Richard Angel Valentine - Academia.edu (original) (raw)

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Papers by Richard Angel Valentine

Research paper thumbnail of The electron transport system in nitrogen fixation by Azotobacter. III. Requirements for NADPH-supported nitrogenase activity

Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1971

Research paper thumbnail of Release of Pb(II) from Monochloramine-Mediated Reduction of Lead Oxide (PbO2)

Environmental Science & Technology, 2008

A contributing factor causing the sudden release of excessive lead into drinking water is believe... more A contributing factor causing the sudden release of excessive lead into drinking water is believed to involve the change in redox conditions occurring when monochloramine (NH 2 Cl) replaces free chlorine as a disinfectant. Studies suggest that NH 2 Cl cannot effectively oxidize Pb(II) to form PbO 2 , a Pb(IV) mineral scale formed from the oxidation of metallic lead and Pb(II) species by free chlorine. Unexpectedly, we observed that NH 2 Cl is actually capable of reducing PbO 2 to form Pb(II). We systematically investigated this reaction by varying important water chemistry factors such as solution pH, total carbonate concentration, and the Cl/N molar ratio to control chloramine speciation and its rate of decomposition via a complex set of autodecomposition reactions. The amount of Pb(II) formed was found to be proportional to the amount of NH 2 Cl that autodecomposed regardless of the rate of this reaction. This implies that the rate of Pb(II) release is proportional to the absolute rate of NH 2 Cl decomposition. It is proposed that the species responsible for the reduction of PbO 2 is likely a reactive intermediate produced during the decay of NH 2 Cl. This finding is the first to report that NH 2 Cl can act as a reductant.

Research paper thumbnail of Role of superoxide anion in contaminant degradation by hydrogen peroxide in the presence of sandy aquifer material

ABSTRACT The decomposition of hydrogen peroxide (H{sub 2}O{sub 2}) in the presence of metal oxide... more ABSTRACT The decomposition of hydrogen peroxide (H{sub 2}O{sub 2}) in the presence of metal oxides has frequently been described by a modified Haber-Weiss mechanism, which involves a redox reaction where H{sub 2}O, can accept an electron from a metal surface site (Eq. 1) or donate an electron to a surface site (Eq.2), S + H{sub 2}O{sub 2} {r_arrow}S{sup +} + OH{sup -} + {lg_bullet}OH S{sup +} + H{sub 2}O{sub 2} {r_arrow} S + H{sup +} + {lg_bullet}HO{sub 2} where [S] represents a reduced surface site and [S+] represents an oxidized surface site respectively. These reactions involve the formation of hydroxyl radical (OH{lg_bullet}) and perhydroxyl radical ({lg_bullet}HO{sub 2}). in addition, superoxide anion (O{sub 2}{sup -}) is present because it is in equilibrium with {lg_bullet}HO according to: {lg_bullet}HO{sub 2}{longleftrightarrow} H{sup +} + O{sub 2}- pKa=4.8. The hydroxyl radical is a non-specific powerful oxidant, capable of reacting with a number of organic compounds at near diffusion-limited reaction rates. Consequently, several researchers have studied surface catalyzed H{sub 2}O{sub 2} decomposition for the purpose of organic contaminant degradation. For example, trichloroethylene and pentachlorophenol were effectively degraded in column experiments with H{sub 2}O{sub 2} and a coarse sand. The purpose of this study was to examine the role of superoxide anion in the degradation of contaminants in materials similar to aquifer materials.

Research paper thumbnail of The electron transport system in nitrogen fixation by Azotobacter. III. Requirements for NADPH-supported nitrogenase activity

Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1971

Research paper thumbnail of Release of Pb(II) from Monochloramine-Mediated Reduction of Lead Oxide (PbO2)

Environmental Science & Technology, 2008

A contributing factor causing the sudden release of excessive lead into drinking water is believe... more A contributing factor causing the sudden release of excessive lead into drinking water is believed to involve the change in redox conditions occurring when monochloramine (NH 2 Cl) replaces free chlorine as a disinfectant. Studies suggest that NH 2 Cl cannot effectively oxidize Pb(II) to form PbO 2 , a Pb(IV) mineral scale formed from the oxidation of metallic lead and Pb(II) species by free chlorine. Unexpectedly, we observed that NH 2 Cl is actually capable of reducing PbO 2 to form Pb(II). We systematically investigated this reaction by varying important water chemistry factors such as solution pH, total carbonate concentration, and the Cl/N molar ratio to control chloramine speciation and its rate of decomposition via a complex set of autodecomposition reactions. The amount of Pb(II) formed was found to be proportional to the amount of NH 2 Cl that autodecomposed regardless of the rate of this reaction. This implies that the rate of Pb(II) release is proportional to the absolute rate of NH 2 Cl decomposition. It is proposed that the species responsible for the reduction of PbO 2 is likely a reactive intermediate produced during the decay of NH 2 Cl. This finding is the first to report that NH 2 Cl can act as a reductant.

Research paper thumbnail of Role of superoxide anion in contaminant degradation by hydrogen peroxide in the presence of sandy aquifer material

ABSTRACT The decomposition of hydrogen peroxide (H{sub 2}O{sub 2}) in the presence of metal oxide... more ABSTRACT The decomposition of hydrogen peroxide (H{sub 2}O{sub 2}) in the presence of metal oxides has frequently been described by a modified Haber-Weiss mechanism, which involves a redox reaction where H{sub 2}O, can accept an electron from a metal surface site (Eq. 1) or donate an electron to a surface site (Eq.2), S + H{sub 2}O{sub 2} {r_arrow}S{sup +} + OH{sup -} + {lg_bullet}OH S{sup +} + H{sub 2}O{sub 2} {r_arrow} S + H{sup +} + {lg_bullet}HO{sub 2} where [S] represents a reduced surface site and [S+] represents an oxidized surface site respectively. These reactions involve the formation of hydroxyl radical (OH{lg_bullet}) and perhydroxyl radical ({lg_bullet}HO{sub 2}). in addition, superoxide anion (O{sub 2}{sup -}) is present because it is in equilibrium with {lg_bullet}HO according to: {lg_bullet}HO{sub 2}{longleftrightarrow} H{sup +} + O{sub 2}- pKa=4.8. The hydroxyl radical is a non-specific powerful oxidant, capable of reacting with a number of organic compounds at near diffusion-limited reaction rates. Consequently, several researchers have studied surface catalyzed H{sub 2}O{sub 2} decomposition for the purpose of organic contaminant degradation. For example, trichloroethylene and pentachlorophenol were effectively degraded in column experiments with H{sub 2}O{sub 2} and a coarse sand. The purpose of this study was to examine the role of superoxide anion in the degradation of contaminants in materials similar to aquifer materials.

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