Catalytic wet air oxidation of bisphenol A aqueous solution in trickle-bed reactor over single TiO2 polymorphs and their mixtures
Graphical abstract
Two series of TiO2 polymorphs (anatase (A and A1), rutile (R and R1), brookite (B and B1)) and nanocomposite mixtures of TiO2 polymorphs (anatase A/rutile R (AR), anatase A/TiO2-B polymorph (ATB), anatase A1/rutile R1/brookite B1 (ARB)) were synthesized and tested in the process of catalytic wet air oxidation (CWAO). The key factor enabling enhanced activity in the CWAO process is high BET specific surface area of the catalysts, which can be compensated either by the presence of appropriate crystallinity, or improved charge separation. Most suitable for long-term use are catalysts A, A1 and ATB.
Introduction
Exposure of humans to the endocrine-disrupting chemical bisphenol A (BPA) has been associated with chronic diseases, such as cardiovascular disease, diabetes and serum markers of liver disease [1]. An important source of human exposure to BPA is the consumption of food and drinks that have been in contact with packaging made out of epoxy resins or polycarbonate plastics [[2], [3], [4], [5], [6], [7], [8]] produced from BPA. Contamination of the aquatic environment with BPA mainly occurs from landfill leachates, where BPA is leached from disposed packaging [9].
A commonly used method for the removal of organic pollutants from wastewaters is biological treatment with active sludge, where the pollutant caught in the sludge can cause secondary contamination of the environment [10]. Advance oxidation processes (AOPs) are an effective alternative in purifying non-biodegradable, toxic and estrogenically active organic pollutants from wastewater. AOPs are divided into different processes such as wet air oxidation (WAO), catalytic wet air oxidation (CWAO), photocatalytic oxidation, the photo-Fenton process and (catalytic) ozonation [[11], [12], [13]]. The high reactivity of hydroxyl radicals in the AOPs is used to achieve complete destruction and mineralization of organic pollutants [[14], [15]].
Application of the WAO process is very attractive for cleaning wastewaters that are too toxic to be treated with biological technology or too dilute for incineration [16], although the use is limited by severe operating conditions and high costs. The addition of the appropriate catalysts into WAO mitigates the operating conditions, enhances the reaction rate, shortens the reaction time and reduces the operating costs [[17], [18]]. In the CWAO process organic pollutants are oxidized by activated O2 species in the presence of solid catalysts at temperatures between 130 and 250 °C and pressures of 10–50 bar into biodegradable intermediate products, or mineralized to CO2, water and associated inorganic salts [19]. Metal oxides, mixed metal oxide systems, cerium-based composite oxides and supported (e.g. titanium oxide (TiO2) [[20], [21]]) noble metal catalysts have been studied in CWAO of various organic pollutants [19]. Only a few studies reported the use of bare TiO2 as a catalyst in the CWAO process. Pintar et al. [22] used TiO2 support for Ru and achieved 40% TOC conversion of phenol from an aqueous solution at 240 °C when bare TiO2 was used. Erjavec et al. [23] achieved complete removal of BPA and 70% TOC conversion at 210 °C when using TiO2 nanotubes annealed at 600 °C. With prolonging the time of CWAO runs, more effective oxidation of organic compounds and reaction intermediates can be achieved [24]. Kaplan et al. [25] recycled the liquid phase (5- to 10-fold recycle) in CWAO over TiO2 nanotubes, which enabled complete BPA conversion and up to 98% TOC conversion at 200 °C.
Studies [[26], [27]] have shown that in heterogeneous photocatalytic oxidation processes different TiO2 polymorphs and combinations of various TiO2 polymorphs exhibit different rates of BPA degradation and mineralization when compared to pure TiO2 polymorphs. As far as we know, no studies have been carried out in which an influence of different TiO2 polymorphs or nanocomposites of TiO2 polymorphs, on the degradation of organic pollutants in the CWAO process was examined. Correspondingly, TiO2 polymorphs (anatase (A and A1), rutile (R and R1), brookite (B and B1)) and nanocomposite mixtures of TiO2 polymorphs (anatase A/rutile R (AR), anatase A/TiO2-B polymorph (ATB), anatase A1/rutile R1/brookite B1 (ARB)) were synthesized in the present study and used in the CWAO process to decompose BPA in a model aqueous solution. A series of TiO2 polymorphs was prepared with a similar synthesis procedure and same TiO2 precursor so that the influence of these factors on the properties and performance of solids in the CWAO process could be discarded. The CWAO process was carried out in a continuous-flow trickle-bed reactor to investigate the possibility of long-term catalyst use as well as efficient mineralization of BPA and its intermediates. Various characterization techniques were used to determinate surface, textural and morphological properties of catalysts before and after the use in CWAO process. To investigate whether the catalysts could be used for long-term mineralization of water dissolved BPA, CHNS elemental analysis of the catalysts was carried out to quantify the accumulation of carbonaceous deposits on the catalyst surface during the CWAO process.
Section snippets
Catalyst preparation
Hydrothermal procedure described in detail in our previous work [23] was used to prepare TiO2 nanotubes, which were used as a precursor to synthesize anatase (A) TiO2. The obtained TiO2 nanotubes were dispersed in H3PO4 and further hydrothermally treated in a Teflon-lined autoclave. Rutile (R) TiO2 was prepared using a low-temperature synthesis route proposed by Zhang et al. [28], in which titanium isopropoxide (TTIP, Sigma-Aldrich, ≥97.0%) was used as starting material. Pure brookite (B) was
Catalyst characterization
XRD diffraction patterns of synthesized catalysts are presented in Fig. 2. Average crystallite sizes were calculated by means of the Scherrer equation for the main diffraction peak of anatase (101) (2θ = 25.3°), rutile (110) (2θ = 27.4°) and brookite (121) (2θ = 30.8°). The XRD diffraction patterns of single TiO2 polymorphs samples (A, A1, R, R1, B and B1) show diffraction peaks, which belong to corresponding TiO2 polymorphs. The XRD diffractogram of the AR composite confirms the presence of
Conclusions
In the present study, TiO2 polymorphs (A and A1, R and R1, B and B1) and nanocomposite materials containing TiO2 polymorphs (AR, ATB, ARB) were used as catalysts for the removal of BPA from aqueous solution in the CWAO process conducted in the continuous-flow trickle-bed reactor. The performance of catalysts is prevailingly governed by their BET specific surface area: higher catalytic activity is obtained in the presence of solids with higher surface area. Results of BPA removal and
Acknowledgements
The authors acknowledge the financial support from the Slovenian Research Agency (research core funding No. P2-0150). We thank Dr. Petar Djinović for kindly interpreting the FT-IR spectra.
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Present address: Lek Pharmaceutical Company d.d., Verovškova 57, SI-1526, Ljubljana, Slovenia.