Application of a novel rGO-CuFeS2 composite catalyst conjugated to microwave irradiation for ultra-fast real textile wastewater treatment
Graphical abstract
Introduction
Dyes are compounds used in the manufacturing of various products such as textiles, detergents, papers, food, cosmetics and paints. This versatility for industrial applications generates large amounts of dye contaminated effluents, which can cause environmental and public health problems. Dyes must be stable to the light, to washing processes and uniformly fixate to the substrate, being also difficult to treat. The high concentration of dyes in the discharged industrial wastewaters causes a decrease of the sunlight penetration in water and reduction of the solubility of gases in the aquatic environments, lead to the modification of the biota [[1], [2], [3], [4]]. Also, since these residues might be carcinogenic, an adequate treatment before discharged into the environment is of highest priority [5].
In the last years, advanced oxidation processes have been used as a safe and effective alternative for the treatment of complex and hazardous wastewaters [6]. The use of energy sources in these processes, such as ultrasound and ultraviolet light irradiation, improve degradation reaction rates. As a novel trend, the use of microwave (MW) irradiation is presented as a mild energy source, capable of providing thermal and non-thermal effects [7]. MW assisted reactions becomes an even more attractive method for degrading organic pollutants when combined with a catalyst containing excellent MW absorption properties, where synergistic effects can be observed. Furthermore, MW irradiation contribute with uniform heating and formation of hot spots, a specific form of energy absorption [8].
Previous studies have reported successful use of iron [9] and copper based [10] catalysts in MW assisted treatments. Chalcopyrite is a mineral that combines both Fe and Cu in a crystalline structure, such as CuFeS2. As a catalyst, it leads to the formation of non-selective hydroxyl radicals (OH), by accelerating H2O and H2O2 decomposition under acidic medium in short time frames, which already occurs by MW irradiation itself [8,9,[11], [12], [13], [14]]. Moreover, CuFeS2 presents low optical energy band gap of 0.5–0.6 eV, being very interesting for these applications [[15], [16], [17], [18], [19], [20]]. However, CuFeS2 presents small specific surface area, about 3 to 5 m2 g−1 [21], which can affect hot spots formation as well as the whole process. The hot spots are generated by the temperature difference at the reaction site and in the medium. This facilitates the destruction of complex chemical bonds, decreasing the activation energy and increasing the reaction rate [22].
An alternative to solve this problem consists on coupling of CuFeS2 to other material containing large pores and high surface area. An excellent material is reduced graphene oxide (rGO), due to its high electronic mobility and conductivity, high chemical stability and specific surface area, leading to greater adsorption capacity [[23], [24], [25]]. When combined with inorganic semiconductor catalysts, the different Fermi levels of the rGO and inorganic semiconductors generate a strong heterojunction electric field at the interfaces. The interfacial electric field promotes the rapid transfer of generated electrons to the surface of the rGO and, therefore, prolongs the life of the electron-hole generated pairs [23,24,26].
Many studies have described the combination of inorganic semiconductor catalysts and rGO, resulting in formation of the graphene-based composites with superior catalytic activity [18,[23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]]. However, the preparation of rGO-CuFeS2 composite as well as its performance combined to MW irradiation towards textile wastewater treatment has not been reported yet. Therefore, rGO-CuFeS2 composites with different weight ratios of rGO and CuFeS2 were prepared. Its catalytic efficiencies were evaluated for degradation of real textile wastewater (RW) from a factory in Mar del Plata (Argentina), containing the azo dye Direct Black 22 (DB22), under different experimental conditions (pH, H2O2, concentration, composite amount) in coupling with MW irradiation, in order to obtain optimal reaction parameters. Finally, a phytotoxicity test of the treated RW to Lactuca sativa was applied.
Section snippets
Synthesis of rGO-CuFeS2 composite
Graphite oxide was synthesized from graphite flake (Sigma-Aldrich) by a modified Hummers method [34]. Exfoliation of graphite oxide to graphene oxide (GO) was achieved by sonication for particles dispersion. The obtained brown dispersion was then subjected to centrifugation to remove any unexfoliated graphite oxide. GO reduction was obtained by the addition of N2H4·H2O. The as-prepared GO solution was diluted in deionized water under ultrasonication and vigorous stirring. The product obtained
rGO-CuFeS2 characterization
XRD patterns of rGO, CuFeS2 and composites are shown in Fig. 1. A typical diffraction broad peak with low intensity appears at 26° for the rGO sample [23,32,40]. In addition, as it is present in a low proportion, only small shifts can be observed in the patterns obtained for the composites. The CuFeS2 presents characteristics peaks at 29.37, 48.97 and 48.68°, corresponding to those given in the reference pattern (ICDD PDF 35−0752), as in published research [[17], [18], [19],41].
Results obtained
Conclusions
In this work, a new rGO-CuFeS2 composite was synthesized and successfully applied to the treatment of RW under MW irradiation. The novelty of this work lies in the use of real textile wastewater, without a pretreatment, without additional carbon sources and undiluted, using an unprecedented composite for this application coupling microwave irradiation. MW energy is insufficient to produce the cleavage of chemical azo bonds of the DB22 dye present in the RW. However, MW energy in acidic medium
CRediT authorship contribution statement
Yasmin Vieira: Conceptualization, Formal analysis, Investigation, Methodology, Validation, Writing - original draft. María Belén Ceretta: Conceptualization, Formal analysis, Funding acquisition, Investigation, Methodology, Validation, Writing - original draft. Edson Luiz Foletto: Writing - review & editing. Erika Alejandra Wolski: Funding acquisition, Investigation, Methodology, Project administration, Supervision, Writing - original draft, Writing - review & editing. Siara Silvestri:
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are immensely grateful to Júlia da Silveira Salla for providing the CuFeS2 and the financial support provided by CNPq, CAPES/FAPERGS (Bolsista CAPES/BRASIL: 88887.195036/2018-00), CONICET (PIP 0101/2015), National University of Mar del Plata (15/G560-2019) and Montevideo Group by Graduate Student Scale Program scholarship (PEEPg). Maria Belén Ceretta is a fellow of CONICET.
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