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The Chemistry of Soils$
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Garrison Sposito

Print publication date: 2016

Print ISBN-13: 9780190630881

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780190630881.001.0001

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Soil Particle Surface Charge

Soil Particle Surface Charge

7 Soil Particle Surface Charge
The Chemistry of Soils

Garrison Sposito

Oxford University Press

Structural charge arises on the surfaces of soil mineral particles in which either cation vacancies or isomorphic substitutions of cations by cations of lower valence occur. The principal minerals bearing structural charge are therefore the micas (Section 2.2), the 2:1 clay minerals (Section 2.3), or the Mn(IV) oxide, birnessite (Section 2.4). These three classes of mineral are all layer type and the cleavage surface on which their structural charge is manifest is a plane of O ions. The plane of O ions on the cleavage surface of a layer-type aluminosilicate is called a siloxane surface.This plane is characterized by hexagonal symmetry in the configuration of its constituent O ions, as shown at the top of Fig. 2.3 and, more explicitly, on the right side of Fig. 2.4, where a portion of the siloxane surface of the micas is depicted. Reactive molecular units on the surfaces of soil particles are termed surface functional groups. The functional group associated with the siloxane surface is the roughly hexagonal (strictly speaking, ditrigonalbecause the hexagonal symmetry is distorted when the tetrahedral sheet is fused to an octahedral sheet to form a layer) cavity formed by six corner-sharing silica tetrahedra. This cavity has a diameter of about 0.26 nm. The reactivity of the siloxane cavity depends on the nature of the electronic charge distribution in the layer structure. If there are no nearby isomorphic cations substitutions to create a negative charge, the O ions bordering the siloxane cavity function as an electron cloud donor that can bind molecules weakly through the van der Waals interaction. These interactions are akin to those underlying the hydrophobic interaction, discussed in Section 3.5, because the O in the siloxane surface can form only very weak hydrogen bonds with water molecules. Therefore, uncharged patches on siloxane surfaces may be considered hydrophobic regions to a certain degree, with, accordingly, an attraction for hydrophobic organic molecules. However, if isomorphic substitution of Al3+ by either Fe2+ or Mg2+ occurs in the octahedral sheet, the resulting structural charge is manifest on the siloxane cavities, as discussed in Section 2.3.

Keywords:   Lewis acid site, PZC theorems, Schindler diagram, Stern layer, adsorbed charge, boron adsorption, calcareous soil, diffuse ion swarm, exchangeable ions, indifferent electrolyte, negative adsorption

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