Effect of the ionic strength and temperature on the arsenic(V) -Fe3+and -Al3+interactions in aqueous solution

Interactions between arsenic(V) and Fe3þ and Al3þ in aqueous solution were studied by potentiometry and, for Fe3þ system, by UV spectrophotometry too. Potentiometric measurements have evidenced the formation of FeAsO4 0 , AlAsO4 0 and Al(AsO4)OH species with formation constant values log b ¼ 17.77, 12.68 and 8.29, respectively, at I ¼ 0.1 mol L1 and T ¼ 298.15 K. In order to applied speciation study to different kind of natural waters, the effect of the ionic strength on the thermodynamic parameters was analysed in the 0.1 I 0.7 mol L1 range and modeled by a Debye-Hückel type equation. In addition, the formation enthalpy changes of the species were determined by direct calorimetric titrations, in order to define the effect of temperature on the strength of interactions. Examples of distribution of species were reported in different experimental conditions. Finally, the ability of arsenate “to sequester” Fe3þ and Al3þ cations was evaluated by pL0.5 empiric parameter, that represents the ligand concentration required to sequester 50% of the metal cation present in traces.