the interaction of natural organic matter

8/17/2019 The Interaction of Natural Organic Matter

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Aquatic Geochemistry 5: 207–223, 1999.

© 1999 Kluwer Academic Publishers. Printed in the Netherlands. 207

The Interaction of Natural Organic Matter with Ironin a Wetland (Tennessee Park, Colorado) Receiving

Acid Mine Drainage

STEFAN PEIFFER1, KATHERINE WALTON-DAY2 and DONALD L.

MACALADY31 Limnological Station, Department of Hydrology, University of Bayreuth, D-95440 Bayreuth;2United States Geological Survey, Denver Federal Center, Box 25046, M5415, CO 80225, USA;3 Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, CO, 80315, USA

(Received November 1998)

Abstract. Pore water from a wetland receiving acid mine drainage was studied for its iron and natural

organic matter (NOM) geochemistry on three different sampling dates during summer 1994. Samples

were obtained using a new sampling technique that is based on screened pipes of varying length (sev-

eral centimeters), into which dialysis vessels can be placed and that can be screwed together to allow

for vertical pore-water sampling. The iron concentration increased with time (through the summer)

and had distinct peaks in the subsurface. Iron was mainly in the ferrous form; however, close to the

surface, significant amounts of ferric iron (up to 40% of 2 mmol L−1 total iron concentration) were

observed. In all samples studied, iron was strongly associated with NOM. Results from laboratory

experiments indicate that the NOM stabilizes the ferric iron as small iron oxide colloids (able to pass

a 0.45-µm dialysis membrane). We hypothesize that, in the pore water of the wetland, the high NOM

concentrations (>100 mg C L−1) allow formation of such colloids at the redoxcline close to the

surface and at the contact zone to the adjacent oxic aquifer. Therefore, particle transport along flow

paths and resultant export of ferric iron from the wetland into ground water might be possible.

Key words: natural organic matter, ferrous iron, ferric iron, wetland, colloidal iron, acid mine

drainage, pore water

1. Introduction

Iron is one of the main products of the weathering of pyrite (Theobald et al.,

1963). Together with high acidity, elevated iron concentrations characterize the

water chemistry of surface water that receives acid mine drainage. Such waters

also frequently contain elevated concentrations of trace metals (Chapman et al.,1983; Karlsson et al., 1988; Nordstrom and Alpers, in press).

On entering a wetland, some metal ions are immobilized by sorption to or

precipitation as metal oxides or sulfides (Machemer and Wildeman, 1991; Walton-

Day, 1991). Such processes are mainly linked to the redox chemistry of iron in the

wetland:


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208 STEFAN PEIFFER ET AL.

– The precipitation of ferric iron as amorphous ferric oxides results in adsorp-

tion of trace metals (Tessier et al., 1985).

– The reduction of ferric iron to ferrous iron and the subsequent formation

of pyrite may result in incorporation of trace metals into the mineral phase(Huerta-Diaz and Morse, 1992). This process is the reverse of the metal re-

lease that occurs during weathering of pyrite.

A previous field study about the effect of a natural wetland receiving acid mine

drainage (Tennessee Park, Colorado) on ground-water chemistry, however, indi-

cated a possible net export of filterable (


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THE INTERACTION OF NATURAL ORGANIC MATTER WITH IRON 209

Figure 1. Location of the wetland and the monitoring wells.


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emanates from tailings piles located about 2 km upstream from the discharge of

St. Kevin Gulch into the wetland. This acid mine drainage has a pH of 2.7 and

contains elevated concentrations of many constituents (Smith, 1991). The acid

mine drainage is diluted by the water of St. Kevin Gulch to a pH ranging from

3.5 to 4.5 before entering the wetland.

The ground-water system of Tennessee Park consists of a sand and gravel aquifer

overlain by the wetland (Paschke and Harrison, 1995). The hydrology of the wet-

land is characterised by a rise in the water table between October and June (Walton-

Day, 1991) caused by accumulation of in-situ snow melt, and by excess surface-

water runoff. During the runoff period (May and June), slight upward gradients

cause minor ground-water flow into the base of the wetland sediments from the

underlying aquifer. As runoff wanes during the summer (July and August), the

water table in the wetland gradually lowers and the ground-water gradients reverse:

Water from the wetland sediments recharges the underlying aquifer.

3. Material and Methods

3.1. SAMPLING DESIGN

3.1.1. Shallow ground-water sampling

Samples were collected from the shallow ground-water monitoring wells MW-10

and MW-13 and from St. Kevin Gulch near MW-13 (cf. Figure 1) at four times

during the period of decreasing water table in the wetland (June 13, July 6, July

28 and Aug. 22, 1994). Additional ground water was sampled from wells MW-8

and MW-6 on June 13. Ground-water samples were collected after at least three

casing volumes of water had been pumped from each well or after stabilization of

conductivity. All monitoring wells are screened over a 60-cm interval at a depthgreater than 1 meter below the ground surface. For a detailed description of the

well construction, see Walton-Day (1991). The samples were analyzed for pH,

conductivity, alkalinity, dissolved sulfide, NOM, and total and ferrous iron (cf.

Section 3.3 for the details).

3.1.2. Pore-water sampling

To gain information about the redox transformations close to the water table in the

wetland, dialysis pipes were constructed that allowed the sampling of pore water

at different depths close to well MW-13 (Figure 2). Screened polyvinylchloride

(PVC) pipes, with a diameter of 3.75 cm and of different length (6, 12, 18 and

30 cm) were threaded on both ends (Figure 3). Into one end, a solid PVC tap wasglued, to which a second PVC pipe could be screwed. A series of such pipes thus

forms a longer pipe with the individual segments sealed against each other. In the

field, a single pipe segment was pushed into predrilled holes until the segment

was covered with water, and a glassy serum bottle (height 3.5 cm, 1.5 cm)

was placed into the segment. Then, a second pipe segment was screwed onto the


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Figure 2. Location of the pore-water well. The inset shows the decreasing water level in the

well.

first one and pushed deeper into the hole and so on. Like dialysis chambers (e.g.,

Höpner, 1981) the serum bottles were filled with deionized and deaerated water in

the laboratory. Instead of the rubber septum, the vials were covered with a cellulose

acetate membrane (0.45 µm), which allowed diffusive transport of ions from the

pore water entering the PVC pipe.

After a period of equilibration, on three dates during the summer of 1994 (July6, July 28 and August 22), the pipes were removed and the membranes of the

serum bottles were opened. The samples were either analyzed directly in the field

using field kits (cf. Section 3.3 for the details) or filled into glass vials for TOC

analysis in the laboratory. No conservation of the TOC samples was performed.

The equilibration times were 22, 22, and 25 days.


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Figure 3. Scheme of two segments of the dialysis pipes for pore-water sampling. More

segments can be added to the top. See text for a detailed description of the device.


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The great advantage of this construction is its flexibility with respect to the

variable water table. Unlike dialysis chambers, which usually cover only a short

vertical distance (40 cm), this system permits one to follow the declining water

table and to sample with a higher resolution (here 6 cm) at this depth, whereas at

deeper levels, larger PVC segments can be used.

3.2. FRACTIONATION OF NATURAL ORGAN IC MATTER

Fractionation of NOM was studied in a sample from pore water close to well MW-

13 and from ground water from well MW-6. In this sample, which had been stored

in the refrigerator for approximately 1.5 years, all dissolved iron was in the ferric

form (see Section 4.1). To study the effect of colloid stabilization by NOM, the

fractionation was performed before and after ferric iron in solution was flocculated

using 0.1 mol L−1 MgCl2.

The XAD-fractionation technique as described by Thurman (1984) was used.

In brief, an acidified sample is pumped at a constant flow rate of 1 mL min−1 intoa column that is packed with a nonionic acrylic ester (XAD-8). In a first step, the

humic substances of the NOM adsorb to XAD-8, whereas the hydrophilic efflu-

ent is collected and analyzed for its organic matter content. In a second step, the

column is back-eluted with NaOH, and the eluate with the humic substances also

is collected and analyzed for its organic matter content. In this article we present

results from the first step only.

3.3. ANALYTICAL TECHNIQUES

Field kits (HACH) were used to measure alkalinity, dissolved sulfide, total iron

and ferrous iron in the field directly after sampling. The time span between the

opening of the vials and the fixation of a sample aliquot in the field kits never

exceeded 1 minute so that interferences by degassing of hydrogen sulfide or ox-

idation of ferrous iron can be excluded. Except for alkalinity, all concentrations

were determined colorimetrically, and all concentrations are mean values (SD =


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Table I. Natural organic matter (NOM) concentration and percentage of humic substances

retained after XAD-8 fractionation of NOM from ground water samples from well MW-6

before and after flocculation of ferric iron with 0.1 mol L−1MgCl2 and from pore waters of

two different depths sampled close to well MW-13 (cf. Figure 2). All NOM concentrations

given are in mg Carbon L

−1

. The well MW-6 sample was stored in the refrigerator for 1.5years.

Sample Untreated Hydrophilic Percentage of humic

effluent from the substances retained

XAD column

Well MW-6 with iron 22.4 ± 0.9 12.0 ± 0.7 46 ± 3

Well MW-6 after flocculation 14.6± 0.1 16.6 ± 0.5 –

Pore water 53 cm below 64.1± 0.1 25.6 ± 0.9 60 ± 2

surface (Aug. 22, 1994)

Pore water 60 cm below 28.5± 0.6 20.3 ± 1.6 29 ± 2

surface (Aug. 22, 1994)

The bias due to reduction of the colorless ferric iron – phenanthroline com-

plex in the presence of NOM and subsequent color formation was negligible (<

1 µmol L−1) within the one minute reaction time before reading. Natural Organic

Matter was measured using a TOC-analyzer after addition of 1 volume percent

concentrated HCl. The deionized water had relatively high blank values of 2.5 mg

C L−1. Detection limits were 0.03 mmol L−1 for iron, 5 µmol L−1 for S(-II),

0.1 mmol L−1 for alkalinity and 0.5 mg L−1 for NOM.

4. Results

4.1. FRACTIONATION OF NATURAL ORGAN IC MATTER

Table I shows the results from the XAD-8 fractionation procedure. The data in-

dicate that 40 to 60% of the NOM in the pore water close to well MW-13 and

in the ground water from well MW-6 was of hydrophilic nature. The initial iron

concentration in the well MW-6 sample we used was 442 µmol L−1 in the ferric

form and was filterable through an 0.45-µm cellulose acetate filter. It could bealmost completely flocculated on addition of 0.1 mol L−1 MgCl2. This procedure

resulted in a decrease of the total iron content to 14 µmol L−1 and the NOM content

to 65% of the initial value. Flocculation of iron also resulted in a complete removal

of the NOM fraction containing the humic substances from the solution, which

then contained only hydrophilic compounds of the NOM.


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4.2. PORE- AND GROUND-WATER COMPOSITION

Table II presents an overview of the water chemistry in the shallow ground-water

wells. All wells are reduced and contain dissolved sulfide; well MW-6 contained

the greatest concentrations. Ferric iron was present in all wells, except in well MW-6 where sulfide concentrations were the greatest. The pH increases slightly towards

the outflow from the wetland. Although the water table fell during the summer, the

water chemistry in wells MW-13 and MW-10 was fairly stable. In contrast, St.

Kevin Gulch clearly reflects the seasonal variations. The pH, alkalinity, and NOM

concentration increase towards the end of summer. In addition, at St. Kevin Gulch

on July 28, one-third of the total iron was in the ferrous form, and on Aug. 22 40%

was in the ferrous form, which is due to photoreductive processes (McKnight et

al., 1988).

Seasonal variation was also observed in the pore waters. Figure 4 shows a plot of

the total iron concentration versus depth in the pore-water well. Although we could

not analyze the pore water in the upper few centimeters on July 6, it is clear that

the total iron concentration is highest on July 28, except at 50-cm depth. The iron

concentration peaks within 15 cm of the surface on July 28 and Aug. 22, which

coincides with the decreasing water level at these sampling dates (cf. Figure 2).

On July 6, the wetland was still flooded at this well. It seems that the decreasing

water level coincides with an increase in concentration of species in the pore water

relative to the well water by a factor of three for dissolved iron and even higher for

the NOM (cf. Figure 4 and Figure 5).

The decreasing water level also affects the redox conditions in the system with

the relative portion of ferric iron highest on July 28 comprising more than 40%

of the total iron content (Figure 6). It further seems that the iron concentration is

closely related to the high organic carbon content (>100 mg L−1, Figure 5), which

also is highest on July 28. A linear-regression analysis between these two variableswas calculated with data from the sampling dates July 28 and Aug. 22 and from all

depths, which yields (r2 = 0.89, p > 0.999, c(NOM) and c(Fetot) in mmol L−1):

c(Fetot) = 0.31 ± 0.083 + (0.17 ± 0.017) · c(NOM) (3)

5. Discussion

As was pointed out in Section 4.1, the ground-water sample from well MW-6 was

stored in the refrigerator for more than a year and still had an iron concentration of

442 µmol L−1 at a pH of 6.4, which was all in the ferric form. Such a concentration

distinctly exceeds the solubility of ferric oxides at this pH,

Fe(OH)3 ⇔ Fe3+ + 3 OH− (4)

which would be 10−15.2 mol L−1 (Ksp(FeOH3) ≈ 10−38 mol4 L−4, Stumm and Mor-

gan, 1996). Iron could not be removed by filtration through a 0.45- µm filter, which


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Table II. Composition of ground water and water from St. Kevin Gulch close to well MW-13 at

during summer 1994 (n. d. = not detectable; blanks indicate that the parameter was not measu

S-(II)t ot = total sulfide; el. cond. = electrical conductivity.

Well # Date NOM Fe2+ Fe3+ Alk S(-II)tot

(mg C L−1) (mmol L−1) (mmol L−1) (meq L−1) (µmol L−

MW-8 Jun 13 0.20 n. d. 0.5 4.7

MW-6 Jun 13 0.54 n. d. 1.2 46.0

MW-13 Jun 13 0.28 0.09 1.2 5.3

Jul 6 20.0 0.28 0.06 0.8 n. d.

Jul 28 10.0 0.36 0.04 0.9 n. d.

Aug 22 8.2 0.36 0.06 0.8 n. d.

MW-10 Jun 13 0.60 n. d. 1.5 6.2

Jul 6 26.6 0.38 0.18 1.3 n. d.

Jul 28 16.2 0.60 n. d. n. d. n. d.

Aug 22 11.8 0.54 0.09 1.3 15.6

SK Gulch Jul 6 6.1 n. d. n. d. n. d. n. d.

Jul 28 4.3 n. d. n. d. 0.04 n. d.

Aug 22 35.3 n. d. n. d. 0.85 n.d.


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Figure 4. Vertical pore-water profiles of total dissolved iron on three different dates. The

concentration increases during summer and shows maxima close to the water level and at

40-cm depth.

indicates that the ferric iron stays in solution either complexed by the NOM or as

very small colloids.

The NOM content of the well MW-6 sample is 24.6 mg C L−1. Therefore, the

molar Fe/C-ratio is 0.22, which is similar to the ratio described by Equation (3).

Such a ratio implies that extremely small organic ligands complex the ferric iron,

which seems unreasonable. McKnight et al. (1985) reported that the fulvic acids

isolated from a Sphagnum bog with similarly high NOM concentrations have a

molecular mass range from 1000 to 5000 g mol−1 and a mean carbon content of

48.5%. Even with a molecular mass of 1000 g mol

−1

, the resulting total ligand con-centration would not exceed (24.6 mg L−1 /0.485/1000 g mol−1) ≈ 50 µmol L−1.

Given the dissolved-iron concentrations of 442 µmol L−1, ferric iron in the stored

well MW-6 sample would be excessive compared to organic ligands, and therefore,

precipitation of iron as ferric hydroxides would be favoured. It seems, however,

that fulvic acids are associated with the ferric oxides. From Table I it becomes


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Figure 5. Vertical pore-water profiles of NOM at two different dates. The NOM concentration

correlates well with the total iron concentration (Figure 4).

clear that the addition of MgCl2 coprecipitates the humic substance fraction of the

NOM together with the iron. In addition, the concentration of carboxylic groups

of the NOM exceeds that of reactive surface sites at the ferric oxide surface as the

following rough estimate indicates:

c(—COOH) = c(NOM) · CSC ∼ 120 µmol L−1 (5)

where c(-COOH) is the concentration of carboxylic groups in solution; c(NOM) is

the concentration of NOM in solution, 0.0246 g C L−1; CSC is the coordinating

site content, 4.5–4.9 mmol (g NOM)−1 (Buffle, 1988)

c(sites) = c(Fe) MMFe · SSA · SCRS ∼ 20 µmol L−1

(6)

where c(Fe) is the concentration of iron, 442 µmol L−1; MMF e the molecular mass

of iron, 55.9 g mol−1; SSA the specific surface area, 300 m2 g−1 (Dzombak and

Morel, 1990); SCRS the surface concentration of reactive sites, 2.2 µmol m−2

(Dzombak and Morel, 1990)


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One might envison a surface reaction between the carboxylic groups of the

fulvic acids and the ferric oxides (Tipping, 1981; Balistrieri and Murray, 1987)

that results in a negative surface charge of the particles (Tipping and Cooke, 1982).

We, therefore, hypothesize that the suspended ferric oxides are stabilized by the

adsorbed NOM.

Fulvic acids have a positive effect on the colloid stability of hematite, pro-

vided that the pH is close to neutral (between 6 and 7) and the concentration of

Ca2+ and Mg2+ is low (Liang and Morgan, 1990; Amirbahman and Olson, 1995).

Moreover, oxidation of ferrous iron in the presence of high amounts of organic

compounds seems to promote the formation of extremely small ferric oxide

particles (von Gunten and Schneider, 1991). At fulvic acid concentrations

>0.1 mg L−1, colloid stability may arise from repulsion between negatively

charged goethite particles to which humic substances are adsorbed (Liang and

Morgan, 1990). In hard water, the negative charge is balanced by Ca 2+ and Mg2+

ions (Tipping and Cooke, 1982) resulting in reduction of colloid stability (Amir-

bahman and Olson, 1995). Therefore, conditions in the shallow groundwater of the Tennessee Park wetland are ideal for forming stable colloids with

its soft water (c(Ca), c(Mg) < 1 mmol L−1, Walton-Day, 1991), circum-neutral

pH, high ferrous iron, and NOM concentrations (Table II).

The high percentage of dialysable ferric iron in the pore waters of the wetland

during summer (Figure 6) may be explained by the combined effect of oxida-

tion of ferrous iron and colloid formation in the presence of high amounts of

NOM. Enhanced breakdown of organic matter and excretion of NOM from the

higher plants is concomitant with the increase of primary productivity of the bio-

mass in the wetland during summer (Hemond, 1982; Wetzel, 1983) and results in

high surface concentrations of NOM in the wetland. McKnight et al. (1985) also

observed greater NOM concentrations at the surface of a Sphagnum bog with amaximum of 62 mg L−1 at late summer. Additionally, evapotranspiration affects

the concentration of dissolved compounds (Mulholland and Kuenzler, 1979) so

that a seasonal increase of NOM in the pore waters of wetlands with a maximum

during mid-summer is a common phenomenon in wetlands (Marin et al., 1990,

Yavitt, 1994). Also, the oxygen production is enhanced close to the surface. As

Wetzel (1983) states, higher plants in wetlands are colonized by epiphytic algae,

whose productivity and oxygen production often exceed that of the macrophytes.

The high redox potential then favours oxidation of ferrous iron. In the presence of

excess negative surface charge from the NOM, ferric oxide colloids then are able

to form. A time scale analysis according to Webster et al. (1998) demonstrated that

the equilibration time in the field is sufficient to allow diffusion of such colloids

into the serum bottles. The time necessary to establish 90% equilibrium of the ironcolloids (size of the iron colloids: 5 nm; diffusion coefficient: 4 · 10−7 cm2 s−1)

would be 27 days, which is somewhat longer than the equilibration time we used

(cf. Section 3.1).


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Figure 6. Vertical profile of the ratio: ferric to total iron in the pore water. The high ratio close

to the surface on July 28 reflects changing redox conditions during summer.

Liang et al. (1993) observed similar effects of NOM that was injected with

oxygen-containing water into a sandy aquifer containing Fe(II). Because of the

presence of NOM, small colloids (


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6. Conclusion

The pore-water sampling device designed for this study was an important tool to

elucidate the dynamics of the interaction between iron and NOM in a wetland of

varying water levels. There is strong evidence from this study that the elevatedNOM concentrations in this wetland have a significant mobilizing effect on ferric

iron. Our data indicate that the following processes occur at the boundary between

oxic and anoxic pore water close to the surface of the wetland. During summer,

evapotranspiration results in a subsurface concentration of dissolved substances,

such as ferrous iron, in the pore water. This effect coincides with high photo-

synthetic activity that produces large amounts of organic carbon and also pumps

oxygen into the pore water; therefore, oxidation of ferrous iron can occur in the

presence of high NOM concentrations. Given the low hardness of the pore water,

conditions are ideal for the formation of small iron oxide colloids stabilized by

surficially bound organic matter. This effect is reflected by the high fraction of

ferric iron compared to the total dissolved iron concentration found in these porewaters.

Because the wetland is underlain by an oxic deeper ground-water aquifer, we

hypothesize that similar processes occur at the bottom of the wetland. Because of

reducing conditions in the wetland, ferrous iron is transported during summer to

the redoxcline at the oxic aquifer. Oxidation to ferric iron occurs in the presence of

NOM and, therefore, part of the ferric oxide is stabilized as colloidal iron. These

small particles then can be transported with the ground water, which could explain

the net export of iron from the wetland hypothesized by Walton-Day (1991).

Acknowledgments

This study was performed as part of a grant given by the Deutsche Forschungsge-

meinschaft (Pe 438/2-1) to the first author of this paper. We are grateful to Suzanne

Paschke, Marion Peiffer, Jim Ranville, and Ning Li Zu for their field assistance, to

Diane McKnight for her support in the XAD fractionation and to Silke Bär for the

measurements of the NOM. We thank Aria Amirbahman and Diane McKnight and

two anonymous reviewers for their critical comments on an earlier version of this

manuscript. Vern Tate assisted in the design of the pore-water samplers.

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