Cation Exchange Chemistry of Hexadecyltrimethylammonium in a Subsoil Containing Vermiculite

Substituting native inorganic exchangeable cations of soil clays with cationic surfactants like hexadecyltrimethylammonium (HDTMA) greatly increases the sorption of organic contaminants, and hence reduces their transport potential. Both static and kinetic studies were conducted to probe the inorganic‐HDTMA exchange processes in a subsoil containing vermiculite and to determine the chemical stability of the resultant HDTMA‐soil complexes. The HDTMA generally had much higher affinity for the clay surface than inorganic cations native to soil and thus HDTMA saturation of soil resulted in a low residual HDTMA concentration in solution (≈ 10 −6 to 10 −5 M ). Conditions leading to flocculation of soil clays (e.g., high ionic strength or divalent exchangeable cations) resulted in inorganic cation entrapment in the interlayers of clays and decreased cation selectivity coefficients of inorganic‐HDTMA exchange. Under these conditions, a portion of HDTMA was adsorbed via nonpolar interactions (hydrophobic bonding) resulting in a higher residual concentration of HDTMA in solution (≈ 10 −4 M ) and a greater tendency for HDTMA desorption. The change in electrophoretic mobility of soil clays and turbidity of soil suspensions as HDTMA loading increased suggested that HDTMA, if adsorbed via cation exchange mechanism, caused clay aggregation and tended to distribute on the internal sites of the aggregates, leaving inorganic cations on external surfaces. The kinetic study revealed that inorganic‐HDTMA exchange was fast on external surfaces but slow on internal sites of Al‐hydroxy‐interlayered vermiculites or flocculated vermiculites where the inorganic cation entrapment had been found. We concluded that the most stable HDTMA‐soil complexes form at HDTMA loading of 0.6 to 0.7 cation‐exchange capacity for nonsodic soils.