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Adsorption of Organic Herbicides by Montmorillonite: Role of pH and Chemical Character of Adsorbate
Author(s) -
Bailey G. W.,
White J. L.,
Rothberg T.
Publication year - 1968
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/sssaj1968.03615995003200020021x
Subject(s) - adsorption , freundlich equation , chemistry , langmuir , montmorillonite , inorganic chemistry , organic chemistry
Samples of well‐characterized 1‐0.2µ montmorillonite clay adjusted to two pH values, pH 3.35 and 6.80, were reacted with members of such herbicide families as s ‐triazines, substituted ureas, phenylcarbamates, aniline, anilides, phenylalkanoic acids, benzoic acids, and picolinic acids. Graphical methods were used to plot adsorption isotherms and to determine conformity to the Freundlich or Langmuir equations. Calculations were made to determine: (i) the percent cation exchange capacity satisfied at the maximum amount adsorbed; (ii) the percent surface coverage at the maximum amount adsorbed; and (iii) the partial molar‐free energy for each concentration level; also determined graphically was the Freundlich “ K ” and “1/ n ” value. Conformity to the Freundlich adsorption equation was found for all organic compounds studied for both the H‐clay and the Na‐clay systems with two possible exceptions: the phenoxyacetic acid‐H‐clay system and the substituted urea‐Na‐Clay systems. Conformity to the Langmuir adsorption equation was not found for any family or group studied on either H‐ or the Na‐saturated clay. Regardless of chemical character, adsorption occurred to the greatest extent on the highly acid H‐montmorillonite compared to the near neutral Na‐montmorillonite. Within a family or within an analog series basic in chemical character, the magnitude of adsorption is related to and governed by the degree of water solubility. The major factor governing the magnitude of adsorption of different basic chemical families is the dissociation constant of the adsorbate. The magnitude of adsorption of organic compounds with widely different chemical character is governed by the degree of water solubility, the dissociation constant of the adsorbate, and the pH of the clay system. The adsorption of basic organic compounds by the montmorillonite type clay system appears to be dependent upon surface acidity and not upon the pH of the suspension while the converse appears to hold true for the adsorption of acidic type compounds. The surface acidity of montmorillonite appears to be 3–4 pH units lower than the suspension pH. Mechanisms for the adsorption of basic and acidic organic compounds are discussed.