Mineralogy of Iron Precipitates in a Constructed Acid Mine Drainage Wetland

Abstract

Precipitates forming at various stages of acid mine drainage treatment in a high metal load (≈1000 mg L−1 Fe) and low pH (≈3) constructed wetland were characterized by chemical dissolution, x‐ray diffraction, thermal analysis, and scanning electron microscopy. Minerals precipitating in flumes and in entry wetland cells lacking vegetation included poorly crystalline ferrihydrite, lepidocrocite, goethite, possibly an akaganéite‐like mineral, and high Fe/S ratio Fe oxyhydroxysulfates (similar to schwertmannite). Within vegetated wetland cells lined with crushed limestone, well‐crystallized gypsum, lepidocrocite, and Fe‐oxyhydroxysulfate minerals with low Fe/S ratios were accompanied by gradual reductions in ferrihydrite and akaganéite. The Fe/S molar ratios of Fe oxyhydroxysulfates in flume precipitates averaged 5.2 ± 0.3, while those of cell precipitates averaged ≈3.5 ± 0.5. The oxalate‐extractable (Feox) to total (Fet) Fe fraction of the precipitates was considerably higher in wetland cells (1.1 ± 0.3), where organic C was 10‐fold higher than in entry flumes (0.7 ± 0.1). Scanning electron micrographs of flume precipitates showed a fiber‐like morphology of densely aggregated spherical particles, 1.5 to 2.0 mm in diameter, with a closely packed microcrystalline matrix. Precipitates collected from vegetated wetland cells formed aggregates of somewhat smaller diameter spherical particles with grassy surfaces or finger‐like projections entangled with bacterial cells. The overall composition of the precipitates suggested that the Fe chemistry is controlled primarily by the solubility of Fe oxyhydroxides in flumes and by S‐enriched Fe oxyhydroxysulfates inside the wetland cells. Although jarosite and goethite are thermodynamically favored in the wetland cells, their formation appeared to be inhibited by the presence of organics and the precipitation of Fe oxyhydroxysulfates and gypsum.

Application, Chemical Interaction and Fate of Iron Minerals in Polluted Sediment and Soils

“…Lepidocrocite (γ-FeOOH) is less common and is typically found in sub-micron scale association with goethite [27], and akaganeite (β-FeOOH) is confined to unique chloriderich environments (e.g. acid sulphate wetland sediment [28] and acid mine drainage sediment [29]). The adsorption of contaminants to these Fe (oxy-hydr)oxides is governed by the ambient pH conditions and by the reactivity, structure and chemistry of the hydroxyl groups on the mineral surfaces [30].…”

Review of Constructed Subsurface Flow vs. Surface Flow Wetlands

“…Aluminum, iron, and manganese can form insoluble compounds -oxides, oxyhydroxides, and hydroxides -via hydrolysis and/or oxidation (Karathanasis and Thompson 1995). Oxidation and hydrolysis are generally the main removal mechanisms for these three metals (ITRC 2003).…”

Field Study on Heavy Metal Accumulation in a Natural Wetland Receiving Acid Mine Drainage

“…Second, the residue was extracted in 8 cm 3 of 1 mol dm À3 NaOAc (pH 5.0) for 5 h at room temperature to release carbonate-bound species. Third, the residue was extracted in 20 cm 3 The final residue was decomposed in a Mile Stone ETHOS PLUS microwave digester. 0.5 g of the residual sample was placed inside of a teflon vessel, with 7 cm 3 of concentrated HNO 3 , 2.5 cm 3 of 49.5% HF, and 0.5 cm 3 of 30% H 2 O 2 .…”

“…Accordingly, the information necessary to predict long-term efficiency and to improve planning and management in constructed wetlands is lacking. 3,4) A natural wetland in southern part of Hokkaido, Japan, receives acidic heavy metal bearing water from the portal of abandoned mine workings. The drainage from the portal flows in two streams, one flowing through the wetland and one not (Fig.…”