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Mobilization and speciation of arsenic from hydrothermally altered rock in laboratory column experiments under ambient conditions

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Title: Mobilization and speciation of arsenic from hydrothermally altered rock in laboratory column experiments under ambient conditions
Authors: Tabelin, Carlito Baltazar Browse this author →ORCID
Igarashi, Toshifumi Browse this author →KAKEN DB
Takahashi, Ryohei Browse this author
Keywords: Leaching
redox reactions
column experiments
Issue Date: Jan-2012
Publisher: Elsevier
Journal Title: Applied Geochemistry
Volume: 27
Issue: 1
Start Page: 326
End Page: 342
Publisher DOI: 10.1016/j.apgeochem.2011.10.007
Abstract: This paper describes the mobilization and speciation of arsenic (As) found in hydrothermally altered rock under oxic column conditions. The altered rock sample was obtained from a tunnel project located in the Nakakoshi area of Hokkaido, Japan, whose geology is represented by slate, shale and sandstone. This area underwent silicification, pyritization and argillic alteration resulting in As-enrichment of the rock. Results of the column experiments show that the infiltration rate, bulk density and rock bed thickness affected the duration of water residence, which in turn influenced the pH of the rock-water system. Coexisting ions most notably calcium (Ca2+) at amounts greater than ca. 50 mg/L retarded the mobilization of As. Mobilization of As from the rock with time occurred in two stages: stage 1 (weeks 1-20) with higher As leaching and stage 2 (weeks 20-76) characterized by nearly constant As release. In addition, pore water As concentrations revealed that the columns developed into two regions: the top half where most of the leaching occurred and the bottom part dominated by adsorption. Thus, the mechanisms controlling the mobilization of As from the rock is a combination of one or more of the following processes: dissolution of soluble As-bearing fractions, pyrite oxidation and adsorption reactions. Arsenite (As[III]) was the dominant species in the effluent at the start of the experiment in columns with shorter water residence time and lower pH conditions (<8). On the other hand, arsenate (As[V]) was the major inorganic species released from the rock at higher pH (8-9.5) and when the system was close to equilibrium. Speciation of As with depth also indicated that As[III] disappeared around the bottom half of the columns, probably as a result of adsorption and/or oxidation. Arsenic speciation is partially controlled by the pH dependent adsorption of As species. The important adsorbent phases in the rock included Fe-Al oxides/oxyhydroxides, clay minerals and organic matter, which permitted the columns to attenuate additional As loadings including As[III]. Implications of these results on the design of a novel disposal method for these altered rocks include the enhancement of As adsorption through the addition of natural or artificial adsorbents and the utilization of a covering soil with low permeability to minimize rainwater infiltration into the rock.
Type: article (author version)
Appears in Collections:工学院・工学研究院 (Graduate School of Engineering / Faculty of Engineering) > 雑誌発表論文等 (Peer-reviewed Journal Articles, etc)

Submitter: Tabelin, Carlito Baltazar

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