Si-Hyun Do

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.

Persulfate activation by iron oxide-immobilized MnO2 composite: identification of iron oxide and the optimum pH for degradations.

Iron oxide-immobilized manganese oxide (MnO2) composite was prepared and the reactivity of persulfate (PS) with the composite as activator was investigated for degradation of carbon tetrachloride and benzene at various pH levels. Brunauer-Emmett-Teller (BET) surface area of the composite was similar to that of pure MnO2 while the pore volume and diameter of composite was larger than those of MnO2. Scanning electron microscopy couples with energy dispersive spectroscopy (SEM-EDS) showed that Fe and Mn were detected on the surface of the composite, and X-ray diffraction (XRD) analysis indicated the possibilities of the existence of various iron oxides on the composite surface. Furthermore, the analyses of X-ray photoelectron (XPS) spectra revealed that the oxidation state of iron was identified as 1.74. In PS/composite system, the same pH for the highest degradation rates of both carbon tetrachloride and benzene were observed and the value of pH was 9. Scavenger test was suggested that both oxidants (i.e. hydroxyl radical, sulfate radical) and reductant (i.e. superoxide anion) were effectively produced when PS was activated with the iron-immobilized MnO2.

Synthesis of iron composites on nano-pore substrates: Identification and its application to removal of cyanide

Two types of nano-pore substrates, waste-reclaimed (WR) and soil mineral (SM) with the relatively low density, were modified by the reaction with irons (i.e. Fe(II):Fe(III)=1:2) and the applicability of the modified substrates (i.e. Fe-WR and Fe-SM) on cyanide removal was investigated. Modification (i.e. Fe immobilization on substrate) decreased the BET surface area and PZC of the original substrates while it increased the pore diameter and the cation exchange capacity (CEC) of them. XRD analysis identified that maghemite (γ-Fe(2)O(3)) and iron silicate composite ((Mg, Fe)SiO(3)) existed on Fe-WR, while clinoferrosilite (FeSiO(3)) was identified on Fe-SM. Cyanide adsorption showed that WR adsorbed cyanide more favorably than SM. The adsorption ability of both original substrates was enhanced by the modification, which increased the negative charges of the surfaces. Without the pH adjustment, cyanide was removed as much as 97% by the only application of Fe-WR, but the undesirable transfer to hydrogen cyanide was possible because the pH was dropped to around 7.5. With a constant pH of 12, only 54% of cyanide was adsorbed on Fe-WR. On the other hand, the pH was kept as 12 without adjustment in Fe-WR/H(2)O(2) system and cyanide was effectively removed by not only adsorption but also the catalytic oxidation. The observed first-order rate constant (k(obs)) for cyanide removal were 0.49 (± 0.081) h(-1). Moreover, the more cyanate production with the modified substrates indicated the iron composites, especially maghemite, on substrates had the catalytic property to increase the reactivity of H(2)O(2).

Reductive dechlorination of chlorinated hydrocarbons as non-aqueous phase liquid (NAPL): Preliminary investigation on effects of cement doses

The reactivities of various types of iron mixtures to degrade chlorinated hydrocarbons (PCE, TCE and 1,1,1-TCA) in the form of non-aqueous phase liquids were investigated. The iron mixtures included a mixture of Fe(II) and Portland cement (Fe(II)-C), a mixture of Fe(II), Fe(III) and Ca(OH)(2) (Fe(II/III)-L), and a mixture of Fe(II), Fe(III), Ca(OH)(2), and Portland cement (Fe(II/III)-C). When the same amount of Fe(II) was used, Fe(II)-C was more reactive with chlorinated ethylenes (i.e. PCE and TCE) than Fe(II/III)-L. The reductive pathway for high concentrations of total PCE (i.e. above solubility) with Fe(II)-C was determined to be a combination of two-electron transfer, β-elimination and hydrogenolysis. Increasing the cement dose from 5% to 10% in Fe(II)-C did not affect PCE dechlorination rates, but it did favor the β-elimination pathway. In addition, when Fe(II/III)-C with 5%C was used, PCE dechlorination was similar to that by Fe(II)-C, but this mixture did not effectively degrade TCE. A modified second-order kinetic model was developed and shown to appropriately describe degradation of TCE at high concentrations. Fe(II/III)-L effectively degraded high concentrations of 1,1,1-TCA at rates that were similar to those obtained with Fe(II)-C using 10% C. Moreover, both increasing cement doses and the presence of Fe(III) increased dechlorination rates of 1,1,1-TCA, which was mainly through the hydrogenolysis pathway. The reactivity of Fe(II/III)-L was strongly dependent on the target compound (i.e. less reactivity with TCE, more with 1,1,1-TCA). Therefore, Fe(II/III)-L could be a potential mixture for degrading 1,1,1-TCA, but it should be modified to degrade TCE more effectively.

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.

Degradation of TCE by Persulfate Oxidation with Various Activation Methods (heat, Fe 2+ , and UV) for ex-situ Chemical Oxidation Processes

Rreactivity of persulfate (PS) for oxidation of TCE under various conditions such as heat, , and UV was investigated. It was found that degradation rate of TCE increased with increasing temperature from 15 to . At pH 7.0, the rate constants (k) at 15, 25, 30, and were 0.07, 0.30, 0.74, and , respectively. For activation by , removal efficiency of TCE increased with increasing concentration from 1.9 mM to 11 mM. The maximum removal efficiency of TCE was approximately 85% when pH of the solution dropped from 7.0 to 2.5. Degradation of TCE by UV-activated PS was the most effective, showing that the degradation rate of TCE increased with inreasing PS dosage; the rate constants (k) at 0.5, 2.5, and 10 mM were 34.2, 40.5, and , respectively. Our results suggest that PS activation by UV/PS process could be the most effective in activation processes tested for TCE degradation. For oxidation process by PS, however, pH should be observed and adjusted to neutral conditions (i.e., 5.8-8.5) if necessary.