Effects of monorhamnolipid and dirhamnolipid on sorption and desorption of triclosan in sediment–water system

Chemosphere 90 (2013) 581–587

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Effects of monorhamnolipid and dirhamnolipid on sorption and desorption of triclosan in sediment–water system Xiaoyu Zhang a,1, Qian Guo a,2, Yongyou Hu a,b,⇑, Hui Lin a a College of Environmental Science and Engineering, South China University of Technology, The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters, Ministry of Education, Guangzhou 510006, China b State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

" The solubilization enhancement of

trilosan (TCS) by rhamnolipids was investigated. " The sorption of rhamnolipids by the sediments was adequately described by Langmuir-type isotherm. " The sorption and desorption of TCS mainly depend on the sediment composition and rhamnolipid structure. " Dirhamnolipid was more effective than monorhamnolipid for TCS desorption from sediments.

a r t i c l e

i n f o

Article history: Received 14 April 2012 Received in revised form 13 August 2012 Accepted 20 August 2012 Available online 6 October 2012 Keywords: Rhamnolipid Triclosan Sorption Desorption Sediment

a b s t r a c t The effects of monorhamnolipid (RL-F1) and dirhamnolipid (RL-F2) on the sorption and desorption of triclosan (TCS) in sediment–water system were investigated in this study. Results of the bath equilibrium experiments showed that RL-F2 provided much higher solubilization enhancement for TCS than RL-F1. Sorption of both rhamnolipids by the sediments was highly correlated with the sediment clay content.   Moreover, the apparent distribution coefficients of TCS K d decreased with the increase of rhamnolipid concentration (0.05–7.5 mM), and RL-F2 presented a larger distribution capacity of TCS into the aqueous phase at relatively higher concentrations (>2.5 mM). Further results also indicated that the release of TCS from sediment could be enhanced by both rhamnolipids. RL-F2 was more efficient than RL-F1 in desorb  ing TCS from the sediment with low clay content. The TCS desorption percentages Rd of RL-F2 (5 mM) was 1.8–2.4 times that of RL-F1. These findings could provide useful guidelines for the application of rhamnolipid-enhanced remediation technologies for TCS contaminated sediment. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Triclosan [TCS, 5-chloro-2-(2,4-dichlorophenoxy)-phenol], one of the most common antimicrobial agents, is widely used in house-

hold and personal care products. Owing to its relatively low aqueous solubility and hydrophobic property with a log Kow of 4.7 (Ying et al., 2007), TCS has a tendency to accumulate in sludge (Heidler et al., 2006; Ying and Kookana, 2007) and sediment (Zhao et al.,

⇑ Corresponding author. Address: School of Environmental Science and Engineering, South China University of Technology, Guangzhou 510006, China. Tel.: +86 020 39380506; fax: +86 020 39380508. E-mail addresses: [email protected] (X. Zhang), [email protected] (Q. Guo), [email protected] (Y. Hu). 1 Tel.: +86 13631379879. 2 Tel.: +86 15013030031. 0045-6535/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2012.08.036

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2010) where it can persist with concentrations at mg kg1 levels. Recently, TCS has been globally detected in surface water (Lindstrom et al., 2002; Donna et al., 2004; Hua et al., 2005; Wu et al., 2007). Studies on the occurrence and fate of TCS in aquatic environment indicate that TCS is potentially toxic to various organisms (Orvos et al., 2002; Chalew and Halden, 2009) and was found to be accumulated in human milk and blood (Adolfsson-Erici et al., 2002; Allmyr et al., 2006). Laboratory studies also showed that TCS can be degraded by microbial processes in soil under aerobic conditions, but it was highly resistant to biodegradation in soil anaerobic conditions (Ying et al., 2007). Therefore, it is extremely necessary to investigate the distribution of TCS in solid–water system for the elimination of TCS. Due to the high sorption affinity to the solid phase, TCS is very difficult to remove by conventional pump-and-treat technologies (Kommalapati et al., 1997). It is well known that surfactant can increase the solubility of hydrophobic organic compounds (HOCs) or lower the interfacial tension to enhance the mobility of HOCs. Surfactant-enhanced remediation (SER) is suggested as a promising technology that has recently been extensively exploited for the remediation of organic contaminated soil and water (groundwater or surface water) (West and Harwell, 1992; Mulligan et al., 2001; Paria, 2008). The interactions of surfactants with organic contaminants in soil are very complex and are influenced by various factors including CMC, sorption behavior of the surfactant and contaminant, solubility of the contaminant and the soil composition (Haigh, 1996). As a result of lower cellular toxicity and ready biodegradability, biosurfactants have recently gained more attention and are extensively used in many environmental applications including remediation of PAHs, pesticides, heavy metals and so on (Mulligan, 2005). It has been suggested to be more attractive from an environmental perspective for SER processes (Clifford et al., 2007). However, compared with many investigations of the distribution of PAHs or pesticides in soil–water–surfactant systems (Edwards et al., 1994; Zhou and Zhu, 2005, 2007; Wang and Keller, 2008, 2009; Zhu and Zhou, 2008), much less information is known about that of TCS within a soil–water system containing surfactant or biosurfactant (Lin et al., 2011). As one of the typical biosurfactant, rhamnolipid has been well defined in pollutant remediation, and as many as 28 different homologues of rhamnolipid have been reported (Nitschke et al., 2005). Rhamnolipid dosage is an important parameter in mobilization of HOCs in contaminated soil. It has been reported that pesticide desorption from soil was only enhanced when rhamnolipid concentrations were high enough to reach soil saturation and form micelles in aqueous phase (Mata-Sandoval et al., 2002). In another study (Chen et al., 2005), PAHs had linear sorption isotherms on silica sand with partition coefficients decreasing with the increase of rhamnolipid concentration until the CMC, while the partition coefficients increased with increasing rhamnolipid concentration above the CMC. Moreover, the different structures of biosurfactants may have different surface and micelle properties (Zhang et al., 1997). While most current research has focused on the applications of rhamnolipid mixtures, little knowledge is available on the effects of different forms of rhamnolipid for remediation. Thus, more information about the effect of rhamnolipid structure and concentration on solubilization and distribution of HOCs in soil– water system are certainly needed for the development of effective biosurfactant-enhanced remediation technologies. The present study was based on previous investigations of TCS sorption onto sediments and its distribution in sediment–water system by a rhamnolipid mixture (Lin et al., 2011). It reported that the rhamnolipid mixture was more efficient in distributing TCS into aqueous phase in sediments with large volume of pores and low organic matter. The rhamnolipid mixture could be further purified into two fractions, monorhamnolipid (RL-F1) and dirhamnolipid

(RL-F2). The purpose of this study was to further quantify the effect of both RL-F1 and RL-F2 on TCS sorption and desorption in sediment–water systems. A series of batch experiments was performed to determine TCS solubilization in aqueous solutions by both rhamnolipids. The portioning of TCS within sediment– water–rhamnolipid system is also discussed. The results of this study could provide a better understanding on the distribution behavior of TCS in sediment–water–rhamnolipid system, and offer valuable information for the application of rhamnolipid in the remediation of organic contaminated sediment. 2. Materials and methods 2.1. Materials Triclosan, with a purity of 99%, was purchased from Alfa Aesar (Ward Hill, MA). Sodium azide was obtained from Solarbio. All analytical solvents including methanol and acetonitrile (HPLC grade) were obtained from Merck (Hannover, Germany). RL-F1 and RL-F2 were produced and purified from Pseudomonas aeruginosa mutant strain MIG-N146 as described previously (Guo et al., 2009). The average molecular weight of RL-F1 is 509, and the CMC is 0.09 mM. RL-F2 has an average molecular weight of 656 and CMC of 0.18 mM. 2.2. Sediment preparation Three sediments were collected from different sites of Pearl River Delta in southern China. Sediment samples were air-dried and sieved to obtain particles less than 1.0 mm. Organic matter in sediment was determined by combustion method of loss on ignition at 450 °C for 4 h. Particle size analysis was determined by hydrometer method. The physic-chemical properties of sediment samples are shown in Table 1 (Lin et al., 2011). 2.3. Rhamnolipid solubilization of TCS The solubility enhancement of TCS by rhamnolipids was conducted in duplicate. 10 mL RL-F1 and RL-F2 solutions each with a series of concentrations were placed in 15 mL glass centrifuge tubes with Teflon-lined screw cap. TCS was subsequently added to each tube in an amount slightly greater than required to saturate the solution. These tubes were shaken in the dark at 22 ± 0.1 °C, 225 rpm for 48 h and then centrifuged at 2000 rpm for 30 min. An appropriate aliquot of supernatant was then carefully taken and the concentrations of TCS were analyzed by HPLC. 2.4. Sorption of rhamnolipids onto sediments Batch sorption experiments were conducted in duplicate to determine rhamnolipid equilibrium sorption isotherms in a series of 15 mL glass centrifuge tubes. A set of RL-F1 and RL-F2 solutions with varying concentrations (0.05–10 mM) were prepared in deionized water with 0.01 M NaCl to minimize ionic strength change and 0.01% (w/v) NaN3 to inhibit microbial growth. All solutions were buffered at pH 6.8 with 15 mM NaHCO3. A mass of 0.2 g sediments and 10 mL rhamnolipid solutions were added to each tube. The tubes were then equilibrated at 22 ± 1 °C and 225 rpm for 48 h Table 1 Select physic-chemical properties of sediment samples in this study. Sediment samples

Organic content (%)

Clay/silt/sand (%)

Sediment A Sediment B Sediment C

2.4 3.0 1.1

26.4/46.4/27.2 19.5/35.3/45.20 13.3/14.9/71.8

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in a gyratory shaker. The solutions and sediments were then separated by centrifugation at 2500 rpm for 40 min. 1.0 mL of the supernatant was then taken and analyzed for rhamnolipid by the orcinol colorimetric assay.

2.0

2.5. Distribution of TCS in sediment–water–rhamnolipid system The sorption of TCS onto sediment by RL-F1 and RL-F2 was measured by batch experiments using the same method of Lin et al. (2011). The amount of TCS adsorbed onto sediment was determined by calculating the difference between the amount of TCS initially added to the system and that remained in the solution after equilibration.

Apparent Solubility of TCS (mM)

2.6. TCS desorption experiment 1.6

Batch experiments were conducted using the same experimental procedures of the sorption experiments, except that sediments were previously amended with TCS standard solution prepared in acetone. Acetone was then evaporated and the sediments were aged for about 1 month before the experiments. Extraction of sediments aged with TCS gave the maximum desirable concentration of 76.67, 96.07 and 99.21 lg g1 on Sediments A, B and C, respectively. Rhamnolipid solutions were prepared at concentrations of 0.05, 0.1, 0.3, 0.5, 1.0, 1.5, 2.5 and 5 mM.

1.2

0.8

0.4

RL-F1 RL-F2

0.0 0

1

2

3

4

5

6

2.7. HPLC analysis

Rhamnolipid Concentration (mM)

A Shimadzu HPLC system (Japan) fitted with a SPD-M20A diode array detector and Agilent Eclipse C-18 column (4.6  250 mm, 5 lm) Plus C18 reverse phase column was used to quantify

Fig. 1. Solubility enhancement of TCS by rhamnolipids.

200

1.0

RL-F1

80

RL-F1 0mM 0.05mM 0.5mM 2.5mM 5.0mM 7.5mM

0.8

Sorbed Amount ( µmol g-1)

-1

120

surf

Cs ( µmol g )

160

Sediment A

40

0 2000

4000

6000

8000

0.4

0.2

Sediment A Sediment B Sediment C 0

0.6

0.0

10000

0

2

4

Cwsurf ( µM)

6

8

10

12

1.0

40

Sediment A

RL-F2

RL-F2 0mM 0.05mM 0.5mM 2.5mM 5.0mM 7.5mM

0.8

Sorbed Amount ( µmol g-1)

Cssurf (µmol g-1)

30

20

10

Sediment A Sediment B Sediment C

0.6

0.4

0.2

0.0

0 0

2000

4000

6000 surf w

C

14

Equilibrium Concentration (µM)

8000

( µM)

Fig. 2. Sorption isotherms of RL-F1 and RL-F2 onto sediments.

10000

0

2

4

6

8

10

12

14

16

18

Equilibrium Concentration (µM) Fig. 3. Sorption isotherms of TCS on sediment a with different concentrations of RLF1 and RL-F2.

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Table 2 Langmuir isotherm parameters for the sorption of RL-F1 and RL-F2 onto sediments. Sediment B

Qmax (lmol g RL-F1 RL-F2

)

192.56 29.45

1

Klang (L lmol 0.00109 0.00095

)

Qmax (lmol g

3. Results and discussion 3.1. Solubilization of TCS by rhamnolipids To quantify the effectiveness of solubilization of HOCs in aqueous solution by rhamnolipid, the molar solubilization ratio (MSR) and the micelle phase/aqueous phase partition coefficient (Km) were used. MSR is defined as the ratio of the moles of organic compound solubilized to the moles of surfactant present as micelles, which can be determined from the slope of the solubility curve above the CMC (Li and Chen, 2002; Laha et al., 2009). Km is a parameter that represents the partitioning of organic compound between aqueous and micellar phases, and can be calculated (Edwards et al., 1991; Bhat et al., 2009) as

3.2. Sorption of rhamnolipids onto sediments The sorption isotherms of both rhamnolipids onto the three sediments are shown in Fig. 2. All the isotherms can be well fitted by the Langmuir model

C surf s

1

)

0.00128 0.00094

Qmax (lmol g1)

Klang (L lmol1)

103.57 17.04

0.00133 0.00163

et al., 2002; Ochoa-Loza et al., 2007). With the increase of rhamnolipid concentration, the sorption of rhamnolipid increased and a plateau was exhibited. Langmuir isotherm parameters for RL-F1 and RL-F2 are listed in Table 2. Although the organic content of these three sediments followed the order of Sediment B > Sediment A > Sediment C, sediment B did not give the highest value of Qmax for both rhamnolipids. It turned out that the tendency of both rhamnolipids to adsorb onto sediments corresponded to the soil clay content (Sediment A > Sediment B > Sediment C). Previous studies (MataSandoval et al., 2002; Zhou and Zhu, 2007) showed the similar results that the sorption of surfactant were better correlated to the percent of clay content of sediment or soil rather than to the percent of organic content. Compared to the sorption of RL-F2 to sediments, the maximum sorption amounts of RL-F1 are nearly six times higher than that of RL-F2 for all three sediments. The adsorptivity of rhamnolipids is related to the rhamnolipid species and the degree of their hydrophobicity. RL-F1 with a single

ð1Þ

where SCMC is the apparent solubility of HOC (M) at the CMC and Vw is the molar volume of water. Results of the TCS solubility enhancement by rhamnolipids are presented in Fig. 1, which indicated that both rhamnolipids significantly increased the solubility of TCS above the CMC due to the partition of TCS into rhamnolipid micelles. At concentrations below the CMC of either rhamnolipid, solubility enhancement of TCS by rhamnolipid monomers are insignificant. The MSR values of TCS in RL-F1 solution and RL-F2 solution were found to be 0.254 and 0.350, respectively. The Km values were also calculated and given results of 391.3 and 645.0 for RL-F1 and RL-F2, respectively. The MSR and Km values of TCS were much higher for RL-F2 than that for RL-F1, which indicated that RL-F2 had greater solubilization capacity for TCS than RL-F1. The reason for the greater solubilization of RL-F2 was mainly attributed to the larger micellar volume of RL-F2 than that of RL-F1 (Guo et al., 2009), and more TCS molecules can partition into the hydrophobic micellar core of RL-F2. The micellar solubilization also depends on the interaction between rhamnolipid micelles or aggregation and solubilizates. Comparing to PAHs and alkane, TCS is moderately polar with polar groups which can interact with hydrophilic groups of micelles and then additionally solubilized by attaching to the micellar shells of surfactants.

.  1 þ C surf ¼ C surf w K lang Q max w K lang

Klang (L lmol

)

158.37 24.84

aqueous TCS at a detection wavelength of 230 nm. Acetonitrile/ water (v/v, 75:25) was used as the mobile phase at a flow rate of 1.0 mL min1. The column temperature was maintained at 35 °C.

K m ¼ MSR=fð1 þ MSRÞSCMC V w g

Sediment C 1

2000

RL-F1

Sediment A Sediment B Sediment C

1500

K d* (L kg-1)

1

1000

500

0 0

2

4

6

8

Initial Rhamnolipid Concentration (mM) 2000

RL-F2

Sediment A Sediment B Sediment C

1500

K d* (L kg-1)

Sediment A

1000

500

ð2Þ

where C surf (lmol g1) and C surf (lM) are rhamnolipid concentras w tions in the solid phase and the aqueous phase, Qmax (lmol g1) is the maximum value of rhamnolipid adsorbed in sediment, and Klang is the equilibrium constant. Similar sorption isotherms of surfactant onto soils were reported in some previous studies (Mata-Sandoval

0 0

2

4

6

8

Initial Rhamnolipid Concentration (mM) Fig. 4. The apparent distribution coefficient (K d ) for TCS with RL-F1 and RL-F2 on three different sediments in relation to the initial concentration of rhamnolipids.

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Rd* (%)

40

100

RL-F1

Sediment A Sediment B Sediment C

80

30

60

20

40

10

20

0

0

20

1

Sediment A Sediment B Sediment C

2

3

4

5

6

0

1

2

3

4

5

6

50

RL-F1

Sediment A (CEDC=2.09mM) Sediment B (CEDC=0.85mM) Sediment C (CEDC=0.38mM)

0

RL-F2

40

15

RL-F2

Sediment A (CEDC=0.30mM) Sediment B (CEDC=0.21mM) Sediment C (CEDC=0.19mM)

REC

30 10 20 5

0

10

0

1

2

3

4

5

6

0

0

1

2

3

4

5

6

Initial Rhamnolipid Concentration (mM) Fig. 5. The Rd and REC values for TCS with RL-F1 and RL-F2 on three different contaminated sediments in relation to the initial concentration of rhamnolipids.

rhamnose ring and relatively lower CMC value is more hydrophobic than RL-F2 containing double rhamnose ring in the polar head. The sorption capacity of surfactants onto sediments would be expected to increase as the degree of hydrophilicity decreased. Noordman et al. (2000) also found the similar results that the more hydrophobic rhamnolipid components were preferentially adsorbed. 3.3. Distribution of TCS in sediment–water–rhamnolipid system As an example, the sorption isotherms of TCS in sediment– water system with and without rhamnolipids are shown in Fig. 3. All the sorption isotherms for different concentrations of rhamnolipids were well fitted with linear regression equations   (R2 = 0.914–0.988). The apparent distribution coefficients K d of TCS in sediment–water–rhamnolipid systems were determined from the slope of the isotherms. The values of K d as a function of initial rhamnolipid concentration in solution were presented in Fig. 4. TCS in all sediment–water systems containing rhamnolipids shared a similar sorption behavior that the value of K d decreased with the increasing concentration of both RL-F1 and RL-F2. The higher the value of K d , the greater the sorption capacity of TCS onto sediment in these systems. At low concentration of rhamnolipid, the value of K d slightly decreased with the increase of rhamnolipid concentration, which probably is due to the rhamnolipid equilibrium concentration in the aqueous phase that was below CMC after the sorption of rhamnolipid onto sediment. Once the sorption saturation of rhamnolipid onto the solid phase and the CMC in the aqueous phase were attained, rhamnolipid micelles were able to be formed as an important partition phase for TCS. As a result of solubilization by micelles, more TCS molecules distributed into the aqueous

phase than the solid phase, thereby leading to a large decrease in K d at high concentrations of rhamnolipids (2.5, 5.0, and 7.5 mM). RL-F2 performed a larger distribution capacity of TCS into the aqueous phase than RL-F1 at high concentrations. This could be due to the much larger maximum sorption amounts of RL-F1 onto sediments and greater solubilization enhancement of RL-F2 micelles for TCS. Although previous study (Lin et al., 2011) has shown that Sediment C had the largest sorption capacity of TCS without rhamnolipid, K d had smaller values in Sediment C systems than in the other two sediments for both RL-F1 and RL-F2. This results probably depend on that TCS molecules are stored in micropores of sediment which was likely much easier to transfer into the aqueous phase. 3.4. TCS desorption by rhamnolipids The effect of rhamnolipid on TCS desorption from contaminated sediment can be determined by the relative efficiency coefficient of desorption (REC), which can be defined (Zhou and Zhu, 2007) as

REC ¼ Rd =Rd Rd

ð3Þ

where and Rd are the desorption percentage of TCS with and without rhamnolipids respectively. The larger the REC value, the greater the efficiency of rhamnolipids in enhancing TCS desorption. The values of Rd and REC for TCS as a function of the initial rhamnolipid concentration were presented in Fig. 5. A value of REC > 1 indicates enhanced TCS desorption, while REC < 1 represents an inhibition of desorption. When REC = 1, the corresponding rhamnolipid concentration is the critical enhanced desorption concentration (CEDC). The values of Rd and REC for TCS in all sediment–water–rhamnolipid systems showed a similar trend that the values of Rd and REC

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initially decreased to reach a minimum value and then increased with the increasing initial rhamnolipid concentration. As NaN3 was added into the solutions to avoid the biodegradation of rhamnolipid, the initial decrease was most likely caused by the sorption of rhamnolipid onto sediments and the partition of TCS into the sorbed rhamnolipid. And the similar tendency with the increasing surfactant concentration was also found in the study of Zhou and Zhu (2007), which owed the decrease in the Rd and REC values to the strong sorption of surfactant onto solid phase and the functionality of the sorbed surfactant as the sorptive phase for PHAs. Rhamnolipids began to desorb TCS from the three sediments at a relative low rhamnolipid concentration. When the maximum sorption of rhamnolipid onto solid phase and its CMC in solution were reached, the formation of micelles greatly enhanced desorption of TCS from sediments, then the values of Rd rapidly increased. The efficiency of rhamnolipid in enhancing desorption of TCS from contaminated sediments mainly depends on sediment composition, rhamnolipid structure and dosage. The CEDC values of both rhamnolipids followed the order of Sediment A > Sediment B > Sediment C. This order corresponded to the clay content of sediment, which also played an important role on the sorption of rhamnolipid onto the solid phase. The more rhamnolipid absorbed onto sediment, the lower the rhamnolipid concentration in the aqueous phase. Thus this resulted in the greater demand of rhamnolipid dose for enhancing TCS desorption. Owing to the lower capacity of sorption onto the three sediments and greater solubilization enhancement for TCS, RL-F2 was more efficient than RL-F1 in enhancing desorption of TCS from contaminated sediments. This was consistent with the results of TCS distribution of in sediment–water systems with high rhamnolipid concentrations.

4. Conclusions The sediment clay content contributed to the sorption of rhamnolipid, which was adequately described by Langmuir-type isotherm. With the increase of rhamnolipid concentration (0.05– 7.5 mM), the amount of TCS in the aqueous phase increased in the sediment–water system. At high concentrations (>2.5 mM), the distribution capacity of TCS into the aqueous phase by RL-F2 was higher than that by RL-F1 due to its greater solubilization enhancement for TCS. Moreover, TCS desorption was only enhanced by rhamnolipid at concentrations above the corresponding CEDC values. The efficiency of rhamnolipids in enhancing TCS desorption from sediments was strongly relative to the sediment composition and rhamnolipid structure. For sediments with lower clay content, the relatively hydrophilic rhamnolipid was more efficient in desorbing TCS. In particular, the TCS desorption percentages Rd of RL-F2 (5 mM) was 1.8–2.4 times that of RL-F1. Thus, RL-F2 appears to be a potential desorption agent for TCS bioremediation. Acknowledgment This study were supported by the National S&T Major Project Foundation of China (No. 2008ZX07211-005) and National Natural Science Foundation of China (21277050).

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere. 2012.08.036.

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