title: Effect of organic matter and pH on the adsorption of metalaxyl and penconazole by soils.
abstract: Soil organic matter (SOM) is considered to be the primary adsorbent of non-ionic pesticides, and it is therefore thought to determine the concentration of such pesticides in the soil solution and how they are transported throughout the medium. It is generally assumed that the sorption capacity of different soils is the same per unit mass of SOM; however, the reactivity also depends on the SOM composition and the pH of the medium. We carried out experiments to study the effects of pH and ionic strength on the adsorption of the non-ionic fungicides metalaxyl and penconazole on four soils containing different amounts of organic carbon. The adsorption isotherms fitted a Freundlich equation. For pH>5, partitioning of the fungicides between the solid phase and the soil solution did not vary with the pH, while at lower pH, the fraction adsorbed on the solid phase increased as the pH decreased. The response was related to the effect of pH on the ionization of the carboxylic groups of the SOM and therefore to the hydrophilic nature of the SOM. Analysis of the charge effect on the partitioning of both fungicides revealed a common response in all four soils. Adsorption appears to be related to the magnitude of the charge developed at the SOM due to ionization of the carboxylic acid groups.
authors.affiliation: Departamento de Química Física, Facultad de Química, Universidad de Santiago de Compostela, E-15782 Santiago de Compostela, Spain.
authors.forename: Dora
authors.initials: D
authors.lastname: Gondar
authors.forename: Rocío
authors.initials: R
authors.lastname: López
authors.forename: Juan
authors.initials: J
authors.lastname: Antelo
authors.forename: Sarah
authors.initials: S
authors.lastname: Fiol
authors.forename: Florencio
authors.initials: F
authors.lastname: Arce
mesh_terms: D000327:Adsorption; D000409:Alanine / Q000031:analogs & derivatives* / Q000737:chemistry; D005659:Fungicides, Industrial / Q000032:analysis / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007477:Ions; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D009930:Organic Chemicals / Q000737:chemistry*; D010575:Pesticides / Q000032:analysis / Q000737:chemistry*; D012987:Soil / Q000737:chemistry; D012989:Soil Pollutants / Q000032:analysis*; D014230:Triazoles / Q000737:chemistry*
chemical_list: D005659:Fungicides, Industrial; D007477:Ions; D009930:Organic Chemicals; D010575:Pesticides; D012987:Soil; D012989:Soil Pollutants; D014230:Triazoles; C028175:metalaxyl; C087058:penconazole; D000409:Alanine
keywords: Adsorption; Charge effect; Metalaxyl; Penconazole; Soil organic matter; pH effect