Summary of Research
Summary of Research Accomplishment of Prof. Hoffmann: Areas related to environmental science and technology.
In the subject area of pollution control technologies, Hoffmann and his group have made advances in our understanding of a) the basic chemistry and applications of homogeneous and heterogeneous catalysis involving metal-phthalocyanine complexes, b) in the chemistry and application of hydrogen peroxide for the oxidative elimination of hydrogen sulfide and other reduced sulfur compounds in water and wastewater, c) in the chemistry and application of quantum-sized semiconductor colloids for the oxidative and reductive elimination of chlorinated hydrocarbons, d) in the examination of the underlying chemistry and physics of the application of ultrasonic irradiation in water for the destruction of chemical contaminants, e) for the development and advancement of the pulsed-power plasma process for water treatment, f) for the development and advancement of metal-doped semiconductor electrodes for the electrochemical production of hydroxyl radical from water, g) for the development and characterization of semiconductor-coated fiber optic cable reactors for the heterogeneous photochemical destruction of chemical contaminants in water, and for h) the application of photochemically-activated periodate solutions for chemical compound oxidation.
Hoffmann’s detailed investigations of the kinetics and mechanism of the oxidation of dissolved sulfur dioxide established the state-of-the-art for our understanding of important pathways for the oxidation of sulfur dioxide to sulfuric acid in the atmosphere. This fundamental reaction is now recognized as the single most important pathway for the conversion of sulfur dioxide to sulfuric acid on a global basis. As much as 80% of the total sulfur dioxide on a global basis is oxidized via the cloud-processing pathway originally proposed by Hoffmann in 1975.
Hoffmann and his group carried out a number of field-oriented studies of the chemistry of aerosols, clouds and fogs. The primary focus of these studies was to obtain solid field-based observations of dynamic chemical changes within liquid water droplets and to very predictions of chemical kinetic models for cloudwater acidification. In this regard, Hoffmann and co-workers developed several patented devices for the automatic time-series collection of cloud and fog water from ground-based sampling stations. In 1982, Hoffmann group reported in Science the detailed chemical composition of hyper-acidic clouds and fogs in which the ambient pH values were found to be as low as pH 1.5 in coastal marine clouds and fogs.
During the course of their field studies, Hoffmann and his students discovered that many cloud systems in near urban environments were enriched in dissolved sulfur dioxide (i.e., S(IV)) and aldehydes such as formaldehyde, methyl glyoxal, glyoxal, and hydroxyacetaldehyde. Based on their initial observations the Hoffmann group proposed that bisulfite and the aldehydes react I within cloud droplets via a classical reaction to form reversibly aldehyde-bisulfite adducts or hydroxyalkylsulfonates (i.e., these compounds also form in sulfite preserved beer and wine) as reservoir species for S(IV). Hoffmann and his group developed a new analytical method for the direct chromatographic determination of the hydroxyalkylsulfonates in ambient samples. They also reported this discovery in Science in 1986. The Hoffmann group continues to investigate the environmental and health implications of the chemistry and physics of aerosols and atmospheric water droplets such as haze aerosols51-67.
Hoffmann and his research group carried out a detailed series of investigations in the photochemistry and photophysics of colloidal metal oxide semiconductor systems that could be used for the effective elimination of chemical contaminants from water or for the in situ production of hydrogen peroxide. This research led to the development of hybrid photocatalytic systems involving the chemical coupling of Co(II)tetrasulfophthalocyanine to the surface of TiO2 to produce an unusually high reactivity photocatalysts.
Research in the area of metal- ion doping of the quantum-size semiconductor photocatalysts led directly to the successful development of niobium (V)-doped TiO2-coated titanium anodes for the direct electrochemical production of hydroxyl radical from the oxidation of hydroxide ion and water with current efficiencies approaching 98%. This novel electrochemical system led to the granting of 4 patents and to direct commercialization of the electrochemical reactor system for water and wastewater treatment applications.
Further advances in the application of semiconductor photocatalysis by the Hoffmann group; include the development of TiO2-coated fiber optic cable reactors for solar-light photocatalysis. These novel photoreactor systems employ quartz fiber-optic cables to deliver focused solar irradiation over long-distances before entering into an active reaction zone in which the quartz of plastic optical fibers or wave guides are coated with quantum-dot TiO2 or visible-light absorbing metal-doped TiO2. As the propagated light reaches the semiconductor coating, the UV and visible light is refracted out of the coated fibers and into the successive layers of TiO2. This invention has been patented. Potential applications for this type of photocatalytic reactor include the in situ treatment of contaminated groundwater, for the treatment of contaminated air streams, and for recycled air purification on commercial airliners. This technology has also been explored for chemical and biowarfare agent control.
The Hoffmann group continues to explore the applications of electrohydraulic cavitation for the elimination of hazardous chemical compounds from water. Acoustic cavitation and electrohydraulic discharges have been applied by the Hoffmann group for the elimination of chemical contaminants from water such as carbon tetrachloride, pentachlorophenol, TNT, parathion, triethanolamine, and methyl tertiary-butyl ether. His research efforts have shown that the chemical pathways resulting from the violent collapse of cavitation bubbles include direct pyrolytic decomposition within the vapor-phase of the collapsing bubbles, oxidation by hydroxyl radical produced from the pyrolytic decomposition of water, and by transient supercritical water reactions resulting from the extremely high temperatures and pressures generated within a bubble during acoustic cavitation.
For a number of years, the Hoffmann group also explored the use of various microbial systems employing such as Thiobacillus ferroxidans and Thiobacillus thiooxidans for the desulfurization of coal and for the extraction of iron from iron-ores using dissimilatory iron-reducing bacteria. The Hoffmann group carried out detailed kinetic and mechanistic studies of the removal of inorganic sulfur such as iron pyrite and organic sulfur in the form of thiophenes from high-sulfur coals and the extraction of ferrous iron from a variety of iron ores using iron-reducing bacteria such as Shewanella putrifaciens.