TECHNICAL REPORTS 1909 Unique forms of manufactured nanomaterials, nanoparticles, and their suspensions are rapidly being created by manipulating properties such as shape, size, structure, and chemical composition and through incorporation of surface coatings. Although these properties make nanomaterial development interesting for new applications, they also challenge the ability of colloid science to understand nanoparticle aggregation in the environment and the subsequent effects on nanomaterial transport and reactivity. his review briefly covers aggregation theory focusing on Derjaguin-Landau-Verwey-Overbeak (DLVO)-based models most commonly used to describe the thermodynamic interactions between two particles in a suspension. A discussion of the challenges to DLVO posed by the properties of nanomaterials follows, along with examples from the literature. Examples from the literature highlighting the importance of aggregation effects on transport and reactivity and risk of nanoparticles in the environment are discussed. Nanoparticle Aggregation: Challenges to Understanding Transport and Reactivity in the Environment Ernest M. Hotze, Tanapon Phenrat, and Gregory V. Lowry* Carnegie Mellon University A s of the submission date of this review, approximately 4700 articles on nanoparticle (NP)-related topics have been published in the calendar year 2009 alone (hompson Reuters, 2009). Many of these papers discuss novel particles, and applica- tions of nanomaterials along with their use in consumer products are expanding rapidly (Project on Emerging Nanotechnologies, 2009). For example, nanomaterials are used for environmental clean-up and biomedical applications (Alivisatos, 2001; Liu et al., 2005). his expansion in applications, and the incorporation of “nano” ingredients into products that may be released during the life cycle of those products, will increase the occurrence of manu- factured nanomaterials in the environment. For example, the leaching of silver NPs used as antimicrobial agents in cloth was recently reported (Benn and Westerhoff, 2008). Unintentionally or intentionally, manufactured NPs might come into contact with biological receptors. For this reason, the risk of nanomaterials to humans and to the environment garners attention from the scien- tific community, governmental agencies, and public stakeholders. Nanoparticless in engineered or environmental systems could be thought of as dispersions of primary particles. However, pri- mary NPs tend to aggregate into clusters up to several microns in size (Brant et al., 2007; Chen and Elimelech, 2007; Jiang et al., 2009; Limbach et al., 2005; Phenrat et al., 2008). herefore, NP aggregation plays an important role in determining reactivity, toxicity, fate, transport, and risk in the environment. he critical role of aggregation in environmental implications had occasion- ally been ignored (Baveye and Laba, 2008), but many recent stud- ies have begun applying colloid science principles, based around Derjaguin-Landau-Verwey-Overbeak (DLVO) theory, to under- stand NP aggregation under various conditions. Manufactured NPs challenge the limits of colloid science, however, due to their small size, variable shape, structure, composition, and poten- tial presence of adsorbed or grafted organic macromolecules. Laboratory aggregation studies, which are covered in detail in this review, have demonstrated that these challenges, acting simulta- neously, will ultimately affect how NPs behave in the environ- ment. Additionally, particles released to the environment undergo Abbreviations: CT, carbon tetrachloride; D f , fractal dimension; DLVO, Derjaguin-Landau- Verwey-Overbeak; EDL, electrostatic double layer; MWCNT, multi-walled carbon nanotubes; NOM, natural organic matter; NP, nanoparticle; NZVI, nano zero valent iron; PZC, point of zero charge; ROS, reactive oxygen species; V elas , elastic-steric potential; V OSM , osmotic potential; XDLVO, extended Derjaguin-Landau-Verwey-Overbeak. Center for Environmental Implications of NanoTechnology (CEINT) and Deps. of Civil & Environmental Engineering, Carnegie Mellon Univ., Pittsburgh, PA 15213-3890. Assigned to Associate Editor Dionysios Dionysiou. Copyright © 2010 by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including pho- tocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. J. Environ. Qual. 39:1909–1924 (2010) doi:10.2134/jeq2009.0462 Published online 20 May 2010. Received 18 Nov. 2009. *Corresponding author (glowry@andrew.cmu.edu). © ASA, CSSA, SSSA 5585 Guilford Rd., Madison, WI 53711 USA SPECIAL SUBMISSIONS