Magnetic titanium/carbon nanotube nanocomposite catalyst for oxidative degradation of Bisphenol A from high saline polycarbonate plant effluent using catalytic wet peroxide oxidation
Graphical abstract
Introduction
Polycarbonate plant wastewater (PCW) is typically characterized by high levels of aromatic hydrocarbons and phenolic substances, such as Bisphenol A, as well as high concentrations of salinity, inorganic dissolved salts and total dissolved solids (TDS) [1], [2], [3]. Bisphenol A (2, 2-bis (4-hydroxyphenyl) propane, BPA), characterized by two phenolic rings joined together through isopropylidene as bridging group, is a main industrial chemical widely used in plastic industry as intermediate in the manufacturing of polycarbonate and in the production of epoxy resins and corrosion-resistant unsaturated polystyrene resins [4], [5]. The worldwide BPA production is estimated about 5.4 million tons in 2015 and exceeding 3.8 million tons every year [5], [6]. Due to its wide application and slow biodegradation, BPA concentrations are reported in the influent and effluents of wastewater treatment plants (WWTPs) in the range of 1.0–100 µg/L and 1–100 ng/L, respectively [4], [5], [6], [7], [8]. BPA is classified as “moderately toxic” to aquatic organisms and considered an endocrine disrupting chemical (EDC) because of the similar structure with the estrogen receptors. Bearing this in mind, some studies reported that owing to its tumor promoting properties, BPA may be reasonably anticipated to be a human carcinogen [5], [6], [9], [10].
The complexity and toxicity of the presence of highly toxic organic compounds, as well as the high salinity of PCW, justifies the inefficiency of conventional biological treatments. Thus, finding innovative and cost-effective alternative technologies for the treatment of these types of industrial wastewater is of utmost importance. Among the currently available alternative technologies, the so-called advanced oxidation processes (AOPs) have already shown to play a key role in the degradation of organic pollutants in the green wastewater treatment scenario. AOPs technologies are considered among the most efficient water purification technologies based on the ability to generate in-situ highly reactive free radical species (such as HO and SO4− radicals) for decomposition of recalcitrant compounds. The advantages of AOPs can be summarized to the simplicity of the required equipment, the operation at mild conditions and the capability to degrade a wide range of organic pollutants to the extent of mineralization [11], [12], [13], [14], [15], [16], [17].
In this sense, catalytic wet peroxide oxidation (CWPO) is an AOP based on the transfer of electrons from an appropriate catalyst to hydrogen peroxide (H2O2) molecules in order to accelerate its catalytic decomposition into hydroxide ions (OH−) and hydroxyl radicals (HO) (E0 = 2.73 V), as given in Eq. (1). Hydroxyl radical is considered a robust and non-selective oxidant in the AOPs technologies, responsible by the degradation of a wide range of organic compounds to harmless end-products, such as CO2 and H2O. CWPO was not only considered an economical viable and low-cost destructive technology, but also able to proceed with simple operation equipment under mild conditions (typically at low temperature and atmospheric pressure) and non-threatening to the environment. CWPO has been successfully applied in the degradation of a huge range of recalcitrant organic contaminants in wastewater [11], [18], [19], [20], [21], [22], [23], [24].
The classical Fenton oxidation process, one of the well-known and most effective homogenous AOPs, is considered a particular type of CWPO, which in this case, using the specific catalyst at the specific operating conditions, based on Eqs. (2), (3), the chemical reaction between homogenous Fe (II) and H2O2 under strong acidic condition (pH = 2.5–4) results in the formation of HO radicals.
The Fenton process suffer from important shortcomings, including the unavoidable loss of catalyst, requiring to further process the iron sludge generated at the end of the treatment, and a complicated final chemical and physical separation step for recovery or elimination of Fe (II)/Fe (III) ions in the effluent. Therefore, in order to overcome the abovementioned important shortcomings, application of heterogeneous catalysts have important advantages compared to the homogenous catalyst [21], [24], [25]. Among iron-containing heterogeneous catalysts (such as Fe3O4, Fe2O3, Fe0 and FeOOH), Fe3O4 magnetic nanoparticles (MNPs) are considered one of the most efficient heterogeneous nanocomposites to use in CWPO [14], [24]. In a similar mechanism to that shown in Eq. (2), Fe3O4 MNPs are capable to generate HO through decomposition of H2O2. Nevertheless, due to intra-particle interaction, i.e. Van der Waals and intrinsic magnetic interaction, Fe3O4 MNPs have a strong tendency for particle agglomeration, leading to a decrease of the surface/volume ratio of the particles, as well as dispersion of their stability in the reaction solution. As a consequence, its catalytic activity is eventually reduced [24], [26], [27]. Taking this into consideration, in the recent two decades, different support materials, such as alumina, pillared clays, zeolites, silica, and ion-exchange resins have been used to prepare transition-metal-supported catalysts, mainly iron, for application in CWPO [24], [28], [29]. However, due to the leaching phenomenon, these catalysts mainly suffer from limited stability [28], [30]. On the other hand, carbon materials with easily tuned properties, like activated carbons [20], [30], [31], graphite and graphene-based materials [32], [33], activated carbon xerogels [11], [21], [34], carbon blacks [33], glycerol-based carbon materials [35] and carbon nanotubes (CNTs) [36], have been used as active and efficient catalysts for degradation of pollutants by CWPO, however with lower efficiency compared to metal based catalysts [23]. Among the reported supported catalysts, nanomaterials such as CNTs have attracted attention due to their unique and interesting properties, important for superior catalyst support materials, including their distinctive tubular structure, low mass-transfer limitations, high mechanical strength, superior electrical properties, large specific surface area and relatively high thermal stability in oxidizing conditions [23], [36], [37]. More recently, titanium dioxide nanotubes (TNTs) were successfully used as efficient nanocomposites in the CWPO/CWAO for oxidative degradation of organic pollutants [4]. According to reported studies, TNTs have also interesting properties, such as high specific surface area and adsorption capacity, good ion-exchange property and low recombination rate by long electron transport distance along the tubular structure [4], [38].
Therefore, the synthesis of TNT/CNT porous nanocomposite has gained research interest due to the provided advantages of two nanotubular structures, which could enhance the efficiency of organic pollutants degradation. On the other hand, the synthesis of magnetite TNT/CNT nanocomposite present important features due to the easily separation of the nanocomposite from the reaction solution by an external magnet field and the elimination of the high cost for final chemical and physical separation process. In the present study, a magnetite TNT/CNT nanocomposite was synthesized using an alkaline hydrothermal method and its catalytic activity performance in CWPO for the degradation of BPA, as well as the reductions of chemical oxygen demand (COD), total organic carbon (TOC), and the five-day biochemical oxygen demand (BOD5) in the highly toxic and high salinity of PCW were assessed.
Section snippets
Chemicals
All the materials used in the experiments were of analytical reagent grade and applied without further purification. Double distilled water was used to prepare the experimental solutions. Bisphenol A, (BPA, C15H16O2, ≥99 wt%), Multi-Walled Carbon Nanotubes (MWCNT, >99% carbon basic, OD = 60–100 nm, ID = 5–10 nm, and length = 0.5–500 µm), Titanium (IV) oxide (anatase, nanopowders, <25 nm particle size, 99.7%), Titanium (IV) oxysulfate (TiOSO4·xH2O, 15 wt% in diluted sulphuric acid, 99.99%) and
Characterization of the synthesized magnetite TNT/CNT nanocomposite
The morphologies of the prepared Fe3O4 MNPs, TNTs and magnetite TNT/CNT nanocomposite were revealed by SEM, as shown in Fig. 2a–d. With respect to Fig. 2a, it is observed that the Fe3O4 MNPs have uniform spherical shape with high homogeneity. Due to the magnetic properties of the nanoparticles and their interactions, the agglomeration of Fe3O4 MNPs may be occurred [40], [47], [48]. As can be seen in Fig. 2a, the average size of the Fe3O4 MNPs is in the range between 20 and 80 nm, confirming the
Conclusion
In the present study, a magnetite TNT/CNT nanocomposite was developed, characterized and applied towards oxidative degradation of organic pollutants. The developed catalyst shows high catalytic activity in the CWPO in terms of BPA degradation alone or present in the PCW. The influence of operating parameters, including solution pH, H2O2 dosage, reaction temperature and catalyst loading was optimized in the CWPO process for degradation of BPA in the PCW. The COD, BOD5 and TOC of the PCW was also
Conflict of interests
The authors of this research declare no conflict of interest.
Acknowledgments
The present work was a part of a Ph.D. thesis of Seyyed Abbas Mirzaee in Ahvaz Jundishapur University of Medical Sciences (AJUMS), Ahvaz, Iran. This study was financially supported by Environmental Technologies Research Center, AJUMS (grant No. ETRC-9612). The authors would also like to grateful to Mrs. Atashin, Mrs. Azizi and Mrs. Khodadadi for running the TOC, HPLC and GC/MS analyzer, respectively.
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