Transformations of ZnO Nanoparticles in Waste Water Treatment, Biosolids and Soil.

Nano zinc oxide (ZnO) is among the highest volume/mass manufactured nanomaterials (MNMs) currently used in consumer products. Therefore understanding its fate in the environment is of critical importance. Life-cycle inspired risk analysis has indicated that soil will be a primary repository for nano ZnO resulting from its likely entry into the wastewater stream, ultimately ending up in wastewater treatment derived biosolids, a majority of which are land applied.  In order to properly evaluate the ecological and human health risks of MNMs released to the environment, it is critical to understand the likely transformation products of manufactured metal and metal oxide nanomaterials in the wastewater treatment process and in soils receiving biosolids.

                ZnO MNMs will likely react with both phosphate ions and natural organic matter (NOM) in the wastewater stream and in the wastewater treatment process. Although the solubility of Zn-phosphates are low (Ksp ~9X 10^-33), little is known regarding the reaction kinetics of ZnO MNMs with phosphate, NOM, and in mixed phosphate-NOM systems. To address this critical knowledge gap, we examined the transformation reactions of 30nm ZnO MNMs in the presence of phosphate and NOM as a function of time and pH. Samples were analyzed using various techniques, including dynamic light scattering, phase analysis light scattering, inductively coupled plasma mass spectrometry, spatially resolved synchrotron X-ray absorption near edge structure spectroscopy, Phosphorus-31 nuclear magnetic resonance spectroscopy, as well as transmission electron microscopy coupled with energy dispersive x-ray spectroscopy. Preliminary data reveals that ZnO MNMs react with phosphate at concentrations expected in wastewater and transform into two distinct phases; a micron sized zinc phosphate mineral phase (hopeite) and a nano-sized phosphate rich ZnO phase. TEM imaging with energy dispersive X-ray spectroscopy and XANES spectroscopy provide evidence that the nano- sized phosphate rich ZnO phase is likely comprised of a ZnO core and a Zn-phosphate shell.