Investigation of multicomponent sorption and desorption rates in saturated ground-water systems
Year: 1991
Type: report
Abstract: Laboratory experiments were performed and mathematical models were developed and used to analyze sorption and desorption processes of hydrophobic organic solutes to subsurface materials.The work included consideration of both single and multisolute systems, while experiments were performed in both batch and one-dimensional column reactor systems.The solutes used covered a range of hydrophobicity, which fell within a log octanolwater partition coefficient range of 2.18 to 4.30.One of the solutes, 7-hexachlorocyclo- hexane (lindane), was observed to degrade by an E2 elimination process, which occurred at a rate that was dependent upon the pH of the system.Two sandy subsurface materials were used in the experimental work, which had organic carbon contents that covered a relatively wide range, 0.026 to 1.200%.Mathematical model development included a variety of batch, one-dimensional, and twodimensional models for simulating single and multicomponent sorption processes described by nonlinear equilibrium expressions and diffusional rate models.Petrov-Galerkin finite element methods were used to accommodate sharp-front problems, while sorption rates were simulated using a method of lines solution procedure.Numerical procedures used also included operator splitting methods and consideration of parallel processing.A set of experiments was performed to analyze desorption equilibrium and rate behavior for lindane on the high total organic carbon subsurface material.The results indicated that rates of sorption and desorption are important factors contributing to apparent desorption hysteresis.Rate modeling of experimental data showed that a substantial fraction of the hysteresis could be explained by non-attainment of equilibrium in pseudo-equilibrium batch experiments.This result is especially important in view of the finding that time scales on the order of a few months to a year are needed to approach equilibrium in the systems investigated.This time far exceeds the period over which most experimental sorption work is performed.A complete and consistent set of multicomponent sorption rate and equilibrium experiments was performed for both solid materials using the solute lindane and 1,2,4trichlorobenzene.These experiments reinforced the findings that time scales for equilibration are on the order of months, and sorption equilibrium relationships are nonlinear.Competition was not observed in batch experiments, either with regard to sorption equilibrium or with regard to the rate of sorption.This lack of competition did not agree well with predictions made using ideal adsorbed solution theory.However, significant competition was observed in a set of column experiments.It is concluded that column experiments provide a more semi tive indicator of competitive sorption than batch experiments.
Language: English
Subfield: Environmental Engineering
Field: Environmental Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W16143860