Effect of organic matter and iron oxides on quaternary herbicide sorption-desorption in vineyard-devoted soils

Herbicide soil/solution distribution coefficients (K(d)) are used in mathematical models to predict the movement of herbicides in soil and groundwater. Herbicides bind to various soil constituents to differing degrees. The universal soil colloid that binds most herbicides is organic matter; however metallic hydrous oxides might also have some influence. The adsorption-desorption of three quaternary ammonium herbicides on soils with different chemical-physical characteristics was determined using a batch equilibration method before and after the following sequential selective dissolution procedures: removal of organic matter, and removal of organic matter plus free iron oxides. The experimentation involved paraquat (PQ), diquat (DQ) and difenzoquat (DFQ) herbicides. The distribution coefficients (K(d)) of the molecules and their correlation to the soil components were determined and a significant negative correlation with organic carbon was highlighted (r<-0.610, p<0.035, n=12). All quats cations experiment high adsorption in the control soils with a Zeta potential at about -21 mV. The order of adsorption on soils (based on K(d)) was the following: PQ>DQ>>DFQ. The adsorption isotherms of these three herbicides on the natural and processed soils were satisfactorily fitted with the Freundlich equation, and a significant correlation with organic carbon was highlighted for quats K(F) (r<-0.696, p<0.012, n=12). The removal of organic matter from soils seems to leave free new adsorption sites for quats on the clay surface, which is no longer occluded by organic matter. This work shows that the amount and nature of the surface that remains available after the removal of single soil constituents is a critical parameter in determining the sorptive behavior of cationic contaminants.