ORCID Profile
0000-0002-3657-2882
Current Organisation
UNSW Sydney
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Publisher: Elsevier BV
Date: 06-2014
DOI: 10.1016/J.FREERADBIOMED.2014.03.021
Abstract: Although quinones represent a class of organic compounds that may exert toxic effects both in vitro and in vivo, the molecular mechanisms involved in quinone species toxicity are still largely unknown, especially in the presence of transition metals, which may both induce the transformation of the various quinone species and result in generation of harmful reactive oxygen species. In this study, the oxidation of 1,4-naphthohydroquinone (NH2Q) in the absence and presence of nanomolar concentrations of Cu(II) in 10 mM NaCl solution over a pH range of 6.5-7.5 has been investigated, with detailed kinetic models developed to describe the predominant mechanisms operative in these systems. In the absence of copper, the apparent oxidation rate of NH2Q increased with increasing pH and initial NH2Q concentration, with concomitant oxygen consumption and peroxide generation. The doubly dissociated species, NQ(2-), has been shown to be the reactive species with regard to the one-electron oxidation by O2 and comproportionation with the quinone species, both generating the semiquinone radical (NSQ(·-)). The oxidation of NSQ(·-) by O2 is shown to be the most important pathway for superoxide (O2(·-)) generation with a high intrinsic rate constant of 1.0×10(8)M(-1)s(-1). Both NSQ(·-) and O2(·-) served as chain-propagating species in the autoxidation of NH2Q. Cu(II) is capable of catalyzing the oxidation of NH2Q in the presence of O2 with the oxidation also accelerated by increasing the pH. Both the uncharged (NH2Q(0)) and the mono-anionic (NHQ(-)) species were found to be the kinetically active forms, reducing Cu(II) with an intrinsic rate constant of 4.0×10(4) and 1.2×10(7)M(-1)s(-1), respectively. The presence of O2 facilitated the catalytic role of Cu(II) by rapidly regenerating Cu(II) via continuous oxidation of Cu(I) and also by efficient removal of NSQ(·-) resulting in the generation of O2(·-). The half-cell reduction potentials of various redox couples at neutral pH indicated good agreement between thermodynamic and kinetic considerations for various key reactions involved, further validating the proposed mechanisms involved in both the autoxidation and the copper-catalyzed oxidation of NH2Q in circumneutral pH solutions.
Publisher: Elsevier BV
Date: 07-2015
Publisher: CSIRO Publishing
Date: 2008
DOI: 10.1071/SR07119
Abstract: Acid sulfate soils (ASS) are distributed worldwide on coastal floodplains, presenting a great challenge to coastal development and urbanisation. Upon oxidation, these soils become stratified with visibly distinguishable soil strata that are progressively less oxidised with depth. In this study, the geotechnical properties, quantified by hydraulic conductivity and consolidation coefficient, of an ASS profile from the Tweed River floodplain, north-eastern New South Wales, Australia, were investigated at a laboratory scale and compared with results obtained from the field. Measurements were conducted with a Rowe cell (or hydraulic consolidometer) by controlled compressive and pore water pressures. The results indicated that hydraulic conductivity and consolidation coefficient values gradually decreased with increasing consolidation pressure or decreasing void ratio, but were significantly higher for the more oxidised ASS horizons. These results suggest that controlled soil consolidation along ASS drainage banks may prove to be effective at reducing acid discharge. Passing low pH (pH 3) or high cation concentration (50 mm CaCl2) solutions through intact consolidated potential ASS s les did not induce changes in the hydraulic conductivity or consolidation coefficient of this material indicating that ASS soil ripening involves more than acidification reactions, and the practice of flushing drains with high ionic strength estuarine tidal waters is unlikely to induce soil subsidence as a result of ASS structural change and clay flocculation.
Publisher: Elsevier BV
Date: 08-2014
DOI: 10.1016/J.JINORGBIO.2014.03.018
Abstract: Spontaneous oxidation of dopamine (DA) and the resultant formation of free radical species within dopamine neurons of the substantia nigra (SN) is thought to bestow a considerable oxidative load upon these neurons and may contribute to their vulnerability to degeneration in Parkinson's disease (PD). An understanding of DA oxidation under physiological conditions is thus critical to understanding the relatively selective vulnerability of these dopaminergic neurons in PD and may support the development of novel neuro-protective approaches for this disorder. In this study, the oxidation of dopamine (0.2-10μM) was investigated both in the absence and the presence of copper (0.01-0.4μM), a redox active metal that is present at considerable concentrations in the SN, over a range of background chloride concentrations (0.01-0.7M), different oxygen concentrations and at physiological pH7.4. DA was observed to oxidize extremely slowly in the absence of copper and at moderate rates only in the presence of copper but without chloride. The oxidation of DA however was significantly enhanced in the presence of both copper and chloride with the rate of DA oxidation greatest at intermediate chloride concentrations (0.05-0.2M). The variability of the catalytic effect of Cu(II) on DA oxidation at different chloride concentrations can be explained and successfully modeled by appropriate consideration of the reaction of Cu(II) species with DA and the conversion of Cu(I) to Cu(II) through oxygenation. This model suggests that the speciation of Cu(II) and Cu(I) is critically important to the kinetics of DA oxidation and thus the vulnerability to degradation of dopaminergic neuron in the brain milieu.
Publisher: Elsevier BV
Date: 07-2015
Publisher: American Chemical Society (ACS)
Date: 06-2012
DOI: 10.1021/JP300995H
Abstract: The kinetics of Cu(II) reduction by Suwannee River fulvic acid (SRFA) at concentrations from 0.25 to 8 mg L(-1) have been investigated in 2 mM NaHCO(3) and 0.7 M NaCl at pH 8.0. In the absence of oxygen, SRFA reduced Cu(II) to Cu(I) in a biphasic manner, with initial rapid formation of Cu(I) followed by a much slower increase in Cu(I) concentration over time. When present, oxygen only had a noticeable effect on Cu(I) concentrations in the second phase of the reduction process and at high [SRFA]. In both the absence and presence of oxygen, the rate of Cu(I) generation increased with increasing [SRFA]. At 8 mg L(-1) [SRFA], nearly 75% of the 0.4 μM Cu(II) initially present was reduced to Cu(I) after 20 min, although the yield of Cu(I) relative to [SRFA] decreased at [SRFA] > 1 mg L(-1). Two plausible kinetic modeling approaches were found to satisfactorily describe the experimental data over a range of [SRFA]. Despite some uncertainty as to which approach is correct, common features of both approaches were complexation of Cu(II) by SRFA and reduction of Cu(II) by two different electron donor groups within SRFA: a relatively labile electron donor (with a concentration of 1.1 × 10(-4) equiv of e(-) (g of SRFA)(-1)) that reduced Cu(II) relatively rapidly and a less labile donor (with a concentration of 3.1 × 10(-4) equiv of e(-) (g of SRFA)(-1)) that reduced Cu(II) more slowly.
Publisher: Royal Society of Chemistry (RSC)
Date: 2018
DOI: 10.1039/C7EM00497D
Abstract: Release of dopamine by Ulvaria obscura var. blyttii may, allelopathically, assist Ulvaria growth by H 2 O 2 generation and benefit the bloom region through alleviation in iron stress and by mitigating the effects of other oxidative metabolites.
Publisher: Elsevier BV
Date: 08-2011
Publisher: Elsevier BV
Date: 07-2018
Publisher: American Chemical Society (ACS)
Date: 29-05-2008
DOI: 10.1021/JP801126P
Abstract: The kinetics of Fe(II) oxidation in the presence of low concentrations of citrate and salicylate have been investigated in aqueous solutions over the pH range 6.0-8.0 using colorimetry. A kinetic model has been developed to describe the oxidation of Fe(II) with specific attention given to the oxidation of inorganic Fe(II), formation and dissociation of Fe(II) complexes and the oxidation of these complexes. At low concentrations of salicylate, both experimental data and model show that the common approach to modeling Fe(II) oxidation that assumes pre-equilibrium between metal and ligand prior to their oxidation is not valid. Complexation of Fe(II) by salicylate is found to be relatively slow, and oxidation of the complex formed occurs rapidly. Citrate, on the other hand was found to be in rapid equilibrium with Fe(II) but the complex formed was oxidized slowly. Both citrate and salicylate complexes are found to dissociate at a rate much faster than previously thought. A model of the oxidation kinetics of Fe(II) species that incorporates the formation and dissociation kinetics of Fe(II) and Fe(III) complexes of citrate and salicylate as well as the reactions of these species with oxygen and reduced oxygen species including superoxide and hydrogen peroxide provides an excellent description of data obtained over a wide range of concentration and pH conditions.
Publisher: American Chemical Society (ACS)
Date: 25-01-2011
DOI: 10.1021/ES103757C
Abstract: Contemporary studies indicate that reactive oxygen species (ROS) such as superoxide play a key role in the toxicity and behavior of silver nanoparticles (AgNPs). While there have been suggestions that superoxide is able to reduce silver(I) ions with resultant production of AgNPs, no experimental evidence that this process actually occurs has been produced. Here we present definitive experimental evidence for the reduction of silver(I) by superoxide. A second-order rate constant of 64.5 ± 16.3 M(-1)·s(-1) is determined for this reaction in the absence of AgNPs. The overall rate constant, however, increases by at least 4 orders of magnitude in the presence of AgNPs. A model based on electron charging and discharging of AgNPs satisfactorily describes the kinetics of this process. The ability for AgNPs to undergo catalytic cycling provides a pathway for the continual generation of ROS and the regeneration of AgNPs following oxidation.
Publisher: American Chemical Society (ACS)
Date: 04-03-2011
DOI: 10.1021/JP111275A
Publisher: Elsevier BV
Date: 05-2009
Publisher: Elsevier BV
Date: 11-2018
DOI: 10.1016/J.JINORGBIO.2018.08.007
Abstract: The oxidation of hydroquinones is of interest both due to the generation of reactive oxygen species (ROS) and to the implications to trace metal redox state. Menadione (MNQ), a typical toxicant quinone used extensively for studying the mechanisms underlying oxidative stress, is known to be an effective source of exogenous ROS. In this study, the kinetics and mechanism of the oxidation of menadiol (MNH
Publisher: Frontiers Media SA
Date: 26-11-2018
Publisher: Elsevier BV
Date: 02-2006
Publisher: Elsevier BV
Date: 08-2008
Publisher: American Chemical Society (ACS)
Date: 18-10-2016
Abstract: Increasing concentrations of dissolved silicate progressively retard Fe(II) oxidation kinetics in the circum-neutral pH range 6.0-7.0. As Si:Fe molar ratios increase from 0 to 2, the primary Fe(III) oxidation product transitions from lepidocrocite to a ferrihydrite/silica-ferrihydrite composite. Empirical results, supported by chemical kinetic modeling, indicated that the decreased heterogeneous oxidation rate was not due to differences in absolute Fe(II) sorption between the two solids types or competition for adsorption sites in the presence of silicate. Rather, competitive desorption experiments suggest Fe(II) was associated with more weakly bound, outer-sphere complexes on silica-ferrihydrite compared to lepidocrocite. A reduction in extent of inner-sphere Fe(II) complexation on silica-ferrihydrite confers a decreased ability for Fe(II) to undergo surface-induced hydrolysis via electronic configuration alterations, thereby inhibiting the heterogeneous Fe(II) oxidation mechanism. Water s les from a legacy radioactive waste site (Little Forest, Australia) were shown to exhibit a similar pattern of Fe(II) oxidation retardation derived from elevated silicate concentrations. These findings have important implications for contaminant migration at this site as well as a variety of other groundwater/high silicate containing natural and engineered sites that might undergo iron redox fluctuations.
Publisher: American Chemical Society (ACS)
Date: 28-10-2019
Abstract: Flow-electrode capacitive deionization (FCDI) is an emerging electrochemically driven technology for brackish and/or sea water desalination with merits of large salt adsorption capacity, high flow efficiency, and easy electrode management. While FCDI holds promise for continuous operation, there are very few investigations with regard to the regeneration/reuse of flowable electrodes and the separation of brine from electrodes with these operation prerequisites for real nonintermittent water desalination. In this study, we propose a novel module design to achieve these critical steps involving integration of an FCDI cell and a ceramic microfiltration (MF) contactor. Our investigations reveal that the brine discharge rate is the dominant factor for stable and efficient operation of the integrated module. Results obtained show that the integrated FCDI/MF system can be used to successfully separate brackish water (of salinities 1, 2 and 5 g L
Publisher: Wiley
Date: 12-05-2016
DOI: 10.1111/JNC.13615
Publisher: American Chemical Society (ACS)
Date: 10-07-2013
DOI: 10.1021/ES4014344
Abstract: A detailed kinetic model has been developed to describe the oxidation of Cu(I) by O2 and the reduction of Cu(II) by 1,4-hydroquinone (H2Q) in the presence of O2 in 0.7 M NaCl solution over a pH range of 6.5-8.0. The reaction between Cu(I) and O2 is shown to be the most important pathway in the overall oxidation of Cu(I), with the rate constant for this oxidation process increasing with an increasing pH. In 0.7 M NaCl solutions, Cu(II) is capable of catalyzing the oxidation of H2Q in the presence of O2 with the monoanion, HQ(-), the kinetically active hydroquinone form, reducing Cu(II) with an intrinsic rate constant of (5.0 ± 0.4) × 10(7) M(-1) s(-1). Acting as a chain-propagating species, the deprotonated semiquinone radical (SQ(•) (-)) generated from both the one-electron oxidation of H2Q and the one-electron reduction of 1,4-benzoquinone (BQ) also reacts rapidly with Cu(II) and Cu(I), with the same rate constant of (2.0 ± 0.5) × 10(7) M(-1) s(-1). In addition to its role in reformation of Cu(II) via continuous oxidation of Cu(I), O2 rapidly removes SQ(•) (-), resulting in the generation of O2(•) (-). Agreement between half-cell reduction potentials of different redox couples provides confirmation of the veracity of the proposed model describing the interactions of copper and quinone species in circumneutral pH saline solutions.
Publisher: Royal Society of Chemistry (RSC)
Date: 2018
DOI: 10.1039/C7DT04373B
Abstract: While the application of Asc alone may aggravate the progression of PD in view of the possible peroxidation of Asc bound Fe( ii ), a combination therapy of Asc and strong clinically appropriate iron chelator would appear to be a promising direction for the treatment of PD as a result of the enhanced iron chelation and attenuation in oxidative stress and toxicity induced by DA derived quinones.
Publisher: American Chemical Society (ACS)
Date: 2008
DOI: 10.1021/JP077219L
Abstract: The kinetics of Fe(II) oxidation in the presence of various citrate concentrations have been investigated in aqueous solutions over the pH range 6.0-8.0 using colorimetry and speciation modeling. Oxidation of Fe(II) was interpreted and quantitatively modeled in terms of oxidation of various Fe(II)-citrate species. Using the model, it is possible to predict whether the presence of citrate would dominate the Fe(II) oxidation and thus enhance/retard the oxidation rate of Fe(II) and vice versa. The study also supports the presence of other Fe(II)-citrate species rather than just the monomeric species at circumneutral pH. At low pH and in a system where complexation of Fe(II) by citrate is dominant, oxidation of Fe(II) is controlled by the oxidation of both Fecit- and Fecit24-. As the pH increases, the oxidation of Fe(OH)cit25- becomes increasingly important and dominates the oxidation of Fe(II) at pH 8.0. Rate constants for the oxidation of all five suggested Fe(II)-citrate species have been estimated and may be used to predict the rate of Fe(II) oxidation at any combination of pH and citrate concentration.
Publisher: American Chemical Society (ACS)
Date: 30-01-2018
DOI: 10.1021/ACSCHEMNEURO.7B00478
Abstract: Elevation in iron content as well as severe depletion of dopamine (DA) as a result of iron-induced loss of dopaminergic neurons has been recognized to accompany the progression of Parkinson's disease (PD). To better understand the mechanism of the mitigating effect of the iron chelator deferiprone (DFP) on PD, the interplay between iron and DFP was investigated both in the absence and presence of DA. The results show that DFP was extremely efficient in scavenging both aqueous iron and iron that was loosely bound to DA with the entrapment of iron in Fe-DFP complexed form critical to halting the iron catalyzed degradation of DA and associated generation of toxic metabolites. The DFP related scavenging of dopamine semiquinone (DA
Publisher: Elsevier BV
Date: 05-2013
Publisher: Elsevier BV
Date: 05-2012
Publisher: American Chemical Society (ACS)
Date: 13-01-2012
DOI: 10.1021/ES203394K
Abstract: The oxidation kinetics of nanomolar concentrations of Cu(I) in NaCl solutions have been investigated over the pH range 6.5-8.0. The overall apparent oxidation rate constant was strongly affected by chloride, moderately by bicarbonate, and to a lesser extent by pH. In the absence of bicarbonate, an equilibrium-based speciation model indicated that Cu(+) and CuClOH(-) were the most kinetically reactive species, while the contribution of other Cu(I) species to the overall oxidation rate was minor. A kinetic model based on recognized key redox reactions for these two species further indicated that oxidation of Cu(I) by oxygen and superoxide were important reactions at all pH values and chloride concentrations considered, but back reduction of Cu(II) by superoxide only became important at relatively low chloride concentrations. Bicarbonate concentrations from 2 to 5 mM substantially accelerated Cu(I) oxidation. Kinetic analysis over a range of bicarbonate concentrations revealed that this was due to formation of CuCO(3)(-), which reacts relatively rapidly with oxygen, and not due to inhibition of the back reduction of Cu(II) by formation of Cu(II)-carbonate complexes. We conclude that the simultaneous oxygenation of Cu(+), CuClOH(-), and CuCO(3)(-) is the rate-limiting step in the overall oxidation of Cu(I) under these conditions.
No related grants have been discovered for An Ninh Pham.