Environ. Sci. Technol. 1988, 22, 432-437
Partitioning of Arsenic and Metals in Reducing Sulfidic Sediments Johnnie N. Moore* Department of Geology, University of Montana, Missoula, Montana 5981 2
Walter H. Flcklin Branch of Exploration Geochemistry, U S . Geological Survey, MS 955, Box 25046, DFC, Denver, Colorado 80225
Carolyn Johns Department of Geology, University of Montana, Missoula, Montana 5980 1
The sediment in a reservoir on the Clark Fork River in western Montana is contaminated with arsenic, copper, zinc, and other elements. This sediment is the source of groundwater contamination in the adjacent alluvial aquifer, and this study elucidates the processes transferring arsenic to the groundwater by the formation of diagenetic sulfides in the sediment. Vertical trends in a core through oxidized surface sediment into reducing sediment below show that concentration and partitioning of metals and arsenic are controlled by the redox interface. Solid phases of arsenic, copper, and zinc change from dominantly oxyhydroxide and organic phases to sulfide phases across the interface. Arsenic, copper, zinc, manganese, and iron in the pore water are controlled by the solubility of iron and mahganese oxyhydroxides in the oxidized zone and metal sulfides in the reduced zone. The change in redox conditions upon burial results in a system where the growth of diagenetic copper, zinc, and arsenic sulfides controls the distribution and partitioning of metals and arsenic in the sediment and the speciation and release of arsenic to the adjacent groundwater system. Introduction
Recent studies have addressed mechanisms and processes of trace element partitioning in oxidized sediment (1-5) and the significance of oxide precipitation in controlling the oxidation state of arsenic (6, 7). Although the importance of reducing conditions in sediments is wellestablished for the migration and fixation of iron and manganese, there is less information for other metals and arsenic, especially in freshwater systems. Generalized models have been developed for metal mobilization within and between reduced and oxidized sediment on the basis of experimental and theoretical studies (8, 9). These models emphasize the importance of redox conditions but have not specifically addressed the role of early diagenetic monosulfides in fixation, mobilization, and chemical speciation processes for trace elements (ref 10, p 235). In contaminated sediments, trace elements stored by the formation of authigenic sulfides are potential sources of secondary contamination if sulfides (11)are moved into oxidizing environments where they are unstable (12). Chemolithotrophic bacteria assist this oxidation process when sulfides are removed from anaerobic environments. This process releases iron and any trace elements associated with the sulfides, analogous to release during acidmine drainage (13,14). The significanceof these reactions is that under reducing conditions trace elements and metals in contaminated sediments can be coprecipitated with or sorbed onto iron sulfide and later released to the surface and groundwater if the sediments are oxidized. The formation of authigenic sulfides during burial can also alter oxidation states of trace elements in pore water and sediment (ref 15, p 91), possibly mobilizing more toxie forms [e.g., As(III)]. 432
Environ. Sci. Technol., Vol. 22, No. 4, 1988
A small reservoir in western Montana has been useful for elucidating such a complex system. Milltown Reservoir (Figure 1)has been intensely studied because of the contamination of the community groundwater supply (16) in the adjacent town of Milltown. Analyses of sediment in the reservoir found significant enrichment over local background concentrations for arsenic (32 times), manganese (7 times), copper (62 times), zinc (67 times), lead (11times), and cadmium (37 times), and calculated loads predicted that tens of thousands of tons of these elements are available in the reservoir sediment (16). These studies linked the reservoir sediment to the groundwater contamination, where manganese and arsenic were elevated, but did not address the mechanisms of trace element fixation and remobilization. Analyses by Ficklin showed that the groundwater system was dominated by arsenic(II1). We used the extensive data base available to locate a site suitable for studying the processes that fix trace elements in the sediment and release arsenic and manganese to the groundwater. Our results emphasize the importance of sulfides in retaining and remobilizing trace elements from contaminated, reduced sediment and support the role of sediment in arsenic speciation in natural systems. Experimental Section
Initial work by Moore and Johns identified specific areas in Milltown Reservoir containing high values of total metals and arsenic (16). One site close to the contaminated wells was chosen for detailed study because the sediment contained an upper oxidizing zone overlying reduced sediment. One core was taken through this zonation by using a segmented hand-coring device. The core liner is constructed of a stack of poly(viny1 chloride) (PVC) sections 10 cm long. Each section seals tightly against its neighbors to retain sediment and pore water. When the corer was extracted, each section was removed and immediately capped after the pH, Eh, and dissolved oxygen were measured by insertion of electrodes and probe into the interior of the core section. The core sections were stored on ice and returned to the laboratory within 1 h of collection. The pore water of each section was then displaced by nitrogen gas supplied through the core section. During this procedure only the ends of the core sections were exposed to the atmosphere for the brief intervals of capping and uncapping to minimize oxidation effects. During extraction, pore water ran through 0.45-pm filters directly into acidified containers and was either bathed in nitrogen or in the weak vacuum of the filter apparatus. The water was stabilized with hydrochloric acid to 0.12 M immediately following the extraction. The concentration of dissolved arsenic [arsenic(III) and arsenic(V)] was determined in the pore water with the methods developed by Ficklin (17). The concentration of total dissolved arsenic was so low in the first five sections of the core that arsenic(II1) and arsenic(V) could not be determined for every section. The ion-exchange separa-
0013-936X/88/0922-0432$0 1.50/0
0 1988 American Chemical Society
Core 5
Clayey silt 30
40 50
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Silt-mud Sandy mud Muddy sand
60
Figure 1. Location map.
tions of arsenic(II1) from arsenic(V) were begun within 1 day following extraction of the pore water. Ferrous iron concentrations were determined immediately after extraction by the bipyridine method (18). The total iron, manganese, and zinc concentrations were determined by flame atomic absorption spectrometry (AAS); copper and arsenic were analyzed by graphite furnace AAS. Sulfate concentrations were determined by ion chromatography. After pore water was removed, sediment from the core section was extruded and split vertically into two subsamples. One was immediately freeze-dried and then stored in an air-tight, acid-washed container. Wet subsamples were taken from the other split for grain size analyses by wet sieving for percent sand and pipet analysis for percent silt and percent clay using sodium lignosulfate as a dispersant. Sequential chemical extractions were performed on subsamples of freeze-dried sediment to separate major, operationally defined, metal-binding phases. First, a 0.25 M hydroxylamine hydrochloride in 25% acetic acid (v/v) solution (AA-HA) was used to remove the metals and arsenic bound to oxyhydroxides of iron and manganese (19). This extraction would also remove trace elements bound in carbonates, but the carbonate content of all the samples was so low that this was likely an unimportant phase. This slurry was shaken for 1 2 h before it was centrifuged and the supernatant decanted and filtered for analysis. A preliminary experiment was done to determine the effect of any carbonate in the sediments on this extractant, although no carbonate was found in surface sediment. Ten sediment samples were extracted for 24 h, and the pH of the extractants was measured at 1 , 2 , 4 , 6 , 9, and 24 h. During that time, the pH changed by only 0.15 pH unit (average). The minimal change in pH of the extracting solutions indicated that small quantities of carbonate present would not significantly alter the effectiveness of the extract. The sediment remaining after the AA-HA extraction was rinsed with distilled-deionized water; then the organic fraction was determined by a 12-h extraction with 0.1 M sodium pyrophosphate at pH 10 (19). Again, the remaining sediment was rinsed. Finally, the sulfide phase was dissolved by using potassium chlorate with hydrochloric acid (20). This extraction was repeated on the same sediment and extractant solutions combined for analysis. The double potassium chlorate/hydrochloric acid extraction has been found to dissolve 86-100% of galena, chalcopyrite, cinnabar, orpiment, stibnite, sphalerite, and tetrahedrite and 71436% of pyrite present (20). The total metal content of the sediment was determined by hydrofluoric-perchloric acid dissolution of separate subsamples of freeze-dried sediment. All chemical analyses of extractions were done by inductively coupled plasma spectrometry.
'
70
..
Sand
e0
90 100 110
120
Base of geochemistry
130 140 150
Figure 2. Stratigraphic column of sequence sampled by the core used for chemical analysis.
Results and Discussion
Physical-Field Measurements. The sediment core contained a complex assemblage of bed-load and suspension-load sediment encompassing interlayers of sand, muddy sand, sandy mud, and clayey silt (Figure 2). Organic material occurred throughout the core as layers and isolated pieces of wood, and organic carbon ranged from approximately 3.8 to 0.5%. The sediment contained mostly from 75 to 97% mud (grains less than 63 pm) with one sandy interval from 50 to 60 cm, composed of only 30-40% mud (Figure 3A). Clay (grains less than 4 pm) content increased from 15 to 20% in the lower part of the core, concentrated in layers of very cohesive, light gray, clayey, micaceous silt. Comparison of elemental data with grain size shows no textural effects (Figure 3). The major decrease in mud content at 50-60 cm is not reflected by significant decreases in trace element concentrations. The upper 70 cm of the sediment column is mottled yellowish brown within light olive gray sandy mud and muddy sand. Small roots penetrate the upper part of this zone. The concentrations of dissolved oxygen (do,) are low (less than 0.5 ppm) to a depth of about 20 cm (Figure 3D), below 20 cm it increases to over 1.8 ppm and then steadily decreases to undetectable values below 70 cm. The change from detectable d o 2 to no d o 2 corresponds to a visual change from yellowish brown mottled sediment to olive-black and dark gray sediment. Changes in Eh and concentration of ferrous iron (Table I) occur just above this boundary. This boundary represents the interface between an oxidizing zone above and reducing conditions below 70 cm. The pH decreases continuously across this zone, more or less uniformly, from 8.0 at the surface to 7.0 below 80 cm (Figure 3D). The downward decrease in pH appears to stabilize at approximately 80 cm a few centimeters below the d o 2 minimum. Chemical. Total particulate arsenic concentrations in the upper 60 cm of sediment are relatively constant around Environ. Sci. Technol., Vol. 22, No. 4, 1988
433
A
9
B
Percent gr. size IO 2 0 30 40 50 60 70 8: 90 I? I
I
I
I
I
I
0
1.19 A s / l PW.
1.19 As/g sed. 100
200
300
4?0
5r)O
600
700
800
I
l
l
600
700
800
I
0 IO 20 30 40 50 60 70 80 90 100
0
D
100
200
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500
20
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5 15 35
g Organic
55
100
200
300
400
500
600
20
40
60
80
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40
60
80
100
35 45 55
Sulfide
65 75
85 95
Resistant
85 95 105
115
115 IO
30
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70
90
20
Relative
Relative '10
PH
6yO
25
65 75
105
do2
5?0
15
AA-HA
45
8.0
4yO
5
25
7.5
l
F
0
7.0
300
l
I
E I.o
200
100
YO
Figure 3. Grain size, field measurements, and arsenic trends in sediment with depth.
Table I. Measured Eh and Ferrous Iron in Sediment with Depth depth, measured ferrous cm Eh," mV iron,bmg/L 5 15 25 35 45 55
114 349 365 484 369 224
