Elsevier

Engineering Geology

Volumes 139–140, 22 June 2012, Pages 76-84
Engineering Geology

Combined neutralization–adsorption system for the disposal of hydrothermally altered excavated rock producing acidic leachate with hazardous elements

https://doi.org/10.1016/j.enggeo.2012.04.006Get rights and content

Abstract

Hydrothermally altered rock excavated in a tunnel project produces acidic leachate containing hazardous elements that include arsenic (As), lead (Pb), copper (Cu) and zinc (Zn). To mitigate this problem, this paper evaluated a combined neutralization–adsorption system that used readily available and cheap reagents like calcium carbonate (CaCO3) and partly-weathered volcanic ash. Batch neutralization experiments showed that CaCO3 was effective in raising the pH of the leachate around neutral while the batch adsorption experiments illustrated that the volcanic ash sample collected near the tunnel project area was highly capable of adsorbing arsenate (As[V]), Pb, Cu and Zn. Under column conditions, the amount of hazardous elements released from the rock increased by several folds and their breakthrough curves had flushing-out trends. The mechanisms of As and heavy metals release probably include the dissolution of soluble phases and pyrite oxidation. Addition of CaCO3 in the column experiments based on estimates from the batch results underestimated the amount of neutralizer needed to adjust the effluent pH to around 8, resulting only in slight increase of the pH. Nevertheless, the presence of CaCO3 drastically reduced the amount of hazardous elements released from the altered rock especially during the initial stages of the column experiments. Combining neutralization and adsorption effectively reduced the amount of As and heavy metals in the effluent throughout the duration of the column experiments, which is attributed to the slight neutralizing effect of volcanic ash that raised the pH around circumneutral as well as its rich Al and Fe oxyhydroxide/oxide contents. The combined system immobilized the hazardous elements through a combination of co-precipitation and adsorption reactions and showed potential as an alternative method for the disposal of altered rocks producing acidic leachate.

Highlights

► Mechanisms of hazardous element release include dissolution and pyrite oxidation. ► Volcanic ash sampled near the site was capable of adsorbing As(V), Pb, Cu and Zn. ► The combined system effectively minimized the release of As, Pb, Cu and Zn. ► Immobilization of the toxic elements occurred via co-precipitation and adsorption.

Introduction

Hydrothermally altered rocks, which are widely distributed in geologically active volcanic regions like Japan, are formed due to the migration of superheated fluid/water called hydrothermal solutions through fractures and fissures in rocks. Enrichment of these rocks with toxic elements like Pb and As preferentially occurs in and around precipitated pyrite grains, which are oxidized upon exposure to the atmosphere producing acidic leachate containing these toxic elements (Igarashi et al., 2008, Tabelin and Igarashi, 2009). Tunnels for roads, railways and other projects in the island of Hokkaido have excavated these kinds of rocks, which are potential sources of soil and groundwater contamination. At the moment, excavated altered rocks are being disposed of by applying landfill liners to prevent the interaction of the rock and rainwater similar to those utilized in the disposal of municipal and industrial wastes (Lundgren and Soderblom, 1985, Katsumi et al., 2001, Wijeyesekera et al., 2001, Malusis et al., 2003, Rapti-Caputo et al., 2006), but this method is very expensive and impractical so that alternative methods are being explored.

Leaching of As and Pb from altered rocks and their mechanisms of release are both strongly pH dependent (Tabelin and Igarashi, 2009, Tabelin et al., 2012a). In addition, minor and trace minerals that have strong effects on the pH of the rock when in contact with water (e.g., calcite and pyrite) are important in the mobilization of these hazardous elements (Tabelin et al., 2010, Tabelin et al., 2012a, Tabelin et al., 2012b). The best way to immobilize As and Pb from altered rocks is through adsorption and precipitation, respectively, and these processes are most effective in the circumneutral pH range (Tabelin and Igarashi, 2009, Tabelin et al., 2010, Tabelin et al., 2012a). Based on these previous studies, we have developed a disposal method called the neutralization–adsorption system as a simple and low cost alternative to special landfilling. The principles behind this system are simple: identify the mechanisms of As and heavy metal release from the altered rock, minimize the extent of these mechanisms, and provide additional countermeasures to immobilize any hazardous element released from the rock. In contrast to traditional landfills that prevent the interaction of wastes with water through the use of special barriers, the concept of this system more closely resembles a permeable reactive barrier (PRB). In this proposed system, water is allowed to percolate into the waste, albeit at a lower infiltration rate, and the “loaded” leachate would then pass through the adsorption layer where immobilization of the hazardous elements would take place. A conceptual model of this new neutralization–adsorption system is presented in Fig. 1.

The Teine mine area, which is located northwest of Sapporo, Hokkaido, Japan, consists of Late Miocene andesite tuff breccia and mudstone, extruded by altered andesite (propylite). The veins occur mainly in altered andesite, which are grouped into Mitsuyama, Koganezawa and Bannozawa (Imai, 1999). The mine produced gold (Au), silver (Ag) and copper (Cu) from 1893 until 1971 (Watanabe, 1936, Watanabe, 1943, Watanabe, 1944, Sugimoto, 1952, Imai, 1978). A new tunnel was constructed near the Mitsuyama deposit in 2006 to collect acid mine drainage for the new water treatment system installed in this mine. The construction of this new tunnel excavated hydrothermally altered rock rich in hazardous elements like As, Pb, Cu and Zn.

The main goal of this study is to evaluate the effectiveness of the neutralization–adsorption system in the immobilization of toxic elements like As and heavy metals such as Pb, Cu and Zn. The neutralizer and adsorbent selected for our experiments are calcium carbonate (CaCO3) and partly-weathered volcanic ash, respectively. To achieve our goal, we first evaluated the effects of CaCO3 on the leachate chemistry through batch neutralization experiments. Second, batch adsorption experiments using the ash sample with high amorphous aluminum (Al) and iron (Fe) mineral contents were carried out to characterize its adsorption capacity and affinity for As, Pb, Cu and Zn. Finally, column experiments were conducted using different neutralization and adsorption configurations to compare the effects of using neutralization only and a combined neutralization–adsorption system. If this alternative disposal system is effective, it could provide a more economical, practical and safe way of disposing these hazardous excavated rocks.

Section snippets

Hydrothermally altered rock, volcanic ash and calcium carbonate

The hydrothermally altered rock sample used in this study was collected from the bulk excavated rock stored in an interim disposal site that had already been exposed to the atmosphere for ca. 6 months. This interim disposal site is used until the final disposal of the rock and/or while waiting for the thawing of snow in winter. The test material (i.e., partly-oxidized altered rock) was selected because it would most likely represent the actual waste rock for disposal. The rock sample was air

Properties of the altered rock and volcanic ash

The chemical and mineralogical compositions of the altered rock and partly-weathered volcanic ash used in this study are listed in Table 2, Table 3, respectively. The altered rock sample contains significant amounts of S at 10.6 wt.%, which could be attributed to the presence of pyrite. It also has significant As, Pb, Cu and Zn contents at 150, 375, 68 and 62 mg/kg, respectively. Results of the leaching experiment of the altered rock sample using deionized water illustrated that its pH was acidic

Dissolution of soluble phases and pyrite oxidation

Exposure of pyrite to the atmosphere results in its rapid oxidation commencing with the oxidation of S2− species (Schaufuss et al., 1998). This process is further enhanced in the presence of water or moisture that could strip reaction products exposing new sites on the pyrite surface. Reaction products from the atmospheric oxidation of pyrite include Fe-sulfates, Fe oxyhydroxides and oxides, which are more soluble than pyrite especially under acidic conditions (De Donato et al., 1993, Schaufuss

Conclusions

This paper describes the mitigation potential of a combined neutralization–adsorption system for excavated altered rocks producing acidic leachate. The findings of this paper are summarized as follows:

  • (1)

    Addition of at least 2% CaCO3 in the batch experiments effectively neutralized the leachate of the altered rock and immobilized most of the As and heavy metals dissolved in it.

  • (2)

    The partly-weathered volcanic ash used in this study adsorbed both As and the heavy metals, and the corresponding KF

Acknowledgments

The authors wish to acknowledge Eco Management Co., Ltd. for the preparation of samples and Horonobe Research Institute for the Subsurface Environment for the mineralogical and chemical analyses of the samples. Part of this research was supported by the Japan Society for the Promotion of Science (JSPS) grants-in-aid for scientific research. Finally, the authors wish to thank the anonymous reviewers for their valuable inputs to this paper.

References (33)

Cited by (72)

  • Potentially toxic elements (As, Cd, Cr, Hg, and Pb), their provenance and removal from potable and wastewaters

    2023, Current Trends and Future Developments on (Bio-) Membranes: Membrane Technologies in Environmental Protection and Public Health: Challenges and Opportunities
  • Bioremediation of acid mine drainage – Review

    2023, Alexandria Engineering Journal
View all citing articles on Scopus
View full text