Journal of Environmental Chemical Engineering
Dyes decolorization using silver nanoparticles supported on nanometric diamond as highly efficient photocatalyst under natural Sunlight irradiation
Graphical abstract
Introduction
Advanced oxidation processes (AOPs) are among the most useful technologies for the remediation of waste waters containing recalcitrant contaminants or resisting pathogens [1], [2], [3]. AOPs are based on the generation of reactive oxygen species such as highly aggressive hydroxyl radicals, superoxide or others, able to oxidize organic contaminants to less-toxic biodegradable intermediates or even to produce their partial or complete mineralization [1]. For example, AOPs have been extensively applied to the treatment of textile waste waters characterized by the production of large volumes of waste water containing colored, toxic and/or mutagenic dyes [4], [5]. It has been estimated that about of 15% of the produced dyes are released in textile waste waters [6]. Color and chemical oxygen demand (COD) removal represents one major environmental concern in textile waste waters due the inefficacy of conventional biological treatments [4], [7].
The photo-Fenton reaction is one of the most popular AOPs due its easy implementation and effectiveness in the production of hydroxyl radicals [8], [9]. The reaction proceeds via a redox cycle according to Eqs (1) and (2) leading to the formation of HO· radicals. The main drawbacks the photo-Fenton reaction increasing the operation costs and, therefore, hampering its general application to waste water remediation, are: i) the need of acidic pH values to obtain the correct iron speciation (Eqs (1) and (2)) and to avoid iron oxides precipitation; ii) the need of iron removal after the chemical treatment with the subsequent sludge formation; iii) the need of artificial UV irradiation source to promote the photolysis of the Fe(OH)2+ complex (Eq. (2)) and the existence of undesirable side reactions decreasing the selectivity towards HO· formation.
Nowadays, there is an increasing interest on developing heterogeneous photo-Fenton catalysts able to reduce the operating cost as well as to overcome some of the commented drawbacks of the homogeneous process. Currently, the solid-based catalysts employed as heterogeneous photo-Fenton materials include zeolites [8], [10], clays [10], pillared clay-based materials [10], [11], metal oxides [8], nanosized iron oxides [12], supported metal[13], [14] or metal oxide nanoparticles[15] and graphenes [16], [17]. Nevertheless, there is still room for improvement basic aspects of the heterogeneous process including minimization of the H2O2 excess reducing undesirable decomposition, optimization of H2O2 and catalyst to substrate molar ratios, maximization the H2O2 to catalyst molar ratio by developing highly efficient (photo)catalysts, expanding the operation pH towards neutral or slightly basic values, use of natural Sunlight irradiation, among others [8]. In this context, we have reported that Au or Ag NPs supported on treated commercial diamond nanoparticles (D) modified by introducing surface hydroxyl groups is an efficient photocatalyst for degradation of phenol as well as its reaction intermediates (hydroquinone, catechol and p-benzoquinone) using less than 5.5 equivalents of H2O2 under natural Sunlight irradiation[13], [18] Therefore, it is of interest to evaluate the scope of these Fenton catalysts by testing their efficiency in the degradation of other recalcitrant contaminants, particularly decolorization of synthetic dyes that are frequently employed in the textile industry [10].
In the present work we describe the use of Ag NPs supported on surface hydroxylated D as photocatalyst to effect the decolorization and COD removal of four common synthetic dyes present in textile waste water effluents namely Methylene Blue (MB), Orange II (OII), Acid Red 1 (AR1) and Rhodamine B using H2O2 as oxidant and natural Sunlight irradiation. The activity of this catalyst is compared with analogous Au NPs supported on the same hydroxylated D or Ag NPs supported on nanometric diamond NPs, graphite, activated carbon as well as on the benchmark TiO2 photocatalyst under natural or simulated sunlight. The purpose is to establish how the pretreatment of the support influences the photocatalytic activity of AgNPs by controlling the average particle size and the surface functional groups. It would be shown that treating the supports under Fenton conditions followed by reduction with H2 renders the most active supports due to the small particle size and the lower density of surface OH groups. The catalytic data presented compare favorably with those reported in the literature for iron based catalysts that undergo extensive metal leaching and typically operate with H2O2 excesses of one thousand equivalents or more. In the case of Ag, the absence of metal leaching and its high activity can make the use of this metal, in low loading, an attractive alternative compared to other metals [19].
Section snippets
Materials
MB, OII, AR1, RhB, H2O2 solution in water (30%, w/w), HNO3 (65%), NaOH (>97%) were supplied by Sigma-Aldrich. The other reagents or solvents were of analytical or HPLC grade
Catalyst preparation
The catalysts employed in the present work have been previously prepared and fully characterized [18]. The carbonaceous materials employed as support of gold and/or silver NPs include commercial diamond NPs, activated carbon and graphite. The commercial carbon supports were also functionalized by a homogeneous Fenton
Photocatalysts tested
The present work includes the use of eleven photocatalyts based on Au or Ag NPs supported on different carbonaceous supports as well as on TiO2-P25 as benchmark photocatalyst (Table 1 and S1). In the case of Ag NPs supported on TiO2, the catalytic activity of Ag promoting generation of OH· from H2O2 would be combined with the well-known photocatalytic activity of semiconducting TiO2 generating ROS. In contrast to the activity of TiO2, the other solid supports do not exhibit any photocatalytic
Conclusions
In the present work we have demonstrated that Ag NPs supported on commercial D NPs previously functionalized by a homogeneous Fenton treatment followed by H2 annealing at high temperature is an efficient photo-Fenton catalyst to promote the decolorization of different dyes including MB, OII, AR and RhB under natural Sunlight irradiation with minimal H2O2 consumption and achieving remarkable detoxification levels. Ag/D3 as photocatalyst can be reused without undergoing metal leaching and with
Acknowledgements
Financial support by the Spanish Ministry of Economy and Competitiveness (Severo Ochoa, CTQ2012-32315 and CTQ2014-53292-R) is gratefully acknowledged. Generalidad Valenciana is also thanked for funding (Prometeo 2012/013).
European Commission has been gratefully acknowledged for granting Erasmus Mundus Action-2 (SVAAGATA) fellowship to the first author for carrying out this work in Department of Chemistry, Universitat Politecnica de Valencia, Valencia, Spain.
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