Nanotechnology and the Environment - Chapter 6

6 Environmental Fate and Transport Chris E. Mackay and Kim M. Henry AMEC Earth & Environmental CONTENTS 6.1 Introduction .................................................................................................124 6.2 Nature of Nanomaterials in the Environment .............................................125 6.2.1 Physical Manifestation of Nanomaterials: Particle Size Distribution and Formation of Mobile Suspensions ........................125 6.2.2 Chemical Forces Acting on Nanomaterials .....................................128 6.2.2.1 Electrostatic or Coulomb Force..........................................130 6.2.2.2 van der Waals Forces..........................................................131 6.2.2.3 Solvency Force ...................................................................132 6.2.3 Implications of Polymorphism .........................................................132 6.3 Predicting the Behavior of Nanomaterials in the Environment ..................133 6.3.1 Predicting Temporal Reaction Rates: Chain Interactions................134 6.3.2 Predicting Temporal Reaction Rates: Estimating Particle Affinities ..........................................................................................139 6.3.3 Nanoparticle Affinity and Inter-Particle Force Fields .....................140 6.3.3.1 Coulomb Energy.................................................................140 6.3.3.2 van der Waals Energy ......................................................... 141 6.3.4 Prediction of Probability of Product Formation ..............................143 6.3.5 Summary ..........................................................................................144 6.4 Research Results .........................................................................................145 6.4.1 Surface Water and Sediment ............................................................146 6.4.2 Groundwater ....................................................................................148 6.5 Conclusions .................................................................................................150 6.6 List of Symbols ...........................................................................................151 References ..............................................................................................................152 The movement and transformation of materials within an environmental setting is a very important consideration when evaluating the risks associated with their release. The greater a material’s stability, in terms of low chemical reactivity and ready sus-pension in f luid environmental media, the greater its potential for distribution and therefore the wider the potential scope of exposure (area, number of receptors, types of habitats, etc.). 123 © 2009 by Taylor & Francis Group, LLC 124 Nanotechnology and the Environment 6.1 INTRODUCTION The environmental fate and transport of a given chemical can usually be charac-terized or predicted based on a relatively small set of characteristics. These typi-cally include phase properties (boiling point, melting point, vapor pressure); aff inity properties (air/water, water/soil, etc.); media reactivity (hydrolysis, oxidoreduction, photoreactivity); and biological degradation rates. Most models of environmental fate and transport use a combination of some or all of these properties to predict concentrations within various environmental media. The potential for environmen-tal risk can then be determined from these predicted concentrations based on the toxicity of the materials. This chapter examines the fate and transport of free nanomaterials in the envi-ronment. In some cases, nanomaterials may be considered in a manner identical to smaller molecular materials. Other cases require special methods to account for differences in the physical and chemical properties of nanomaterials as well as their peculiar phase properties. (See Chapter 2 for a discussion of the critical properties of nanomaterials.) Figure 6.1 illustrates the primary forces that determine the fate and transport of nanoparticles in suspension. Upon an initial release of disperse nanoparticles, buoy-ancy suspends the nanoparticles in the fluid. Van der Waals forces, relatively weak forces resulting from transient shifts in electron density, cause the nanoparticles to FIGURE 6.1 Conceptual model of primary forces determining fate and transport of nanoparticles in solution. © 2009 by Taylor & Francis Group, LLC Environmental Fate and Transport 125 be attracted to one another and to other environmental constituents. (The term “phy-sisorption” refers to adsorption as a result of van der Waals forces.) Nanoparticles will tend to agglomerate unless this physisorption is inhibited. As the size of the agglomerates increases, buoyancy is reduced and the force of gravity causes the particles to settle out of suspension. If the nanoparticles have similar electrostatic surface charges, however, the repulsive force will counter the attraction resulting from van der Waals forces and keep particles in suspension. Nanoparticles also can adsorb to natural organic matter. That may either increase the particles’ buoyancy or disrupt subsequent agglomeration, thereby allowing the nanoparticles to remain suspended. Other environmental interactions such as dissolution or biodegradation also can reduce the concentration of nanoparticles in suspension. As a result of the various forces acting on nanoparticles, which become even more complex than this simple conceptual model when considering transport through soil, the concentration of nanoparticles in solution does not remain at equilibrium but changes over time and over distance from the discharge point. Sections6.2and6.3describetheforcesthataffectthefateandtransportofnanopar-ticles.(Section6.6liststhesymbolsusedinmathematicalequationsinthosesections.) As with any model, the mathematics can approximate only real-world complexities. The nanoparticles’ characteristics such as a shape or variance in composition will affect the material’s chemical properties. Further, the environmental characteristics of the suspending medium such as the pH, hardness, mineral content, ionic strength, types and amounts of dissolved organic matter, and especially the characteristics of sediment/soil will affect the environmental fate and transport of nanomaterials. Sec-tion 6.4 summarizes research f indings regarding the fate and transport of the target nanomaterials, which account for the effects of some of those characteristics. 6.2 NATURE OF NANOMATERIALS IN THE ENVIRONMENT Special considerations unique to predicting the fate and transport of nanomaterials can be divided into two general groups: (1) those related to the physical manifesta-tion of the materials, and (2) those related to special chemical properties that affect their reactivity and interactions with their surroundings. Each is discussed below. 6.2.1 PHYSICAL MANIFESTATION OF NANOMATERIALS: PARTICLE SIZE DISTRIBUTION AND FORMATION OF MOBILE SUSPENSIONS Nanoparticles can form suspensions in air or water, and can be transported through the environmentinsuchsuspensions.Thesuspensionofnanoparticlesisnotanequilibrium phenomenon, but depends in part on the particle size and changes in particle size that result from collisions and reactions in the environment, as discussed below. Other fac-tors that affect the suspension of nanoparticles are discussed in subsequent sections. With few exceptions, preparations of nanomaterials are not of uniform particle size. Rather, nanopreparations consist of a distribution of varying particle sizes. When a nanomaterial is released into a fluid environment, such as air or water, the sizedistributionwillbeginimmediatelytochangeastheresultofdifferentialsettling © 2009 by Taylor & Francis Group, LLC 126 Nanotechnology and the Environment based on the particle size. This results from the vector settling force (Fr), which is a function of buoyancy and gravity (g). F ∀WxVxg Gravity F ∀(WfVxg) Buoyancy (6.1) ΑF ∀VxgWx Wf Settling Force When expressed as force vectors, it becomes clear that the smaller the nanoparticle’s volume (V ), the lower the force vector, regardless of the difference in either particu-late (Wx) or f luid W(f) densities. The extremely small particle size of nanomaterials results in a very low settling force due to the small magnitude of Vx. In short, over time, the concentration of suspended nanoparticles will decline as the larger par-ticles settle out of suspension while the smaller particles remain in suspension. The rate at which particles settle out of suspension determines the potential for transport through the environment and the ease of removal through air or water treat-ment processes. The settling or terminal velocity (v ) is a function of the settling force and the f luid’s resistance to passage or viscosity (Μ) as follows: vx ∀ 2 š r2g šWx Wf (6.2) where r is the effective particle radius. Table 6.1 provides examples of the effect of particle radius on the settling rate of titanium dioxide in air and water. These examples show that as the particle size decreases, the rate of settling decreases sub-stantially and thus the particles can stay in suspension more readily. At particle sizes below 100 nm, the settling velocity has a magnitude akin to rates of Brownian motion, which is the random movement of small particles sus-pended in a fluid resulting from the thermal velocity of the particles in the suspend-ing medium. As a result, the particles can form a stable suspension. Such systems, referred to as sols, can occur in fluids such as water (hydrosol) or gases such as atmospheric air (aerosol). Suspensions of nanoparticles may not be true solutions. This is because the sus-pension is not the result of an equilibrium condition, but rather the result of very TABLE 6.1 Sedimentation Rate for TiO2 Spheres ofVarying Size in Water and Air (cm/hr) Particle Diameter 1 mm 1 μm 100 nm 10 nm Settling Rate in Water (vx) 7 × 102 7 × 10−4 7 × 10−6 7 × 10−8 Settling Rate in Air (vx) 3 × 104 3 × 10−2 3 × 10−4 3 × 10−6 Note: Pressure = 1 atm; Temperature = 25°C. © 2009 by Taylor & Francis Group, LLC Environmental Fate and Transport 127 slow settling kinetics. As a result, nanoparticles can be said to possess an apparent solubility (kas) that can be described in a manner similar to that for a solution as follows: [X]f as [X]s (6.3) where [X]f represents the concentration of nanoparticle X in sol and [X]s represents the concentration in the solid, non-sol form. If it is assumed that the material is ini-tially introduced into the f luid medium in the nanoparticulate form, the settling rates are within a range of thermal kinetics, and hence absolute temperature (T) becomes a factor in determining the equilibrium concentration of the particles in the sol. An expression for kas can be derived using the Boltzmann equation as follows: lnkas ∀ ln[X]f ∀ µVxg(kTWf )šhdh (6.4) where k is the Boltzmann constant, T is absolute temperature, and h is the linear measure of particle separation. At saturation, the amount in non-suspension (i.e., [X]s) will have no real effect on the amount in suspension, Hence the equilibrium equation can be expressed solely based on the aqueous concentration of the nanopar-ticle as follows: lnkas ∀ µVxg(WX Wf )šhdh (6.5) The integration of the Boltzmann equation allows a f irst approximation of the total suspended nanoparticulate concentration at equilibrium as follows: lnkas ∀ x∀0.01m Vxg(Wx Waq)šhdh x∀0m Vxg(WX Waq) 2 2kT Therefore: [X]aq ∀ eVxg(WxTWaq)š0.012 (6.6) This derivation shows that the particulate concentration and temporal stability of heterogeneous sols depend on the size of the particles. If the nanoparticles’ size is stable, then the suspension will be stable (excluding disruption by outside forces). Thus, nanoparticles can form metastable suspensions. However, if the particles agglomerate with like particles or other constituents in air or water, then the suspen-sion will not be stable. This phenomenon is discussed further in Section 6.2.2. This method provides a means to predict the concentration of nanomaterials in a hydrosol or aerosol based on the physical properties of the materials and the interplay of particle size and density (Figure 6.2). For materials with a density less © 2009 by Taylor & Francis Group, LLC ... - tailieumienphi.vn