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 Fe and HO, and reaction time) and of Cl and SO 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 Fe 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 Fe, 78 mg L HO, and 20 min of reaction), 94 % of decolorization was achieved. However, even under the these conditions, but in the presence of Cl and SO , 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 (Cl and SO). The results show that the presence of Cl is more deleterious, since sulfate-iron complexes are more reactive towards HO, and the SO 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. 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.

Strategy for Treating a Landfill Leachate by Integration of Physico-Chemical and Photo-Fenton Processes

“…36 The complexation of Fe 2+ /Fe 3+ by chloride and sulfate has been observed during degradation of the azo dye Direct Red 81 by Fenton process, causing a decrease in the efficiency of degradation when compared with experiments carried out in the absence of these inorganic anions. 40 Furthermore, Silva et al 7 reported the strong influence of chloride and sulfate on the consumption of H 2 O 2 , extending the time required for the treatment of sanitary landfill leachate by the photo-Fenton process. Meanwhile, no influence of nitrate ions has been observed during the treatment of sanitary landfill leachate using the photo-Fenton process.…”

Matrix Effects on the Degradation of Gemfibrozil, Hydrochlorothiazide, and Naproxen by Heterogeneous Photocatalysis

“…• ). 35 Thus, in the STP effluent matrix, the addition of H 2 O 2 is unnecessary. (19) Furthermore, using the optimal experimental conditions for the different aqueous matrices, new experiments were carried out by extending the irradiation time to 240 min for the DW and SW and 360 min for the STP effluent, aiming for higher degradation efficiencies (Figure 3).…”

Degradação de cloranfenicol por fotocatálise heterogênea em diferentes matrizes aquosas: avaliação dos parâmetros operacionais, toxicidade e identificação dos produtos de transformação

“…Este fato, assim como a necessidade de maior tempo de irradiação, demonstra a influência da composição da matriz no processo. LIN, 2017;MARSON et al, 2017;ISMAIL et al, 2018).…”

“…O ânion persulfato (S2O8 2¯) , o qual também pode ser gerado no meio na presença de radicais sulfato (Equação 52), também age como aceptor de elétrons (MARSON et al, 2017;ISMAIL et al, 2018). A adição de um agente oxidante ao meio como o H2O2 também é utilizada para prevenção da recombinação de elétron/ lacuna, uma vez que este reagente também é um forte aceptor de elétrons (AHMED et al, 2011).…”

“…Por estarem adsorvidos na superfície do TiO2, isto pode interferir nas reações do peróxido de hidrogênio com o elétron da banda de condução, uma vez que há também uma competição pelos sítios ativos do fotocatalisador, resultando em uma alta concentração de peróxido de hidrogênio no meio (ISMAIL et al, 2018). Desta forma, o consumo de H2O2 (Figura 34b) pode ser atribuído a reações paralelas entre este reagente e espécies radicalares, como os radicais hidroxila (Equação 24), radicais sulfato (Equação 53) ou radicais derivados de cloro (Equações 54 e 55), resultando assim em radicais menos reativos, como os radicais hidroperoxila (HO2 • ) e, portanto, em menores eficiências de degradação (MARSON et al, 2017).…”