Effects of myo-inositol hexakisphosphate and orthophosphate adsorption on aggregation of CeO2 nanoparticles: roles of pH and surface coverage

Abstract

Environmental context To understand the behaviour and fate of nanoparticles (NPs) in the natural environment requires knowledge of their aggregation state under environmentally relevant conditions. This work investigates the influence of myo-inositol hexakisphosphate and orthophosphate on the colloidal stability of CeO2 NPs, and shows that adsorption of organic and inorganic phosphorus plays an important role on the interaction between the nanoparticles. Surface phosphorus coverage should be considered when predicting the fate of CeO2 NPs and other similar NPs in the environment. Abstract The effects of myo-inositol hexakisphosphate (IHP) and orthophosphate (Pi) adsorption on aggregation and dispersion of CeO2 nanoparticles (NPs) in suspension and the underlying mechanism were investigated. The results show that IHP and Pi play a significant role in the colloidal chemistry behaviour of CeO2 NPs through inner-sphere complex formation on adsorption as indicated by concurrent hydroxyl release, zeta (ζ) potential measurements and in situ attenuated total reflectance Fourier-transform infrared spectroscopy. The ratio of IHP/Pi adsorption density suggests that IHP may complex on the CeO2 surface through four of its six phosphate groups with the other two free and dissociated. Sedimentation and aggregation of CeO2 NPs in suspension are mainly dependent on their surface potentials, which are greatly regulated by pH and surface phosphorous coverage. At pH 3.0, IHP led to aggregation of CeO2 NPs at a loading lower than 0.15μmolL–1, whereas a higher IHP loading made them disperse again. However, Pi adsorption merely caused an increase of the aggregate size of CeO2 NPs. At pH 7.0, both IHP and Pi can apparently stabilise the suspension of CeO2 NPs from aggregation by alteration of the zeta potential from near zero down to –38mV. The effect of IHP on the aggregation and dispersion of CeO2 NPs is much greater than that of Pi, which agrees well with calculations from Derjaguin–Landau–Verwey–Overbeek (DLVO) theory.