Characterization, properties, transformations and behavior of ZnO nanoparticles in aquatic systems

Thesis

Reference

Characterization, properties, transformations and behavior of ZnO nanoparticles in aquatic systems

MOHD OMAR, Fatehah Binti

Abstract

Les objectifs de ce travail de thèse concernent une meilleure compréhension des différents facteurs qui vont gouverner le comportement, le transport et les transformations de nanoparticules d'oxyde de Zinc, ZnO. L'oxyde de Zinc est très largement utilisé dans de nombreuses applications (cosmétiques, peintures, industrie automobile, etc). Nous nous sommes particulièrement focalisés sur l'effet du pH, de la force ionique, de la concentration en matière organique naturelle sur la modification de la charge de surface de nos nanoparticules et leur stabilité en solution à travers la mesure de tailles moyennes. Au final, les résultats présentés dans ce travail et à travers les nombreuses publications obtenues constituent des éléments de réponses importants quant au comportement des nanoparticules manufacturées du type oxyde de Zinc. Ils permettent de mettre en avant les paramètres importants à mesurer afin de pouvoir prédire le comportement de ces nanoparticules dans les systèmes aquatiques. Ces résultats montrent également que chaque nanoparticule manufacturée est unique.

MOHD OMAR, Fatehah Binti. Characterization, properties, transformations and behavior of ZnO nanoparticles in aquatic systems . Thèse de doctorat : Univ. Genève, 2015, no. Sc. 4284

URN : urn:nbn:ch:unige-800952

DOI : 10.13097/archive-ouverte/unige:80095

Available at:

http://archive-ouverte.unige.ch/unige:80095

Disclaimer: layout of this document may differ from the published version.

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UNIVERSITÉ DE GENÈVE FACULTÉ DES SCIENCES Section des Sciences de la Terre et de l’Environnement Docteur Serge STOLL

UNIVERSITI SAINS MALAYSIA SCHOOL OF CIVIL ENGINEERING Department of Environmental Engineering Professor Dr. Hamidi ABDUL AZIZ

Characterization, Properties,

Transformations and Behavior of ZnO Nanoparticles in Aquatic Systems

THÈSE

présentée à la Faculté des Sciences de l’Université de Genève et School of Civil Engineering de l’Universiti Sains Malaysia pour obtenir le grade de Docteure ès Sciences,

mention Sciences de l’Environnement.

par

Fatehah MOHD OMAR de

Malaisie (MY)

Thèse No. 4284

GENÈVE

Atelier d’impression Repro Mail

2015

UNIVERSITÉ DE GENÈVE FACULTÉ DES SCIENCES Section des Sciences de la Terre et de l’Environnement Docteur Serge STOLL

UNIVERSITI SAINS MALAYSIA SCHOOL OF CIVIL ENGINEERING Department of Environmental Engineering Professor Dr. Hamidi ABDUL AZIZ

Characterization, Properties,

Transformations and Behavior of ZnO Nanoparticles in Aquatic Systems

THÈSE

présentée à la Faculté des Sciences de l’Université de Genève et School of Civil Engineering de l’Universiti Sains Malaysia pour obtenir le grade de Docteure ès Sciences, mention Sciences de

l’Environnement.

par

Fatehah MOHD OMAR de

Malaisie (MY)

Thèse No. 4284 GENÈVE

Atelier d’impression Repro Mail

2015

iii

Table of Contents

Remerciements

Résumé en Français

Chapter I - Introduction 1

I.1. Introduction

I.2. Nanoparticle Behavior and Fate in Aquatic Systems

I.3. Components of the Natural Aquatic System

I.4.Types of Transformations and Nanoparticle Interactions

I.5. Thesis Objectives

I.6. List of Scientific Publications

I.7. References

Chapter II - Nanoparticle Properties, Behavior, Fate in Aquatic Systems and Characterization Methods : Review

15

II.1. Main Introduction

II.2. Paper I

II.2.1. Introduction

II.2.1.1. Zinc oxide Nanoparticles and Environmental Risk

II.2.1.2. Influence of Natural Organic Matter on Fate and Transport of ZnO

Nanoparticles

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II.2.1.3. Influence of pH and Zeta Potential on Stability of ZnO Nanoparticles

II.2.1.4. Nanoparticles and their Removal from the Environment

II.2.1.5. Scope and Objectives

II.2.2. Literature Review

II.2.2.1. Nanoparticles

Natural Nanoparticles

Manufactured Nanoparticles

Natural Aquatic Colloids

Humic Acid

Biopolymers

Alginate

II.2.2.2. Nanoparticle Properties

Physico-chemical Properties

Zeta Potential and Surface Charge

Electrical Double Layer

Particle Size and Shape

Surface Area

Point of Zero Charge and Isoelectric Point

Nanoparticle Structure and Morphology

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II.2.2.3. Occurrence, Fate and Transport of Nanoparticles in Aquatic Systems

Sources and Routes of Nanoparticles into the Aquatic System

Interactions of Nanoparticles in Aquatic Systems

Particle-Particle and Particle-Surface Interactions

Interaction with Natural Colloids

Interaction with Organisms and Pollutants

Nanoparticle Stability

Nanoparticle Dissolution

Nanoparticle Toxicity

II.2.2.4. Zinc Oxide

Amphoteric Nature of ZnO

ZnO Behavior as a Nanoparticle

II.2.2.5. Methods and Principles of Nanoparticle Characterization

Particle Morphology

Scanning Electron Microscopy

Transmission Electron Microscopy

Atomic Force Microscopy

Environmental Scanning Electron Microscopy

Nanoparticle Stability

Nanoparticle Structure

X-Ray Diffraction

Fourier Transform Infrared Spectroscopy

ATR-FTIR Spectroscopy

Separation Methods

Flow Field-Flow Fractionation

Size Exclusion Chromatography

Fluorescence Correlation Spectroscopy

Dynamic Light Scattering

Nanoparticle Tracking Analysis

II.2.2.6. Challenges of Nanoparticle Characterization in the Environment

II.2.3. Conclusion

II.2.4. References and Notes

Chapter III - Stability of ZnO Nanoparticles in Solution. Influence of pH, Dissolution, Aggregation and Disaggregation Effects

49

III.1. Introduction

III.2. Main Important Results

Paper II

III.3. Conclusion

III.4. References

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Chapter IV - Aggregation and Disaggregation of ZnO Nanoparticles: Influence of pH and Adsorption of Suwannee River Humic Acid

65

IV.1. Introduction

IV.2. Main Important Results

Paper III

IV.3. Conclusion

IV.4. References

Chapter V – Stability of ZnO Nanoparticles in Aquatic Systems: Influence of pH, Natural Organic Matter and Ionic Strength

81

V.1. Introduction

V.2. Results and Discussion

V.3. Conclusion

V.4. References

Chapter VI - Conclusions and Perspectives ¹¹⁵

Annexes ¹²¹

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Remerciements

Cette thèse a été réalisée en collaboration entre l’Université de Genève et l’Universiti Sains Malaysia et a été financée par le Gouvernnement de Malaisie.

Je voudrais tout d’abord remercier grandement Dr. Serge Stoll et Professor Hamidi Abdul Aziz pour avoir accepté d’être mes directeurs de thèse. Je n’aurais jamais pu élaborer ce travail sans leur aide si compétente. J’ai beaucoup apprécié la collaboration avec Dr. Stoll, sa patience, ses connaissances approfondies, ses conseils, sa motivation, sa disponibilité et surtout son optimisme qui m’a toujours motivé de bien travailler. Professor Hamidi m’a toujours soutenu et encouragé, sa motivation et ses conseils m’ont aider à finier cette thèse à terme.

Je tiens à exprimer en particulier ma profonde reconnaissance à Professor David Hunkeler pour m’avoir accordé sa confiance à faire le doctorat et pour m’avoir introduit à Dr. Serge Stoll et le programme des études en Suisse, pour ses connaissances de la caractérisation des colloïdes par AQUATECH ainsi que ses conseils concernant mon travail et mon bien-être. Aussi à Professor Abdelhamid Elaissari, Directeur de Recherche de génie pharmacotechnique à l’Université Lyon 1, Professor Amane Jada de l’institut de Science des Matériaux de Mulhouse et Professor Dennis McGinnis de l’Université de Genève pour avoir accepté de lire mon travail et pour leur participation à mon jury de thèse.

Ce travail n’aurait pu être mené à bien sans l’aide de différents financiers qui, à travers de leur soutien matériel, ont reconnu mon travail et m’ont fait confiance: Le Ministre des Etudes de la Gourvernement de Malasie, La Fondation Internationale pour la Science (IFS), Suède et la Secrétariat d'Etat à l'Education, de la Recherche et de l'Innovation (SERI), Suisse.

Je tiens à remercier tous mes collègues avec qui j’ai partagé mes études et notamment des années de

thèse. Au Moulin: Frédéric, Arnaud, Fabrice, Olena, Nadia, Giulia, Coralie, Dorothea et Severine. Ils

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ont pris le temps de m’écouter et de discuter avec moi. Les discussions m’ont permis de voir mon travail sous un autre angle. Les <<éphémères>> de l’Institut: Zhai, Monica, Moussa, Blanca, Christine, Christopher, Elodie, Enrique, Luca, Weiwei, Neil et Anh Dao. Ainsi que ceux de la

“ferme”: Alexandra, Andrea, Claudia, Elena, John, Naresh, Katia, Nicole, Philippe et Rebecca. Pour finir les piliers de l’Institut: Jean-Luc Loizeau, Janusz Dominik, Walter Wildi, ainsi que Vera Slaveykova et Bastiaan Ibelings. Ils m’ont beaucoup apporté et m’ont encouragé dans chaque situation durant ma recherche.

Il m’est impossible d’oublier ceux dans l’adminstritation de School of Civil Engineering, Professor Ahmad Farhan Mohd Sadullah, le directeur, et tous les administratif : Cik Nur’Asyikin, Pn. Zaharah, Pn. Khajidah et Pn. Juhana, les technicians au labo: En. Zaini, En. Nizam, En. Naim, Pn. Shamsiah et bien sûr mes collègues Shaylinda, Fateha, Noor Ainee et tout le mond là-bas.

Mes remerciements vont aussi à ma grande famille, mes frères: Abdullah, Taha et Ka’ab, mes belles sœurs: Nik Zaitun, Nik Noriza, Nik Zakida, Nik Fatimah, Wan Fariza, Shakila, Shie, Noriana et Halimah, mes belles nièces qui m’ont contacté par skype toutes les semaines: Nadia, Angah, Achik, Khaleesa et Baby Afrina, qui, avec cette question recurrente, <<quand est-ce que vas-tu soutenir ta thèse?>>, bien qu’angoissante en période fréquente de doutes, m’ont permis de ne jamais dévier de mon objectif final. Merci à l’ancien ambassade de la Mission Permanente de Malaisie à Genève, Datuk Othman Hashim et son épouse, Datin Rohayazam Kamaruzam, les diplomat officielles: En.

Aziz, En. Johan, En. Ismail, Pn. Azie, Pn. Kamelia, Pn. Nony, En. Amri, Pn. Fiza, Pn. Saimon et tout le monde qui m’ont accuillé bien depuis mon arrivé à Genève jusqu’à mon départ.

Merci à Madame Farida et sa belle famille, la famille de Mumenthaler, la famille de Jolidon, Dina et

Syazwana, Adlin, Zaidan, Surina, Shazeera, Safra, Pn. Mazian, Faiz et Hafidah pour leurs accueils

reguliers sur la capitale; en m’offrant tout leur hospitalité chaleureuse, ils m’ont permis d’effectuer

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mes recherches dans les meilleures conditions possibles. Un merci tout particulier à Farida, Fred, Micaela, Guillaume, Jullianne et Aurelie qui m’ont appris le français et m’ont aidé avec la traduction.

Je remercie mon cher époux, Nik Mohd Zailani Nik Salleh et mes beaux enfants, Danish, Aliyn, Hariz, Aliya et Hilman pour ses quotidiens indefectibles et ses enthousiasmes contagieux à l’égard de mes travaux comme de la vie en général. Notre couple a grandi en meme temps que mon projet scientifique, le premier servant de socle solide a l’épanouissement du second.

Ces remerciements ne peuvent s’achever, sans une pensée pour ma première fan (et correctrice des fautes d’orthographes de cette thèse!): ma belle mère, Professor Nik Norulaini Nik Ab. Rahman. Sa présence et ses encouragements sont pour moi les piliers fondateurs de ce que je suis et de ce que je fais. Merci toujours à mon père, Professor Mohd Omar Ab. Kadir ainsi que ma grandmère, Nik Fauziah Wan Mahmood, qui m’ont encouragé et m’ont supporté fortement dans toutes mes entreprises.

Enfin, je remercie tous ceux que je n’ai pas cité avec lesquels j’ai pu échanger de bons moments.

x

xi

“ Pour mes cœurs...mes cher parents,

mon époux et mon Danish”

xii

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Résumé en Français

De l'industrie pharmaceutique aux cosmétiques, de l'aéronautique à la chimie, de la médecine à l’énergie, les champs d'application des nanotechnologies apparaissent chaque jour plus nombreux.

Malheureusement les nano-objets et nanoparticules manufacturées, liés aux nanotechnologies, possèdent des réactivités chimiques à la fois exceptionnelles ! et inquiétantes. A l’échelle du nanomètre (milliardième de mètre), leurs propriétés changent radicalement et leurs réactivités chimiques augmentent de façon spectaculaire !

Or ces dernières années le nombre de produits de consommation courante contenant des nanoparticules manufacturées n’a pas cessé d’augmenter, avec leur production en masse, et leur lente diffusion dans notre environnement (l’air, l’eau, les sols) est déjà une réalité aux conséquences encore inconnues qu’il est désormais important d’étudier.

La recherche sur les nanoparticules dans ce travail de thèse se focalise sur un compartiment

environnemental essentiel et concerne la transformation, le comportement et la circulation de des

nanoparticules manufacturées dans les systèmes aquatiques. L’utilisation, par exemple, de crèmes

solaires (souvent composées de nanoparticules) « contamine » les eaux. Il devient alors important de

connaître les processus, souvent très complexes, de transport et aussi de transformation de ces

nanoparticules dans les systèmes aquatiques naturels. Dans ces systèmes, la composition physico-

chimique de l’eau va jouer un rôle important (pH, force ionique) au même titre que la présence

d’autres composés chimiques naturels (colloïdes, matière organique, polysaccharides, etc) déjà

présents dans la colonne d’eau. Ces interactions vont donc agir sur la stabilité des nanoparticules, plus

particulièrement sur l’agrégation, et les propriétés de surfaces de ces dernières à travers des processus

d’adsorption et de recouvrement de surface. Globalement ces nanoparticules pourront se retrouver sous

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forme d’agrégats ou agglomérats, facilement éliminés par la sédimentation, soit sous forme d’objets isolés, plus réactifs, plus mobiles et donc potentiellement plus toxiques.

Les objectifs de ce travail de thèse concernent une meilleure compréhension des différents facteurs qui vont gouverner le comportement, le transport et les transformations de nanoparticules d’oxyde de Zinc, ZnO. L’oxyde de Zinc est très largement utilisé dans de nombreuses applications (cosmétiques, peintures, industrie automobile, etc). Nous nous sommes particulièrement focalisés sur l’effet du pH, de la force ionique, de la concentration en matière organique naturelle sur la modification de la charge de surface de nos nanoparticules et leur stabilité en solution à travers la mesure de tailles moyennes.

Le chapitre I constitue une introduction générale à la problématique des nanoparticules dans les milieux aquatiques et fait le point sur les transformations possibles sur le plan de la physico-chimie.

Les objectifs et l’articulation générale du travail sont ensuite présentés, ainsi qu’une liste de publications écrites dans le cadre de ce travail.

Le chapitre II constitue sous la forme d’un article de revue (sur invitation) une discussion sur les propriétés et les transformations des nanoparticules manufacturées en général tout en se focalisant sur les propriétés des ZnO. Les méthodes et principes de caractérisation des nanoparticules y sont discutés de façon objective et une discussion des avantages et des inconvénients des différentes techniques est présentée. Les différents challenges à couvrir dans le domaine sont également présentés notamment en matière de détection des nanoparticules dans des matrices environnementales. Il est également montré que de nombreux efforts doivent être encore effectués en matière d’harmonisation des différentes techniques de préparation de dispersions de nanoparticules, de caractérisation et de mise en œuvre de tests écotoxicologiques.

Le chapitre III constitue un pas important dans la compréhension du comportement des nanoparticles

du type ZnO. En effet, malgré de nombreuses études utilisant ces nanoparticules, très peu concernent

le comportement physicochimique de ces dernières en fonction des paramètres de la solution. Nous

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avons fait varier ici de façon systématique le pH afin de mesurer les variations des charges de surface et nous avons montré que les variations associées étaient complexes. En effet ces nanoparticules possèdent non seulement un point de charge nulle mais également un point isoélectrique qui résulte d’un processus de dissolution. Ici non seulement l’agrégation mais aussi la dissolution des ZnO vont jouer un rôle important sur la stabilité de ces nanoparticules. Il est également montré que le domaine de charge de surfaces positives existe bien mais est extrêmement réduit dans une fenêtre de pH faible de 6.4 à 9.4. Il est à noter que deux techniques importantes ont été comparées ici à savoir la diffusion de lumière dynamique et le traçage de nanoparticules. Ceci a permis de définir les avantages et inconvénients des deux techniques et de valider les résultats obtenus. A travers ce chapitre, le comportement de nanoparticules du type ZnO a été élucidé et publié.

Dans le chapitre IV, le comportement des nanoparticules de ZnO est étudié en présence d’un composé

environnemental important ; les acides humiques. En effet ces acides sont largement présents dans les

systèmes aquatiques et représentent un large part de la matière organique naturelle. Ils sont suspectés

de jouer un rôle important dans le comportement de nombreux polluants (métaux) et la matière

colloïdale en suspension. Le comportement des nanoparticules est étudié pour trois valeurs de pH

représentatives de la charge de surface des nanoparticules (négative à haut pH, positive à faible pH et

au point de charge nulle). Pour chaque valeur de pH, la concentration en humiques est ajustée et la

variation de la charge de surface ainsi que la taille moyenne des nanoparticules sont mesurées. Il est

montré que la concentration des humiques joue un rôle important en contrôlant le processus

d’agglomération des nanoparticules. Au point de charge nulle, correspondant à des conditions

favorables pour l’agglomération des nanoparticules, nous montrons que la présence de composés

humiques provoque une désagglomération (ou fragmentation) des agrégats formés au point de charge

nulle. Ce résultat est un résultat original et très important dans le domaine car il montre clairement que

des agglomérats de nanoparticules entrant dans les systèmes aquatiques ont la possibilité de se

fragmenter et ainsi de libérer des nanoparticules isolées (plus mobiles). Un diagramme de phase est

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également proposé pour mettre en avant la complexité du système. Ces résultats ont abouti à une publication dans un journal international de haut rang dans le domaine.

Dans le chapitre V, l’agglomération des nanoparticules est étudiée en fonction du pH, de la concentration en matière organique et de la force ionique de la solution. Deux modèles de matière organique sont utilisés ; l’alginate qui correspond à un polysaccharide linéaire chargé et hydrophile et des acides humiques qui correspondent à des structures macromoléculaires amphiphiles (hydrophobes et hydrophiles). La force ionique est un élément important dans l’étude du comportement des oxydes en solution dans la mesure où elle rend compte de la composition ionique des eaux naturelles. Une force ionique faible correspondra à une eau douce tandis qu’une force ionique élevée correspondra à une eau saline. Dans ce chapitre nous montrons que l’élévation de la force ionique va dans le sens de l’agglomération des nanoparticules. La concentration en matière organique joue également un rôle important, de la neutralisation des charges de surfaces des nanoparticules (agglomération) à l’inversion de charge de ces dernières (stabilisation). L’autre résultat important est de montrer de la présence de matière organique limite les effets de sels. En effet, l’adsorption de matière organique à la surface des nanoparticules stabilise ces dernières devant l’effet de sel.

Au final, les résultats présentés dans ce travail et à travers les nombreuses publications obtenues constituent des éléments de réponses importants quant au comportement des nanoparticules manufacturées du type oxyde de Zinc. Ils permettent de mettre en avant les paramètres importants à mesurer afin de pouvoir prédire le comportement de ces nanoparticules dans les systèmes aquatiques.

Ces résultats montrent également que chaque nanoparticule manufacturée est unique.

Chapter I

Introduction

2

I.1. Introduction

Nanoparticles are materials with a nanometer size consisting unique physico-chemical properties that has captured the interest of a majority of researchers, scientists including industrialists worldwide. The properties of these minute materials have been developed to advance stages in order to serve human needs through various applications and commercial products [1-2]. Commonly known as manufactured or engineered nanoparticles (ENPs), among the few examples such as fullerene, TiO

, ZnO, Ag and carbon nanotubes have become the main components in diods, biomedical, electrical appliances, etc [3]. The increasing usage of ENPs in almost every aspect of our communal lives has sparked a great concern globally. Extensive research relevant to behavior, fate, transport, occurrence as well as toxicity are currently ongoing to grasp an in depth understanding of their fate once they enter the environment and the potential impacts towards human health and the aquatic system [4-5]. Despite all extensive benefits ENPs initially provide towards humans being part of the main ingredient in commercial products and applications, these advanced materials will create a 360 degree turn of becoming a pollutant as soon as they enter the environment which leads to a great concern among humanity for their health.

Industrial products and wastes tend to end up in waterways (e.g. drainage ditches, rivers, lakes, estuaries and coastal waters). Hence, it is inevitable that nanoscale products and by-products will enter the aquatic environment. Other human activities such as accidental spillages or permitted release of industrial effluents in waterways and aquatic systems may yield water aerosols and direct ingestion of contaminated drinking water. Other direct ingestion could be via particles that are adsorbed onto vegetables as well as other foodstuff.

Studies have shown that nanoparticles do act as potential aquatic pollutants [1,4]. Upon

entering the environment, natural nanoscale or microscale particles such as colloids (humic

4

substances) will associate or adsorb with larger biotic and non-biotic particles in porous medium, both suspended and deposited sediments. The hydrodynamic and morphological characteristics of bodies of water and coastal zones will largely determine the distribution of bound nanoparticles. It is also known that suspended sediment particles are significant in sequestering and transporting contaminant chemicals over long distances [6]. It is imperative to assess the potential risks that nanoparticles pose to the environment and human health.

Hence it is important to understand their mobility in a typical aquatic environment.

I.2. Nanoparticle Behavior and Fate in Aquatic Systems

In aquatic systems, nanoparticles undergo many transformations due to their high surface area to volume ratio leading to highly reactive and dynamic reactions with aquatic colloids. Among these transformations involve interactions with humic substances, biopolymers and chemical processes such as aggregation, redox reactions and dissolution [4].

I.3. Components of the Natural Aquatic System

In the environment, both naturally occurring and manufactured nanoparticles can be found ubiquitously. The particles can be further separated based on their chemical composition into carbon containing and inorganic NPs [5]. In contrast to manufactured NPs which are intentionally designed and comprised of nanofilms/nanoplates (one dimension), nanowires and nanotubes/nanofibres (two dimensions) and engineered nanoparticles (three dimension) [7], natural or environmental nanoparticles are formed by natural geochemical (abiotic) and biogeochemical (biotic) processes in water and also results from anthropogenic impacts (e.g. flocculation of nanometer-scale metal oxides in acid mine drainage) [4].

Natural NPs also exist in soil as clays, organic matter, iron oxides while in the air [1],

volcanic eruption, physico-chemical wearing of rocks and dust volatilization [3]. ENPs can be classified into five groups which are carbon nanomaterials (NMs), metal oxides NPs, zero- valence NPs, quantum dots and dendrimers where natural nanoparticles are categorized into major mineral classes such as Fe-oxides (goethite hematite), Mn-oxides (vernadite, birnessite), heavy metal oxides (uraninite) and metal sulfides (sephalerite, pyrite, galena) [2].

I.4. Types of Transformations and Nanoparticle Interactions

Despite the various origins, both natural and manufactured NPs exhibit similar properties in the nanoscale size regime. The surface area of a nanoparticle plays one the most important roles in part of the interactions of NPs and other surfaces of other particles or colloids as they will increase chemical reactivity, causing the NPs to act as carriers for other contaminants and thereby providing rapid long-range transport [3].

Figure 1 represents of the different physicochemical processes that nanoparticles can

undergo upon entering the aquatic system. The surface charge of the ENPs will determine

how it is affected by the aggregation and disaggregation leading to their subsequent behavior,

fate and transport. Nanoparticles have the highest possibility of interacting with natural clay

in aquatic systems [8]. The general tendency of metals to sorb to high-specific-surface area

small colloids will aggregate and later deposit themselves [9]. Colloids largely dominate the

aggregation behavior of ENPs. Colloids in fresh water microcosms, dissolved organic matter

(DOM) that are released by plants into the water column readily bound free metal ions from

ENPs to undergo dissolution as in the case of released Ag

ions that bound on sediment and

plant surfaces [10]. In situations where NOM is present, the ions are able to bridge adsorbed

NOM layers on several particles, thereby enhancing the aggregation process. In other cases,

the occurrence of nanoparticles undergo physical transformation such as disaggregation will

6

form suspended sediment particles which are known to be important in sequestering and transporting contaminant chemicals [11-12].

AGGREGATION / DISPERSION

DISSOLUTION

REACTIONS WITH HUMIC SUBSTANCES

REACTIONS WITH BIOPOLYMERS

Freshwaters Marine Waters

Heteroaggregation

Flocculation Flocculation & heteroaggregation of

NPs, ICs & BPs

Sedimentation

2+

2+

2+

Dissolution

Disaggregation Aggregation

Sedimentation

Low ionic strength High ionic strength

Inorganic colloids (Ics) Manufactured Nanoparticles (NPs)

Humic substances (HS) Collasped biopolymers (BPs)

Extended biopolymers (BPs)

Fig. 1. Illustration of the different physicochemical processes that nanoparticles can undergo

upon entering the aquatic system.

I.5. Thesis Objectives

In this thesis research, commercial ZnO nanoparticles are chosen as a subject study to represent the metal oxide NPs generally in order to gain a better insight of the behavior of manufactured nanoparticles in aquatic systems. As known, ZnO is widely used in cosmetics, sunscreens, electronic and optoelectronic devices which will end up in the environment. It has been found that ZnO and other contaminants released into the environment at unknown concentrations are capable of causing ecotoxicity on various organisms [6,7,10]. The fate and transport of ZnO in the environment, particularly aqueous medium must be investigated and understood to ensure the ecotoxicity impact can be controlled. The presence of ZnO in the environment is exacerbated by their nano size. ZnO due to its nanoscale, shape and consequently huge surface area may interact with the biological systems producing toxic material [11]. In addition, the surface area is directly correlated to many other physicochemical properties such as chemical reactivity, surface adsorption ability, surface charge and therefore strongly dominates nanotoxicological behavior in vivo. When the particle size decreases, there is a tendency to increase the toxicity, even if the same material is relatively inert in bulk form. Based on Fig. 1, investigations are conducted to comprehend the transformations of ZnO NPs which subsequently determines their behavior, fate and transport in the aquatic environment.

A methodical framework is constructed accordingly to achieve the following objectives, (i) to examine the behavior of ZnO nanoparticles in solution to understand their characteristics, dissolution, dispersion and stability as a function of pH and time; (ii) to observe the effect on ZnO NPs aggregation and disaggregation behavior with the presence of natural organic matter (in this study Suwannee River Humic Acid) at increasing levels of concentration and at different pH regions with the pH

of ZnO as the point of reference;

(iii) to investigate the influence of natural polyelectrolytes (e.g. alginates) on the aggregation

8

and disaggregation behavior of ZnO NPs in similar controlled conditions as applied for natural organic matter (NOM); (iv) to consider and determine the role of electrolytes (monovalent ions) with the presence of NOM (Suwannee River Humic Acid and alginate) at various ionic strengths simulating fresh and marine waters.

In the first part of this study, a full scale approach of examining the individual influence of pH on the stability of a ZnO suspension, zeta potential and aggregation without increasing the ionic strength or adding polyelectrolytes is conducted. This work targets to establish the effect of pH on zeta potential and the preponderance of ZnO nanoparticles to aggregate in suspension. A methodical attempt is made to examine the changes in pH of ZnO suspension as a function of time and the influence of dissolution effect on the ZnO suspension. In addition, the pH and zeta potential stability including aggregation processes were determined and subsequently, we demonstrated that it was difficult for the ZnO suspension to sustain stability. The affiliation between pH, zeta potential, dissolution and aggregation behaviour of ZnO nanoparticles are elucidated.

In the next step of this research, the behaviour and fate of ZnO and its interactions

with NOM in solution mixtures were studied. A systematic study was conducted both on the

aggregation and disaggregation of ZnO NPs over a wide range of pH values in the presence

of Suwannee River humic acids (SRHAs) at variable concentrations. Unlike other studies,

we started our experimental work with nanoparticles which readily appeared in partial

aggregate forms. The aggregation and disaggregation study of the nanoparticles was

structured in an analytical procedure to observe the interactions with NOM standard SRHAs

as a function of concentration representing environmental matrix. The pH effect on the

interaction properties between ZnO NPs and SRHAs such as particle size and surface charge

were observed as well. An assessment of zeta potential and z-average particle size was

conducted to reveal the propensity of the ZnO nanoparticles to aggregate or undergo disaggregation in simulated aqueous environments.

The final part of this study was conducted on ZnO agglomerates in the presence of alginate, a negatively charged polyelectrolyte. The ZnO nanoparticles were initially dispersed via ultrasonication bath and followed by a systematic investigation on the impact of surface charge and z-average agglomerate size as a function of alginate concentration.

Similar to the second part of the study, three different pH domains were studied by

considering the pH of the point of zero charge (PZC) of ZnO as the point of reference: i)

below the PZC; ii) at the PZC and iii) above the PZC. We have also investigated the

aggregation behavior of ZnO with the absence and presence of two different types of natural

organic matter (i.e. alginate and SRHA), representing a major component of NOM, in

monovalent sodium salt solutions at variable ionic strengths. The hydrodynamic diameter of

the nanoparticles was measured via dynamic light scattering under a variety of experimental

conditions.

10

Table 1 Graphical abstracts summarizing main results obtained in each chapter.

CHAPTER TWO Nanoparticle properties, fate, behavior and transport in the environment.

Nanoparticle and its interactions with natural organic matter.

Principles and methods of nanoparticle characterization .

ZP (mV)

3 5 8 10 11

Aggregation at PZC Aggregation at IEP

Initial pH Partial dissolution

Disaggregation

pH (+mV)

(- mV)

CHAPTER THREE Influence of pH on the stability of ZnO

nanoparticles, surface charge and

hydrodynamic

diameter.

<PZC PZC >PZC

pH

SRHA

0.2

0.4

++ + ++

++ +++

++ ++++ ++ ++ ++

+

+ +++ +

+++

++ + +

++ +++ +++

++ +++ ++

++ +

-

- - - --

- -

- - - - --

- --

- - --

- --

- - - - -

- -- -- - -- - - -

-- -- - - -

- ---

AGGREGATION DOMAIN

PARTIAL DISAGGREGATION

PARTIAL DISAGGREGATION

PARTIAL DISAGGREGATION +++++ ++

+ +++

++++++ ++++++

++ ++ ++++

++ + ++ ++++

+ ++

+++

-

---- --- - - - - -- - - - -- --- - -- - ---- -- - -

- - - - -

- -- - - -

- - ---

-- -- - -

--- -

- --- --

- -

- -- - -

- - - -

- ---

-- - - - -- - --- - - -- -

-- -- -- -

- -

- - -

+ + +++++ +++

++ ++

+ +

+ +++ ++

++

+ ++

++ ++ ++ + + + +++

+ ++ +++

+ ++++

+++ ++ +

++++ +

AGGREGATION DOMAIN PARTIAL AGGREGATES PARTIAL AGGREGATES

[mg/L]

CHAPTER FOUR

Influence of pH and Suwannee River humic acids on ZnO nanoparticles behavior.

CHAPTER FIVE

Influence of pH and alginate ZnO

nanoparticles.

12

I.6. List of Scientific Publications

Paper I

F. Mohd Omar, H. Abdul Aziz and S. Stoll, “Nanoparticle Properties, Behavior, Fate in Aquatic Systems and Characterization Methods,” Journal of Colloid Science and Biotechnology, vol. 3, no. 2, 2014, pp.1-30.

Paper II

F. Mohd Omar, H. Abdul Aziz and S. Stoll, “Stability of ZnO Nanoparticles in Solution.

Influence of pH, Dissolution, Aggregation and Disaggregation Effects,” Journal of Colloid Science and Biotechnology, vol. 3, no. 1, 2014, pp.1-10.

Paper III

M.O. Fatehah, H. Abdul Aziz and S. Stoll, “Aggregation and Disaggregation of ZnO Nanoparticles: Influence of pH and Adsorption of Suwannee River Humic Acid,” Science of the Total Environment, vol. 468-469, 2014, pp.195-201.

Paper IV

M.O. Fatehah, H. Abdul Aziz and S. Stoll, “Aggregation, Disagglomeration and Stabilization of ZnO Nanoparticles: Influence of pH, Natural Organic Matter and Ionic Strength, Journal of Environmental Chemistry, in preparation.

Published Papers Included at the end of the Manuscript Paper V

F. Mohd Omar, H. Abdul Aziz and S. Stoll, “Behavior of ZnO Nanoparticles in Aqueous Environments: Influence of pH and Adsorption of Humic Acid,” Advanced Material Research, vol. 832, 2014, pp. 728-733.

Paper VI

M.O. Fatehah, H. Abdul Aziz and S. Stoll, “Aggregation and Disaggregation of ZnO

Nanoparticles: Influence of pH and Alginate Concentration,” International Journal of

Chemical and Environmental Engineering, vol. 5, 2014, pp. 172-175.

Paper VII

Frédéric Loosli, Fatehah Mohd Omar, Fabrice Carnal, Olena Oriekhova, Arnaud Clavier, Zhi Chai and Serge Stoll, “Manufactured Nanoparticle Behavior and Transformations in Aquatic Systems. Importance of Natural Organic Matter,” Chimia, vol. 68, no. 11, 2014, pp.783-787.

I.7. References

[1] S.J. Klaine, P.J.J. Alvarez, G.E. Batley, T.F. Fernandes, R.D. Handy et al., D.Y.

Lyon, S. Mahendra, M.J. McLaughlin, J.R. Lead, 2008, Nanomaterials in the Environment:

Behavior, Fate, Bioavailability and Effects, Environmental Toxicology and Chemistry, 27, pp. 1825-1851.

[2] N.S. Wigginton, K.L. Haus, M.F. Hochella Jr., 2007, Aquatic Environmental Nanoparticles, Journal of Environmental Monitoring, 9 (12), pp. 1306-1316.

[3] M. Farré, J. Sanchí, D. Barceló, 2011, Analysis and Assessment of the Occurrence, the Fate and the Behavior of Nanomaterials in the Environment, Trends in Analytical Chemistry, 30 (3), pp. 517-527.

[4] G.V. Lowry, K.B. Gregory, S.C. Apte, J.R. Lead, 2012, Transformations of Nanomaterials in the Environment, 46, pp. 6893-6899.

[5] B. Nowack, T.D. Bucheli, 2007, Occurrence, Behaviour and Effects of Nanoparticles in the Environment, Environmental Pollution, 150, pp. 5-22.

[6] M.N. Moore, 2006, Do Nanoparticles Present Ecotoxicological Risks for the Health of the Aquatic Environment?, Environment International, 32, pp. 967-976.

[7] N. von Moos, V.I. Slaveykova, 2014, Oxidative Stress Induced by Inorganic Nanoparticles in Bacteria and Aquatic Microalgae – State of the Art and Knowledge Gaps, Nanotoxicology, 8 (6), pp. 1-26.

[8] D. Zhou, S.W. Bennett, A.A. Keller, 2012, Increased Mobility of Metal Oxide Nanoparticles Due to Photo and Thermal Induced Disagglomeration, PLos ONE, e37363.

[9] M.O. Fatehah, A.A. Hamidi, S. Stoll, 2014, Nanoparticle Properties, Behavior, Fate in Aquatic Systems and Characterization Methods, Colloid Science and Biotechnology, 3 (1), pp. 1-30.

[10] J.M. Unrine, B.P. Colman, A.J. Bone, A.P. Gondikas, 2012, Biotic and Abiotic

Interactions in Aquatic Microcosms Determine Fate and Toxicity of Ag Nanoparticles. Part

1. Aggregation and Dissolution, Environmental, Science and Technology, 46, pp. 6915-

6924.

14

[11] R. Brayner, S.A. Dahoumane, C. Yepremian, C. Djediat, M. Meyer, A. Coute, F.

Fievet, 2010, ZnO Nanoparticles: Synthesis, Characterization, and Ecotoxicological Studies, Langmuir, 26 (9), pp. 6522-6528.

[12] K. Van Hoecke, K.A.C. De Schamphelaere, P. Van der Meeren, G. Smagghe, C.R.

Janssen, 2011, Aggregation and Ecotoxicity of CeO

Nanoparticles in Synthetic and Natural

Waters with Variable pH, Organic Matter Concentration and Ionic Strength, Environmental

Pollution, 159, pp. 970-976.

Chapter II

Nanoparticle Properties, Behavior, Fate in Aquatic Systems and

Characterization Methods

Fatehah, M. O., Abdul Aziz, H., and Stoll, S., 2014, Nanoparticle properties,

behavior, fate in aquatic systems and characterization methods, published in

Journal of Colloid Science and Biotechnology, vol. 3, no. 2, p. 1-30

16

Chapter II

II.2.1 Main Introduction

This chapter details out the classification of nanoparticles (NPs) that are categorized into two large groups, natural and engineered NPs. Further elaboration is given on the key properties of nanoparticles, with respect to their physical properties, zeta potential and surface charge, electrical double layer and etc. ZnO is taken as an example to generally describe nanoparticles and as a representative of metal oxides as it is among the widely used NPs by humans. The mass production of engineered nanoparticles (ENPs) by manufacturers worldwide has led to unintentional release of manufactured nanoparticles as trace pollutants into the environment. Apart from this, other sources and routes of ENPs into the natural aquatic systems through landfills, soil remediation techniques utilizing NPs and accidental releases from production and transport means are discussed. ENPs are prone to interact with natural aquatic colloids both physically and chemically. The transport and behavior of ENPs strongly depend on the interaction with major colloids groups as it will determine the type of binding energy between and ENPs and natural colloids components. In most natural aquatic systems, natural organic matter (NOM) plays a major role in stabilizing ENPs and is highly influential on the transport and behavior of ENPs by dominating the interaction processes such as aggregation, dissolution, sedimentation which causes the ENPs to continually persist to occur in the environment.

This review also presents an overview of the available conceptual approaches for

analyzing, characterizing or even fractionation that have been, or are potential to be applied

on ENPs in the environment. Methods and principles of nanoparticle characterization from

different perspectives i.e. particle morphology, particle stability, particle structure are

reviewed. The advantages and disadvantages of each approach is discussed and later

18

summarized in a table form. Crucial information of ENPs pertaining to zeta potential, physic-

chemical properties, point of zero charge, structure and morphology are discussed.

R EVI EW

Copyright © 2014 American Scientific Publishers All rights reserved

Printed in the United States of America

Journal of Colloid Science and Biotechnology Vol. 3, 1–30, 2014

Nanoparticle Properties, Behavior, Fate in Aquatic Systems and Characterization Methods

Mohd Omar Fatehah

, Hamidi Abdul Aziz

, and Serge Stoll

1F.-A. Forel Institute, University of Geneva, 10 route de Suisse, Versoix, 1209, Switzerland

2School of Civil Engineering, Universiti Sains Malaysia, Nibong Tebal, 14300, Pulau Pinang, Malaysia

The global demand for a wide variety of applications based on engineered nanoparticles (ENPs) has expanded the worldwide industrial scale production and inevitably released these materials into the environment. The increasing existence of NPs and its impact towards human health and the environment especially the aquatic system has sparked a great concern among both the scientific community and the public. It is therefore crucial to gain an in depth understanding of the properties the manufactured nanoparticles possess along with the various transformations they undergo that determine their behaviour and mobility. This review begins by addressing the fundamental physico- chemical aspects of manufactured oxide nanoparticles with detailed attention given specifically to ZnO as a representative example in a separated section. The literature collected is summarized and focused on the essential point of view to evaluate their occurrence, fate and transport in the natural aquatic environment as a result of their interactions with other nanoparticles or natural colloids. Key methods and principles of nanoparticle characterization are also presented.

Keywords: Nanoparticles, ZnO, Fate, Transport, Transformation, Nanoparticle Stability, pH Effects, Nanoparticle Characterization, Aquatic Systems.

CONTENTS

1. Introduction . . . . 1 1.1. Zinc Oxide Nanoparticles and Environmental Risk . . . . 1 1.2. Influence of Natural Organic Matter on Fate and

Transport of ZnO Nanoparticles . . . . 3 1.3. Influence of pH and Zeta Potential on Stability of

ZnO Nanoparticles . . . . 3 1.4. Nanoparticles and Their Removal

from the Environment . . . . 4 1.5. Scope and Objectives . . . . 4 2. Literature Review . . . . 4 2.1. Nanoparticles . . . . 4 2.2. Occurrence, Fate and Transport of

Nanoparticles in Aquatic Systems . . . . 11 2.3. Zinc Oxide . . . . 15 2.4. Methods and Principles of Nanoparticle

Characterization . . . . 18 2.5. Challenges of Nanoparticle Characterization

in the Environment . . . . 25 3. Conclusion . . . . 26 Acknowledgments . . . . 27 References and Notes . . . . 27

1. INTRODUCTION

Engineered nanoparticles (ENPs) are produced by human activities on a relatively large scale and have at least one

∗Author to whom correspondence should be addressed.

dimension in the size range of 1 to 100 nm.¹² These nanoparticles exist in groups of carbon-based materials and inorganic nanoparticles including metal oxides, met- als and quantum dots.³The increasing use of nanoparticles in a gamut of applications comprehending industrial and households, will inadvertantly see large release of these nanomaterials into the environment. This is supported by an increasing body of scientific evidence which suggest that nanoparticles have been found to end up in the envi- ronment and that their fate and transformation processes are difficult to evaluate and control.⁴⁵As a result of their nanometric dimensions and interactions with the surround- ing environment, these manufactured nanoparticles will become mobile due to their dissolution and disaggregation behaviour.²⁶⁷Abiotic factors that affect the mobility and transport of nanoparticles are pH, ionic strength, particle surface chemistry, interactions of nanoparticles with other pollutants⁷and natural organic molecules.^18–10

1.1. Zinc Oxide Nanoparticles and Environmental Risk

Some archetypes of nanoparticles are iron oxide, titanium dioxide, fullerene, cerium oxide, carbon nanotubes and others.¹¹¹²One of the most studied manufactured nanopar- ticles, in particular for its characteristics and behavior is zinc oxide.^13–17 Nanosized ZnO has shown potential

J. Colloid Sci. Biotechnol. 2014, Vol. 3, No. 2 2164-9634/2014/3/001/030 doi:10.1166/jcsb.2014.1090 1

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Nanoparticle Properties, Behavior, Fate in Aquatic Systems and Characterization Methods Fatehah et al.

Mohd Omar Fatehahreceived her B.Tech and M.Sc in environmental technology from Universiti Sains Malaysia, Penang, Malaysia. She is currently pursuing her Ph.D in Environ- mental Science at the F. A. Forel Institute, Université de Genève, Switzerland and Schoold of Civil Engineering, Universiti Sains Malaysia, Malaysia and is in her final year. Her research area extends to water quality analysis, semiconductor wastewater treatment with natural coagulants, characterization of nanoparticle metal oxides and natural organic matter (humic acids), fate and transport of nanoparticles and interactions with the environment which is subsequently followed by the treatment and removal of nanoparticles in water treatment with application of biopolymers. She has 5 international peer-reviewed publications to date related to wastewater treatment, supercritical fluid extraction and she recently received an international research grant from the International Foundation of Science (IFS), Sweden to support her research in the fate, behavior and transport of nanoparticles.

Hamidi Abdul Aziz is a professor in environmental engineering in the School of Civil Engineering of Universiti Sains Malaysia. Professor Aziz received his Ph.D degree in civil engineering (environmental engineering) from the University of Strathclyde in Scotland in 1992. To date, he has published over 200 refereed articles in professional journals and pro- ceedings. Dr. Aziz continues to serve as a peer reviewer for more than 30 international journals. Professor Aziz currently serves as the editor-in-chief of the International Journal of Scientific Research in Environmental Sciences (IJSRES). He also serves as the managing editor of the International Journal of Environment and Waste Management, IJEWM and the International of Journal of Environmental Engineering, IJEE. Aside from these, he is a member of the editorial board of 10 other international journals in the environmental dis- cipline. Dr. Aziz’s research focuses on alleviating problems associated with water pollution issues from industrial wastewater discharges and from solid waste management via landfilling, such as landfill leachate.

Advanced oxidation processes is one of his research focuses. He also has strong interest in biodegradation and bioreme- diation of oil spills. Dr. Aziz has been involved in numerous consultancy works and industrial testing projects related to water and wastewater treatment, solid waste management, landfill leachate treatment, curriculum design, environmental impact assessment, and environmental management planning, to name a few. Professor Aziz has also been involved in international projects, especially in Saudi Arabia. Since 2011, he has also served as a member of the International Advisory Committee of the Center of Excellence in Environmental Science (CEES) in King Abdul Aziz University in Jeddah, Saudi Arabia. The details of his published work can be checked via Researcher ID: F-6836-2010 and SCOPUS ID: 7005960760.

Serge Stollis actually responsible of the group of Environmental Physical-Chemistry at the Institute Forel, Switzerland. He received his Ph.D in Physical Chemistry at the University Louis Pasteur in 1992, France. After joining the department of analytical, inorganic and applied chemistry at the University of Geneva, Switzerland, he was promoted senior lecturer in 1999. He has co-authored more than 90 publications in referred international journals, 4 book chapters, supervised Ph.D thesis works, and was chosen for his work to represent Switzerland in 1998 at the CERC3 Workshop on Colloidal Synthesis and Characterisation, Bristol UK. His teaching activities are related to courses in colloid and polymer chem- istry, advanced courses in environmental sciences (aquatic chemistry, chemical processes in the environment, water quality and management, atmosphere chemistry) at the Institute of environmental sciences (ISE) and University of Geneva. Numerical modelling as well as experimental studies using state-of-the-art techniques are performed in his group to understand the factors controlling the reactivity, fate, transport of colloids, biopolymers and manufactured nanoparticles in aquatic systems. Fundamental research is also being applied within environmental chemistry and for industrial process control such as the rational design of flocculants used in water treatment and drinking water production, as well as to better understand the risks associated to the release of manufactured nanoparticles in aquatic systems. His research is supported by the Swiss national foundation, European Union Seventh Framework Programme and International collaborative programs. He has an active part in international projects, international editorial and expertise activities.

2 J. Colloid Sci. Biotechnol. 3, 1–30,2014

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Fatehah et al. Nanoparticle Properties, Behavior, Fate in Aquatic Systems and Characterization Methods toxicity with its existence in the environment which has

sparked a great concern from both the scientific commu- nity and the public. In the past few years various adverse effects of nanosized ZnO on plants, phytoplanktons, mam- mals, and even human cell lines have been reported.^18–22 The mechanism of ZnO toxicity has been discussed by Djuriši´c et al.²³ ZnO NPs in the aquatic systems have been revealed to potentially cause harm to aquatic organ- isms, especially if dissolved Zn²⁺ions are released.²⁴The solubilized ZnO NP can exert stress on cells and have adverse impacts on different organisms.^25–27 This is evi- dent in the ecotoxicity studies on ZnO NPs conducted on bacteria such asEscherichia coli,²¹²⁸Bacillus subtilis, Streptococcus aureus,²⁹ and marine algae.³⁰ It is there- fore essential to comprehend the behaviour of ZnO NPs because their fate, transport, behaviour and ecotoxicol- ogy are closely related to their intrinsic properties such as particles in suspension, surface energy and colloid stabilisation.⁵³¹ Additionally, understanding how exter- nal factors such as physicochemical conditions e.g., pH,³² physicochemistry of the particles,⁸ and interactions with other molecules¹⁰³²will provide a clearer view on the com- plex system of ZnO NP and its behaviour. This is relevant because the NP mobility is dependent on the physicochem- ical transformation they undergo such as surface modifi- cation, aggregation, disaggregation and dissolution.²⁶The transformation is a function of abiotic factors including pH, ionic strength, particle surface chemistry, the interactions of nanoparticles with other pollutants and natural organic molecules.^{7–81032–34}

Based on previous research on NP environmental tox- icology, the above factors like redox conditions, light, natural organic matter (NOM), and the presence of microorganisms may result in chemical and/or biological transformations of ZnO nanomaterials and induce their mobilization in the environment where they can poten- tially exert noxious effects on aquatic organisms and humans.^1235–37

1.2. Influence of Natural Organic Matter on Fate and Transport of ZnO Nanoparticles

The role of NOM in the fate and transport of ZnO nanopar- ticles have been broadly studied.^38–42 NOM in natural aquatic systems mostly comprise of humic substances (HS) and polysaccharides.⁴³HS are macromolecule structures⁴⁴ and consists of 30–50% of dissolved organic carbon (DOC), humic and fulvic acids. DOC is found naturally in water with a concentration rarely exceeding 5 mg/L.

The typical DOC molecular weight ranges from 102 to 106 Da.⁴⁵They carry potentially important functions in the environment as they can control the pH balance, govern the mobility of contaminants through absorption, aggrega- tion, and disaggregation and they can coat other surfaces to give them an overall negative charge through charge stabilization.³⁹⁴⁶

Humic acid (HA), on the other hand, is insoluble and will precipitate out in water under acidic conditions, especially below pH 2. It is then otherwise water soluble at alkaline pH.⁴⁵ The presence of HA is found ubiquitous in the natural environment⁴⁷ and has entailed with sev- eral investigations conducted on nanoparticles to observe their aggregation and disaggregation behavior with HA adsorption. The interaction of HA and negatively charged ions is mainly due to the van der Waals interactions with the NPs in the solution. Electrostatic and steric stabi- lizations have also been demonstrated in other indepen- dent studies involving NPs and NOM when they are in suspensions.¹⁹⁴⁵ The NOM surface coatings around the NPs indicate disaggregation through charge and steric sta- bilization mechanisms.^41–48

Polysaccharides constitute 10–30% of the NOM in nat- ural waters⁴⁹ and in marine environments. Alginates are naturally occurring polysaccharides, released by microor- ganisms such as algae, bacteria and plant roots⁵⁰⁵¹ com- monly found in the marine environments.⁵²Some alginates may also be found in nature as components of some algae cell walls, and are likely to be excreted by the algae in the form of extracellular organic matter.⁵³ Alginates have the tendency to promote and enhance particle aggregation and deposition via bridging process.⁵⁴ The interaction caused by these macromolecules will have a profound effect on the surface chemistry and transport of nanoparticles in aquatic systems.⁵⁵⁵⁶

1.3. Influence of pH and Zeta Potential on Stability of ZnO Nanoparticles

ZnO is an amphoteric oxide and can easily dissolve in both acids and bases.⁵⁷ At acidic pH values of <63, ZnO is hydrated to form Zn²⁺ cations and subsequently forms hydroxide layers in water at basic pH values, where Zn(OH)₂ is in equilibrium with the Zn²⁺, Zn(OH)⁻₃, and Zn(OH)²⁻₄ species. At pH>12, the latter two zincate ions become the dominant species in solution.⁵⁸ The major problem of ZnO nanoparticles arises from their poor sta- bility in water⁵⁹⁶⁰ which leads to the formation of aggre- gates as it approaches the point of zero charge (PZC) or pH_PZC.⁶¹ In aqueous suspensions of ZnO, certain pH regions can strongly affect the stability electrostatically due to the transformation of colloidal Zn(OH)_2Sparticles to Zn(OH)_2aqas the suspension stability is highly depen- dent on the surface charge of the constituent oxides.⁶²⁶³It is unknown to which extent that nanoparticles will agglom- erate depending on the processing conditions and the bal- ance between the attractive and repulsive forces among the nanoparticles as well as in between them.⁴¹

pH has a huge influence on ZnO nanoparticles and has led researchers to further investigate its rheological and electrophoretic properties based on measuring the viscos- ity versus the pH and amount of dispersant.⁵⁸ One of the earliest studies on zeta potential and pH was done by

J. Colloid Sci. Biotechnol. 3, 1–30,2014 3