Article pubs.acs.org/jced
Aminomethylphosphonate Chelating Ligand and Octadecyl Alkyl Chain in a Resin for Simultaneous Removal of Co(II) Ions and Organic Contaminants Tawfik A. Saleh,*,† Auwal M. Muhammad,‡ Bassam Tawabini,‡ and Shaikh A. Ali† †
Department of Chemistry, and ‡Department of Geosciences; King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia ABSTRACT: The efficiency of a hydrophobic cross-linked polyzwitterionic acid for the simultaneous removal of Co(II) and organic dyes from water was examined. The synthesis of the resin was conducted via cyclotetrapolymerization of N,Ndiallyl-N-aminomethylphosphonic acid as a hydrophilic monomer, N,N-diallyl-N-octadecylammonium chloride as a hydrophobic monomer, and SO2 in the presence of a crosslinker using the initiator α,α′-azoisobutyronitrile. The efficiency of the resin for the removal of Co(II) was evaluated under the effects of metal ion concentration, temperature, adsorption contact time, and medium pH. The results show the adsorption data fitted a pseudo-second-order model. The standard enthalpy change ΔHo was 23.2 kJ/mol, suggesting an endothermic adsorption process. The adsorption from the mixture of the binary systems of Co(II) with eriochrome Black T, methyl orange, phenol, or methylene blue exhibited a distinct bimodal behavior that could be ascribed to the chelating and hydrophobic nature of the prepared resin. The resin, by virtue of having the advantages of high capacity and good ability to adsorb Co(II) ions and organic contaminants simultaneously, is indeed a promising candidate for water purification.
1. INTRODUCTION
materials have been developed for the adsorption of organic or toxic metals. Since the adsorption performance is mainly limited by the material’s design and properties, strong efforts have been made in exploring the structural properties of various materials. The use of inorganic materials, in combination with polymer hybrid materials for adsorption, was considered as promising, as it is effective, economical and environmentally friendly in removing pollutants from the water.11,12 Zwitterionic hybrid polymeric materials have excellent properties in term of structural flexibility, and mechanical and thermal stability.13,14 A cross-linked polyaminocarboxylate was used for the removal of Cu2+, and the resins showed an excellent ability to adsorb the metal ions.15 The activity of a zwitterionic cross-linked polymer for the sorption of Sr2+ was reported.16 The utilization of zwitterionic hybrid polymers for metal ion removal from aqueous solution was reported.17 In this study, a cross-linked polyzwitterion was synthesized utilizing Butler’s homo-18−20 and tetra-cyclopolymerization protocol21−23 involving hydrophilic monomer 2, hydrophobic monomer 3, and cross-linker 4, in the presence of SO2 to obtain a functionalized hydrophobic cross-linked polyzwitterionic acid (HCPZA) 5 as a sorbent for the Co(II) removal
Environmental pollution is considered one of the key areas that continues to contribute to the deterioration of the environment worldwide. Looking at the dangers to health posed by this growing problem, a serious effort is being employed to curb this menace, particularly in the case of heavy metals pollution. Industrial wastes from mining, electroplating, nuclear plants, and textile mills, to mention a few, contain a huge amount of heavy metals. The toxic nature, health issues, and the damaging effect of these elements within the environment are well documented.1,2 Cobalt is among the common toxic metals that are used extensively in various human endeavors, among which are its use in power plants (nuclear) and applications in electronic equipment.3 Cobalt occurs naturally in the universe but is found mostly in rocks and soils. Soil and sediment contaminated by industrial pollution may contain high levels of cobalt. Some of the health problems related to cobalt toxicity are paralysis, diarrhea, lung irritation, and bone defects. Chronic exposure to cobalt may cause goiter in human beings.4,5 The allowable level of cobalt in water for consumption is 0.002 mg/ L, but in some cases values of 0.107 mg/L were noted.6 Although different treatment techniques, such as precipitation, membrane technology, ion exchange, and electrocoagulation were reported to remove effluents in aqueous media, adsorption seems to have great potential for such an application.7−10 Recently, more and more novel adsorption © XXXX American Chemical Society
Received: June 9, 2016 Accepted: August 2, 2016
A
DOI: 10.1021/acs.jced.6b00475 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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1-bromooctadecane, and allyl chloride with piperazine, respectively. After refluxing a solution of 1-bromooctadecane (18.4 g, 55.0 mmol) and diallylamine 1 (8.28 g, 85 mmol) in toluene (6 cm3) under N2 for 24 h followed by base treatment with NaOH (2.65 g, 66 mmol) in water (100 cm3), the product was extracted with ether (75 cm3). The organic layer was dried (Na2SO4) and distilled to obtain a colorless liquid of N,Ndiallyl-N-octadecylamine (15.3 g, 79.4%); bp0.05 mbarHg, 170− 175 °C.24 Hydrophobic monomer 3 was obtained as a white powder by passing HCl through a solution of diallyloctadecylamine (10.0 g, 28.6 mmol) in ether (75 cm3).24 To a stirring saturated solution of K2CO3 (15.0 g, 0.108 mol) containing piperazine (4.3 g, 0.050 mol) was added allyl chloride (8.42 g, 0.110 mol) dropwise (1 h) with continued stirring at 20 °C for 24 h. The product was extracted with dichloromethane (3 × 15 cm3). Then, the organic layer was dried (Na2S04) and distilled under vacuum to give 1,4diallylpiperazine (6.5 g, 79%): colorless liquid (bp6 mbar Hg, 74 °C). The diallylpiperazine (6.0 g, 0.036 mol) was quaternized by alkylation with allyl chloride (8.3 g, 0.108 mol) in acetone (50 cm3) in a closed vessel at 87 °C for 72 h to give 4 which was crystallized from methanol (9.1 g, 80%).25 Hydrophilic monomer, diallylaminomethylphosphonic acid 2, was synthesized as described in the literature.26 Thus, under occasional ice cooling, diallylamine (1) (0.35 mol) was added slowly in drops during 45 min to H3PO3 (0.35 mol), H2O (35 mL), and 37 wt % HCl (35 mL) mixture, followed by reflux for 60 min, and solid paraformaldehyde (0.70 mol) and H2O (35 mL) were added portionwise (ca. 20 min). The mixture was then refluxed for a further 1 h and finally freeze-dried to obtain monomer 2 along with excess formaldehyde. The crude product was taken up in water (100 mL); the precipitated formaldehyde powder was filtered out, and the filtrate was evaporated at 80 °C. The residual thick liquid was further dried under vacuum to obtain 2 as a light yellow viscous liquid (72 g, 90%). An 1H NMR spectra of a mixture that contains known masses of monomer 2 and ethanol in D2O was analyzed to estimate the product’s purity. Integrating the protons’ signals of CH2P- of 2 at δ3.11 versus CH2O of ethanol at δ3.40 helped us to determine the purity and molar mass of the product. The ratio of the integrated area of the proton signals of CH2P- of 2 at δ3.11 versus CH2O of ethanol at δ3.40 gave the monomer 2/ ethanol mole ratio. The known mass or mole of ethanol led us determining the amount (mole) of the crude product for its known mass present in the NMR tube. The obtained molar mass of 230.3 g mol−1 as against the calculated value of 227.63 g mol−1 confirmed the structure of 2 as the HCl salt. Monomer 2 was thus found to be pure. 2.2.2. Synthesis of Resin 5 via Tetrapolymerization of Monomers 2, 3, 4, and SO2. To a solution of 2 (9.83 g, 43.2 mmol), 3 (1.82 g, 4.71 mmol), and 4 (1.13 g, 3.54 mmol) in DMSO (21 g) in a RB flask (50 mL) was absorbed SO2 (3.52 g, 55 mmol) by blowing a gentle stream of the gas over the agitated surface of the solution. The amount of SO2 absorbed was frequently checked by an analytical balance. The polymerization was thus carried out using 2/3/4 mol ratio of ≈84:9:7. After addition of the initiator (AIBN, 326 mg), the mixture was properly stirred in the closed flask (65 °C, 48 h). At every interval of ≈12 h, decomposition product N2 was released by opening the flask. The resulting gel was soaked in water; the white resin thus obtained was washed with water, then with acetone for several times. During the washing, HCl is depleted from the monomer unit 2 as confirmed by elemental
from the solution, as shown in Figure 1. Under the influence of pH, resin 5 is expected to be in equilibration with the
Figure 1. Syntheses of N-allyl-N-(phosphonomethyl)prop-2-en-1aminium chloride (2) via diallylamine (1), N,N-diallyl-N-octadecylammonium chloride (3), N,N,N′,N′-tetraallylpiperazinium dichloride (4) and their cyclopolymerization with SO2 to yield poly[(hydrogen(diallylammonio)methyl)phosphonate-alt-SO 2 )]-ran-net-poly(N,N,N′,N′-tetraallylpiperazinium dichloride-alt-SO2)]-ran-poly[(N,Ndiallyl-N-octadecylammonium chloride-alt-SO2)] (5) and poly[(sodium((diallylammonio)methyl)phosphonate-alt-SO2)]-ran-netpoly(N,N,N′,N′-tetraallylpiperazinium dihydroxide-alt-SO2)]-ran-poly[(N,N-diallyl-N-octadecylamine-alt-SO2)] (6).
hydrophobic cross-linked dianionic polyelectrolyte (HCDAP) 6 that has several ligand centers rich the lone pairs of electrons required for chelation activities. The presence of the hydrophobic C18 alkyl chain in 5 may also aid the simultaneous removal of organic contaminants from waters.
2. EXPERIMENTAL SECTION 2.1. Materials. Solutions of analytical grade Co (II) (1000 mg/L), HNO3, HCl, and NaOH were purchased from SigmaAldrich, USA. The stock solutions were diluted to the required concentrations. Azoisobutyronitrile was crystallized from chloroform−ethanol. Calcium hydride−dried dimethyl sulfoxide (DMSO) was distilled at 64−65 °C (4 mmHg). Diallylamine, piperazine, allyl chloride from Fluka (AG) with respective purity of ≥98, 97, and 98% were used as received. 2.2. Synthesis. 2.2.1. Monomer Syntheses. N,N-diallyl-Noctadecylammonium chloride as hydrophobic monomer 3 and 1,1,4,4-tetraallylpiperazinium dichloride as cross-linker 4 were synthesized as described24,25 by reacting diallylamine (1) with B
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experiments were carried out using 20 mL of each system, with the addition of 30 mg of the resin, and shaken at 150 rpm until the equilibrium was achieved. The concentrations of Co(II) and the organic components were monitored by the atomic absorption spectrometer and UV−vis spectrophotometer, respectively. 2.5. Data Analysis. The percentage (%) of removed Co(II) after the equilibrium was calculated as
analysis described below. The polymer was dried under vacuum at 60 °C (13.2 g, 89.6%). The composition of the polymer was found to be C, 36.2; H, 6.4; N, 5.2; S, 11.6%. The incorporated monomers as in HCPZA 5 containing [(2-HCl)·SO2] (84.0 mol %), [3·SO2] (9.15 mol %), and [4·(SO2)2] (6.88 mol %) requires C, 36.47; H, 6.06; N, 5.32; S, 12.18%. Note that crosslinker 4 having two pairs of double bonds can react with two SO2 units. νmax (KBr): 3420, 2925, 2853, 1646, 1467, 1308, 1128, 915, 756, and 535 cm−1. Figure 1 illustrates the syntheses process of N-allyl-N(phosphonomethyl)prop-2-en-1-aminium chloride (2) via diallylamine (1), N,N-diallyl-N-octadecylammonium chloride (3), N,N,N′,N′-tetraallylpiperazinium dichloride (4), and their cyclopolymerization with SO2 to yield poly[(hydrogen(diallylammonio)methyl)phosphonate-alt-SO2)]-ran-net-poly(N,N,N′,N′-tetraallylpiperazinium dichloride-alt-SO2)]-ranpoly[(N,N-diallyl-N-octadecylammonium chloride-alt-SO2)] (5). The poly[(sodium((diallylammonio)methyl)phosphonate-alt-SO 2 )]-ran-net-poly(N,N,N′,N′-tetraallylpiperazinium dihydroxide-alt-SO2)]-ran-poly[(N,N-diallyl-Noctadecylamine-alt-SO2)] (6) was obtained by treating (5) with NaOH solution. In this synthesis process, (2) was used as a hydrophilic monomer, and (3) was used as a hydrophobic monomer. The compound (4) was used as a cross-linker and AIBN (α,α′-azoisobutyronitrile) was used as an initiator to produce (5) as a hydrophobic cross-linked polyzwitterionic acid (HCPZA). 2.3. Analytical Methods. The morphology of the resin was characterized by scanning electron microscopy (SEM). The elemental spectrum was obtained by energy-dispersive X-ray spectroscopy (EDX) to obtain the elemental analysis of the pristine and spent polymer. A Thermo Scientific iCE 3000 atomic absorption spectrometer (AAS) was employed to determine the cobalt ions’ concentration. Inductively coupled plasma, equipped with a mass spectrometer (ICP−MS), was employed to analyze the real wastewater aliquot. A Fourier transform infrared spectroscopy (FTIR) Nicolet 6700 Thermo Scientific was used for the identification of functional groups. Thermogravimetric analysis (TGA) for the resin was conducted using a thermal analyzer SDT Q600, (TA Instruments, USA) by raising the temperature by 10 °C/min over the range of 20−800 °C, with an air flow of 100 mL/min. A Zeta-Sizer Nano series ZEN2600 instrument (Malvern, UK) was used to measure the zeta potentials of the samples. 2.4. Batch Experiments. The adsorption efficiency of the prepared resin was evaluated by batch experiments carried out as follows: 30 mg, of the HCPZA 5 was mixed in an aqueous Co(II) solution prepared using cobalt nitrate, 20 mL each, and then stirred for various periods of 10, 15, 20, 30, 40, 50, 60, 90, and 120 min at 298 K, keeping the initial Co(II) concentration in 5−20 mg L−1 range. The resultant solution was filtered and analyzed using AAS to determine the amount of Co(II) ion uptake. The pH was monitored during the adsorption and adjusted appropriately using 0.1 M nitric or sodium hydroxide. For the effect of the temperature on adsorption behavior, the kinetic and thermodynamic data were obtained at 296, 316, and 336 K. The experiments on the binary systems were carried out by preparing standard stock solutions of (i) methylene blue (MB) and Co(II), (ii) phenol and Co(II), (iii) methyl orange (MO) and Co(II), and (iv) eriochrome Black-T (EBT) and Co(II). The concentration of each component was 10 ppm. The
% removal =
Co − Ce × 100 Co
(1)
In eqs 2 and 3, the adsorption capacity qe (mg g−1) and qt (mg g−1) are the amount of Co(II) adsorbed per unit mass of the resin at equilibrium and at time t. The adsorption capacity was calculated using the equations:
qe = (Co − Ce) ×
V m
(2)
qt = (Co − C t) ×
V m
(3)
where C0 (mg L−1), Ct and Ce denote the initial concentration and the concentration at time t and equilibrium, respectively. The V (L) and m (g) stand for the volume and resin mass.
3. RESULTS AND DISCUSSION 3.1. Characterization. 3.1.1. Synthesis of Resin 5 and Its FT-IR Characterization. Hydrophilic chelating monomer 2 (84 mol %) (formed via the intermediate shown in the inset box of Figure 1), hydrophobic monomer 3 (9 mol %), cross-linker 4 (7 mol %), and the equivalent proportion of SO2 in solvent DMSO underwent cyclocopolymerization-initiated by AIBN-to yield the cross-linked tetrapolymer HCPZA 5, with an excellent yield of 90% (Figure 1). The calculation takes into account that cross-linker 4, by virtue of having two pairs of double bonds, can copolymerize with two units of SO2. The elemental analysis revealed the incorporation of monomers 2, 3, and 4 into HCPZA 5 in an approximate respective mole ratio of 84:9:7, which is the same as the feed ratio. This is expected for a high conversion, as in the current case. It is amply demonstrated in the literature21−23 that SO2 undergoes alternate copolymerization with diallylammonium salts and the reactivity ratios do not permit their homopolymerizations under the reaction conditions. Elemental analyses of HCPZA 5 confirmed the mol ratio of (2+3+4)/SO2 as 1:1. In the current context, the Butler’s cyclopolymerization protocol18−20 is instrumental in the synthesis of a plethora of industrially significant cyclopolymers having the eighth most important structural type of pyrrolidine ring-embedded backbone architecture.27 Resin 5, having chelating aminophophonate motifs, an unquenched nitrogen valency and a long C18 alkyl chain in the hydrophobic repeat unit, is anticipated to demonstrate interesting chelation properties in impounding both toxic metal ions and organic contaminants. The IR absorptions of HCPZA 5 around ≈1308 and ≈1127 cm−1 (Figure 2a) show the presence of SO2 and phosphonate groups in the produced polymer.22 The C−N absorption band appears at around ≈1467 cm−1. The H−O−H bending vibration was displayed around 1646 cm−1. The FTIR spectrum of the resin after being loaded with Co(II) (Figure 2b) displayed the disappearance of the peaks at 1720 and 1467 cm−1, which indicates that cobalt ions were adsorbed on the carboxylic sites.15,16 The decrease in the intensity of the peaks C
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loaded polymer with the pristine polymer, it is clear that the cobalt is shown in the peaks at 0.776 and 6.924 kV in the spectrum, as shown in Figure 4b. The tables in the insets of Figure 4a,b depict the semiquantitative analysis of the elements in the samples. It is important to mention that these values represent the surface analysis of the samples and not the bulk. 3.2. Sorption Performance Evaluation. 3.2.1. Effect of pH. The pH of the media where the adsorption takes place is a key factor in the sorption performance, because the pH influences both the sorbent surface charge and the speciation of Co(II). There is an influence of the various functional groups (acidic or basic) which can lead to both protonation and deprotonation. The pH effect was investigated using solutions of pH values of 3, 4, 5, 6, and 7. Solutions of pH > 7 were not conducted because of the precipitation of the Co(II) ion as metal hydroxides.6 As depicted in Figure 5, the highest percent adsorption was obtained at a pH between 5 and 6. This is attributed to the lessened competition between the depleted hydro H3O+ and the cobalt for the resin surface. The low adsorption of Co(II) at a lower pH could be explained by the excess H+ overcoming the competition from the Co(II). The optimum value of pH 6 was chosen for further experiments in the following sections. 3.2.2. The Influence of the Contact Time. The batch experiments with initial concentrations of 5, 10, and 20 ppm of Co(II) were carried out at 23 °C to examine the effect of contact time on the metal’s removal. The adsorption was rapid, and equilibrium was obtained within 20 min. This can be attributed to the negatively charged sites on the surface of the adsorbent, which are attractive toward the positively charged cobalt species. 3.3. Kinetics of the Adsorption. Three kinetic models were applied to examine the controlling mechanism of cobalt ions adsorption from aqueous solution. The first was Lagergren pseudo-first-order model using28
Figure 2. FTIR spectra of (a) HCPZA 5 and (b) HCPZA 5 after Co(II)adsorption.
at 1127 cm−1 indicates that cobalt ions were also adsorbed on the phosphate sites. 3.1.2. Thermogravimetric Analysis (TGA). The first (18.6%) and the second (57.7%) major weight losses in the TGA, as depicted in Figure 3, were a result of the loss of SO2 and
ln(qe − qt) = ln qe − k1t
(4)
where qe and qt stand for the amount of Co(II) (mg/g) sorbed at a equilibrium and any time t, while k1 represents the rate constant. Values of k1 and adsorption density qe, as obtained from the ln(qe − qt) versus t plots (Figure 6b), are given in Table 1. Disagreement between the experimental (qe,exp) and computed value (qe,cal), and the poor correlation coefficients (R2) rule out the adsorption process obeying the Lagergren’s pseudo-first-order model. The pseudo-second-order kinetics model was used and the following equation was used to plot the experimental data:29,30
Figure 3. TGA curve of HCPZA 5.
pendant carboxylate groups with the release of carbon dioxide, nitrogen oxides and water gases resulting from the decomposition of nitrogenated fractions, respectively. The loss also included the decomposition of moisture embedded in the prepared cross-linked polymer. The remnant mass at 800 °C was established to be 34.7%, and is assigned to P2O5.16 The cross-linked polymer (HCPZA) 5 and HCPDA 6 (obtained by treating 5 with equivalent amount of NaOH) were determined to have zeta potentials of +1.39 and −8.01 mV, respectively, thereby confirming the pH responsiveness of the surface charges on the polymer backbone. Note that the structural inspection revealed the presence of the excess positive and negative charges on the polymer surfaces of resin 5 and 6, respectively. From an electrostatic perspective, the negative charge under the condition would favor the adsorption of positively charged species, which in our case is the metal ions or the cationic dyes. 3.1.3. Scanning Electron Microscopy and Energy Dispersive X-ray Microanalysis. The surface morphology and structure of the prepared polymer and spent polymer was characterized by using SEM and EDX. The results are shown in Figure 4 a,b. Comparing the elemental spectra of the Co(II)-
dqt dt
= k 2(qe − qt)2
(5)
where k2 denotes the rate constant and qe and qt represent the adsorption capacity of cobalt ions at the equilibrium and at time t. In linear form, the equation is written as t 1 t = + 2 qt qe k 2qe
(6)
where k2 is obtained from a t/qt versus t plot (Figure 6c). The values of R2 > 0.99 as well as the close agreement between the qe,cal and the qe,exp indicate the adsorption process following the pseudo-second-order model. Co(II) is mostly adsorbing on the resin via chemical interaction (Table 1).31,32 D
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Figure 4. EDX spectra and the SEM images of (a) the prepared HCPZA and (b) Co(II)-loaded resin; with the tables depicting the semiquantitative microanalysis of the same.
sorption on sorbent surfaces.35 The linear equation of this model is34 Ce C 1 = + e qe kLqm qm
(8)
where kL is the equilibrium constant (L/mg), and qm (mg/g) represents the maximum adsorption capacity. The plot of the experimental data using this equation is depicted in Figure 7a, which led to the determination of the Langmuir constants, qm and kL from the slope and intercept of the plot, which were found to be 50.8 mg/g and 0.7 L/mg. The R2 value of 0.995 attests that the experimental results fit this model. Further, the dimensionless equilibrium parameter RL was used as defined by Weber and Chakravorti:30 Figure 5. Influence of pH on the cobalt adsorption on HCPZ A 5 (resin, 30 mg; solution (20 mL) containing 10 ppm of Co(II).
RL =
(9)
where C0 is the initial sorbate concentration. RL provides an indication of unfavorable (RL > 1), linear (RL = 1), and favorable (0 < RL< 1) or irreversible (RL = 0) adsorption. The RL of 0.23 in this study indicates favorable adsorption. The capacity (qm) of the present resin (Table 2) was comparable to that of other sorbents reported for the removal of Co(II) (Table 3). To explain the mechanism of the cobalt interaction with the active sites on the polymer, we have proposed the scheme in Figure 8. The evidence of interactions from IR (Figure 2) and EDX analyses (Figure 4) suggests the mechanism of the possible interactions between the functional groups on the resin and the cobalt ions during adsorption. The aminomethylphosphonate functionality is potentially a tridentate ligand, having a coordination site at the nitrogen and other two bonding sites at the phosphonate motif.15,41−43 The cobalt ions can interact with the resin via the tridentate ligand as shown in Figure 8a. In the weak acid range, the formation of a cobaltcomplex using the aminophosphonate motif as a tridentate ligand and a bidentate ligand is depicted in Figure 8.
The intraparticle diffusion model normally identifies the adsorption mechanism by fitting the experimental results in a plot of qt versus t1/2. Using the following equation:33 qt = k idt 1/2 + C
1 1 + KLCo
(7)
where the model rate constant is kid, while the intercept (mg/g) is C. The experimental data of the cobalt adsorption on the prepared resin indicating a multilinear plot of qt versus t1/2. This means that the intraparticle diffusion seems not to be the only rate-determining step.34 Three stages were observed: (i) initial curved representing the bulk diffusion, (ii) the subsequent linear stage due to the intraparticle diffusion, and (iii) plateau stage representing the equilibrium. 3.4. Isotherms of the Adsorption. Three isotherm models were used to evaluate the experimental results: Langmuir, Freundlich, and Temkin. The first model assumes the formation of a monolayer on an energetically homogeneous resin surface. The model assumes a physical or chemical E
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Figure 6. (a) qt with contact time for Co(II) adsorption; (b) plot of the Lagergren’s pseudo first-order kinetic model; (c) plot of the pseudo-second-order kinetic model.
Freundlich isotherm model is applied to define the adsorption nature on heterogeneous surfaces considering the interaction between the sorbent molecules.26 The equation is expressed as
qe = K f Ce1/ n
(10)
Figure 7. (a) Langmuir, (b) Freundlich, and (c) Temkin adsorption isotherms for Co(II) from aqueous solution (20 mL) adsorption on resin HCPZA 5 (30 mg) at 296 K.
where KF (mg/g) and n represent the adsorption capacity and adsorption intensity, respectively. The equation of this model is 1 ln qe = ln K f + ln Ce (11) n
of the strength of the adsorption 1/n < 1 reveals a normal adsorption while 1/n >1 reveals a possible cooperative sorption. Here the result of our work, 1/n = 0.56, indicates a favorable adsorption process of Co (II) on the developed polymer.
The plot of ln qe versus ln Ce (Figure 7b), KF, and n were computed, and the values were included in Table 2. While the n value indicates the favorability of the adsorption. The function
Table 1. Results of the Kinetics Obtained for Co(II) Adsorption on the Resin Lagergren’s first-order
pseudo-second-order
intraparticle diffusion
Ci
qe, exp
k1
qe, cal
k2
qe, cal
kid
C
mg/L
mg/g
min−1
mg/g
R2
g/mg·min
mg/g
R2
mg/g·min
mg/g
R2
5 10 20
3.19 4.33 8.67
0.001 0.007 0.038
0.85 0.12 0.05
0.623 0.762 0.685
0.0375 0.385 1.899
3.04 4.33 8.63
0.958 0.999 0.999
0.68 0.85 0.62
0.187 2.89 3.5
0.999 0.988 0.996
F
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Table 2. Langmuir, Freundlich, and Temkin Isotherm Parameters for Co(II) Adsorption on the Polymer Langmuir isotherm
Freundlich isotherm
Temkin isotherm
qm (mg/g)
kL (L/mg)
RL
R2
1/n
n
kf (mg/g)
R2
KT (L/g)
bT (kJ/mol)
R2
50.8
0.70
0.23
0.995
0.56
1.78
19.8
0.988
7.082
0.22
0.989
Table 3. Capacity of the Adsorption of the Current Resin with That of Different Sorbents for Co(II) at 296 K adsorption capacity qm(mg/g)
adsorbent almond green hull grafted poly(ethylene terephthalate) fiber clinoptilolite sulfurized activated carbon EDTA-modified silica gel DTPA-modified silica gel present resin
ref
45.5 at pH 4 27.2 at pH 4−6
5 36
14.1 40.5 20.0 16.1 50.8
37 38 39
at at at at at
pH pH pH pH pH
6−7 6 3 3 6
this study
Figure 8. Cobalt complex aminomethylphosphonate as (a) tridentate and (b) bidentate ligand. Figure 9. (a) A plot of ln Kc versus 1/T and (b) Arrhenius plot of ln k2 vs 1/T for Co(II) adsorption on HCPZA 5.
On the side, the Temkin model is one of the reported isotherms that takes into account the adsorbent to adsorbate interaction and assumes a linear decrease in the heat of adsorption with increasing coverage. The model is expressed as qe =
RT RT ln KT + ln Ce bT bT
(Table 4). The positive ΔHo of 23.2 kJ/mol suggests endothermic adsorption of Co(II) on the resin. Table 4. Thermodynamic Related Parameters for the Co(II) Adsorption on HCPZA
(12)
The parameters of this model are defined as the sorption heat in joule/mol as bT; the model equilibrium binding constant in L/g or the maximum binding energy as kT. The plot of qe versus ln Ce led to the determination of the isotherm constants (Figure 7c). 3.5. Thermodynamic Studies. The energy and entropy are considered to determine if the adsorption process occurs spontaneously. The thermodynamic parameters of ΔG o (standard free energy), ΔHo (enthalpy change), and ΔSo (entropy change) are considered indicators for practical application of a process. These parameters were computed by plotting ln Kc versus 1/T (Figure 9a) and using the Van’t Hoff equation: ln Kc =
ΔS ° ΔH ° − R RT
ΔGo (kJ/mol)
ΔHo (kJ/mol)
ΔSo (J/mol·K)
296 316 336
−2.52 −2.69 −2.86
23.2
8.6
3.6. Activation Energy. The Ea in kJ/mol is used to express the activation energy by the following Arrhenius equation: ln k 2 = −
Ea ⎛ 1 ⎞ ⎜ ⎟ + constant R ⎝T ⎠
(15)
where the second-order rate constant k2 is in g/mg·h, and R equals 8.314 J/mol·K. Values of k 2 were determined considering the slopes of t/qt against t at 296, 316, and 336 K. Then, the Arrhenius plot of ln k2 against 1/T for the adsorption of Co(II) was obtained, as shown in Figure 9b. From the slope, the activation energy was calculated and found to be 5.7 kJ/mol. This value, which is in agreement with the rapid equilibrium, indicates a physisorption process with an energy requirement ranging between 5 and 40 kJ/mol. 3.7. Study of Binary Interference. The hydrophobic branch of C18 hydrocarbons in resin 5 was incorporated to capture the aromatic pollutants as well as the Co(II) from water solutions. A sorption evaluation of the resin in the presence of
(13)
where R is equal to 8.314 J/mol·K, and K c is the thermodynamic constant calculated as qe/Ce in L/mg.40 Then the ΔGo was obtained as ΔG° = ΔH ° − T ΔS °
T (K)
(14)
The negative values of ΔG indicate the favorability of adsorption of cobalt ions on the resin. At higher temperatures, the removal of the ion by the resin becomes more favorable 0
G
DOI: 10.1021/acs.jced.6b00475 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
given in Table 5, indicate the high efficiency of the polymer in cobalt removal (≈99%), even in a matrix of other metal ions.
another organic pollutant was carried out to understand the efficiency of the material. Standard stock solutions of eriochrome Black-T dye (EBT) were diluted to predetermined concentrations. The mixed solutions of 10 ppm of Co(II) and 10 ppm of EBT were prepared and the adsorption tests were performed under optimum conditions. Then, the concentrations of Co(II) and EBT were monitored by the atomic absorption spectrometer and UV−vis spectrophotometer. Similar experiments were conducted for the binary systems, consisting of 10 ppm each of Co(II)−methyl orange (MO), Co(II)−phenol, and Co(II)−methylene blue (MB). Figure 10
Table 5. Comparing Co(II) Concentration and Other Metals in Industrial Wastewaters with Adsorption Treatment Using the Prepared Polymera metal
original sample (μg L−1)
original sample treated with the prepared polymer
original sample spiked with 10000 (μg L−1) Co(II) and then treated with the prepared polymer
Co Cu Zn As Sb Pb
0.468 21.81 10.16 7.89 0.045 0.682
