Chung-Seop Lee

Hierarchically structured manganese oxide-coated magnetic nanocomposites for the efficient removal of heavy metal ions from aqueous systems.

In this study, hierarchical MnO2-coated magnetic nanocomposite (Fe3O4/MnO2) was synthesized by a mild hydrothermal process, and its application for removing heavy metal ions from contaminated water systems was examined. Structural characterization showed that the Fe3O4 nanoparticle core was coated with amorphous MnO2 shell with flowerlike morphology. The as-prepared nanocomposite had a large surface area and high magnetic saturation value, which ensured its good sorption ability and convenience of separation. Fe3O4/MnO2 exhibited a greatly improved removal capacity toward four different heavy metals (Cd(II), Cu(II), Pb(II), and Zn(II)) compared to unmodified Fe3O4 nanoparticles. The adsorption property of Fe3O4/MnO2 was studied with Cd(II) in more detail. The sorption equilibrium data were well fitted to the Langmuir model, and the maximum adsorption capacity toward Cd(II) was 53.2 mg g(-1). Fe3O4/MnO2 retained over 80% of its adsorption capacity under various solution conditions that are typically encountered in natural waters. This nanocomposite was easily recovered and reused through consecutive adsorption-desorption experiments with the assistance of an external magnetic field. Overall, the findings propose that Fe3O4/MnO2 could be used as an effective recyclable adsorbent for heavy metal ions.

Degradation of synthetic pollutants in real wastewater using laccase encapsulated in core–shell magnetic copper alginate beads

Immobilization of laccase has been highlighted to enhance their stability and reusability in bioremediation. In this study, we provide a novel immobilization technique that is very suitable to real wastewater treatment. A perfect core-shell system composing copper alginate for the immobilization of laccase (Lac-beads) was produced. Additionally, nFe2O3 was incorporated for the bead recycling through magnetic force. The beads were proven to immobilize 85.5% of total laccase treated and also to be structurally stable in water, acetate buffer, and real wastewater. To test the Lac-beads reactivity, triclosan (TCS) and Remazol Brilliant Blue R (RBBR) were employed. The Lac-beads showed a high percentage of TCS removal (89.6%) after 8h and RBBR decolonization at a range from 54.2% to 75.8% after 4h. Remarkably, the pollutants removal efficacy of the Lac-beads was significantly maintained in real wastewater with the bead recyclability, whereas that of the corresponding free laccase was severely deteriorated.

Fabrication of novel oxygen-releasing alginate beads as an efficient oxygen carrier for the enhancement of aerobic bioremediation of 1,4-dioxane contaminated groundwater

Oxygen-releasing alginate beads (ORABs), a new concept of oxygen-releasing compounds (ORCs) designed to overcome some limitations regarding the fast oxygen release rate and the high pH equilibrium of ORCs, were fabricated to promote the stimulation of aerobic biodegradation in anaerobic groundwater. Slow oxygen-releasing rate and maintenance of constant pH were achieved by changing the parameters (ionic radius and valence) related to the cross-linking ions composing ORABs, and the best results were obtained for ORABs cross-linked with Al (Al-ORABs). Furthermore, the mechanism of the improved aerobic biodegradation using Al-ORABs under oxygen-limiting groundwater conditions was elucidated in batch and column studies with 1,4-dioxane and Mycrobacterium sp. PH-06 as a model contaminant and aerobic microbes, respectively. Maximum 1,4-dioxane degradations of 99% and 68.1% were achieved when Al-ORABs were applied in batch and column conditions, respectively, whereas 34.3% and 18% of 1,4-dioxane were degraded without Al-ORABs in batch and column conditions, respectively.

Mechanical and microstructural analysis on the superplastic deformation behavior of Ti–6Al–4V Alloy

Abstract The present investigation has been made to study the superplastic deformation behavior of Ti–6Al–4V alloy based on the theory of inelastic deformation, and to analyze the boundary sliding characteristics using transmission electron microscopy. Flow characteristics for the microstructures of 2.5–16 μm grain sizes were analyzed by the load relaxation tests at various temperatures ranging from 600 to 927°C. The results showed that at relatively low temperatures such as 600°C the grain matrix deformation was dominant and found to be consistent with the state equation based on the dislocation dynamics. On the contrary, above the temperature of 800°C, the grain boundary sliding became dominant resulting in the change of curvature in the stress–strain rate curves, which was more pronounced in the finer microstructures. However, the deformation mode changes from grain boundary sliding to grain matrix deformation with the increase in grain size as evidenced by transmission electron microscopy.

Novel self-assembled bimetallic structure of Bi/Fe0: The oxidative and reductive degradation of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)

A novel self-assembled bimetallic zero-valent bismuth/iron (Bi/Fe(0)) composite was synthesized, characterized, and used successfully to remove hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) from wastewater. To assess the oxidative and reductive reactivities of Bi/Fe(0) nanoparticles (NPs), RDX degradation experiments were conducted in either ambient or anaerobic conditions, respectively. The best RDX degradation was achieved using 4%-Bi/Fe(0) (atomic ratio) NPs. In ambient conditions, concentrations of Fe(2+) ions and H2O2 were lower in the Bi/Fe(0) solution than in the Fe(0) solution; this difference indicates that most Fe(2+) ions and H2O2 reacted to produce hydroxyl radicals (*OH) and superoxide radical anions (O2(*-)), thereby resulting in the remarkable degradation of RDX. In anaerobic conditions, the presence of Bi increased the electron generation rate from the surfaces of the Bi/Fe(0) NPs. This increase was responsible for the excellent reductive degradation of RDX. Based on Density Functional Theory (DFT) calculations, the adsorption of water was endothermic on Fe(0) NPs and exothermic on Bi/Fe(0) NPs. Therefore, only the dissociation reactions of H2O in the Bi/Fe(0) system were spontaneous, and these reactions resulted in the prominent reactivity of the Bi/Fe(0) NPs.

Superparamagnetic nalidixic acid grafted magnetite (Fe3O4/NA) for rapid and efficient mercury removal from water

A new nanomaterial, nalidixic acid grafted magnetite (Fe3O4/NA), was synthesized via a chemical reaction with nano sized magnetite particles. The Fe3O4/NA was superparamagnetic at room temperature and could be separated by an external magnetic field. The presence of mercury in groundwater in wide scale industrial areas of the world has been a huge problem and the prepared Fe3O4/NA nanoparticles showed a high adsorption capacity towards Hg(II) as compared to bare magnetite particles. The high adsorption capacity of NA grafted Fe3O4 (9.52 mg g−1) was due to the increased adsorption sites in the magnetite-nalidixic acid (Fe3O4/NA). The sorption equilibrium data obeyed the Langmuir model while kinetic studies demonstrated that the sorption process of Hg(II) followed well the pseudo second order model. Since the Fe3O4/NA showed (over 99.8%) removal of the initial 1000 ppb Hg(II) within 60 min, it should be practically usable for Hg(II) contaminated water. The desorption of Hg(II) loaded on Fe3O4/NA could be successfully achieved with 0.001 M HCl containing 0.3 M thiourea, and the sorbent exhibited excellent reusability.

Enhancing the reactivity of bimetallic Bi/Fe(0) by citric acid for remediation of polluted water.

In this study, the environmentally benign citric acid (CA) was utilized to improve the aerobic degradation of 4-chlorophenol (4-CP) over bismuth modified nanoscale zero-valent iron (Bi/Fe(0)). The characterization results revealed the existence of bismuth covering on the Fe(0) surface under zero-valent state. And, the Bi/Fe(0)-CA+O2 system performed excellent reactivity in degradation of 4-CP due to the generation of reactive oxygen species (ROS), which was confirmed by electron spin resonance (ESR) spectroscopy. After 30min of reaction, 80% of 4-CP was removed using Bi/Fe(0)-CA+O2 accompanying with high dechlorination rate. The oxidative degradation intermediates were analyzed by HPLC and LC-MS. We found that CA could promote the bismuth-iron system to produce much reactive oxygen species ROS under both aerobic and anaerobic conditions due to its ligand function, which could react with Fe(3+) to form a ligand complex (Fe(III)Cit), accompanying with a considerable production of Fe(2+) and H2O2. This study provides a new strategy for generating ROS on nZVI and suggests its application for the mineralization of many recalcitrant pollutants.

Self-Generation of Reactive Oxygen Species on Crystalline AgBiO3 for the Oxidative Remediation of Organic Pollutants

In this study, we synthesized a novel perovskite nanomaterial consisting of AgBiO3 nanoparticles (NPs) via an ion-exchange method for remediation of polluted environments. The AgBiO3 NPs could self-produce significant amounts of reactive oxygen species (ROS) without light illumination or any other additional oxidant due to the controllable release of lattice oxygen from the crystalline AgBiO3, resulting in the formation of ROS somehow. The self-produced 1O2, O2•-, and •OH were confirmed by electron spin resonance spectroscopy using a spin trap technique. We found that the AgBiO3 NPs could be reused for the mineraliztion of most recalcitrant organic compounds alone, including Rhodamine B (RhB), phenol, 4-chlorophenol, 2,4-dichlorophenol, and bisphenol A. After the repeated eight cycles of continious treatment of RhB, AgBiO3 NPs still achieved 79% of degradation after 30 min of treatment. Characterization results revealved that the lattice oxygen inside AgBiO3 was activated to form active oxygen (O*), which resulted in consecutive formation of ROS. This study provides new insight on the lattice oxygen activation mechanism of silver bismuthate and its application to the remediation of polluted waters.

Evaluation of Nanoscale Zero-valent Iron for Reductive Degradation of Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX): Batch and Column Scale Studies

ABSTRACT Reductive degradation of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) by nanoscale zero-valent iron (nZVI) wasinvestigated to evaluate the feasibility of using it for in-situ groundwater remediation. Batch experiments were conductedto quantify the kinetics and efficiency of RDX removal by nZVI, and to determine the effects of pH, dissolved oxygen(DO), and ionic strength on this process. Experimental results showed that the reduction of RDX by nZVI followedpseudo-first order kinetics with the observed rate constant (k obs ) in the range of 0.0056-0.0192 min −1 . Column tests wereconducted to quantify the removal of RDX by nZVI under real groundwater conditions and evaluate the potential efficacyof nZVI for this purpose in real conditions. In column experiment, RDX removal capacity of nZVI was determined to be82,500 mg/kg nZVI. pH, oxidation-reduction potential (ORP), and DO concentration varied significantly during thecolumn experiments; the occurrence of these changes suggests that monitoring these quantities may be useful in evaluationof the reactivity of nZVI, because the most critical mechanisms for RDX removal are based on the chemical reductionreactions. These results revealed that nZVI can significantly degrade RDX and that use of nZVI could be an effectivemethod for in-situ remediation of RDX-contaminated groundwater.Key words : Nanoscale Zero-Valent Iron, RDX, Column, Groundwater, Reduction