Influence Of Phosphates On Colloidal Stability And Transformation Of Several Nano Oxides

With the increasing usage of insoluble nano-TiO₂/nano-CeO₂ and soluble nano-ZnO in both commercial industries and daily life, it is inevitable for them to be released into natural environment, directly or indirectly, thus resulting in the potential risk to the ecosystem. Given the high surface energy and a large number surface site of the engineered nanoparticles, and the abundant ligand diversity of organic acid,humic acid and phosphorus（P） in the environment, phosphorus like such as myo-inositol hexakisphosphate（IHP） and orthophosphate（Pi）, might strongly bind to engineering nanoparticles at their surfaces, and thus alter interaction between them,their surface chemistry, dissolution, transformation and fate in the ecosystem.Furthermore, aggregation behavior of TiO₂/CeO₂ nanoparticles and chemical transformation of ZnO nanoparticles play a key role in determining their mobility and persistence in natural environment. However, adsorption of P, especially organic phosphate, at the surface of nanoparticles and its influence on the colloidal chemistry behavior and environmental transformation of the engineered nanoparticles has not been adequately addressed.We investigated the influence of surface IHP/Pi coverage 1) on colloidal stability of TiO₂ and CeO₂ nanoparticles and the pH dependence of IHP/Pi coordination on TiO₂/CeO₂ surface, and 2) on dissolution and transformation of ZnO nanoparticles in the presence of P via batch experiments, Zeta potential, dynamic light scattering（DLS）techniques, Derjaguin–Landau–Verwey–Overbeek（DLVO） theory, in situ attenuated total reflectance Fourier transform infrared spectroscopy（ATR-FTIR）, high-resolution transmission electronic microscopy（HRTEM）, solution/solid-state 31 P nuclear magnetic resonance（NMR） spectroscopy and X-ray adsorption spectroscopy（XAS）.The crystal structure and micro morphology of TiO₂, CeO₂, and ZnO nanoparticles were characterized by X-ray diffraction（XRD） and transmission electron microscope（TEM）. TiO₂ nanoparticles are spherical in shape without impurity phase, its average particle size is around 20 nm and the Brunauer-Emmett-Teller（BET） surface area is 83.4 ± 3 m2 g–1. CeO₂ nanoparticles are mainly spherical in shape with an average particle size around 20 nm, and its BET surface area is 82 ± 3 m2 g–1. ZnO nanoparticles are mainly rhombohedral with a particle size ranging from 15 to 35 nm（an average particle size of 30 nm）, and theBrunauer-Emmett-Teller（BET） surface area of ZnO nanoparticles is 37.9 ± 3 m2 g–1.The adsorption isotherms of IHP/Pi onto TiO₂ and CeO₂ nanoparticles can be fitted well by the Langmuir equation; surface IHP/Pi coverage is the behavior depending on solution pH levels, namely, the adsorption density of IHP/Pi on TiO₂ and CeO₂ nanoparticles decreased with the increase of solution pH. The results of batch experiment and spectroscopy indicated formation of inner-sphere complex occurs to the surface of TiO₂ and CeO₂ nanoparticles via ligand exchange between surface hydroxyl and phosphate groups. At low pH, there are two species of Pi adsorbed onto TiO₂ surface: protonated bidentate binuclear surface species≡Ti2–HPO4 and nonprotonated bidentate binuclear surface species ≡Ti2–PO4–. At high pH, two species of Pi adsorbed onto TiO₂ surface are ≡Ti2–PO4– and ≡TiHPO4–,respectively. The protonated extent of phosphate group ions dissociated from the surface of TiO₂ nanoparticles increase with the decrease of solution pH levels based on evidence of 31 P NMR. The mainly species of Pi adsorbed on CeO₂ surface are protonated bidentate binuclear surface species or possibly monodentate surface complexes stabilized by hydrogen bonding to adjacent sites and a monoprotonated bridging bidentate surface complex. In addition, IHP is multi-acid with six phosphate groups, IHP may bind to CeO₂ surface by four of its six phosphate groups and to TiO₂ surface by three of its six phosphate groups, and the other dissociated phosphate groups were the ionization states.Zeta potential measurement, sedimentation experiment, DLS measurement and DLVO calculations suggested that interaction between TiO₂/CeO₂ nanoparticles was highly dependent on pH and surface P coverage. Zeta potentials close to zero will result in unstable suspension and nanoparticle aggregation, vice versa. IHP/Pi played a significant role in colloidal chemistry behavior of TiO₂ and CeO₂ nanoparticles, and the effect of IHP was much greater than Pi due to the high density of negative charge of IHP. It is possible that owing to the strong affinity of TiO₂ and CeO₂ nanoparticles surfaces for low weight organic acid or ions, adsorption of IHP/Pi might modify the surface charge of TiO₂ and CeO₂ nanoparticles and thus change their colloidal stability.The results of IHP interacted with ZnO nanoparticles showed that IHP could substantially enhance the dissolution of ZnO nanoparticles. The IHP-induced dissolution and transformation of ZnO nanoparticles（< 0.5 h） was strikingly faster than that in the presence of phosphate（Pi, > 3.0 h） at pH 7.0, and the reaction rateincreased with decreasing pH and increasing concentration of IHP. Multi-technique analyses indicated that the interaction of ZnO nanoparticles with IHP induced rapid transformation of ZnO nanoparticles into poorly crystalline zinc phytate-like phase（Zn-IHP） and the interaction of ZnO nanoparticles with Pi transforms ZnO nanoparticles into crystalline hopeite（Zn3（PO4）2·4H2O）, which may lower the risk of ZnO Nps ecological and environmental toxicity. Our results also indicated that ZnO nanoparticles preferentially reacted with IHP when Pi and IHP coexist due to the stronger chelating ability of IHP than that of Pi. Additionally, during the dissolution and transformation of ZnO, significant amount of OH– was released from phytate.The mechanism of IHP in mediating the dissolution and transformation of ZnO nanoparticles was investigated using 31 P NMR, in situ ATR-FTIR, XRD, HRTEM,and synchrotron-based extended X-ray absorption fine structure（EXAFS）spectroscopy. Dissolved Zn2+ is complexed with aqueous IHP to form soluble zinc phytate complexes, followed by the precipitation of poorly crystalline zinc phytate phase. These reactions result in the decrease of free Zn2+ ions in the solution,continuous release of Zn2+ from ZnO, and thus subsequent transformation of ZnO nanoparticles into Zn-IHP complexes and stable zinc phytate precipitate phase.In order to better understand the engineering nanoparticles’ behavior in the environment enriched with P and properly predict the environmental fate and ecological risk of TiO₂/CeO₂/ZnO nanoparticles or other similar nanoparticles, it is essential to consider the impacts of organic phosphate and inorganic phosphate due to its influence on colloidal stability and transformation property of their nanoparticles.