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

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 = KCN, 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). Ns was significantly smaller than Nf 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.