博士 / 國立成功大學 / 地球科學系碩博士班 / 101 / Acid mine drainage (AMD) often occurs as a result of weathering of mine wastes in mining areas. Migration of high concentrations of toxic elements and subsequent precipitation of hydrous iron oxides in AMD areas could pose great threats on environmental quality, ecosystem, and human health. Analyzing the characteristics and reactions of AMDs and their precipitates is fundamentally important for understanding geochemical reactions in such an environment. It can also provide parameters essential for contamination assessment and remediation crucial to environmental sustainability.
Variations of water chemistry and precipitate mineralogy during mixing of acid mine drainage (AMD) (pH = 2.8) with a creek (pH = 6.0) and seawater (pH = 8.0) were investigated by XRD, SEM, TEM, IR, ICP analysis to discuss the relationships between metal mobility and precipitation of micro-nanometric iron oxyhydroxysulfate minerals in the Chinkuashih AMD area in consideration of reaction kinetics, waterfall effects, and seasonal variations. In terms of Fe(II) oxidation and Fe(III) precipitation in the creek section, the summer rates were 4-5 times higher than the winter rates, largely attributed to a temperature effect. In contrast, the seasonal differences in rate and rate constant were small in the waterfall section due to waterfall aeration which enhanced the Fe precipitation rate when the flow rate was large in the winter.
Radiating aggregates of schwertmannite with an overall hedgehog morphology occurred as a principal constituent of the surface precipitates on the bedrocks of Golden Falls and downstream Lian-Dong Creek and a channel at the Penshan 7th adit. Remarkable downstream reduction of Fe and As concentrations with respect to other metals in the water suggested in-situ precipitation of schwertmannite associated with an arsenate sorption process. Under the influence of water aeration, the waterfall section showed up to 1-2 orders of magnitude faster rates and higher model rate constants of Fe(II) oxidation (6.1-6.7×10-6 mol L-1 sec-1and 2.7-2.9×10-2 sec-1) and Fe (schwertmannite) precipitation (1.7-2.1×10-6 mol L-1 sec-1and 3.5-4.1×10-7 mol L-1 sec-1) than the creek section, and had a high As sorption rate (4.7-6.3×10-9 mol L-1 sec-1).
The schwertmannite in the surface precipitate at Golden Falls had an chemical formula of Fe16O16(OH)11(SO4)2.5•16H2O, and contained 2794 ppm As, 439 ppm Al, 115 ppm Cu, and 23 ppm Cr on average. With increasing depth, a transformation from schwertmannite, poorly crystalline goethite, to better crystallized goethite was characterized. In addition, bulk analyses indicated decreases of Al/Fe, Cr/Fe and S/Fe molar ratios and an increase of Cu/Fe and with rather small variations in As/Fe as the sample depth increased. The data suggested that Al, Cr and sulfate were released, As was retained, and Cu was accumulated, and that the formation of poorly crystalline goethite had a strong effect on the mobility and attenuation of elements during the transformation.
Schwertmannite precipitation was the main cause for downstream attenuation of As and metal concentrations in the Chinkuashih AMD and creek waters. However, there were still about 3235 metric tons per year (t yr-1) of Fe, 737 t yr-1 Al, 191 t yr-1 Cu, and 2.4 t yr-1 As discharged into Lian-Dong Bay, which produced abundant suspended particulates in the surface seawater near the estuary. The estuarine suspended particulates formed at pH 〈 4 were short nanorods consisting of schwertmannite and subordinate goethite, and the inshore suspended particulates formed at pH 〉 5.5 were composed of non-crystalline hydrous iron and aluminum oxides (HFO and HAO) with a trace of schwertmannite, in a form of nanoballs. As the pH increased in the water, the suspended particulates were enriched in Al, Si, Cu, Zn, Mn, Ni, Co, and Cd relative to As, Cr, Pb, Fe, and S.
The water chemistry and precipitate constituents in areas affected by AMDs at Chinkuashih were mainly influenced by the formation of iron oxyhydroxysulfate minerals and HFO/HAO and sequential co-precipitation or adsorption of trace elements interrelated to changes in solution pH at various stages of water mixing and neutralization. These processes may have reduced and/or deferred environmental impacts made by the Chinkuashih AMDs. However, further investigations and continuous monitoring of the potential influences of desorption, transformation, and/or dissolution of the suspended particulates in Lian-Dong Bay are apparently important.
| Identifer | oai:union.ndltd.org:TW/101NCKU5135139 |
| Date | January 2013 |
| Creators | Chun-JungChen, 陳君榮 |
| Contributors | Wei-Teh Jiang, 江威德 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 166 |
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