An isotope exchange technique to assess mechanisms of sorption hysteresis applied to naphthalene in kerogenous organic matter

The sorption of organic compounds to natural sorbents is often found to show hysteresis. The objective of this study was to develop an experimental technique based on the use of C-14 isotopes to distinguish hysteresis due to experimental artifacts from true hysteresis due to thermodynamically irreversible processes. The study was also designed to investigate causation of true hysteresis (irreversible sorption). The technique determines the rates and the degree of isotope exchange (IE) on equilibrated sorption and desorption points at different constant bulk chemical concentrations. The technique was applied to the sorption of naphthalene (NAPH) on Beulah-Zap lignite, a low rank reference coal composed mainly of kerogen. Sorption of bulk was found to be reversible below 10(-5) g L-1, but irreversible above 10(-4) g L-1. Complete isotope exchange on sorption and desorption points that defined an irreversible cycle demonstrated that hysteresis was true. A comparison of normalized uptake and release kinetics of labeled and bulk NAPH at different concentrations revealed slow structural deformation processes of the sorbent during bulk sorption and desorption. This is taken as corroborating evidence for the pore deformation hypothesis of hysteresis in which incoming sorbate molecules induce quasi-reversible changes in the organic matter that lead to different pathways for sorption and desorption. Although unable to rule it out completely, the data demonstrate that physical entrapment of sorbate molecules plays a minor, if any, role to the observed hysteresis in this system.