Processes of Iron and Manganese Retention in Laboratory Peat Microcosms Subjected to Acid Mine Drainage

Abstract

Despite increasing use of constructed wetlands for treatment of metalenriched acid coal mine drainage (AMD), the biotic and abiotic mechanisms of metal retention in such wetlands are poorly understood. The present study was conducted to evaluate the processes responsible for Fe and Mn retention in peat and the effects of microbial activity, pH, temperature, and metal concentration in AMD on these processes. Experimental units consisted in 30 g (wet wt.) of fresh Sphagnum peat, which was repeatedly flushed with synthetic AMD at pH 3.5. Of the four major processes of metal cation retention in peat (cation exchange, complexation with peat organics, precipitation as oxides, and precipitation as sulfides), Fe oxidation and Fe binding on peat organics were predominant, with Fe oxides and organically bound Fe making up, respectively, 62 and 22% of the total Fe in the peat at the end of the experiment. Whereas Fe complexation was a finite process, reaching saturation at 12 mg Fe g' dry peat, Fesxide concentration in peat increased steadily throughout the experiment. At pH 3.5, Fe-oxide precipitation was depressed by the addition of an antiseptic (formaldehyde) to AMD, suggesting that the process was microbially mediated. Iron oxide precipitation was higher at pH 5.5 than 3.5 and less depressed at pH 5.5 than 3.5 by the presence of formaldehyde in AMD. The efficiency of peat to remove Fe from AMD was diminished at low temperature (<15 "C) and high Fe concentration in AMD (>lo0 mg L-I). Manganese retention in peat was small compared with that of Fe, and Mn was retained in peat almost exclusively as exchangeable Mn2+. Retention oPFe2+ in peat was not affected by the presence of Mn2+ in AMD. Iron oxides that had accumulated in peat subjected to AMD were not readily resolubilized by any of three processes investigated: photoreduction, microbial Fe(II1) reduction under reducing conditions, and exposure to simulated acid precipitation. These findings suggest that constructed wetlands may be an appropriate technology to remove Fe from AMD with low soluble Fe concentration, but are inadequate for treating drainage waters rich in soluble Mn. CIDIC drainage from active and abandoned coal A mining operations is formed when sulfide-bearing minerals (principally pyrite) associated with coal and underlying bedrock are exposed to air and oxidized by chemical and microbiological processes, generating H+ and soluble species of S (SO:-) and Fe (Fez+).As water transporting sulfuric acid runs over the mine pavement and through pyrite-bearing overburden, it dissolves other minerals containing Mn. As a result, acid mine drainage (AMD) has characteristically high metal concentrations, with Fe and Mn in the range of 39 to 7300 and 4 to 126 mg L-', respectively, and pH values often ranging from 2 to 4.5 (Appalachian Regional Commission, 1969; Watzlaf, 1988).Federal and state regulations require that all water discharged from an active coal surface mine meets effluent limits: total Fe 7.0 mg L-I, total Mn 4.0 mg Both authors,

Concentration and chemical species of iron in soils from groundwater/surface water ecotones

“…KCl-extractable Fe, was of minor importance, which is in agreement with other studies (Wieder, 1988;Henrot & Wieder, 1990;Tarutis et al, 1992). However, it was shown in an earlier study (Norrstrôm, unpublished results) that the concentration in H3 varied greatly between separate soil samples.…”

“…Fe P was 24-84% of the total pool of Fe. Other studies have reported organically-bound Fe to be 47-90% (Wieder & Lang, 1986) and 42-43% of total Fe (Henrot & Wieder, 1990). Henrot & Wieder (1990) found that the binding of Fe to peat organics was the predominant mechanism of Fe retention up to saturation of the organic binding sites.…”

“…Other studies have reported organically-bound Fe to be 47-90% (Wieder & Lang, 1986) and 42-43% of total Fe (Henrot & Wieder, 1990). Henrot & Wieder (1990) found that the binding of Fe to peat organics was the predominant mechanism of Fe retention up to saturation of the organic binding sites. Tarutis et al (1992) demonstrated that oxidizable Fe (defined as Fe bound to organic matter and sulphides) increased in a nonlinear fashion with Soil water was water sampled with suction lysimeters in the peat layer.…”

“…Studies on Fe in wetlands have mostly been restricted to ones influenced by acid coal mine drainage (AMD). Wieder & Lang (1986), Wieder (1988), Henrot & Wieder (1990) and Tarutis et al (1992) all tried to identify characteristics and properties optimal for a maximal retention of Fe. Studies in non-polluted wetlands have measured total and exchangeable Fe only from the botanical point of view (Maimer, 1962;Sonesson, 1970;Danman, 1978).…”

Relationships between environmental variables and vegetation across mountain wetland sites, N. Iran

“…A high positive correlation between Fe +2 and OM indicates the importance of OM for retention of Fe +2 , which is consistent with other studies (Henrot & Wieder 1990;Tarutis et al 1992;Grybos et al 2007). In an investigation of Welsh wet meadow soils, the concentration of Fe 2+ and OM were positively correlated with acidity in the soil (Blackstock et al 1998).…”