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Paper ID 50 Mineralogy and Leaching Behavior of Mineralized Rocks Excavated Near Tunnel Construction Sites Miyuki HIROTA1, Nohara YOKOBORI1, Toshifumi IGARASHI2 and Tetsuro YONEDA2 1 Graduate school of Engineering, Hokkaido University, Sapporo, Japan 2 Faculty of Engineering, Hokkaido University, Sapporo, Japan e-mail: [email protected] ABSTRACT Several tunnels for the Hokkaido Bullet Train Line are being planned for construction in mineralized areas widely distributed in Hokkaido, Japan. Mineralized rocks when exposed to surface oxidizing conditions are potential sources of acid rock drainage (ARD) and heavy metals/toxic metalloids. Mineralogical analyses and leaching experiments using the mineralized rocks collected near the construction sites were carried out to characterize the rock samples. The results showed that they contained substantial amounts of sulfide and zinc minerals, and the leaching concentrations of Cd and Pb were related to zinc or sulfur content. This indicates that zinc or sulfur will be a good indicator of leaching of hazardous elements. KEY WORDS: Leaching / Mineralized rocks / Mineralogy 1. INTRODUCTION Contamination of soil and groundwater by toxic elements, such as arsenic (As), cadmium (Cd) or lead (Pb), is a serious problem around the world. Chronic exposure to As may cause skin pigmentation, hyperkeratosis, cardiovascular disease and cancer [1][2]. The mining and metal processing industry is a major anthropogenic contributor of As to the environment. Lead contaminations of soils have also been reported in mining areas, and chronic Pb poisoning causes paralysis of the peripheral motor nerves, anemia, kidney damage, abnormal fetal development, and abnormal neurological development and function [3][4]. The biogeochemical and ecological impacts of Cd have been two of the fastest growing areas of research since the 1960s, because of the discovery of a painful bone disease (itai-itai disease) caused by Cd pollution in Fuchu, Toyama Prefecture, Japan [5][6]. Anthropogenic sources of Cd include ash from coal combustion, atmospheric fallout, urban refuse, fertilizer, smelting and refining of non-ferrous metals, and manufacturing processes of metals and chemicals [6]. Cadmium is toxic because it causes irreversible damage to most cell types via the inhibition of cell respiration and some key enzymedependent processes. The specific target systems include the lungs, erythrocytes, spleen, endocrine glands, liver and kidneys [7]. These toxic elements are relatively rare in nature, but could be found concentrated into soils or rocks due to certain natural phenomena, such as weathering of sulfide-rich parent materials, volcanic activity and forest fires. Mineralized rocks formed due to active volcanic strata are important sources of these toxic elements. These rocks contain sulfide minerals, such as pyrite (FeS2), which is of primary interest because of its importance in the generation of acid rock drainage (ARD) as well as a source of heavy metals/toxic metalloids. Sulfide minerals are unstable under oxidizing conditions so that once these rocks are exposed to surface conditions, ARD and heavy metals/toxic metalloids are simultaneously released into the surrounding environment. Hokkaido in northern Japan has many active volcanoes, mineralized areas and abandoned mine sites. Extension of the Hokkaido Bullet Train Line from Honshu to Hokkaido requires the excavation of several new tunnels. Many of the tunnels being planned for excavation would transect across some of the mineralized areas of the island. To understand the leaching behavior of these heavy metals/toxic metalloids from the rocks, we collected mineralized rock samples near the tunnel construction site and investigated the relation between the mineralogy of the rocks and leaching behavior of the toxic trace elements contained. Furthermore, the information obtained here will be useful to evaluate the risks involved in disposing of heavy metals/metalloid-rich excavated rocks. Environmental Concerns 33 2. MATERIALS AND METHODS 3. RESULTS 2.1 Materials 3.1 Mineralogical analysis Nine mineralized rock samples were collected in areas close to tunnel construction site. M-samples (M1 and M2), K-samples (K1, K2, K3, K4 and K5), O-sample (O1) and T-sample (T1) were collected. These nine rocks were formed during the Middle Miocene. The surface of the rocks was weathered since the rocks were collected near the ground surface. Thus, the oxidized surface of the rocks were removed by washing with deionized water. Table 1 lists the identified mineralogical composition of the rock samples by XRD. Sample M1 contains pyrite while those from K-site have pyrite, galena (PbS) and/or sphalerite (ZnS). Figure 1 shows photomicrographs of a sphalerite crystal using polarized light microscopy, indicating the presence of significant impurities in sphalerite. 2.2 Methods 2.2.1 Mineralogical analysis The rocks collected were air dried, crushed using a jaw crusher or agate mortar and pestle, and sieved through a 2 mm aperture screen. The crushed rocks passing the screen were further ground to <50 μm in preparation for the mineralogical and chemical analyses. Polished thin sections of the rock samples were also prepared for the microscopic analysis using a microscope and an electron probe micro analyzer (EPMA). Microscopic observation and X-ray diffraction spectrometry (XRD) were carried out to characterize the rock samples. On the other hand, Xray fluorescence spectrometry (XRF), scanning electron microscopy/energy dispersive spectroscopy (SEM-EDS) and EPMA were conducted to determine the relative abundance of the major and trace elements in the samples. 2.2.2 Batch leaching experiments and chemical analyses Batch leaching experiments using crushed rock samples (<2 mm) were carried out. These were done by mixing 5 grams of the rock samples and 50 ml of deionized water for 6, 24 and 168 h at 120 rpm. After the predetermined shaking time, the pH, electrical conductivity (EC) and oxidation-reduction potential (Eh) of the suspensions were measured. This was followed by filtration of the suspensions through 0.45 μm Millex® membrane filters (Merck Millipore, USA). Dissolved As, Cd, Pb and Zn in the filtrates were determined using an inductively coupled plasma atomic emission spectrometer (ICP-AES) (ICPE-9000, Shimadzu Corporation, Japan) or an inductively coupled plasma mass spectrometer (ICPMS) (iCAP Qc, Thermo Scientific, USA). Sulfate (SO42-) concentration was quantified by anion chromatography (ICS-1000, Dionex Corporation, USA). Table 1 The mineralogical properties of the rock samples by XRD Samples M1 M2 K1 K2 K3 K4 K5 O1 T1 Identified minerals Quartz, pyrite Quartz Sphalerite, galena, anglesite, susannite, barite Quartz, pyrite, muscovite Quartz, sphalerite, barite Sphalerite, galena, cerussite, barite Quartz Quartz, muscovite Quartz, hedenbergite impurities impurities sphalerite Figure 1 Photomicrographs of a sphalerite crystal of sample K4 under polarized light The sulfuer (S) and heavy metal/metalloid contents of the rock samples are summarized in Table 2. Although some samples contained substanial amounts of As and Cd, As-bearing and Cd-bearing minerals were not detected by the XRD analysis. This suggests that these toxic elements are found in the rocks most probably as in the crystal matrix of other minerals like sphalerite or pyrite. Table 2 Sulfur and heavy metal/metalloid contents of the rock samples M1 S (wt %) 2.82 M2 0.171 113 13.0 4.82 7.43 K1 33.5 287,000 254,000 1.00 1,120 K2 0.925 124 690 7.20 5.30 K3 13.5 456 270,000 438 919 K4 22.2 42,600 418,000 1,580 3,510 K5 1.15 2,890 4640 16.8 33.8 O1 1.34 4,720 14,600 32.0 44.4 T1 0.108 309 523 5.14 5.68 Samples Pb (mg/kg) 22.0 Zn (mg/kg) 48.0 As (mg/kg) 32.0 Cd (mg/kg) 6.20 Environmental Concerns 34 Figure 2 shows the elemental maps of one sphalerite crystal of sample K4. The results show that Cd is concentrated inside the sphalerite crystal. In addition, As is also found in the sphalerite crystal, but in contrast to Cd, a part of As may exist as the mineral tennantite ((Cu, Fe, Zn)12As4S13). 4. DISCUSSION The rock samples with sulfide minerals released a significant amount of heavy metals. Among sulfide minerals, sphalerite had the strongest effect on the leaching concentration of Cd. On the other hand, although some samples considerably contained amounts of As, its leaching concentration was very low. This could be attributed to the As immobilization via coprecipitaion with ferric hydroxide (Fe(OH)3), which starts to precipitate above pH 3 or 4 [8]. The Zn content of the rocks correlated well with the S content except for samples with pyrite (Figure 3). Moreover, the leaching concentrations of Zn had a positive correlation with those of SO42- for most of the rock samples while the correlation of Zn and SO42- concentrations in samples with pyrite were not statistically significant (Figure 4). This suggestes that the oxidation of sphalerite is the primary source of Zn and SO42- in most of the rock samples. The results of batch leaching experiments showed that the pH of the leachate of most of the rock samples was around neutral, except for those samples containig pyrite (Table 3). The concentrations of Cd leached from Ksamples were significantly higher than the environmental standards of Japan (10 µg/L) (Table 3). However, those from the other sites were lower than the K-samples. The concentrations of Pb in the leachate of M-, K-samples and O1 were higher than that of T1. In contrast to Cd and Pb, the concentrations of As were very low in all samples and were not detected by ICP-MS. The concentrations of heavy metals in the leachate did not change dramatically with time, reaching apparent equilibrium after just 6 h. 100000 10000 1000 with pyrite without pyrite 100 10 Samples M1 M2 K1 K2 K3 K4 K5 O1 T1 pH 3.94 4.95 5.18 4.15 6.00 6.06 5.29 7.21 7.62 As (mg/L) ND ND ND ND ND ND ND ND ND Cd (mg/L) 0.0037 0.0016 0.243 0.0275 0.0187 0.436 0.0252 0.0051 0.0048 Pb (mg/L) 1.06 0.198 7.78 0.255 0.126 7.53 4.42 0.119 ND 1000 10000 100000 1000000 Figure 3 Zn content vs. S content of the rock samples 1000 with pyrite without pyrite 100 10 1 0.1 10 Table 3 Leachate chemistry of the rock samples 100 S (mg/L) Zn (mg/L) 3.2 Leaching behaviour of arsenic, cadmium and lead Zn content (mg/kg) 1000000 Figure 2 Elemental maps of S, As, Cd and Zn of the rock sample (K4) at a magnification of 50 μm (The sphalerite grain is the part of the elemental map of Zn with color.) 100 1000 SO42- (mg/L) Figure 4 Zn concentration vs. SO42- concentration in the leachate The concentrations of Cd and Zn had a strong positive correlation (Figure 5), indicating that the processes responsible for their release are similar. Since majority of Zn came from the oxidation of sphalerite and Cd was mostly found as an impurity in sphalerite crystal, it follows that the main release mechanism of Cd is the oxidation of sphalerite under oxidizing conditions. Environmental Concerns 35 5. CONCLUSION 1 R2 = 0.84742 Cd (mg/L) 0.1 0.01 1E-3 0.1 1 10 100 Zn (mg/L) Figure. 5 Cd concentration in the leachate vs. Zn concentration in the leachate. R2 is the determination coefficient. The total amounts of Pb and Cd leached from the rocks strongly depended on their initial contents of these elements (Figures 7 and 8). The moderate positive correlations between the dissolved concentrations of Pb and Cd to their initial content of the rock indicate that the leaching concentrations of these toxic elements would increase proportional to the their initial contents in the solidphase. The mineralogical properties of several mineralized rock samples from Hokkaido as well as the leaching behaviors of As, Cd and Pb found in these rocks were elucidated. The findings are summarized as follows: (1) The leaching of heavy metals/metalloid was caused by the oxidation of sulfide minerals, especially sphalerite. (2) We can roughly estimate the leaching concentrations of Cd and Pb from the Cd and Pb contents of the rocks. REFERENCES [1] S. Kapaj, H. Peterson, K. Liber, and P. Bhattacharya (2006), Human health effects from chronic arsenic poisoning - a review, J. Environ. Sci. Health, Part A, Vol. 41, pp. 2399-2428 [2] C. H. Rich, M. L. Biggs and A. H. Smith (1998), Lung and kidney cancer mortality associated with arsenic in drinking water in Cordoba, Argentina, International Journal of Epidemiology, Vol. 27, pp. 561-569 [3] B. M. Rabinowitz (2005), Lead isotopes in soils near five historic American lead smelters and refineries, Science of the Total Environment, Vol. 346, pp. 138-148 1 Cd (mg/L) 0.1 [4] M. R. Moore, and A. Goldberg (1985), Health implications of the hematopoietic effect of lead, In Dietary and Environmental Lead: Human Health Effects, K.R. Mahaffey (Ed.), Elsevier, Amsterdam, pp. 260-314 0.01 R2 = 0.58543 1E-3 1 10 100 1000 10000 Cd content (mg/kg) Figure 6 Cd concentration in leachate vs. Cd content of rock sample. R2 is the determination coefficient. [6] J. L. Pan, J. A. Plant, N. Voulvoulis, C. J. Oates and C. Ihlenfeld (2010), Cadmium levels in Europe: Implications for human health, Environmental Geochemistry and Health, Vol. 32, pp. 1–12 100 10 Pb (mg/L) [5] G. F. Nordberg (2009), Historical perspectives on cadmium toxicology, Toxicology and Applied Pharmacology, Vol. 238, pp. 192-200 1 0.1 R2 = 0.21219 0.01 1 10 100 1000 10000 100000 1000000 Pb content (mg/kg) Figure 7 Pb concentration in leachate vs. Pb content of rock sample. R2 is the determination coefficient. [7] E. R. Plunkett (1987), Handbook of industrial toxicology, 3rd Edition, Chemical Publishing Company Incorporation, New York, NY [8] A. Violante, S. Del Gaudio, M. Pigna, M. Ricciardella and D. Banerjee (2007), Coprecipitation of arsenate with metal oxides. 2. Nature, mineralogy, and reactivity of iron(III) precipitates, Environmental Science and Technology, Vol. 41(24), pp. 8275-8280 Environmental Concerns 36