Pergamon Phys. Chem. Eurrh, Vol. 23, No. 2, pp. 179-185, 1998

0 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain

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 1997

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 co- sorption 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 physico- chemical parameters (ionic strength, composition, pH), (ii) DOM of different origin, and (iii) aging of a contamination on HOC release.

Q 1998 Elsevier Science Ltd.

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 ad-

mixtures in contaminated soils.

Correspondence to: I. K(lgel-Knabner

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

179

180 I. Kogel-Knabner and K. U. Totsche

al., 1993) is thought to have a decisive infhience on

the transport of hydrophobic contaminants

(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 -<S.l 4.7

4.6 _ 5.0 3.6 4.0 4.7 - 5.1 4.2 4.8 - 5.2 5.2 5.2 4.5

b=oIelPyrene 4.0 - 4.3 4.5 4.7 5.2

benzo[a]pyrene 4.2 - 5.0 5.1 4.6 - 6.0 5.3 5.2 6.3 6.3

benzo[k]fluoranthene 4.6 _ 4.7 4.6 - 4.7 5.0 5.1

benzo[g,h,i]perylene 4.9 - 5.0 5.5 5.8

A B B B

B B B B B B B B B

A A A A

A C C A A C C

A A A A

A 12 A

DOM (soil, organic layer) Raber et al. (1997) DOM (mineral soil) Magee et al. (1991) DOM (groundwater) Backhus and Gschwend (1990) humic acid (soil) Gauthier et al. (1986)

DOM (sod. organic layer) Raber et al. (1997) DOM (mineral soil) Herbert et al. (1993) fulvic acid (soil) Herbert et al. (1993) fulvic acid (soil) Gauthier et al. (1986) fulvic acid (stream) Schlautman and Morgan (1993) DOM (marine sediment) Chin and Gschwend (1992) humic acid (soil) Gauthier et al. (1986) humic acid (soil) Herbert et al. (1993) humic acid (stream) Schlautman and Morgan (1993)

DOM (mineral soil) DOM (mineral soil) DOM (mineral soil) humic acid (Aldrich)

DOM (surface water) DOM (stream) DOM (surface /groundwater) humic acid (Aldrich) humic acid (Aldrich) humic acid (Aldrich) humic acid (Aldrich)

DOM (mineral soil) DOM (mineral soil) DOM (mineral soil) humic acid (Aldrich)

DOM (mineral soil) DOM (mineral aoil) humic acid (Aldrich)

Raber et ai. (1997) Maxin and K~gel-Knabner (1995) Raber and Kagel-Knabner ( 1997) Maxin and Kbgel-Knabner (1995)

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)

Raber et sl. (1997) Maxin and KOgel-Knabner (1995) Raber and Kfigel-Knabner (1997) Maxin and KBgel-Knabner (1995)

Maxin and Kbgel-Knabner (1995) Raber and KdgelXnnlx~er (1997) Maxin and KGgel-Knabner (1995)

Influence of Dissolved and Colloidal Phase Humic Substances 181

The partition coefficient for the binding of a Desorption linearly increased with increasing DOM

distinct PAH compound to soil DOM can vary more concentrations up to >lOOO mg L-l. Partition

than one order of magnitude, depending on the coefficients (log Koc) for the desorption of “C-PAHs

source of DOM. K~oc values for DOM from mineral are 4.2 for pyrene and 5.0 for benzo[a]pyrene in the

arable soils are markedly lower compared to DOM presence of DOM from plant waste compost (200 mg

from acid forest soils. The hydrophobic and C L-l). These values are about 3.5 (pyrene) and 25

hydrophilic composition of DOM depends on the (benzo[a]pyrene) times lower than in the aqueous

origin of the soil material. DOM from mineral soils control solutions of similar ionic strength. The

under agricultural use contains higher proportions of enhancement of PAH desorption between various

hydrophobic components compared to DOM from types of DOM, from composts and waste disposal site

acid forest floor materials. This partially explains the leachates, seems to be influenced by the molecular

differences in binding capacity for PAH (Raber et al., weight distribution of DOM (Raber and Kogel-

1997). Knabner, 1998).

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 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).

The presence of dissolved organic matter (DOM) in

the soil solution has an enhancing effect on the

desorption of PAH (Fig. 1).

PAH sorbed (pg kg-‘) 5.m .._.

Benzo[a]pyrene in solution pglL

plant waste 1

-.. -I

..‘- domestic waste 1

..... 1

.‘. .- domestic waste 2

.,_... +..,.‘I

.,,,,.... *...“‘”

~ _..,..... ./‘-

,,,, ~

, t ’ , f ’ , 8.

0 250 500 750 1000

DOM mgC L”

. ..* ~. ,.._ .~...

/

. ~. ~...~

200 . . . . . . . . . . . . . . . . . . . . . . . l . . . . . . . . c ~...~.~..

,@) ..f t . . . . . . . _. ..~.. ~..~

0,Ol 0,05 0.1 05 1 5 10 50

PAH in solution (pg L-l)

Fig. 2. Desorption isotherms for the desorption of %-PAH

(benzo(a)pyrene----, pyrene) from a mineral soil (Ap, Gleyic

Cambisol, artificially contaminated) in the presence of a soil solution

composed of bidistilled water l , 2.8 mM CaCll 0, and DOM m

(200 mg L-l, concentration of CaC12 2.8 mM); from Raber and

Kbgel-Knabner (1998);

Fig. 1. Desorption of “C-benzo(a)pyrene from the Ap horizon of an

agricultural soil (artificially contaminated with 1.91 mg kg-l) in the

presence of varying DGC concentrations obtained from different

types of compost material. 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.

182 I. Kogel-Knabner and K. U. Totsche

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~gel-

Khmer, 1997).

DOM KOC log Koc

Control erperiments ?

(L kg-‘) (Lkg-‘)

compost plant waste compost 1 plant waste compost 2

2.8 mM C&l 2

domestic waste compost 1 domestic waste compost 2 seepage water ofwaste dqmal site domestic waste storage industrial waste storage

10.3 mM CaCI_

0.994 92930 4.97 0.995 113962 5.06 0.999 2458045 6.39

0.999 103549 5.02 0.992 134969 5.13

0.993 169959 5.93 0.989 887141 5.95 0.963 2378643 6.38

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 in the presence of

different soil solutions (similar to the experiment described in Fig. 2); from Raber and K6gelXnabner (1998).

Age of contamhtion (days) b r2 Kd %C log& log SE b

exchange solution a (W) (Lw)

1 HZ? -54.6 0.992 2317 236461 5.37 0.04

2.8 mM C&l2 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

a DOM normalized to 200 mg C/L, addition of C&l2 in the Same ionic strength as in the experiments in tbe presence of DOM. b SE = standard

error of means; deviation of J&

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

PV

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

- DOM II fitted

n D0Mlob.s

--.. DOM I fitted

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

0.8

0.6

$ 0.4

0.2

0,

anthracene A without DOM A with DOM

1

0 100 200 300 400 500 600

PV

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.

0.3

0.2

P

o 0.1

0

pyrene * without DOM * with DOM

183

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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185