Selenium chemical speciation in natural waters: Protonation and complexation behavior of selenite and selenate in the presence of environmentally relevant cations

Selenium is an essential trace element. Selenium speciation in natural environmental conditions has been poorly studied, mostly because its relevance has been only recently acknowledged. In this work, a complete thermodynamic study on selenium(IV) and selenium(VI) was carried out. It includes the protonation equilibria of selenium anions in aqueous solution at 20 °C. The interaction of selenium anions with some metal ions that are normally encountered in environmental scenarios was also studied. The strategy involves the potentiometric study of all the equilibria using different ionic strength values of a non interacting electrolyte. Distribution species diagrams allow the prediction of the predominant species of selenium both under reducing or oxidizing conditions and in the presence of these metal ions. The dependence of the thermodynamic magnitudes on the ionic strength value is also evaluated. The speciation of Se(IV) or Se(VI), 10− 7 M, in the presence of mean concentrations of Ca2+ (16.5 mM) and Mg2+ (6.4 mM) shows the predominance of HSeO3−, [Ca(HSeO3)]+ and [Mg(HSeO3)]+, or of SeO42−, respectively. Results show that, depending on media pH and pE, interaction with metal ions can be either negligible or very important.