Adsorption, Desorption and Bioavailability of Tungstate in Mediterranean Soils

The adsorption and desorption process of the tungstate ion was studied in three soils
characteristic of the Mediterranean area, with particularly reference to bioavailability pathways.
In the three soils examined, the tungstate adsorption was described by a Langmuir-type equation,
while the desorption process showed that not all the adsorbed tungstate was released, probably due
to the formation of dierent bonds with the adsorbing soil surfaces. The pH was found to be the main
soil property that regulates the adsorption/desorption: The maximum adsorption occurred in the soil
with the acidic pH, and the maximum desorption in the most basic soil. In addition, the organic matter
content played a fundamental role in the adsorption of tungstate by soils, being positively correlated
with the maximum of adsorption. These results indicate that the lowest bioavailability should be
expected in the acidic soil characterized by the highest adsorption capacity. This is confirmed by the
trend of the maximum buer capacity (MBC) of soils which is inversely related to bioavailability,
and was the highest in the acidic soil and the lowest in the most basic soil. Our data could contribute
in drafting environmental regulations for tungsten that are currently lacking for Mediterranean soils.