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Seasonal variations of total organic carbon, iron, and aluminium on the Svartberget catchment in northern Sweden
Environment International, Vol. 22, No. 5, pp. 541-549, 1996 Copyright 01996 Else&r Science Ltd Printed in the USA. All tights reserved 0160-4120/96 $15.00+.00
Pergamon
PIISO160-4120(96)00050-5
SEASONAL VARIATIONS OF TOTAL ORGANIC CARBON, IRON, AND ALUMINIUM ON THE SVARTBERGET CATCHMENT IN NORTHERN SWEDEN Catharina Pettersson Department of Water and Environmental Studies, Linkbping University, S-581 83 Linkbping, Sweden
Kevin Bishop Department of Forest Ecology, Swedish University of Agricultural Sciences, S-901 83 Umea, Sweden
El 9507-387 M (Received 26 April 1996; accepted 28 April 1996)
The variation in the concentration of organic carbon, especially humic substances, and two metals (Fe and Al) was studied in a mire and two tributaries on a boreal catchment in northern Sweden during one year. Water samples were collected monthly, except during the spring flood when more intensive sampling was made. The fraction of Fe and Al that was associated with the humic substances was estimated using an ion exchange technique. The total concentrations of Fe and Al increased as the concentrations of organic matter and humic substances increased. The fractionation study indicated that Fe has a larger tendency to form complexes with the humic fraction than Al has. The amount of the metals that were bound to the humic substances was relatively constant for the water from each location throughout the year, but the values varied within a broad range between the different sampling locations: 50-90% of the Fe and 20-80% of the Al was found in the humic fraction. Increased fractions of both metals were bound to the humic substances during the spring flood, i.e., 80-90% for both Fe and Al. Conditional stability constants for the formation of complexes, calculated from the field data, varied in the range 4.8-5.2 for Fe-humic complexes and 4.1-4.8 for Al-humic complexes.
INTRODUCTION Humic substances have a tendency to form complexes with metals, which influence their mobility and redistribution in soil and water. The complex formation is controlled by hydrochemical parameters such as pH, Eh, ionic strength and concentration of metals, and complexing agents. Hydrological processes determine the sequence of chemical environments through which the water passes, and hence the chemical forms of the metals. So far, most speciation studies have focussed on
trace metal speciation (M&night et al. 1983; John et al. 1988; Ephraim and Xu 1989; Pettersson et al. 1993). However, iron (Fe) and aluminium (Al) are often present in relatively high concentrations in humic-rich water and might, therefore, dominate the humic complexes. It has been observed that in surface water containing large concentrations of Fe and Al, an unexpectedly small fraction of copper was associated to the humic fraction (Pettersson et al. 1993). This was inter541
542
C. Pettersson and K. Bishop
Hillslope Research Area Mire Perennial Stream Intermittent Stream , 200m ,
Fig. 1. The Svartberget catchment.
preted as a competition between the different metals for the available binding sites on the humic matter. The binding of Fe (Ephraim 1992) and Al (Liivgren et al. 1987; Clarke et al. 1995) to humic substances has been studied in a few laboratory studies. Although fulvic acids isolated from surface water (Ephraim 1992; Clarke et al. 1995) or natural bogwater (Lovgren et al. 1987) were used in those experiments, it is not possible to directly transfer the results to processes in the natural environment. The aim of this study was to determine and compare the size of the fractions of Fe and Al associated to humic matter in surface waters with different character, i.e., different concentrations of humic substances, Fe, Al, and pH. The variation in the speciation was followed during one year and special attention was paid to the spring flood to observe the effects of high flow and rapid decline in pH on the Fe and Al speciation. Based on the results from the field study, it should be possible to calculate conditional stability constants for the formation of Fe-humic and Al-humic complexes in the waters.
EXPERIMENTAL
The study was performed on the Svartberget catchment (50 ha; Fig. 1) in northern Sweden. The catchment is covered with coniferous forest on podzol soils. The sampling was made in two tributaries (Kallki-illblcken and Vastrabacken, sites K and V, respectively) and the mire outlet (site M) from which Kallklllbacken originates. The area is thoroughly described elsewhere (Bishop 199 1). Weekly sampling of stream and mire water was conducted during the period 1 January to 3 1 December 1993. All samples were analysed for total organic carbon (TOC) (Shimadzu TOC-5000 utilizing catalytic combustion) and absorbance at 254 nm as a measure of the concentration of humic substances (Beckman DU-8 spectrophotometer). Monthly water samples were collected for analysis of total concentration of Fe and Al as well as the humicbound fraction of these metals. Major anions and cations and pH were also analysed. Samples for metal analysis were acidified immediately after sampling,
Seasonal variations of total organic carbon, iron, and aluminium in northern Sweden
whereas the other samples were stored in a freezer until analysis. The humic fraction of the organic matter, including the associated metals, was isolated in the field on a weak anion-exchange resin, diethylaminoethylSephadexA25 (Pharmacia Fine Chemicals) in a batch procedure (Pettersson et al. 1993). The resin was pretreated with HCl (suprapur) and rinsed with Milli-Qwater. After a contact time of 20 min, the supernatant, which contained the non-humic fraction, was decanted and collected. The concentration of Fe and Al in the supematant and the original water sample was determined on ICP-MS (Perkin Elmer Elan 5000). The fraction of Fe and Al bound to the humic fraction was calculated as the difference between these analyses. Polyethylene bottles were used in the metal fractionation procedure and were acid washed (20% I-INO, + 1% HCl) for at least 24 h and thoroughly rinsed with MilliQ-water prior to use. Samples were taken more frequently during the spring flood (April 24-29) and a rain storm in the summer (July 23-28). Those samples were analysed following the same procedure as the monthly samples. RESULTS AND DISCUSSION pH and organic mafter The flow pattern and runoff per unit area were similar in both tributaries, although the runoff from the smaller Wstrabticken tributary comprised only 18% of the cat&rent runoff, whereas KallkZllbZckenaccounted for the other 82%. The pH was relatively high (5.5-6.0) in both creeks during low flow in the winter and decreased during the spring flood to 4.0 in Kallk~llb&ken and 4.5 in Ustrabticken. A slow increase in pH followed during summer and autumn. The pH of the mire water was 0.31.O pH units below that of KallklllbZicken during the whole year except during the spring flood when the difference was less than 0.3 pH units (Pettersson et al. 1995). The mire outlet and Kallklllb~cken below the mire had a fairly high TOC content (30-40 mg/L and 1832 mg/L, respectively), while Wstrabiicken had a lower TOC content (8-20 mg/L) during the year, except for periods of high flow. During the spring flood, the TOC exhibited different patterns in the mire and the two tributaries. The TOC was relatively constant in the mire from January until the spring flood, when a rapid decrease occurred due to di ution with melted snow which comprised 60% of runo fI/(Bishop et al. 1995). Increased water flow during the spring flood and the heavy rains in July lead to an increase in TOC in both tributaries,
543
although the increase was less pronounced in Kallklllbgcken where the diluted mire water neutralized the otherwise expected increase in TOC (Bishop et al. 1995). Estimations of the concentration of humic substances, based on I-IV-absorbance measurements, showed that the mire and KallklllbZcken contain a larger fraction of humic matter than VZistrab~cken, except for one month just before the spring flood when both creeks have a larger traction of humic matter than the mire (Pettersson et al. 1995). Aluminium and iron The total concentrations of Fe (Fe-tot) and Al (Al-tot) as well as the concentration of metals in the humic fraction (Fe-hum and Al-hum) were different in the different waters, with Al being the most variable (Fig. 2). The concentration of Fe was within the range 1.O-1.7 mg/L most of the year in the mire and Kallktillbgcken, but a rapid decrease was observed during the spring flood. During periods of low flow (i.e. January - March and November - December), the Fe concentration was higher in the mire than in Kallkgllbgcken, whereas during the rest of the year the opposite was observed. The total concentration of Al was low in the mire and slightly increased in Kallkiillbgcken. VlstrabZcken has a different character than Kallkiillb&ken. The concentration of Fe was lower but the concentration of Al was higher in Ustrabgcken than in Kallklllticken. In contrast to the mire and its tributary, though, there was a rapid increase to a peak in the concentration ofFe and Al (a maximum of 1.1 and 0.9 mgL, respectively, was observed) in V&nab&ken during high flow, i.e., during the spring flood and the rain storm. A large part of the Fe and Al was bound to humic substances (Fig. 2). The humic fraction of Fe and Al followed the total concentration, but the size of the fraction varied between the different waters for both metals. There was also an increase in the total concentration of both Fe and Al with increasing content of organic matter, and, thus, the concentration of humic substances, although the metals showed different patterns in the different waters (Fig. 3). To estimate the seasonal variation in the traction of Fe and Al which were associated to the humic substances, the ratios Fe-hum/Fe-tot and Al-hum/Al-tot, respectively, were calculated (Fig. 4). The overall picture of the partitioning of Fe and Al was that a larger fraction of Fe was bound to humic substances. In the humic-rich Kallktillb&ken, 70-90% of the Fe was associated with the humic matter and the size of the Fe-humic fraction
C. Pettersson and K. Bishop
544
Kallkallb&ken
a)
Kallk2illbZvzken
b, 1.2 -
Mire
V&.trab%ken
Vthtrab&ken 2.0 -
1.2 -
0.8 d ;,I.0
8
-
V
8
‘0 1 00.O
0.0
00
0
G
0 ?? ??4
E”
8 .6’
0.4 -
0 0
?? 00
Jan
Mar May Jul Sep
0.0
Nov
Fig. 2. Concentration of a) Fe and b) Al in the sampled waters.
0
?? .
0
0
0
T
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00
I I I I I I I 1 1
0.0
.
Jan
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4
NOV
??= total concentration, 0 = humic-bound fraction during the period January December 1993.
was relatively constant throughout the year. Almost the same fraction, 60-80%, of the Fe formed humic complexes in VIstrabacken, despite a lower concentration of humic matter, but the Fe-humic fraction showed a peak (>90%) during the spring flood in this creek. In the mire, the Fe-humic fraction varied from 50-70% except during the spring flood when almost all Fe was bound to humic matter. The size of the Al-humic fraction in the three waters varied within a wide range (Fig. 4), which seems to be due to the variation in the total concentration of Al
combined with the variation in the concentration of humic substances. Thus, during the main part of the year, 50-80% of Al in the mire was found in the humic fraction whereas only 20-30% of Al in Vgistrabacken and 40-60% in Kallklllbticken was associated to humic matter. During the spring flood, the size of the humic fraction increased to 70-90% of the total Al concentration in the mire as well as in the two tributaries, indicating that the rapid outflow of Al was related to easily mobilized Al-humic complexes. The extremely large Fe-humic and Al-humic fractions during the spring
Seasonal variations of total organic carbon, iron, and aluminium in northern Sweden
2.00
545
1
1.60 -
1.20 3 2 z 0.80 -
++
*
0
++
+ c+
10
4
20
30
40
60
40
60
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+
: c4
0.60
+++
$
+
++ + +
z
+ ++
0.40
0
10
20
30
TOC (mg/L)
Fig. 3. Total concentration of a) Fe and b) Al vs. TOC in all samples.
??=
Kallk%llb&ken, 0 = the mire, + = Vhtrabtlcken.
C. Pettersson and K. Bishop
546
KallkBllbBcken
b)
a)
Y g
80
g
60
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40
E 2
20
$ if
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??
g
80
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80
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40
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Wstrabhken
$ ??
Jan
*
?? ??
??
??
?? ??
Mar May Jul Sep
?? ??
Nov
Jan Mar May Jul Sep
NOV
Fig. 4. The ratio a) Fe-hum/Fe-tot and b) Al-hum/Al-tot in the sampled waters during the period January - December 1993.
flood suggests that the humic matter which was released had different binding properties during this period than during the rest of the year. Conditional stability constants
To compare the strengths of the Fe-humic and the Alhumic complexes, conditional stability constants for the complexation were calculated from the field data. The calculation was based on a simplified equilibrium reaction M + A- * MA, where M denotes Fe or Al and
MA represents the complex. The concentration of ionized humic substances, A-, was estimated under the assumption that the humic substances have a carboxylic content of 5 peq/mg which are dissociated to 50% in those waters (pH is close to pK, for the humic substances). The stability constant was calculated as p = [MA]/[M][A-1. In the calculation of log p for Fe, A- includes all ionized humic substances that have not complexed with Fe, i.e., it includes complexes with other metals as well as free A‘. A similar approach was used for Al. Log l3was calculated for all
Seasonal variations of total organic carbon, iron, and aluminium in northern Sweden
547
Table 1. Mean values of log 0 in the mire, Kallktilbilcken, and V&t&&ken.
Site
1% P (Fe)
log P (Al)
The mire
4.76
4.78
Kallktilbilcken
4.76
4.07
Vilstrabticken
5.17
4.74
sampling occasions during 1993. The analysis of Fe did not differentiate between Fe(I1) and Fe@), which might have different complexing ability towards humic substances. Assuming Fe to be dominated by either Fe(I1) or Fe(III), expressed in peq/L, log p was calculated for each case. The log /3 based on Fe(I1) was however, on average, only 0.06-o. 16 units below the values based on Fe(II1) for the three waters. Small differences in log l3 for Fe(II)-humic and Fe(III)-humic complexes, which support this finding, have been reported by Ephraim (1992). In the discussion, the results from Fe(I1) were chosen as being more likely to represent the oxidation state in the waters. The mean values of log p for the mire and the two tributaries are summarized in Table 1. The variation in log l3 with pH was also calculated (Fig. 5). The stability constants were slightly higher for Fe than for Al in both tributaries, compared to the mire where log l3was similar for the two metals. The mean value of log l3for Fe had the same value in Kallkiillbacken as in the mire but the variation in the values was larger in the mire (Fig. 5). Log p values for both the Fe-humic and Al-humic complexes were relatively independent of pH in the creeks, but increased sharply in the mire during the spring flood when the pH was low. In general, log p increased with increasing pH (Liivgren et al. 1987; Ephraim 1992; Pettersson et al. 1993), indicating that other factors are involved in the complexation during the spring flood. Thus, as expected, the fairly low pH values during the summer months (July to August) did not result in high log l3values. Calculations of pK, and degree of dissociation indicated that the organic acids tended to be more acidic during the spring flood than during the rest of the year (Bishop et al. 1996), a result that may explain the increase in log l3during the spring flood. Moreover, the increase in log l3during the spring flood coincided with the observed large fraction of the total amount of Fe and Al which was bound to the humic fraction on this occasion (Fig. 4). The log p-values for Al-complexation obtained in this study are comparable to results based on studies of
humic-rich bogwater (log p = 4.2-4.4 at pH 3.3-5.0; Lovgren et al. 1987) and laboratory experiments with complexation of Al to a fulvic acid (log p = 5.13 at pH 4.6-4.7; Clarke et al. 1995). Also, log p-values for Fecomplexation studies with a soil fulvic acid (Ephraim 1992) are in agreement with the results of this study. CONCLUSION
This study focussed on the variation of the fraction of Fe and Al associated to humic substances throughout a year in a mire and two tributaries (Kallkallbacken and Vastrabiicken) on a boreal catchment in northern Sweden. The size of the metal-humic traction was relatively constant within each water type, except during the spring flood when the humic fraction increased. The size of the fraction varied also between the sites, even though they were located in the same catchment. A larger part of Fe (60-90%) than Al (20-50%) was found in the humic fraction in the two tributaries. In the mire, though, a larger part of Al (60-80%) than Fe (50-70%) was bound to the humic fraction. The reason for this was probably the different relations between the concentrations of humic substances, Fe and Al in the different waters. The Fe content was similar in all three waters, whereas the concentration of Al was high in Wstrabbken and low in the mire. Moreover, the TOC and the concentration of humic matter varied between the different waters. During the spring flood, a rapid increase of the size of the Al-humic fraction was observed, particularly in the creeks. Also, the Fe-humic fraction in the mire was larger, whereas only small changes were found in the creeks. Conditional stability constants, p, were calculated from the field data. They were relatively independent of pH except during the spring flood when pH dropped l-2 units and, surprisingly, the log p-values increased. This effect was most pronounced in the mire for both metals. Despite the similarity of the calculated conditional stability constants for Fe and Al, the results indicated
C. Pettersson and K. Bishop
548
a)
7.0
1
b)
B
6.0
5.0
4.0
m z p
4.0
5.0
6.0
8 2
5.0
?z 4.0
0
5.0 4.0 3.0 -‘,
5.0
6.0
7.0 -l
6.0 -
14 2
5.0 -
-‘-Gv+ <+sk’ ~+ + + ++
++* s
I 4.0
I
I 5.0
I
6.0 ,/;--., &.t_.-_-+; + + + ++++
5.0
4.0 3.0
6.0
6.0
6.0
7.0 -
z
5.0
7.0 7
4.0
% =
4.0
T
3.0 I-’
Q
1
3.0 1,
3.0 *I
7.0
7.0
I 6.0
4.0 3.0
1
I
I
5.0
Fig. 5. Conditional stability constants, log 6, for a) Fe-humic complexes and b) Al-humic complexes vs pH. + = V&strabbken. Data from the spring flood are within the dotted line.
Acknowledgment-The authors would like to thank Bengt Andersson at the Environmental Quality Laboratory, Ume& for metal analyses. This study was financed by the Swedish Geological Survey.
+ ++
I
1 6.0
PH
PH
that Fe had a larger ability to form complexes with humic substances. A comparison between the two creeks showed that the high concentration of Al combined with the slightly decreased concentration of Fe in ViistrabZickenresulted in only a small decrease in the size of the Fe-humic fraction. Moreover, it seems the organic matter, Fe and Al originate from the same sources, since the increase in the concentration of organic matter was accompanied by an increase in the concentration of Fe and Al. Future studies will further elucidate the pathways of humic substances, Fe and Al from the soil to the surface waters.
I
4.0
+
??=
Kallktlllbacken, 0 = the mire,
REFERENCES Bishop, K.H. Episodic increases in stream acidity, catchment flow pathways and hydrograph separation. Ph.D. thesis. Cambridge, England: Cambridge University; 1991. Bishop, K.H.; Lee, Y.H.; Pettersson, C.; Allard, B. Methylmercury output from the Svartberget catchment in northern Sweden during the spring flood. Water Air Soil Pollut. 80: 445-454; 1995. Bishop, K.H.; Pettersson, C. Organic carbon in the boreal spring flood from adjacent subcatchments. Environ. Int. 22: 535-540; 1996. Clarke, N.; Danielsson, L.-G.; Spar&t, A. Studies of aluminium complexation to humic and fulvic acids using a method for the determination of quickly reacting aluminium. Water Air Soil Pollut. 84: 103-l 16; 1995. Ephraim, J.; Xu, H. The binding of cadmium by an aquatic fulvic acid: A comparison of ultrafiltration with ion-exchange distribution and ion-selective electrode techniques. Sci. Total Environ. 81182: 625-634; 1989.
Seasonal variations of total organic carbon, iron, and aluminium in northern Sweden
Ephraim, J.H. Iron interaction with a soil fulvic acid: Studies via potentiometric titrations, ultrafiltration and dialysis techniques. Ghana J. Chem. 1: 300-312; 1992. John, J.; Salbu, B.; Gjessing, E.T.; Bjsmstad, E. Effect of pH, humus concentration and molecular weight on conditional stability constants of cadmium. Water Res. 11: 1381-1388; 1988. Lovgren, L.; Hedlund, T.; Ghman, L.-O.; Sjoberg, S. Equilibrium approaches to natural water systems - 6. Acid-base properties of a concentrated bog-water and its complexation reactions with aluminium(II1). Water Res. 21: 1401-1407; 1987.
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M&night, D.M.; Feder, G.L.; Thurman, E.M.; Wershaw, R.L. Complexation of copper by aquatic humic substances from different environments. Sci. Total Environ. 28: 65-76; 1983. Pettersson, C.; H&kansson, K.; Karlsson, S.; Allard, B. Metal speciation in a humic surface water system polluted by acidic leachates from a mine deposit in Sweden. Water Res. 27(5): 863871; 1993. Pettersson, C.; Bishop, K.; Lee, Y.-H.; Allard, B. Relations between organic carbon and methylmercury in humic rich surface waters from Svartberget catchment in northern Sweden. Water Air Soil Pollut. 80: 971-979; 1995.
Pergamon
PIISO160-4120(96)00050-5
SEASONAL VARIATIONS OF TOTAL ORGANIC CARBON, IRON, AND ALUMINIUM ON THE SVARTBERGET CATCHMENT IN NORTHERN SWEDEN Catharina Pettersson Department of Water and Environmental Studies, Linkbping University, S-581 83 Linkbping, Sweden
Kevin Bishop Department of Forest Ecology, Swedish University of Agricultural Sciences, S-901 83 Umea, Sweden
El 9507-387 M (Received 26 April 1996; accepted 28 April 1996)
The variation in the concentration of organic carbon, especially humic substances, and two metals (Fe and Al) was studied in a mire and two tributaries on a boreal catchment in northern Sweden during one year. Water samples were collected monthly, except during the spring flood when more intensive sampling was made. The fraction of Fe and Al that was associated with the humic substances was estimated using an ion exchange technique. The total concentrations of Fe and Al increased as the concentrations of organic matter and humic substances increased. The fractionation study indicated that Fe has a larger tendency to form complexes with the humic fraction than Al has. The amount of the metals that were bound to the humic substances was relatively constant for the water from each location throughout the year, but the values varied within a broad range between the different sampling locations: 50-90% of the Fe and 20-80% of the Al was found in the humic fraction. Increased fractions of both metals were bound to the humic substances during the spring flood, i.e., 80-90% for both Fe and Al. Conditional stability constants for the formation of complexes, calculated from the field data, varied in the range 4.8-5.2 for Fe-humic complexes and 4.1-4.8 for Al-humic complexes.
INTRODUCTION Humic substances have a tendency to form complexes with metals, which influence their mobility and redistribution in soil and water. The complex formation is controlled by hydrochemical parameters such as pH, Eh, ionic strength and concentration of metals, and complexing agents. Hydrological processes determine the sequence of chemical environments through which the water passes, and hence the chemical forms of the metals. So far, most speciation studies have focussed on
trace metal speciation (M&night et al. 1983; John et al. 1988; Ephraim and Xu 1989; Pettersson et al. 1993). However, iron (Fe) and aluminium (Al) are often present in relatively high concentrations in humic-rich water and might, therefore, dominate the humic complexes. It has been observed that in surface water containing large concentrations of Fe and Al, an unexpectedly small fraction of copper was associated to the humic fraction (Pettersson et al. 1993). This was inter541
542
C. Pettersson and K. Bishop
Hillslope Research Area Mire Perennial Stream Intermittent Stream , 200m ,
Fig. 1. The Svartberget catchment.
preted as a competition between the different metals for the available binding sites on the humic matter. The binding of Fe (Ephraim 1992) and Al (Liivgren et al. 1987; Clarke et al. 1995) to humic substances has been studied in a few laboratory studies. Although fulvic acids isolated from surface water (Ephraim 1992; Clarke et al. 1995) or natural bogwater (Lovgren et al. 1987) were used in those experiments, it is not possible to directly transfer the results to processes in the natural environment. The aim of this study was to determine and compare the size of the fractions of Fe and Al associated to humic matter in surface waters with different character, i.e., different concentrations of humic substances, Fe, Al, and pH. The variation in the speciation was followed during one year and special attention was paid to the spring flood to observe the effects of high flow and rapid decline in pH on the Fe and Al speciation. Based on the results from the field study, it should be possible to calculate conditional stability constants for the formation of Fe-humic and Al-humic complexes in the waters.
EXPERIMENTAL
The study was performed on the Svartberget catchment (50 ha; Fig. 1) in northern Sweden. The catchment is covered with coniferous forest on podzol soils. The sampling was made in two tributaries (Kallki-illblcken and Vastrabacken, sites K and V, respectively) and the mire outlet (site M) from which Kallklllbacken originates. The area is thoroughly described elsewhere (Bishop 199 1). Weekly sampling of stream and mire water was conducted during the period 1 January to 3 1 December 1993. All samples were analysed for total organic carbon (TOC) (Shimadzu TOC-5000 utilizing catalytic combustion) and absorbance at 254 nm as a measure of the concentration of humic substances (Beckman DU-8 spectrophotometer). Monthly water samples were collected for analysis of total concentration of Fe and Al as well as the humicbound fraction of these metals. Major anions and cations and pH were also analysed. Samples for metal analysis were acidified immediately after sampling,
Seasonal variations of total organic carbon, iron, and aluminium in northern Sweden
whereas the other samples were stored in a freezer until analysis. The humic fraction of the organic matter, including the associated metals, was isolated in the field on a weak anion-exchange resin, diethylaminoethylSephadexA25 (Pharmacia Fine Chemicals) in a batch procedure (Pettersson et al. 1993). The resin was pretreated with HCl (suprapur) and rinsed with Milli-Qwater. After a contact time of 20 min, the supernatant, which contained the non-humic fraction, was decanted and collected. The concentration of Fe and Al in the supematant and the original water sample was determined on ICP-MS (Perkin Elmer Elan 5000). The fraction of Fe and Al bound to the humic fraction was calculated as the difference between these analyses. Polyethylene bottles were used in the metal fractionation procedure and were acid washed (20% I-INO, + 1% HCl) for at least 24 h and thoroughly rinsed with MilliQ-water prior to use. Samples were taken more frequently during the spring flood (April 24-29) and a rain storm in the summer (July 23-28). Those samples were analysed following the same procedure as the monthly samples. RESULTS AND DISCUSSION pH and organic mafter The flow pattern and runoff per unit area were similar in both tributaries, although the runoff from the smaller Wstrabticken tributary comprised only 18% of the cat&rent runoff, whereas KallkZllbZckenaccounted for the other 82%. The pH was relatively high (5.5-6.0) in both creeks during low flow in the winter and decreased during the spring flood to 4.0 in Kallk~llb&ken and 4.5 in Ustrabticken. A slow increase in pH followed during summer and autumn. The pH of the mire water was 0.31.O pH units below that of KallklllbZicken during the whole year except during the spring flood when the difference was less than 0.3 pH units (Pettersson et al. 1995). The mire outlet and Kallklllb~cken below the mire had a fairly high TOC content (30-40 mg/L and 1832 mg/L, respectively), while Wstrabiicken had a lower TOC content (8-20 mg/L) during the year, except for periods of high flow. During the spring flood, the TOC exhibited different patterns in the mire and the two tributaries. The TOC was relatively constant in the mire from January until the spring flood, when a rapid decrease occurred due to di ution with melted snow which comprised 60% of runo fI/(Bishop et al. 1995). Increased water flow during the spring flood and the heavy rains in July lead to an increase in TOC in both tributaries,
543
although the increase was less pronounced in Kallklllbgcken where the diluted mire water neutralized the otherwise expected increase in TOC (Bishop et al. 1995). Estimations of the concentration of humic substances, based on I-IV-absorbance measurements, showed that the mire and KallklllbZcken contain a larger fraction of humic matter than VZistrab~cken, except for one month just before the spring flood when both creeks have a larger traction of humic matter than the mire (Pettersson et al. 1995). Aluminium and iron The total concentrations of Fe (Fe-tot) and Al (Al-tot) as well as the concentration of metals in the humic fraction (Fe-hum and Al-hum) were different in the different waters, with Al being the most variable (Fig. 2). The concentration of Fe was within the range 1.O-1.7 mg/L most of the year in the mire and Kallktillbgcken, but a rapid decrease was observed during the spring flood. During periods of low flow (i.e. January - March and November - December), the Fe concentration was higher in the mire than in Kallkgllbgcken, whereas during the rest of the year the opposite was observed. The total concentration of Al was low in the mire and slightly increased in Kallkiillbgcken. VlstrabZcken has a different character than Kallkiillb&ken. The concentration of Fe was lower but the concentration of Al was higher in Ustrabgcken than in Kallklllticken. In contrast to the mire and its tributary, though, there was a rapid increase to a peak in the concentration ofFe and Al (a maximum of 1.1 and 0.9 mgL, respectively, was observed) in V&nab&ken during high flow, i.e., during the spring flood and the rain storm. A large part of the Fe and Al was bound to humic substances (Fig. 2). The humic fraction of Fe and Al followed the total concentration, but the size of the fraction varied between the different waters for both metals. There was also an increase in the total concentration of both Fe and Al with increasing content of organic matter, and, thus, the concentration of humic substances, although the metals showed different patterns in the different waters (Fig. 3). To estimate the seasonal variation in the traction of Fe and Al which were associated to the humic substances, the ratios Fe-hum/Fe-tot and Al-hum/Al-tot, respectively, were calculated (Fig. 4). The overall picture of the partitioning of Fe and Al was that a larger fraction of Fe was bound to humic substances. In the humic-rich Kallktillb&ken, 70-90% of the Fe was associated with the humic matter and the size of the Fe-humic fraction
C. Pettersson and K. Bishop
544
Kallkallb&ken
a)
Kallk2illbZvzken
b, 1.2 -
Mire
V&.trab%ken
Vthtrab&ken 2.0 -
1.2 -
0.8 d ;,I.0
8
-
V
8
‘0 1 00.O
0.0
00
0
G
0 ?? ??4
E”
8 .6’
0.4 -
0 0
?? 00
Jan
Mar May Jul Sep
0.0
Nov
Fig. 2. Concentration of a) Fe and b) Al in the sampled waters.
0
?? .
0
0
0
T
I 3 I 1 I I I I I 1
00
I I I I I I I 1 1
0.0
.
Jan
+@* **
Mar May Jul Sep
4
NOV
??= total concentration, 0 = humic-bound fraction during the period January December 1993.
was relatively constant throughout the year. Almost the same fraction, 60-80%, of the Fe formed humic complexes in VIstrabacken, despite a lower concentration of humic matter, but the Fe-humic fraction showed a peak (>90%) during the spring flood in this creek. In the mire, the Fe-humic fraction varied from 50-70% except during the spring flood when almost all Fe was bound to humic matter. The size of the Al-humic fraction in the three waters varied within a wide range (Fig. 4), which seems to be due to the variation in the total concentration of Al
combined with the variation in the concentration of humic substances. Thus, during the main part of the year, 50-80% of Al in the mire was found in the humic fraction whereas only 20-30% of Al in Vgistrabacken and 40-60% in Kallklllbticken was associated to humic matter. During the spring flood, the size of the humic fraction increased to 70-90% of the total Al concentration in the mire as well as in the two tributaries, indicating that the rapid outflow of Al was related to easily mobilized Al-humic complexes. The extremely large Fe-humic and Al-humic fractions during the spring
Seasonal variations of total organic carbon, iron, and aluminium in northern Sweden
2.00
545
1
1.60 -
1.20 3 2 z 0.80 -
++
*
0
++
+ c+
10
4
20
30
40
60
40
60
TOC (mg/L)
+
: c4
0.60
+++
$
+
++ + +
z
+ ++
0.40
0
10
20
30
TOC (mg/L)
Fig. 3. Total concentration of a) Fe and b) Al vs. TOC in all samples.
??=
Kallk%llb&ken, 0 = the mire, + = Vhtrabtlcken.
C. Pettersson and K. Bishop
546
KallkBllbBcken
b)
a)
Y g
80
g
60
C .o
40
E 2
20
$ if
Mire 100
??
g
80
g C .o
60
?? ??
g ??
+
??
??
??? ? ?
80
?? ??
40
E J 20
c
Eistrabiicken
Wstrabhken
$ ??
Jan
*
?? ??
??
??
?? ??
Mar May Jul Sep
?? ??
Nov
Jan Mar May Jul Sep
NOV
Fig. 4. The ratio a) Fe-hum/Fe-tot and b) Al-hum/Al-tot in the sampled waters during the period January - December 1993.
flood suggests that the humic matter which was released had different binding properties during this period than during the rest of the year. Conditional stability constants
To compare the strengths of the Fe-humic and the Alhumic complexes, conditional stability constants for the complexation were calculated from the field data. The calculation was based on a simplified equilibrium reaction M + A- * MA, where M denotes Fe or Al and
MA represents the complex. The concentration of ionized humic substances, A-, was estimated under the assumption that the humic substances have a carboxylic content of 5 peq/mg which are dissociated to 50% in those waters (pH is close to pK, for the humic substances). The stability constant was calculated as p = [MA]/[M][A-1. In the calculation of log p for Fe, A- includes all ionized humic substances that have not complexed with Fe, i.e., it includes complexes with other metals as well as free A‘. A similar approach was used for Al. Log l3was calculated for all
Seasonal variations of total organic carbon, iron, and aluminium in northern Sweden
547
Table 1. Mean values of log 0 in the mire, Kallktilbilcken, and V&t&&ken.
Site
1% P (Fe)
log P (Al)
The mire
4.76
4.78
Kallktilbilcken
4.76
4.07
Vilstrabticken
5.17
4.74
sampling occasions during 1993. The analysis of Fe did not differentiate between Fe(I1) and Fe@), which might have different complexing ability towards humic substances. Assuming Fe to be dominated by either Fe(I1) or Fe(III), expressed in peq/L, log p was calculated for each case. The log /3 based on Fe(I1) was however, on average, only 0.06-o. 16 units below the values based on Fe(II1) for the three waters. Small differences in log l3 for Fe(II)-humic and Fe(III)-humic complexes, which support this finding, have been reported by Ephraim (1992). In the discussion, the results from Fe(I1) were chosen as being more likely to represent the oxidation state in the waters. The mean values of log p for the mire and the two tributaries are summarized in Table 1. The variation in log l3 with pH was also calculated (Fig. 5). The stability constants were slightly higher for Fe than for Al in both tributaries, compared to the mire where log l3was similar for the two metals. The mean value of log l3for Fe had the same value in Kallkiillbacken as in the mire but the variation in the values was larger in the mire (Fig. 5). Log p values for both the Fe-humic and Al-humic complexes were relatively independent of pH in the creeks, but increased sharply in the mire during the spring flood when the pH was low. In general, log p increased with increasing pH (Liivgren et al. 1987; Ephraim 1992; Pettersson et al. 1993), indicating that other factors are involved in the complexation during the spring flood. Thus, as expected, the fairly low pH values during the summer months (July to August) did not result in high log l3values. Calculations of pK, and degree of dissociation indicated that the organic acids tended to be more acidic during the spring flood than during the rest of the year (Bishop et al. 1996), a result that may explain the increase in log l3during the spring flood. Moreover, the increase in log l3during the spring flood coincided with the observed large fraction of the total amount of Fe and Al which was bound to the humic fraction on this occasion (Fig. 4). The log p-values for Al-complexation obtained in this study are comparable to results based on studies of
humic-rich bogwater (log p = 4.2-4.4 at pH 3.3-5.0; Lovgren et al. 1987) and laboratory experiments with complexation of Al to a fulvic acid (log p = 5.13 at pH 4.6-4.7; Clarke et al. 1995). Also, log p-values for Fecomplexation studies with a soil fulvic acid (Ephraim 1992) are in agreement with the results of this study. CONCLUSION
This study focussed on the variation of the fraction of Fe and Al associated to humic substances throughout a year in a mire and two tributaries (Kallkallbacken and Vastrabiicken) on a boreal catchment in northern Sweden. The size of the metal-humic traction was relatively constant within each water type, except during the spring flood when the humic fraction increased. The size of the fraction varied also between the sites, even though they were located in the same catchment. A larger part of Fe (60-90%) than Al (20-50%) was found in the humic fraction in the two tributaries. In the mire, though, a larger part of Al (60-80%) than Fe (50-70%) was bound to the humic fraction. The reason for this was probably the different relations between the concentrations of humic substances, Fe and Al in the different waters. The Fe content was similar in all three waters, whereas the concentration of Al was high in Wstrabbken and low in the mire. Moreover, the TOC and the concentration of humic matter varied between the different waters. During the spring flood, a rapid increase of the size of the Al-humic fraction was observed, particularly in the creeks. Also, the Fe-humic fraction in the mire was larger, whereas only small changes were found in the creeks. Conditional stability constants, p, were calculated from the field data. They were relatively independent of pH except during the spring flood when pH dropped l-2 units and, surprisingly, the log p-values increased. This effect was most pronounced in the mire for both metals. Despite the similarity of the calculated conditional stability constants for Fe and Al, the results indicated
C. Pettersson and K. Bishop
548
a)
7.0
1
b)
B
6.0
5.0
4.0
m z p
4.0
5.0
6.0
8 2
5.0
?z 4.0
0
5.0 4.0 3.0 -‘,
5.0
6.0
7.0 -l
6.0 -
14 2
5.0 -
-‘-Gv+ <+sk’ ~+ + + ++
++* s
I 4.0
I
I 5.0
I
6.0 ,/;--., &.t_.-_-+; + + + ++++
5.0
4.0 3.0
6.0
6.0
6.0
7.0 -
z
5.0
7.0 7
4.0
% =
4.0
T
3.0 I-’
Q
1
3.0 1,
3.0 *I
7.0
7.0
I 6.0
4.0 3.0
1
I
I
5.0
Fig. 5. Conditional stability constants, log 6, for a) Fe-humic complexes and b) Al-humic complexes vs pH. + = V&strabbken. Data from the spring flood are within the dotted line.
Acknowledgment-The authors would like to thank Bengt Andersson at the Environmental Quality Laboratory, Ume& for metal analyses. This study was financed by the Swedish Geological Survey.
+ ++
I
1 6.0
PH
PH
that Fe had a larger ability to form complexes with humic substances. A comparison between the two creeks showed that the high concentration of Al combined with the slightly decreased concentration of Fe in ViistrabZickenresulted in only a small decrease in the size of the Fe-humic fraction. Moreover, it seems the organic matter, Fe and Al originate from the same sources, since the increase in the concentration of organic matter was accompanied by an increase in the concentration of Fe and Al. Future studies will further elucidate the pathways of humic substances, Fe and Al from the soil to the surface waters.
I
4.0
+
??=
Kallktlllbacken, 0 = the mire,
REFERENCES Bishop, K.H. Episodic increases in stream acidity, catchment flow pathways and hydrograph separation. Ph.D. thesis. Cambridge, England: Cambridge University; 1991. Bishop, K.H.; Lee, Y.H.; Pettersson, C.; Allard, B. Methylmercury output from the Svartberget catchment in northern Sweden during the spring flood. Water Air Soil Pollut. 80: 445-454; 1995. Bishop, K.H.; Pettersson, C. Organic carbon in the boreal spring flood from adjacent subcatchments. Environ. Int. 22: 535-540; 1996. Clarke, N.; Danielsson, L.-G.; Spar&t, A. Studies of aluminium complexation to humic and fulvic acids using a method for the determination of quickly reacting aluminium. Water Air Soil Pollut. 84: 103-l 16; 1995. Ephraim, J.; Xu, H. The binding of cadmium by an aquatic fulvic acid: A comparison of ultrafiltration with ion-exchange distribution and ion-selective electrode techniques. Sci. Total Environ. 81182: 625-634; 1989.
Seasonal variations of total organic carbon, iron, and aluminium in northern Sweden
Ephraim, J.H. Iron interaction with a soil fulvic acid: Studies via potentiometric titrations, ultrafiltration and dialysis techniques. Ghana J. Chem. 1: 300-312; 1992. John, J.; Salbu, B.; Gjessing, E.T.; Bjsmstad, E. Effect of pH, humus concentration and molecular weight on conditional stability constants of cadmium. Water Res. 11: 1381-1388; 1988. Lovgren, L.; Hedlund, T.; Ghman, L.-O.; Sjoberg, S. Equilibrium approaches to natural water systems - 6. Acid-base properties of a concentrated bog-water and its complexation reactions with aluminium(II1). Water Res. 21: 1401-1407; 1987.
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M&night, D.M.; Feder, G.L.; Thurman, E.M.; Wershaw, R.L. Complexation of copper by aquatic humic substances from different environments. Sci. Total Environ. 28: 65-76; 1983. Pettersson, C.; H&kansson, K.; Karlsson, S.; Allard, B. Metal speciation in a humic surface water system polluted by acidic leachates from a mine deposit in Sweden. Water Res. 27(5): 863871; 1993. Pettersson, C.; Bishop, K.; Lee, Y.-H.; Allard, B. Relations between organic carbon and methylmercury in humic rich surface waters from Svartberget catchment in northern Sweden. Water Air Soil Pollut. 80: 971-979; 1995.