The Hoffmann research group made major contributions to our understanding of the role of water in the atmosphere controlling the chemical transformation pathways for acidic gas-precursors, sulfur dioxide and nitrogen oxides, and the subsequent acidification of the atmosphere. They also defined the role of ammonia as an atmospheric base in neutralizing the impact of inorganic acid formation (sulfuric and nitric acids) leading to the formation of hygroscopic ammonium sulfate and ammonium nitrate aerosols that lead to haze aerosol formation in polluted urban environments (e.g., New Delhi, Beijing, Mexico City). The Hoffmann group’s discovery of hyper acidic clouds and fogs in the Los Angeles, San Francisco, and San Joaquin Valley, led to major California state laws requiring 1) a switch from using high-sulfur fuels for power generation to the use of natural gas (i.e., CH4) as the only acceptable fossil fuel source statewide, and 2) to California and national United States regulations requiring more stringent control of the emission of SO2 and NOx in order to control atmospheric acidification at urban, rural, and continental scales. In addition, the Hoffmann research group made seminal contributions in unraveling the kinetics and detailed mechanisms of transformation of SO2 oxidation by ozone, hydrogen peroxide, and oxygen in water as controlled by the pH of clouds and other atmospheric water droplets. On a global basis, we now know that up to 90% of total sulfur dioxide oxidation to sulfuric acid occurs in clouds, fogs, and haze aerosols. In the past, it was thought that sulfur dioxide was controlled exclusively by the gas-phase reaction with hydroxyl radical.
Professor Hoffmann was elected to the US National Academy of Engineering in recognition of his major contributions to the exploration and implementation for advanced oxidation and reduction technologies for the remediation and control recalcitrant environmental pollutants. Professor Hoffmann advocated for the use of hydrogen peroxide, ozone, peroxydisulfate, peroxydiphosphate, peroxone, peroxymonosulfate, and semiconductor photocatalysis as major options for pollution control based on his fundamental laboratory studies. Before 1980, the principal oxidants for water pollution control in the United States were chlorine in the form hypochlorus acid/hypochlorite and potassium permanganate. Today, hydrogen peroxide, ozone, peroxone, and persulfate are routinely used for groundwater remediation and for tertiary water treatment and for direct potable water reuse of wastewater in Orange County/Los Angeles, CA and Singapore. Many of the Hoffmann research group most highly cited publications are in the area of the use of nano-particulate semiconductors for the photocatalytic oxidation and reduction of water and atmospheric contaminants. Professor Hoffmann was one of a small group of investigators advocating for the use of titanium dioxide photocatalysis in the late 1970’s and early 1980’s. Since that time there has been an explosion of university and industrial research into the use of inexpensive metal oxide semiconductors for pollution control. For example, during the timeframe of 2014-2016, more than 2200 ISI publications appeared in the primary literature under the search words "titanium dioxide photocatalysis" for water pollution control.
The Hoffmann research group has developed small-scale human wastewater treatment systems involving a combination of biochemical and electrochemical treatment in order to achieve onsite decentralized pollution control. These units can be used for human waste treatment in urban, rural, peri-urban, and remote regions of the world. Since 2011, the Hoffmann group has received major funding from the Gates Foundation to further develop the concept of photovoltaic-powered treatment systems for use in the developing world in order to reduce the problem of open defecation and consequent transmission of water-borne diseases due to surface and groundwater contamination, which often lead to numerous early deaths in Africa and India. More than one billion people in the developing world practice open defecation due to the lack of access to proper sanitary facilities. In recognition of the progress that the Hoffmann group has made in utilizing integrated engineering approaches for human wastewater treatment on a small scale, they were award a Gates Foundation Prize for Reinventing the Toilet in 2012. Since that time, Professor Hoffmann along with colleagues at Tsinghua have been able to form a joint-venture company based in Yixing, China for the further development and manufacturing of decentralized onsite wastewater treatment systems. The joint-venture company, Eco-San, is now manufacturing integrated toilet and waste treatment systems for use by the Chinese National Tourism Authority in China at tourist sites in Southern China and for use in South Africa at elementary, middle, and high schools in remote locations of the East London district. In addition, the semiconductor electrodes developed in the Hoffmann Lab and licensed by Eco-San are now being produced in Yixing for worldwide sales and distribution. The joint venture company, the Yixing Eco-san Sanitation Manufacturing Co., Ltd involves support from Caltech, Tsinghua, the City of Yixing, and the Jiangsu Institute for Environmental Technology. The integrated biological and electrochemical treatment strategy employed by Eco-San allows for the onsite treatment of human waste by near total elimination of the chemical oxidation demand (COD), complete denitrification, and disinfection of microorganisms via the electrochemical formation of reactive chlorine species. After treatment, clean water is disinfected and recycled internally as flushing water for toilets.