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Photocatalytic properties of zinc sulfide nanocrystals biofabricated by metal-reducing bacterium Shewanella oneidensis MR-1
Accepted Manuscript Title: Photocatalytic properties of zinc sulfide nanocrystals biofabricated by metal-reducing bacterium Shewanella oneidensis MR-1 Author: Xiang Xiao Xiao-Bo Ma Hang Yuan Peng-Cheng Liu Yu-Bin Lei Hui Xu Dao-Lin Du Jian-Fan Sun Yu-Jie Feng PII: DOI: Reference:
S0304-3894(15)00090-4 http://dx.doi.org/doi:10.1016/j.jhazmat.2015.02.009 HAZMAT 16580
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
6-8-2014 16-11-2014 3-2-2015
Please cite this article as: Xiang Xiao, Xiao-Bo Ma, Hang Yuan, PengCheng Liu, Yu-Bin Lei, Hui Xu, Dao-Lin Du, Jian-Fan Sun, Yu-Jie Feng, Photocatalytic properties of zinc sulfide nanocrystals biofabricated by metalreducing bacterium Shewanella oneidensis MR-1, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2015.02.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Photocatalytic properties of zinc sulfide nanocrystals biofabricated by metal-reducing bacterium Shewanella oneidensis MR-1 Xiang Xiaoa,c Xiao-Bo Maa, Hang Yuanb, Peng-Cheng Liua, Yu-Bin Leia, Hui Xua, Dao-Lin Dua,c,*, Jian-Fan Suna, Yu-Jie Fengc,* a
School of The Environment and Safety Engineering, Jiangsu University, Zhenjiang, 212013, China
b
Key Laboratory of Ion Beam Bioengineering, Institute of Technical Biology & Agriculture Engineering, Chinese Academy of Sciences, Hefei, 230031, China
c
State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China
* Corresponding authors: Prof. Yu-Jie Feng, Fax: (+86)-451-86283068; E-mail: [email protected] Prof. Dao-Lin Du, Fax: +86-511-88790955; E-mail: [email protected]
Highlights ► ► S. oneidensis MR-1 biofabricated ZnS nanocrystals using artificial wastewater. ►
ZnS nanocrystals were 5 nm in diameter and aggregated extracellularly. ► ZnS had good catalytic activity in the degradation of RHB under UV irradication. ► >Photogenerated holes mainly contributed to the degradation of RhB.
Abstract
Accumulation and utilization of heavy metals from wastewater by biological treatment system has aroused great interest. In the present study, a metal-reducing bacterium Shewanella oneidensis MR-1 was used to explore the biofabrication of ZnS
nanocrystals from the artificial wastewater. The biogenic H2S produced via the reduction of thiosulfate precipitated the Zn(II) as sulfide extracellularly. Characterization by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and field emission scanning electron microscope (FESEM) confirmed the precipitates as ZnS nanocrystals. The biogenic ZnS nanocrystals appeared spherical in shape with an average diameter of 5 nm and mainly aggregated in the medium and cell surface of S. oneidensis MR-1. UV−vis DRS spectra showed ZnS nanoparticles appeared a strong absorption below 360 nm. Thus, the photocatalytic activity of ZnS was evaluated by the photodegradation of rhodamine B (RhB) under UV irradiation. The biogenic ZnS nanocrystals showed a high level of photodegradation efficiency to RhB coupled with a significant blue-shift of maximum adsorption peak. A detailed analysis indicated the photogenerated holes, rather than hydroxyl radicals, contributed to the photocatalytic decolorization of RhB. This approach of coupling biosynthesis of nanoparticles with heavy metal removal may offer a potential avenue for efficient bioremediation of heavy metal wastewater. Keywords: ZnS nanocrystal; Biofabrication; Shewanella oneidensis MR-1; Photocatalysis
1. Introduction Heavy metal in wastewater has cause serious harm to the environment and human health, due to toxicity, non-biodegradation and tendency to accumulate as metalloorganic complexes in living organisms [1, 2]. Generally, zinc wastewater is treated by chemical or physical approaches, including precipitation, adsorption, membrane filtration, and ion exchange [3]. Recently, biological methods are considered to be more environmentally-benign and cost-effective for treatment of such wastewater and have attracted great interest. Unlike traditional bioflocculation and biosorption, sulfate-reducing bacteria (SRB) could immobilize the metal as the sulfide precipitates in anaerobic sediments [4, 5]. However, difficulties in recycling of treated products limit its actual application. Thus, it is interesting to explore a new
approach to treat and utilize the heavy metals in wastewater. The genus Shewanella is an important kind of dissimilatory metal reducing bacteria (DMRB), known as its special anaerobic respiratory capacity [6]. Shewanella is found widely distributing in the wastewater and has been intensively used for the bioremediation of environmental contaminants [7]. Recently, the special metal reducing ability of Shewanella species have been utilized to produce and assemble various of metal nanomaterials such as Pd [8], Pt [9], Au [10-12], Ag [13], and even alloy [14]. Although lack of the sulfate reducing capacity of SRBs, Shewanella still can utilize thiosulfate, sulfite, and even element sulfur as the electron acceptors to anaerobic respiratory coupled with the production of S2- [15, 16]. Recently, Shewanella has been used to produce As-S nanotubes [17] and MnS nanoparticles [18]. Thus, Shewanella has a potential to precipitate the heavy metal from wastewater as sulfide nanomaterials. However, relevant reports are rather scarce. Metal sulfide nanomaterials have attracted great attention due to their excellent properties and promising applications in electronic, optical and optoelectronic devices [19]. In which, zinc sulfide (ZnS), an important wide band gap semiconductor, is considered to have important applications in areas such as field emitters, field effect transistors (FETs), p-type conductors, catalyzators, UV-light sensors, chemical sensors (including gas sensors), biosensors, and nanogenerators [20]. In addition, ZnS nanomaterials are helpful to the remediation of environment pollutants by absorption or photocatalytic degradation [21, 22]. Up to now, zinc sulfide nanomaterials are mainly synthesized through various chemical approaches. Taking into account the environmental hazards and high costs, biosynthesis is considered as a potential alternative and has attracted growing attention. The photosynthetic bacterium Rhodobacter sphaeroides has an ability of utilizing sulfate as the sulfur source to produces S2- and biofabricate the ZnS nanomaterials [23]. However, no study about biosynthesis of ZnS nanomaterials by metal-reducing bacterium Shewanella was reported. Considering that sulfite and thiosulfate are also common pollutants in wastewaters and need to be removed [24, 25], biofabrication of ZnS mediated by Shewanella can bring extra environmental benefits.
Therefore, this study aims to explore a new approach to treat and utilize the heavy metal from wastewater. For this purpose, a typical metal-reducing bacterium S. oneidensis MR-1 was conducted to investigating the immobilization of zinc ions from artificial wastewater and biofabricating ZnS nanomaterials under anaerobic condition. The distribution and characterization of as-synthesized nanomaterials were analyzed by transmission electron microscopy (TEM), high resolution TEM (HRTEM), field emission scanning electron microscope (FESEM), energy-dispersive analysis of X-rays (EDX), powder X-Ray diffractometer (XRD), and UV-Vis diffuse reflectance spectra (DRS). Finally, the bioremediation activity of biosynthesized ZnS nanoparticles was evaluated by photocatalytic degradation of rhodamine B (RhB). This work may facilitate a better understanding on the biologically-mediated sulfide precipitation of heavy metal wastewater, and is beneficial to their application in bioremediation.
2. Materials and methods 2.1. Biosynthesis of ZnS nanoparticles S. oneidensis MR-1used in this study is kindly provided by Prof. K.H. Nealson from the University of Southern California. The strain was routinely cultivated Luria-Bertani medium at 30oC until the late stationary phase. The bacterial cells were collected by centrifugation at 5000 rpm for 15 min, and then washed 2 times with sterile distilled water. The growth medium without phosphate was used for the anaerobic biofabrication of ZnS experiments [26]. 18 mM lactate was used as the sole electron donor and carbon source. 15 mM Na2S2O3 were added as the electron acceptor to maintain the anaerobic growth of Shewanella cells. The pH was adjusted to 7.0 buffered with 50 mM 4-(2-hydroxyerhyl) piperazine-1-erhanesulfonic acid (HEPES). Aliquots of 200 ml mixed solution were filled into 250 ml serum bottles, bubbled with N2 for 10 min to ensure anaerobic condition, then sealed with butyl bubber stoppers. The initial concentration of cells was adjusted to 4~6×106 CFU ml-1. After incubation on a rotary
shaker (200 rpm) at 30oC for 2 days, 1 mM ZnSO4 was added. Followed incubation for 4 days, the precipitate was collected by centrifugation at 5000 rpm for 10 min. 2.2. Characterizations The distribution and morphology of the biomass-supported nanomaterials were analyzed by TEM/HRTEM (JEOL, Model JEM-2100). To accurately analyze the element component and photocatalytic performance, the biogenic nanomaterials were washed twice with Milli-Q water, and then purified with 50% ethanol, 75% ethanol, 100% ethanol and acetone, gradually. Finally, these nanoparticles were dried in a nitrogen atmosphere. The morphological characterization was analyzed by FESEM (S-4800, Hitachi High-Technologies Corporation, Japan). The elemental analysis of the sample was performed by FESEM coupled with EDX. Samples were also characterized by means of XRD using a Discover D8 X-ray diffractometer with a Xe/Ar gas–filled Hi Star area detector, operated at 40 kV and 40 mA (Bruker AXS Company, Germany). Light absorption properties were measured by UV–vis diffuse reflectance spectra (DRS) (UV-2450PC, Shimadzu Co., Japan) using BaSO4 powder as a reference. The energy of band gap (Eg) was calculated via a Tauc plot of (αhν)2 versus (hν) and extrapolation of the linear part of the curve to the energy axis according to [27-29]: αhν = B(hν-Eg)1/2
(1)
where α is the absorption coefficient, hν is the photon energy, Eg is the direct band gap energy, and B is a constant. 2.3. Photocatalytic activity tests To examine the photocatalytic activity of as-prepared samples, photodegradation of rhodamine B (RhB), which is a typical dye resistant to biodegradation, was investigated in aqueous solution under ultraviolet light irradiation. A 50 mg of powdered photocatalyst was added into 50 mL RhB aqueous solution (20 mg/L). Before UV light irradiation, the mixed solution was magnetically stirred in dark for 40 min to reach an adsorption/desorption equilibrium. A 300 W UV lamp was used as the light source to trigger the photocatalytic reaction. At each sampling time, 1 ml of the mixed solution was sampled to evaluate the photodegradation efficiency by using a
UV-Vis spectrophotometer (UV-2501PC, Shimadzu Co., Japan). To elucidate the reaction mechanism involved into the RhB degradation, tert-butanol (TBA) and ammonium oxalate (AO) were used as the scavengers of hydroxyl radicals and holes, respectively [30, 31].
3. Results and discussion 3.1. Synthesis and characterization of ZnS nanoparticles After Shewanella cells adapted to the anaerobic environment for 2-d, 1 mM ZnSO4 was added. White precipitates were clearly observed after 6-d incubation (Fig. S1). The white precipitates were collected and then analyzed by FESEM-EDX after purification. As shown in Fig. 1, the precipitates appear spherical shape and aggregate. EDX of the synthesized products determine that these nanoparticles are composed of the Zn and S, and the atomic ratio of Zn:S is 0.98:1, which is very close to the theoretic expectation (Fig. 1). Meantime, the C and O signatures are also observed. This should contribute to biological molecules absorbed on the surface of nanoparticles. The FTIR analysis also indicates the existence of biological molecules (Fig. S2). Fig. 2 displays the XRD pattern of the ZnS synthesized by S. oneidensis MR-1 after 6-d incubation. XRD data show that all of the diffraction peaks can be well-indexed to the cubic phase of ZnS (JCPDS card No. 79-0043). The dominated diffraction peaks at around 2θ= 29.06°, 48.36°, and 57.42° are corresponded to the (110), (220), and (311), respectively, which indicates that ZnS nanocrystals have been successfully biofabricated by S. oneidensis MR-1. Although synthesis of sulfide nanomaterials, including MnS, Ag2S, and AsxSy, by Shewanella under anaerobic condition has been previously reported [17, 18, 32], this is the first report about ZnS nanoparticles synthesized by S. oneidensis MR-1. The broadening of the XRD pattern implies a small particle size of the products. The average diameter of these particles reach 5.1 nm calculated by Scherrer's equation. The subcellular localization of ZnS precipitates was analyzed by using TEM after
6-d incubation. Loosely packed aggregates of particles are observed on the cell surface and in the medium (Fig. 3A and 3B), which is similar with loosely extracellular distribution of UO2 nanoparticles biosynthesized by S. oneidensis MR-1. However, the form of densely packed particles in association with an extracellular polymeric substance is not observed in our experiment, indicating that EPS is not an essential component for the extracellular formation of ZnS nanomaterial [33]. This phenomenon may be due to the different formation mechanism involved in the synthesis of biogenic nanomaterials. The special electrical conductivity properties of EPS secreted by Shewanella may contribute to the reduction of U(IV) and formation of U(IV) nanoparticles, acting as a reductant. But for the formation of ZnS nanomaterials, the reduction of thiosulfate is occurred in the periplasm of S. oneidensis MR-1 and then the produced S2- precipitates Zn2+ as ZnS nanocrystals extracellularly. All process need not an additional reductant. Considering that EPS as a template is not necessary for the formation of ZnS nanoparticles, it is not surprising that ZnS precipitates are not densely coupled with the EPS. Similarly, no densely packed particles in association with EPS are observed in the biosynthesis of other sulfide nanomaterials by Shewanella [17, 18, 32]. The ZnS nanocrystals produced by S. oneidensis have an average diameter of approximately 5 nm measured by TEM images, which is consistent with the value calculated by Scherrer's equation. This average size of as-synthesed ZnS nanoparticles is significantly smaller than that biosynthesized by Rhodobacter sphaeroides (average 105 nm) [23]. This may be due to the different reduction capacity of sulfur source in two bacterial strains. Selected area electron diffraction (SAED) pattern clearly indicates the Scherrer ring pattern characteristic of face centered cubic (fcc), showing nanostructures are polycrystalline (inset of Fig. 3C). HRTEM and lattice images reveal that the nanocrystals are cubic with a d-spacing of 0.31 nm, corresponding to the (111) plane of cubic ZnS (Fig. 3D). To determine light absorption characterization and band gap energy of as-prepared ZnS nanoparticles, UV-vis DRS were measured. As shown in Fig. 4A, the optical absorption spectra of ZnS nanoparticles appear a strong absorption below 360
nm. Fig. 4B showed the Tauc plots of ZnS nanoparticles. The direct bandgap energy of ZnS synthesized by Shewanella estimated from the intercept of the tangent to the plots is 3.53 eV, higher than ZnS synthesized by chemical methods [34, 35], suggesting as-biosynthesized ZnS nanocrystal has a larger redox potential for photocatalytic decomposition of organic contaminants under UV irradiation [20]. Additionally, the room temperature photoluminescence (PL) indicates that biogenic ZnS nanoparticles can emit blue fluorescence using an excitation wavelength of 278 nm (Fig. S3). These results suggest this prepared semiconductor nanomaterial can be excited by ultraviolet irradiation and has a potential application in the photodegradation of pollutants.
3.2. Photocatalytic properties of biogenic ZnS After reaching the adsorption equilibrium in dark condition, RhB was photodegraded by ZnS nanocrystals under UV light irradiation. As shown in Fig. 5A, ZnS displays a strong photocatalytic activity to RhB. After irradiation for 3 h, the absorption peak of RhB at 554 nm disappears. A detailed analysis by UV-Vis adsorption spectra shows a significant blue-shift of maximum absorption peak from 554 nm to 498 nm coupled with the gradual decolorization of RhB (Fig. 5B). This degradation process by as-prepared ZnS nanoparticles is similar with the reports about the partial photodegradation of RhB [35, 36], suggesting a gradual de-ethylation of the N,N,N’,N’-tetraethylrhodamine structure [37].
3.3. Photocatalytic mechanism for RhB degradation Photocatalytic mechanism involved in the RhB photodegradation was elucidated by addition of TBA and AO as the scavengers of hydroxyl radicals and holes, respectively. It is observed that the presence of TBA not quench the degradation rate of RhB obviously, while addition of AO severely inhibits the decolorization of RhB (Fig. 6A). The plots of the photocatalytic degradation of RhB over ZnS nanoparticles against irradiation time were calculated. As shown in Fig. 6B, a straight line is obtained when -ln(C/C0) is plotted against irradiation time, suggesting that the
degradation of RhB obeys the rule of first-order kinetic reaction. When AO is added, the reaction rate constant k decreases significantly, indicating that photogenerated holes play an important role in the photodegradation process of RhB. For ZnS nanocrystals, the valence band edges (Ev) is 1.92 V versus SCE, lower than the normal potential of the OH-/·OH couple (2.7 V versus SCE), which means that the holes photogenerated by ZnS under UV irradiation have not enough energy to react with OH-/H2O to form ·OH radicals [35]. Thus, it is not surprising that the scavenger of ·OH radicals do not display a strong inhibitory efficiency in the photochemical process of the RhB/ZnS system. Therefore, the photocatalytic degradation of RhB in the presence of ZnS is due to the direct participation of photogenerated holes rather than hydroxyl radicals. 4. Conclusions Environmental concerns encourage one to look for new strategies to fabricate nanomaterials. In the present study, a typical metal-reducing bacterium S. oneidensis MR-1 was explored to biosynthesize the ZnS nanocrystals using artificial heavy metal wastewater. The biogenic ZnS nanocrystals were spherical in shape and mainly aggregated extracellularly. The average diameter reached approximately 5 nm. Biofabricated ZnS had a strong absorption below 360 nm and the band gap energy was 3.53 eV. Further assessments revealed that ZnS nanoparticles exhibited an obviously photocatalytic activity towards the decolorization of RhB in an aqueous solution under UV irradiation. Furthermore, a detailed analysis indicated that the photogenerated holes, rather than hydroxyl radicals (·OH), contributed to the photocatalytic decolorization of RhB and resulted in the RhB photodecolorization by gradual de-ethylation. In the future, more work should be conducted to investigating the actual role of electricigens in the biosynthesis of nanomaterials coupled with the bioremediation of heavy metal wastewater.
Acknowledgements This study was partially supported by the National Basic Research Program of
China (2013CB934302), National Natural Science Foundation of China (51478208 and 31200317), China Postdoctoral Science Foundation (2012T50470) and Open Project of State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No. ESK201201).
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Figure Legends
Fig. 1 FESEM-EDX analysis of ZnS nanostructures biofabicated by S. oneidensis MR-1.
Fig. 2 X-ray diffraction pattern of ZnS nanoparticles synthesized by S. oneidensis MR-1.
Fig. 3 TEM analysis of the biogenic ZnS nanoparticles. (A, B) TEM images of cell
suspensions (C) TEM images of biogenic ZnS nanoparticles and the corresponding SAED pattern (inset); (D) high resolution TEM (HRTEM) images of ZnS.
Fig. 4 (A) UV-vis diffuse reflection spectra and (B) Tauc plots of the as-prepared ZnS nanoparticles.
Fig. 5 (A) Photocatalytic activity of as-synthesized ZnS nanocrystal for the photodegradation of RhB aqueous solution (20 mg/L) in air. C0 and C represent the initial concentration and residual concentrations of RhB, respectively. (B) Absorption changes of an RhB aqueous solution at room temperature in the presence of ZnS nanoparticles under UV irradiation.
Fig. 6 (A) Effects of hydroxyl radical and hole scavengers on the degradation of RhB dye over ZnS; (B) Kinetic fit of RhB degradation by ZnS under different reaction conditions.
Fig. 1 FESEM-EDX analysis of ZnS nanostructures biofabicated by S. oneidensis MR-1
300 ZnS(111)
Intensity (a. u.)
250 200 150
ZnS(220) ZnS(311)
100 50 0
20
40
60 2θ/degree
80
Fig. 2 X-ray diffraction pattern of ZnS nanoparticles synthesized by S. oneidensis MR-1
Fig. 3 TEM analysis of the biogenic ZnS nanoparticles. (A, B) TEM images of cell suspensions (C) TEM images of biogenic ZnS nanoparticles and the corresponding SAED pattern (inset); (D) high resolution TEM (HRTEM) images of ZnS.
Absorbance (a.u.)
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Fig. 4 (A) UV-vis diffuse reflection spectra and (B) Tauc plots of the as-prepared ZnS nanoparticles.
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Fig. 5 (A) Photocatalytic activity of as-synthesized ZnS nanocrystal for the photodegradation of RhB aqueous solution (20 mg/L) in air. C0 and C represent the initial concentration and residual concentrations of RhB, respectively. (B) Absorption changes of an RhB aqueous solution at room temperature in the presence of ZnS nanoparticles under UV irradiation.
A 1.0 CK ZnS ZnS + AO ZnS + TBA
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Fig. 6 (A) Effects of hydroxyl radical and hole scavengers on the degradation of RhB dye over ZnS; (B) Kinetic fit of RhB degradation by ZnS under different reaction conditions.
S0304-3894(15)00090-4 http://dx.doi.org/doi:10.1016/j.jhazmat.2015.02.009 HAZMAT 16580
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
6-8-2014 16-11-2014 3-2-2015
Please cite this article as: Xiang Xiao, Xiao-Bo Ma, Hang Yuan, PengCheng Liu, Yu-Bin Lei, Hui Xu, Dao-Lin Du, Jian-Fan Sun, Yu-Jie Feng, Photocatalytic properties of zinc sulfide nanocrystals biofabricated by metalreducing bacterium Shewanella oneidensis MR-1, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2015.02.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Photocatalytic properties of zinc sulfide nanocrystals biofabricated by metal-reducing bacterium Shewanella oneidensis MR-1 Xiang Xiaoa,c Xiao-Bo Maa, Hang Yuanb, Peng-Cheng Liua, Yu-Bin Leia, Hui Xua, Dao-Lin Dua,c,*, Jian-Fan Suna, Yu-Jie Fengc,* a
School of The Environment and Safety Engineering, Jiangsu University, Zhenjiang, 212013, China
b
Key Laboratory of Ion Beam Bioengineering, Institute of Technical Biology & Agriculture Engineering, Chinese Academy of Sciences, Hefei, 230031, China
c
State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China
* Corresponding authors: Prof. Yu-Jie Feng, Fax: (+86)-451-86283068; E-mail: [email protected] Prof. Dao-Lin Du, Fax: +86-511-88790955; E-mail: [email protected]
Highlights ► ► S. oneidensis MR-1 biofabricated ZnS nanocrystals using artificial wastewater. ►
ZnS nanocrystals were 5 nm in diameter and aggregated extracellularly. ► ZnS had good catalytic activity in the degradation of RHB under UV irradication. ► >Photogenerated holes mainly contributed to the degradation of RhB.
Abstract
Accumulation and utilization of heavy metals from wastewater by biological treatment system has aroused great interest. In the present study, a metal-reducing bacterium Shewanella oneidensis MR-1 was used to explore the biofabrication of ZnS
nanocrystals from the artificial wastewater. The biogenic H2S produced via the reduction of thiosulfate precipitated the Zn(II) as sulfide extracellularly. Characterization by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and field emission scanning electron microscope (FESEM) confirmed the precipitates as ZnS nanocrystals. The biogenic ZnS nanocrystals appeared spherical in shape with an average diameter of 5 nm and mainly aggregated in the medium and cell surface of S. oneidensis MR-1. UV−vis DRS spectra showed ZnS nanoparticles appeared a strong absorption below 360 nm. Thus, the photocatalytic activity of ZnS was evaluated by the photodegradation of rhodamine B (RhB) under UV irradiation. The biogenic ZnS nanocrystals showed a high level of photodegradation efficiency to RhB coupled with a significant blue-shift of maximum adsorption peak. A detailed analysis indicated the photogenerated holes, rather than hydroxyl radicals, contributed to the photocatalytic decolorization of RhB. This approach of coupling biosynthesis of nanoparticles with heavy metal removal may offer a potential avenue for efficient bioremediation of heavy metal wastewater. Keywords: ZnS nanocrystal; Biofabrication; Shewanella oneidensis MR-1; Photocatalysis
1. Introduction Heavy metal in wastewater has cause serious harm to the environment and human health, due to toxicity, non-biodegradation and tendency to accumulate as metalloorganic complexes in living organisms [1, 2]. Generally, zinc wastewater is treated by chemical or physical approaches, including precipitation, adsorption, membrane filtration, and ion exchange [3]. Recently, biological methods are considered to be more environmentally-benign and cost-effective for treatment of such wastewater and have attracted great interest. Unlike traditional bioflocculation and biosorption, sulfate-reducing bacteria (SRB) could immobilize the metal as the sulfide precipitates in anaerobic sediments [4, 5]. However, difficulties in recycling of treated products limit its actual application. Thus, it is interesting to explore a new
approach to treat and utilize the heavy metals in wastewater. The genus Shewanella is an important kind of dissimilatory metal reducing bacteria (DMRB), known as its special anaerobic respiratory capacity [6]. Shewanella is found widely distributing in the wastewater and has been intensively used for the bioremediation of environmental contaminants [7]. Recently, the special metal reducing ability of Shewanella species have been utilized to produce and assemble various of metal nanomaterials such as Pd [8], Pt [9], Au [10-12], Ag [13], and even alloy [14]. Although lack of the sulfate reducing capacity of SRBs, Shewanella still can utilize thiosulfate, sulfite, and even element sulfur as the electron acceptors to anaerobic respiratory coupled with the production of S2- [15, 16]. Recently, Shewanella has been used to produce As-S nanotubes [17] and MnS nanoparticles [18]. Thus, Shewanella has a potential to precipitate the heavy metal from wastewater as sulfide nanomaterials. However, relevant reports are rather scarce. Metal sulfide nanomaterials have attracted great attention due to their excellent properties and promising applications in electronic, optical and optoelectronic devices [19]. In which, zinc sulfide (ZnS), an important wide band gap semiconductor, is considered to have important applications in areas such as field emitters, field effect transistors (FETs), p-type conductors, catalyzators, UV-light sensors, chemical sensors (including gas sensors), biosensors, and nanogenerators [20]. In addition, ZnS nanomaterials are helpful to the remediation of environment pollutants by absorption or photocatalytic degradation [21, 22]. Up to now, zinc sulfide nanomaterials are mainly synthesized through various chemical approaches. Taking into account the environmental hazards and high costs, biosynthesis is considered as a potential alternative and has attracted growing attention. The photosynthetic bacterium Rhodobacter sphaeroides has an ability of utilizing sulfate as the sulfur source to produces S2- and biofabricate the ZnS nanomaterials [23]. However, no study about biosynthesis of ZnS nanomaterials by metal-reducing bacterium Shewanella was reported. Considering that sulfite and thiosulfate are also common pollutants in wastewaters and need to be removed [24, 25], biofabrication of ZnS mediated by Shewanella can bring extra environmental benefits.
Therefore, this study aims to explore a new approach to treat and utilize the heavy metal from wastewater. For this purpose, a typical metal-reducing bacterium S. oneidensis MR-1 was conducted to investigating the immobilization of zinc ions from artificial wastewater and biofabricating ZnS nanomaterials under anaerobic condition. The distribution and characterization of as-synthesized nanomaterials were analyzed by transmission electron microscopy (TEM), high resolution TEM (HRTEM), field emission scanning electron microscope (FESEM), energy-dispersive analysis of X-rays (EDX), powder X-Ray diffractometer (XRD), and UV-Vis diffuse reflectance spectra (DRS). Finally, the bioremediation activity of biosynthesized ZnS nanoparticles was evaluated by photocatalytic degradation of rhodamine B (RhB). This work may facilitate a better understanding on the biologically-mediated sulfide precipitation of heavy metal wastewater, and is beneficial to their application in bioremediation.
2. Materials and methods 2.1. Biosynthesis of ZnS nanoparticles S. oneidensis MR-1used in this study is kindly provided by Prof. K.H. Nealson from the University of Southern California. The strain was routinely cultivated Luria-Bertani medium at 30oC until the late stationary phase. The bacterial cells were collected by centrifugation at 5000 rpm for 15 min, and then washed 2 times with sterile distilled water. The growth medium without phosphate was used for the anaerobic biofabrication of ZnS experiments [26]. 18 mM lactate was used as the sole electron donor and carbon source. 15 mM Na2S2O3 were added as the electron acceptor to maintain the anaerobic growth of Shewanella cells. The pH was adjusted to 7.0 buffered with 50 mM 4-(2-hydroxyerhyl) piperazine-1-erhanesulfonic acid (HEPES). Aliquots of 200 ml mixed solution were filled into 250 ml serum bottles, bubbled with N2 for 10 min to ensure anaerobic condition, then sealed with butyl bubber stoppers. The initial concentration of cells was adjusted to 4~6×106 CFU ml-1. After incubation on a rotary
shaker (200 rpm) at 30oC for 2 days, 1 mM ZnSO4 was added. Followed incubation for 4 days, the precipitate was collected by centrifugation at 5000 rpm for 10 min. 2.2. Characterizations The distribution and morphology of the biomass-supported nanomaterials were analyzed by TEM/HRTEM (JEOL, Model JEM-2100). To accurately analyze the element component and photocatalytic performance, the biogenic nanomaterials were washed twice with Milli-Q water, and then purified with 50% ethanol, 75% ethanol, 100% ethanol and acetone, gradually. Finally, these nanoparticles were dried in a nitrogen atmosphere. The morphological characterization was analyzed by FESEM (S-4800, Hitachi High-Technologies Corporation, Japan). The elemental analysis of the sample was performed by FESEM coupled with EDX. Samples were also characterized by means of XRD using a Discover D8 X-ray diffractometer with a Xe/Ar gas–filled Hi Star area detector, operated at 40 kV and 40 mA (Bruker AXS Company, Germany). Light absorption properties were measured by UV–vis diffuse reflectance spectra (DRS) (UV-2450PC, Shimadzu Co., Japan) using BaSO4 powder as a reference. The energy of band gap (Eg) was calculated via a Tauc plot of (αhν)2 versus (hν) and extrapolation of the linear part of the curve to the energy axis according to [27-29]: αhν = B(hν-Eg)1/2
(1)
where α is the absorption coefficient, hν is the photon energy, Eg is the direct band gap energy, and B is a constant. 2.3. Photocatalytic activity tests To examine the photocatalytic activity of as-prepared samples, photodegradation of rhodamine B (RhB), which is a typical dye resistant to biodegradation, was investigated in aqueous solution under ultraviolet light irradiation. A 50 mg of powdered photocatalyst was added into 50 mL RhB aqueous solution (20 mg/L). Before UV light irradiation, the mixed solution was magnetically stirred in dark for 40 min to reach an adsorption/desorption equilibrium. A 300 W UV lamp was used as the light source to trigger the photocatalytic reaction. At each sampling time, 1 ml of the mixed solution was sampled to evaluate the photodegradation efficiency by using a
UV-Vis spectrophotometer (UV-2501PC, Shimadzu Co., Japan). To elucidate the reaction mechanism involved into the RhB degradation, tert-butanol (TBA) and ammonium oxalate (AO) were used as the scavengers of hydroxyl radicals and holes, respectively [30, 31].
3. Results and discussion 3.1. Synthesis and characterization of ZnS nanoparticles After Shewanella cells adapted to the anaerobic environment for 2-d, 1 mM ZnSO4 was added. White precipitates were clearly observed after 6-d incubation (Fig. S1). The white precipitates were collected and then analyzed by FESEM-EDX after purification. As shown in Fig. 1, the precipitates appear spherical shape and aggregate. EDX of the synthesized products determine that these nanoparticles are composed of the Zn and S, and the atomic ratio of Zn:S is 0.98:1, which is very close to the theoretic expectation (Fig. 1). Meantime, the C and O signatures are also observed. This should contribute to biological molecules absorbed on the surface of nanoparticles. The FTIR analysis also indicates the existence of biological molecules (Fig. S2). Fig. 2 displays the XRD pattern of the ZnS synthesized by S. oneidensis MR-1 after 6-d incubation. XRD data show that all of the diffraction peaks can be well-indexed to the cubic phase of ZnS (JCPDS card No. 79-0043). The dominated diffraction peaks at around 2θ= 29.06°, 48.36°, and 57.42° are corresponded to the (110), (220), and (311), respectively, which indicates that ZnS nanocrystals have been successfully biofabricated by S. oneidensis MR-1. Although synthesis of sulfide nanomaterials, including MnS, Ag2S, and AsxSy, by Shewanella under anaerobic condition has been previously reported [17, 18, 32], this is the first report about ZnS nanoparticles synthesized by S. oneidensis MR-1. The broadening of the XRD pattern implies a small particle size of the products. The average diameter of these particles reach 5.1 nm calculated by Scherrer's equation. The subcellular localization of ZnS precipitates was analyzed by using TEM after
6-d incubation. Loosely packed aggregates of particles are observed on the cell surface and in the medium (Fig. 3A and 3B), which is similar with loosely extracellular distribution of UO2 nanoparticles biosynthesized by S. oneidensis MR-1. However, the form of densely packed particles in association with an extracellular polymeric substance is not observed in our experiment, indicating that EPS is not an essential component for the extracellular formation of ZnS nanomaterial [33]. This phenomenon may be due to the different formation mechanism involved in the synthesis of biogenic nanomaterials. The special electrical conductivity properties of EPS secreted by Shewanella may contribute to the reduction of U(IV) and formation of U(IV) nanoparticles, acting as a reductant. But for the formation of ZnS nanomaterials, the reduction of thiosulfate is occurred in the periplasm of S. oneidensis MR-1 and then the produced S2- precipitates Zn2+ as ZnS nanocrystals extracellularly. All process need not an additional reductant. Considering that EPS as a template is not necessary for the formation of ZnS nanoparticles, it is not surprising that ZnS precipitates are not densely coupled with the EPS. Similarly, no densely packed particles in association with EPS are observed in the biosynthesis of other sulfide nanomaterials by Shewanella [17, 18, 32]. The ZnS nanocrystals produced by S. oneidensis have an average diameter of approximately 5 nm measured by TEM images, which is consistent with the value calculated by Scherrer's equation. This average size of as-synthesed ZnS nanoparticles is significantly smaller than that biosynthesized by Rhodobacter sphaeroides (average 105 nm) [23]. This may be due to the different reduction capacity of sulfur source in two bacterial strains. Selected area electron diffraction (SAED) pattern clearly indicates the Scherrer ring pattern characteristic of face centered cubic (fcc), showing nanostructures are polycrystalline (inset of Fig. 3C). HRTEM and lattice images reveal that the nanocrystals are cubic with a d-spacing of 0.31 nm, corresponding to the (111) plane of cubic ZnS (Fig. 3D). To determine light absorption characterization and band gap energy of as-prepared ZnS nanoparticles, UV-vis DRS were measured. As shown in Fig. 4A, the optical absorption spectra of ZnS nanoparticles appear a strong absorption below 360
nm. Fig. 4B showed the Tauc plots of ZnS nanoparticles. The direct bandgap energy of ZnS synthesized by Shewanella estimated from the intercept of the tangent to the plots is 3.53 eV, higher than ZnS synthesized by chemical methods [34, 35], suggesting as-biosynthesized ZnS nanocrystal has a larger redox potential for photocatalytic decomposition of organic contaminants under UV irradiation [20]. Additionally, the room temperature photoluminescence (PL) indicates that biogenic ZnS nanoparticles can emit blue fluorescence using an excitation wavelength of 278 nm (Fig. S3). These results suggest this prepared semiconductor nanomaterial can be excited by ultraviolet irradiation and has a potential application in the photodegradation of pollutants.
3.2. Photocatalytic properties of biogenic ZnS After reaching the adsorption equilibrium in dark condition, RhB was photodegraded by ZnS nanocrystals under UV light irradiation. As shown in Fig. 5A, ZnS displays a strong photocatalytic activity to RhB. After irradiation for 3 h, the absorption peak of RhB at 554 nm disappears. A detailed analysis by UV-Vis adsorption spectra shows a significant blue-shift of maximum absorption peak from 554 nm to 498 nm coupled with the gradual decolorization of RhB (Fig. 5B). This degradation process by as-prepared ZnS nanoparticles is similar with the reports about the partial photodegradation of RhB [35, 36], suggesting a gradual de-ethylation of the N,N,N’,N’-tetraethylrhodamine structure [37].
3.3. Photocatalytic mechanism for RhB degradation Photocatalytic mechanism involved in the RhB photodegradation was elucidated by addition of TBA and AO as the scavengers of hydroxyl radicals and holes, respectively. It is observed that the presence of TBA not quench the degradation rate of RhB obviously, while addition of AO severely inhibits the decolorization of RhB (Fig. 6A). The plots of the photocatalytic degradation of RhB over ZnS nanoparticles against irradiation time were calculated. As shown in Fig. 6B, a straight line is obtained when -ln(C/C0) is plotted against irradiation time, suggesting that the
degradation of RhB obeys the rule of first-order kinetic reaction. When AO is added, the reaction rate constant k decreases significantly, indicating that photogenerated holes play an important role in the photodegradation process of RhB. For ZnS nanocrystals, the valence band edges (Ev) is 1.92 V versus SCE, lower than the normal potential of the OH-/·OH couple (2.7 V versus SCE), which means that the holes photogenerated by ZnS under UV irradiation have not enough energy to react with OH-/H2O to form ·OH radicals [35]. Thus, it is not surprising that the scavenger of ·OH radicals do not display a strong inhibitory efficiency in the photochemical process of the RhB/ZnS system. Therefore, the photocatalytic degradation of RhB in the presence of ZnS is due to the direct participation of photogenerated holes rather than hydroxyl radicals. 4. Conclusions Environmental concerns encourage one to look for new strategies to fabricate nanomaterials. In the present study, a typical metal-reducing bacterium S. oneidensis MR-1 was explored to biosynthesize the ZnS nanocrystals using artificial heavy metal wastewater. The biogenic ZnS nanocrystals were spherical in shape and mainly aggregated extracellularly. The average diameter reached approximately 5 nm. Biofabricated ZnS had a strong absorption below 360 nm and the band gap energy was 3.53 eV. Further assessments revealed that ZnS nanoparticles exhibited an obviously photocatalytic activity towards the decolorization of RhB in an aqueous solution under UV irradiation. Furthermore, a detailed analysis indicated that the photogenerated holes, rather than hydroxyl radicals (·OH), contributed to the photocatalytic decolorization of RhB and resulted in the RhB photodecolorization by gradual de-ethylation. In the future, more work should be conducted to investigating the actual role of electricigens in the biosynthesis of nanomaterials coupled with the bioremediation of heavy metal wastewater.
Acknowledgements This study was partially supported by the National Basic Research Program of
China (2013CB934302), National Natural Science Foundation of China (51478208 and 31200317), China Postdoctoral Science Foundation (2012T50470) and Open Project of State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No. ESK201201).
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Elucidating
the
structure-dependent photocatalytic properties of Bi2WO6: a synthesis guided investigation, Dalton T, 41 (2012) 7939-7945.
Figure Legends
Fig. 1 FESEM-EDX analysis of ZnS nanostructures biofabicated by S. oneidensis MR-1.
Fig. 2 X-ray diffraction pattern of ZnS nanoparticles synthesized by S. oneidensis MR-1.
Fig. 3 TEM analysis of the biogenic ZnS nanoparticles. (A, B) TEM images of cell
suspensions (C) TEM images of biogenic ZnS nanoparticles and the corresponding SAED pattern (inset); (D) high resolution TEM (HRTEM) images of ZnS.
Fig. 4 (A) UV-vis diffuse reflection spectra and (B) Tauc plots of the as-prepared ZnS nanoparticles.
Fig. 5 (A) Photocatalytic activity of as-synthesized ZnS nanocrystal for the photodegradation of RhB aqueous solution (20 mg/L) in air. C0 and C represent the initial concentration and residual concentrations of RhB, respectively. (B) Absorption changes of an RhB aqueous solution at room temperature in the presence of ZnS nanoparticles under UV irradiation.
Fig. 6 (A) Effects of hydroxyl radical and hole scavengers on the degradation of RhB dye over ZnS; (B) Kinetic fit of RhB degradation by ZnS under different reaction conditions.
Fig. 1 FESEM-EDX analysis of ZnS nanostructures biofabicated by S. oneidensis MR-1
300 ZnS(111)
Intensity (a. u.)
250 200 150
ZnS(220) ZnS(311)
100 50 0
20
40
60 2θ/degree
80
Fig. 2 X-ray diffraction pattern of ZnS nanoparticles synthesized by S. oneidensis MR-1
Fig. 3 TEM analysis of the biogenic ZnS nanoparticles. (A, B) TEM images of cell suspensions (C) TEM images of biogenic ZnS nanoparticles and the corresponding SAED pattern (inset); (D) high resolution TEM (HRTEM) images of ZnS.
Absorbance (a.u.)
A 0.6 0.5 0.4 0.3 0.2 240 B 10
280 320 360 Wavelength (nm)
(ahν)
2
8 6 4 2 0
3.5
4.0
4.5 hν (eV)
5.0
5.5
Fig. 4 (A) UV-vis diffuse reflection spectra and (B) Tauc plots of the as-prepared ZnS nanoparticles.
A 1.0 0.8 C/C0
0.6 0.4 0.2 0.0 0.0
0.5
1.0 1.5 2.0 Time (h)
B 3.0
3.0
0h 0.5 h 1.0 h 1.5 h 2.0 h 2.5 h 3.0 h
2.5 Absorbance
2.5
2.0 1.5 1.0 0.5 0.0 200
300
400 500 600 Wavelength(nm)
700
800
Fig. 5 (A) Photocatalytic activity of as-synthesized ZnS nanocrystal for the photodegradation of RhB aqueous solution (20 mg/L) in air. C0 and C represent the initial concentration and residual concentrations of RhB, respectively. (B) Absorption changes of an RhB aqueous solution at room temperature in the presence of ZnS nanoparticles under UV irradiation.
A 1.0 CK ZnS ZnS + AO ZnS + TBA
C/C0
0.8 0.6 0.4 0.2 0.0 0.0
B 3.0
1.0
1.5 2.0 Time (h)
2.5
3.0
2.5
3.0
CK ZnS ZnS + AO ZnS + TBA
2.5 2.0 -ln (C/C0)
0.5
1.5 1.0 0.5 0.0 0.0
0.5
1.0
1.5 2.0 Time (h)
Fig. 6 (A) Effects of hydroxyl radical and hole scavengers on the degradation of RhB dye over ZnS; (B) Kinetic fit of RhB degradation by ZnS under different reaction conditions.