11:45 AM - 12:00 PM
[HRE13-10] Leaching characteristics of heavy metals in borehole core samples from waste rock dump of an abandoned mine.
Keywords:abandoned mine, waste rock dump, heavy metals, Leaching test
[Background] Mine and waste rock dump drainage contains high levels of heavy metals such as Cu, Pb, and Zn, causing pollution of rivers, farmland, and groundwater. In Japan, there are many abandoned mines and mine drainage needs to be managed for a long time. In order to reduce costs and improve efficiency, it is essential to understand the characteristics of drainage and the elution mechanism of heavy metals. In this study, to clarify the mechanisms of waste rock dump discharge and the factors affecting the behavior of heavy metals, borehole investigations and leaching tests on core samples of waste rock dump from abandoned mine were carried out.
[Sample and method] The sampling site is a epithermal deposit contain copper, lead, and zinc, and the waste rock dump generate drainage containing Pb and Zn in amounts exceeding effluent standards with a pH of about 4. Borehole investigations were carried out at the upper (No.1), middle (No.2) and lower(No.3) parts of the waste rock dump. The geology consisted of topsoil (layer thickness: 0-2m), waste rock (6-7m), sandy gravel (1-2m) from old river sediments and tuff or andesite at the deepest part. The core samples were divided at intervals of up to 30 cm, and some of them were air-dried and then classified using a sieve with a mesh opening of 2 mm. The sieved samples were sealed in a PP container with ultrapure water at a liquid-solid ratio of 10 and shaken horizontally for 6 hours. After shaking, perform centrifugation and the supernatant liquid was filtered through a 0.45 μm membrane filter. pH and EC (electrodes), concentration of sulfate ion (IC), and heavy metal (ICP-MS) were measured. This test was conducted on split core samples from each site, 124 samples in total (n=3).
[Result and discussion] Leaching test results showed that Cr, (maximum concentration 0.06 mg/L), Mn (17.5 mg/L), Fe (16.8 mg/L), Co (0.80 mg/L), Ni (2.4 mg/L), Cu (15.8 mg/L), Zn (152.2 mg/L), As (0.14 mg/L), Cd (1.3 mg/L), and Pb (11.5 mg/L) were leached. In particular, As, Cd, and Pb were eluted at concentrations exceeding the soil elution standards. The maximum concentration of sulfate ion was 2010 mg/L, and the pH was in the range of 3-8.25. Looking at the results for each site, the amount of heavy metals leaching from No.3 was the highest, especially Cr, Mn, Ni, Cu, Zn, and Cd. Next, the leaching from No.2 was high, especially Co and Pb. The amount of Fe and As from No.1 was the largest among the three sites, but other heavy metals were small, ranging from 0.1 to 0.4 times that of No.3.
Correlation analysis was performed to investigate the relationship between heavy metal concentrations and pH or sulfate ion concentrations. Among the heavy metal concentrations obtained from all the sites, Mn, Co, Ni, Cu, Zn, Cd and Pb were positively correlated (r > 0.52) with each other. Negative correlation (r < -0.45) was observed for pH and heavy metals other than As. Positive correlation (0.42 < r < 0.78) was observed between sulfate ion and heavy metals such as Mn, Co, Ni, Cu, Zn, Cd, and Pb. But the relationship between heavy metals and sulfate ions for each site were different from for all sites. A weak positive correlation was observed between sulfate ions and heavy metals other than Cr, Fe, and As at No.1 and 2 (0.07 < r < 0.6), while a strong positive correlation was observed at No.3 (0.44 < r < 0.94). The mechanism of heavy metals leaching into mine drainage is known to be oxidation of sulfide. The relatively strong correlation between heavy metals, sulfate ions, and pH at No.3 suggests that the dissolution of sulfide is one of the factors that define the leaching of heavy metals. But the absence of a strong correlation between sulfate ions and heavy metals at No.1 and 2 suggests that the heavy metals in No.1 or 2 are present in forms other than sulfide. These results suggest that the chemical forms of heavy metals in waste rock vary from site to site.
[Sample and method] The sampling site is a epithermal deposit contain copper, lead, and zinc, and the waste rock dump generate drainage containing Pb and Zn in amounts exceeding effluent standards with a pH of about 4. Borehole investigations were carried out at the upper (No.1), middle (No.2) and lower(No.3) parts of the waste rock dump. The geology consisted of topsoil (layer thickness: 0-2m), waste rock (6-7m), sandy gravel (1-2m) from old river sediments and tuff or andesite at the deepest part. The core samples were divided at intervals of up to 30 cm, and some of them were air-dried and then classified using a sieve with a mesh opening of 2 mm. The sieved samples were sealed in a PP container with ultrapure water at a liquid-solid ratio of 10 and shaken horizontally for 6 hours. After shaking, perform centrifugation and the supernatant liquid was filtered through a 0.45 μm membrane filter. pH and EC (electrodes), concentration of sulfate ion (IC), and heavy metal (ICP-MS) were measured. This test was conducted on split core samples from each site, 124 samples in total (n=3).
[Result and discussion] Leaching test results showed that Cr, (maximum concentration 0.06 mg/L), Mn (17.5 mg/L), Fe (16.8 mg/L), Co (0.80 mg/L), Ni (2.4 mg/L), Cu (15.8 mg/L), Zn (152.2 mg/L), As (0.14 mg/L), Cd (1.3 mg/L), and Pb (11.5 mg/L) were leached. In particular, As, Cd, and Pb were eluted at concentrations exceeding the soil elution standards. The maximum concentration of sulfate ion was 2010 mg/L, and the pH was in the range of 3-8.25. Looking at the results for each site, the amount of heavy metals leaching from No.3 was the highest, especially Cr, Mn, Ni, Cu, Zn, and Cd. Next, the leaching from No.2 was high, especially Co and Pb. The amount of Fe and As from No.1 was the largest among the three sites, but other heavy metals were small, ranging from 0.1 to 0.4 times that of No.3.
Correlation analysis was performed to investigate the relationship between heavy metal concentrations and pH or sulfate ion concentrations. Among the heavy metal concentrations obtained from all the sites, Mn, Co, Ni, Cu, Zn, Cd and Pb were positively correlated (r > 0.52) with each other. Negative correlation (r < -0.45) was observed for pH and heavy metals other than As. Positive correlation (0.42 < r < 0.78) was observed between sulfate ion and heavy metals such as Mn, Co, Ni, Cu, Zn, Cd, and Pb. But the relationship between heavy metals and sulfate ions for each site were different from for all sites. A weak positive correlation was observed between sulfate ions and heavy metals other than Cr, Fe, and As at No.1 and 2 (0.07 < r < 0.6), while a strong positive correlation was observed at No.3 (0.44 < r < 0.94). The mechanism of heavy metals leaching into mine drainage is known to be oxidation of sulfide. The relatively strong correlation between heavy metals, sulfate ions, and pH at No.3 suggests that the dissolution of sulfide is one of the factors that define the leaching of heavy metals. But the absence of a strong correlation between sulfate ions and heavy metals at No.1 and 2 suggests that the heavy metals in No.1 or 2 are present in forms other than sulfide. These results suggest that the chemical forms of heavy metals in waste rock vary from site to site.