Hoon Young Jeong

X-ray absorption and photoelectron spectroscopic study of the association of As(III) with nanoparticulate FeS and FeS-coated sand.

Iron sulfide (FeS) has been demonstrated to have a high removal capacity for arsenic (As) in reducing environments. However, FeS may be present as a coating, rather than in nanoparticulate form, in both natural and engineered systems. Frequently, the removal capacity of coatings may be different than that of nanoparticulates in batch systems. To assess the differences in removal mechanisms between nanoparticulate FeS and FeS present as a coating, the solid phase products from the reaction of As(III) with FeS-coated sand and with suspensions of nanoparticulate (NP) FeS were determined using x-ray absorption spectroscopy and x-ray photoelectron spectroscopy. In reaction with NP FeS at pH 5, As(III) was reduced to As(II) to form realgar (AsS), while at pH 9, As(III) adsorbed as an As(III) thioarsenite species. In contrast, in the FeS-coated sand system, As(III) formed the solid phase orpiment (As(2)S(3)) at pH 5, but adsorbed as an As(III) arsenite species at pH 9. These different solid reaction products are attributed to differences in FeS concentration and the resultant redox (pe) differences in the FeS-coated sand system versus suspensions of NP FeS. These results point to the importance of accounting for differences in concentration and redox when making inferences for coatings based on batch suspension studies.

Microscopic and spectroscopic characterization of Hg(II) immobilization by mackinawite (FeS)

This study investigated the solid-phase Hg formed by reacting 0.005 or 0.01 M Hg(II) with 10 g/L mackinawite (FeS) as a function of pH in 0.2 M chloride solutions using X-ray diffraction (XRD), transmission electron microscopy (TEM), and extended X-ray absorption fine structure (EXAFS) analyses. Under all experimental conditions, XRD analysis showed formation of metacinnabar (β-HgS) as a bulk-phase sorption product, in agreement with the results from high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and selected area electron diffraction (SAED) in TEM analysis. HAADF-STEM and energy dispersive X-ray (EDX) analyses also suggested formation of Hg(II) surface precipitates. EXAFS analysis indicated that metacinnabar was the dominant product under most conditions, with Hg(II) chlorosulfide-like surface precipitates having increased contribution at lower Hg(II) concentration and higher pH. This finding is consistent with the results of desorption experiments using Hg(II)-complexing ligands. Considering the low solubility and high stability of metacinnabar, our results support the potential application of mackinawite for sequestering Hg(II) in anoxic environments.

Abiotic reductive dechlorination of cis -dichloroethylene by Fe species formed during iron- or sulfate-reduction

This study investigated reductive dechlorination of cis-dichloroethylene (cis-DCE) by the reduced Fe phases obtained from in situ precipitation, which involved mixing of Fe(II), Fe(III), and S(-II) solutions. A range of redox conditions were simulated by varying the ratio of initial Fe(II) concentration ([Fe(II)](o)) to initial Fe(III) concentration ([Fe(III)](o)) for iron-reducing conditions (IRC) and the ratio of [Fe(II)](o) to initial sulfide concentration ([S(-II)](o)) for sulfate-reducing conditions (SRC). Significant dechlorination of cis-DCE occurred under highly reducing IRC and iron-rich SRC, suggesting that Fe (oxyhydr)oxides including green rusts are highly reactive with cis-DCE but that Fe sulfide as mackinawite (FeS) is nonreactive. Relative concentrations of sulfate to chloride were also varied to examine the anion impact on cis-DCE dechlorination. Generally, slower dechlorination occurred in the batches with higher sulfate concentrations. As indicated by higher dissolved Fe concentration, the slower dechlorination in the presence of sulfate was probably due to the decreased surface-complexed Fe(II). This study demonstrates that the chemical form of reduced Fe(II) is critical in determining the fate of cis-DCE under anoxic conditions.

X-ray Absorption and X-ray Photoelectron Spectroscopic Study of Arsenic Mobilization during Mackinawite (FeS) Oxidation

In this study we investigated the speciation of the solid-phase As formed by reacting 2 x 10(-4) M As(III) with 1.0 g/L mackinawite and the potential for these sorbed species to be mobilized (released into the aqueous phase) upon exposure to atmospheric oxygen at pH 4.9, 7.1, and 9.1. Before oxygen exposure, X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) analyses indicated that As(III) was removed from the aqueous phase by forming As(0), AsS, and surface precipitates as thioarsenites at pH 4.9 and As(0) and thioarsenite surface precipitates at pH 7.1 and 9.1. When oxygen was introduced, XAS analysis indicated that As(0) and the surface precipitates were quickly transformed, whereas AsS was persistent. During intermediate oxygen exposure times, dissolved As increased at pH 4.9 and 7.1 due to the rapid oxidation of As(0) and the slow precipitation of iron (oxyhydr)oxides, the oxidation products of mackinawite. This indicates that oxidative mobilization is a potential pathway for arsenic contamination of water at acidic to neutral pH. The mobilized As was eventually resorbed by forming edge-sharing and double-corner-sharing surface complexes with iron (oxyhydr)oxides.

Kinetic study of cis-dichloroethylene (cis-DCE) and vinyl chloride (VC) dechlorination using green rusts formed under varying conditions

Abiotic degradation of cis-dichloroethylene (cis-DCE) and vinyl chloride (VC) was investigated using Fe hydroxides obtained by hydrolyzing Fe(II) salts over a pH range of 7.7-8.0. Within this narrow pH range, a green rust (GR) precipitated. The dechlorination reactivity of the resulting GR precipitates increased with the dissolved Fe(II) concentration remaining in solution after precipitation. Controls run using only the dissolved Fe(II) supernatant were not reactive, suggesting the relative amount of Fe(II) on the surface of precipitated GRs was the causative agent in the relative reactivity. To test this, a series of GR batches with varying dissolved Fe(II) concentrations were prepared by acid-base titration and examined for cis-DCE and VC dechlorination kinetics under reducing conditions. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses of these batches were performed to characterize the bulk mineralogy and the excess surface Fe(II), respectively. Cis-DCE and VC dechlorination results along with solid phase characterization show that different surface Fe(II)/Fe(III) compositions are responsible for the different reactivity of GRs formed within the GR precipitation zone.

Sorption of a nonionic surfactant Tween 80 by minerals and soils.

Batch experiments were conducted to evaluate Tween 80 sorption by oxides, aluminosilicates, and soils. For oxides, the sorption by silica and alumina follow linear isotherms, and that by hematite follows a Langmuir isotherm. Considering isotherm type and surface coverage, Tween 80 may partition into the silica/alumina-water interface, whereas it may bind to hematite surface sites. Among aluminosilicates, montmorillonite shows the greatest sorption due to the absorption of Tween 80 into interlayers. For other aluminosilicates, it sorbs to surfaces, with the sorption increasing as plagioclase<vermiculite<kaolinite. This results from the relative reactivity among surface sites: ≡NaOH, ≡CaOH<<≡SiOH<≡AlOH. Experiments using dry- and wet-sieved soils reveal that fine-grained clay minerals, difficult to separate by dry-sieving, contribute significantly to Tween 80 sorption. The greater sorption by untreated soils than H2O2-treated soils indicates that soil organic matter is a vital sorbent. The sorption hysteresis, contributed to by clay minerals and soil organic matter, is characterized by the greater sorption during the desorption than the sorption stages. This suggests the potential difficulty in removing surfactants from soils. Also, sorption of surfactants can adversely affect surfactant-enhanced remediation by decreasing the aquifer permeability and the availability of surfactants for micellar solubilization.

A Technical Note on Monitoring Methods for Volcanic Gases

The monitoring methods for volcanic gases are divided into remote sensing and direct gas sampling approaches. In the remote sensing approach, COSPEC and Li-COR are used to measure and , respectively, with FT-IR for detection of a range of volcanic gases. However, the remote sensing approach is not applicable to Mt. Baegdu, where the atmospheric contents of volcanic gases are very low as a result of the strong interaction of volcanic gases with the nearby surface water and groundwater. On the other hand, the direct gas sampling approach involves the collection of volcanic gases from volcanic vents or fumaroles and the subsequent laboratory analysis, thus making it possible to measure even very low levels of volcanic gases. The direct sampling approach can be subdivided into the evacuated bottle method and the flow-through bottle method. In applying both methods, sampling bottles typically contain reaction media to trap specific volcanic gases. For example, NaOH solution(Giggenbach bottle), solution, and acid condensates have been experimented for volcanic gas sampling. Once taken from vents and fumaroles, the samples of volcanic gases are pretreated and subsequently analyzed for volcanic gases using GC, IC, HPLC, titrimetry, TOC-IC, or ICP-MS. Recently, there has been the increasing number of evidences on the potential volcanic activity of Mt. Baegdu. However, little technical development has been made for the sampling and analysis of volcanic gases in Korea. In the present work, we reviewed various volcanic gas monitoring methods, and provided the detailed information on the monitoring methods applied to Mt. Baegdu.

Beam-induced redox transformation of arsenic during As K-edge XAS measurements: availability of reducing or oxidizing agents and As speciation

During X-ray absorption spectroscopy (XAS) measurements of arsenic (As), beam-induced redox transformation is often observed. In this study, the As species immobilized by poorly crystallized mackinawite (FeS) was assessed for the susceptibility to beam-induced redox reactions as a function of sample properties including the redox state of FeS and the solid-phase As speciation. The beam-induced oxidation of reduced As species was found to be mediated by the atmospheric O2 and the oxidation products of FeS [e.g. Fe(III) (oxyhydr)oxides and intermediate sulfurs]. Regardless of the redox state of FeS, both arsenic sulfide and surface-complexed As(III) readily underwent the photo-oxidation upon exposure to the atmospheric O2 during XAS measurements. With strict O2 exclusion, however, both As(0) and arsenic sulfide were less prone to the photo-oxidation by Fe(III) (oxyhydr)oxides than NaAsO2 and/or surface-complexed As(III). In case of unaerated As(V)-reacted FeS samples, surface-complexed As(V) was photocatalytically reduced during XAS measurements, but arsenic sulfide did not undergo the photo-reduction.

Geochemical interactions of mine seepage water with an aquifer: laboratory tests and reactive transport modeling

Laboratory tests and reactive transport modeling were conducted to evaluate the geochemical interactions between the seepage water from a mine waste rock dump and the nearby aquifer. In laboratory tests, the reaction of the mine seepage water with the aquifer materials increased pH, alkalinity, and dissolved Ca and Mg, whereas it decreased dissolved Fe, SO42−, and metals (Al, Zn, Cd, Cu, Cr, and Mn). Such results were mainly due to dissolution of carbonate minerals and precipitation of secondary minerals. The geochemical processes inferred from the laboratory tests (i.e., acid neutralization via dissolution of carbonates and retention of metals via precipitation of secondary minerals) were incorporated into a reactive transport model to predict the evolution of a mine seepage plume along a groundwater flow path below the waste rock dump site. The model simulations showed that dissolved metals within the plume were sequestered below non-detectable levels as a result of interactions with aquifer materials. The decreased mobility of metals was closely related to the neutralization of the acidic plume mostly due to dissolution of carbonate minerals, thus resulting in favorable geochemical conditions for the formation of secondary minerals incorporating metals (hydroxides, carbonates, and sulfides). This study helps to understand the geochemical processes governing the fate and transport of acid mine drainage in aquifers.

Removal of hexavalent chromium using mackinawite (FeS)-coated sand

This study investigates the feasibility of mackinawite (FeS)-coated sand in permeable reactive barrier applications to treat Cr(VI)-contaminated groundwater under anoxic conditions. For this, Cr(VI) sorption experiments were conducted using both coated and uncoated sands. Solution-phase Cr speciation and Cr K-edge X-ray absorption near-edge structure (XANES) analysis indicated the complete reduction of Cr(VI) to Cr(III) by coated sand. At pH 4.7, substantial amounts of Cr(III) remained in solution due to its unfavorable cationic adsorption at acidic pH. At pH 7.1 and 9.8, it was quantitatively immobilized by forming Cr(III)-bearing precipitates. In contrast, uncoated sand showed the decreasing Cr(VI) sorption with pH. In uncoated sand, magnetite impurities would mediate the partial reduction of Cr(VI). Thus, the pH-dependent sorption by uncoated sand was due to both unfavorable anionic Cr(VI) adsorption and its lesser reduction to Cr(III) with pH. Compared to uncoated sand, coated sand showed significantly increased Cr(VI) sorption at neutral to basic pH. By Fe K-edge XANES analysis, FeS was mainly responsible for Cr(VI) reduction by coated sand, with a green rust-like phase being the major Fe product. Since Fe(OH)3 is not thermodynamically stable under the redox conditions favoring formation of green rust, Fe(III)-substituted Cr(OH)3 likely represents a Cr(III)-bearing phase.