The stability of engineered nanoparticles in natural aquatic systems is of high interest for environmental risk assessment since an already important quantity of these reactive species is entering aquatic systems. In the present study an important issue is addressed by investigating i) the influence of divalent cations and water hardness (Mg2+ and Ca2+) in agglomerate formation and ii) alginate concentration effect on the stability TiO2 agglomerates formed in environmental freshwater conditions (pH and total hardness) representative of Lake Geneva, Switzerland. Our results indicate that the presence of alginate at typical natural organic matter concentration strongly modifies the stability of TiO2 nanoparticle agglomerates by inducing their partial disagglomeration. Significant TiO2 nanoparticles redispersion and formation of small fragments is expected to be induced by alginate adsorbed layer formed at the nanoparticle surfaces within the agglomerates.