Environ. Sci. Technol. 1993, 27, 1563-1571
Elution of Aged and Freshly Added Herbicides from a Soil Joseph J. Plgnatello,' Francls J. Ferrandlno, and Lee 0. Huang
Connecticut Agricultural Experiment Station, P.O. Box
1 106,
New Haven, Connecticut
Elution of atrazine and metolachlor residues from a longcontaminated soil under saturated flow was compared to elution of freshly-injected compounds from the same soil. The mobility of the injected herbicide was far greater than the native. A two-compartment diffusion sorption model-having a fast compartment SI in rapid exchange with water and a slow compartment SZwith exchange by radial diffusion kinetics-gave good simultaneous fits to native and injected elution curves and predicted flow rate effects and postleaching soil herbicide profiles. An analogous model with first-order kinetics was less successful. The diffusion model parameters indicated that ( i ) at apparent equilibrium, the bulk (82-92%) of the sorbate was in S2; (ii)the short-term (24-h) batch partition coefficient greatly underestimates the apparent true value and instead reflects partitioning into SI;and (iii)the time scale for sorption is many months. The absence of particlesize effects on desorption rates suggests that the diffusive medium of S2 is microparticles or microstructures (I1 pm) distributed among all particle-size fractions. Introduction
Recent studies indicate that sorption/desorption of organic contaminants in soil and sediment can be ratelimiting to their transport,biodegradation, and biouptake (1, 2). Both polar and nonpolar compounds show nonequilibrium behaviors, such as hysteresis, prolonged uptake or release, and asymmetric breakthrough curves on elution from columns (1-16). Long-contaminated (aged) topsoil, subsoil, or aquatic sediments have been found to contain the bulk of contaminant in a slow desorbing and biorecalcitrant state (6, 17-27). Slow sorption is thought to have contributed to the temporal slowdown of injected halocarbon plumes in groundwater relative to nonsorbing tracers (28,291. These behaviors cast doubt on the assumption that sorption is rapid and reversible at the grain scale-i.e., the local equilibrium assumption. Slow sorption has far-reaching implications for fate modeling and risk assessment. In addition, it is recognized as a serious obstacle to soil vapor extraction (30),pump-and-treat remediation of aquifers (31), and bioremediation of soil (32). Although slow sorption seems to apply to many types of compounds and natural sorbents, there is yet no way to predict its magnitude or impact on contaminant fate, and the mechanism is incompletely understood. Total sorbate may be partitioned, albeit indistinctly, between fast and slow compartments relative to the time scale of a given fate process. Models are needed that can predict sorption rates as a function of contaminant and soil properties and environmental conditions. Paradoxically, conclusions about the causes of slow sorption have often been based on experiments carried out over a few hours-probably a small fraction of the total time scale for sorption. Also, less is known about sorption kinetics of polar than nonpolar compounds. Slow sorption has 0013-936X/93/0927-1563$04.00/0
0 1993 Amerlcan Chemical Society
06504
been attributed to adsorption bond energies (33),interstitial micropore diffusion retarded by (rapid) sorption to pore walls (7, 34-36), or diffusion through soil organic matter (SOM) ( I , 2 , 8 , 1 0 , 1 3 ) ,or a combination of these (5,37).Intraorganic matter diffusion is consistent with a popular model of sorption as an entropy-driven, hydrophobic partitioning into a solvent-like SOMphase (38). We recently studied the sorptive status of the herbicides atrazine (AT; 2-chloro-4-ethylamino-6-isopropylamino1,3,5-triazine) and metolachlor (MET; 2-chloro-N-[2ethyl-6-methylphenyll -N-[2-methoxy-l-methylethyllacetamide) in several contaminated topsoils collected 2-15 mon after herbicide application (17). The apparent soilwater partition coefficient (KpaPP)of the native AT and MET varied between 2.3 and 42 times greater than that from short-term 24-h sorption isotherms (Kp24)using freshly added compounds, suggesting that a large fraction of the native forms was in a "slow" state at the time of sample collection. Furthermore, the ratio KpaPP/KpZ4 increased with time in the environment. This paper describes the column leaching of native AT and MET from one of the contaminated field samples. The sample was collected 7 mon after the last herbicide application and contained 591 pglkg MET and 212 pg/kg AT, corresponding to about half of the values expected in the topsoil from a single application. The Kpapp/KPz4 ratio was 13 (AT)and 14 (MET),indicating that the bulk of the residues (87 and 92 5% , respectively) were nonlabile on a 24-h time scale. Column studies generally test freshly added compounds. This is apparently the first published detailed study on the elution of aged contaminants. Our objective was to help elucidate their transport behavior and to provide a needed link between laboratory and actual field behavior. The specific objectives were to determine the time scales for elution of native vs freshly injected AT and MET, to evaluate sorption kinetic models, and to address the mechanism to the extent possible. Experimental Section
Soil. The soil was a Merrimac fine sandy loam (Typic Dystrochrept) from a corn plot at the University of Connecticut Experimental Farm in Coventry, CT, and was from the same lot used in the batch studies (previously called Soil CVa) (17). To obtain material suitable for the column study, it was air dried and passed through a 500pm sieve. The resulting material contained 2.6% organic carbon (OC). Analytical Methods. Herbicides in water samples were extracted with CHzClz and quantified by gas chromatography/mass spectrometry (GC/MS) using selectedion monitoring (39, 40). Herbicides in the soil were extracted by a batch hot-solvent method using methanolwater (8020) at 75 'C, which had been optimized for the extraction of aged MET and A T (40). Following extraction, analytes were phase-transferred into CHzC12 prior to GC/MS analysis. Envlron. Scl. Technol., Vol. 27, No. 8, 1993
1689
5
Table I. Column Conditions
soil dry weight (9) column length, L (cm) column cross-sectional area, (cm2) porosity, 0 (mL cm-3) bulk density, 0 (g cm-3) eluent, M normal volumetric flow rate, q (mL min-1) normal flow velocity, u (cm h-1) Peclet number, uL/D 300
-
250
-
200
-
150
-
100
-
50
-
column 1
column 2
511 24.5 19.6 0.65 1.072 0.01 CaClz 0.2 1.0 34-35
498 24.5 19.6 0.60 1.046 0.01 CaClz 0.2 1.0 34-35
4
3
ot 0
-3
I
h
60 50
1
/
FIOH
10
20
i"
30
40 Pore Volumes
naused-
50
I
****** 5
10
I\
MET
15
- 4
1
- 3
20
- 2
Pore Volumes
- 1
1
350, 300
200 250
I
0
0 0
10
20
30
40
50
60
, ' 0 70
Pore Volumes
t
50
70
- 5
** 0
I
H
60
%
0
\
***
Figure 2. Elution of native herbicides and tritium tracer from column 1. Note change in ordinate scale at 30 pore voiumes. Solid curves are predicted based on simultaneous soiution of injected and native curves using the DIF model.
aa
,. -
* * ,
5
10
15
I .****
J
20
Pore Volumes Figure 1. Elution of injectedherbicidesand tritium tracer from column 2.
Overview of Column Experiments. Two nearly identical columns were constructed. Column 1was used to elute native herbicide. Elution was terminated when effluent concentrations approached detection limits. The column was then sectioned and extracted to determine the distribution of the remaining herbicide in the soil. Column 2 was eluted until the native herbicides in the effluent were very low (
