Environ. Sci. Technol. 1994, 28, 1698-1705
Speciation of Fe( I I ) and Fe( I I I ) in Contaminated Aquifer Sediments Using Chemical Extraction Techniques Gorm Heron,'gt Catherine Crouzet,* Alain C. M. Bourg,$ and Thomas H. Christensent Institute of Environmental Science and Engineering, Groundwater Research Centre, Technical University of Denmark, Building 115, DK-2800 Lyngby, Denmark, and Department of Geochemistry, National Geological Survey, BRGM, BP 6009, F-45060 Orl6ans Cedex 02, France
The iron mineralogy of aquifer sediments was described by chemical extraction techniques. Single-step extractions including 1 M CaC12, NaAc, oxalate, dithionite, Ti(II1)EDTA, 0.5 M HC1,5 M HC1, hot 6 M HC1, and a sequential extraction by HI and CrIIHC1 were tested on standard iron minerals and nine aquifer sediments from different redox environments sampled in a landfill leachate plume. Ion-exchangeable Fe(I1) is easily quantified by anaerobic CaClz extraction. A rapid indication of the redox status of a sediment sample can be achieved by a 0.5 M HC1 extraction. This extraction gives an indication of the content of amorphous Fe(II1) and reduced Fe(I1) species such as FeS and FeC03, though the fractions are not quantified. A good estimate of the iron(II1) oxide content contributing to the oxidation capacity (OXC) of the sediment is given by the Ti(II1)-EDTA extraction. The iron(I1) sulfide species are distinguished as AVS (acid volatile sulfide, hot 6 M HC1 extraction) and pyrite (sequential HI and CrIIHC1 extraction). By including a cold 5 M HC1extraction, the total distribution of the major reactive Fe(I1) and Fe(II1) fractions in aquifer sediments can be assessed.
Introduction
The attenuation of pollutants in landfill leachate plumes will depend, among other things, on the redox conditions in the plume. Recent studies at the Vejen Landfill (Denmark) have shown that Fe(II1)-reducing conditions were prevalent in a large part of the plume ( 1 ) and that substantial attenuation of specific organic pollutants took place in this part of the plume (2). Many other leachate plumes described in the literature appear to host Fe(II1)reducing zones, although this issue may not have been addressed specifically (reviewed in ref 3). The identification of Fe(II1)-reducing conditions is usually based on the appearence of elevated concentrations of dissolved Fe(I1) in groundwater samples, since the solubility of Fe(I1) is much higher than that of Fe(II1) species. Dissolution, adsorption, and precipitation reactions might control the dissolved iron content, and the sediment-associated iron typically makes up the major pool of iron. Iron species likely to be found in aquifers are listed in Table 1. In order to characterize the apparently important Fe(II1)-reducing environments, methods for determining the amount and reactivity of the iron species in aquifer sediments are needed. Several techniques have been used to study iron mineralogy in sediments and soils. Iron oxides are typically yellow or red. Although Torrent et al. (7)showed that the
* Corresponding author. + Technical University of Denmark. t
National Geological Survey.
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Environ. Sci. Technol., Vol. 28, No. 9, 1994
Table 1. Iron Species Likely To Be Found in Oxidized and Reduced Aquifers (4-6) name
formula
dissolvediadsorbedl ferrous iron complexed ferric iron solid, Fe(II1) ferrihydrite goethite akageneite lepidocrocite hematite maghemite solid, Fe(I1) siderite mackinawite, troilite pyrite, marcasite Fe(I1) silicates, clays solid. mixed green rust Fe(I1)-Fe(II1) magnetite greigite
color
Fez+ Fe3+
Fe(OH)3 a-Fe00H 6-FeOOH y-FeOOH ci-Fe203 y-FezO3 FeC03 FeS FeSz Fe(I1)-X Fes(0H)s FesOd FesS4
reddish-brown yellowish-brown yellowish-brown orange bright red reddish-brown white, brownish black golden, black green black
redness correlated well with the hematite content of soil, the color is of no use when the samples contain a mixture of several iron species. Surface-oriented techniques such as scanning electron microscopy and bulk-oriented techniques such as X-ray diffraction are frequently used to study iron minerals (8). These techniques are only semiquantitative and are difficult to use for total iron contents smaller than 1% (10 mgig) often encountered in aquifers. Other techniques are therefore needed. Chemical extraction techniques for iron determination in soils and surface water sediments have been reviewed (9-13). Based on these reviews, we have selected the most promising methods and tested them on well-defined standard minerals and on aquifer sediment representing a variety of redox conditions in a contaminated aquifer. The chemical extraction techniques are used to quantify the various iron pools associated with the aquifer sediment. Methodology
Sediment Sampling. Fresh sediment samples were collected in the leachate pollution plume downgradient from the municipal Vejen Landfill in Denmark ( I , 2 , 1 4 ) . The plume is located in a sandy glaciofluvial aquifer consisting of reddish-gray medium- to coarse-grained sand. Over a period of approximately 15years, landfill leachate rich in organic matter and various organic contaminants has entered the groundwater. A series of redox zones has previously been identified in the originally aerobic aquifer (Figure 1) based on the analysis of water samples for a number of redox-active species ( I ) . For this study, sediment samples were collected from the same sandy layer hosting environments spanning the entire redox sequence (Figure 1). Sediment samples along with the corresponding pore water were obtained (anaerobically) using a Waterloo piston sampler (15) and transferred to an anaerobic glovebox (Coy). Inside the glovebox, the sediment subsamples (representing 20-cm sections) were 0013-936X/94/0928-1698$04.50/0
0 1994 American Chemical Society
in the extract was determined by atomic absorption spectrophotometry (AAS). The amount of S(-11) released during extraction was quantified using a digestiondistillation apparatus with nitrogen purging to transport the evolved H2S into a zinc acetate trapping solution (18, 19). The precipitated ZnS was quantified by spectrophotometry a t 670 nm using the reagents N,N-dimethylp-phenylenediamine sulfate and ferric ammonium sulfate. The extractions used are listed in Table 3. More detailed information (where needed) is given below. AVS (Acid Volatile Sulfide) Determination. Fresh sediments (1-3 g) along with 5 mL of demineralized water were put into the distillation flask previously purged with nitrogen. Five milliliters of 1 2 M HC1 was then added, and the reaction (proton-catalyzed dissolution) took place for 15 min at room temperature and for 1 h a t boiling temperature. The dissolved iron and sulfide produced were then analyzed. HI-Reducing Extraction (First of Two Steps for Sequential Pyrite Determination). The mixture (150 mL of 57% HI, 32.5 mL of 50% H3P02, 75 mL of 97% CH202) was simmered gently (115-117 "C) for 30 min under nitrogen atmosphere and stored under subdued light. The sediment (1-3 g) and the reagent (8 mL) were then boiled for 1h under nitrogen purging. This extraction (reductive and proton-catalyzed dissolution) removes FeS, SO, Sod2-, and ester S042-, permitting the subsequent determination of the pyrite content using the Cr"HC1 attack. CrIIHCl-Reducing Attack (Second Step for Pyrite Determination). The residual sediment (from HIreducing extraction) and the reagents (2 mL of ethanol, 12 mL of 1M Cr(I1) and 0.5 M HC1,3 mL of 12 M HC1) were boiled for 1.5 h avoiding air contact. The dissolved
Distance f r o m landfill [m]
-
100
0
200
400
300
I
t
E
Water Level 7
38 -1
z
34
v)
a,
8
30
m
26 S
+-m
2 22 Iron/manganese reducing Methanogenic
u
Sulfate reducing
0 Aerobic
Nitrate reducing
Figure 1. Redoxzones identified by the analysis of groundwater samples downgradient of the Vejen Landfill ( r) and locationof the nlne sediment sampling points.
transferred to air-tight glass bottles and stored at 10 "C. The characteristics of the sediment samples and adjacent groundwater chemistry are given in Table 2. Chemical Extraction Techniques. All wet extraction techniques involve the mixing of the sediment with an extractant followed by determination of the extractant composition. The extracts were either centrifuged (particle cutoff calculated from ref 16) or filtered in order to remove particles greater than 0.25 pm (0.1 pm in the case of dithionite extraction). The Fe(I1) concentration in extracts was determined using ferrozine in the presence of an acetate buffer at pH 5 (17). The total amount of Fe
Table 2. General Sediment Characteristics and Groundwater Parameters Measured at Nine Locations in the Vejen Landfill Leachate Plume sample
redox (water)
Munsell conductivity soil pH dissolved 0 2 NOSdissolved Fe Sod2S(-11) dissolved CH4 (mg/L) (mg of SIL) (smell) (mg/L) soil colora (&/cm) (CaC12) (% saturation) (mg of NIL)
2.5 Y 412 methanogenic sulfate-reducing 2.5 Y 612 Fe(II1)-reducing 2.5 Y 712 Fe(II1)-reducing 2.5 Y 713 Fe(II1)-reducing 2.5 Y 613 Fe(II1)-reducing1 10 YR 616 precipitating nitrate-reducing 10 YR 714 aerobic 10 YR 714 aerobic 10 YR 516
6720 1090 480 410 350 270
6.45 5.62 5.33 5.63 7.14 5.95
95 % magnetite with impurities of siderite (
