A newly green photocatalyst support for azo dye remediation under UV light irradiation
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A new cellulosic material “corn silk” was modified with titanium dioxide nanoparticles as a novel photocatalyst support. In this study, the prepared support was tested for the removal of Reactive Black 5 (RB5) as an azo dye pollutant candidate from synthetic samples. High capability of decolorization (> 99%) was achieved after 30 s using the corn silk/TiO2 photo-biocatalyst. The effect of important parameters such as pH of the medium, the amount of photocatalyst, mixing rate and dye concentration was investigated and modified. UV–Vis spectroscopy, scanning electron microscopy (SEM), X-ray powder diffraction and Fourier-transform IR spectrometry were applied to characterize the effect of functionalization, structure, surface morphology and photocatalyst properties of the support and mineralization of pollutants. It was observed that the maximum decolorization of RB5 occurred at pH 3.0, 25 °C, 300 rpm, 30 s using the corn silk/TiO2 composite material for this study. The results reveal that corn silk/TiO2 composite has high and significant photocatalytic activity.
KeywordsCorn silk Titanium (IV) oxide NPs Textile dye Reactive Black 5 UV photodegradation
Excessive production of pollutants with increasing rate threatens the global environment seriously. On the other hand, by increasing public awareness, revolutionary approach to overcome to the problems of the environment is taking place. Finding appropriate solutions and valuable ideas that utilize economic policies, healthy and environmentally friendly methods, without destructive side effects with maximum response in removing pollutants of ecosystem is on the agenda of scientists. Among the wide spectrum of environmental pollutants, especially water contaminants, textile dyes have a larger stake. Remediation of dye-contaminated wastewater released from the textile and other dye industries is necessary to prevent pollution of soil, surface and ground water (Erturk 2010).
More than 0.7 million tons of organic dyes are produced each year globally. It is noted that there are over 10,000 commercially accessible dyes that are classified by their utilization fields, namely acid, reactive, disperse, vat, metal complex, mordant, direct, basic and sulfur dyes. Reactive dyes have been identified as the most environmentally problematic compounds in textile dye effluents for several reasons. First, reactive dyes are intensively used due to their preferable performance and their increasing market share. Second, they are very soluble and approximately 10–15% of the weight of applied reactive dyes is discharged from the dye houses. Third, conventional wastewater treatment units, which reliance on sorption and aerobic biodegradation, have a low removal output for reactive and other anionic soluble dyes. Consequently, they cause to be colored water channels and public complaints, and the pollutants are being transferred to another phase and not being eliminated (Dojcinovic et al. 2012).
Currently, various methods such as physical, chemical, physicochemical and biological treatments (Erturk 2010; Dojcinovic et al. 2012; Laohaprapanon et al. 2015), advanced oxidation (Wu et al. 2008) and electrochemical oxidation such as anodic oxidation (Dhaouadi et al. 2009) and electro-Fenton processes (Ramirez et al. 2013) are used to decolorization of this wastewater. Biological techniques are the most common used methods in textile industries as they have low processes cost. Nevertheless, due to complex molecular structure of dyes, aerobic degradation can only remove lesser degree of color. In addition, poor anaerobic degradation of azo dye releases toxic and potentially carcinogenic aromatic amine compounds in the treated waste (Laohaprapanon et al. 2015).
Enzymatic process is another method used for the removal of dyes. However, this technique is not effective as enzymes are inhibited at different pHs, temperatures and by inhibitors (Yanmis et al. 2013). Advanced oxidation processes (AOPs) are widely recognized as highly effective methods for obstinate wastewater treatment. AOPs destroy organic pollutants by forming hydroxyl radicals (Shoabargh et al. 2014). Titanium dioxide nanoparticles have attracted particular interest in AOPs due to their wide applications in various fields and extra benefits such as low cost, high photocatalytic activity and high stableness in comparing with other photocatalysts such as SnO2 and ZnO (Shoabargh et al. 2014). Immobilization of nanoparticles on a proper support can avoid the requirement for the separation of the surplus amount of photocatalyst from sewage. Supports may be organized into macro-particles, such as sand and glass beads (Khataee et al. 2009), glass tubes surrounding the light source in photoreactor (Ling et al. 2004) and some polymers (Mahmoodi et al. 2006; Kasanen et al. 2011).
Cellulosic materials can be a good support for immobilization of photosensitive nanoparticle, such as TiO2, ZnO and SnO2 due to their porosity, abundance, low cost, ease of usage, nontoxic, reusability and replacement. Despite these advantages, application of cellulosic material as support has been understudied. Luffa sponge as a cellulosic source was introduced and modified by ZnO nanoparticles (Nadaroglu et al. 2017). In here, we report the immobilization of TiO2 NPs on corn silk as a novel cellulosic photosensitive support. Corn silk is a fibrous, biodegradable and nontoxic material. We investigated the effect of operational parameters, including dye concentration, mixing speed, amount of CS/TiO2 nanophotocatalyst and pH on dye decolorization. Moreover, to evaluate the performance of prepared photosensitive support, decolorization of Reactive Black 5 under UV-C irradiation was studied in batch reactor and compared with previously reported processes.
Materials and methods
General characteristics of Reactive Black 5 dye
Reactive Black 5 (RB15)
Molar mass (g mol−1)
The UV–Vis spectra of dye solutions were recorded from 300 to 900 nm using an Epoch Microplate Spectrophotometer. Decolorization of dye was determined by flowing the dye concentration using their maximum absorbance (597 nm) in a UV–Vis spectrophotometer (Epoch Microplate Spectrophotometer). pH of samples was adjusted by Crison model pH meter. Immobilization of TiO2 nanoparticles on the texture of corn silk was performed in the Ultrasonic bath (Kudos SK 10GT Model). Bachman Coulter model centrifuge was used to separate the solid photocatalyst from solution. The photocatalytic degradation of RB5 on the CS immobilized nano-TiO2 was performed under a 15 W/50 HZ UV-C lamp. The distance between the lamp and the phocatalyst reactor was 15 cm. The surface morphology of CS, TiO2, CS/TiO2 NPs and CS/TiO2/RB5 was monitored by scanning electron microscope (Zeiss Sigma 300 field emission SEM). X-ray diffractometer (XRD) of CS and TiO2 IML-CS, before and after dye treatment, was undertaken using a PANalytical Empyrean model XRD at Cu-Kα radiation (λ = 1.54 Å). The analysis of dried CS, TiO2 NPs-IML-CS and TiO2 IML-CS/RB5 was carried out by continuous scans from 10° to 100° at 2° scan rate at 2θ min−1 in ambient pressure.
FTIR analysis of RB5 dye, TiO2 and IML-CS with TiO2 NPs, before and after tests, was recorded using Vertex 80 Model FTIR Frontier spectrophotometer with attenuated total reflection (ATR) technique in the 4000–400 cm−1 region.
Supply and preparation of corn silk
Corn silk was supplied from local vendors. 25 g corn silk sample was weighed and added to 250 mL, 0.5 M NaClO solution. Then, this mixture was treated in a water bath (80 °C) for 1 h. Corn silk was washed thoroughly with 500 mL of pure water and then incubated in the same conditions with 1 M NaOH solution. Then, corn silk was completely washed with double distilled water and dried in oven at 60 °C for 8 h. The dried corn silk sample was fractionated with pure water in a steel blender. 0.02 g TiO2 NPs was then added to the obtained mixture and treated for 2 h in an ultrasonic bath at 60 °C for immobilizing TiO2 nanoparticles onto the cellulosic structure of corn silk. TiO2 immobilized corn silk was separated from the supernatant and washed 5 times with distilled water to remove unbound TiO2 NPs. The TiO2 immobilized corn silk support was dried in oven at 60 °C for 8 h and used in all the experiments.
Preparation of dye solutions
The stock solutions of RB5 were prepared in 50 mg L−1 concentration and diluted with deionized water. The pH of the solution was adjusted with diluted HCl or NaOH solutions.
Removal (%) is dye removal efficiency, C0 (mg L−1) is the initial dye concentration, and Ct (mg L−1) is concentration of dyes at t time.
Effect of various parameters on dye removal
Degradation of RB5 dye was monitored by measuring absorbance at 597 nm. In order to determine the contact time, photodegradation reaction in UV system was followed for 10 min. Samples were taken at regular intervals from the reaction medium, measured against distilled water and degradation % was calculated using Eq. (1).
Also, the effects of stirring speed and medium pH on the photodegradation of RB5 were investigated. For this purpose, pH of RB5 azo dye was adjusted using 0.01 N HCl/NaOH solutions in the range of 3–10. At each pH, the change in absorbance was monitored at 597 nm by establishing the same experiments.
Effect of stirring rate
To study the effect of stirring speed on degradation of dye, 0.4 g corn silk was added to 25 mL RB5 azo dye at 50 mg L−1 concentration solution and reactions were occurred at 25 °C pH 3.0 at 100, 200 and 300 rpm. Absorbance was recorded for each speed at 597 nm against distilled water.
Effect of amount of support material
The different reactions were performed to investigate the effect of support material onto decolorization of RB5 using 0.1 and 0.8 g TiO2 NPs-IML-CS.
Effect of dye concentration
The effect of dye concentration on the photocatalytic degradation of RB5 azo dye was investigated by setting up the reactions using the same conditions (pH 3.0, 25 °C, 300 rpm) and at the broad spectrum of RB5 dye concentrations: 25, 50, 75, 100, 150 and 200 mg L−1.
Reusability tests for TiO2 NPs immobilized corn silk
The elimination of RB5 was performed in the presence of 0.4 g CS/TiO2 NPs photocatalyst in 10 cycles reactions to assess the potential reusability of TiO2 IML-CS-NPs at an initial dye concentration of 50 mg L−1. Before each cycle, TiO2 IML-CS-NPs pieces were washed three times with distilled water. All experiments were carried out in triplicates, and the data are presented as mean value.
Results and discussion
Characterization of support material
Effect of reaction time
Effect of pH
Effect of stirring rate
Effect of dye concentration
Effect of photocatalyst amount on degradation efficiency
Reusability of bio-photocatalyst
A novel cellulosic photocatalyst material corn silk–TiO2 was introduced and successfully applied for degradation of Reactive Black 5 (RB5) as a model azo dye under UV irradiation in batch reactors. High decolorization percentage (99.96%) was achieved in a very short time (30 s). XRD patterns showed the high loaded amount of TiO2 nanoparticle on cellulosic texture of corn silk and good treated with dye during photoreaction. Based on the UV–Vis and FTIR spectroscopy results, decolorization of RB5 confirmed the cleavage of azo bonds (N=N) of dye molecule. According to these results, the best reaction condition for RB5 degradation (%) was determined as 0.4 g/50 mL TiO2 NPs-IML-CS, 50.0 mg L−1 initial dye concentration, pH 3.0, mix rate 300 rpm and 25.0 °C temperature. The recycling of TiO2 NPs-IML-CS was performed and found to be adequately used up to six times. This new photocatalyst, in comparison with others, offers higher quality and efficiency.
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