Decolorization and mineralization of an azo reactive dye using loaded nano-photocatalysts on spacer fabric: Kinetic study and operational factors

Highlights

• The potential of loaded TiO₂ and ZnO nano-photocatalysts for the decolorization of an azo reactive dye was investigated.

• Optimum decolorization conditions have been determined.

• Complete decolorization and high efficient mineralization were achieved at 120 min treatment in the case of ZnO:TiO_2.

• Photocatalytic decolorization rates obeyed the first-order kinetic.

Abstract

In this study, the photocatalytic decolorization and mineralization of Remazol Black B (RBB), an azo reactive dye, in aqueous solutions was investigated using UV/H₂O₂/ZnO, UV/H₂O₂/TiO₂ and UV/H₂O₂/ZnO:TiO₂ systems. ZnO and TiO₂ nanoparticles were loaded on 3-dimensional polyethylene terephthalate fabrics (spacer fabrics). Morphology of the spacer fabrics and the presence of the nanoparticles were studied by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), respectively. Furthermore, the effects of key operational parameters on the efficiency of the decolorization were investigated. These parameters included initial pH value, initial hydrogen peroxide concentration, initial dye concentration, the loaded nanoparticle ratio and the presence of anions (sulfate, chloride and bicarbonate). Zero-, first- and second-order reaction kinetics were evaluated. Complete decolorization and high efficient mineralization with 90% total organic carbon (TOC) reduction were achieved at 120 min treatment in the case of ZnO:TiO₂ under optimum condition. The results proved that the novel heterogeneous photocatalytic process is capable of decolorizing and mineralizing azo reactive dyes in textile wastewater.

Introduction

The discharge of textile wastewater containing various organic and mineral pollutants such as dyes and pigments into natural streams and rivers causes serious environmental problems. In fact, most of the dyes owing to their conjugated aromatic compounds and complicated chemical structure are toxic to the aquatic living organisms [1], [2], [3], [4], [5].

Among the broad range of the dyes used in textile industry, reactive dyes due to their favorable characteristics such as low energy consumption in the dyeing process, color brightness and water fastness, have found extensive application and they constitute almost 50% of the annual worldwide production of the dyes [6].

Some reactive dyes, such as RBB which are composed of one or more azo bonds (NN) and aromatic rings are categorized as xenobiotic compounds. Additionally, because the fixation efficiency of azo dyes is estimated to fall in the range of 60% to 90%, their loss in the dyeing process causes environmental concerns [7], [8], [9].

The conventional methods for wastewater decolorization such as chemical, physical or biological methods only transform dyes from aqueous to another phase, which consequently results in secondary pollution [10], [11], [12], [13], [14], [15], [16], [17]. Therefore, post-treatments of the solid wastes, which are costly operations, are required [18], [19].

Currently, advanced oxidation processes (AOPs) have been developed as alternative methods for decolorization of dyes and other contaminated wastewaters. The main purpose of AOPs is to transfer all organic compounds to water, carbon dioxide and mineral acids [20]. Among different methods of AOPs, heterogeneous photocatalytic processes which use metal oxide semiconductor (MOS) nanoparticles such as TiO₂ and ZnO are considered as the most effective approach in degrading and mineralizing organic compounds in wastewater [21]. The fundamental of this technique is based on the irradiation of ultraviolet (UV) light on MOS to produce reactive species such as hydroxyl radical (OH^•) which oxidize a broad range of organic pollutants non-selectively [22], [23].

Although TiO₂ is the most frequently used photocatalyst for a wide range of organic compounds, it exhibits some disadvantages [24], [25]. Among other semiconductors, ZnO represents an apt alternative candidate owing to the similarity of its photo-degradation mechanism to TiO₂. Furthermore, it has been substantiated by some reports that ZnO is more efficient than TiO₂ under certain conditions whose reason might be attributed to its higher quantum efficiency [26]. Photocatalytic processes incorporating semiconductor nanoparticles involve two typical approaches including suspension of the nanoparticles in solution and immobilization of the nanoparticles on surface. Amidst those, the immobilization method is preferred, because the catalyst separation and recycling from the treated wastewater prior to discharge are eliminated [27], [28].

There has been some reports on the immobilization of nano-photocatalysts onto solid materials such as glass wool, glass beads, glass reactors, microporous cellulosic membranes, ceramic membranes, monoliths, zeolites, and stainless steel [29]. However, the immobilization of nanoparticles on 3-dimensional polyethylene terephthalate fabrics (spacer fabrics) for photocatalytic processes has not been reported yet.

Spacer fabrics can be defined as complex 3-dimensional constructions composed of two separated layers of fabric which are connected with spacer yarns for whose production weaving and knitting technologies are generally required. The 3-dimensional structures of these fabrics make them markedly different from their conventional counterparts. Owing to the low possibility of physical or chemical absorption of the reactive dyes on polyethylene terephthalate (PET), spacer fabrics made of PET were selected in the current study. Furthermore, it is noteworthy to mention that passing of wastewater through the two layers of the spacer requires more time which leads to provide sufficient time for contact with the nanoparticles and consequently the photocatalytic process [30], [31], [32].

The main purpose of this study is to investigate the photocatalytic decolorization and mineralization of RBB using ZnO and TiO₂ nanoparticles loaded on spacer fabric. The effects of the operational parameters on efficiency of the decolorization were evaluated to determine optimum condition. Moreover, the kinetics of the photocatalytic decolorization were investigated.