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, K
D, of 1.28 × 103 and 1.37 × 103 L kg-1, respectively, and a Freundlich-type isotherm
for DQ with a linearized partition coefficient, K
D*, of 1.01 × 103 L kg-1. On the other hand, soil 2
exhibited curved isotherms and smaller K
D* 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.
Keywords: Paraquat, diquat, difenzoquat, copper effect, adsorption−desorption, vineyard acid soils
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