Die Angewandte Makromolekulare Chemie 201 (1993) 1-8 (Nr. 3445)

IMRE, University of Havana, 10400, Cuba Faculty of Chemistry, University of Havana, 10400, Cuba

Preparation and characterization of a mercaptan derivative of chitosan for the removal of mercury

from brines

W. Argiielles-Monal' , C. Peniche-Covas2*

(Received 25 September 1991)

SUMMARY: Chitosan, [( 1,4)-2-amin0-2-deoxy-b-D-glucan], was reacted consecutively with epi-

chlorohydrin and thiourea. Subsequent hydrolysis of the isothiouronium salt yielded the mercaptan derivative of chitosan, which scavenges mercury ions from solution in the presence of a considerably high concentration of chloride ions.

Sulfur contents varied with reacting conditions and reached values up to 7.3%. The derivative is insoluble due to partial crosslinking of chitosan during the reaction with epichlorohydrin and was characterized by IR spectroscopy, solid-state I3C NMR, thermal analysis and pyrolysis-mass spectrometry.

ZUSAMMENFASSUNG: Chitosan [( 1,4)-2-Amino-2-desoxy-~-D-glucan] wurde nacheinander mit Epichlor-

hydrin und Thioharnstoff umgesetzt. Durch nachfolgende Hydrolyse des Isothiuroni- umsalzes wurde das Mercaptoderivat des Chitosans erhalten, welches Quecksilberio- nen aus Wsungen in Gegenwart erheblicher Mengen von Chloridionen adsorbiert . Der Schwefelgehalt hangt von den Reaktionsbedingungen ab und erreicht bis zu 7,3%. Das Derivat ist unloslich, da das Chitosan wahrend der Reaktion mit Epichlorhydrin teil- weise vernetzt. Es wurde mittels IR- und Festphasen-'3C-NMR-Spektroskopie, thermi- scher Analyse und Pyrolyse-Massenspektroskopie charakterisiert.

Introduction

In the last decades there has been an increased interest in the preparation of polymers with a high capacity as scavengers for heavy metal ions so that they may be employed in the decontamination of industrial waste waters.

* Correspondence author.

0 1993 Hiithig & Wepf Verlag, Basel CCC OOO3-3146/93/$05.00 1

W. Arguelles-Monal, C. Peniche-Covas

Chitosan, the N-deacetylation product of chitin, is a chelating polymer which is very effective for the removal of transition and post-transition metal ions, mainly due to the regular distribution of its aliphatic primary amino groups along the chain'.

The high uptake capacity of chitosan for mercuric ions has already been reported2, but removal becomes inefficient in the presence of high concen- tration of chloride ions.

A number of chitosan derivatives have been prepared in order to enhance its chelating ability, among these N-carboxymethyl chitosan3 and chitosan dithi~carbamate~. The latter exhibits high adsorption capacity for mercury ions, even at elevated concentration of chlorides, but the interaction product decomposes according to the reaction:

S

R-NH-C-S-Hg+ (1) I1

R-N=C=S + HgS + H +

which constitutes a serious drawback for the application in columns. In the present work, thiol groups are introduced in the chitosan chain by

successively reacting the polysaccharide with epichlorohydrin and thiourea, followed by hydrolysis of the isothiouronium salt. The mercaptan derivative of chitosan thus obtained is characterized and its ability as scavenger of mercury ions is evaluated.

Experimental

Materials

Chitosan was obtained from shells of lobsters (Panulirus argus) by an established method5, and used without further purification. The polymer was 90% deacetylated as determined potentiometrically. All the other reagents were used as purchased.

Methods

The mercaptan derivative of chitosan was prepared in the following way: 2 g of chitosan were swollen in 30 ml of water for at least 4 h, after which 20 ml of ethanol and 6 ml of epichlorohydrin were added. The reaction mixture was stirred for several hours at 35 -40 O C , after which the solid was separated from the liquid and washed with ethanol. Thiourea (4.5 g) dissolved in an ethanol : water (40: 60, v/v) mixture was added

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Mercaptan derivative of chitosan

to the solid and the reaction proceeded steeply at 50 and 70°C for some hours. Hydrolysis of the isothiouronium salt was accomplished at 70 "C with 50 ml of aqueous 2.5 M NaOH. The product was washed with acetone, water, dimethylformamide and ethanol. NaN,-I, test was used to reveal the presence of divalent sulfur in the product6.

The ability of the products to adsorb mercury ions was calculated determining the metal ion concentration after 10 ml aqueous solution (8% chloride, 17 mg Hg2+ * 1-I) were shaken with 100 mg of the polymer during 3 h . The mercury ion concentration was measured by atomic absorption spectrometry in a Philips SP-8 equipment.

Thermal analyses were carried out in a MOM derivatograph at a heating rate of 10 K . min-' with 100 mg of sample placed in a platinum crucible under air.

Elemental analyses were performed in a Perkin-Elmer analyzer at the Institute of Macromolecular Chemistry of Prague. Cross-polarization-magic angle spinning I3C-NMR spectra of the solid probes were recorded in a Bruker CXP-200 spectrometer at 50.32 MHz , rotation frequency: 3.8-4 kHz. Spectral width and data points were 20 kHz and 8 K, respectively.

Pyrolysis-mass spectra were obtained in a MX-132 equipment by direct sample introduction and 70 eV ionization voltage (EI). IR spectra were registered in a Specord M-80 spectrophotometer.

Results and discussion

Epichlorohydrin has been frequently used as a crosslinking reagent for chitin and chitosan'? *. To this end, chitosan is treated with epichlorohydrin in strong alkaline medium, and the reaction proceeds through the hydroxy groups as well as the amino groups of the polysaccharide. The resulting epichloro- hydrin chitosan is insoluble in both acidic and basic media.

Nevertheless, if the reaction of epichlorohydrin with chitosan is carried out in a slightly basic, aqueous alcoholic medium, the reaction of the oxirane ring of epichlorohydrin with the primary amino groups of the polysaccharide:

OH 0 I

H2C L.2, CH-CH,Cl + R-NH, - R-NH-CH,-CH-CH2Cl(2)

is favoured9. The active chlorides attached to the polymer can then be converted into -SH groups by reaction with thiourea followed by hydrolysis of the isothiouronium salt, giving rise to the N-(2-hydroxy-3-mercaptopropyl)- chitosan.

Reaction conditions such as time, epichlorohydrin concentration and temperature were varied and their effect on the capacity of the resin to adsorb

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W. Argiielles-Monal, C. Peniche-Covas

mercury ions in the presence of a considerably high concentration of chloride ions was evaluated. The results are shown in Tab. 1.

The color of samples increased with the increase in sulfur content, going from pale yellow (M-1) to intense yellow (M-5). Sample M-5 exhibited the largest S/N ratio, as well as the highest capacity to scavenge Hg2+. Therefore, a further characterization of this sample was carried out.

Tab. 1 . Preparation of derivatives.

Probe ECHa Temp. Time S S/N Uptakeb ("C) (h) (Yo) (Yo)

M- 1 1.5 25 3 2.75 0.42 72 M-2 1.5 35 3 3.41 0.51 78 M-3 2.5 38 24 6.90 1.12 99.7 M-4 2.5 38 30 7.39 1.23 99.8 M-5 3.0 38 24 7.32 1.29 99.96

a Epichlorohydrin in ml per gram of chitosan. Percent of mercuric ions removed from solution.

The IR spectrum of the mercaptan derivative (Fig. 1) exhibits stronger absorption bands, in comparison with chitosan, around 3400 cm-' (0-H str.) and 2900 cm-I (C-H str.). There is also a significant increase in the intensities of the bands around 1030-1075 cm-' (C-0 str.). All these changes should be expected from the structure resulting from the addition reaction mentioned above.

Thermoanalytical curves of both chitosan and its derivative are shown in Fig. 2. It is clearly seen that the mercaptan derivative is thermally less stable than chitosan: While chitosan has a maximum rate of weight loss at 290"C, the mercaptan derivative exhibits the maximum decomposition rate at 238 "C followed by a smaller one at 290°C.

This behaviour could be a consequence of the heterogeneous conditions employed to prepare the derivative, giving rise to a structure resembling that of a block copolymer of substituted and non-substituted chitosan units, so that the effects at 290°C and 238°C are associated to sequences of glucosamine and substituted glucosamine moieties, respectively.

On the basis of the weight loss associated to each of the thermal effects, it can be calculated that 65% of the glucosamine units have been substituted, which is consistent with the composition found from elemental analysis:

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Mercaptan derivative of chitosan

MW 203.19 mol-Yo: 10 Average MW

161.15

214.93 35

CHzOH

HO -0, NH

H2C' >CH-OH I CH2-SH

251.29 55

4000 3000 2000- 1600 1200 800 400 v (cm-1)

Infrared spectra of (a) chitosan and (b) the mercaptan derivative. Fig. 1.

Fig. 3 shows the solid-state 13C-NMR spectra. Signals from the chitosan spectrum are unequivocally assigned, according to data previously reportedlo. However, the spectrum of the mercaptan derivative exhibits more wide signals and a diffuse region between 25 and 55 ppm.

The signal-broadening in the spectrum of the mercaptan derivative is attributed to partial crosslinking of chitosan during the reaction with epi- chlorohydrin, which is also responsible of the insolubility of the product. The observed diffuse patterns in the 25 to 55 ppm zone are to be expected from the proposed structure of the derivative, since in this region the signals due to the lateral groups (--CH2-N < and -CH,-S-) should appear.

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W. Arguelles-Monal, C. Peniche-Covas

30

Fig. 2. Thermoanalytical curves of the mercaptan derivative (-, heavy line) and chitosan (- ). The sample was placed in a platinum crucible under air and heated at 10 K * min-'.

1: ,A Chitosan

50 loo 6 (ppm) 200 150

I C1 d C 6 C 2 jl ,dk Chitosan

50 loo 6 (ppm) 200 150

Fig. 3. I3C CP-MAS NMR spectra (50.32 MHz) of the mercaptan derivative and chitosan.

The pyrolisis-mass spectra for both chitosan (reference peak at m/z = 42) and the mercaptan derivative (reference peak at m/z = 32) can be seen in

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Mercaptan derivative of chitosan

Fig. 4. Peaks below m/z = 30 were omitted. As in the thermal analysis, the lower stability of the derivative in comparison with chitosan is here also apparent.

100 100

5 )r Derivative c

50 50 + .- A

d

0 0 30 50 70 90,1z110 130 150 170 30 40 50 60m1z 70 80 90 100

Fig. 4. Pyrolysis-mass spectra of the mercaptan derivative (reference peak at m/z = 32) and chitosan (reference peak at m/z = 42).

In the mass spectrum of the mercaptan derivative, peaks at m/z = 55 and 67 are absent, while the intensities of peaks at m/z = 29, 59 and 80 decrease in comparison to chitosan. It is important to point out that the major contributors to all these peaks in the spectrum of chitosan are the fragments involving the primary amino groups of the glucosamine moieties".

DO

Fig. 5 . Breakthrough curve of the mercaptan resin with a mercury chloride solution (8% chloride, 17 mg Hg2+ el-') at a flow rate of 0.55 ml.min-'. Dimensions of column: 1.4 x 4 cm.

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W. Argiielles-Monal, C. Peniche-Covas

The peak at m/z = 34 is the highest in the spectrum of the mercaptan derivative. This peak is not observed for chitosan and is attributed to the elimination of H2S originated from the thiol groups present in the derivative. Moreover, the appearance of two new peaks at m/z = 47 and 76 in the spectrum of the derivative can be assigned to fragments CH,S and C2H40S.

The adsorption capacity of the mercaptan derivative was evaluated under dynamic conditions, using a column of dimensions 1.4 x 4 cm, fed with a mercury chloride solution (8% chloride, 17 mg Hg2+ * 1 - l ) at a flow rate of 0.55 ml- min-*. Results are illustrated in Fig. 5 .

The concentration of the efluent was lower than the limit of detection up to the breakthrough point. A capacity of 8 mg of mercury per ml of resin was calculated from the breakthrough curve. This result evidences the suitability of this product for the treatment of mercury-polluted water.

R. A. A. Muzzarelli, “Chitin”, Pergamon Press, Oxford 1977 R. A. A. Muzzarelli, A. Isolati, Water, Air, Soil Pollut. 1 (1971) 65 R. A. A. Muzzarelli, Carbohydr. Polym. 8 (1988) 1 R. A. A. Muzzarelli, F. lhnfani, S. Mariotti, M. Emanuelli, Carbohydr. Res. 104 (1982) 235 CU 35844 (1983), Invs.: I. Garcia, D. Oviedo, J. M. Nieto, C. Peniche, R. D. Henriques F. Feigl, “Spot Test”, Elsevier-Maruzen, Amsterdam 1961, Vol. 2, p. 242 J. Noguchi, Kogyo Kagaku Zasshi 72 (1969) 796 Jpn. 39322 (1971), Nippon Suisan Kaisha, Ltd., Inv.: H. Haga, C. A. 76 (1972) 100682m J. Kalal, F. Svec, V. Marougek, J. Polym. Sci., Polym. Symp. 47 (1974) 155

lo H. Saito, R. Tabeta, K. Ogawa, Macromolecules 20 (1987) 2424 ‘ I J. Mattai, E. R. Hayes, J. Anal. Appl. Pyrolysis 3 (1982) 327

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