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DOM-enhanced mobilization of benzo(a)pyrene in a contaminated soil under different chemical conditions
Phys. C&m. Earth, Vol. 23, No. 2, pp. 199-203, 1998 0 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0079- 1946/98 $19.00 + 0.00 PII: SOO79-1946(98)00013-5
Pergamon
DOM-Enhanced Mobilization of Benzo(a)pyrene in a Contaminated Soil Under Different Chemical Conditions B. Marschner Technical University Berlin, Institute of Ecology and Biology, Department, Salzufer 11-12, 10587 Berlin, Germany Received
25 April 1997; accepted
15 December
Soil Science
1997
(Kooc). It has been shown that this can depend on DOMorigin (eg. aquatic or pedogenic), DOM-aromaticity or -hydrophobicity and on size or mass of the DOM-molecules (Chiou et al., 1987; McCarthy et al., 1989; Maxin and Kbgel-Knabner, 1995). Due to the flexible and polyectrolytic nature of DOM, some of these characteristics are influenced by the chemical environment. Well documented are pH- and electrolyte effects on DOM-solubility, -size and configuration (Ghosh and Schnitzer, 1980; Murphy et al., 1994). Depending on type and magnitude of these effects, HOC-solubility may be either decreased or enhanced. Since most of the above studies have been carried out in solutions with no or only model solid-phase organic matter present, little is known about the effects of different chemical environments on DOM-enhanced solubility and mobility of HOCs in soils where interactions in the three-phase system are more complex. The aim of this study therefore was to investigate the effects of changes in pH and Caconcentrations in the soil on the mobilization of benzo(a)pyrene in a long-term column experiment.
Abstract. The effects of acidification and liming on the mobilization of benzo(a)pyrene (BaP) were studied in a long-term column experiment with a sandy topsoil spiked with tbis hydrophobic compound at a concentration of 100 mg kg-‘. Soil acidification to pH 3.3 initially caused a decreased release of BaP, but in the course of the 14months study, BaP-concentrations in the leachates surpassed control levels IO-15-fold. This is attributed to DOM-enhanced solubility of BaP, although no correlation with DOC-concentrations was found. Instead, increased DOM-sorptivity is attributed to the increases in DOMaromaticity and DOM-size which occurs in response to decreasing Ca-concentrations and increasing pH in the solutions. A short term BaP-mobilization in the lime treatment is not related to these parameters but may indicate the participation of heavy metals in DOM-BaP-interactions. Q 1998 Elsevier Science Ltd.
1 Introduction Hydrophobic organic compounds (HOCs) such as high molecular polycyclic aromatic hydrocarbons and many pesticides are strongly sorbed by organic matter in soils (Schwarzenbach et al., 1993). Sorption also occurs to dissolved organic matter (DOM), so that the solubility and mobility of these compounds can be enhanced in the presence of DOM (Murphy and Zachara, 1995). Consequently, the DOM-sorbed fraction must be considered as a third phase in the partitioning process of HOCs between the solid and dissolved phase, as it has been modelled by Gschwend and Wu (1985) and several other workers. However, the prediction of DOM-effects on HOC-behaviour is not only complicated by the fact that the exact sorption mechanisms are not fully understood (cf. Murphy et al., 1994; Pignatello and Xing, 1996), but also by the variability of DOM-properties and sorption coefficients
2 Materials and Methods All experiments were conducted with a sandy topsoil from a field plot that had received untreated sewage waters for over 100 years. The untreated soil material had pH 5.3, 1.7% C,, and showed moderate heavy metal, PAH- and PCBaccumulations (Cd: 5.9 mgkg”, Cu: 60 mgkg-‘, Cl6 PAH: 1.33 mg.kg-‘, C6 PCB: 0.13 mgkg”). For the artificial contamination, reagent grade benzo(a)pyrene (BaP) dissolved in acetone was mixed with pure quartz sand and the solvent allowed to evaporate. The spiked sand was then mixed into a subsample of the collected topsoil at about 1 % (w/w) to give a final BaP concentration of 100 mgkg-’ in the soil. After storage in an open shed for 9 months at field moisture, two different treatments were prepared horn subsamples of the spiked material according to previously determined buffering curves. For the acid treatment, HCl was added to give a final pH of 3.3. In the lime treatment, the final pH of 6.4 was well below the target pH of 7.5,
Correspondence to: B. Marschner 199
200
B. Marshner
although CaC03 had been added as an aqueous suspension. From each treatment, three replicates of 5.5 kg soil material were filled into steel columns (40 cm height, 14.3 cm diameter) equipped with a porous base and an outlet leading into glass collection bottles. Over a period of 14 months, the columns received periodical irrigations of about 50 mm Hz0 at 14-16 mm he’at irregular intervals. Leachates were collected within 24 h, by applying a 30 hPa vacuum to the base, thus ensuring unsaturated conditions throughout the irrigation as shown by tensiometer and TDR-readings in selected columns. In total, the columns received 11 irrigations with 594 mm and the leachate volume amounted to 5 18-525 mm. Due to evaporation, the top soil layer dried out considerably between irrigations and had to be carefully wetted 24 h prior to irrigation to avoid preferential flow due to reduced wettability. The experiment was conducted at 18-22°C in the dark. Chemical analysis of the leachates for pH, electric conductivity, DOC and BaP was performed immediately after sampling, cations and anions were determined after storage at -18°C with standard procedures. BaP was extracted from unfiltered and from 0.45 pm membrane filtered 100 mL solution samples by solid phase extraction (C18, Isolute) at a flow rate of 1 mLmin_‘. After drying in the air stream, BaP was eluted with 20 mL toluene and transferred into 0.5 mL acetonitrile after evaporating the toluene at -200 hPa at 25’C and drying with N2. Preliminary experiments with radiolabelled BaP had shown that this procedure ensures high recovery rates (> 90 %) of the compound from aqueous solutions containing DOM (Doting, pers. comm.). The analysis of BaP was conducted by HPLC (Gynkothek) equipped with a fluorescence detector. Dissolved organic matter (DOM) was measured as DOC with an elemental CN-analyzer (ANA 1500, Carlo Erba) and characterized by UVNIS-spectra of the solutions. For selected samples, DOM size distribution was determined with high pressure size exclusion chromatograpy (HPSEC) using Nucleogel or HEMA-Bio separation columns with detection at 210 mn.
3 Results and Discussion In the column leachates, BaP-concentrations in the acid treatment were initially below levels of the control and lime treatment (Fig. 1). But with subsequent irrigations BaPconcentrations in the acid leachates increased drastically and surpassed the control levels of 0.1-0.8 ugL_’ during the last 30 weeks 10-l 5-fold (Fig. 1). After week 3 1, maximum BaP-concentrations reached 13 ugL_’ in the filtered and almost 50 ugL ’ in unfiltered samples, which far exceeds its water solubility of l-2 ugL“ (Schwarzenbach et al., 1993). The lime treatment also caused a short-term concentration increase between week 39 and 59 with up to 9 ugL_’ (Fig. 1). These high BaP-concentrations in the leachates suggest the presence of colloidal or soluble sorbents in the solutions, since organic cosolvents are not present in the system. The most likely sorbent for BaP in the column leachates is DOM, since inorganic colloids are expected to be released
100.00 -control -lime 10.00 -
&acid
0.10 -
1
0.01 -I 0
10
20
30
40
50
60
t [weeks]
Fig. 1 Benzo(a)pyrene in membrane-filtered column leachates from the different treatments of the contammated so11 durmg the 59 week study period (mean and extremes of three replicates).
horn this sandy soil and BaP has a much higher affinity to organic matter than to mineral surfaces (Schwarzenbach et al., 1993). This is also shown by the following calculations, where the fraction of DOM-associated BaP (BaPooM) relative to total solution concentrations (BaP& is estimated from equation 1 (from Gschwend and Wu, 1985). BaP DOM BaP S0l
K DOC * DOC
(1)
l+Kooc*DOC
Based on Kooc-values of 1O4 - 1OSL kg-’ determined in separate experiments (Doring and Marschner, 1997) and typical DOC-concentrations of 80-120 mgL_’ in the leachates, the DOM-associated BaP-fraction would be calculated to account for 9 l-99% in all treatments. But despite this high affinity for DOM, BaPconcentrations show no significant positive correlation with DOC in any treatment (Tab. 1). The negative correlation coefficient even indicates an inverse relationship which for the acid treatment is explained by the strong DOMmobilization during the first 20 weeks of the experiment, where DOC-concentrations reached 360 mg L-’ (Fig. 2) while BaP-concentrations were at a minimum. This high DOM-release in response to the acidification is probably due to partial hydrolysis of soil organic matter and to lysis products horn dead microbial biomass. During the same period, electrolyte concentrations and acidity were also highly elvated as shown for the main cation Ca and for pH in Fig. 2, thus reflecting buffering reactions in the soil in response to the presence of HCl. With subsequent irrigations, these initial reaction products were apparently leached from the soil together with surplus acidity, so that DOC approached control levels by week 25 and Ca-concentrations decreased far below control values (Fig. 2). During the same time period, the concentrations of Mg, K, NO3 and the heavy metals Zn, Cu and Cd decreased similar to Ca, which is reflected in the close correlation among these inorganic solution parameters (R > 0.9). Only S04-concentrations remained constant in this treatment
DOM-Enhanced
Mobilization
of Benzo(a)pyrene
Table 1. Pearson correlation coefficients between log BaP concentrations and other solution parameters in the column leachatas from the three treatments (* denotes p < 0.01).
201
1000.0 , a
control
y = 4.91x”
R’ = 0.80
0 lime 100.0
control
log BaP lime
log DOC
-0.31
-0.53
-0.52
loa EC
-0 01
-0.44
-0 81*
acid
. acid
-7 .:
.3 10.0 i
log Ca
6:
PH
-0.09
020
0.83:
log e212§
-0.06
0 26
0.79’
m” 1.0
1specific absobance of DOC at 272 nm [L mg.’ rn-‘1 0.1
’
0.0
4
0.5
1.0
1.5
2.0
2.5
3.0
S+ [L.mg”.m”]
400
Fig. 3 RelatIonship between the specific absorbance of DOC at 272 nm (~272)and BaP:DOC-ratios in the leachates from the different treatments of the contaminated soil from the column study. The regression is only valid for the acid treatment.
-control -lime
300
--t acid
7 -r E 200 : 100
0 0
IO
20
30
46
66
66
10ooo
-control -lime -c acid 1000 7 4 E 0” 100
10 0
IO
20
30
46
66
66
0
IO
20
30 t [weeks]
46
66
60
8 7
Fig. 2. DOC, Ca and pH m column leachates from the different treatment9 of the contaminated soil during the 59 week study period (mean and extremes of three replicates).
throughout the experiment, while they were slightly elevated in the control and lime leachates during the first 20 weeks. The pH increased steadily in the leachates from the acid treatment and almost reached control values at the end of the experiment (Fig. 2). The close correlation between inorganic solution parameters and BaP in the acid treatment (Tab. 1) can not be explained by direct effects on the solubility of this nonpolar organic compound. Therefore, the above correlations suggest indirect effects of the chemical background on sorptive properties of the solid-phase or dissolved organic matter. Due to the calculated dominance of DOM-associated BaP in the leachates, an estimation of this association can be derived from the BaP:DOC-ratios. In the acid treatment, these ratios rise gradually during the experiment and surpass control values 10-l 5-fold in the second half of the experiment, thus suggesting that changes in the sorptive properties of DOM are responsible for the strong BaPmobilization. Similarily, the specific absorbance of DOC at 272 mn (E& as a measure for DOM-aromaticity (Chin et al., 1994) increases so that a close correlation exists between these two parameters in the acid treatment (Fig. 3). Similar relationships have been reported by other authors and are attributed to the low polarity of aromatic structures which should therefore have a stronger affinity for hydrophobic compounds (Chiou et al., 1987; McCarthy et al., 1989). The increase in DOM-aromaticity is related to the changing inorganic solution parameters, since the a2r2values correlate positively with pH and negatively with Ca (/RI >0.85). Thi s indicates, that low pH and high Caconcentrations may reduce the release or dissolution of aromatic DOM from the soil matrix. The low contaminant mobilization despite high DOC-concentrations in the acid treatment during the initial phase would therefore be explained by the low aromatic@ of the dissolved organic substances.
B. Marshner
202
However, the greatly increased BaP-mobilization in the acid treatments (Fig. 1) can not be attributed to DOMaromatic&y alone, since s,,,-values in these leachates do not exceed those in the control treatments (Fig. 3). Instead, DOM-size seems to play the decisive role for the differences in contaminant concentrations. In the beginning of the experiment (week 12), the HPSEC-plots in Fig. 4 indicate that DOM in the acid treatment is smaller than in the other treatments because it elutes later from the columns. Since srr2-values are lower too at that time, these findings are in agreement with results of Chin et al. (1994) who found a close correlation between DOM-aromaticity and molecular weight. Towards the end of the experiment (week 51) the leachates from the acid treatment are dominated by an
10
5
t [min]
15
20
macromolecules rather than on DOM-surfaces. In the acid treatment, the release or formation of these larger structural units is probably mainly due to the low electrolyte concentrations, which decreased to less than 20 % of control values after week 25. Here, Ca’+ as the dominant cation in the solutions is considered most important, since it can control DOM-solubility and configuration by binding the organic macromolecules to the solid phase through ternary surface complexes and by causing DOM-molecules to condense through intramolecular cation bridging (Ghosh and Schnitzer, 1980, Murphy et al., 1994). At low Ca-concentrations, organic matter therefore becomes more soluble and expands to form larger structures. This is additionally favoured by a higher pH (Murphy et al., 1994). While this explains the phenomena observed in the acid treatment, the temporarily elevated BaP-concentrations in the lime treatment (Fig. 1) must have different causes, since no changes in electrolyte or DOM-properties were detected during that period. Only Cd-concentrations also peak in that phase and their highly significant correlation to the BaP:DOC-ratios (R = 0.88) may indicate, that this heavy metal affects DOM-sorptive properties through structural alterations of the macromolecules as described by Blaser and Sposito (1987) for trace metal effects on DOM from litter extracts. But the reasons for the temporary comobilization of BaP and Cd remain obscure and therefore await further investigations.
-control
b
4 Conclusions
30
40
35
45
t [min] Fig. 4. HPSEC chromatograms
of DOM in the leachates from the dtfferently treated soils at two sampling dates from the column study (a) week 12 (HEMA-Bio separation column); (b) week 51 (Nucleogel separation column)
early (=larger) DOM-fraction, while the other treatments also contain two smaller size classes (Fig. 4). Due to the use of different separation columns, the results from the two sampling dates can only be compared qualitatively. However, the dominance of larger DOM-molecules in the acid treatment during that phase is also reflected in significantly lower E4:E6-ratios of 2-7 in comparison to the control or lime where ratios ranged between 10 and 45 during the last 20 weeks of the experiment. The importance of DOM-size for the sorption of HOCs was also stressed in the findings of Raber and KiigelKnabner (1996) and supports the conceptual model proposed by Engebretson and Wandruszka (1994) where sorption occurs mainly in hydrophobic cavities of organic
In this study, the mobilization of BaP in the acid treatment is mainly attributed to the strong release and subsequent depletion of Ca from the soil due to buffering reactions and leaching, resulting in a chemical environment that favors the release and formation aromatic DOMof larger macromolecules with stronger sorptive properties for the hydrophobic compounds. This shows that at least under these extreme experimental conditions, the chemical background can influence the solubilization and mobilization of a hydrophobic organic compound such as BaP in soils. But the unexplained reactions in the lime treatment also illustrate, that high Ca-concentrations not necessarily guarantee an immobilization of this compound in the soil. Even if this were due to some unknown interaction between heavy metals and BaP, this must be controlled by parameters not analyzed in this experimental setup. Due to the participation of biological processes in organic matter turn-over and DOM-formation, further investigations where biological activity is monitored or manipulated may therefore be promising. Achnavledgemenfs This study was part of an interdisciplinary research project on the behaviour and effects of PAHs and PCBs in soils, financed by the German Ministry of Education, Research, Science and Technology (BMBF).
DOM-Enhanced
Mobilization
References Blaser, P. and Sposito, G., Spectrofluorometric investigation of trace metal complexation by an aqueous chestnut litter extract, Soil Sci. Sot Am. J. 51, 612-619, 1987. Chin, Y.-P., Aiken, G.. and O’Loughlin, E., Molecular weight, polydispersity. and spectroscopic properties of aquatic humic substances, Envrron. Sci. Tech&. 28, 1853- 1858, 1994. Chiou, C T , Kile, D. E., Brinton, T. l., Malcolm, R. E., Leenheer, J. A., and McCarthy, P , A comparison of water solubility enhancement of organic solutes by aquatic humic materials and commercial humic acids, Envrron. SCI. Technol. 21, 1231-1234, 1987 Doring, U., Exogene Einfliisse auf Sorptron und Vertedung van Benzo(a)pyren und 2.2’#5, S’-Tetrachlorbrphenyl ber B&ien anthropogen beernfufiter Standorte, Dissertation TU Berlin, 1997. Ddring, U. and Marschner, B , Water solubility enhancement of benzo(a)pyrene and 2,2’,5,5’-tetrachlorbiphenyl by dissolved organic matter (DOM), Phys. Chem. Earth (this issue). Eadie. B. J., Morehead, N. R., and Landrum, P. F., Three-phase partnioning of hydrophobic organic compounds in great lake waters, Chemosphere 20, 161-178, 1990. Engebretson, R. R. and Wandruszka, R. von, Microorganization in dissolved organic acrds, Envrron. Sci. Technol. 28, 1934- 194 1, 1994 Ghosh, K. and Schnitzer, M., Macromolecular structures of humic substances, Soil Ser. 129,266-276, 1980. Gschwend, P M. and Wu, S-C., On the constancy of sediment-water partttion coefficients of hydrophobtc organic pollutants, Envrron. Ser. Tech&.
19,90-96,
1985
Maxin, C R and Kogel-Knabner, I., Parttttoning of polycyclic aromatic hydrocarbons (PAH) to water soluble soil organic matter, Europ. J. Sod Ser. 46, 193-204, 1995. McCarthy, J. F., Roberson, L. E., and Burrus, L. W., Association of benzo(a)pyrene with dissolved organic matter: predtction of Kdom from structural and chemtcal properties of the organic matter, Chemosphere 19. 1911-1920, 1989 Murphy, E. M and Zachara, J. M., The role of sorbed humic substances on the distribution of organic and inorganic contaminants m groundwater, Geoderma 67, 103-124, 1995. Murphy, E. M., Zachara, J. M., Smith, S. C., Phillips, J. L., and Wietsma, T. W., Interaction of hydrophobtc organic compounds with mineralbound humic acid, Environ. Sci. Tech&. 28, 1291- 1299, 1994. Pignatello, J J. and Xing, B., Mechanism of slow sorption of organic chemicals to natural particles, Environ. Six Technol. 30, I - 11, 1996. Raber, B. and Kogel-Knabner. I., Abschatzung des Verhaltens von PAK in Boden unter dem Einflu8 von DGM unterschiedlicher Herkunft, Mrt. Deut. Bode&II.
Schwarzenbach, Envrronmental
I993
Ges. 80, 93-96, 1996.
R. P., Gschwend, Organic
Chemwtry,
P. M. and hnboden, D. M., John Wiley & Sons, New York,
of Benzo(a)pyrene
203
Pergamon
DOM-Enhanced Mobilization of Benzo(a)pyrene in a Contaminated Soil Under Different Chemical Conditions B. Marschner Technical University Berlin, Institute of Ecology and Biology, Department, Salzufer 11-12, 10587 Berlin, Germany Received
25 April 1997; accepted
15 December
Soil Science
1997
(Kooc). It has been shown that this can depend on DOMorigin (eg. aquatic or pedogenic), DOM-aromaticity or -hydrophobicity and on size or mass of the DOM-molecules (Chiou et al., 1987; McCarthy et al., 1989; Maxin and Kbgel-Knabner, 1995). Due to the flexible and polyectrolytic nature of DOM, some of these characteristics are influenced by the chemical environment. Well documented are pH- and electrolyte effects on DOM-solubility, -size and configuration (Ghosh and Schnitzer, 1980; Murphy et al., 1994). Depending on type and magnitude of these effects, HOC-solubility may be either decreased or enhanced. Since most of the above studies have been carried out in solutions with no or only model solid-phase organic matter present, little is known about the effects of different chemical environments on DOM-enhanced solubility and mobility of HOCs in soils where interactions in the three-phase system are more complex. The aim of this study therefore was to investigate the effects of changes in pH and Caconcentrations in the soil on the mobilization of benzo(a)pyrene in a long-term column experiment.
Abstract. The effects of acidification and liming on the mobilization of benzo(a)pyrene (BaP) were studied in a long-term column experiment with a sandy topsoil spiked with tbis hydrophobic compound at a concentration of 100 mg kg-‘. Soil acidification to pH 3.3 initially caused a decreased release of BaP, but in the course of the 14months study, BaP-concentrations in the leachates surpassed control levels IO-15-fold. This is attributed to DOM-enhanced solubility of BaP, although no correlation with DOC-concentrations was found. Instead, increased DOM-sorptivity is attributed to the increases in DOMaromaticity and DOM-size which occurs in response to decreasing Ca-concentrations and increasing pH in the solutions. A short term BaP-mobilization in the lime treatment is not related to these parameters but may indicate the participation of heavy metals in DOM-BaP-interactions. Q 1998 Elsevier Science Ltd.
1 Introduction Hydrophobic organic compounds (HOCs) such as high molecular polycyclic aromatic hydrocarbons and many pesticides are strongly sorbed by organic matter in soils (Schwarzenbach et al., 1993). Sorption also occurs to dissolved organic matter (DOM), so that the solubility and mobility of these compounds can be enhanced in the presence of DOM (Murphy and Zachara, 1995). Consequently, the DOM-sorbed fraction must be considered as a third phase in the partitioning process of HOCs between the solid and dissolved phase, as it has been modelled by Gschwend and Wu (1985) and several other workers. However, the prediction of DOM-effects on HOC-behaviour is not only complicated by the fact that the exact sorption mechanisms are not fully understood (cf. Murphy et al., 1994; Pignatello and Xing, 1996), but also by the variability of DOM-properties and sorption coefficients
2 Materials and Methods All experiments were conducted with a sandy topsoil from a field plot that had received untreated sewage waters for over 100 years. The untreated soil material had pH 5.3, 1.7% C,, and showed moderate heavy metal, PAH- and PCBaccumulations (Cd: 5.9 mgkg”, Cu: 60 mgkg-‘, Cl6 PAH: 1.33 mg.kg-‘, C6 PCB: 0.13 mgkg”). For the artificial contamination, reagent grade benzo(a)pyrene (BaP) dissolved in acetone was mixed with pure quartz sand and the solvent allowed to evaporate. The spiked sand was then mixed into a subsample of the collected topsoil at about 1 % (w/w) to give a final BaP concentration of 100 mgkg-’ in the soil. After storage in an open shed for 9 months at field moisture, two different treatments were prepared horn subsamples of the spiked material according to previously determined buffering curves. For the acid treatment, HCl was added to give a final pH of 3.3. In the lime treatment, the final pH of 6.4 was well below the target pH of 7.5,
Correspondence to: B. Marschner 199
200
B. Marshner
although CaC03 had been added as an aqueous suspension. From each treatment, three replicates of 5.5 kg soil material were filled into steel columns (40 cm height, 14.3 cm diameter) equipped with a porous base and an outlet leading into glass collection bottles. Over a period of 14 months, the columns received periodical irrigations of about 50 mm Hz0 at 14-16 mm he’at irregular intervals. Leachates were collected within 24 h, by applying a 30 hPa vacuum to the base, thus ensuring unsaturated conditions throughout the irrigation as shown by tensiometer and TDR-readings in selected columns. In total, the columns received 11 irrigations with 594 mm and the leachate volume amounted to 5 18-525 mm. Due to evaporation, the top soil layer dried out considerably between irrigations and had to be carefully wetted 24 h prior to irrigation to avoid preferential flow due to reduced wettability. The experiment was conducted at 18-22°C in the dark. Chemical analysis of the leachates for pH, electric conductivity, DOC and BaP was performed immediately after sampling, cations and anions were determined after storage at -18°C with standard procedures. BaP was extracted from unfiltered and from 0.45 pm membrane filtered 100 mL solution samples by solid phase extraction (C18, Isolute) at a flow rate of 1 mLmin_‘. After drying in the air stream, BaP was eluted with 20 mL toluene and transferred into 0.5 mL acetonitrile after evaporating the toluene at -200 hPa at 25’C and drying with N2. Preliminary experiments with radiolabelled BaP had shown that this procedure ensures high recovery rates (> 90 %) of the compound from aqueous solutions containing DOM (Doting, pers. comm.). The analysis of BaP was conducted by HPLC (Gynkothek) equipped with a fluorescence detector. Dissolved organic matter (DOM) was measured as DOC with an elemental CN-analyzer (ANA 1500, Carlo Erba) and characterized by UVNIS-spectra of the solutions. For selected samples, DOM size distribution was determined with high pressure size exclusion chromatograpy (HPSEC) using Nucleogel or HEMA-Bio separation columns with detection at 210 mn.
3 Results and Discussion In the column leachates, BaP-concentrations in the acid treatment were initially below levels of the control and lime treatment (Fig. 1). But with subsequent irrigations BaPconcentrations in the acid leachates increased drastically and surpassed the control levels of 0.1-0.8 ugL_’ during the last 30 weeks 10-l 5-fold (Fig. 1). After week 3 1, maximum BaP-concentrations reached 13 ugL_’ in the filtered and almost 50 ugL ’ in unfiltered samples, which far exceeds its water solubility of l-2 ugL“ (Schwarzenbach et al., 1993). The lime treatment also caused a short-term concentration increase between week 39 and 59 with up to 9 ugL_’ (Fig. 1). These high BaP-concentrations in the leachates suggest the presence of colloidal or soluble sorbents in the solutions, since organic cosolvents are not present in the system. The most likely sorbent for BaP in the column leachates is DOM, since inorganic colloids are expected to be released
100.00 -control -lime 10.00 -
&acid
0.10 -
1
0.01 -I 0
10
20
30
40
50
60
t [weeks]
Fig. 1 Benzo(a)pyrene in membrane-filtered column leachates from the different treatments of the contammated so11 durmg the 59 week study period (mean and extremes of three replicates).
horn this sandy soil and BaP has a much higher affinity to organic matter than to mineral surfaces (Schwarzenbach et al., 1993). This is also shown by the following calculations, where the fraction of DOM-associated BaP (BaPooM) relative to total solution concentrations (BaP& is estimated from equation 1 (from Gschwend and Wu, 1985). BaP DOM BaP S0l
K DOC * DOC
(1)
l+Kooc*DOC
Based on Kooc-values of 1O4 - 1OSL kg-’ determined in separate experiments (Doring and Marschner, 1997) and typical DOC-concentrations of 80-120 mgL_’ in the leachates, the DOM-associated BaP-fraction would be calculated to account for 9 l-99% in all treatments. But despite this high affinity for DOM, BaPconcentrations show no significant positive correlation with DOC in any treatment (Tab. 1). The negative correlation coefficient even indicates an inverse relationship which for the acid treatment is explained by the strong DOMmobilization during the first 20 weeks of the experiment, where DOC-concentrations reached 360 mg L-’ (Fig. 2) while BaP-concentrations were at a minimum. This high DOM-release in response to the acidification is probably due to partial hydrolysis of soil organic matter and to lysis products horn dead microbial biomass. During the same period, electrolyte concentrations and acidity were also highly elvated as shown for the main cation Ca and for pH in Fig. 2, thus reflecting buffering reactions in the soil in response to the presence of HCl. With subsequent irrigations, these initial reaction products were apparently leached from the soil together with surplus acidity, so that DOC approached control levels by week 25 and Ca-concentrations decreased far below control values (Fig. 2). During the same time period, the concentrations of Mg, K, NO3 and the heavy metals Zn, Cu and Cd decreased similar to Ca, which is reflected in the close correlation among these inorganic solution parameters (R > 0.9). Only S04-concentrations remained constant in this treatment
DOM-Enhanced
Mobilization
of Benzo(a)pyrene
Table 1. Pearson correlation coefficients between log BaP concentrations and other solution parameters in the column leachatas from the three treatments (* denotes p < 0.01).
201
1000.0 , a
control
y = 4.91x”
R’ = 0.80
0 lime 100.0
control
log BaP lime
log DOC
-0.31
-0.53
-0.52
loa EC
-0 01
-0.44
-0 81*
acid
. acid
-7 .:
.3 10.0 i
log Ca
6:
PH
-0.09
020
0.83:
log e212§
-0.06
0 26
0.79’
m” 1.0
1specific absobance of DOC at 272 nm [L mg.’ rn-‘1 0.1
’
0.0
4
0.5
1.0
1.5
2.0
2.5
3.0
S+ [L.mg”.m”]
400
Fig. 3 RelatIonship between the specific absorbance of DOC at 272 nm (~272)and BaP:DOC-ratios in the leachates from the different treatments of the contaminated soil from the column study. The regression is only valid for the acid treatment.
-control -lime
300
--t acid
7 -r E 200 : 100
0 0
IO
20
30
46
66
66
10ooo
-control -lime -c acid 1000 7 4 E 0” 100
10 0
IO
20
30
46
66
66
0
IO
20
30 t [weeks]
46
66
60
8 7
Fig. 2. DOC, Ca and pH m column leachates from the different treatment9 of the contaminated soil during the 59 week study period (mean and extremes of three replicates).
throughout the experiment, while they were slightly elevated in the control and lime leachates during the first 20 weeks. The pH increased steadily in the leachates from the acid treatment and almost reached control values at the end of the experiment (Fig. 2). The close correlation between inorganic solution parameters and BaP in the acid treatment (Tab. 1) can not be explained by direct effects on the solubility of this nonpolar organic compound. Therefore, the above correlations suggest indirect effects of the chemical background on sorptive properties of the solid-phase or dissolved organic matter. Due to the calculated dominance of DOM-associated BaP in the leachates, an estimation of this association can be derived from the BaP:DOC-ratios. In the acid treatment, these ratios rise gradually during the experiment and surpass control values 10-l 5-fold in the second half of the experiment, thus suggesting that changes in the sorptive properties of DOM are responsible for the strong BaPmobilization. Similarily, the specific absorbance of DOC at 272 mn (E& as a measure for DOM-aromaticity (Chin et al., 1994) increases so that a close correlation exists between these two parameters in the acid treatment (Fig. 3). Similar relationships have been reported by other authors and are attributed to the low polarity of aromatic structures which should therefore have a stronger affinity for hydrophobic compounds (Chiou et al., 1987; McCarthy et al., 1989). The increase in DOM-aromaticity is related to the changing inorganic solution parameters, since the a2r2values correlate positively with pH and negatively with Ca (/RI >0.85). Thi s indicates, that low pH and high Caconcentrations may reduce the release or dissolution of aromatic DOM from the soil matrix. The low contaminant mobilization despite high DOC-concentrations in the acid treatment during the initial phase would therefore be explained by the low aromatic@ of the dissolved organic substances.
B. Marshner
202
However, the greatly increased BaP-mobilization in the acid treatments (Fig. 1) can not be attributed to DOMaromatic&y alone, since s,,,-values in these leachates do not exceed those in the control treatments (Fig. 3). Instead, DOM-size seems to play the decisive role for the differences in contaminant concentrations. In the beginning of the experiment (week 12), the HPSEC-plots in Fig. 4 indicate that DOM in the acid treatment is smaller than in the other treatments because it elutes later from the columns. Since srr2-values are lower too at that time, these findings are in agreement with results of Chin et al. (1994) who found a close correlation between DOM-aromaticity and molecular weight. Towards the end of the experiment (week 51) the leachates from the acid treatment are dominated by an
10
5
t [min]
15
20
macromolecules rather than on DOM-surfaces. In the acid treatment, the release or formation of these larger structural units is probably mainly due to the low electrolyte concentrations, which decreased to less than 20 % of control values after week 25. Here, Ca’+ as the dominant cation in the solutions is considered most important, since it can control DOM-solubility and configuration by binding the organic macromolecules to the solid phase through ternary surface complexes and by causing DOM-molecules to condense through intramolecular cation bridging (Ghosh and Schnitzer, 1980, Murphy et al., 1994). At low Ca-concentrations, organic matter therefore becomes more soluble and expands to form larger structures. This is additionally favoured by a higher pH (Murphy et al., 1994). While this explains the phenomena observed in the acid treatment, the temporarily elevated BaP-concentrations in the lime treatment (Fig. 1) must have different causes, since no changes in electrolyte or DOM-properties were detected during that period. Only Cd-concentrations also peak in that phase and their highly significant correlation to the BaP:DOC-ratios (R = 0.88) may indicate, that this heavy metal affects DOM-sorptive properties through structural alterations of the macromolecules as described by Blaser and Sposito (1987) for trace metal effects on DOM from litter extracts. But the reasons for the temporary comobilization of BaP and Cd remain obscure and therefore await further investigations.
-control
b
4 Conclusions
30
40
35
45
t [min] Fig. 4. HPSEC chromatograms
of DOM in the leachates from the dtfferently treated soils at two sampling dates from the column study (a) week 12 (HEMA-Bio separation column); (b) week 51 (Nucleogel separation column)
early (=larger) DOM-fraction, while the other treatments also contain two smaller size classes (Fig. 4). Due to the use of different separation columns, the results from the two sampling dates can only be compared qualitatively. However, the dominance of larger DOM-molecules in the acid treatment during that phase is also reflected in significantly lower E4:E6-ratios of 2-7 in comparison to the control or lime where ratios ranged between 10 and 45 during the last 20 weeks of the experiment. The importance of DOM-size for the sorption of HOCs was also stressed in the findings of Raber and KiigelKnabner (1996) and supports the conceptual model proposed by Engebretson and Wandruszka (1994) where sorption occurs mainly in hydrophobic cavities of organic
In this study, the mobilization of BaP in the acid treatment is mainly attributed to the strong release and subsequent depletion of Ca from the soil due to buffering reactions and leaching, resulting in a chemical environment that favors the release and formation aromatic DOMof larger macromolecules with stronger sorptive properties for the hydrophobic compounds. This shows that at least under these extreme experimental conditions, the chemical background can influence the solubilization and mobilization of a hydrophobic organic compound such as BaP in soils. But the unexplained reactions in the lime treatment also illustrate, that high Ca-concentrations not necessarily guarantee an immobilization of this compound in the soil. Even if this were due to some unknown interaction between heavy metals and BaP, this must be controlled by parameters not analyzed in this experimental setup. Due to the participation of biological processes in organic matter turn-over and DOM-formation, further investigations where biological activity is monitored or manipulated may therefore be promising. Achnavledgemenfs This study was part of an interdisciplinary research project on the behaviour and effects of PAHs and PCBs in soils, financed by the German Ministry of Education, Research, Science and Technology (BMBF).
DOM-Enhanced
Mobilization
References Blaser, P. and Sposito, G., Spectrofluorometric investigation of trace metal complexation by an aqueous chestnut litter extract, Soil Sci. Sot Am. J. 51, 612-619, 1987. Chin, Y.-P., Aiken, G.. and O’Loughlin, E., Molecular weight, polydispersity. and spectroscopic properties of aquatic humic substances, Envrron. Sci. Tech&. 28, 1853- 1858, 1994. Chiou, C T , Kile, D. E., Brinton, T. l., Malcolm, R. E., Leenheer, J. A., and McCarthy, P , A comparison of water solubility enhancement of organic solutes by aquatic humic materials and commercial humic acids, Envrron. SCI. Technol. 21, 1231-1234, 1987 Doring, U., Exogene Einfliisse auf Sorptron und Vertedung van Benzo(a)pyren und 2.2’#5, S’-Tetrachlorbrphenyl ber B&ien anthropogen beernfufiter Standorte, Dissertation TU Berlin, 1997. Ddring, U. and Marschner, B , Water solubility enhancement of benzo(a)pyrene and 2,2’,5,5’-tetrachlorbiphenyl by dissolved organic matter (DOM), Phys. Chem. Earth (this issue). Eadie. B. J., Morehead, N. R., and Landrum, P. F., Three-phase partnioning of hydrophobic organic compounds in great lake waters, Chemosphere 20, 161-178, 1990. Engebretson, R. R. and Wandruszka, R. von, Microorganization in dissolved organic acrds, Envrron. Sci. Technol. 28, 1934- 194 1, 1994 Ghosh, K. and Schnitzer, M., Macromolecular structures of humic substances, Soil Ser. 129,266-276, 1980. Gschwend, P M. and Wu, S-C., On the constancy of sediment-water partttion coefficients of hydrophobtc organic pollutants, Envrron. Ser. Tech&.
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Maxin, C R and Kogel-Knabner, I., Parttttoning of polycyclic aromatic hydrocarbons (PAH) to water soluble soil organic matter, Europ. J. Sod Ser. 46, 193-204, 1995. McCarthy, J. F., Roberson, L. E., and Burrus, L. W., Association of benzo(a)pyrene with dissolved organic matter: predtction of Kdom from structural and chemtcal properties of the organic matter, Chemosphere 19. 1911-1920, 1989 Murphy, E. M and Zachara, J. M., The role of sorbed humic substances on the distribution of organic and inorganic contaminants m groundwater, Geoderma 67, 103-124, 1995. Murphy, E. M., Zachara, J. M., Smith, S. C., Phillips, J. L., and Wietsma, T. W., Interaction of hydrophobtc organic compounds with mineralbound humic acid, Environ. Sci. Tech&. 28, 1291- 1299, 1994. Pignatello, J J. and Xing, B., Mechanism of slow sorption of organic chemicals to natural particles, Environ. Six Technol. 30, I - 11, 1996. Raber, B. and Kogel-Knabner. I., Abschatzung des Verhaltens von PAK in Boden unter dem Einflu8 von DGM unterschiedlicher Herkunft, Mrt. Deut. Bode&II.
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