Journal of Colloid and Interface Science 417 (2014) 131–136
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Simple preparation of aminothiourea-modified chitosan as corrosion inhibitor and heavy metal ion adsorbent Manlin Li a, Juan Xu a, Ronghua Li b, Dongen Wang a, Tianbao Li a, Maosen Yuan a, Jinyi Wang a,⇑ a b
College of Science, Northwest A&F University, Yangling, Shaanxi 712100, PR China College of Natural Resources and Environment, Northwest A&F University, Yangling, Shaanxi 712100, PR China
a r t i c l e
i n f o
Article history: Received 27 October 2013 Accepted 20 November 2013 Available online 26 November 2013 Keywords: Chitosan Modification Aminothiourea Inhibitor Adsorbent
a b s t r a c t By a simple and convenient method of using formaldehyde as linkages, two new chitosan (CS) derivatives modified respectively with thiosemicarbazide (TSFCS) and thiocarbohydrazide (TCFCS) were synthesized. The new compounds were characterized and studied by Fourier transform infrared spectroscopy, elemental analysis, thermal gravity analysis and differential scanning calorimetry, and their surface morphologies were determined via scanning electron microscopy. These CS derivatives could form pH dependent gels. The behavior of 304 steel in 2% acetic acid containing different inhibitors or different concentrations of inhibitor had been studied by potentiodynamic polarization test. The preliminary results show that the new compound TCFCS can act as a mixed-type metal anticorrosion inhibitor in some extent; its inhibition efficiency is 92% when the concentration was 60 mg/L. The adsorption studies on a metal ion mixture aqueous solution show that two samples TSFCS and TCFCS can absorb As (V), Ni (II), Cu (II), Cd (II) and Pb (II) efficiently at pH 9 and 4. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction As one of the most important and abundant natural polysaccharides next to cellulose and starch, chitosan (CS) has advantages of non-toxic, biocompatible and biodegradable, which is partially deacetylated derivatives obtained from chitin. This polysaccharide is composed of b-D-glucosamine and N-acetyl-b-D-glucosamine unit with a 1, 4-linkage. Deacetylated degree determines the amino amounts [1–3]. CS and its derivatives can be used as insecticidal [4], antifungal [5], antimicrobial materials [6] and anticoagulant [7]. In the field of tissue engineering, CS presents excellent biocompatibility [8] and enhances the biological performance of a biomaterial [9]. Some CS derivatives can act as potential carriers for gene transfection [10,11]. CS and their derivatives may be expansively utilized in many more fields [12,13]. Furthermore, the presence of a considerable percentage of free amine and hydroxyl groups on this natural polymer endows it anticorrosion properties [14] and good capability in the adsorption of pollutant [15–19]. CS is such an attractive material that many scientific teams have devoted themselves in searching new modification methods and new application of it. The key purpose of modification is to alter the functional groups of CS to match specific applications [20]. Modification procedures for CS were often handled at the two chemical active sites: hydroxy group at 6-site and amino group ⇑ Corresponding author. Fax: +86 29 87082520. E-mail address: [email protected] (J. Wang). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.11.053
at 2-site. In order to amplify its application capability, new derivatives containing novel functional groups in CS structure await to be synthesized efficiently [21]. Recently, the development of inhibitors or adsorbents has been the subject of great interest especially from the point of view of their efficiency and applications [22,23]. Compounds containing heteroatom in the conjugated system have been particularly reported as efficient corrosion inhibitors [22,24], and these kinds of compounds can be used as chelate sorbents in dealing with waste water [25]. Thiosemicarbazide (TS) and thiocarbohydrazide (TC) are two useful compounds containing N@C@S heteroatom conjugated structure. They were widely used in pharmaceutical and materials science as chemical intermediate [26–29]. The derivatives of them were also employed as corrosion inhibitor and metal ion adsorbents in treated with heavy metal pollutants [30–33]. Literature has reported that chitosan derivatives modified with TC could stabilize silver and copper nanoparticles in their matrix [34,35]. By the methods of using formaldehyde as linkages, adsorbents containing TS groups were synthesized [36], and water soluble CS derivatives could be synthesized smoothly [37]. Based on the analysis above, we attempted to find a simple and convenient approach to synthesize some new CS derivatives containing TS or TC groups to cater for specific applications such as anticorrosion and heavy metal sorbents. In this work, we chose formaldehyde as linkages for its cheapness and easy accessibility to get active intermediates, and then the two new CS derivatives
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M. Li et al. / Journal of Colloid and Interface Science 417 (2014) 131–136
S HO
HO
O
H2CO
O
N H
O
O HO
H2N
HO
NH2
N
S
or H2N N
CH2
H
O
HO
NH2
OH H2 C S N HN HN H N N H H OH
O
O
or
NH H2C
N H
OH
O HO
NH2
HO
S NH N H
NH2
TSFCS
H2 C N H
O O
TCFCS
Scheme 1. Synthesis of TSFCS and TCFCS by using formaldehyde as linkages.
solution (37–40%) was purchased from Xilong Chem. Co. Ltd. (Chengdu, China). Deionized (DI) water (Milli-Q, Millipore, Bedford, MA) was used to prepare aqueous solutions. A heavy metal mixture solution contained As (V) (2.0 mg/L), Ni (II) (2.0 mg/L), Cu (II) (1.6 mg/L), Cd (II) (0.8 mg/L), Zn (II) (0.8 mg/L) and Pb (II) (8.0 mg/L) was prepared by diluting the standard stock solution purchased from Sigma–Aldrich (St. Louis, MO, USA). 2.2. Synthesis of modified CS 2.2.1. Synthesis of TS modified chitosan (TSFCS) Modification of CS with TS was accomplished by using formaldehyde as coupling link reagent. In a 100-mL round bottomed flask, CS (0.8 g) was added into a solution of 10 mL of ice acetic acid diluted in 30 mL DI water to form viscous solution. Subsequently, TS (0.45 g, 1 mol equivalent to pyranose ring) was added and stirred until the mixture changed to clear. After about 45 min, formaldehyde (1 mL) was added and mixed thoroughly for 12 h at room temperature to get gel like mixture. The obtained product TSFCS was neutralized with aqueous NaOH to form precipitates. The precipitates were filtrated and washed with DI water and ethanol several times, and then dried under vacuum to give TSFCS as pale powder.
Fig. 1. Comparative FT-IR spectra of CS, TCFCS and TSFCS.
Table 1 Elemental analysis results of TSFCS TCFCS and CS. Sample
TSFCS TCFCS CS
Elemental analysis (%)
C/N
C
H
N
S
36.23 37.62 44.98
5.70 6.12 6.79
15.07 12.69 8.52
5.36 2.37 –
2.40 2.96 5.28
were synthesized and characterized. The preliminary tests of anticorrosion and heavy metal adsorption were also investigated. 2. Experimental 2.1. Materials All compounds were of analytical grade and used as received. Chitosan (CS) was purchased from Sinopharm Chemical Reagents Co. Ltd. (Shanghai, China; the deacetylation degree was 90%). Thiosemicarbazide (TS) and thiocarbohydrazide (TC) were purchased from Solarbio Sci. & Tech. Ltd. (Beijing, China). Formaldehyde
2.2.2. Synthesis of TC modified CS (TCFCS) TCFCS was synthesized using CS (0.8 g) with TC (0.53 g, 0.5 mol equivalent to pyranose ring), formaldehyde (2 mL) as the reactants by the same methods mentioned above to give product as rufous powder. 2.3. Characterization The Fourier transform infrared spectra (FT-IR) of CS, TSFCS and TCFCS were recorded in powder form by using a Nicolet 5700 instrument (Thermo Company, USA) over the wave number range of 4000–400 cm1 in KBr disks. Software OPUS viewer from Bruker Optics was used to analyze the spectra. The elemental analysis (C, H, N and S) was performed on a Thermo Scientific FLASH 2000 organic elemental analyzer (Thermo Fisher, Italy). Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed with a TGA/DSCI analyzer (Mettler, Switzerland) between 20 and 500 °C with a 10 °C/min heating rate under a nitrogen flow rate of 20.0 mL/min. The samples were put into
M. Li et al. / Journal of Colloid and Interface Science 417 (2014) 131–136
133
Fig. 2. Gel formation of CS, TSFCS and TCFCS in 10 mg/mL 2% HAc aqueous solutions: (a) after adding 1 mL 2% HAc aqueous solutions; (b) simultaneous inversion tests; (c) after 30 min.
at about 30 °C at different periods of time. According to the method reported previously [38], samples were prepared and tested. The swelling ratio of these samples was calculated by using the equation as follows:
Swelling ratio ¼
Ws Wd Wd
ð1Þ
where Ws and Wd are the weights of the swollen and dry samples, respectively.
2.6. Anticorrosion tests Fig. 3. TGA thermographs for TCFCS and TSFCS.
The electrochemical measurements were employed for the preliminary tests of anticorrosion. The electrochemical measurements were performed in a typical three-compartment glass cell consisted a working electrode (WE), a platinum wire as the counter electrode (PE) and a saturated calomel electrode (SCE) as the reference electrode (RE). The electrochemical experiments were performed using a CH1660 electrochemical station (CH instrument Co., Shanghai, China). For potentiodynamic polarization experiments, the potential was scanned from 0.7 to 0.6 V with scan rate of 1.0 mV s1. Three hundred and four steel sheets as working electrodes for electrochemical measurements were prepared from olefin sealed metal sheets with an exposed total area of 1 cm2. Prior to the experiments, the exposed metal surfaces were abraded smoothly with a series of emery papers of different grades (1200–2000), degreased with AR-grade ethanol and acetone, rinsed with DI water and then dried in air [14]. Acetic acid (2% HAc) solutions were prepared by dilution of analytical grade ice acetic acid with DI water.
Fig. 4. DSC curves for TCFCS and TSFCS.
aluminum cup and sealed. An empty pan was used as reference in the test. 2.4. Surface morphology Images of the surface morphology of the products were observed using an S-4800 (Hitachi, Japan) field emission scanning electron microscope (SEM) method. Samples were sputter-coated with gold and were imaged. 2.5. Swelling studies To measure the swelling properties, samples TSFCS and TCFCS were studied by immersing in the pH controlled (pH = 6) DI water
2.7. Batch adsorption studies Batch adsorption experiments were conducted to examine the properties of samples TSFCS and TCFCS in removing heavy metal ions from aqueous solutions. In a typical experiment, 50 mg of sample was added to a plastic centrifuge tube containing 10 mL of heavy metal ion mixture solution diluted from a standard stock solution. pH of the solution was then adjusted with 0.1 mol/L or 0.01 mol/L HNO3 and NaOH; the test tube was kept standing for 36 h at room temperature. At the end of the experiment, sample was filtrated through a 0.10 lm membrane filter and concentration of heavy metal ions in the filtrate was analyzed with ICP–MS (Agilent 7700, Agi. tech., US). The heavy metal ion removal by adsorption of the sample was calculated by mass balance as of the concentration of heavy metal ion before and after adsorption.
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M. Li et al. / Journal of Colloid and Interface Science 417 (2014) 131–136
Fig. 5. SEM micrographs of (a) CS, (b) TCFCS, (c) TSFCS and (d) nanoparticles of TCFCS.
Fig. 6. Swelling ratios of TSFCS and TCFCS at different times with a controlled pH 6.
3. Results and discussion 3.1. Synthesis and characterization The synthesis of modified products TSFCS and TCFCS was outlined as Scheme 1. The reactions were performed in aqueous acetic acid, since both raw materials were easily dissolved in acid aqueous conditions. Schiff’s base intermediates were produced by free amino groups of aminoglucose units of deacetyl CS with formaldehyde, and readily undergo addition reaction with the amino groups of TS or TC [36]. Since two amino groups locating on both ends of TC could separately react with two different intermediates of Schiff’s bases on CS, a cross-linking structure of TCFCS will be formed. This phenomenon could be observed at the end of the reaction. At this time, the reactants changed from viscous solution to gel. The FT-IR of CS and products TSFCS and TCFCS are shown in Fig. 1. The TCFCS and TSFCS show all characteristic peaks of CS along with two new peaks. The new peak appears at about
Fig. 7. Potentiodynamic polarization curves for the corrosion of 304 steel in 2% HAc at 30 ± 2 °C: (a) in the absence and presence of different inhibitors (30 mg/L); (b) in the absence and presence of different concentrations of inhibitor TCFCS.
1545 cm1 is attributed to thiourea group [39]. The peak appears at about 1250 cm1 should be attributed to C@S bonds’ stretching vibration [40]. The formation of TSFCS and TCFCS was further
M. Li et al. / Journal of Colloid and Interface Science 417 (2014) 131–136 Table 2 Corrosion parameters calculated from polarization measurements for 304 steel in 2% HAc at 30 ± 2 °C. Samples
Ecorr /mV
icorr/ (lA/cm2)
ba (mv dec1)
bc (mv dec1)
IE/%
(a) Blank CS TSFCS TCFCS
274 205 202 174
5.824 2.930 2.602 0.8167
398 390 192 481
741 590 409 801
– 50 55 86
(b) 20 mg/L 30 mg/L 50 mg/L 60 mg/L
197 174 155 132
2.773 0.8167 0.5824 0.4659
379 481 395 393
541 801 692 832
52.3 86 90 92
(a) In the absence and presence of different inhibitors (30 mg/L); (b) In the presence of different concentrations of inhibitor TCFCS.
135
different cross-linking degrees of CS, TSFCS and TCFCS and this result could further prove the successful synthesis of target products. TGA curves of TSFCS and TCFCS are presented in Fig. 3. The TGA curves show two different stages of weight loss. The first weight losses at about 80 °C and 70 °C are mainly due to the evaporation of water and moisture content in the structure of TCFCS and TSFCS, respectively. The second stages of weight losses at about 293 °C and 278 °C for TCFCS and TSFCS are might due to the TC and TS groups in polymer. DSC thermograms of both TCFCS and TSFCS are shown in Fig. 4. DSC curves of TCFCS show two endothermic peaks at 86 °C (Tg) and 195 °C (Tm) and two exothermic peaks at 220 °C and 269 °C. The DSC thermograms of TSFCS show two endothermic peaks at 73 °C (Tg) and 177 °C (Tm) and one exothermic peak at 193 °C. Based on the DSC and TGA analysis, TCFCS has higher thermal stability than TSFCS since the former has higher thermal decomposition temperature than the latter. 3.2. Surface morphology The SEM images showed different surfaces of CS, TCFCS and TSFCS in Fig. 5. CS had a nonporous and flat lamellar phase surface (Fig. 5a), while TCFCS and TSFCS had a complicated three dimensional structures with the highly porous (Figs. 5b and c). Moreover, the Nanoparticles could be prepared easily by the dilute aqueous solution (pH 4–5) of TCFCS (Fig. 5d). 3.3. Swelling studies Both TSFCS and TCFCS were insoluble in organic solvents but easily dissolved in an acidic aqueous solution due to the protonation of amino groups at C-2 position in CS structure along with TS and TC groups. In comparison with CS, modified products were hard to dissolve. The possible reason may be that the introduction of TS or TC to CS structure does not reduce the number of free amino groups but increase the strong interand intra-hydrogen bonding [41]. The swelling ratio of TSFCS and TCFCS, however, depended on pH value as that of pure CS. Fig. 6 shows the swelling ratio change in TSFCS and TCFCS with time. In a solution with a controlled pH 6.0, the swelling ratios (g/g) increased with the time prolonging and reached equilibrium within 5 h. 3.4. Anticorrosion tests
Fig. 8. The heavy metal ion adsorption at different pH by (a) TSFCS and (b)TCFCS.
confirmed by elemental analysis and the results are listed in Table 1. As shown in Table 1, the changing trends of the C/N values which represented the different degrees of substitution of CS accorded with the different chemical structures of products. Moreover, the results showed that S element was detected in both products, implying the formation of products. The impressive gelation ability of products was observed in acidic aqueous solution. As a general procedure, equal quality of CS, TSFCS and TCFCS were separately added with equal volume of 2% acetic acid solution. TCFCS could form a gel immediately while CS and TSFCS kept their fluidity (Figs. 2a and b). After 30 min, the vessel of TSFCS was turned upside down and the fluidity was absent (Fig. 2c). This phenomenon might be caused by the
Previous studies have reported that CS can be used as a copper corrosion inhibitor in acid medium [14]. Hence, CS was employed as reference to evaluate the anticorrosion capability of the prepared products. Potentiodynamic anodic and cathodic polarization scans were firstly carried out at 30 ± 2 °C in 2% HAc solution with different inhibitors. Then, the more effective inhibitor among them will be chosen as a target for the further study. Fig. 7 presents the cathodic and anodic polarization curves of various inhibitors and the polarization curves in the absence and in the presence of different concentrations of TCFCS, in which a decrease in both cathodic and anodic currents appeared. The corrosion potentials shifted to more positive values with the change in inhibitors from CS to TSFCS to TCFCS or by addition of inhibitor TCFCS. These phenomena indicate that the inhibitors have a strong influence on the steel dissolution or hydrogen evolution reaction. Electrochemical parameters including corrosion potential (Ecorr) and corrosion current density (icorr) determined by extrapolation of the Tafel curves to the open circuit corrosion potentials (OCP) are summarized as Table 2. The inhibition efficiency (IE) of each sample was then calculated by using the equation as follows:
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IEð%Þ ¼
M. Li et al. / Journal of Colloid and Interface Science 417 (2014) 131–136
icorr iinh 100 icorr
ð2Þ
where icorr and iinh are the corrosion current densities without and with different inhibitors or different concentrations of inhibitor, respectively [24]. The results of IE reveal that CS, TSFCS and TCFCS can used as inhibitors for 304 steel in acidic medium, and TCFCS is more effective (Table 2a). Furthermore, with the increase in TCFCS concentration, the inhibition efficiencies can increase up to 92% at 60 mg/L (Table 2b). Moreover, the values of Ecorr changing less than 85 mv mean that inhibitors must be classified as mixed inhibitors that act preferably on the anodic site [24,42,43]. 3.5. Batch adsorption studies The preliminary batch adsorption experiments were conducted to examine the adsorption properties of samples TSFCS and TCFCS. Adsorption percentages for each metal ion in the sample were established by using the equation as follows:
Að%Þ ¼
C0 Ce 100 C0
ð3Þ
in which, C0 and Ce are the initial and equilibrium concentrations (mg/L) for each metal cation in solution, respectively [44]. To the target adsorbents TSFCS and TCFCS, the main functional groups are amino at 2-sites of CS and amino along with thiourea groups of TS or TC. The pH of solution was known as one of the most important variables affecting the adsorption amount of heavy metal ions. It could influence the protonation of the functional groups on the adsorbents as well as the solution chemistry of the heavy metal ions. Adsorption histograms of As (V), Ni (II), Cu (II), Cd (II) and Pb (II) on the two samples TSFCS and TCFCS at pH 4.0, 7.0 and 9.0 are shown in Fig. 8. As shown in Fig. 8, it is evident that both TSFCS and TCFCS cannot adsorb Zn (II) ion; the removal of the As (V), Ni (II), Cu (II), Cd (II) and Pb (II) are around 66.4–99.9%, and 71.5– 99.9% for the two adsorbents, respectively. The removal of the As (V), Ni (II), Cu (II), Cd (II) and Pb (II) at pH 9 and 4 is more effective than that at pH 7. These results partially accorded with conclusions available in literature [44–46], and might be caused by the introduction of chelating active groups TS or TC in CS structures [39]. In general, it seemed that the adsorption of TSFCS and TCFCS for metal ions was effective in some extent. This might can be explained by the fact that the metal ion adsorption strongly depends on the structures of the adsorbents since the two samples TSFCS and TCFCS have many thiourea and amino groups contained in the structures. In order to clarify the adsorption mechanism and adsorption capacity, the further study need to be conducted.
4. Conclusions A convenient and simple method for the synthesis of aminothiourea-modified chitosan derivatives has been developed by using formaldehyde as linkages. Using this method, two target compounds TCFCS and TSFCS were synthesized and characterized. This one pot approach has advantages of saving time, ease of work up, avoiding solvent and energy consuming procedures during the isolation and purification of intermediates. These CS derivatives can form pH dependent gels and act as effective corrosion inhibitors as well as heavy metal ion adsorbents. All the results imply these two products TSFCS and TCFCS might be having potential applications in anticorrosion and treatment of heavy metal ion contamination.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Simple preparation of aminothiourea-modified chitosan as corrosion inhibitor and heavy metal ion adsorbent Manlin Li a, Juan Xu a, Ronghua Li b, Dongen Wang a, Tianbao Li a, Maosen Yuan a, Jinyi Wang a,⇑ a b
College of Science, Northwest A&F University, Yangling, Shaanxi 712100, PR China College of Natural Resources and Environment, Northwest A&F University, Yangling, Shaanxi 712100, PR China
a r t i c l e
i n f o
Article history: Received 27 October 2013 Accepted 20 November 2013 Available online 26 November 2013 Keywords: Chitosan Modification Aminothiourea Inhibitor Adsorbent
a b s t r a c t By a simple and convenient method of using formaldehyde as linkages, two new chitosan (CS) derivatives modified respectively with thiosemicarbazide (TSFCS) and thiocarbohydrazide (TCFCS) were synthesized. The new compounds were characterized and studied by Fourier transform infrared spectroscopy, elemental analysis, thermal gravity analysis and differential scanning calorimetry, and their surface morphologies were determined via scanning electron microscopy. These CS derivatives could form pH dependent gels. The behavior of 304 steel in 2% acetic acid containing different inhibitors or different concentrations of inhibitor had been studied by potentiodynamic polarization test. The preliminary results show that the new compound TCFCS can act as a mixed-type metal anticorrosion inhibitor in some extent; its inhibition efficiency is 92% when the concentration was 60 mg/L. The adsorption studies on a metal ion mixture aqueous solution show that two samples TSFCS and TCFCS can absorb As (V), Ni (II), Cu (II), Cd (II) and Pb (II) efficiently at pH 9 and 4. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction As one of the most important and abundant natural polysaccharides next to cellulose and starch, chitosan (CS) has advantages of non-toxic, biocompatible and biodegradable, which is partially deacetylated derivatives obtained from chitin. This polysaccharide is composed of b-D-glucosamine and N-acetyl-b-D-glucosamine unit with a 1, 4-linkage. Deacetylated degree determines the amino amounts [1–3]. CS and its derivatives can be used as insecticidal [4], antifungal [5], antimicrobial materials [6] and anticoagulant [7]. In the field of tissue engineering, CS presents excellent biocompatibility [8] and enhances the biological performance of a biomaterial [9]. Some CS derivatives can act as potential carriers for gene transfection [10,11]. CS and their derivatives may be expansively utilized in many more fields [12,13]. Furthermore, the presence of a considerable percentage of free amine and hydroxyl groups on this natural polymer endows it anticorrosion properties [14] and good capability in the adsorption of pollutant [15–19]. CS is such an attractive material that many scientific teams have devoted themselves in searching new modification methods and new application of it. The key purpose of modification is to alter the functional groups of CS to match specific applications [20]. Modification procedures for CS were often handled at the two chemical active sites: hydroxy group at 6-site and amino group ⇑ Corresponding author. Fax: +86 29 87082520. E-mail address: [email protected] (J. Wang). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.11.053
at 2-site. In order to amplify its application capability, new derivatives containing novel functional groups in CS structure await to be synthesized efficiently [21]. Recently, the development of inhibitors or adsorbents has been the subject of great interest especially from the point of view of their efficiency and applications [22,23]. Compounds containing heteroatom in the conjugated system have been particularly reported as efficient corrosion inhibitors [22,24], and these kinds of compounds can be used as chelate sorbents in dealing with waste water [25]. Thiosemicarbazide (TS) and thiocarbohydrazide (TC) are two useful compounds containing N@C@S heteroatom conjugated structure. They were widely used in pharmaceutical and materials science as chemical intermediate [26–29]. The derivatives of them were also employed as corrosion inhibitor and metal ion adsorbents in treated with heavy metal pollutants [30–33]. Literature has reported that chitosan derivatives modified with TC could stabilize silver and copper nanoparticles in their matrix [34,35]. By the methods of using formaldehyde as linkages, adsorbents containing TS groups were synthesized [36], and water soluble CS derivatives could be synthesized smoothly [37]. Based on the analysis above, we attempted to find a simple and convenient approach to synthesize some new CS derivatives containing TS or TC groups to cater for specific applications such as anticorrosion and heavy metal sorbents. In this work, we chose formaldehyde as linkages for its cheapness and easy accessibility to get active intermediates, and then the two new CS derivatives
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S HO
HO
O
H2CO
O
N H
O
O HO
H2N
HO
NH2
N
S
or H2N N
CH2
H
O
HO
NH2
OH H2 C S N HN HN H N N H H OH
O
O
or
NH H2C
N H
OH
O HO
NH2
HO
S NH N H
NH2
TSFCS
H2 C N H
O O
TCFCS
Scheme 1. Synthesis of TSFCS and TCFCS by using formaldehyde as linkages.
solution (37–40%) was purchased from Xilong Chem. Co. Ltd. (Chengdu, China). Deionized (DI) water (Milli-Q, Millipore, Bedford, MA) was used to prepare aqueous solutions. A heavy metal mixture solution contained As (V) (2.0 mg/L), Ni (II) (2.0 mg/L), Cu (II) (1.6 mg/L), Cd (II) (0.8 mg/L), Zn (II) (0.8 mg/L) and Pb (II) (8.0 mg/L) was prepared by diluting the standard stock solution purchased from Sigma–Aldrich (St. Louis, MO, USA). 2.2. Synthesis of modified CS 2.2.1. Synthesis of TS modified chitosan (TSFCS) Modification of CS with TS was accomplished by using formaldehyde as coupling link reagent. In a 100-mL round bottomed flask, CS (0.8 g) was added into a solution of 10 mL of ice acetic acid diluted in 30 mL DI water to form viscous solution. Subsequently, TS (0.45 g, 1 mol equivalent to pyranose ring) was added and stirred until the mixture changed to clear. After about 45 min, formaldehyde (1 mL) was added and mixed thoroughly for 12 h at room temperature to get gel like mixture. The obtained product TSFCS was neutralized with aqueous NaOH to form precipitates. The precipitates were filtrated and washed with DI water and ethanol several times, and then dried under vacuum to give TSFCS as pale powder.
Fig. 1. Comparative FT-IR spectra of CS, TCFCS and TSFCS.
Table 1 Elemental analysis results of TSFCS TCFCS and CS. Sample
TSFCS TCFCS CS
Elemental analysis (%)
C/N
C
H
N
S
36.23 37.62 44.98
5.70 6.12 6.79
15.07 12.69 8.52
5.36 2.37 –
2.40 2.96 5.28
were synthesized and characterized. The preliminary tests of anticorrosion and heavy metal adsorption were also investigated. 2. Experimental 2.1. Materials All compounds were of analytical grade and used as received. Chitosan (CS) was purchased from Sinopharm Chemical Reagents Co. Ltd. (Shanghai, China; the deacetylation degree was 90%). Thiosemicarbazide (TS) and thiocarbohydrazide (TC) were purchased from Solarbio Sci. & Tech. Ltd. (Beijing, China). Formaldehyde
2.2.2. Synthesis of TC modified CS (TCFCS) TCFCS was synthesized using CS (0.8 g) with TC (0.53 g, 0.5 mol equivalent to pyranose ring), formaldehyde (2 mL) as the reactants by the same methods mentioned above to give product as rufous powder. 2.3. Characterization The Fourier transform infrared spectra (FT-IR) of CS, TSFCS and TCFCS were recorded in powder form by using a Nicolet 5700 instrument (Thermo Company, USA) over the wave number range of 4000–400 cm1 in KBr disks. Software OPUS viewer from Bruker Optics was used to analyze the spectra. The elemental analysis (C, H, N and S) was performed on a Thermo Scientific FLASH 2000 organic elemental analyzer (Thermo Fisher, Italy). Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed with a TGA/DSCI analyzer (Mettler, Switzerland) between 20 and 500 °C with a 10 °C/min heating rate under a nitrogen flow rate of 20.0 mL/min. The samples were put into
M. Li et al. / Journal of Colloid and Interface Science 417 (2014) 131–136
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Fig. 2. Gel formation of CS, TSFCS and TCFCS in 10 mg/mL 2% HAc aqueous solutions: (a) after adding 1 mL 2% HAc aqueous solutions; (b) simultaneous inversion tests; (c) after 30 min.
at about 30 °C at different periods of time. According to the method reported previously [38], samples were prepared and tested. The swelling ratio of these samples was calculated by using the equation as follows:
Swelling ratio ¼
Ws Wd Wd
ð1Þ
where Ws and Wd are the weights of the swollen and dry samples, respectively.
2.6. Anticorrosion tests Fig. 3. TGA thermographs for TCFCS and TSFCS.
The electrochemical measurements were employed for the preliminary tests of anticorrosion. The electrochemical measurements were performed in a typical three-compartment glass cell consisted a working electrode (WE), a platinum wire as the counter electrode (PE) and a saturated calomel electrode (SCE) as the reference electrode (RE). The electrochemical experiments were performed using a CH1660 electrochemical station (CH instrument Co., Shanghai, China). For potentiodynamic polarization experiments, the potential was scanned from 0.7 to 0.6 V with scan rate of 1.0 mV s1. Three hundred and four steel sheets as working electrodes for electrochemical measurements were prepared from olefin sealed metal sheets with an exposed total area of 1 cm2. Prior to the experiments, the exposed metal surfaces were abraded smoothly with a series of emery papers of different grades (1200–2000), degreased with AR-grade ethanol and acetone, rinsed with DI water and then dried in air [14]. Acetic acid (2% HAc) solutions were prepared by dilution of analytical grade ice acetic acid with DI water.
Fig. 4. DSC curves for TCFCS and TSFCS.
aluminum cup and sealed. An empty pan was used as reference in the test. 2.4. Surface morphology Images of the surface morphology of the products were observed using an S-4800 (Hitachi, Japan) field emission scanning electron microscope (SEM) method. Samples were sputter-coated with gold and were imaged. 2.5. Swelling studies To measure the swelling properties, samples TSFCS and TCFCS were studied by immersing in the pH controlled (pH = 6) DI water
2.7. Batch adsorption studies Batch adsorption experiments were conducted to examine the properties of samples TSFCS and TCFCS in removing heavy metal ions from aqueous solutions. In a typical experiment, 50 mg of sample was added to a plastic centrifuge tube containing 10 mL of heavy metal ion mixture solution diluted from a standard stock solution. pH of the solution was then adjusted with 0.1 mol/L or 0.01 mol/L HNO3 and NaOH; the test tube was kept standing for 36 h at room temperature. At the end of the experiment, sample was filtrated through a 0.10 lm membrane filter and concentration of heavy metal ions in the filtrate was analyzed with ICP–MS (Agilent 7700, Agi. tech., US). The heavy metal ion removal by adsorption of the sample was calculated by mass balance as of the concentration of heavy metal ion before and after adsorption.
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Fig. 5. SEM micrographs of (a) CS, (b) TCFCS, (c) TSFCS and (d) nanoparticles of TCFCS.
Fig. 6. Swelling ratios of TSFCS and TCFCS at different times with a controlled pH 6.
3. Results and discussion 3.1. Synthesis and characterization The synthesis of modified products TSFCS and TCFCS was outlined as Scheme 1. The reactions were performed in aqueous acetic acid, since both raw materials were easily dissolved in acid aqueous conditions. Schiff’s base intermediates were produced by free amino groups of aminoglucose units of deacetyl CS with formaldehyde, and readily undergo addition reaction with the amino groups of TS or TC [36]. Since two amino groups locating on both ends of TC could separately react with two different intermediates of Schiff’s bases on CS, a cross-linking structure of TCFCS will be formed. This phenomenon could be observed at the end of the reaction. At this time, the reactants changed from viscous solution to gel. The FT-IR of CS and products TSFCS and TCFCS are shown in Fig. 1. The TCFCS and TSFCS show all characteristic peaks of CS along with two new peaks. The new peak appears at about
Fig. 7. Potentiodynamic polarization curves for the corrosion of 304 steel in 2% HAc at 30 ± 2 °C: (a) in the absence and presence of different inhibitors (30 mg/L); (b) in the absence and presence of different concentrations of inhibitor TCFCS.
1545 cm1 is attributed to thiourea group [39]. The peak appears at about 1250 cm1 should be attributed to C@S bonds’ stretching vibration [40]. The formation of TSFCS and TCFCS was further
M. Li et al. / Journal of Colloid and Interface Science 417 (2014) 131–136 Table 2 Corrosion parameters calculated from polarization measurements for 304 steel in 2% HAc at 30 ± 2 °C. Samples
Ecorr /mV
icorr/ (lA/cm2)
ba (mv dec1)
bc (mv dec1)
IE/%
(a) Blank CS TSFCS TCFCS
274 205 202 174
5.824 2.930 2.602 0.8167
398 390 192 481
741 590 409 801
– 50 55 86
(b) 20 mg/L 30 mg/L 50 mg/L 60 mg/L
197 174 155 132
2.773 0.8167 0.5824 0.4659
379 481 395 393
541 801 692 832
52.3 86 90 92
(a) In the absence and presence of different inhibitors (30 mg/L); (b) In the presence of different concentrations of inhibitor TCFCS.
135
different cross-linking degrees of CS, TSFCS and TCFCS and this result could further prove the successful synthesis of target products. TGA curves of TSFCS and TCFCS are presented in Fig. 3. The TGA curves show two different stages of weight loss. The first weight losses at about 80 °C and 70 °C are mainly due to the evaporation of water and moisture content in the structure of TCFCS and TSFCS, respectively. The second stages of weight losses at about 293 °C and 278 °C for TCFCS and TSFCS are might due to the TC and TS groups in polymer. DSC thermograms of both TCFCS and TSFCS are shown in Fig. 4. DSC curves of TCFCS show two endothermic peaks at 86 °C (Tg) and 195 °C (Tm) and two exothermic peaks at 220 °C and 269 °C. The DSC thermograms of TSFCS show two endothermic peaks at 73 °C (Tg) and 177 °C (Tm) and one exothermic peak at 193 °C. Based on the DSC and TGA analysis, TCFCS has higher thermal stability than TSFCS since the former has higher thermal decomposition temperature than the latter. 3.2. Surface morphology The SEM images showed different surfaces of CS, TCFCS and TSFCS in Fig. 5. CS had a nonporous and flat lamellar phase surface (Fig. 5a), while TCFCS and TSFCS had a complicated three dimensional structures with the highly porous (Figs. 5b and c). Moreover, the Nanoparticles could be prepared easily by the dilute aqueous solution (pH 4–5) of TCFCS (Fig. 5d). 3.3. Swelling studies Both TSFCS and TCFCS were insoluble in organic solvents but easily dissolved in an acidic aqueous solution due to the protonation of amino groups at C-2 position in CS structure along with TS and TC groups. In comparison with CS, modified products were hard to dissolve. The possible reason may be that the introduction of TS or TC to CS structure does not reduce the number of free amino groups but increase the strong interand intra-hydrogen bonding [41]. The swelling ratio of TSFCS and TCFCS, however, depended on pH value as that of pure CS. Fig. 6 shows the swelling ratio change in TSFCS and TCFCS with time. In a solution with a controlled pH 6.0, the swelling ratios (g/g) increased with the time prolonging and reached equilibrium within 5 h. 3.4. Anticorrosion tests
Fig. 8. The heavy metal ion adsorption at different pH by (a) TSFCS and (b)TCFCS.
confirmed by elemental analysis and the results are listed in Table 1. As shown in Table 1, the changing trends of the C/N values which represented the different degrees of substitution of CS accorded with the different chemical structures of products. Moreover, the results showed that S element was detected in both products, implying the formation of products. The impressive gelation ability of products was observed in acidic aqueous solution. As a general procedure, equal quality of CS, TSFCS and TCFCS were separately added with equal volume of 2% acetic acid solution. TCFCS could form a gel immediately while CS and TSFCS kept their fluidity (Figs. 2a and b). After 30 min, the vessel of TSFCS was turned upside down and the fluidity was absent (Fig. 2c). This phenomenon might be caused by the
Previous studies have reported that CS can be used as a copper corrosion inhibitor in acid medium [14]. Hence, CS was employed as reference to evaluate the anticorrosion capability of the prepared products. Potentiodynamic anodic and cathodic polarization scans were firstly carried out at 30 ± 2 °C in 2% HAc solution with different inhibitors. Then, the more effective inhibitor among them will be chosen as a target for the further study. Fig. 7 presents the cathodic and anodic polarization curves of various inhibitors and the polarization curves in the absence and in the presence of different concentrations of TCFCS, in which a decrease in both cathodic and anodic currents appeared. The corrosion potentials shifted to more positive values with the change in inhibitors from CS to TSFCS to TCFCS or by addition of inhibitor TCFCS. These phenomena indicate that the inhibitors have a strong influence on the steel dissolution or hydrogen evolution reaction. Electrochemical parameters including corrosion potential (Ecorr) and corrosion current density (icorr) determined by extrapolation of the Tafel curves to the open circuit corrosion potentials (OCP) are summarized as Table 2. The inhibition efficiency (IE) of each sample was then calculated by using the equation as follows:
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IEð%Þ ¼
M. Li et al. / Journal of Colloid and Interface Science 417 (2014) 131–136
icorr iinh 100 icorr
ð2Þ
where icorr and iinh are the corrosion current densities without and with different inhibitors or different concentrations of inhibitor, respectively [24]. The results of IE reveal that CS, TSFCS and TCFCS can used as inhibitors for 304 steel in acidic medium, and TCFCS is more effective (Table 2a). Furthermore, with the increase in TCFCS concentration, the inhibition efficiencies can increase up to 92% at 60 mg/L (Table 2b). Moreover, the values of Ecorr changing less than 85 mv mean that inhibitors must be classified as mixed inhibitors that act preferably on the anodic site [24,42,43]. 3.5. Batch adsorption studies The preliminary batch adsorption experiments were conducted to examine the adsorption properties of samples TSFCS and TCFCS. Adsorption percentages for each metal ion in the sample were established by using the equation as follows:
Að%Þ ¼
C0 Ce 100 C0
ð3Þ
in which, C0 and Ce are the initial and equilibrium concentrations (mg/L) for each metal cation in solution, respectively [44]. To the target adsorbents TSFCS and TCFCS, the main functional groups are amino at 2-sites of CS and amino along with thiourea groups of TS or TC. The pH of solution was known as one of the most important variables affecting the adsorption amount of heavy metal ions. It could influence the protonation of the functional groups on the adsorbents as well as the solution chemistry of the heavy metal ions. Adsorption histograms of As (V), Ni (II), Cu (II), Cd (II) and Pb (II) on the two samples TSFCS and TCFCS at pH 4.0, 7.0 and 9.0 are shown in Fig. 8. As shown in Fig. 8, it is evident that both TSFCS and TCFCS cannot adsorb Zn (II) ion; the removal of the As (V), Ni (II), Cu (II), Cd (II) and Pb (II) are around 66.4–99.9%, and 71.5– 99.9% for the two adsorbents, respectively. The removal of the As (V), Ni (II), Cu (II), Cd (II) and Pb (II) at pH 9 and 4 is more effective than that at pH 7. These results partially accorded with conclusions available in literature [44–46], and might be caused by the introduction of chelating active groups TS or TC in CS structures [39]. In general, it seemed that the adsorption of TSFCS and TCFCS for metal ions was effective in some extent. This might can be explained by the fact that the metal ion adsorption strongly depends on the structures of the adsorbents since the two samples TSFCS and TCFCS have many thiourea and amino groups contained in the structures. In order to clarify the adsorption mechanism and adsorption capacity, the further study need to be conducted.
4. Conclusions A convenient and simple method for the synthesis of aminothiourea-modified chitosan derivatives has been developed by using formaldehyde as linkages. Using this method, two target compounds TCFCS and TSFCS were synthesized and characterized. This one pot approach has advantages of saving time, ease of work up, avoiding solvent and energy consuming procedures during the isolation and purification of intermediates. These CS derivatives can form pH dependent gels and act as effective corrosion inhibitors as well as heavy metal ion adsorbents. All the results imply these two products TSFCS and TCFCS might be having potential applications in anticorrosion and treatment of heavy metal ion contamination.
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