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Proton Surface‐Charge Density in Soils with Structural and pH‐Dependent Charge
Author(s) -
Anderson Sharon J.,
Sposito Garrison
Publication year - 1992
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1992.03615995005600050017x
Subject(s) - surface charge , adsorption , chemistry , titration , ionic strength , soil water , charge density , vermiculite , proton , analytical chemistry (journal) , point of zero charge , inorganic chemistry , materials science , aqueous solution , environmental chemistry , geology , soil science , physics , quantum mechanics , composite material
Nearly all soils contain both variable‐ and permanent‐charge surface sites. The objective of this research was to quantify the variable‐charge properties of three soils that contain both permanent structural charge (primarily on external siloxane surfaces of mica and collapsed vermiculite) and variable (pH‐dependent) charge. When the three soils were suspended in 0.02 or 0.05 mol kg −1 NaCl solutions at eight equilibrium pH values between 4 and 8, the net ion adsorption was approximately two times greater at pH 8 than at pH 4 and decreased with increasing ionic strength. The point of zero net proton charge (PZNPC), which was calculated from proton titration data, net NaCl adsorption data, and previously measured values of the structural charge density, ranged from 4 to 5. When the soils were suspended in 0.05 mol kg −1 CsCl solutions at equilibrium pH values between 4 and 9, the PZNPC was at least 1 pH unit lower than for NaCl suspensions. Cesium adsorption was greater than Na adsorption at low pH and was less pH dependent; the net proton surface‐charge density also was less pH dependent in CsCl suspensions. The three soils contain <10 g kg −1 organic C, yet the proton‐titration, ion‐adsorption, and PZNPC data suggest that organic matter is extremely imortant in the surface chemistry of the three soils.