Surface complexation modeling of interactions between freshwater and marine diatom species and trace elements (Mo, W, Cr, Ge, Ga, Al)

Towards a better understanding of photosynthetic algae interaction with less common trace elements, we performed adsorption experiments of Al, Ga, Ge, Cr (VI), Mo, and W onto four diatom species typical for marine, estuarine, and freshwater aquatic systems. Using a surface complexation approach, we tentatively identified the nature of functional groups involved in the complexation of trace elements with diatom surface moieties and we quantified the stability of surface complexes formed at the diatom - solution interface. The differences in binding properties between freshwater and marine species are linked to variable proportion of three main binding groups at the diatom surfaces, namely amine, carboxylate and silanol. Under acidic conditions, the oxyanion (MoO42−, WO42−, CrO42−) sorption occurred electrostatically with a 1:1 ratio to the protonated amino groups. Trivalent cations (Ga3+ and Al3+) were complexed by deprotonated carboxyl groups, while the anionic hydroxylated forms of these metals (Ga(OH)4− and Al(OH)4−) formed mononuclear monodentate complex with positively charged amino groups at higher pH. Finally, Ge(IV) sorption was controlled by interaction with neutral carboxylic, silanol and amino groups. Taken together, the surface complexation model developed in this study allows rigorous description of anions and trivalent hydrolysates adsorption onto diatom surfaces and demonstrates potential importance of diatoms for the control of trace element transfer in aquatic settings.