Interactions between organic model compounds and pesticides with water-soluble soil humic substances

Interactions in solution of both water-soluble and sodium hydroxide extract able humic substances with four model organic compounds of different polari ty and four pesticides of different solubility were investigated by size-ex clusion chromatography. At neutral or alkaline pH, association occurred onl y for the most hydrophobic compounds (acridine orange and atrazine) and for bromocresol green, at low pH after charge suppression. The role of humic s ubstances in promoting mobility of xenobiotic substances through the soil a nd their effect on interaction of xenobiotics with soil components was also examined. Dissolved humic substances enhance removal of hydrophobic xenobi otic compounds from solution by aluminium and iron hydroxides. The presence of humic substances in the solid phase (humified organic matter) also favo urs sorption of hydrophobic compounds, whereas predominance of non-humic so il organic matter components favour sorption of polar or negatively charged xenobiotic compounds.