Do-Gun Kim

Mechanism study of nitrate reduction by nano zero valent iron.

This study investigates the fate of nitrogen species during nitrate reduction by nano-scale zero valent iron (NZVI) and related reaction mechanisms. The NZVI used for the experiments was prepared by chemical reduction without a stabilizing agent. NZVI has great ability to reduce nitrate. However, the question of what end-product results from nitrate reduction by NZVI has sparked controversy. Establishing nitrogen mass balance by quantitative analysis of aqueous phase and gas-phase nitrogen species, this study clearly determines that nitrate was converted to ammonium ion followed by ammonia stripping under a strong alkaline condition, which leads to a decrease in the total aqueous nitrogen amount. Moreover, some of the major reactions, which consisted of nitrate reduction, ammonia production, and ammonia stripping were modelled by pseudo first-order kinetics. According to the model estimation results, additional reaction mechanisms would exist in an early stage of reaction. This might be due to the adsorption and desorption reaction which could be explained by the core-shell structure model.

Fate of nitrogen species in nitrate reduction by nanoscale zero valent iron and characterization of the reaction kinetics

This study investigates the fate of nitrogen species during nitrate reduction by nanoscale zero valent iron (NZVI) as well as the related kinetics. The NZVI used for the experiments was prepared by chemical reduction without a stabilizing agent. The pseudo first order kinetic constant of nitrate reduction at 30 degrees C with an NZVI/nitrate ratio of 1.25:1, which were the reference conditions of this study, was 4.08 h(-1) (R(2)=0.955). A nitrogen mass balance was established by quantitative analysis of aqueous-phase and gas-phase nitrogen species. The results confirm that the nitrate was converted to ammonium ion, that ammonia stripping subsequently occurred under a strong alkaline condition, and that the total amount of aqueous nitrogen was consequently reduced. The nitrate reduction rate also increased with a lower pH and a higher temperature when microscale ZVI was used. However, in contrast to the reaction by microscale ZVI, the nitrate reduction rate by NZVI was higher for an unbuffered condition, possibly due to the abundance of surface atoms and the smaller size.

Effects of ozonation and coagulation on effluent organic matter characteristics and ultrafiltration membrane fouling.

Effluent organic matter (EfOM) is the major cause of fouling in the low pressure membranes process for wastewater reuse. Coagulation and oxidation of biological wastewater treatment effluent have been applied for the fouling control of microfiltration membranes. However, the change in EfOM structure by pre-treatments has not been clearly identified. The changes of EfOM characteristics induced by coagulation and ozonation were investigated through size exclusion chromatography, UV/Vis spectrophotometry, fluorescence spectrophotometry and titrimetric analysis to identify the mechanisms in the reduction of ultrafiltration (UF) membrane fouling. The results indicated that reduction of flux decline by coagulation was due to modified characteristics of dissolved organic carbon (DOC) content. Total concentration of DOC was not reduced by ozonation. However, the mass fraction of the molecules with molecular weight larger than 5 kDa, fluorescence intensity, aromaticity, highly condensed chromophores, average molecular weight and soluble microbial byproducts decreased greatly after ozonation. These results indicated that EfOM was partially oxidized by ozonation to low molecular weight, highly charged compounds with abundant electron-withdrawing functional groups, which are favourable for alleviating UF membrane flux decline.

Inhibition of Nitrate Reduction by NaCl Adsorption on a Nano-Zero-Valent Iron Surface during a Concentrate Treatment for Water Reuse

Nanoscale zero-valent iron (NZVI) has been considered as a possible material to treat water and wastewater. However, it is necessary to verify the effect of the matrix components in different types of target water. In this study, different effects depending on the sodium chloride (NaCl) concentration on reductions of nitrates and on the characteristics of NZVI were investigated. Although NaCl is known as a promoter of iron corrosion, a high concentration of NaCl (>3 g/L) has a significant inhibition effect on the degree of NZVI reactivity towards nitrate. The experimental results were interpreted by a Langmuir–Hinshelwood–Hougen–Watson reaction in terms of inhibition, and the decreased NZVI reactivity could be explained by the increase in the inhibition constant. As a result of a chloride concentration analysis, it was verified that 7.7–26.5% of chloride was adsorbed onto the surface of NZVI. Moreover, the change of the iron corrosion product under different NaCl concentrations was investigated by a surface analysis of spent NZVI. Magnetite was the main product, with a low NaCl concentration (0.5 g/L), whereas amorphous iron hydroxide was observed at a high concentration (12 g/L). Though the surface was changed to permeable iron hydroxide, the Fe(0) in the core was not completely oxidized. Therefore, the inhibition effect of NaCl could be explained as the competitive adsorption of chloride and nitrate. GRAPHICAL ABSTRACT

Humic acid characteristics and effects on the reactivity of nano-scale zero-valent iron particles during nitrate reduction

ABSTRACT Humic acid (HA) comprises a significant fraction of natural organic matter (NOM), greatly influencing the performance of water and wastewater treatment processes. HA is expected to significantly affect the reactivity of nano-scale zero-valent iron (NZVI), which is receiving increasing attention due to its high reactivity. The effects of HA on nitrate reduction by NZVI were investigated to evaluate the potential of NZVI in practical applications. HA was characterized to identify the mechanism whereby HA affects. Nitrate reduction was enhanced at low HA concentration, but inhibited as HA concentration increased. HA decreased to reach a plateau and considerable amount of Fe3+ was detected when HA was present in dissolved phase. The increase in the degree of condensation of HA was verified by UV–vis spectroscopy, fluorescence spectroscopy, and size exclusion chromatography. Fourier transform infrared spectra confirmed that the Fe3+–HA complexes were formed in both the dissolved and solid fractions, w...

Characterization of sludge generated by electrocoagulation for the removal of heavy metals

AbstractIn this work, heavy metal removal by electrocoagulation (EC) was evaluated, and the characteristics of the EC sludge were investigated to understand the behavior of heavy metals during EC. It was found that iron electrodes are superior to aluminum due to the negatively charged surface. Using iron electrodes, the removal rate of heavy metals increased as the current density increased and as the total initial heavy metal amount, either single or multiple heavy metals, decreased, suggesting that the efficiency is closely related to the floc amount. The X-ray diffraction (XRD) patterns showed that the chemical species in the EC sludge are different from those obtained by chemical equilibrium analysis, indicating that many electrochemical and chemical reactions occur during EC. The pH variation during EC and the XRD patterns of the sludge indicate that precipitation, adsorption, and chemical oxidation/reduction in the vicinity of the electrodes contribute much to Pb(II), Cd(II), and Cu(II) removal, res...

Deactivation of nanoscale zero-valent iron by humic acid and by retention in water

The effects of the deactivation of nanoscale zero-valent iron (NZVI), induced by humic acid (HA) and by the retention of NZVI in water, on nitrate reduction were investigated using a kinetic study. Both the nitrate removal and generation of ammonia were significantly inhibited as the HA adsorption amount and retention time were increased. However, HA removal was greatly enhanced when the NZVI was used after 1 d or 25 d of retention in water. The results are caused by the formation of iron oxides/hydroxides, which increased the specific surface area and the degree of NZVI aggregation which was observed by transmission electron microscopy (TEM). However, the nitrate reduction was greater at the beginning of reaction in the presence of HA when fresh NZVI was used, because of the enhanced electron transfer by the HA in bulk phase and on NZVI surface as train sequences. The pseudo second order adsorption kinetic equation incorporating deactivation and a Langmuir-Hinshelwood (LH) type kinetic equation provided accurate descriptions of the nitrate removal and ammonia generation, respectively. The deactivation constant and the reaction rate constant of the LH type kinetic equation were strongly correlated with the HA amount accumulated on NZVI. These results suggest that the HA accumulation on the NZVI surface reactive sites plays the dominant role in the inhibition and the inhibition can be described successfully using the deactivation model. The HA accumulation on NZVI was verified using TEM.

Nonsacrificial Template Synthesis of Magnetic-Based Yolk–Shell Nanostructures for the Removal of Acetaminophen in Fenton-like Systems

Recently, yolk-shell structured materials with active metal cores have received considerable attention in heterogeneous Fenton-like systems, which have excellent catalytic performance. In this study, we initially attempted the nonsacrificial template synthesis of yolk-shell structured nanoparticles with magnetite cores encapsulated in a mesoporous silica shell (Fe3O4@SiO2) via a modified sol-gel process and then evaluated their catalytic activity for acetaminophen degradation in Fenton-like systems. Second, copper nanoparticles were decorated on the surface of the Fe3O4@SiO2 microspheres (Fe3O4@SiO2@Cu) to enhance the catalytic activity. The morphological, structural, and physicochemical properties of the prepared materials were characterized via X-ray diffraction, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, field emission transmission electron microscopy, nitrogen adsorption-desorption isotherms, specific surface area, ζ-potential, magnetic properties, and Fourier transform infrared spectroscopy. The results demonstrated a successful fabrication of the targeted materials. The yolk-shell structured materials possess a spherical morphology with an active core, protective shell, and hollow void. The Fe3O4@SiO2 and Fe3O4@SiO2@Cu variants showed acetaminophen removal rates significantly higher compared to those of their counterparts, i.e., the Fe3O4 and Fe3O4@Cu core-shell structures. Fe3O4@SiO2@Cu showed that the copper nanoparticles were firmly immobilized on the mesoporous silica shell, dramatically improving the catalytic performance. Both the yolk-shell structured Fe3O4@SiO2 and Fe3O4@SiO2@Cu exhibited good separation and satisfactory regeneration properties, which could be recycled six times without any obvious decline in catalytic activity. Overall, the results of this study suggested that Fe3O4@SiO2 and Fe3O4@SiO2@Cu yolk-shell nanostructures could be promising catalysts for a heterogeneous Fenton-like system by which the removal of emerging contaminants can be greatly improved.

Pollutant characteristics of road deposited sediments collected by road sweeping

Road deposited sediments (RDS) swept from highways in South Korea were characterized to quantitatively evaluate the reduction in non-point source pollutants by sweeping. The swept RDS consisted primarily of sand (63 μm to 2 mm) particles (80.34 ± 8.33% of total weight) highly contaminated by organics, nutrients and heavy metals. The average concentrations of total organic carbon (TOC), biochemical oxygen demand (BOD), volatile solids (VS), total nitrogen (T-N), and total phosphorus (T-P) were 20.17 ± 9.13, 1.04 ± 0.62, 39.92 ± 16.55, 1.99 ± 0.96, and 0.54 ± 0.19 g kg(-1) (±one standard deviation), respectively, for 63 μm to 2 mm RDS. The concentrations of the pollutants were high for RDS smaller than 63 μm, but most of the mass was associated with the 63 μm to 2 mm RDS. The results suggest that the pollutants associated with RDS swept from highways originated mainly from engine wear, exhaust emissions, and tire wear. These results were different from the RDS on roads in residential and commercial areas, where natural particles and brake wear contribute significantly to RDS. In addition, the reductions in TOC, BOD, VS, T-N, T-P, Cu, Pb, Zn, Fe, and As based on the swept RDS measurements were calculated to be 3,355.3, 175.1, 6,621.4, 323.0, 88.3, 30.3, 13.7, 1.0, 303.4, 11,198.7, and 0.4 g km(-1), respectively.

Removal of road deposited sediments by sweeping and its contribution to highway runoff quality in Korea.

Highway runoff is known to be an important non-point source (NPS), increasing the load of pollutants in receiving water. For reducing NPS pollutants in runoff, removal of road deposited sediment (RDS) by sweeping is considered effective. However, the contribution of sweeping to the improvement of runoff quality has not been clearly and quantitatively demonstrated so far. In this study, a field test was carried out on a section of operating highway in Korea to investigate the effectiveness of sweeping on improving the quality of highway runoff. Results showed that the average reduction in the load of RDS by sweeping was 61.10% with a standard deviation of 1.74%. RDS removal efficiency decreased when the sweeping speed increased from 4–8 to 20 km h−1, the load decreased from 12.5 to 1.25 g m−2 and particle size decreased from sand to silt/clay size ranges. Runoff was induced by applying a 15 mm h−1 artificial rainfall to both swept and non-swept sections. Analysis of runoff quality showed that the event mean concentrations of total suspended solid, biological oxygen demand, chemical oxygen demand, nutrients and most of the heavy metals were reduced by 31–87% after sweeping. In addition, field tests for RDS build-up indicated a sweeping frequency of once every four or five days to prevent re-suspension of RDS. The results of this study suggest that sweeping can be the best management practice for effectively reducing RDS on highways and improving the quality of highway runoff.