Process balance and product quality in the production of natural indigo from Polygonum tinctorium Ait. applying low-technology methods

Bioresource Technology 81 (2002) 171±177

Process balance and product quality in the production of natural indigo from Polygonum tinctorium Ait. applying low-technology methods T. Bechtold a

a,*

, A. Turcanu a, S. Geissler b, E. Ganglberger

b

Institute for Textile Chemistry and Textile Physics, Leopold±Franzens-University Innsbruck, Hoechsterstrasse 73, A-6850 Dornbirn, Austria b Institute for Applied Ecology, Seidengasse 13, A-1070 Vienna, Austria Received 30 July 2001; received in revised form 5 August 2001; accepted 16 August 2001

Abstract Indigo is the most important blue component in the class of natural dyes for cellulose and protein ®bres. In the moderate European climate Polygonum tinctorium Ait. could be an interesting source for natural indigo (Vat blue 1). Following a cultivation of the plant material a simple procedure for the extraction of the indigo precursor indican was investigated with regard to crop and quality of dye obtained. The dependence of the crop on the storage conditions of the harvested plant material was investigated. The results quantify the distinct sensitivity of the fresh material to the time of storage before extraction with regard to the amount of natural indigo obtained, the photometrically determined indigo content in the product and the shade and colour depth observed in standardised dyeing experiments. A basic set of data is presented, which describes the process in terms of consumption of energy, water and chemicals and organic waste released from the extraction step. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Indigo; Natural dye; Textiles; Dyeing; Indican; Polygonum tinctorium Ait.; Dyer's Knotweed; Colour

1. Introduction For textile dyeing with natural dyes, indigo has an almost unique position as the most important blue natural dye (CI Vat Blue 1). Woad (Isatis tinctoria L.) and Dyer's Knotweed (Polygonum tinctorium Ait.) were cultivated in Europe for the production of indigo up to the 17th century. After this time indigo from the Indigoplant (Indigofera tinctoria L.) was used because of the superior quality of the dyestu€ (Schmidt, 1997). At the end of the 19th century the synthetic indigo almost completely displaced natural indigo. In 1913 the production of synthetic indigo had reached more than 33,000 tons (Schweppe, 1993). At this time mainly woollen clothes were dyed for marine blue textiles with excellent fastness properties. At present the main market for indigo is dyeing of cotton yarn for the production of denim (Essl, 1999, 2000a,b). The annual production of synthetic indigo is estimated as 22,000 tons of dyestu€ (Schrott, 2001).

*

Corresponding author. Tel.: +43-5572-28533; fax: +43-5572-28629. E-mail address: [email protected] (T. Bechtold).

Due to the importance of indigo considerable research has been performed to replace the chemical synthesis of the dye by an application of biotechnological methods (O'Connor and Hartmans, 1998; Itoh et al., 1999; Bhushan et al., 2000; Young-Am et al., 2000; Shim et al., 1998). At present there is still considerable demand for research to optimise these methods of biosynthesis with regard to their production costs (Schrott and Saling, 2000). For the production of eco-textiles based on natural dyes an evaluation of economic and ecologic sources for natural dyes is of growing interest (Geissler and Ganglberger, 2001). Yellow, red, brown, green and dark grey shades can be obtained from various sources of plant material e.g. buds from Canadian Golden Rod, Madder roots, barks from Sticky Alder tree, Ash-tree (Bechtold et al., 2001). Despite the various shades that can be obtained with such dyes the need for indigo as a blue component is persistent. As a result, with the historical disappearance of indigo farming in Europe there is a considerable lack in knowledge with regard to simple production of natural indigo from plant material with consideration of the ecological standards applied at present. For an ecient

0960-8524/02/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 0 1 ) 0 0 1 4 6 - 8

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and ecological production of natural indigo a simple extraction method is required (Th uringer Landesanstalt, 1999). From the historical literature it is known that the freshly harvested plant material containing the indigo precursors e.g. Isatan B (Indoxyl-5-ketogluconat, Isatis tinctoria L.) or indican (Indoxy-b-D -glucosid, Polygonum tinctorium Ait.) exhibits a distinct sensitivity on the conditions of storage and extraction (Schweppe, 1993). Detailed information about the time dependence of the crop of indigo is needed as a basis to optimise harvesting, storage of plant material, extraction of indigo precursors and dye formation. In many papers dealing with the production of indigo from natural sources the amount of indigo formed was determined by analysis of precursor substances e.g. indican or indoxyl or by chemical extraction methods (O'Connor and Hartmans, 1998; Itoh et al., 1999; Young-Am et al., 2000; Shim et al., 1998). For an evaluation of the overall crop of indigo the formation of indigo dye should be considered because losses during the (bio)chemical transformation of the precursors of the dye have to be considered. In this study Dyers' Knotweed (Polygonum tinctorium Ait.) was chosen as a representative plant for further investigation. A simple standard procedure, considering both basic technical equipment available to a farmer at moderate costs and ecological demands of a dyestu€ extraction, is presented. Results of an experimental study to determine the in¯uence of the storage and the conditions of indican extraction on the total amount of indigo obtained per kg plant material and on the dyestu€ content in the dried product are given. The crude indigo dye obtained with the proposed extraction method was

analysed for its indigo content by spectrophotometry and in addition standard dyeings were performed to de®ne the obtained shade and colour depth in comparison to synthetic indigo. The given results form a basic set of data required for optimisation and further scale up of the extraction process, including methods for standardisation and quality control of the product. 2. Methods 2.1. Plant material The seed material for Polygonum tinctorium Ait. was obtained from the Th uringer Landesanstalt f ur Landwirtschaft Jena (Dornburg, Germany). Cultivation was performed in the moderate climate of Vorarlberg (Austria) during the summer period of 2000. The plant material was harvested in the end of September 2000 to avoid damage of plants by possible frost during nights. The total mass (leaves and stem) of wet plant material was approximately 250 kg. To study the in¯uence of storage conditions the material was separated within 24 h after harvesting into samples of 15±20 kg mass. The sample identi®cation and storage conditions are shown in Table 1. After a de®ned storage time the samples were extracted with application of a simple standard procedure (Fig. 1). Storage at room temperature was chosen as a general method. To estimate di€erent strategies for longer storage, two samples were air dried with absence of direct sunlight (Samples 12, 13, Table 1) one sample was stored frozen (14) and one sample was stored in water (11).

Table 1 Sample characteristics: storage conditions, pH and redox potential (E) at the end of indican hydrolysis, absolute amount of crude dyestu€ and crude dye calculated per 100 kg plant material Sample

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 a

Storage

Indican hydrolysis

Mass

Time (d)

Temperature (°C)

pH

E (mV)

Plant material extracted (kg)

Crude dye (g)

Crude dye/100 kg plant material (kg)

0.7 1 1 2 4 5 6 7 8 9 49 49 53 54 3a

ca. 20 ca. 20 ca. 20 ca. 20 ca. 20 ca. 20 ca. 20 ca. 20 ca. 20 ca. 20 ca. 20 10±15 10±15 < 20 ca. 10

4.35 3.99 4.23 4.00 3.85 3.87 3.89 4.04 4.07 n.d. 4.36 4.59 4.56 3.73 4.16

‡70 )80 )80 )20 )170 )180 )150 )80 )190 n.d. ‡50 )120 )70 )50 ‡20

7.7 17.8 7.2 19.2 18.5 16.8 15.1 17.1 20.8 20.3 20 18.9 16.9 17.0 2

31.4 95.6 55.3 53.2 19.8 16.4 45.2 55.5 101.1 69.9 259.2 40.2 18.1 20.2 12.9

0.44 0.54 0.79 0.28 0.11 0.10 0.30 0.32 0.49 0.34 1.30 0.21 0.11 0.12 0.64

Samples 1±14: cultivation in Batschuns (660 m over sea level) and Tufers (520 m), hills, soil Braunerde; sample 15: cultivation approx. 6 weeks later in Hohenems, (410 m), ¯at valley, soil: river sediment.

T. Bechtold et al. / Bioresource Technology 81 (2002) 171±177

173

Fig. 1. Production scheme for a basic process for natural indigo.

2.2. Extraction of indican and formation of crude indigo An amount of 15±20 kg of plant material was extracted in two stages with soft water (<20 ppm Ca2‡ ). The ®rst extraction step was performed for 4 h with a volume 40 l at 50±70 °C, followed by a second extraction for 2 h with 20 l water at 40 °C. The combined extracts were stored for 4±5 days in open polyethylene containers. During this time the solution was stirred for some minutes per day to introduce air oxygen. pH and redox potential of the solution were measured at the end of the storage period by means of a glass electrode and a Pt-electrode, respectively (Ag/ AgCl, 3M KCl reference, Orion 720 A, Orion Research, Boston, MA). To improve ¯occulation and precipitation of indigo approx. 1:5 g=l CaCl2
2H2 O (technical grade) was added before ®ltration. The sediment was collected and ®ltered through a cotton fabric. After drying in a laboratory dryer at 70 °C the amount of crude indigo was weighed. The material then was pulverised and used in dyeing experiments and for analytical tests. 2.3. Analytical determination of indigo The indigo content in the crude dyestu€ was determined by photometry (Bechtold et al., 1992; Merritt et al., 2001). An aqueous alkaline solution of Fe(II)triethanolamine was used as reducing agent. In a 100 ml volumetric ¯ask 0.1 g of pulverised crude dyestu€ was

dispersed in 20 ml of a solution containing 33 g/l NaOH, 333 g/l triethanolamine (85% o.w.), 50 g=l FeSO4
7H2 O and 0.8 g/l citric acid. The ¯ask then was ®lled to 100 ml with a solution containing 10 g/l NaOH, 50 g/l triethanolamine (85% o.w.) and 5 g=l FeSO4
7H2 O. To complete reductive extraction and dissolution of the dyestu€ the solution was held at 40 °C for 60 min with gentle shaking every 15 min. The spectrum of the solution was recorded from 350 to 700 nm using a diode-array spectraphotometer (Zeiss CLH 500/MCS521 UV±Vis, Carl Zeiss (Jena) Germany, 1 mm cuvette). For the determination of the content of indigo the absorbance at 410 nm was used. Synthetic indigo (Indigo Plv. DyStar, Frankfurt a. Main, Germany) was used as a standard dyestu€ for calibration. P.a. quality NaOH was used, FeSO4
7H2 O, Fe2 …SO4 †3
7±8H2 O, CaCl2
2H2 O, triethanolamine, citric acid, and Na2 S2 O4 were technical grade chemicals. 2.4. Determination of the colour strength The colourimetric determination of the colour strength of the natural dye was determined in a series of standardised exhaust dyeing experiments (Mathis Labomat BFA-8, Mathis AG Niederhasli, Z urich, Switzerland). Depending on the estimated indigo content of the product, an amount of 2±25 g dyestu€ was pulverised and brought to 100 ml with a solution containing 10 g=l Na2 S2 O4 , 80 ml/l NaOH 50% and 1 g/l dispersing

174

T. Bechtold et al. / Bioresource Technology 81 (2002) 171±177

agent (lignosulfonate, Setamol WS, BASF, Ludwigshafen, Germany). Complete reduction was achieved by gently shaking the solution for 12±18 h at 40 °C. A volume of 20 ml of this stock vat was then used for a dyeing experiment: 10 g of bleached cotton fabric were dyed in a liquor ratio of 1:10. 5 g=l Na2 S2 O4 , 7 ml/l NaOH 50%, 10 g=l Na2 SO4 and 1 g/l dispersing agent (Setamol WS) were added to the dyebath. Within 20 min the dyebath temperature was raised to 60 °C and maintained at 60 °C for 45 °C. To compensate for the low substantivity of indigo the dyebath was cooled to 25 °C before removal of the samples. Additional liquor was removed by squeezing the samples with a laboratory padder followed by air oxidation for at least 1 h. After complete oxidation the samples were rinsed in water and neutralised in a bath containing 1 ml/l acetic acid (80%). Drying of the samples was performed at 80 °C in a laboratory drying unit (Mathis Labdryer, Mathis AG, Niederhasli, Z urich, Switzerland). The CIELab coordinates of the dyeings were measured with a tristimulus colorimeter (Minolta Chroma-meter CR 210, sample diameter 50 mm, D65). The indigo content was determined according to the Kubelka±Munk function, the K=S values were calculated from the re¯ectance determined at 660 nm (Pye Unicam SP 8±100 double beam spectrophotometer, di€use re¯ectance sphere 0°/d). Standardisation was performed with Indigo Plv. (DyStar) using identical procedures for dyestu€ reduction and dyeing. 3. Results and discussion The analysis of the indigo concentration in the crude dyestu€ obtained was performed by photometry and by standardised dyeing experiments. Assuming an equal content of indican in the plant material, the amount of crude indigo used for the dyeing experiments was increased in proportion to the amount of crude product obtained per kg of extracted plants. The natural dyeings were compared to standard dyeings obtained with Indigo Plv. BASF (Exp. 16 1% and exp. 17 3% in colour depth). From a set of standard dyeings the colour depth was determined for dyeings 1±15 according to the Kubelka±Munk function. The results of the indigo content as function of storage time are given in Table 2. As shown in Table 2 the mass of crude dyestu€ and the indigo content were directly dependent on the time of storage. A dramatic decrease in colour strength and indigo content of the products within 2±3 days of storage at RT was observed. During the ®rst two days of storage at room temperature the extractable amount of indigo decreased from 50 g/100 kg (expts. 1, 3) to 9 g/100 kg (expt. 4). After a storage time of more than 4 days the extractable indigo content generally dropped to less than

3 g/100 kg. Only sample 9 showed a remarkable content of indigo after a storage period of 8 days. In this sample the photometrically determined indigo content seemed to exceed the results found in the dyeing experiments, which can be explained by an increased content of extractable yellow components. A subtraction of the light absorption of coloured by-products extracted was not attempted because in alkaline medium a partial reduction of the indigo is also possible, due to reducing components present in the crude product. Sample 15 was harvested from a di€erent site of cultivation so the initial indican content was di€erent. The material was stored outside for 72 h at 5±10 °C. Despite the longer duration of storage a remarkably higher indigo content was found, indicating that a reduction in temperature permits prolongation of storage before extraction. On the basis of the analytical data and from the results of the dyeing experiments both the indigo content in the crude dyestu€ and the crop of dye per 100 kg plant material were calculated (Table 2). In the purest samples an indigo content of approximately 10% was determined. Related to the total mass of wet plant material up to 70 g indigo were obtained per 100 kg of plants, which corresponds to an indigo content of 0.070%. In experiments 12 and 13 the material had been air dried before extraction. In experiment 12, 3.36 kg air dry material was obtained from 16.9 kg of plant matter and in experiment 13, 3.50 kg was obtained after drying starting from 18.9 kg material. Considering a water content of 80.8% the indigo content in the dry material can be given as 0.35%. In some historical reports only the leaves were used for the extraction which would require an additional separation step to remove the stems. The stems represent approximately 50% of the total mass, so results given in such reports tend to higher values of indigo content (Schweppe, 1993). The tendency of the analytical determination of the indigo content corresponds to the ®ndings from the dyeing results. The di€erence in the calculated indigo content between the two di€erent methods can be explained with the di€erent methods for dyestu€ extraction and applied instrumental methodology. Besides the colour strength the shade of the dyeings are of particular interest to determine changes in the dyeing results as a function of the conditions of dyestu€ preparation particularly with regard to the storage of the plant material. In comparison to the standard dyeings shown in Table 2, dyeings with natural indigo tended towards more greenish shades which can be seen in the more negative a-values. Sample 2 shows only minimal di€erences of Lab-coordinates compared to the standard dyeings, indicating that a good quality of dye can be obtained without further puri®cation. The CIELab-

T. Bechtold et al. / Bioresource Technology 81 (2002) 171±177

175

Table 2 Indigo content in the samples determined by photometry and standard dyeing experiments calculated as crop pure natural indigo per 100 kg plant material, K=S values of the dyeings (Kubelka±Munk value at 660 nm) and Lab-coordinates (CIELab colour coordinates) of the dyeings obtained Sample

Photometry Purity (%)

Dyeing Indigo per 100 kg (g)

K=S

L

a

b

Purity (%)

Indigo per 100 kg (g)

10.1 10.9 8.3 8.3 13.3 13.3 4.5 4.4 1.1 1.2 0.5 0.5 1.0 1.1 2.1 2.0 4.9 4.7 1.6 1.6 0.9 1.2 0.4 0.4 0.2 0.2 0.2 0.1 7.1 7.1 4.3 4.0 8.1 8.1

28.22 27.22 28.75 29.46 27.23 27.03 39.28 39.29 56.91 56.54 65.77 65.77 57.28 56.97 50.49 50.97 37.90 38.36 52.61 53.34 56.88 53.21 69.01 69.17 78.03 77.45 78.76 78.46 33.17 32.44 40.91 42.72 30.85 30.29

)2.81 )2.73 1.17 1.04 )1.76 )1.92 )4.36 )4.34 )5.22 )5.24 )4.52 )4.52 )4.78 )4.70 )4.22 )4.19 )5.19 )4.97 )6.03 )6.23 )4.83 )5.47 )5.76 )5.51 )7.40 )6.96 )7.11 )7.60 )4.45 )4.27 )1.74 )2.04 0.30 0.50

)14.84 )15.11 )20.29 )20.39 )17.33 )16.92 )16.67 )16.96 )13.60 )13.69 )10.35 )10.35 )12.28 )12.50 )18.12 )17.71 )16.49 )16.41 )13.04 )12.62 )5.12 )5.20 )8.35 )9.34 )4.78 )4.48 )6.65 )7.09 )15.82 )15.85 )22.52 )21.64 )21.59 )21.65

12.3

53.6

4.2

22.4

6.29

49.9

3.2

9.0

1.4

1.4

0.6

<1.0

0.5

1.5

1.0

3.3

2.1

10.0

0.7

2.4

0.2

3.0

0.0

<1.0

2.2

2.4

1.9

2.3

10.8

69.4

1

9.2

40.0

2

2.1

11.3

3

2.5

19.9

4

3.5

8.4

5

2.4

2.5

6

<2.0

±

7

<2.0

±

8

<2.0

±

9

3.5

17.0

10

2.0

6.9

11

<2.0

±

12

<2.0

±

13

<2.0

±

14

<2.0

±

15

6.2

16 Indigo 1% 17% Indigo 3%

39.9

coordinates are in good agreement to the data given for dyeings with synthetic indigo (Bechtold et al., 1997; Xin et al., 2000).

The observed decrease in dyestu€ content in the products was accompanied by a distinct shift in shade towards yellow/green. The photometric curves given in Fig. 2 also prove this ®nding. The spectra of the natural dyes samples 1, 9, 15 show higher absorption at wavelengths above 440 nm indicating an increased content of yellow±brown components. For comparison the spectra of the Fe(II)triethanolamine complex solution and of the corresponding Fe(III)-complex also are shown in Fig. 2.

4. Summary

Fig. 2. Absorption spectra: indigo samples 1, 9, 15 (see Table 1), standards 0.05 and 0.1 g/l indigo in Fe(II)triethanolamine complex solution and spectra of the Fe(III)-form and the Fe(II)-form of the complexes.

The extraction of the plant material was performed as a simple hot water extraction to de®ne a simple basic technology which could be handled by a farmer immediately after harvesting. A production scheme for the extraction of indigo precursors from Dyer's Knotweed and the following up

176

T. Bechtold et al. / Bioresource Technology 81 (2002) 171±177

treatment to form indigo is shown in Fig. 1, important data for water consumption, energy and chemicals used and for the required storage volume for a certain production capacity are given in Table 3. For an estimation of the indigo crop the technique of harvesting and extraction of the plant material has to be considered very carefully because the amount of indigo ®nally obtained for a given indigo content directly is in¯uenced by these steps. An optimisation of the farming conditions will increase the overall amount of indigo obtained at the end of the treatment, but the relation of the data for energy, chemicals and water consumption for the treatment of a certain amount of plant material will remain constant and be mainly independent of the indigo content of the plant. The data in Table 3 indicate several important factors of the process: The rapid decrease in the amount of indigo determined in the product proves the importance of performing the extraction in a short time after harvesting the plant material. The estimated energy consumption for the process is mainly determined by the hot water extraction. For a scale-up the extraction at lower temperature should be considered to optimise the energy consumption of the process. As a rather high volume of water has to be used an extraction in cold water would be of particular interest. Optimisation of the water consumption in the process by appropriate recycling techniques will be of particular interest. Identifying the extraction as an essential step with regard to the amount and quality of the dyestu€, a semi-continuous harvesting/extraction is favourable because an intermediate storage should be avoided.

In this case the volume of extract to be handled during hydrolysis of indican and indigo formation has to be optimised with regard to the very large volume of liquid that has to be stored during this period. After formation of indigo during the hydrolysis, an addition of CaCl2 facilitates precipitation and speeds up ®ltration of the solids. The addition of chemicals during extraction has to be avoided because of the contamination of the plant residue, which represents the major part of the organic mass which has to be disposed o€ and to avoid any further treatment of the wastewater released from the precipitation/®ltration step. The extraction has to concentrate on soluble precursors e.g. indican. Losses during storage can be explained by formation of indoxyl and irreversible oxidation of indoxyl to isatin but also can be caused by formation of insoluble indigo. Indigo formation is observed following mechanical damage of leaves. In case of indigo formation an extraction by use of solvents or reducing chemicals (NaOH, Na2 S2 O4 ) is theoretically possible, but it has to be excluded because of the amount of chemicals released with the extracted residue and because of expected chemical consumption per kg dyestu€. The results of the dyeing experiments indicate that the shade of the natural dye tends towards more greenish shades and a careful standardisation of the production process will be required to obtain a constant quality of the natural dye. Data describing indigo formation on the basis of analytical results obtained with precursors should be treated with great care because these values describe a theoretical maximum content and will be too optimistic in many cases.

Table 3 Production scheme and basic requirements for daily processing of 100, 1670 and 167,000 kg/d freshly harvested plant material, corresponding to a production capacity of 0.060, 1, and for 100 kg/d natural indigo from Polygonum tinctorium Ait. Process step

Production capacity of indigo per day 0.060 kg/d

1.0 kg/d

100 kg/d

Harvesting of fresh plant material per day (kg/d)

100

1670

167,000

60 °C extraction of (Indican) Water consumption per day (m3 =d) Energy consumption per day (k W h/d) Residue for disposal per day (kg/d)

0.3 14 ca. 100

5 5230 ca. 1700

500 23,000 ca. 170,000

Hydrolysis of Indoxyl to form Indigo required storage volume (m3 )

1.2

20.0

2000

Precipitation of indigo Waste water released per day (m3 =d) Chemicals consumption per day (CaCl2 ) (kg/d)

0.3 0.45

5.0 7.5

500 750

Filtration: amount of wet ®lter cake formed per day (kg/d)

3

50.0

5000

Drying: energy consumption per day (k W h/d)

3

50

5000

Crop of crude dye per day (kg/d)

0.60

10.0

1000

T. Bechtold et al. / Bioresource Technology 81 (2002) 171±177

For a technical product standardised dyeing experiments should be used, while photometric methods are favourable for a comparison of the indigo concentration under well-standardised conditions. Acknowledgements The authors are indebted to K. Nenning (School for agriculture, Hohenems, A), A. Riedmann (School for gardening, Lebenshilfe, Batschuns, A) for cultivation of the plant material. Authors thank the Th uringer Landesanstalt f ur Landwirtschaft Jena for providing seed material of Dyer's Knotweed and DyStar for indigo dyestu€. References Bechtold, T., Burtscher, E., Bobleter, O., 1992. Konzentrationsbestimmung von Indigo in Ansatz-und Prozessbadern. Text.-Prax. Int. 47, 44±49. Bechtold, T., Burtscher, E., K uhnel, G., Bobleter, O., 1997. Electrochemical reduction processes in indigo dyeing. J. Soc. Dyers Colour 113, 135±144. Bechtold, T., Turcanu, A., Geissler, S., Ganglberger, E., 2001. Natural dyes in modern textile dyehouses ± How to combine experiences of two centuries to meet the demands of the future? In: Proceedings of the 7th ERCP (European Roundtable on Cleaner Production), Lund, Sweden, 2±4 May, 2001, pp. 45±46. Bhushan, B., Samantha, S.K., Jain, R.K., 2000. Indigo production by napthalene-degrading bacteria. Lett. Appl. Microbiol. 31, 5±9. Essl, H., 1999. Jeans ± das blaue Phanomen (Teil 1). Textilveredlung 34 (1/2), 26±31.

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