Environ. Sci. Technoi. 1994, 28, 153-158
Oxidation Capacity of Aquifer Sediments Gorm Heron,’ Thomas H. Chrlstensen, and Jens Chr. Tjell
Department of Environmental Engineering, Groundwater Research Centre, Technical University of Denmark, Bulldlng 115, DK-2800 Lyngby, Denmark A laboratory extraction method (Ti3+-EDTA extraction) for the determination of the oxidation (electron accepting) capacity related to oxides and hydroxides of aquifer sediments was developed. At room temperature, titanous ions (0.008 M Ti39 in a solution of 0.05 M ethylenediaminetetraacetic acid (EDTA) reduced oxidized aquifer species. This operationally defined oxidation capacity (OXC) was determined as pmoles of electrons accepted per gram of sediment sample. Well-described oxidized iron and manganese minerals were reduced (ferrihydrite 98 % , akageneite loo%, goethite and hematite 93 % , magnetite 9 % , pyrolusite 99%) whereas organic matter in the sediments was not reduced significantly. The method was applied to reduced as well as oxidized aquifer sediments from a well-described Danish sandy aquifer. The OXC (23-31 pequiv/g) of the oxic sediments could be attributed to the content of extractable Fe(II1). The OXC was significantly lower for the reduced sediments (4-10 pequivlg), approaching the detection limit of the method. Drying of the sediments was shown to be unacceptable because of the oxidation of Fe(I1) compounds, whereas storing of sediments inside an anaerobic chamber for up to 1 month was acceptable. The oxidation capacity of aquifer sediments may be a key parameter in predicting contaminant plume development and size and in understanding aquifer redox zones in general. Introduction The entrance of organic matter and pollutants into aerobic aquifers may affect the water quality of downgradient wells and surface waters. The organic load on the aquifer, e.g., from landfill leachate, nonaqueous phase liquids and wastewater, typically leads to the development of reduced environments as the organic matter is microbially degraded. The ability of the aquifer to restrict the development of reduced environments depends on the availability of oxidized species (electron acceptors) in the aquifer, referred to as the oxidation capacity (OXC). In an aquifer with low oxidation capacity, large zones of very low redox levels (e.g., methanogenic conditions) will develop, while an aquifer with high oxidation capacity will limit the methanogenic zone and allow for development of a sequence of different redox zones dependent on the species contributing to the available oxidation capacity. Hence the oxidation capacity of the aquifer is an important parameter in controlling the geochemistry of the pollution plume and thereby the fate of the pollutants. The dominating aquifer redox reactions include both aqueous and solid species as reviewed (1-3), The potential electron-accepting reactions (half-reactions) in the aquifer are listed in Table 1. Reduced organic matter is expected to be the electron donor, but reduced inorganic species may also contribute. As microbially mediated redox processes may decrease the redox potential to values as low as -300 mV in natural even aquifer organic matter may groundwater systems (4, 0013-936X/94/0928-0153804.50/0
0 1993 American Chemical Society
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Table 1. Electron-Accepting Reactions in Aquifers O2 + 4H+ + 4e- 2Hz0 NO3- + 6H+ + 5e- 0.5Nz + 3H20 Fe(OH)a + 3H+ + e- Fez+ + 3Hz0 FeOOH + 3H+ + e- Fez+ + 2H20 Fez03 + 6H+ + 2e- 2Fe2+ + 3H20 MnOz 4H+ + 2e- Mn2++ 2Hz0 SO4” 9H+ + 8e- HS- + 4Hz0 CH2O + 4e- + 4H+ CHI + HzO
+ +
-
(aq) (as). (solid) (so1id) (solid) (solid) (aq + solid) (aq + solid)
Table 2. Content of Oxidized Species and Their Estimated Potential Contribution to Oxidation Capacity of Two Aerobic Sandy Aquiferse
Vejen (Denmark) content
oxc
(equiv/m3)
10 mg/L 0.44 1.9 15 mg/L 0.2 mg/g 12 6 mdg 175 40 mg/L 1.2 3 mg/L 0.35 110 0.2 mg/g refs 8,9, this study Porosities of 0.35 and bulk densities assumed.
Sand Ridge (Illinois)
oxc
content
(equiv/m3)
9 mg/L 0.95mg/L 0.39 mg/g 6.8 mg/g 36 mg/L 0.85 mg/L 0.42 mg/g
0.39 0.12 23 200 1.1
0.10 230
597 of 1.6 kg/L have been
be partly reduced. Currently, it is unknown to what extent the aquifer organic matter can be reduced by methanogenic bacteria, and therefore the contribution to the actual oxidation capacity is unknown. Assuming that aquifer organic matter is fully reduced from the oxidation state 0 (e.g., glucose) to the oxidation state -4 (methane), the totaloxidation capacity of an aquifer volume can be written as (modified after ref 5 and in context with ref 6): OXC = 4[02] + 5[NO;l
+ [Fe(III)I + 2[Mn(IV)l+ 8[S0,2-1 + 4[TOCl
(1)
All concentrations are converted to moles per cubic meter of the aquifer. The end products are assumed as in Table 1. The content of oxidized species and the corresponding maximum OXC per cubic meter were calculated for two well-described aerobic aquifers (Table 2). Porosities of 0.35 and bulk densities of 1.6 kg/L have been assumed for both aquifers. In this case, solid sulfur species (elemental S and sulfate containing solids as jarosite (KFes(0H)s(SOd~5H20)) have been neglected, assuming insignificant solid sulfur concentrations for these sediments. Sediments containing large amounts of oxidized sulfur, e.g., gypsum (CaSOq2HzO), may need special consideration. Table 2 shows that iron species and organic matter (if reducible) are the dominating contributors to the oxidation capacity of these aerobic aquifers. In support of the view that Fe(II1) is an important electron acceptor for microbial activity in aquifers, large iron reducing zones have been identified in polluted aquifers (&IO). Only a fraction of Environ. Scl. Technoi., Vol. 28, No. 1, 1994
153
Table 3. Oxidized Iron and Manganese Species name
formula
ferrihydrite, rust lepidocrocite akageneite maghemite magnetite hematite goethite pyrolusite birnessite braunite manganite hausmannite
color reddish-brown orange yellowish-brown reddish-brown black bright red yellowish-brown reddish-brown brown/black brown brown/black brown/black
the sediment Fe(III), Mn(1V) and organic matter may be reducible under the actual field conditions. For example, the Fe(II1) in magnetite and structurally bound elements supposedly will be unavailable for reduction. Therefore, the reducible fraction of the oxidized species are in focus, and not the total concentration of the oxidized metals. Table 3 lists the most commonly found oxidized iron and manganese species. Currently, the availability of aquifer organic matter to reduction is very poorly understood. This makes the quantification of the associated OXC impossible at this time. Supposedly some functional groups of humic and fulvic acids are reducible under strongly reducing conditions. In this study, reduction of sediment organic groups was not adressed as an important contributor to the oxidation capacity of sediments. The method was optimized for iron and manganese compounds. This paper presents a wet extraction technique designed for the quantification of the oxidation capacity of aquifer sediments primarily related to oxidized Fe and Mn minerals. Key elements of the chemical reactions are discussed, and the method is tested on individual minerals as well as oxidized and reduced aquifer sediments. The method is well suited for measuring aquifer oxidation capacity related to Fe and Mn compounds. Selection of Extraction Solution: Theoretical Considerations
Quantification of the electron transfer between the employed reductant and the aquifer species is essential. Prerequisites are that the reductant concentration should be measurable, and that the consumption should be significant relative to the initial concentration. Selecting Reductant and Ligand. Various reducing agents and complexing ligands have been used for the reductive dissolution of oxidized iron species as lately discussed in an excellent review (11). None of these combinations were deemed suited for this study. Either because of difficulties in the quantification of the reducing agent or because the concentration of reducing agent could not be decreased to a level low enough for an appropiate quantification of the electron transfer. Lately, a method for the determination of the oxidation capacity of aquifer sediments was developed by Barcelona and Holm (5). In our opinion, the reductant used (Cr2+) was not acceptable for this purpose due to the fact that Cr2+is very unstable and reactive even in the absence of oxidized aquifer species. In support of this, oxidation capacities 15-20 times greater than accounted for by the identified species (assuming a very limited reduction of 154
Envlron. Scl. Technol., Vol. 28, No. 1, 1994
aquifer organic matter) were recorded, and the instability of Cr2+was discussed (5). A method for the reductive dissolution of iron oxides from aquifer sediment developed by Ryan and Gschwend (12)showed promising results. In that study, a large excess (0.05 M) of Ti3+ was used along with combinations of complexing agents (citrate, EDTA) and pH buffering. The solutions of Ti3+-EDTA (unbuffered) and Ti3+-citrateEDTA-bicarbonate proved very selectiveand similar. The latter was tested on reduced and oxidized sediments and extracted a well-defined fraction of the iron oxides. In addition, the method was highly selective as the fraction of iron extracted from the reduced sediment was very low (12). For this work complexationof the reductant with asingle ligand (EDTA) was preferred because of the simplicity in understanding the actual reactions during extraction and quantification. In addition, the pH buffering capacity of EDTA itself was preferred, as bicarbonate might complicate the chemical reactions in question and appeared to affect the concentration of Ti3+ in special cases. On this basis, Ti3+ and EDTA were selected as the reductant and the complexing ligand, respectively. The concentration of Ti3+in the reagent was lowered to a level (0.008M) allowing for the consumption of approximately 213 by the most oxidized iron-rich sediments. The EDTA concentration was maximized to 0.05 M (slightly below the solubility) to ensure an appreciable excess of complexing ligand preventing the release of Ti3+from the Tis+EDTA complex by competition by other cations (Fez+, Mn2+,Ca2+,Mgz+etc.) derived from the sediments. A pH of 6 was selected in order to reach a strong buffering capacity of the EDTA and to ensure that a minimum of reduced iron and manganese species would dissolve at low PH. Titanous ions (Ti3+)have been used for the reduction and determination of several organics as nitro compounds, oximes, quinones, azo compounds and sulfoxides (13-15). To our knowledge, the ability of the titanous EDTA complex to reduce natural aquifer organic groups has never been addressed. Therefore, the possible reduction of some organic compounds should be tested. EHof Extractant. Considering the short laboratory contact time of sediment extractant (hours or days) relative to the hctual field conditions (months or years) a very aggressivetreatment is required. This includes low redox potential and strong complexing ability of the solution. The EH of the Ti3+-EDTA complex could not be calculated due to the lack of data. Instead, we directly measured the redox potential ( E H = -330 mV) of the extractant at pH 6. Theoretically, this should allow for thermodynamically favorable reduction of the aquifer species oxygen, nitrate, iron oxides, manganese oxides, sulphate and reducible parts of the organic matter. However, the degree of reduction of the selected species (Fe, Mn and TOC) should be thoroughly tested, as redox reactions typically are slow and redox systems seldom are at equilibrium. Quantification of Ti3+. The quantification of the Tis+-EDTA was done by redox titration with the strong oxidizing agent acid dichromate. It is essential that only Ti3+ is oxidized in this step. Therefore, a redox indicator with a color shift at a redox potential slightly above that of the Ti3+-EDTA/Ti4+-EDTA couple was needed. The redox indicator Neutral Red was selected experimentally, as the shift occurred prior to the oxidation of Fe(I1) or
Mn(I1). Oxidation of dissolved gases such as hydrogen sulfide and nitrogen was not significant.
Methodology Field Site. Fresh sediments were collected in the leachate pollution plume downgradient of the Vejen Landfill in Denmark (8, 9, 16). The plume is located in a sandy glaciofluvial aquifer consisting of reddish-gray medium to coarse-grained sand. A series of well-described redox zones have been identified in the originally aerobic aquifer. For this study, sediments were collected from the highly reduced methanogenic zone (M samples), from the active iron-reducing zone (F samples), and from the weakly polluted aerobic sections of the sandy aquifer (A samples). The selected sediment samples originate from the same sandy layer of the aquifer. Sediment Sampling and Handling. Sediments were sampled anaerobically with a Waterloo piston sampler (17) without contact to the atmosphere and with the preservation of the pore water. The cores were sealed and stored at 10 "C, the approximate temperature of the aquifer, for a maximum of 10 days prior to transfer to an anaerobic glovebox. The cores were cut into 10cm sections, immediately flushed at the ends with nitrogen and sealed. Inside the glovebox, the sediments were transferred from the center of each core piece to 15mL serum bottles which were sealed with butyl rubber stoppers and crimp caps. The weight of the wet sample (0.6-1.0 g) was determined. Split 50-g samples of selected sediments were transferred to glass bottles and, for comparison, either stored anaerobically inside the chamber or taken out for freeze-drying or heat-drying exposed to air. Extraction Solution. A 0.008 M solution of Ti3+ in 0.05 M EDTA was prepared from Tic13 (15% w/v, low in iron, in 15% HC1, The British Drug Houses Ltd., Poole, U.K., Prod 30447) dissolved in anoxic deionized water. The pH of the resulting solution was adjusted to 6.0 with NaOH added primarily prior to the addition of Tic13 to avoid precipitation of EDTA at low pH. The pH remained between 6 and 5.6 during the extractions except when large amounts of synthetic minerals were added as standard additions. The Ti3+-EDTA extractant was stored under oxygen free headspace inside an anaerobic chamber to prevent any oxidation of the Ti3+by atmospheric oxygen. The effect of storage was found to be negligible during at least 14 days for each freshly made solution of Ti3+. OXC Determination. Anaerobically, 10 mL of Ti3+EDTA extractant was transferred to the serum bottles containing the wet sample. The bottles were incubated at 20 "C in the dark and rotated a t 2 rpm. Following the extraction period, the bottles were centrifuged to remove particles greater than 0.25 pm (calculated according to ref 18). The extractant was bright violet with a very regular absorption spectrum with a maximum at 550 nm (19).In weakly polluted sediments, the absorbance at 550 nm closely matched the concentration of Tis+-EDTA determined by redox titration. In strongly reduced sediments, the interference by presumably organic matter prevented an accurate quantification by spectrophotometry. Therefore, the remaining nonreacted Ti3+ in the extract was quantified by immediate titration under anaerobic conditions with a solution of 0.006 N KzCr207 in 10% HzS04 prepared from KzCrz07 (99.8-100.2 % pure, Riedel de Haen, Germany). A total of 4 mL of the extract was used.
During the titration, the color shifted from purple to light blue. Then the redox indicator Neutral Red (E, = -0.29 V at pH 7; +0.24 V at pH 0) was added, and the titration was completed with the shift from blank to dark violet. The amount of Ti3+ was determined according to 6Ti3++ Crz0,2- 4- 14H+
-
6Ti4+ 2Cr3+4- 7Hz0
(2)
The maximum OXC detectable (100% reductant consumption) corresponds to 150pequivlg for 0.6 g of sample. The quantification was checked by titrating standard amounts of Ti3+ in a 0.05 M EDTA suspension. The extracted Fe and Mn were determined from split samples of the extracts by atomic absorption spectrophotometry. Fe(I1) Content. To test for the oxidation of the Fe(I1) species during sediment handling, the amount of 0.5 M HC1-extractable Fe(I1) was determined using a slight modification of the method described by Lovley and Phillips (20). Briefly, 0.60 g of fresh sediment was extracted anaerobically for 24 h by 0.5 M HC1. After centrifugation as above the amount of Fez+ in the extract was determined by the ferrozine method (21). An acetate buffer at pH 5.0 was used. The spectrophotometric determinations were conducted after 15 s of mixing the extract and the ferrozine solution. Total Extractable Fe and Mn Content. As an estimate of the total nonstructural iron and manganese content, sediments were extracted with 5 M HC1 at 90 "C for 8 h. The extracted amounts of iron and manganese were determined by atomic absorption spectrophotometry. Standard Minerals. Amorphous, noncrystalline ferrihydrite was synthesized as described in ref 22. Briefly, 40 g of Fe(N03)~9HzOin 500 mL of redistilled water was mixed with 330 mL of 1 M KOH to a final pH of 7-8. Ferrihydrite formed rapidly and was washed until the nitrate concentration in the equilibrated water dropped below 0.1 mg/L. Akageneite was synthesized by the aging of ferrihydrite synthesized from FeCl3 and KOH (23) at 20 "C prior to washing. Well-crystallized goethite was synthesized as described by Schwertmann and Cornel1 (22): 100 mL of 1M Fe(N03)~9HzOwas mixed with 180 mL of 5 M KOH in a polyethylene flask. Red-brown ferrihydrite precipitated. The volume was rapidly made up to 2 L, and the suspension was heated at 70 "C for 60 h. Goethite formed during the heating and was washed as described above. Hematite (Fez03,98% pure, Merck 3924) and pyrolusite (MnOz, >90% pure, Merck 5957) were purchased as powder and used without further treatment. Magnetite (Fe& 98% pure, Aldrich 31,0069) was purchased as rocks. The rocks were ground. The resulting powder contained >95 % magnetite with impuquartz (
