time-dependent isotherm shape of organic compounds in soil organic matter: implications for sorption...

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Environmental Toxicology and Chemistry, Vol. 15, No. 8, pp. 1282–1288, 1996q 1996 SETAC

Printed in the USA0730-7268/96 $6.00 1 .00

TIME-DEPENDENT ISOTHERM SHAPE OF ORGANIC COMPOUNDS IN SOIL ORGANICMATTER: IMPLICATIONS FOR SORPTION MECHANISM

BAOSHAN XING and JOSEPH J. PIGNATELLO*The Connecticut Agricultural Experiment Station, P.O. Box 1106, New Haven, Connecticut 06504, USA

(Received 28 August 1995; Accepted 6 February 1996)

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. Theisotherms were fitted to the Freundlich model, S 5 KCN, where S and C are the sorbed and solution-phase concentrations and K andN 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 unityin all cases and decreased with increasing sorption. Also, the isotherms were operationally separated into a ‘‘fast’’ fraction (amountsorbed after 1 d) and a ‘‘slow’’ fraction (amount sorbed thereafter). Ns was significantly smaller than Nf in all systems tested. Theresults show that partitioning in soil organic matter (SOM) is appreciably less ideal for the slow fraction. It is concluded that SOMhas both partition and adsorption domains analogous to the dual-mode sorption model of glassy polymers. The adsorption componentis more prominent for the slow fraction, indicating that the adsorption sites are internal to the SOM matrix and unevenly distributedwith respect to access by sorbing molecules. Sorption by these natural materials was compared with sorption by polyvinylchloride, aglassy 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.

Keywords—Nonlinear sorption Soil organic matter Dual-mode sorption Nonequilibrium sorption Organic matterdiffusion

INTRODUCTION

Sorption mechanism and kinetics in natural particles are cen-tral to the fate, bioavailability, and remediation of organic pol-lutants. The principal sorbent of hydrophobic compounds underhydrated conditions is soil organic matter (SOM) if it is suf-ficiently abundant [1–3]. Stevenson [4] defined SOM (humus)as the ‘‘total of the organic compounds in soil exclusive ofundecayed plant and animal tissues, their partial decompositionproduct, and the soil biomass.’’ The same definition is used herefor SOM. Sorption to SOM is widely believed to occur bypartition, which is the uniform, concentration-independent dis-solution of solute into its three-dimensional matrix; thus, linearisotherms and noncompetitive sorption are expected [3, refer-ences therein]. But recent data suggest that sorption to SOM ismore complex. First, the isotherms of polar and nonpolar com-pounds alike are often nonlinear when constructed over a wideconcentration range [5–9]. In such cases, values of the exponentN lying between 0.8 and 1.0 are commonly obtained when iso-therms are fit to the Freundlich equation,

S 5 KCN (1)

where S is the sorbed concentration (mg·g21), C is the solution-phase concentration (mg·mL21), K is the Freundlich sorptioncoefficient (mLN·mg12N·g21 ), and N (dimensionless) is the ex-ponential order in C. Second, there are now examples of com-petitive sorption between hydrophobic compounds in soils[5,8,10], humic acid-coated clays [11], and humic acid particles[8].

The penetrability of SOM is an underlying assumption in thepartition model—and probably a valid one considering thatsorption in rubbery organic polymers occurs by a partition

* To whom correspondence may be addressed.

mechanism and is linear under dilute conditions [12]. In soilswhere sorption is predominantly by SOM, this raises the ques-tion of what the source of nonlinearity and competitive effectsis.

Moreover, sorption (or desorption) to natural particles is ki-netically controlled, often exhibiting a relatively fast phase onthe order of hours, followed by a slower phase that can takeweeks or much longer to complete [13–16]. Sorption may there-fore become rate-determining to transport and bioavailability.Since most experiments are carried out over a few days at most,there is a paucity of information about long-term sorption be-havior. A proposed cause of slow sorption is organic matterdiffusion, i.e., restricted molecular diffusion through SOM[13,16,17]. Hence, the fundamental thermodynamic interactionsof organic molecules with SOM are likely to bear directly onthe factors underlying slow kinetics.

Transport and biodegradation models often employ a two-compartment conceptualization of sorption in which the totalsorbate (ST) consists of a fast compartment (Sf) in equilibriumwith solution and a slow compartment (Ss) that is not [7,16–20]. Thermodynamically, an important consequence of N , 1is that sorption becomes greater as the concentration declines.This results in a corresponding reduction in transport, degra-dation, or uptake, as the case may be. The effect of nonlinearityon the fast fraction is handled by the expression for the retar-dation factor R, i.e., the time for transport of a sorbing soluterelative to water [17].

rKNN21R 5 1 1 C (2)

u

where r is the bulk density (g·cm23) and u is the moisture content(cm3·cm23). Thus, when N , 1, R is inversely dependent on C[17]. The effect of nonlinearity on the slow fraction, however,

Time-dependent isotherm shape in soil organic matter Environ. Toxicol. Chem. 15, 1996 1283

Table 1. Chemical formulas, structures, and physicochemical properties of sorbates

Sorbatea Formula StructureSol.b

(mg/L) logKOWc KH

d

a DCB 5 1,3-Dichlorobenzene; DCP 5 2,4-dichlorophenol; metolachlor 5 2-chloro-N-[2-ethyl-6-methylphenyl]-N-[2-methoxy-1-methyle-thyl]acetamide.

b Sol. 5 solubility in water.c Kow 5 octanol/water partition coefficient.d KH 5 dimensionless Henry’s Law constant.

is unclear since it may have kinetic as well as thermodynamicsignificance. An isotherm with N , 1 implies a distributedinteraction potential such that sorption decreases as concentra-tion increases.

The purpose of this study was to examine the concentrationdependence of sorption (i.e., N) as a function of time, partic-ularly in regard to differences between an operationally defined‘‘fast’’ and ‘‘slow’’ sorption state. By this we hoped to gaininsight into the nature of SOM as a sorbent of organic pollutants.Prior to this study there was no direct experimental evidencethat isotherm linearity was variable with time. Miller and Pedit[21] used model simulations to suggest that N of lindane in asoil measured at 732 h adequately described sorption thereafter.Other evidence suggests that the Ns , Nf; in particular, desorp-tion of small, nonpolar molecules from soils [22,23], silica [24],or humic-coated silica [25] left a ‘‘resistant’’ fraction (i.e., Ss)that became progressively more important to total sorption (i.e.,Ss expressed as percentage of total sorption increases) as theapplied concentration was reduced.

The test compounds of this study include weakly polar 1,3-dichlorobenzene (DCB) and two compounds that contain bothhydrophobic and polar/hydrogen-bonding surface area, 2,4-dichlorophenol (DCP) and metolachlor (2-chloro-N-[2-ethyl-6-methylphenyl]-N-[2-methoxy-1-methylethyl] acetamide). Thenatural sorbents include a mineral soil containing ;3% SOMand a peat soil that consisted almost entirely (93%) of SOM.The pH of the soil suspensions (5.6) was three orders of mag-nitude below the pKa of DCP, ensuring that its behavior cor-responds to the uncharged form. We also examined the sorptionof one of the compounds (DCB) to polyvinyl chloride micro-spheres as a model for SOM [8].

MATERIALS AND METHODS

[Ring-UL-14C] Metolachlor was provided by Ciba-Geigy(Greensboro, NC, USA), and unlabeled metolachlor was ob-tained from the Crescent Chemical Company (Hauppage, NY,USA). Both DCB and DCP were from Aldrich Chemical Com-pany (Milwaukee, WI, USA), and [Ring-UL-14C] DCP was fromSigma Chemical Company (St. Louis, MO, USA). Propertiesand structures of these sorbates are shown in Table 1. Peat(44.6% C, 6.9% ash, and 1.7% Si as SiO2) was purchased fromthe International Humic Substance Society. The C/N ratio (14.4)of the peat indicates that it is highly humified [26]. The soil(56% sand, 36% silt, 8% clay, and 1.4% organic carbon) wasa fine sandy loam collected from Lockwood Farm in Hamden,Connecticut, USA. It was passed through a 2-mm sieve at fieldmoisture content and stored at 58C. Plant residues were notapparent in the sieved soil sample. Polyvinyl chloride (PVC)was purchased from Aldrich Chemical Company and consistedof spherical particles with a median diameter of 110 mm and arange of 60 to 150 mm and had a specific surface area of 0.363m2·g21 (characterization by Quantachrome Corp., BoyntonBeach, FL, USA).

Sorption was conducted in 8-ml screw-cap vials (minimalheadspace) with Teflont-lined septa (for DCB) or 7-ml flame-sealed glass ampules (for DCP and metolachlor) [19]. The sol-vent was 0.01 M CaCl2 containing 200 mg/L HgCl2 as biocide,which we have used previously [10,27], to prevent long-termdegradation. After examining various sterilization methods,Wolf et al. [28] concluded that HgCl2 was highly effective(equivalent to 33 autoclaving) and, in contrast to other che-mosterilants, had little impact on cation exchange capacity, ex-tractable metal ions, and pH. The Freundlich parameters of me-

1284 Environ. Toxicol. Chem. 15, 1996 B. Xing and J.J. Pignatello

Fig. 1. Sorption isotherms of 1,3-dichlorobenzene (DCB) in peat. (a)1- and 30-d isotherms. (b) Slow fraction isotherm.

tolachlor in a 2-d experiment were identical within uncertaintylimits for 200 mg/L HgCl2 and 200 mg/L NaN3 (data not shown).

The water to solids ratio was 3.5 for DCB in soil, 140 forDCB in peat, 2.5 for metolachlor in soil, 35 for metolachlor inpeat, 3 for DCP in soil, 110 for DCP in peat, and 95 for DCBin PVC. The initial concentrations in water ranged from 0.1 to150 mg/L for metolachlor, 0.06 to 60 mg/L for DCB, and 0.1to 1,600 mg/L for DCP. Two controls without sorbent were runat each initial concentration. The samples were shaken brieflyupon mixing and then stored in the dark at 24 6 28C for 1, 30,or 180 d. Subsequently, the samples were shaken by hand for5 to 8 min every hour during the first 12 h and every 4 h inthe daytime thereafter. Before sampling, the samples were shak-en again and centrifuged at 850 g for 20 min.

1,3-Dichlorobenzene was hexane-extracted from 1- to 2-mlsupernatant samples and analyzed by gas chromatography on a0.53 mm 3 30 m DB 624 capillary column (J & W Scientific,Folsom, CA, USA) using electron capture detection. Metolach-lor and DCP were determined by scintillation counting of 1.0to 1.3 g of the supernatant in 15 ml Opti-Fluor cocktail (PackardInstrument Co., Meriden, CT, USA). Sorbed concentrations inthe soils were calculated by mass balance. Sorption of DCB tovials was generally less than 8% and corrected with controlsby the method of McGinley et al. [5]. Sorption of metolachlorand DCP to ampules was negligible.

The parameters K and N were determined by linear regressionof log-transformed data (Eqn. 4 or 6 in Results). The 90 and99% confidence limits on N were determined using standardstatistical methods [29]. Linear fit of log-transformed data isjustified over direct nonlinear curve fitting in this study for tworeasons: (1) Concentrations are spread evenly over the log scale;thus, nonlinear curve fitting would underestimate the importanceof the low concentration data. (2) The relative error in the mea-surement is not greatly dependent on concentration.

For convenience the full isotherm was constructed in threesets of experiments initiated days or weeks apart. The highregression (r2) values and the absence of discontinuities in theisotherms testify to the quality of the data.

RESULTS

For the natural sorbents isotherms were constructed at soluteconcentrations ranging between three and five orders of mag-nitude. Incubation times of 1, 30, and 180 d were used. Fastand slow states were defined on an operational basis. Theamount sorbed after 1 d was taken to represent the fast state;i.e.,

N Nf 1S 5 k C 5 S 5 K C (3)f f 1 1 1

A linear regression of the log-transformed expression was usedto obtain Kf and Nf:

log S1 5 log Kf 1 Nflog C1 (4)

We assume establishment of equilibrium for the fast state after1 d; thus, Kf and Nf remain constant though the solute concen-tration changes (due to redistribution to slow state) with time.The difference between the amount sorbed after 1 d and thetotal amount sorbed (St) after 30 d (for DCB) or 180 d (for DCPor metolachlor) was taken to represent the slow state (Ss):

N Ns fS 5 K C 5 S 2 S 5 S 2 K C (5)s s t f t f t

The log-transformed expression of Equation 5 is

Nflog S 5 log(S 2 K C ) 5 log K 1 N log C (6)s t f t s s t

where the value of t can be either 30 or 180 d depending oncompound.

The isotherms of DCB in peat after 1 and 30 d contact timeare shown in Figure 1a. During this time interval K increasedand N decreased. The increase in K was anticipated based onnumerous studies documenting slow sorption kinetics. The 99%confidence limits of N1 are 0.815 to 0.863 and of N30 are 0.787to 0.791. A plot of Equation 6 relating Ss to C is shown in Figure1b. The 99% confidence limits of Ns are 0.560 to 0.720. Theseresults show that Ns is significantly smaller than Nf (5 N1).Similar trends occur for DCB in soil (Table 2).

Analogous results were obtained for DCP and metolachlorsorption in both peat and soil. These results are summarized inTable 2, and examples are shown in Figures 2 and 3 for DCPin peat and soil, respectively. In all cases K increases between1 and 30 d contact times. The largest relative increase (2.7-fold) occurs for DCP in soil. However, there is little or noincrease in K between 30 and 180 d, suggesting that apparentequilibrium is reached or uptake is too slow on this timescaleto measure. In all cases, N decreases with equilibration time.The changes are usually greater between 1 and 30 d than theyare between 30 and 180 d. For DCP sorption in soil and peat,N30 and N180 are both smaller than N1 at the 99% confidencelevel, but N30 and N180 are not significantly different. For me-tolachlor in peat, N180 is smaller than both N30 and N1 at the99% level. For metolachlor in soil, the downward trend holds,but the values are not significantly different. In general, Ns issmaller than Nf; the difference is significant at the 99% levelin all cases except metolachlor in soil, for which the differenceis significant at the 90% level.


Table 2. Freundlich parameters, K and N, for sorption of DCB, DCP,and metolachlor in soil and peat

Sorption Fractiona K N b r 2

DCBSoil Fast

30 dSlow

8.0211.1

2.86

0.858 6 0.007a0.801 6 0.006b0.665 6 0.021c

0.9990.9990.985

Peat Fast30 dSlow

253321

61.5

0.839 6 0.008a0.799 6 0.004b0.640 6 0.027c

0.9990.9990.974

DCPSoil Fast

30 d180 dSlow

7.8621.121.612.2

0.806 6 0.006a0.720 6 0.011b0.715 6 0.010b0.637 6 0.019c

0.9990.9960.9960.982

Peat Fast30 d180 dSlow

319486494154

0.777 6 0.005a0.736 6 0.003b0.727 6 0.002b0.601 6 0.011c

0.9990.9990.9990.994

MetolachlorSoil Fast

30 d180 dSlow

1.622.462.921.26

0.852 6 0.005a*0.841 6 0.005a*,**0.838 6 0.008a*,**0.809 6 0.017a**

0.9990.9990.9990.993

Peat Fast30 d180 dSlow

76.4107115

35.9

0.890 6 0.006a0.875 6 0.006a0.837 6 0.004b0.731 6 0.012c

0.9990.9990.9990.996

a,b,c 5 values of N are significantly different at the 99% confidencelevel.*,** Values of N are significantly different at the 90% confidence level.a Fast and slow fractions calculated by Eqn. 3–6 (see text).b Mean 6 1 SD.

Fig. 2. Sorption isotherms of 2,4-dichlorophenol (DCP) in peat. (a)1- and 180-d isotherms. (b) Slow fraction isotherm.

We also looked at sorption of DCB in 110-mm PVC particles.Polyvinyl chloride is a glassy polymer (glass transition tem-perature, 858C) that we have considered as a model for SOM[8]. Figure 4b shows that uptake of DCB by PVC is slow;equilibrium had not been achieved even after 128 d. Figure 4ashows the sorption isotherms constructed after 1 and 15 d. Thisfigure shows, first, that the isotherms are nonlinear, indicatingthat sorption is less ideal than simple partitioning, and, second,that N is identical for isotherms constructed at different posi-tions of equilibrium.

DISCUSSION

The sorption isotherms of DCB, DCP, and metolachlor in bothsoil and peat are nonlinear even after 1 d and become increas-ingly so with time. While the Freundlich equation is an equi-librium, not a kinetic, expression, we may nevertheless use itto interpret sorption at intermediate times by regarding it simplyas an expression relating the sorbed concentration to the aqueousconcentration at any given time. We now examine the cause ofnonlinearity and its time-dependency.

It is first worth considering whether nonlinearity per se is anartifact of insufficient contact time. The wide concentrationrange used to construct the isotherms invites the question ofwhether the approach to equilibrium is concentration-dependent.Assuming a linear isotherm at equilibrium, if high-concentrationsamples reached equilibrium faster than low-concentration sam-ples, we would expect a change in isotherm shape from con-cave-up (N . 1) at short times tending toward linear at longtimes. Conversely, if low-concentration samples reached equi-librium faster than high-concentration samples, we would expect

a change in shape from concave-down (N , 1) to linear. How-ever, sorption by the peat and soil clearly did not tend towardlinearity. Therefore, nonlinearity is not an artifact due to in-sufficient contact time.

It may also be concluded that if mass transfer is governedsolely by Fickian diffusion, the approach to equilibrium shouldbe independent of position along the concentration curve. ForFickian diffusion the relative amount sorbed is a function ofthe diffusion coefficient (D) and time (t) and is independent oftotal mass of sorptive added to the system (To), provided thatthe properties of the sorbent remain unchanged over that rangeand the sorbent matrix is homogeneous [30], i.e.,

Mt 5 f (D, t) ± f (M ) (7)0M`

where Mt is mass sorbed at time t and M` is mass sorbed attime infinity. (The highest applied concentrations of the sorp-tives used here are probably insufficient to alter the sorbentproperties of SOM.) The ratio of sorbed- to aqueous-phase con-centrations embodied in the Freundlich isotherm is related toMt/M` by

211 21NS M V M21t t t1 2N N5 1 1 M (KW) 2 (8)`N F GC M W Mt ` `

where W is the mass of solids, V is the volume of liquid, andK and N are the Freundlich parameters. All terms in the rightside of Equation 8 are independent of Mo except M`

(1/N21)(KW)21/N.For linear isotherms that term becomes a constant (1/KW); fornonlinear isotherms it is weakly dependent on To but always


Fig. 3. Sorption isotherms of 2,4-dichlorophenol (DCP) in soil. (a)1- and 180-d isotherms. (b) Slow fraction isotherm.

Fig. 4. Sorption of 1,3-dichlorobenzene (DCB) in polyvinyl chloride.(a) 1- and 15-d isotherms. (b) Sorption kinetics.

K1 and so may be ignored. The data displayed in Figure 4provide an example of a system characterized by a nonlinearisotherm and a slow approach to equilibrium due to diffusionlimitation of DCB in the PVC polymer. We see that for thisexample the value of N is independent of position of equilib-rium, which is consistent with the above conclusion.

Nonlinearity, which appears to be real in the systems of soiland peat, implies heterogeneity in the interaction potential with

the sorbents. The peat is composed almost totally of SOM. Themineral content is only 6.9% ash, of which 1.7% is silicatemineral. Thus, virtually all sorption on peat must occur to theSOM. What little mineral surface is available would almostcertainly be blocked with humic coatings. Therefore, the iso-therm nonlinearity observed for the peat cannot easily be at-tributed to adsorption to mineral surfaces. Since sorption in thesoil is qualitatively similar to that in peat it is reasonable toattribute the same mechanism to both.

The results support our proposal that SOM is a dual-modesorbent [8]. In this model SOM has both partition and adsorptiondomains. The partition domain is characterized by linear sorp-tion, while the adsorption domain is characterized by nonlinearsorption as a result of having ‘‘sites’’ which are distributed inenergy. The nature of the sites is unknown, but these sites appearmore concentrated within the SOM matrix. Additional evidenceconsistent with the dual-mode model for SOM has been detailedelsewhere [8] and will be given in future reports.

Dual-mode sorption was originally proposed for sorption ofpermanent gases and organic vapors in glassy polymers (e.g.,PVC or polystyrene) [31,32]. We see such dual-mode behaviorin the isotherm of DCB in PVC (Fig. 4). The surface area ofPVC is too small (0.363 m2·g21) for there to be significant con-tribution from surface adsorption. Sorption in rubbery polymers,which have structures that are more expanded and flexible thanthat of glassy polymers, is solely by partition. The nature ofthe adsorption sites in glassy polymers is not well understood,but they have been described as ‘‘voids’’ [33,34] internal to thepolymer matrix. Partitioning combined with an array of Lang-muir adsorption sites having different energies can lead to anisotherm that is overall Freundlich in shape [35]. Whether theadsorption domain becomes saturated at high concentrationsdepends on how the energies are distributed. Mobile and im-mobile fractions of tri-n-butyl phosphate were detected in glassypolystyrene by 31P nuclear magnetic resonance [36]; the mobilityof the latter fraction was widely variable.

The important new information that this study contributes isthat the adsorption component is more important for the slowfraction, as reflected in the general finding that Ns , Nf. Thisimplies that the adsorption ‘‘sites’’ are less abundant on thesurface or in external regions of SOM that are readily availableto bulk solution and are more abundant in internal regions (i.e.,within the SOM matrix). Said another way, the adsorption sitesare not evenly distributed in SOM. In contrast, the adsorptionsites of PVC are uniformly distributed judging from the con-stancy of N with extent of equilibrium. Uniformity is expectedfor a material like PVC that has been synthesized by a homo-geneous process. Soil organic matter, on the other hand, is het-erogeneous. Humic substances have condensed and expandedregions [37], which may be analogous to the glassy and rubberyregions of organic polymers. Our results suggest that at earlytimes sorption is limited predominantly by diffusion into themore penetrable rubbery regions of SOM where sorption islinear. As time goes on penetration into the glassy regions con-taining the adsorption sites occurs. No sharp division existsbetween the two, so sorption is nonlinear even after 1 d. In fact,the isotherms in the natural particles may have been even closerto linearity at very early times.

It is also significant that the slow fraction becomes larger asthe total contaminant concentration declines. For example, Ss

of DCB in peat is 36% of S30 at a solution concentration of 0.02mg·ml21 compared to 13% at 6 mg·ml21 (calculated from Fig.1). For another example, Ss of DCP in soil is 77% of S180 at a


solution concentration of 0.02 mg·ml21 compared with 36% at300 mg·ml21 (Fig. 3). This behavior is consistent with previousfindings mentioned above that the ‘‘resistant’’ fraction is in-versely related to the applied concentration [22–25]. This isimportant from the perspective of bioavailability and remedi-ation because it may provide one mechanism for the often-observed drop-off in degradation rate [38,39].

In summary, both the Freundlich coefficient, K, and exponent,N, are time-dependent. The N of the slow fraction is significantlysmaller than that of the fast fraction for three compounds (DCB,DCP, and metolachlor) of different polarity in two soils, onebeing almost entirely organic matter. The results are consistentwith SOM being a dual-mode sorbent in which partitioning aswell as site-directed interactions occur. The latter is more im-portant for the slow than the fast fraction.

NOTE ADDED IN PROOF: Weber and Huang [42] reportthe same effect of equilibration time on the Freundlich param-eters that we observe using different soil-contaminant systems.

Acknowledgement—This work was supported by the U.S. Departmentof Agriculture National Research Initiative Competitive Grants Pro-gram, Agreement 93-37102-8975.

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time-dependent isotherm shape of organic compounds in soil organic matter: implications for sorption...

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