Time‐dependent isotherm shape of organic compounds in soil organic matter: Implications for sorption mechanism

Abstract

Abstract-Batch 1-, 30-, and 180-d sorption isotherms were constructed for 1,3-dichlorobenzene, 2,4-dichlorophenol, and metolachlor (2-chloro-N-[2-ethyl-6-methylphenyl]-N-[2-methoxy-1-methylethyl] acetamide) in aqueous suspension of a fine sandy loam soil (3% organic matter) and a peat soil (93% organic matter) at sorptive concentrations ranging over three to five orders of magnitude. The isotherms were fitted to the Freundlich model, S ϭ KC N , where S and C are the sorbed and solution-phase concentrations and K and N are constants. Both K and N were time-dependent. K increased by as much as 2.7-fold beyond the 1-d period. N was less than unity in all cases and decreased with increasing sorption. Also, the isotherms were operationally separated into a ''fast'' fraction (amount sorbed after 1 d) and a ''slow'' fraction (amount sorbed thereafter). N s was significantly smaller than N f in all systems tested. The results show that partitioning in soil organic matter (SOM) is appreciably less ideal for the slow fraction. It is concluded that SOM has both partition and adsorption domains analogous to the dual-mode sorption model of glassy polymers. The adsorption component is more prominent for the slow fraction, indicating that the adsorption sites are internal to the SOM matrix and unevenly distributed with respect to access by sorbing molecules. Sorption by these natural materials was compared with sorption by polyvinylchloride, a glassy polymer that exhibits dual-mode sorption. That system gave nonlinear isotherms with an N that was invariant with time, consistent with its nature as a homogeneous polymer having evenly distributed adsorption sites.

Studies on the sorption behaviors of phenanthrene on marine sediments

“…The rate of diffusion on the hole-filling domain is much slower than that on the partition domain. The rate of diffusion directly determined the equilibrium time of adsorption [31,32]. Different media can affect the adsorption on two domains of organic matter, which may result in the difference of adsorption equilibrium time between DW and NSW.…”

Heinrich‐Heine‐Universität Düsseldorf

“…Despite the large number of studies supporting "partition" model sorption through linear sorption of HOCs to HA (90,96,97), many recent studies give evidence for partition character by reporting nonlinear sorption isotherms (89,(98)(99)(100)(101)(102)(103)(104), solute-solute competition (88,95,98), and desorption hysteresis (105)(106)(107)(108)(109). These observations cannot be by attributing "dual-mode" sorption properties to SOM (110)(111)(112)(113)(114)(115).…”

Modeling desorption kinetics of a persistent organic pollutant from field aged sediment using a bi-disperse particle size distribution

“…All but one of these models requires at least two (mathematical) compartments to fit the experimental data. These compartments can then define a combination of an instantaneous compartment where no mass transfer limitation is assumed with a dynamic compartment where mass transfer is limited (Ball and Roberts 1991;Sabbah et al 2005), two dynamic compartments (Brusseau et al 1991;Weber et al 1992;Xing and Pignatello 1996;Cornelissen et al 1998;Ghosh et al 2000;Shor et al 2003;Gamst et al 2004) or a continuum of compartments with various parameters (Culver et al 1997;Werth et al 2000;Werth and Hansen 2002). Although sometimes excellent fits of experimental and modeled data were demonstrated in the different papers, the physical explanation of the desorption process and its limitations is only briefly elaborated and do not include the effect of particle-size distribution.…”