Acid neutralization mechanisms and metal release in mine tailings: a laboratory column experiment

Mining and milling of base metal ore deposits can result in the release of metals to the environment. When sulfide minerals contained in mine tailings are exposed to oxygen and water, they oxidize and dissolve. Two principal antagonistic geochemical processes affect the migration of dissolved metals in tailings impoundments: sulfide oxidation and acid neutralization. This study focuses on acid neutralization reactions occurring in the saturated zone of tailings impoundments. To simulate conditions prevailing in many tailings impoundments, 0.1 mol/L sulfuric acid was passed continuously through columns containing fresh, unoxidized tailings, collected at Kidd Creek metallurgical site. The results of this column experiment represent a detailed temporal observation of pH, Eh, and metal concentrations. The results are consistent with previous field observations, which suggest that a series of mineral dissolution-precipitation reactions control pH and metal mobility. Typically, the series consists of carbonate minerals, Al and Fe(III) hydroxides, and aluminosilicates. In the case of Kidd Creek tailings, the dissolution series consists of ankerite-dolomite, siderite, gibbsite, and aluminosilicates. In the column experiment, three distinct pH plateaus were observed: 5.7, 4.0, and 1.3. The releases of trace elements such as Cd, Co, Cr, Cu, Li, Ni, Pb, V, and Zn were observed to be related to the pH buffering zones. High concentrations of Zn, Ni, and Co were observed at the first pH plateau (pH 5.7), whereas Cd, Cr, Pb, As, V, and Al were released as the pH of the pore water decreased to 4.0 or less.     

Damage evaluation and conservation treatment of the tenth century Korean rock-carved Buddha statues

This study focuses on the lithological characterization, quantitative deterioration assessment and conservation treatment of the tenth century rock-carved Buddha statues in Korea. The Buddha statues were carved on light gray macrocrystalline biotite granite, and features microcline phenocrysts and pegmatite patches. The rock-forming minerals are quartz, plagioclase, microcline and biotite showing micrographic and porphyritic textures. Feldspars and biotite in the host rock have been partly altered into sericite and chlorite by weathering. The surface of the Buddha statues is remarkably irregular due to granular disintegration and the differential weathering of the quartz and microcline. In addition, horizontal and vertical cracks in the host rock have promoted mechanical weathering. Biological colonization of the statues was serious, and featured dark gray and yellowish green lichen, dark green bryophyte and some plants. The Buddha statues were evaluated as the highly weathered rock by ultrasonic survey. The lower parts under the influence of water and areas with severely broken surfaces showed a much more advanced weathering grade. Therefore, scientific conservation treatment was carried out for the long-term maintenance and conservation of the Buddha statues. After pretests, dry, wet and chemical cleanings were applied to the statues. Next, joining of cracked parts and surface consolidating were executed. Finally, the site environment was improved by installation of drain and trimming trees around the statues to ensure more stable long-term conservation.     

Chemical and isotopic compositions of groundwater and stream water in a heavy agricultural basin of Korea

The chemical and isotopic compositions of groundwater and stream water in the Haean basin, Korea, which have been intensively used for agricultural irrigation, were examined. The field parameters and chemical compositions were typical of relatively uncontaminated waters, except for the agricultural impact. The low EC levels indicated that the waters had not experienced any significant geochemical processes such as water-rock interaction and evaporation. The very similar chemical compositions and ionic ratios were also indicative of fast circulation and good hydraulic connection. The oxygen and hydrogen isotopic compositions of most of the samples in wet and dry seasons showed very similar range which indicated a single source for both waters without any significant geochemical alteration. However, in the wet season, evaporation trends were observed in some groundwaters because of inflow of irrigation return flow from a rice paddy field. The isotopes of both waters were affected by the altitude and amount of precipitation.     

29Si and 27Al magic-angle-spinning NMR studies of the thermal transformation of kaolinite

Thermal transformations of kaolinite of different degree of crystallinity have been monitored by ²⁷Al and ²⁹Si high-resolution NMR with magic-angle spinning (MAS NMR), X-ray diffraction, Fourier transform infrared, atomic absorption spectrophotometry and thermogravimetric analysis. NMR shows differences in the dehydroxylation process of kaolinites with different degree of crystallinity and reveals the presence of short-range order in metakaolinite. ²⁹Si NMR spectra acquired with a 30 s recycle delay of poorly and highly crystalline samples heated at 480 and 500° C, respectively, contain three distinct signals; we discuss their assignment in the light of experiments involving leaching of the samples with aqueous KOH. Ca. 40% of Si sites retain their original Q ³ symmetry just above the onset of dehydroxylation and the Q ⁴ environment is present showing that a small amount of amorphous silica has already segregated. The spectrum of samples treated at 1000° C contains a signal at -110ppm (from Q ⁴ silicons) and a faint resonance, from mullite, at ca. -87 ppm. ²⁹Si NMR also shows that cristobalite germs are already present at 950–1000° C. The ²⁷Al MAS NMR spectra of metakaolinite reveal the presence of 4-, 5-and 6-coordinated Al. Changes in the three Al populations as a function of temperature have been monitored quantitatively. Below 800° C, 4-and 5-coordinated Al appears at the expense of 6-coordinated Al, but above 800° C the amount of 6-coordinated Al increases again. We suggest a dehydroxylation scheme which accounts for the presence of 4-and 5 coordinated Al. Above 900–950° C the latter signal is no longer present in the ²⁷Al NMR spectra and new 4-and 6-coordinated Al species (mullite and γ-alumina) appear. We propose new ideas for the structure of metakaolinite.