Retention of propiconazole and terbutryn on acid sandy-loam soils with different organic matter and Cu concentrations.

Equilibrium propiconazole and terbutryn adsorption characteristics were evaluated in ten acid vineyard soils with a wide range of organic matter and copper concentrations using batch experiments. Adsorption data from equilibrium adsorption experiments were generally well described by linear and Freundlich models. Parameters from these models showed that soil organic matter played a key role of propiconazole adsorption processes, but also the amorphous Fe oxides content in soils. Soil organic matter positively influenced terbutryn adsorption, whereas increases in exchangeable copper decreased terbutryn adsorption. Desorption experiments showed that both, propiconazole and terbutryn adsorption in soils was quite irreversible, i.e. the amount of pesticides desorbed after its adsorption was always less than 50%.

Phosphorus effect on Zn adsorption-desorption kinetics in acid soils.

The adsorption-desorption kinetics of Zn in the absence and presence of P was studied by using the stirred flow chamber technique. The results thus obtained were compared with those previously obtained for Cu. As with copper, the simultaneous addition of P and Zn in a 1:1 mole ratio to soil was found to significantly increased Zn adsorption relative to the absence of P. Unlike Cu, however, Zn was only adsorbed at fast adsorption sites in the absence of P. In any case, the increased adsorption of Zn in the presence of P was largely due to slow adsorption sites, where Zn(2+) ion acted as a bridging element between P and organic matter. Following adsorption in both the presence and absence of P, Zn was desorbed to a much higher extent than was Cu. However, the proportion of Zn desorbed after adsorption in the presence of P was significantly lower than in the absence of P. This indicates that Zn binds more strongly to adsorbing surfaces in the presence of P than in its absence.

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.

Influence of copper on the adsorption and desorption of paraquat, diquat, and difenzoquat in vineyard acid soils.

Retention of the cationic herbicides paraquat (PQ), diquat (DQ), and difenzoquat (DFQ) in two vineyard soils with a different management history and retention capacity was examined. The influence of copper on the ability of the soils to retain the herbicides was determined by comparing the results of adsorption and desorption tests on untreated and Cu-enriched soil samples, and also on soils that were previously treated with EDTA to extract native copper. The three herbicides were strongly adsorbed by both soils. Soil 1 exhibited linear adsorption isotherms for PQ and DFQ with partition coefficients, KD, of 1.28 x 103 and 1.37 x 103 L kg-1, respectively, and a Freundlich-type isotherm for DQ with a linearized partition coefficient, KD*, of 1.01 x 103 L kg-1. On the other hand, soil 2 exhibited curved isotherms and smaller KD* values (viz. 106, 418, and 28 L kg-1 for PQ, DQ, and DFQ, respectively). Using EDTA to extract copper from the soils released new sites for the herbicides to bind. The three herbicides exhibited strong hysteresis in the adsorption-desorption process. Extracting copper decreased the percent desorption of PQ and DQ; on the other hand, it decreased the affinity of DFQ for the resulting vacant adsorption sites. Similarly, competitive adsorption tests with copper and the herbicides revealed that the metal was only capable of displacing DFQ from adsorption sites. The behavior of this herbicide in the soils was consistent with a specific adsorption model. The disparate behavior of the two soils toward the herbicides was a result of the adsorption sites in soil 1 being less extensively occupied than those of soil 2 in the adsorption tests. The effect of copper on the adsorption of DFQ in the two soils was acceptably reproduced by an adsorption model involving Coulombic and specific sorption with competition from the metal.