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biochar nanocomposites as catalysts
Ultrasonics - Sonochemistry 57 (2019) 22–28
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Ultrasound-assisted heterogeneous Fenton-like process for bisphenol A removal at neutral pH using hierarchically structured manganese dioxide/ biochar nanocomposites as catalysts
T
Kyung-Won Junga, Seon Yong Leeb, Young Jae Leeb, Jae-Woo Choia,c,
⁎
a
Water Cycle Research Center, Korea Institute of Science and Technology, 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea Department of Earth and Environmental Sciences, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea c Division of Energy and Environmental Engineering, KIST School, Korea University of Science and Technology, 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea b
ARTICLE INFO
ABSTRACT
Keywords: Bisphenol A Ultrasound-assisted heterogeneous Fenton-like Urchin-like α-MnO2 Biochar Nanocomposites
Bisphenol A (BPA) is an important emerging contaminant with endocrine-disrupting potential that has frequently been detected in aquatic environments. In this study, two types of hierarchically structured manganese dioxide/biochar nanocomposites (MnO2/BCs) were prepared for the first time via facile hydrothermal synthesis. The hydrothermal reaction was maintained at 100 °C for 6 h or 12 h, after which an ultrasound-assisted heterogeneous Fenton-like process was used to catalyze the removal of BPA under neutral pH condition. The characterization results indicated that MnO2 nanoparticles were successfully formed on the nanocomposite surfaces and had flower-like (δ-MnO2, 6 h) and urchin-like (α-MnO2, 12 h) morphology. This enabled a significant improvement in the catalytic activity of BPA removal by the reversible redox reaction. A series of experiments confirmed that the crystalline properties of the nanocomposites affected their catalytic activity. In particular, the α-MnO2/BCs exhibited catalytic activity in the ultrasound-assisted heterogeneous Fenton-like process and completely removed BPA within 20 min under the following conditions: [BPA]0 = 100 μM; [H2O2]0 = 10 mM; [catalyst]0 = 0.5 g/L; ultrasound = 20 kHz (130 W) at 40% amplitude; pH = 7.0 ± 0.1; and temperature = 25 ± 1 °C. This efficiency may have been due to the synergistic effect of ultrasound and αMnO2/BCs, which simultaneously induce the effective generation of reactive free radicals and increase the mass transfer rate at the solid-liquid interface. Overall, these results demonstrated that hierarchical urchin-like αMnO2/BCs have significant potential as an efficient and low-cost catalyst in ultrasound-assisted heterogeneous Fenton-like systems.
1. Introduction Because emerging organic contaminants pose a serious threat to the environment and human health, their presence in natural waters has become a global concern. Bisphenol A [BPA; 2,2-bis (4-hydroxyphenyl) propane], a suspected endocrine disrupting compound, is a phenolic chemical that is widely used as an intermediate material for the manufacture of paints, polycarbonate plastics, paper, adhesives, and epoxy resins [1,2]. The extensive demand for and widespread use of BPA in vital industries and commercial products has resulted in its continuous release into the environment in large amounts. As a result, BPA is frequently detected in a wide variety of aquatic environments, including surface waters, groundwater, and even drinking water [3–5]. The major
concern regarding the widespread distribution of BPA in water systems is that it has adverse effects on the endocrine systems of living organisms even at concentrations as low as nanograms per liter by mimicking the activity of natural hormones or occupying hormone receptors in the body, thus limiting or completely inhibiting gene expression [6]. Advanced oxidation processes (AOPs) based on the in situ generation of non-selective and powerful oxygen species, particularly hydroxyl radicals (HO%), have been considered a promising and practical technique for removing non-biodegradable organic contaminants owing to their robustness, simple operation, versatility, and environmental friendliness [7–10]. Although the classical homogeneous Fenton process, which uses ferrous salts as the catalyst and hydrogen peroxide (H2O2) as the oxidant, is one of the most widely adopted AOPs, its
⁎ Corresponding author at: Water Cycle Research Center, Korea Institute of Science and Technology, 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea. E-mail address: [email protected] (J.-W. Choi).
https://doi.org/10.1016/j.ultsonch.2019.04.039 Received 15 March 2019; Received in revised form 17 April 2019; Accepted 27 April 2019 Available online 29 April 2019 1350-4177/ © 2019 Elsevier B.V. All rights reserved.
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practical application for environmental remediation is greatly hindered by the narrow pH range required (acidic condition) and the large volumes of sludge that are produced during the reaction [8,11]. To address these weaknesses, metal-oxide-nanoparticles-based heterogeneous Fenton-like processes were developed and are recognized to be a more promising and practical technique owing to their wide operating pH range and reduced sludge generation [12–14]. Manganese dioxide (MnO2), which is ubiquitous in natural soils and sediments, has received increasing attention as an effective heterogeneous Fenton-like catalyst material because of its low environmental toxicity and unique physicochemical properties, such as dynamic redox potential, large surface area, and multiple oxidation states [15]. However, the high surface reactivity of nanoparticles such as MnO2 often leads to strong agglomeration owing to their attractive interactions as a result of the van der Waals force. This results in a lower active surface area, and thus the suppression of catalytic activity [16]. Biochar, which is obtained by the thermal decomposition of carbonrich biomass residuals under oxygen-limited conditions, has attracted significant interest in recent years as a highly promising low-cost adsorbent to potentially replace activated carbon for removing organic and inorganic contaminants from aqueous solutions [17]. In addition to its use as an adsorbent, because of its redox activity and electron shuttling capacity, biochar can also act as a catalyst for the generation of HO% by activating H2O2 [18]. Furthermore, the application of biochar as a support material for nanocomposite synthesis can effectively prevent the agglomeration of MnO2, thus enabling its efficient utilization. Considering their advantages, biochar-supported MnO2 nanocomposites are thus expected to have potential as a highly efficient and cost-effective catalyst in the heterogeneous Fenton-like process, which would significantly expand the applicability of biochar in environmental remediation. In recent years, to simultaneously enhance degradation rates and reduce operation time, a modified AOP consisting of a combination of a Fenton or Fenton-like process with ultrasound has attracted considerable attention [19–22]. When ultrasound irradiation is introduced, the resulting collapse of cavitation microbubbles generates localized conditions of extreme pressure (∼500 bar) and temperature (∼5000 K). This simultaneously facilitates the formation of reactive free radicals and a reduction in the mass transfer resistance, as well as improving the catalyst dispersibility, thereby enabling effective degradation of organic pollutants [23]. To date, very few studies have reported using biochar as a Fenton-like catalyst to remove organic pollutants [24–26], and no previous studies have focused on the synthesis of hierarchically structured MnO2/biochar nanocomposites (MnO2/BCs) or their application in ultrasound-assisted heterogeneous Fenton-like processes. Herein, two types of three-dimensional hierarchically structured (MnO2/BCs) were prepared via hydrothermal synthesis with a controlled duration. To compare the effect of their crystalline structures on the catalytic activity, a series of experiments was conducted using single (H2O2, ultrasound, or catalyst alone), dual (catalyst + H2O2 and ultrasound + H2O2 or catalyst), and triple (ultrasound + catalyst + H2O2) systems.
2.2. Preparation of MnO2/biochar nanocomposites Rice husk was used as a precursor for the biochar preparation, which was obtained from a rice field in Chungnam Province in the Republic of Korea. Before synthesis, the rice husk was ground and passed through a 0.5-mm sieve, washed several times with DI water, and oven-dried at 60 °C for 24 h. The dried samples were then heated in a horizontal electric furnace to 600 °C for 1 h at a heating rate of 7 °C/ min under N2 at a flow rate of 25 mL/min. After cooling to room temperature, the resultant materials were washed several times with DI water and then oven-dried at 60 °C for 24 h. The rice husk-derived biochar obtained from this process is subsequently referred to as pristine biochar (PBC). Next, two types of three-dimensionally structured MnO2/BCs were synthesized following a hydrothermal method. Briefly, 0.45 g of KMnO4 was dissolved in 80 mL of DI water at room temperature, and 1.0 mL of HCl was added dropwise with magnetic stirring. Thereafter, 0.45 g of PBC was placed in the prepared solution, which was stirred for 1 h. The mixture was transferred into a 100 mL Teflonlined stainless-steel autoclave and heated at 100 °C for 6 or 12 h. After cooling to room temperature, the samples were collected by vacuum filtration and rinsed several times using DI water. Finally, the samples were dried at 60 °C for 24 h and kept in a sealed container until use. 2.3. Experimental procedure Ultrasound-assisted heterogeneous Fenton-like catalytic removal of BPA was evaluated by injecting 0.5 g/L of catalyst into a 500 mL aluminum-foil-wrapped jacketed glass reactor containing 200 mL of BPA solution. The temperature in the reactor was controlled at 25 ± 1 °C using a cooling water circulator. An ultrasonicator with an ultrasonic probe (VCX-130, Sonics and Materials, USA) was operated with 130 W of power, a frequency of 20 kHz, and an amplitude of 40%. The other experimental conditions were as follows: initial BPA concentration = 100 μM; H2 O2 concentration = 10 mM; pH = 7.0 ± 0.1; and reaction time = 120 min. In the experiments without H2O2 or ultrasound, the as-synthesized samples were added to an Erlenmeyer flask and agitated at 150 rpm using an orbital shaking incubator (WIS02011, Wise-Cube®, Germany), and the other conditions were set as described above. All experiments were conducted in triplicate. 2.4. Analytical methods The BPA concentration in the solutions was determined using highperformance liquid chromatography (Phenomenex, Torrance, CA, USA). X-ray diffraction (XRD) patterns were analyzed using a D8 Advance Sol-X (Bruker Co., USA) with Cu Kα radiation at 40 kV (λ = 0.15418 nm). The surface morphologies and localized elemental compositions were simultaneously analyzed by scanning electron microscopy (SEM, FEI Quanta 200FEG, Thermo Scientific Quanta, USA) with energy dispersive X-ray spectroscopy (EDX) operated at 15.0 kV. Transmission electron microscopy (TEM) images were also obtained using a Talos F200X Field-Emission transmission electron microscope (FEI, USA) with EDX operated at 200 kV. The surface area of the assynthesized samples was evaluated by N2 adsorption-desorption at 77 K using a nanoporosity system (NP-XQ, Mirae Scientific Instruments, Republic of Korea). The point of zero charge (pHpzc) of the as-synthesized materials was determined using a zeta potential analyzer (Zetasizer Nano ZS, Malvern Instruments Ltd., UK). The elemental compositions before and after BPA removal were determined using Xray photoelectron spectroscopy (XPS, X-tool, ULVAC-PHI, Japan).
2. Materials and methods 2.1. Materials All chemicals and reagents used in this study were of analytical grade. Standard BPA (> 99%), potassium permanganate (KMnO4), and hydrogen peroxide (H2O2, 30%) were purchased from Sigma-Aldrich Korea and used as received without further purification. All aqueous solutions were prepared using distilled deionized (DI) water (18.2 MΩ, Milli-Q Plus, Merck Millipore Co., Germany).
3. Results and discussion The crystalline structure of the as-synthesized nanocomposites was examined via powder XRD analysis, and the results are shown in Fig. 1. 23
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MnO2 in the hydrothermal process can be explained by the mechanisms of nucleation and crystal growth [27]. A number of tiny crystalline MnO2 nuclei form initially, and heterogeneous crystal growth of nanosheets on the nuclei is then initiated under hydrothermal conditions with the aid of temperature and autogenous pressure; thus, a hierarchical flower-like structure is formed [28]. As the duration is extended, the system enters a thermodynamically stable state, which induces dissolution of the inner cores and recrystallization of the outer surface of the spherical nanoparticles owing to the energy difference between the inner and outer components; thus, an urchin-like structure is formed [27,29,30]. A comparative study of the BPA removal efficiency was performed in different systems: single (H2O2, ultrasound [US], and catalyst [PBC, α-MnO2/BCs, or δ-MnO2/BCs]), dual (catalyst + H2O2 and US + H2O2 or catalyst), and triple (US + α-MnO2/BCs + H2O2 and US + δ-MnO2/ BCs + H2O2) systems. Fig. 3 shows the BPA removal performance (Ct/ C0, where Ct and C0 are the BPA concentration at time t and t = 0, respectively) as a function of reaction time in the various systems. Overall, the experimental results indicated that BPA removal was insignificant when each system was employed separately, whereas BPA removal was remarkably enhanced in the combined systems, especially in the ultrasound-assisted heterogeneous Fenton-like process. As shown in Fig. 3(A), when only H2O2 existed in the reaction systems, no obvious BPA removal was observed after 120 min, indicating that activation of H2O2 is negligible without the use of ultrasound and a catalyst. In addition, Fig. 3(A) shows that, in the presence of a single catalyst in the reaction system, BPA removal was approximately 12.5%, 21.5%, and 41.9% in the PBC, α-MnO2/BCs, and δ-MnO2/BCs systems after 120 min, respectively. This pattern could be mainly attributed to a surface adsorption mechanism. The PBC exhibited the lowest BPA adsorption performance, possibly because it has the smallest surface area (53.98 m2/g; see Fig. S3(A)) and because of electrostatic repulsion between the positively charged BPA (pKa = 9.6) [31] and the positively charged biochar surface (pHpzc = 8.43; see Fig. S4) under the tested conditions. Although the surface charges of α-MnO2/BCs and δ-MnO2/ BCs are both negative (pHpzc = 5.94 and 3.64, respectively; see Fig. S4) at the given pH, δ-MnO2/BCs exhibited better adsorption performance. This was likely owing to their more negative surface charge, which induces a stronger binding affinity of δ-MnO2/BCs toward BPA compared to α-MnO2/BCs. In contrast, as shown in Fig. 3(A), in the heterogeneous Fenton-like systems (dual: catalyst + H2O2), complete BPA removal was achieved within 100 min in the α-MnO2/BCs + H2O2 system, whereas BPA removal approximated 68.9% and 96.5% for the PBC + H2O2 and δMnO2/BCs + H2O2 systems, respectively, after 120 min. To determine the kinetic rate constants of BPA removal, a first-order kinetic model
Fig. 1. XRD patterns of the as-synthesized nanocomposites (black: 6 h, blue: 12 h). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The results indicate that two different types of MnO2/BCs were successfully formed on the PBC surface by controlling the duration of the hydrothermal processes. The sample prepared at 100 °C for 6 h exhibited diffraction peaks at 2θ values of approximately 12.1°, 24.3°, 36.9°, and 65.9°, which correspond to those of δ-MnO2 (JCPDS No. 801098). Furthermore, the sample prepared at 100 °C for 12 h exhibited diffraction peaks at 2θ values of approximately 12.7°, 18.2°, 28.7°, 37.5°, 42.1°, 49.8°, 56.4°, 60.1°, and 65.6°, which are indexed to αMnO2 (JCPDS No. 44-0141). No other peaks were observed, indicating high purity. Based on the XRD results, the as-synthesized materials obtained after 6 h and 12 h were identified as δ-MnO2/BCs and αMnO2/BCs, respectively. The morphological properties of these two types of nanocomposites were further analyzed by SEM and TEM analyses, as shown in Fig. 2. The SEM and TEM images clearly demonstrate that three-dimensional frameworks were successfully formed on the biochar surfaces. These consisted of urchin-like α-MnO2 with a diameter of 5–15 nm and a length of 20–200 nm (Fig. 2(A1–4)) and flowerlike δ-MnO2 with a diameter of 1–2 μm (Fig. 2(B1–4)). The elemental distribution mapping and the corresponding EDX spectrum (see Figs. S1 and S2) further confirmed the presence of O and Mn in both samples, where the atomic percent of O and Mn contents in α-MnO2/BC and δMnO2/BC were determined as 41.01 ± 0.52 atom% and 44.39 ± 0.36 atom%, and 44.80 ± 0.44 atom% and 45.03 ± 0.36 atom%, respectively. The time-dependent crystalline and morphological structures of
Fig. 2. SEM and TEM images of (A) α-MnO2/BCs and (B) δ-MnO2/BCs. 24
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Fig. 3. BPA removal in different systems. Error bars indicate the standard deviation of the mean value (n = 3). Experimental conditions: [BPA]0 = 100 μM; [H2O2]0 = 10 mM; [catalyst]0 = 0.5 g/L; ultrasound = 20 kHz (130 W) at 40% amplitude; pH = 7.0 ± 0.1; and temperature = 25 ± 1 °C.
was applied, which can be expressed as follows:
ln
Ct = C0
kt
(C), respectively). The activity of highly crystallized catalysts is reportedly superior to that of their poorly crystallized or amorphous counterparts [35,36]. In addition, δ-MnO2 has a higher Mn–O bond strength and a much smaller interlayer separation than α-MnO2, and these factors are relatively unfavorable for effective catalytic activity [37]. Thus, the differences in the catalytic activity among the MnO2/ BCs may result from the differences in their surface areas, crystallinities, and structural interlayer properties. As shown in Fig. 3(C), when ultrasound (US) alone was employed, a low BPA removal of approximately 15% was obtained after 120 min. This indicated that BPA could not be efficiently removed by ultrasound alone because the sonolysis efficiency of the aqueous solution (required to generate reactive radicals from water molecules in the absence of an oxidant) was insufficient [21,38]. However, when ultrasound and H2O2 were both present in the reaction system (US + H2O2), BPA removal was significantly enhanced. As shown in Fig. 3(C), approximately 75% BPA removal was obtained after 120 min, which was approximately five times that of the ultrasound-only system. This can be attributed to the effective generation of reactive HO% by simultaneous decomposition of H2O2 and dissociation of water molecules in localized hot spots, which are ultrasonically induced by the collapse of cavitation microbubbles [19,20]. In addition, Fig. 3(C) shows that although BPA removal by the ultrasound + catalyst systems (US + PBC, α-MnO2/BCs, or δ-MnO2/BCs) increased compared to the single systems (PBC, αMnO2/BCs, or δ-MnO2/BCs), this was nearly the same as for the single systems. This indicates that BPA removal in the absence of H2O2 resulted mainly from adsorption mechanisms. However, BPA removal was notably improved in the ultrasound-assisted heterogeneous Fentonlike systems, especially in the US + α-MnO2/BCs + H2O2 system,
(1) −1
where k denotes the measured kinetic rate constant (min ). Fig. 3(B) shows plots of ln (Ct/C0) versus reaction time, which demonstrate that the BPA removal processes are well fitted by the firstorder kinetics with satisfactory correlation coefficients (R2 > 0.98). The obtained kinetic rate constants of the H2O2 alone, PBC + H2O2, αand δ-MnO2/BCs + H2O2 systems were MnO2/BCs + H2O2, 0.0002 min−1, 0.0055 min−1, 0.0465 min−1, and 0.0227 min−1, respectively. The significantly improved BPA removal rates in both the MnO2/BCs + H2O2 systems compared to the PBC + H2O2 system were attributed to their higher adsorption performance compared to PBC, because the oxidative degradation process of organic pollutants generally follows an adsorption–oxidation mechanism [32]. Furthermore, the surface Mn(IV)/Mn(III) reversible redox reaction in the MnO2/BCs matrix, which accounts for the easier and higher decomposition activity of H2O2 for generating free radical species, resulted in higher catalytic BPA degradation [33]. Interestingly, although the δ-MnO2/BCs exhibited higher adsorption of BPA than the α-MnO2/BCs, the kinetic rate constant of the α-MnO2/BCs + H2O2 system was approximately twice that of the δ-MnO2 counterpart. This tendency may result from the difference in catalytic activity resulting from differences in their structural properties. In general, the structure of MnO2 consists of different interlinked MnO6 octahedra as a fundamental building unit with a chain-like tunnel (α-MnO2) structure and a sheet or layered (δ-MnO2) structure [34] that affects the crystallinity and surface area (173.42 m2/ g for α-MnO2/BCs and 111.87 m2/g for δ-MnO2/BCs; see Fig. S3(B) and 25
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where 100% BPA removal was obtained within 20 min, which exceeds other such systems. To evaluate the synergistic effect of the α-MnO2/ BCs-based ultrasound-assisted heterogeneous Fenton-like system on BPA removal, the synergy factor (SF) was calculated on the basis of the determined kinetic rate constants of BPA removal (Eq. (1)). This can be expressed as follows:
SF =
ktriple (2)
ksingle + kdual
where ksingle, kdual, and ktriple are the measured kinetic rate constants in the single (H2O2 and US alone), dual (α-MnO2/BCs + H2O2, US + H2O2, and US + α-MnO2/BCs), and triple (US + α-MnO2/ BCs + H2O2) systems, respectively. As presented in Fig. 3(B) and (D), the kinetic rate constants of the H2O2, α-MnO2/BCs + H2O2, US, US + H2O2, US + α-MnO2/BCs, and US + α-MnO2/BCs + H2O2 systems were determined as 0.0002 min−1, 0.0465 min−1, 0.0008 min−1, 0.0021 min−1, 0.0082 min−1, and 0.1565 min−1, respectively, with corresponding R2 values of more than 0.97. The determined SF value of approximately 2.71 demonstrates a significant synergistic effect of the ultrasound-assisted heterogeneous Fenton-like process for BPA removal. This result may be attributable to both enhanced microturbulence, as the driving force of BPA to overcome the mass transfer resistance at the solid-liquid interface [39] and the synergistic effects of ultrasound and α-MnO2/BCs on the effective decomposition of H2O2 into reactive free radicals, such as HO% and superoxide (O2%−) [40,41], as follows: H2O2 + ))) (ultrasound) → 2HO%
(3)
H2O + ))) (ultrasound) → HO +H %
≡Mn(IV) + H2O2 → ≡Mn(III) +
(4)
%
HO2%
+H
+
(5)
≡Mn(III) + H2O2 → ≡Mn(II) + HO2% + H+
(6)
HO2% + H2O2 → H2O + O2 + HO%
(7)
O2 + ))) (ultrasound) → 2O
(8)
%
O + H2O → 2HO %
HO2%
(9)
%
+
→H
+ O2%−
(10) −
≡Mn(III) + HO → ≡Mn(IV) + OH %
(11) −
≡Mn(II) + H2O2 → ≡Mn(III) + HO + OH %
(12)
First, H2O2 and water molecules were simultaneously decomposed by ultrasound irradiation to generate HO% (Eqs. (3) and (4)). Simultaneously, catalytic reactions between the α-MnO2/BCs and H2O2 would induce oxidation of lattice oxygen, leading to the reduction of oxidation states on the α-MnO2/BCs surface [≡Mn(IV) to ≡Mn(III)) or ≡Mn(III) to ≡Mn(II)] and the formation of perhydroxyl radicals (HO2%) (Eqs. (5) and (6)). These phenomena are indirectly supported by the results of the XPS analysis of the α-MnO2/BCs before and after BPA removal. As shown in Fig. 4(A), the Mn 2p spectrum showed two typical peaks located at binding energies of 654.00 eV and 642.25 eV, which correspond to Mn 2p1/2 and Mn 2p3/2, respectively, with a spin-energy separation of 11.75 eV. In addition, two peaks in the Mn 2p3/2 peak appeared at binding energies of 642.7 eV and 643.0 eV, which correspond to Mn(III) and Mn(IV), respectively [42]. These observations are consistent with the presence of α-MnO2 and indicate the presence of the Mn(IV)/Mn(III) oxidation state in the α-MnO2/BCs matrix [43]. As shown in Fig. 4(B), after BPA removal, the binding energy of Mn 2p3/2 shifted slightly to a lower value of 641.88 eV with changes in the ratios of the integral area of Mn(IV) and Mn(III), likely because of electron transfer between the α-MnO2/BCs and H2O2. In addition, as shown in Fig. 4(C), the O 1s spectrum of the α-MnO2/BCs exhibited three main peaks at binding energies of 529.5 eV, 531.1 eV, and 533 eV, where the ratios of the integral areas are 64.69%, 27.24%, and 8.07%,
Fig. 4. (A) Mn 2p, (B) Mn 2p3/2, and (C) O 1s XPS spectra of α-MnO2/BCs before and after BPA removal. Experimental conditions: [BPA]0 = 100 μM; [H2O2]0 = 10 mM; [catalyst]0 = 0.5 g/L; ultrasound = 20 kHz (130 W) at 40% amplitude; reaction time = 20 min; pH = 7.0 ± 0.1; and temperature = 25 ± 1 °C.
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Fig. 5. Schematic diagram of chain reactions in the α-MnO2/BCs-based ultrasound-assisted heterogeneous Fenton-like process.
respectively; these peaks are attributed to Mn–O–Mn, Mn–O–H, and H–O–H, respectively. After BPA removal, the ratio of the integral area of Mn–O–Mn decreased to 52.15% and that of Mn–O–H increased to 42.69%, indicating an increase in the lower Mnx+ oxidation state. Thus, these results indicate that the oxidation states on the α-MnO2/BCs surfaces were reduced by the consumption of lattice oxygen in the decomposition of H2O2 into HO2%, thereby promoting the reversion of ≡Mn(IV) to ≡Mn(III) or ≡Mn(III) to ≡Mn(II) [44]. Accordingly, in addition to cleavage of H2O2, the radical chain reactions of HO2% with H2O2, and the decomposition of the generated O2 molecules by ultrasound irradiation were also responsible for generating HO% and O2%−, respectively (Eqs. (7)−(10)). Alternatively, the ≡Mn(III) or ≡Mn(II) on the surfaces of α-MnO2/BCs could be recovered to its original form [≡Mn(IV) or ≡Mn(III)] by reaction with some of the remaining unreacted HO% and undecomposed H2O2 in the reaction system (Eqs. (11) and (12)). On the basis of these described reactions, a series of chain reactions is illustrated in Fig. 5 for the α-MnO2/BCs-based ultrasoundassisted heterogeneous Fenton-like process. Lastly, as presented in Fig. S5, the α-MnO2/BCs-based ultrasoundassisted heterogeneous Fenton-like process exhibited stable and efficient BPA removal during the entire recycling reactions; BPA removal was approximately 93% in the fourth run, suggesting that α-MnO2/BCs exhibit highly reusable catalytic activity in ultrasound-assisted heterogeneous Fenton-like process for the removal of BPA. Despite its high stability and reusability, total organic carbon (TOC) removal was limited, with approximately 63.7% and 45.1% removal after the first and fourth runs, respectively. This indicates the mineralization of BPA molecules and the requirement for a prolonged reaction time and process optimization to achieve the complete removal of the generated intermediates. Therefore, more detailed research, including the optimization of the solution chemistry parameters (e.g., catalyst dosage, initial pH, H2O2 concentration, ultrasound intensity, temperature, etc.) and the determination of the possible pathway for BPA removal will be performed in the near further. In addition, the structural stability and reusability of hierarchical urchin-like α-MnO2/BCs as a catalyst in ultrasound-assisted heterogeneous Fenton-like processes under such optimized conditions will be evaluated.
4. Conclusion Two types of three-dimensionally structured MnO2/BCs were synthesized via a duration-controlled hydrothermal process, and their ultrasound-assisted heterogeneous Fenton-like catalytic activity was evaluated. The results showed that urchin-like α-MnO2/BCs exhibited the highest ultrasound-assisted heterogeneous Fenton-like catalytic activity, achieving complete BPA removal within 20 min under the following conditions: [BPA]0 = 100 μM; [H2O2]0 = 10 mM; [catalyst]0 = 0.5 g/L; ultrasound = 20 kHz (130 W) at 40% amplitude; pH = 7.0 ± 0.1; and temperature = 25 ± 1 °C. Overall, the findings suggest that hierarchical urchin-like α-MnO2/BCs can be expected to exhibit excellent catalytic performance in ultrasound-assisted heterogeneous Fenton-like processes, giving them wide applicability to the environmental remediation of various organic pollutants. Acknowledgement This work was supported by an institutional program grant (Project No.: 2E29660) from the Korea Institute of Science and Technology. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ultsonch.2019.04.039. References [1] W. Liu, H. Zhang, B. Cao, K. Lin, J. Gan, Oxidative removal of bisphenol A using zero valent aluminum–acid system, Water Res. 45 (2011) 1872–1878. [2] Y. Zhou, X. Fang, T. Wang, Y. Hu, J. Lu, Chelating agents enhanced CaO2 oxidation of bisphenol A catalyzed by Fe3+ and reuse of ferric sludge as a source of catalyst, Chem. Eng. J. 313 (2017) 638–645. [3] Z.-H. Liu, Y. Kanjo, S. Mizutani, Removal mechanisms for endocrine disrupting compounds (EDCs) in wastewater treatment — physical means, biodegradation, and chemical advanced oxidation: a review, Sci. Total Environ. 407 (2009) 731–748. [4] A.R. Bakr, M.S. Rahaman, Removal of bisphenol A by electrochemical carbon-nanotube filter: influential factors and degradation pathway, Chemosphere 185
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Ultrasound-assisted heterogeneous Fenton-like process for bisphenol A removal at neutral pH using hierarchically structured manganese dioxide/ biochar nanocomposites as catalysts
T
Kyung-Won Junga, Seon Yong Leeb, Young Jae Leeb, Jae-Woo Choia,c,
⁎
a
Water Cycle Research Center, Korea Institute of Science and Technology, 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea Department of Earth and Environmental Sciences, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea c Division of Energy and Environmental Engineering, KIST School, Korea University of Science and Technology, 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea b
ARTICLE INFO
ABSTRACT
Keywords: Bisphenol A Ultrasound-assisted heterogeneous Fenton-like Urchin-like α-MnO2 Biochar Nanocomposites
Bisphenol A (BPA) is an important emerging contaminant with endocrine-disrupting potential that has frequently been detected in aquatic environments. In this study, two types of hierarchically structured manganese dioxide/biochar nanocomposites (MnO2/BCs) were prepared for the first time via facile hydrothermal synthesis. The hydrothermal reaction was maintained at 100 °C for 6 h or 12 h, after which an ultrasound-assisted heterogeneous Fenton-like process was used to catalyze the removal of BPA under neutral pH condition. The characterization results indicated that MnO2 nanoparticles were successfully formed on the nanocomposite surfaces and had flower-like (δ-MnO2, 6 h) and urchin-like (α-MnO2, 12 h) morphology. This enabled a significant improvement in the catalytic activity of BPA removal by the reversible redox reaction. A series of experiments confirmed that the crystalline properties of the nanocomposites affected their catalytic activity. In particular, the α-MnO2/BCs exhibited catalytic activity in the ultrasound-assisted heterogeneous Fenton-like process and completely removed BPA within 20 min under the following conditions: [BPA]0 = 100 μM; [H2O2]0 = 10 mM; [catalyst]0 = 0.5 g/L; ultrasound = 20 kHz (130 W) at 40% amplitude; pH = 7.0 ± 0.1; and temperature = 25 ± 1 °C. This efficiency may have been due to the synergistic effect of ultrasound and αMnO2/BCs, which simultaneously induce the effective generation of reactive free radicals and increase the mass transfer rate at the solid-liquid interface. Overall, these results demonstrated that hierarchical urchin-like αMnO2/BCs have significant potential as an efficient and low-cost catalyst in ultrasound-assisted heterogeneous Fenton-like systems.
1. Introduction Because emerging organic contaminants pose a serious threat to the environment and human health, their presence in natural waters has become a global concern. Bisphenol A [BPA; 2,2-bis (4-hydroxyphenyl) propane], a suspected endocrine disrupting compound, is a phenolic chemical that is widely used as an intermediate material for the manufacture of paints, polycarbonate plastics, paper, adhesives, and epoxy resins [1,2]. The extensive demand for and widespread use of BPA in vital industries and commercial products has resulted in its continuous release into the environment in large amounts. As a result, BPA is frequently detected in a wide variety of aquatic environments, including surface waters, groundwater, and even drinking water [3–5]. The major
concern regarding the widespread distribution of BPA in water systems is that it has adverse effects on the endocrine systems of living organisms even at concentrations as low as nanograms per liter by mimicking the activity of natural hormones or occupying hormone receptors in the body, thus limiting or completely inhibiting gene expression [6]. Advanced oxidation processes (AOPs) based on the in situ generation of non-selective and powerful oxygen species, particularly hydroxyl radicals (HO%), have been considered a promising and practical technique for removing non-biodegradable organic contaminants owing to their robustness, simple operation, versatility, and environmental friendliness [7–10]. Although the classical homogeneous Fenton process, which uses ferrous salts as the catalyst and hydrogen peroxide (H2O2) as the oxidant, is one of the most widely adopted AOPs, its
⁎ Corresponding author at: Water Cycle Research Center, Korea Institute of Science and Technology, 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea. E-mail address: [email protected] (J.-W. Choi).
https://doi.org/10.1016/j.ultsonch.2019.04.039 Received 15 March 2019; Received in revised form 17 April 2019; Accepted 27 April 2019 Available online 29 April 2019 1350-4177/ © 2019 Elsevier B.V. All rights reserved.
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practical application for environmental remediation is greatly hindered by the narrow pH range required (acidic condition) and the large volumes of sludge that are produced during the reaction [8,11]. To address these weaknesses, metal-oxide-nanoparticles-based heterogeneous Fenton-like processes were developed and are recognized to be a more promising and practical technique owing to their wide operating pH range and reduced sludge generation [12–14]. Manganese dioxide (MnO2), which is ubiquitous in natural soils and sediments, has received increasing attention as an effective heterogeneous Fenton-like catalyst material because of its low environmental toxicity and unique physicochemical properties, such as dynamic redox potential, large surface area, and multiple oxidation states [15]. However, the high surface reactivity of nanoparticles such as MnO2 often leads to strong agglomeration owing to their attractive interactions as a result of the van der Waals force. This results in a lower active surface area, and thus the suppression of catalytic activity [16]. Biochar, which is obtained by the thermal decomposition of carbonrich biomass residuals under oxygen-limited conditions, has attracted significant interest in recent years as a highly promising low-cost adsorbent to potentially replace activated carbon for removing organic and inorganic contaminants from aqueous solutions [17]. In addition to its use as an adsorbent, because of its redox activity and electron shuttling capacity, biochar can also act as a catalyst for the generation of HO% by activating H2O2 [18]. Furthermore, the application of biochar as a support material for nanocomposite synthesis can effectively prevent the agglomeration of MnO2, thus enabling its efficient utilization. Considering their advantages, biochar-supported MnO2 nanocomposites are thus expected to have potential as a highly efficient and cost-effective catalyst in the heterogeneous Fenton-like process, which would significantly expand the applicability of biochar in environmental remediation. In recent years, to simultaneously enhance degradation rates and reduce operation time, a modified AOP consisting of a combination of a Fenton or Fenton-like process with ultrasound has attracted considerable attention [19–22]. When ultrasound irradiation is introduced, the resulting collapse of cavitation microbubbles generates localized conditions of extreme pressure (∼500 bar) and temperature (∼5000 K). This simultaneously facilitates the formation of reactive free radicals and a reduction in the mass transfer resistance, as well as improving the catalyst dispersibility, thereby enabling effective degradation of organic pollutants [23]. To date, very few studies have reported using biochar as a Fenton-like catalyst to remove organic pollutants [24–26], and no previous studies have focused on the synthesis of hierarchically structured MnO2/biochar nanocomposites (MnO2/BCs) or their application in ultrasound-assisted heterogeneous Fenton-like processes. Herein, two types of three-dimensional hierarchically structured (MnO2/BCs) were prepared via hydrothermal synthesis with a controlled duration. To compare the effect of their crystalline structures on the catalytic activity, a series of experiments was conducted using single (H2O2, ultrasound, or catalyst alone), dual (catalyst + H2O2 and ultrasound + H2O2 or catalyst), and triple (ultrasound + catalyst + H2O2) systems.
2.2. Preparation of MnO2/biochar nanocomposites Rice husk was used as a precursor for the biochar preparation, which was obtained from a rice field in Chungnam Province in the Republic of Korea. Before synthesis, the rice husk was ground and passed through a 0.5-mm sieve, washed several times with DI water, and oven-dried at 60 °C for 24 h. The dried samples were then heated in a horizontal electric furnace to 600 °C for 1 h at a heating rate of 7 °C/ min under N2 at a flow rate of 25 mL/min. After cooling to room temperature, the resultant materials were washed several times with DI water and then oven-dried at 60 °C for 24 h. The rice husk-derived biochar obtained from this process is subsequently referred to as pristine biochar (PBC). Next, two types of three-dimensionally structured MnO2/BCs were synthesized following a hydrothermal method. Briefly, 0.45 g of KMnO4 was dissolved in 80 mL of DI water at room temperature, and 1.0 mL of HCl was added dropwise with magnetic stirring. Thereafter, 0.45 g of PBC was placed in the prepared solution, which was stirred for 1 h. The mixture was transferred into a 100 mL Teflonlined stainless-steel autoclave and heated at 100 °C for 6 or 12 h. After cooling to room temperature, the samples were collected by vacuum filtration and rinsed several times using DI water. Finally, the samples were dried at 60 °C for 24 h and kept in a sealed container until use. 2.3. Experimental procedure Ultrasound-assisted heterogeneous Fenton-like catalytic removal of BPA was evaluated by injecting 0.5 g/L of catalyst into a 500 mL aluminum-foil-wrapped jacketed glass reactor containing 200 mL of BPA solution. The temperature in the reactor was controlled at 25 ± 1 °C using a cooling water circulator. An ultrasonicator with an ultrasonic probe (VCX-130, Sonics and Materials, USA) was operated with 130 W of power, a frequency of 20 kHz, and an amplitude of 40%. The other experimental conditions were as follows: initial BPA concentration = 100 μM; H2 O2 concentration = 10 mM; pH = 7.0 ± 0.1; and reaction time = 120 min. In the experiments without H2O2 or ultrasound, the as-synthesized samples were added to an Erlenmeyer flask and agitated at 150 rpm using an orbital shaking incubator (WIS02011, Wise-Cube®, Germany), and the other conditions were set as described above. All experiments were conducted in triplicate. 2.4. Analytical methods The BPA concentration in the solutions was determined using highperformance liquid chromatography (Phenomenex, Torrance, CA, USA). X-ray diffraction (XRD) patterns were analyzed using a D8 Advance Sol-X (Bruker Co., USA) with Cu Kα radiation at 40 kV (λ = 0.15418 nm). The surface morphologies and localized elemental compositions were simultaneously analyzed by scanning electron microscopy (SEM, FEI Quanta 200FEG, Thermo Scientific Quanta, USA) with energy dispersive X-ray spectroscopy (EDX) operated at 15.0 kV. Transmission electron microscopy (TEM) images were also obtained using a Talos F200X Field-Emission transmission electron microscope (FEI, USA) with EDX operated at 200 kV. The surface area of the assynthesized samples was evaluated by N2 adsorption-desorption at 77 K using a nanoporosity system (NP-XQ, Mirae Scientific Instruments, Republic of Korea). The point of zero charge (pHpzc) of the as-synthesized materials was determined using a zeta potential analyzer (Zetasizer Nano ZS, Malvern Instruments Ltd., UK). The elemental compositions before and after BPA removal were determined using Xray photoelectron spectroscopy (XPS, X-tool, ULVAC-PHI, Japan).
2. Materials and methods 2.1. Materials All chemicals and reagents used in this study were of analytical grade. Standard BPA (> 99%), potassium permanganate (KMnO4), and hydrogen peroxide (H2O2, 30%) were purchased from Sigma-Aldrich Korea and used as received without further purification. All aqueous solutions were prepared using distilled deionized (DI) water (18.2 MΩ, Milli-Q Plus, Merck Millipore Co., Germany).
3. Results and discussion The crystalline structure of the as-synthesized nanocomposites was examined via powder XRD analysis, and the results are shown in Fig. 1. 23
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MnO2 in the hydrothermal process can be explained by the mechanisms of nucleation and crystal growth [27]. A number of tiny crystalline MnO2 nuclei form initially, and heterogeneous crystal growth of nanosheets on the nuclei is then initiated under hydrothermal conditions with the aid of temperature and autogenous pressure; thus, a hierarchical flower-like structure is formed [28]. As the duration is extended, the system enters a thermodynamically stable state, which induces dissolution of the inner cores and recrystallization of the outer surface of the spherical nanoparticles owing to the energy difference between the inner and outer components; thus, an urchin-like structure is formed [27,29,30]. A comparative study of the BPA removal efficiency was performed in different systems: single (H2O2, ultrasound [US], and catalyst [PBC, α-MnO2/BCs, or δ-MnO2/BCs]), dual (catalyst + H2O2 and US + H2O2 or catalyst), and triple (US + α-MnO2/BCs + H2O2 and US + δ-MnO2/ BCs + H2O2) systems. Fig. 3 shows the BPA removal performance (Ct/ C0, where Ct and C0 are the BPA concentration at time t and t = 0, respectively) as a function of reaction time in the various systems. Overall, the experimental results indicated that BPA removal was insignificant when each system was employed separately, whereas BPA removal was remarkably enhanced in the combined systems, especially in the ultrasound-assisted heterogeneous Fenton-like process. As shown in Fig. 3(A), when only H2O2 existed in the reaction systems, no obvious BPA removal was observed after 120 min, indicating that activation of H2O2 is negligible without the use of ultrasound and a catalyst. In addition, Fig. 3(A) shows that, in the presence of a single catalyst in the reaction system, BPA removal was approximately 12.5%, 21.5%, and 41.9% in the PBC, α-MnO2/BCs, and δ-MnO2/BCs systems after 120 min, respectively. This pattern could be mainly attributed to a surface adsorption mechanism. The PBC exhibited the lowest BPA adsorption performance, possibly because it has the smallest surface area (53.98 m2/g; see Fig. S3(A)) and because of electrostatic repulsion between the positively charged BPA (pKa = 9.6) [31] and the positively charged biochar surface (pHpzc = 8.43; see Fig. S4) under the tested conditions. Although the surface charges of α-MnO2/BCs and δ-MnO2/ BCs are both negative (pHpzc = 5.94 and 3.64, respectively; see Fig. S4) at the given pH, δ-MnO2/BCs exhibited better adsorption performance. This was likely owing to their more negative surface charge, which induces a stronger binding affinity of δ-MnO2/BCs toward BPA compared to α-MnO2/BCs. In contrast, as shown in Fig. 3(A), in the heterogeneous Fenton-like systems (dual: catalyst + H2O2), complete BPA removal was achieved within 100 min in the α-MnO2/BCs + H2O2 system, whereas BPA removal approximated 68.9% and 96.5% for the PBC + H2O2 and δMnO2/BCs + H2O2 systems, respectively, after 120 min. To determine the kinetic rate constants of BPA removal, a first-order kinetic model
Fig. 1. XRD patterns of the as-synthesized nanocomposites (black: 6 h, blue: 12 h). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The results indicate that two different types of MnO2/BCs were successfully formed on the PBC surface by controlling the duration of the hydrothermal processes. The sample prepared at 100 °C for 6 h exhibited diffraction peaks at 2θ values of approximately 12.1°, 24.3°, 36.9°, and 65.9°, which correspond to those of δ-MnO2 (JCPDS No. 801098). Furthermore, the sample prepared at 100 °C for 12 h exhibited diffraction peaks at 2θ values of approximately 12.7°, 18.2°, 28.7°, 37.5°, 42.1°, 49.8°, 56.4°, 60.1°, and 65.6°, which are indexed to αMnO2 (JCPDS No. 44-0141). No other peaks were observed, indicating high purity. Based on the XRD results, the as-synthesized materials obtained after 6 h and 12 h were identified as δ-MnO2/BCs and αMnO2/BCs, respectively. The morphological properties of these two types of nanocomposites were further analyzed by SEM and TEM analyses, as shown in Fig. 2. The SEM and TEM images clearly demonstrate that three-dimensional frameworks were successfully formed on the biochar surfaces. These consisted of urchin-like α-MnO2 with a diameter of 5–15 nm and a length of 20–200 nm (Fig. 2(A1–4)) and flowerlike δ-MnO2 with a diameter of 1–2 μm (Fig. 2(B1–4)). The elemental distribution mapping and the corresponding EDX spectrum (see Figs. S1 and S2) further confirmed the presence of O and Mn in both samples, where the atomic percent of O and Mn contents in α-MnO2/BC and δMnO2/BC were determined as 41.01 ± 0.52 atom% and 44.39 ± 0.36 atom%, and 44.80 ± 0.44 atom% and 45.03 ± 0.36 atom%, respectively. The time-dependent crystalline and morphological structures of
Fig. 2. SEM and TEM images of (A) α-MnO2/BCs and (B) δ-MnO2/BCs. 24
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Fig. 3. BPA removal in different systems. Error bars indicate the standard deviation of the mean value (n = 3). Experimental conditions: [BPA]0 = 100 μM; [H2O2]0 = 10 mM; [catalyst]0 = 0.5 g/L; ultrasound = 20 kHz (130 W) at 40% amplitude; pH = 7.0 ± 0.1; and temperature = 25 ± 1 °C.
was applied, which can be expressed as follows:
ln
Ct = C0
kt
(C), respectively). The activity of highly crystallized catalysts is reportedly superior to that of their poorly crystallized or amorphous counterparts [35,36]. In addition, δ-MnO2 has a higher Mn–O bond strength and a much smaller interlayer separation than α-MnO2, and these factors are relatively unfavorable for effective catalytic activity [37]. Thus, the differences in the catalytic activity among the MnO2/ BCs may result from the differences in their surface areas, crystallinities, and structural interlayer properties. As shown in Fig. 3(C), when ultrasound (US) alone was employed, a low BPA removal of approximately 15% was obtained after 120 min. This indicated that BPA could not be efficiently removed by ultrasound alone because the sonolysis efficiency of the aqueous solution (required to generate reactive radicals from water molecules in the absence of an oxidant) was insufficient [21,38]. However, when ultrasound and H2O2 were both present in the reaction system (US + H2O2), BPA removal was significantly enhanced. As shown in Fig. 3(C), approximately 75% BPA removal was obtained after 120 min, which was approximately five times that of the ultrasound-only system. This can be attributed to the effective generation of reactive HO% by simultaneous decomposition of H2O2 and dissociation of water molecules in localized hot spots, which are ultrasonically induced by the collapse of cavitation microbubbles [19,20]. In addition, Fig. 3(C) shows that although BPA removal by the ultrasound + catalyst systems (US + PBC, α-MnO2/BCs, or δ-MnO2/BCs) increased compared to the single systems (PBC, αMnO2/BCs, or δ-MnO2/BCs), this was nearly the same as for the single systems. This indicates that BPA removal in the absence of H2O2 resulted mainly from adsorption mechanisms. However, BPA removal was notably improved in the ultrasound-assisted heterogeneous Fentonlike systems, especially in the US + α-MnO2/BCs + H2O2 system,
(1) −1
where k denotes the measured kinetic rate constant (min ). Fig. 3(B) shows plots of ln (Ct/C0) versus reaction time, which demonstrate that the BPA removal processes are well fitted by the firstorder kinetics with satisfactory correlation coefficients (R2 > 0.98). The obtained kinetic rate constants of the H2O2 alone, PBC + H2O2, αand δ-MnO2/BCs + H2O2 systems were MnO2/BCs + H2O2, 0.0002 min−1, 0.0055 min−1, 0.0465 min−1, and 0.0227 min−1, respectively. The significantly improved BPA removal rates in both the MnO2/BCs + H2O2 systems compared to the PBC + H2O2 system were attributed to their higher adsorption performance compared to PBC, because the oxidative degradation process of organic pollutants generally follows an adsorption–oxidation mechanism [32]. Furthermore, the surface Mn(IV)/Mn(III) reversible redox reaction in the MnO2/BCs matrix, which accounts for the easier and higher decomposition activity of H2O2 for generating free radical species, resulted in higher catalytic BPA degradation [33]. Interestingly, although the δ-MnO2/BCs exhibited higher adsorption of BPA than the α-MnO2/BCs, the kinetic rate constant of the α-MnO2/BCs + H2O2 system was approximately twice that of the δ-MnO2 counterpart. This tendency may result from the difference in catalytic activity resulting from differences in their structural properties. In general, the structure of MnO2 consists of different interlinked MnO6 octahedra as a fundamental building unit with a chain-like tunnel (α-MnO2) structure and a sheet or layered (δ-MnO2) structure [34] that affects the crystallinity and surface area (173.42 m2/ g for α-MnO2/BCs and 111.87 m2/g for δ-MnO2/BCs; see Fig. S3(B) and 25
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where 100% BPA removal was obtained within 20 min, which exceeds other such systems. To evaluate the synergistic effect of the α-MnO2/ BCs-based ultrasound-assisted heterogeneous Fenton-like system on BPA removal, the synergy factor (SF) was calculated on the basis of the determined kinetic rate constants of BPA removal (Eq. (1)). This can be expressed as follows:
SF =
ktriple (2)
ksingle + kdual
where ksingle, kdual, and ktriple are the measured kinetic rate constants in the single (H2O2 and US alone), dual (α-MnO2/BCs + H2O2, US + H2O2, and US + α-MnO2/BCs), and triple (US + α-MnO2/ BCs + H2O2) systems, respectively. As presented in Fig. 3(B) and (D), the kinetic rate constants of the H2O2, α-MnO2/BCs + H2O2, US, US + H2O2, US + α-MnO2/BCs, and US + α-MnO2/BCs + H2O2 systems were determined as 0.0002 min−1, 0.0465 min−1, 0.0008 min−1, 0.0021 min−1, 0.0082 min−1, and 0.1565 min−1, respectively, with corresponding R2 values of more than 0.97. The determined SF value of approximately 2.71 demonstrates a significant synergistic effect of the ultrasound-assisted heterogeneous Fenton-like process for BPA removal. This result may be attributable to both enhanced microturbulence, as the driving force of BPA to overcome the mass transfer resistance at the solid-liquid interface [39] and the synergistic effects of ultrasound and α-MnO2/BCs on the effective decomposition of H2O2 into reactive free radicals, such as HO% and superoxide (O2%−) [40,41], as follows: H2O2 + ))) (ultrasound) → 2HO%
(3)
H2O + ))) (ultrasound) → HO +H %
≡Mn(IV) + H2O2 → ≡Mn(III) +
(4)
%
HO2%
+H
+
(5)
≡Mn(III) + H2O2 → ≡Mn(II) + HO2% + H+
(6)
HO2% + H2O2 → H2O + O2 + HO%
(7)
O2 + ))) (ultrasound) → 2O
(8)
%
O + H2O → 2HO %
HO2%
(9)
%
+
→H
+ O2%−
(10) −
≡Mn(III) + HO → ≡Mn(IV) + OH %
(11) −
≡Mn(II) + H2O2 → ≡Mn(III) + HO + OH %
(12)
First, H2O2 and water molecules were simultaneously decomposed by ultrasound irradiation to generate HO% (Eqs. (3) and (4)). Simultaneously, catalytic reactions between the α-MnO2/BCs and H2O2 would induce oxidation of lattice oxygen, leading to the reduction of oxidation states on the α-MnO2/BCs surface [≡Mn(IV) to ≡Mn(III)) or ≡Mn(III) to ≡Mn(II)] and the formation of perhydroxyl radicals (HO2%) (Eqs. (5) and (6)). These phenomena are indirectly supported by the results of the XPS analysis of the α-MnO2/BCs before and after BPA removal. As shown in Fig. 4(A), the Mn 2p spectrum showed two typical peaks located at binding energies of 654.00 eV and 642.25 eV, which correspond to Mn 2p1/2 and Mn 2p3/2, respectively, with a spin-energy separation of 11.75 eV. In addition, two peaks in the Mn 2p3/2 peak appeared at binding energies of 642.7 eV and 643.0 eV, which correspond to Mn(III) and Mn(IV), respectively [42]. These observations are consistent with the presence of α-MnO2 and indicate the presence of the Mn(IV)/Mn(III) oxidation state in the α-MnO2/BCs matrix [43]. As shown in Fig. 4(B), after BPA removal, the binding energy of Mn 2p3/2 shifted slightly to a lower value of 641.88 eV with changes in the ratios of the integral area of Mn(IV) and Mn(III), likely because of electron transfer between the α-MnO2/BCs and H2O2. In addition, as shown in Fig. 4(C), the O 1s spectrum of the α-MnO2/BCs exhibited three main peaks at binding energies of 529.5 eV, 531.1 eV, and 533 eV, where the ratios of the integral areas are 64.69%, 27.24%, and 8.07%,
Fig. 4. (A) Mn 2p, (B) Mn 2p3/2, and (C) O 1s XPS spectra of α-MnO2/BCs before and after BPA removal. Experimental conditions: [BPA]0 = 100 μM; [H2O2]0 = 10 mM; [catalyst]0 = 0.5 g/L; ultrasound = 20 kHz (130 W) at 40% amplitude; reaction time = 20 min; pH = 7.0 ± 0.1; and temperature = 25 ± 1 °C.
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Fig. 5. Schematic diagram of chain reactions in the α-MnO2/BCs-based ultrasound-assisted heterogeneous Fenton-like process.
respectively; these peaks are attributed to Mn–O–Mn, Mn–O–H, and H–O–H, respectively. After BPA removal, the ratio of the integral area of Mn–O–Mn decreased to 52.15% and that of Mn–O–H increased to 42.69%, indicating an increase in the lower Mnx+ oxidation state. Thus, these results indicate that the oxidation states on the α-MnO2/BCs surfaces were reduced by the consumption of lattice oxygen in the decomposition of H2O2 into HO2%, thereby promoting the reversion of ≡Mn(IV) to ≡Mn(III) or ≡Mn(III) to ≡Mn(II) [44]. Accordingly, in addition to cleavage of H2O2, the radical chain reactions of HO2% with H2O2, and the decomposition of the generated O2 molecules by ultrasound irradiation were also responsible for generating HO% and O2%−, respectively (Eqs. (7)−(10)). Alternatively, the ≡Mn(III) or ≡Mn(II) on the surfaces of α-MnO2/BCs could be recovered to its original form [≡Mn(IV) or ≡Mn(III)] by reaction with some of the remaining unreacted HO% and undecomposed H2O2 in the reaction system (Eqs. (11) and (12)). On the basis of these described reactions, a series of chain reactions is illustrated in Fig. 5 for the α-MnO2/BCs-based ultrasoundassisted heterogeneous Fenton-like process. Lastly, as presented in Fig. S5, the α-MnO2/BCs-based ultrasoundassisted heterogeneous Fenton-like process exhibited stable and efficient BPA removal during the entire recycling reactions; BPA removal was approximately 93% in the fourth run, suggesting that α-MnO2/BCs exhibit highly reusable catalytic activity in ultrasound-assisted heterogeneous Fenton-like process for the removal of BPA. Despite its high stability and reusability, total organic carbon (TOC) removal was limited, with approximately 63.7% and 45.1% removal after the first and fourth runs, respectively. This indicates the mineralization of BPA molecules and the requirement for a prolonged reaction time and process optimization to achieve the complete removal of the generated intermediates. Therefore, more detailed research, including the optimization of the solution chemistry parameters (e.g., catalyst dosage, initial pH, H2O2 concentration, ultrasound intensity, temperature, etc.) and the determination of the possible pathway for BPA removal will be performed in the near further. In addition, the structural stability and reusability of hierarchical urchin-like α-MnO2/BCs as a catalyst in ultrasound-assisted heterogeneous Fenton-like processes under such optimized conditions will be evaluated.
4. Conclusion Two types of three-dimensionally structured MnO2/BCs were synthesized via a duration-controlled hydrothermal process, and their ultrasound-assisted heterogeneous Fenton-like catalytic activity was evaluated. The results showed that urchin-like α-MnO2/BCs exhibited the highest ultrasound-assisted heterogeneous Fenton-like catalytic activity, achieving complete BPA removal within 20 min under the following conditions: [BPA]0 = 100 μM; [H2O2]0 = 10 mM; [catalyst]0 = 0.5 g/L; ultrasound = 20 kHz (130 W) at 40% amplitude; pH = 7.0 ± 0.1; and temperature = 25 ± 1 °C. Overall, the findings suggest that hierarchical urchin-like α-MnO2/BCs can be expected to exhibit excellent catalytic performance in ultrasound-assisted heterogeneous Fenton-like processes, giving them wide applicability to the environmental remediation of various organic pollutants. Acknowledgement This work was supported by an institutional program grant (Project No.: 2E29660) from the Korea Institute of Science and Technology. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ultsonch.2019.04.039. References [1] W. Liu, H. Zhang, B. Cao, K. Lin, J. Gan, Oxidative removal of bisphenol A using zero valent aluminum–acid system, Water Res. 45 (2011) 1872–1878. [2] Y. Zhou, X. Fang, T. Wang, Y. Hu, J. Lu, Chelating agents enhanced CaO2 oxidation of bisphenol A catalyzed by Fe3+ and reuse of ferric sludge as a source of catalyst, Chem. Eng. J. 313 (2017) 638–645. [3] Z.-H. Liu, Y. Kanjo, S. Mizutani, Removal mechanisms for endocrine disrupting compounds (EDCs) in wastewater treatment — physical means, biodegradation, and chemical advanced oxidation: a review, Sci. Total Environ. 407 (2009) 731–748. [4] A.R. Bakr, M.S. Rahaman, Removal of bisphenol A by electrochemical carbon-nanotube filter: influential factors and degradation pathway, Chemosphere 185
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