Interaction of organic compounds with carbonate mineral surfaces in seawater and related solutions

Abstract

The selective adsorption of the C[4] to C[14] fatty acid homologs, twelve amino acids, and a series of synthetic polyglutamic acids of variable monomer unit size with the surfaces of calcite, aragonite, dolomite, and magnesite were examined. Factors considered in this study, not commonly investigated previously, included: the influence of solution composition on sorption, the effect of solid to solution ratio, importance of carbonate mineral type, the effect of organic concentration on adsorption, the influence of inorganic and organic cosorption, and the reversibility of adsorption. The adsorption of the saturated n-fatty acid homologs as a function of alkyl chain length was investigated. Butyrate, octanoate, and laurate did not adsorb over the conditions examined. Myristate, palmitate, and stearate adsorbed onto calcite, aragonite, dolomite, and magnesite surfaces displaying an increasing affinity for carbonate surfaces with increasing alkyl chain length. This behavior is consistent with Traube's Rule. The influence of solution composition on fatty acid sorption at levels characteristic of those observed in natural systems favored sorption onto carbonate surfaces in distilled water over that in synthetic seawater and the effect increased with increasing alkyl chain length. Adsorption-desorption experiments determined that sorption was characterized by a large resistant component as indicated by nonsingular isotherms. A partition coefficient for the reversible and resistant component of sorption was determined using the model of Ditoro and Horzempa (1982). Myristate adsorption was enhanced in the presence of stearate cosorption and interpreted as resulting from hydrophobic bonding between the two sorbates. Orthophosphate displaced fatty acid from the surfaces of calcite and aragonite, presumably by site competition. Adsorption isotherms were interpreted as belonging to a variant of the S-shaped family of isotherms. A model for fatty acid sorption with carbonate surfaces was developed following the model of Wakamatsu and Fuerstenau (1968). No adsorption onto CaCO3 surfaces was observed for any of the twelve free amino acids, including the acidic amino acids, over a wide range of experimental conditions. Free glutamate did not adsorb but when polymerized in peptide linkages sorption did occur, presumably because of multiple surface complexes. These results are consistent with the model proposed by Mitterer and Cunningham (1985).