HEAVY METAL BINDING BY H Y D R O P H O B I C AND H Y D R O P H I L I C

D I S S O L V E D ORGANIC CARBON FRACTIONS IN A S P O D O S O L A

AND B H O R I Z O N

GEORG G U G G E N B E R G E R * , BRUNO GLASER, and W O L F G A N G ZECH

Institute of Soil Science and Soil Geography, University of Bayreuth, 95440 Bayreuth, Germany

(Received November 6, 1992; accepted February 9, 1993)

Abstract. For a one year period intact Spodosol soil columns were percolated weekly with H2Odeion, 1.58 mmol H2SO 4 L -~, and 0.79 mmol H2SO 4 L -1 + 0.64 mmol HNO3 L -1, respectively, Decompo- sition rates, soil organic carbon (OC) solubilization, dissolved organic carbon (DOC) fractions, and Cr-, Cu-, and Cd-binding by dissolved hydrophobic and hydrophilic acids were studied. Acid treat- ment reduced significantly OC respiration as well as OC solubilization in the humic layers. The re- duced OC solubility at acid addition was more pronounced for the less polar hydrophobic com- pounds, resulting in a decrease of the hydrophobie acids (from ca. 65 to 40-45% of DOC), and in an increase of the hydrophilic acids (from ca. 25 to 40-45% of DOC). For B horizon leachates, DOC increased at acid treatment. Generally, hydrophobic acids were retained preferentially in the B hori- zon. Also in the B horizon output there was an increase of the hydrophilic acids as acidity increased (from ca. 40 to 50% of DOC). Differences between the two acid treatments were negligible. The degree of metal-organic complexes decreased in the order Cr > Cu > Cd, from A to B horizon leachates, and with increasing acidity. Hydrophilic acids were found to be the dominating ligands in complexing Cr and Cu. Actual Cr- and Cu-binding by hydrophilic acids exceeded that by hydrophobic acids 2-8 times. As the hydrophilic acids represented the most mobile DOC components in the soil columns, in particular with increasing acidity, significant amounts of Cr and Cu in the B horizon leachates were organically complexed, although a great proportion of the hydrophobic acids was retained in the B horizon.

1. Introduction

In the last decades forest soils of industrialized countries have been increasingly contaminated with heavy metals. Some heavy metals like copper and lead are highly selectively bonded by organic carbon (OC) (McBride, 1989), which results in a pronounced accumulation of heavy metals in the forest floor. Ruppert (1986) for instance reported that nearly all of the Cd and Pb and a part of Zn, Ni, and Cu in the humic layers of soils of the Fichtelgebirge (NE Bavaria, Germany) originate from anthropogenic immissions. The mobilization, transport and speciation of trace metals in the soil solution is controlled by the interaction of the metals with protons, inorganic anions, organic ligands, redox potential, surfaces, and organisms. The knowledge of the chemical speciation is essential for understanding transport mechanisms and biological interactions of metals in soils and natural waters. As environmental acidification strongly promotes the mobilization of most heavy metals, it has been the objective of many studies (e.g. Tyler, 1981; Bergkvist, 1986). The

* Corresponding author.

Water, Air, and Soil Pollution 72:111-127, 1994. �9 1994 Kluwer Academic Publishers, Printed in the Netherlands.

112 GEORG GUGGENBERGER ET AL.

important role of dissolved organic carbon (DOC) as ligands for metals has been reviewed by Stevenson and Fitch (1986).

Among the different techniques for speciation of heavy metals in natural waters, cation exchange fractionation is one of the most widely used (Berggren, 1989). Hiraide et al. (1987) introduced the use of a nonionic Amberlite XAD-2 resin in the separation of heavy metals complexed by humic substances from ionic species. Generally, it has been found that mobilization of Pb, Cr, Cu, Ni, and V in organic horizons is favoured under conditions of high DOC concentrations, whereas Zn and Cd are more susceptible to pH changes (Tyler, 1981; Bergkvist, 1986; K6nig et al., 1986).

The studies of Pohlman and McColl (1988) and Tamm and McColl (1990) showed that structural composition of the organic molecules has a profound influence on the metal-binding affinity of DOC. Recently, Kuiters and Mulder (1992) studied Cu-binding by different molecular weight fractions of DOC obtained by gel permeation chromatography. In order to obtain physico-chemically different DOC fractions, Leenheer (1981) developed a fractionation and isolation method that separates DOC into hydrophobic and hydrophilic acids, bases, and neutrals. This method can be used for a further characterization of specific properties of DOC components after isolation of the dominant fractions. Vance and David (1989) and David et aL (1989) reported a variable distribution of the DOC fractions in dependence of the soil horizon and soil acidification. They concluded that this alteration of DOC will have significant consequences for the solubilization and transport of metals.

The purpose of this paper is to provide information on the response of OC dynamics, DOC composition, and heavy metal speciation on acidic precipitation. The metals included in this study were Cu, Cr, and Cd. Specific objectives of this study were (i) to determine the effects of dilute acid addition on CO2 production; (ii) to study the response of OC solubilisation and DOC transport in Spodosol soil columns on acid treatment; (iii) to investigate the DOC composition in terms of DOC fractionation of A and B horizons solutions as affected by increasing acidity; and (iv) to examine the heavy metal binding by hydrophobic and hydrophilic DOC fractions.

2. Material and Methods

2.1. SAMPLES

The soil examined in this study consisted of an Entic Haplorthod collected at the Oberwarmensteinach Norway spruce experimental forest located in the Fichtel- gebirge (NE Bavaria, Germany). The Oberwarmensteinach forest research site has an elevation of 750 m a.s.1.; annual precipitation varies around 1130 ram; and total deposition of protons and sulfate amounts to 1.96 kmolc ha -1 yr -1 and 2.67 kmolc ha -1 yr -1, respectively (Tiirk, 1992). The experimental plot received intensive

HEAVY METAL BINDING BY DOC FRACTIONS

TABLE I

Some physical and chemical characteristics of the soil samples

113

Bulk Horizon Depth Texture density PHcaCl 2 OC Nto t

(cm) (g cm -3) (g kg -1) (g kg -1)

Cation exchange capacity (mmol c kg -1)

O 9- 0 - 0.15 3.2 441 15.8 n.d. Abe 0-18 loamy 1.20 3.2 17 0.9 86

sandy silt Bhs 18-38 loamy 1.15 3.8 15 0.8 66

sandy silt Bv 38-54 loamy 1.44 4.2 6 0.6 53

sandy silt

attention because of the sensitivity of this soil to acid precipitation (Schulze et al., 1989). Some soil characteristics are given in Table I.

Intact soil cores were removed from the field in August 1990. This was done by pushing 19 cm wide polymetylmethacrylate pipes over manually prepared soil columns. On the bottom the columns were equipped with porous P 52 ceramic suction plates. To ensure contact of the soil cores with the suction plates and to minimize marginal effects at the pipe walls, quartz powder (silt fraction) was washed between the soil cores and the suction plates as well as the polymetylmethacrylate pipes. Collection included nine 20 cm (O + A horizon) soil columns, and nine 60 cm (O + A + B horizon) soil columns. In the laboratory a water potential o f - 6 0 cm was installed by using a hanging water column. The temperature of the microcosms was set between 2 ~ (winter) and 15 ~ (summer). Prior to sampling the soil cores, ceramic suction plates had been equilibrated for one week with 0.1 M HC1 and for three months with soil solution sampled in the field.

2.2. EXPERIMENTAL DESIGN

From October 1990 to October 1991 the microcosms were subjected to weekly 35 mm storm events. Nine O + A horizon soil columns and nine O + A + B hori- zon soil columns were distributed at random into the three treatment groups:

- precipitation with H2Odeion, - precipitation with 1.58 mmol H 2 S O 4 L -1, equiv, to ca 60 kmolc H + ha -1 yr -1, - and precipitation with 0.79 mmol H z S O 4 L -1 and 0.64 mmol HNO3 L-l; equiv.

to ca 42 kmol c H + ha -1 yr -~, respectively. The samples were watered on Fridays and allowed to drain over the weekend. On Mondays, the column leachates were silver-membrane filtered (0.45 ~m), and analyzed for anions (SO42-, NO3-, C1-), cations (Ca 2+, Mg 2+, K +, Na+),

total Fe, A1, NH4 +, pH, and DOC. Samples were stored at 2 ~ in the dark for a maximum of two weeks. Additionally, CO2 production in the soil columns was measured cumulatively by reaction with soda-lime (Edwards, 1982). Only CO2, DOC,

114 GEORG GUGGENBERGER ET AL.

TABLE II

General composition and chemical properties of hydrophobic and hydrophilic DOC fractions (adapted from Leenheer, 1981; Vance and David, 199 la; Guggenberger and Zech (unpublished data))"

DOC fraction Structural composition Chemical properties

hydrophobic acids

neutrals

bases

hydrophilic acids

neutrals

bases

lignocellulose degradation products; mixture of complex polyfunctional aliphatic and aromatic acids

non-carbohydrate aliphatics; carbonyl compounds

polynuclear amines, ethers and quinones

lignocellulose degradation products; mixture of complex polyfunctional aliphatic and aromatic acids; more carboxyl groups than hydrophobic acids

carbohydrates; polyfunctional alcohols

amphoteric proteinaceous compounds; amino acids and sugars

exchange acidity 8.5-11.8 mmol c g-1 C; pH dependent charge density

exchange acidity 10.6-14.3 mmol c g-1 C; pH dependent charge density

and pH data are reported here. Leachate solutions were analyzed weekly for DOC using a Dohrmann DC-90

TOC analyzer. In a two-weekly sequence DOC-fractionation analyses using XAD- 8, cation (AG-MP 50), and anion (Duolite A-7) exchange resins were performed (Leenheer, 1981; Vance and David, 1991a). A general description of the different types of organic constituents in the DOC fractions is given in Table II.

2.3. HEAVY METAL SPECIATION PROCEDURE

Also every two weeks the heavy metals Cr, Cu, and Cd were analyzed on a Varian SpectrAA 400 equipped with a Zeeman Graphite Furnace-AAS. Heavy metals were speciated by using a combination of the low-energy physical sorption procedure given by Hiraide et aL (1987), and the cation exchange procedure (e.g. Driscoll, 1984). Heavy metals complexed by hydrophobic acids were removed from solution by Amberlite XAD-8 absorber resin (Hiraide et aL, 1987), whereas the AG-MP 50 cation exchange resin bound flee and labile bonded metal species (Driscoll, 1984; K6nig et al., 1986). Hence, the solution in the effluent of both resins contained heavy metals complexed by hydrophilic substances.

Resin pre-treatment and re-conditioning were conducted according to Leenheer (1981). Briefly, XAD-8 resin was washed with 0.1 N NaOH, soxhlet-extracted with acetone and hexane 24 h each, and slurried in methanol. Prior each run XAD- 8 resin was re-conditioned with subsequent percolation of H2Odeion, methanol, H2Odeion, 0.1 M HC1, H2Odeion, 0.1 M NaOH, and H2Odeio n. AG-MP 50 cation

HEAVY ME T A L BINDING BY DOC FRACTIONS 115

exchange resin was also soxhlet-extracted with acetone, and slurried in H2Odeio n. Re-conditioning included percolation of H2Odeion, 1 M NaOH, H2Odeion, 1 M HC1, and H2Odeio n. Each resin was filled into glass columns (100 • 7 mm) which were directly connected. The samples were passed through the resins using a peristaltic pump, analogous to the DOC fractionation procedure. Application rate was adjusted to 2 mL min -1. To avoid sorption of cationic heavy metal species on polar impurities of the XAD-8 adsorber resin, cation-exchange sites were saturated by percolation of an indium solution (10 mg In L -1) ove r the resin (Hiraide et al., 1987). All vessels, bottles and tubes were pretreated with dilute HNO3 and with the indium solution to avoid contamination.

Sorption of hydrophobic acids by XAD-8 is quantitative at pH 2, but dissociation of metal-hydrophobic acid complexes is high at such a low pH. At original sample pH dissociation of the metal-hydrophobic acids complexes is minimized, but sorption of hydrophobic acids is low. Therefore, the heavy metal speciation was carried out at pH 2 and at original solution pH so that the following binding forms could be differentiated: - ionic, inorganic and very labile organically complexed (retained by the cation

exchange resin at original sample pH); - labile bonded to hydrophobic and hydrophilic acids (retained by the cation

exchange resin at pH 2 but not at original sample pH); - stable bonded to hydrophobic acids (retained by XAD-8 resin at pH 2); - stable bonded to hydrophilic acids (effluent at pH 2).

It must be emphasized that heavy metal speciation by cation exchange and absorber resins strongly depends on the operational procedure, i.e. Na + or H + resin, pH, application rate (Berggren, 1989). In particular, at a application rate of 2 mL min -1 organic complexation tends to be underestimated due to dissociation of the very labile organic complexes during their passage through the cation exchange resin (Driscoll, 1984; K6nig et al., 1986). Therefore, this study is mainly a comparative investigation of metal complexation.

3. Results and Discussion

3.1. OC MINERALIZATION

Cumulative soil respiration was determined for the periods 11/90-2/91, 3/91-5/91, and 6/9 I - 9 / 9 I, respectively (Table Ill). It is obvious that OC mineralization increases with increasing temperature from winter (ca. 2 ~ over springtime (ca. 8 ~ to summer (ca. 15 ~ There are no significant (p~0.05) differences in CO2 release between the O + A horizon columns and the O + A + B horizon columns of each treatment, indicating that mineralization occurs primarily in the O and A horizons. Both acid treatments result in significant (p~0.001) lower respiration rates than the H2Odeion treatment, indicating a reduced biotic mineralization of organic carbon.

These results are in accordance with data of a long-term field experiment in

116 GEORG GUGGENBERGER ET AL.

TABLE Ill

Cumulative CO z production in the soil columns; mean values with standard deviations are reported

Soil horizons Treatment 11/90-2/91 3/91-5/91 6/91-9/91 COz output

- - g CO2 m -2 d -l (g C column -1 yr -1)

O +A horizon H2Odeio n 0.9_+0.1 1 .4- -0 .1 4.3_+0.5 3.9+0.4 H2SO4 0.3_+0.1 0.6• 2.0+0.7 1,7 + 0.5 H2SO4 + HNO 3 0.3_+0.1 0.6+0.3 2.0+0.7 1.7+0.5

O +A +B horizon H2Odeio n 0.7_+0.1 1.3_+0.1 3.8• 3.5_+0.5 H2SO 4 0.4_+0.2 0.6 • 0,5 2.5_+ 1.0 2.1 _+0.8 H2SO4 + HNO 3 0.4_+0.0 0.6_+0.1 2.1_+0.5 1.8-+0.4

the 'H6glwald ' (Southern Bavaria) performanced by Kreutzer and his group. Kreutzer

and Zelles (1986) reported a decrease of the microbial activity after application of dilute sulfuric acid (pH 2.7), and Agerer (1990) found a 55% decrease of the ectomykorrhiza fungi activity at the plots receiving acid precipitation. However,

Cronan (1985) reported that precipitation acidity down to pH 3.5 has no consistent quantitative effect upon soil respiration.

3.2. OC SOLUBILIZATION

Results of the DOC measurements in the column leachates indicate that acid

precipitation has a pronounced influence (p < 0.001) on the OC solubilization in the short microcosms (O + A horizons) (Figure 1, Table IV). DOC concentrations

are highly reduced in both of the acid application experiments, and run parallel with a decline of the pH. According to Hayes and Swift (1978) increasing acidity

decreases the solubility of OC, because electrostatic repulsion between negative charges of dissociated groups is required for dissolution of OC. Replacement of

readily dissociated counterions by H § gives rise to contraction and precipitation of OC by reduction of the electrostatic repulsion, thus reducing the DOC con- centration. Guggenberger and Zech (1993) elucidated that the release of DOC in

the forest floor is also a function of the microbial activity. Hence, percolation of dilute acid decreases OC solubilization also indirectly by the reduction of the

microbial activity as determined by OC mineralization (cf. Table III). Compared with most other studies (e.g. Hay et al., 1985; Vance and David, 1991b)

the effects of acid precipitation on H+-buffering and DOC release are remarkably high. This is probably due to the fact, that the buffering capacity of the forest floor and the Ah horizon of the soil used in this experiment is relatively low caused by the intensive acid deposition the spruce stand received in the last decades (Schulze et al., 1989). Furthermore, applied acid concentrations are rather high in our expe- riment.

The low DOC concentrations in the leachates of the long columns (O + A + B horizons) stress the importance of the B horizons in retaining DOC. Similar results

150

125

I ,_..q 100

75

50 O

25

0

HEAVY METAL BINDING BY DOC FRACTIONS

O+A horizon

�9 H2Odei~ I: �9 HzS04 / iT [ ] H2S04 + HN03 "~"I ~ i

"'..(

t I I I I i ' I

6 12 20 28 35 45 50

Weeks

117

150

- 125

100

75

5O

25

0

O+A+B horizon

I

0

20

16

12

8

4

0

�9 H 2 O d e i o n - _

�9 H2SO 4

�9 . . . e " e ' o . o - e ... ~ r - - , . - - - - - - * * . ; = . ; , . . .o �9 �9 . . . , . . �9

l L i I I I I

0 6 12 20 28 35 43 50

20

16

12

4

Weeks Fig. 1. Time plot of the DOC concentrations in the column leachates; mean values with s tandard

deviations are given.

118 GEORG GUGGENBERGER ET AL.

TABLE IV

pH, DOC concentrations, and annual DOC output in the effluent of the soil columns; mean values with standard deviations are reported

Soil horizons Treatment pH DOC concentration DOC output (mg C L -1) (g C column -1 yr -1)

O + A horizon H2Odeio n 4.7 -+ 0.6 58.6 • 22.8 3.2 + 1.2 n 2 s o 4 3.4_+0.4 13.5-+5.5 0.7+0.3 H2SO 4 + HNO 3 3.9 _+ 0.5 14.5 + 7.3 0.8 + 0.4

0 + A + B horizon H2Odeio n 4.9_+0.5 4.7+1.0 0.3• HzSO4 4.1_+0.5 6.9+2.0 0.4+0.1

H2SO4 + HNO 3 3.9+0.3 7.0_+2.5 0.4_+0.1

were obtained by Jardine et al. (1989) and David et al. (1989). Leaching of DOC from the forest floor and the A horizon and the subsequent retention in the B horizon leads to a calculated annual accumulation of OC in the B horizon from

120 kg ha -1 ( H z S O 4 treatment) to 1010 kg ha -1 (HzOdeio n treatment). The significant (p<0.01) higher DOC concentrations at the acid irrigated treatments compared with the deionized water treatment suggest a reduced affinity of the B horizon for DOC at low pH. Also Vance and David (1989) and Jardine et al. (1989) observed a decline of the DOC retention in B horizons with increasing acidity of the soil solution. This is probably due to the release of metal-organic complexes by proton interactions. H + mediated contraction and precipitation of OC is supposed to be of minor importance, because generally pH increases and ion strength decreases from the O + A horizon leachates to the effluent of the O + A + B horizons. Therefore, OC must overcome a much stronger contractive effect for solubilization

in the O + A horizon as it is exposed in the B horizon.

3.3. DOC FRACTIONATION

The distribution of the DOC fractions in the leachates is given in Figure 2. The leachate of the O + A horizon percolated with deionized water is dominated by hydrophobic acids (ca. 65% of DOC), with hydrophilic acids being the second largest fraction (ca. 25% of DOC). The other fractions are of minor importance. There is a pronounced acid treatment effect on the concentrations of the single DOC fractions in the O + A horizon leachates (Table IV). Acid precipitation results in a decrease of the mean hydrophobic/hydrophil ic acid ratio from 2.37 for the

HEOdeio n treatment to 0.95 for the HzSO4 treatment and to 1.04 for the H2SQ + HNO 3 treatment. This indicates that the declining mobilization of OC in the humic layers at low pH is predominantly due to the reduced solubility of the less polar hydrophobic acids. They are more susceptible for a H + mediated contraction and precipitation than the more polar hydrophilic acids. Similar effects of aci- dification of the soil solution on the DOC composition have been reported by

Vance and David (1989, 1991b).

HEAVY METAL BINDING BY DOC FRACTIONS 119

%

D 0 C

0 + A horizon; H20deio n

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0 + A + B horizon; H2Odeto n

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Month

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O + A horizon; H2SO 4 + HNO 3 0 + A + B horizon; H2SO 4 + HNO 3

%

D 0 C

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Fig. 2. Time plot of the DOC distributions in the column leachates; HiN = hydrophilic neutrals, HiA = hydrophilic acids, HiB -- hydrophilic bases, HoA = hydrophobic acids, HoN = hydrophobic neutrals.

As the so lu t ions pe rco la t e t h rough the minera l B hor izons , the hydroph i l i c acids

genera l ly are the d o m i n a n t D O C f rac t ion (ca. 40-52% of D O C ) , ind ica t ing a more

quan t i t a t ive re ten t ion o f the h y d r o p h o b i c acids. Aga in , Vance and D a v i d (1989,

1991b) f o u n d s imi lar results. J a rd ine et al. (1989) showed tha t this selective re ten t ion

o f the h y d r o p h o b i c acids is due to their phys ica l a d so rp t i on on the soil ma t r ix

dr iven by favourab le en t ropy changes. The response of the D O C d i s t r ibu t ion in

120 GEORG GUGGENBERGER ET AL.

TABLE V

Concentrations of the D0C fractions in the effluent of the soil columns (HoA = hydrophobic acids; HiA = hydrophilic acids; HoN = hydrophobic neutrals; HiN = hydrophilic neutrals; HiN = hydrophilic

bases); mean values with standard deviations are reported

Soil horizons Treatment HoA HiA HoN HiN HiB

(mg C L -I)

O + A horizon H2Odeio n 37.5 + 4.7 15.8 +_ 4.7 0.6 • 1.1 4.1 + 2.3 4.1 +_ 2.3 H2SO 4 5.7• 5.9+0.7 0.4+_0.5 1.1• 1.1+0.4 H2SO4 + HNO3 6.4• 1.1 6.1 _+ 1.0 0.2+0.4 1.2• 1.2+0.5

O +A+ B horizon H2Odeio n 1.6+_0.4 1.9• 0.1+_0.2 0.6+_0.4 0.6 • 0.3 H2SO4 2.1+0.5 3.6 • 0.6 0.1+0.2 0.8+_0.4 0.5+0.2 H2SO4 + HNO3 2.3+0.4 3.4_+0.4 0.1• 0.7+0.4 0.6+_0.2

the O + A + B horizon leachates on addition of dilute acids is similar to that in the O + A horizon leachates, although the hydrophobic/hydrophil ic ratios are

lower. Mean ratios obtained in the effluent of the long columns are 0.85, 0.60,

and 0.54 for the H2Odeion, the H2SO4, and the H2SO4 + HNO3 leaching treatments, respectively. There is only a weak absolute retention of the hydrophilic acids in the B horizons of the soil columns receiving acid precipitation (Table V).

The data indicate that the acidity of the soil solution as well as the occurrence

of a DOC retaining B horizon are significant factors in controlling both the amount

and composition of DOC in forest soil leachates. These observations of the microcosms experiments are also found to occur in the field (Guggenberger and Zech, 1993). In the following the influence of the different DOC composition and

retention in the B horizons on the mobility of heavy metals will be discussed.

3.4. HEAVY METAL ADSORPTION BY AG-MP 50 CATION EXCHANGE RESIN

It is assumed that metals complexed by DOC pass through the ion exchange column, while inorganic metal species are retained on the resin. Since the dissociation of

metal-organic complexes depends on pH, sorption of DOC, Cr, Cu, and Cd from an O + A horizon solution by the AG-MP 50 cation exchange resin was tested

as a function of pH. Figure 3 shows that sorption of DOC by the cation exchange resin amounts

to only approximately 3% of the DOC added irrespective of pH. Sorption of DOC is restricted to organic bases. Also Cr passes the cation exchange resin irrespective

of p H to about 95%. This indicates that Cr is almost entirely complexed by DOC. The independence f rom pH furthermore suggests, that the Cr-organic complexes

are very stable and do not dissociate, even at pH 2. Cu complexation by DOC amounts for at least 50% at original sample pH. According to Driscoll (1984) the degree of complexation can be even higher. At a application rate of 2 mL rain -1 labile organic complexes can be partly dissociated by the resin. Increasing acid•

HEAVY METAL BINDING BY D O C FRACTIONS 121

100 -

DOC

15.0

Cr

I

80

60

40

20

0

m

ORIG 4- 3

pH

2

T

12.5

10.0

7.5

5 .0

2.5

0.0

ORIG. r ,3 2

pH

C u C d

30 1.40

........................................ ii~iii12111111211111~/ 25 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.20

T 20 T I .oo

15 ~ / ~ ~. O.OB | 1o j

(?.04 5

0 0 .o0 ~ t t I

ORIG 4 ,..3 2 ORIG. 4 3 2

pH pH

Fig. 3. Sorption of DOC, Cr, Cu, and Cd from a leachate of the H2Odeio n percolated O + A horizon by AG-MP 50 cation exchange resin at original solution pH, pH 4, pH 3, and pH 2; values shown are concentrations in the effluent of the resin compared to the total concentrations in the leachate;

mean values and standard deviations are given.

fication of the solution reduces the concentrations in the effluent of the Ag-MP 50 resin to approximately 20% of the total Cu. This can be interpreted that about 20% of Cu is kinetically stable complexed and at least 30% of Cu is weakly bonded to DOC and dissociates at higher proton activity. In contrast to the other two heavy metals, Cd was found by this method to occur almost entirely as inorganic species, also at relatively high pH. These results are in accordance with K6nig et al. (1986) who used an Amberlite IR-120 cation exchange resin: Cr showed the highest degree of complexation and also the highest complex stability; Cu was dominated by thermodynamic labile organic complexes; and Cd occurred pre- dominantly in its inorganic form.

122 G E O R G G U G G E N B E R G E R E T A L .

Original solution pH pH 2

c

0 jl/ij, l 1~1/91 ~ ft'lt~tl ~4/l) I

Month

11~ �9 < �9 r t �9 f r r r r ~" r : J l l i ~ f f

Month

[ [-- '] ieaie [~'~l ~ tmd by ItiA ~ e l bound by ItoA ]

Fig. 4. Time plot of the Cr distribution in inorganic, bound by hydrophilic acids, and bound by hydrophobic acids species in the leachates of the H2Odeio n treated O + A horizon at original solution

pH and at pH 2 adjusted solution.

3.5. HEAVY METAL BINDING BY HYDROPHOBIC AND HYDROPHILIC ACIDS

Figure 4 shows an exemplary time plot of the operationally defined binding forms of Cr in the leachates of the H2Odeio n irrigated O + A horizon. Since there is no seasonal trend, the plot only represents the variability. Application of the sample at original pH and pH 2 is necessary in order to distinguish between ionic, inorganic species, labile organically complexed species, stable by hydrophobic acids bonded species, and stable by hydrophilic acids bonded species (cf. Section 2.3.). Mean values of the operationally defined binding forms obtained from ca 20 measure- ments in the course of the year are given in Table VI.

Mean annual Cr concentrations in the effluents of the microcosms vary between 0.8 and 3.0 ~g L -I. The highest Cr concentrations were measured in the leachates of the HzOdeio n percolated O + A horizon, which also shows the highest DOC concentrations. It is obvious from Table VI that 90% of Cr is organically complexed. Due to a partly dissociation of the labile organic complexes during their passage through the cation exchange resin column (Driscol], 1984; Kbnig et al., 1986), values again indicate minumum organic complexation. Hydrophilic acids represent the dominating organic ligands, since 56% of total Cr is stable complexed by this DOC fraction. Decreasing DOC concentrations in the O + A + B horizon solutions at the acid treatments go significantly (13 < 0.01) along with a decrease of the total Cr concentrations. Simultaneously the proportions of the ionic, inorganic Cr species increase from at most I0% to at most 20-36%. The percentages of the stable Cr- hydrophobic acid complexes decrease from 26% to 8-17%, whereas those of the stable Cr-hydrophilic acid complexes remain rather constant. The proportions of the pH sensitive labile bonded Cr species are generally low. As Cr is mainly bonded to the hydrophilic acids, the 88% retention of the hydrophilic acids in the B hori- zon of the H2Odeio n treatment group results in a pronounced decline of the Cr concentration in the O + A + B horizon leachate. In contrast, the only weak abso- lute adsorption of the hydrophilic acids in the B horizon of the acid-treated soil

TA

BL

E V

I

Cr,

Cu,

an

d C

d co

nce

ntr

atio

ns

in t

he e

fflu

ent

of

the

soil

co

lum

ns,

an

d p

erce

nta

ge

dis

trib

uti

on

in

to t

he b

ind

ing

fo

rms

(i)

ioni

c,

(ii)

lab

ile

bo

nd

ed b

y h

yd

rop

ho

bic

an

d h

yd

rop

hil

ic a

cids

(la

bile

), (

iii)

stab

le b

on

ded

by

hy

dro

ph

ob

ic a

cids

(st

able

Ho

A),

an

d (

iv)

stab

le b

on

ded

by

hy

dro

ph

ilic

aci

ds (

stab

le H

iA);

mea

n v

alue

s w

ith

stan

dar

d d

evia

tio

ns

are

rep

ort

ed

Spe

ciat

ion

Soil

ho

rizo

ns

Tre

atm

ent

Ele

men

t C

on

cen

trat

ion

Io

nic

L

abil

e S

tabl

e H

oA

S

tabl

e H

iA

--

(~zg

L -

1)

--

%

O +

A h

ori

zon

H

2Ode

io n

Cr

3.0

_+ 1

.4

10 ±

7

8 ±

6

26 ±

13

56 ±

11

Cu

12

.5 ±

6.3

29

+ 1

0 26

+ 1

2 26

± 1

2 23

-+

9 C

d 0

.4+

0.1

8

6±

10

- 7_

+7

7±

5

O +

A h

ori

zon

H

2SO

4 C

r 1.

7 ±

0.9

36

_+

11

5 ±

2

9 +

8

50 ±

17

Cu

7

.5±

4.1

75

_+ 1

6 8_

+4

6±

4

11 ±

7

Cd

3.9

±0

.9

93

±6

-

5+

6

2+

1

O+

A

ho

rizo

n

H2S

O4

+ H

NO

3

Cr

1.5

±0

.9

20

±1

6

2+

1

9+

5

69

±2

0

Cu

7

.3±

3.0

56

_+18

20

_+6

8±

6

16_+

8 C

d 2.

3_+

0.7

91_+

6 -

5±

6

4±

3

O+

A+

B

ho

rizo

n

H2O

deio

n C

r 0.

8 +

0.5

26

± 1

3 1

+ 1

1

7+

10

56

± 1

6 C

u

11.1

_+

4.0

72

± 1

5 1

2±

7

3±

4

13

±7

C

d 1

.0±

0.4

91

±9

2

±2

4

±3

3

+4

O +

A +

B h

ori

zon

H

zSO

4

Cr

1.1

± 0

.4

27

±1

5

- i2

±7

61

_+21

C

u

12

.0±

3.4

6

8+

27

1

8±

17

3_+

6 1

1±

8

Cd

4.9

+ 1

.3

95

± 5

1

_+ 1

2

±3

2

±2

O +

A+

B h

ori

zon

H

2SO

4 +

HN

O 3

C

r 1

.2±

0.3

2

4±

20

1

7±

18

8

±4

5

1+

19

C

u

7.1

±2

.9

80

+ 1

1 1

0±

5

1 ±

2

9_+

5 C

d 5.

1 +

1.4

9

5±

5

- 4

± 3

1

± 1

124 GEORG GUGGENBERGER ET AL.

columns goes along with reduced retention of Cr in the illuvial horizon. Cu speciation in the effluent of the H2Odeio n percolated O + A horizon shows

an almost even distribution into the four different species. With decreasing DOC concentrations in the leachates of the O + A + B horizon and the acid percolated soil columns, the proportions of the ionic, inorganic species increase from at most 29% to at most 56-80%, while total Cu concentrations do not change significantly. This can be interpreted that with increasing acidity the Cu concentrations in the leachates are no longer determined by Cu complexation with DOC, but more and more by the increasing proton activity. So K6nig et al. (1986) reported that Cu is highly susceptible to minor pH changes at pH < 3.7. The decrease of the Cu- organic complexes with increasing soil depth and increasing acidity of the soil solu- tion is much more pronounced for the hydrophobic acids than for the hydrophilic acids. This indicates that under these conditions the importance of the hydrophilic acids in binding Cu is much higher than that of the hydrophobic acids. In all treatments the proportion of Cu, which is labile bonded to hydrophobic and hydro- philic acids is relatively high.

Cd is regarded to be mobilized mainly by increasing acidity (Tyler, 1981). Also according to Table VI the acid treatment groups generally show higher Cd con- centrations in the soil solution than the HzOdeio n treatment group. In spite of having the highest DOC concentration, the effluent of the H2Odeio n percolated O + A hori- zon shows the lowest Cd concentration. Complexation of Cd by hydrophobic and hydrophilic acids is poor in all treatments.

Using the concentrations of the hydrophobic and hydrophilic acids, the heavy metal concentrations, and the percentage of the heavy metal distribution, it is possible to calculate the actual heavy metal complexation by the two DOC fractions (Table VII). Cu complexation by DOC amounts to 5.7-39.1 ~mol Cu mo1-1 DOC for the hydrophobic acids, and to 26.7-143.5 ~mol Cu mo1-1 DOC for the hydrophilic acids. Therefore, the hydrophilic acids are much more effective in binding Cu than the hydrophobic acids. Because metal binding by organic matter can be viewed as an ion exchange process between H + and metal ions on acidic functional groups (McBride, 1989), the greater degree of Cu complexation by hydrophilic acids is probably due to their higher exchange acidity (Table II). Except for the leachate of the HzOdeion-percolated O + A horizon, there is a trend of decreasing Cu-binding by the hydrophobic and hydrophilic acids with decreasing solution pH. It is sug- gested, that this effect is due to the protonation of the acid functional groups at low pH. K6nig et al. (1986) and Kuiters and Mulder (1992) showed that Cu is preferentially bonded to the low molecular DOC fraction. Because low molecular organic molecules are characterized by higher carboxyl contents, i.e. are more hydrophilic (Thurman, 1985), the results of K6nig et al. (1986) and Kuiters and Mulder (1992) are in accordance with the data reported here.

The actual complexation of Cu by the hydrophobic and hydrophilic acids is much lower than the maximum Cu-binding capacity of DOC. Luster et al. (1989) reported a Cu-binding capacity of an aqueous larch litter extract of 7.56 mmol

TA

BL

E V

II

Co

mp

lex

atio

n o

f th

e h

eav

y m

etal

s C

r, C

u, a

nd

Cd

by

hy

dro

ph

ob

ic a

cids

(H

oA

) an

d h

yd

rop

hil

ic a

cids

(H

iA)

Soi

l h

ori

zon

s

Hea

vy

met

als

com

ple

xed

by

Tre

atm

ent

pH

E

lem

ent

Ho

A

HiA

H

oA

H

iA

O +

A h

ori

zon

O +

A h

ori

zon

O +

A h

ori

zon

O +

A +

B h

ori

zon

O +

A +

B h

ori

zon

O +

A +

B h

ori

zon

--

Ix

g L

~ -

-

--

p,

mo

l m

ol -

~

H2O

deio

n 4.

71

Cr

0.78

1.

68

4.8

24.5

C

u

3.25

2.

88

16.4

34

.5

Cd

0.

03

0.03

0.

08

0.2

H2S

O 4

3.40

C

r 0.

15

0.85

6.

0 33

.4

Cu

0.

45

0.83

14

.7

26.7

C

d 0.

20

0.08

3,

7 1.

5

HzS

O4

+ H

NO

3

3.85

C

r 0.

14

1.04

5.

1 39

.2

Cu

0.

58

1.17

21

.0

36.1

C

d 0.

12

0.09

2.

0 1.

6

H2O

deio

n 4.

93

Cr

0.14

0.

45

20.2

54

.8

Cu

0.

33

1.44

39

.1

143.

5 C

d

0.04

0.

03

2.6

1.7

H2S

O 4

4.10

C

r 0.

13

0.67

14

.3

42.9

C

u

0.36

1.

32

32.4

69

.3

Cd

0.10

0.

10

5.1

3.0

HzS

O4

+ H

NO

3 3.

88

Cr

0.10

0.

6t

10.0

41

.5

Cu

0.

07

0.64

5.

7 35

.6

Cd

0.20

0.

05

9.3

1.6

C

--

lI:

>

Z O

>

O

126 G E O R G G U G G E N B E R G E R ET AL.

Cu mo1-1 DOC. Kuiters and Mulder (1992) observed Cu-complexing capacities of various litter extracts from 9.00 to 10.92 mmol Cu mo1-1 DOC. In both cases samples were desalted so that there was no competition between Cu and other cations for complexing sites, and excess of Cu was added. Dietze (1987) found similar maximum Al-complexing capacities from 5.04 to 17.28 mmol mo1-1 DOC for beech leaf extracts. In contrast to the determination of the total metal-binding capacities, actual metal complexation data reported in this paper are highly influenced by the solution pH, complexing inorganic anions and competing inorganic cations, the redox potential, the organic and inorganic soil matrix, and the heavy metal concentration itself.

Due to the low total Cr concentrations, actual Cr-complexation by hydrophobic and hydrophilic acids is lower than for Cu. As already reported for Cu, Cr is preferentially bonded to the hydrophilic acids as well. Again high proton activities reduce the metal complexation by hydrophobic and hydrophilic acids.

In contrast to Cu and Cr, Cd shows a higher binding affinity to the hydrophobic acids, although the Cd complexation by both DOC fractions is low. Generally, Cd complexation by the hydrophobic and hydrophilic acids is better at higher total Cd concentrations in the acid treatments.

4. Summary and Conclusions

Microcosms studies with intact soil cores suggest that OC mineralization is lower at simulated acid precipitation. OC solubilization in the humic horizons is also reduced at the acid treatments. In contrast, DOC retention in the B horizon is impeded at the acid treated soil columns, leading to higher DOC concentrations in the B horizon leachate. Acid inputs also affect the composition of DOC with a relative reduction of the hydrophobic acids by (i) decreasing amounts of hydro- phobic acids mobilized in the humic layers and by (ii) decreasing retention of the hydrophilic acids in the B horizon.

The concentrations of Cu and in particular Cr in the column leachates are strongly influenced by the DOC concentration. Composition of DOC has a profound influence on the heavy metal complexation. The hydrophilic acids are responsible for most of the Cr- and Cu-binding in the leachates, while heavy metal complexation by the hydrophobic acids is much lower. Acidification increases the percentage of the ionic metal species, but only reduces the proportions of metal-hydrophobic acid complexes, and not that of the metal-hydrophilic acid complexes. The hydrophilic acids are the most mobile DOC fraction in the investigated soil columns, and their percentage on total DOC increases with decreasing solution pH. As a result, Cu and in particular Cr are exported from the B horizon in significant amounts as organic complexes (i.e. metal-hydrophilic acids complexes), although great proportions of the hydrophobic acids are retained in the B horizons. The results suggest that DOC composition and alterations in DOC composition e.g. by acid precipitation must be considered in assessing the heavy metal mobilization and transport in forest soils.

HEAVY METAL BINDING BY DOC FRACTIONS 127

Acknowledgement

We w o u l d l ike to t h a n k the 'Baye r i s ches S t a a t s m i n i s t e r i u m fo r L a n d e s e n t w i c k l u n g

u n d U m w e l t f r a g e n ' fo r f inanc ia l suppo r t .

References

Agerer, R.: 1990, Agric. Ecosyst. 28, 3. Berggren, D.: 1989, lntern. J. Environ. Anal Chem. 35, 1. Bergkvist, B.: 1986, Water, Air, and Soil Pollut. 31,901. Cronan, C. S.: 1985, Plant Soil88, 101. David, M. B., Vance, G. F., Rissing, J. M., and Stevenson, F. J.: 1989, 3". Environ. QuaL 18, 212. Dietze, G.: 1987, in International Symposium of Heavy Metals in the Environment, VoL II, CEP Consultants

Ltd., London, p. 482. Driscoll, C. T.: 1984, lntern. J. Environ. Anal Chem. 16, 267. Edwards, N. T.: 1982, Pedobiologia 23, 231. Guggenberger, G. and Zech, W.: 1993, Geoderma (in press). Hay, G. W., James, J. H., and Vanloon, G. W.: 1985, Soil Sci. 139,422. Hayes, M. H. B. and Swift, R. S.: 1978, in: Greenland, D. J. and Hayes, M. H. B. (eds.), The Chemistry

of Soil Constituents. Wiley, New York, p. 179. Hiraide, M., Arima, Y., and Mizuike, A.: 1987, Anal Chim. Acta 200, 171. Jardine, P. M., Weber, N. L., and McCarthy, J. F.: 1989, SoilScL Soc. Am. 3". 53, 1378. K6nig, N., Baccini, P., and Ulrich, B.: 1986, Ber. Forschungszentr. Wald6kosysteme/Waldsterben, Reihe

B 3, 84. Kreutzer, K. and Zelles, L.: 1986, Forstw. CbL 105, 314. Kuiters, A, T. and Mulder, W.: 1992, Geoderma. 52, 1. Leenheer, J. A: 1981, Environ. ScL Technol. 15, 578. Luster, J., Blaser, P., and Magyar, B.: 1989, Mitteilgn. Dtsch. Bodenkundl. Gesellsch. 59, 411. McBride, M. B.: 1989, in Stewart, B. A. (ed,), Advances in Soil Science, Vol. 10, Springer, New York,

p. 1. Pohlman, C. M. and McColl, J. G.: 1988, SoilSci. Soc. Am. J. 52, 265. Ruppert, H.: 1986, Mitteilgn. Dtsch. BodenkundL Gesellsch. 49, 166. Schulze, E. D., Lange, O., and Oren, R. (eds.): 1989, 'Acid Rain and Forest Decline of Picea Abies

- A Case Study on Acid Soils', Ecological Studies 77, Springer, Berlin. Stevenson, F. J. and Fitch, A.: 1986, in Huang, P. M. and Schnitzer, M. (eds.). Interaction of Soil

Minerals with Natural Organics and Microbes, ASA and SSSA Special Publication 17, SSSA, Madison. Tamm, S.-C. and McColl, J. G.: 1990, Soil ScL Soc. Am. J. 55, 1421. Thurman, E. M.: 1985, Organic Geochemistry of Natural Waters, W. Junk, Boston. Ttirk, T.: 1992, Bayreuther BodenkundL Ber. 22, 1. Tyler, G.: 1981, Water, Air, and Soil Pollut. 9, 137. Vance, G. F. and David, M. B.: 1989, Soil Sci. Soc. Am. J. 53, 1242. Vance, G. F. and David, M. B.: 1991a, Geochim. Cosmochim. Acta 55, 3611. Vance, G. F. and David, M. B.: 1991b, Soil Sci. 151,297.