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Innovative encapsulated oxygen-releasing beads for bioremediation of BTEX at high concentration in groundwater
Journal of Environmental Management 204 (2017) 12e16
Contents lists available at ScienceDirect
Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Research article
Innovative encapsulated oxygen-releasing beads for bioremediation of BTEX at high concentration in groundwater Chi-Wen Lin a, Chih-Hung Wu b, Pei-Yu Guo a, Shih-Hsien Chang c, d, * a
Department of Safety, Health and Environmental Engineering, National Yunlin University of Science and Technology, Yunlin, 64002, Taiwan, ROC School of Resources & Chemical Engineering, Sanming University, Sanming City, 365004, China c Department of Public Health, Chung-Shan Medical University, Taichung, 402, Taiwan, ROC d Department of Family and Community Medicine, Chung Shan Medical University Hospital, Taichung, 402, Taiwan, ROC b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 September 2016 Received in revised form 1 May 2017 Accepted 10 May 2017
Both a low concentration of dissolved oxygen and the toxicity of a high concentration of BTEX inhibit the bioremediation of BTEX in groundwater. A novel method of preparing encapsulated oxygen-releasing beads (encap-ORBs) for the biodegradation of BTEX in groundwater was developed. Experimental results show that the integrality and oxygen-releasing capacity of encap-ORBs exceeded those of ORBs. The use of polyvinyl alcohol (PVA) with high M.W. to prepare encap-ORBs improved their integrality. The encap-ORBs effectively released oxygen for 128 days. High concentration of BTEX (480 mg L1) inhibited the biodegradation by the free cells. Immobilization of degraders in the encap-ORB alleviated the inhibition. Scanning electron microscope analysis reveals that the BTEX degraders grew on the surface of encap-ORB after bioremediation. The above results indicate that the encap-ORBs were effective in the bioremediation of BTEX at high concentration in groundwater. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Encapsulated oxygen-releasing bead BTEX Immobilization Polyvinyl alcohol Bioremediation
1. Introduction Groundwater is frequently contaminated by BTEX that has leaked from underground petroleum tanks and gas stations. BTEX poses a high risk for human health and the ecosystem (Pinedo et al., 2013). The various techniques for treating groundwater that contains BTEX include physical, chemical, and biological methods (Erto et al., 2014; Yeh et al., 2010). Of these, bioremediation is popular because it is low-cost and environmentally friendly (Gul et al., 2015). However, the bioremediation of BTEX at a high concentration in groundwater is commonly often limited by its toxicity and the low dissolved oxygen concentration in groundwater (Cao and Harris, 2010; Mohan et al., 2014). Oxygen-releasing material (ORM) can provide dissolved oxygen for groundwater remediation (Thiruvenkatachari et al., 2008). Numerous factors affects its effectiveness in bioremediation, including the compatibility of the ORM with the environment (Cao and Harris, 2010; Severino et al., 2015), the integrality of the ORM (Meng et al., 2009), and its effective oxygen-releasing characteristics (Nykanen et al., 2012; Qian et al., 2013). Traditionally, cement is
* Corresponding author. Department of Public Health, Chung-Shan Medical University, Taichung, 402, Taiwan, ROC. E-mail address: [email protected] (S.-H. Chang). http://dx.doi.org/10.1016/j.jenvman.2017.05.035 0301-4797/© 2017 Elsevier Ltd. All rights reserved.
mixed with oxygen-releasing chemicals, such as CaO2 and MgO2 to produce ORM (Ahmad et al., 2007). However, cement is nonbiodegradable and persists in the aquifer after use (Daghighi et al., 2013). Recently, we used the green materials Polyvinyl alcohol (PVA), alginate, CaO2, citrate, and biochar to prepare ORBs (Liang et al., 2011) and found that it effectively provides dissolved oxygen that enables indigenous degraders to remediate BTEX in groundwater (Jadhav et al., 2013; Lu et al., 2008). Several studies report that a high concentration of BTEX is toxic to degraders, and so inhibits its own bioremediation (Wu et al., 2012). Encapsulation can alleviate the toxicity of BTEX at high concentration (Lin et al., 2010; Tsai et al., 2013; Xin et al., 2013)) and improve its biodegradation (Lassinantti Gualtieri et al., 2015). In this work, encapsulated oxygen-releasing beads (encapORBs) were synthesized and used for the bioremediation of BTEX at high concentration in groundwater. SEM analysis was conducted to observe the growth of BTEX degraders on the encap-ORB surface after bioremediation. 2. Materials and methods 2.1. Preparation of encap-ORB Encap-ORB was prepared as follows. A gelling agent (PVA/
C.-W. Lin et al. / Journal of Environmental Management 204 (2017) 12e16
alginate ¼ 3:1 w/w) was heated in an autoclave (121 C, 1 h) and was sprayed into a CaO2 solution (0.1 M) to form capsules of various sizes (0.18e0.22 mm in diameter). After cooling, the capsules were mixed with an oxygen-releasing agent (CaO2, 5 g), a pH-buffering agent (citrate, 2.5 g), a supporter (biochar, 13.24 g), 10 mL of BTEX degrader (Mycobacterium sp. CHXY119 and Pseudomonas sp. YATO411, A600 nm 2.0 abs) (Xin et al., 2013), and PVA, to form the ORBs (with a diameter of 1 cm). The ORBs were mixed with PAV to prepare encap-ORBs. Fig. 1 displays the procedure for preparing encap-ORBs. The measurement method for integrality ratio of ORBs (Wu et al., 2015) was provided in the Supplementary Material. 2.2. Oxygen-releasing characteristics of encap-ORBs An oxygen release experiment was performed in an acrylic column reactor (diameter: 4 cm, H: 25.5 cm, total volume: 314 mL) at hydraulic retention time of1.24 day. The experimental condition for the oxygen release experiment was provided in the Supplementary Material. To simulate the low level of dissolved oxygen (DO) in common groundwater, 0.3 g L1 of Na2SO3 was added to the inflow to maintain the DO concentration below 0.14 mg L1. A 75 g mass of encap-ORBs was added to the column to begin the experiment. The oxygen concentration was measured using a DO meter (CLEAN DO500, USA). 2.3. Batch bioremediation experiment A batch bioremediation experiment was conducted to study the effect of adding ORBs on the bioremediation of BTEX. Benzene (99.9%, Echo Chemicals, Taiwan), toluene (99.9%, Fisher Scientific, USA), ethylbenzene (99.7%, Tedia, USA) and xylene (99.7%, Tedia), all of analytic grade, were dissolved in the mineral solution to yield a total BTEX concentration of 120e480 mg/L (B: T: E: X ¼ 1: 1: 1: 1, w/w). A 2 g mass of ORBs was placed in the flask to start the bioremediation experiment. At the determined time, 250 mL of solution was sampled and analyzed using a GC apparatus that was equipped with a flame ionization detector (GC-FID, Shimadzu, GC-14B, Japan) and a Stabliwax column (30 m 0.53 mm id 1 mm film thickness, Restek, USA). The oven temperature was set to 105 C. The injector and detector temperatures were maintained at) 200 C and 250 C, respectively. A set of BTEX standard solutions with concentrations of between 1 and 60 mg L1 was used to obtain a calibration curve. This curve allowed the integrals of the sample peaks in the chromatograms to be converted to concentrations. 2.4. Microscope and scanning electron microscopic analysis An optical microscope (0.63e4 time magnification, Z6, Leica, Bensheim, Germany) and a scanning electron microscope (SEM, JEOL, JSM-6700F, Japan) were used to observe the surfaces of ORB and encap-ORB before and after each experiment.
BTEX degrader Biochar
3. Results and discussion 3.1. Integrality of ORBs and encap-ORBs The integralities of ORBs and encap-ORBs were compared. PVA with a molecular weight of 61600 g mol1 was used in this experiment. Table 1 shows that integrality of encap-ORBs (88.5 ± 0.3%) exceeded that of ORBs (84.5 ± 0.4%). The effect of the molecular weight of the PVA on the integrality of encap-ORBs was examined. Table 1 indicates that increasing the molecular weight of the PVA improved the integrality of the encap-ORBs. Table 1 also reveals that the capsule size (0.018e0.022 mm) slightly influenced the integrality. In this work, the integrality of encap-ORBs exceeded that of ORBs, because adding capsules to ORBs increased their compactness (Freire-Nordi et al., 2006; Jiang et al., 2014). Additionally, Fig. 2b also indicates that the porosity in the cross-section of the encap-ORB was smaller than that of the ORB, resulting in the higher compactness of encap-ORBs. 3.2. Effects of molecular weight of PVA on release of oxygen by encap-ORBs The effect of the molecular weights of the PVA (61600, 105600, and 114400 g mol1) on the oxygen-releasing capacities of encapORBs was evaluated. Fig. 3 reveals that the encap-ORBs that were prepared with high M.W. PVA had a substantially greater oxygenreleasing capacity than that those that were prepared with low M.W. PVA. Table 1 also demonstrates that the particle sizes of the capsule slightly affected the integrality. In this work, increasing the molecular weight of PVA improved the oxygen-releasing capacities of encap-ORBs, because PVA with a higher molecular weight yielded encap-ORBs with a greater tendency to gel and a greater strength (Hassan and Peppas, 2000; Lozinsky et al., 2003; Park et al., 2011). 3.3. BTEX remediation in batch experiment The effects of adding ORBs and encap-ORBs to groundwater on the remediation of BTEX were evaluated in a batch experiment. The experiment comprised three phases (Phase 1: day 0e9; Phase 2: day10e28; Phase 3: day 29e48). At the end of each phase, the treated solution was removed and fresh BTEX solution was added at a variable concentration. The initial BTEX concentrations in Phases 1, 2, and 3 were 120, 240, and 480 mg L1, respectively. The operating conditions were ORB (or encap-ORB) dose of 1.057 g L1 and a DO0 concentration of less than 0.1 mg L1. Four sets were used in this experiment, including the control set (without ORB and degraders), the ORBs set, the ORBs with free degraders set, and the encapORBs set. Since the encap-ORBs had immobilized degraders in themselves (Fig. 1), free degraders were not added to the encapORBs set. Fig. 4 reveals that the loss of BTEX in the control set was negligible during the experiment. In contrast, BTEX was partially
CaO2 + Citrate PVA-alginate capsule
Stir
13
1 cm Granulate into 1cm
Fig. 1. The preparation procedure of encap-ORBs flowchart.
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C.-W. Lin et al. / Journal of Environmental Management 204 (2017) 12e16 Table 1 Integrality of encap-ORB by the various sizes of microcapsules. Items
Capsule size (mm)
PVA model (molecular weight, g/mol)
Integrality (%)
ORB Encap-ORBs Encap-ORBs Encap-ORBs Encap-ORBs
e 1.3 1.3 1.3 0.018e0.022
61,600 61,600 105,600 114,400 114,400
84.5 88.5 91.9 93.7 94.6
± ± ± ± ±
0.4 0.3 0.2 0.3 0.4
Fig. 2. Microscopy and scanning electron microscope of ORB (magnification factor, a1: x 5, a2: x 100, a3: x 500) and encap-ORB (b1: x 5, b2: x 100, b3: x 500).
~ oz et al., 2007). However, when encap-ORBs (Lin et al., 2013; Mun were added to bio-remediate the BTEX at 480 mg L1 (the encapORB set, Phase 3), the inhibition was alleviated. This can be attributed that the micropores of encap-ORBs caused concentration gradient of BTEX inside the beads. Thus, the concentrations of BTEX in the encap-ORBs might be lower than that in the bulk solution (Banerjee and Ghoshal, 2011; Chung et al., 2003). The SEM was used to observe the surfaces of encap-ORBs before and after the bioremediation experiment. SEM analysis reveals that the BTEX degraders grew on the surface of encap-ORBs after biodegradation (Fig. 5). The growth of degraders on the surface of encap-ORBs favored the biodegradation of BTEX in the subsequent phases. The above results show that encap-ORB is a potential oxygen-releasing material for the bioremediation of toxic pollutants at high concentrations.
18 ORB Encap-ORBs
DO (mg/L)
15 12 9 6 3 0
20
40
60
80
100
120
140
Time (d) Fig. 3. Variations in release of dissolved oxygen from encap-ORC in a long-term study.
removed in the ORBs set, because the biochar in the ORBs adsorbed BTEX (Wu et al., 2015). Fig. 4 also shows that the removal rate of BTEX in the ORB with free degraders set exceeded that in the ORBs set, because the oxygen that was released from the ORBs promoted the biodegradation of BTEX. The inhibition of biodegradation caused by high concentration of BTEX (480 mg L1) was observed in the ORB with free degraders set (Phase 3). This is because BTEX and their intermediates, such as catechol, are reportedly toxic to degraders. Catechol can retard the metabolism of benzene to catechol and its further transformation
4. Conclusions In this work, a novel method for preparing encap-ORBs was developed. The encap-ORBs were used to remediate high concentration of BTEX in groundwater. The integrality and oxygenreleasing capacity of encap-ORBs exceeded those of ORBs. The use of high M.W. PVA to prepare encap-ORBs improved their integrality and oxygen-releasing capacity. Encap-ORBs effectively released dissolved oxygen for 128 days. The bioremediation was inhibited by a high concentration of BTEX (480 mg/L) when free degraders were used. Immobilization of degraders in encap-ORBs alleviated the inhibition. Adding encap-ORBs effectively promoted the bioremediation of BTEX at 480 mg/L. SEM analysis indicates that the BTEX degraders grew on the surface of encap-ORBs following biodegradation. The above results show that encap-ORB
C.-W. Lin et al. / Journal of Environmental Management 204 (2017) 12e16
15
(a) 140 Control ORBs ORB with free degeaders Ecap-ORBs
Benzene (mg/L)
120 100 80 60 40 20 0 0
10
20
30
40
50
Time (d)
(b) 140 Control ORBs ORB with free degeaders Ecap-ORBs
Toluene (mg/L)
120 100
Fig. 5. Scanning electron micrographs of encap-ORB surface after experiment.
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is a potential oxygen-releasing material for the bioremediation of toxic pollutants at high concentrations.
60 40
Acknowledgements
20 0 0
10
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40
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Time (d)
(c)
Appendix A. Supplementary data
140 Control ORBs ORB with free degeaders Ecap-ORBs
Ethylbenzene (mg/L)
120 100
Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jenvman.2017.05.035.
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References
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The authors would like to thank the Ministry of Science and Technology of the Republic of China, Taiwan, for financially supporting this research under Contract No. MOST 102-2221-E-224003-MY3.
100 80 60 40 20 0 0
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Time (d) Fig. 4. Effect of ORB and encap-ORB on removal of BTEX by degraders. (a) benzene; (b) toluene; (c) ethylbenzene; (d) xylene.
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Park, H.K., Kong, B.S., Oh, E.S., 2011. Effect of high adhesive polyvinyl alcohol binder on the anodes of lithium ion batteries. Electrochem. Commun. 13, 1051e1053. 2013. Assessment of soil pollution ~ ez, R., Lijzen, J.P.A., Irabien, A., Pinedo, J., Ib an based on total petroleum hydrocarbons and individual oil substances. J. Environ. Manage. 130, 72e79. Qian, Y., Zhou, X., Zhang, Y., Zhang, W., Chen, J., 2013. Performance and properties of nanoscale calcium peroxide for toluene removal. Chemosphere 91, 717e723. Severino, P., Chaud, M.V., Shimojo, A., Antonini, D., Lancelloti, M., Santana, M.H.A., Souto, E.B., 2015. Sodium alginate-cross-linked polymyxin B sulphate-loaded solid lipid nanoparticles: antibiotic resistance tests and HaCat and NIH/3T3 cell viability studies. Colloids Surf. B 129, 191e197. Thiruvenkatachari, R., Vigneswaran, S., Naidu, R., 2008. Permeable reactive barrier for groundwater remediation. J. Ind. Eng. Chem. 14, 145e156. Tsai, S.L., Lin, C.W., Wu, C.H., Shen, C.M., 2013. Kinetics of xenobiotic biodegradation by the Pseudomonas sp. YATO411 strain in suspension and cell-immobilized beads. J. Taiwan Inst. Chem. Eng. 44, 303e309. Wu, C.H., Chang, S.H., Lin, C.W., 2015. Improvement of oxygen release from calcium peroxide-polyvinyl alcohol beads by adding low-cost bamboo biochar and its application in bioremediation. Clean. e Soil, Air, Water 43, 287e295. Wu, Y., Hong, R., Fu, T., 2012. Use inhibition rate of vertical movement ability of Daphnia carinata as an indicator of chromium acute bio-toxicity. Energy Procedia 16 (Part A), 383e390. Xin, B.P., Wu, C.H., Wu, C.H., Lin, C.W., 2013. Bioaugmented remediation of high concentration BTEX-contaminated groundwater by permeable reactive barrier with immobilized bead. J. Hazard. Mater. 244e245, 765e772. Yeh, C.H., Lin, C.W., Wu, C.-H., 2010. A permeable reactive barrier for the bioremediation of BTEX-contaminated groudwater: microbial community distribution and removal efficiencies. J. Hazard. Mater. 178, 74e80.
Contents lists available at ScienceDirect
Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Research article
Innovative encapsulated oxygen-releasing beads for bioremediation of BTEX at high concentration in groundwater Chi-Wen Lin a, Chih-Hung Wu b, Pei-Yu Guo a, Shih-Hsien Chang c, d, * a
Department of Safety, Health and Environmental Engineering, National Yunlin University of Science and Technology, Yunlin, 64002, Taiwan, ROC School of Resources & Chemical Engineering, Sanming University, Sanming City, 365004, China c Department of Public Health, Chung-Shan Medical University, Taichung, 402, Taiwan, ROC d Department of Family and Community Medicine, Chung Shan Medical University Hospital, Taichung, 402, Taiwan, ROC b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 September 2016 Received in revised form 1 May 2017 Accepted 10 May 2017
Both a low concentration of dissolved oxygen and the toxicity of a high concentration of BTEX inhibit the bioremediation of BTEX in groundwater. A novel method of preparing encapsulated oxygen-releasing beads (encap-ORBs) for the biodegradation of BTEX in groundwater was developed. Experimental results show that the integrality and oxygen-releasing capacity of encap-ORBs exceeded those of ORBs. The use of polyvinyl alcohol (PVA) with high M.W. to prepare encap-ORBs improved their integrality. The encap-ORBs effectively released oxygen for 128 days. High concentration of BTEX (480 mg L1) inhibited the biodegradation by the free cells. Immobilization of degraders in the encap-ORB alleviated the inhibition. Scanning electron microscope analysis reveals that the BTEX degraders grew on the surface of encap-ORB after bioremediation. The above results indicate that the encap-ORBs were effective in the bioremediation of BTEX at high concentration in groundwater. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Encapsulated oxygen-releasing bead BTEX Immobilization Polyvinyl alcohol Bioremediation
1. Introduction Groundwater is frequently contaminated by BTEX that has leaked from underground petroleum tanks and gas stations. BTEX poses a high risk for human health and the ecosystem (Pinedo et al., 2013). The various techniques for treating groundwater that contains BTEX include physical, chemical, and biological methods (Erto et al., 2014; Yeh et al., 2010). Of these, bioremediation is popular because it is low-cost and environmentally friendly (Gul et al., 2015). However, the bioremediation of BTEX at a high concentration in groundwater is commonly often limited by its toxicity and the low dissolved oxygen concentration in groundwater (Cao and Harris, 2010; Mohan et al., 2014). Oxygen-releasing material (ORM) can provide dissolved oxygen for groundwater remediation (Thiruvenkatachari et al., 2008). Numerous factors affects its effectiveness in bioremediation, including the compatibility of the ORM with the environment (Cao and Harris, 2010; Severino et al., 2015), the integrality of the ORM (Meng et al., 2009), and its effective oxygen-releasing characteristics (Nykanen et al., 2012; Qian et al., 2013). Traditionally, cement is
* Corresponding author. Department of Public Health, Chung-Shan Medical University, Taichung, 402, Taiwan, ROC. E-mail address: [email protected] (S.-H. Chang). http://dx.doi.org/10.1016/j.jenvman.2017.05.035 0301-4797/© 2017 Elsevier Ltd. All rights reserved.
mixed with oxygen-releasing chemicals, such as CaO2 and MgO2 to produce ORM (Ahmad et al., 2007). However, cement is nonbiodegradable and persists in the aquifer after use (Daghighi et al., 2013). Recently, we used the green materials Polyvinyl alcohol (PVA), alginate, CaO2, citrate, and biochar to prepare ORBs (Liang et al., 2011) and found that it effectively provides dissolved oxygen that enables indigenous degraders to remediate BTEX in groundwater (Jadhav et al., 2013; Lu et al., 2008). Several studies report that a high concentration of BTEX is toxic to degraders, and so inhibits its own bioremediation (Wu et al., 2012). Encapsulation can alleviate the toxicity of BTEX at high concentration (Lin et al., 2010; Tsai et al., 2013; Xin et al., 2013)) and improve its biodegradation (Lassinantti Gualtieri et al., 2015). In this work, encapsulated oxygen-releasing beads (encapORBs) were synthesized and used for the bioremediation of BTEX at high concentration in groundwater. SEM analysis was conducted to observe the growth of BTEX degraders on the encap-ORB surface after bioremediation. 2. Materials and methods 2.1. Preparation of encap-ORB Encap-ORB was prepared as follows. A gelling agent (PVA/
C.-W. Lin et al. / Journal of Environmental Management 204 (2017) 12e16
alginate ¼ 3:1 w/w) was heated in an autoclave (121 C, 1 h) and was sprayed into a CaO2 solution (0.1 M) to form capsules of various sizes (0.18e0.22 mm in diameter). After cooling, the capsules were mixed with an oxygen-releasing agent (CaO2, 5 g), a pH-buffering agent (citrate, 2.5 g), a supporter (biochar, 13.24 g), 10 mL of BTEX degrader (Mycobacterium sp. CHXY119 and Pseudomonas sp. YATO411, A600 nm 2.0 abs) (Xin et al., 2013), and PVA, to form the ORBs (with a diameter of 1 cm). The ORBs were mixed with PAV to prepare encap-ORBs. Fig. 1 displays the procedure for preparing encap-ORBs. The measurement method for integrality ratio of ORBs (Wu et al., 2015) was provided in the Supplementary Material. 2.2. Oxygen-releasing characteristics of encap-ORBs An oxygen release experiment was performed in an acrylic column reactor (diameter: 4 cm, H: 25.5 cm, total volume: 314 mL) at hydraulic retention time of1.24 day. The experimental condition for the oxygen release experiment was provided in the Supplementary Material. To simulate the low level of dissolved oxygen (DO) in common groundwater, 0.3 g L1 of Na2SO3 was added to the inflow to maintain the DO concentration below 0.14 mg L1. A 75 g mass of encap-ORBs was added to the column to begin the experiment. The oxygen concentration was measured using a DO meter (CLEAN DO500, USA). 2.3. Batch bioremediation experiment A batch bioremediation experiment was conducted to study the effect of adding ORBs on the bioremediation of BTEX. Benzene (99.9%, Echo Chemicals, Taiwan), toluene (99.9%, Fisher Scientific, USA), ethylbenzene (99.7%, Tedia, USA) and xylene (99.7%, Tedia), all of analytic grade, were dissolved in the mineral solution to yield a total BTEX concentration of 120e480 mg/L (B: T: E: X ¼ 1: 1: 1: 1, w/w). A 2 g mass of ORBs was placed in the flask to start the bioremediation experiment. At the determined time, 250 mL of solution was sampled and analyzed using a GC apparatus that was equipped with a flame ionization detector (GC-FID, Shimadzu, GC-14B, Japan) and a Stabliwax column (30 m 0.53 mm id 1 mm film thickness, Restek, USA). The oven temperature was set to 105 C. The injector and detector temperatures were maintained at) 200 C and 250 C, respectively. A set of BTEX standard solutions with concentrations of between 1 and 60 mg L1 was used to obtain a calibration curve. This curve allowed the integrals of the sample peaks in the chromatograms to be converted to concentrations. 2.4. Microscope and scanning electron microscopic analysis An optical microscope (0.63e4 time magnification, Z6, Leica, Bensheim, Germany) and a scanning electron microscope (SEM, JEOL, JSM-6700F, Japan) were used to observe the surfaces of ORB and encap-ORB before and after each experiment.
BTEX degrader Biochar
3. Results and discussion 3.1. Integrality of ORBs and encap-ORBs The integralities of ORBs and encap-ORBs were compared. PVA with a molecular weight of 61600 g mol1 was used in this experiment. Table 1 shows that integrality of encap-ORBs (88.5 ± 0.3%) exceeded that of ORBs (84.5 ± 0.4%). The effect of the molecular weight of the PVA on the integrality of encap-ORBs was examined. Table 1 indicates that increasing the molecular weight of the PVA improved the integrality of the encap-ORBs. Table 1 also reveals that the capsule size (0.018e0.022 mm) slightly influenced the integrality. In this work, the integrality of encap-ORBs exceeded that of ORBs, because adding capsules to ORBs increased their compactness (Freire-Nordi et al., 2006; Jiang et al., 2014). Additionally, Fig. 2b also indicates that the porosity in the cross-section of the encap-ORB was smaller than that of the ORB, resulting in the higher compactness of encap-ORBs. 3.2. Effects of molecular weight of PVA on release of oxygen by encap-ORBs The effect of the molecular weights of the PVA (61600, 105600, and 114400 g mol1) on the oxygen-releasing capacities of encapORBs was evaluated. Fig. 3 reveals that the encap-ORBs that were prepared with high M.W. PVA had a substantially greater oxygenreleasing capacity than that those that were prepared with low M.W. PVA. Table 1 also demonstrates that the particle sizes of the capsule slightly affected the integrality. In this work, increasing the molecular weight of PVA improved the oxygen-releasing capacities of encap-ORBs, because PVA with a higher molecular weight yielded encap-ORBs with a greater tendency to gel and a greater strength (Hassan and Peppas, 2000; Lozinsky et al., 2003; Park et al., 2011). 3.3. BTEX remediation in batch experiment The effects of adding ORBs and encap-ORBs to groundwater on the remediation of BTEX were evaluated in a batch experiment. The experiment comprised three phases (Phase 1: day 0e9; Phase 2: day10e28; Phase 3: day 29e48). At the end of each phase, the treated solution was removed and fresh BTEX solution was added at a variable concentration. The initial BTEX concentrations in Phases 1, 2, and 3 were 120, 240, and 480 mg L1, respectively. The operating conditions were ORB (or encap-ORB) dose of 1.057 g L1 and a DO0 concentration of less than 0.1 mg L1. Four sets were used in this experiment, including the control set (without ORB and degraders), the ORBs set, the ORBs with free degraders set, and the encapORBs set. Since the encap-ORBs had immobilized degraders in themselves (Fig. 1), free degraders were not added to the encapORBs set. Fig. 4 reveals that the loss of BTEX in the control set was negligible during the experiment. In contrast, BTEX was partially
CaO2 + Citrate PVA-alginate capsule
Stir
13
1 cm Granulate into 1cm
Fig. 1. The preparation procedure of encap-ORBs flowchart.
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C.-W. Lin et al. / Journal of Environmental Management 204 (2017) 12e16 Table 1 Integrality of encap-ORB by the various sizes of microcapsules. Items
Capsule size (mm)
PVA model (molecular weight, g/mol)
Integrality (%)
ORB Encap-ORBs Encap-ORBs Encap-ORBs Encap-ORBs
e 1.3 1.3 1.3 0.018e0.022
61,600 61,600 105,600 114,400 114,400
84.5 88.5 91.9 93.7 94.6
± ± ± ± ±
0.4 0.3 0.2 0.3 0.4
Fig. 2. Microscopy and scanning electron microscope of ORB (magnification factor, a1: x 5, a2: x 100, a3: x 500) and encap-ORB (b1: x 5, b2: x 100, b3: x 500).
~ oz et al., 2007). However, when encap-ORBs (Lin et al., 2013; Mun were added to bio-remediate the BTEX at 480 mg L1 (the encapORB set, Phase 3), the inhibition was alleviated. This can be attributed that the micropores of encap-ORBs caused concentration gradient of BTEX inside the beads. Thus, the concentrations of BTEX in the encap-ORBs might be lower than that in the bulk solution (Banerjee and Ghoshal, 2011; Chung et al., 2003). The SEM was used to observe the surfaces of encap-ORBs before and after the bioremediation experiment. SEM analysis reveals that the BTEX degraders grew on the surface of encap-ORBs after biodegradation (Fig. 5). The growth of degraders on the surface of encap-ORBs favored the biodegradation of BTEX in the subsequent phases. The above results show that encap-ORB is a potential oxygen-releasing material for the bioremediation of toxic pollutants at high concentrations.
18 ORB Encap-ORBs
DO (mg/L)
15 12 9 6 3 0
20
40
60
80
100
120
140
Time (d) Fig. 3. Variations in release of dissolved oxygen from encap-ORC in a long-term study.
removed in the ORBs set, because the biochar in the ORBs adsorbed BTEX (Wu et al., 2015). Fig. 4 also shows that the removal rate of BTEX in the ORB with free degraders set exceeded that in the ORBs set, because the oxygen that was released from the ORBs promoted the biodegradation of BTEX. The inhibition of biodegradation caused by high concentration of BTEX (480 mg L1) was observed in the ORB with free degraders set (Phase 3). This is because BTEX and their intermediates, such as catechol, are reportedly toxic to degraders. Catechol can retard the metabolism of benzene to catechol and its further transformation
4. Conclusions In this work, a novel method for preparing encap-ORBs was developed. The encap-ORBs were used to remediate high concentration of BTEX in groundwater. The integrality and oxygenreleasing capacity of encap-ORBs exceeded those of ORBs. The use of high M.W. PVA to prepare encap-ORBs improved their integrality and oxygen-releasing capacity. Encap-ORBs effectively released dissolved oxygen for 128 days. The bioremediation was inhibited by a high concentration of BTEX (480 mg/L) when free degraders were used. Immobilization of degraders in encap-ORBs alleviated the inhibition. Adding encap-ORBs effectively promoted the bioremediation of BTEX at 480 mg/L. SEM analysis indicates that the BTEX degraders grew on the surface of encap-ORBs following biodegradation. The above results show that encap-ORB
C.-W. Lin et al. / Journal of Environmental Management 204 (2017) 12e16
15
(a) 140 Control ORBs ORB with free degeaders Ecap-ORBs
Benzene (mg/L)
120 100 80 60 40 20 0 0
10
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50
Time (d)
(b) 140 Control ORBs ORB with free degeaders Ecap-ORBs
Toluene (mg/L)
120 100
Fig. 5. Scanning electron micrographs of encap-ORB surface after experiment.
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is a potential oxygen-releasing material for the bioremediation of toxic pollutants at high concentrations.
60 40
Acknowledgements
20 0 0
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Time (d)
(c)
Appendix A. Supplementary data
140 Control ORBs ORB with free degeaders Ecap-ORBs
Ethylbenzene (mg/L)
120 100
Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jenvman.2017.05.035.
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References
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The authors would like to thank the Ministry of Science and Technology of the Republic of China, Taiwan, for financially supporting this research under Contract No. MOST 102-2221-E-224003-MY3.
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