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Repairing Nature
Published in Daniel T. Rogers, Environmental Compliance Handbook, 2023
Chemical dehalogenation is a remedial method that removes halogens from contaminants. As covered in Chapter 3, halogens are contaminants with halogen atoms within their atomic structure. Halogens include fluorine, chlorine, bromine, and iodine. Common halogenated contaminants include several DNAPL VOCs, PCBs, and dioxins. The process of chemical dehalogenation generally involves the excavation of contaminated soil. The soil is typically sifted and crushed to remove larger objects and provide for better remedial treatment. The sifted soil is then mixed with chemical agents and heated in a reactor. During this process, a chemical reaction occurs that removes the halogen atom from the molecular structure of the contaminant, thereby destroying the contaminant or rendering it less harmful depending on the completeness of the reaction. There are two common types of chemical dehalogenation: (1) glycolate dehalogenation and (2) base-catalyzed dehalogenation (USEPA 2001g).
Chemistry of Contaminants
Published in Daniel T. Rogers, Environmental Compliance Handbook, 2023
Dense nonaqueous phase liquids are liquids denser than water and do not mix or dissolve readily in water (USGS 2006a). DNAPL compounds include many common solvents and coal tar (Suthersan and Payne 2005). They are also commonly referred to as chlorinated solvents, halogenated VOCs, or chlorinated VOCs because chlorine is in the atomic structure, and the most common uses of these compounds are for cleaning and degreasing (USGS 2006a). Chlorinated VOCs have been in use for nearly 100 years and have been widely used by industry, and many household products contain them. Figure 2.10 shows some household products containing chlorinated solvents (USGS 2006a). Halogenated VOCs are a group of organic compounds with a halogen atom as part of their molecular structure. Halogens include the elements fluorine (F), chlorine (Cl), bromine (Br), or iodine (I). Part of the uniqueness of halogenated VOCs is they tend to have a very weak tendency to form hydrogen bonds with water. This lack of affinity for water means halogenated VOCs—especially those with fluorine or chlorine—tend to be hydrophobic and have low solubility (Suthersan and Payne 2005). Common halogenated VOCs include:
Metal-Organic Frameworks with Immobilized Nanoparticles for Hydrogen Generation
Published in Anish Khan, Mohammad Jawaid, Abdullah Mohammed Ahmed Asiri, Wei Ni, Mohammed Muzibur Rahman, Metal-Organic Framework Nanocomposites, 2020
Kabelo E. Ramohlola, Tshaamano C. Morudu, Thabiso C. Maponya, Gobeng R. Monama, Edwin Makhado, Phuti S. Ramaripa, Mpitloane J. Hato, Emmanuel I. Iwouha, Anish Khan, Kwena D. Modibane
Very recently, Li et al. [140] prepared 2D polyhalogenated Co (II) MOFs for HER. Halogens have the electron-withdrawing characteristic, and in this case they have the ability to enhance the affinity for the reactants to intermediate. Halogen-substituted linker consisted of a phthalic acid (X4-H2pta = tetrahalogenphthalic acid and X is F, Cl, Cl, and Br). The three synthesized MOFs, Co-Br4-MOF, Co-Cl4-MOF, and Co-F4-MOF with general representation [CoCX4-pta) (bpy) (H2O)2]n (bpy = 4.4-dipyridyl) were prepared via hydrothermal methods. The HER activities were investigated in 0.50 M H2SO4 and from the three Co-X4-MOFs, Co-Cl4-MOF displayed excellent HER electrocatalytic activity with overpotential of 24 mV at 10 mA.cm−2 and Tafel slope of 125 mV.dec−1. The high active Co-Cl4-MOF was then subjected to acetylene black (AB) and the corresponding HER results are shown in Figure 12.9a−d.
A mini-review on mechanochemical treatment of contaminated soil: From laboratory to large-scale
Published in Critical Reviews in Environmental Science and Technology, 2018
Giovanni Cagnetta, Jun Huang, Gang Yu
Differently from clay minerals and organic matter, oxides are the most active species in soil under the action of mechanical forces and are responsible for pollutant effective degradation. They can be classified in three groups according their activation mechanism: covalent oxides, low valent ionic oxides, and high valent ones. Silica (SiO2) is an example of covalent oxide. Its activation is due to accumulation of defects, amorphization, and crystal crushing (Kosobudskii et al., 2015). These transformations induce homolytic rupture of covalent bonds, so the newly formed surfaces are rich of unpaired electrons (Fig. 6a) (Delogu, 2011; Kaupp, 2009). Such electrons rapidly detach atoms (in particular, halogens) from organic compounds, starting molecular degradation through radical mechanism (Zhang, Wang, et al., 2014). Concerning low valent ionic oxides, like CaO, MgO, etc., their activation passes through another common effect of high energy ball milling: the accumulation of defects in the crystal lattice (Fig. 6b). In particular it has been proved that formation of oxygen vacancies generates trapped electrons on particle surfaces (Cagnetta, Huang, et al., 2017; Ikoma et al., 2001), according to the following equilibrium: where VO indicates an oxygen vacancy. In aerobic environment, aerial oxygen can re-oxidize the vacancy to regenerate the oxide anion. However, electrons of the vacancy can also be transferred to other compounds such as the organic pollutants. Halogens in organic pollutants stimulate electron capture, due to their electron-withdrawing effect, and generate a halides and organic radicals as products. Finally, high-valent oxides such as MnO2 differ from the low-valent ones because of their oxidant capability. Reasonably, the oxygen vacancy formation occurs during high energy milling of such oxides, but the massive presence of cations in high oxidation state prevents the accumulation of electrons on particle surface because they are captured by cations themselves. Compounds with high and low valent cations, such as magnetite (Fe3O4) can be a source of both unpaired electrons and oxidant centers (i.e. Fe3+) (Samara, Nasser, & Mingelgrin, 2016). The main effect of milling is the improvement of the contact between oxides and pollutants, facilitating the adsorption of mineral surfaces. Then, cations can oxidize the organic molecule; the reduced species may be regenerated through oxidation by aerial oxygen (Nasser, Sposito, & Cheney, 2000; M. D. Pizzigallo, Napola, Spagnuolo, & Ruggiero, 2004).