Goethite Dispersibility in Solutions of Variable Ionic Strength and Soluble Organic Matter Content
The degree of flocculation of aqueous suspensions of microcrystalline goethite was measured in salts of monovalent, divalent and trivalent cations at pH 6.0–6.5 over a range of ionic strengths using light scattering measurements at 650 nm. Varying concentrations of soluble humic material as well as the organic ligands, salicylate and citrate, were tested for their effect on flocculation. It was found that KCl and NaCl induced flocculation at lower ionic strength than CaCl2, while AlCl3 favored dispersion at all ionic strengths tested. The simple organic ligands promoted flocculation at low concentration, with citrate having a more pronounced effect than salicylate. At higher concentrations, these ligands reversed their effect, inducing a more dispersed state of the oxide. The organic ligand effect on dispersibility was modified by the particular metal cation present, with Ca2+ being more conducive to flocculation than K+. Soluble humic materials affected goethite flocculation in a qualitatively similar way to that of the simple organic ligands, that is low concentrations favored flocculation while high concentrations induced dispersion. This dispersing effect was partially suppressed by the presence of Ca2+, and completely suppressed by Al3+. Thus, soluble humic substances at relatively high concentrations appear to have a marked dispersing effect on goethite in the absence of polyvalent cations, and a strongly flocculating effect in their presence.
The results can be explained qualitatively by a simple oxide surface charge model, in which chemi-sorption of multivalent cations or organic ligands alters the surface charge. Reactions that increase the magnitude of positive or negative surface charge favor dispersion, while those that reduce the magnitude of charge favor flocculation.
Key WordsCitrate Dispersion Flocculation Goethite Humic Acid Light Scattering Salicylate Soluble Organic Matter
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- Fontes MPF, Alvarenga RC, Gjorup GB, Nascif PS. 1992a. Influence of calcium salts and mechanical stresses in the water dispersible clay (WDC) of Brazilian oxisols. Agron Abstr p. 370.Google Scholar
- Greenberg AE, Trussell RR, Clesceri LS. 1992. Standard methods for the examination of water and wastewater, including bottom sediment and sludges. American Public Health Association, American Water Works Association. New York: Water Pollution Central Federation, p. 16.Google Scholar
- Oades JM. 1989. An introduction to organic matter in mineral soils. In:Dixon JB, Weed SB, editors. Minerals In Soil Environments. 2nd ed. Madison, WI: Soil Sci Soc Am. p.89–159.Google Scholar
- O’Melia CR. 1987. Particle-particle interactions. In: Stumm W, editor. Aquatic Surface Chemistry. New York: John Wiley and Sons. p. 385–403Google Scholar
- Sletten RS, Benjamin MM. 1994. Mobilization of Fe- and Al-hydroxides by fulvic and humic acids. Agron Abstr p. 256.Google Scholar
- Steel RGD, Torrie JH. 1980. Principles and Procedures of Statistics. 2nd ed. New York: McGraw-Hill. 633p.Google Scholar
- Stumm W, Kummert R, Sigg L. 1980. A ligand exchange model for the adsorption of inorganic and organic ligands at hydrous oxide interfaces. Croatica Chem Acta 53:291–312.Google Scholar
- Stumm W. 1992. Chemistry of the Solid-Water Interface, Processes at the Mineral-Water and Particle-Water Interface in Natural Systems. New York: J. Wiley and Sons. 428p.Google Scholar
- van der Marel HW, Beutelspacher H. 1976. Atlas of Infrared Spectroscopy of Clay Minerals and their Admixtures. Amsterdam: Elsevier. 396p.Google Scholar
- van Olphen H. 1977. An Introduction to Clay Colloid Chemistry. 2nd edition. New York: John Wiley and Sons. 318p.Google Scholar
- Wilson MJ. 1987. A Handbook of Determinative Methods in Clay Mineralogy. New York: Chapman and Hall. 320p.Google Scholar