Use of model soil colloids to evaluate adsorption of phenanthrene and its mobilization by different solutions

Abstract

Inorganic (a montmorillonite; a complex of montmorillonite and aluminium hydroxide) and organo-mineral model soil colloids (a complex of montmorillonite, aluminium hydroxide and humic acid) were tested for their capacity to adsorb and desorb phenanthrene. The adsorption-desorption isotherms demonstrated that the inorganic surfaces were able to bind the pollutant, although to a lesser extent than the organo-mineral complex. The capacity of different aqueous solutions to desorb phenanthrene after 1, 7, and 30 days' contact with the model colloids was tested in order to assess their efficacy with respect to the aging effect. The solutions used were 0.01 N CaCl₂, dissolved soil organic matter (DOM), a model root exudates solution (aqueous solution of saccharose, fructose, glucose, fumaric acid, ferrulic acid, and leucine), and three commercial surfactants (sodium dodecyl sulfate, Tween 20, and didecyldimethylammonium bromide). The desorptive capacity of these solutions were compared with that of organic solutions (water-ethanol and n-hexane–ethanol). More than 85% phenanthrene was desorbed after 30 days aging by organic solutions. Among the aqueous solutions, the best results were obtained with dissolved soil organic matter solution and the model root exudates solution which were able to desorb up to 50% phenanthrene after 30 days aging. Such naturally occurring aqueous solutions should be preferred to synthetic compounds to mobilize phenanthrene in contaminated sites without further contaminating the environment.

Keywords:

• Phenanthrene
• montmorillonite
• aluminium hydroxide
• humic acid
• adsorption
• aging
• dissolved organic matter
• root exudates
• surfactants

Acknowledgments

The authors thank Prof. Claudio Colombo of the Università degli Studi del Molise for providing the model soil colloids and Daniela Vindrola for her useful help in the experimental part. This research was supported by a grant of the “Ministero dell'Istruzione e della Ricerca” (Progetto Prin 2001, prot. 2001072713).