title: Degradation of Direct Red 81 mediated by Fenton reactions: multivariate optimization, effect of chloride and sulfate, and acute ecotoxicity assessment.
abstract: The role of different operational parameters related to Fenton reactions (pH, concentration of Fe2+and H2O2, and reaction time) and of Cl-and SO4-was investigated in the degradation of the azo dye Direct Red 81, expressed in terms of its decolorization. The factorial design and Pareto's charts showed that only Fe2+concentration and pH influence the decolorization under the conditions evaluated. So, only these parameters were optimized using the response surface model. Under the best experimental conditions (initial pH 2.5, 11 mg L-1Fe2+, 78 mg L-1H2O2, and 20 min of reaction), 94 % of decolorization was achieved. However, even under the these conditions, but in the presence of Cl-and SO4-, a striking loss of efficiency was observed as the concentration of these ions was increased, due the formation of chloride- and sulfate-iron complexes and less reactive inorganic radicals (Cl2•-and SO4•-). The results show that the presence of Cl-is more deleterious, since sulfate-iron complexes are more reactive towards H2O2, and the SO4•-turns out to favor the degradation. On the other hand, the  negative effect of Cl-can be compensated by increasing the chloride concentration up to 300 mmol L-1. In addition, although a high degradation level has been obtained by monitoring the dye absorbance and by HPLC-UV, a low mineralization occurred, being generated degradation products of higher ecotoxicity to Vibrio fischeri, showing the need of subsequent studies to identify these compounds as well as the application of additional treatments aiming the complete mineralization of the dye.
authors.affiliation: Universidade Federal de Uberlândia, Instituto de Química, 38400-902, Uberlândia, MG, Brazil.
authors.forename: Eduardo O
authors.initials: EO
authors.lastname: Marson
authors.affiliation: Universidade Federal de Uberlândia, Instituto de Química, 38400-902, Uberlândia, MG, Brazil.
authors.forename: Vinícius A B
authors.initials: VAB
authors.lastname: de Paiva
authors.affiliation: Universidade Federal de Uberlândia, Instituto de Química, 38400-902, Uberlândia, MG, Brazil.
authors.forename: Bárbara R
authors.initials: BR
authors.lastname: Gonçalves
authors.affiliation: Universidade Federal de Uberlândia, Instituto de Química, 38400-902, Uberlândia, MG, Brazil.
authors.forename: Oswaldo
authors.initials: O
authors.lastname: Gomes Júnior
authors.affiliation: Universidade Federal de Uberlândia, Instituto de Química, 38400-902, Uberlândia, MG, Brazil.
authors.forename: Waldomiro
authors.initials: W
authors.lastname: Borges Neto
authors.affiliation: Universidade Federal de Uberlândia, Instituto de Química, 38400-902, Uberlândia, MG, Brazil.
authors.forename: Antonio E H
authors.initials: AEH
authors.lastname: Machado
authors.affiliation: Universidade Federal de Uberlândia, Instituto de Química, 38400-902, Uberlândia, MG, Brazil. alamtrovo@ufu.br.
authors.forename: Alam G
authors.initials: AG
authors.lastname: Trovó
mesh_terms: D048908:Aliivibrio fischeri / Q000187:drug effects; D001391:Azo Compounds / Q000737:chemistry* / Q000633:toxicity*; D002712:Chlorides / Q000737:chemistry*; D054750:Ecotoxicology; D004785:Environmental Pollutants / Q000737:chemistry / Q000633:toxicity; D006861:Hydrogen Peroxide / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000737:chemistry*; D015999:Multivariate Analysis; D010084:Oxidation-Reduction; D013431:Sulfates / Q000737:chemistry*
chemical_list: D001391:Azo Compounds; D002712:Chlorides; D004785:Environmental Pollutants; C045076:Fenton's reagent; D013431:Sulfates; C062246:Direct Red 81; D006861:Hydrogen Peroxide; D007501:Iron
keywords: Advanced oxidation process; Decolorization; Effluents; Inorganic radicals; Response surface methodology; Vibrio fischeri