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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 INCOAL MINE DRAINAGE WETLANDMICROCOSMS

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 StateUniversity, University Park; PA 16802, USA

ABSTRACT

The primary mechanisms responsible for the removal and retention of iron, manganese, and sulfate inconstructed wetlands receiving acidic mine drainage (AMD) include the formation of metal oxides andsulfides within the sediments. This study was initiated to determine the kinetics of metal ion liberation. underreducing conditions, from synthetic and naturally occurring iron and manganese oxides typically found inAMD precipitates. Rates of metal ion liberation were determined during time series incubations of an organicsubstrate (spent mushroom compost) to which five metal oxides of varying crystallinity (amorphous andcrystalline oxides of iron and manganese; natural AMD oxide) were added. All experiments were carried outin silicone-sealed polycarbonate centrifuge tubes incubated at nee for a period of 3. 7. 10. 14. 21 or 28days. 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 increasingconcentrations of iron and manganese. Liberation of iron and manganese decreased with increasing mineralcrystallinity (amorphous> natural AMD » crystalline). The results suggest that metal ion liberation fromoxide minerals may be an important source of iron and manganese within constructed wetlands receivingAMD.

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 constructedwetlands receiving acidic mine drainage (AMD) depend on the rate at which their respective biogeochemicalcycles operate as well as the degree to which these cycles interact. Recent research has demonstrated theimportance of the formation of metal oxides (Wieder et at.. 1990) and metal sulfides (Machemer andWildeman, 1992) for the retention of metals in AMD wetlands. The diagenetic remobilization of iron andmanganese during suboxic diagenesis (Froelich et at.• 1979) results in the diffusion of soluble metals fromthe subsurface into the overlying water where they may ultimately be discharged in the wetland effluent. Ithas been suggested (Tarutis et al., 1992) that long-term metal retention in AMD wetlands would beenhanced by active sulfate reduction and the subsequent formation of metal sulfides. The chemistry of theiron-sulfur system in freshwater. estuarine. and marine environments is well documented (Morse et al.,

187

188 W. J. TARUTIS, Jr and R. F. UNZ

1987), but the interactions of iron, manganese, and sulfide on metal retention in the wetland environmenthave not been adequately studied under laboratory conditions. Furthermore, the importance of suboxic ironand 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 naturallyoccurring iron and manganese oxides under anoxic conditions typical in constructed wetlands for AMDtreatment.

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 ofdistilled water were added. Diagenesis was allowed to proceed in unamended incubations of SMC to theanoxic, sulfate-reducing stage at a constant temperature of 22°C. Appropriate amounts of sulfate (asNa2S04) were added to all tubes to give a final concentration of 500 rng/l. Natural or synthetic oxide wasthen 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 amixture of amorphous oxides. Tubes without metal oxides or SMC were employed as controls. No effortwas made to exclude oxygen during tube preparation. Centrifuge tubes were then sealed with a bead ofsilicone 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 thesupernatant was analyzed for redox potential, Pt (combination electrode), pH (combination electrode),sulfide (ion-specific electrode), and iron and manganese concentrations (flame atomic absorptionspectrophotometry).

Metal oxide forms

Five metal oxides of varying crystallinity were used. Natural AMD oxide rich in reducible iron andmanganese was obtained from the surficial soil near the influent of a wetland constructed to treat coal minedrainage (Tarutis, 1993). Amorphous iron oxide was formed by neutralizing a 0.4 M solution of FeCI 3 topH 7 with NaOH (Lovley and Phillips, 1986a). Amorphous manganese oxide was prepared by the oxidationof MnCI 2 by KMn04 under basic conditions (Balistrieri and Murray, 1982). Amorphous forms wererepeatedly washed with distilled water to remove contaminants. The crystalline iron and manganese oxidesused were commercially available hematite (U-Fe203) and pyrolusite (B-MnOV powders. Suspensions of alloxides, except crystalline pyrolusite, were prepared and stored at 8°C in the dark until needed. Appropriatevolumes of each suspension were transferred to incubation tubes to give a final metal concentration of Immol metal; pyrolusite was weighed directly into tubes.

RESULTS

The liberation of iron and manganese from natural and synthetic metal oxides during incubations in whichsulfate reduction was allowed to occur is shown in Figs 1-2. Values of pH were relatively constant near pH 7throughout 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. WithinI week, the inner walls of control tubes began to blacken, which corresponded to the accumulation ofdissolved 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, thensharply 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,

Coal mine drainage wetland microcosms 189

100 140

::::;(a)

::::; 1200. 80 0. s.§.. s .§.. 100

Fe---- Feen 60 en 80 Ml:;; ---<>- Ml :;;C 40 C 60::0 ::0

III40

20 IIIu, u,20

00 5 10 15 20 25 30 00 5 10 15 20 25 3D

Time (days) Time (days)

200 35(e) S (d)

Fe 3D::::; 150 ::::;

25 S0. 0. Fe.§.. .§.. 20en 100 Ul... ... 150 0

III III 10u, 50 u,

5

0 00 5 10 15 20 25 3D 0 5 10 15 20 25 30

Time (days) Time (days)

Figure I. Metal liberation during times series incubations: (a) control (no metal added); (b) natural AMD oxide; (c)amorphous Fe oxide; (d) crystalline Fe oxide. Points are means of duplicate samples and error bars present one

standard deviation.

The release of dissolved iron from amorphous iron oxide (Fig. Ic) was much greater than from crystallineiron oxide (Fig. ld) and iron concentration began to decrease after about 10 days. No dissolved sulfide wasdetected in the amorphous iron oxide incubations. but dissolved sulfide did appear in solution after I weekin the crystalline oxide incubations and steadily increased (Figs Ic, ld). The amorphous tubes were muchmore black relative to tubes in which crystalline oxide was added. Similar behaviour was observed formanganese, although without blackening of tubes. Dissolved sulfide was produced in tubes to whichamorphous manganese oxide was added. but not until after 3 weeks of incubation (Fig. 2a). Much lessmanganese was released from the crystalline oxide, and dissolved sulfide began to accumulate after only Iweek (Fig. 2b).

The release of iron in tubes to which a mixture of amorphous iron and manganese oxides was added wasmuch less than tubes to which only amorphous iron was added (Figs lc, 2c). No dissolved sulfide wasmeasured 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 oxidetubes (Figs 2a. 2c).

190 W. J. TARUTIS, Jr and R. F. UNZ

150 30(a) (b)

~ 12025

.... ::;t;, 0, 20.§. 90 .§.(fl

S sn 15-0--- M1 :;;:;; 60c c 10::; ::;

30 5

000

0 5 10 15 20 25 30 5 10 15 20 25 30

Time (days) TIme (days)

140(c) S

:J' 120 Fe0> M1.§. 100

(fl 80...0

C60

::;40

oju,

20

DO 5 10 15 20 25 30

Time (days)

Figure 2. Metal liberation during time series incubations: (a) amorphous Mn oxide; (1)) crystalline Mn oxide; (c)amorphous Fc-Mn oxide mixture. Points are means of duplicate samples and error bars represent one standard

deviation.

DISCUSSION

Mobilization of iron and manl:anese

Time-series laboratory incubations allowed examination of the anoxic liberation of iron and manganeseunder more controlled conditions than is possible in the field. The term metal liberation is used instead ofmetal reduction because some of the metal released may adsorb, causing a potential underestimate of actualmetal reduction rates. Control tubes not amended with metal oxides revealed that sulfate reduction occurredwithin 7 days, indicated by the accumulation of dissolved sulfide and the deposition of black iron sulfide onthe interior walls of the tubes (Fig. Ia).

The bacterial reduction of iron oxides may be an important process in iron geochemistry and organic matterdecomposition in waterlogged soils and sediments (Lovley and Phillips, 1986a), but only a portion of thetotal iron present is able to be reduced by microorganisms. It has been shown that microorganismspreferentially reduce amorphous iron oxides over the more crystalline forms during organic matterdecomposition (Munch and Ottow, 1980; Lovley and Phillips, 1986b). The time-release of iron from naturalAMD oxide is similar to that from amorphous iron oxide (Figs Ib. Ic). Most, but not all, of the iron presentin 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 fromthe crystalline oxide (Fig. Id). The less reactive nature of the crystalline form is most probably due to lowersurface area (Canfield, 1989). The dissolution of Fe(III) oxides has been shown to be controlled by the

Coal mine drainage wetland microcosms 191

detachment of iron from the surface (Zinder et al., 1986), and it is thought that direct contact with the oxidesurface for microbial enzymatic reduction (Lovley, 1991). In addition, Gorby and Lovley (1991) concludedthat iron is reduced by a membrane-bound Fe(III) reductase. Thus, it is likely that the higher surface ofamorphous oxide was responsible for the much higher liberation of iron relative to the crystalline oxideincubations.

A similar argument can be made for the liberation of manganese (Fig. 2). Burdige et al. (1992), in theirstudy of the effects of manganese oxide mineralogy on manganese reduction, found that manganesereduction 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 mineralogyand 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 oxidemixture (Fig. 2c). Lovley and Phillips (1988) demonstrated that the primary mechanisms responsible forpreventing 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. Similarobservations were noted by Krishnamurti and Huang (1988) who found that manganese oxide minerals wereunstable in the presence of ferrous iron, resulting in the precipitation of iron oxides concomitant with theaccumulation of soluble reduced manganese. This seems to explain the much lower accumulation ofdissolved 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 andmanganese oxides as terminal electron acceptors, or indirectly, through the production of microbialmetabolites (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 ofmanganese oxides (Stone, 1987) and iron oxides (Lindsay, 1991) by organic acid metabolites does occur tosome 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 the28-day incubation period. In tubes to which either natural AMD oxides or amorphous iron oxides wereadded, sulfide generated by sulfate reduction quickly reacted with the oxides to form black iron sulfide withconcomitant removal of iron from solution, and an absence of dissolved sulfide existed over the entireincubation period (Figs 1b, lc). In contrast, tubes in which crystalline iron oxide were added turned blackvery slowly, and dissolved sulfide began to accumulate after 7 days (Fig. Id). The natural AMD andamorphous 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 ofsulfate reduction compared to rates of sulfide reaction with iron minerals. For example, when the rate of ironreaction with sulfide is greater than the rate of sulfate reduction, dissolved sulfide is effectively titrated fromsolution, 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, andiron sulfide formation is limited by iron (Canfield et al., 1992). The kinetics of iron oxide sulfidation hasbeen shown to depend on the iron oxide surface area (Pyzik and Sommer, 1981), which accounts for "thehigher reactivity of sulfide toward the more amorphous forms. This explains them absence of dissolvedsulfide in the presence of reactive iron (natural AMD and amorphous oxides) and its accumulation in thepresence of crystalline hematite (Figs 1-2).

Much less manganese was released from the natural AMD oxide than the amorphous form, owing to themuch lower reducible manganese concentration of the former (Tarutis, 1993). The reactivity of manganesetoward sulfide was much different than that of iron, however. Manganese release into solution was similar tothat of iron, but appeared to be unaffected by the presence of dissolved sulfide since no decrease in dissolvedsulfide since no decrease in dissolved manganese was observed (Fig. 2). The sulfidation of manganeseoxides results not in the precipitation of manganese sulfide (alabandite). but rather the oxidation of sulfide toMST ll-)-N

192 W. J. TARUTlS, Jr and R. F. UNZ

either elemental sulfur or sulfate accompanied by the release of soluble manganese into solution (Aller andRude, 1988). Alabandite is a relatively uncommon mineral and would not be expected to form under mostconditions (Saunders and Swann, 1992). This accounts for the gradual increase in dissolved manganese andthe appearance of dissolved sulfide later in the incubation period (Fig. 2).

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