Tetsuro Yoneda

Slake durability and mineralogical properties of some pyroclastic and sedimentary rocks

Abstract Slake durability of rocks is an important property of rock-mass and rock-materials in geotechnical practice. The slake durability of rocks is closely related to their mineralogical composition. In this paper, mineralogical examinations and slake durability tests for argillaceous clastic rocks, especially pyroclastic rocks, sandstones and mudstones of Neogene Tertiary age from Japan, were performed in order to assess the slake durability and rock alteration process of these rocks as well as to understand the relationship between mineralogy and durability. The mineral composition and textural features of the rocks were studied by means of optical microscopy (OM), X-ray diffractometry (XRD), electron microprobe analysis (EPMA), and scanning electron microscopy (SEM). In addition, the slake durability test was carried out by using the standard testing method of ISRM [Int. J. Rock Mech. Min. Sci. 16 (1979) 148] in distilled water and in the aqueous solutions with dissolved electrolytes of NaCl and CaCl 2 . The pyroclastic rocks and tuffaceous sandstone, rich in di-octahedral and tri-octahedral Fe smectite, respectively, show distinctively different slaking behaviors. The pyroclastic rocks show relatively high slaking (Id 2 =55.5% and Id 10 =10.5%) than the tuffaceous sandstone (Id 2 =94.1% and Id 10 =87.8%, refer to text for Id 2 and Id 10 ). This difference in the slake durability observed in these rocks is due to the microscopic occurrences of smectite present in the interspaces between the particles (pyroclastic rocks) and zeolite cementing the interspaces (tuffaceous sandstone) as alteration minerals. In addition, the durability results of tuffaceous sandstone show that the slake durability decreases as the degree of weathering increases (weathered material Id 2 =88.7% and Id 10 =65.3%). Furthermore, two mudstones of Miocene and Pliocene ages, having different clay mineral compositions (smectite vs. illite+chlorite), show the lowest and the highest slake durability among the tested clastic rocks. Hard mudstone shows the highest (Id 2 =98.1% and Id 10 =95.5%) while the soft mudstone shows the lowest (Id 2 =33.9% and Id 6 =0.4%.) slake durability. Thus, the slake durability of pyroclastic and sedimentary rocks is greatly affected by their mineral composition and texture, and is closely related to their alteration history. Slake durability is also affected by the kind of dissolved electrolyte and its concentration in the aqueous solution, providing some useful information for geotechnical practice.

Adsorption and co-precipitation behavior of arsenate, chromate, selenate and boric acid with synthetic allophane-like materials.

Pollution caused by boric acid and toxic anions such as As(V), Cr(VI) and Se(VI) is hazardous to human health and environment. The sorption characteristics of these environmentally significant ionic species on allophane-like nanoparticles were investigated in order to determine whether allophane can reduce their mobility in the subsurface environment at circum-neutral pH condition. Solutions containing 100 or 150 mmol of AlCl(3)x6H(2)O were mixed to 100 mmol of Na(4)SiO(4) and the pH were adjusted to 6.4+/-0.3. The mineral suspensions were shaken for 1h and incubated at 80 degrees C for 5 days. Appropriate amounts of As, B, Cr and Se solutions were added separately during and after allophane precipitation. The results showed that As(V) and boric acid can be irreversibly fixed during co-precipitation in addition to surface adsorption. However, Cr(VI) and Se(VI) retention during and after allophane precipitation is mainly controlled by surface adsorption. The structurally fixed As(V) and boric acid were more resistant to release than those bound on the surface. The sorption characteristics of oxyanions and boric acid were also influenced by the final Si/Al molar ratio of allophane in which Al-rich allophane tend to have higher uptake capacity. The overall results of this study have demonstrated the role of allophane-like nanoparticles and the effect of its Si/Al ratio on As, B, Cr and Se transport processes in the subsurface environment.

Leaching of boron, arsenic and selenium from sedimentary rocks: II. pH dependence, speciation and mechanisms of release.

Sedimentary rocks excavated in Japan from road- and railway-tunnel projects contain relatively low concentrations of hazardous trace elements like boron (B), arsenic (As) and selenium (Se). However, these seemingly harmless waste rocks often produced leachates with concentrations of hazardous trace elements that exceeded the environmental standards. In this study, the leaching behaviors and release mechanisms of B, As and Se were evaluated using batch leaching experiments, sequential extraction and geochemical modeling calculations. The results showed that B was mostly partitioned with the residual/crystalline phase that is relatively stable under normal environmental conditions. In contrast, the majority of As and Se were associated with the exchangeable and organics/sulfides phases that are unstable under oxidizing conditions. Dissolution of water-soluble phases controlled the leaching of B, As and Se from these rocks in the short term, but pyrite oxidation, calcite dissolution and adsorption/desorption reactions became more important in the long term. The mobilities of these trace elements were also strongly influenced by the pH of the rock-water system. Although the leaching of Se only increased in the acidic region, those of B and As were enhanced under both acidic and alkaline conditions. Under strongly acidic conditions, the primarily release mechanism of B, As and Se was the dissolution of mineral phases that incorporated and/or adsorbed these elements. Lower concentrations of these trace elements in the circumneutral pH range could be attributed to their strong adsorption onto minerals like Al-/Fe-oxyhydroxides and clays, which are inherently present and/or precipitated in the rock-water system. The leaching of As and B increased under strongly alkaline conditions because of enhanced desorption and pyrite oxidation while that of Se remained minimal due to its adsorption onto Fe-oxyhydroxides and co-precipitation with calcite.

Surface complexation reactions of inorganic anions on hydrotalcite-like compounds

Complexation reactions of environmentally important inorganic anions such as nitrate, chloride, sulfate, arsenate, and phosphate on the surface of hydrotalcite-like compounds (HT) were investigated to understand the role of HT in the immobilization of hazardous anions in an alkaline environment. The effects of surface complexation reactions on the solid state properties of HT were also evaluated to understand their stability. Synthetic HT was used for the adsorption and post-adsorption experiments. The obtained adsorption isotherms showed that the order of selectivity of HT for anions was NO(3)<Cl<SO(4)<<AsO(4)<PO(4). To distinguish the adsorption mechanisms (inner-sphere complexes or outer-sphere complexes) of these anions, zeta potential measurements and infrared absorption spectroscopic analysis were performed. The results indicated that NO(3) and Cl were adsorbed as diffuse ions on the outer surfaces of the HT, while SO(4) formed outer-sphere complexes with a strong electrostatic interaction. Moreover, AsO(4) and PO(4) formed inner-sphere complexes via a ligand substitution reaction on the HT surfaces. And it was suggested that oxyanions with low ionic potential, such as AsO(4) and PO(4), had a tendency to form inner-sphere complexes with the HT surfaces. The formation of inner-sphere complexes shifted the isoelectric point and the surface charge of the HT. Furthermore, the solubility of the HT was reduced by the inner-sphere complexes with PO(4) and AsO(4). It was revealed that the formation of inner-sphere complexes on the HT surfaces contributed to the stabilization of the HT, as well as a decrease in the mobility of these anions.

Leaching of boron, arsenic and selenium from sedimentary rocks: I. Effects of contact time, mixing speed and liquid-to-solid ratio.

Sedimentary rocks of marine origin excavated in tunnel projects were recently identified as potentially hazardous because they could release significant amounts of toxic trace elements when exposed to the environment. This study investigated the leaching characteristics of B, As, Se and the major coexisting ions under various conditions to identify the factors and processes controlling their evolution in the leachate. In addition, we evaluated whether the parameters of the currently used leachability test for excavated rocks were adequate. Although the leachabilities of B, As and Se similarly increased at longer contact times, only those of B and As were influenced by the mixing speed and/or liquid-to-solid ratio (L/S). The majority of trace elements dissolved in the leachate originated from the dissolution of soluble salts formed from seawater of the Cretaceous trapped during the formation of the sedimentary rocks. Moreover, the alkaline pH of the leachates could be attributed to the simultaneous dissolutions at varying degrees of the mineral components of the rocks as well as the precipitation of clay minerals. In the leaching test of excavated rocks for regulatory purposes, the best values of contact time and mixing speed should represent conditions of the highest trace element extractabilities, which in this study were found at longer contact times (>48 h) and the fastest mixing speed (200 rpm). The most appropriate L/S for the leaching test is 10 because it was around this L/S that the extractabilities and leaching concentrations of the trace elements were simultaneously observed at their highest values.

Removal of Arsenic, Boron, and Selenium from Excavated Rocks by Consecutive Washing

This paper describes the leaching behavior and release mechanisms of arsenic (As), boron (B), and selenium (Se) from excavated rocks using sequential extraction for solid-phase fractionation, batch experiments with pH variation, and consecutive batch experiments with changes in the solid–liquid mixing ratios. Arsenic in the excavated rock was mostly found with the sulfides/organic matter fraction while majority of the leachable B and Se were associated with the exchangeable phases. The leaching of As was strongly pH dependent, Se was pH dependent only around the acidic region, and B was pH independent. Consecutive washing technique with deionized water effectively lowered the B and Se concentrations in the leachate below the drinking water standards of Japan, but was inefficient in the removal of As. Arsenic exhibited non-conservative leaching behavior and its movement was affected by processes like dissolution, precipitation, and pyrite oxidation. In contrast, B and Se behaved more conservatively, resulting in their easy removal from the excavated rock by simple washing and dilution.

Sorption Behavior of Arsenate by Mg-Bearing Minerals at Hyperalkaline Condition: Implications for Oxyanions Sequestration During the Use and Disposal of Alkaline Wastes

The utilization and disposal of alkaline waste materials such as slag and coal fly ash as cement aggregates and raw materials in cement manufacturing can pose environmental and health hazards because these waste materials usually contain elevated concentration of toxic elements. This study examined the possibility of controlling the pore water chemistry of these waste materials in order to induce the secondary mineral formation of Mg-bearing minerals as major sorbing solids for oxyanions during the utilization and disposal of alkaline wastes. The formation of Mg-bearing minerals was examined at ambient temperature and alkaline pH conditions in the Mg–Si–Al system. The interaction of Mg-bearing minerals with oxyanions using arsenate as an analog was examined during and after mineral formation. The results revealed that the generated Mg-bearing mineral phases were smectite and brucite in Mg–Si system and hydrotalcite and serpentine in Mg–Si–Al system. Moreover, hydrotalcite, serpentine, brucite, and smectite phases formed under low Si ratio showed high sorption capacity for arsenate, but only high Al content hydrotalcite and serpentine showed substantial irreversible fraction of sorbed arsenate. Hence, the generation of these kinds of hydrotalcite and serpentine phases as scavengers for oxyanions must be considered during the utilization and disposal of alkaline wastes.

A novel method for remediation of nickel containing wastewater at neutral conditions

Heavy metals contained in wastewater are generally removed by adding antalkaline to increase the pH, and Ni is commonly precipitated as Ni-hydroxides at pH 10. However, a more sustainable remediation method of treatment at neutral conditions would be attractive due to the high cost of chemical reagents and inefficient treatment at present. Based on natural attenuation, the method of adding Al ions has been used in wastewater treatment to precipitate layered double hydroxides (LDH). Here, we investigated the use of Al ion addition in the Ni containing wastewater treatment, experimentally and thermodynamically. By co-precipitation experiments adding Al ions to Ni-containing water, Ni was selectively incorporated into the structure of LDH, and the removal efficiency of Ni was close to 100% even in pH 7 and 8 samples (lower pH than conventional methods) with initial Ni concentrations of 200-10,000mg/L. Geochemical modeling results replicate the experimental results well when the Al/Ni ratio of LDH is assumed to be 0.33. This model makes it possible to estimate the amount of Al ions and additive agents necessary for use in treatment of wastewater containing different Ni concentrations.

Leaching Characteristics of Heavy Metals from Mineralized Rocks Located Along Tunnel Construction Sites

Soil and groundwater pollution caused by acid rock drainage (ARD) containing heavy metals leached from mineralized rocks is a serious environmental problem. Mineralized rocks are widespread throughout Hokkaido, Japan, and several tunnels for the Hokkaido Bullet Train Line are planned to be constructed through these mineralized areas. In this study, batch and column leaching experiments were conducted to investigate the leaching characteristics of heavy metals from the mineralized rock samples collected in these areas. The results showed that the mineralized samples contained substantial amounts of sulfide minerals (e.g., sphalerite (ZnS)), and that cadmium (Cd) was incorporated in some of these sulfide minerals. Moreover, the leaching concentrations of lead (Pb), Cd and arsenic (As) were higher than the Japanese environmental standards. The results of the column leaching experiment showed that these mineralized rocks could continuously release high concentrations of heavy metals for a long time. Therefore, such rocks should be disposed of properly to prevent the contamination of the surrounding environment.

Mineralogical aspects of interstratified chlorite-smectite associated with epithermal ore veins: A case study of the Todoroki Au-Ag ore deposit, Japan

Abstract Chlorite (C)-corrensite (Co)-smectite (S) seriesminerals occur as vein constituents in the two epithermal ore veins, the Chuetsu and Shuetsu veins of the Todoroki Au-Ag deposit. The characteristics of the C-Co-S seriesminerals indicate that the clays may be products of direct precipitation from hydrothermal fluids and subsequent mineralogical transformations during and/or after vein formation. The minerals from the Chuetsu vein are characterized by ‘monomineralic’ corrensite showing an extensive distribution throughout the vein, and trioctahedral smectite occurring locally. The Shuetsu vein minerals are characterized by C-Co series minerals which can be divided into three different types: a I type including discrete chlorite with minor amounts of S layers, a II type comprising interstratified C/Co and discrete chlorite, and a III type characterized by segregation structures of C and Co layers. The C-Co series minerals show slightly different spatial distributions in the Shuetsu vein. Different epithermal environments during the vein formations and possible kinetic effects may have played a role in the formation and conversion of Co-C series at the Shuetsu vein and S-Co series at the Chuetsu vein.