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

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 the 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 (Ca 2+) 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.

Mobilization and speciation of arsenic from hydrothermally altered rock containing calcite and pyrite under anoxic conditions

“…Since the authors' previous column experiments were limited only to ambient conditions, the effects of atmospheric O 2 and CO 2 and water residence time on these release mechanisms still remain unclear. This is because changes in the water residence time (i.e., infiltration rate) under ambient conditions also caused variations in the concentrations of dissolved gases in the pore water (Tabelin et al, 2012b). Thus, it was speculated from these previous results that gaseous O 2 and 4 CO 2 probably played a greater role in the release of As than was previously thought (Tabelin et al, 2012b).…”

“…There were indeed temporal and spatial As release mechanisms from these sources. In addition, speciation of As was an important factor that influenced the release mechanisms especially during the early part of the experiments when most of the leaching took place (Tabelin et al, 2012b). Since the authors' previous column experiments were limited only to ambient conditions, the effects of atmospheric O 2 and CO 2 and water residence time on these release mechanisms still remain unclear.…”

“…The hydrothermally altered rock used in the experiments was obtained from a road-tunnel project constructed on the island of Hokkaido, Japan, which is the same altered rock sample utilized in the previous study of the authors (Tabelin et al, 2012b). This excavated rock is mainly composed of hydrothermally altered slate, sandstone and mudstone from the Hidaka 5 metamorphic belt and a detailed discussion of the geology, alteration and characterization of the tunnel area is found elsewhere (Takahashi et al, 2011).…”

“…This excavated rock is mainly composed of hydrothermally altered slate, sandstone and mudstone from the Hidaka 5 metamorphic belt and a detailed discussion of the geology, alteration and characterization of the tunnel area is found elsewhere (Takahashi et al, 2011). The sample used in this study is a mixture of altered and unaltered rocks collected from the bulk excavated rock, which is primarily composed of silicate minerals (i.e., quartz and plagioclase) with calcite, kaolinite and chlorite as minor minerals and trace amounts of pyrite (Tabelin et al, 2012b). Arsenic content of the sample is 23.6 mg/kg, with low sulfur content of 0.20% by weight (Tabelin et al, 2012b).…”

“…Although these data are useful as a reference, they provide little information about the temporal and spatial mechanisms controlling the movement of these toxic elements that is of primary importance in the disposal of these hazardous waste rocks. The most recent work of the authors had partly tackled this problem utilizing calcite-pyrite-bearing altered rock in laboratory column experiments under ambient conditions (Tabelin et al, 2012b). There were indeed temporal and spatial As release mechanisms from these sources.…”