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

Interactions in solution of both water-soluble and sodium hydroxide extractable humic substances with four model organic compounds of different polarity and four pesticides of different solubility were investigated by size-exclusion chromatography. At neutral or alkaline pH, association occurred only for the most hydrophobic compounds (acridine orange and atrazine) and for bromocresol green, at low pH after charge suppression. The role of humic substances in promoting mobility of xenobiotic substances through the soil and their effect on interaction of xenobiotics with soil components was also examined. Dissolved humic substances enhance removal of hydrophobic xenobiotic compounds from solution by aluminium and iron hydroxides. The presence of humic substances in the solid phase (humified organic matter) also favours sorption of hydrophobic compounds, whereas predominance of non-humic soil organic matter components favour sorption of polar or negatively charged xenobiotic compounds.