Multitechnique Investigation of the pH Dependence of Phosphate Induced Transformations of ZnO Nanoparticles Sewwandi Rathnayake, †,‡ Jason M. Unrine,* ,†,‡ Jonathan Judy, †,‡ Anne-Frances Miller, § William Rao, † and Paul M. Bertsch †,‡,∥ † Department of Plant and Soil Sciences, University of Kentucky, Lexington, Kentucky 40546-0312, United States ‡ Center for Environmental Implications of Nanotechnology, Duke University, Durham, North Carolina 27708, United States § Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055, United States ∥ Division of Land and Water, CSIRO, Ecosciences Precinct, Brisbane, Queensland 4102, Australia * S Supporting Information ABSTRACT: In order to properly evaluate the ecological and human health risks of ZnO manufactured nanomaterials (MNMs) released to the environment, it is critical to understand the likely transformation products in various environments, such as soils, surface and ground waters, and wastewater treatment processes. To address this knowledge gap, we examined the transformation of 30 nm ZnO MNMs in the presence of different concentrations of phosphate as a function of time and pH using a variety of orthogonal analytical techniques. The data reveal that ZnO MNMs react with phosphate at various concentrations and transform into two distinct morphological/structural phases: a micrometer scale crystalline zinc phosphate phase (hopeite-like) and a nanoscale phase that likely consists of a ZnO core with an amorphous Zn 3 (PO 4 ) 2 shell. The P species composition was also pH dependent, with 82% occurring as hopeite-like P at pH 6 while only 15% occurred as hopeite-like P at pH 8. These results highlight how reactions of ZnO MNMs with phosphate are influenced by environmental variables, including pH, and may ultimately result in structurally and morphologically heterogeneous end products. ■ INTRODUCTION Manufactured nanomaterials (MNMs) are increasingly being employed in consumer products such as pharmaceuticals, cosmetics, and electronics, exploiting their novel physical and chemical properties. 1,2 The commercialization of nanotechnol- ogy is expanding rapidly and is expected to be a $1 trillion (U.S. dollars) industry by 2015. 3 However, MNMs can enter the environment during their manufacturing, transport, use, and disposal and several studies have demonstrated that MNMs will be released into wastewater streams during their use from consumer items, such as textiles and personal care products. 4−6 Life-cycle inspired material flow analysis has indicated many MNMs will partition to biosolids during wastewater treatment and that soil will be a primary repository for MNMs in areas where biosolids are land applied. 2,6 Other possible routes of environmental exposure are by direct application of MNMs to agricultural fields as agrochemicals, 7,8 or direct release to surface waters from personal care products such as sunscreens 9 or as wastewater treatment effluents. 2 The potential risks to environmental and human health posed by MNMs deposited into aquatic and terrestrial ecosystems in this manner are not fully understood. Once discharged into the environment, MNMs will be subjected to dynamic physical and chemical conditions, which will result in transformation to largely unknown end products. 10 For example, several studies have shown that silver (Ag) MNMs are readily transformed to Ag 2 S during the wastewater treatment process and Ag 2 S nanoparticles have been found in wastewater effluent and sludge in a pilot wastewater treatment plant, 11 as well as in field sample sewage sludge. 12 However, the structure, morphology, and mobility of Ag 2 S may differ in biosolids receiving Ag + as compared to Ag MNMs. 13 Recently, two studies reported that ZnO MNMs rapidly transformed to ZnS and Zn 3 (PO 4 ) 2 during anaerobic digestion of wastewater and post-treatment processing of sewage sludge. 14,15 These transformations are expected to dramatically alter the fundamental properties of the MNMs and will likely impact their bioavailability, toxicity, and mobility in terrestrial ecosystems. Therefore, characterizing the nature and mechan- sims of these transformations is essential in assessing the potential risk to the environment. 16 Zinc oxide nanoparticles are among the highest volume MNMs used in consumer products due to their widespread use in semiconductors, pharmaceuticals, paints, personal care products, and sunscreens. 17−20 Modeling efforts predict ZnO Received: October 10, 2013 Revised: March 18, 2014 Accepted: April 2, 2014 Article pubs.acs.org/est © XXXX American Chemical Society A dx.doi.org/10.1021/es404544w | Environ. Sci. Technol. XXXX, XXX, XXX−XXX