The role of Na-montmorillonite in the evolution of copper, nickel, and vanadyl geoporphyrins during diagenesis

Sorption of a series of toluene-solvated porphyrins with differing structural characteristics and metallic centers [i.e. free-base, Cu(II), Ni(II) and VO(II)] onto Na-montmorillonite was measured to determine how this clay mineral might influence porphyrin pathways during diagenesis. In most instances, adsorption was adequately described by the Langmuir isotherm equation. The adsorption results were compared in two ways: (1) by the metallating ion, where predicted trends of the relative degree of adsorption are based on Buchler stability indices, and (2) by the structural type, where predicted trends of the relative degree of adsorption are based on the number and type of substituent groups and their corresponding steric and functional characteristics. The implications of macrocycle shape (planar, ruffle, and wave) are also discussed. In general, sorption affinity based on metallating ion followed the order: VO(II)≈Ni(II)