Advances in Agronomy

1. Auflage 2015
ISBN: 978-0-12-802346-4
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, Band Volume 130, 444 Seiten

Reihe: Advances in Agronomy

Advances in Agronomy continues to be recognized as a leading reference and a first-rate source for the latest research in agronomy. Each volume contains an eclectic group of reviews by leading scientists throughout the world. Five volumes are published yearly which ensures that authors' contributions are disseminated to the readership in a timely manner. As always, the subjects covered are varied and exemplary of the myriad of subject matter dealt with by this long-running serial. - Timely and state-of-the-art reviews - Distinguished, well recognized authors - A venerable and iconic review series - Timely publication of submitted reviews

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Leseproben

3. Formation
3.1. Adsorption Reactions of Organic Ligands
3.1.1. Definition of Terms Adsorption refers to the accumulation of solutes (adsorptives) present in the bulk solution at the mineral–water interface, where these molecular entities are termed “adsorbates” (Essington, 2004). Adsorption is said to be specific when short-range atomic interactions between the adsorbate and the interface become important. In contrast, the term nonspecific adsorption is used when an adsorbate interacts with interfaces via long-range electrostatic interactions while fully or partially retaining its hydration shell (Trasatti and Parsons, 1986). The term “specific adsorption” is frequently used interchangeably with “innersphere complexation.” Innersphere complexes are characterized by the formation of polar covalent bonds between structural surface atoms and the adsorbate (“chemisorption”). Bond energies of polar covalent bonds are high and in the order of several hundred kilojoules per mole (Luo, 2007). When an adsorbate is electrostatically held at the surface by long-range Coulomb forces while retaining at least one solvent molecule between itself and the surface, we refer to “outersphere complexation” (Essington, 2004). In the literature, this term is synonymous to “nonspecific adsorption” or “physisorption.” The energies of electrostatic bonds are much lower (<20 kJ mol-1) than those associated with polar covalent bonds, and the attractive electrostatic forces between oppositely charged ions decrease nonlinearly with increasing charge distance (1/r). Other binding modes that contribute to the stabilization of organic ligands at mineral surfaces include H-bond formation, van der Waals interactions, and hydrophobic interactions. In H-bonds, a hydrogen atom—polar covalently bound to a more electronegative atom within a molecular entity—interacts with an adjacent, partially negatively charged atom X (e.g., O or N), resulting in interatomic H-X distances of approximately 1.5–2.6 Å. The energies associated with this type of bond lie in the range of approximately 4–13 kJ mol-1 (Berg et al., 2002). Van der Waals interactions occur between all molecules and involve Coulomb forces between transient-induced or permanent electric dipoles (permanent dipole–permanent dipole, permanent dipole–induced dipole, and induced dipole–induced dipole (“London dispersion forces”) interactions). With approximately 2–4 kJ mol-1 per atom pair (Berg et al., 2002), these interactions are considerably weaker than H-bonds, and their strength decreases rapidly with increasing distance (1/r4–1/r6). The contribution of van der Waals interactions to the adsorption process depends largely on the contact surface area and polarizability of the interacting electron systems. Hydrophobic interactions (“hydrophobic effect”) denote the association of apolar, that is, hydrophobic molecules at the mineral–water interface. This binding mode does not refer to intermolecular forces (which are dispersion forces) but rather to the energy released as water molecules gain entropy upon their removal from the space between apolar adsorbates or between the adsorbates and a hydrophobic surface (Berg et al., 2002; Lodish et al., 2000). Adsorption energies associated with physical adsorption processes such as van der Waals and hydrophobic interactions are only a few multiples of RT (~2.48 kJ mol-1 at 25 °C), where R and T are the molar gas constant and absolute temperature, respectively. For example, a value of 8 kJ mol-1 has been used to separate physisorption from chemisorption in the 2-ketogluconate-goethite system (Journey et al., 2010). Since OM in the aqueous phase tends to be both polydisperse and multifunctional and minerals often exhibit surfaces with different charge properties (compare Chapter 2), several adsorption mechanisms may operate simultaneously (multimode adsorption). Although noncovalent bonds are only effective over short distances and associated with small interaction energies, the extensive formation of such bonds can result in a strong MOA (cooperativity). In addition, for noncovalent interaction mechanisms to operate effectively, the binding sites of a mineral surface and a given ligand must possess complementary properties (complementarity). 3.1.2. Adsorption to Metal Oxides and Phyllosilicates Information on the mode of organic ligand interactions with mineral surfaces can be gained from a number of methodologies, such as macroscopic batch adsorption experiments (Arnarson and Keil, 2000; Gu et al., 1994; Schlautman and Morgan, 1994; Tipping, 1981; Yuan et al., 2000), calorimetry (Gu et al., 1994; Benoit et al., 1993; Wershaw et al., 1995), spectroscopy (Ainsworth et al., 1998; Chernyshova et al., 2011; Del Nero et al., 2013; Hug and Bahnemann, 2006; Johnson et al., 2004a; Persson and Axe, 2005), as well as molecular and surface complexation modeling (Boily et al., 2000; Journey et al., 2010; Kubicki et al., 1997, 1999; Weng et al., 2006, 2007; Nair et al., 2006). The adsorption of simple low-molecular weight organic ligands such as aliphatic and aromatic carboxylates, amino acids, hydroxamate ligands, or phospholipids has been extensively studied for variable-charge metal oxide surfaces (Table 4). The rationale behind these studies is that natural OM features similar functional groups (mainly carboxyl and alcoholic/phenolic OH groups) and hence reacts with mineral surfaces in a similar fashion. Results obtained from Fourier transform infrared (FTIR) spectroscopy show that surface hydroxyls of metal oxides undergo rapid ligand-exchange reactions with carboxyl or hydroxamate groups of OM, resulting in the formation of polar covalent metal–O–C bonds and the release of OH- ions or water (Ainsworth et al., 1998; Boily et al., 2000; Borer and Hug, 2014; Borer et al., 2009; Chernyshova et al., 2011; Duckworth and Martin, 2001; Greiner et al., 2014; Guan et al., 2007, 2006; Norén et al., 2008; Persson and Axe, 2005; Yoon et al., 2004). Phosphoester groups of phospholipids found in EPS also participate in ligand-exchange reactions on metal oxide surfaces (Cagnasso et al., 2010; Omoike and Chorover, 2004; Omoike et al., 2004). The innersphere complexes formed may have different geometries depending on the ligand's structure and environmental conditions. Monodentate–mononuclear, bidentate–mononuclear, and monodentate–binuclear binding geometries are most commonly observed (Figure 11, Table 4). Unfortunately, the notation of surface complexes according to their denticity and nuclearity is not handled consistently in the literature. Ideally, the “n-dentate” designation refers to the number n of donor atoms of a ligand being attached to the same (structural) metal atom (IUPAC, 2005), whereas the term “m-nuclear” gives the total number of (structural) metal atoms m a ligand is attached to. Hence, a bidentate–mononuclear complex is formed when a structural metal atom is coordinated with two ligand donor atoms (Figure 11(B)). A further complication arises from the fact that different binding modes can be present in a single surface complex. For example, both inner- and outersphere complexes may be stabilized by H-bonds (Chernyshova et al., 2011; Norén et al., 2008). In addition to innersphere complexation, carboxylate ligands bind to metal oxide surfaces through outersphere complexation and H-bond formation (Cagnasso et al., 2010; Chernyshova et al., 2011; Das and Mahiuddin, 2007; Hwang et al., 2007; Johnson et al., 2004a; Norén and Persson, 2007). The discovery of these comparatively weak but quantitatively important interaction mechanisms by in situ FTIR spectroscopy may have major implications for the preservation of adsorbed OM in soils. Table 4 Selected spectroscopy-based studies on the interaction between organic ligands and mineral surfaces Low-molecular weight organic ligands Alumina (d-Al2O3) Salicylate BA, F S: 1.0 g L-1 (100.6 m2 g-1), [L]tot: 10-7–10-3 M, B: 0.001–1 M NaClO4, pH: 4.5–8.5, T: 25 °C Innersphere (bidentate and monodentate-mononuclear) and outersphere complexation Ainsworth et al. (1998) Alumina (a-Al2O3) Maleate ATR-FTIR, BA, BS, M S:...