships have been observed between the organic carbon contents of extracted humic and fulvic fractions and the sediment content of chlorinated hydrocarbons (2). More generally, the observed partition coefficient for a whole sediment has been shown to be the weighted sum of a series of partition coefficients applicable to different particle size fractions within a whole sediment (11). The treatment of organic matter as a “solvent” for organic molecules implies a uniform penetrability of the macromolecular organic matter which is itself present primarily as the adsorbed phase in organomineral particles. Humic components are commonly associated with clay-size particles (13-15), and a lower proportion of these components are extractable with alkali (13), than from the total humic content of the sediment. Some low molecular weight organic matter can even be present in clay interlamellar spaces (16). The association of organic matter with mineral particles can alter the surface charge characteristics of the particles (17). The effect on the organic matter itself may be to apply some constraint to conformational changes that can otherwise occur freely when humic particles are present in true solution (18). It is important to see if such constraints have a significant influence on the sorption properties of organic matter. The treatment of sorption as a distribution process also assumes that the mechanism is reversible, but where it has 0013-936X/84/0918-0043$01.50/0
been studied the results are varied. Carbonyl and parathion sorption by homoionic soil organic matter preparations is completely reversible (9),whereas PCB sorbed by lake sediment material consists of both a reversible and a nonreversible fraction (19). Halobenzene and haloalkane sorption by aquifer sorbents is reversible in terms of material balance over complete sorption-desorption cycles, but differences may exist in the kinetics of sorption and desorption (6). The aim of this study was to examine the sorption and desorption of phenol, 2-chlorophenol, and 2,4-dichlorophenol by lake sediment material. These compounds were chosen as representatives of widely used organic chemicals, or of possible organic degradation products (20). In addition, they have the potential for interaction by hydrogen-bond mechanisms, which contribute a significant part of the inter- and intramolecular forces governing the conformational structure of humic compounds (21). The interactions of the phenols with the sediment-associated organic matter are investigated in order to examine the sensitivity of sorption and desorption to organic matter structure and “penetrability”.
Experimental Section The terms sorptive, sorbate, and sorbent are used to refer to the free solute in solution, solute sorbed by sediment material, and the sediment material, respectively. Sorptives. Phenol, 2-chlorophenol, and 2,4-dichlorophenol were obtained from commercial sources and used without further purification. Sorbents. Surface sediment from Lake Zoar, CT, was sampled, by using an Eckman dredge and was stored in glass containers at 4 O C . Subsamples were separated into two particle size fractions, coarse (>2 pm) and fine (
