molecules Article
Synthesis and Characterization of Glutamic-Chitosan Hydrogel for Copper and Nickel Removal from Wastewater Huda E. Abdelwahab, Seham Y. Hassan, Mohamed A. Mostafa and Mohamed M. El Sadek * Chemistry Department, Faculty of Science, Alexandria University, Alexandria 21231, Egypt;
[email protected] (H.E.A.);
[email protected] (S.Y.H.);
[email protected] (M.A.M.) * Correspondance:
[email protected] or
[email protected]; Tel.: +20-010-065-446-17 Academic Editor: Massimiliano Fenice Received: 24 March 2016; Accepted: 20 May 2016; Published: 25 May 2016
Abstract: Chitosan was reacted with four concentrations (2.5, 5, 10 and 20 mmol) of glutamic acid resulting in four types of glutamic-chitosan hydrogels (GCs), the activity of the resulted compounds on the removal of copper(II) and nickel(II) from wastewater were tested. The results indicated that by increasing glutamic acid concentration from GCs-1 to GCs-4, the efficiency of removing Cu(II) and Ni(II) were decreased, which may be due to a decrease in the pore size of the hydrogels as a result of the increased degree of crosslinking. Keywords: chitosan; wastewater; copper(II); nickel(II); glutamic acid
1. Introduction The huge increase in the use of heavy metals over the past few decades has resulted in an unwanted increased presence of heavy metals in the environment, for example, industrial wastewater which contains high amount of heavy metals can pollute water resources. Heavy metals which include zinc, copper, nickel, mercury, cadmium, lead and chromium are one of the most toxic types of water pollutants. At least 20 metals are considered to be toxic and approximately half of these metals are emitted to the environment in quantities that are risky to the surroundings, in addition to the human health. A majority of heavy metals are non-biodegradable and highly toxic [1], so their concentrations have to be reduced to acceptable levels before discharge into the environment; otherwise, they can pose a threat to the health of animals and humans. Wastewater containing heavy metals results mainly from metal plating facilities, mining operations, batteries, paper, fertilizer, tanneries, pesticide industries, stabilizers, thermoplastics, and pigment manufacture [2]. These industries discharge heavy metals directly or indirectly into the environment, especially in developing countries. Heavy metals tend to accumulate in living organisms [3] causing numerous diseases and disorders due to their toxicity and non-biodegradability [4]. Nickel is a chemically active metal used for preparing a large number of nickel alloys and also used in many other industries. The excessive intake of nickel may cause carcinogenesis, mutagenesis and dermatogenic effects as a result of bioaccumulation [5,6]. Copper (Cu(II)) is micronutrient element that plays an important role in bone formation together with certain proteins and enzymes [7]. However, the consumption of food or water containing high copper concentrations can cause several diseases such as gastrointestinal symptoms, liver toxicity, osteoporosis, Wilson’s, and Alzheimer’s diseases [8–10]. Excessive intake of copper can also cause hemolysis, hepatotoxicity, nephrotoxicity, vomiting, cramps, and convulsions [4]. The high cost and complexity, high energy consumption and secondary pollution problems [11] of most of the treatment processes that are used to remove heavy metals from wastewater are
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considered to be major problems [12]; for those reasons, a number of studies were carried out on the use of low-cost adsorbents for the removal of heavy metals from natural resources [13] such as chitosan, which also of Molecules 2016, 21, 684 is a biodegradable and biocompatible polymer. Chitosan consists mainly 2 of 14 β-(1Ñ4)-2-acetamido-2-deoxy-D-glucose units which are produced by deacetylation of chitin [14]. chitosan, also is aabundant biodegradable and biocompatible polymer. Chitosan mainly of Chitosan is thewhich second most biopolymer on Earth after cellulose [15], itconsists is widely distributed β-(1→4)-2-acetamido-2-deoxyD-glucose units which are produced by deacetylation of chitin [14]. in crustacean shells and cell walls of fungus. Several methods have been reported for the chemical Chitosan is the second most abundant biopolymer on Earth after cellulose [15], it is widely distributed modification of chitosan, one of which is the crosslinking of chitosan with various substances such as in crustacean shells and cell walls of fungus. Several methods have been reported for the chemical dialdehydes and dicarboxylic acids. Chitosan was modified with several dicarboxylic acids including modification of chitosan, one of which is the crosslinking of chitosan with various substances such as glutamic acid byand reacting both carboxylic acidwas groups withwith theseveral aminodicarboxylic groups of chitosan [6]. In the dialdehydes dicarboxylic acids. Chitosan modified acids including present work, chitosan was reacted with four different amounts of glutamic acid, resulting in four glutamic acid by reacting both carboxylic acid groups with the amino groups of chitosan [6]. In the typespresent of glutamic-chitosan (GC) hydrogels, thedifferent resulting crosslinking polymers were tested for the work, chitosan was reacted with four amounts of glutamic acid, resulting in four types glutamic-chitosan (GC) hydrogels, the resulting crosslinking polymers were tested for the removal ofofcopper(II) and nickel(II) from wastewater. removal of copper(II) and nickel(II) from wastewater.
2. Results and Discussion 2. Results and Discussion
2.1. Synthesis of Glutamic-Chitosan Cross Linked Hydrogels (GCs) 2.1. Synthesis of Glutamic-Chitosan Cross Linked Hydrogels (GCs)
Chitosan was modified with glutamic acid, whereby the carboxylate groups of glutamic acid Chitosan was modified glutamic(Scheme acid, whereby the amount carboxylate groups of acid glutamic reacted with the amino groupswith of chitosan 1). The of glutamic withacid respect reacted with the amino groups of chitosan (Scheme 1). The amount of glutamic acid with respect to to chitosan was varied to produce four new cross-linked glutamic-chitosan hydrogels (Scheme 1) chitosan was varied to produce four new cross-linked glutamic-chitosan hydrogels (Scheme 1) designated designated as: glutamic-chitosan-1 (GCs-1), glutamic-chitosan-2 (GCs-2), glutamic-chitosan-3 (GCs-3), as: glutamic-chitosan-1 (GCs-1), glutamic-chitosan-2 (GCs-2), glutamic-chitosan-3 (GCs-3), and and glutamic-chitosan-4 (GCs-4)with withincreasing increasing degrees of cross linking, respectively. Allprepared the prepared glutamic-chitosan-4 (GCs-4) degrees of cross linking, respectively. All the derivatives are produced in a nearly quantitative yield (89%–96.4%). derivatives are produced in a nearly quantitative yield (89%–96.4%). O
O OH *
O HO
O
H3COCHN HO O
NH2
HO
+
O O
HO
NH2
OH
OH *
*
O
O HO
n
NH O
1
H3COCHN HO O HO
O O
* n
NH2
O NH OH *
O O
O OH
OH O O
NHCOCH3
*
HO n
Scheme Synthesisof of glutamic-chitosan glutamic-chitosan hydrogel. Scheme 1. 1.Synthesis hydrogel.
The modification of chitosan with crosslinking reactions leads to formation of chitosan derivatives
The of chitosan with media crosslinking reactions to formation of the chitosan derivatives withmodification better resistance in extreme conditions [16]. leads On the other hand, crosslinking with modification better resistance in extreme media conditions [16]. On the other hand, the crosslinking slightly decreases the adsorption capacity of chitosan; to overcome this difficulty we modification slightlyacid decreases the adsorption capacity chitosan; overcome this2)difficulty choose glutamic as crosslinker since glutamic acidofwill increasetothe amino (-NH and the we carbonyl (C=O) [17]. Increasing the abundance of increase these groups in the (-NH target2 )molecule will choose glutamic acidgroups as crosslinker since glutamic acid will the amino and the carbonyl facilitate complex formation with Ni(II) and Cu(II). The adsorption mechanism (C=O) groupsincreased [17]. Increasing the abundance of these groups inproposed the target molecule will facilitate of Ni(II) and Cu(II) on chitosan-glutamic hydrogel is illustrated in Scheme 2. increased complex formation with Ni(II) and Cu(II). The proposed adsorption mechanism of Ni(II) and Cu(II) on chitosan-glutamic hydrogel is illustrated in Scheme 2. 2.2. Fourier Transform Infrared Spectroscopy (FTIR) Characterization of G-Cs
Glutamic-chitosan formation was confirmed using Fourier transform 2.2. Fourier Transform Infrared Spectroscopy (FTIR) Characterization of G-Csinfrared spectroscopy. The FTIR spectrum of chitosan showed four strong absorption peaks at 1157.8, 1076.6, 1030, and 895.7 cm−1
Glutamic-chitosan formation was confirmed using Fourier transform infrared spectroscopy. The which are characteristic peaks of the saccharide structure, where the OH and NH functions showed a FTIR very spectrum of chitosan showed four strong3600–3200 absorption peaks at 1157.8, 1030, 895.7 cm´1 strong broad absorption peak around cm−1 . Primary amines1076.6, showed twoand absorption −1, the whichpeaks are characteristic peaks saccharide structure, the OH andofNH functions[18]. showed at 1650.4 and 1598.9 cmof which indicated that chitosanwhere had a high degree deacetylation ´1 −1 due a very strong absorption around 3600–3200 amines showed two The broad FTIR spectra of the peak GC hydrogels showed thecm broad. Primary band between 3450 and 3470 cmabsorption peakstoatthe 1650.4 andNH 1598.9 cm´In1 ,addition, which indicated that chitosan had aofhigh deacetylation at 1650.4ofand 1598.9 cm−1 [18]. OH and groups. the characteristic absorbance NH2 degree was also seen. The spectra also showed a broad absorption band around 1637 cm−1 which corresponds
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The FTIR spectra of the GC hydrogels showed the broad band between 3450 and 3470 cm´1 due 3 of ´114 In addition, the characteristic absorbance of NH2 at 1650.4 and 1598.9 cm was also seen. The spectra also showed a broad absorption band around 1637 cm´1 which corresponds to the (CONH) amide group, and the intensity of this band increased with increasing cross-linking to the (CONH) amide group, and the intensity of this band increased with increasing cross-linking densityofofthe thehydrogels, hydrogels,i.e., i.e.,from fromGCs-1 GCs-1totoGCs-4 GCs-4(see (seeTable Table1).1). density 684groups. toMolecules the OH2016, and21, NH
2+2+for Scheme2.2.The Theproposed proposedmechanism, mechanism,MM for(A) (A)nickel nickeland and(B) (B)copper. copper. Scheme
Table1.1.IRIRspectral spectraldata dataofofcompounds compounds1–4. 1–4. Table Compound Compound GCs-1 GCs-2 GCs-1 GCs-3 GCs-2 GCs-3 GCs-4 GCs-4
IR (γ, cm−1) IR (γ,3465 cm´1 ) 1635 (CONH), (NH), 3465 cm−1 (OH) −1 (OH) ´1cm 1637 (CONH), 3458 (NH), 3458 1635 (CONH), 3465 (NH), 3465 cm (OH) ´1 −1 (OH) 1637 (CONH), 3467 (NH), 3467 cm 1637 (CONH), 3458 (NH), 3458 cm (OH) ´1 −1 (OH) 1637 (CONH), 3467 (NH), 3467 cm (OH) 1638 (CONH), 3467 (NH), 3467 cm 1638 (CONH), 3467 (NH), 3467 cm´1 (OH)
2.3. Elemental Characterization of GCs 2.3. Elemental Characterization of GCs Elemental analyses of the GC derivatives is another confirmation of GC formation; the elemental analysis data are shownofinthe Table Elemental analyses GC 2. derivatives is another confirmation of GC formation; the elemental analysis data are shown in Table 2. Table 2. Elemental analyses and % yield of G-chitosan hydrogels. Table 2. Elemental analyses and % yield of G-chitosan hydrogels. Elemental Analyses Compound Yield % %C %H %N Elemental Analyses Yield % Cs Compound 45.10 6.77 8.43 %H % N 7.69 GCs-1 47.80 % C 7.15 96.4 Cs 47.84 45.10 6.77 8.43 7.72 GCs-2 7.24 92.5 GCs-147.86 47.80 7.15 7.69 7.7496.4 GCs-3 7.27 90.2 GCs-2 47.84 7.24 7.72 92.5 GCs-4 47.88 7.35 7.77 89 GCs-3 47.86 7.27 7.74 90.2 GCs-4 47.88 7.35 7.77 89
2.4. 1H-NMR Characterization of GC Hydrogels
The structure of compounds GCs-1 to GCs-4 is further proved by 1H-NMR spectroscopy, which showed the two (NH) protons as a singlet at δ 7.77, the 1′-OH proton at 4.00, and the rest of the sugar protons at the range 3.29–3.33 ppm, as well as the appearance of the two (NH2) protons f at 3.29 (see Experimental). After shaking of compounds 1–4 with D2O, their 1H-NMR spectra showed the disappearance of the (NH2) and (NH) protons as well as (OH) protons [18].
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2.4. 1 H-NMR Characterization of GC Hydrogels The structure of compounds GCs-1 to GCs-4 is further proved by 1 H-NMR spectroscopy, which showed the two (NH) protons as a singlet at δ 7.77, the 11 -OH proton at 4.00, and the rest of the sugar protons at the range 3.29–3.33 ppm, as well as the appearance of the two (NH2 ) protons f at 3.29 (see Experimental). After shaking of compounds 1–4 with D2 O, their 1 H-NMR spectra showed the Molecules 2016, 21, 684 4 of 14 disappearance of the (NH2 ) and (NH) protons as well as (OH) protons [18]. Scanning Electron Microscopy Observations of G-Chitosan Hydrogels 2.5. Scanning
Microstructures of the hydrogels surface were investigated by scanning electron microscopy as presented in Figure 1. It could be seen that the hydrogels have a similar surface appearance, but the distribution and the size of their pores are different. The porosity distribution became more uniform and dense with increasing concentration of glutamic acid.
Figure 1. 1. Scanning Scanning electron electron microscopy microscopy of of G-chitosan G-chitosan hydrogels. hydrogels. Figure
The extremely extremelyporous poroussurface surfacestructure structure hydrogels could to high surface The ofof thethe hydrogels could leadlead to high surface areas.areas. The pore pore size the hydrogels decreased with increasing the cross-linking density the hydrogels from size of theofhydrogels decreased with increasing the cross-linking density of the of hydrogels from GCs-1 GCs-1 to GCs-4 hydrogel. to GCs-4 hydrogel. 2.6. Solubility Solubility of G-Chitosan Hydrogels The solubility solubilityofofthe thenew newhydrogels hydrogels was studied in different solvents at room temperature. was studied in different solvents at room temperature. The The results show hydrogels insoluble aceticacid acidsolution solution(1% (1% v/v), v/v),dimethylformamide, dimethylformamide, results show thatthat the the hydrogels areare insoluble inin acetic dimethyl sulfoxide, tetrahydrofuran, N-methylpyrrolidone, chloroform, methylene chloride, acetone and methanol since no soluble fractions of the hydrogels were obtained. This indicates a successful formation of crosslinked networks in these hydrogels.
2.7. Sorption Studies of Ni(II) and Cu(II) 2.7.1. Influence of G-chitosan Amount The dependence of Ni(II) sorption on G-chitosan amount was studied by varying the amount of
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2.7. Sorption Studies of Ni(II) and Cu(II) 2.7.1. Influence of G-chitosan Amount The dependence of Ni(II) sorption on G-chitosan amount was studied by varying the amount of the adsorbent from 1 g to 5 g while keeping the other parameters such as pH, metal solution volume (100 mL), concentration (200 mg/L), and contact time (60 min) constant. Figure 2A shows that the percentage removal of nickel increases with increasing adsorbent dose from 48% to 95%. Molecules 2B 2016,shows 21, 684 that the removal efficiency of copper was improved on increasing adsorbent 5 of 14 Figure doses; this may occur due to the fact that the higher dose of adsorbents in the solution provides the greater availability of exchangeable sites for the ions. The maximum % removal of Cu(II) was 95.17% at theMolecules dosage2016, of 21, 250684mg. 5 of 14
Figure 2. Influence of GCs amount on (A) nickel and (B) copper sorption.
Figure 2B shows that the removal efficiency of copper was improved on increasing adsorbent doses; this may occur due to the fact that the higher dose of adsorbents in the solution provides the Figure 2. Influence GCsamount amount on (A) nickel and copper sorption. Figure Influence ofofGCs on (A) nickel and(B)(B) sorption. greater availability of2.exchangeable sites for the ions. The maximum %copper removal of Cu(II) was 95.17% at the dosage of 250 mg. Figure 2B shows that the removal efficiency of copper was improved on increasing adsorbent 2.7.2. Influence of pH doses; this may of occur 2.7.2. Influence pH due to the fact that the higher dose of adsorbents in the solution provides the greater availability sites for The maximum % removal of Cu(II) wasthe 95.17% The effect of pH of onexchangeable the adsorption of the Ni ions. is presented in Figure 3A. The pH of aqueous The effect of pH on the adsorption of Ni is presented in Figure 3A. The pH of the aqueous at the dosage of 250 mg. solution is an important parameter in the adsorption process because it affects the concentration of solution is an important parameter in the adsorption process because it affects the concentration of the counter ionsions on the functional groups of the adsorbent, the solubility of the metal ions and the the counter 2.7.2. Influence ofon pHthe functional groups of the adsorbent, the solubility of the metal ions and the degree of ionization of the adsorbate during [12].The Theactive active sites adsorbent can either degree of ionization of the adsorbate duringreaction reaction [12]. sites onon an an adsorbent can either The effect of pH on thedepending adsorption on of Ni ispH presented inthe Figure 3A. Thethe pHadsorbate of the aqueous be protonated or deprotonated the while at same time speciation be protonated or deprotonated depending on the pH while at the same time the adsorbate speciation solution is an important parameter in the adsorption process because it affects the concentration of in solution depends on the pHpH too. in solution depends on the too. the counter ions on the functional groups of the adsorbent, the solubility of the metal ions and the degree of ionization of the adsorbate during reaction [12]. The active sites on an adsorbent can either be protonated or deprotonated depending on the pH while at the same time the adsorbate speciation in solution depends on the pH too.
Figure 3. InfluenceofofpH pHon on(A) (A) nickel nickel and sorption. Figure 3. Influence and(B) (B)copper copper sorption.
At low pH (2–4), less metal ion uptake was observed due to the competitive adsorption of the
At (2–4), ion uptake was observed dueAt to the adsorption of is the H+ H+ low and pH Ni(II) ionsless onmetal the G-chitosan compounds surface. lowcompetitive pH values, the adsorbent Figure 3. Influence of pH on (A) nickel and (B)the copper sorption. + ion concentration, and Ni(II) ionscharged on the G-chitosan compounds surface. At low pH values, adsorbent is metal positively positively with higher H reducing number ofthe binding sites for + charged higher ion concentration, reducing number of binding for metal ion (Figure 4). ion with (Figure 4). In H addition, the protonation of aminothe groups in acidic solutionsites induces an electrostatic At low pH (2–4), less metal ion uptake was observed due to the competitive adsorption of the repulsion ofprotonation metal cationsof that reduces the number of binding available for metallic ions [13]. of In addition, the amino groups in acidic solutionsites induces an electrostatic repulsion H+ and Ni(II) ions on the G-chitosan compounds surface. At low pH values, the adsorbent is However, Ni(II) uptake increased as the pH increased to pH 9, as most active sites on the adsorbent metalpositively cations that reduces the number of binding sites available for metallic ions [13]. However, Ni(II) charged with higher H+ ion concentration, reducing the number of binding sites for metal areincreased deprotonated resulting to a moreto net attractive forceactive whichsites is responsible for high nickeldeprotonated removal uptake as pH increased pH 9,amino as most on the adsorbent ion (Figure 4). In the addition, the protonation of groups in acidic solution induces anare electrostatic from aqueous solution. The optimum adsorption takes place at pH 5. Further increase in pH leads to resulting to a more netcations attractive force which is responsible high nickelfor removal aqueous repulsion of metal that reduces the number of bindingfor sites available metallicfrom ions [13]. the precipitation of nickel hydroxide complexes which inhibits the adsorption process. solution. The optimum adsorption takes place at pH 5. Further in pHsites leads the precipitation However, Ni(II) uptake increased as the pH increased to pH 9, increase as most active onto the adsorbent Figure 3B as shown above illustrated that pH obviously influenced the removal efficiency of the are deprotonated resulting towhich a moreinhibits net attractive force which process. is responsible for high nickel removal of nickel hydroxide complexes the adsorption copper ions in the aqueous solution; the results indicated that Cu(II) removal was increased to from aqueous solution. The optimum adsorption takes place at pH 5. Further increase in pH leads to maximum and then decreased with pH variation from 4 to 9 at 25°C and agitation speed of 100 rpm. the precipitation of nickel hydroxide complexes which inhibits the adsorption process. The maximum % removal of Cu(II) was 95% at pH 5. The dominant species of copper was free Cu(II) Figure 3B as shown above illustrated that pH obviously influenced the removal efficiency of the and was mainly involved in the adsorption process when the pH was lower than 5. With the pH copper ions in the aqueous solution; the results indicated that Cu(II) removal was increased to greater than 5, copper ions started to precipitate as Cu (OH) [13]. Increases in metal removal with
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Figure 3B as shown above illustrated that pH obviously influenced the removal efficiency of the copper ions in the aqueous solution; the results indicated that Cu(II) removal was increased to maximum and then decreased with pH variation from 4 to 9 at 25˝ C and agitation speed of 100 rpm. The maximum % removal of Cu(II) was 95% at pH 5. The dominant species of copper was free Cu(II) and was mainly involved in the adsorption process when the pH was lower than 5. With the pH Molecules 21, 684 ions started to precipitate as Cu (OH) [13]. Increases in metal removal 6 of 14 with greater than2016, 5, copper increased pH can be explained on the basis of the decrease in competition between proton and metal increased pH can be explained on the basis of the decrease in competition between proton and metal cations, which results in a lower electrostatic repulsion between surface and metal ions. Decrease in cations, which results in a lower electrostatic repulsion between surface and metal ions. Decrease in adsorption at higher (>5) is is due of soluble solublehydroxyl hydroxyl complexes Molecules 2016, 684pHpH adsorption at21,higher (>5) duetotothe theformation formation of complexes [15].[15]. 6 of 14 increased pH can be explained on the basis of the decrease in competition between proton and metal cations, which results in a lower electrostatic repulsion between surface and metal ions. Decrease in adsorption at higher pH (>5) is due to the formation of soluble hydroxyl complexes [15].
Figure 4. Metalion ionuptake uptake (A) (A) at low at at high pH.pH. Figure 4. Metal lowpH pHand and(B) (B) high
2.7.3. Influence of Contact Time
2.7.3. Influence of Contact Time
Contact time is one of the effective factors in the batch adsorption process. Keeping other parameters
Figure Metal ion uptake (A) at low and (B) at high pH. process. Keeping other Contact is one(25 of°C), the4.pH effective factors thepH batch adsorption including time temperature 5, adsorbent dosein (1 g/100 mL), initial nickel concentration (200 mg/L) ˝ parameters including temperature (25 the C), adsorption pH 5, adsorbent dose (1 g/100 mL), initial nickel and agitation speed (250 rpm) constant, of nickel on glutamic-chitosan compounds 2.7.3. Influence of Contact Time concentration (200 mg/L) agitation speed rpm)time constant, theadsorption adsorption of nickel was studied in the rangeand 10–360 min. The effect (250 of contact on nickel efficiency is on Contact time is one of the effective factors inincreased the adsorption process. Keeping other parameters glutamic-chitosan compounds wasrate studied in thebatch range 10–360and min. effect of contact time on shown in Figure 5A. Adsorption initially rapidly, the The optimal removal efficiency including temperature (25min. °C),No pHchange 5, adsorbent doseconcentration (1 g/100 mL), after initial280–360 nickel concentration (200 mg/L) reached within 280 in nickel minincreased was observed. The and nickelwas adsorption efficiency is shown in Figure 5A. Adsorption rate initially rapidly, and agitation speed (250 rpm) constant, the adsorption of nickel on glutamic-chitosan compounds availability of sufficient vacant adsorbing sites in the beginning of the removal process is possibly the after the optimal removal efficiency was reached within 280 min. No change in nickel concentration was studied in the initial range removal; 10–360 min. The effect of contactpercent time on nickel adsorption efficiency is cause of the higher afterwards, the removal rate decreased due to the limited 280–360 mininwas observed. The availability of sufficient vacant adsorbing sites in the beginning of the shown Figure 5A. Adsorption rate initially increased rapidly, and the optimal removal efficiency vacant adsorption sites. removal process is possibly the cause of the higher initial removal; afterwards, the removal percent was reached within 280 min. No change in nickel concentration after 280–360 min was observed. The rate decreased to the limited vacant adsorption availabilitydue of sufficient vacant adsorbing sites in thesites. beginning of the removal process is possibly the Figure that removal; Cu(II) removal wasthe increased 50% todecreased 92% as the contact time varied cause of5B theindicates higher initial afterwards, removalfrom percent rate due to the limited vacant adsorption sites. from 10 min to 360 min. The optimum contact time for maximum removal (92%) of Cu(II) was 300 min.
Figure 5. Influence of contact time on (A) nickel and (B) copper sorption.
Figure 5B indicates that Cu(II) removal was increased from 50% to 92% as the contact time varied from 10 min to 360 min. The optimum contact time for maximum removal (92%) of Cu(II) was 300 min. Figure 5. Influence ofofcontact (A) nickel nickeland and(B) (B) copper sorption. Figure 5. Influence contacttime time on (A) copper sorption. 2.7.4. Influence of Initial Concentration Figure of 5B Initial indicates that Cu(II) removal was increased from 50% to 92% as the contact time varied 2.7.4. Influence Concentration
experimental results of the contact effect of initial nickel concentration on of removal efficiency are from The 10 min to 360 min. The optimum time for maximum removal (92%) Cu(II) was 300 min. presented in Figure 6A. The experiment was conducted using the volumes of solutions as 100 mL, initial The experimental results of the effect of initial nickel concentration on removal efficiency are 2.7.4. Influenceof ofmetal InitialasConcentration concentrations 50, 100, 300, 500, 900 and 1000 mg/L solution of Ni(II) in conical as flasks, presented in Figure 6A. The5,experiment was700, conducted using the volumes of solutions 100 mL, were gently shaken with 1 g of G-chitosan compounds for 60 min with 250 rpm in the orbit mechanical initial concentrations of metal asof5,the 50,effect 100, of 300, 500,nickel 700, concentration 900 and 1000onmg/L solution of Ni(II) Thewith experimental initial removal efficiency are in shaker, initial pH results of the solution 5. Figure 6A shows the nickel removal efficiency decreased conical flasks, in were gently shaken with 1was g of G-chitosan compounds for 60 min with 250 rpm in the presented Figure 6A. The experiment conducted using the volumes of solutions as 100 mL, initial with the increase in initial nickel concentration. In the case of low nickel concentrations, the ratio of the orbit concentrations mechanical shaker, with initial pH of the solution 5. Figure 6A shows the nickel removal efficiency of metal as 5, 50, 100, 300, 500, 700, 900 and 1000 mg/L solution of Ni(II) in conical flasks, initial number of moles of nickel ions to the available surface area of adsorbent is large. However, at were shaken with g of G-chitosan for 60 min with 250 rpm the in the orbit mechanical highergently concentrations, the1available sites ofcompounds adsorption become fewer and hence percentage removal shaker, with initial pH of the solution 5. Figure 6A shows the nickel removal efficiency decreased of metal ions which depends on the initial concentration decreases [2]. The removal percentage with the increase in initial nickel concentration. the case of low nickel concentrations, the ratio of the decreases from 96% to 66% as the concentrationInincreases. initial number of moles of nickel ions to the available surface area of adsorbent is large. However, at higher concentrations, the available sites of adsorption become fewer and hence the percentage removal of metal ions which depends on the initial concentration decreases [2]. The removal percentage
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decreased with the increase in initial nickel concentration. In the case of low nickel concentrations, the ratio of the initial number of moles of nickel ions to the available surface area of adsorbent is large. However, at higher concentrations, the available sites of adsorption become fewer and hence the percentage removal of metal ions which depends on the initial concentration decreases [2]. The removal percentage decreases from 96% to 66% as the concentration increases. The experimental results of the effect of initial copper concentration on removal efficiency were presented in Figure 6B, which showing that the copper removal efficiency decreased with the increase in initial copper concentration [19]. The removal percentage decreases from 92%–55%, 93%–56%, Molecules 2016, 21, 684 7 of 14 95%–58% and 97%–60% as the concentration increases.
Figure 6. Influence of the initial concentration on (A) nickel and (B) copper sorption. Figure 6. Influence of the initial concentration on (A) nickel and (B) copper sorption.
The experimental results of the effect of initial copper concentration on removal efficiency were
2.8. Sorption Isotherm Studies presented in Figure 6B, which showing that the copper removal efficiency decreased with the increase
in initial copper concentration [19]. The removal percentage decreases from 92%–55%, 93%–56%,
2.8.1.95%–58% Langmuir Sorption Isotherm and 97%–60% as the concentration increases.
The maximum amount of Ni(II) or Cu(II) ion adsorption on the modified polymer is defined by the 2.8. Sorption Isotherm Studies corresponding adsorption isotherms [17]. The Langmuir isotherm is based on the assumption that the adsorbent surface has sites with identical energy and has equal affinities for the adsorbate molecules, 2.8.1. Langmuir Sorption Isotherm which mean that each adsorbate molecule is assumed to be located on a single site. Langmuir model The maximum amount of Ni(II) or Cu(II) ion adsorption on the modified polymer is defined by predicts the formation of the monolayer of the adsorbate [20]. The experimental adsorption data are the corresponding adsorption isotherms [17]. The Langmuir isotherm is based on the assumption that fitted according to the Langmuir isotherm models, from the equation: the adsorbent surface has sites with identical energy and has equal affinities for the adsorbate molecules, which mean that each adsorbate molecule is assumed to be located on a single site. Langmuir model Ce {Qe “of C Qm qexperimental adsorption data are (1) e {Q m ` 1{pK L ˆThe predicts the formation of the monolayer the adsorbate [20]. fitted according to the Langmuir isotherm models, from the equation:
where Ce is the equilibrium concentration of the adsorbate (mg/L), Qm is the maximum adsorption e/Qe = Ce/Qm + 1/(KL × Qm) (1) and capacity of the adsorbent and KL Cis the Langmuir adsorption constant related to capacity energy of adsorption, respectively; Qe isofthe adsorption quantity (mg/g) which calculated by the where Ce is the equilibrium concentration the adsorbate (mg/L), Qm is the maximum adsorption capacity of the equation: adsorbent and KL is the Langmuir adsorption constant related to capacity and energy of adsorption, following respectively; Qe is the adsorption quantity (mg/g) calculated by the following equation: Qe “ rpC0 which ´ CqVs{W (2) Qe = [(C0 – C)V]/W
(2)
where C0 is the initial Ni(II) or Cu(II) concentration (mg/L), C is final concentration after the adsorption; where C0 is volume the initial or is Cu(II) concentration (mg/L), C is final V is the solution (L);Ni(II) and W the weight of the used adsorbent (g).concentration after the adsorption; V is thethe solution volume (L); and W is the weight the used adsorbent (g).ions on chitosan Figure 7 shows Langmuir adsorption isotherm ofofadsorption of nickel Figure 7 shows the Langmuir adsorption isotherm of adsorption of nickel ions on chitosan derivatives GCs-1, GCs-2, GCs-3 and GCs-4 using Ni(II) concentrations (100–1300 mg/L) at pH 5 and derivatives GCs-1, GCs-2, GCs-3 and GCs-4 using Ni(II) concentrations (100–1300 mg/L) at pH 5 and 1 g of adsorbent, where Figure 8 shows the Langmuir adsorption isotherm of adsorption of copper ions 1 g of adsorbent, where Figure 8 shows the Langmuir adsorption isotherm of adsorption of copper ions on chitosan derivatives GCs-1, GCs-2, GCs-3 and GCs-4 using Cu(II) concentrations (75–1300 mg/L) at on chitosan derivatives GCs-1, GCs-2, GCs-3 and GCs-4 using Cu(II) concentrations (75–1300 mg/L) at pH 5 pH and5 1and g of 1 gadsorbent. of adsorbent. By plotting Ce /Q Ce eas in Figures Figures77and and8 8forfor nickel copper, respectively; By plotting Cee/Qversus e versus C asshown shown in nickel andand copper, respectively; it wasit was foundfound that that the experimental adsorption accordingtotothe the Langmuir isotherm models the experimental adsorptiondata data are are fitted fitted according Langmuir isotherm models with with correlation coefficient values R2R2==(0.998, 0.974,0.947) 0.947)and and (0.999, 0.901, 0.967, 0.993) correlation coefficient values (0.998, 0.951, 0.951, 0.974, (0.999, 0.901, 0.967, 0.993) for for Cu(II), respectively.Both BothQ Qmm and bebe calculated fromfrom the slope and the intercept of Ni(II)Ni(II) and and Cu(II), respectively. andKKL Lcan can calculated the slope and the intercept m and intercept = 1/Q m × K L . The fitting result showed that the the linear plot in which; slope = 1/Q of the linear plot in which; slope = 1/Qm and intercept = 1/Qm ˆ KL . The fitting result showed that maximum sorption capacity Qm of polymer adsorbent reached 103.4 mg/g in case of Ni(II) and 83.33 mg/g in case of Cu(II), these results confirm the applicability of Langmuir model which suggests that the adsorption was taken place as mono layer adsorption.
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the maximum sorption capacity Qm of polymer adsorbent reached 103.4 mg/g in case of Ni(II) and 83.33 mg/g in case of Cu(II), these results confirm the applicability of Langmuir model which suggests that the adsorption was taken place as mono layer adsorption. Molecules 2016, 21, 684 8 of 14 Molecules 2016, 21, 684
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Figure 7. Langmuir Langmuir isothermfor forNi(II) Ni(II)adsorption adsorption using using (A) GCs-1; (B) GCs-2; (C)(C) GCs-3 andand (D)(D) GCs-4. Figure 7. Langmuir isotherm using(A) (A)GCs-1; GCs-1;(B) (B) GCs-2; GCs-3 GCs-4. Figure 7. isotherm for Ni(II) adsorption GCs-2; (C) GCs-3 and (D) GCs-4.
Figure 8. Langmuir isotherm for Cu(II) adsorption using (A) GCs-1; (B) GCs-2; (C) GCs-3 and (D) GCs-4.
Figure Langmuir isotherm forfor Cu(II) adsorption using (A) GCs-1; GCs-2;(B) (C) GCs-2; GCs-3 and GCs-4.and Figure 8. 8.Langmuir isotherm Cu(II) adsorption using (A)(B) GCs-1; (C)(D)GCs-3 (D) GCs-4. 2.8.2. Freundlich Freundlich Sorption Sorption Isotherm Isotherm 2.8.2.
Surface heterogenty of the sorbent is indicated from Freundlich model which represented by the following equation: following equation:
Surface heterogenty of the sorbent is indicated from Freundlich model which represented by the 2.8.2. Freundlich Sorption Isotherm
Surface heterogenty of the sorbent is indicated from Freundlich model which represented by the log Q Qee == log log K Kff ++ 1/n 1/n ×× log log C Cee (3) log (3) following equation: where Q Qee is is the the adsorbed adsorbed amount amount log at equilibrium equilibrium (mg/g), Ceeˆ is log the equilibrium equilibrium concentration concentration of of Cu(II) Cu(II) (3) where at (mg/g), C is the Qe “ log K C e f ` 1{n or Ni(II) Ni(II) (mg/L), (mg/L), K Kff is is Freundlich Freundlich constant, constant, 1/n 1/n is is Freundlich Freundlich exponent. exponent. Both Both constants constants were were calculated calculated or
from thethe slope and intercept intercept of the the plotting between between log Q QC e and log C C e. Linear plotconcentration of log Ce vs. logofQCu(II) e where Qe the is adsorbed amount at equilibrium (mg/g), equilibrium e is the from slope and of plotting log e and log e. Linear plot of log Ce vs. log Qe confirm the applicability of Freundlich model as shown in Figures 9 and 10. It means that the or Ni(II) (mg/L), K is Freundlich constant, 1/n is Freundlich exponent. Both constants were calculated f confirm the applicability of Freundlich model as shown in Figures 9 and 10. It means that the adsorption was taken place of at aathe heterogeneous surface;log however nickel and copper adsorption onC vs. fromadsorption the slope was andtaken intercept plotting between Qe and log and Ce . copper Linearadsorption plot of log place at heterogeneous surface; however nickel on e glutamic-chitosan are fitted to both models since the correlation coefficients values are very close. are fitted to of both models since the correlation close.that the log Qglutamic-chitosan Freundlich model as shown coefficients in Figures values 9 and are 10. very It means e confirm the applicability
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adsorption was taken place at a heterogeneous surface; however nickel and copper adsorption on glutamic-chitosan Molecules 2016, 21, are 684 fitted to both models since the correlation coefficients values are very9 close. of 14 Molecules 2016, 21, 684 9 of 14
Figure 9. Freundlich plotplot forfor Ni(II) (A)GCs-1; GCs-1;(B) (B)GCs-2; GCs-2; GCs-3 (D) GCs-4. Figure 9. Freundlich Ni(II)adsorption adsorption on on to to (A) (C)(C) GCs-3 andand (D) GCs-4. Figure 9. Freundlich plot for Ni(II) adsorption on to (A) GCs-1; (B) GCs-2; (C) GCs-3 and (D) GCs-4.
Figure 10. Freundlich plot for Cu(II) adsorption on to (A) GCs-1; (B) GCs-2; (C) GCs-3 and (D) GCs-4.
Figure 10. Freundlich plotfor forCu(II) Cu(II) adsorption to to (A)(A) GCs-1; (B) GCs-2; (C) GCs-3 (D)and GCs-4. Figure 10. Freundlich plot adsorptiononon GCs-1; (B) GCs-2; (C) and GCs-3 (D) GCs-4.
2.9. 2.9. Kinetics Kinetics Studies Studies
2.9. Kinetics Studies
The The kinetic kinetic parameters parameters for for the the adsorption adsorption process process were were studied studied for for contact contact time time from from 10 10 min min to to
360 by separately the of of and The kinetic parameters for the adsorption process were studied for contact timePseudo-first from 10 min to 360 min min by monitoring monitoring separately the percentages percentages of removal removal of the the Cu(II) Cu(II) and Ni(II). Ni(II). Pseudo-first order kinetics are represented by Equation (4), while the pseudo-second order ones are represented 360 min bykinetics monitoring separately percentages ofthe removal of the Cu(II) Pseudo-first order are represented by the Equation (4), while pseudo-second order and ones Ni(II). are represented Equation [20]: by Equation (5) [20]: orderby kinetics are(5) represented by Equation (4), while the pseudo-second order ones are represented by Equation (5) [20]: ln (4) ln (Q (Qee −− Q Qtt)) == −k −k11tt ++ ln ln Q Qee (4) 2 t/Q t/Qtt == 1/k 1/k22 ×× Q Qee2 ×× tt ++ t/Q t/Qee
ln pQe ´ Qt q “ ´k1 t + ln Qe
(5) (5)
(4)
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t{Qt “ 1{k2 ˆ Qe 2 ˆ t + t/Qe
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(5)
where Qe and Qt : are the amount of metal adsorbed (mg/g) at equilibrium and at time t (min), k1 1 ) and ´1the ´1 ) are where Qekand Qt: are amount of metal adsorbed (mg/g) at equilibrium and at timeorder, t (min), k1 (min−1 ) (min´ ¨ min the adsorption rate constant of pseudo-first pseudo-second 2 (g¨Qmg where Qe and t: are the amount of metal adsorbed (mg/g) at equilibrium and at time t (min), k1 (min−1) −1 −1 and k2 (g·mg −1⋅min −1) are the adsorption rate constant of pseudo-first order, pseudo-second order orderand adsorption respectively. Therate values of k1 can be determined from the slope oforder the linear k2 (g·mg kinetics, ⋅min ) are the adsorption constant of pseudo-first order, pseudo-second adsorption kinetics, respectively. The values of k1 can be determined from the slope of the linear plot adsorption values of k1 from can bethe determined the slope linear plot plot of ln (Qe ´kinetics, Qt ) vs. t,respectively. and k2 canThe be calculated slope of from the linear plotofofthe t/Q vs. t. t of ln (Qe − Qt) vs. t, and k2 can be calculated from the slope of the linear plot of t/Qt vs. t. of ln (Q e − Q t ) vs. t, and k 2 can be calculated from the slope of the linear plot of t/Q t vs. t. The The linear plots of of thethe two ofNi(II) Ni(II)are are presented in Figures 1112, and 12 respectively. linear plots twokinetic kinetic models models of presented in Figures 11 and respectively. The linearthe plots of the of twokkinetic models ofthe Ni(II) are presented in Figures 112 )and 12, respectively. TableTable 3 showed values , k , Q and correlation coefficient (R from the linear 2 e 1 2 3 showed the values of k1, k2, Qe and the correlation coefficient (R 2) from the linear plots. The plots. Table 3 showed the values of k1, k2, Qe and the correlation coefficient (R ) from the linear plots. The 2 values 2 The pseudo-second order linear plots of Ni(II) resulted in higher R than the pseudo-first pseudo-second order linear plots of Ni(II) resulted in higher R 2 values than the pseudo-first order. Theseorder. pseudo-second order linear plots of Ni(II) resulted in higher R values than the pseudo-first order. These Theseindicated indicated better applicability ofpseudo-second the pseudo-second order model, which on the assumption better applicability of the order model, which relies onrelies the assumption that indicated better applicability of the pseudo-second order model, which relies on the assumption that chemisorptions are the rate limiting step.step. that chemisorptions are the rate limiting chemisorptions are the rate limiting step.
Figure 11. Pseudo-first order kinetics models of nickel adsorption using (A) GCs-1;(B) (B) GCs-2;(C) (C) Figure 11. Pseudo-first order kinetics models of nickel adsorption using (A) GCs-1; Figure 11. Pseudo-first order kinetics models of nickel adsorption using (A) GCs-1; (B)GCs-2; GCs-2; (C)GCs-3 GCs-3 and (D) GCs-4. and (D) GCs-4. GCs-3 and (D) GCs-4.
Figure 12. Pseudo-second order kinetics models of nickel adsorption using (A) GCs-1; (B) GCs-2; (C)
Figure 12. Pseudo-secondorder order kinetics kinetics models of of nickel adsorption usingusing (A) GCs-1; (B) GCs-2; Figure 12. and Pseudo-second models nickel adsorption (A) GCs-1; (B)(C) GCs-2; GCs-3 (D) GCs-4. GCs-3 and (D) GCs-4. (C) GCs-3 and (D) GCs-4.
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Table 3. Constants and correlation coefficient of pseudo-first order and pseudo-second order kinetics of nickel adsorption. Molecules 2016, 21, 684 Molecules 2016, Pseudo-First 21, 684 Order Model
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11 of 14 Experimental Value Pseudo-Second Order Model Metal Ion ´1 2 ´1 ¨kinetics Table Constantskand correlation coefficient ofQ pseudo-first order Q and pseudo-second order Qe3.(mg/g) R k2 (g¨ mg min´1 ) R2 e (mg/g) e (mg/g) 1 (min ) Table 3. Constants and correlation coefficient of pseudo-first order and pseudo-second order kinetics of nickel adsorption. GCs-1 of nickel 20.32 0.930 18.08 18.67 0.005 0.956 adsorption. 0.013 GCs-2 25.00 19.06 Value 17.81Pseudo-Second Order 0.004 Model 0.988 Pseudo-First 0.009 Order Model0.973 Experimental Metal Pseudo-First Order Experimental Value Pseudo-Second Metal GCs-3 0.012 17.68 0.004 −1) Model0.961 −1·minModel −1) Ion Qe23.26 (mg/g) k1 (min R2 Q e (mg/g) Q20.05 e (mg/g) k2 (g·mgOrder R2 0.966 −1) −1·min−1) Ion GCs-4 0.008 0.940 17.46 0.002 Qe27.03 (mg/g) k1 (min R2 Q e (mg/g) Q18.43 e (mg/g) k2 (g·mg R2 0.949 GCs-1 20.32 0.013 0.930 18.08 18.67 0.005 0.956 GCs-1 20.32 GCs-2 25.00 GCs-2 25.00 GCs-3 23.26 GCs-3 The plots GCs-4linear23.26 27.03 GCs-4 27.03
0.013 0.009 0.009 0.012 0.012 the 0.008 0.008
0.930 0.973 0.973 0.961 0.961 kinetic 0.940 0.940
18.08 19.06 19.06 17.68 models 17.68 of Cu(II) 17.46 17.46
18.67 17.81 17.81 20.05 20.05 presented 18.43 18.43 2
0.005 0.004 0.004 0.004 in0.004 Figures 0.002 0.002
0.956 0.988 0.988 0.966 130.966 and 0.949 0.949
of two are 14. Table 4 showed the values of k1 , k2 , Qe and the correlation coefficient (R ) from the linear plots. The linear plots the two kinetic models of Cu(II) presented in Figures 13 and 14. Table 4order. The pseudo-second orderoflinear plots of Cu(II) resulted inare higher R2 values than the pseudo-first Thethe linear plots the two kinetic models of Cu(II) are presented in Figures 13 and 14. Table 4 showed values of kof 1, k2, Qe and the correlation coefficient (R2) from the linear plots. The pseudo-second showed the values of k 1, k2, Qe and the correlation coefficient (R2)the from the linear plots. The pseudo-second order 4. linear plots of Cu(II) resultedcoefficient in higher of R2 pseudo-first values than pseudo-first order. Table Constants and correlation order and pseudo-second order kinetics order linear plots of Cu(II) resulted in higher R2 values than the pseudo-first order. of copper adsorption.
Table 4. Constants and correlation coefficient of pseudo-first order and pseudo-second order kinetics Table 4. Constants and correlation coefficient of pseudo-first order and pseudo-second order kinetics of copper adsorption. Pseudo-First Order Model Experimental Value Pseudo-Second Order Model of copper adsorption. Metal Ion Pseudo-First Order Model Experimental Value Pseudo-Second Metal Q (mg/g) ´1 2 Qe (mg/g) Qe (mg/g) k1 (min ) R k2 (g¨Order mg´1Model min´1 ) e Metal Ion GCs-1GCs-1 Ion GCs-2GCs-1 GCs-2 GCs-3GCs-2 GCs-3 GCs-4GCs-3 GCs-4 GCs-4
Pseudo-First Order −1) ModelR2 Qe (mg/g) k1 (min 12.99 0.974 Qe12.99 (mg/g) k0.007 1 (min−1) R2 0.007 0.974 14.73 0.009 0.895 12.99 0.007 0.974 14.73 0.009 0.895 15.26 0.009 0.829 14.73 0.009 0.895 15.26 0.009 0.829 12.31 0.005 0.890 15.26 0.009 0.829 12.31 0.005 0.890 12.31 0.005 0.890
Experimental Qe (mg/g)Value Q18.8 e (mg/g) 18.8 18.4 18.8 18.4 18.06 18.4 18.06 17.84 18.06 17.84 17.84
Pseudo-Second Model −1min−1 Qe (mg/g) k2 (g·mgOrder ) R2 −1min −1) 19.01 0.009 Qe19.01 (mg/g) k2 (g·mg R2 0.009 0.989 21.20 19.01 0.009 0.989 21.20 0.0060.006 0.975 20.73 21.20 0.006 0.975 20.73 0.0050.005 0.963 21.30 20.73 0.005 0.963 21.30 0.0040.004 0.954 21.30 0.004 0.954
R2 0.989 0.975 0.963 0.954
Figure 13. Pseudo-firstorder orderkinetics kinetics models copper adsorption usingusing (A) GCs-1, (B) GCs-2, Figure 13. 13. Pseudo-first modelsofof copper adsorption (A) GCs-1; (B)(C) GCs-2; Figure Pseudo-first GCs-3 and (D) GCs-4. order kinetics models of copper adsorption using (A) GCs-1, (B) GCs-2, (C) (C) GCs-3 and (D) GCs-4. GCs-3 and (D) GCs-4.
Figure 12. Cont.
Figure Cont. Figure 14. 12. Cont.
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Figure 14. ofof copper adsorption using (A)(A) GCs-1, (B) (B) GCs-2, (C) Figure 14. Pseudo-second Pseudo-secondorder orderkinetics kineticsmodels models copper adsorption using GCs-1; GCs-2; GCs-3 and (D) GCs-4. (C) GCs-3 and (D) GCs-4.
2.10. Desorption Studies of Ni(II) and Cu(II) 2.10. Desorption Studies of Ni(II) and Cu(II) The sorbed sorbed Ni Ni and and Cu Cu were were dried dried and and preserved. preserved. For For desorption and Ni-sorbed The desorption Cu Cu and Ni-sorbed chitosan chitosan were were shaken with dilute hydrochloric acid (0.5 M) for 1 h at room temperature and filtered. The filtrate was shaken with dilute hydrochloric acid (0.5 M) for 1 h at room temperature and filtered. The filtrate was analyzed for the oxidation states of Cu and Ni using UV-Visible spectrophotometry in the wavelength analyzed for the oxidation states of Cu and Ni using UV-Visible spectrophotometry in the wavelength ranges 190 nm–1100 nm. ranges 190 nm–1100 nm. 3. Materials and Methods 3. Materials and Methods 3.1. Materials 3.1. Materials Chitosan was purchased from Acros Organics (Morris Plains, NJ, USA). Its deacetylation degree Chitosan was purchased from Acros Organics (Morris Plains, NJ, USA). Its deacetylation degree is is 88% and its average molecular weight is 100,000–300,000 Da. Glutamic acid, acetic acid, and methanol, 88% and its average molecular weight is 100,000–300,000 Da. Glutamic acid, acetic acid, and methanol, were of analytical grade from Aldrich (Saint Louis, MO, USA) and used as received. were of analytical grade from Aldrich (Saint Louis, MO, USA) and used as received. 3.2. Measurements 3.2. Measurements Fourier transform transform infrared infrared (FTIR) using KBr KBr discs discs on on aa Perkin Perkin Elmer Elmer Fourier (FTIR) spectra spectra were were recorded recorded using spectrometer (Perkin-Elmer, Waltham, MA, USA) at room temperature in the range of 4000–400 cm´−11.. spectrometer (Perkin-Elmer, Waltham, MA, USA) at room temperature in the range of 4000–400 cm Elemental analyses analysesofofthethe prepared derivatives in a Model 2410-Series H, N, S Elemental prepared derivatives werewere donedone in a Model 2410-Series II C, H, II N, C, S Analyzer 1H-NMR spectra were recorded using a Gemini-300 MHz Analyzer (Perkin-Elmer, Shelton, CT, USA). 1 (Perkin-Elmer, Shelton, CT, USA). H-NMR spectra were recorded using a Gemini-300 MHz instrument instrument (Gemini, PaloUSA), Alto, in CA, USA), inasDMSO-d 6 as a solvent at 25 °C.shifts Chemical shifts (δ) are (Gemini, Palo Alto, CA, DMSO-d a solvent at 25 ˝ C. Chemical (δ) are expressed 6 expressed inmillion parts per million (ppm) from tetramethylsilane as an internal standard. The morphology in parts per (ppm) from tetramethylsilane as an internal standard. The morphology of the of the cross-linking gel was analyzed with a JEOL-JSM 5300 Scanning electron microscope (JEOL, cross-linking gel was analyzed with a JEOL-JSM 5300 Scanning electron microscope (JEOL, Tokyo, Tokyo, metal Japan);solutions metal solutions were analyzed using aU-2800 ModelUV-Visible U-2800 UV-Visible spectrophotometer Japan); were analyzed using a Model spectrophotometer (Hitachi, (Hitachi, Schaumburg, IL, USA). Ni(II) and Cu(II) concentrations were measured using a ThermoSchaumburg, IL, USA). Ni(II) and Cu(II) concentrations were measured using a Thermo-Scientific Scientific Atomic ICE-3300 Atomic adsorption spectrometer AAs (Thermo-Scientific, ICE-3300 adsorption spectrometer AAs (Thermo-Scientific, Waltham, Waltham, MA, USA).MA, USA). 3.3. General for Chitosan-Glutamic Chitosan-Glutamic Synthesis Synthesis 3.3. General Procedures Procedures for A solution solutionofofglutamic glutamic acid in distilled water (20 was mL)added was added to chitosan (20inmmol) in A acid in distilled water (20 mL) to chitosan (20 mmol) distilled distilled The mixture wasfor stirred 60 °C.cooling, After cooling, the homogenous crosswater (30water mL). (30 ThemL). mixture was stirred 4 h atfor 604˝hC.atAfter the homogenous cross-linked linked hydrogels which formed was submerged in methanol 12 h for dewatering to give white hydrogels which formed was submerged in methanol for 12 h for for dewatering to give white product; ˝ product; the dewatering wasand filtered dried 60 °C to weight. constant weight. the dewatering hydrogel hydrogel was filtered driedand at 60 C toatconstant G-chitosan-1: Obtained 96.4% yield; yield; IR IR (KBr): (KBr): 1568 1568 (CONH), (CONH), 1631 1631 G-chitosan-1: Obtained from from glutamic glutamic acid acid (2.5 (2.5 mmol) mmol) in in 96.4% −1 1 1 (NH), 1 H-NMR (CONH), 2867, 3437 cm ´(NH), (OH); H-NMR (δ, ppm) (DMSO-d 6) δ: 1.88 (q, 2H, CH2(CONH), 2867, 2925 2925 (NH (NH2), (OH); (δ, ppm) (DMSO-d 2 ), 3437 cm 6 ) δ: 1.88 (q, 2H, 1 1 1 glutamic), 2.11 (m, 1H, H-5′), 2.29 (m, 2H, H-1′, H-2′), 2.35 (t, 2H, CH 2 CO), 2.60 (m, 2H, H-3′, CH2 -glutamic), 2.11 (m, 1H, H-5 ), 2.29 (m, 2H, H-1 , H-2 ), 2.35 (t, 2H, CH2 CO), 2.60 (m, 2H,H-4′), H-31 , 1 1 1 2.61 ), (m, 2H,(m, H-6a′, H-6b′), 3.27 (t, 1H, CH-CO), 3.29 (bs,3.29 NH(bs, 2, OH’s; with D2O), 4.00 H-4 2.61 2H, H-6a , H-6b ), 3.27 (t, 1H, CH-CO), NH2exchangeable , OH’s; exchangeable with D2(m, O), 1H, OH; exchangeable with D 2 O), 7.77 (s, 2H, 2NH; exchangeable with D 2 O). Anal. Found: C, 47.80; 4.00 (m, 1H, OH; exchangeable with D2 O), 7.77 (s, 2H, 2NH; exchangeable with D2 O). Anal. Found: H, 47.80; 7.15; N, C, H,7.69. 7.15; N, 7.69. G-chitosan-2: Obtained from glutamic acid (5 mmol) in 92.5% yield; IR (KBr): 1638 (CONH), (NH2), 3445 cm−1 (NH), (OH); Anal. Found: C, 47.84; H, 7.24; N, 7.72.
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G-chitosan-2: Obtained from glutamic acid (5 mmol) in 92.5% yield; IR (KBr): 1638 (CONH), (NH2 ), 3445 cm´1 (NH), (OH); Anal. Found: C, 47.84; H, 7.24; N, 7.72. G-chitosan-3: Obtained from glutamic acid (10 mmol) in 90.2% yield; IR (KBr): 1636 (CONH), 2925 (saturated C-H), (NH2 ), 3728, 3434 cm´1 (NH), (OH); Anal. Found: C, 47.86; H, 7.27; N, 7.74 G-chitosan-4: Obtained from glutamic acid (20 mmol) in 89% yield; IR (KBr): 1637 (CONH), 2925 (saturated C-H), NH2 , 3438 cm´1 (NH), (OH); Anal. Found: C, 47.88; H, 7.35; N, 7.77. 3.4. Stock Solution Preparation Stock solution of 10 mg/L Cu(II) ion was prepared by dissolving copper sulphate pentahydrate (CuSO4 ¨ 5H2 O, 39.28 mg)in distilled water (1000 mL)contained in a volumetric flask. Ni(II) stock solution of 10 mg/L concentration was also prepared by dissolving nickel sulphate hexahydrate (NiSO4 ¨ 6H2 O, 43.96 mg) in 1000 mL distilled water. Hydrochloric acid and sodium hydroxide were used to adjust the solution pH. Distilled water was used throughout the experimental work. 3.5. Adsorption Experiments Sorption experiments were conducted in 250 mL conical flasks containing 100 mL of various concentrations of Ni(II) or Cu(II) solution using accurately weighed chitosan. The flasks were agitated in an orbit shaker at 100 rpm and at room temperature (25 ˝ C). The initial and final concentrations of the solutions were measured by atomic adsorption (AAS) at the maximum adsorption wavelength and the adsorption capacities of the adsorbent were calculated. The percent removal of metals from the solution was calculated by the following equation [20]: % removal “
C0 ´ C ˆ 100 C0
(6)
where C0 (mg/L) is the initial metal ion concentration and C (mg/L) is the final metal ion concentration in the solution. The effect of sorbent dosage was studied from 1 g to 5 g for 1 h contact time. Effect of initial pH on the sorption capacity of sorbent for Ni(II) and Cu(II) was studied by varying solution pH from 3 to 9 at the sorbent dosage of 1 g/100 mL for 1 h contact time using 200 mg/L initial Ni and Cu concentration; the solution pH was adjusted with dilute HCl or NaOH solution. The effect of contact time on the sorption capacity of sorbent was studied in the range 25–350 min at an initial Ni and Cu concentration of 200 mg/L. The effect of initial concentration on the sorption capacity of sorbent was studied using the volume of solutions as 100 mL, and initial concentration of nickel as 5, 50, 100, 300, 500, 700, 900 and 1000 mg/L; and copper as 5, 50, 75, 125, 250, 500, 750 and 1000 mg/L. Adsorption isotherms were studied at different initial Ni, Cu concentrations at room temperature. 4. Conclusions Chitosan-glutamic hydrogels were synthesized by modification of chitosan with glutamic acid which was characterized with FTIR, 1 H-NMR, elemental analysis and SEM. The influence of adsorbent dosage, solution pH value, reaction time and initial concentration of Ni(II) and Cu(II) on the adsorption capacity were investigated. The results indicate that chitosan-glutamic derivatives are good adsorbents of Ni(II) and Cu(II). Acknowledgments: This research was partially supported by the Chemistry Department, Faculty of Science, Alexandria University. Authors also wish to thank Sh. El Shazly for his valuable discussions. Thanks are also for the reviewers and editor for their helpful suggestions and enlightening comments. Author Contributions: Mohamed M. El Sadek suggested and supervised the work and performed the article editing. Mohamed A. Mostafa and Seham Y. Hassan conceived and designed the experiments and analyzed the data. Huda E. Abdelwahab performed the experiments. Conflicts of Interest: The authors declare no conflict of interest.
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