Environ. Sci. Technol. 2003, 37, 2120-2126
Environmental, Mineralogical, and Genetic Characterization of Ochreous and White Precipitates from Acid Mine Drainages in Taebaeg, Korea JEONG JIN KIM* AND SOO JIN KIM School of Earth and Environmental Sciences, Seoul National University, Seoul 151-742, Korea
X-ray diffraction, energy-dispersive X-ray fluorescence, thermal analysis, scanning electron microscopy, inductively coupled plasma emission spectroscopy, and ion chromatography were used for the environmental, mineralogical, and genetic characterization of brownish yellow, reddish brown, and white precipitates from acid mine drainage in Taebaeg, Korea. Ferrihydrite+goethite, schwertmannite, and Al-sulfate were precipitated under different chemical environments on the stream bottom of acid mine drainages. The brownish yellow precipitates (Munsell color 9.5YR hues) consist mainly of schwertmannite with traces of quartz, illite, pyrophyllite, goethite, lepidocrocite, and gypsum. The reddish brown precipitates (Munsell color 3.5YR hues) consist mainly of ferrihydrite with small amount of goethite. The white precipitates consist mainly of poorly crystalline Al-sulfate with small amounts of quartz, gypsum, and calcite. Thermal decomposition due to dehydration of ferrihydrite and schwertmannite takes place at approximately 120 °C and 140 °C, respectively. Alsulfate converts to γ-alumina at 850 °C. SEM study shows that the spheroid and rod-shaped precipitates characteristic of Gallionella consist of iron hydroxide with varying chemical compositions.
Introduction Many coal and metal mines in Korea have been closed due to economical or environmental reasons during past decade. The Taebaeg coal field, one of the largest coal fields, located in the middle eastern part of the Korean peninsula, has been abandoned since late 1980s. Thereafter, a significant amount of acid mine drainage (AMD) was discharged from the mine adits and dumps, acidifying the water system. One of the most significant environmental issues is acidic drainage which gives rise to the many environmental problems in the mining and surrounding areas. The geology of the study area is dominated by the Joseon and Pyeongan supergroup metasedimentary rocks of Paleozoic to early Mesozoic period (Figure 1). The Joseon Supergroup consists of the lower Paleozoic sedimentary sequence, namely, Jangsan quartzite, Myobong formation, Pungchon limestone, Hwajeol formation, and Maggol limestion. The Pyeongan supergroup consists of the Manhang, Keumcheon, Jangsung, and Hambaeksan formations which * Corresponding author phone: +82-2-877-3073; fax: +82-8773062; e-mail:
[email protected]. 2120
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consist predominantly of carbonates and subordinate sandstone and shale. The coal dumps in the mine area are composed of coal, chlorite, quartz, and pyrite. Limestone contains small amounts of clay minerals, whereas shale consists of quartz and aluminum-silicate minerals such as kaolinite, pyrophyllite, and chlorite. The Donghae mine in the Taebaeg area consists of small coal mines such as Samgyeong, Hanyeong, Namil, Deogyong, Pungsan, and Taegeug that had been actively mined from early 1960s to early 1990s. These mines were closed from 1989 to 1991 due to industrial reason by the Korea Coal Industry Promotion Board. The altitude of the waste dump site in the mine is approximately 1000 m above sea level. Underground working was done to a depth of 3000 m. Several hundred square meters of the surface are covered with waste rocks. The AMD produced from the Donghae mine and the leachate through waste dumps enters directly into local streams. Two major creeks, Soro and Sanae in the study area, are contaminated with colored precipitates from coal mine drainages. Both creeks are approximately 3-4 km in length and 10-20 m in width. The Sanae creek has two unpolluted tributaries joining the main creek at 2.5 and 2 km, respectively, from the outlet of the acid water. Artificial ponds were constructed near the mining water outlet in the Soro creek in order to neutralize the acid water and settle the metallic compounds. The neutralized mine waters after precipitation of iron oxides and/or sulfates flow automatically into the Soro creek. Precipitates in the both creek underwent a color change from brownish yellow, reddish brown, and white depending on the chemical change of the streamwaters with month and distance of AMD source. The chemical precipitates associated with AMD include various mineral phases, such as schwertmannite, ferrihydrite, goethite, and jarosite (13). Mixing with downstream tributaries causes dilution, progressive neutralization of AMD, and the formation of Al colloids on the stream bed (4, 5). The ochreous precipitates contain abundant cocci and bacilli and show characteristic shapes of twisted stalks of Gallionella ferruginea. The genesis of ochreous precipitates at AMD is also strongly related to microbial process together with chemical process. Microbially, the precipitates are formed by acidophilic bacteria that oxidize the reduced iron and sulfur in pyrite (6, 7). The objectives of this study are to examine the environmental, mineralogical, and genetic characteristics of ochreous and white precipitates from the acid mine drainage.
Experimental Section Sampling. During the period from April to October 2000, water and sediment samples were collected from the sampling sites shown in Figure 1. Each water sample was filtered through 0.45 µm membrane filter and then stored in 1000 mL polyethylene bottles. Filtered water samples were acidified with concentrated HNO3 and bottled in 250 mL polyethylene bottle and stored in a refrigerator at 4 °C prior to chemical analysis in the laboratory. pH and electrical conductivity (EC) were measured at each sampling site. Precipitate samples were collected from the stream bottom where sediments accumulated. Ochreous and white bottom sediments were sieved in laboratory using the 63 µm plastic sieve to remove large detrital materials, concentrated by gravity settling and dried at room temperature for X-ray diffraction analysis and scanning electron microscope 10.1021/es026353a CCC: $25.00
2003 American Chemical Society Published on Web 04/12/2003
FIGURE 1. Geological map and sampling sites in the hydrological system of the Taebaeg area. observation and other experiments. The dry color of precipitates are reported according to the Munsell soil color chart. Analytical Methods. Chemical analyses of water samples were made using Perkins-Elmer Optima 3000XL inductively coupled plasma emission spectrometry (ICP-AES) for cations and Dionex 4000i ion chromatograph (IC) for anions. The mineralogy of precipitates was analyzed using X-ray powder diffractometer using Rigaku Geigerflex RAD3-C equipped with CoKR radiation, a scintillation counter, and graphite monochromator. All the XRD samples were step-scanned in step interval of 0.05°2θ using 1° divergence slit and 10 s scanning time because they are poorly crystalline. The morphological features of precipitates were studied using a JEOL-JSM5200LV scanning electron microscope (SEM), equipped with an energy-dispersive X-ray spectrometer. Chemical compositions of precipitates were analyzed using a JSX-3200 energy-dispersive X-ray fluorescence (ED-XRF) at 30 kV and scanning time for 600 s. Setaram LABTGA derivative thermogravimetry (DTG) and thermogravimetry (TG) of precipitates were run on heating rate of 10 °C/min. Weight loss was determined for 20-35 mg samples in alumina crucibles at temperatures ranging from 50 to 1200 °C. Water Chemistry. Table 1 shows that field measurement data and chemical compositions of water samples from the Sanae and Soro creeks. The water shows pH ranging from 3.18 to 8.30 and EC ranging from 272 to 1458 µS/cm. The content of SO4 ranges from 47.87 to 1017.90 mg/L. Concentrations of cations such as Mg, Al, Si, Ca, and Fe are relatively high showing the range from 8.44-49.96, 0.0443.11, 0.00-12.28, 36.55-138.44, 0.00-20.22 mg/L, respectively. Figure 2 shows water chemistry in the stream with time and distances from the AMD sources. Salt content decreases with distance from the waste piles or outlets. The abrupt decrease in total dissolved salts often encountered may be
the result of their removal through the precipitation of Al and/or Fe compounds and dilution by unpolluted water (811). For instance, the variations in the Al concentration can be explained as a function of pH. As the pH increases downstream, white Al components gradually precipitate, resulting in the removal of Al from waters at about pH>5.0. The field measurements show that brownish yellow and reddish brown precipitates dominated in the range of pH 3.0-4.5 and 5.3-6.9, respectively, whereas white precipitates at pH 4.5-6.0.
Results and Discussion Brownish Yellow Precipitate (Schwertmannite: Munsell Color 9.5YR Hues). The XRD patterns of brownish yellow precipitates consist of eight broad diffraction bands (4.89, 3.36, 2.54, 2.26, 1.96, 1.66, 1.51, and 1.46 Å) of schwertmannite with traces of quartz, illite, pyrophyllite, goethite, lepidocrocite, and gypsum (Figure 3). They show agreement with the distinctive diffraction patterns previously reported for schwertmannite. Table 2 shows that the brownish yellow precipitates on the stream bottom contain approximately 55-60% Fe, 1115% SO3, 18-25% H2O, 0.4-1.7% Al2O3, 1.0-3.9% SiO2, and 0.1-3.1% CaO. The Fe:S ratios are ranges from 3.77 to 5.04. Though Fe:S ratio for ideal schwertmannite, Fe8O8(OH)6SO4, is 8, the lower values (ranging to
