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Evangelou, V. P. (2001). Pyrite microencapsulation technologies: Principles and potential field application. Ecological Engineering, 17(2-3), 165–178.
Abstract: In nature, pyrite is initially oxidized by atmospheric O2, releasing acidity and Fe2+. At pH below 3.5, Fe2+ is rapidly oxidized by T. ferrooxidans to Fe3+, which oxidizes pyrite at a much faster rate than O2. Commonly, limestone is used to prevent pyrite oxidation. This approach, however, has a short span of effectiveness because after treatment the surfaces of pyrite particles remain exposed to atmospheric O2 and oxidation continuous abiotically. Currently, a proposed mechanism for explaining non-microbial pyrite oxidation in high pH environments is the involvement of OH- in an inner-sphere electron-OH exchange between pyrite/surface-exposed disulfide and pyrite/surface-Fe(III)(OH)n3-n complex and/or formation of a weak electrostatic pyrite/surface-CO3 complex which enhances the chemical oxidation of Fe2+. The above infer that limestone application to pyritic geologic material treats only the symptoms of pyrite oxidation through acid mine drainage neutralization but accelerates non-microbial pyrite oxidation. Therefore, only a pyrite/surface coating capable of inhibiting O2 diffusion is expected to control long-term oxidation and acid drainage production. The objective of this study was to examine the feasibility in controlling pyrite oxidation by creating, on pyrite surfaces, an impermeable phosphate or silica coating that would prevent either O2 or Fe3+ from further oxidizing pyrite. The mechanism underlying this coating approach involves leaching mine waste with a coating solution composed of H2O2 or hypochlorite, KH2PO4 or H4SiO4, and sodium acetate (NaAC) or limestone. During the leaching process, H2O2 or hypochlorite oxidizes pyrite and produces Fe3+ so that iron phosphate or iron silicate precipitates as a coating on pyrite surfaces. The purpose of NaAC or limestone is to eliminate the inhibitory effect of the protons (produced during pyrite oxidation) on the precipitation of iron phosphate or silicate and to generate iron-oxide pyrite coating, which is also expected to inhibit pyrite oxidation. The results showed that iron phosphate or silicate coating could be established on pyrite by leaching it with a solution composed of: (1) H2O2 0.018-0.16 M; (2) phosphate or silicate 10-3 to 10-2 M; (3) coating-solution pH [approximate]5-6; and (4) NaAC as low as 0.01 M. Leachates from column experiments also showed that silicate coatings produced the least amount of sulfate relative to the control, limestone and phosphate treatments. On the other hand, limestone maintained the leachate near neutral pH but produced more sulfate than the control.
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Ye, Z. H. (2001). Removal and distribution of iron, manganese, cobalt, and nickel within a Pennsylvania constructed wetland treating coal combustion by-product leachate. Journal of Environmental Quality, 30(4), 1464–1473.
Abstract: A flow-through wetland treatment system was constructed to treat coal combustion by-product leachate from an electrical power station at Springdale, Pennsylvania. In a nine-compartment treatment system, four cattail (Typha latifolia L.) wetland cells (designated Cells I through 4) successfully removed iron (Fe) and manganese (Mn) from the inlet water; Fe and Mn concentrations were decreased by an average of 91% in the first year (May 1996-May 1997), and by 94 and 98% in the second year (July 1997-June 1998), respectively. Cobalt (Co) and nickel (Ni) were decreased by an average of 39 and 47% in the first year, and 98 and 63% in the second year, respectively. Most of the metal removed by the wetland cells was accumulated in sediments, which constituted the largest sink. Except for Fe, metal concentrations in the sediments tended to be greater in the top 5 em of sediment than in the 5- to 10- or 10- to 15-cm layers, and in Cell I than in Cells 2, 3, and 4. Plants constituted a much smaller sink for metals; only 0.91, 4.18, 0.19, and 0.38% of the Fe, Mn, Co, and Ni were accumulated annually in the aboveground tissues of cattail, respectively. A greater proportion of each metal (except Mn) was accumulated in cattail fallen litter and submerged Chara (a macroalga) tissues, that is, 2.81, 2.75, and 1.05% for Fe, Co, and Ni, respectively. Considerably higher concentrations of metals were associated with cattail roots than shoots, although Mn was a notable exception.
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Chung, I. J. (2001). Immobilization of arsenic in tailing by using iron and hydrogen peroxide. Environ. Technol., 22(7), 831–835.
Abstract: Under environmental conditions, arsenic (As) reveals anionic behavior and is converted into various forms in accordance with the Eh/pH condition. This causes the difficulty of treating As with other heavy metals in tailing. This study was carried out to develop the immobilization method of arsenic in tailing as ferric arsenate (FeAsO4) using hydrogen peroxide. According to experimental results, the extracted concentrations of arsenic and iron (Fe) from tailing were reduced up to 84% and 93%, respectively. In the experiment using pure Pyrite (FeS2) and As solution, As concentration decreased with an increase of hydrogen peroxide dosage. The experimental results of re-extraction showed that only 10% of As and 20% of Fe were extracted in the case of using hydrogen peroxide. As a result, the long-term stability of this method was clarified.
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Goulet, R. R. (2001). Changes in dissolved and total Fe and Mn in a young constructed wetland: Implications for retention performance. Ecological Engineering, 17(4), 373–384.
Abstract: Surface-flow wetlands are generally considered sinks for Fe and Mn but they may also export and affect the partitioning of these metals. This study was undertaken to evaluate the effect of a young constructed wetland on the retention and transformation of both dissolved and particulate Fe and Mn. Duplicate water samples were collected every three days at the inlet and outlet structures of the Monahan Wetland, Kanata, Ontario, from spring of 1997 to 1999. While on a yearly basis the wetland showed significant retention of che dissolved phase, the retention of total Fe and Mn was poor. There were strong seasonal differences in retention and, during the winter, the wetland was a source. The wetland transformed dissolved into particulate Fe and Mn from spring to fall whereas during the winter, dissolved Fe and Mn were released. Changes in pH, alkalinity and temperature could explain 11% and 40% of the outlet variation in the ratio of dissolved to total Fe and Mn respectively. Furthermore, from spring to late summer, planktonic algal biomass was negatively related to the ratio of dissolved to total Fe and Mn implying a role in Fe and Mn transformations in young wetlands where emergent and submerged vegetation have yet to dominate the system. (C) 2001 Elsevier Science B.V. All rights reserved.
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Goulet, R. R. (2001). The evaluation of metal retention by a constructed wetland using the pulmonate gastropod Helisoma trivolvis (Say). Archives of Environmental Contamination and Toxicology, 40(3), 303–310.
Abstract: Constructed wetlands are built because they can act as sinks fur many pollutants, thereby protecting the water quality of downstream ecosystems. The treatment performance is generally assessed using mass balance calculations. Along with the mass balance approach, we compared the metal content of populations of a common pond snail (Helisoma trivolvis Say) collected upstream and downstream of a 3-year-old constructed wetland. Snails were collected in early May, June, and August 1998. At the same time, water samples for particulate and dissolved metals were taken every 3 days for the duration of the experiment. Overall, the wetland retained most dissolved metals, including Fe, Mn, Cu, Zn, Ni, and Pb, but released dissolved As. However, the wetland released particulate Fe and Mn. With the exception of Zn, the metal concentrations of the downstream snails were on average higher than those measured in the upstream population. The higher metal content of downstream snails was likely related to the significant export of particulate metals by the wetland, despite the overall retention of dissolved metals. This study points to the need for biological as well as chemical monitoring to determine the treatment efficiency and toxicological risk associated with constructed wetlands.
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