The mining industry produces large volumes of wastes, including sterile rocks and, in particular, tailings, having high potential to release acidic and metal contaminated water (acid mine drainage, AMD). In South Korea, among the approximately 5,000 abandoned metal mines, 92% out of 310 recently investigated (2007-2009) already exceed the restrictions on the soil and water quality standards relative to pH and metal concentrations. To reduce the metal leaching potential of these mine wastes, prevention and/or treatment technologies are urgently required. One effective method uses mixtures of organic and inorganic materials to inhibit sulfide oxidation by limiting oxygen contact and/or water contact with reactive minerals. Food waste-based compost is a largely available organic amendment in South Korea because of the prohibition of food waste disposal in landfills (2005) and the upcoming ban on dumping the wastewater from processing leftover food into the sea (2013), which requires securing alternative markets. The present study aims to assess the hazardous potential of weathered tailings collected on an abandoned mine site in South Korea and the evaluation of the effectiveness of a mixture of food waste-based compost and zeolite on the reduction of metal leaching potential. At the same time, the involved biological processes were assessed. Three amendments, including two food waste-based composts and zeolite, as well as weathered tailings, were thoroughly characterized. In addition, simple chemical extractions and a three-step standard sequential extraction procedure (SEP) were performed for the assessment of metal leaching potential in tailings. Leaching tests were then performed in six 0.6L reactors filled with various mixtures of tailings and amendments. To simulate rainy conditions, water was semi-continuously fed over a 4-week testing period. The collected leachate was analyzed for several parameters, especially pH and heavy metal concentrations (Al, As, Cd, Cr, Cu, Fe, Pb, Mn, and Zn), in order to evaluate the amendments’ efficiency. After leaching tests, yellow-reddish to black colored residues from the bottom side of all columns were used for microbial testing, such as the quantifying of general enzymatic activity and counting of heterotrophic bacteria, including sulfate-reducing bacteria (SRB) and iron-reducing bacteria (IRB). Physicochemical characterization indicated acidic pH (tailings, 3.03) vs. slightly alkaline pH (compost, 7.76-8.44), high TVS content of compost (42-46%), as well as high metal concentrations of tailings (14 mg kg-1 Cr to 50g kg-1 Fe). Hence, apparently composts could be effective for reducing metal leaching potential in tailings. The results of SEP showed the high metal leaching potential of tailings as indicated by the high mobile fraction of Cd, Mn, and Zn, as well as a high residual (stable) fraction of Al, As, Pb, and Cr. Overall, in amended reactors, the pH increased from 3.3 up to 7.9 and metal immobilization showed the following ranking: Zn (44-91%) > Mn (4-76%) > Cr (20-53%) > Fe (34-44%) > Cd (17-43%) > Al (0.5-24%). However, Pb and As leaching potential was increased up to 43% and 158%, respectively, relative to unamended tailings. The combination of compost and zeolite showed the best efficiency, as indicated by the highest pH (7.5-8.1) values and metal immobilization data. Results on microbial tests confirmed that, in addition to chemical processes, metals were also immobilized due to the biological precipitation induced by heterotrophic anaerobic bacteria. The activity of SRB, which leads to sulfate reduction, and of IRB, which reduces Fe3+ to Fe2+, could lead to FeS formation and precipitation, a dark-colored precipitate such as the one in some amended columns after the leaching test. General microbial enzymatic activity, as indicated by dehydrogenase (DHs) values (up to 41mg g-1), and the presence of large counts of two specific heterotrophic anaerobic bacteria (SRB: up to 1.6×107Cells 100mL-1 and IRB: up to 1.6×107Cells 100mL-1) were interpreted as a confirmation of microbial involvement in sulfides’ precipitation. Therefore, the study proves the effectiveness of food waste-based compost and zeolite on increasing the pH and immobilization of several metals (Al, Cd, Cr, Fe, Mn, and Zn) in tailings. These beneficial effects provide a promising solution for an AMD prevention and treatment system, as well as alternative uses of food waste compost in South Korea. Further research is necessary for the optimization of the mixture of amendments for an effective immobilization of As and Pb.
광업으로부터 발생되는 폐기물 중 특히 광미는 풍화작용을 통해 산성광산배수를 발생시키고 중금속을 용출시킬 수 있는 높은 잠재력을 가지고 있다. 한국에는 약 5천여개의 폐광산 들이 산재되어 있으며 환경부 조사결과에 따르면 대부분의 지역에서 토양 및 수질기준을 초과하는 것으로 나타났다. 따라서, 광산배수 발생 및 중금속 용출 방지 및 처리기술이 시급한 실정이다.
한가지 대안으로 유기물 및 무기물의 혼합물을 이용하여 광미의 풍화작용을 제한시키는 방법이 있다. 본 연구에서는 음식물 쓰레기 기반의 퇴비가 유기물질로 사용되었는데 이는 한국에서 음식물 쓰레기는 2005년 매립금지, 2013년 음식물쓰레기 처리 수의 해양투기 금지에 따라 다른 이용방안 및 처리방안이 필요한 실정이기 때문이다.
본 연구의 목적은 한국의 폐광산에서 채취한 광미를 이용하여 위험 잠재력을 분석하고 음식물 쓰레기 기반의 퇴비와 zeolite의 혼합물의 위험 잠재력 저감 효과를 평가하는데 있다. 또한 위험 잠재력 저감 효과에 관련된 생물학적 공정도 함께 평가되었다. 광미의 위험 잠재력은 단일 추출법과 연속추출법을 이용하여 분석하였고 용출실험을 통해 포집된 침출수를 이용하여 혼합물의 효과를 평가하였다.
실험 결과 광미시료의 광산배수 발생 및 중금속 용출 잠재력을 확인하였고 퇴비와 zeolite의 혼합물은 pH 증가와 환원환경의 조성, 금속 고정화 (metal immobilization)에 효과가 있는 것으로 나타났다. 또한 생물학적 실험 결과 금속 고정화는 화학적 침전뿐만 아닌 종속영양 박테리아 (heterotrophic bacteria), 특히 황산염환원균과 철환원균에 의해서도 황화물형태의 침전이 일어 났음을 알 수 있었다. 따라서, 본 연구는 산성광산배수 방지 및 처리에 전도유망한 결과와 한국의 음식물 쓰레기 퇴비의 다른 이용방안을 제시하였다고 여겨진다.
