Effect of humic acids on the adsorption of paraquat by goethite.
The adsorption of the herbicide paraquat (PQ(2+)) on goethite and on the binary system humic acid-goethite has been studied in batch experiments by performing adsorption isotherms under different conditions of pH, supporting electrolyte concentration and temperature. The results were completed with capillary electrophoresis (CE) in order to measure the binding isotherm between PQ(2+) and humic acid (HA) molecules in solution. PQ(2+) adsorption is negligible on the bare goethite surface but important on the HA-goethite adsorbent. In this last case, the adsorption increases by increasing pH and decreasing electrolyte concentration. There are no significant effects of temperature on the adsorption. The adsorption takes place by direct binding of PQ(2+) to adsorbed HA molecules leading to the formation of surface species of the type goethite-HA-PQ(2+). The results are consistent with a mechanism where PQ(2+) binds negatively charged groups of HA (carboxylates and phenolates) forming ionic pairs or outer-sphere complexes. Since goethite in nature usually contains adsorbed HA molecules, it may act as a good adsorbent for cationic herbicides. This will not only benefit the deactivation of the herbicides but also reduce their leaching and transport through groundwater.
Brigante
Maximiliano
M
INQUISUR, Departamento de Química, Universidad Nacional del Sur, Av. Alem 1253, 8000 Bahía Blanca, Argentina. Electronic address: brigante@uns.edu.ar.
Zanini
Graciela
G
INQUISUR, Departamento de Química, Universidad Nacional del Sur, Av. Alem 1253, 8000 Bahía Blanca, Argentina.
Avena
Marcelo
M
INQUISUR, Departamento de Química, Universidad Nacional del Sur, Av. Alem 1253, 8000 Bahía Blanca, Argentina.
D000327:Adsorption; D019075:Electrophoresis, Capillary; D006540:Herbicides / Q000737:chemistry*; D006812:Humic Substances; D058085:Iron Compounds / Q000737:chemistry*; D008903:Minerals / Q000737:chemistry*; D010269:Paraquat / Q000737:chemistry*; D017550:Spectroscopy, Fourier Transform Infrared; D014961:X-Ray Diffraction
D006540:Herbicides; D006812:Humic Substances; D058085:Iron Compounds; D008903:Minerals; C094886:goethite; D010269:Paraquat