Titanate Nanotubes As a Novel Catalyst for Removal of Toxicity and

Article pubs.acs.org/IECR

Titanate Nanotubes As a Novel Catalyst for Removal of Toxicity and Estrogenicity of Bisphenol A in the CWAO Process Boštjan Erjavec,†,‡ Tatjana Tišler,*,† Renata Kaplan,† and Albin Pintar†,‡ †

Laboratory for Environmental Sciences and Engineering, National Institute of Chemistry, Hajdrihova 19, SI-1001 Ljubljana, Slovenia Centre of Excellence for Low Carbon Technologies, Hajdrihova 19, SI-1001 Ljubljana, Slovenia

‡

ABSTRACT: Catalytic wet air oxidation (CWAO) of aqueous bisphenol A (BPA) solution was studied in a continuous-flow trickle-bed reactor in the presence of titanate nanotube-based catalysts. These active metal-free solids were prepared by alkaline hydrothermal synthesis followed by heat treatment at temperatures ranging from 300 to 700 °C, so that they would have varying physicochemical properties. The aim of the study was to investigate the removal efficiency of titanate nanotube-based catalysts used in the CWAO process to remove toxicity and estrogenicity of BPA at a very short space time of 0.6 min·gCAT·g−1. In addition, noncatalytic BPA degradation in the presence of inert SiC particles was also performed. As a result, significantly higher estrogenicity and toxic effects of the liquid-phase sample compared to the catalytic oxidation experiments were observed. Complete removal of estrogenicity and high reduction of toxicity from aqueous solution were achieved at 210 °C over 0.5 g of catalyst that had been annealed at 600 °C.

1. INTRODUCTION

Among available AOPs, the CWAO process is regarded as an efficient oxidation route for achieving a meaningful extent of organic pollutant mineralization within a short residence time (0.1−0.3 min) of liquid phase in the catalytic bed.13 Therefore, a proper catalyst has to be selected, not only to diminish the severity of reaction conditions, but also to enhance the oxidation capacity of oxidants, to ensure lower investment and operating costs.17,18 Most CWAO processes are performed in the presence of either supported mixed metal oxides of Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, and Ce or supported noble-metal catalysts, such as Ru, Rh, Pd, Ir, and Pt.19 Both types of catalysts exhibit inherent imperfections; the former are often subjected to leaching and thus deteriorate the quality of treated waters, whereas the latter, though very efficient, increase the investment cost. Generally, the active metals are supported over TiO2, ZrO2, CeO2, γ-Al2O3, and carbon materials with metal loadings of less than 5%.20 Particularly, TiO2 is often applied to support noble metals; however, there are few reports available regarding CWAO processes carried out over bare TiO2 catalysts. Pintar et al.13 reported 40% conversions of total organic carbon (TOC) obtained during the oxidation of aqueous phenol solution in the presence of bare TiO2 support. Furthermore, Bistan et al.21,22 showed notable removals of estrogens and xenoestrogens in the presence of pure TiO2 extrudates (Degussa). Consequently, the toxicity and estrogenicity of treated aqueous samples were significantly reduced in comparison to those of the feed solutions. Because pure TiO2 permits satisfactory conversions of priority pollutants under oxidative conditions, it can be regarded as an inexpensive and environmentally innocuous catalyst candidate.

Bisphenol A (BPA) is a well-known endocrine-disrupting chemical (EDC) with estrogen-like effects, which frequently induce adverse effects in the endocrine systems of humans and wildlife.1,2 For this reason, the efficient removal of EDCs from wastewaters, before the wastewaters are discharged into an aquatic environment, is necessary. It has been reported in many studies that estrogens and xenoestrogens (BPA) are not completely broken down during conventional biological treatment in a wastewater treatment plant (WWTP),3,4 as estrogenic activity has been detected in treated wastewaters, as well as in river waters after WWTP discharges.5 In addition, considerable amounts of EDCs are, in fact, adsorbed onto activated sludge,6 which can cause further problems in sludge deposition.7 Several possibilities for EDC removal from water are available, and among them, advanced oxidation processes (AOPs), such as catalytic wet air oxidation (CWAO), heterogeneous photocatalysis, ozone-based technologies, and ultrasound oxidation, seem to be the most promising.8,9 Nevertheless, many studies of these approaches include data about only the amounts of remaining estrogens and xenoestrogens but not the toxicity and estrogenic activity of treated aqueous solutions. However, some studies have reported that, during BPA oxidation processes, intermediates such as p-hydroquinone, phenol, and acidic compounds, were produced.10,11 To detect the presence of possible remaining estrogenic activity and toxicity in aqueous samples treated by AOPs, bioassays should be used in addition to chemical analytical measurements.12,13 Aquatic test species from different taxonomic groups are used for the assessment of possible toxic effects of pollutants, whereas various bioassays [e.g., yeast estrogen screen (YES) assay] are commonly applied for the assessment of the estrogenicity of treated wastewaters and surface waters.14−16 © 2013 American Chemical Society

Received: Revised: Accepted: Published: 12559

June 3, 2013 July 30, 2013 August 5, 2013 August 5, 2013 dx.doi.org/10.1021/ie401752c | Ind. Eng. Chem. Res. 2013, 52, 12559−12566

Industrial & Engineering Chemistry Research

Article

file (PDF) standards from the International Centre for Diffraction Data (ICDD). To measure the pH of the point of zero charge (pHPZC), above which the surface is negatively charged, carefully weighed amounts of solid were added sequentially to 50 mL of aqueous 0.005 M NaCl until the pH of the solution did not change with further solid additions. The equilibrium pH at the plateau of the curve corresponds to the pHPZC. The well-mixed cell in which the measurements were performed was thermostatted to 25 °C. The carbon contents of fresh and spent titanate nanotubebased catalysts were determined by CHNS elemental analyzer (Perkin-Elmer, model 2400 Series II) to evaluate the amount of accumulated carbonaceous deposits on the catalyst surface. Leaching of Ti from the catalysts during the CWAO reaction was verified by inductively coupled plasma mass spectrometry (ICP-MS) analysis of collected liquid-phase samples. 2.4. CWAO Experiments. CWAO experiments were performed in a Microactivity-Reference unit (PID Eng&Tech, Madrid, Spain), which is a fully automated and computercontrolled continuous-flow trickle-bed reactor for catalytic tests. A detailed apparatus description can be found elsewhere.13 It should be noted that a typical continuous-flow CWAO test lasted for 40 h, with approximately 20 h needed to reestablish dynamic equilibrium between phases in the catalyst layer. After the initial nonequilibrium period, a steady-state performance was obtained, giving rise to good reproducibility of the oxidation capacity. Therefore, all acute toxicity and estrogenicity tests, as well as the final TOC and BPA removals, were determined from the representative liquid-phase samples of about 700 mL that were collected during the steady-state period. Additionally, representative liquid-phase samples were also collected continuously (10 mL fraction per hour) from the liquid/gas (L/G) condenser/separator during the entire course of the CWAO experiments by a microcomputer-controlled fraction collector (ADVANTEC, model CHF122SC) and further analyzed for residual BPA/TOC content. In a typical CWAO experiment, the BPA oxidation reaction was conducted at 200 °C over 300 mg of a titanate nanotube-based catalyst. The properties of the catalyst bed and operating conditions can be found in our previous article.24 2.5. Standard Preparation and Chemical Analyses. The aqueous feed solution was prepared daily using ultrapure water with a BPA concentration of 10.0 mg/L (≥99%, Aldrich). Determinations of residual BPA content, performed on an HPLC apparatus (Spectra system), were carried out in isocratic analytical mode using a 100 mm × 4.6 mm BDS Hypersil C18 2.4 μm column thermostatted at 30 °C and equipped with an universal column protection system [UV detection at λ = 210 nm with a mobile phase of methanol (70%) and ultrapure water (30%) at a flow rate of 0.5 mL/min]. The level of mineralization (i.e., the total amount of removed organic substances in the withdrawn aqueous-phase samples) was determined in terms of total organic carbon (TOC), using a TOC analyzer (Teledyne Tekmar, model Torch) equipped with a high-pressure nondispersive infrared (NDIR) detector. A high-temperature catalytic oxidation (HTCO) method was applied (at 750 °C), in which the measured inorganic carbon content (removed from a sample by purging the acidified sample with a purified gas) was subtracted from the measured total carbon (TC) content, which, in this case, corresponds to the TOC content. The composition of CWAO reaction intermediates in the collected liquid-phase samples was

The main objective of this study was to synthesize and investigate the potential of several titanate nanotube-based catalysts applied in a continuous-flow trickle-bed reactor to remove the toxicity and estrogenic activity of BPA from aqueous solutions. Herein, we report a novel approach for the destruction of estrogenic active organic compounds in the CWAO process over bare titanate nanotube-based catalysts (without surface decoration with active metals). The properties of the catalysts before and after CWAO runs were comparatively investigated by means of CHNS elemental analysis, N2 porosimetry, and textural measurements. The efficiency of oxidative destruction of BPA in treated samples was investigated by chemical analyses [high-performance liquid chromatography (HPLC), TOC]; toxicity tests with marine bacteria Vibrio fischeri, water fleas Daphnia magna, unicellular green algae Desmodesmus subspicatus, and zebrafish embryos Danio rerio; and YES assay using the genetically modified yeast strain Saccharomyces cerevisiae.

2. MATERIAL AND METHODS 2.1. Synthesis of Titanate Nanotubes. Titanate nanotubes were synthesized using a procedure similar to that reported by Kasuga et al.23 First, 2 g of TiO2 powder (Degussa P25) was dispersed in 10 M NaOH (V = 150 mL) solution using an ultrasonic homogenizer. A Teflon-lined autoclave was then filled with the reaction mixture and held at 130 °C for 24 h. The resulting white precipitates were separated from the reaction solution by filtration and then rinsed thoroughly with deionized water. In the next step, the wet cakes were rinsed with 0.1 M HCl solution, to promote proton exchange (Na+ exchange with H+ occurs at low pH), and then neutralized with deionized water. Finally, the samples were dried in a vacuum under cryogenic conditions. 2.2. Titanate Nanotube-Based Catalyst Preparation. The as-prepared protonated titanate nanotubes were heattreated at 300, 400, 500, 600, and 700 °C for 1 h in air, to vary the particle size, crystal structure, morphology, and surface properties of the catalysts. 2.3. Characterization of Titanate Nanotube-Based Catalysts. After the protonated titanate nanotubes had been annealed at temperatures up to 700 °C, diverse titanate nanotube-based catalysts were obtained. The surface morphologies of the synthesized catalysts were examined by fieldemission scanning electron microscopy (FE-SEM, SUPRA 35VP, Carl Zeiss). For transmission electron microscopy (TEM) investigations, the materials were deposited on a copper-grid-supported perforated transparent carbon foil. A field-emission electron-source transmission electron microscope (JEOL 2010 F) was used. The specific surface areas, total pore volumes, and average pore widths of the titanate nanotubes were determined from the adsorption and desorption isotherms of N2 at −196 °C using a Micromeritics TriStar II 3020 instrument. This characterization was performed after gradual degassing of the samples under a N2 stream (purity 6.0) and programmed bilevel heating, starting with a first heating stage at 90 °C for 60 min followed by a second heating stage at 180 °C for 240 min. The heating rate was set to 10 °C/min for both heating stages. The X-ray powder diffraction patterns of the catalysts were collected on a PANalytical X’pert PRO MPD diffractometer using Cu Kα1 radiation (1.54056 Å) in reflection geometry. The data were collected in the range of 10−90° in steps of 0.034°. Crystalline phases were identified by comparison with powder diffraction 12560

dx.doi.org/10.1021/ie401752c | Ind. Eng. Chem. Res. 2013, 52, 12559−12566


Article

exposure, the immobile daphnids were counted, and the percentages were calculated.29 Adult zebrafish were bred in a temperature-controlled room in an aquarium (60 × 30 × 30 cm) containing 45 L of tap water at constant temperature (26 °C) and photoperiod (12 h light/ 12 h dark). Fish were fed three times per day with commercially available dried fish food. A detailed description of zebrafish breeding to obtain eggs was published by Kammann et al.30 In each hole of 24-well plates, 1 mL of feed solution of BPA, treated sample, or synthetic ISO medium (a control) was dispensed in 10 replicates, and then fertilized eggs in four to eight cell stages were placed.31 After 48 h of exposure at 26 °C, lethal (egg coagulation, missing heartbeat, and missing tail detachment from the yolk sac) and sublethal (missing body and eye pigmentation, spine deformation, yolk sac edema) malformations were observed, and the percentages for each tested sample were calculated. The green, unicellular alga Desmodesmus subspicatus Chodat 1926 (SAG 86.81) was obtained from the Collection of Algal Cultures, University of Göttingen, Göttingen, Germany. A stock culture of algae was maintained in a nutrient solution according to the method of Jaworski32 at a constant room temperature of 21 ± 1 °C under continuous fluorescent illumination (4000 lx) provided by four 20-W cool-white fluorescent lights (Osram). Flasks with algae were agitated at 150 rpm for 15 min, alternating with 15 min of resting, on an orbital shaker. To perform toxicity tests, test flasks containing feed solution of BPA, treated sample, and a control (growth medium) were constantly shaken at the same frequency as stock flasks; they were illuminated with four 40-W cool-white fluorescent lights giving an illumination of 7000 lx. After 72 h of exposure, the algal densities of the control and treated samples were determined by counting cells in a Bürker counting chamber. The percentage inhibition of specific growth rates for each tested sample was calculated in comparison to the control.33

analyzed by ion chromatography (IC) using DX-120 Dionex apparatus. 2.6. Estrogenicity. The estrogenic activities (EAs) of the BPA feed solution and treated samples were determined by yeast estrogen screen (YES) assay.25 The human estrogen receptor-transfected yeast strain Saccharomyces cerevisiae BJ1991 was used under agreement with John P. Sumpter (Brunel University, London, U.K.). First, aqueous samples collected at the outlet of the continuous-flow CWAO reactor were concentrated using Oasis HLB 6 cc (500 mg) solid-phase extraction (SPE) cartridges (Milford, MA) and methanol as the eluting solvent, according to the approach of Bistan et al.26 To find a relationship between estrogenic activity and successive dilutions of SPE concentrates, serial dilutions were prepared in ethanol, and 10-μL aliquots of these solutions were transferred in duplicates to 96-well optical flat-bottom microtiter plates (TPP, MIDSCI, St. Louis, MO) under sterile conditions. The actual concentrations of treated liquid-phase samples were also tested for estrogenicity by transferring 2 μL of the concentrated samples in quadruplicate into the holes of microtiter plates. After the ethanol had been evaporated to dryness, 200 μL of the assay medium containing yeast and chromogenic substrate CPRG (chlorophenol red-β-D-galactopyranoside) was dispensed to each hole on the microtiter plate. Each experiment was repeated twice. The validity of the YES assay was confirmed with a positive control (2.6 ng/L−1.36 μg/L of 17β-estradiol), a negative control (0.025−1.57 μg/L of progesterone), and a blank control (yeast exposed to the growth medium and CPRG). The plates were shaken vigorously and incubated in a ventilated heat chamber (WTW, TS 606/2i) at 34 °C for 60−72 h. The absorbance at 575 and 620 nm was measured on a PowerWave XS microtiter plate reader (BioTek, Winooski, VT). The estrogenic activity of diluted SPE concentrates was expressed in percentages relative to the BPA maximum response following the correction of measured absorbances for turbidity. The relative estrogenic activities (REAs) of outlet samples with a concentration factor of 1.0 (actual concentration) were determined by comparing the EAs of treated samples and BPA feed solution. 2.7. Toxicity. The freeze-dried luminescent bacterium Vibrio f ischeri NRRL-B-11177, obtained from the manufacturer (Dr. Lange GmbH, Düsseldorf, Germany), was exposed to the feed solution of BPA (10 mg/L), and treated samples in two replicates for 30 min at 15 ± 0.2 °C. The luminescence of V. f ischeri was measured on a LUMIStox 300 luminometer at the start of the experiment (before sample addition) and after 30 min of exposure. The percentages of luminescence inhibition were calculated for each treated sample relative to the luminescence of the control samples.27 Daphnia magna Straus 1820 (Clone A) was obtained from the ECT Oekotoxikologie, Flörsheim, Germany. Twenty water fleas were placed into 3-L aquariums covered with glass plates containing 2.5 L of modified M4 medium at 21 ± 1 °C and illuminated with fluorescent bulbs (approximately 1800 lx) in a 16 h light/8 h dark regime. The water fleas were fed twice per week with TetraMin (20 mg blended in deionized water per aquarium), once per week with instant yeast (5 μg per aquarium), and four times per week with algae Desmodesmus subspicatus Chodat 1926 corresponding to 0.13 mg of C/ daphnia.28 In the acute toxicity test with water fleas, neonates less than 24 h old were exposed to the feed solution of BPA and undiluted treated samples in two replicates. After 24 h of

3. RESULTS AND DISCUSSION 3.1. Characterization of Titanate Nanotube-Based Catalysts. Transmission electron microscopy (TEM; Figure 1) confirmed the successful transformation of the initial powdered TiO2 precursor (Degussa P25), which is in the form of agglomerated spheroidal particles, into well developed and randomly oriented titanate nanotubes during the alkaline

Figure 1. TEM micrograph of the as-prepared titanate nanotubes. 12561



Article

The actual anatase crystallite sizes were estimated from the corresponding width of the (101) diffraction peak (2θ = 25.3°) by Scherrer’s formula (Figure 3). As expected, the largest

hydrothermal treatment. The as-prepared titanate nanotubes were several hundred nanometers in length and had an outer diameter of about 10−15 nm. The structure of protonated titanate nanotubes is metastable, which means that the nanotubular morphology can undergo a transformation into solid nanorods accompanied by a decrease in the surface area and the appearance of anatase phase when calcined at temperatures above 300 °C.34 These outcomes were encountered by means of scanning electron microscopy (Figure 2), which revealed that the heat treatment at elevated

Figure 3. Specific surface area (SBET) and average anatase crystallite size of the as-prepared and heat-treated NT4 samples (catalysts) at temperatures ranging from 300 to 700 °C.

crystallites (35.9 nm) were obtained for the sample that was heat-treated at the highest temperature (i.e., 700 °C), whereas the smallest crystallite size that could be determined was 10.8 nm, corresponding to the anatase crystallites of sample NT4_400. The trend in average particle size is consistent with the trend in decreased Brunauer−Emmett−Teller (BET) surface area (Figure 3). The initial BET surface area (383 m2/ g) gradually decreased with increasing calcination temperature, until the ultimate BET surface area of 49 m2/g was reached (at 700 °C). Similar behavior was observed with the total pore volume (not shown here), which was reduced by a factor of almost 3 after calcination at 700 °C (in comparison to the starting total pore volume), indicating excessive recrystallization and sintering processes. The trend in decreased total pore volume is likewise fully consistent with the trend in increased average anatase crystallite size. The experimental results of the pHPZC determinations for the as-prepared and heat-treated titanate nanotubes are given in Figure 4. It is immediately apparent from the illustrated data that the as-prepared material (NT4) and the sample annealed at 300 °C (NT4_300) were acidic solids, because the

Figure 2. SEM micrographs of the (a) as-prepared and (b−f) heattreated titanate nanotubes at (b) 300, (c) 400, (d) 500, (e) 600, and (f) 700 °C.

temperatures, ranging from 300 to 700 °C, provoked extensive recrystallization (especially at higher temperatures) noticeable as an evident deviation from the morphology of the as-prepared titanate nanotubes (Figure 2a). An exception among the calcined samples can be recognized in the sample prepared at 300 °C (Figure 2b), because its morphology resembles that of the initial sample, confirming that only minute structural changes were induced at this temperature. At higher temperatures, simultaneous particle growth was observed, indicating evolution of the anatase phase and deterioration of the titanate nanotube tubular morphology. The phase transformation and crystal growth were clearly discernible from X-ray powder diffraction measurements, but only for samples annealed at 400 °C or higher. The intensities of the corresponding anatase Bragg reflections were in agreement with increasing calcination temperature, whereas the peak widths decreased as the temperature rose. Eventually, at 700 °C (sample NT4_700), the anatase structure was thoroughly developed, and the initial layered structure was ultimately transformed into the thermodynamically more stable crystal structure. Narrow Bragg reflections and high peak intensities of the NT4_700 sample coincide with the SEM image (Figure 2f), revealing large anatase crystallites as a result of extensive recrystallization.

Figure 4. Mass titration curves for titanate nanotube-based catalysts in 0.005 M NaCl. 12562



Article

BPA. Obviously, the NT4_600 catalyst sample meets this criterion better than the other catalysts. Liquid-phase sample S9 was obtained during the CWAO process conducted at higher operating temperature (210 °C) and catalyst loading (0.5 g of NT4_600), which resulted in the complete removal of BPA. However, 32% of TOC was still present in this sample, mainly in the form of short-chain aliphatic acids (IC analysis). CHNS elemental analysis of the fresh and spent solids confirmed that negligible amounts of carbonaceous deposits were accumulated on the catalyst surface during the CWAO runs. Considering the carbon-based elemental analysis of the catalysts (Table 2) and

equilibrium pH values were at about 2.2 and 3.3, respectively. Titanate nanotubes are initially abundant with surface MOH and MOH2+ groups, which give acidic character to these solids. Heat treatment at temperatures higher than 300 °C gradually switched from acidic to basic surface properties of nanotubes, as a result of further stripping of the H+ protons. Eventually, at the highest calcination temperatures, namely, 600 and 700 °C, the pHPZC increased to values of 9.5 and 10, respectively, because of the prevailing MO− groups. The annealing of noble-metal-free titanate nanotubes at different temperatures enabled the formation of heterogeneous catalysts with different physicochemical properties. Such a dissimilarity in these high-surface-area solids should be reflected in different BPA oxidation rates during CWAO process, which, consequently, could influence the toxicity and estrogenicity of treated samples. 3.2. Chemical Analyses. The values of the remaining BPA concentrations and the BPA and TOC content measurements of the initial BPA sample (feed solution) and samples treated by means of the CWAO process in the continuous-flow tricklebed reactor are presented in Table 1.

Table 2. Carbon Contenta on the Surface of Fresh and Spent Catalyst Samples Used in the CWAO Process of BPA TC (mg/g)

Table 1. BPA and TOC Conversion and BPA Remaining in the End-Product Solutions Derived from Corresponding CWAO of BPA catalysta

sample S1 BPAb S2 S3 S4 S5 S6 S7 S8 S9c S10 a c

BPA removal (%)

BPA remaining (mg/L)

TOC removal (%)

−

−

10

−

TiO2, Degussa P25 NT4 NT4 (300) NT4 (400) NT4 (500) NT4 (600) NT4 (700) NT4 (600) SiC

80.0

2.0

56

85.0 76.0 82.0 78.0 87.0 79.0 100.0 49.0

1.5 2.4 1.8 2.2 1.3 2.1 0.0 5.1

42 64 61 58 69 47 68 20

Temperature of heat treatment in parentheses. Treaction = 210 °C, mcatalyst = 0.5 g.

b

sample

fresh

spent

TOCtrue removalb (%)

NT4 NT4_300 NT4_400 NT4_500 NT4_600 NT4_700 Degussa P25

6.5 3.4 2.5 1.9 1.7 1.5 0.4

3.3 3.3 3.2 2.8 1.9 2.7 2.7

− − 57 53 68 39 43

a

Measured by means of CHNS elemental analysis. bTrue TOC removals were calculated as TOC accumulated subtracted from TOC removal: TOCtrue re (%) = TOCre − TOCac.

related TOC removals (TOCre), one can estimate the amounts of TOC accumulated (TOCac) on the catalyst surface. If TOCac is then subtracted from TOCre, the true TOC conversion (i.e., mineralization to CO2 and H2O) can be determined. As expected, the corrected TOC removals (true conversions) were lower than the measured values (TOCre). However, the differences between the measured and correct values were significant, especially when comparing the commercial sample with sample NT4_600. The true TOC removal (Table 2) of the latter (68%) was almost identical to the measured value (Table 1), indicating that the catalyst surface was not covered with carbonaceous deposits during the CWAO run. In this respect, a long-term oxidation reaction can be conducted in the presence of NT4_600 catalyst without fear of catalyst deactivation. On the other hand, Degussa P25 catalyst exhibited a high TOCac value and a markedly decreased true TOC value (43%) as a consequence. Furthermore, no leaching of titanium (to the detection limit of 0.01 mg/L) was detected in the CWAO experiments performed in the presence of the solids examined in this study. Consequently, the titanate nanotube-based materials examined in this work can be regarded as promising heterogeneous catalysts for the degradation of hazardous organic compounds without concern over environmental risks because of active metal leaching from the reactor system. 3.3. Estrogenicity. The removals of the estrogenic activity of the end-product solutions with a concentration factor (CF) equal to 1.0 (actual concentration) that were treated in the continuous-flow trickle-bed reactor and diluted SPE concentrates in comparison to the estrogenic activity of BPA feed solution are presented in Figure 5. The YES assay of the as-collected aqueous samples that were treated under oxidative conditions in the presence of titanate nanotube-based catalysts revealed that the estrogenic activities of samples S3−S8 (CF = 1.0) dropped by at least 66% (sample

Feed solution.

Analyses of treated BPA aqueous-phase samples revealed moderate conversions of BPA (49%) and TOC (20%) in sample S10, which was collected continuously during the oxidative reaction over inert SiC particles (SBET < 0.1 m2/g). The latter experiment resembles the thermal decomposition of the model pollutant, which, as expected, was far less pronounced in comparison to the catalytically promoted BPA oxidation. In the presence of commercial catalyst (TiO2, Degussa P25), the BPA and TOC removals were nearly 80% and 56%, respectively, whereas the conversions in the presence of the most promising titanate nanotube-based catalyst (NT4_600) were even further improved (87% for BPA and 69% for TOC). These observations (Table 1) confirmed that, in the given range of operating conditions, BPA oxidation involves both noncatalytic and catalytic oxidative routes. The trend in TOC conversion is consistent with the trend in BPA conversion determined by HPLC measurements. Based on thorough analysis of the prepared catalysts and their performance in the CWAO process, it can be concluded that balanced physicochemical properties (e.g., specific surface area, crystallinity, surface acidity, etc.) are required for high conversions of 12563



Article

Figure 5. Estrogenic activity of the end-product solutions with a concentration factor of 1.0 (actual concentration) and diluted SPE concentrates treated by means of the CWAO process. See Table 1 for sample notation.

S9 did not reach 100%, the remaining organic molecules were not recognized as estrogenically active. In addition, the YES bioassay results confirmed that subsequent estrogenic intermediates were not produced during the CWAO process in the continuous-flow trickle-bed reactor. The CWAO of aqueous BPA solution conducted in the presence of various titanate nanotube-based catalysts led to high conversion rates, so reduced estrogenic activity was observed. The same observations were reported in a study by Benotti et al.,35 as no products revealing estrogenic activity were produced during the treatment of surface water polluted with pharmaceuticals and endocrine-disrupting compounds in a photocatalytic reactor membrane pilot system. In contrast, Chen et al.36 speculated that a possible reason for the slower removal of estrogenic activity measured by YES bioassay in comparison to HPLC measurements of BPA concentrations was the production of estrogenic degradation products and synergistic or additive interactions between remaining BPA and degradation products during photolysis treatments. It seems that the AOP method and experimental conditions employed for the oxidation process play an important role in the degradation pathways of BPA in aqueous samples. 3.4. Toxicity to Aquatic Organisms. The toxicities of BPA feed solution (S1) and end-product solutions treated by means of CWAO process in the continuous-flow trickle-bed reactor are reported in Table 3. The highest toxicity of BPA (10 mg/L) was observed when bacteria and water fleas were exposed to the feed solution, as 69% luminescence inhibition and 55% immobility, respectively, were determined. Lethal effects (coagulation of embryo, missing tail detachment) were observed for 30% of exposed zebrafish embryos, and sublethal effects (i.e., weaker pigmentation of surviving embryos) were discerned in comparison to the control group. The toxicity results obtained for the initial BPA solution are comparable to literature data indicating that BPA is not highly toxic to aquatic organisms during acute exposure.37,38 Slight decreases in toxicity to bacteria, algae, and zebrafish were determined in the WAO experiment performed over inert SiC particles (run S10) in comparison to the feed solution, whereas a more pronounced decrease of toxic effects was observed when S10 sample was exposed to water fleas. It is evident that the titanate nanotube-based catalysts applied in the CWAO process enhanced the removal of BPA from the aqueous solution and significantly reduced the toxicity levels of the tested samples in comparison to that of the BPA feed

S4) and up to 87% (sample S3) (Figure 5a). This is in agreement with the removal of BPA from the feed solution, measured by HPLC, as the conversions of BPA always exceeded 75%, irrespective of the catalyst used in the process (Table 1). On the contrary, sample S10 was obtained in the presence of inert SiC, resulting in a low conversion of model aqueous pollutant and a high level of remaining estrogenic activity (67%) as a consequence. The latter experiment demonstrated a WAO process, where BPA molecules were oxidized by a noncatalytic oxidation route, thus leaving a notably higher BPA concentration (5.1 mg/L) in the end-pipe solution in comparison to the catalytic destruction of BPA in the rest of the experiments. To confirm the relationship between estrogenic activity (determined by YES assay) and amount of remaining BPA (determined by HPLC) in the treated aqueous samples, the CWAO experiment was conducted under slightly more severe conditions (210 °C) and with a higher catalyst loading (500 mg), which resulted in complete BPA removal. In this way, the oxidation capacity was enhanced together with the residence time, which enabled higher conversion rates. The related S9 sample showed no estrogenic activity, as expected according to the 100% conversion of BPA. Similar noticeable EA removals of sequential dilutions of the SPE concentrates were determined in the outlet samples treated in the CWAO process over titanate nanotube-based catalysts (Figure 5b). A high estrogenic activity of serial dilutions of the sample S10 concentrate acquired in the presence of inert SiC was determined. In contrast, the estrogenic activities of serial dilutions of the SPE concentrates that had been treated in the presence of various protonated titanate nanotube-based catalysts (S3−S8) and commercial TiO2 (S2) decreased markedly in comparison to that of the feed solution (S1). The estrogenic activity of the SPE concentrate at a CF of 1.25, which is of similar concentration to the actual outlet samples from the reactor, was significantly lower than that of the S1 (feed solution) SPE concentrate with the same CF. However, the noncatalytic oxidation of BPA (sample S10) was less efficient in removing the high estrogenic activity of diluted samples of SPE concentrate in comparison to the SPE concentrates of samples treated in the presence of various protonated titanate nanotubebased catalysts (S3−S8) and commercial TiO2 (S2). The highest removal efficiency of estrogenic activity was obtained with sample S9, as an estrogenic activity only 10% was determined for the sample with the highest tested concentration (CF = 10). Even though the TOC conversion of sample 12564



Article

the presence of titanate nanotube-based catalyst NT4_600 at a short space time of about 0.6 min·gCAT·g−1. When this catalyst was applied in a higher dosage (0.5 instead of 0.3 g) than usual, complete BPA removal and thus estrogenicity was attained, and no estrogenically active intermediates were determined during the CWAO process by means of YES bioassay. Furthermore, a remarkable extent of detoxification of BPA feed solution was achieved, when bacteria and zebrafish embryos were exposed to the catalytically treated aqueous samples in comparison to the thermally treated sample (S10). Interestingly, the toxicity of sample S9 (treated in the presence of 0.5 g of NT4_600 catalyst) to water fleas remained to a limited extent despite the complete removal of BPA. IC analysis revealed the presence of nonoxidized and refractory short-chain carboxylic acids, which are commonly toxic to water fleas. The obtained results confirmed the importance of implementing bioassays for the evaluation of the toxicity and estrogenicity of reaction intermediates, which can often cause more severe effects to aquatic organisms.

Table 3. Toxicities of BPA Feed Solution (S1) and EndProduct Solutions Treated by the CWAO Process to Bacteria Vibrio fischeri, Algae Desmodesmus subspicatus, Water Fleas Daphnia magna, and Zebrafish Danio rerio sample

Vibrio f ischeri luminescence inhibition (%)

D. subspicatus growth inhibition (%)

D. magna immobility (%)

D. rerio lethal effects (%)

69

13

55

30

21 17 33 19 23 19 24 13 44

11 14 11 12 12 17 14 0 0

0 0 15 15 10 0 0 40 0

10 0 10 0 10 0 0 0 20

S1 BPAa S2 S3 S4 S5 S6 S7 S8 S9 S10 a

feed solution.

■

solution. In the toxicity tests with luminescence bacteria, the inhibition of luminescence is well correlated with the remaining concentration of BPA. The same trend was observed in the acute toxicity tests with daphnids, as the efficient removal of BPA from the feed solution led to the removal of toxic effects of samples treated in WAO runs in which catalysts were present. In these samples (S2−S8), the percentages of immobile daphnids dropped below 20%. However, the toxicity to water fleas of sample S9 (treated in the presence of 0.5 g of NT4_600 catalyst) did not drop significantly, despite complete removal of BPA from aqueous sample. The TOC conversion of S9 sample reached 70%, whereas the residual organic content can be attributed to the presence of nonoxidized and refractory shortchain carboxylic acids (mainly acetic acid), whose presence was confirmed by IC analyses. The concentration of acetic acid in sample S9 was 1.91 ppm, which was about 3.5 times higher than that in noncatalytically treated sample S10. It is known from our previous study that short-chain carboxylic acids could be toxic to water fleas.13 Similar production of acidic degradation products during the process of UV photolysis of BPA in the presence of H2O2 was reported by Chen et al.,36 resulting in embryonic lethality of Japanese medaka (Oryzias latipes). The results obtained in toxicity tests with zebrafish embryos showed that the percentages of lethal effects in all samples treated under oxidative conditions over synthesized catalysts were 10% or lower. In sample S10 (WAO over inert SiC), lethal effects in 20% of the exposed embryos and weaker pigmentation of embryo bodies were observed, which is attributed to the BPA remaining in the aqueous sample.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Ministry of Higher Education, Science, Culture and Sport of the Republic of Slovenia through Research Program P2-0150.

■

REFERENCES

(1) Gross, T. S.; Arnold, B. S.; Sepulveda, M. S.; Mc Donald, K. Endocrine disrupting chemicals and endocrine active agents. In Handbook of Ecotoxicology, 2nd ed.; Hoffman, D. J., Ratnner, B. A., Burton, G. A., Jr., Cairns, J., Jr., Eds.; CRC Press: Boca Raton, FL, 2003; Chapter 39, p 1033. (2) Woodruff, T. J. Bridging epidemiology and model organisms to increase understanding of endocrine disrupting chemicals and human health effects. J. Steroid Biochem. Mol. Biol. 2011, 127, 108−117. (3) Ternes, T. A.; Stumpf, M.; Mueller, J.; Haberer, K.; Wilken, R. D.; Servos, M. Behavior and occurrence of estrogens in municipal sewage treatment plantsI. Investigations in Germany, Canada and Brazil. Sci. Total Environ. 1999, 225, 81−90. (4) Ying, G. G.; Kookana, R. S.; Kumar, A. Fate of estrogens and xenoestrogens in four sewage treatment plants with different technologies. Environ. Toxicol. Chem. 2008, 27, 87−94. (5) Pawlowski, S.; Ternes, T. A.; Bonerz, M.; Rastall, A. C.; Erdinger, L.; Braunbeck, T. Estrogenicity of solid phase-extracted water samples from two municipal sewage treatment plant effluents and river Rhine water using the yeast estrogen screen. Toxicol. In Vitro 2004, 18, 129− 138. (6) Clara, M.; Strenn, B.; Saracevic, E.; Kreuzinger, N. Adsorption of bisphenol-A, 17β-estradiole and 17α-ethinylestradiole to sewage sludge. Chemosphere 2004, 56, 843−851. (7) Mohapatra, D. P.; Brar, S. K.; Tyagi, R. D.; Surampalli, R. Y. Physico-chemical pre-treatment and biotransformation of wastewater and wastewater sludgeFate of bisphenol A. Chemosphere 2010, 78, 923−941. (8) Belgiorno, V.; Rizzo, L.; Fatta, D.; della Rocca, C.; Lofrano, G.; Nikolaou, C.; Naddeo, V.; Meric, S. Review on endocrine disruptingemerging compounds in urban wastewater: Occurrence and removal by photocatalysis and ultrasonic irradiation for wastewater reuse. Desalination 2007, 215, 166−176.

4. CONCLUSIONS Titanate nanotube-based catalysts with different psychochemical properties were used to facilitate the conversion of BPA under oxidative conditions in the CWAO process. These bare solids (i.e., without surface decoration with noble metals and/ or mixed metal oxides) can be regarded as promising inexpensive catalysts for the degradation of hazardous organic compounds, owing to their negligible environment risk due to active metal leaching from the reactor system and their longterm performance in the CWAO process, because of negligible accumulation of carbonaceous deposits during CWAO runs. The most pronounced removal (87%) of BPA was achieved in 12565



Article

aquatic invertebrates Daphnia magna and Chironomus riparius. Aquatic Toxicology 2010, 97, 304−313. (29) Water Quality − Determination of the Inhibition of the Mobility of Daphnia magna Straus (Cladocera, Crustacea) − Acute Toxicity Test; ISO 6341; International Organization for Standardization: Geneva, Switzerland, 1996. (30) Kammann, U.; Biselli, S.; Hühnerfuss, H.; Reineke, N.; Theobald, N.; Vobach, M.; Wosniok, W. Genotoxic and teratogenic potential of marine sediment extracts investigated with comet assay and zebrafish test. Environ. Pollut. 2004, 132, 279−287. (31) Water Quality − Determination of the Acute Toxicity of Waste Water to Zebrafish Eggs (Danio rerio); ISO 15088; International Organization for Standardization: Geneva, Switzerland, 2007. (32) Thompson, A. S.; Rhodes, J. C.; Pettman, I. Culture Collection of Algae and Protozoa, Catalogue of Strains; Culture Collection of Algae and Protozoa Freshwater Biological Association: Cumbria, U.K., 1988. (33) Water Quality − Freshwater Algal Growth Inhibition Test with Unicellular Green Algae; ISO 8692; International Organization for Standardization: Geneva, Switzerland, 2004. (34) Zhang, H.; Li, G. R.; An, L. P.; Yan, T. Y.; Gao, X. P.; Zhu, H. Y. Electrochemical lithium storage of titanate and titania nanotubes and nanorods. J. Phys. Chem. C 2007, 111, 6143−6148. (35) Benotti, M. J.; Stanford, B. D.; Wert, E. C.; Snyder, S. A. Evaluation of a photocatalytic reactor membrane pilot system for the removal of pharmaceuticals and endocrine disrupting compounds from water. Water Res. 2009, 43, 1513−1522. (36) Chen, P. J.; Linden, K.; Hinton, D. E.; Kashiwada, S.; Rosenfeldt, E. J.; Kullman, S. W. G. Biological assessments of bisphenol A degradation in water following direct photolysis and UV advanced oxidation. Chemosphere 2006, 65, 1094−1102. (37) Alexander, H. C.; Dill, D. C.; Smith, L. W.; Guiney, P. D.; Dorn, P. Bisphenol A: Acute aquatic toxicity. Environ. Toxicol. Chem. 1988, 7, 19−26. (38) Mihaich, E. M.; Friederich, U.; Caspers, N.; Hall, A. T.; Klecka, G. M.; Dimond, S. S.; Staples, C. A.; Ortego, L. S.; Hentges, S. G. Acute and chronic toxicity testing of bisphenol A with aquatic invertebrates and plants. Ecotoxicol. Environ. Saf. 2009, 72, 1392−1399.

(9) Liu, Z.; Kanjo, Y.; Mizutani, S. Removal mechanisms for endocrine disrupting compounds (EDCs) in wastewater treatment Physical means, biodegradation, and chemical advanced oxidation: A review. Sci. Total Environ. 2009, 407, 731−748. (10) Katsumata, H.; Kawabe, S.; Kaneco, S.; Suzuki, T.; Ohta, K. Degradation of bisphenol A in water by the photo-Fenton reaction. J. Photochem. Photobiol. A 2004, 162, 297−305. (11) Chiang, K.; Lim, T. M.; Tsen, L.; Lee, C. C. Photocatalytic degradation and mineralization of bisphenol A by TiO2 and platinized TiO2. Appl. Catal. A 2004, 261, 225−237. (12) Tišler, T.; Zagorc Končan, J.; Cotman, M.; Drolc, A. Toxicity potential of disinfection agent in tannery wastewater. Water Res. 2004, 383, 3503−3510. (13) Pintar, A.; Batista, J.; Tišler, T. Catalytic wet-air oxidation of aqueous solutions of formic acid, acetic acid and phenol in a continuous-flow trickle-bed reactor over Ru/TiO2 catalysts. Appl. Catal. B 2008, 84, 30−41. (14) Campbell, C. G.; Borglin, S. E.; Green, F. B.; Grayson, A.; Wozei, E.; Stringfellow, W. T. Biologically directed environmental monitoring, fate, and transport of estrogenic endocrine disrupting compounds in water: A review. Chemosphere 2006, 65, 1265−1280. (15) Stuer-Lauridsen, J.; Kjølholt, F.; Høibye, L.; Hinge-Christensen, S.; Ingerslev, F.; Hansen, M.; Andersen Krogh, K.; Andersen, H. R.; Halling-Sørensen, B.; Hansen, N.; Køppen, B.; Bjerregaard, P.; Frost, B. Survey of Estrogenic Activity in the Danish Aquatic Environment; Danish Environmental Protection Agency: København, Denmark, 2005; available at http://www2.mst.dk/udgiv/publications/2005/877614-505-0/pdf/87-7614-506-9.pdf (accessed Nov. 20, 2007). (16) Nelson, J.; Bishay, F.; van Roodselaar, A.; Ikonomou, M.; Law, F. C. P. The use of in vitro bioassays to quantify endocrine disrupting chemicals in municipal wastewater treatment plant effluents. Sci. Total Environ. 2007, 374, 80−90. (17) Oliviero, L.; Barbier, J.; Duprez, D. Wet air oxidation of nitrogen-containing organic compounds and ammonia in aqueous media. Appl. Catal. B 2003, 40, 163−184. (18) Luck, F. Wet air oxidation: Past, present and future. Catal. Today 1999, 53, 81−91. (19) Kim, K. H.; Ihm, S. K. Heterogeneous catalytic wet air oxidation of refractory organic pollutants in industrial wastewaters: A review. J. Hazard. Mater. 2011, 186, 16−34. (20) Roy, S.; Vashishtha, M.; Saroha, A. K. Catalytic wet air oxidation of oxalic acid using platinum catalysts in bubble column reactor: A review. J. Eng. Sci. Technol. Rev. 2010, 3, 95−107. (21) Bistan, M.; Tišler, T.; Pintar, A. Ru/TiO2 catalyst for efficient removal of estrogens from aqueous samples by means of wet-air oxidation. Catal. Commun. 2012, 22, 74−78. (22) Bistan, M.; Tišler, T.; Pintar, A. Catalytic and photocatalytic oxidation of aqueous bisphenol A solutions: Removal, toxicity, and estrogenicity. Ind. Eng. Chem. Res. 2012, 51, 8826−8834. (23) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Titania nanotubes prepared by chemical processing. Adv. Mater. 1999, 11, 1307−1311. (24) Erjavec, B.; Kaplan, R.; Djinović, P.; Pintar, A. Catalytic wet air oxidation of bisphenol A model solution in a trickle-bed reactor over titanate nanotube-based catalysts. Appl. Catal. B 2013, 132−133, 342− 352. (25) Routledge, E. J.; Sumpter, J. P. Estrogenic activity of surfactants and some of their degradation products assessed using a recombinant yeast screen. Environ. Toxicol. Chem. 1996, 15, 241−248. (26) Bistan, M.; Marinšek Logar, R.; Tišler, T. Detection of estrogenic activity in Slovenian wastewaters by bioassay. Cent. Eur. J. Biol. 2011, 6, 829−837. (27) Water Quality − Determination of the Inhibitory Effect of Water Samples on the Light Emission of Vibrio fischeri (Luminescent Bacteria Test) − Part 2: Method Using Liquid-Dried Bacteria; ISO 11348-2; International Organization for Standardization: Geneva, Switzerland, 2007. (28) Schweitzer, N.; Fink, G.; Ternes, T. A.; Duis, K. Effects of ivermectin-spiked cattle dung on a water−sediment system with the 12566


Titanate Nanotubes As a Novel Catalyst for Removal of Toxicity and

Recommend Documents