Photo-FentonRemediationofWastewatersContainingSilicones: ExperimentalStudyandNeuralNetworkModeling By AntonioCarlosS.C.Teixeira, Roberto Guardani, and ClaudioA.O.Nascimento* The present work is aimed at the investigation of the photo-Fenton technology with regard to the remediation of diluted aqu- eous emulsions containing an aminosilicone polymer, in a bench-scale photochemical reactor. The experimental results show a strong interaction between temperature, light, Fe(II) and H 2 O 2 concentrations on the degradation process, which generates substances that might be readily biodegradable and/or a solid phase that is easily separated by simple mechanical operations. The neural network technique is an effective, simple approach to successfully modeling the photo-Fenton degradation system, in which thermal and photochemical reactions and related phenomena (such as solid precipitation) take place. The model might therefore be useful in process optimization, as well as in the design and scaleup of photochemical reactors for industrial application. 1Introduction Polydimethylsiloxanes {CH 3 -[Si(CH 3 ) 2 O] n -Si(CH 3 ) 3 } be- long to the class of compounds known as poly(organosilox- anes) or silicones. The presence of different monomer units, side chains and functional groups gives these polymers suit- able properties for a wide number of industrial processes and consumer products. Silicone-in-water emulsions are used as materials for fabric impregnation and water repel- lency treatments, as antifoaming agents, and in many person- al and household care products [1]. Silicones are found to be very resistant to enzymatic and oxidative attack under conventional wastewater treatment [2±7]. Monitoring of sewage sludge in municipal and indus- trial wastewater treatment facilities revealed high silicone concentrations (up to a few thousands parts per million on a dry weight basis), mainly associated with inputs from indus- trial sources [2, 4]. Silicone biodegradation is not expected to occur during sewage treatment in an aqueous system [2]. As a consequence, waterborne silicones will end up and accu- mulate in the environment, adsorb and deposit in sediments [8]. Silicone polymer fluids are therefore considered as en- vironmentally persistent substances [5, 9]. Their increased use and the need for effective treatment processes for sili- cone-containing wastewater has therefore become a matter of environmental importance. Several reactions are known that can rather unselectively oxidize a wide range of compounds with a diversity of chem- ical structures and functional groups. When applied to the degradation of pollutants, these reactions are usually termed advanced oxidation technologies (AOT) [10,11]. The com- mon AOT use low to moderate concentrations of inexpen- sive, environmentally compatible chemical reagents and, in favorable cases, lead to the complete oxidation of organic pollutants into carbon dioxide, water, and inorganic com- pounds of all heteroatoms other than oxygen. Among AOT, the Fenton reaction (based on hydrogen peroxyde and an Fe(II) salt) and especially the photochemically enhanced- Fenton (photo-Fenton) reaction are considered most prom- ising for the remediation of wastewater containing a variety of nonbiodegradable organic compounds [12, 13±15]. Oxida- tion of organic pollutants implies in most cases generation and subsequent reactions of hydroxyl radicals (HO ), which are short-lived, powerful oxidizing agents, capable of oxidiz- ing organic compounds by hydrogen abstraction, addition to unsaturated compounds, and electron transfer. In a simplified scheme, in the Fenton reaction Fe(II) is oxidized to Fe(III) and H 2 O 2 is reduced to hydroxide ion and hydroxyl radical: Fe 2 H 2 O 2 ! Fe 3 HO OH (R1) The hydroxyl radical is widely accepted in the literature as the main primary oxidizing species of the Fenton reaction [16]. In the absence of light, the ferric ion produced in reac- tion R1 is reduced back to ferrous ion: Fe 3 H 2 O 2 H 2 O ! Fe 2 H 3 O HO 2 (R2) The thermal reduction given by reaction R2 ultimately de- termines the overall rate at which the process proceeds. A more detailed description of these reactions, considering hy- drated iron complexes, including rate constants and redox potentials, is presented by Bossmann etal. [17]. Although chemically very efficient for the removal of or- ganic pollutants, the Fenton reaction slows appreciably after the initial conversion of Fe(II) to Fe(III). However, expo- sure to UV-visible light strongly accelerates both Fenton (H 2 O 2 /Fe(II)) and Fenton-like (H 2 O 2 /Fe(III)) reactions, im- proving the degradation rates of a variety of organic pollu- tants [10,18,19]. This enhancement has been explained by 800 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/ceat.200401976 Chem. Eng. Technol. 2004, 27, No. 7 ± [*] Dr. A. C. S. C. Teixeira, Prof. Dr. R. Guardani, Prof. Dr. C. A. O. Nascimento (oller@usp.br), Chemical Engineering Department, Univer- sity of So Paulo ± Av. Prof. Luciano Gualberto, travessa 3, 380, CEP 05508±900, So Paulo, SP, Brazil. Full Paper