Showing papers by "Michael R. Hoffmann published in 1993"

Photocatalytic degradation of pentachlorophenol on titanium dioxide particles: identification of intermediates and mechanism of reaction

Abstract: The photoassisted oxidation of pentachlorophenol (PCP) in TiO_2 particle suspensions was investigated. Complete dechlorination of 47 µM PCP was achieved after 3 h of illumination at high intensity with apparent quantum efficiencies (ΦPCP, ΦCI, ΦH+, ΦH_2O_2) ranging from 1 to 3%. p-Chloranil, tetrachlorohydroquinone, H_2O_2, and o-chloranil were formed as the principal intermediates. Formate and acetate were formed as products during the latter stages of photooxidation. The mechanism for photooxidation of PCP proceeds via hydroxyl radical attack on the para position of the PCP ring to form a semiquinone radical which in turn disproportionates to yield p-chloranil and tetrachlorohydroquinone. Under high-intensity illumination, the reaction intermediates are attacked further by •OH to yield HCO_2^-, CH_3CO_2^-, CO_2, H^+, and Cl^-.

Photoreduction of iron oxyhydroxides in the presence of important atmospheric organic compounds

Abstract: The photolytic reduction of amorphous iron hydroxide [am-Fe(OH)_3], lepidocrocite (γ-FeOOH), goethite (α-FeOOH), hematite (α-Fe_2O_3), and natural iron-containing aerosol particles in the presence of formaldehyde, formate, acetate, oxalate, and butyrate has been investigated. Important parameters in the photoreduction experiments are the pH, wavelength of the irradiating light, nature of the electron donor, characteristics of the iron phase. The present results show that the fastest rates of photoreduction of Fe(III) to Fe(II) are achieved with am-Fe(OH)_3 as the electron acceptor and formate as the electron donor. Maximum rates of photoreduction were observed at 330 nm with a continuous decrease to 405 nm. Natural iron-containing aerosol particles show photochemical behavior similar to am-Fe(OH)_3 and γ-FeOOH. These results suggest that a significant fraction of the reactive atmospheric iron in urban aerosol could be present as am-Fe- (OH)_3 and γ-FeOOH. Ambient iron-containing aerosol particles with oxalate as the electron donor resulted in a significant photochemical production of H_2O_2.

Redox chemistry of iron in fog and stratus clouds

Abstract: The redox chemistry of Fe in fog and cloudwater has been investigated at coastal and inland locations in the Los Angeles basin, in Bakersfield California, and in Delaware Bay. Samples were collected and analyzed for Fe (Fe(II)), Fe(III), total(Fe), sulfur (S(IV), S(VI)), organic ligands (formate, acetate, oxalate), total organic carbon (TOC), pH, major cations (sodium, calcium, magnesium, potassium, ammonium), chloride, sulfate, nitrate, peroxides, and aldehydes (HCHO); the amount of sunlight was also measured. The ratio Fe(II)/Fe(total) varied between 0.02 and 0.55. The concentration of Fe(II) varied between 0.1 and 5 micromole, and the concentration of total Fe varied between 2 and 27 micromole. The atmospheric redox cycle of Fe involves both dissolved and aerosol surface species and appears to be related to the presence of organic compounds which act as electron donors for the reduction of Fe(III). Fe(III) reduction is enhanced by light but significant Fe(II) levels were observed in the dark. We suggest that reduction of Fe(III) species by organic electron donors may be an important pathway that affects the speciation of Fe in both urban and rural atmospheres. It is possible that reactions involving Fe and organic compounds might be an important source of carboxylic acids in the troposphere.

Chemical Pollution of the Environment: Past, Present and Future

Abstract: In an era of ‘global environmental change’ people are concerned about emissions of CO_2, CH_4, N_2O and chlorofluorocarbons (CFCs) to the atmosphere because of their direct impact on global warming and their stratospheric ozone-depleting effects. Unprecedented efforts have been made to reduce the global emissions of CFCs. Major industries, which are competing within the modern global economy, have recognized the importance of maintaining a ‘green’ perspective. Future operations will be designed to reduce the direct emissions of chemical by-products to air, water and soil, and to recycle and to reuse critical solvents such as water. ‘Star Wars’ technologies for the rapid, economical and effective elimination of industrial and domestic wastes will be developed and employed on a large scale. Advanced technologies for the control and monitoring of chemical pollutants on regional and global scales will be developed and implemented. Satellite-based instruments will be able to detect, to quantify, and to monitor a wide range of chemical pollutants. Our understanding of the fate and consequences of chemicals in the environment will increase dramatically such that we shall be able to predict the environmental, ecological and biochemical consequences of novel synthetic molecules with much greater precision.