Chemosphere 197 (2018) 20e25
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Carboxymethyl cellulose stabilized ZnO/biochar nanocomposites: Enhanced adsorption and inhibited photocatalytic degradation of methylene blue Shengsen Wang a, b, c, Yanxia Zhou a, Shuwen Han a, Nong Wang c, Weiqin Yin a, Xianqiang Yin d, Bin Gao e, Xiaozhi Wang a, Jun Wang f, * a
College of Environmental Science and Engineering, Yangzhou University, Yangzhou, 225127, China Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Guangdong, China Key Laboratory of Original Agro-Environmental Pollution Prevention and Control, Ministry of Agriculture/Tianjin Key Laboratory of Agro-environment and Safe-product, Tianjin, 300191, China d College of Natural Resources and Environment, Northwest A&F University, Yangling, 712100, China e Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL, 32611, USA f College of Resources and Environment, Key Laboratory of Agricultural Environment in Universities of Shandong, Shandong Agricultural University, Taian, 271018, PR China b c
h i g h l i g h t s CMC reduced ZnO crystallite size but increased band gap of ZnO/biochar composite. UV-irradiated ZnO and biochar generated ROS in MB degradation. CMC increases MB sorption by electrostatic attraction and other mechanisms. CMC can react with ROS and reduce ROS availability for MB degradation. CMC capping is not suitable for MB degradation by ZnO.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 August 2017 Received in revised form 25 November 2017 Accepted 6 January 2018 Available online 8 January 2018
Biochar(BC)-supported nanoscaled zinc oxide (nZO) was encapsulated either with (nZORc/BC) or with no (nZOR/BC) sodium carboxymethyl cellulose (CMC). The X-ray diffraction and ultraviolet (UV)-visible-near infrared spectrophotometry revealed that nZO of 16, 10, and 20 nm with energy band gaps of 2.79, 3.68 and 2.62 eV were synthesized for nZOR/BC, nZORc/BC and nZO/BC, respectively. The Langmuir isotherm predicted saturated sorption of methylene blue (MB) was 17.01 g kg1 for nZORc/BC, over 19 times greater than nZOR/BC and nZO/BC. Under UV irradiation, 10.9, 61.6, 83.1, and 41.6% of MB were degraded for nZORc/BC, nZO/BC, nZOR/BC and BC. The scavenging experiment revealed hydroxyl radical dominated CMC degradation. Exogenous CMC (2 g L1) increased MB sorption from 10.6% to 73.1%, but decreased MB degradation from 80.7% to 41.1%, relative to nZOR/BC. Thus, CMC could increase MB sorption by electrostatic attraction and other possible mechanisms. The compromised MB degradation may be ascribed to reduced availability of hydroxyl and superoxide radicals to degrade MB, and increased band gap energy of ZnO. © 2018 Published by Elsevier Ltd.
Handling Editor: Chang-Ping Yu Keywords: Sodium carboxymethyl cellulose Nanoscaled zinc oxide Methylene blue Sorption Degradation Hydroxyl radical
1. Introduction Methylene blue (MB), known as methylthioninium chloride, is a cationic type model dye compound. The excessive discharge of
* Corresponding author. E-mail address: [email protected]
(J. Wang). https://doi.org/10.1016/j.chemosphere.2018.01.022 0045-6535/© 2018 Published by Elsevier Ltd.
MB in textile efﬂuents is hazardous to human health and ecological safety (Houas et al., 2001; Zhang et al., 2011). Both sorption and degradation are two commonly-used techniques for MB removal (Zhang et al., 2011). Sorption is a facile and efﬁcient route to recycle MB(Yan et al., 2011), whereas degradation is capable of complete removal of MB and intermediate products (Houas et al., 2001).
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Nanoscaled ZnO (nZnO) exhibits tunable wide band gap (3.2e3.4eV) as semiconductor materials for energy conversion (Hanada, 2009; Qin et al., 2011; Eskizeybek et al., 2012). Recently, nZO has been extended for in-situ degradation remediation of resistant organic contaminants or dyes. Excellent ultraviolet light (UV) sensitivity has conferred nZO photosensitizer the good photocatalytic capacity of organic dyes (Ercan et al., 2016). However, challenges pertaining to nZO fabrication using wet-chemistry methods may include less dispersion, poor uniformity and poor conductivity of nanoparticles (Qin et al., 2011). Not surprisingly, high tendency of agglomeration of nanoscaled particles could increase the particle size and reduce reactivity of nZO. Numerous work has demonstrated that nZO aggregation may be antagonized by immobilization with porous carbonaceous matrix (graphene, resins, carbon nanotubes) (Ponder et al., 2000; Jabeen et al., 2011; Lv et al., 2011). Biochar was practiced to support Fe3O4/BiVO4 which exhibited enhanced photocatalytic capacity of nanoscaled catalysts for organic dyes (Pathania et al., 2016; Kumar et al., 2017). As an alternative, coating with surfactant has been practiced to antagonize aggregation of metallic nanoparticles such as sodium carboxymethyl cellulose (CMC) (Hashem et al., 2013) and polyvinyl-alcohol (PVA) (Qin et al., 2011). Thus, efforts have been undertaken to synthesize and characterize CMC coated nZO nanocomposites. CMC is an anionic polysaccharide surfactant and nanoparticle stabilizer, which are widely used for preparation of nanoparticles (He and Zhao, 2007). Alleviated aggregation of CMCaided nanoparticles is mainly ascribed to enhanced intraparticle repulsion to overcome the attractive forces between nanoparticles (He and Zhao, 2007). Both well-dispersed nZO particles and CMC itself would then affect the removal of MB from aqueous solutions. However, few works have demonstrated the potential impact of CMC on simultaneous sorptive and photocatalytic performance of nZO. The anionic CMC can remove MB via electrostatic attraction and ion exchange (Yan et al., 2011; Zhang et al., 2011). To date, the effects of CMC on nZO photocatalytic capacity have not been well documented. Two possible mechanisms may participate. First, CMC may affect nZO crystallinity and photocatalytic property. Previous work showed PVA encapsulation can enhance UV sensitivity of nZO (Qin et al., 2011). Second, CMC is subject to attack by free radicals, which would affect the degradation capacity of CMC capped nZO. The dominant MB degradation mechanisms by UV-irradiated nZO involve activation of reactive oxygen species (ROS), e.g., superoxide (O 2 ) and hydroxyl ( OH) radicals(Houas et al., 2001). The generated ROS can be quenched CMC. For example, organic substance can scavenge and compete for ROS generated by UV-irradiated nZO (Ercan et al., 2016). In this work, the nZO was prepared by wet-chemistry method using CMC as surfactant and the resulting nanocomposites were examined for adsorptive and photocatalytic removal of MB in aqueous solutions. The objectives of this study were to (1) compare the properties of as-prepared nZO composites with or without CMC, (2) examine sorption and degradation of MB by the sorbents, and (3) reveal the possible mechanisms associated with MB removal by the CMC facilitated sorbents. 2. Materials and methods 2.1. Reagents All chemicals were dissolved with ultrapure water (Nanopure water, Barnstead) denoted as DI water (18.2 MU). Zinc chloride (ZnCl2), sodium borohydride (NaBH4), sodium carboxymethyl cellulose (CMC), methylene blue (MB), isopropyl alcohol (IPA) and pbenzoquinone (PBQ) of analytical grade were purchased from Sigma-Aldrich chemical supply.
2.2. Preparation of nZO and biochar nanocomposites 6 mmol ZnCl2 with or with no 0.05 g CMC was dissolved in 100 mL DI water with bamboo-derived biochar. The ZnCl2 was reduced by 60 mM NaBH4 at 90 C. The resulting catalysts were denoted as nZOR/BC and nZORc/BC (with CMC). As a comparison, a traditional precipitation approach was adopted to prepared BC impregnated nZO nanocomposite (See SI for details). 2.3. MB adsorption and degradation experiments MB adsorption was carried out by adding 0.04 g sorbents to 20 mL (2 g L1) of 25, 50, 100, 200 and 300 mg L1 MB solutions in 68 mL digestion vessels. The photocatalytic degradation of MB was undertaken in an UV light photochemical reactor (XP, Nanjing, China) with 300 W mercury lamp irradiation. To identify the participation of free radicals in MB degradation, both IPA and PBQ were introduced to scavenge hydroxyl and peroxide radicals, respectively (Han et al., 2017). To examine CMC effects on MB removal mechanisms, nZOR/BC with exogenous 2 g L1 CMC was tested for MB sorption as well as photocatalytic degradation (See SI for details). 2.4. Characterization of catalysts The as-prepared catalysts were characterized with a scanning electron microscope (SEM) (JEOL JSM-6400 Scanning Microscope), transmission electron microscopy (TEM) (Tecnai 12, Philips Electronic Instruments) equipped with an energy dispersive spectrometer (EDS) (Thermo Electron Corporation), X-ray diffractometer (XRD) (Ultima IV X-Ray Diffractometer, Rigaku Corporation, Japan), Ultravioletevisibleenear infrared spectrophotometry (UVIS) (Cary-500, Varian, US) and paramagnetic resonance spectrometer (A300-10/12, Bruker, Germany). 3. Results and discussion 3.1. Characterization of sorbents TEM/EDS elemental mapping analysis indicated that abundant Zn bearing minerals appeared in three Zn doped catalysts, concurrent with appearance of O (Figure S1, supporting information (SI)). SEM topography suggested that Zn-bearing crystals were embedded in the irregular pores or dispersed on rough carbonaceous surfaces of the biochars (Figure S2 a-c, SI). The diffraction patterns of Zn-bearing crystals were obtained with an XRD diffractometer (Fig. 1). The characteristic diffraction peaks at d spacings of 2.8244, 2.613 and 2.4847 Å for nZOR/BC and nZO/BC can be indexed to zincite (ZnO). Regarding nZORc/BC, d spacings at 2.8244 and 2.4847 Å corresponded to zincite, whereas 2.702 Å represented CMC crystal (Qi et al., 2015). The appearance of CMC can also be infered from TGA analysis (Figure S3, SI). DebyeScherrer equation showed average crystallite sizes of nZO were 16, 10, and 20 nm for nZOR/BC, nZORc/BC and nZO/BC, respectively (Patterson, 1939; Holzwarth and Gibson, 2011). The smaller crystallite size of nZO in nZORc/BC collaborated with previous ﬁnding of TiO2 in presence of CMC (Ali et al., 2015). Coupled with distribution of nZO on BC surfaces (Figure S2 d-f, SI), the CMC was able to reduce agglomeration and particle size of nZO particles. nZO has direct bandgap energy (Eg) (Hanada, 2009; Ali et al., 2015), which was 2.79, 3.68 and 2.62 eV for nZOR/BC, nZORc/BC and nZO/BC, respectively (Fig. 2). Both nZOR/BC and nZO/BC exhibit lower Eg values relative to bare nZO reported previously. This may be related to the biochar introduction (Dhiman et al., 2017). Kumar et al. (2017) reported biochar supported Fe3O4/BiVO4 has lower Eg
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Fig. 1. XRD diffraction patterns for nZOR/BC, nZORc/BC and nZO/BC.
than bare Fe3O4/BiVO4. Greater Eg of nZORc/BC relative to nZOR/BC was ascribed to CMC. CMC as a capping agent greatly increased Eg of ZnS2 from 3.6 eV to 4.13e4.26 eV, varying with different capping ratio(Ahemen et al., 2013). The lower Eg values of nZO particles without CMC may be ascribed to appearance of crystal defects and vacancies (Dhiman et al., 2017).
3.2. MB sorption and degradation The sorption isotherms revealed that sorption increased rapidly with increasing equilibrium sorbate concentrations at lower MB concentrations and much more slowly at higher MB concentrations (Fig. 3). The batch sorption data were ﬁtted with Langmuir and Freundlich models with reasonable coefﬁcients of determination (Table S1 in SI). The langmuir model assumes monolayer coverage of a homogeneous sorbent surface with ﬁnite identical sites (Hu et al., 2015). The good ﬁt of Langmuir model suggested that the MB sorption was governed by the monolayer sorption mechanism. Both BC and nZOR/BC have minimal MB sorption, while nZO/BC increased MB sorption by more than two times. The maximal MB sorption occurred for nZORc/BC (17.01 g kg1), which was 19 times greater MB sorption relative to BC. In Freundlich models, all n values are below unity, indicative of chemisorption (Hu et al., 2015). For example, organic dyes retention by biochars may occur through surface sorption process, i.e., van der Waals interactions, electrostatic interactions and pi-pi interactions (Xu et al., 2017). In MB degradation experiment, the MB removal efﬁciency showed same trend as in isotherm study after 30 min sorption (Fig. 4). That is, the order of sorptive removal efﬁciency was nZORc/ BC> nZO/BC> nZOR/BC>BC. After sorption, the degradation was then initiated and MB degradation efﬁciency kept increasing until maximal MB degradation was achieved. The results clearly showed that the percent of MB degraded (out of total MB) was 10.9, 61.6, 83.1, and 41.6% for nZORc/BC, nZO/BC, nZOR/BC and BC, respectively. The UV scan of MB after sorption (30 min) and degradation (2 h) was performed (Figure S4, SI). At 30 min, the peaks at characteristic wavelengths maintained same styles with reduced intensity. The greatest reduction occurred for nZORc/BC, indicating the highest MB removal. After 2 h, the peak pattern of MB for nZORc/BC was barely changed, whereas the peak almost disappeared for both nZO/BC and nZOR/BC. This suggests MB was degraded to different intermediates.
Fig. 2. UVIS analysis for energy band gap of nZOR/BC (a), nZORc/BC (b) and nZO/BC (c).
3.3. MB removal mechanisms by nZO associated with CMC encapsulation In this work, CMC was used to manipulate the particle size and dispersion of nZO on carbonaceous surfaces. nZORc/BC exhibited greatest MB removal capacity but least MB degradation efﬁciency. However, MB removal trend was reversed in both nZO/BC and nZOR/BC, which may be associated with introduction of CMC. To conﬁrm impact of CMC, MB degradation by nZOR/BC plus CMC were performed under same conditions. The results showed exogenous CMC at 2 g L1 increased sorptive removal of MB from 10.6% to 73.1%, but decreased MB degradation from 80.7% to 41.1%, relative to nZOR/BC (Fig. 5). This implied that CMC may well explain the difference for sorption and degradation between nZO sorbents prepared with or without CMC. The excellent sorptive capacity of MB by CMC was documented as much as 300 g kg1 (Yan et al., 2011), which may involve diverse mechanisms, i.e., electrostatic attraction and ion exchange (Yan et al., 2011; Sharma et al., 2017). On one hand, cationic MB can be
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Fig. 3. MB sorption isotherm data and ﬁtted models for BC, nZO/BC, nZOR/BC and nZORc/BC. Insert table is the best-ﬁt parameters for isotherm models.
Fig. 5. The effects of CMC on sorption and photocatalytic degradation of MB (initial MB concentration ¼ 40 mg L1).
Fig. 4. Degradation of MB (20 mg L1) by BC, nZO/BC, nZOR/BC and nZORc/BC as a function of time. The photocatalytic degradation was initiated 30 min after reaction.
Fig. 6. The free radicals scavengers (IPA and PBQ) on MB degradation by nZOR/BC.
attracted by the deprotonated carboxylic groups of CMC (Yan et al., 2011). Previous work has conﬁrmed the electrostatic attraction plays important functions for removal of anionic dye R-250 and cationic MB by starch/poly(alginic acid-cl-acrylamide) and clay minerals/CMC, respectively (Yan et al., 2011; Sharma et al., 2017). The pHpzc for BC, nZOR/BC, nZORc/BC and nZO/BC were approximately 7.82, 7.85, 7.57 and 9.22, respectively (Figure S5, SI). Lowest pHpzc for nZORc/BC was thus more favorable for adsorptive removal of cationic MB relative to other three sorbents. This can partially explain the best MB sorption by nZORc/BC, and much lower sorption by nZOR/BC and BC. On other hand, other mechanisms such as monolayer chemical sorption and ion exchange process may be involved (Yan et al., 2011). Thus, the dominant MB sorption mechanisms by CMC were non-speciﬁc chemical process, such as electrostatic attraction and ion exchange. Besides, smallersized nZO in CMC treatment can also contribute to a higher MB sorption removal capacity for nZORc/CMC (Al-Khateeb and Mahmood1, 2016). MB degradation was initiated by free radicals generated by photogenerated species (hþ and e-, see equations (1)e(5)) (Thomas et al., 2016) (see details in SI). To better understand degradation
mechanisms, the scavenging experiment was further performed. The MB photocatalytic degradation revealed that both IPA and PBQ reduced MB degradation for nZOR/BC and nZORc/BC to some extent (Fig. 6). As a OH radical scavenger, IPA decreased MB degradation by 19.4% and 9.7% for nZOR/BC and nZORc/BC, respectively. Instead, PBQ, a O$2 scavenger, decreased MB degradation by 6.3% and 1.6% for nZOR/BC and nZORc/BC, respectively. The greater extent of inhibition of IPA indicates OH radical mainly contributed to MB degradation. The involvement of OH radicals was conﬁrmed with EPR analysis. The OH radical was successfully identiﬁed in nZOR/ BC, but not detected in nZORc/BC as well as nZOR/BC plus CMC (Fig. 7). In this work, MB degradation by nZOR/BC decreased from 80.7 to 41.1% after exogenous CMC was introduced in the reaction system (Fig. 5). The decreased MB degradation by CMC capped nZO may be ascribed to two aspects. First, amine and carboxylic groups of CMC can react with ROS and thus compete OH and O$2 radicals for MB degradation by nZORc/BC (see equations (6) and (7)). This can be inferred by low antioxidant activity of CMC (Fan et al., 2014; Dashipour et al., 2015), and ability of OH to degrade alkali cellulose (Rosenau et al., 2006). Second, greater Eg values of nZORc/BC
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Fig. 7. PRS analysis of nZOR/BC (a), nZORc/BC (b) and nZO/BC (c).
relative to nZOR/BC reduced photo-absorption properties at the visible light region, suggesting lower photocatalytic capacity (Yoon et al., 2017). CMC shifted upwards the bandgap of nZO, and thus may decrease production of photo-generated electrons (Li et al., 2015; Yoon et al., 2017). ZnO þ hg/ ZnO þ hþ þe-,
hþ þ H2O/ Hþ þ$OH,
e- þ O2/ O$2,
MB þ $OH/ CO2 þ H2O þ byproducts,
MB þ O$2 / CO2 þ H2O þ byproducts,
CMC þ $OH / byproducts,
CMC þ O2 / byproducts,
degradation by nZO was compromised by polyaniline encapsulation (Eskizeybek et al., 2012). However, some work showed CMC encapsulation decreased bandgap energy of TiO2 (Ali et al., 2015) and enhanced photocatalytic degradation of Congo red dye by TiO2 (Thomas et al., 2016). The contrasting results may be associated with difference in NP species and target organic contaminants. More work may be needed to investigate this phenomenon in detail. 4. Conclusions
This work suggests CMC capping was not suitable for MB degradation under UV irradiation. Similarly, the UV-irradiated MB
In this work, nZO and biochar nanocomposites were synthesized with presence of CMC. Both sorptive and photocatalytic capacity of as-prepared nZO based catalysts were compared for MB removal. CMC reduced nZO crystalite size by 37.5%, but increased energy band gap by 31.9%. Compared to nZOR/BC, nZORc/BC increased maximal MB sorption from 0.823 to 37.01 mg kg1, but decreased MB degradation from 83.1% to 10.9%. Both hydroxyl and superoxide radicals were involved in MB degradation, and hydroxyl radical is the major contributor. Scavenging and EPR experiments conﬁrmed that CMC is likely to quench hydroxyl radicals, attributing to reduced MB degradation. Besides, the CMC-induced
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change in morphology of catalysts may also contribute to the decreased MB degradation by nZO. Acknowledgments This research was supported by National Natural Science Foundation of China (41771349 & 31772394), Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Laboratory Research Fund from Key Laboratory of Original Agro-Environmental Pollution Prevention and Control, Ministry of Agriculture/Tianjin Key Laboratory of Agroenvironment and Safe-product, Startup Fund for distinguished scholars of Yangzhou University (5016/137011014), Industry-University-Research-Application Cooperative Innovation Key Program of Yangling Agricultural Hi-tech Industries Demonstration Zone (2017CXY-10). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.chemosphere.2018.01.022. References Ahemen, I., Meludu, O., Odoh, E., 2013. Effect of sodium carboxymethyl cellulose concentration on the photophysical properties of zinc sulﬁde nanoparticles. Br. J. Appl. Sci. Technol. 3, 1228e1245. Al-Khateeb, I.K., Mahmood1, M.S., 2016. Adsorption, thermodynamics and kinetics of methylene blue on nano structured ZnO crystals. Am. Chem. Sci. J. 13, 1e9. Ali, H.E., Atta, A., Senna, M.M., 2015. Physico-chemical properties of carboxymethyl cellulose (CMC)/nanosized titanium oxide (TiO2) gamma irradiated composite. Arab J. Nuclear Sci. Appl. 48, 44e52. Dashipour, A., Razavilar, V., Hosseini, H., Shojaee-Aliabadi, S., German, J.B., Ghanati, K., Khakpour, M., Khaksar, R., 2015. Antioxidant and antimicrobial carboxymethyl cellulose ﬁlms containing Zataria multiﬂora essential oil. Int. J. Biol. Macromol. 72, 606e613. Dhiman, P., Naushad, M., Batoo, K.M., Kumar, A., Sharma, G., Ghfar, A.A., Kumar, G., Singh, M., 2017. Nano FexZn1xO as a tuneable and efﬁcient photocatalyst for solar powered degradation of bisphenol A from aqueous environment. J. Clean. Prod. 165, 1542e1556. Ercan, D., Cossu, A., Nitin, N., Tikekar, R.V., 2016. Synergistic interaction of ultraviolet light and zinc oxide photosensitizer for enhanced microbial inactivation in simulated wash-water. Innovat. Food Sci. Emerg. Technol. 33, 240e250. Eskizeybek, V., Sarı, F., Gülce, H., Gülce, A., Avcı, A., 2012. Preparation of the new polyaniline/ZnO nanocomposite and its photocatalytic activity for degradation of methylene blue and malachite green dyes under UV and natural sun lights irradiations. Appl. Catal. B Environ. 119e120, 197e206. Fan, L., Peng, M., Zhou, X., Wu, H., Hu, J., Xie, W., Liu, S., 2014. Modiﬁcation of carboxymethyl cellulose grafted with collagen peptide and its antioxidant activity. Carbohydr. Polym. 112, 32e38. Han, S., Yu, H., Yang, T., Wang, S., Wang, X., 2017. Magnetic [email protected]
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