Saturday, 15 July 2006

Sorption of Chlorinated Phenols by Allophanic Soils.

Mara X. Cea, Barbara S. Fuentes, M. Cristina. Diez, and María de la Luz Mora. Univ de La Frontera, Av. Francisco Salazar 01145, Temuco, Chile

Chlorophenols sorption behavior as a function of solution pH, ionic strength, and organic carbon content was investigated using an Andisol belonging to Temuco Series located in southern Chile. Pentachlorophenol (PCP), 2,4,6 trichlorophenol (TCP), and 2,4 dichlorophenol (DCP) are the major wastewater contaminants and were selected for this study. Sorption isotherms were determined for the untreated soil and after removal of organic matter by treatment with hydrogen peroxide. The isothems were fitted using the Freundlich soption model. Marked differences were observed for the various chlorophenols. Sorption increased as the number of chlorines increased for both treated and untreated soil in the following order: PCP>2,4-DCP=2,4,6-TCP. The reduction in the sorption of PCP and TCP was small but significant in soil without organic matter. These differences in sorption behavior can be explained by differences in hydrophobicity and reactivity of the phenolic hydroxy group and suggests sorption of chlorophenols was due to interactions with mineral/oxide surfaces and soil organic matter. Partitioning into soil organic matter is thought to be the main mechanism for sorption of chlorophenols; however these results suggest sorption on mineral and oxide surfaces may also be important. Chlorophenol sorption generally decreased with increasing pH. PCP and TCP sorption were maximal in the pH range 4-6, then decreasing rapidly from pH 6 to 8. DCP sorption was constant within the measured pH range. PCP was most affected by pH changes due to its pKa of 4.75 and thus, existly mainly in ionic form. The small influence of the pH on the sorption of TCP and the negligible effect on DCP is due to their higher pKa values (5.99 and 7.85, respectively) which results in the predominance of protonated molecular species at the pH (4-8) of this study. Soil sorption capacity for the protonated neutral molecules is higher than for the dissociated ionized form. The quantity of chlorophenol sorbed at low pH values decreased from PCP to DCP. On the contrary, at high pH values an increase of negative charge associated with soil surfaces decreased the PCP adsorption by electrostatic repulsion; therefore, the quantity of chlorophenol sorbed at higher pH decreased from DCP to PCP. Only PCP sorption was influenced by ionic strength and increased with increasing ionic strength since PCP was mainly in the ionic form in the pH range examined. Acknowledgment: Proyecto Fondecyt Nº1050614.

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