Inorganic Nanoparticles and the Environment: Balancing Benefits and Risks

Chapter 8

Inorganic Nanoparticles and the Environment: Balancing Benefits and Risks Eudald Casals*, Edgar Gonza´lez* and Victor Puntes*,{ *

CIN2(ICN-CSIC), Catalan Institute of Nanotechnology, and Universitat Aut onoma de Barcelona (UAB), Bellaterra, Barcelona, Spain { Institut Catala` de Recerca i Estudis Avanc¸ats (ICREA), Barcelona, Spain

1. INTRODUCTION: ENGINEERED INORGANIC NANOPARTICLES IN THE ENVIRONMENT Industry and society are beginning to use nanomaterials in greater quantities and in consumer products. The National Science Foundation estimates that by 2015 the impact on the global economy will be $1 trillion and that 2 million workers will be employed in nanotechnology [1]. Also, over 1000 nanotechnologyenabled products have been made available to consumers around the world, according to the Project on Emerging Nanotechnologies [2]. Therefore, the proper knowledge of NPs’ behaviour once produced and exposed to humans and the environment, together with the mechanisms of interaction between them, is specially needed at this time to allow a sustainable development and a proper implementation of nanotechnologies. Besides, conventional remediation technologies have shown a restricted capacity for removal of pollutants from air, water and soil or in response to challenges of massive cleanup. For instance, current water filtration and purification systems to obtain drinking water usually yield partial results because materials employed are of limited effectiveness. Moreover, the intrinsic properties of some materials when they reach the nanometric scale make them promising candidates to work effectively in this field [3–8]. Nanoparticles (NPs) are able to obtain significantly higher efficiency for environmental remediation than larger particles of the same chemical composition due to their increased surface area, reactivity (in part due to their higher radius of curvature), and tunable morphologies and surface states. In fact, these Comprehensive Analytical Chemistry, Vol. 59. http://dx.doi.org/10.1016/B978-0-444-56328-6.00008-6 # 2012 Elsevier B.V. All rights reserved.

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properties have been already extensively used in the petrochemical industry in designing better heterogeneous catalysts. Nowadays, several nanotechnology applications for environmental remediation are being demonstrated in the laboratory. However, in this early stage of the field, published scientific papers usually are limited to the description of the phenomena while still lacking the detailed mechanisms of interaction between NPs and pollutants. Moreover, the fruitful transfer, effective and safe, to the real ground of application or at least to a pilot plant is not yet fully acquired. To design more controlled and effective remediation strategies, it must be coupled with both the proper knowledge of NPs’ properties and a better understanding of the natural mechanisms of water and soil remediation through the interaction between mineral layers (strata), the soil and resident microorganisms. Research on the interactions between pollutants and NPs have shown that, as expected, increasing surface area through nanostructuring yields an increased adsorption of the contaminants [7,8]. Also, at these minute sizes, particles better explore all the water volume, meeting the pollutants more efficiently. Thus, removal of toxic metals with NPs is a fast developing field of research. The ability of inorganic NPs to purify drinking water is well known for waters contaminated with cations such as As, Cr, Ni, Cd, Hg, etc. (Sections 2.3.1–2.3.4). Also, the use of NPs as efficient photocatalysts to promote the complete degradation of the organic matter (to CO2 or CH4) is being explored (Section 2.3.5). The use of NPs could improve environmental remediation technologies and help to reduce their cost, but benefits obtained have to be balanced with potential risks (Table 1). Apart from exposure through intended use, unwanted dispersion (spill) or (nano) waste management also has to be considered as a critical pathway to introduce NPs in the environment. Following the dispersability, persistence and evolution of NPs in the environment will be a key parameter for risk assessment since it will determine how living organisms will be exposed to them. The dispersion in the environment strongly depends on the ability of NPs to remain independent, avoiding absorption, sedimentation, agglomeration or disintegration. Inorganic NPs are not very common in nature due to their instability. Thus, NPs’ fate is to aggregate with other materials, change nature (e.g. iron dextran was found to be transformed in the liver to both akagane´ite NPs and ferrihydrite NPs, two different iron oxides [9]) or disintegrate into atomic and molecular species (as ZnO [10] or CdSe [11]), which in turn will be transformed into stable species or incorporated into other materials, in any case resulting in deactivation of the NPs. Of course, the aggregate or disintegrated species can be toxic, as micrometric fibres known to cause a broad range of granulomatosis, in the case of aggregation, or the Cd cations released from CdSe NPs, in the case of dissolution. Therefore, in some cases, NP should be considered as a protoxin rather than a toxin itself. The NP toxicological and safety aspects are developed in Section 3.

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TABLE 1 Nanotechnology Used as Remediation Technology Location

Media Treated

Contaminants

NP Used 0

Result

Canada (City of Ontario)

Groundwater

Trichloroethylene Perchlorethylene

Fe

Reduction of 70%

Canada (military installation in Quebec)

Sands and clayey silts

Dichloroethylene Trichloroethylene Vinyl chloride

Fe0 and Pd–Fe0

Reduction of 98% for TCE and 10% for DCE

Czech Republic (industrial plant)

Groundwater

Dichloroethylene Trichloroethylene Perchlorethylene

Fe0

After 180 days, total chlorinated solvents were one order of magnitude lower

Czech Republic (solvent manufacturing plant)

Groundwater

Dichloroethylene Trichloroethylene Perchlorethylene

Fe0

After 1 month, chlorinated solvents’ concentration in groundwater was more than one order of magnitude lower

Italy (City of Biella)

Groundwater

Dichloroethylene Trichloroethylene

Fe0

20–50% Reductions in total chlorinated solvent concentrations after 1 month

United States (City of Rochester, NY)

Groundwater in bedrock

Methylene chloride 1,2Dichloropropane 1,2Dichlorethane

Fe0

Chlorinated solvents concentration in groundwater was reduced about one order of magnitude

United States (military installation in Rockaway Township, NJ)

Groundwater

Carbon tetrachloride Trichloroethylene

Fe0, Ferragel NPs

After 1 year, chlorinated solvents concentration in groundwater continued

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TABLE 1

Nanotechnology Used as Remediation Technology—Cont’d

Location

Media Treated

Contaminants

NP Used

Result was about 30% lower

United States (manufacturing site in Passaic, NJ)

Groundwater

Trichloroethylene

Fe0

90–100% Reduction in TCE concentrations

United States (manufacturing site in Edison, NJ)

Fractured bedrock

Trichloroethane Dichloroethylene Trichloroethylene Chloroethane Vinyl chloride

Fe0

Decreased to a level below minimum detection limit

United States (Hamilton Township, NJ)

Groundwater

Dichloroethane Trichloroethane Dichloroethylene Trichloroethylene

Nanoiron slurry

Reduction in dissolved chlorinated contaminants at concentrations by up to 90%

United States (manufacturing site in Trenton, NJ)

Soil and groundwater

Dichloroethylene Trichloroethylene Perchlorethylene Vinyl chloride Chloroform Carbon tetrachloride

BNP (Fe/ Pd NPs)

Contaminant concentrations reduced by 1.5–96.5%

Taiwan (petroleum manufacturing plant in Kaohsiung)

Groundwater

Dichloroethane Trichloroethane Dichloroethylene Trichloroethylene Vinyl chloride

Pd–Fe0

Reduction of 20–90%

List of sites, as an example, where NPs have been employed to clean different contaminated sites. Source: Nanoremediation Map: http://www.nanotechproject.org/inventories/remediation_map/. http://www.nanowiki.info/#%5B%5BContaminated%20site%20nanoremediation%5D%5D.

2. ENVIRONMENTAL REMEDIATION WITH INORGANIC NPs 2.1 Inorganic NPs: Definition Inorganic materials have shown interesting properties to adsorb and detoxify metal ions. For instance, nickel filtered from industrial areas or natural pyrites

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into groundwaters can be adsorbed by natural calcite (CaCO3) (Section 2.3.3). Similarly, some iron oxide minerals naturally adsorb heavy metals such as chromium and arsenic from water (Section 2.3.2). These properties can be further exploited in the NP form. A proper knowledge of what is an inorganic NP is essential to understand why it is considered a key material in the future of environmental remediation and how it carries out these works. There are lots of proposed definitions of the term nanomaterial, principally driven by the need of providing a framework to the regulations laying down provisions on substances. The International Organization for Standardization have proposed the ISO TS 27687 definition, in which a nano object is considered a material with one, two or three external dimensions in the nanoscale, where nanoscale is defined as the size range from
1 to 100 nm. This is common in most of the definitions, which emphasize the size aspect or even only consider it. However, other voices call for including in the definition the functional properties that enable them to be considered a new class of materials (see e.g. the European Consumers’ Organization to the European Commission public consultation on nanomaterials [12,13]). Certainly, if NPs are of great scientific interest, it is because they are effectively a bridge between bulk and atomic or molecular structures. This means that, at the nanoscale, material properties may change abruptly with respect to those displayed by a material that either is of the same composition but larger (“bulk”) or is a single atom isolated (ion), or is a single atom forming part of a molecule. And, while a bulk material should have constant physical properties regardless of its size, at the nanoscale this is often not the case. The properties of materials change as the percentage of atoms at the surface of a material becomes significant or finite size effects appear. Size-dependent properties observed for nanomaterials include, among others, quantum confinement in semiconductor NPs, surface plasmon resonance in some metal NPs and superparamagnetism in magnetic materials. Thus, as discussed in our review [14], an NP can be considered as a small particle with at least one dimension < 100 nm which presents novel properties that differ from the bulk material. Obviously, there is no strict dividing line between NPs and non-NPs since the size at which materials display different properties to the bulk material is material-dependent. Beyond size discussion, three important features distinguish engineered NPs from other particulate matter: l

l

Their monodispersity. Without entering a discussion about which is the physical limit of monodispersity (i.e. which standard deviation threshold determines that a collection of NPs should be considered monodisperse) from a practical point of view, two NPs can be considered monodisperse if they respond undiscernibly to a determined test. Roughly, this means that in 1 mol of NPs, every NP behaves indistinguishable respect to the others respect to that test. Their morphology. Engineered NPs can be designed with specific geometrical forms as spheres, cubes, tubes, wires, rings, disks, etc. (which are not

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found in inorganic particulate matter from natural origin). Defined NP size and shape is correlated to a particular activity, which reminds the structure– activity relationship of biological macromolecules. Their stability. Obviously, NPs lose their nanoscale properties if aggregate. As NPs need to be kept separate from each other, surface engineering is required in order to provide them with repulsion forces to prevent aggregation, either electrostatic (e.g. creating a double electrical layer of inorganic ions around the surface of the NPs) or by steric means (e.g. attaching to the surface organic or biological molecules). Thus, an inorganic NP must be understood as inorganic core with a stabilization shell (inorganic or organic) that prevents their agglomeration.

2.2 Appealing Properties of NPs for Environmental Remediation Decades ago, the distinct properties of minute matter with respect to bulk counterparts were already being explored and even applied to address environmental harms derived from the petrochemical industry (e.g. as catalysts for oil processing [15]) or in the automotive sector to catalyze detoxification reactions [16], among others. In recent years, the great development of the characterization techniques to study matter at the nanoscale has enabled a greater understanding of the mechanisms of action (and also the emergence of nanotechnology as a widely funded scientific discipline). Thus, nanomaterial properties are nowadays the subject of detailed studies in many laboratories worldwide. A large body of knowledge is being acquired, which is dispersed in a multitude of articles, reviews, books and conferences devoted to this field. Generally, there is an agreement that the natural skills presented by NPs that make them attractive to be applied in environmental remediation are a combination of characteristics that arise from their particular sizes, shapes, surfaces and inherent compositions.

2.2.1 Due to the Size The immediate effects of reducing the size of a material are (i) the increase of number of particles for a given mass and (ii) the exponential increase of the surface-to-volume ratio in a particle (Figure 1). These are well known and entail two important consequences for their environmental applications. First, the increase of the surface-to-volume ratio is correlated with the increase of the curvature radii of the particles. Therefore, NPs become especially reactive due to high density of low-coordinated atoms at the surface, edges and vortex, allowing reactions at these actives sites [17]. In a simplified manner, in an inorganic crystal, the reactivity of an atom increases as its coordination decreases. Therefore, for particle sizes small enough, the curvature radii are so high that all the atoms of the particle lay close or at the surface in kinks, steps and edges. For instance, assuming an icosahedral distribution of atoms, for particles with a diameter of 1 or 2 nm, all the atoms of the particle are at the surface and at the same time on the

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Number of surface atoms

Number of atoms

1,00,000 10,000 1000 100 10 0

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5

0

10

15

20

100 80 60 40 20

25

0

5

10

15

20

25

NP size (nm)

NP size (nm)

NP diameter (nm)

Number of NPs (cm-3)

Surface area (nm-2 cm-3)

5

153,000,000

12,000

20

2,400,000

3016

250

1200

240

5000

0.15

12

FIGURE 1 Surface area increases as size decreases. Table values are calculated assuming NP mass concentration of 10 mg/ml.

edge of the particle. For particles of 3 nm, 80% of the atoms are at the surface, from which 90% are on edges. In the case of 4 nm particles, 50% of the atoms are at the surface and 80% of these atoms are on the edges. For 5 nm particles, 40% of the atoms are at the surface, 75% of those are on the edges, and so on. For larger diameters, the surface of the crystal will show a collection of steps and kinks independent of the size and shape, and the reactivity will then depend on the quality of the surface. This explains that very small crystals (below 5 nm in diameter) are the most used in heterogeneous catalysis [18]. Second, mobility in solution of small NPs is high since Brownian motion overcomes the gravitational force. For instance, the whole volume of a recipient can be quickly scanned with small amounts of NPs (Figure 2). In a rough estimation, a 10-nm gold NP in water at room temperature will experience Brownian relaxation on the order of the nanosecond, and each Brownian step in solution will move it for about 10–20 nm. Therefore, a typical NP concentration (e.g. 1012–1014 NP/ml, otherwise stability is compromised due to supersaturation) will explore the total volume in the order of the centiseconds (assuming a 10% efficiency, i.e., an NP visit up to one new position out of 10 Brownian steps),1 being able to trap pollutants or catalytically degrade them. 1. 1012 NPs in 1 ml. Each NP is responsible to scan a volume of 10–12 ml, which in cubic nanometre is 109 nm3. The volume of one 10-nm AuNP is 523 nm3, so each NP has to take 109/523 steps to visit its whole corresponding volume—about 106 steps; with 10% efficiency, it will take 107 steps, and with each taking 1 ns, it means that the whole volume is explored every 10 2 s. This is also developed in Ref. [8].

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For instance, stable colloidal AuNPs of 10 nm mean diameter can easily arrive to a concentration of 1012 NP/ml. Then, each NP scan a volume of 1/1012 ml, that is 109 nm3 (1ml = 1cm3; 1cm3 = 10–21 nm3). The volume of one AuNP 10 nm mean diameter is 523 nm3 Thus, if each AuNP do 109/523 = 1.91 ⫻106steps, all the volume is visited by the AuNPs. (109/523 = 1.91⫻ 106steps) Considering a 10% efficiency (each 10 steps, a new portion of the volume is visited), ~107steps are needed. If each step takes about 1 ns, every 10–2 s all the volume is explored.

FIGURE 2 NPs for water remediation. A whole volume of a recipient can be quickly scanned with small amounts of NPs.

2.2.2 Due to the Shape Similar to that explained above, atom coordination and surface-to-volume ratio also depend on shape: flat surfaces may be less reactive than curved surfaces, as nanospheres. Moreover, hollow NPs can be manufactured as thin as to have almost all atoms in the surface, thus with improved catalytic behaviour and also with the ability to trap, host and/or degrade the contaminant in their interior. 2.2.3 Due to the Surface NP interactions with its environment occur through its surface, which experiment constant modifications. For instance, “naked” inorganic NP surfaces (e.g. electrostatically stabilized NPs) are immediately coated with biomolecules in a non-specific way when dispersed in biological media. The chemistry of coating inorganic surfaces can be harnessed to design functionalized NPs to trap desired pollutants. For instance, thiols, amines and carboxylic acids bind strongly to Au, Pt, Co, Fe and their oxides (with different and specific affinities). A molecule bearing one of these functional groups in one end can be easily attached to the NP, while the other end is able to carry other active functional groups. 2.2.4 Due to Their Inherent Composition Certain specific minerals from natural origin are known to trap or degrade specific contaminants. Thus, a better understanding of the natural mechanisms of water and soil remediation, coupled with the advantages of nanoparticulate matter, will help to design more effective remediation strategies. As discussed in Section 1, CaCO3 naturally adsorbs nickel [19,20] and some iron oxides adsorb heavy metals such as chromium and arsenic from water [7,21–23].

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Thus, these materials could carry out these processes more efficiently when taking advantage of preparing them as NPs. Last but not least, there is an unclassified NP feature that deserves special attention when analyzing the benefits of using NPs for environmental remediation. The use of NPs allows the easy separation of these cleaning agents from the cleaned medium through the application of mild gravitational or magnetic gradients (Figure 3). In this latter case, it is obvious that magnetic NP, such as iron oxides, maghemite and magnetite, must be used. But the former case can be affordably applied in any type of NP. Centrifugation at 10,000–20,000 g, which is a speed reached, for example, by commonly used small lab centrifuges, efficiently separates (precipitate) small inorganic NPs from aqueous media, and also large NPs will slow sediment spontaneously.2 This NP recovery is indeed valuable when the pollutant is physically or chemically attached to the NP. Moreover, once NPs are recovered, they can be either reused again

FIGURE 3 Magnetic removal of contaminated water using magnetic NPs. (A) Top: contaminated water; bottom: colloidal suspension of Fe3O4NPs. (B) Both suspensions are mixed and NPs trap pollutants. (C) Due to the magnetic character of the NPs, the composite NP contaminants can be separated from liquid driven by a magnetic field. (D) Finally, removing the NP contaminants attached, purified water is obtained.

2. Sedimentation rates depend on the viscosity of the medium where NPs are dispersed, the NP size and NP densities. NP densities vary from lighter NPs as SiO2NPs with a density of 2.7 g/ cm3 to heavier NPs as AuNPs, of 19.3 g/cm3.

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when possible or processed to obtain new NPs by the complete NP degradation into its constituent atoms through corrosion processes and further resynthesis. This recycling procedure would make the use of inorganic NPs more sustainable and would substantially reduce the cost of the overall process.

2.3 Inorganic NPs Used in Environmental Remediation: Model Cases 2.3.1 Metalic Gold and Silver NPs: Catalytic Activity and Mercury Removal Over the past years, the ability of gold nanoparticles (AuNPs) to catalyze a variety of oxidation reactions has attracted great interest. Gold in bulk is chemically inert and has often been regarded to be poorly active as a catalyst. However, when size is reduced to diameters below 5 nm, it is active for many reactions, such as carbon monoxide (CO) oxidation to carbon dioxide (CO2) and propylene epoxidation [24]. Recently, van Bokhoven et al. [25], using high-energy resolution X-ray absorption spectroscopy in the European Synchrotron Radiation Facility, unravelled how oxygen is activated on this “surprising” catalyst. The researchers first applied a flow of oxygen over the AuNPs and observed how the oxygen becomes chemically active when bound on the AuNPs and reacted with CO to form CO2, while without the AuNPs, the reaction did not take place. This could have a variety of applications. For environmental remediation, they include the pollution control such as air cleaning or purification of hydrogen streams used for fuel cells, among others. Also, silver nanoparticles (AgNPs) are investigated for the conversion of ethylene to ethylene oxide, an important industrial precursor [26]. Importantly, these reactions take place at ambient temperature or less and can be even enhanced when NPs are supported on metal oxide substrates [27]. Beyond catalysis, amongst the nanotechnology-enabled environmental remediation procedures, the ability of metal and metal oxide NPs to purify heavy-metal-contaminated waters is one of the most developed fields of activity. An exhaustive review of the use of noble metal NPs for water purification is the work of Pradeep and Anshup [5]. The fast mobility of NPs in aqueous media has been exploited to use Au, Ag, Fe, Pt, FeOx, CeO2 and many other NPs to remove poisonous cations as those of As, Cr, Ni, Cd, etc. from drinking water. The cleaning of these contaminated waters can be either through adsorption of the cation onto the NP surface or because it integrates into the NP structure or because of the modification of the cation oxidation state. A special case that arises from harnessing the benefits of nanotechnology to known natural mechanisms is the removal of mercury by Au or AgNPs through amalgamation (the reaction of mercury with these metals). Mercury presents serious environmental and health hazards. It is released into the environment through a variety of natural (as volcanic eruptions) and

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anthropogenic sources (from fossil fuels to gold mining) [4,28,29]. Released mercury vapour gets converted into soluble forms (Hg2 þ, Hg22 þ) and gets deposited in soil and water by rain. Due to microbial action, inorganic mercury gets converted into methyl mercury and enters the food chain of predatory species, potentially arriving to humans. In the body, methyl mercury affects various organ systems. In adults, it is related to memory loss, decreased rate of fertility, birth of abnormal offspring, etc. In children, the effects include autism, late walking and also deficit in memory and language [30]. The most famous case was the mercury contamination reported in Minamata City (Japan) in 1956 due to the poisoning of the central nervous system caused by methyl mercury which was accumulated in fish and seafood [31]. Due to the severe effects of mercury on mankind, the World Health Organization (WHO) has set the limit for mercury in drinking water to be 0.001 mg/l. The inverse case, that is, the isolation of precious metals using mercury, is known from centuries ago. Already in the sixteenth century, trying to exploit the natural resources of the recently discovered New World, the Patio process was developed to extract silver and gold from their respective ores. It is known as the first form of amalgamation. Briefly, it consisted of the crushing of the mixture of materials extracted from the ore until reaching a fine slime. Afterwards, water, salts and mercury were spread in a layer in a courtyard (the “patio” in Castilian). Horses were driven on the “patio” to mix the ingredients. After weeks of mixing and soaking in the sun, those metals formed an amalgam with mercury. As mercury and gold or silver have very different boiling points, when heating the amalgam, mercury evaporated and the precious metal could be recovered. The natural process of amalgamation was first used in colloids in the early twentieth century by mixing gold and mercury hydrosols or by shaking a mercury-containing liquid mixed with a gold sol. In these cases, complete interpenetration in solution, according to the molar Au/Hg ratio, was observed after chemical analysis [32–34]. Recently, reduction of Hg2 þ in silver sols and simultaneous reduction of Agþ and Hg2 þ ions by NaBH4 [35] or by radiation [36] and similar procedures to remove inorganic mercury from drinking water using AuNPs have been reported [4]. The efficiency of this cleaning process using NPs due to their increased surface area and higher mobility in solution, together with the easy recovery of the amalgamated dense particles by mild centrifugation makes this system one of the most promising methods to purify water from mercury contamination. Synthetic procedures to obtain Au and AgNPs have been developed long ago. However, the synthesis of these NPs with narrow size distributions and well-defined shapes is still one of the most current research topics and the number of publications per year on this subject is still growing for decades. Thus, there are endless recipes to prepare AuNPs and AgNPs that can be employed [37]. To name a few, the most famous procedure is the Turkevich

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method [38] which consists of the fast injection of gold or silver salts (e.g. hydrogen tetrachloroaurate(III), hydrate (HAuCl4) or silver nitrate) to a boiling solution containing trisodium citrate (SC) while stirring. After few minutes, solution acquires the characteristic red colour in the case of AuNPs or yellow colour for AgNPs. This method yields to spherical NPs of 10– 20 nm. Recently, Bastus et al. [39] have extended this citrate reduction method to the synthesis of spherical AuNPs of narrow size distribution from 8 up to 200 nm. Also, smaller NPs can be obtained using the stronger reducing agent sodium borohydride (NaBH4) [40].

2.3.2 Iron Oxide NPs: Chromium and Arsenic The natural mechanism of conversion of “green rust” to “red rouge” is interesting for the protection of groundwater contamination from other heavy metals. Green rust are the iron compounds rich in Fe2 þ with the mineralogical form of fougerite, while red rouge is applied to some iron compounds rich in Fe3 þ, mainly magnetite, maghemite, hematite and goethite. NPs of “green” iron (II) oxide, generated during the oxidation of metallic iron in water, contain Fe2 þ ion. This is effective for removing redox-sensitive elements such as chromium or chlorinated solvents while oxidizing to Fe3 þ-containing species [3,41]. The chromium specie Cr4 þ is one of the heavy metals that appear in the fly ash from burning fireplaces and, when settled down, is easily transported to groundwater by rain. It is also found as a dye residue in the textile industry. It is soluble and causes cancer, genetic mutations and foetal deformities. On the other hand, Cr3 þ is not soluble in water and is an essential trace mineral in the body. High-resolution studies [3] show that chromate (Cr(VI)O42 ) replaces sulphates SO42  naturally present on the surface and interstices of the green oxide. During this process, Fe2 þ becomes Fe3 þ while Cr4 þ is converted to Cr3 þ. As a consequence of this redox process, iron oxide structure is modified to goethite. And a favourable aspect is that the Cr3 þ is incorporated into the goethite structure producing a chromium carrier phase even more insoluble than goethite alone. Of course, this can be controlled and adapted to specific needs by the design of strategies based on synthesized NPs. Similarly, magnetite (Fe3O4) NPs are used to trap arsenic (As). Arsenic is widespread in the earth crust and groundwater contamination occurs via its dissolution from minerals and ores in the subsurface [42]. This is a severe global problem, most notably in Southeast Asia, where a million people suffer from acute and chronic arsenic poisoning. The acute minimal lethal dose of arsenic in adults is estimated to be 70–200 mg or 1 mg/kg/day. However, long-term exposure to minimal amounts of arsenic in drinking water elevates cancer rates of the skin, lungs, urinary bladder and kidneys, in addition to several skin diseases [43]. As a result, the US EPA and the WHO recommend a contaminant limit of 0.01 mg/l (10 ppb) of arsenic in drinking water.

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It is known that Fe3O4 powder efficiently removes As3 þ and As5 þ from water due to the high affinity of both materials at pHs in the range of 4–10 [7,44]. In addition, Fe3O4 is a magnetic natural mineral on earth and can be manipulated by a low-strength magnetic field [7,45]. The efficiency of this removal was shown to be
200 times when the Fe3O4 size decreases from 300 to 12 nm [7,46]. Easily, in those reported experiments, a low magnetic field pulled the NPs out of a solution previously contaminated with arsenic, leaving behind only the purified water. For the obtention of Fe3O4NPs, the Massart’s method is a simple, low-cost and well-known procedure [47,48]. It yields monodisperse
10 nm NPs. It consists of the mixture of FeCl2 and FeCl3 at 1:2 molar ratio dissolved in deoxygenated water and then added drop-wise to a solution at basic pH (aqueous solutions containing either NaOH or ammonia (NH4OH) or tetramethyl ammonium hydroxide (TMAOH) are the most commonly used). After 30 min of vigorous stirring under an N2 stream, the Fe3O4 precipitate must be washed by soft magnetic decantation and then can be preserved as nanopowder or re-dissolved to obtain a stable aqueous ferrofluid.

2.3.3 Calcite NPs and Nickel It is another important case where the understanding of natural mechanisms together with the acquired knowledge of nanoscale properties greatly contribute to design more effective remediation strategies. Nickel is released into groundwater either from industrial areas or filtered from waste dumps and by the natural oxidation of pyrite (FeS2), where nickel is present in traces [20,49]. Even at tens of ppb, Nickel causes painful skin allergies [6,50,51]. Contaminated wells must be carefully cleaned or definitively closed. In addition, it is known that CaCO3 easily accommodates a large number of divalent substitutions in its structure [19]. Macroscopic studies have shown that nickel is rapidly adsorbed by calcite in solution and crystallographic studies showed that nickel is incorporated into calcite precipitates. Observations on the nanoscale have complemented these results, showing that Ni ions substitute Ca ions and, even in conditions in which part of calcite is dissolved, the nickel remains adsorbed. A larger surface area increases speed and amount of nickel adsorption. In dry samples, nickel is incorporated by solid-state diffusion, showing that, in solution, supersaturation is not required for the nickel to enter into the solid from the surface [19]. Once this pollutant has been trapped by the NP, altogether it can be collected by sedimentation or using a gravitational field, thus being eliminated from the environment. There are many strategies to prepare calcite nanoparticles (CaCO3NPs). One of the simplest is the carbonation of calcium hydroxide (Ca(OH)2) in a reactor system H2O/CO2 at constant pressure. CaCO3 nanocrystals are formed and grow in this medium and the reaction is stopped when the pH of the medium reaches a value of 7. Finally, a purification step is needed to discard potential by-products of the synthesis [52].

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2.3.4 Cerium Oxide NPs: A Polyfunctional Type of NP Cerium oxide (CeO2) is one of the main compounds among the useful materials of the rare earth elements. It is applied as catalytic converter in the automotive industry for the reduction of toxic emissions from internal combustion engines, antioxidant in biomedicine to treat disorders caused by oxygen radicals, additive in fuel cells, UV absorber and many others [53]. All these applications rely on the capability of CeO2 to store or release oxygen depending on the surrounding conditions. This capability depends on the crystal structure which, in turn, depends on the process of synthesis. It is known that, as oxide, cerium can form two main crystal structures, cerium (IV) oxide (CeO2) and cerium (III) oxide (Ce2O3), CeO2 being the most stable phase at room temperature and under atmospheric conditions. When the size of the grain is reduced to the nanometric regime, a large amount of surface defects appear in the CeO2 crystal structure, primarily caused by the reversible removal of oxygen atoms from the surface. Electrons left behind by released oxygen localize on empty f states of cerium ions, being reduced from Ce4 þ to Ce3 þ [54]. The nature and density of the oxygen vacancies are also determined by the synthetic procedure and chemicals used (e.g. it is not trivial to use cerium (IV) salts or cerium (III) salts as a precursor reagent) and the surroundings of the crystal (increase of the partial pressure of oxygen re-oxidizes Ce3 þ to Ce4 þ lowering the number of oxygen vacancies) [55]. Anyway, a number of vacant positions in CeO2NPs are significant and the increased reversible oxygen positions and electron movements in the crystal structure determine the unique catalytic and biological activity of CeO2NPs. From one side, there are many processes where CeO2 is used as a catalyst or an active support and oxygen buffer for noble metals in catalysis (see as an example Ref. [56]). Probably, CeO2 as catalytic converter in the automotive industry is the most known and commercially used application. Nanostructured CeO2 serves as oxygen storage diesel additive in vehicle exhaust catalysts. When insufficient oxygen is in the exhaust stream, CeO2 releases the “stored” oxygen and continues oxidizing carbon monoxide (CO) and unburnt hydrocarbons into less-contaminant carbon dioxide (CO2) and water. During this process, Ce4 þ ions are reduced to Ce3 þ that are re-oxidized when there is enough oxygen again in the exhaust gases, allowing more cycles of catalysis. As oxygen buffer, CeO2 is also a component of the three-way catalytic converters because it can add to the two previous reactions the reduction of nitrogen oxides (NOx) into nitrogen and oxygen. Summarizing, the reactions involved in all these processes are 1. Reduction of nitrogen oxides to nitrogen and oxygen: 2NOx ! xO2 þ N2. 2. Oxidation of carbon monoxide to carbon dioxide: 2CO þ O2 ! 2CO2. 3. Oxidation of unburnt hydrocarbons to carbon dioxide and water: CxH2x þ 2þ((3xþ 1)/2)O2 ! xCO2þ(xþ 1)H2O.

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From the other side, the ability to switch valence states between Ce3 þ and Ce4 þ depending on the surroundings, also in physiological conditions, makes CeO2NPs a striking material for biomedicine and environmental remediation. The key factor is their ability to participate in redox processes, especially in the modulation of oxidative stress in living organisms. It is intensively investigated in a variety of medical fields from ophthalmology [57], to cardiology [58], neurology [59] and oncology [60,61]. The same rationale is behind several research projects trying to apply these NPs to degrade redox-sensitive contaminants. For instance, similar to the case shown for iron oxide NPs applied to chromium removal and the conversion of Fe2 þ to Fe3 þ during this process, CeO2NPs have the additional potential to switch between its oxidized and reduced form cyclically, which is being explored [55]. Also, CeO2NPs have been proved as efficient adsorbents to trap and remove a variety of contaminants such as arsenates, phosphates, lead, chromium (VI) and the organic persistent contaminant 2,4-dichlorophenoxyacetic acid [62,63]. An easy and low-cost method, widely used for the obtention of aqueous suspensions of CeO2NPs, consists of the precipitation of cerium salts at basic pH. Briefly, oxidation of the Ce3 þ ions from Ce(NO3)3 salt at basic pH conditions to the insoluble species Ce4 þ using hexamethylenetetramine (HMT). During precipitation, CeO2 nanocrystals are formed and can further stabilize in aqueous medium with the same reagent HMT, that form the double electrical layer to prevent agglomeration [64]. Other basic reagents usually employed are urea, NH4OH and TMAOH [65].

2.3.5 Titanium Oxide NPs: Photocatalytic Degradation of Organic Pollutants Photocatalysis is an active method that uses the sun energy to degrade many different pollutants like nitrogen oxides (NOx), volatile organic compounds or other organic matter. To this, the use of semiconductor NPs has found increasing interest to solve these global pollution problems. Currently, thousands of scientific publications on the photocatalytic degradation of organic compounds employing these materials can be found. So far, among photocatalysts, TiO2 has been the most promising material due to its high photoreactivity, low cost, low or nil toxicity, chemical and biological inertness and photostability. When this semiconductor material receives light of low wavelengths (about 400 nm), an electron jumps from the valence band to the conduction band resulting in an electron–hole pair. As a result, the valence band has gained enough positive potential to generate hydroxyl radicals on the surface, and the conduction band becomes sufficiently negative to reduce molecular O2. The hydroxyl radical is a powerful oxidizing agent that reacts with organic pollutants present at, or near to, the TiO2 surface, completely oxidizing them to CO2, when the process is allowed to continue until the total conversion.

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To name a few examples of the use of titanium oxide nanoparticles (TiO2NPs), it has been demonstrated that photocatalytic destruction of persistent organic pollutants in water such as oestrogen, 17-beta-estradiol, p-nitrophenol and atrazine [66,67]. Atrazine is a potent herbicide widely used to stop weeds in agriculture and has been recently banned in the EU. It is commonly found in surface and groundwaters and its persistence in the environment is due to the presence of s-triazine ring, which limits its biodegradation. Also, TiO2NPs have been proven highly effective against aquatic microbes, including bacterial spores and resistant organisms. The photocatalytic disinfection of water containing pathogenic microorganisms is an effective method to provide drinking water even in media with organisms resistant to bleach, such as Clostridium perfringens, which is an indicator of faecal contamination. Tests on Escherichia coli have shown significantly higher rates of effective disinfection using this photocatalyst than using UVA irradiation alone [68–71]. These results could be even more relevant in the case of developing countries or in emergency situations where diseases caused by contaminated water are a severe threat. In addition, basic principles of photocatalysis are also already being exploited in some commercial products, such as self-cleaning windows and architectonic coatings and building blocks to remove NO from the air [72]. Also TiO2NPs have been incorporated into paints and cement for construction. In laboratory tests, these materials showed an 80% reduction in released levels of NOx [73]. Of significant importance is TiO2 modified with noble metals. This doping has shown a significant increase in the activity of the TiO2 for a variety of catalytic processes. For instance, gold increases the electron donor ability of TiO2 to an extent that the photocatalytic activity of TiO2 is doubled [74– 76]. Additionally, this TiO2–Au catalyst becomes sensitive to photons in the visible spectrum, which is a considerable advantage for water remediation, as water is opaque to UV and transparent to visible light. There are a variety of methods for the obtention of TiO2NPs depending on the desired crystalline phase, anatase, rutile or brookite. Anatase is that which offers better performance for photocatalytic applications. A simple synthesis procedure for its obtention consists of the decomposition of titanium tetrachloride (TiCl4) at acidic pH (from 2 to 6). After that follows a growing step of the nanocrystals, carried out at 70  C, purification by means of centrifugation and resuspension with TMAOH to stabilize. Depending on the pH during the growing step, the obtained size and shape of the TiO2NPs vary from small size and spherical-like (
5 nm) to bigger particles (
10 nm) [77].

3. UNDERSTANDING THE RISK Prior to the introduction of a new product to the market (pharmaceuticals, chemicals, materials, etc.), its safety must be verified. Obviously, nanoscale

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products are not an exception. The same properties that make NPs particularly reactive and appealing for environmental remediation are those that may pose environmental hazards associated with their use. Potential hazards include the higher toxicity of the NP intended to apply than the original pollutant, the introduction of secondary products in the food chain, the impact in microbiology or the possible soil degradation, among others. Potential risks of NPs dispersed in the environment may result from their chemical and physical characteristics, such as minute size, special surface reactivity and/or their composition, and also as a consequence of several physicochemical processes as NP aggregation or corrosion. The awareness of the importance of these studies to appropriately and safely implement the benefits of the use of NPs is reflected in the increasing number of funded projects and research papers about the life cycle of NPs in biological environments [78].3 Next, NPs’ characteristics that could entail environmental hazards are commented.

3.1

The Surface of the NPs and Their Stability

While much of the NPs’ function is due to their core structure, the surface coating defines much of their bioactivity. As discussed in Section 2.1, it is important to note that NPs, inappropriately understood in many cases as solely an inorganic or polymeric core, never travel alone. On the contrary, they are constantly surrounded by a coating of different molecules, intended for a further application (NP functionalization) and/or spontaneous due to molecules present in the environment. These also take part in NP morphology and functionality. NP coating may improve its stability, due to steric repulsion between NPs, and thus increase residence time in the environment. As an example, Huang et al. [79] reported few years ago how multi-walled carbon nanotubes (MWCNTs) mixed with natural organic matter present in water from a mountain river remained suspended for more than a month. Besides, the addition of MWCNTs to organic-free water turns the water completely transparent again in less than 1 h due to rapid CNT sedimentation. However, if the addition was to a sodium dodecyl sulphate solution, the CNTs immediately turn the water dark and cloudy again, and some MWCNTs remained suspended for more than a month, mirroring natural conditions. However, as discussed in Section 1 of this chapter, although some natural processes, such as forest fires or volcanic rashes, may produce NPs, these materials are not common in nature since these are systems far from chemical equilibrium and their final fate once produced is their disintegration (see Section 3.2) or agglomeration towards more stable phases. Agglomeration takes place when the inorganic surfaces get in contact, either because once 3. This reference is a detailed document of the European projects related to safe use of NPs in biological media (from cellular models to the environment) and provides an extensive list of references from scientific literature devoted to the environmental risks of nanotechnology.

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NP is formed it is faster than acquiring a coating or because the organic shell is often degraded or unable to prevent from aggregation. In aqueous environments, NPs destabilized agglomerate, and those agglomerates precipitate. Similarly, higher concentrations of NPs could make some NPs to be kicked-off from the solution by some others, like soluble salts where the excess sediments once an upper concentration threshold is overcome (saturation limit). In such case, once the concentration of particles in solution decreases, the sediment spontaneously re-dissolves (what is commonly observed with NPs) [80]. However, on the other side, in many occasions, agglomerates may end up in irreversible aggregates that cannot be redispersed. These phenomena determine some upper concentration limit for NPs in biological media which must be principally taken into account in order to experimentally assess their in vitro or in vivo toxicity. Considering that colloidal synthesis represents the major technique to get individual size and shape-controlled NPs [81,82], the standard concentrations to still obtain monodisperse collections of stable and morphologically controlled NPs is between 1012 and 1016 NPs/ml (this is not a theoretical limit, but practical) depending on the material. This corresponds to an upper NP concentration of 10 mM that, for instance, in the case of the more dense material used, gold (19.3 g/cm3), may arrive to a few milligrams per millilitre. Nevertheless, it is worth noting that when NPs are dispersed in the environment, such concentrations are very difficult to obtain unless NPs are trapped in impasses. Thus, agglomeration of NPs is specially relevant “indoors” as in the case of accumulation in different places as organs (lungs, gut, etc.). In any case, either because NPs are destabilized in biological media or because they have exceeded their saturation limit, special physicochemical properties that arise at the nanoscale (as catalytic activity) are progressive/partially lost when NPs aggregate. Neither the properties nor the dynamics are any longer the same. Agglomeration leads to specific surfaces, concentrations, mobilities, etc., very different from the parent NP dispersion. And, when evaluating NP toxicity, both aggregation and sedimentation of NPs could be indeed a source of confusion: the larger size of the agglomerates yields a different range of effective size and concentration of particles, and consequently no comparable doses in terms of number and type of NPs. For instance, in some reports, the onset of toxicity in the viability experiments might be related to the onset of agglomeration and thus the higher cytotoxicity of unstable colloidal preparation of NPs could not be attributed to the material but to its final micro- or macrometric size [83]. Finally, it is needless to say that the toxicity of a substance is related to its dose (dosis sola facit venenum) in such a way that an accurate determination of the dose is critical to properly assess the potential toxicity of a material. Regarding detrimental health effects, that NPs tend to aggregate and then they are no more “nano” could lead to a similar scenario to that posed by incidental inorganic microparticles, extensively investigated during the past

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century. Particulate inorganic matter, as burning oil residues, silica from mining or asbestos, has been found in diseased tissues, and it is known to cause various pathologies, such as silicosis, asbestosis or inflammatory reactions [84]. Also, in the case of fine titanium dioxide powder (micrometric and submicronic, thought not to be easily absorbed), detectable amounts were found settled in the blood, glands and some organs, with the highest concentrations being in the lymph nodes and lungs. In addition, large NP agglomerates in the body may be difficult to eliminate causing frustrated phagocytosis and chronic inflammation, leading to diseases related to granulomatosis and cancer. Thus, summarizing even if NPs are not very toxic by themselves, they may be risky because they could be a source of toxic aggregates.

3.2

The Core of the NPs and Its Minute Size

Corrosion or dissolution processes have been widely studied for macro-sized materials of interest in the metallurgic industry, such as the well-known case of iron, the corrosion (rust) of which is associated with degradation of ironbased tools and structures like bridges. Even more, these processes are of evident environmental impact and it has been studied for long the harms derived from the introduction of metal ions in a variety of ecosystems. These processes in inorganic NPs made of Au or Ag or other noble metals are often neglected since they are considered non-degradable. However, it takes place and there is an increase of reports establishing that cations released when NPs corrode are responsible for detrimental effects [10,85–87]. Moreover, the degradation of inorganic “non-degradable” matter is supposed to be magnified at the nanoscale. Due to their reduced size, NPs have a high curvature and surface-to-mass ratio and the corresponding low coordination atoms at the surface which could enhance dissolution. However, there are other many factors to take into account such as the metal solubility within a given environment, NP stability and aggregation states, functionalization of NPs with protective shells or coatings as SAMs or solvent properties such as pH, ionic strength and/or presence of adsorbing species. Thus, corrosion processes known to affect the toxicity of micro- or macroparticles are not straightforwardly applicable to NPs, which convert this research as a huge enterprise to be developed in the near future. Since, despite the importance it has, it still lacks a detailed study of the corrosion process for the different inorganic NPs. This NP disintegration is of special relevance in the case of NPs dispersed in the environment, where NPs are likely to be highly diluted. Under these conditions, a released ion does not return to the NP because the Brownian motions of both species make the encounter highly improbable. And this would drive NPs towards higher or even complete dissolution in such environments over the due time. Thus, NPs may possess environmental risk because they may dissolve becoming a source/reservoir of toxic cations (and/or toxic aggregates, as

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discussed in section 3.1). To name a few examples, Derfus et al. [88] showed that the intracellular oxidation and toxicity of CdSe QDs were due to the release of Cd ions. Cadmium binds to sulphhydryl groups of critical mitochondrial proteins, leading to cell death. Physiological levels of metallothionein, a protein found in the cytoplasm of hepatocytes which detoxifies cadmium by sequestering it into an inert complex, were not sufficient to cells exposed to the high levels of Cd2 þ ions released from the QDs. Similarly, toxicity studies performed on freshwater alga Pseudokirchneriella subcapitata revealed comparable toxicity of ZnO (30 nm) and dissolved ZnCl2 salts [86], thus presuming that effects must be attributed to Zn2 þ ions. Also, it was found that the inhibition of wastewater nitrogen and phosphorus removal induced by higher concentrations of ZnONPs was due to the release of zinc ions from ZnONPs. It seemed that these Zn2 þ ions caused an increase of the ROS production and an inhibitory effect of the responsible microorganisms of that removal [10]. Metabolization of magnetite/maghemite iron oxide NPs has also been described in the rat liver [9]. Also, it has been reported that CNTs can be biodegraded through enzymatic catalysis [89]. Additionally, the effects of silver ions released from AgNPs on bacterial mortality have been reported in many papers [90–94]. And finally, “the most noble of the noble metals”, gold, is recognized as inert and not biodegradable, and that is why it is used in medicine (stents) or dental restoration. However, gold dissolves in biological environments. Larsen et al. [95,96] showed that the extracellular liberation of gold ions from the surface of metallic gold implants reduced microgliosis and neuronal apoptosis and increased neural stem cell response after a focal brain injury. Interestingly, this last case shows that in the same way as the NP corrosion phenomenon could result in biological or environmental damage, this process could be harnessed for different applications, as the delivery of specifically desired compounds to specific targets.

3.3 Environmental Toxicity of Inorganic NPs: A Huge Enterprise In this chapter, we aimed to show key features of inorganic NPs which make them both appealing for environmental remediation and potentially hazardous. The special characteristics of matter at the nanoscale make NPs active either by themselves or as a consequence of the processes of their evolution in the environment (aggregation or disintegration). Thus, toxicological studies involving inorganic NPs may critically report the data on the characterization of the NPs used in toxicity tests. Size, methodology of synthesis, specific surface, solvent and formulation used are factors that definitely contribute to the potential NPs’ toxic effects [97,98]. To date, these studies are carried out in a few species that have been accepted by regulatory agencies as models to define ecotoxicological effects as Daphnia magna or Vibrio fischeri [8]. In the review of Farre´ et al. [99], the main data observed when exposing D. magna to several types of NPs

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are reported. For instance, Zhu et al. [100] reported that ZnO, TiO2 and C60 fullerenes appeared to be very toxic to D. magna, whereas Fe3O4NPs exhibited a low effect. Other studies showed toxicity of alumina (Al2O3) NPs in crops since these NPs perturbed the microbial substrate around the roots [101], reducing root growth. This raised many concerns since most of the food chains depend on the benthic and soil flora and fauna. However, in this same study, the authors observed that Al2O3NPs did not induce detectable effects on seed–root growth using root-elongation tests on C. sativus and, in another study, Zn and ZnONPs significantly inhibited seed germination and root growth likely through Zn2 þ poisoning [102]. Those results are only reported here as an illustration. Many other results are listed in different reviews [8,99]. Clearly, it would be necessary to have a database combining all organisms and NPs and the toxicity effects for each species, but the amount of work necessary could make the costs almost impossible to face. For instance, Choi et al. [103] commented that, despite the importance to gather information about the toxicity of NPs, in order to have proper regulation, it has been estimated for the United States that the cost for testing existing NPs would be about $249 million for optimistic assumptions about NP hazards. Also, if all existing nanomaterials are to be thoroughly tested, the time taken to complete it would be very high (34–53 years). Of course, maturation of the field, understanding of the mechanisms involved and common sense will significantly shorten the time and reduce the cost and the effort to regulate nanomaterials properly (Figure 4).

CeO2

Fe3O4

Ag

Au

TiO2

FIGURE 4 Transmission electron microscopy (TEM) images of model NPs produced in our laboratory and used for the environmental remediation procedures mentioned in the text. In these images, scale bars are 50 nm; CeO2NPs are 12 nm mean diameter, Fe3O4NPs are 7 nm, TiO2 are 6 nm, AuNPs are 12 nm and AgNPs are 25 nm.

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