Bs. Xing et Jj. Pignatello, TIME-DEPENDENT ISOTHERM SHAPE OF ORGANIC-COMPOUNDS IN SOIL ORGANIC-MATTER - IMPLICATIONS FOR SORPTION MECHANISM, Environmental toxicology and chemistry, 15(8), 1996, pp. 1282-1288
Batch 1-, 30-, and 180-d sorption isotherms were constructed for 1,3-d
ichlorobenzene, 2,4-dichlorophenol, and metolachlor -ethyl-6-methylphe
nyl]-N-[2-methoxy-1-methylethyl] acetamide) in aqueous suspension of a
fine sandy loam soil (3% organic matter) and a peal soil (93% organic
matter) at sorptive concentrations ranging over three to five orders
of magnitude. The isotherms were fitted to the Freundlich model, S = K
CN, 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 sho
w that partitioning in soil organic matter (SOM) is appreciably less i
deal for the slow fraction. It is concluded that SOM has both partitio
n and adsorption domains analogous to the dual-mode sorption model of
glassy polymers. The adsorption component is more prominent for the sl
ow 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 sorpt
ion by polyvinylchloride, a glassy polymer that exhibits dual-mode sor
ption. That system gave nonlinear isotherms with an N that was invaria
nt with time, consistent with its nature as a homogeneous polymer havi
ng evenly distributed adsorption sites.