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Chitosan nanoparticle hydrogel based sebacoyl moiety with remarkable capability for metal ion removal from aqueous systems
International Journal of Biological Macromolecules 122 (2019) 578–586
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International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac
Chitosan nanoparticle hydrogel based sebacoyl moiety with remarkable capability for metal ion removal from aqueous systems Nadia G. Kandile ⁎, Hemat M. Mohamed Chemistry Department, Faculty of Women, Ain Shams University, Heliopolis 11757, Cairo, Egypt
a r t i c l e
i n f o
Article history: Received 17 August 2018 Received in revised form 15 October 2018 Accepted 27 October 2018 Available online 30 October 2018 Keywords: Chitosan nanoparticles hydrogels Metal ions uptake Antimicrobial activity
a b s t r a c t In this present study, a new modified chitosan hydrogel, Cts-SC NPs, was prepared from a one pot reaction of sebacoyl chloride (SC) with chitosan in the presence of 1% v/v glacial acetic acid and 1% w/v sodium tripolyphosphate (TPP) using an ionotropic gelation technique. The modified chitosan hydrogel, Cts-SC NPs, was characterized by FTIR, TEM, XRD, TGA, DSC and SEM. The adsorption efficiency of Cts-SC NPs for metal ions Hg2+, Ni2+ and Co2+ from aqueous solution was evaluated. The effects of various parameters such as contact time, pH and initial metal ions concentration were investigated. Adsorption isotherm data were fitted using different two-parameter models. Modified chitosan hydrogel Cts-SC NPs had remarkable adsorption of Hg2+, Ni2+ and Co2+ ions than chitosan hydrogel Cts-NPs. The antimicrobial activity of Cts-NPs towards the bacteria, Bacillus subtilis, Pseudomonas aeruginosa and fungus Aspergillus flavus was improved by modification with sebacoyl chloride to give Cts-SC NPs. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Industrial processes have been the cause of various environmental problems. Among these was the generation of wastewater containing heavy metal contaminants which were highly toxic, persistent, and bioaccumulative. Therefore, this wastewater must be treated before its reuse or disposal on land or in water bodies [1,2]. Various methods and approaches are available for treating wastewater but face many disadvantages. Numerous approaches are being studied for the development of cheaper and more effective technologies, both to decrease the amount of wastewater produced and to improve the quality of the treated effluent [3]. Many technologies had been developed to remove toxic metal ions from water over the years such as electro deposition, chemical precipitation, membrane processing, ion exchange and adsorption [4,5]. Adsorption, through the natural resources especially via biopolymers, is an emerging technology, which uses natural materials as adsorbents involving a process used for the removal of heavy metal ions from the water under treatment and takes the advantage of the three dimensional structure of a molecule to chelate and thus remove an ion with a specific size in the presence of large quantities of other ions. This approach is inherently remarkable since only the toxic metals can be removed while the harmless ions can be released into the environment [3]. Chemical modification of chitosan is of interest because the modification would not change the fundamental skeleton of chitosan, and will improve its properties [6–16]. ⁎ Corresponding author at: Chemistry Department, Faculty of Women, Ain Shams University, Heliopolis, Cairo, Egypt. E-mail address: [email protected] (N.G. Kandile).
https://doi.org/10.1016/j.ijbiomac.2018.10.198 0141-8130/© 2018 Elsevier B.V. All rights reserved.
Some studies have been reported on the chemical modification of chitosan to produce multi-function materials such as the arylation of the amino group via grafting process to enhancing chitosan properties such as chelating properties, bacteriostatic effects, adsorption properties, biocompatibility and biodegradability [17]. The polyfunctional nature of chitosan enables its application not only in polymer technology but also has been shown to be of importance in the field of nanotechnology for the fabrication of a wide spectrum of functional nanomaterials in the biomedical field [18]. Hydrogel nanoparticles or nanogels as a group of nanoparticulate systems have extensive potential and capability for different applications because they contain the characteristic features of the beneficial properties of hydrogels such as hydrophilicity, flexibility, versatility, high water absorptivity, and biocompatibility of these particles and all the benefits of nanoparticles [19,20]. In a continuation of our studies in the synthesis of new hydrogels incorporating the chitosan moiety for metal ions uptake from aqueous systems [6–15], we herein report a simple method for the synthesis of a new modified hydrogel based on nano chitosan entities and sebacoyl chloride (SC). The new hydrogel was characterized and evaluated for metal ions adsorption and its antimicrobial activity.
2. Materials and methods 2.1. Materials Low molecular weight chitosan with the deacetylation degree (DD) ≥ 85.0% and viscosity 20–300 cps was purchased from Aldrich. Sebacoyl chloride, glacial acetic acid, mercuric chloride, nickel sulfate and cobalt
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chloride were obtained from Adwic, Egypt. All aqueous solutions were prepared using distilled water. 2.2. Chitosan degree of deacetylation The percentage of free amino groups on the chitosan was determined by elemental analysis [11,21] using the following Eq. (1): DD ¼ ½1−ððC=NÞ−5:145Þ=6:186−5:145 100
ð1Þ
where 5.145 is related to the completely N deacetylated Cts (C6H11O4N repeat unit) and 6.186 to the fully N acetylated polymer (C8H13O5N repeat unit). 2.2.1. Preparation of chitosan nanoparticles hydrogel Cts-NPs The ionotropic gelation for the formation of chitosan nanoparticles Cts-NPs was performed as previously described [22,23]. Sodium tripolyphosphate TPP, dissolved in distilled water (20 mL, 1% w/v) was added drop wise through a syringe needle (1 mm in diameter) to the Cts solution (0.5 g, 1% w/v), which was obtained by dissolving Cts in dilute acetic acid (50 mL, 1% v/v) at room temperature, thorough mixing by stirring (100 rpm) for 2 h until complete solubility. The hydrogel Cts-NPs was formed instantaneously upon the drop wise addition of TPP solution. The mixture was stirred for 2 h, then poured into a petri dish and left for 24 h under vacuum to dry. Yellowish white nanoparticle pellets of Cts-NPs were obtained, washed with sodium hydroxide solution (1% w/v) to neutralize the excess of acetic acid solution and finally was washed with distilled water. 2.2.2. Preparation of modified chitosan nanoparticles hydrogel Cts-SC Sebacoyl chloride (0.25 mL, 0.5% v/v) was added to the previous acetic acid solution of Cts and left stirring for 1 h at room temperature. Then TPP solution (20 mL, 1% w/v) was added drop wise, as the crosslinker, through a syringe needle (1 mm in diameter) and the reaction mixture was stirred for another 2 h. The reaction mixture was then poured into a petri dish and left for 24 h under vacuum to dry. The brown nanoparticle pellets Cts-SC NPs (1) were formed, washed with sodium hydroxide solution (1% w/v) to neutralize the excess of acetic acid solution and finally were washed with distilled water. 2.3. Instrumentation Fourier Transform Infrared (FTIR) spectra were measured on a Perkin Elmer-1430 Instrument using casting a thin film onto a KBr plate. Transmission Electron Microscopy (TEM) was used to characterize the nanoparticles by means of a JEOLJEM-1200 apparatus. X-ray diffractograms (XRD) of the polymers were obtained with a Phillips Xray unit (Generator PW-1390) and Ni-filtered Cu. The thermal stability was studied with thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), carried out in a nitrogen atmosphere using a Shimadzu TGA-50H and DSC-50 thermal analyzer. Scanning electron microscope (SEM) was studied at 500 μm and at 1000 μm.
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2.5. Adsorption studies of the metal ions uptake For each metal ion, 0.1 g of hydrogel Cts-NPs or Cts-SC NPs was immersed in distilled water and left to swell for 24 h, then filtered and 2 g/L of each ion Hg2+, Ni2+ or Co2+ solution 25 mL was added and stirred with a magnetic stirrer at room temperature. After solutionhydrogel contacted, stirring was continued with time intervals. In each time, after filtration using filter paper, the concentration of nonadsorbed metal ions Hg2+, Ni2+ and Co2+ in the solution was measured from the calibration curve using UV–visible spectrophotometer [9,24], at wavelength of 270, 394 and 512 nm for metal ions Hg2+, Ni2 + and Co2+ respectively. The efficiency of metal ion uptake of Cts-NPs and Cts-SC NPs was calculated using the following Eq. (3): F ¼ ½1−ðC=Co Þ 100
ð3Þ
where F was the efficiency (%), C was the concentration of M2+ ions in the solution after a certain time period and Co was the initial concentration of M2+ ions in solution. Adsorption capacity (X), was the maximum metal ion concentration adsorbed per weight unit of Cts-NPs and Cts-SC NPs (g metal ion/g dry polymer) at time t and was calculated from the following mass balance Eq. (4): X ¼ ½ðCo −Ct Þ V=W
ð4Þ
where Co and Ct (g/L) were the concentrations of the metal ions in the aqueous phase before and after adsorption at time t respectively. V was the volume of the aqueous phase (L) and W was the weight of used dry Cts-NPs and Cts-SC NPs (g). 2.5.1. Effect of contact time The adsorption capacity was studied for 10 h using 0.1 g swollen sorbent dose of Cts-NPs and Cts-SC NPs at room temperature. The adsorption capacity was measured by taking samples from the solution every 2 h for analysis of residual metal ions Hg2+, Ni2+ and Co2+ concentration in solution. 2.5.2. Effect of pH Uptake experiments were performed at 25 °C and controlled at pH ranging from 1 to 5 using 0.01 M HCl or 0.01 M NaOH. Before the metal ions Hg2+, Ni2+ and Co2+ uptake was studied at the different pH values, a 0.05 g sorbent dose of Cts-SC NPs was immersed in distilled water and left to swell for 24 h until equilibrium was reached. Adsorption capacity experiments were carried out at a fixed time 6 h at least three times and the mean value was taken. 2.5.3. Effect of initial metal ions concentration The effect of initial metal ions Hg2+ and Ni2+ concentration on the uptake by Cts-SC NPs was carried out at definite concentrations 10–30 mg L−1, using 0.05 g of swollen hydrogel Cts-SC NPs at pH 5 for 24 h. After adsorption, the residual concentration of the metal ions Hg+2 and Ni+2 in solution was determined.
2.4. Swelling studies 2.6. Antimicrobial activity studies The swelling characteristics of Cts-NPs and Cts-SC NPs hydrogels were determined in controlled pH ranging from 3 to 10 using 0.01 M HCl or 0.01 M NaOH at room temperature. After 24 h, the hydrogels were taken out and were blotted with a filter paper to remove surface absorbed water, and weighed immediately. The swelling ratios Esw (%) of samples were calculated from the following Eq. (2) [13]: Esw ¼ ½ðWs −Wd Þ=Wd 100
ð2Þ
where Ws and Wd were the weights of swollen and dried hydrogels respectively.
Antimicrobial activity of the tested samples was determined using a modified Kirby-Bauer disc diffusion method [25]. The hydrogels were tested in vitro for their antimicrobial activity against Bacillus subtilis as Gram positive bacteria and Pseudomonas aeruginosa as Gram negative bacteria, in nutrient agar medium. Antifungal activity was tested on Aspergillus flavus. Briefly, 100 μL of the test bacteria/fungi were grown in 10 mL of fresh media until they reached a count of approximately 107 cells/mL for bacteria or 105 cells/mL for fungi [26]. 100 μL of microbial suspension was spread onto agar plates corresponding to the broth in which they were maintained. The agar used is Meuller-Hinton agar
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that was rigorously tested for composition and pH. Further the depth of the agar in the plate was a factor to be considered in the disc diffusion method. This method is well documented and standard zones of inhibition have been determined for susceptible and resistant values. 3. Results and discussion 3.1. Synthesis of Cts-NPs and Cts-SC NPs hydrogels Nanogels were fabricated via the ionotropic gelation procedure. This method was based on the ability of chitosan to generate gel structure after contacting with polyanions such as TPP by forming inter- and intra-molecular linkages [19]. In this study, the ionotropic gelation technique [22,23] was applied to the preparation of Cts-NPs, and Cts-SC NPs hydrogels in which the free polycation groups of chitosan Cts interact with a polyanionic sodium tripolyphosphate TPP. The new Cts-SC NPs hydrogel was prepared by acylation reaction of the NH2 groups in chitosan. The acylation reaction occurred via elimination of HCl molecule formed through interaction of two chlorine atoms of sebacoyl chloride and NH2 group of chitosan Cts via covalent linkage to form two amide units in structure of Cts-SC NPs Scheme 1. Chitosan Cts, Cts-NPs and Cts-SC NPs were characterized by FTIR, TEM, X-ray diffraction and thermal analysis. 3.2. Degree of deacetylation of chitosan The presence of the free amine groups on the chitosan backbone increases the potential for interactions with metal ions to a greater extent than the acetamide groups do [27]. Therefore, the degree of deacetylation is a principal feature of chitosan that may influence its sorption properties [11]. Carbon, hydrogen and nitrogen contents of Cts were found to be C, 40.72; H, 7.47; N, 7.68, so the degree of deacetylation (DD) of chitosan used in this study was calculated to be 85%. 3.3. Characterization of Cts-NPs and Cts-SC NPs hydrogels 3.3.1. FTIR spectroscopy FTIR spectra of chitosan Cts, sebacoyl chloride SC, Cts-NPs and Cts-SC NPs hydrogels were shown in Fig. 1a–d. The FTIR spectrum of chitosan Cts Fig. 1a showed a broad band at 3436 cm−1 assigned to \\OH and \\NH2 stretching vibrations which shifted to 3427 cm−1 and the peak was sharper in Cts-NPs. This indicated that hydrogen bonding was enhanced [28].
Two sorption bands at 1663 cm−1 and 1600 cm−1 were shown for the C_O of acetyl group and N\\H bending vibrations of the \\NH2 groups, which could be observed clearly in chitosan Cts, decreased in Fig. 1c for Cts-NPs and two new sorption bands at 1642 cm−1 and1562 cm−1appeared, which showed that the amine groups were crosslinked with TPP [28]. The above two new sorption bands also appeared in Fig. 1d for Cts-SC NPs at 1629 cm−1 and 1563 cm−1. FTIR spectrum of sebacoyl chloride SC Fig. 1b showed sharp two bands at 2933 cm−1 and 1797 cm−1 referred to the aliphatic C\\H stretching vibration in the \\CH2 group of long aliphatic chain and C_O respectively. While in the case of the hydrogel Cts-SC NPs these bands decreased to 2927 cm−1 and 1707 cm−1, indicated successful modification of chitosan Cts with sebacoyl chloride and the formation of amide groups by the reaction between the primary amine groups of chitosan Cts and two chlorine atoms of sebacoyl chloride. 3.3.2. TEM analysis Transmission electron microscopy (TEM) was used to characterize the hydrogels nanoparticles. The samples were prepared by placing a drop of the colloidal solution on a 400 mesh copper grid coated with an amorphous carbon film and then evaporating the solvent in air at room temperature. Fig. 2a, b presented the TEM for the Cts-NPs and Cts-SC NPs hydrogels respectively. TEM images were measured at 100 nm and indicated particles with spherical shapes agreed with Xu et al. [29] which had studied different formulations of chitosan NPs produced by the ionic gelation of TPP and confirmed that chitosan yielded spherical shapes in TEM [20]. Thus the particles shapes were not affected by modification of Cts. 3.3.3. X-ray diffraction X-ray diffraction (XRD) was used to study the amorphous and crystallinity properties of Cts, Cts-NPs and Cts-SC NPs hydrogels. Fig. 3a–c showed the interfering peaks at 2Ө were in the range 4–90. Fig. 3a for Cts gave two sharp peaks at 2Ɵ = 13.6 and 20. The peaks became broad in Cts-NPs which showed two peaks at 2Ɵ = 15 and 22.6 indicating a shift from the Cts peaks and increased amorphous nature, thus decreasing the crystal structure of chitosan after crosslinking with sodium tripolyphosphate TPP [30]. XRD of the hydrogel Cts-SC NPs showed new peaks around 2Ɵ = 4.2 and 26.5 These major changes suggested a more crystalline and possibly more stable organization for Cts-SC NPs than Cts and Cts-NPs hydrogels. This could be due to the chemical modification of chitosan by long sebacoyl chloride aliphatic carbon chain [31]. 3.3.4. Thermal characterization
Scheme 1. Synthesis of modified chitosan nanoparticles hydrogel Cts-SC Nps.
3.3.4.1. Thermogravimetric analysis (TGA). Thermal properties of chitosan Cts, Cts-NPs and Cts-SC NPs hydrogels were evaluated using TGA under a nitrogen atmosphere Fig. 4a and the results are summarized in Table 1. From TGA analysis of chitosan hydrogel Cts Fig. 4a two weight losses were observed. The first stage started between 50 and 150 °C which showed weight loss of 14% at 52 °C, it may be attributed to the removal of water absorbed and the second stage of weight loss between 200 and 400 °C was due to the thermal degradation of chitosan Cts which showed weight losses of 24.9% and 25.11% at 224.6 °C and 267.8 °C [30].TGA of chitosan nano-particles Cts-NPs Fig. 4a showed two significant weight losses, one weight loss at 40–150 °C of 21.5% at 49.4 °C due to moisture vaporization and the other at 200–400 °C was due to the thermal degradation of chitosan nano-particles Cts-NPs which showed a weight loss 23% at 268.7 °C [30]. The hydrogel Cts-SC NPs showed in Fig. 4a two weight losses. The first stage started between 30 and 70 °C showed weight losses of 2.33% at 55.7 °C which was attributed to the removal of water absorbed. The second stage of weight loss between 200 and 400 °C was due to the thermal degradation of the hydrogel Cts-SC NPs showed weight loss of 57.1% at 229.7 °C. The above data indicated that the thermal stability decreased after modification
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SC
%T
%T
Ct
1663
1600 2933
3436
1797
b
a
Wave number (cm-1)
Wave number (cm-1)
Cts-NPs
1642
%T
%T
Cts- SC NPs
1707
1562
1629 2927 3427
1563
3427
d
c
Wave number (cm-1)
Wave number (cm-1)
Fig. 1. FTIR spectra of a) Cts, b) SC, c) Cts-NPs and d) Cts-SC NPs.
3.3.4.2. Differential scanning calorimetry (DSC). The glass transition temperatures (Tg) of Cts-NPs and Cts-SC NPs hydrogels were determined from differential scanning calorimetry (DSC) Fig. 4b which showed a broad band related to glass transition temperatures (Tg) at 56 °C and 76 °C for Cts-NPs and Cts-SC NPs hydrogels respectively. Cts-SC NPs hydrogel showed the highest (Tg) than Cts-NPs which may be due to the
a
b
presence of long aliphatic carbon chain in the chemical structure of Cts-SC NPs [32].and the ability of the chains to undergo segmental motion, which would increase the (Tg) value [9,33]. The crystal
Intensity (Count)
of chitosan Cts by sebacoyl chloride, this may be attributed to the ease of degradation of amide groups [32].
a
b
c
2θ Fig. 2. TEM of a) Cts-NPs and b) Cts-SC NPs.
Fig. 3. X-ray diffraction of a) Cts, b) Cts-NPs and c) Cts-SC NPs.
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Fig. 5. Swelling ratio of Cts-NPs and Cts-SC NPs.
3.4. Metal ions uptake studies
Fig. 4. a. TGA of Cts, Cts-NPs and Cts-SC NP. b. DSC of Cts-NPs and Cts-SC NPs.
temperatures (Tc) for the Cts-SC NPs were recorded at 172 °C. The DSC curve for Cts-SC NPs also showed a melting temperature (Tm) at 232 °C.
3.3.5. Swelling studies The immersion of cross-linked polymers in solvents does not lead to their dissolution because of their chemically bonded hydrocarbon chains; nonetheless, these links do not prevent cross-linked polymers from swelling. Fig. 5 showed swelling studies for nano chitosan CtsNPs and modified chitosan Cts-Sc NPs and indicated greater swelling for hydrogel Cts-Sc NPs as compared to Cts-NPs hydrogel at pH ranging from 3 to 10, this affinity to absorb water was attributed to the presence of hydrophilic groups such as amide group [34]. Cts-Sc NPs hydrogel showed the highest degree of swelling at pH 5. This could be explained by the fact that the swelling was controlled mainly by the amine group (NH2) on the C2 carbon of the chitosan component. The protonation of the chitosan amino groups at low pH leaded to repulsion of the polymeric chains [35]. High swelling at pH = 3 was confirmed the chemical stability of the new hydrogel in high acidic conditions. Raising the pH above 7 would reduce the number of NH3+ groups and increase the number of NH2 groups. Swelling was thus decreased as the positive charge repulsions were removed [13]. Modified hydrogel Cts-Sc NPs showed higher swelling compared to Cts-NPs at pH 10 due to increase of the hydrophilicity of the Cts after modification with sebacoyl chloride.
3.4.1. Effect of contact time The main question to answer for the metal adsorption process was the effect of time on metal ions uptake. Since various ions were present in wastewater, it was essential to investigate the competitive binding affinity of hydrogels for these ions. Different factors such as hydrogel structure, metal combination, level of metal concentration, and uptake time affect the interaction between the metal ions and the reactive sites of hydrogels [6,36–42]. It was considered that there were three different possibilities for metal ion uptake (i) adsorption by the active functional groups that could acquire metal ions, (ii) a simple chelating mechanism, or (iii) adsorption by the open polymer matrix [43]. The structure of hydrogel skeleton was an important factor that would affect adsorption of metal ions [44]. Therefore, in this study we compare between the metal adsorption capacity values of Hg2+, Ni2+ and Co2+ by hydrogels Cts-NPs and CtsSc NPs at optimum conditions for pH, temperature, adsorbent dose and initial metal ions concentration. The adsorption capacity for Hg2+, Ni2+ and Co2+ ions was investigated for 10 h and the results were presented in Fig. 6a,b for hydrogels Cts-NPs and Cts-SC NPs respectively. It was evident from Fig. 6a,b for both nano adsorbents that the adsorption capacity values increased as contact time increased due to the transport of metal ions from the aqueous phase onto the surface of the hydrogels Cts-NPs and Cts-SC NPs. The graphical results for the efficiency % Fig. 6c showed that Cts-SC NPs hydrogel had higher percent removal of Hg2+, Ni2+ and Co2+ than Cts-NPs hydrogel. This could be attributed to the chemical structure of Cts-SC NPs hydrogel which contained long aliphatic carbon chain which given more flexibility and two amide groups participated in chelating with the metal cations thus increased removal of metal ions from aqueous solution. It can be concluded that modification of chitosan with sebacoyl chloride SC was utilized to increase the metal ions uptake from aqueous solution.
3.4.2. Effect of pH pH water contaminated with heavy metal ions and industrial effluents were generally known to have different pH values depending on the type of industrial activities. The pH of aqueous solution was
Table 1 Thermogravimetric analysis (TGA) for hydrogels. Hydrogel code Cts Cts-NPs Cts-SC NPs
Temperature
Weight loss%
Temperature
Weight loss%
Temperature
Weight loss%
52 49.4 55.7
14 21.5 2.33
224.6 – 229.7
24.9 – 57.1
267.8 268.7 –
25.11 23 –
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Fig. 6. Effect of contact time for a) Cts-NPs, b) Cts-SC NPs and c) Efficiency%. d: Effect of pH values on Cts-SC NPs. e: Initial metal ions concentration of Hg2+and Ni 2+ on Cts-SC NPs.
known as one of the most important operational factors in the adsorption process because it affected the solubility of metal ions, concentration of the counter ions, the functional groups of the sorbent and the degree of ionization of the sorbent during reaction. According to the pH value of the contact solution, the active functional groups and sites on the sorbent surface can be either protonated or deprotonated and the sorbate speciation in solution can be also affected [45,46]. Previous studies reported that the maximal sorption efficiency for M2+ ions was observed at pH 5. However, when the pH value was increased up to 6.0, the adsorption capacity decreased due to the formation of soluble hydroxylated complexes of the metal ions which compete for the active sites, and as a consequence, the retention was decreased [6,47–50]. In our study, the effect of pH was studied in range of 1.0–5.0 at a fixed temperature, sorbent dose and initial metal ions concentration for 6 h using the Cts-SC NPs hydrogel which showed the highest percent removal of metal ions uptake. The pH values selected in the
experiments were chosen with respect to the precipitation limits of the three metals ions Hg2+, Ni2+ and Co2+ (pH 5). As shown in Fig. 6d, Cts-SC NPs hydrogel showed low metal ion uptake in acidic conditions (at low pH values) where the H+ ion concentration was very high. Low level of metal ion adsorption may therefore be attributed to the protonation of active groups and the competition of H+ with metal ions for adsorption sites on the hydrogel surface [51]. The abundance of H+ in the medium showed another type of interaction (ion exchange mechanism) through the protonation of the unsubstituted amino groups [16]. From Fig. 6d the removal percentage of metal ions increased with increasing pH values. The H+ ions concentration decreases with increasing pH of the medium, an increased number of M2 + ions get bound and hence the maximum adsorption capacity was observed at pH 5. M2+ ions uptake increased as the pH increased, while most active sites on the adsorbent were deprotonated resulting in a greater net attractive force which was responsible for high M2+ ions uptake removal from aqueous solution [30].
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Table 2 Adsorption isotherm model parameters for the adsorption of Hg2+ and Ni2+ on Cts-Sc NPs. Metal ion
Hg2+ Ni2+
Langmuir isotherm
Freundlich isotherm
Equation
R2
KL
Qm
Equation
R2
N
KF
Ce/qe = 0.0127 Ce+ 0.0505 Ce/qe = 0.0147 Ce+ 0.0766
0.9932 0.9944
0.25 0.192
78.74 68
Log qe = 0.4925 Ce+ 1.2776 Log qe = 0.4703 Ce+ 1.751
0.9724 0.9747
2.03 2.13
18.95 14.97
3.4.3. Effect of initial metal ions concentration Preliminary experiments revealed that the concentration of metal ions could have a significant effect on the adsorption capacity of the Cts-SC Nps hydrogel. The effect of initial metal ions Hg2+ and Ni2+ concentrations on the adsorption capacity of Cts-Sc NPs was studied. In this study, pH, temperature, adsorbent dose were fixed and with a contact time of 24 h. The results were presented in Fig. 6e which indicated that increasing the Hg2+ and Ni2+ concentration accelerated the diffusion of Hg2+ and Ni2+ ions from solution to the adsorbent surface due to the increase in the driving force of the concentration gradient. 3.4.4. Adsorption isotherms The equilibrium adsorption isotherm was fundamental in describing the interactive behavior between solute and adsorbent, and it was very important for designing an adsorption system. Quantification of the sorption capacity of Hg2+ and Ni2+ ions onto Cts-SC NPs was evaluated using two parameter equations namely the Langmuir and Freundlich isotherm models [16,52]. The Langmuir model [53] assumed that the metal ions uptake occurred on a homogeneous surface by monolayer adsorption without any interaction between adsorbed ions. The Langmuir and Freundlich adsorption isotherm models [16,52] were used to determine the appropriate isotherm for Hg2+ and Ni2+ ions adsorption on the Cts-Sc NPs hydrogel. Langmuir Eq. (6) and Freundlich Eq. (7) were employed in the following linear form as follows: Ce =qe ¼ 1=kL Q m þ Ce =Q m
ð6Þ
In Eq. (6) Ce (mg L−1) and qe (mg g−1) were the equilibrium concentration of M2+ ions in the solution and adsorbent, Qm (mg g−1)
a
e
b
f
represented the maximum adsorption amount, kL was the Langmuir constant. The values of Qm and kL were determined from the slope and intercept of the plots of Ce/qe versus Ce and represented in Table 2. The linearized Freundlich isotherm had the general form given in Eq. (7): log qe ¼ log k F þ 1=n log Ce
ð7Þ
In Eq. (7) qe (mg g−1) and Ce (mg L−1) were the equilibrium concentrations of M2+ ions in the adsorbent and solution, kF and n were Freundlich constant and intensity factors respectively. The values of n and kF were calculated from the slope and intercept of the plots of log qe versus log Ce and represented in Table 2. It was obvious that the Langmuir adsorption isotherm was more suitable to describe the adsorption equilibrium (R2 N 0.99). The essential characteristics of Langmuir isotherm could be explained in terms of dimensionless constant separation factor (RL) [54] which was defined by Eq. (8): RL ¼ 1=ð1 þ KL Co Þ
ð8Þ
where, KL (L mg−1) was the Langmuir constant related to the energy of adsorption and Co (mg L−1) was the initial concentration of both Hg2+ and Ni2+ ions. The RL value indicated the type of the isotherm to be either unfavorable (RL N 1), linear (RL = 1), favorable (0 b RL b 1), irreversible (RL = 0). RL values lay between 0.12 and 0.286 for Hg2+ and between 0.15 and 0.34 for Ni2+ indicating the favorable sorption of Hg2+ and Ni2+ ions by the Cts-SC-NPs hydrogel.
c
d
g
h
Fig. 7. a, e SEM of Cts-SC NPs before treatment of metal ions at 500 and 1000 μm, b, c, d SEM of Cts-SC NPS after treatment of metal ions Hg2+, Ni2+ and Co2+ at 500 μm and f, g, h for Hg2+, Ni2+ and Co2+ at 1000 μm.
N.G. Kandile, H.M. Mohamed / International Journal of Biological Macromolecules 122 (2019) 578–586 Table 3 Anti-bacterial and antifungal activity data of Cts-NPs and Cts-Sc NPs. Sample
Inhibition zone diameter (mm/mg sample) Bacterial species
Control: DMSO Standard Ampicillin antibacterial agent Amphotericin B antifungal agent Cts-NPs Cts-Sc NPs
Fungus
(G−)
(G+)
Bacillus subtilis
Pseudomonas aeruginosa
Aspergillus flavus
0.0 26
0.0 26
0.0 –
–
–
17
9 11
9 10
0.0 10
585
be the better one due to a high value of R2. The surface morphology of Cts-SC NPs hydrogel, after metal ions uptake, showed heterogeneous holes and some small openings on the surface. Thus chitosan nanoparticles based on the sebacoyl moiety hydrogel, Cts-SC NPs, had the capability as a metal ions adsorbent for wastewater contaminated with Hg2 + , Ni2+ and Co2+ ions. Finally, the antimicrobial activity data showed that inhibitory effects of Cts-SC NPs hydrogel were always higher in comparison with that of a Cts-NPs hydrogel. Conflict of interest The authors report no conflict of interest. The authors alone are responsible for the content and writing of the article. Acknowledgment
3.5. Surface morphology studies The surface morphology of the Cts-SC NPs hydrogel, before and after metal ions Hg2+, Ni2+ and Co2+ uptake was examined by Scanning Electron Microscopy (SEM) and the images obtained were shown in Fig. 7a–h. The surface morphology of the Cts-SC NPs hydrogel before treatment with metal ions appeared to be smooth in Fig. 7a, e, however, after treatment with metal ions the images in Fig. 7b–h illustrated that the surface morphology of the Cts-SC NPs hydrogel had an irregular structure. Heterogeneous holes and some small openings were seen on the surface for Ni2+ and Co2+ but with Hg2+, the adsorbent surface became abnormal and a great deal of crystal adhered to the surface [6], which would increase the contact area and improve metal ions adsorption.
We thank our department for support with the chemicals used in this research. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
3.6. Antimicrobial activity [12]
The data on antibacterial activity of Cts-NPs and Cts-SC NPs hydrogels on Bacillus subtilis as Gram positive bacteria, Pseudomonas aeruginosa as Gram negative bacteria, and a fungi, Aspergillus flavus were displayed in Table 3. The results showed higher inhibition zone diameters of Cts-SC NPs hydrogel than of Cts-NPs hydrogel due to several mechanisms reported on the antimicrobial activity of chitosan. Among them the three most accepted mechanisms are discussed below. First one considers the electrostatic forces of interaction between the positively charged protonated amino groups of chitosan and the negatively charged microbial cell surface [55]. The second mechanism considers the binding of chitosan with microbial DNA, which inhibits the mRNA and protein synthesis via penetration of chitosan into the nuclei of the microorganisms [56]. The third mechanism considers the chelating of chitosan with metals, resulting in suppression of spore elements and binding to essential nutrients to stop microbial growth [57]. 4. Conclusion Chitosan nanoparticle hydrogel Cts-NPs and modified hydrogel CtsSC NPs were successfully synthesized via ionotropic gelation and characterized. Modified chitosan hydrogel Cts-Sc NPs showed a significantly high swelling ratio in water than Cts-NPs hydrogel from pH 3 to 10. Evaluation of the hydrogels Cts-NPs and Cts-SC NPs as adsorbent agents for Hg2+, Ni2+, and Co2 + ions from aqueous solution revealed that hydrogel Cts-SC NPs showed a more remarkable adsorption capacity for Hg2+, Ni2+ and Co2+ ions than hydrogel Cts-NPs which was attributed to the presence of long aliphatic carbon chain in the modified hydrogel Cts-SC NPs which gave more flexibility and two amide groups that participated in chelating with the metal cations. Also the modified hydrogel Cts-SC NPs showed a higher metal ions uptake capacity at pH values. Adsorption isotherms data were fitted using two different parameter models. The Langmuir model proved to
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Contents lists available at ScienceDirect
International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac
Chitosan nanoparticle hydrogel based sebacoyl moiety with remarkable capability for metal ion removal from aqueous systems Nadia G. Kandile ⁎, Hemat M. Mohamed Chemistry Department, Faculty of Women, Ain Shams University, Heliopolis 11757, Cairo, Egypt
a r t i c l e
i n f o
Article history: Received 17 August 2018 Received in revised form 15 October 2018 Accepted 27 October 2018 Available online 30 October 2018 Keywords: Chitosan nanoparticles hydrogels Metal ions uptake Antimicrobial activity
a b s t r a c t In this present study, a new modified chitosan hydrogel, Cts-SC NPs, was prepared from a one pot reaction of sebacoyl chloride (SC) with chitosan in the presence of 1% v/v glacial acetic acid and 1% w/v sodium tripolyphosphate (TPP) using an ionotropic gelation technique. The modified chitosan hydrogel, Cts-SC NPs, was characterized by FTIR, TEM, XRD, TGA, DSC and SEM. The adsorption efficiency of Cts-SC NPs for metal ions Hg2+, Ni2+ and Co2+ from aqueous solution was evaluated. The effects of various parameters such as contact time, pH and initial metal ions concentration were investigated. Adsorption isotherm data were fitted using different two-parameter models. Modified chitosan hydrogel Cts-SC NPs had remarkable adsorption of Hg2+, Ni2+ and Co2+ ions than chitosan hydrogel Cts-NPs. The antimicrobial activity of Cts-NPs towards the bacteria, Bacillus subtilis, Pseudomonas aeruginosa and fungus Aspergillus flavus was improved by modification with sebacoyl chloride to give Cts-SC NPs. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Industrial processes have been the cause of various environmental problems. Among these was the generation of wastewater containing heavy metal contaminants which were highly toxic, persistent, and bioaccumulative. Therefore, this wastewater must be treated before its reuse or disposal on land or in water bodies [1,2]. Various methods and approaches are available for treating wastewater but face many disadvantages. Numerous approaches are being studied for the development of cheaper and more effective technologies, both to decrease the amount of wastewater produced and to improve the quality of the treated effluent [3]. Many technologies had been developed to remove toxic metal ions from water over the years such as electro deposition, chemical precipitation, membrane processing, ion exchange and adsorption [4,5]. Adsorption, through the natural resources especially via biopolymers, is an emerging technology, which uses natural materials as adsorbents involving a process used for the removal of heavy metal ions from the water under treatment and takes the advantage of the three dimensional structure of a molecule to chelate and thus remove an ion with a specific size in the presence of large quantities of other ions. This approach is inherently remarkable since only the toxic metals can be removed while the harmless ions can be released into the environment [3]. Chemical modification of chitosan is of interest because the modification would not change the fundamental skeleton of chitosan, and will improve its properties [6–16]. ⁎ Corresponding author at: Chemistry Department, Faculty of Women, Ain Shams University, Heliopolis, Cairo, Egypt. E-mail address: [email protected] (N.G. Kandile).
https://doi.org/10.1016/j.ijbiomac.2018.10.198 0141-8130/© 2018 Elsevier B.V. All rights reserved.
Some studies have been reported on the chemical modification of chitosan to produce multi-function materials such as the arylation of the amino group via grafting process to enhancing chitosan properties such as chelating properties, bacteriostatic effects, adsorption properties, biocompatibility and biodegradability [17]. The polyfunctional nature of chitosan enables its application not only in polymer technology but also has been shown to be of importance in the field of nanotechnology for the fabrication of a wide spectrum of functional nanomaterials in the biomedical field [18]. Hydrogel nanoparticles or nanogels as a group of nanoparticulate systems have extensive potential and capability for different applications because they contain the characteristic features of the beneficial properties of hydrogels such as hydrophilicity, flexibility, versatility, high water absorptivity, and biocompatibility of these particles and all the benefits of nanoparticles [19,20]. In a continuation of our studies in the synthesis of new hydrogels incorporating the chitosan moiety for metal ions uptake from aqueous systems [6–15], we herein report a simple method for the synthesis of a new modified hydrogel based on nano chitosan entities and sebacoyl chloride (SC). The new hydrogel was characterized and evaluated for metal ions adsorption and its antimicrobial activity.
2. Materials and methods 2.1. Materials Low molecular weight chitosan with the deacetylation degree (DD) ≥ 85.0% and viscosity 20–300 cps was purchased from Aldrich. Sebacoyl chloride, glacial acetic acid, mercuric chloride, nickel sulfate and cobalt
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chloride were obtained from Adwic, Egypt. All aqueous solutions were prepared using distilled water. 2.2. Chitosan degree of deacetylation The percentage of free amino groups on the chitosan was determined by elemental analysis [11,21] using the following Eq. (1): DD ¼ ½1−ððC=NÞ−5:145Þ=6:186−5:145 100
ð1Þ
where 5.145 is related to the completely N deacetylated Cts (C6H11O4N repeat unit) and 6.186 to the fully N acetylated polymer (C8H13O5N repeat unit). 2.2.1. Preparation of chitosan nanoparticles hydrogel Cts-NPs The ionotropic gelation for the formation of chitosan nanoparticles Cts-NPs was performed as previously described [22,23]. Sodium tripolyphosphate TPP, dissolved in distilled water (20 mL, 1% w/v) was added drop wise through a syringe needle (1 mm in diameter) to the Cts solution (0.5 g, 1% w/v), which was obtained by dissolving Cts in dilute acetic acid (50 mL, 1% v/v) at room temperature, thorough mixing by stirring (100 rpm) for 2 h until complete solubility. The hydrogel Cts-NPs was formed instantaneously upon the drop wise addition of TPP solution. The mixture was stirred for 2 h, then poured into a petri dish and left for 24 h under vacuum to dry. Yellowish white nanoparticle pellets of Cts-NPs were obtained, washed with sodium hydroxide solution (1% w/v) to neutralize the excess of acetic acid solution and finally was washed with distilled water. 2.2.2. Preparation of modified chitosan nanoparticles hydrogel Cts-SC Sebacoyl chloride (0.25 mL, 0.5% v/v) was added to the previous acetic acid solution of Cts and left stirring for 1 h at room temperature. Then TPP solution (20 mL, 1% w/v) was added drop wise, as the crosslinker, through a syringe needle (1 mm in diameter) and the reaction mixture was stirred for another 2 h. The reaction mixture was then poured into a petri dish and left for 24 h under vacuum to dry. The brown nanoparticle pellets Cts-SC NPs (1) were formed, washed with sodium hydroxide solution (1% w/v) to neutralize the excess of acetic acid solution and finally were washed with distilled water. 2.3. Instrumentation Fourier Transform Infrared (FTIR) spectra were measured on a Perkin Elmer-1430 Instrument using casting a thin film onto a KBr plate. Transmission Electron Microscopy (TEM) was used to characterize the nanoparticles by means of a JEOLJEM-1200 apparatus. X-ray diffractograms (XRD) of the polymers were obtained with a Phillips Xray unit (Generator PW-1390) and Ni-filtered Cu. The thermal stability was studied with thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), carried out in a nitrogen atmosphere using a Shimadzu TGA-50H and DSC-50 thermal analyzer. Scanning electron microscope (SEM) was studied at 500 μm and at 1000 μm.
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2.5. Adsorption studies of the metal ions uptake For each metal ion, 0.1 g of hydrogel Cts-NPs or Cts-SC NPs was immersed in distilled water and left to swell for 24 h, then filtered and 2 g/L of each ion Hg2+, Ni2+ or Co2+ solution 25 mL was added and stirred with a magnetic stirrer at room temperature. After solutionhydrogel contacted, stirring was continued with time intervals. In each time, after filtration using filter paper, the concentration of nonadsorbed metal ions Hg2+, Ni2+ and Co2+ in the solution was measured from the calibration curve using UV–visible spectrophotometer [9,24], at wavelength of 270, 394 and 512 nm for metal ions Hg2+, Ni2 + and Co2+ respectively. The efficiency of metal ion uptake of Cts-NPs and Cts-SC NPs was calculated using the following Eq. (3): F ¼ ½1−ðC=Co Þ 100
ð3Þ
where F was the efficiency (%), C was the concentration of M2+ ions in the solution after a certain time period and Co was the initial concentration of M2+ ions in solution. Adsorption capacity (X), was the maximum metal ion concentration adsorbed per weight unit of Cts-NPs and Cts-SC NPs (g metal ion/g dry polymer) at time t and was calculated from the following mass balance Eq. (4): X ¼ ½ðCo −Ct Þ V=W
ð4Þ
where Co and Ct (g/L) were the concentrations of the metal ions in the aqueous phase before and after adsorption at time t respectively. V was the volume of the aqueous phase (L) and W was the weight of used dry Cts-NPs and Cts-SC NPs (g). 2.5.1. Effect of contact time The adsorption capacity was studied for 10 h using 0.1 g swollen sorbent dose of Cts-NPs and Cts-SC NPs at room temperature. The adsorption capacity was measured by taking samples from the solution every 2 h for analysis of residual metal ions Hg2+, Ni2+ and Co2+ concentration in solution. 2.5.2. Effect of pH Uptake experiments were performed at 25 °C and controlled at pH ranging from 1 to 5 using 0.01 M HCl or 0.01 M NaOH. Before the metal ions Hg2+, Ni2+ and Co2+ uptake was studied at the different pH values, a 0.05 g sorbent dose of Cts-SC NPs was immersed in distilled water and left to swell for 24 h until equilibrium was reached. Adsorption capacity experiments were carried out at a fixed time 6 h at least three times and the mean value was taken. 2.5.3. Effect of initial metal ions concentration The effect of initial metal ions Hg2+ and Ni2+ concentration on the uptake by Cts-SC NPs was carried out at definite concentrations 10–30 mg L−1, using 0.05 g of swollen hydrogel Cts-SC NPs at pH 5 for 24 h. After adsorption, the residual concentration of the metal ions Hg+2 and Ni+2 in solution was determined.
2.4. Swelling studies 2.6. Antimicrobial activity studies The swelling characteristics of Cts-NPs and Cts-SC NPs hydrogels were determined in controlled pH ranging from 3 to 10 using 0.01 M HCl or 0.01 M NaOH at room temperature. After 24 h, the hydrogels were taken out and were blotted with a filter paper to remove surface absorbed water, and weighed immediately. The swelling ratios Esw (%) of samples were calculated from the following Eq. (2) [13]: Esw ¼ ½ðWs −Wd Þ=Wd 100
ð2Þ
where Ws and Wd were the weights of swollen and dried hydrogels respectively.
Antimicrobial activity of the tested samples was determined using a modified Kirby-Bauer disc diffusion method [25]. The hydrogels were tested in vitro for their antimicrobial activity against Bacillus subtilis as Gram positive bacteria and Pseudomonas aeruginosa as Gram negative bacteria, in nutrient agar medium. Antifungal activity was tested on Aspergillus flavus. Briefly, 100 μL of the test bacteria/fungi were grown in 10 mL of fresh media until they reached a count of approximately 107 cells/mL for bacteria or 105 cells/mL for fungi [26]. 100 μL of microbial suspension was spread onto agar plates corresponding to the broth in which they were maintained. The agar used is Meuller-Hinton agar
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that was rigorously tested for composition and pH. Further the depth of the agar in the plate was a factor to be considered in the disc diffusion method. This method is well documented and standard zones of inhibition have been determined for susceptible and resistant values. 3. Results and discussion 3.1. Synthesis of Cts-NPs and Cts-SC NPs hydrogels Nanogels were fabricated via the ionotropic gelation procedure. This method was based on the ability of chitosan to generate gel structure after contacting with polyanions such as TPP by forming inter- and intra-molecular linkages [19]. In this study, the ionotropic gelation technique [22,23] was applied to the preparation of Cts-NPs, and Cts-SC NPs hydrogels in which the free polycation groups of chitosan Cts interact with a polyanionic sodium tripolyphosphate TPP. The new Cts-SC NPs hydrogel was prepared by acylation reaction of the NH2 groups in chitosan. The acylation reaction occurred via elimination of HCl molecule formed through interaction of two chlorine atoms of sebacoyl chloride and NH2 group of chitosan Cts via covalent linkage to form two amide units in structure of Cts-SC NPs Scheme 1. Chitosan Cts, Cts-NPs and Cts-SC NPs were characterized by FTIR, TEM, X-ray diffraction and thermal analysis. 3.2. Degree of deacetylation of chitosan The presence of the free amine groups on the chitosan backbone increases the potential for interactions with metal ions to a greater extent than the acetamide groups do [27]. Therefore, the degree of deacetylation is a principal feature of chitosan that may influence its sorption properties [11]. Carbon, hydrogen and nitrogen contents of Cts were found to be C, 40.72; H, 7.47; N, 7.68, so the degree of deacetylation (DD) of chitosan used in this study was calculated to be 85%. 3.3. Characterization of Cts-NPs and Cts-SC NPs hydrogels 3.3.1. FTIR spectroscopy FTIR spectra of chitosan Cts, sebacoyl chloride SC, Cts-NPs and Cts-SC NPs hydrogels were shown in Fig. 1a–d. The FTIR spectrum of chitosan Cts Fig. 1a showed a broad band at 3436 cm−1 assigned to \\OH and \\NH2 stretching vibrations which shifted to 3427 cm−1 and the peak was sharper in Cts-NPs. This indicated that hydrogen bonding was enhanced [28].
Two sorption bands at 1663 cm−1 and 1600 cm−1 were shown for the C_O of acetyl group and N\\H bending vibrations of the \\NH2 groups, which could be observed clearly in chitosan Cts, decreased in Fig. 1c for Cts-NPs and two new sorption bands at 1642 cm−1 and1562 cm−1appeared, which showed that the amine groups were crosslinked with TPP [28]. The above two new sorption bands also appeared in Fig. 1d for Cts-SC NPs at 1629 cm−1 and 1563 cm−1. FTIR spectrum of sebacoyl chloride SC Fig. 1b showed sharp two bands at 2933 cm−1 and 1797 cm−1 referred to the aliphatic C\\H stretching vibration in the \\CH2 group of long aliphatic chain and C_O respectively. While in the case of the hydrogel Cts-SC NPs these bands decreased to 2927 cm−1 and 1707 cm−1, indicated successful modification of chitosan Cts with sebacoyl chloride and the formation of amide groups by the reaction between the primary amine groups of chitosan Cts and two chlorine atoms of sebacoyl chloride. 3.3.2. TEM analysis Transmission electron microscopy (TEM) was used to characterize the hydrogels nanoparticles. The samples were prepared by placing a drop of the colloidal solution on a 400 mesh copper grid coated with an amorphous carbon film and then evaporating the solvent in air at room temperature. Fig. 2a, b presented the TEM for the Cts-NPs and Cts-SC NPs hydrogels respectively. TEM images were measured at 100 nm and indicated particles with spherical shapes agreed with Xu et al. [29] which had studied different formulations of chitosan NPs produced by the ionic gelation of TPP and confirmed that chitosan yielded spherical shapes in TEM [20]. Thus the particles shapes were not affected by modification of Cts. 3.3.3. X-ray diffraction X-ray diffraction (XRD) was used to study the amorphous and crystallinity properties of Cts, Cts-NPs and Cts-SC NPs hydrogels. Fig. 3a–c showed the interfering peaks at 2Ө were in the range 4–90. Fig. 3a for Cts gave two sharp peaks at 2Ɵ = 13.6 and 20. The peaks became broad in Cts-NPs which showed two peaks at 2Ɵ = 15 and 22.6 indicating a shift from the Cts peaks and increased amorphous nature, thus decreasing the crystal structure of chitosan after crosslinking with sodium tripolyphosphate TPP [30]. XRD of the hydrogel Cts-SC NPs showed new peaks around 2Ɵ = 4.2 and 26.5 These major changes suggested a more crystalline and possibly more stable organization for Cts-SC NPs than Cts and Cts-NPs hydrogels. This could be due to the chemical modification of chitosan by long sebacoyl chloride aliphatic carbon chain [31]. 3.3.4. Thermal characterization
Scheme 1. Synthesis of modified chitosan nanoparticles hydrogel Cts-SC Nps.
3.3.4.1. Thermogravimetric analysis (TGA). Thermal properties of chitosan Cts, Cts-NPs and Cts-SC NPs hydrogels were evaluated using TGA under a nitrogen atmosphere Fig. 4a and the results are summarized in Table 1. From TGA analysis of chitosan hydrogel Cts Fig. 4a two weight losses were observed. The first stage started between 50 and 150 °C which showed weight loss of 14% at 52 °C, it may be attributed to the removal of water absorbed and the second stage of weight loss between 200 and 400 °C was due to the thermal degradation of chitosan Cts which showed weight losses of 24.9% and 25.11% at 224.6 °C and 267.8 °C [30].TGA of chitosan nano-particles Cts-NPs Fig. 4a showed two significant weight losses, one weight loss at 40–150 °C of 21.5% at 49.4 °C due to moisture vaporization and the other at 200–400 °C was due to the thermal degradation of chitosan nano-particles Cts-NPs which showed a weight loss 23% at 268.7 °C [30]. The hydrogel Cts-SC NPs showed in Fig. 4a two weight losses. The first stage started between 30 and 70 °C showed weight losses of 2.33% at 55.7 °C which was attributed to the removal of water absorbed. The second stage of weight loss between 200 and 400 °C was due to the thermal degradation of the hydrogel Cts-SC NPs showed weight loss of 57.1% at 229.7 °C. The above data indicated that the thermal stability decreased after modification
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SC
%T
%T
Ct
1663
1600 2933
3436
1797
b
a
Wave number (cm-1)
Wave number (cm-1)
Cts-NPs
1642
%T
%T
Cts- SC NPs
1707
1562
1629 2927 3427
1563
3427
d
c
Wave number (cm-1)
Wave number (cm-1)
Fig. 1. FTIR spectra of a) Cts, b) SC, c) Cts-NPs and d) Cts-SC NPs.
3.3.4.2. Differential scanning calorimetry (DSC). The glass transition temperatures (Tg) of Cts-NPs and Cts-SC NPs hydrogels were determined from differential scanning calorimetry (DSC) Fig. 4b which showed a broad band related to glass transition temperatures (Tg) at 56 °C and 76 °C for Cts-NPs and Cts-SC NPs hydrogels respectively. Cts-SC NPs hydrogel showed the highest (Tg) than Cts-NPs which may be due to the
a
b
presence of long aliphatic carbon chain in the chemical structure of Cts-SC NPs [32].and the ability of the chains to undergo segmental motion, which would increase the (Tg) value [9,33]. The crystal
Intensity (Count)
of chitosan Cts by sebacoyl chloride, this may be attributed to the ease of degradation of amide groups [32].
a
b
c
2θ Fig. 2. TEM of a) Cts-NPs and b) Cts-SC NPs.
Fig. 3. X-ray diffraction of a) Cts, b) Cts-NPs and c) Cts-SC NPs.
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Fig. 5. Swelling ratio of Cts-NPs and Cts-SC NPs.
3.4. Metal ions uptake studies
Fig. 4. a. TGA of Cts, Cts-NPs and Cts-SC NP. b. DSC of Cts-NPs and Cts-SC NPs.
temperatures (Tc) for the Cts-SC NPs were recorded at 172 °C. The DSC curve for Cts-SC NPs also showed a melting temperature (Tm) at 232 °C.
3.3.5. Swelling studies The immersion of cross-linked polymers in solvents does not lead to their dissolution because of their chemically bonded hydrocarbon chains; nonetheless, these links do not prevent cross-linked polymers from swelling. Fig. 5 showed swelling studies for nano chitosan CtsNPs and modified chitosan Cts-Sc NPs and indicated greater swelling for hydrogel Cts-Sc NPs as compared to Cts-NPs hydrogel at pH ranging from 3 to 10, this affinity to absorb water was attributed to the presence of hydrophilic groups such as amide group [34]. Cts-Sc NPs hydrogel showed the highest degree of swelling at pH 5. This could be explained by the fact that the swelling was controlled mainly by the amine group (NH2) on the C2 carbon of the chitosan component. The protonation of the chitosan amino groups at low pH leaded to repulsion of the polymeric chains [35]. High swelling at pH = 3 was confirmed the chemical stability of the new hydrogel in high acidic conditions. Raising the pH above 7 would reduce the number of NH3+ groups and increase the number of NH2 groups. Swelling was thus decreased as the positive charge repulsions were removed [13]. Modified hydrogel Cts-Sc NPs showed higher swelling compared to Cts-NPs at pH 10 due to increase of the hydrophilicity of the Cts after modification with sebacoyl chloride.
3.4.1. Effect of contact time The main question to answer for the metal adsorption process was the effect of time on metal ions uptake. Since various ions were present in wastewater, it was essential to investigate the competitive binding affinity of hydrogels for these ions. Different factors such as hydrogel structure, metal combination, level of metal concentration, and uptake time affect the interaction between the metal ions and the reactive sites of hydrogels [6,36–42]. It was considered that there were three different possibilities for metal ion uptake (i) adsorption by the active functional groups that could acquire metal ions, (ii) a simple chelating mechanism, or (iii) adsorption by the open polymer matrix [43]. The structure of hydrogel skeleton was an important factor that would affect adsorption of metal ions [44]. Therefore, in this study we compare between the metal adsorption capacity values of Hg2+, Ni2+ and Co2+ by hydrogels Cts-NPs and CtsSc NPs at optimum conditions for pH, temperature, adsorbent dose and initial metal ions concentration. The adsorption capacity for Hg2+, Ni2+ and Co2+ ions was investigated for 10 h and the results were presented in Fig. 6a,b for hydrogels Cts-NPs and Cts-SC NPs respectively. It was evident from Fig. 6a,b for both nano adsorbents that the adsorption capacity values increased as contact time increased due to the transport of metal ions from the aqueous phase onto the surface of the hydrogels Cts-NPs and Cts-SC NPs. The graphical results for the efficiency % Fig. 6c showed that Cts-SC NPs hydrogel had higher percent removal of Hg2+, Ni2+ and Co2+ than Cts-NPs hydrogel. This could be attributed to the chemical structure of Cts-SC NPs hydrogel which contained long aliphatic carbon chain which given more flexibility and two amide groups participated in chelating with the metal cations thus increased removal of metal ions from aqueous solution. It can be concluded that modification of chitosan with sebacoyl chloride SC was utilized to increase the metal ions uptake from aqueous solution.
3.4.2. Effect of pH pH water contaminated with heavy metal ions and industrial effluents were generally known to have different pH values depending on the type of industrial activities. The pH of aqueous solution was
Table 1 Thermogravimetric analysis (TGA) for hydrogels. Hydrogel code Cts Cts-NPs Cts-SC NPs
Temperature
Weight loss%
Temperature
Weight loss%
Temperature
Weight loss%
52 49.4 55.7
14 21.5 2.33
224.6 – 229.7
24.9 – 57.1
267.8 268.7 –
25.11 23 –
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583
Fig. 6. Effect of contact time for a) Cts-NPs, b) Cts-SC NPs and c) Efficiency%. d: Effect of pH values on Cts-SC NPs. e: Initial metal ions concentration of Hg2+and Ni 2+ on Cts-SC NPs.
known as one of the most important operational factors in the adsorption process because it affected the solubility of metal ions, concentration of the counter ions, the functional groups of the sorbent and the degree of ionization of the sorbent during reaction. According to the pH value of the contact solution, the active functional groups and sites on the sorbent surface can be either protonated or deprotonated and the sorbate speciation in solution can be also affected [45,46]. Previous studies reported that the maximal sorption efficiency for M2+ ions was observed at pH 5. However, when the pH value was increased up to 6.0, the adsorption capacity decreased due to the formation of soluble hydroxylated complexes of the metal ions which compete for the active sites, and as a consequence, the retention was decreased [6,47–50]. In our study, the effect of pH was studied in range of 1.0–5.0 at a fixed temperature, sorbent dose and initial metal ions concentration for 6 h using the Cts-SC NPs hydrogel which showed the highest percent removal of metal ions uptake. The pH values selected in the
experiments were chosen with respect to the precipitation limits of the three metals ions Hg2+, Ni2+ and Co2+ (pH 5). As shown in Fig. 6d, Cts-SC NPs hydrogel showed low metal ion uptake in acidic conditions (at low pH values) where the H+ ion concentration was very high. Low level of metal ion adsorption may therefore be attributed to the protonation of active groups and the competition of H+ with metal ions for adsorption sites on the hydrogel surface [51]. The abundance of H+ in the medium showed another type of interaction (ion exchange mechanism) through the protonation of the unsubstituted amino groups [16]. From Fig. 6d the removal percentage of metal ions increased with increasing pH values. The H+ ions concentration decreases with increasing pH of the medium, an increased number of M2 + ions get bound and hence the maximum adsorption capacity was observed at pH 5. M2+ ions uptake increased as the pH increased, while most active sites on the adsorbent were deprotonated resulting in a greater net attractive force which was responsible for high M2+ ions uptake removal from aqueous solution [30].
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Table 2 Adsorption isotherm model parameters for the adsorption of Hg2+ and Ni2+ on Cts-Sc NPs. Metal ion
Hg2+ Ni2+
Langmuir isotherm
Freundlich isotherm
Equation
R2
KL
Qm
Equation
R2
N
KF
Ce/qe = 0.0127 Ce+ 0.0505 Ce/qe = 0.0147 Ce+ 0.0766
0.9932 0.9944
0.25 0.192
78.74 68
Log qe = 0.4925 Ce+ 1.2776 Log qe = 0.4703 Ce+ 1.751
0.9724 0.9747
2.03 2.13
18.95 14.97
3.4.3. Effect of initial metal ions concentration Preliminary experiments revealed that the concentration of metal ions could have a significant effect on the adsorption capacity of the Cts-SC Nps hydrogel. The effect of initial metal ions Hg2+ and Ni2+ concentrations on the adsorption capacity of Cts-Sc NPs was studied. In this study, pH, temperature, adsorbent dose were fixed and with a contact time of 24 h. The results were presented in Fig. 6e which indicated that increasing the Hg2+ and Ni2+ concentration accelerated the diffusion of Hg2+ and Ni2+ ions from solution to the adsorbent surface due to the increase in the driving force of the concentration gradient. 3.4.4. Adsorption isotherms The equilibrium adsorption isotherm was fundamental in describing the interactive behavior between solute and adsorbent, and it was very important for designing an adsorption system. Quantification of the sorption capacity of Hg2+ and Ni2+ ions onto Cts-SC NPs was evaluated using two parameter equations namely the Langmuir and Freundlich isotherm models [16,52]. The Langmuir model [53] assumed that the metal ions uptake occurred on a homogeneous surface by monolayer adsorption without any interaction between adsorbed ions. The Langmuir and Freundlich adsorption isotherm models [16,52] were used to determine the appropriate isotherm for Hg2+ and Ni2+ ions adsorption on the Cts-Sc NPs hydrogel. Langmuir Eq. (6) and Freundlich Eq. (7) were employed in the following linear form as follows: Ce =qe ¼ 1=kL Q m þ Ce =Q m
ð6Þ
In Eq. (6) Ce (mg L−1) and qe (mg g−1) were the equilibrium concentration of M2+ ions in the solution and adsorbent, Qm (mg g−1)
a
e
b
f
represented the maximum adsorption amount, kL was the Langmuir constant. The values of Qm and kL were determined from the slope and intercept of the plots of Ce/qe versus Ce and represented in Table 2. The linearized Freundlich isotherm had the general form given in Eq. (7): log qe ¼ log k F þ 1=n log Ce
ð7Þ
In Eq. (7) qe (mg g−1) and Ce (mg L−1) were the equilibrium concentrations of M2+ ions in the adsorbent and solution, kF and n were Freundlich constant and intensity factors respectively. The values of n and kF were calculated from the slope and intercept of the plots of log qe versus log Ce and represented in Table 2. It was obvious that the Langmuir adsorption isotherm was more suitable to describe the adsorption equilibrium (R2 N 0.99). The essential characteristics of Langmuir isotherm could be explained in terms of dimensionless constant separation factor (RL) [54] which was defined by Eq. (8): RL ¼ 1=ð1 þ KL Co Þ
ð8Þ
where, KL (L mg−1) was the Langmuir constant related to the energy of adsorption and Co (mg L−1) was the initial concentration of both Hg2+ and Ni2+ ions. The RL value indicated the type of the isotherm to be either unfavorable (RL N 1), linear (RL = 1), favorable (0 b RL b 1), irreversible (RL = 0). RL values lay between 0.12 and 0.286 for Hg2+ and between 0.15 and 0.34 for Ni2+ indicating the favorable sorption of Hg2+ and Ni2+ ions by the Cts-SC-NPs hydrogel.
c
d
g
h
Fig. 7. a, e SEM of Cts-SC NPs before treatment of metal ions at 500 and 1000 μm, b, c, d SEM of Cts-SC NPS after treatment of metal ions Hg2+, Ni2+ and Co2+ at 500 μm and f, g, h for Hg2+, Ni2+ and Co2+ at 1000 μm.
N.G. Kandile, H.M. Mohamed / International Journal of Biological Macromolecules 122 (2019) 578–586 Table 3 Anti-bacterial and antifungal activity data of Cts-NPs and Cts-Sc NPs. Sample
Inhibition zone diameter (mm/mg sample) Bacterial species
Control: DMSO Standard Ampicillin antibacterial agent Amphotericin B antifungal agent Cts-NPs Cts-Sc NPs
Fungus
(G−)
(G+)
Bacillus subtilis
Pseudomonas aeruginosa
Aspergillus flavus
0.0 26
0.0 26
0.0 –
–
–
17
9 11
9 10
0.0 10
585
be the better one due to a high value of R2. The surface morphology of Cts-SC NPs hydrogel, after metal ions uptake, showed heterogeneous holes and some small openings on the surface. Thus chitosan nanoparticles based on the sebacoyl moiety hydrogel, Cts-SC NPs, had the capability as a metal ions adsorbent for wastewater contaminated with Hg2 + , Ni2+ and Co2+ ions. Finally, the antimicrobial activity data showed that inhibitory effects of Cts-SC NPs hydrogel were always higher in comparison with that of a Cts-NPs hydrogel. Conflict of interest The authors report no conflict of interest. The authors alone are responsible for the content and writing of the article. Acknowledgment
3.5. Surface morphology studies The surface morphology of the Cts-SC NPs hydrogel, before and after metal ions Hg2+, Ni2+ and Co2+ uptake was examined by Scanning Electron Microscopy (SEM) and the images obtained were shown in Fig. 7a–h. The surface morphology of the Cts-SC NPs hydrogel before treatment with metal ions appeared to be smooth in Fig. 7a, e, however, after treatment with metal ions the images in Fig. 7b–h illustrated that the surface morphology of the Cts-SC NPs hydrogel had an irregular structure. Heterogeneous holes and some small openings were seen on the surface for Ni2+ and Co2+ but with Hg2+, the adsorbent surface became abnormal and a great deal of crystal adhered to the surface [6], which would increase the contact area and improve metal ions adsorption.
We thank our department for support with the chemicals used in this research. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
3.6. Antimicrobial activity [12]
The data on antibacterial activity of Cts-NPs and Cts-SC NPs hydrogels on Bacillus subtilis as Gram positive bacteria, Pseudomonas aeruginosa as Gram negative bacteria, and a fungi, Aspergillus flavus were displayed in Table 3. The results showed higher inhibition zone diameters of Cts-SC NPs hydrogel than of Cts-NPs hydrogel due to several mechanisms reported on the antimicrobial activity of chitosan. Among them the three most accepted mechanisms are discussed below. First one considers the electrostatic forces of interaction between the positively charged protonated amino groups of chitosan and the negatively charged microbial cell surface [55]. The second mechanism considers the binding of chitosan with microbial DNA, which inhibits the mRNA and protein synthesis via penetration of chitosan into the nuclei of the microorganisms [56]. The third mechanism considers the chelating of chitosan with metals, resulting in suppression of spore elements and binding to essential nutrients to stop microbial growth [57]. 4. Conclusion Chitosan nanoparticle hydrogel Cts-NPs and modified hydrogel CtsSC NPs were successfully synthesized via ionotropic gelation and characterized. Modified chitosan hydrogel Cts-Sc NPs showed a significantly high swelling ratio in water than Cts-NPs hydrogel from pH 3 to 10. Evaluation of the hydrogels Cts-NPs and Cts-SC NPs as adsorbent agents for Hg2+, Ni2+, and Co2 + ions from aqueous solution revealed that hydrogel Cts-SC NPs showed a more remarkable adsorption capacity for Hg2+, Ni2+ and Co2+ ions than hydrogel Cts-NPs which was attributed to the presence of long aliphatic carbon chain in the modified hydrogel Cts-SC NPs which gave more flexibility and two amide groups that participated in chelating with the metal cations. Also the modified hydrogel Cts-SC NPs showed a higher metal ions uptake capacity at pH values. Adsorption isotherms data were fitted using two different parameter models. The Langmuir model proved to
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