Effect of solute concentration on sorption of polyaromatic hydrocarbons insoil: Uptake rates

The effect of solute concentration on sorption kinetics may be a factor in determining bioavailability and transport of organic pollutants in soils an d sediments, but there is conflict in the literature over whether sorption is concentration-dependent. Sorption of phenanthrene and pyrene to seven so ils ranging in organic carbon (OC) content from 0.18 to 43.9% was studied. Careful analysis revealed that experimentally the normalized rate of approa ch to equilibrium for compounds exhibiting a concave-down (with respect to the solute concentration axis) nonlinear isotherm increases with concentrat ion. However, the effect is rather sma II and is most apparent when the fra ction of total solute finally taken up by the solid (F) is low. The explana tion is rooted in the nonlinearity of the isotherm and the finite-bath cond ition of the experiment and can be expressed in terms of two opposing effec ts. On the one hand, the apparent diffusivity of a (concave-down) nonlinear ly sorbing compound within particles increases with concentration because i ts affinity for the solid phase decreases with increasing concentration. On the other hand, rates in finite-bath reactors carried out at the same liqu id/solid ratio will suffer from a batch process temporal bias called the "s hrinking gradient" effect. It is an artifact of the methodology and is due to gradient driving forces that slow the sorption rate as F declines. In no nlinear cases F declines as concentration increases. The shrinking gradient effect vanishes as the liquid/solid ratio approaches infinity. Although th is effect is self-correcting when an appropriate nonlinear diffusion model is applied, consensus about such models has not yet been achieved. To provi de bounds for the shrinking gradient effect in finite-bath systems semiempi rically, two models that give lower and upper bounds of the characteristic sorption time tin the limit of infinite bath have been employed: (5) a wett ing front model, which assumes sorption is rate-limited by molecular migrat ion, and (b) a fast diffusion model, which assumes a mass-transfer resistan ce at the sorption site. The results are consistent with an intrinsic posit ive concentration dependence of sorption kinetics.