Composite exopolysaccharide-based hydrogels extracted from Nostoc commune V. as scavengers of soluble methylene blue Текст научной статьи по специальности «Химические науки»

Research Article /St Available online at http://jfrm.ru/en

Open Access https://doi.org/10.21603/2308-4057-2024-l-587

https://elibrary.ru/SHFKMQ

Composite exopolysaccharide-based hydrogels extracted from Nostoc commune V. as scavengers of soluble methylene blue

Nora Gabriela Herrera1* , Nelson Adrián Villacrés2 , Lizbeth Aymara1 , Viviana Román1 , Mayra Ramírez1

1 Federico Villarreal National University"0", Lima, Peru 2 National University of Engineering"0", Lima, Peru

* е-mail: [email protected]

Received 26.12.2022; Revised 24.01.2023; Accepted 07.02.2023; Published online 11.07.2023

Abstract:

The industrial water contamination with synthetic dyes is currently a cause for concern. This paper introduces composite hydrogels as alternative scavengers of soluble dyes.

This research used kinetic models and adsorption isotherms to test composite exopolysaccharide hydrogels extracted from Nostoc commune V., pectin, and starch for their ability to remove methylene blue from water.

The exopolysaccharides demonstrated a rather low extraction yield and a crystallinity percentage of 38.21%. However, the crystallinity increased in the composite hydrogels (48.95%) with heterogeneous surface. The pseudo-second-order kinetic model served to explain the adsorption mechanism at pH 8 and pH 11, while the Elovich model explained the adsorption mechanism at pH 5. When in acid fluid, the hydrogels had a heterogeneous surface, whereas alkaline fluid resulted in a homogeneous surface. The Temkin adsorption model showed a good fit in the treatments.

At a basic pH value, composite exopolysaccharide-based hydrogels showed good results as scavengers of low-concentration methylene blue.

Keywords: Hydrogel, removal, methylene blue, adsorption, exopolysaccharide, Nostoc commune V.

Funding: This research was supported by the Vice-Rectorate for Research of the Federico Villarreal National University (UNFV)"0" as part of The Basic and Applied Research Projects Competition CANON 2019, project No. 5784-2019-CU-UNFV.

Please cite this article in press as: Herrera NG, Villacrés NA, Aymara L, Román V, Ramírez M. Composite exopolysaccharide-based hydrogels extracted from Nostoc commune V. as scavengers of soluble methylene blue. Foods and Raw Materials. 2024;12(1):37-46. https://doi.org/10.21603/2308-4057-2024-1-587

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Foods and Raw Materials. 2024;12(1)

ISSN 2308-4057 (Print) ISSN 2310-9599 (Online)

INTRODUCTION

The term microalgae refers to both eukaryotic (microalgae) and prokaryotic (cyanobacteria) microga-nisms that perform oxygenic photosynthesis. These organisms live in aquatic and terrestrial habitats. They produce various compounds, e.g., polyunsaturated fatty acids, pigments, proteins, some enzymes, and exopolysaccharides. These compounds can be applied in various biotechnology sectors, i.e., food, energy, health, and biomaterials [1, 2].

Cyanobacterial exopolysaccharides possess unique biochemical properties due to their high molecular weight, anionic properties, and acidic profile [3]. Exopolysaccharides extracted from Nostok commune V. can be applied in biomedicine and food industry to produce

hydrogels and films. However, the chemical structure of these exopolysaccharides is not yet known [3, 4].

Hydrogels consist of three-dimensional networks of intertwined polymer chains that are able to absorb and retain water molecules and solutes, including such ionic dyes as methylene blue [5].

Methylene blue is a cationic thiazine dye used in textile industries. However, it affects human health by causing asthma, cancer, and mutations [6]. Moreover, it affects the growth of aquatic organisms and generates mutagenic effects in fish [7, 8].

Industrial development facilitates economic prosperity but causes water pollution [9, 10]. This type of pollution occurs because various industries that deal with textile, dyes, and pharmaceuticals discharge

Copyright © 2023, Herrera et al. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/), allowing third parties to copy and redistribute the material in any medium or format and to remix, transform, and build upon the material for any purpose, even commercially, provided the original work is properly cited and states its license.

effluents that usually contain dyes and/or heavy metals [11]. For example, more than 700 000 tons of dyes are produced annually, of which approximately 1-2% are drained during production and around 10-15% are eliminated as effluents during application [12].

The list of modern wastewater treatment methods includes chemical precipitation, filtration, reverse osmosis, and photo-degradation [13-16]. However, not only are all these methods expensive and complex, but they also generate secondary products [17].

As a result, scientists are on the look for new absorbents, such as hydrogels, that could remove contaminants, e.g., dyes, from wastewater [18, 19].

This research extracted exopolysaccharides from the N. commune to prepare a new composite hydrogel that would remove soluble methylene blue.

STUDY OBJECTS AND METHODS

Materials. Pectin, starch, and calcium chloride di-hydrate were purchased from Sigma-Aldrich. Petroleum ether, chloroform, propanol, ethanol, and methylene blue were obtained from Merck. The exopolysaccharides were extracted from cyanobacteria Nostoc commune V. collected in Conococha Lake, Province of Bolognesi, Ancash-Peru. These cyanobacteria were dried, crushed, and stored in an amber jar at room temperature.

Extracting the exopolysaccharide. We defatted 2 g of dry powder by maceration with 100 mL of petroleum ether, followed by filtering and oven-drying. This process was repeated with chloroform and then with etha-nol. The extraction of the exopolysaccharide followed the procedure described by Rodriguez et al. [20]. The extractant was precipitated with propanol for subsequent drying, grinding, and storage in an amber bottle. The exopolysaccharide yield (Ye, %) was calculated as follows:

W

Y - —x 100 eW

(1)

where is the weight of exopolysaccharide, g; and W2 is the weight of N. commune dried powder, g.

Preparation of hydrogels. Wediluted 0.05 g ofexo-polysaccharide in distilled water to mix it with a pectin-starch solution in a ratio of 2:0.5, according to the methodology described by Dafe et al. [21]. After that, we poured the resulting mix drop by drop into a 0.2 M solution of CaCl2-2H 2O under constant stirring at room temperature. The hydrogels were filtered and washed with distilled water. Before each application, the hydrogels were dried at 30°C for 36 h until a constant weight was obtained.

Characterization. The FTIR-ATR spectra (600 to 4000 cm-1) were obtained usinga Nicolet iS10 Thermo Scientific spectrophotometer. The thermogravimetric curves were gathered in an STA 6000 PerkinElmer device using 5.0 ± 0.1 mg of the sample in an N2 atmosphere. The flow rate was 20 mL/min, and the temperature was between 20 and 600°C with a heating rate of 10°C/min. The XRD diffractograms were obtained

using a D2 Phaser (Bruker) equipment in a range of 10° to 60°. The crystallinity (%) index was determined using the ratio between the crystalline area and the total area (Eq. (2)). The specific surface area was determined using a Gemini VII 2390t micrometer with the nitrogen adsorption and desorption technique. The SEM images were obtained withan FEI Inspect S50 microscope. The samples were gold-plated in an 11430E-AX (SPI Supplies) high vacuum metallizer:

Crystalline area

Crystallinity =-x100

Total area

(2)

Removing methylene blue. We added 0.1 g of hydro-gel to 50 mL of methylene blue solution. For isothermal studies, the methylene blue concentration was 1.01.5*10-6 mol/L at 25°C. The experiment involved five-time intervals (15, 30, 60, 90, and 120 min) and three pH levels (5, 8, and 11). The pH values were adjusted with NaOH (0.1 mol/L) find HC1 (0.1 mol/L). The eh-sorbance value s 0gmg = 668 nm) were obtaihod h-ing a Thermo Scientific/Spectronic GENESYS 20 Visible spectrophotometer by quantifying the adsorption capacity (q , mg/g) and r^me^^nal perce neage (%y.

= =

r x(Co - Ce )

W

C - C

Removal - -0-^ x100

C

(3)

(4)

where C0 and Ce are the initial and V equilibrium concentrations of methylene blue, mg/L, respectively; W is the volume of the solution,L; W is the mass of the hydrogel, g.

Isotherm and kinetic models. The adsorption isotherm illustrates tlie mobility or reteniion of a snbsOaooe using a solid phate at a cynsOyni j^l0 and tnmpoeotbre. The Langmuir isotherm (Eq. (5)) is an empirical model that describes the adsoyrrienpooceso on a yomayene-ous surface, forie^ y amgOe ^no wiraeui Sterai iii-teraction between the absorbed meencules. Os the contrary, theFreundlich ioottrm (Eq. (6)) assumes that the adsorption is carrie° out nn e heterogeneous surface via a multilayer prochts, weile ten Tcmkin isotherm (Eq. (7)) considerr ihe interoctioe betwetiihe adsorbent and the adsorbate:

C

1

C

KLX(lm

lnqe -lnb+— lnCe n

q - RRlnC + +RRRlnk

q t e t m

b b

(5)

(6) (7)

where Ce is the adsorbate equilibrium concentration, mg/L; qe is the adsorbedamount at equilibrium, mg/g; q is the maximal amount of adsorbed surfactant, mg/g;

1max ' °

KL is the Langmuirconstant, L/mg; b is the adsorption

capacity, L/mg; Hn is the; adsorption intensity or surface heterogeneity; R is the universal gas constant, J/mol/K; T is the temperaü^ IC; itb is the Temkin constant related to serction haas, -/mo1; kbis the Temkin isotherm constant, L/g.

Finally, tha sepotetion factoa or eqoürt)rium parameter (Eq. (8)), denoted os R + cheoks if the adsorption is favorable (RL < tg er uefevorab+e gRL p 1):

rl =

1

1+ KL x C0

(8)

The pseudo-first-order (Eq. (9)) and pseudo-second-order (Eq. (10)) krnetic moeels dnffisrentiated the kinetic equations according to (he aesorpt-on caaacity affected by the initial concentratton of the dye. The Elovich model (Eq. (11)) assumes thol the adsorbent surfaces are heterogeneous, an= pdsorpltoa is performed in a multilayer proce is:

1

= - k1 x t

---= k2 x t

X Ittt

(9) (10)

(11)

where qe is the amount ofthe adsorbateat equilibrium, mg/g; qt is the maximal uptake of adsorbate, mg/g; kx is the pseudo-first-order rate constant; k2 is the pseudo-second-order rate constant; t is the contact time with

adsorbent, min; a is initial sorption rate, mg/g/min; в is the extent of surface coverage and activation energy for chemisorption, g/mg [22, 23].

RESULTS AND DISCUSSION Exopolysaccharide profile. The exopolysaccharide obtained from the Nostoc commune V. had a brown-amber color (Fig. 1a); the extraction yield was 25% dw. However, Wang et al. managed to obtain a much greater yield of 96.7% [24].

X-ray analysis. The X-ray diffractogram (Fig. 1b) showed a broad peak at 20° and a bun-shaped curve, which suggested the non-crystallinity of the exopoly-saccharides extracted from cyanobacteria [20]. This result was found consistent because the exopolysaccharide had crystallinity of 38.21% (Fig. 2b).

Thermogravimetry of exopolysaccharides. Figure 1c presents the TG thermogravimetric curve of the exopo-lysaccharide with mass losses assigned to the following thermal events: dehydration, depolymerization, degradation, and carbonization [20]. Table 1 shows the percentage of mass loss in each thermal event, with their respective temperature intervals.

FTIR of exopolysaccharides. Figure 1d illustrates the FTIR spectrum of the exopolysaccharide sample. The spectrum showed signals at 3325 cm-1 (hydroxyl groups), 2923 cm-1 (C-H vibrational stretch), 1586 cm-1 (asymmetric stretching of -COO-), 1416 cm-1 (symmetric stretching of -COO-), 1019 cm-1 (C-O-C vibrational stretch in cyclic glucose units), 889 cm-1 (в-glycosidic bond), and 795 cm-1 (glucopyranose units) [3, 25].

100 -,

80

60

öß

1 40

20

0

200 400

Temperature, °C

600

1800 . 1500

Д 1200

1 900

o4

С

Л

£

600

300

10 15 20 25 30 35 40 45 50 55 60 20 (degree) b

100 98 96 94 92 90 88 86

4000 3500 3000 2500 2000 1500 1000 Wavelenght, cm-1 d

Figure 1 Extracted sample (a); XRD diffractogram (b); TG curve (c); and FTIR spectrum (d) of exopolysaccharide from Nostoc commune

a

0

c

Table 1 Mass loss values: thermogravimetric analysis of exopolysaccharide obtained from Nostoc commune

Weight, mg Thermal event AT, °C

Mass loss, %

4.965 Dehydration 20.0-186.0 14.1 Depolymerization 186.0-413.0 49.9 Degradation 413.0-558.0 28.8 _Carbonization 558.0-600.0 2.8

Table 2 Mass loss values: thermogravimetric analysis of exopolysaccharide-based composite hydrogels

Weight, mg Thermal event AT, °C

Mass loss, %

5.297

Dehydration Degradation Carbonization

30.0-150.0

150.0-440.5

440.5-600.0

1.5

3.3

Exopolysaccharide-based composite hydrogel profile. Exopolysaccharide-based composite hydrogels had crystallinity of 48.95% (Fig. 2a). The XRD diffracto-gram (Fig. 2b) showed peaks close to 15 and 22°, which corresponded to the gelatinized starch chains [26]. On the other hand, a peak around 34° corresponded to the crystalline structure of pectin [27]. However, the broad peak at 20°, which corresponded to the exopo-lysaccharides, disappeared, probably because the exopo-lysaccharide structure was destroyed.

Figure 2c presents the TG thermogravimetric curve of compound hydrogels with mass losses assigned to the stages of dehydration, degradation, and carbonization. According to Dash et al, the second stage consists of

two continuous processes that follow pectin (200-280°C) and starch (290-425°C) degradation [28]. Table 2 shows the percentage of mass loss in each thermal event with their respective temperature intervals.

The FTIR spectrum of the hydrogels (Fig. 2d) shows additional signals to the spectrum of the exopolysac-charide (Fig. 1d). These signals corresponded to the C=O carbonyl group (1632 cm-1) for pectin, while the peaks at 1429 and 1098 cm-1 could be attributed to C-O-O stretching and C-O-H bending modes in starch, respectively, and the signal at 719 cm-1 could be correlated with vibrations belonging of the polysaccharide ring [29-31].

The specific surface area of the composite hydro-gel, as obtained from the BET isotherm model, was 0.5616 m2/g. Figure 3 shows the N2 adsorption-desorption process of the hydrogel before methylene blue scavenging. This process was a type VI isotherm, which is typical of solids with a uniform non-porous surface and represents a multilayer adsorption [31].

After the removal process, the composite hydrogels turned blue (Fig. 4a). Figure 4b shows a decrease in the band at 1632 cm-1. However, the increase in pH to basic levels intensified the bands: at pH 11, the removal process probably occurred by electrostatic attraction [32, 33].

The SEM images of the hydrogels revealed the superficial changes in these materials during the methylene blue scavenging at different pH values. At pH 5 (Fig. 5b), the surface of the hydrogel became smoother and more homogeneous, compared to the hydrogel

cN

£

M

'J3

100 90 80 70 60 50 40 30

0 100 200 300 400 Temperature, °C

500 600

10 15 20 25 30 35 40 45 50 55 60 20 (degree)

b

cN

T

100 80 60 40 20 0

4000 3500 3000 2500 2000 1500 1000 Wavelenght, cm-1

d

Figure 2 Samples (a); XRD diffractogram (b); TG curve (c); and FTIR spectrum (d) of exopolysaccharide-based composite hydrogel

a

c

1.2 -i

c 1.0 -

(D

S 0.8 -

o > 0.6 -

d

(D b 0.4 -

c« d 0.2 -

0.0

0.0 0.2 0.4 0.6 0.8 1.0 Partial pressure, P/Pn - Adsorbtion - Desorption

Figure 3 BET sorption-desorption isotherms for the exopolysaccharide-based composite hydrogel

Figure 5 SEM images of hydrogels before (a) and after methylene blue removal at pH 5 (b), pH 8 (c), and pH 11 (d)

50 n

40 30

& 20 io H o

■

rin

15 30 60 90 120

Time, min □ pH5 □ pH8 □ pHll

Figure 6 Methylene blue removal percentages at different pH and time values

100 959085 80 75

4000 3500 3000 2500 2000 1500 1000 Wavelenght, cnr1

- pH 5 - pH 8 - pHll

b

Figure 4 Samples (a) and FTIR spectrum (b) of exopolysaccharide-based composite hydrogel after methylene blue removal

before scavenging (Fig. 5a). At pH 8 (Fig. 5c), the hydrogel improved its surface homogeneity but changed shape. The same pattern occurred at pH 11 (Fig. 5d).

Methylene blue percentage removal. Figure 6 shows an increase in methylene blue scavenging at a basic pH value (pH 11) for 120 min. The increase could be explained by the more negative charge on the adsorbent surface, which generated a greater electrostatic attraction with the positively charged adsórbate [34]. Table 3 summarizes the methylene blue removal percentages at different pH levels and processing times.

Kinetic adsorption models. The pseudo-first-order, pseudo-second-order, and Elovich kinetic models were used to verify the experimental data (Fig. 7). Table 4 shows the values of the constants for the different kinetic models.

The pseudo-second-order model showed a higher R2 value in the scavenging processes at pH 8 and pH 11. On the other hand, the Elovich model demonstrated a higher value at pH 5. Apparently, the mechanism of methylene blue sorption at pH 8 and pH 11 was caused by chemisorption. At pH 5, the surface of the hydrogel was heterogeneous, which was in line with Fig. 5b [35].

Adsorption isotherm models. We employed the Langmuir, Freundlich, and Temkin isotherms to verify the experimental data (Fig. 8). Table 5 shows the values that correspond to the adsorption isotherm models.

Negative RL values were obtained from methylene blue removal at different pH values. At the three

a

b

a

d

c

Table 3 Methylene blue percentage removal and adsorption capacity

Time, min pH Initial methylene blue concentration, mg/L Adsorption capacity, mg/g Methylene blue removal, %

15 5 1.045x10-6 4.669x10-7 8.00

8 1.014x10-6 4.070x10-7 16.57

11 1.019x10-6 3.623x10-7 25.66

30 5 1.151x10-6 4.817x10-7 13.44

8 1.117x10-6 4.520x10-7 17.59

11 1.146x10-6 4.067x10-7 29.02

60 5 1.275x10-6 5.054x10-7 16.50

8 1.209x10-6 4.698x10-7 20.34

11 1.226x10-6 4.231x10-7 32.11

90 5 1.394x10-6 5.587x10-7 19.83

8 1.286x10-6 5.131x10-7 21.48

11 1.358x10-6 4.936x10-7 32.38

120 5 1.459x10-6 6.054x10-7 22.28

8 1.429x10-6 5.917x10-7 22.28

11 1.469x10-6 5.503x10-7 33.18

-14.50 -14.55 -14.60 -14.65 -14.70 -14.75

pH 5

20 40 60 80 100 120 Time (f), min

10s 8.0 6.0 4.0 2.0 0

pH 5

0 20 40 60 80 100 120 Time (f), min

10 1.6 1.4 1.2 1.0 0.8 0.6 0.4

pH 5

0 20 40 60 80 100 120 ln{t)

-14.80 \ pH 8 -15.15

d* -14.82 -15.20

3 -14.84 N. 1 -15.25

Ö -14.86 • N. Ö

i-i -15.30

-14.90

-14.92 -15.35

x

0 20 40 60 80 100 120 Time (f), min a

10s 10.0 8.0 6.0 4.0 2.0 0

pH 8

io-

1.4

1.3

1.2

1.1 1.0 0.95

0 20 40 60 80 100 120 Time (f), min b

pH 8

0 20 40 60 80 100 120 ln{t)

c

pH 11

0 20 40 60 80 100 120 Time (t), min

10s 7.0

5.0

3.0

1.0

0

pH 11

0 20 40 60 80 100 120 Time (f), min

102

2

2

1

pH 11

0 20 40 60 80 100 120 ln(t)

Figure 7 Linear plots for methylene blue adsorption: pseudo-first-order model (a); pseudo-second-order model (b); and Elovich kinetic model (c)

0

pH levels, the methylene blue removal did not fit the Langmuir isotherm [36, 37]. However, the adsorption intensity (1/n) in all the treatments was below one, which suggested that the active centers had less and less free enthalpy [38]. The bT values were negative, so

the adsorption process in all the treatments were en-dothermic [39]. The results indicated a good fit (R2) with the Temkin model; therefore, this model explained the adsorption process between the adsorbate and the adsorbent.

Herrera N.G. et al. Foods and Raw Materials. 2024;12(1):37-46 Table 4 Parameter values of methylene blue removal: kinetic studies

pH value Kinetic model R2 K, K2 a, mg/g/min ß, g/mg

pH 5 Pseudo-first-order 0.9331 -1.633x10-5 / / /

Pseudo-second-order 0.9257 / 3.120x105 / /

Elovich 0.9857 / / 2.180x107 1.059x10-8

pH 8 Pseudo-first-order 0.9324 -8.017x10-6 / / /

Pseudo-second-order 0.9942 / 1.516x106 / /

Elovich 0.9708 / / 5.715x107 3.147x10-7

pH 11 Pseudo-first-order 0.7549 -1.317x10-5 / / /

Pseudo-second-order 0.9980 / 1.724x106 / /

Elovich 0.9464 / / 4.824x107 1.911 x10-6

x106

17.5012.50-^ 12.50110.00 7.50 5.00

oo o h-1

x106

9.909.35-

pq 8.808.257.70 7.15

pH 8

-6.8-6.9-7.0-7.1-7.2 -7.3

8.19 8.58 8.97 9.36 1/C

pH 5

10 1.6. 1.41.2" o.i-o.i-0.1

-5.96 -5.94 -5.92 -5.90

L°g C

pH 5

1.0

1.0 1.0 1/C

e

a

-6.86

^-6.90

o°-6.94 h-l

-6.98 -7.00

1.1

pH 8

-6.027 -6.020 -6.006 -5.999 LogC

b

101.32 , 1-21 1.10

0.99

pH 8

x10-7 6.90 6.60 6.30 6.00 5.70 5.40 5.10

-6.72 -0^-6.76

pH 11

1.2 1.2 1.2 1/C

oo o

I -6.8

-6.84 -

1.3 1.3

pH 11

-6.12 -6.11 -6.10 -6.09 -6.08-6.07 LogC

pH 11

-13.72 -13.68 -13.64 -13.60. -13.56 LnC

-13.88 -13.86 -13.84 -13.82 -13.8 LnC

io-7

22222"

-14.10 -14.07 -14.04 -14.01 -13.9 LnC

Figure 8 Linear plots for methylene blue adsorption: Langmuir model (a); Freundlich model (b); and Temkin model (c) Table 5 Parameter values of methylene blue removal: adsorption studies

Type of isotherm Parameters Methylene blue removal

pH 5 pH 8 pH 11

Langmuir TL, L/mg -9.89X105 -1.273x106 -1.753x106

9max, mg/g 1.3ÜX10-8 2.234x10-8 4.898x10-8

RL -2.65 -1.86 -0.99

R2 0.8495 0.9912 0.9893

Freundlich mg/g 2.09X10-43 8.30x10-33 3.51x10-22

1/n -6.02 -4.18 -2.41

R2 0.9587 0.9978 0.9973

Temkin Km, L/g 7.08x105 8.07x105 8.15x105

J/mol -5.901x10-7 -4.871x10-7 -4.04x10-7

R2 0.9991 0.9999 0.9997

c

CONCLUSION

Exopolysaccharides from Nostoc commune V. yielded composite hydrogels that could act as methylene blue scavengers. These materials had a non-porous and heterogeneous surface, which underwent changes at basic pH values during the removal process. The methylene blue adsorption mechanism depended on chemisorption and endothermic processes. The maximal removal was 33.18%, which proved that these composite hydrogels were not efficient as methylene blue scavengers. The result open pros-

pects for further research of exopolysaccharides with other adsorption materials.

CONTRIBUTION

The authors were equally involved in writing the manuscript and are equally responsible for any potential plagiarism.

CONFLICT OF INTEREST

The authors declare no conflict of interests regarding the publication of this article.

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ORCID IDs

Nora Gabriela Herrera https://orcid.org/0000-0003-0595-8747 Nelson Adrián Villacrés https://orcid.org/0000-0001-9499-3792 Lizbeth Aymara https://orcid.org/0000-0001-9358-7688 Viviana Román https://orcid.org/0000-0002-6614-3632 Mayra Ramírez https://orcid.org/0000-0002-9143-4060