Introduction to Nanoscience and Nanotechnology. Chris Binns
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Название: Introduction to Nanoscience and Nanotechnology
Автор: Chris Binns
Издательство: John Wiley & Sons Limited
Жанр: Отраслевые издания
isbn: 9781119172253
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The most popular type of magnetic nanoparticle is so‐called nano zero‐valent iron (nZVI), which can be produced chemically on a large scale with a relatively simple reaction [25]. The zero‐valent label is used to denote unoxidized or pure metallic iron but the metal is so reactive that the formation of an oxide shell during synthesis is unavoidable. This, however, is beneficial to some processes and it protects the core from rapid oxidation, though the oxide will, over time, eat into the core. Under certain applications, the magnetic nanoparticles can be separated from the water using an applied magnetic field.
The nZVI removes contaminants by forming compounds with them and bonding them to the surface of the nanoparticles, which is greatly facilitated by the large surface area presented. This is typically 25 m2/g for commercially available nZVI powders but can reach 100 m2/g (see Problem 1, Chapter 1). The freshly synthesized nZVI particles have a thin porous oxide shell that allows contaminants to get very close to the iron/oxygen interface where the reactions take place. As they work, the nanoparticles get coated by a shell of the reaction products and eventually become inactive. The first demonstration of the method was in 1997 where it was used to remove chlorinated organic toxins [26] but since then it has been shown to clear a range of contaminants from groundwater including antibiotics, dyes, solvents, pesticides, metals, and radioactive isotopes. For a comprehensive list of contaminants removed see [27].
A common method of application is illustrated in Figure 2.16 for the case of mobile groundwater requiring a static filter. After assessing the geological conditions at the site to decide on the density of nanoparticles required, boreholes are drilled to the required depth and the nVZI powder mixed with water is introduced as a slurry under pressure. This permeates the aquifer (the layer of water‐bearing permeable rock) and forms a static filter through which the groundwater flows. In some applications, it may be more appropriate to inject mobile nanoparticles into the water flow where they are carried to the source of pollution.
The discussion has focused entirely on the chemical properties of iron without considering its magnetism, which, in principle, provides a simple mechanism to separate the contaminant‐holding particles from the water. In the presence of a magnetic field gradient, dB/dz, a magnetic particle with a magnetic moment, μ (see Chapter 1, Section 1.2), experiences a force, F, in the direction of the field gradient given by:
Figure 2.16 Application of nZVI particles to groundwater remediation. In the case of mobile groundwater.
Figure 2.17 Magnetic separation of Pb contamination from water. (a) Suspension of grapheme‐coated nVZI (G‐nVZI) nanoparticles at a density of 1 mg/mi. (b) Separation of the G‐nVZI with a bar magnet.
(2.6)
So for nZVI particles in water, an applied magnetic field gradient will move all the particles through the water in the same direction to a point where they can be collected. On the laboratory scale, this is easy to demonstrate with a simple bar magnet as shown for the case of the removal of lead (Pb) contamination from water [28]. In this experiment, the nZVI nanoparticles were coated with a shell of graphene (see Chapter 4) to increase their effectiveness at adsorbing the Pb ions and the magnetic separation is demonstrated in Figure 2.17. The left image shows a suspension of the grapheme‐coated nZVI particles (labeled G‐nVZI) at a density of 1mg/ml and the right image demonstrates their separation with a bar magnet. Graphene alone is sometimes used as an adsorbent to filter Pb from water and it was found that the G‐nVZI particles are twice as effective in removing the contaminant as it is able to remove the Pb in both the neutral and ionic states. The ability to then separate the magnetic nanoparticles is a bonus.
Magnetic separation of contaminants has only been demonstrated in the laboratory and is much more difficult in field applications. One issue is that it is difficult to apply sufficiently high field gradients over the distance scales required to produce magnetic separation on a workable timescale. In addition, if the particles are carried in a flow, the viscous drag will overcome the magnetic field gradients that can be applied in practice in even slow flows (see Chapter 8, Advanced Reading Box 8.1).
2.7.2 Conversion of Waste Plastics to High‐Grade Materials (Upcycling)
Plastic is a polymer of small hydrocarbons like the CH4 (ethylene) molecule shown in Figure 2.18a. The molecule has two spare bonds ready to combine with the spare bonds from other CH4 molecules and heating ethylene gas at high pressure causes the molecules to form long chains (polymerize) of polyethylene (or polythene) as illustrated in Figure 2.18c. Modern plastics contain thousands to millions of molecules in the polymer chain. Polyethylene was first discovered accidentally in 1898 but was not produced commercially until 1939, though this was interrupted by the second world war when the process was kept secret. In the 1950s the introduction of catalysts enabled production at much lower temperatures and pressures producing a significant decrease in cost and the modern plastics industry emerged. Catalysts were briefly introduced in Chapter 1, Section 1.4 and their role is to provide a surface whose chemical properties reduce the energy required for a specific reaction between two other species to occur. Since it is only surface atoms that contribute, all catalysts are in the form of nanoparticles in order to maximize the surface area presented per gram of material.
Figure 2.18 Polymerization of ethylene to produce polyethylene. (a) Ethylene CH4 molecule consisting of two carbon and four hydrogen atoms. (b) Bonding in CH4 showing two spare bonds for linking to other CH4 molecules. (c) Heating the ethylene gas at high pressure in the presence of a catalyst produces polyethylene.
The properties of the solid polymer are highly controllable by changing the chain length, introducing other atoms and branching the polymer chains. This happens naturally to some extent and can be encouraged to form a low‐density light material (LDPE) or discouraged, СКАЧАТЬ