Sorption kinetics of organic contaminants by sandy aquifer and its kerogen isolate

Long-term sorption behaviors of phenanthrene (Phen) on the Borden sand from 1 min to 365 days and of Phen and 1,2-dichlorobenzene (DCB) on the isolated kerogen from 1 to 120 days were characterized by examining the time dependence of solute phase distribution relationships (PDRs) and compared with the prior reported sorption of tetrachlorobenzene (TeCB) and tetrachloroethene (PCE) on the pulverized and/or acid-treated bulk sand and size fractions. The sorption kinetics for Phen on the bulk sand and its kerogen isolate can well be described by the fractional power kinetics equation (q(e) = kt(b)). The similar rate parameter b for q(e)(t) vs t at 5-7 levels of initial concentrations of Phen on the two sorbents, respectively, ranging from 0.077 to 0.099 and from 0.072 to 0.086, indicates the similar sorption kinetics rate. The modified-Freundlich parameters of TeCB on the pulverized or acid-treated 0.3-mm size fraction match those of Phen on the isolated kerogen, suggesting the same natural organic matter (NOM) property of the two sorbents. As the prior investigation underestimated the Koc value for the TeCB sorption on the acid-treated 0.3-mm size fraction by a factor of 1.76, the estimated time to reach 95% of sorption equilibrium is much longer than the prior estimation (over 10 years vs about 2.5 years). The estimated times to reach 95% of sorption equilibrium at three levels of relative solubility for Phen on the bulk sand and its isolated kerogen are, respectively, longer than one decade, demonstrating the similar diffusion length for Phen on the two sorbents. The observed slow sorption kinetics is related to nanometer-pore diffusion within kerogen matrix. The investigation supplies new clues for explaining the often observed much longer persistence of organic contaminants in soils and sediments than the prediction based on the short-term laboratory experiment.