Biomedical subjects

W Chu

Publications and source records attributed to W Chu.

At least 37 records · Page 2Linked to original sources

PubMed2004

The system design of UV-assisted catalytic oxidation process--degradation of 2,4-D.

Unlike the conventional first- or second-order model, a novel approach to design for the removal of 2,4-dichlorophenoxy (2,4-D) by the UV-catalytic oxidation process (UVCOP) was investigated. Two distinctive parameters, initial decay rate and maximum oxidative capacity, were characterized. By using these parameters, the performance of the degradation of 2,4-D by UVCOP regarding to the reagent dosages could be successfully predicted. Low concentrations of ferrous ion was found to be a rate-limiting factor for the process while the dosage of hydrogen peroxide was concluded as a dominant species in determining the maximum oxidation capacities. This information can be used to optimize the treatment process and achieve the expected performance target; an "optimal-dose model" was developed accordingly. The model is an intelligent and useful tool to evaluate the optimal doses of hydrogen peroxide with the minimum dose of ferrous ion, which leads to a better design of the treatment process.

2,4-Dichlorophenoxyacetic Acid↗

PubMed2004

An unconventional approach to studying the reaction kinetics of the Fenton's oxidation of 2,4-dichlorophenoxyacetic acid.

The Fenton's oxidation kinetics of herbicide 2,4-D at various [Fe(II)] and [H(2)O(2)] combinations was investigated and modelled through an unconventional approach. The reaction kinetics of 2,4-D degradation demonstrated a two-stage pattern of decay, where a very fast reactive stage was followed by a retardation stage due to the depletion of oxidants and to the competitive side-reactions of the intermediates (including 2,4-dichlorophenol, chlorohydroquinone and 2,6-dichlororesorcinol). A model characterized by two newly established constants, the initial decay rate and the maximum oxidative capacity, was proposed and proven capable of describing the two-stage process, which cannot easily be described by conventional first- or second-order kinetics approaches.

2,4-Dichlorophenoxyacetic Acid↗

PubMed2004

The photocatalytic degradation of dicamba in TiO2 suspensions with the help of hydrogen peroxide by different near UV irradiations.

The direct photolysis and the photocatalytic degradations of dicamba in TiO2 suspensions with and without the use of hydrogen peroxide were studied using two different monochromatic UV irradiations (300 and 350 nm). Both the direct photolysis and photocatalytic degradations of dicamba follow pseudo-first-order decay kinetics. Photolysis reactions were slow but the corresponding photocatalysis rates were increased by about 3 and 5 times in the presence of TiO2 at 300 and 350 nm of UV, respectively. Photocatalytic rates were increased with the pH at acidic to neutral ranges because of the increase of hydroxide ions, but the reaction was gradually retarded at the alkaline medium due to the effect of charges repulsion. The different proton sources causing various degrees of rate retardation were due to the presence of the corresponding counter anions. The results of H2O2-assisted photocatalysis experiments showed that a low H2O2 dosage in photocatalysis using UV 300 nm would enhance the decay rate of dicamba by 2.4 times, but an overdose of H2O2 will retard the rate because of the expenditure of hydroxyl radicals. However, this process was found impracticable at UV 350 nm due to the absorption characteristic of H2O2. A neutral initial pH level was found to favour the H2O2-assisted photocatalysis at UV 300 nm. The reactions were highly retarded at the alkaline medium due to the unstable properties of H2O2.

Catalysis↗

PubMed2004

A study of the reaction mechanisms of the degradation of 2,4-dichlorophenoxyacetic acid by oxalate-mediated photooxidation.

The oxidation of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) by FeII/H2O2/UV (FHU) and ferrous-oxalate/H2O2/UV (FOHU) processes was investigated and compared. The initial decay rate and the overall removal percentage were used as the performance indexes. To extensively explore the associated processes, the intermediates and the end products were also examined and compared. The results showed that the degradation of 2,4-D by FHU is slower than that of FOHU. The involvement of ferrous-oxalate in the reaction can greatly improve the initial decay rate of 2,4-D because of the higher light sensitivity of the organometallic complexes. The total removal of 2,4-D and its corresponding intermediates was found to be strongly dependent on the initial hydrogen peroxide concentration; an excessively low initial [H2O2] will terminate the process due to the deficiency of hydroxyl radicals in the solution. All of the major primary intermediates (2,4-dichlorophenol, 2-chloro-4-hydroxyphenoxyacetic acid, 4-chloro-2-hydroxyphenoxyacetic acid, 5-hydroxy-2,4-dichlorophenoxyacetic acid and 6-hydroxy-2,4-dichlorophenoxyacetic acid) have been identified and confirmed by LC-MS. A reaction mechanism was proposed for FHU and FOHU and verified by the evidence presented in this study.

2,4-Dichlorophenoxyacetic Acid↗

PubMed2004

PCR detection of pathogenic viruses in southern California urban rivers.

AIMS: To investigate human viral contamination in urban rivers and its impact on coastal waters of southern California, USA. METHODS AND RESULTS: Three types of human viruses (adeno, entero and hepatitis A) were detected using nested- and RT-PCR from 11 rivers and creeks. Faecal indicator bacteria as well as somatic and F-specific coliphage were also tested. Approximately 50% of the sites were positive for human adenoviruses. However, there was no clear relationship between detection of human viruses and the concentration of indicator bacteria and coliphage. Both faecal indicator bacteria and human viral input at beaches near river mouths were associated with storm events. The first storm of the wet season seemed to have the greatest impact on the quality of coastal water than following storm events. CONCLUSIONS: This study provides the first direct evidence that human viruses are prevalent in southern California urban rivers. Urban run-off impacts coastal water quality most significantly during the storm season. SIGNIFICANCE AND IMPACT OF THE STUDY: To protect human health during water recreational activities, it is necessary to develop effective strategies to manage urban run-off during storm events.

Adenoviridae↗

PubMed2004

Photooxidation of 2,4-dichlorophenoxyacetic acid by ferrous oxalate-mediated system.

The photodegradation of a herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) by ferrous oxalate/UV/H2O2 was studied. Ferrous oxalate, the more photoactive and reactive species, was found to react faster with hydrogen peroxide for hydroxyl radical production than ferrous ions under UV irradiation. About 2.9 times greater rate enhancement was found with the addition of 0.3 mM oxalate than that of a solution containing 0.1 mM Fe2+ and 1 mM H2O2 in the presence of UV light at 253.7 nm. The kinetics dependence of hydrogen peroxide concentration and initial solution pH were also investigated. A threefold increase in peroxide concentration could accelerate the removal of 2,4-D and nearly complete the reaction in 30 min of illumination. The initial decay rate of 2,4-D treated by ferrous oxalate/UV/H2O2 accelerated from 0.0029 to 0.0034 s(-1) and the overall removal of the starting material increased from 78% to 99.9%. The 2,4-D transformation at lower initial pH had higher reaction rates than that at higher pH and different reaction mechanisms were identified. The efficiency of the herbicide decomposition was retarded 2.6 times and 9.6 times as initial pH increased from 2.8 to 5.1 and 9.0, respectively.

2,4-Dichlorophenoxyacetic Acid↗

PubMed2003

The mechanism of the surfactant-aided soil washing system for hydrophobic and partial hydrophobic organics.

A surfactant-aided soil washing mechanism is proposed in this study by examining nine common organic contaminants, which were divided into two groups, hydrophobic compounds and partial hydrophobic compounds, depending on the respective soil partitioning of contaminants, K(oc). The presence of a free non-aqueous phase liquid in the soil washing system is critical to determine the soil washing performance curves. A mathematical model is proposed to describe soil-washing performance at various surfactant concentrations. The resulting slopes and intercepts from the model for the performance prediction are linearly related to K(oc). In addition, a transition zone between the hydrophobic and partial hydrophobic compounds was observed, and has been used in verifying the proposed mechanism successfully by overdosing and underdosing the contaminants in the system.

Journal Article↗

PubMed2003

The hydrogen peroxide-assisted photocatalytic degradation of alachlor in TiO2 suspensions.

Photocatalytic degradations of alachlor in TiO2 suspensions with and without the use of hydrogen peroxide were studied using two different monochromatic UV irradiations (300 and 350 nm). Direct photolysis of alachlor was a rather slow process, but the addition of TiO2 enhanced the reaction rates by 12 and 26 times using 300 and 350 nm UV irradiation, respectively. The results showed that a low H2O2 dosage in photocatalysis using 300 nm UV would enhance the rates by 3.3 times, but an overdose of H2O2 will retard the rate due to the hydroxyl radicals are consumed. However, this process is impracticable at 350 nm due to the absorption characteristic of H2O2. A neutral initial pH level was found to favor the H2O2 assisted photocatalysis at 300 nm UV illumination. Eleven major intermediates were identified by liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS) and MS/MS. The major degradation mechanisms of H2O2-assisted alachlor photocatalysis include dechlorination, dealkylation, hydroxylation, cyclization, scission of C-O bond, and N-dealkylation. Bell-shaped evolution profiles of different intermediates were observed. Degradation pathways were proposed accordingly to illustrate series of degradation steps. The TOC analysis revealed the different stages of the reaction.

Acetamides↗

PubMed2003

The vertical distribution and diversity of marine bacteriophage at a station off Southern California.

Sixty-two bacteriophages were isolated on eight indigenous bacteria from a Pacific Ocean station spanning 887-m vertical depth, on two occasions between 1999 and 2000. On the basis of 16S rRNA sequences, six hosts were tentatively identified to be in the genus Vibrio and the other two were closely related to Altermonas macleodii (W9a) and Pseudoalteromonas spp. (W13a). Restriction fragment length polymorphism (RFLP) analysis of phage genomes using AccI and HapI showed that 16 phages infecting host C4a (Vibrio) displayed 14 unique RFLP patterns. However, identical phages infecting host C4b, C6a, and C6b (all Vibrio) were obtained from both the surface layer and the hypoxic zone at 850 m. Most phage isolates from the second year had a different RFLP pattern but shared genetic similarity to the phages infecting the same host from the previous year based on a hybridization study using phage genome probes. Cluster analysis of RFLP patterns and hybridization results also indicated that phages infecting the same or genetically related hosts, in general, shared higher degrees of homology in spite of the diverse RFLP patterns. Pulsed field gel electrophoresis (PFGE) analysis of native viral genomes indicated a range in genome size from less than 40 to 200 kb, and the dominant band shifted up by about 5-10 kb in the deep samples compared to the shallow ones. Hybridization of phage genome probes with total viral community DNA from various depths suggests these isolates, or at least some of their genes, represent a detectable portion of the natural viral community and were distributed throughout the water column. Thus, the results of this study demonstrated that the genetic diversity of bacteriophage in the ocean is far greater than that of their bacterial hosts. However, host range may have contributed to the evolution of the diverse phage population in the marine environment.

Bacteriophages↗

PubMed2003

Modeling the ozonation of 2,4-dichlorophoxyacetic acid through a kinetic approach.

The ozonation of herbicide 2,4-dichlorophoxyacetic acid (2,4-D) was studied and modeled at different initial pH levels and initial concentrations. The degradation rate of 2,4-D was found to increase with the initial pH level and decrease with initial 2,4-D concentration. A two-stage ozonation of 2,4-D was observed, in which the initial stage of ozonation was slightly faster than the second stage due to the higher initial (saturated) ozone concentration; under these circumstances, the direct molecular ozone oxidation was likely to predominate. At the second stage, the indirect radical oxidation became more significant in determining the overall 2,4-D oxidation, and the reaction was more pH-dependent. A mathematic model associated with the analytical solutions consisting of direct and indirect oxidations contributed by the molecular ozone and free radical is proposed. The proposed model is capable of predicting the decay of 2,4-D quantitatively by ozonation at different initial pH levels and initial 2,4-D concentrations.

2,4-Dichlorophenoxyacetic Acid↗

PubMed2003

Reactor design and kinetics study of 4,4'-dichlorobiphenyl photodecay in surfactant solution by using a photosensitizer and hydrogen source.

The system design based on the photodegradation kinetics of 4,4'-dichlorobiphenyl (4,4'-DCB) in surfactant solution with the aid of solvents (acetone and/or squalane) has been studied. Organic solvents acetone and squalane were added as a photosensitizer and a hydrogen source, respectively, to achieve better photolysis performance. The quantum yield of 4,4'-DCB photodecay in Tween 80 micellar solution in the presence of added acetone was increased from 0.032 to 0.043 at its optimal condition. Acetone was shown to be an effective photosensitizer at low concentration, but an overdose would quench the reaction. Furthermore, the addition of squalane could further promote the photodechlorination of 4,4'-DCB in the Tween 80/acetone solutions for an additional 59% of rate improvement. However, an overdose of either solvent was found to cause UV light attenuation and to reduce the observed quantum yield. This effect has been justified and quantified in this study by a proposed light attenuation model, which has also been incorporated into the kinetic equation so that the resulted formula can be used to design the UV reactors for water and wastewater treatment works.

Facility Design and Construction↗

PubMed2003

Treatment of trichlorophenol by catalytic oxidation process.

The oxidation of 2,4,6-trichlorophenol (TCP) by ferrous-catalyzed hydrogen peroxide was quantified and modeled in the study. TCP was effectively degraded by hydroxyl radicals that were generated by Fe(II)/H(2)O(2) in the oxidation process. The oxidation capacity (OC) of the process depends on the concentrations of oxidant (hydrogen peroxide) and oxidative catalyst (ferrous ion). Up to 99.6% of TCP removal can be achieved in the process, provided the doses of Fe(II) and H(2)O(2) are selected correctly. The OC of the process was successfully predicted through a kinetic approach in a two-stage model with some simple and measurable parameters, which makes the model useful for predicting, controlling and optimizing the catalyzed oxidation process in the degradation of TCP.

Catalysis↗

PubMed2003

The system design of atrazine oxidation by catalytic oxidation process through a kinetic approach.

A design method for the removal of atrazine (ATZ) by catalytic oxidation process (COP) was developed though examination of the reaction kinetics in various conditions. The reaction kinetics of ATZ decay in COP are found to consist of a reaction and a stagnant stage, while two characteristic constants, the initial decay rate and the oxidative capacity of the COP, are successfully derived and used to describe the two stages, respectively. The proposed model and the way to solve the two characteristic constants (the initial decay rate and the oxidative capacity) are capable of describing the ATZ degradation precisely in COP at different Fe(II) concentrations and [Fe(II)]/[H(2)O(2)] ratios, where the marginal [Fe(II)]/[ATZ] ratio at 10 is found to be the critical point to achieve a cost-effective treatment of ATZ by COP. A series of equations for practical completely mixed batch reactor design is also developed based on the two characteristic constants.

Atrazine↗

PubMed2003

Photodegradation of 2,4-dichlorophenoxyacetic acid in various iron-mediated oxidation systems.

The oxidation of herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) by different iron-mediated processes, with or without the presence of ultraviolet (at 253.7 nm) and oxalate, was investigated and compared. The initial decay rate and the overall removal percentage were used as the performance indexes. To extensively explore the associated processes, the following combinations or blank systems were investigated: UV radiation only, Fe(2+)/H(2)O(2), Fe(2+)/H(2)O(2)/UV, ferrous oxalate/H(2)O(2), ferrous oxalate/H(2)O(2)/UV, Fe(3+)/H(2)O(2), Fe(3+)/H(2)O(2)/UV, ferrioxalate/H(2)O(2), and ferrioxalate/H(2)O(2)/UV. This study showed that the degradation of 2,4-D by sole UV or dark processes (without UV) is generally slow, except by the conventional Fenton's process (Fe(2+)/H(2)O(2)). However, these slow reactions can be accelerated by exposure to UV irradiation, which can increase the initial 2,4-D decay rate from ten to more than one hundred times. Furthermore, if the reaction is initiated by ferrous oxalate or ferrioxalate instead of Fe(2+) or Fe(3+) ions, the rates can be further improved, because of the higher light sensitivity of the organometallic complexes. These reactions were also found sensitive to the initial hydrogen peroxide concentration. The competition of hydroxyl free radicals by the primary intermediate, 2,4-dichlorophenol, was also observed.

2,4-Dichlorophenoxyacetic Acid↗

PubMed2003

A disappearance model for the prediction of trichlorophenol ozonation.

The disappearance and modeling of the ozonation of 2,4,6-trichlorophenol (TCP) was studied under different initial TCP concentrations and initial pH levels. The ozonation of TCP was found to follow a pseudo-first-order reaction. The degradation rates increased with the initial pH, and decreased with initial TCP concentration. 2,6-Dichlorohydroquinone was identified as the major intermediate, indicating that dechlorination and hydroxylation co-occurred during TCP ozonation. A model was proposed to quantitatively predict the pseudo-first-order rate constants under different initial TCP concentration and different initial pH levels. The proposed model can successfully describe the reaction; therefore another practical equation was proposed to predict the TCP removal rate at any detention time, which has high potential for practical applications and reactor design.

Chlorophenols↗

PubMed2003

The direct photolysis and photocatalytic degradation of alachlor at different TiO2 and UV sources.

Direct photolysis and photocatalytic degradations of alachlor, a widely used herbicide, were studied using three different monochromatic UV lamps (254, 300 and 350 nm) and two TiO(2) sources. Both the direct photolysis and photocatalytic degradations of alachlor follow pseudo-first-order decay kinetics. TiO(2)-P25 was found to be an effective photocatalyst compared to TiO(2)-BDH. The direct photolysis of alachlor was dominant at 254 nm even if TiO(2) was present in the solution. Among the three UV wavelengths used, the highest photocatalysis quantum yield was obtained at 300 nm. The photocatalytic degradation rate of alachlor increased with the dosages of TiO(2), but an overdose of TiO(2) would retard the reaction due to light attenuation. Photocatalytic reactions were slightly enhanced in an alkaline medium, and the different proton sources causing various degrees of rate retardation were due to the presence of the corresponding counter anions. This effect was diminished at a later stage after the reaction intermediates were formed.

Acetamides↗

PubMed2003

Modeling the reaction kinetics of Fenton's process on the removal of atrazine.

The degradation of pesticide, atrazine (ATZ), 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine, by Fenton's reagent (FR) was investigated as a function of reagents' concentrations and ratios in a batch reactor. The degradation of ATZ was effectively achieved by hydroxyl radicals, which were generated in the FR process. The decay rates of ATZ and the oxidation capacities of FR were found to depend on the concentrations of hydrogen peroxide and ferrous ion. The removal kinetics of ATZ are initiated by a rapid decay and then followed by a much slower one. After an extended reaction time (5-10 min), the reactions ceased because the Fe(II) and H(2)O(2) were consumed and would be deactivated in the process. A mathematical model was successfully developed to describe the two-stage reaction kinetics by using two simple but critical parameters: the initial ATZ decay rate and the final oxidation capacity of Fenton's process. In general, higher [Fe(II)] or H(2)O(2) concentrations result in faster initial decay rate and higher oxidation capacity. However, the oxidation capacity is more sensitive to the initial [Fe(II)] due to the presence of side reactions as discussed in the paper.

Atrazine↗

PubMed2003

Remediation of contaminated soil by a solvent/surfactant system.

This study investigates a new approach using a solvent/surfactant-aided soil-washing process to improve the performance of conventional surfactant-aided soil remediation. Three surfactants (Brij 35, Tween 80, and SDS) and three organic solvents (acetone, triethylamine, and squalane) were used to evaluate the desorption performances of 4,4'-dichlorobiphenyl (DCB) out of three soils with different sorption characteristics. The performance improvement is likely due to better dissolution of the hydrophobic contaminants from the soil assisted by the solvent, and the formation of solvent-incorporated surfactant micelles, which increases both the size (i.e. capacity) and affinity of micelles for more effective contaminant extraction. The foc of soils were found to be important in determining the performance of a solvent/surfactant-aided soil-washing process. Judging from the experimental data and as verified by the two constants in the proposed soil-washing model, as the organic solvent is coexisting with the surfactant micelles, both the marginal soil-washing performance (right after the use of a very small amount of solvent compared to that of none) and the final soil-washing capacity are increased compared to those of a pure surfactant-aided washing process.

Detergents↗