Several authors have developed two-site (two-compartment) models in which rate-limited sorption is explained by mass transfer and diffusion resistance of the contaminant from the bulk solution to the solid phase. Many published kinetic studies have shown that a significant fraction of the pollutant is adsorbed rapidly onto soil particles, the rest being adsorbed more progressively, usually following first-order kinetics. In addition, for highly hydrophobic organic polluants, sorption studies in soil–water systems are difficult because of the very low water solubility, especially for desorption studies. However, the apparent hysteresis effect may be due to experimental artefacts attributed to biotic or abiotic degradation of the pollutant or changes in the physical characteristics of the soil during the contact with the organic pollutant.
Results of many studies suggest the existence of a hysteresis phenomenon that tends to be significant when soil–contaminant contact time increases. The desorption of organic pollutants from soils has been studied in batch systems by numerous investigators. The study has been conducted in two steps:batch studies to determine the states of apparent equilibrium and evaluate the kinetics and studies of naphthalene adsorption-desorption in water-saturated soil columns. Several soil fractions and model soils have been used to investigate the effects of particles' size and organic matter distribution in the soil microagregates. Naphthalene has been selected as a test pollutant because it is the most soluble PAH and may be present in groundwater contaminated by manufactured gas plant sites, for example. In the present work, the sorption of naphthalene in soils containing more than 1% organic matter has been studied in water-saturated conditions. Organic matter plays a major role in the sorption of hydrophobic organic pollutants in soils, especially in water-saturated conditions. Sorption may therefore strongly reduce the availability of the pollutants to the soil microorganisms. In general, microbial degradation of pollutants in soil and water systems is possible only if the organic compounds are present in the liquid phase. Sorption phenomena are one of the key factors controlling the fate in soils of hydrophobic neutral organic compounds, such as polynuclear aromatic hydrocarbons (PAHs), through the partition between the aqueous (mobile) and solid (immobile) phases. Abiotic interactions between organic pollutants and the soil may reduce the efficiency of the treatments. The site distribution depends on the nature and degree of humification of the soil organic matter.Īlthough in situ bioremediation technologies are attractive for the treatment of contaminated soils, their application is not easy because of the difficulty of predicting their efficiency from laboratory feasibility studies. The adsorption was described using a double-compartment model, considering a nonuniform distribution of the adsorption sites (organic matter) between the external surface of soil microaggregates (with instantaneous adsorption) and the internal compartment inside the microaggregates (with rate-limited adsorption). Data obtained from soil column experiments conducted with water as mobile phase were in good agreement with the results obtained in batch. At higher sorbed concentrations, a slight hysteresis effect was observed in batch studies. Naphthalene adsorption was found to be well reversible when the pollutant–sorbent contact time was short (24 h) and the sorbed concentration was relatively low.
Adsorption is directly related to the organic content of the soil fractions and not to the size of the particles with which the organic matter is associated. Adsorption of naphthalene mainly takes place on the organic matter of the soils or soil fractions. A significant fraction of naphthalene was found to be adsorbed almost instantaneously, the rest being adsorbed more progressively, following first-order kinetics.
Apparent equilibrium was reached in batch within 10 to 15 h and could be described with simple linear isotherms in the ranges of concentrations studied. Batch experiments, in which the sorbents were suspended in aqueous solutions of naphthalene at different initial concentrations, were used to determine the states of apparent equilibrium and approach the kinetics of adsorption and desorption. The adsorption–desorption of naphthalene on several soil fractions and model soils has been studied in batch and column experiments to investigate the complementarity between the two methods.