Treating industrial wastewater with solar energy

In the so-called Fenton reaction, an iron cation, Fe(II), gets oxidized to Fe(III), while hydrogen peroxide (H₂O₂) gets reduced to OH⁻ and the hydroxyl radical HO^•, which reacts with a great many organic substances. In water, the Fe(III) eventually converts back to Fe(II), and the process can begin again. Although chemically very efficient at removing organic pollutants from water, the Fenton reaction requires UV or visible radiation (below 400 nm) to efficiently reconvert Fe(III) to Fe(II) in the last step. A team of researchers in Brazil, led by Claudio Nascimento of the University of São Paulo, has now explored the feasibility of using sunlight instead of expensive UV lamps to expedite that crucial step in the process. Using demonstration reactors, such as the parabolic-trough reactor shown here, the scientists ran a series of experiments, typically for several hours in the afternoon under various sky conditions. They found that in the photo-Fenton process, sunlight worked effectively at degrading various pollutants, including silicone polymers from the textile industry, pesticides, phenols, and hydrocarbons. Overall, no new pollutants are created, residual Fe(III) can be precipitated as iron hydroxide, and residual H₂O₂ spontaneously decomposes into water and molecular oxygen. (C. A. O. Nascimento et al. , ASME J. Sol. Energy Eng. 129 , 45, 2007 .)