Iron and manganese release in coal mine drainage wetland microcosms

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Wet. Sci. Tech. Vol. 32, No.3, pp. 187-192, 1995. Copyright © 19951AWQ

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IRON AND MANGANESE RELEASE IN COAL MINE DRAINAGE WETLAND MICROCOSMS W. J. Tarutis, Jr* and R. F. Unz** * Department of GeoEnvironmental Sciences and Engineering, Wilkes University, Wilkes-Barre, PA 18766, USA ** Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park; PA 16802, USA

ABSTRACT The primary mechanisms responsible for the removal and retention of iron, manganese, and sulfate in constructed wetlands receiving acidic mine drainage (AMD) include the formation of metal oxides and sulfides within the sediments. This study was initiated to determine the kinetics of metal ion liberation. under reducing conditions, from synthetic and naturally occurring iron and manganese oxides typically found in AMD precipitates. Rates of metal ion liberation were determined during time series incubations of an organic substrate (spent mushroom compost) to which five metal oxides of varying crystallinity (amorphous and crystalline oxides of iron and manganese; natural AMD oxide) were added. All experiments were carried out in silicone-sealed polycarbonate centrifuge tubes incubated at nee for a period of 3. 7. 10. 14. 21 or 28 days. Tubes were sacrificed after each incubation period and were analyzed for redox potential. pH. sulfide. and metals. All tubes exhibited reducing potentials within 3 days coupled with rapidly increasing concentrations of iron and manganese. Liberation of iron and manganese decreased with increasing mineral crystallinity (amorphous> natural AMD » crystalline). The results suggest that metal ion liberation from oxide minerals may be an important source of iron and manganese within constructed wetlands receiving AMD.

KEYWORDS Coal mine drainage; iron reduction; manganese reduction; microcosms; sulfate reduction; wetlands. INTRODUCTION The mechanisms responsible for the removal and retention of iron. manganese, and sulfate in constructed wetlands receiving acidic mine drainage (AMD) depend on the rate at which their respective biogeochemical cycles operate as well as the degree to which these cycles interact. Recent research has demonstrated the importance of the formation of metal oxides (Wieder et at.. 1990) and metal sulfides (Machemer and Wildeman, 1992) for the retention of metals in AMD wetlands. The diagenetic remobilization of iron and manganese during suboxic diagenesis (Froelich et at.• 1979) results in the diffusion of soluble metals from the subsurface into the overlying water where they may ultimately be discharged in the wetland effluent. It has been suggested (Tarutis et al., 1992) that long-term metal retention in AMD wetlands would be enhanced by active sulfate reduction and the subsequent formation of metal sulfides. The chemistry of the iron-sulfur system in freshwater. estuarine. and marine environments is well documented (Morse et al., 187

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1987), but the interactions of iron, manganese, and sulfide on metal retention in the wetland environment have not been adequately studied under laboratory conditions. Furthermore, the importance of suboxic iron and manganese remobilization with respect to metal retention in AMD wetlands has not been determined. The purpose of this paper is to determine the kinetics of metal ion liberation from synthetic and naturally occurring iron and manganese oxides under anoxic conditions typical in constructed wetlands for AMD treatment. MATERIALS AND METHODS

Rates of metal ion liberation were determined during time series incubations of spent mushroom compost (SMC) to which different forms of iron and manganese oxides were added. Five grams of air-dried, sieved (2 mm mesh) SMC were weighed into duplicate 50 ml polycarbonate centrifuge tubes to which ca. 10 ml of distilled water were added. Diagenesis was allowed to proceed in unamended incubations of SMC to the anoxic, sulfate-reducing stage at a constant temperature of 22°C. Appropriate amounts of sulfate (as Na2S04) were added to all tubes to give a final concentration of 500 rng/l. Natural or synthetic oxide was then added, and each tube was completely filled with distilled water. Six treatments were performed: addition of natural oxide, amorphous iron or manganese oxide, crystalline iron or manganese oxide, and a mixture of amorphous oxides. Tubes without metal oxides or SMC were employed as controls. No effort was made to exclude oxygen during tube preparation. Centrifuge tubes were then sealed with a bead of silicone cement, mixed daily, and incubated in the dark at 22°C for a period of 3, 7, 10, 14, 21, or 28 days. Tubes were sacrificed at the end of the incubation period, centrifuged at 12,000 g for 10 min, and the supernatant was analyzed for redox potential, Pt (combination electrode), pH (combination electrode), sulfide (ion-specific electrode), and iron and manganese concentrations (flame atomic absorption spectrophotometry). Metal oxide forms Five metal oxides of varying crystallinity were used. Natural AMD oxide rich in reducible iron and manganese was obtained from the surficial soil near the influent of a wetland constructed to treat coal mine drainage (Tarutis, 1993). Amorphous iron oxide was formed by neutralizing a 0.4 M solution of FeCI 3 to pH 7 with NaOH (Lovley and Phillips, 1986a). Amorphous manganese oxide was prepared by the oxidation of MnCI 2 by KMn04 under basic conditions (Balistrieri and Murray, 1982). Amorphous forms were repeatedly washed with distilled water to remove contaminants. The crystalline iron and manganese oxides used were commercially available hematite (U-Fe203) and pyrolusite (B-MnOV powders. Suspensions of all oxides, except crystalline pyrolusite, were prepared and stored at 8°C in the dark until needed. Appropriate volumes of each suspension were transferred to incubation tubes to give a final metal concentration of I mmol metal; pyrolusite was weighed directly into tubes. RESULTS The liberation of iron and manganese from natural and synthetic metal oxides during incubations in which sulfate reduction was allowed to occur is shown in Figs 1-2. Values of pH were relatively constant near pH 7 throughout the 28 day incubation period for all treatments. Redox potential quickly dropped to anoxic levels «-250 mV) within the first 7 days of incubation. Control tubes without metal oxide showed relatively low, but measurable, dissolved iron and manganese (Fig. la), probably released from decomposing SMC. Within I week, the inner walls of control tubes began to blacken, which corresponded to the accumulation of dissolved sulfide. Sulfide concentration increased throughout the incubation period. Dissolved iron released from natural AMD oxide rapidly increased to 120 mg/l during the first 10 days, then sharply declined (Fig. 1b), These tubes became very black within I week, much more so than control tubes. However, no dissolved sulfide was detected. Dissolved manganese was relatively constant at 5 mg/l,

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The release of dissolved iron from amorphous iron oxide (Fig. Ic) was much greater than from crystalline iron oxide (Fig. ld) and iron concentration began to decrease after about 10 days. No dissolved sulfide was detected in the amorphous iron oxide incubations. but dissolved sulfide did appear in solution after I week in the crystalline oxide incubations and steadily increased (Figs Ic, ld). The amorphous tubes were much more black relative to tubes in which crystalline oxide was added. Similar behaviour was observed for manganese, although without blackening of tubes. Dissolved sulfide was produced in tubes to which amorphous manganese oxide was added. but not until after 3 weeks of incubation (Fig. 2a). Much less manganese was released from the crystalline oxide, and dissolved sulfide began to accumulate after only I week (Fig. 2b). The release of iron in tubes to which a mixture of amorphous iron and manganese oxides was added was much less than tubes to which only amorphous iron was added (Figs lc, 2c). No dissolved sulfide was measured and, in contrast to the amorphous iron oxide tubes, no blackening of tubes was observed. Dissolved manganese concentration was only slightly lower relative to the amorphous manganese oxide tubes (Figs 2a. 2c).

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DISCUSSION Mobilization of iron and manl:anese Time-series laboratory incubations allowed examination of the anoxic liberation of iron and manganese under more controlled conditions than is possible in the field. The term metal liberation is used instead of metal reduction because some of the metal released may adsorb, causing a potential underestimate of actual metal reduction rates. Control tubes not amended with metal oxides revealed that sulfate reduction occurred within 7 days, indicated by the accumulation of dissolved sulfide and the deposition of black iron sulfide on the interior walls of the tubes (Fig. Ia). The bacterial reduction of iron oxides may be an important process in iron geochemistry and organic matter decomposition in waterlogged soils and sediments (Lovley and Phillips, 1986a), but only a portion of the total iron present is able to be reduced by microorganisms. It has been shown that microorganisms preferentially reduce amorphous iron oxides over the more crystalline forms during organic matter decomposition (Munch and Ottow, 1980; Lovley and Phillips, 1986b). The time-release of iron from natural AMD oxide is similar to that from amorphous iron oxide (Figs Ib. Ic). Most, but not all, of the iron present in the natural AMD oxide used in this study was found to be in reducible (amorphous) form (Tarutis, 1993). The liberation of iron from both the natural AMD and amorphous iron oxides was much higher than from the crystalline oxide (Fig. Id). The less reactive nature of the crystalline form is most probably due to lower surface area (Canfield, 1989). The dissolution of Fe(III) oxides has been shown to be controlled by the

Coal mine drainage wetland microcosms

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detachment of iron from the surface (Zinder et al., 1986), and it is thought that direct contact with the oxide surface for microbial enzymatic reduction (Lovley, 1991). In addition, Gorby and Lovley (1991) concluded that iron is reduced by a membrane-bound Fe(III) reductase. Thus, it is likely that the higher surface of amorphous oxide was responsible for the much higher liberation of iron relative to the crystalline oxide incubations. A similar argument can be made for the liberation of manganese (Fig. 2). Burdige et al. (1992), in their study of the effects of manganese oxide mineralogy on manganese reduction, found that manganese reduction showed a strong dependence on the type of oxide being reduced, which they attributed to surfacearea effects. However, they also noted that, while surface area is important, other factors such as mineralogy and crystal structure may affect manganese reduction, particularly at slower reduction rates. The presence of amorphous manganese greatly inhibited the liberation of iron in the amorphous oxide mixture (Fig. 2c). Lovley and Phillips (1988) demonstrated that the primary mechanisms responsible for preventing the accumulation of dissolved iron in suboxic sediments was the oxidation of Fe(II) by Mn(IV), although the preferential reduction of MN(IV) by iron-reducing bacteria may also be important. Similar observations were noted by Krishnamurti and Huang (1988) who found that manganese oxide minerals were unstable in the presence of ferrous iron, resulting in the precipitation of iron oxides concomitant with the accumulation of soluble reduced manganese. This seems to explain the much lower accumulation of dissolved iron in the presence of added amorphous manganese oxide (Fig. 2c). It should be noted that microorganisms may reduce metal oxides either directly, by using iron and manganese oxides as terminal electron acceptors, or indirectly, through the production of microbial metabolites (organic acids) which are capable of chemically reducing metal oxides abiotic ally (Lovley (1991) alluded that abiotic reduction is much less important than biotic reduction; however, reduction of manganese oxides (Stone, 1987) and iron oxides (Lindsay, 1991) by organic acid metabolites does occur to some extent. Thus, metal liberation observed in this study likely resulted from both mechanisms. Effect of sulfate reduction As mentioned above. in the absence of metal oxides, sulfate reduction occurred within the first 7 days of the 28-day incubation period. In tubes to which either natural AMD oxides or amorphous iron oxides were added, sulfide generated by sulfate reduction quickly reacted with the oxides to form black iron sulfide with concomitant removal of iron from solution, and an absence of dissolved sulfide existed over the entire incubation period (Figs 1b, l c). In contrast, tubes in which crystalline iron oxide were added turned black very slowly, and dissolved sulfide began to accumulate after 7 days (Fig. Id). The natural AMD and amorphous iron oxide forms were more reactive toward sulfide than the crystalline form. Canfield (1989) obtained similar results in marine sediments. The formation of iron sulfides depends on the relative rates of sulfate reduction compared to rates of sulfide reaction with iron minerals. For example, when the rate of iron reaction with sulfide is greater than the rate of sulfate reduction, dissolved sulfide is effectively titrated from solution, and iron sulfide formation is limited by the availability of dissolved sulfide. On the other hand, when the rate of iron reaction with sulfide is less than the rate of sulfate reduction, sulfide accumulates, and iron sulfide formation is limited by iron (Canfield et al., 1992). The kinetics of iron oxide sulfidation has been shown to depend on the iron oxide surface area (Pyzik and Sommer, 1981), which accounts for "the higher reactivity of sulfide toward the more amorphous forms. This explains them absence of dissolved sulfide in the presence of reactive iron (natural AMD and amorphous oxides) and its accumulation in the presence of crystalline hematite (Figs 1-2). Much less manganese was released from the natural AMD oxide than the amorphous form, owing to the much lower reducible manganese concentration of the former (Tarutis, 1993). The reactivity of manganese toward sulfide was much different than that of iron, however. Manganese release into solution was similar to that of iron, but appeared to be unaffected by the presence of dissolved sulfide since no decrease in dissolved sulfide since no decrease in dissolved manganese was observed (Fig. 2). The sulfidation of manganese oxides results not in the precipitation of manganese sulfide (alabandite). but rather the oxidation of sulfide to MST ll-)-N

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either elemental sulfur or sulfate accompanied by the release of soluble manganese into solution (Aller and Rude, 1988). Alabandite is a relatively uncommon mineral and would not be expected to form under most conditions (Saunders and Swann, 1992). This accounts for the gradual increase in dissolved manganese and the appearance of dissolved sulfide later in the incubation period (Fig. 2). REFERENCES Aller, R. C. and Rude, P. D. (\ 988). Complete oxidation of solid phase sulfides by manganese and bacteria in anoxic marine sediments. Geochim. Cosmochim. Acta, 52,751-765. Balistrieri, L. S. and Murray, J. W. (1982). The surface chemistry of Mn02 in major ion seawater. Geochim. Cosmochim. Acta. 46, 1041-1052. Burdige, D. J., Dhakar, S. P. and Nealson, K. H. (\992). Effects of manganese oxide mineralogy on microbial and chemical manganese reduction. Geomicrobiol. J., 10,27-48. Canfield, D. E. (1989). Reactive iron in marine sediments. Geochim. Cosmochim. Acta, 53,619-632. Canfield, D. E., Raiswell, R. and Bottrell, S. (1992). The reactivity of sedimentary iron minerals toward sulfide. Am. J. Sci., 292, 659-683. Froelich, P. N., Klinkhammer, G. P., Bender, M. L., Luedtke, N. A., Heath, G. R., Cullen, D., Dauphin, P., Hammond, D., Hartman, B. and Maynard, V. (1979). Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: suboxic diagenesis. Geochim. Cosmochim. Acta, 43,1075-1090. Gorny, Y. A. and Lovley, D. R. (l99\). Electron transport in the dissirnatory iron reducer, GS-15. Appl. Environ. Microbiol., 57, 867-870. Krishnamurti, G. S. R and Huang, P. M. (1988). Influence of manganese oxide minerals on the formation of iron oxides. Clays & Clay Min.• 36,467-475. Lindsay, W. L. (\991). Iron oxide solubilization by organic matter and its effect on iron availability. Plant and Soil. 130,27-34. Lovley, D. R. (\991). Dissimilatory Fe(III) and Mn(IV) reduction. Microbial: Rev.• 55, 259-287. Lovley, D. R. and Phillips, E. J. P. (l986a). Organic matter mineralization with reduction of ferric iron in anaerobic sediments. Appl. Environ. Microbiol .. 51, 683-689. Lovley, D. R. and Phillips, E. J. P. (l986b). Availability of ferric iron for microbial reduction in bottom sediments of the freshwater tital Potomac River. App!. Environ. Microbiol., 52, 751-757. Lovley, D. R and Phillips, E. J. P. (1988). Manganese inhibition of microbial iron reduction in anaerobic sediments, Geomicrobiol. J.. 6, 145-155. Machemer, S. D. and Wildeman, T. R. (1992). Adsorption compared with sulfide precipitation as metal removal processes from acid mine drainage in a constructed wetland. J. Contam. Hydrol., 9, 115-13 I. Morse, J. W., Millero, F. J., Cornwell, J. C. and Rickard, D. (1987). The chemistry of the hydrogen sulfide and iron sulfide systems in natural waters. Earth Sci. Rev., 24, 1-42. Munch, J. C. and Ottow, J. C. G. (1980). Preferential reduction of amorphous to crystalline iron oxides by bacterial activity. Soil Sci., 129, 15-21. Pyzik, A. J. and Sommer, S. E. (1981). Sedimentary iron monosulfides: kinetics and mechanism of formation. Geochim. Cosmochtm. Acta .. 45.687-698. Saunders, J. A. and Swann, C. T. (1992). Nature and origin of authigenic rhodochrosite and siderite from the Paleozoic aquifer, northeast Mississippi, U.S.A. Appl. Geochem.. 7,375-387. Stone, A. T. (1987). Microbial metabolites and the reductive dissolution of manganese oxides: oxalate and pyruvate. Geochim. Cosmochim. Acta. 51,919-925. Taruus, W. J. (\993), iron and Manganese Diagenesis Constructed Wetlands Receiving Mine Drainage. Ph.D. Dissertation, The Pennsylvania State University, University Park, PA, USA. Tarutis, W. J., Unz, R. F. and Brooks, R. P. (1992). Behaviour of sedimentary Fe and Mn in a natural wetland receiving acidic mine drainage, Pennsylvania, USA. Appl. Geochem. , 7,77-85. Wieder, R. K., Linton, M. N. and Heston, K. P. (1990). laboratory mesocosm studies of Fe, AJ, Mn, Ca, and Mg dynamics in wellands exposed to synthetic acid coal mine drainage. Warer Air Soil Pollut., 51, 181-196. Zinder, B., Furrer, G. and Stumm, W. (1986). The coordination chemistry of weathering: II. Dissolution of Fe(ll!) oxides. Geochim. Cosmochim. Acta., 50, 1861-1869.