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Influence of dissolved and colloidal phase humic substances on the transport of hydrophobic organic contaminants in soils
Phys. Chem. Eurrh, Vol. 23, No. 2, pp. 179-185, 1998 0 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain
Pergamon
0079-1946/98 $19.00 + 0.00 PII: SOO79-1946(98)00010-X
Influence of Dissolved and Colloidal Phase Humic Substances on the Transport of Hydrophobic Organic Contaminants in Soils I.
Received
25 April 1997; accepted
15 December
Abstract. Dissolved and colloidal size organic matter (DOM) controls the mobility of hydrophobic organic chemicals (HOC) in soils by affecting the sorptive interaction between the soil matrix and the solution phase. The association between HOC and DOM in the soil solution leads to an increase in the water solubility of HOC. DOM is a reactive component of the soil solution with respect to the immobile solid phase. Therefore the overall mobility of HOC in soils is enhanced due to co-transport with DOM as the mobile carrier or reduced due to cosorption or cumulative sorption. The specific processes relevant to the DOM-mediated fate of HOC in natural and contaminated soils are discussed, with special consideration to the effect of (i) soil physicochemical parameters (ionic strength, composition, pH), (ii) DOM of different origin, and (iii) aging of a contamination on HOC release. Q 1998 Elsevier Science Ltd.
1997
The controlling process is a partition between the soil matrix and the solution phase, where the partition equilibrium is dominated by the solid phase. In recent years growing attention has been given to the effect of mobile sorbents, i.e., dissolved or colloidal-size aqueous phase components, on the behaviour of polycyclic aromatic hydrocarbons (PAHs) and other hydrophobic pollutants in soils and sediments (Means et al., 1978; McCarthy and Zachara, 1989). Several types of materials were identified as mobile sorbents and shown to increase the water solubility of organic and inorganic pollutants: inorganic colloids, such as clay and silt minerals or iron oxides, and mobile organic colloids (dissolved organic matter, DOM) (Short et al., 1988; Nakayama et al., 1986; Chiou, 1989; McCarthy and Zachara, 1989). Contaminant mobility is affected by processes controlling the total water solubility, i.e., the aqueous-phase concentration of the free and of all mobile sorbent associated contaminants. The interaction of a contaminant with the mobile sorbent reduces the sorption to the solid phase, resulting in increased contaminant mobility (McCarthy and Zachara, 1989). DOM has been shown to specifically enhance the mobility of hydrophobic organic contaminants (HOC) in aquifers and soils. These effects may be due to the strong atlinity of hydrophobic chemicals to DOM. Thermodynamic studies with humic acids as organic sorbents suggest that this process is driven by entropy (Kile and Chiou, 1989; Jota and Hassett, 1991). The mobility of DOM in natural systems, which depends largely on its molecular size (McCarthy et
1 Introduction The role of soils as filters and chemical reactors is beginning to dominate our conception of how to manage soil and groundwater resources to meet quality and quantity aims. For this purpose we have to identity the relevant soil physical and chemical processes within the unsaturated zone. The fate of hydrophobic organic chemicals (HOC) in soils is predominantly determined by the interactions with soil organic matter. HOC accumulation is found in soil horizons high in organic matter (0 / A horizons). This is the consequence of hydrophobic interactions of HOC with the organic phase of natural soils and / or with soot, tar or coal admixtures in contaminated soils. Correspondence to: I. K(lgel-Knabner 179
I.
180
Kogel-Knabner and K. U. Totsche
al., 1993) is thought to have a decisive infhience on of hydrophobic contaminants the transport (McCarthy and Zachara, 1989). Column experiments confirmed that organic macromolecules can facilitate transport of hydrophobic compounds in aquifers (Dunnivant et al., 1992a; Johnson and Amy, 1995; Magee et al., 1991). However, transport in mineral soil horizons most frequently takes place under unsaturated rather than saturated flow regimes. Observations of high PAH contents in subsoil have been interpreted as being due to PAHs interacting with DOM as a mobile carrier (Jones et al., 1989; Deschauer et al., 1994). In soils DOM can be immobilized in significant amounts (David and Vance, 1991; Dunnivant et al., 1992b; Guggenberger and Zech, 1993). DOM sorption in soils was described by Leenheer (1981), Schnitzer (1986) and Jardine et al. (1989), who suggested different binding mechanisms, e.g., physisorption or partitioning (driven by favorable entropy changes) or electrostatic interactions (anion exchange).
2
Table 1: Log KDOC values obtained from literature for the partition of PAH to dissolved organic substances from different origin. Data obtained by (A) reversed phase separation, (B) fluorescence quenching, and (C) dialysis methods (from Raber et al., 1997). PAH
log KDOC
M&Xi
Type and origin of dissolved organic substances
Reference
phenanthrene
4.1 - 4.6 4.6 c4.1 -
A B B B B B B B B B B B B
DOM (soil, organic layer) DOM (mineral soil) DOM (groundwater) humic acid (soil) DOM (sod. organic layer) DOM (mineral soil) fulvic acid (soil) fulvic acid (soil) fulvic acid (stream) DOM (marine sediment) humic acid (soil) humic acid (soil) humic acid (stream)
Raber et al. (1997) Magee et al. (1991) Backhus and Gschwend (1990) Gauthier et al. (1986) Raber et al. (1997) Herbert et al. (1993) Herbert et al. (1993) Gauthier et al. (1986) Schlautman and Morgan (1993) Chin and Gschwend (1992) Gauthier et al. (1986) Herbert et al. (1993) Schlautman and Morgan (1993)
b=oIelPyrene
4.0-4.3 4.5 4.7 5.2
A A A A
DOM (mineral soil) DOM (mineral soil) DOM (mineral soil) humic acid (Aldrich)
Raber et ai. (1997) Maxin and K~gel-Knabner (1995) Raber and Kagel-Knabner ( 1997) Maxin and Kbgel-Knabner (1995)
benzo[a]pyrene
4.2- 5.0 5.1 4.6-6.0 5.3 5.2 6.3 6.3
A C C A A C C
DOM (surface water) DOM (stream) DOM (surface /groundwater) humic acid (Aldrich) humic acid (Aldrich) humic acid (Aldrich) humic acid (Aldrich)
Morehead et al. (1986) Kukonnen et al. (1990) McCarthy et al. (1989) Morehead et al. (1986) Landrum et al. (1984) McCarthy et al. (1989) McCarthy and Jim&z (1985)
benzo[k]fluoranthene
4.6 _ 4.7 4.6 - 4.7 5.0 5.1
A A A A
DOM (mineral soil) DOM (mineral soil) DOM (mineral soil) humic acid (Aldrich)
Raber et sl. (1997) Maxin and KOgel-Knabner (1995) Raber and Kfigel-Knabner (1997) Maxin and KBgel-Knabner (1995)
benzo[g,h,i]perylene
4.9 - 5.0 5.5 5.8
A 12 A
DOM (mineral soil) DOM (mineral aoil) humic acid (Aldrich)
Maxin and Kbgel-Knabner (1995) Raber and KdgelXnnlx~er (1997) Maxin and KGgel-Knabner (1995)
Influence of Dissolved and Colloidal Phase Humic Substances The partition coefficient for the binding of a distinct PAH compound to soil DOM can vary more than one order of magnitude, depending on the source of DOM. K~oc values for DOM from mineral arable soils are markedly lower compared to DOM from acid forest soils. The hydrophobic and hydrophilic composition of DOM depends on the origin of the soil material. DOM from mineral soils under agricultural use contains higher proportions of hydrophobic components compared to DOM from acid forest floor materials. This partially explains the differences in binding capacity for PAH (Raber et al., 1997). More DOC can be released from composts and sewage sludges than the DOC content of the soil solution, and therefore the amount of hydrophobic compounds in the soil solution can be increased considerably when these materials are added to soil (Raber and Kbgel-Knabner, 1997). The binding capacities of DOM from composts were found to be similar or slightly less than those from soil. The sorption of PAHs to DOM obtained from sewage sludges was less and varied considerably for different types of sludge treatment. 3 Desorption of PAHs from soils The presence of dissolved organic matter (DOM) in the soil solution has an enhancing effect on the desorption of PAH (Fig. 1). Benzo[a]pyrene
in solution
Desorption linearly increased with increasing DOM concentrations up to >lOOO mg L-l. Partition coefficients (log Koc) for the desorption of “C-PAHs are 4.2 for pyrene and 5.0 for benzo[a]pyrene in the presence of DOM from plant waste compost (200 mg C L-l). These values are about 3.5 (pyrene) and 25 (benzo[a]pyrene) times lower than in the aqueous control solutions of similar ionic strength. The enhancement of PAH desorption between various types of DOM, from composts and waste disposal site leachates, seems to be influenced by the molecular weight distribution of DOM (Raber and KogelKnabner, 1998). The composition of the soil solution has a decisive influence on the sorption and desorption of PAH in soils. Figure 2 and Table 2 show the desorption isotherms of benzo(a)pyrene and pyrene from soil in the presence of different soil solutions. The desorption of “C-benzo[a]pyrene and 14C-pyrene is strongly influenced by the properties of the aqueous phase. Generally the desorption coefficients are similar to the sorption coefficients Koc for these compounds. Whereas the presence of DOM has an enhancing effect on PAH desorption, high concentrations of Cat& lead to a reduced desorption (salting out effect). PAH sorbed (pg kg-‘)
5.m
.._.
pglL . ..*
plant waste 1 -.. ..‘-I
domestic waste 1
..... .‘..- domestic waste 2 1
~ _..,..... ./‘-
200 .. . . . . .. . . . . . . . . . . . . . . . l . . . . . . . . ,@) ..f
t .. . .. ..
05
isotherms pyrene)
composed of bidistilled
250
500 DOM
soil (artificially
presence of varying
f
750
’
,
8.
1000
from the Ap horizon of an
contaminated with 1.91 mg kg-l) in the
DGC concentrations
types of compost material.
1
5
10
50
for the desorption
water
(200 mg L-l, concentration
l,
of %-PAH
soil (Ap, Gleyic
in the presence of a soil solution 2.8 mM CaCll
0, and DOM m
of CaC12 2.8 mM); from Raber and
Kbgel-Knabner (1998);
mgC L”
Fig. 1. Desorption of “C-benzo(a)pyrene agricultural
,
~...~.~..
..~.. ~..~
from a mineral
Cambisol, artificially contaminated)
’
~. ~...~
c _.
0,050.1
Fig. 2. Desorption (benzo(a)pyrene----,
t
.
PAH in solution (pg L-l)
.,,,,.... *...“‘”
,
~. ,.._ .~... /
0,Ol
,,,,~ .,_... +..,.‘I
0
181
obtained
from different
The error bars represent the standard
deviation of triplicates; from Raber and KOgel-Knabner (1998);
Another factor controlling the desorption of PAHs from soil is the time elapsed since contamination. Table 3 shows the effect of aging of a contamination on the desorption of benzo(a)pyrene in the presence of different soil solutions.
I. Kogel-Knabner and K. U. Totsche
182
Table 2. LIesorption of 14C-benzo(a)pyrene from mineral soil material (Ap, Gleyic Cambisol) with DOM (200 mg L-l), regression (r2) and slope (+) of isotherm. Control experiments with C&l2 in the same ionic strength without addition of DOM (from Raber and K~gelKhmer, 1997). DOM Control
KOC
log Koc
(L kg-‘)
(Lkg-‘)
0.994 0.995 0.999
92930 113962 2458045
4.97 5.06 6.39
0.999 0.992
103549 134969
5.02 5.13
0.993 0.989 0.963
169959 887141 2378643
5.93 5.95 6.38
?
erperiments
compost plant waste compost 1 plant waste compost 2 2.8 mM C&l domestic domestic seepage domestic industrial
waste compost 1 waste compost 2 water ofwaste dqmal waste storage waste storage
2
site
10.3 mM CaCI_
Again, an effect of the soil solution composition on the desorption is found, similar to the previous experiments. In the first 36 days following a contamination a decrease of the PAH concentration in the solution phase was observed, indicated by the increasing partitioning coeffkient Koo Monitoring the contamination for up to 183 days showed no further decrease in the fraction that can be desorbed,
suggesting a two step process. An “easily” exchangeable, surface bound fraction of the PAH compounds has reached an equilibrium already after 1 month. The further alteration of the contamination, which may be due to intra-organic matter diffusion processes, does not change the “easily” exchangeable fraction of the compound.
Table 3. Parameters for the desorption isotherms for desorption of ‘k%enzo(a)pyrene
at different times after contamination
different soil solutions (similar to the experiment described in Fig. 2); from Raber and K6gelXnabner Age of contamhtion
(days)
b
r2
exchange solution a 1 HZ?
Kd (W)
(1998). log&
%C
in the presence of
log SE b
(Lw)
-54.6
0.992
2317
236461
5.37
0.04
17.0
0.999
11177
1140544
6.06
0.04
plant waste compost 1
-84.0
0.983
737
75175
4.88
0.05
8 H20.
-10.1
0.997
2645
268851
5.43
0.05
2.8 mM C&l2
71.4
0.994
12599
1285575
6.11
0.06
plant waste compost 1
-68.5
0.984
761
77672
4.89
0.04
36 H20.
-49.9
0.997
3333
340131
5.53
0.03
2.8 mM CaC12
81.4
0.990
25366
2588358
6.41
0.03
plant waste compost I
-81.4
0.972
986
100582
5.00
0.03
89 H20.
-70.3
0.995
3431
350055
5.54
0.05
2.8 mM CaC12
-14.6
0.973
24089
2458045
6.39
0.04
plant waste compost 1
-79.6
0.984
998
101829
5.01
0.04
183 HZ?
-35.3
0.998
3421
349125
5.54
0.05
2.8 mM CatI2
32. I
1.000
24389
2488647
6.40
0.04
plant waste compost 1
44.1
0.997
1050
107192
5.03
0.04
2.8 mM C&l2
a DOM normalized to 200 mg C/L, addition of C&l2 error of means; deviation of J&
in the Same ionic strength as in the experiments in tbe presence of DOM.
b SE = standard
Influence
of Dissolved and Colloidal Phase Humic Substances
4 PAH mobility in the presence of DOM: soil column experiments A series of experiments was carried out to determine the breakthrough of DOM, PAHs and PAHs in the presence of DOM. All miscible displacement experiments were performed employing a laboratory soil column system specifically designed for experiments with hydrophobic substances (To&he et al., 1998). All experiments were conducted under unsaturated, low convection-dominated flow
183
conditions. Natural DOM was used in concentrations typically observed in mineral soil solutions. The soil materials were not pre-equilibrated with DOM, thus allowing for sorptive interactions with the bulk material for both DOM and PAHs. Miscible displacement experiments were carried out with DOM alone (Fig. 3) with PAHs alone (anthracene, pyrene, benzo(e)pyrene), and with a mixture of the PAHs and DOM (Fig. 4). Two different sandy materials were used, a spodic B horizon and a commercially available seasand. l-
*
DOM Ilobs
n
DOM II fitted D0Mlob.s DOM I fitted
--..
PV Fig. 3. Breakthrough of DOM through spodic B material. (left) reduced concentration of DOM and chloride. (O/m) DOM I/II. (O/O)
C1’ in
experiment DOM I/11(), (right) scaled breakthrough of the mobile fraction of DOM (from Totsche et al., 1997).
DOM transport (Fig. 3) can be understood by assuming that DOM is composed of at least two physicochemical different fractions: A mobile fraction, composed of the hydrophilic moieties of DOM, and an immobile fraction, composed of the hydrophobic moieties of DOM. The mobile fraction
1
1
anthracene A without DOM
0.8
0.3
0.2
A with DOM
0.6
$
shows a transport behavior comparable to that of very low-reactive tracers illustrated by the similar dispersion lengths and retardation parameters as chloride. The immobile fraction is sorbed completely by the spodic B material.
pyrene * without DOM * with DOM
P o
0.4
0.1
0.2 0 0, 0
100
200
300
400
500
600
PV Fig. 4. Bnxkthrougb ofanthracene and pyreoe through spodic B material in the absence and presence of DOM; (from Totsche et al., 1997)
184
I. Kbgel-Knabner and K. U. Totsche The breakthrough behaviour of PAHs (Fig. 4) under conditions typical for soil environments is dominated by the complex interaction of DOM with the PAHs and the bulk soil material. In contrast to studies referring to aquifer environments, DOM-mediated transport of PAHs in our experiments does not result in increased but in reduced mobility (Totsche et al., 1997). For both sandy materials, the presence of DOM resulted in (i) reduced PAH effluent concentration, (ii) reduced PAH mobility and (iii) an increased tailing of the PAR breakthrough curve. The DOM-mediated retention of PAHs can be explained by two different scenarios. Co-sorption describes the sorption of the DOM-PAH associate to the bulk phase. Cumulative sorption results from increased sorptive capacity of the bulk phase due to sorption of DOM and thus increased OC content. The differentiation between both retention processes is essential for the estimation of experimental mobility parameters, such as Kd values and sorption isotherm parameters. Results obtained from experiments representing aquifer conditions do not necessarily cover the flow conditions and sorptive properties for DOM and PAHs in unsaturated soil materials. The experimental conditions used for such column experiments are decisive for the observation of enhanced or reduced mobility of HOC in the presence of DOM. The estimation of PAH transport behaviour in soils has to take into account the controlling effect of the bulk soil and solution properties, controlling the dissipation of PAHs in soils. Acknowledgments. Financial support is acknowledged from the Deutsche Forschungsgemeinscha!? (Ko 1035/6, 103517, 1035/8) and the EU environment and climate programme (EVSV-CT94-0536).
References Backhus, DA and Gschwend, P.M., Fluorescent polycyclic aromatic hydrocarbons aa probes for studying the impact of colloids on pollutant transport in groundwater, Enwon. Ser. Technol., 24. 1214-1223, 1990. Chin, Y.P. and Gschwend, P.M., Partitioning of polcyclic aromatic hydrocarbons to marine porewater organic colloids. Envrron. Ser. Technol. 26, 1621-1626, 1992. Chiou, CT., Theoretical considerations of the partition uptake of nonionic organic compounds by soil organic matter, In: Sawhney, N.K. and Brown, K. (Eds.), Reoctrons and movement oforgomc chemrcols m soils, Special Publication No 22, l-19, ASA, Madison, WI, 1989. David, M.B. and Vance, G.F., Chemical character and origin of organic acids in streams and seepage of central maine, Bmgeochem.. 12, 17-41, 1991. Deschauer, H., Hartmann, R., K(igel-Knabner, I., and Zech W., The influence of dissolved organic matter (DOM) on the transport of
polycyclic aromatic hydrocarbons (PAHs) in a forest soil under Pinus silvestris, In: Senesi, N. and Miano, T.M. (Eds.), Humrc substances rn rhe Global Envrronment and Implicatrons m HumanIIealfh, 1063-1070, Elsevier, Amsterdam, 1994. Dunnivant, F.M., Jardine, P.M., Taylor, D.L., and McCarthy J.F., Cotransport of cadmium and hexachlorbiphenyl by dissolved organic carbon through columns containing aquifer material, Environ. Sci. Technol.. 26,360-368, 1992a. Dunnivant, F.M., Jardine, P.M., Taylor, D.L., and McCarthy, J.F., Transport of naturally ocurring dissolved organic carbon in laboratory columns containing aquifer material, Sorl Ser. Sot. Am. J., 56, 437-444, 1992b. Gauthier, T.D., Shane, E-C., Guerin, W.F., Seitz, W.R.. and Grant, C.L., Fluorescence quenching method for determining equilibrium constants for polycyclic aromatic hydrocarbons binding to dissolved humic materials, Environ. Ser. Technol., 20, 1162-l 166, 1986. Guggenberger, G., and Zech, W., Dissolved organic carbon control in acid forest soils of the Fichtelgebirge (Germany) as revealed by distribution patterns and structural composition analysis, Geoderma. _59,109-129, 1993. Herbert, B.E., Bertsch, P.M., and Novak, J.M. Pyrex sorption by water-soluble organic carbon, Envrron. Ser. TechnoL. 27, 398-403, 1993. Jardine, P.M., Weber, N.L., and McCarthy, J.F., Mechanism of dissolved organic carbon adsorption on soil, Sorl Scl. Sot. Am. J.. 53, 1378-1385, 1989. Johnson, W.P. and Amy, G.L., Facilitated transport and enhanced desorption of polycyclic aromatic hydrocarbons by natural organic matter in aquifer sediments, Envrron. %I. TechnoL. 29, 807-817, 1995. Jones, K.C., Stratfort, J.A., Titridge. P., and Waterhouse, K.S., Polynuclear aromatic hydrocarbons in an agricultural soil: Long term changes in profile distribution, Envrronmental Pollubon, 56, 337-351, 1989. Jota, M.A. and Hasset, J.P., Effects of environmental variables on binding of a PCB congener by dissolved humic substances, Envrronmental Toxrcology and Chemtry, IO, 483-491, 1991. Kile, D.E. and Chiou, C.T., Water solubility enhancement of nonionic organic contaminants, In: Suffet, I.H. and Ma&&thy, P. (Eds.), Aquatrc Humrc Substances: Influence on Fare and Trearment ofPollukmls, American Chemical Society Symposium Series, 219, 129-l 57, American Chemical Society, Washington D.C., 1989. Landrum, P.F., Nihart, S.R., Eadie, B.J., and Gardner, W.S., Reverse-phase separation method for determining pollutant binding to aldrich humic acid and DOC of natural waters, Enwron. Ser. TechnoL, 18, 187-192, 1984. Leenhecr, J.k, Comprehensive approach to preparation isolation and fractionation of dissolved organic carbon from natural waters and wastewaters, Envrron. Ser. Technol.. lS, 578-587, 1981. Magee, B., Lion, L.W., and Lemley, AT., Transport of dissolved organic macromolecules and their effect on the transport of phenantrene in porous media, Enwon. Ser. Technol., 25, 323-331, 1991. Malcolm, R.L. and MacCarthy, P., Limitations in the use of commercial humic acids in water and soil research, Envrron. Scr Technol.. 20,904-9 11, 1986. Maxin, C. and Kijgel-Knabner, I., Partitioning of PAH to DOM: Implications on PAH mobility in soils, Eur. J So11 Ser., 46, 193. 204, 1995. McCarthy, J. F. and Zachara, J.M., Subsurface transport of contaminants, Envrron. Ser. Techno/., 23. 496-504, 1989. McCarthy, J.F. and Jimenez, B.D., Interactions between polycyclic aromatic hydrocarbons and dissolved humic materials: binding and dissociation, Envrron. Sci. Technol.. 19, 1067-1072, 1985. McCarthy, J.F., Laurence, E.R., and Bonus, L.W.. Association 01 benzo(a)pyrene with dissolved organic matter: prediction of KDGM from structural and chemical properties of the organic matter, Chemosphere. 19, 1911-1920, 1989. McCat?by, J.F., Williams, T.M., Liang, L., Jardine, P.M., Jolley, L.W., Taylor, D.L., Palumbo, A.V., and Cooper, L.W., Mobility of natural organic matter in a sandy aquifer, Enwron. Ser. Technol.. 27.667-676, 1993. Means, J.C., Crerar, D.A., and Duguid, J.O., Migration of radioactive wastes: Radionuclide mobilization by complexing agents, Scwxe. 200,1477-1481,1978
Influence of Dissolved
and Colloidal Phase Humic Substances
Morehead, N.R., Eadie, B.J., Lake, B., Landrum, P.F., and Bemer, D., The Sorption of PAH onto dissolved organic matter in Lake Michigan waters, Chemosphere, 15,403-412, 1986. Nakayama, S., Moriyama, H., Arimoto, H., and Higashi, K., Behaviour of Americium in aqueous solutions containing iron, J. Nucl. Ser. Technol.. 23.73 l-739, 1986. Raber, B. and Keg&Knabner, I., Desorption of PAH from soil in the presence of dissolved organic matter, Envrron. Sci. Technol., submitted, 1998. Raber B. and Kbgel-Knabner, I., Influence of origin and properties of dissolved organic matter on the partition of PAH, European J. Sorl SCL, 48,443455, 1997. Raber, B., KOgel-Knabner, I., Stein, C., and Klem, D., Partitioning of polycyclic aromatic hydrocarbons to dissolved organic matter from different soils, Chemosphere, in press, 1997. Schlautman, MA. and Morgan, J.J., Effects of aqueous chemistry on the binding of polycyclic aromatic hydrocarbons by dissolved humic materials, Environ. Ser. Technol.. 27, 961-969, 1993. Schnitzer, M., Binding of humic substances by soil mineral colloids, In: Hung, P.M. and Schnitzer, M. (Ed.%), Interaclrons of soil mrnerals with nohual orgonics and microbes, Spec. Pub]. No. 17, 77-101, ASA, Madison, WI, 1986. Short, S.A., Lowson, R.T., and Ellis, J., 234U/238U and 23OTh/234U activity ratios in the colloidal phases of aquifers in la&tic weathered zones, Geochim. Cosmochrm. Acta. 52, 2555-
2563,1988. Totsche, K.U., K&gel-Knabner, I, and Danzer J., Dissolved organic matter-enhanced retention of polycyclic aromatic hydrocarbons in soil miscible displacement experiments, J. Emwon. Quo/., 26. 1090-1100,1997. To&he, K.U., Scheibke, R., Kdgel-Knabner, I., and Huwe B., A computer-controlled soil column system for saturated/unsaturated and steady state/transient miscible displacement experiments, Eur. J. Soil .%I., submitted, 1998.
185
Pergamon
0079-1946/98 $19.00 + 0.00 PII: SOO79-1946(98)00010-X
Influence of Dissolved and Colloidal Phase Humic Substances on the Transport of Hydrophobic Organic Contaminants in Soils I.
Received
25 April 1997; accepted
15 December
Abstract. Dissolved and colloidal size organic matter (DOM) controls the mobility of hydrophobic organic chemicals (HOC) in soils by affecting the sorptive interaction between the soil matrix and the solution phase. The association between HOC and DOM in the soil solution leads to an increase in the water solubility of HOC. DOM is a reactive component of the soil solution with respect to the immobile solid phase. Therefore the overall mobility of HOC in soils is enhanced due to co-transport with DOM as the mobile carrier or reduced due to cosorption or cumulative sorption. The specific processes relevant to the DOM-mediated fate of HOC in natural and contaminated soils are discussed, with special consideration to the effect of (i) soil physicochemical parameters (ionic strength, composition, pH), (ii) DOM of different origin, and (iii) aging of a contamination on HOC release. Q 1998 Elsevier Science Ltd.
1997
The controlling process is a partition between the soil matrix and the solution phase, where the partition equilibrium is dominated by the solid phase. In recent years growing attention has been given to the effect of mobile sorbents, i.e., dissolved or colloidal-size aqueous phase components, on the behaviour of polycyclic aromatic hydrocarbons (PAHs) and other hydrophobic pollutants in soils and sediments (Means et al., 1978; McCarthy and Zachara, 1989). Several types of materials were identified as mobile sorbents and shown to increase the water solubility of organic and inorganic pollutants: inorganic colloids, such as clay and silt minerals or iron oxides, and mobile organic colloids (dissolved organic matter, DOM) (Short et al., 1988; Nakayama et al., 1986; Chiou, 1989; McCarthy and Zachara, 1989). Contaminant mobility is affected by processes controlling the total water solubility, i.e., the aqueous-phase concentration of the free and of all mobile sorbent associated contaminants. The interaction of a contaminant with the mobile sorbent reduces the sorption to the solid phase, resulting in increased contaminant mobility (McCarthy and Zachara, 1989). DOM has been shown to specifically enhance the mobility of hydrophobic organic contaminants (HOC) in aquifers and soils. These effects may be due to the strong atlinity of hydrophobic chemicals to DOM. Thermodynamic studies with humic acids as organic sorbents suggest that this process is driven by entropy (Kile and Chiou, 1989; Jota and Hassett, 1991). The mobility of DOM in natural systems, which depends largely on its molecular size (McCarthy et
1 Introduction The role of soils as filters and chemical reactors is beginning to dominate our conception of how to manage soil and groundwater resources to meet quality and quantity aims. For this purpose we have to identity the relevant soil physical and chemical processes within the unsaturated zone. The fate of hydrophobic organic chemicals (HOC) in soils is predominantly determined by the interactions with soil organic matter. HOC accumulation is found in soil horizons high in organic matter (0 / A horizons). This is the consequence of hydrophobic interactions of HOC with the organic phase of natural soils and / or with soot, tar or coal admixtures in contaminated soils. Correspondence to: I. K(lgel-Knabner 179
I.
180
Kogel-Knabner and K. U. Totsche
al., 1993) is thought to have a decisive infhience on of hydrophobic contaminants the transport (McCarthy and Zachara, 1989). Column experiments confirmed that organic macromolecules can facilitate transport of hydrophobic compounds in aquifers (Dunnivant et al., 1992a; Johnson and Amy, 1995; Magee et al., 1991). However, transport in mineral soil horizons most frequently takes place under unsaturated rather than saturated flow regimes. Observations of high PAH contents in subsoil have been interpreted as being due to PAHs interacting with DOM as a mobile carrier (Jones et al., 1989; Deschauer et al., 1994). In soils DOM can be immobilized in significant amounts (David and Vance, 1991; Dunnivant et al., 1992b; Guggenberger and Zech, 1993). DOM sorption in soils was described by Leenheer (1981), Schnitzer (1986) and Jardine et al. (1989), who suggested different binding mechanisms, e.g., physisorption or partitioning (driven by favorable entropy changes) or electrostatic interactions (anion exchange).
2
Table 1: Log KDOC values obtained from literature for the partition of PAH to dissolved organic substances from different origin. Data obtained by (A) reversed phase separation, (B) fluorescence quenching, and (C) dialysis methods (from Raber et al., 1997). PAH
log KDOC
M&Xi
Type and origin of dissolved organic substances
Reference
phenanthrene
4.1 - 4.6 4.6 c4.1 -
A B B B B B B B B B B B B
DOM (soil, organic layer) DOM (mineral soil) DOM (groundwater) humic acid (soil) DOM (sod. organic layer) DOM (mineral soil) fulvic acid (soil) fulvic acid (soil) fulvic acid (stream) DOM (marine sediment) humic acid (soil) humic acid (soil) humic acid (stream)
Raber et al. (1997) Magee et al. (1991) Backhus and Gschwend (1990) Gauthier et al. (1986) Raber et al. (1997) Herbert et al. (1993) Herbert et al. (1993) Gauthier et al. (1986) Schlautman and Morgan (1993) Chin and Gschwend (1992) Gauthier et al. (1986) Herbert et al. (1993) Schlautman and Morgan (1993)
b=oIelPyrene
4.0-4.3 4.5 4.7 5.2
A A A A
DOM (mineral soil) DOM (mineral soil) DOM (mineral soil) humic acid (Aldrich)
Raber et ai. (1997) Maxin and K~gel-Knabner (1995) Raber and Kagel-Knabner ( 1997) Maxin and Kbgel-Knabner (1995)
benzo[a]pyrene
4.2- 5.0 5.1 4.6-6.0 5.3 5.2 6.3 6.3
A C C A A C C
DOM (surface water) DOM (stream) DOM (surface /groundwater) humic acid (Aldrich) humic acid (Aldrich) humic acid (Aldrich) humic acid (Aldrich)
Morehead et al. (1986) Kukonnen et al. (1990) McCarthy et al. (1989) Morehead et al. (1986) Landrum et al. (1984) McCarthy et al. (1989) McCarthy and Jim&z (1985)
benzo[k]fluoranthene
4.6 _ 4.7 4.6 - 4.7 5.0 5.1
A A A A
DOM (mineral soil) DOM (mineral soil) DOM (mineral soil) humic acid (Aldrich)
Raber et sl. (1997) Maxin and KOgel-Knabner (1995) Raber and Kfigel-Knabner (1997) Maxin and KBgel-Knabner (1995)
benzo[g,h,i]perylene
4.9 - 5.0 5.5 5.8
A 12 A
DOM (mineral soil) DOM (mineral aoil) humic acid (Aldrich)
Maxin and Kbgel-Knabner (1995) Raber and KdgelXnnlx~er (1997) Maxin and KGgel-Knabner (1995)
Influence of Dissolved and Colloidal Phase Humic Substances The partition coefficient for the binding of a distinct PAH compound to soil DOM can vary more than one order of magnitude, depending on the source of DOM. K~oc values for DOM from mineral arable soils are markedly lower compared to DOM from acid forest soils. The hydrophobic and hydrophilic composition of DOM depends on the origin of the soil material. DOM from mineral soils under agricultural use contains higher proportions of hydrophobic components compared to DOM from acid forest floor materials. This partially explains the differences in binding capacity for PAH (Raber et al., 1997). More DOC can be released from composts and sewage sludges than the DOC content of the soil solution, and therefore the amount of hydrophobic compounds in the soil solution can be increased considerably when these materials are added to soil (Raber and Kbgel-Knabner, 1997). The binding capacities of DOM from composts were found to be similar or slightly less than those from soil. The sorption of PAHs to DOM obtained from sewage sludges was less and varied considerably for different types of sludge treatment. 3 Desorption of PAHs from soils The presence of dissolved organic matter (DOM) in the soil solution has an enhancing effect on the desorption of PAH (Fig. 1). Benzo[a]pyrene
in solution
Desorption linearly increased with increasing DOM concentrations up to >lOOO mg L-l. Partition coefficients (log Koc) for the desorption of “C-PAHs are 4.2 for pyrene and 5.0 for benzo[a]pyrene in the presence of DOM from plant waste compost (200 mg C L-l). These values are about 3.5 (pyrene) and 25 (benzo[a]pyrene) times lower than in the aqueous control solutions of similar ionic strength. The enhancement of PAH desorption between various types of DOM, from composts and waste disposal site leachates, seems to be influenced by the molecular weight distribution of DOM (Raber and KogelKnabner, 1998). The composition of the soil solution has a decisive influence on the sorption and desorption of PAH in soils. Figure 2 and Table 2 show the desorption isotherms of benzo(a)pyrene and pyrene from soil in the presence of different soil solutions. The desorption of “C-benzo[a]pyrene and 14C-pyrene is strongly influenced by the properties of the aqueous phase. Generally the desorption coefficients are similar to the sorption coefficients Koc for these compounds. Whereas the presence of DOM has an enhancing effect on PAH desorption, high concentrations of Cat& lead to a reduced desorption (salting out effect). PAH sorbed (pg kg-‘)
5.m
.._.
pglL . ..*
plant waste 1 -.. ..‘-I
domestic waste 1
..... .‘..- domestic waste 2 1
~ _..,..... ./‘-
200 .. . . . . .. . . . . . . . . . . . . . . . l . . . . . . . . ,@) ..f
t .. . .. ..
05
isotherms pyrene)
composed of bidistilled
250
500 DOM
soil (artificially
presence of varying
f
750
’
,
8.
1000
from the Ap horizon of an
contaminated with 1.91 mg kg-l) in the
DGC concentrations
types of compost material.
1
5
10
50
for the desorption
water
(200 mg L-l, concentration
l,
of %-PAH
soil (Ap, Gleyic
in the presence of a soil solution 2.8 mM CaCll
0, and DOM m
of CaC12 2.8 mM); from Raber and
Kbgel-Knabner (1998);
mgC L”
Fig. 1. Desorption of “C-benzo(a)pyrene agricultural
,
~...~.~..
..~.. ~..~
from a mineral
Cambisol, artificially contaminated)
’
~. ~...~
c _.
0,050.1
Fig. 2. Desorption (benzo(a)pyrene----,
t
.
PAH in solution (pg L-l)
.,,,,.... *...“‘”
,
~. ,.._ .~... /
0,Ol
,,,,~ .,_... +..,.‘I
0
181
obtained
from different
The error bars represent the standard
deviation of triplicates; from Raber and KOgel-Knabner (1998);
Another factor controlling the desorption of PAHs from soil is the time elapsed since contamination. Table 3 shows the effect of aging of a contamination on the desorption of benzo(a)pyrene in the presence of different soil solutions.
I. Kogel-Knabner and K. U. Totsche
182
Table 2. LIesorption of 14C-benzo(a)pyrene from mineral soil material (Ap, Gleyic Cambisol) with DOM (200 mg L-l), regression (r2) and slope (+) of isotherm. Control experiments with C&l2 in the same ionic strength without addition of DOM (from Raber and K~gelKhmer, 1997). DOM Control
KOC
log Koc
(L kg-‘)
(Lkg-‘)
0.994 0.995 0.999
92930 113962 2458045
4.97 5.06 6.39
0.999 0.992
103549 134969
5.02 5.13
0.993 0.989 0.963
169959 887141 2378643
5.93 5.95 6.38
?
erperiments
compost plant waste compost 1 plant waste compost 2 2.8 mM C&l domestic domestic seepage domestic industrial
waste compost 1 waste compost 2 water ofwaste dqmal waste storage waste storage
2
site
10.3 mM CaCI_
Again, an effect of the soil solution composition on the desorption is found, similar to the previous experiments. In the first 36 days following a contamination a decrease of the PAH concentration in the solution phase was observed, indicated by the increasing partitioning coeffkient Koo Monitoring the contamination for up to 183 days showed no further decrease in the fraction that can be desorbed,
suggesting a two step process. An “easily” exchangeable, surface bound fraction of the PAH compounds has reached an equilibrium already after 1 month. The further alteration of the contamination, which may be due to intra-organic matter diffusion processes, does not change the “easily” exchangeable fraction of the compound.
Table 3. Parameters for the desorption isotherms for desorption of ‘k%enzo(a)pyrene
at different times after contamination
different soil solutions (similar to the experiment described in Fig. 2); from Raber and K6gelXnabner Age of contamhtion
(days)
b
r2
exchange solution a 1 HZ?
Kd (W)
(1998). log&
%C
in the presence of
log SE b
(Lw)
-54.6
0.992
2317
236461
5.37
0.04
17.0
0.999
11177
1140544
6.06
0.04
plant waste compost 1
-84.0
0.983
737
75175
4.88
0.05
8 H20.
-10.1
0.997
2645
268851
5.43
0.05
2.8 mM C&l2
71.4
0.994
12599
1285575
6.11
0.06
plant waste compost 1
-68.5
0.984
761
77672
4.89
0.04
36 H20.
-49.9
0.997
3333
340131
5.53
0.03
2.8 mM CaC12
81.4
0.990
25366
2588358
6.41
0.03
plant waste compost I
-81.4
0.972
986
100582
5.00
0.03
89 H20.
-70.3
0.995
3431
350055
5.54
0.05
2.8 mM CaC12
-14.6
0.973
24089
2458045
6.39
0.04
plant waste compost 1
-79.6
0.984
998
101829
5.01
0.04
183 HZ?
-35.3
0.998
3421
349125
5.54
0.05
2.8 mM CatI2
32. I
1.000
24389
2488647
6.40
0.04
plant waste compost 1
44.1
0.997
1050
107192
5.03
0.04
2.8 mM C&l2
a DOM normalized to 200 mg C/L, addition of C&l2 error of means; deviation of J&
in the Same ionic strength as in the experiments in tbe presence of DOM.
b SE = standard
Influence
of Dissolved and Colloidal Phase Humic Substances
4 PAH mobility in the presence of DOM: soil column experiments A series of experiments was carried out to determine the breakthrough of DOM, PAHs and PAHs in the presence of DOM. All miscible displacement experiments were performed employing a laboratory soil column system specifically designed for experiments with hydrophobic substances (To&he et al., 1998). All experiments were conducted under unsaturated, low convection-dominated flow
183
conditions. Natural DOM was used in concentrations typically observed in mineral soil solutions. The soil materials were not pre-equilibrated with DOM, thus allowing for sorptive interactions with the bulk material for both DOM and PAHs. Miscible displacement experiments were carried out with DOM alone (Fig. 3) with PAHs alone (anthracene, pyrene, benzo(e)pyrene), and with a mixture of the PAHs and DOM (Fig. 4). Two different sandy materials were used, a spodic B horizon and a commercially available seasand. l-
*
DOM Ilobs
n
DOM II fitted D0Mlob.s DOM I fitted
--..
PV Fig. 3. Breakthrough of DOM through spodic B material. (left) reduced concentration of DOM and chloride. (O/m) DOM I/II. (O/O)
C1’ in
experiment DOM I/11(), (right) scaled breakthrough of the mobile fraction of DOM (from Totsche et al., 1997).
DOM transport (Fig. 3) can be understood by assuming that DOM is composed of at least two physicochemical different fractions: A mobile fraction, composed of the hydrophilic moieties of DOM, and an immobile fraction, composed of the hydrophobic moieties of DOM. The mobile fraction
1
1
anthracene A without DOM
0.8
0.3
0.2
A with DOM
0.6
$
shows a transport behavior comparable to that of very low-reactive tracers illustrated by the similar dispersion lengths and retardation parameters as chloride. The immobile fraction is sorbed completely by the spodic B material.
pyrene * without DOM * with DOM
P o
0.4
0.1
0.2 0 0, 0
100
200
300
400
500
600
PV Fig. 4. Bnxkthrougb ofanthracene and pyreoe through spodic B material in the absence and presence of DOM; (from Totsche et al., 1997)
184
I. Kbgel-Knabner and K. U. Totsche The breakthrough behaviour of PAHs (Fig. 4) under conditions typical for soil environments is dominated by the complex interaction of DOM with the PAHs and the bulk soil material. In contrast to studies referring to aquifer environments, DOM-mediated transport of PAHs in our experiments does not result in increased but in reduced mobility (Totsche et al., 1997). For both sandy materials, the presence of DOM resulted in (i) reduced PAH effluent concentration, (ii) reduced PAH mobility and (iii) an increased tailing of the PAR breakthrough curve. The DOM-mediated retention of PAHs can be explained by two different scenarios. Co-sorption describes the sorption of the DOM-PAH associate to the bulk phase. Cumulative sorption results from increased sorptive capacity of the bulk phase due to sorption of DOM and thus increased OC content. The differentiation between both retention processes is essential for the estimation of experimental mobility parameters, such as Kd values and sorption isotherm parameters. Results obtained from experiments representing aquifer conditions do not necessarily cover the flow conditions and sorptive properties for DOM and PAHs in unsaturated soil materials. The experimental conditions used for such column experiments are decisive for the observation of enhanced or reduced mobility of HOC in the presence of DOM. The estimation of PAH transport behaviour in soils has to take into account the controlling effect of the bulk soil and solution properties, controlling the dissipation of PAHs in soils. Acknowledgments. Financial support is acknowledged from the Deutsche Forschungsgemeinscha!? (Ko 1035/6, 103517, 1035/8) and the EU environment and climate programme (EVSV-CT94-0536).
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2563,1988. Totsche, K.U., K&gel-Knabner, I, and Danzer J., Dissolved organic matter-enhanced retention of polycyclic aromatic hydrocarbons in soil miscible displacement experiments, J. Emwon. Quo/., 26. 1090-1100,1997. To&he, K.U., Scheibke, R., Kdgel-Knabner, I., and Huwe B., A computer-controlled soil column system for saturated/unsaturated and steady state/transient miscible displacement experiments, Eur. J. Soil .%I., submitted, 1998.
185