Adsorption, Desorption and Bioavailability of Tungstate in Mediterranean Soils

The adsorption and desorption process of the tungstate ion was studied in three soilscharacteristic 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 dueto the formation of dierent bonds with the adsorbing soil surfaces. The pH was found to be the mainsoil property that regulates the adsorption/desorption: The maximum adsorption occurred in the soilwith the acidic pH, and the maximum desorption in the most basic soil. In addition, the organic mattercontent played a fundamental role in the adsorption of tungstate by soils, being positively correlatedwith the maximum of adsorption. These results indicate that the lowest bioavailability should beexpected in the acidic soil characterized by the highest adsorption capacity. This is confirmed by thetrend 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 contributein drafting environmental regulations for tungsten that are currently lacking for Mediterranean soils.