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The influence of different Tween surfactants on biodesulfurization of ground tire rubber by Sphingomonas sp.
Accepted Manuscript The influence of different Tween surfactants on biodesulfurization of ground tire rubber by Sphingomonas sp. Minghan Hu, Suhe Zhao, Chao Li, Bingwu Wang, Chu Yao, Yaqin Wang PII:
S0141-3910(14)00180-3
DOI:
10.1016/j.polymdegradstab.2014.04.025
Reference:
PDST 7330
To appear in:
Polymer Degradation and Stability
Received Date: 9 January 2014 Revised Date:
21 April 2014
Accepted Date: 26 April 2014
Please cite this article as: Hu M, Zhao S, Li C, Wang B, Yao C, Wang Y, The influence of different Tween surfactants on biodesulfurization of ground tire rubber by Sphingomonas sp., Polymer Degradation and Stability (2014), doi: 10.1016/j.polymdegradstab.2014.04.025. 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.
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The influence of different Tween surfactants on biodesulfurization of ground tire rubber by Sphingomonas sp.
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Minghan Hua, Suhe Zhaoa, b*, Chao Lia, Bingwu Wang b, Chu Yaoa, Yaqin Wang b a State Key Laboratory of Organic-Inorganic Composites, Beijing University of
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Chemical Technology, Beijing 100029, China
b Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer
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Materials, Beijing University of Chemical Technology, Beijing 100029, China *Corresponding author. Tel.: +86 (0)10 6444 2621; fax: +86 (0)10 6443 3964. E-mail address: E-mail address: [email protected] (S. Zhao).
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Abstract
In present work, three non-ionic surfactants (Tween 20, Tween 60 and Tween 80) were used to improve the affinity between the lipophilic ground tire rubber (GTR) and
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hydrophilic microbes. The growth characteristic of the Sphingomonas in the
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co-culture process was studied. The effects of different Tween surfactants on the biodesulfurization of GTR were investigated. Tween 20, among these three surfactants, showed best effect on enhancement of biodesulfurization. Results of SEM-EDS showed that the amount of sulfur in rubber surface layer of 0-4 µm was significantly decreased by 67%. XPS analysis data showed that the area of S-S bonds and S-C bonds were decreased while the S-O bonds were obviously increased. The mechanical properties of desulfurized-GTR/styrene butadiene rubber composite were
ACCEPTED MANUSCRIPT improved. Also, the mechanism of the surfactants in improving the affinity between the GTR and Sphingomonas was proposed.
Sphingomonas sp.
Ground tire rubber
Tween surfactants
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Keywords:
Biodesulfurization
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1. Introduction
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With rapid development of world industry, annual demand of rubber products increases gradually. However, spent rubber cannot be degraded through nature because cross-linked rubber products have a stable three-dimensional network structure. Therefore, recycling of rubber material has been an issue of environmental
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concern since 20th century. Many researchers utilize microwave [1], ultrasonic [2, 3] and chemical agents [4, 5] to recycle rubbers. By breaking the cross-linked bonds in
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rubbers, their chains become re-flow and easily processable. The desulfurized-rubber is then re-vulcanized, either as it is, or mixed with virgin material to make rubber
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products with common performance. In recent years, biodesulfurization has become a potentially attractive way to
recycle rubber. This method is an effective and selective process. Microorganisms, exhibiting ability to produce desulfurized enzymes, can selectively break crosslinked sulfur bonds on rubber surface while remain the main chains intact [6]. In 1945, it was firstly reported that sulfur oxidizing microorganisms might be capable of oxidizing sulfur in rubber. Thaysen et al. [7] noted that sulfuric acid was found in water
ACCEPTED MANUSCRIPT remaining in the fire hoses after use. Eventually microbes were identified as the cause of the acid formation. Romine, R. A. and Romine, M. F. [8] used three bacterial strains
(Thiobacillus
ferrooxidans,
Thiobacillus
thiooxidans,
Sulfolobus
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acidocaldarius) to desulfurize dibenzothiophene and studied the mechanism of biodesulfurization. They proposed a “4S” metabolic pathway through which the sulfur crosslinks were metabolized by S.acidocaldarius into sulfoxide/ sulfone/ sulfonate/
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sulfate. Meanwhile, researchers indicated that treated ground tire rubber (GTR) would
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achieve good chemistry reactive if bioprocess was stayed at the first three steps. Thus the modified GTR could add into virgin rubber with high loading amounts and perform with good mechanical properties. Fliermans [9] screened a thermophilic bacterium from a hot spring, which was used to modify GTR surface. After microbial
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treatment, GTR also exhibited excellent surface chemistry reactive. The additives in rubber had some adverse effect on bacterial growth [10]. Ethanol was used to remove these toxic additives. Some fungi were also applied to rubber detoxification [11]. Li et
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al. reported that T. ferrooxidans [12], Thiobacillus [13] and Sphingomonas [14] were
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used respectively to recycle GTR, and Sphingomonas showed the strongest biodesulfurized activity among these three bacteria. However, rubber is lipophilic thus it can’t dissolve in aqueous phase, which
allows microbes to stay on the rubber surface for a short time. In other words, biodesulfurization effect still needs to improve. Therefore, it is important to increase the affinity between rubber and microorganism, raising the chance of microbial contacting with rubber. Surfactant exhibits a unique ability to increase two-phase
ACCEPTED MANUSCRIPT affinity. Thus it has a wide range of applications in biodegradation field. Kim et al. [15] evaluated the effect of several surfactants (Brij-30, Tween 80 and Triton X-100) on the biodegradation of polycyclic aromatic hydrocarbons (PAHs). The results
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showed that the PAHs solubility was linearly proportional to the surfactant concentration when above the critical micelle concentration. Thus the formation of micelle was the key to PAHs degradation. Bautista et al. [16] investigated the effect of
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Enterobacter sp., Pseudomonas sp. and Stenotrophomonas sp. on degradation of
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PAHs. Three non-ion surfactants, i.e. Tween 80, Triton X-100 and Tergitol NP-10, were respectively added into culture medium to compare their influence on biodegradation process. Results showed that Tween 80 had some toxicity for bacterium growth at early times but after the first 24 hours bacterium growth rapidly
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and exhibited good biodegradation effects for PAHs. On the other hand, Triton X-100 and Tergitol NP-10 had severe toxicity for bacterium growth and biodegradation effects for PAHs didn’t exhibit. Yao et al. [6] mixed Tween 80 with waste latex, and
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then the mixture was added into Alicyclobacillus culture medium. This technique
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avoided surfactant toxicity on microbial growth. However, to the extent of our knowledge, there are few reports on how to select
proper surfactants to enhance biodesulfurization effect. The specific mechanism and their relation to surfactants and rubber surface properties still remain unclear. The effect of three non-ionic surfactants (Tween 20, Tween 60 and Tween 80) respectively on the biodesulfurization of ground tire rubber (GTR) by Sphingomonas sp. Strain was studied. The proper adding technique of surfactant was confirmed. The
ACCEPTED MANUSCRIPT desulfurized effect was evaluated through measuring desulfurized depth, swelling value and sulfur content of desulfurized ground tire rubber (DGTR) sheet. The physical properties of desulfurized ground tire rubber/styrene butadiene rubber
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(DGTR/SBR) composite were measured. The mechanism of surfactants effect in co-culture-desulfurization process was discussed based on analysis of lipophilic
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chains length of the Tween molecule.
2. Materials and Methods
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2.1 Materials
The ground tire rubber (GTR) used in this study was supplied by Puyang Rubber Factory (Henan, China). Styrene-Butadiene rubber (SBR 1502) was supplied by Qilu
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Branch of China Petrochemical Co. (Shandong, China). Carbon Black N330 was provided by Dolphin Carbon Black Development Co. (Tianjin, China). Three non-ion surfactants (Tween 20, Tween 60 and Tween 80) were provided by Yili Fine Chemical
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Co. (Tianjin, China). Relevant chemical names and molecular formulas of these
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surfactants are shown in Table 1. Other chemicals were bought locally. All chemicals used for this work were analytical grade.
2.2 Microorganism and culture medium
Sphingomonas sp. was isolated from coal mine soil in Sichuan Province, China. It was cultured in a medium that contained KH2PO4 4.0 g/L, K2HPO4•3H2O 4.0 g/L, MgSO4•7H2O 0.8 g/L, NH4Cl 0.4 g/L, CaCl2 0.01 g/L, Na2S2O3•5H2O 10.0 g/L,
ACCEPTED MANUSCRIPT glucose 2 g/L, peptone 1.0 g/L and yeast extract powder 0.1 g/L. The cultured temperature was 30 ℃ and pH was 6.5.
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2.3 Biodesulfurization Process
Sphingomonas sp. was cultured in 250 ml flask in shaker incubators. Each flask was filled with 100mL medium. The culture medium of Sphingomonas sp. had been
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autoclaved at 115 ℃ . Microbe was incubated into the medium after cooling down to room temperature. The inoculum of Sphingomonas sp. strain was 10 % (v/v). And the
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cultivation temperature was 30 ℃ . stirring speed was 200 rpm.
Before desulfurization, GTR and GTR sheets were immersed in 75% ethanol (v/v) for 24 h in order to kill the microbes attached to it and remove harmful additives.
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Detoxicated GTR was filtered out and dried in a sterile cabinet. And Tween surfactants were mixed with GTR and GTR sheets. After 48 hours incubation, the mixture (surfactants, GTR and GTR sheets) was added into the culture medium. The
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amount of GTR and surfactants were 2.5% and 0.1% (w/v), respectively. And the
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control group did not add surfactants. After desulfurization for 20 days, DGTR samples were filtered out and washed
by distilled water for 1 h, dried at room temperature. GTR represents the ground tire rubber was immersed for 20 days under the same
culture conditions without inoculation. DGTRctr, DGTR20, DGTR60 and DGTR80 represent the ground tire rubber desulfurized by Sphingomonas sp. without Tween, with Tween 20, Tween 60 and Tween 80, respectively.
ACCEPTED MANUSCRIPT 2.4 Preparation of GTR or DGTR Sheets and SBR Composites
The GTR without virgin rubber stock was molded into a rubber sheet at 15 MPa and 150 ℃ after being passed through a two-roll mill with a roller spacing of 0.5
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mm for 20 min. The cured rubber sample was cut into small squares (10*5*1.2 mm, 0.043±0.001g).
Raw SBR was masticated on the two-roll mill, blended with process additives
ZnO 4.0 phr
Stearic 2.0 phr
carbon black N330 30 phr,
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SBR-1502 100 phr
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and 20 phr GTR or various DGTR for 10 min. The basic formulation included
Accelerator D 0.6 phr, Accelerator DM 1.2 phr, sulfur 2.0 phr.
About 35 g of the GTR/SBR or DGTR/SBR compounded rubber stock after 24 h of storage were placed in a mold and pressed by the platens press (Shanghai, Model
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XLB-DQ). The each samples was cured at 150 ℃ , 15 MPa for the optimum cure times (t = t90). The t90 was obtained from an Oscillating Disk Rheometer (Beijing,
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Model P3555B).
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2.5 Characterizations and Measurements
GTR size distribution and morphology observation The size distribution of GTR was obtained by the OMEC LS-Pop3 laser particle size distribution analyzer, China. The morphology of GTR particles was observed by the Hitachi S-4800 SEM, Japan. The samples were vacuum plated with platinum for electrical conduction.
ACCEPTED MANUSCRIPT Microorganism biomass Microbe growth development was monitored during incubation in this study. The method was as followed: taken 5 mL liquid from a culture flask, then diluted it to a
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certain concentration by distilled water, lastly its optical density at 600 nm (OD600) was measured on a Spectrophotometer (Chongqing, China, Model XSZ-H3).OD600 of the sample represented biomass.
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Water contact angle
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The contact angle change of GTR and various DGTR sheets was tested by a contact angle tester (OCA15EC, Dataphysics, Germany). The measurement was applied at 25 ℃ and 50% relative humidity. The volume of tested water droplet was 3 µL. Each experimental value was averaged by 5 different droplets.
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Swelling value
GTR and various DGTR sheets were immersed in 100 mL toluene solution for 72
2008.
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hours at 30 ℃ . Then their swelling values were measured according GB/T 14797.3–
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Crosslink density
The crosslink density of GTR and various DGTR sheets was tested by magnetic resonance crosslink density spectrometer (XLDS-15, IIC, Germany). The testing temperature was 60 ℃ . Gel content GTR and various DGTR sheets were immersed in toluene for 3 days at 30 ℃ for gel determination. Gel was filtered and dried in an vacuum oven at 60 ℃ for 48 hours to
ACCEPTED MANUSCRIPT constant weight (W1). Gel content was determined by the following equation:
Gel content = × 100%
(1)
where W0 is the initial weight of rubber samples and W1 is the dry weight of rubber
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samples. Element contents of sulfur and oxygen on GTR sheets surface
As shown in Fig.1, the GTR or DGTR sheet was cut into two pieces. And the freshly
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surface was plated with gold to measure its sulfur (S) and oxygen (O) content by
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energy dispersive X-ray spectrometer (EDS). Applied SEM-EDS technique, the S and O amount were obtained along a line across the specimen (direction of thickness). Scan points were 0, 2, 4, 600, 1196, 1198, and 1200 µm.
As different rubber sheets may have different S contents. The amount of sulfur was
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normalized towards S content at 600 µm, assuming this S content level to be constant throughout the sample.
XPS analysis of GTR and DGTR sheet
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The sulfur bonding states of GTR and DGTR sheets were characterized by an X-ray
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photoelectron spectroscopy analyzer (Escalab 250, Thermo Electron, USA). Physical mechanical properties of SBR composites The mechanical properties of GTR/SBR and DGTR/SBR composites were tested following ASTM D412 via a tensile testing machine (SANS, CMT 4104, China). Observation of bacterial attachment on GTR sheet surface GTR sheet was firstly taken from liquid medium at 1-day co-culture periods and fixed with 3% glutaraldehyde for 2 hours at 4 ℃ . Secondly, the GTR sheet was put into
ACCEPTED MANUSCRIPT dehydrated in graded ethanol (30%, 50%, 70%, 90%, and absolute ethanol) for 15 min at 4 ℃ . Thirdly, the dehydrated sample was immersed in isoamyl acetate at ambient temperature for exchange ethanol. The treated GTR sheet was subjected to dry out at
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ambient temperature. Lastly, specimen was plated with platinum and observed by the Hitachi S-4800 SEM, Japan.
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3. Results and Discussion
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3.1 Morphology, diameter and its distribution of GTR particles
Fig.2a shows the shape of GTR particles applied in this work. And Fig. 2b shows the diameter size of GTR. It is found that these GTR are of irregularly shaped particles. The particle diameter of GTR is from 50 to 200 µm and its average value is
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117 µm.
3.2 Effect of surfactants on growth of Sphingomonas sp.
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The influence of three surfactants on the growth curves of Sphingomonas sp. in
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culture is shown in Fig.3a. It is found that bacterial lag phase would be prolonged if surfactant was directly mixed with culture medium before inoculation. Compared with biomass of Sphingomonas sp. without Tween, the decrease in biomass of Sphingomonas with Tween is almost 17% at stationary phase. The results indicated that Tween surfactants can inhibit bacterial growth at a small amount. Fig.3b shows growth curves of Sphingomonas sp. with mixture (premixed GTR and Tween surfactant) in co-culture-desulfurization process. It can be found that
ACCEPTED MANUSCRIPT growth rate of Sphingomonas sp. with Tween surfactants is as the same as the control curve (without Tween). The inhibition of Tween on Sphingomonas sp. disappeared. According to the results, the toxicity of Tween can effectively reduce if surfactant was
3.3 Evaluation of biodesulfurization effect
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3.3.1 Water contact angle
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premixed with GTR before adding into culture media.
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The change curves of water contact angle on GTR surface with increasing incubation time are shown in Fig.4. The curve of DGTRctr indicates that water contact angle slightly decreased from 124°to 81°during culture period. That is to say, rubber surface property changed gradually from lipophilic to hydrophilic. It proves
metabolism.
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that hydrophilic property of GTR surface can be effectively improved by microbial
The contact angle of DGTR20, DGTR60 and DGTR80 also decreased with
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increasing incubation time. Compared with the value of DGTRctr, the contact angle
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values of GTR treated with surfactant (DGTR20, DGTR60 and DGTR80) decreased remarkably from 124°to 65°. It shows that Tween 20 exhibits best effect on improvement of surface interaction between microbes and GTR.
3.3.2 Swelling value of GTR and DGTR sheets
Table 2 shows that compared with swelling value of GTR sheet, those of DGTRctr and DGTR20 increase by 3.30% and 7.19%, respectively. These results further indicate that Tween 20 could significantly enhance biodesulfurization effects of GTR sheet.
ACCEPTED MANUSCRIPT Crosslink network on the rubber surface was loosened significantly thus it was easily re-sulfurized.
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3.3.3 Crosslink density of GTR and DGTR sheets
As shown in Fig.5, the crosslink density of various DGTR sheets decrease after desulfurization. The crosslink density of DGTRctr, DGTR20, DGTR60, and DGTR80
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decreases by 3.50%, 7.61%, 5.73%, and 6.29%, respectively. It shows that Tween 20
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shows best effect on biodesulfurization among these three surfactants.
3.3.4 Gel content of GTR and DGTR sheets
The change in gel content of GTR after desulfurization is shown in Fig.6. The gel content of DGTRctr and DGTR20 decreases by 3.48% and 5.44%, respectively.
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However, the gel content of GTR is relatively high after desulfurization (more than 90%). The gel content is high because biodesulfurization only occurred on GTR
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surface [13]; therefore, the inner part of GTR remained unchanged.
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3.3.5 Sulfur and oxygen content of GTR and DGTR sheets surface
Microbes can only metabolize on rubber particle surface; thus, it is difficult to
decrosslink sulfide bonds of rubber interior [17, 18]. In this case, analyzing sulfur content of GTR and DGTR from the surface to interior is necessary. Line-point scan mold was used to analyze sulfur (S) and oxygen (O) element content with SEM-EDS technique. As is shown in Fig.7a, S content of GTR sheet transection (0-1200µm) didn’t
ACCEPTED MANUSCRIPT change almost. S content of DGTR sample transaction from the interior layer to surface (4-0 µm or 1196-1200 µm) markedly decreased, and S content on the surface of DGTR20 sheet was lowest, and decreased by 67% compared with S content on
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center layer. The results indicate that biodesulfurization of Sphingomonas sp. Strain for GTR sheet only reacted on its surface, and desulfurized depth was 2-4 µm. Tween surfactants are good for increasing biodesulfurization effect, especially Tween 20.
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It could be seen from Fig.7b, O content of GTR sheet almost stayed unchanged.
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And O content of desulfurized samples significantly increased on surface position. It indicates that some polar groups with oxygen were formed to improve surface chemistry reactive and hydrophilic. O content on the surface of DGTR20 was highest.
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3.3.6 XPS analysis for GTR and DGTR sheet
Fig.8 shows the change of sulfur bonding states of GTR and DGTR respectively. Three second splits of the S 2p peaks at 162.3,163.7, and 165.7 eV are assigned as the
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bonding energies for the S-C, S-S, and S-O bonds, respectively [19].
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It could be seen from Fig.7 the surface of GTR sheet mainly exists S-S and S-C covalent bonds,. S-O bonds increase and S-S and S-C bonds decreased markedly in the surface of DGTR sheet. The results show that Sphingomonas sp. could oxidize the crosslinked sulfide bonds to sulfur-oxide groups. These results were in agreement with results obtained by SEM-EDS. Compared with the DGTRctr, DGTR20 had the smaller area of S-S and S-C peaks, while the bigger area of S-O peak. It shows that Tween surfactants are good for
ACCEPTED MANUSCRIPT improvement of biodesulfurization effect.
3.3.7 Mechanical properties of SBR Composites
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As is shown in Table 3, tensile strength and elongation of GTR/SBR composite are lower than those of DGTR/SBR composites. Because Sphingomonas sp. could broke sulfide network on GTR surface, leading to improvement of stronger
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permeability of inter-molecular chains and interfacial bonding between the matrix and DGTRctr.
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It was clear that DGTR20/SBR, DGTR60/SBR or DGTR80/SBR composites had better mechanical properties than that of DGTRctr/SBR composites. Compared with the tensile strength and elongation of DGTR/SBR composite, those of DGTR20/SBR
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significantly increase by 11.6% and 23.5%, respectively. These results are further proof that Tween 20 had best effect for improving GTR biodesulfurization.
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3.4 Mechanism of surfactant in enhancing desulfurization effect
Biodesulfurization process consists of three steps. Firstly, microbes grow on
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GTR surface. Secondly, it produced some desulfurized enzymes and lastly these enzymes reacted on GTR surface. During the bioprocess, microorganisms contact with rubber particles in a bioreactor, e.g. shake flask or fermentation reactor. However, microbes cannot stay on GTR particles surface for a long time because of incompatibility between these two matters. Fig.9 shows the effects of different Tween surfactants on bacterial attachment. It is obvious that there was some bacteria adhesive on GTR sheet surface without Tween
ACCEPTED MANUSCRIPT (Fig.9a and Fig.9b), while there was large amount of bacteria growth on GTR sheet surface with Tween, even some analogous colonies formation (Fig.9c – Fig.9h). These phenomena illustrate that these Tween surfactants could effective improve affinity
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between Sphingomonas sp. and GTR. After premixing with surfactant, GTR particle surface was coated by Tween molecule. Oil-soluble tails contacted with the surface of GTR, while the water-soluble
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ends extended to the culture medium and contacted with the microbes. Desulfurized
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enzymes produced by microbes were water soluble, and the enzymes needed to pass a surfactant molecule to react with sulfide bonds in GTR particles. Thus the shorter the length of hydrophobic part was; the easier enzymes were to reach the rubber surface. Molecule of Tween 20, among the three surfactants used, had the shortest length of
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hydrophobic part. It explains the reason why the Tween 20 exhibited best improvement for biodesulfurization effect of GTR. But it needs further research to
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confirm.
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4. Conclusion
Premixing surfactants with GTR could reduce Tween toxicity on microbial
growth during the co-culture desulfurization process. Tween surfactants could improve the affinity between microbes and GTR. During the co-culture process, the amount of sulfur in GTR surface layer of 0-4 µm was significantly decreased. It was found that S-S bonds and S-C bonds were diminished while the S-O bonds were formed on GTR surface. Experiment performed with surfactant exhibited more
ACCEPTED MANUSCRIPT obvious changes. Therefore, Tween surfactant is a good candidate to improve the biodesulfurization of GTR by Sphingomonas sp.. Tween 20 exhibits best enhancement owing to its shortest oil-soluble chain among three surfactants.
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Acknowledgments
The authors are thankful to the financial support from the Natural Science Foundation
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of Beijing (No. 8122032).
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ACCEPTED MANUSCRIPT Figure Captions
Fig.1 Schematic diagram over how the GTR sheet was cut and analyzed with
Fig.2 Morphology and particle size distribution of GTR
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SEM-EDS
(a)- SEM picture of GTR particles; (b) - Particle diameter of GTR.
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Fig.3 Growth curves of the Sphingomonas sp. with different adding technique
(a) - Surfactants was added into culture medium before inoculation; (b) - Surfactants
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premixed with GTR, and then GTR/Tween mixture was added into culture medium when microbial growth reached the stationary phase.
Fig.4 Change of water contact angle on various DGTR sheets during incubation processing
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Fig.5 Change in crosslink density of GTR sheets after desulfurization Fig.6 Change in gel content of GTR sheets after desulfurization
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Fig.7 Sulfur and oxygen element content of GTR sheets along a detected line (a) - Relative sulfur concentration; (b) - Oxygen concentration.
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Fig.8 XPS S2p analysis of GTR and DGTR samples (a) -The origin XPS spectra of S element; (b) - Sulfide bonds on the surface of GTR and DGTR respectively. Fig.9 Bacterial attachment on GTR sheets immersed in different culture media Culture media: (a),(b) -without Tween; (c),(d) -premixed with Tween 20; (e),(f) -premixed with Tween 60; (g),(h) -premixed with Tween 80.
ACCEPTED MANUSCRIPT Table 1 Molecular formula C58H114O26 C64H128O26 C64H124O26
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Chemical names and molecular formulas of Tween surfactants Surfactants Chemical name Tween 20 Polyoxyethylene (20) sorbitan monolaurate Tween 60 Polyoxyethylene (20) sorbitan monostearate Tween 80 Polyoxyethylene (20) sorbitan monooleate
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Table 2 Swelling values of GTR sheet treated by different Tween surfactants Samples GTR DGTRctr DGTR20 DGTR60 Swelling 2.46 2.54 2.64 2.60 Value
DGTR80 2.61
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Table 3 Mechanical properties of SBR vulcanizates filled with GTR or various DGTR Modulus at Modulus at Tensile Elongation at 100% 300% Samples strength /MPa break /% elongation elongation /MPa /MPa GTR/SBR 16.4 370 2.3 12.1 DGTRctr/SBR 17.1 428 2.1 11.7 18.3 457 2.0 10.3 DGTR20/SBR DGTR60/SBR 17.7 430 2.1 10.9 DGTR80/SBR 17.6 439 2.1 11.0
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S0141-3910(14)00180-3
DOI:
10.1016/j.polymdegradstab.2014.04.025
Reference:
PDST 7330
To appear in:
Polymer Degradation and Stability
Received Date: 9 January 2014 Revised Date:
21 April 2014
Accepted Date: 26 April 2014
Please cite this article as: Hu M, Zhao S, Li C, Wang B, Yao C, Wang Y, The influence of different Tween surfactants on biodesulfurization of ground tire rubber by Sphingomonas sp., Polymer Degradation and Stability (2014), doi: 10.1016/j.polymdegradstab.2014.04.025. 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.
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The influence of different Tween surfactants on biodesulfurization of ground tire rubber by Sphingomonas sp.
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Minghan Hua, Suhe Zhaoa, b*, Chao Lia, Bingwu Wang b, Chu Yaoa, Yaqin Wang b a State Key Laboratory of Organic-Inorganic Composites, Beijing University of
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Chemical Technology, Beijing 100029, China
b Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer
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Materials, Beijing University of Chemical Technology, Beijing 100029, China *Corresponding author. Tel.: +86 (0)10 6444 2621; fax: +86 (0)10 6443 3964. E-mail address: E-mail address: [email protected] (S. Zhao).
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Abstract
In present work, three non-ionic surfactants (Tween 20, Tween 60 and Tween 80) were used to improve the affinity between the lipophilic ground tire rubber (GTR) and
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hydrophilic microbes. The growth characteristic of the Sphingomonas in the
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co-culture process was studied. The effects of different Tween surfactants on the biodesulfurization of GTR were investigated. Tween 20, among these three surfactants, showed best effect on enhancement of biodesulfurization. Results of SEM-EDS showed that the amount of sulfur in rubber surface layer of 0-4 µm was significantly decreased by 67%. XPS analysis data showed that the area of S-S bonds and S-C bonds were decreased while the S-O bonds were obviously increased. The mechanical properties of desulfurized-GTR/styrene butadiene rubber composite were
ACCEPTED MANUSCRIPT improved. Also, the mechanism of the surfactants in improving the affinity between the GTR and Sphingomonas was proposed.
Sphingomonas sp.
Ground tire rubber
Tween surfactants
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Keywords:
Biodesulfurization
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1. Introduction
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With rapid development of world industry, annual demand of rubber products increases gradually. However, spent rubber cannot be degraded through nature because cross-linked rubber products have a stable three-dimensional network structure. Therefore, recycling of rubber material has been an issue of environmental
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concern since 20th century. Many researchers utilize microwave [1], ultrasonic [2, 3] and chemical agents [4, 5] to recycle rubbers. By breaking the cross-linked bonds in
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rubbers, their chains become re-flow and easily processable. The desulfurized-rubber is then re-vulcanized, either as it is, or mixed with virgin material to make rubber
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products with common performance. In recent years, biodesulfurization has become a potentially attractive way to
recycle rubber. This method is an effective and selective process. Microorganisms, exhibiting ability to produce desulfurized enzymes, can selectively break crosslinked sulfur bonds on rubber surface while remain the main chains intact [6]. In 1945, it was firstly reported that sulfur oxidizing microorganisms might be capable of oxidizing sulfur in rubber. Thaysen et al. [7] noted that sulfuric acid was found in water
ACCEPTED MANUSCRIPT remaining in the fire hoses after use. Eventually microbes were identified as the cause of the acid formation. Romine, R. A. and Romine, M. F. [8] used three bacterial strains
(Thiobacillus
ferrooxidans,
Thiobacillus
thiooxidans,
Sulfolobus
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acidocaldarius) to desulfurize dibenzothiophene and studied the mechanism of biodesulfurization. They proposed a “4S” metabolic pathway through which the sulfur crosslinks were metabolized by S.acidocaldarius into sulfoxide/ sulfone/ sulfonate/
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sulfate. Meanwhile, researchers indicated that treated ground tire rubber (GTR) would
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achieve good chemistry reactive if bioprocess was stayed at the first three steps. Thus the modified GTR could add into virgin rubber with high loading amounts and perform with good mechanical properties. Fliermans [9] screened a thermophilic bacterium from a hot spring, which was used to modify GTR surface. After microbial
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treatment, GTR also exhibited excellent surface chemistry reactive. The additives in rubber had some adverse effect on bacterial growth [10]. Ethanol was used to remove these toxic additives. Some fungi were also applied to rubber detoxification [11]. Li et
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al. reported that T. ferrooxidans [12], Thiobacillus [13] and Sphingomonas [14] were
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used respectively to recycle GTR, and Sphingomonas showed the strongest biodesulfurized activity among these three bacteria. However, rubber is lipophilic thus it can’t dissolve in aqueous phase, which
allows microbes to stay on the rubber surface for a short time. In other words, biodesulfurization effect still needs to improve. Therefore, it is important to increase the affinity between rubber and microorganism, raising the chance of microbial contacting with rubber. Surfactant exhibits a unique ability to increase two-phase
ACCEPTED MANUSCRIPT affinity. Thus it has a wide range of applications in biodegradation field. Kim et al. [15] evaluated the effect of several surfactants (Brij-30, Tween 80 and Triton X-100) on the biodegradation of polycyclic aromatic hydrocarbons (PAHs). The results
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showed that the PAHs solubility was linearly proportional to the surfactant concentration when above the critical micelle concentration. Thus the formation of micelle was the key to PAHs degradation. Bautista et al. [16] investigated the effect of
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Enterobacter sp., Pseudomonas sp. and Stenotrophomonas sp. on degradation of
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PAHs. Three non-ion surfactants, i.e. Tween 80, Triton X-100 and Tergitol NP-10, were respectively added into culture medium to compare their influence on biodegradation process. Results showed that Tween 80 had some toxicity for bacterium growth at early times but after the first 24 hours bacterium growth rapidly
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and exhibited good biodegradation effects for PAHs. On the other hand, Triton X-100 and Tergitol NP-10 had severe toxicity for bacterium growth and biodegradation effects for PAHs didn’t exhibit. Yao et al. [6] mixed Tween 80 with waste latex, and
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then the mixture was added into Alicyclobacillus culture medium. This technique
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avoided surfactant toxicity on microbial growth. However, to the extent of our knowledge, there are few reports on how to select
proper surfactants to enhance biodesulfurization effect. The specific mechanism and their relation to surfactants and rubber surface properties still remain unclear. The effect of three non-ionic surfactants (Tween 20, Tween 60 and Tween 80) respectively on the biodesulfurization of ground tire rubber (GTR) by Sphingomonas sp. Strain was studied. The proper adding technique of surfactant was confirmed. The
ACCEPTED MANUSCRIPT desulfurized effect was evaluated through measuring desulfurized depth, swelling value and sulfur content of desulfurized ground tire rubber (DGTR) sheet. The physical properties of desulfurized ground tire rubber/styrene butadiene rubber
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(DGTR/SBR) composite were measured. The mechanism of surfactants effect in co-culture-desulfurization process was discussed based on analysis of lipophilic
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chains length of the Tween molecule.
2. Materials and Methods
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2.1 Materials
The ground tire rubber (GTR) used in this study was supplied by Puyang Rubber Factory (Henan, China). Styrene-Butadiene rubber (SBR 1502) was supplied by Qilu
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Branch of China Petrochemical Co. (Shandong, China). Carbon Black N330 was provided by Dolphin Carbon Black Development Co. (Tianjin, China). Three non-ion surfactants (Tween 20, Tween 60 and Tween 80) were provided by Yili Fine Chemical
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Co. (Tianjin, China). Relevant chemical names and molecular formulas of these
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surfactants are shown in Table 1. Other chemicals were bought locally. All chemicals used for this work were analytical grade.
2.2 Microorganism and culture medium
Sphingomonas sp. was isolated from coal mine soil in Sichuan Province, China. It was cultured in a medium that contained KH2PO4 4.0 g/L, K2HPO4•3H2O 4.0 g/L, MgSO4•7H2O 0.8 g/L, NH4Cl 0.4 g/L, CaCl2 0.01 g/L, Na2S2O3•5H2O 10.0 g/L,
ACCEPTED MANUSCRIPT glucose 2 g/L, peptone 1.0 g/L and yeast extract powder 0.1 g/L. The cultured temperature was 30 ℃ and pH was 6.5.
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2.3 Biodesulfurization Process
Sphingomonas sp. was cultured in 250 ml flask in shaker incubators. Each flask was filled with 100mL medium. The culture medium of Sphingomonas sp. had been
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autoclaved at 115 ℃ . Microbe was incubated into the medium after cooling down to room temperature. The inoculum of Sphingomonas sp. strain was 10 % (v/v). And the
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cultivation temperature was 30 ℃ . stirring speed was 200 rpm.
Before desulfurization, GTR and GTR sheets were immersed in 75% ethanol (v/v) for 24 h in order to kill the microbes attached to it and remove harmful additives.
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Detoxicated GTR was filtered out and dried in a sterile cabinet. And Tween surfactants were mixed with GTR and GTR sheets. After 48 hours incubation, the mixture (surfactants, GTR and GTR sheets) was added into the culture medium. The
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amount of GTR and surfactants were 2.5% and 0.1% (w/v), respectively. And the
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control group did not add surfactants. After desulfurization for 20 days, DGTR samples were filtered out and washed
by distilled water for 1 h, dried at room temperature. GTR represents the ground tire rubber was immersed for 20 days under the same
culture conditions without inoculation. DGTRctr, DGTR20, DGTR60 and DGTR80 represent the ground tire rubber desulfurized by Sphingomonas sp. without Tween, with Tween 20, Tween 60 and Tween 80, respectively.
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The GTR without virgin rubber stock was molded into a rubber sheet at 15 MPa and 150 ℃ after being passed through a two-roll mill with a roller spacing of 0.5
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mm for 20 min. The cured rubber sample was cut into small squares (10*5*1.2 mm, 0.043±0.001g).
Raw SBR was masticated on the two-roll mill, blended with process additives
ZnO 4.0 phr
Stearic 2.0 phr
carbon black N330 30 phr,
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SBR-1502 100 phr
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and 20 phr GTR or various DGTR for 10 min. The basic formulation included
Accelerator D 0.6 phr, Accelerator DM 1.2 phr, sulfur 2.0 phr.
About 35 g of the GTR/SBR or DGTR/SBR compounded rubber stock after 24 h of storage were placed in a mold and pressed by the platens press (Shanghai, Model
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XLB-DQ). The each samples was cured at 150 ℃ , 15 MPa for the optimum cure times (t = t90). The t90 was obtained from an Oscillating Disk Rheometer (Beijing,
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Model P3555B).
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2.5 Characterizations and Measurements
GTR size distribution and morphology observation The size distribution of GTR was obtained by the OMEC LS-Pop3 laser particle size distribution analyzer, China. The morphology of GTR particles was observed by the Hitachi S-4800 SEM, Japan. The samples were vacuum plated with platinum for electrical conduction.
ACCEPTED MANUSCRIPT Microorganism biomass Microbe growth development was monitored during incubation in this study. The method was as followed: taken 5 mL liquid from a culture flask, then diluted it to a
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certain concentration by distilled water, lastly its optical density at 600 nm (OD600) was measured on a Spectrophotometer (Chongqing, China, Model XSZ-H3).OD600 of the sample represented biomass.
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Water contact angle
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The contact angle change of GTR and various DGTR sheets was tested by a contact angle tester (OCA15EC, Dataphysics, Germany). The measurement was applied at 25 ℃ and 50% relative humidity. The volume of tested water droplet was 3 µL. Each experimental value was averaged by 5 different droplets.
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Swelling value
GTR and various DGTR sheets were immersed in 100 mL toluene solution for 72
2008.
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hours at 30 ℃ . Then their swelling values were measured according GB/T 14797.3–
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Crosslink density
The crosslink density of GTR and various DGTR sheets was tested by magnetic resonance crosslink density spectrometer (XLDS-15, IIC, Germany). The testing temperature was 60 ℃ . Gel content GTR and various DGTR sheets were immersed in toluene for 3 days at 30 ℃ for gel determination. Gel was filtered and dried in an vacuum oven at 60 ℃ for 48 hours to
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Gel content = × 100%
(1)
where W0 is the initial weight of rubber samples and W1 is the dry weight of rubber
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samples. Element contents of sulfur and oxygen on GTR sheets surface
As shown in Fig.1, the GTR or DGTR sheet was cut into two pieces. And the freshly
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surface was plated with gold to measure its sulfur (S) and oxygen (O) content by
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energy dispersive X-ray spectrometer (EDS). Applied SEM-EDS technique, the S and O amount were obtained along a line across the specimen (direction of thickness). Scan points were 0, 2, 4, 600, 1196, 1198, and 1200 µm.
As different rubber sheets may have different S contents. The amount of sulfur was
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normalized towards S content at 600 µm, assuming this S content level to be constant throughout the sample.
XPS analysis of GTR and DGTR sheet
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The sulfur bonding states of GTR and DGTR sheets were characterized by an X-ray
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photoelectron spectroscopy analyzer (Escalab 250, Thermo Electron, USA). Physical mechanical properties of SBR composites The mechanical properties of GTR/SBR and DGTR/SBR composites were tested following ASTM D412 via a tensile testing machine (SANS, CMT 4104, China). Observation of bacterial attachment on GTR sheet surface GTR sheet was firstly taken from liquid medium at 1-day co-culture periods and fixed with 3% glutaraldehyde for 2 hours at 4 ℃ . Secondly, the GTR sheet was put into
ACCEPTED MANUSCRIPT dehydrated in graded ethanol (30%, 50%, 70%, 90%, and absolute ethanol) for 15 min at 4 ℃ . Thirdly, the dehydrated sample was immersed in isoamyl acetate at ambient temperature for exchange ethanol. The treated GTR sheet was subjected to dry out at
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ambient temperature. Lastly, specimen was plated with platinum and observed by the Hitachi S-4800 SEM, Japan.
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3. Results and Discussion
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3.1 Morphology, diameter and its distribution of GTR particles
Fig.2a shows the shape of GTR particles applied in this work. And Fig. 2b shows the diameter size of GTR. It is found that these GTR are of irregularly shaped particles. The particle diameter of GTR is from 50 to 200 µm and its average value is
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117 µm.
3.2 Effect of surfactants on growth of Sphingomonas sp.
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The influence of three surfactants on the growth curves of Sphingomonas sp. in
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culture is shown in Fig.3a. It is found that bacterial lag phase would be prolonged if surfactant was directly mixed with culture medium before inoculation. Compared with biomass of Sphingomonas sp. without Tween, the decrease in biomass of Sphingomonas with Tween is almost 17% at stationary phase. The results indicated that Tween surfactants can inhibit bacterial growth at a small amount. Fig.3b shows growth curves of Sphingomonas sp. with mixture (premixed GTR and Tween surfactant) in co-culture-desulfurization process. It can be found that
ACCEPTED MANUSCRIPT growth rate of Sphingomonas sp. with Tween surfactants is as the same as the control curve (without Tween). The inhibition of Tween on Sphingomonas sp. disappeared. According to the results, the toxicity of Tween can effectively reduce if surfactant was
3.3 Evaluation of biodesulfurization effect
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3.3.1 Water contact angle
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premixed with GTR before adding into culture media.
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The change curves of water contact angle on GTR surface with increasing incubation time are shown in Fig.4. The curve of DGTRctr indicates that water contact angle slightly decreased from 124°to 81°during culture period. That is to say, rubber surface property changed gradually from lipophilic to hydrophilic. It proves
metabolism.
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that hydrophilic property of GTR surface can be effectively improved by microbial
The contact angle of DGTR20, DGTR60 and DGTR80 also decreased with
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increasing incubation time. Compared with the value of DGTRctr, the contact angle
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values of GTR treated with surfactant (DGTR20, DGTR60 and DGTR80) decreased remarkably from 124°to 65°. It shows that Tween 20 exhibits best effect on improvement of surface interaction between microbes and GTR.
3.3.2 Swelling value of GTR and DGTR sheets
Table 2 shows that compared with swelling value of GTR sheet, those of DGTRctr and DGTR20 increase by 3.30% and 7.19%, respectively. These results further indicate that Tween 20 could significantly enhance biodesulfurization effects of GTR sheet.
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3.3.3 Crosslink density of GTR and DGTR sheets
As shown in Fig.5, the crosslink density of various DGTR sheets decrease after desulfurization. The crosslink density of DGTRctr, DGTR20, DGTR60, and DGTR80
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decreases by 3.50%, 7.61%, 5.73%, and 6.29%, respectively. It shows that Tween 20
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shows best effect on biodesulfurization among these three surfactants.
3.3.4 Gel content of GTR and DGTR sheets
The change in gel content of GTR after desulfurization is shown in Fig.6. The gel content of DGTRctr and DGTR20 decreases by 3.48% and 5.44%, respectively.
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However, the gel content of GTR is relatively high after desulfurization (more than 90%). The gel content is high because biodesulfurization only occurred on GTR
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surface [13]; therefore, the inner part of GTR remained unchanged.
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3.3.5 Sulfur and oxygen content of GTR and DGTR sheets surface
Microbes can only metabolize on rubber particle surface; thus, it is difficult to
decrosslink sulfide bonds of rubber interior [17, 18]. In this case, analyzing sulfur content of GTR and DGTR from the surface to interior is necessary. Line-point scan mold was used to analyze sulfur (S) and oxygen (O) element content with SEM-EDS technique. As is shown in Fig.7a, S content of GTR sheet transection (0-1200µm) didn’t
ACCEPTED MANUSCRIPT change almost. S content of DGTR sample transaction from the interior layer to surface (4-0 µm or 1196-1200 µm) markedly decreased, and S content on the surface of DGTR20 sheet was lowest, and decreased by 67% compared with S content on
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center layer. The results indicate that biodesulfurization of Sphingomonas sp. Strain for GTR sheet only reacted on its surface, and desulfurized depth was 2-4 µm. Tween surfactants are good for increasing biodesulfurization effect, especially Tween 20.
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It could be seen from Fig.7b, O content of GTR sheet almost stayed unchanged.
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And O content of desulfurized samples significantly increased on surface position. It indicates that some polar groups with oxygen were formed to improve surface chemistry reactive and hydrophilic. O content on the surface of DGTR20 was highest.
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3.3.6 XPS analysis for GTR and DGTR sheet
Fig.8 shows the change of sulfur bonding states of GTR and DGTR respectively. Three second splits of the S 2p peaks at 162.3,163.7, and 165.7 eV are assigned as the
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bonding energies for the S-C, S-S, and S-O bonds, respectively [19].
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It could be seen from Fig.7 the surface of GTR sheet mainly exists S-S and S-C covalent bonds,. S-O bonds increase and S-S and S-C bonds decreased markedly in the surface of DGTR sheet. The results show that Sphingomonas sp. could oxidize the crosslinked sulfide bonds to sulfur-oxide groups. These results were in agreement with results obtained by SEM-EDS. Compared with the DGTRctr, DGTR20 had the smaller area of S-S and S-C peaks, while the bigger area of S-O peak. It shows that Tween surfactants are good for
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3.3.7 Mechanical properties of SBR Composites
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As is shown in Table 3, tensile strength and elongation of GTR/SBR composite are lower than those of DGTR/SBR composites. Because Sphingomonas sp. could broke sulfide network on GTR surface, leading to improvement of stronger
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permeability of inter-molecular chains and interfacial bonding between the matrix and DGTRctr.
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It was clear that DGTR20/SBR, DGTR60/SBR or DGTR80/SBR composites had better mechanical properties than that of DGTRctr/SBR composites. Compared with the tensile strength and elongation of DGTR/SBR composite, those of DGTR20/SBR
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significantly increase by 11.6% and 23.5%, respectively. These results are further proof that Tween 20 had best effect for improving GTR biodesulfurization.
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3.4 Mechanism of surfactant in enhancing desulfurization effect
Biodesulfurization process consists of three steps. Firstly, microbes grow on
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GTR surface. Secondly, it produced some desulfurized enzymes and lastly these enzymes reacted on GTR surface. During the bioprocess, microorganisms contact with rubber particles in a bioreactor, e.g. shake flask or fermentation reactor. However, microbes cannot stay on GTR particles surface for a long time because of incompatibility between these two matters. Fig.9 shows the effects of different Tween surfactants on bacterial attachment. It is obvious that there was some bacteria adhesive on GTR sheet surface without Tween
ACCEPTED MANUSCRIPT (Fig.9a and Fig.9b), while there was large amount of bacteria growth on GTR sheet surface with Tween, even some analogous colonies formation (Fig.9c – Fig.9h). These phenomena illustrate that these Tween surfactants could effective improve affinity
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between Sphingomonas sp. and GTR. After premixing with surfactant, GTR particle surface was coated by Tween molecule. Oil-soluble tails contacted with the surface of GTR, while the water-soluble
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ends extended to the culture medium and contacted with the microbes. Desulfurized
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enzymes produced by microbes were water soluble, and the enzymes needed to pass a surfactant molecule to react with sulfide bonds in GTR particles. Thus the shorter the length of hydrophobic part was; the easier enzymes were to reach the rubber surface. Molecule of Tween 20, among the three surfactants used, had the shortest length of
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hydrophobic part. It explains the reason why the Tween 20 exhibited best improvement for biodesulfurization effect of GTR. But it needs further research to
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confirm.
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4. Conclusion
Premixing surfactants with GTR could reduce Tween toxicity on microbial
growth during the co-culture desulfurization process. Tween surfactants could improve the affinity between microbes and GTR. During the co-culture process, the amount of sulfur in GTR surface layer of 0-4 µm was significantly decreased. It was found that S-S bonds and S-C bonds were diminished while the S-O bonds were formed on GTR surface. Experiment performed with surfactant exhibited more
ACCEPTED MANUSCRIPT obvious changes. Therefore, Tween surfactant is a good candidate to improve the biodesulfurization of GTR by Sphingomonas sp.. Tween 20 exhibits best enhancement owing to its shortest oil-soluble chain among three surfactants.
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Acknowledgments
The authors are thankful to the financial support from the Natural Science Foundation
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of Beijing (No. 8122032).
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[13] Li Y, Zhao S, Wang Y. Improvement of the properties of natural rubber/ground
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[14] Li Y, Zhao S, Wang Y. Microbial desulfurization of ground tire rubber by Sphingomonas sp.: a novel technology for crumb rubber composites. J Polym Environ 2012; 20(2): 372-380. [15] Kim I.S. et al. Enhanced biodegradation of polycyclic aromatic hydrocarbons using nonionic surfactants in soil slurry. Appl Geochem 2001; 16(11): 1419–
ACCEPTED MANUSCRIPT 1428. [16] Fernando Bautista L. et al. Effect of different non-ionic surfactants on the biodegradation of PAHs by diverse aerobic bacteria. Int Biodeter Biodegr 2009;
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63(7): 913–922. [17] Löffler M. et al. Microbial surface desulfurization of scrap rubber crumb-a
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[18] Christiansson M. et al. Reduction of surface sulphur upon microbial devulcanization of rubber materials, Biotechnol Lett 1998; 20(7): 637-642. [19] Moulder JF, Stickle WF, Sobol PE, Bomben KD. Handbook of X-ray photoelectron spectroscopy. Eden Prairie, MO: Perkin Elmer Corporation
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ACCEPTED MANUSCRIPT Figure Captions
Fig.1 Schematic diagram over how the GTR sheet was cut and analyzed with
Fig.2 Morphology and particle size distribution of GTR
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SEM-EDS
(a)- SEM picture of GTR particles; (b) - Particle diameter of GTR.
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Fig.3 Growth curves of the Sphingomonas sp. with different adding technique
(a) - Surfactants was added into culture medium before inoculation; (b) - Surfactants
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premixed with GTR, and then GTR/Tween mixture was added into culture medium when microbial growth reached the stationary phase.
Fig.4 Change of water contact angle on various DGTR sheets during incubation processing
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Fig.5 Change in crosslink density of GTR sheets after desulfurization Fig.6 Change in gel content of GTR sheets after desulfurization
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Fig.7 Sulfur and oxygen element content of GTR sheets along a detected line (a) - Relative sulfur concentration; (b) - Oxygen concentration.
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Fig.8 XPS S2p analysis of GTR and DGTR samples (a) -The origin XPS spectra of S element; (b) - Sulfide bonds on the surface of GTR and DGTR respectively. Fig.9 Bacterial attachment on GTR sheets immersed in different culture media Culture media: (a),(b) -without Tween; (c),(d) -premixed with Tween 20; (e),(f) -premixed with Tween 60; (g),(h) -premixed with Tween 80.
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Chemical names and molecular formulas of Tween surfactants Surfactants Chemical name Tween 20 Polyoxyethylene (20) sorbitan monolaurate Tween 60 Polyoxyethylene (20) sorbitan monostearate Tween 80 Polyoxyethylene (20) sorbitan monooleate
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Table 2 Swelling values of GTR sheet treated by different Tween surfactants Samples GTR DGTRctr DGTR20 DGTR60 Swelling 2.46 2.54 2.64 2.60 Value
DGTR80 2.61
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Table 3 Mechanical properties of SBR vulcanizates filled with GTR or various DGTR Modulus at Modulus at Tensile Elongation at 100% 300% Samples strength /MPa break /% elongation elongation /MPa /MPa GTR/SBR 16.4 370 2.3 12.1 DGTRctr/SBR 17.1 428 2.1 11.7 18.3 457 2.0 10.3 DGTR20/SBR DGTR60/SBR 17.7 430 2.1 10.9 DGTR80/SBR 17.6 439 2.1 11.0
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