New Chelating Ion-Exchange Resin Synthesized via the

Article pubs.acs.org/IECR

New Chelating Ion-Exchange Resin Synthesized via the Cyclopolymerization Protocol and Its Uptake Performance for Metal Ion Removal Shaikh A. Ali,* Izzat W. Kazi, and Nisar Ullah Chemistry Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia S Supporting Information *

ABSTRACT: Azoisobutyronitrile-initiated cycloterpolymerization of [(diallylamino)propyl]phosphonic acid hydrochloride (90 mol %), cross-linker 1,1,4,4-tetraallylpiperazinium dichloride (10 mol %), and SO2 (100 mol %) afforded a new pH-responsive cross-linked polyzwitterionic acid which, upon alkaline treatment, was transformed to a cross-linked dianionic polyelectrolyte (CDAPE). Chelating ion-exchange resin CDAPE was tested for the uptake of Pb2+ and Cu2+ ions; the adsorption process followed pseudo-second-order kinetics with respective Ea values of 13.4 and 13.8 kJ mol−1. The adsorption data fitted well to the Langmuir, Freundlich, and Temkin as well as Dubinin−Radushkevich isotherm models. The maximum uptakes of Pb2+ and Cu2+ were determined to be 3.83 and 10.1 mmol g−1, respectively. The scanning electron microscopy images and energy-dispersive Xray spectroscopy analysis confirmed that the CDAPE adsorbed the metal ions on the surface as well as throughout the polymer. The negative ΔG° and positive ΔH° ensured the adsorption process as favorable and endothermic in nature. The excellent adsorption and desorption efficiencies demonstrated by the resin would enable its use for the removal of toxic metal ions from wastewater. A comparison of CDAPE with several other adsorbents in recent works ascertains the excellent efficiency of the current resin for the removal of toxic metal ions.

1. INTRODUCTION The deteriorating water resources because of industrial discharge of nonbiodegradable toxic metal ions have captured public attention. For instance, wastewaters from refinery, storage battery, paint, and various other industries contain lead, which is a very important pollutant. Chronic exposure to lead causes serious damage to the vital organs of humans and animals. For drinking water quality, the Chinese standard1 has set a concentration limit of 0.01 mg L−1 for Pb as the extremely severe maximum contaminant level. The U.S. Environmental Protection Agency’s (EPA) goal, on the other hand, is to achieve its concentration approaching zero.2 As per demands of the precarious situation, various innovative techniques have been developed to remove Pb2+ ions.3,4 Copper, a nutritional element, is associated with various enzymes and is an important component of human blood. However, above a certain concentration limit, it can cause liver and lung damage and gastrointestinal disturbances. Municipal and industrial wastewaters as well as copper in the form of sulfides in the earth’s crust are important sources of copper pollution. According to the WHO standards, the maximum limit in drinking water and agriculture irrigation is set to be 0.2 mg/L,5 while it is less than 2 ppm for ponds for fish farming.6 The water scarcity in the present day world demands the remediation of the toxic metal pollution to recycle and reuse the industrial wastewaters. In response to the serious environmental concerns, there is tremendous interest in the development of technologies based on biological treatment, adsorption, and ion exchange, as well as advanced physicochemical methods such as oxidation technologies.7 The mobile counterions such as H+ or Na+ in ion-exchange resins containing SO3−, PO32−, or CO2− functionalities can be © XXXX American Chemical Society

exchanged for toxic metal ions from aqueous solutions. An interesting paper describes the effective use of graphene oxide nanosheets where the carboxyl motifs make strong complexes with metal ions.8 Amphoteric exchangers, which contain zwitterionic motifs such as aminocarboxylate, offer a greater latitude of pH-controlled exchanges of either cations or anions.9 The utilization of a chelating ion-exchange protocol to remove toxic metal ions is becoming increasingly attractive as the chelating ligands exhibit high affinity for heavy metal ions.10,11 The insertion of chelating ligands in a resin overcomes the typical lack of the selectivity of usual ion exchangers by exhibiting higher affinity for heavy metal ions than for alkali-metal and alkaline-earth-metal ions. Effective formation of chelate rings requires the presence of functional motifs containing oxygen, nitrogen, and sulfur on the polymer surface. The nature, density, relative positions, spatial configuration, and distance from the matrix of chelating ligands along with the steric factors and solution pH control the capacity and selectivity of chelating ion exchangers.12 Research on ion-exchange resins, which has been an important scientific development in the 20th century, remains a fertile area because of the strong emphasis on wastewater treatment and catalytic applications.11 The ability to design better ion-exchange resins requires a better understanding of substrate−ligand interactions with the help of hard−soft acid− base theory. A literature query search has revealed that, among various chelating ligands, the (aminomethyl)phosphonate Received: June 22, 2015 Revised: September 27, 2015 Accepted: September 28, 2015

A

DOI: 10.1021/acs.iecr.5b02267 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

peak at δ 67.4 as the external standard. Thermogravimetric analyses (TGA) were performed using an SDT analyzer (Q600, TA Instruments, United States) over 20−800 °C with a uniform rate of increase of 15 °C/min under N2 (flowing at a rate of 50 cm3/min). The Brunauer−Emmett−Teller (BET) N2 method with a Micrometrics ASAP 2020 BET surface area analyzer was used to measure the surface area of the synthesized resin 5. The particle size of the synthesized resin was determined in a Microtrac (United States) model S3500 instrument using an aqueous medium. X-ray diffraction analysis was carried out in a Rigaku model MiniFlex II diffractometer (Japan) equipped with a Cu K radiation source. Inductively coupled plasma mass spectrometry (ICP-MS) (model XSERIES-II, Thermo Scientific) was used to determine the concentrations of some metal ions in real wastewater samples. 2.2. Materials. 2,2′-Azoisobutyronitrile (AIBN), purchased from Fluka AG, was purified by crystallizing it from a chloroform−ethanol mixture. Dimethyl sulfoxide (DMSO) was purified by drying (CaH2) and distilling at 64−65 °C (4 mmHg). 2.3. Synthesis of the Monomers. 2.3.1. 3-(N,NDiallylamino)propanephosphonic Acid (2). The monomer precursor amine diethyl 3-(N,N-diallylamino)propanephosphonate (1) was prepared as described.18 A solution of amine 1 (20 g, 72.6 mmol) in 6 M HCl (50 mL) was heated at 95 °C for 48 h. After removal of the solvent, the residual liquid 2 was dried under vacuum at 60 °C to a constant weight (18.5 g, ∼100%). A solution of a known quantity of ethanol (23.9 mg) and the monomer (137.5 mg) in D2O (2 mL) was prepared and its 1H NMR spectrum recorded; careful integration of several nonoverlapping proton signals revealed the molar mass of the monomer as 252 g mol−1, which is close to the actual molar mass of 255.68 g mol−1. The monomer was thus pure and as such used without further purification. Anal. Found: C, 42.0; H, 7.6; N, 5.4. C9H19ClNO3P requires C, 42.28; H, 7.49; N, 5.48. νmax (neat): 3365, 3097, 2952, 2756, 2658, 1646, 1456, 1427, 1234, 1163, 994, 952, 772, and 713 cm−1. δH (D2O): 1.61 (2 H, m), 1.80 (2 H, m), 3.03 (2 H, m), 3.59 (4 H, d, J 7.0 Hz), 5.40 (4 H, m), 5.70 (2 H, m) (HOD signal at δ 4.65 ppm). δC (125 MHz, D2O): 18.23 (s, 1C, PCH2CH2), 24.10 (d, 1C, d, PCH2, 1J(PC) = 136 Hz), 52.77 (1C, d, PCH2CH2CH2, 3J(PC) = 18.6 Hz), 55.75 (s, 2C,  CHCH2), 126.25 (s, 2C, CH), 127.60 (s, 2C, CH2) (dioxane signal at δ 67.4 ppm). δP (202 MHz, D2O): 26.98 (m, 1P). DEPT 135 NMR analysis was carried out to confirm the 13 C spectral assignments. 2.3.2. 1,1,4,4-Tetraallylpiperazinium Dichloride (3). Crosslinker 3 was synthesized using a known procedure.19 2.4. Synthesis of Cross-Linked Resins. 2.4.1. Synthesis of Cross-Linked Polyzwitterionic Acid (CPZA) 4. A known mass of SO2 (3.91 g, 61 mmol) was absorbed onto a solution of monomer 2 (12.8 g, 50 mmol) and cross-linker 3 (1.77 g, 5.56 mmol) in DMSO (22 g) in a 100 mL round-bottomed flask. The polymerization was initiated by adding AIBN (430 mg) to the continuously stirred mixture at 65 °C under N2 for 24 h. The transparent gel of CPZA 4 was then soaked in water (12 h), filtered, washed with water and acetone, and dried at 60 °C for 6 h under vacuum (13.7 g, 78%). Thermal decomposition: the resin did not change its color up to 350 °C; thereafter, the color was changed to yellow. Anal. Found: C, 38.1; H, 6.6; N, 4.8; S, 11.1. A resin containing monomer 2 (−HCl) (C9H18NO3P) (0.9 mol %), monomer 3 (C16H28Cl2N2) (0.1 mol %), and SO2 (1.1 mol %) (each mole of cross-linker is

functionality offers remarkable selectivity toward heavy metal ions.10−17 The purpose of the current work includes the synthesis and application of a new resin containing pHresponsive chelating (aminopropyl)phosphonate motifs capable of sequestering toxic metal ions from industrial wastewaters or industrial wastes (Scheme 1). The synthesized resin would Scheme 1. Synthesis of Cross-Linked Poly[(aminopropyl)phosphonate]

enable us to examine for the first time the effect of the chelating ligands attached to the terminals of a propyl spacer on the removal of toxic Pb2+ and Cu2+ ions as model cases from aqueous solution and industrial wastewater. The propylene (−CH2CH2CH2−) spacer being longer than a methylene (−CH2−) would increase the distance of the phosphonate ligand from the polymer matrix, thus making it free from steric encumbrance to interact strongly with metal ions. We anticipate an exciting outcome from the current endeavor.

2. EXPERIMENTAL SECTION 2.1. Physical Methods. A PerkinElmer elemental analyzer, series 11, model 2400 (Waltham, MA), was used for elemental analysis, while IR analyses were performed on a Thermo Scientific FTIR spectrometer (Nicolet 6700, Thermo Electron Corp., Madison, WI). A TESCAN LYRA 3 (Czech Republic) equipped with an energy-dispersive X-ray (EDX) spectroscopy detector, model X-Max, was used for scanning electron microscopy (SEM) images and EDX spectra. NMR spectra were obtained on a JEOL LA 500 MHz spectrometer using CDCl3 with tetramethylsilane (TMS) or D2O with deuterated TSP (i.e., sodium 3-(trimethylsilyl)propionate-2,2,3,3-d4) as the internal standard (1H signal at δ 0 ppm) and the dioxane 13C B


Article


out by stirring a mixture of the resin (250 mg) and 0.1 M M2+ (100 mL) in a buffer of pH 5 for Cu2+ and pH 4 for Pb2+ at different temperatures. The concentration of the metal ions was determined by taking a small amount of filtered aliquots at various time intervals. The adsorption capacities of the resin at metal ion concentrations in the range of 0.02−0.1 M at different temperatures were used to construct various adsorption isotherms and calculate the thermodynamic parameters ΔG°, ΔH°, and ΔS°. 2.7. Desorption Experiment. The issue of recycling and reuse of an adsorbent needs to be addressed for its effective use in industry. The resin loaded with metal ions was prepared in centrifuge tubes using CDAPE 5 (50 mg) in buffered solutions of 0.1 M Cu(NO3)2 or Pb(NO3)2 (20 mL). After 24 h, the mixture was centrifuged, and the supernatant liquid was used to determine the qe values. The loaded resin left in the centrifuge tube, after washing with buffer solutions of pH 4 (for Pb2+) and pH 5 (for Cu2+), was used for the desorption experiment in 0.5 M HNO3 (50 mL) at 22 °C for 2 h. The amount of metal ions desorbed was then determined (vide supra) to find the efficiency of the desorption process. The resin left in the centrifuge tube was washed with deionized water; as described above, the adsorption/desorption procedure was repeated three times. It is worth mentioning that the procedure ensured the use of a fixed amount of the same resin three times. Centrifugation ensured no loss of the resin as may happen in the usual filtration procedure.

assumed to react with 2 mol of SO2) requires C, 38.87; H, 6.39; N, 5.14; S, 11.77. The molecular formula of each repeat unit is thus taken as (C7H13NO3P)0.9, (C16H28Cl2N2)0.1, and (SO2)1.1 with an approximate empirical formula of C48.5H95N5.5O24.5P4.5S5.5Cl. The amount of H is found to be slightly higher than the calculated value, presumably indicating the presence of moisture or associated water molecules whose amount cannot be specified with certainty. νmax (KBr): 3734, 3381, 2291, 1662, 1460, 1414, 1305, 1127, 1047, 973, 883, 781, 618, 563, and 438 cm−1. 2.4.2. Conversion of CPZA 4 to Cross-Linked Dianionic Polyelectrolyte (CDAPE) 5. Resin 4 (9.00 g 30.7 mmol) was treated with NaOH (3.36 g, 84 mmol) in water (100 mL); after 1 h at room temperature, a mixture of methanol (50 mL) containing NaOH (1 g) and acetone (200 mL) was added to the gel. The resultant CDAPE 5 was then filtered, washed with acetone, and dried under vacuum for 6 h at 65 °C (9.2 g, 91%). Thermal decomposition: the resin did not change its color up to 350 °C; thereafter, the color was changed to pale yellow. Anal. Found: C, 33.9; H, 5.6; N, 4.3; S, 9.9. Requires C, 34.53; H, 5.22; N, 4.51; S, 10.33. 2.5. Ion-Exchange Capacity (IEC). After immersion of resin 5 (100 mg) in 0.1 M HCl (50 mL) for 24 h, the IEC was calculated from the decrease in acidity in the filtrate by titration with 0.1 M NaOH using the following equation: IEC =

mmol i − mmol f W

(1)

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of Cross-Linked Resins. Butler’s linear cyclopolymerization protocol22−24 was used to synthesize the cross-linked polymer using SO2, and monomers 2 and 3 prepared from inexpensive starting materials.18,19 Thus, a feed consisting of monomer 2, crosslinker 3, and SO2 in a mole ratio of 0.9:0.1:1.1 underwent AIBN-initiated terpolymerization to give CPZA 4 in 78% yield (Scheme 1). The resultant polymer architecture is included among the most important structural types.25 It is important to delineate the importance of the polymerization technique: poly(diallyldimethylammonium chloride) alone is the subject of over 1000 publications and patents, and over 35 million pounds per year is sold for the use of water purification and personal care formulation.22 Taking into account that cross-linker 3 would consume 2 equiv of SO2, the terpolymer is calculated to have repeating units of 2, 3, and SO2 in a mole ratio of 0.9:0.1:1.1 to match the feed ratio. This composition, as confirmed by the elemental analysis, is expected in the event of high conversion to the polymer. The resin has an alternate incorporation of SO2 with random placement of cross-linker 3. Monomer 2 and cross-linker 3 have functionality averages of 2 and 4, respectively. As such, the polymerization process at high conversion would lead to cross-linking. Initially, the linear backbone of the terpolymer is expected to have repeating units of 2 and 3 in an approximate ratio of 9:1. The presence of the unreacted diallyl moiety as pendants on the other half of the cross-linker would initiate further polymerization, leading to cross-linking. CPZA 4, on treatment with NaOH, was converted into CDAPE 5 containing three ligand centers in each repeating unit: one in nitrogen and two in the phosphonate motifs. To our knowledge, the synthesis of CPZA 4 represents a unique example of a cross-linked cycloterpolymer containing (aminopropyl)phosphonate motifs and repeating units of SO2.

where the difference between the initial (mmoli) and final (mmolf) amounts of HCl is divided by the polymer mass W in grams. 2.6. Adsorption Capacities. The uptake of Pb2+ and Cu2+ ions by CDAPE 5 was determined by titration. The procedure for Pb2+ adsorption was as follows: A mixture of CDAPE 5 (50 mg) in 0.1 M Pb(NO3)2 (20 mL) in acetate buffer solutions of different pH values was stirred for 24 h, filtered, and washed with the buffer solution. The combined filtrate was titrated with 0.1 M EDTA solution using xylenol orange as the indicator to find the amount of Pb2+ remaining; its adsorption capacity (qPb2+) was then calculated using the following equation: qPb2+ =

(C0 − Cf )V W

(2)

where the initial and final concentrations of Pb2+ ions (mol L−1) are denoted by C0 and Cf, respectively, and V and W represent the volume (mL) and mass of the polymer (g). The determination of the concentrations of Pb2+ was carried out using a protocol reported in a paper which described the quantification of 0.01 M Pb(NO3)2 using a 0.01 M EDTA solution.20 Note that, for the current work, much higher concentrations of Pb2+ ions were used. Stock solutions of known concentrations of Pb2+ in the range of 0.01−0.1 M were found to be accurate to ±2%. Experiments ran in triplicate for the test solutions provided the data, which varied in the range of 1−2%. The adsorption study for Cu2+ ions was carried out using a 0.1 M Cu(NO3)2 solution; the iodometric titration method was used to determine the leftover ions in the filtrate.21 Stock solutions of known concentrations of Cu2+ in the range of 0.1− 0.01 M were found to be accurate to ±2%. Experiments ran in triplicate for the test solutions provided the data, which varied in the range of 1−3%. Adsorption kinetic studies were carried C


Article


Figure 1. (a) TGA curve of CDAPE 5. (b) Dependency of the adsorption capacity on the pH using 0.1 M metal ions at 295 K.

Scheme 2. (Aminopropyl)phosphonates as Chelating Ligands

with increasing pH values owing to a decrease in the H+ concentration (Figure 1b). Increased negative charge density on the polymer backbone would impart a stronger electrostatic force, which encourages ion exchange as well as complexation via formation of coordination bonds with the metal ions (Scheme 2).27 There is also the possibility that, at higher metal loads, the adsorption capacities may be improved; the bidentate ligand may switch to act as an ion exchanger to capture two metal ions. On the other hand, at lower pH values, the increased amount of cationic motifs would discourage the adsorption of metal cations owing to the electrostatic repulsion. In addition, formation of zwitterion motifs such as in CPZA 4 makes it a monodentate ligand with lower adsorption capacities. It is worth mentioning the possibility of sulfone motifs joining in the metal-chelation activities to assist the sorption process. It is well documented that DMSO ((CH3)2SO) as well as tetramethylene sulfone ((CH2)4SO2) are known to act as ligands in metal ion complexes.28,29 3.3. Ion-Exchange Properties of CDAPE 5 and IR. The high IEC value of 5.02 mmol g−1 for CDAPE 5 ascertained the excellent adsorption ability for metal ions. The presence of amine and phosphonate (−P(O)O22−) ligands is considered to be responsible for the chemical adsorption (Scheme 2). The IR spectrum of zwitterionic resin 4 displayed a band at 1062 cm−1 owing to the vibration of the PO3H− group (Figure S1a). Likewise, the spectrum of sodium salt 5 and the spectra after its subsequent treatment with Cu2+ and Pb2+ ions showed

The TGA curve of CDAPE 5 (Figure 1a) showed gradual weight loss of moisture (5%) up to 100 °C. Continued gradual loss of 13% happened in the range of 100−280 °C followed by a sharp loss of 20% between 280 and 310 °C owing to the release of SO2. Note that the actual SO2 content in the resin is calculated to be 20.9%. Another gradual loss of 20% in the range of 310−510 °C is attributed to the loss of the diallylamine moiety. Presumably, a 35% residual mass at 800 °C accounted for the sodium phosphonate fraction. The surface area of the resin 5 was found to be 6.3 m2 g−1. The average particle size of the resin was determined to be 9.25 μm, with the size ranging from 3 to 37 μm. 3.2. Effect of the pH on Adsorption. To ascertain the pH dependence on the uptake of Pb2+ and Cu2+ ions, experiments were carried out in the pH range of 2.8−5.3 by using acetate buffer. The optimum pH for the uptake of Pb2+ was found to be 4. The pH has a very strong effect on the adsorption capacities for Cu2+. As evident in Figure 1b, the adsorption for Cu2+ at pH 2.8 was almost zero but was enhanced to 4.1 mmol g−1 at pH 5.3. This finding may be exploited advantageously for carrying out adsorption and desorption processes at a higher and a lower pH, respectively. Therefore, the adsorption studies for Pb2+ and Cu2+ were carried out at pH 4 and 5, respectively. pH values higher than 5 were not employed to avoid the formation of insoluble metal hydroxides, which might have led to erroneous data on the adsorption by the resin.26 Since both H+ and metal ions compete for the adsorption sites on the resin, the adsorption capacity for Cu2+ increased D


Article


Figure 2. Adsorption on CAPE 5: (a) kinetic curves of Cu2+ (0.1 M, pH 5) at (●) 295, (▲) 308, and (■) 323 K and Pb2+ (0.1 M, pH 4) at (○) 295, (Δ) 308, and (□) 323 K; (b) Lagergren first-order kinetic model; (c) Lagergren second-order kinetic model; (d) intraparticle diffusion model.

absorption bands at ∼970 and ∼1032 cm−1 attributed to the symmetric and asymmetric stretching vibrations of the PO32− group (Figure S1b−d).30−32 The SO2 vibrations appeared at 1128 and 1304 cm−1 (Figure S1). A comparison of the spectra (Figure S1b versus Figure S1c,d) revealed strong perturbations of the P−O peaks upon coordination to Cu2+ and Pb2+ ions, thus implying a direct bond between the metal ions and the phosphonate group.33,34 The presence of new bands around 1383 cm−1 due to a nitrate group30suggests the ability of the resin to act as an anion exchanger (Figure S1c,d).34 It is pertinent to mention that the adsorption experiments were carried out using solutions of the metal nitrates; in acidic pH, the protonated nitrogens (i.e., NH+) by virtue of their electrostatic attractions are expected to bind the nitrates. At lower loads, the chelating functionalities may act as tridentante ligands as depicted in panel D of Scheme 2 or as bidentante ligands as shown in panels B and C. At larger loads, ionic bonding in the presence of nitrate ions may lead to the structure shown in panel A.35 3.4. Adsorption Kinetics and Effect of Temperature on Adsorption. 3.4.1. Lagergren First-Order and PseudoSecond-Order Kinetics. The plots of metal uptake versus time at various temperatures in Figure 2a show the attainment of the adsorption equilibria M2+ + resin ⇋ resin → M2+ in about 1.5 h. The Lagergren adsorption kinetic model is of utmost importance to shed light on the adsorption properties of a polymer.21 The following equation expresses the first-order kinetic equation for the Lagergren model: log(qe − qt) = log qe −

k1t 2.303

denoted as qt and qe, respectively.36Although Pb2+ and Cu2+ both gave very good regression values (R2) for the first-order kinetic model (Figure 2b), there is a significant difference between the experimental adsorption capacity (qexp) and its calculated value (qcalc) (Table S1). The plots in Figure 2c represent the pseudo-second-order kinetic model37 as described by the following equation: t 1 t = + 2 qt q k 2qe e

(4)

where k2 is the second-order rate constant. The data fitted well with excellent values of R2; the qcalc values for Pb2+ and Cu2+ were found to be very close to their qexp values (Table S1), thereby suggesting the process as a chemical adsorption.38 As evident in Table S1, the rate constant k2 and qexp for the uptake of Cu2+ are greater than those of Pb2+ (Figure 2c). This is opposite the findings of several earlier reports;14,15,39 the higher uptake of Pb2+ ions was attributed to its lower hydrated ionic radius of 4.01 Å as compared to 4.19 Å for Cu2+.40 It is argued that the smaller hydrated ionic radius of Pb2+ allows it to approach more closely to the adsorbing surface for favorable adsorption. However, this does not seem to be a convincing explanation as a result of the opposite finding in the current work. Even though Cu2+ has a higher hydrated radius, its nonhydrated ionic radius of 0.73 Å is much lower than 1.19 Å for Pb2+. Since the three-carbon spacer of (CH2)3 keeps the chelating phosphonate ligand away from the sterically crowded surface of resin CDAP 5, the approach of Cu2+ with its larger hydration shell would be less difficult. Chelation with the smaller Cu2+ having a higher charge density is expected to be more tempting with the added entropic advantage of releasing a larger number of water molecules from its hydration shell.

(3)

where k1 is the first-order rate constant, while the adsorption capacities of the metal ions at time t and at equilibrium are E


Article

Industrial & Engineering Chemistry Research 3.4.2. Intraparticle Diffusion Model. Several adsorption diffusion models were developed to understand the mechanism of adsorption, including its rate-limiting step, among the three important consecutive steps of (1) film diffusion, (2) intraparticle diffusion, and (3) mass action. The first step involves diffusion of a metal ion through the liquid film surrounding the resin particles, while the second step deals with its diffusion in the liquid contained in the resin pores. The final step is described by adsorption of metal ions onto the active sites in the resin. For an intraparticle diffusion step, the relation of the metal uptake versus time is expressed by the following equation:41 qt = x i + k pt 0.5

all adsorption sites are energetically equivalent and identical and there are no interactions among adsorbed molecules.45 The mechanism envisages a monolayer adsorption on the resin surface; once a molecule occupies a site, no further adsorption can take place.46 The ratio of the adsorption and desorption rates as represented by the Langmuir constant (b) and the maximum specific metal uptake (Qm) can be calculated by the following linearized Langmuir isotherm equation:47

Ce C 1 = e − qe Qm Q mb

where qe is the millimoles of metal adsorbed per gram of CDAPE 5 and Ce is the residual metal concentration in solution at equilibrium (Table S2). Figure S3a represents the plot of Ce/ qe versus Ce. For the isotherm model, RL, known as the separation factor, can be can be calculated by the following equation:48 1 RL = 1 + bC0 (9)

(5)

where qt describes the metal uptake at time t, kp is the rate constant, and xi is proportional to the boundary layer thickness.42 For a qt versus t0.5 plot with zero intercept (xi = 0), the intraparticle diffusion becomes the rate-limiting step.43,44 For the current study, however, the initial linear plots did have the intercept values, thereby implying that a certain amount of the adsorption happened very rapidly (Figure 2d). This observation leads to the conclusion that the sorption process is controlled by both the film and intraparticle diffusion. The process is thus described by a rapid initial adsorption step followed by a slower process as dictated by intraparticle diffusion. The metal ions are exchanged on the outermost layer of the particle, and as time passes, the reaction sites are gradually moved toward the core of the particle. The initial adsorption factor (Ri), as defined in terms of xi, is expressed by the following equation: x Ri = 1 − i qe (6)

where b represents the Langmuir constant. The RL values fall into the preferred region (0 < RL < 1) (Table 1). Note that, for Table 1. RL Values at Various Initial Metal Ion Concentrations RL value

where xi is the amount initially adsorbed rapidly and qe describes the final capacity at eqilibrium. For Cu2+ at 323 K (Table S1), xi and qe values of 0.919 and 4.28 mmol g−1, respectively, gave an Ri value of 0.785, which translates into 21.5% rapid adsorption followed by 78.5% adsorption governed by intraparticle diffusion (Figure 2d). Larger xi values in the case of Cu2+ suggest that the rate-limiting step is greatly controlled by film diffusion. The sign and magnitude of xi are found to be temperature dependent; at the lowest temperature, the negative values of xi reflect the irrelevance of the boundary layer on the diffusion rate (Table S1). 3.4.3. Adsorption Activation Energy. Using Arrhenius eq 7, the ln k2 versus 1/T plots (Figure S2a) gave activation energies (Ea) of 13.8 and 13.4 kJ mol−1 for the respective adsorptions of Cu2+ and Pb2+. These low Ea values indicate the favorability of the adsorption process.26 ln k 2 = −

Ea + constant 2.303RT

(8)

C0 (mol dm−3)

Pb2+

Cu2+

0.02 0.04 0.06 0.08 0.1

0.827 0.705 0.615 0.545 0.489

0.820 0.694 0.602 0.532 0.476

RL ≥ 1 and RL = 0, the adsorption process is unfavorable and irreversible, respectively. As evident from the table, the RL value depends on the initial concentration; its lower values make the ion exchange more favorable at higher initial concentration. The slightly lower values of RL for Cu2+ are suggestive of its adsorption being more favorable than that of Pb2+ ions. It is interesting to note that the resin (1 g, 3.0 mmol repeat unit) has a very high maximum adsorption capacity (Qm) value of 10.1 mmol of Cu2+ ions (Table S2). Let alone the weak chelating ability of SO2 unit, every millimole of repeat unit has 3 mmol of chelation centers belonging to one nitrogen and two oxygens in the phosphonate motifs. Thereby every gram of the resin can provide 9.0 mmol of chelation centers, which matches well with the experimental finding of Qm as 10.1 mmol g−1. 3.5.2. Freundlich Isotherm Model. This isotherm model describes heterogeneous adsorption systems with uniform energy as expressed by the following equation: 1 log qe = log k f + log Ce (10) n

(7)

3.5. Adsorption Capacity versus Initial Concentration of the Metal Ions: Adsorption Isotherms. The qe of CDAPE 5 was found to be directly proportional to the initial concentrations (Ci) of the metal ions (Figure S2b). The q−Ci dependency can be expressed by several isotherm models (vide infra) which are instrumental in the exploration of the adsorption mechanism. 3.5.1. Langmuir Isotherm. This isotherm model is based on the assumptions that, on a structurally homogeneous adsorbent,

where the Freundlich constants kf and n are calculated from the log qe versus log Ce plot (Figure S3b and Table S2). The n values of 1.45 and 1.42 for Pb2+ and Cu2+, respectively, lie in the range of 1−10, which is considered as favorable adsorption.49 The slopes (1/n) of the plots (0.690 for Pb2+ and 0.704 for Cu2+) are found to be in the range of 0−1, which is indicative of a chemisorption process, whereas 1/n above 1 implies cooperative adsorption.50 F


Article


ΔH° and ΔS° as calculated from the slope and intercept, respectively, of the log(qe/Ce) versus 1/T plots (Figure S4b) gave ΔG° (= ΔH° − TΔS°). The results are given in Table 2.

3.5.3. Temkin Isotherm. This model assumes that the heat of adsorption decreases with an increase in the degree of surface coverage by the adsorbate and is expressed by the following equation:51 RT qe = ln(aCe) b

Table 2. ΔG°, ΔH°, and ΔS° for the Adsorption of Pb2+ and Cu2+ Ions

(11)

metal ion

which can be linearized as qe =

Pb

RT RT ln A + ln Ce b b

2+

(12)

qe = B ln A + B ln Ce

Cu2+

(13)

where R is the gas constant (8.314 J mol−1 K−1), T is temperature (K), A is the equilibrium binding constant (L g−1) corresponding to the maximum binding energy, and the constant B = RT/b is related to the heat of adsorption ((kJ mol−1), which reflects the adsorption potential of the metal ions (Table S2). Constants A and B are obtained from the plot of qe versus ln Ce (Figure S3c). Excellent fitting of the adsorption data with all three isotherm models presented so far (Figure S3a−c) illustrates the metal uptake as a monolayer adsorption on a heterogeneous surface. 3.5.4. Dubinin−Radushkevich (D−R) Isotherm Model. This model, which helps to estimate the porosity of the resin and the apparent energy of adsorption, is represented by the following equation:52,53 qe = qD exp(−BD[RT ln(1 + 1/Ce)]2 )

(14)

ln qe = ln qD − BD[(RT ln(1 + 1/Ce)]2 = ln qD − BDϵ2 (15)

The plot of ln qe against ϵ gave the values of qD and BD calculated from the intercept and slope, respectively (Figure S3d and Table S2). The apparent energy of adsorption (E), calculated by using eq 16, is also reported in Table S2. 2

(16)

The higher values of qD signify higher adsorption capacity. The value of the absorption capacity qD for Cu2+ is higher than that of Pb2+ as confirmed by the experimental results (Table S2). Interestingly, the qD values obtained by the D−R model match the Qm values from the Langmuir isotherm model; both the models gave very high sorption capacity of the resin for Cu2+. Experimental data fitted very well with the D−R isotherm. The low values of BD confirm the favorability of the ion-exchange process (Table S2).54 3.6. Adsorption Thermodynamics. As evident from Figure S4a, the adsorption process is endothermic since the increase in temperature leads to an increase in the adsorption capacities. At a higher temperature, the diffusion of the metal ions becomes faster in a swelled gel.31 The following Van’t Hoff equation was used to calculate the thermodynamic parameters ΔG°, ΔH°, and ΔS°: ⎛q ⎞ ΔH ° ΔS° log⎜ e ⎟ = − + 2.303RT 2.303R ⎝ Ce ⎠

ΔG° (kJ mol−1)

ΔH° (kJ mol−1)

ΔS° (J K−1 mol−1)

R2

295 308 323 295 308 323

−7.66 −8.23 −8.88 −9.31 −9.77 −10.3

5.16

43.5

0.9799

1.43

36.4

0.9985

The positive sign of ΔH° suggests an endothermic adsorption process, which makes it more favorable with increasing temperatures as confirmed by more negative ΔG° values. This is also confirmed by the increase of adsorption capacities with increasing temperatures (Figure S4a). At elevated temperatures, the greater dissociation shifts the equilibrium NH+···P(O)(OH)O− ⇋ NH+···P(O)(O−)2 + H+ to the right. The increased charge imbalance in the polymer backbone in favor of the negative sign increases the electrostatic field force, which encourages binding with the metal cations. The positive ΔS° values indicate an increase of randomness owing to the release of water molecules from the hydration shells of the metal ions (Table 2). 3.7. XRD Analysis. Unloaded resin 4 and Cu2+ and Pb2+ loaded resins were subjected to X-ray diffraction analysis (Figure S5). The absence of any distinct sharp peak and the presence of a broad diffraction hump at about 2θ = 20° indicated the amorphous nature of the samples.55 The presence of metal ions has no effect on the amorphous structure of the resin except in the increase in the broadness of the humps. However, a color change from white to green (for Cu2+) and to off-white cream (for Pb2+) was suggestive of the sorption of the metal ions (Figure S6). 3.8. SEM Images and EDX. CAPE 5 was soaked in 0.1 M Pb(NO3)2 (at pH 4) and 0.1 M Cu(NO3)2 (at pH 5) for 24 h, filtered, and dried under vacuum. A thin film of gold was used to sputter-coat the dried samples for 6 min, and then they were subjected to scanning. The SEM images were used to examine the surface morphology. A rough cracked surface of the unloaded sample (Figure 3a) allows a greater area for adsorption, whereas smoother and uniform surfaces of the loaded samples (Figure 3b,c) are due to the adsorption of metal ions. EDX spectroscopy of unloaded resin indicated the presence of the expected elements, especially Na (Figure 3a), while its absence in loaded resins confirmed the cationic exchange with Cu2+ and Pb2+ (Figure 3b,c). 3.9. Treatment of Real Wastewater Samples. A matrix of industrial wastewater having a pH of 7.1 was used to examine the efficiency of resin 5 for the removal of the metal ions. The samples (20 mL) were spiked with 0.0, 10.0, and 20 mg/L concentrations each of Cu2+ and Pb2+ ions and then treated with the resin (50 mg) and left to equilibrate for 24 h. The analysis of the original sample revealed the presence of Cu, Ni, Cr, Cd, and Pb (Table 3). However, after treatment with the resin, the concentrations of all the metal ions (except Cr) were found to be below the detection limit of our analytical method. In the spiked samples, the resin treatment led to a considerable

where BD is related to the free energy of sorption per mole of the sorbate and qD is the D−R isotherm constant related to the degree of adsorption. The linear form of eq 14 in terms of the Polanyi potential ϵ is given as

E = 1/(2BD)1/2

temp (K)

(17) G


Article


Figure 3. EDX and SEM images for CDAPE 5 (a) unloaded, (b) loaded with Pb2+, and (c) loaded with Cu2+.

Table 3. Comparison of Cu2+ and Pb2+ Concentrations in Wastewater Samples before and after Treatment with Resin 5

3.10. Reuse of the Polymer. Reusability of the resin is important for economic development. In addition, the disposal of the exhausted sorbent loaded with the adsorbates may cause environmental damage. As described in the Experiment Section, the adsorption/desorption was repeated three times. The desorption efficiency of the first cycle was determined to be 91% for Cu2+ and 90% for Pb2+ ions. The resin shows good recovery with almost stable efficiency for the second and third adsorption/desorption cycles.

concn (μg L−1) after treatment (original sample spiked with Cu2+ and Pb2+ and then treated with adsorbent 5) metal

original sample concn (μg L−1)

0

Cr Ni Cu Cd Pb

0.766 1.83 5.98 0.102 0.313

0.446

New Chelating Ion-Exchange Resin Synthesized via the

Recommend Documents