Hongkyun Lee

Hydrogen peroxide decomposition on manganese oxide (pyrolusite): kinetics, intermediates, and mechanism.

The objective of this study is the kinetic interpretation of hydrogen peroxide decomposition on manganese oxide (pyrolusite) and the explanation of the reaction mechanism including the hydroperoxide/superoxide anion. The decomposition of hydrogen peroxide on manganese oxide at pH 7 was represented by a pseudo first-order model. The maximum value of the observed first-order rates constants (k(obs)) was 0.741 min(-1) at 11.8 of [H(2)O(2)]/[triple bond MnO(2)] when [H(2)O(2)]/[triple bond MnO(2)] were ranged from 58.8 to 3.92. The pseudo first-order rate constants (kMnO(2)) approximated as the average value of 0.025 (min mM)(-1) with a standard deviation of 0.003 at [H(2)O(2)]/[triple bond MnO(2)] ranged from 39.2 to 11.8. When [H(2)O(2)]/[triple bond MnO(2)] was 3.92, the rate constants (kMnO(2)) was 0.061 (min mM)(-1) as maximum. Oxygen production showed that the initial rates increased with decreasing [H(2)O(2)]/[triple bond MnO(2)] and the total amounts of oxygen was slightly less than the stoichiometric value (0.5) in most experiments. However, oxygen was produced at more than 0.5 in low [H(2)O(2)]/[triple bond MnO(2)] (i.e. 3.92 and 9.79). The relative production of hydroperoxide/superoxide anion implied that the production increased with low [H(2)O(2)]/[triple bond MnO(2)], and the existence of anions suggested that the mechanism includes propagation reactions with intermediates such as hydroperoxide/superoxide anion in solution. In addition, both [H(2)O(2)] decomposition and the production of anion were accelerated in alkaline solution. Manganese ion dissolved into solution was negligible in neutral and alkaline conditions, but it greatly increased in acidic conditions.

Application of a peroxymonosulfate/cobalt (PMS/Co(II)) system to treat diesel-contaminated soil

We investigated the feasibility of using peroxymonosulfate (PMS) with transition metals (PMS/M(+) system) for remediation of diesel-contaminated soils. To the best of our knowledge, this is the first attempt to apply a PMS/M(+) system for the treatment of diesel-contaminated soils. Two well-known transition metals, Fe(II) and Co(II), used to activate PMS including the effect of co-existence of counter anions (Cl(-) and SO(4)(2-)) were tested and it revealed that the most effective degradation of diesel was achieved with cobalt chloride. The effect of PMS (i.e. 0-500 mM) indicated that the increasing the molar ratio of PMS/diesel increased degradation of diesel on soils. The effect of Co(II) (i.e. 0-4mM) showed that at least 2mM of Co(II) was needed to degrade above 30% of diesel. Moreover, a maximum diesel degradation of 47% was achieved at a single injection of PMS/Co(II) (i.e. 500 mM/2mM). Assessments of system pH showed that diesel degradation was higher under acidic conditions (pH 3) possibly due to the dissolution of metal ions from soils that are not possible at other pHs (pH 6 and 9). Sequential injections of both PMS and Co(II) were employed to improve the level of remediation (approximately 90% degradation). The degradation of diesel increased as much as 88% when PMS/Co(II) was sequentially injected. This indicates that PMS/Co(II) systems are applicable for remediation of soil contaminated with diesel fuel as an aspect of in situ chemical oxidation.

WidmanstÄtten Type Solidification in Squeeze Casting of Mg-Li-Al Alloys

The addition of 4--16 wt% Li to Mg lowers the density from 1.7 g/cm{sup 3} to 1.3--1.6 g/cm{sup 3}. Further this results in the h.c.p. {alpha} phase being replaced by the b.c.c. {beta} phase, leading to considerable low temperature formability. Detailed examination of the Mg-Li binary phase diagram shows that the {alpha} phase can precipitate in the {beta} phase matrix due to the decrease of Mg solubility in the {beta} phase as the temperature decreases. The addition of Al to Mg-Li alloys can improve strength by forming a uniform distribution of second phase particles. From the results of the previous study, Mg-8--10 wt% Li-3 {approximately} 5 wt% Al alloys consist of a mixture of {alpha} and {beta} grains, and fine second phase particles, showing the optimum combination of mechanical properties. Main emphasis in the present study was placed on investigating the effects of Widmanstaetten type solidification on the strengthening behavior in squeeze cast Mg-8 wt% Li-3 wt% Al and Mg-10 wt%-3 wt% Al alloys.

Effect of food wastewater on biomass production by a green microalga Scenedesmus obliquus for bioenergy generation.

Effect of food wastewater (FW) on the biomass, lipid and carbohydrate production by a green microalga Scenedesmus obliquus cultivated in Bolds Basal Medium (BBM) was investigated. Different dilution ratios (0.5-10%) of BBM either with FW or salt solution (NaCl) or sea water (SW) were evaluated. S. obliquus showed the highest growth (0.41 g L(-1)), lipid productivity (13.3 mg L(-1) day L(-1)), carbohydrate productivity (14.7 mg L(-1) day L(-1)) and nutrient removal (38.9 mg TN L(-1) and 12.1 mg TP L(-1)) with 1% FW after 6 days of cultivation. The FW promoted algal autoflocculation due to formation of inorganic precipitates at an alkali pH. Fatty acid methyl ester analysis revealed that the palmitic and oleic acid contents were increased up to 8% with FW. Application of FW improved the growth, lipid/carbohydrate productivity and biomass recovery efficiency of S. obliquus, which can be exploited for cost effective production of microalgae biomass.

As(III) and As(V) removal from the aqueous phase via adsorption onto acid mine drainage sludge (AMDS) alginate beads and goethite alginate beads

Acid mine drainage sludge (AMDS) is a solid waste generated following the neutralization of acid mine drainage (AMD). This material entrapped in calcium alginate was investigated for the sorption of As(III) and As(V). Three different adsorbent materials were prepared: AMDS alginate beads (AABs), goethite alginate beads (GABs), and pure alginate beads. The effects of pH and the adsorption kinetics were investigated, and the adsorption isotherms were also evaluated. The optimum pH range using the AABs was determined to be within 2-10 for As(III) and 2-9 for As(V). Adsorption equilibrium data were evaluated using the Langmuir isotherm model, and the maximum adsorption capacity qmax was 18.25 and 4.97 mg g(-1) for As(III) on AAB and GAB, respectively, and 21.79 and 10.92 mg g(-1) for As(V) on AAB and GAB, respectively. The adsorption of As(III) and As(V) was observed to follow pseudo-second order kinetics. The As K-edge X-ray absorption near-edge structure (XANES) revealed that the adsorbed As(III) on the AABs was oxidized to As(V) via manganese oxide in the AMDS.

Removal of metal from acid mine drainage using a hybrid system including a pipes inserted microalgae reactor.

In this study, the microalgae culture system to combined active treatment system and pipe inserted microalgae reactor (PIMR) was investigated. After pretreated AMD in active treatment system, the effluent load to PIMR in order to Nephroselmis sp. KGE 8 culture. In experiment, effect of iron on growth and lipid accumulation in microalgae were inspected. The 2nd pretreatment effluent was economic feasibility of microalgae culture and lipid accumulation. The growth kinetics of the microalgae are modeled using logistic growth model and the model is primarily parameterized from data obtained through an experimental study where PIMR were dosed with BBM, BBM added 10 mg L(-1) iron and 2nd pretreatment effluent. Moreover, the continuous of microalgae culture in PIMR can be available. Overall, this study indicated that the use of pretreated AMD is a viable method for culture microalgae and lipid accumulation.

Structure and decomposition behaviour of MgLiAl alloys

The addition of 4--16 wt% Li to Mg lowers the density from 1.7g/cm[sup 3] to 1.3--1.6g/cm[sup 3] and the h.c.p. [alpha]-Mg phase is replaced by the b.c.c. [beta] phase, leading to considerable low temperature formability. Addition of Al to Mg-Li alloys can improve strength by forming a distribution of intermetallic particles such as MgLi[sub 2]Al and AlLi. However, the microstructural characteristics of Mg-Li-Al alloys have not yet been investigated in detail. The aim of the present work was to study the microstructure of Mg-Li-Al alloys, mainly by transmission electron microscopy (TEM).

Effect of applied pressure during solidification on the microstructural refinement in an AlCu alloy

The solidification microstructure can be refined by increasing the cooling rate during solidification. Applying high pressure during solidification can promote the solidification rate by increasing the heat transfer coefficient at the metal/mold interface and changing thermodynamic properties such as solid-liquid transition temperature. Based upon these advantages of high pressure solidification, various investigators have reported substantial refinement of microstructures and improvement of mechanical properties. However, the cooling rate and the degree of the microstructural refinement have not been reported consistently. The purpose of the present study is to investigate the effects of pressure up to 1.7GPa during solidification of an hypoeutectic Al-Cu alloy. Experimental observation of the microstructural refinement in high pressure solidified Al-5.4wt%Cu was compared with the mathematical simulation of the heat transfer behavior during high pressure solidification.

Formation of spherical primary silicon crystals during semi-solid processing of hypereutectic Al-15.5wt%Si alloy

Semi-solid state processing is considered as an attractive processing technique because of it offers several potential advantages such as energy saving for forming, reduction of microsegregation and porosity, especially for near net shape manufacturing of components. This technique is frequently applied to make sound casting products of hypoeutectic Al-Si alloys. On the other hand, hypereutectic Al-Si alloys have limited applications because of their inherent problems associated with poor machinability and mechanical properties, although the commercial potential for their outstanding resistance to wear and corrosion, higher hardness and low coefficient of thermal expansion is well recognized. To exploit the full potential of hypereutectic Al-Si alloys, semi-solid processing is utilized in an attempt to improve the mechanical properties through the spheroidization of primary Si crystals. The purpose of the present work is to provide the experimental evidence of the fragmentation of primary Si crystals in hypereutectic Al-15.5wt%Si alloy during semi-solid processing. Microstructural evolution during the isothermal shearing above the eutectic temperature of this alloy as a function of processing time is also reported.

PCE DNAPL degradation using ferrous iron solid mixture (ISM)

Ferrous iron solid mixture (ISM) containing Fe(II), Fe(III), and Cl was synthesized for degradation of tetrachloroethene (PCE) as a dense non-aqueous phase liquid (DNAPL), and an extraction procedure was developed to measure concentrations of PCE in both the aqueous and non-aqueous phases. This procedure included adding methanol along with hexane in order to achieve the high extraction efficiency, particularly when solids were present. When PCE was present as DNAPL, dechlorination of PCE was observed to decrease linearly with respect to the total PCE concentration (aqueous and non-aqueous phases) and the concentration of PCE in the aqueous phase was observed to be approximately constant. In the absence of DNAPL, the rate of PCE degradation was observed to be the first-order with respect to the concentration in the aqueous phase. A kinetic model was developed to describe these observations and it was able to fit experimental data well. Increasing the concentration of Fe(II) in ISM increased the values of rate constants, while increasing the concentration of PCE DNAPL did not affect the value of the rate constant. The reactivity of ISM for PCE dechlorination might be close to that of Friedels salt, and the accumulation of trichloroethylene (TCE) might imply the lower reactivity of ISM for degradation of TCE or the necessity of large amount of Fe(II) in ISM. TCE (the major chlorinated intermediate), ethene (the major non-chlorinated compound), acetylene and ethane were detected, which implied that both hydrogenolysis and beta-elimination were pathways of PCE DNAPL degradation on ISM.