Investigation of naphthalene sorption in soils and soil fractions using batch and column assays

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. 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. 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. A significant fraction of naphthalene was found to be adsorbed almost instantaneously, the rest being adsorbed more progressively, following first-order kinetics. Adsorption of naphthalene mainly takes place on the organic matter of the soils or soil fractions. 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. 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. At higher sorbed concentrations, a slight hysteresis effect was observed in batch studies. Data obtained from soil column experiments conducted with water as mobile phase were in good agreement with the results obtained in batch. 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). The site distribution depends on the nature and degree of humification of the soil organic matter.