Characterization of Nanomaterials in Complex Environmental and Biological Media

By Elsevier Science

Characterization of Nanomaterials in Complex Environmental and Biological Media covers the novel properties of nanomaterials and their applications to consumer products and industrial processes.

The book fills the growing gap in this challenging area, bringing together disparate strands in chemistry, physics, biology, and other relevant disciplines. It provides an overview on nanotechnology, nanomaterials, nano(eco)toxicology, and nanomaterial characterization, focusing on the characterization of a range of nanomaterial physicochemical properties of relevance to environmental and toxicological studies and their available analytical techniques.

Readers will find a multidisciplinary approach that provides highly skilled scientists, engineers, and technicians with the tools they need to understand and interpret complicated sets of data obtained through sophisticated analytical techniques.

• Addresses the requirements, challenges, and solutions for nanomaterial characterization in environmentally complex media
• Focuses on technique limitations, appropriate data collection, data interpretation, and analysis
• Aids in understanding and comparing nanomaterial characterization data reported in the literature using different analytical tools
• Includes case studies of characterization relevant complex media to enhance understanding

• Language: English
• Publisher: Elsevier Science
• Release date: Jun 1, 2015
• ISBN: 9780080999500

Book preview

Frontiers of Nanoscience

Characterization of Nanomaterials in Complex Environmental and Biological Media

Volume 8

Editors

Mohammed Baalousha and Jamie R. Lead

Center for Environmental Nanoscience and Risk, Arnold School of Public Health Department of Environmental Health Sciences University of South Carolina, Columbia, South Carolina, USA

Table of Contents

Cover image

Title page

Frontiers of Nanoscience

Copyright

Contributors

Preface

Part I Nanomaterials and Characterization

Chapter 1. Ecotoxicology of Nanomaterials in Aquatic Systems

1. Introduction

2. Types, Uses and Properties of NMs

3. Sources, Entry and Fate of NMs in Aquatic Environments

4. Dose-Metrics and Features of NMs That Elicit Toxicity

5. NM Characterisation

6. Ecotoxicity of NMs in Aquatic Environments

7. Evidence for Toxicological Effects of NMs in Aquatic Organisms

8. Gaps in Our Knowledge and Future Challenges in NM Aquatic Ecotoxicology

Chapter 2. Overview of Nanomaterial Characterization and Metrology

1. Introduction

2. Physical Characterization

3. Chemical and Elemental Characterization

4. Behavioral Characterization

5. Combined Physical–Chemical Characterization

6. Conclusions

Part II Physicochemical Characterization

Chapter 3. Size Distributions

1. Introduction

2. General Considerations for Size Analysis

3. Descriptors of Size Distributions

4. Measurement Methods

5. Comparison and Interconversion between PSDs

6. General Considerations for Environmental Systems

7. Summary

Chapter 4. Analytical Transmission Electron Microscopy and Scanning Transmission Electron Microscopy Techniques for the Characterization of Nanomaterial Composition, Phase and Crystallinity

1. Introduction

2. TEM and STEM Instruments

3. Specimen Preparation: Ion-Milled Foils, Loose Grains, Ultramicrotome Slices and Cryofixed Samples

4. Conclusions

Chapter 5. Methods for Measuring Concentration (Mass, Surface Area and Number) of Nanomaterials

1. Introduction

2. Concentration Metrics and Their Environmental and Toxicological Relevance

3. Measurement of Mass Concentration

4. Measurement of Surface Area Concentration

5. Measurement of Number Concentration

6. Conclusion

Chapter 6. Nanomaterials: Dispersion, Dissolution and Dose

1. Introduction

2. Detection

3. Dispersion

4. Dissolution

5. Dose

6. Final Summary

Chapter 7. Surface Properties (Physical and Chemical) and Related Reactions: Characterization via a Multi-Technique Approach

1. Introduction: Relationship between Surface Reactivity, Surface Energy and Surface Atoms

2. Surface Properties and Surface Reactions

3. Surface Properties Characterization in Complex Media: (X-ray, Electron, … Based Techniques)

4. Concluding Remarks

Part III Case Studies

Chapter 8. Control of Nanomaterials Used in Chemical Mechanical Polishing/Planarization Slurries during On-site Industrial and Municipal Biological Wastewater Treatment

1. Introduction to Nanomaterials in Chemical Mechanical Polishing/Planarization Fluids

2. Materials and Methods

3. Analytical Methods

4. Results

5. Conclusions

Chapter 9. Case Study – Characterization of Nanomaterials in Food Products

1. Introduction

2. Analytical Techniques Used for the Detection and Characterization of NMs in Foods

3. Challenges Associated with the Characterization of NMs in Food

4. Conclusions and Future Trends

Index

Frontiers of Nanoscience

Series Editor: Richard E. Palmer

The Nanoscale Physics Research Laboratory, The School of Physics and Astronomy,

The University of Birmingham, UK

Vol. 1 Nanostructured Materials edited by

Gerhard Wilde

Vol. 2 Atomic and Molecular Manipulation edited by

Andrew J. Mayne and Gérald Dujardin

Vol. 3 Metal Nanoparticles and Nanoalloys edited by

Roy L. Johnston and J.P. Wilcoxon

Vol. 4 Nanobiotechnology edited by

Jesus M. de la Fuente and V. Grazu

Vol. 5 Nanomedicine edited by

Huw Summers

Vol. 6 Nanomagnetism: Fundamentals and Applications edited by

Chris Binns

Vol. 7 Nanoscience and the Environment edited by

Jamie R. Lead and Eugenia Valsami-Jones

Vol. 8 Characterization of Nanomaterials in Complex Environmental and Biological Media edited by

Mohammed Baalousha and Jamie R. Lead

Copyright

Elsevier

Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands

The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, UK

225 Wyman Street, Waltham, MA 02451, USA

Copyright © 2015 Elsevier Ltd. All rights reserved.

No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions.

This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein).

Notices

Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary.

Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility.

To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein.

ISBN: 978-0-08-099948-7

ISSN: 1876-2778

For information on all Elsevier publications visit our website at http://store.elsevier.com/

Contributors

Mohammed Baalousha,     Center for Environmental Nanoscience and Risk (CENR), Department of Environmental Health Sciences, Arnold School of Public Health, University of South Carolina, Columbia, SC, USA

Xiangyu Bi,     School of Sustainable Engineering and the Built Environment, Arizona State University, Tempe, AZ, USA

Alistair Boxall,     Environment Department, University of York, Heslington, York, UK

Andy Brown,     Institute for Materials Research, School of Chemical and Process Engineering, University of Leeds, Leeds, West Yorkshire, UK

Rik Brydson,     Institute for Materials Research, School of Chemical and Process Engineering, University of Leeds, Leeds, West Yorkshire, UK

Agnieszka Dudkiewicz,     The Food and Environment Research Agency, Sand Hutton, York, UK

Anne A. Galyean

Bioprocess Measurements Group, Biomolecular Measurement Division, Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD, USA

Gillings School of Global Public Health, Department of Environmental Sciences and Engineering, University of North Carolina, Chapel Hill, NC, USA

Rhys Goodhead,     Biosciences, University of Exeter, Exeter, UK

Justin M. Gorham,     Nano Materials Research Group, Materials Measurement Science Division, Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD, USA

Andrew Herzing,     Materials Structure and Data, Materials Measurement Science Division, Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD, USA

Michael F. Hochella Jr.

Department of Geosciences, Virginia Tech, Blacksburg, VA, USA

Institute for Critical Technology and Applied Science, Environmental Nanoscience and Technology Laboratory, Virginia Tech, Blacksburg, VA, USA

R. David Holbrook,     Nano Materials Research Group, Materials Measurement Science Division, Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD, USA

Nicole Hondow,     Institute for Materials Research, School of Chemical and Process Engineering, University of Leeds, Leeds, West Yorkshire, UK

Victoria Jennings,     Biosciences, University of Exeter, Exeter, UK

Bojeong Kim,     Department of Earth and Environmental Science, College of Science and Technology, Temple University, Philadelphia, PA, USA

Jamie R. Lead,     Center for Environmental Nanoscience and Risk (CENR), Department of Environmental Health Sciences, Arnold School of Public Health, University of South Carolina, Columbia, SC, USA

John Lewis,     The Food and Environment Research Agency, Sand Hutton, York, UK

Manuel D. Montano,     Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, CO, USA

Soubantika Palchoudhury,     Center for Environmental Nanoscience and Risk (CENR), Department of Environmental Health Sciences, Arnold School of Public Health, University of South Carolina, Columbia, SC, USA

John Pettibone,     Nano Materials Research Group, Materials Measurement Science Division, Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD, USA

James Ranville,     Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, CO, USA

Robert Reed,     School of Sustainable Engineering and the Built Environment, Arizona State University, Tempe, AZ, USA

Jerome Rose,     CNRS-Aix Marseille University, IRD, UM34, UMR 7330, Europole de l'arbois, Aix en Provence, France

Karen Tiede,     The Food and Environment Research Agency, Sand Hutton, York, UK

Charles R. Tyler,     Biosciences, University of Exeter, Exeter, UK

Paul Westerhoff,     School of Sustainable Engineering and the Built Environment, Arizona State University, Tempe, AZ, USA

Preface

In spite of a decade of research in determining the fate, behavior and biological impact of nanomaterials, the precise role of their properties in these processes is still poorly understood. This is due to the multidisciplinary nature of the problem in hand requiring experts in material science, chemistry, physics, biology and toxicology to be brought together. In addition, the complexity of nanomaterial properties, behaviours and dynamic transformations in environmental and biological media makes the problem extremely challenging. The problem is further exacerbated by the lack of nanomaterial characterization methods for complex media, standard sample-preparation methods, technical expertise and understanding and interpretation of the measured parameters. As a result, there are significant discrepancies between measurements performed on different nanomaterial batches by different analytical techniques and among different research groups. Despite the significant progress in the quality of material characterization over the last few years, the majority of research papers describing environmental and toxicological behaviours of nanomaterials do not fully address nanomaterial characterization, and put little emphasis on linking nanomaterial properties to their environmental behaviours. As a result, there are huge discrepancies in the available literature regarding, for instance, the role of size, particles and ions, surface coatings, etc. Therefore, this book was written in part as a survey of the state of the art in nanomaterial characterization for experts as well as for those investigating the fate and effects of nanomaterials. In part, it is also an effort to bridge this knowledge gap and to better understand nanomaterial characterization and the role of nanomaterial properties in controlling their environmental and toxicological behaviour.

The book was organized to accommodate the vision outlined above by summarizing the current state of the art in nanomaterial characterization, with a focus on modern and novel application of techniques that have not been previously examined in detail or techniques that have seen vast methodological improvements in recent years. The book is divided into three parts; the first part (Chapters 1 and 2) reviews the current state of the art of nanomaterial toxicity to aquatic organisms and nanomaterial characterization techniques; the second part (Chapters 3–7) carefully and critically describes characterization of specific properties of nanomaterials relevant to their environmental behaviours applying a multi-method approach, with specific attention to sample preparation and comparability of measurements performed by different analytical techniques and the third part (Chapters 8 and 9) presents two case studies of nanomaterial characterization in consumer products and food stuffs.

More specifically, Chapter 1 gives a critical review on current nanomaterial toxicity data in aquatic organisms, nanomaterial dose-metric and nanomaterial features that elicit toxicity, the importance of nanomaterial characterization in underpinning nanomaterial health and safety research. Chapter 2 provides the reader with an overview of the basic concepts of the plethora analytical techniques that can be applied to measure nanomaterial properties of relevance to their environmental behaviour, fate and effects.

Chapter 3 gives an account of nanomaterial size characterization based on microscopy techniques, diffusion coefficient and other advanced approaches such as single particle inductively coupled plasma–mass spectrometry. This chapter presents the general consideration for size analysis, the descriptors of size distributions, the comparison and interconversion between distributions and ends with a discussion of the general considerations for nanomaterial size characterization in environmental systems. Chapter 4 discusses how to identify and characterize chemical composition and crystal structure (and therefore the exact phase or phases) of nanomaterials (parent forms) as well as their transformed ones (daughter forms) by using analytical transmission electron microscopy and scanning transmission electron microscopy. This chapter gives special attention to data acquisition and analysis methods, as well as sample-preparation procedures in great detail. Chapter 5 examines the characterization of the three most commonly measured particle concentration metrics, namely mass, surface area and number. The chapter discusses the methods used to measure these metrics, and sample preparation for selected methods, including microscopy and inductively coupled plasma–mass spectrometry. Available analytical techniques for mass, surface area and number concentration measurements are systematically evaluated in terms of sensitivity/detection range for nanomaterial mass, representing nanomaterial surfaces in dispersion, distinguishing nanomaterials in complex medium and minimizing sampling artefacts in number distributions. Chapter 6 highlights the complex interrelationship between: the form of a nanoparticulate material dispersed in a particular medium; the resultant dissolution or chemical change of the nanomaterials in that delivery medium and the nature and degree of uptake of the nanomaterials by a particular cell or organism. Thus, the chapter reviews the various techniques which are appropriate to determine nanomaterial dispersion, dissolution and dose. Chapter 7 discusses the importance of nanomaterial surface properties and surface reactions such as surface area, surface atomic arrangements, and adsorption, photocatalytic and redox reactions. This is followed by describing a multi-method approach for the characterization of nanomaterial surface properties and reactions, with special attention given to sample preparation to preserve surface properties. The chapter intentionally distinguishes between surface-specific and non-surface-specific techniques.

Chapter 8 presents a case study on the characterization of common chemical mechanical polishing/planarization nanomaterials such as silica, cerium and aluminium oxides. The chapter investigates nanomaterial removal efficiency using a common industrial on-site treatment strategy (chemical softening and precipitation) and off-site treatment at biological wastewater treatment plants. Chapter 9 focuses on the characterization of nanomaterials in foodstuffs and food contact materials, and covers approaches for sample preparation prior to analysis; the analytical techniques available for nanomaterial detection and characterization in food matrices and the appropriateness of various QA and QC methodologies. The chapter ends by discussing the specific challenges associated with nanomaterial detection, with analysis in food, beverages and nutraceuticals also discussed.

As the reader will appreciate, due to the breadth of the topic and the skills needed to put this book together, this book would not have been realized without the contributions of the authors of each chapter. Accordingly, we would like to express our gratitude to all authors who have contributed to this work and recognize the efforts that made this publication possible. We would like also to thank various funding bodies supporting the editors in their general research that led the editors to develop this book, in particular, the U.S. National Science Foundation, the Center for Environmental Nanoscience and Risk and the Arnold School of Public Health at the University of South Carolina. Lastly, we would like to thank the publishing team at Elsevier whose help was vital for this book.

Mohammed Baalousha and Jamie R. Lead

Part I Nanomaterials and Characterization

Outline

Chapter 1. Ecotoxicology of Nanomaterials in Aquatic Systems

Chapter 2. Overview of Nanomaterial Characterization and Metrology

Chapter 1

Ecotoxicology of Nanomaterials in Aquatic Systems

Victoria Jennings, Rhys Goodhead and Charles R. Tyler¹     Biosciences, University of Exeter, Exeter, UK

¹ Corresponding author: E-mail: [e-mail]

Abstract

The unique properties of manufactured nanomaterials (NMs), conveyed by their small size that are exploited for novel uses, may also confer different toxicological effects compared with their larger (bulk) counterparts. In this first chapter we provide a critical review on current toxicity data for NMs in aquatic organisms and illustrate thorough characterisation of NMs, which is essential for making comparisons on effects data. Adverse effects of NMs have been shown for a wide range of NMs and in diverse aquatic organisms but, with a few exceptions, only for concentrations that exceed current levels in the natural environment predicted by modelling studies. For some NMs, there appear to be particle-induced biological effectsthat include oxidative stress. For some metal NMs most evidence indicates that biological effects derive principally from metal ions due to dissolution. For carbon-based NMs some effects have been shown to derive from materials associated with the preparation of those NMs. The physicochemistry of the aquatic environment fundamentally effects the form of NMs altering their toxicity, and any standardised tests for assessing ‘nano’-related toxicity in aquatic wildlife will need comprehensive characterisation on the dynamics of NMs in relevant test media. Some nanomedicines are being designed specifically to penetrate cell membranes and induce altered biological function and they may warrant particular attention for risk analysis in the near future.

Keywords

Aquatic; Characterisation; Ecotoxicology (nanoecotoxicology) nanotoxicology; Environment; Nanomaterials (NMs)

1. Introduction

In recent years the development and production of manufactured nanomaterials (NMs), particles with dimensions between 1 and 100  nm, has increased rapidly to supply the expanding nanotechnology industry. Global markets for NMs are expected to exceed $1  trillion by 2015.¹ The small size of NMs results in different and unique properties compared with their larger counterparts and their applications to date are extremely diverse. Indeed, there are currently in excess of 1600 commercially available products containing NMs² (Figure 1). Increasing use of NMs inevitably results in increased discharges into the environment, particularly into aquatic environments. The data on environmental levels of NMs, however, are currently scarce, and there has been a strong reliance on modelling for informing on possible exposure scenarios in humans and wildlife. Modelling approaches may not necessarily reflect real-world exposures of ecotoxicological significance, however, because NMs can change fundamentally with the physicochemical conditions of the immediate environment and this can alter their potential for toxicological effects.

Figure 1  Number of products containing commercial NMs (bar chart) and NMs in products as defined by product categories (pie chart) (As listed on the Consumer Products Inventory, The Project on Emerging Nanotechnologies ² ).

Considerable efforts have recently been directed towards research into the toxicological properties of a wide range of NMs to help better understand their potential risks to human health. For the most part these studies have not considered real-world exposure scenarios, but they are nevertheless starting to advance understanding on the toxicology of some NMs. Less attention has been directed at the potential impacts of NMs in wildlife. The aquatic environment acts as a sink for most contaminants discharged, and, as NMs become more prevalent in aquatic systems, it will become increasingly important to assess their potential risks to aquatic wildlife. Current toxicity data for bulk materials cannot be extrapolated in a straightforward manner to nanosized counterparts because both the physicochemical properties associated with nanoscale differ. Furthermore, NMs are often modified using coatings and functional groups that will affect their toxicological properties. Seas, estuaries, streams, rivers, lakes and ponds have very different physicochemical characteristics that will impact the fate and behaviour, and thus the bioavailability and toxicity of NMs (Figure 2).

Figure 2  Sources of NM to, and their transformations in, aquatic environments.

There are many sources of NMs into aquatic environments (see Section 3) and NMs are likely to be taken up into a range of aquatic organisms across diverse phyla. Bioavailability, and thus potential for toxicity, however, will differ depending on morphologies, habitats and behaviours of these organisms and the physicochemical conditions of their habitat. Developing comprehensive understanding on the risks of NMs and providing appropriate environmental protection for NMs will require understanding of the exposure scenarios for specific wildlife organisms in the different aquatic environments. Understanding on the ecotoxicology of NMs also requires thorough characterisation of the NMs in the exposure medium.

In this chapter, after a brief overview of different NMs, we consider the fate of NMs in aquatic systems and detail the current knowledge on their ecotoxicity for a range of organisms in both freshwater and marine environments. We also identify common effects across these aquatic organisms. To better understand the nature and form of NMs in their natural waters, which will affect their toxicity, thorough characterisation of NMs is essential. We introduce NM characterisation methods to assess physical, chemical and behavioural features, which is covered in subsequent chapters in more detail. We also provide a viewpoint on the challenges we face in linking laboratory-based toxicology for NMs with real-world exposures and effects. Finally, in this chapter we highlight some key emerging issues for NM ecotoxicology research.

2. Types, Uses and Properties of NMs

The physical and chemical characteristics of materials with dimensions in the nanoscale differ to their bulk counterparts. It is these differences that are exploited in a wide range of applications spanning ultraviolet (UV) protection in sunscreens, fuel additives to enhance combustion efficiency, as antimicrobial agents, and as electrical conductors, to name just a few. These differing properties, however, may also have implications when considering the fate and toxicity of these materials. Smaller particulate size gives NMs a higher surface area to volume ratio which can lead to higher reactivity and for some metal-based materials enhanced rates of dissolution of ions from the NM surface. NM particle size and shape can dictate the route of uptake into cells and reactivity. Furthermore, the small size of NMs means that some can cross biological barriers through routes not normally accessible for their larger counterparts. Some NMs are produced in diverse forms, for example, silver NMs are produced as spherical, rod-shaped and flower-structured materials. Carbon NMs can be in forms of carbon nanotubes (CNT), sheets or spheres, each with the potential for different interactions and behaviour, and thus different toxicities.

There are many forms of NMs entering aquatic environments, including carbon-based, metal/metal oxide, quantum-dots (Q-dots), dendrimers and nanocomposites. The most commonly used NMs currently are metal, metal oxide and carbon-based NMs.² Carbon NMs are used for a wide range of applications including gene therapy, in fuel cells utilising their conductive and catalytic properties and in the manufacture of products such as sports racquets and car bodies because of their high tensile strength and light weight.

Metal/metal oxide NMs are similarly used for a wide range of applications, spanning antibacterial agents in medical dressings, brightening agents in paints, fuel catalysts, imaging contrast agents and for groundwater remediation. These materials can be produced with relative ease by reduction or hydrolysis reactions. Nanosized silver (Ag-NMs) is the most commonly used metal NM, predominantly for its antibacterial property. This property is generally associated with ionic silver as the silver NMs (Ag-NMs) undergo dissolution, although toxicity associated with the particle itself is also reported. Antibacterial products that incorporate Ag-NMs include wound dressings, sportswear fabrics, washing machines and air filters. Coating or capping agents are often used to stabilise NMs, and given the nature of some of these capping agents, these can be an additional source of pollutants to aquatic environments. Gold (Au) NMs have size-dependent optical properties, where free electrons follow quantum mechanical rules. Gold NMs are used for a large number of industrial and medical applications, including as nontoxic drug carriers and in both imaging and biodevices for diagnostics. Copper (Cu) NMs too have been used for their antibacterial properties in bioactive antifouling coatings and also in textiles. The optical and semi-conductor properties of copper (I) oxide NMs are being investigated for use in solar energy cells and for catalysis applications (as both catalyst and catalyst support material). Commercially, Cu-NMs have also been used in lubricants as an antifriction agent, in cosmetic products and in batteries.

The photocatalytic properties of metal oxide NMs, such as titanium dioxide (TiO2), zinc oxide (ZnO) and cerium dioxide (CeO2) are exploited in solar cells and in water and air purification systems. They also have been exploited for use in personal care products, including sunscreens and cosmetics to provide UV protection in transparent formulations. TiO2-NMs are also used in paints to act as a whitening pigment. CeO2-NMs are commonly used as catalysts and in fuels in the automotive industry to increase fuel efficiency and reduce exhaust emissions. This use is likely to contribute to increases in environmental levels of CeO2-NMs in both the atmosphere and aquatic systems (e.g. via road runoff). Aluminium NMs (usually Al2O3) have a wide range of uses including as catalytic agents to improve ignition probability in fuel and to enhance the properties of polymer composites used in electronic circuitry.

Nano-zero-valent iron (nZVI) is used in remediation of waters, sediments and soils to remove organic pollutants. Concentrations of nZVI in slurry used in remediation applications in Europe are in the range of 1 and 30  g  L−¹ and a mass in excess of 1  ton can be applied at any one site.³ This makes this the material of immediate environmental relevance, including for aquatic environments. The magnetic properties of nanosized magnetite (Fe3O4) have been utilised for labelling, magnetic separation of biological materials and MRI contrast agents.

Other types of NMs entering aquatic environments include Q-dots, dendrimers and nanocomposites. Q-dots are colloidal semiconductor crystals made from group II and VI elements or group III and V elements. Designed for their optical properties, Q-dots are used in biological imaging, drug delivery, and anticounterfeit pigments. Dendrimers are nano-sized polymers with a branched structure and they can be used to create unique 3D structures for use in biological applications, including drug delivery, sensors and structural processes such as tissue repair. Nanocomposites are materials that contain NMs or nanoscale modifications to enhance specific properties of the material. Currently products classed as related to ‘health and fitness’ are the most common category containing NMs. Silver NMs are used very widely in sportswear products to provide antibacterial properties.

3. Sources, Entry and Fate of NMs in Aquatic Environments

Materials can occur naturally in the environment at the nanometre-size range and have existed for billions of years. Natural occurrences that derive from sources such as volcanic eruptions and forest fires can generate airborne NMs and nanosized components of natural organic macromolecule (NOM) in water and soil colloids. This has raised debate around the necessity for ecotoxicological testing for nanosized materials because organisms will have inevitably evolved over time to cope with these naturally occurring NMs. In stark contrast with natural NMs, however, engineered NMs have been designed to have specific properties to enable, for example, persistence and in turn are likely to pose different challenges to exposed organisms. Furthermore, for some materials, it is likely that the concentrations of NMs will exceed those found for these naturally occurring materials (e.g. silver).

Aquatic environments act as sinks for anthropogenic discharges and receive NMs from divergent sources, including via wastewater treatment works (WWTWs) and surface runoff into rivers. This will be notable for NMs, such as CeO2 from fuels and TiO2 used in exterior paints, for ZnO and TiO2 entering directly into marine waters from the use of sunscreens, and also accidental releases from industrial plants producing these materials. Aquatic environments also receive NMs indirectly, for example, from soils where NMs are applied through treatments with sewage sludge, via leachate from landfill and/or via precipitation of materials released into the air.

Importantly, no safety guidelines exist currently for NM release into aquatic environments. For metals, the UK and US governments have guidelines for water quality (UK Environmental Quality Standards and US EPA Aquatic Life Criteria)⁴,⁵ based on metal ions using the biotic ligand model (BLM). Recently the BLM has been applied to metal NMs but this approach is limited due to the high variability in the rate of dissolution for different sized NMs, which is also affected by the nature of the aqueous environment and capping agents present on the NMs. Furthermore, the BLM considers only the dissociating ions from the NMs and does not take into account possible NM-specific biological effects.⁶ When considering the ecotoxicity of NMs, we would emphasise that there is not sufficient information available currently to link any adverse effects to individuals or populations at concentrations likely to be found in most aquatic environments. Furthermore, information on adverse effects in individuals for any exposures that approach those with (predicted) environmental relevance are still very limited.

3.1. NM Sources and Entry into Aquatic Environments

WWTW discharges are expected to be one of the major sources of NMs into aquatic systems. WWTWs receive significant amounts of NMs from both domestic and industrial sources, and although some are expected to precipitate into the sludge (but may find their way back into aquatic systems via sludge applied to land as fertiliser), the remaining NMs in effluents will enter directly into both freshwater and marine environments. Data on measured levels of NMs in WWTWs influent and effluent are limited and releases of NMs predicted by modelling are highly variable depending on particle type and processes within the specific WWTWs. Measured releases of NMs in WWTW have been reported for C60 and C70 carbon NMs and some metal-based materials. For C60 and C70, carbon NM levels can reach the parts per billion (ppb) range.⁷ In a study using a model WWTW, 6% (by weight) of the CeO2-NMs supplied to the WWTWs were subsequently released in the effluent discharge and addition of associated stabilizing agents increased the amount of CeO2 passing through the WWTWs into the effluent stream.⁸ Predicted effluent concentrations for TiO2-NMs in WWTW effluents have been reported at between 0.7 and 16  μg  L−¹. The predicted no effect concentration (PNEC) for TiO2-NMs is < 1  μg  L−¹.⁹ One study has reported concentrations of titanium containing NMs (<0.7  μm) in WWTW effluents at concentrations in the range <5–15  μg  L−¹, exceeding the PNEC value.¹⁰

The fate and behaviour of silver NMs in WWTWs will depend on many variables including influent concentrations, NM coating, and NM transformation within the WWTW (e.g. sulphidation). Only a relatively small percentage of Ag-NMs pass through WWTWs into the effluent discharged as Ag-NMs, and most are transformed via sulphidation into Ag2S.¹¹ The fate of Ag-NMs however is also size-dependent.¹² Sulphidation of Ag-NMs reduces their solubility and toxicity, but the environmental impact of other transformations of Ag-NMs, such as oxidised silver sulphide, has not been established. A modelling approach has predicted effluent concentrations of uncoated Ag-NMs at less than 0.24  μg  L−¹ but higher concentrations for coated Ag-NMs.¹³ In a study in Germany, Ag-NMs concentrations in influents were measured at <1.5  μg  L−¹ and at <10  ng  L−¹ in effluents.¹⁴ Predicted environmental concentrations (PECs) for Ag-NMs indicate that at current levels of use they are unlikely to be an environmental problem for most aquatic environments, but their future use (and thus discharge into these environments) is set to increase significantly.¹¹,¹³,¹⁵

Other direct sources of NMs into aquatic systems include via industry and hospital discharges, runoff from roads and applications such as agriculture (nanopesticides), exterior paintwork and sunscreens, and via use in soils remediation. There are little data available for the industrial production and release of NMs, and as a consequence studies modelling production levels of NMs have relied on extrapolated data.¹⁶ Hospitals use NMs extensively for health care, for example in imaging (gold and Q-dot NMs), antimicrobial wound dressings (silver and copper NMs) and for drug delivery and gene therapy (polymer/liposome-based NMs). Modelling approaches used to assess gold NM PECs, however, indicate at current usages; even at hot spot discharges from hospitals, they are unlikely to be an environmental problem.¹¹ In one study the highest predicted water concentration of CeO2-NMs from road runoff into rivers was 0.02  ng  L−¹. It is not clear however how representative this is of runoff in waters in close proximity to roads, and after rain storm events.¹⁷ Measured concentrations of TiO2-NMs in runoff derived from newly painted (experimental) facades, have been measured up to 600  μg  L−¹.¹⁸ Studies to assess NM release from NM-containing products include the release of up to 650  μg  L−¹ of silver from socks (washed in 500  mL distilled water)¹⁵ and an average concentration of 11  μg  L−¹ of silver in washing machine effluent.¹⁹ For ZnO and TiO2 in sunscreens up to 25% of sunscreen is washed off on immersion²⁰