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. 2020 Oct 23;5(43):27833–27845. doi: 10.1021/acsomega.0c02860

Design and Synthesis of a Dual-Purpose Superadsorbent Containing a High Density of Chelating Motifs for the Fast Mitigation of Methylene Blue and Pb(II)

Shuaib A Mubarak 1, Shaikh A Ali 1,*, Ibrahim Y Yaagoob 1, Mohammad A J Mazumder 1,*
PMCID: PMC7643114  PMID: 33163766

Abstract

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Maleic acid underwent alternate copolymerization with diallylaminomethylphosphonic acid·HCl [(CH2=CHCH2)2NH+CH2PO3H2 Cl–] and a cross-linker to give a new pH-responsive resin. Methylene blue (MB) removal from its 3000 ppm solution by the resin at pH 7 followed second-order kinetics with an Ea of 34.8 kJ mol–1. MB removal was achieved very fast (10 min), attaining over 98.5% at 328 K. The qe obtained using MB concentrations in the range 100–8000 ppm fitted the Langmuir nonlinear isotherm model to give ΔGo, ΔHo, and ΔSo values of ≈ −21 kJ, 36.5 kJ mol–1, and 185 J mol–1 K–1, respectively. The resin is a superadsorbent with a qmax value of 2445 mg g–1. The adsorbent also removed 97% Pb(II) within 5 min from its 10 000 ppb solution. The resin reduced the Pb(II) concentration from 200 to 3.8 ppb. The resin also demonstrated its ability to remove contaminants from industrial wastewater, reducing priority metal contaminants to ppb and sub-ppb levels. The resin can be recycled with stable efficiency. The outstanding performance places the resin in a top position in a list of recently reported sorbents.

1. Introduction

The presence of commonly used methylene blue (MB) dye in effluents from the textile industry1 causes serious damage to the ecological environment.2 Aquatic life in rivers is threatened owing to the discharge of a significant portion of ≈7 × 105 tons of annually used dyes.3,4 The presence of dyes in river streams is detrimental to the food web as they diminish light penetration into water, thereby reducing the photosynthetic activity.5 Moreover, most of the dyes are carcinogenic and mutagenic; their low biodegradability6 makes biological procedures7 for dye removal very inefficient. The adsorption technique,8,9 superior to many other techniques, has been extensively used for the remediation of wastewater.10,11

Carbon materials like activated carbons,12−14 carbon nanotubes,15 graphenes,16,17 nano-FeS anchored graphene,18 and biochars19,20 have been used for adsorptive removal of MB. The relatively expensive nature of activated carbon hampers its use, and as such, efforts have been directed in search of low-cost adsorbents.5 Among other materials, chitosan21 and chitosan-decorated nanocomposites,22,23 MOF materials,24,25 metal oxides,26−29 biomimetic polydopamine,30 etc. have been found to be effective in MB removal.

Severely toxic lead is widely used in industries like battery manufacturing, metal plating, printing, and tanning. Lead, being toxic and not essential for the body, bears serious health implications. Greater efficiency and simplicity make adsorption a preferred method for lead removal from aqueous systems.31−35

The current article involves the synthesis of a pH-responsive resin using industrially important Butler’s cyclopolymerization protocol.36,37 A plethora of diallylamine salts has been cyclopolymerized to generate the eighth most important polymer architecture whose backbone is embedded with five-membered pyrrolidine rings.38−40 It is worth mentioning that millions of pounds of poly(dimethyldiallylammonium chloride), alone accounting for over 1000 patents and publications, is produced annually for water treatment, the paper industry, and personal care formulations.41 Almost zero reactivity ratios for diallylamine salts/maleic acid (MA) copolymerization ensure the synthesis of a variety of alternate copolymers.42 For the current work, readily synthesized diallylamine salt 1, containing motifs of aminomethylphosphonic acid (H3N+CH2PO3H–), MA 2, and cross-linker 3, would generate the 3D network of a new resin 5 decorated with CO2– ligands and R2NCH2PO3= chelating motifs (Scheme 1). The alternate terpolymerization, to our knowledge, represents the first example of the synthesis of a resin bearing aminophosphonate as well as succinate ligands in the same framework, whose high negative charge-density is anticipated to trap both organic and inorganic cationic pollutants from wastewater. Herein, we describe the use of the resin for the adsorptive mitigation of MB and Pb(II) (as model cases for cationic species) from aqueous systems. As per Environmental Protection Agency (EPA) guidelines,43 while the permissible level of lead in drinking water is 15 μg L–1 (i.e., 15 ppb), the ultimate goal would be to reach a level of zero concentration. While numerous studies report Pb(II) removal at higher concentration regimes, its mitigation at ppb and sub-ppb levels is limited and remains challenging. Keeping in view the limit set by the EPA, the mitigating power of the newly synthesized resin would be examined for MB removal at higher concentration regimes and Pb(II) removal at ppb levels.

Scheme 1. Syntheses of Resins 4 and 5.

Scheme 1

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2. Results and Discussion

2.1. Resin Synthesis

Diallylammonium salts undergo polymerization to generate numerous industrially important cyclopolymers.36,41 Diallylammonium salts and maleic acid (MA) copolymerization are reported to lead to alternate copolymers since the monomers have reactivity ratios of almost zero.42 The APS-initiated alternate copolymerization of monomers 1 (38.9 mol %), 2 (53.7 mol %), and 3 (7.4 mol %) gave resin (PZAR) 4, which was transformed to anionic resin 5 upon treatment with NaOH (Scheme 1). Resin 5 loaded with anionic– groups, as anticipated, has a high IEC value of 8.5 mmol g–1.

2.2. BET Analyses of Resins 4 and 5

Table 1 lists the textural parameters of resin 5 as obtained by BET analysis. The surface area of 1.74 m2 g–1 is more than the usual value of <0.1 m2 g–1 for typical ionic resins.44,45 Note that anionic resin 5 has a greater surface area owing to the expanded morphology in the absence of zwitterionic interactions as in resin 4.

Table 1. Analysis of BET Surface Area of Resins PZAR 4 and PAR 5.

resin BET surface area (SBET) (m2 g–1) total pore volume (Vt) (cm3 g–1) average pore diameter (APD) (nm)
4 1.74 7.56 × 10–3 6.2
5 9.15 5.30 × 10–2 13.3
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2.3. TGA Analyses of Resins

Figure 1a displays the TGA curves for resins 4, 5, and MB-loaded resin. The curves reveal almost no loss of materials up to 100 °C, while losses of ≈5 wt %, observed up to 200 °C, were attributed to the removal of moisture. For basic resin 5, a gradual loss of 11% in the range 200–400 °C and an accelerated loss of 34% in the range 400–500 °C were attributed to the elimination of phosphonate and carboxy groups in the pendants. Curves for the acidic resin 4 and MB-loaded resins followed similar patterns; both lost ≈54 wt % in the range 200–500 °C owing to the release of MA as its anhydride46,47 and the phosphonate pendants. The TGA curves demonstrated the stability of the resins well over 200 °C.

Figure 1.

Figure 1

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(a) TGA curves of basic resin 5, acidic resin 4, and MB-loaded resin. (b) Plots of qMB, pH at 298 K. (c) Resin (50 mg) 20 ppm MB (20 mL) at the desired pH after 3 h at 298 K. (d) Percent MB removal by resin 5 versus time at various temperatures. [Experimental conditions: for (a), (b), and (d): {Resin 4 (50 mg), 3000 ppm (Co) MB (20 mL)].

2.4. Effect of pH on qe

The qe versus pH plot in Figure 1b reveals a steep increase of qe with increasing pH in the range 4–6; thereafter, it remains constant in the pH range 7–10. The pKa values of the two CO2H groups in the succinic acid motifs are expected to be 2.6 and 5.8,42 while the pK1 and pK2 values of the PO3H2 group are estimated to be 2.0 and 6.0.48 Therefore, in the pH range 7–9, all of the acidic groups would be transformed predominantly to the anionic forms so as to impart electrostatic interaction with the positive charge center in MB (vide infra) (Scheme 2). The progressive discoloration of MB in the presence of resin at pH values of 3, 5, and 7 can be seen in Figure 1c. The MB-loaded resin became blue and settled down at the bottom.

Scheme 2. Resin’s Interaction with MB and Pb(II).

Scheme 2

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2.5. Kinetics of MB Adsorption

The adsorption process was found to be very fast; qt values almost attained the equilibrium value of qe within 10 min, whereby the percent removals of MB from its 3000 ppm solutions were determined to be 91, 96, and 98.6% at 298, 313, and 328 K, respectively (Figures 1d and 2a). The adsorption process is endothermic as revealed by the increase of q values with increasing temperatures. The adsorption kinetics of MB removal, as expected, is faster at higher temperatures.

Figure 2.

Figure 2

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MB removal by resin 4 (250 mg) from its 3000 ppm solutions (100 mL) at blue circle solid 298, red circle solid 313, and green circle solid 328 K (final pH ≈7.5): plots of (a) qt versus time; (b) first-order, (c) second-order, and (d) intraparticle diffusion.

Adsorption data were analyzed using first-order (rate constant, k1) (eq 1)44 (Figure 2b), second-order (rate constant, k2) (eq 2) (Figure 2c), and intraparticle diffusion kinetics (rate constant, kp) (eq 3) (Figure 2d):

2.5. 1
2.5. 2
2.5. 3

The better fitting (R2) of the kinetic data for the second-order model and similarity of qcal and qexp values suggest a chemical adsorption process (Table 2).49 Chemisorption may involve sharing or exchange of electrons between the CO2– or PO3= ligands as electron donors and the cationic aromatic rings of MB as electron acceptors.50,51 The adsorption processes usually go through the three stages of (i) fast film diffusion, (ii) a slower intraparticle diffusion, and (iii) mass action involving adsorption/desorption equilibrium. The adsorption process would be solely governed by a linear qt versus t0.5 plot with an intercept (xi) value of 0.52 Since all of the plots (Figure 2d) have considerable intercept values (i.e., xi ≠ 0), which are proportional to boundary-layer thickness,53 film diffusion becomes a major part of the adsorption process. The plots are bilinear;54 the initial ones with a higher slope (kp) and final horizontal lines describe the intraparticle diffusion (Table 2)14 and the mass action equilibrium, respectively (Figure 2d).

Table 2. Kinetic Parameters for MB Adsorption on PZAR 4.

pseudo-second-order
temp (K) qe,exp (mg g–1) k2 (h–1 g mg–1) ha (h–1 mg g–1) qe,calc (mg g–1) R2 Ea (kJ mol–1)
298 1137 0.07489 97 276 1140 0.9999 34.8
313 1166 0.1409 192 308 1168 0.9999  
328 1189 0.2714 384 615 1190 0.9999  
pseudo-first-order
temp (K) qe,exp (mg g–1) k1 (h–1)   qe,calc (mg g–1) R2  
298 1137 14.08   553.6 0.9854  
313 1166 25.26   711.4 0.9612  
328 1189 25.08   448.4 0.9930  
intraparticle diffusion model
temp (K) kp (mg g–1 h–1/2) xi (mg g–1)   Ri R2  
298 1325 555   0.512 0.9783  
313 1487 608   0.479 0.9379  
328 1672 672   0.435 0.9452  
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a

Initial adsorption rate h = k2qe2.

The initial adsorption factor (Ri) is determined using eq 4

2.5. 4

The intercept (xi) values (i.e., instantaneous adsorption at time zero) were found to be 555, 608, and 672 mg g–1 at 298, 313, and 328 K, respectively. The adsorption of MB at 298, 313 and 328 K has Ri values of 0.512 [i.e., 1 – (555/1137)], 0.479 [i.e., 1 – (608/1166)], and 0.435 [i.e., 1 – (672/1189)] (Table 2), which implies that the instantaneous adsorption accounts for 49, 52, and 56.5%, respectively. The increase in the order x(298 K) > x(313 K) > x(328 K) suggests that the external diffusion resistance decreases with increasing temperatures.14

2.6. Infrared Spectra

The MB-loaded resin was prepared by stirring resin 4 (100 mg) in a 3000 ppm MB solution (40 mL) at pH 7.5 for 3 h. The loaded resin, after washing with deionized water, was dried in vacuo at 50 °C.

The IR spectra are displayed in Figure 3. The band at 1712 cm–1 is attributed to the C=O stretch of COOH in PZAR 4. For the basic resin 5, the vibrations of CO2– appeared at 1405 and 1571 cm–1.55 For MB·3H2O (Figure 3a), the peak at 1601 cm–1 was assigned to the aromatic ring.56Figure 3d shows the shift of the C=O band away from that of COOH in Figure 3b, thereby confirming the CO2–/MB interactions. The absence of a band around 1712 cm–1 indicates the absence of COOH but the presence of the CO2– form.

Figure 3.

Figure 3

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FTIR spectra of (a) MB·3H2O, (b) 4, (c) 5, (d) MB-loaded, and (e) Pb(II)-loaded resins.

2.7. Activation Energy (Ea) for MB Adsorption

The Arrhenius (eq 5) plot using second-order rate constants k2 (Table 2) gave the Ea value of 34.8 kJ mol–1; the relatively large magnitude points toward a chemisorption process (Figure 4a).

2.7. 5

Figure 4.

Figure 4

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Plot of (a) Arrhenius and (b) initial Co (for MB) versus qe of resin 4 at 298, 313, and 328 K.

2.8. Adsorption Isotherms of MB

To examine the mechanism of adsorption, several qe and Ce values were determined using the initial MB concentration (Co) in the range 100–8000 ppm at 298, 313, and 328 K. The qe values are shown to increase with increasing Co plot as displayed in Figure 4b. The qe and Ce were fitted into linear Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich (D–R) isotherms,57 and the plots were found to have R2 values of 0.9993, 0.9290, 0.9610, and 0.5595, respectively, at 298 K. The data thus fitted the linear Langmuir isotherm best at all temperatures. The isotherm plots are not displayed here. The qm value of 2498 mg g–1, obtained from the Langmuir isotherm, demonstrates the outstanding performance by the resin with this staggering MB uptake.

However, to obtain thermodynamic parameters, the Langmuir nonlinear isotherm should be constructed using eq 6 to get the dimensionless equilibrium constant Keo (58−60)

2.8. 6

The units are described in parentheses for qe (mg g–1), Ce (mg L–1), KL the Langmuir equilibrium constant (L mg–1), and qmax (mg g–1). In the van’t Hoff equation, Keo must be dimensionless, which is obtained from KL using eq 7.

2.8. 7

Using dimensionless unitary γ (assumed) as the coefficient of activity, the standard concentration [MB]o as 1 mol L–1, and the molar mass (MW) of MB (319.85 g mol–1), followed by multiplication of KL by 1000 to convert its unit (L mg–1) into L g–1, makes Keo dimensionless. Several parameters are inserted in the figure. The Radj2 values of 0.9967, 0.9943, and 0.9839 at 298, 313, and 328 K, respectively, indicate the highly satisfactory fitting of the data. The qmax values were determined to be 2445, 2508, and 2550 mg g–1 at 298, 313, and 328 K, respectively (Figure 5).

Figure 5.

Figure 5

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Nonlinear Langmuir isotherm curve for the adsorption of MB by resin 4 at (a) 298 K, (b) 313 K, and (c) 328 K. [Resin (50 mg), 20 mL of MB having Co values of 100, 300, 500, 1000, 2000, 3000, 4000, 5000, 7000, and 8000 ppm, final pH 7.5].

The negative ΔGo indicates the favorability of the adsorption. Because of the endothermic nature, qMB increases with increasing temperature (Figure 6a). The van’t Hoff plot (eq 8) provided the ΔHo, ΔSo, and ΔGo (= −RT ln Keo) values (Figure 6b) (Table 3). The greater number of desorbed water molecules during MB adsorption from the hydration shell of the anionic centers of the resin led to an increase in randomness at the sorbent–sorbate interface, presumably giving a positive value for ΔSo.

2.8. 8

Figure 6.

Figure 6

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Plots of (a) qe versus temperature and (b) van’t Hoff. [Resin (50 mg), 3000 ppm (Co) MB (20 mL)].

Table 3. Thermodynamic (ΔGo, ΔHo, ΔSo) Data for MB Adsorption on PZAR 4.

temperature (K) KLa (L mg–1) Keo b (unit less) ΔGo c (kJ mol–1) ΔHo (kJ mol–1) ΔSo (J mol–1 K–1) R2
298 0.00560 1791 (−)18.6 (+)36.5 (+)185 0.9960
313 0.0122 3902 (−)21.5      
328 0.0215 6877 (−)24.1      
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a

From Langmuir nonlinear isotherms.

b

Using eq 11.

c

ΔGo = −RT ln Keo.

2.9. MB Uptake in the Presence of NaCl

The presence of saltlike NaCl in industrial effluents may affect the efficacy of adsorptive removal of dye contaminants. The effect of NaCl on MB adsorption from its 2000, 1000, and 500 ppm solutions in the absence and presence of 0.1 and 0.5 M NaCl by resin 4 is presented in Figure 7a; the figure demonstrates the stable efficiency of dye removal. For instance, percent removals from 2000 ppm MB solution in 0, 0.1, and 0.5 M NaCl were determined to be 95.7, 94.6, and 93.7%, respectively. For a Co value of 1000 ppm, removal efficiencies become 96.2, 94.5, and 93% at 0, 0.1, and 0.5 M NaCl. It is gratifying to see that the presence of NaCl has no adverse effects on MB removal capacities of the resin. Na+ ions cannot compete with the cationic MB for adsorption onto the resin. As stated earlier, sharing or exchange of electrons between the CO2– or PO3= ligands and aromatic rings of MB in an electron-donor–acceptor capacity in a chemisorption process is energetically more favorable than a simple electrostatic attraction between the sorbent and the Na+ ions.50,51

Figure 7.

Figure 7

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(a) Effect of NaCl on the qe and % removal of MB [resin 4: (50 mg), MB solution (20 mL) of Co ppm: blue box solid 2000, green box solid 1000, and red box solid 500 ppm]. (b) Efficiencies of recycling of resin 4 for MB removal [resin 4 (100 mg), MB (40 mL) of Co: 500 ppm, pH 7.5]. (c) Elovich kinetic model for fitting kinetic adsorption data of MB adsorption using 100 mL of 3000 ppm (Co) MB and resin 4 (250 mg) at pH 7.5 at various temperatures. (d) pH versus ΔpH plot to determine the pH of point of zero charge.

2.10. Recycling of the Adsorbent

In the four cycles of regeneration and reuse, the resin demonstrated remarkable efficacies of ≈96–97% (Figure 7b). The pH-responsive resin 4 decorated with numerous PO3H–, CO2H, and NH+ can be induced to undergo 4 ⇋ 5 equilibration; the presence of HCl or NaOH would locate the position of the equilibrium. The resin has a robust chemical backbone and pendants owing to the absence of labile motifs like ester, amide, etc.

2.11. Mechanism of Adsorption

A good fitting of the kinetic data to the Elovich model (eq 9) is suggestive of a chemisorption process (Figure 7c).61 The high initial adsorption rate (α) values, calculated to be 6.86 × 105, 1.31 × 106, and 5.26 × 106 mg g–1 h–1 at 298, 313, and 328 K, respectively, and the corresponding low desorption rate (β) values of 0.00611, 0.00630, and 0.00723 g mg–1, make it essentially an irreversible adsorption process.14 The endothermic adsorption, as reflected by the positive ΔHo value of 36.5 kJ mol–1 (Table 3), also supports the process as chemsorption.62 The ultrahigh adsorption capacity for Pb(II) removal may be attributed to the multilayer dye molecule adsorption behavior.63

2.11. 9

The points of zero charge (pH PZC values) of resin 4 were determined by the pH drift method.64 CO2–-free deionized water was prepared by bubbling N2 through it at 90 °C for 15 min. A series of 0.1 M NaNO3 (10 mL) solutions was adjusted to pH values of 1.50, 2.06, 3.07, 5.03, 6.97, and 9.0 using 0.1 M NaOH or 0.1 M HCl. Resin 4 (50 mg) was then immersed in each of the solutions and stirred at 298 K for 24 h. The ΔpH (difference between initial and final pH) versus initial pH plot gave the PZC value of 2.2 at which ΔpH is zero (Figure 7d). At pH > 2.2, the charge density on the resin, being on the negative side, would encourage binding to the cationic MB. Simple washing with water or ethanol was unsuccessful in dislodging the dye from the MB-loaded resin, thereby implying the adsorption by chemisorption rather than physisorption. The dye was desorbed by a strong acid like HCl.

2.12. Comparative MB Adsorption Capacities (qm) of Some Recent Sorbents

A comparison of MB adsorption capacities (qm) of some recent sorbents with current resin 4 is tabulated in Table 4, which reveals its outstanding performance. The finding paves the way for its application in mitigating industrial dye-wastewater.

Table 4. Comparison of Maximum MB Adsorption Capacities (qm) of Some Similar Recent Sorbents with Current Resin 4.

adsorbents qm (mg g–1) references
metal–organic frameworks (MOF-235) 187 (65)
polyacrylamide-grafted xanthan gum//nanosilica 498 (66)
xanthate-functionalized magnetic graphene oxide 526 (67)
agar/graphene oxide composite 578 (68)
nickel-based MOF 694 (30)
activated carbon from sucrose 704 (14)
magnetic halloysite nanotube nanohybrid 714 (69)
polyamine-functionalized graphene 741 (70)
β-cyclodextrin-/chitosan-functionalized graphene oxide 1134 (29)
WOx/C nanowire networks 1188 (71)
chitosan/poly(acrylic acid) (PAA)/montmorillonite 1859 (22)
chitosan/PAA/attapulgite 1873 (23)
cross-linked poly(acrylic acid)-based hydrogel 2100 (72)
aminocarboxylate/MA resin 2101 (60)
titanate nanosheets 3937 (63)
resin with phosphonate/carboxylate motifs 2445 current work
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2.13. Pb(II) Removal

2.13.1. Kinetics of Pb(II) Adsorption

The adsorption was very fast; qt values almost attained the equilibrium value of qe within 5 min, whereby the percent removals of Pb(II) from its 10 000 ppb (i.e., 10 000 μg L–1) solutions were determined to be 96.3, 97.1, and 97.5% at 298, 313, and 328 K, respectively (Figure S1a). Thereafter, the q values remained constant (Figure S1b). The adsorptive removal of Pb(II) is endothermic as revealed by the increase of equilibrium qe values with increasing temperatures.

As in the case of MB removal, the adsorption data were analyzed using first-order (eq 1), second-order (eq 2), and intraparticle diffusion (eq 3) kinetics. The data do not fit the first-order kinetics, and as such, its figure is not shown here. The data fitted the second-order and intraparticle diffusion kinetics very well and are displayed in Figure S1c,d, respectively.

The better fitting (R2) of the kinetic data and the similarity of qcal and qexp values are indicative of second-order kinetics, thereby suggesting a chemical adsorption process (Table S1).49

All of the intraparticle diffusion kinetic plots (Figure S1d) have considerable intercept values (i.e., xi ≠ 0); film diffusion becomes a major part of the adsorption process. The plots are bilinear. The initial adsorption factor (Ri) is determined using eq 4. The intercept (xi) values (i.e., instantaneous adsorption at time zero) were found to be 3458, 3442, and 3392 μg g–1 at 298, 313, and 328 K, respectively. The adsorption of Pb(II) at 298, 313, and 328 K has Ri values of 0.123 [i.e., 1 – (3458/3944)], 0.128, and 0.142 (Table 2), which implies that the instantaneous adsorption accounts for 88, 87, and 86%, respectively.

A good fitting of the kinetic to the Elovich model (eq 9) is suggestive of a chemisorption process (Figure S2). The high initial adsorption rate (α), calculated to be ≈3.8 × 1054 μg g–1 h–1 at temperatures 298, 313, and 328 K, and the corresponding low desorption rate (β) of ≈0.030 g μg–1 make it an essentially irreversible adsorption process.49

2.13.2. Activation Energy (Ea) for Pb(II) Adsorption

The Ea for Pb(II) adsorption as calculated from the Arrhenius (eq 5) plot (Figure S3a) using rate constants k2 (Table S1) was found to be a relatively small value of 13.3 kJ mol–1. The adsorptive removal of Pb(II) was very fast; at 298 K, 95.6% removal was achieved within 3 min (Figure S1a).

2.13.3. Adsorption Isotherms of Pb(II)

The qe and Ce values were determined using resin 4 and the initial Pb(II) concentration (Co) in the range 200–10 000 ppb (i.e., 0.2–10 ppm) at pH 6.0 for 6 h at 298, 313, and 328 K. The experimental procedure was similar to that of MB adsorption. An ICP-MS (model: ICP-MS XSERIES-II) instrument was used to determine the Pb(II) concentrations. The qe values (for Co in the range 200–10 000 ppb) are shown to increase with increasing Co as displayed in Figure S3b. The qe and Ce (for Co in the range 200–2000 ppb) were fitted into linear Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich (D–R) isotherms, and the plots were found to have R2 values of 0.9976, 0.9780, 0.9647, and 0.7340, respectively, at 298 K. The data thus fitted the Langmuir isotherm best at all temperatures. The isotherm plots are not displayed here.

As in the case of MB adsorption (vide supra), the Langmuir nonlinear isotherm (qe versus Ce plots) (Figure S4) was constructed using eq 6 to obtain the dimensionless equilibrium constant Keo (by replacing [MB]o by [Pb]o and using an atomic mass of Pb in eq 7). Several parameters are inserted in the figure. The Radj2 values of 0.9985, 0.9962, and 0.9993 at 298, 313, and 328 K, respectively, indicate the excellent fitting of the data. The qmax values were determined to be 1127, 1198, and 1231 μg g–1 at 298, 313, and 328 K, respectively (Figure S4).

The dimensionless equilibrium constants Keo were used to construct the van’t Hoff plot (eq 8) (Figure 5a). The thermodynamic parameters as extracted from the plot are given in Table S2. The adsorption process is mildly endothermic as reflected by the positive ΔHo value of 5.35 kJ mol–1, indicating the chemical nature of the adsorption process,62 and the negative ΔGo indicates the favorability of the adsorption. Because of the endothermic nature, qMB increases with increasing T (Figure S5b).

2.13.4. Removal of Contaminants from Wastewater

Table 5 reveals the effectiveness of resin 5 (50 mg) in removing several metal ion contaminants from industrial wastewater (20 mL) (pH 5.5) spiked with 0.0 and 10 000 μg L–1 Pb(II) ions at 298 K for 12 h. The concentrations of all of the metal contaminants in the original wastewater matrix were drastically reduced; especially those of toxic metal ions Cr(III), Co(II), and Hg(II) ions were reduced to sub-ppb levels in the nonspiked sample. For the spiked samples, the Pb(II) concentration was reduced from 10 000 mg L–1 (ppb) to 106 mg L–1 (i.e., 99% Pb removal). Note that an aqueous solution of a mixture of MB (10 000 ppb) and Pb(II) (10 000 ppb) upon treatment with resin 4 reduced their concentrations by 31 and 97.6%, respectively. The resin is thus more effective in removing Pb(II) presumably because of its much smaller size as compared to MB.

Table 5. Wastewater Remediationa by PAR 5 and Removal of MB and Pb(II)b by PZAR 4 at 298 K.
    after treatment (μg L–1)
metal original sample (μg L–1) original sample treated with 5 original sample spiked with Pb(II) (10 000 μg L–1) and then treated with 5
Al 34.9 9.78 11.1
Cr 5.30 0.91 1.49
Fe 42.1 14.8 17.2
Co 2.66 0.43 0.59
Ni 17.8 3.24 4.48
Cu 23.1 8.97 11.1
As 0.26 0.11 0.13
Cd 0.51 0.47 0.41
Sn 4.28 1.36 2.16
Hg 1.74 0.89 1.09
Pb 4.16 1.98 106
metal original sample (μg L–1) aqueous solution containing MB (10 000 μg L–1) and Pb(II) (10 000 μg L–1) treated with 4 at pH 6.0
MB 10 000 6870
Pb 10 000 236
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a

Wastewater (20 mL) treated with 5 (50 mg).

b

Aqueous solution (20 mL) of MB and Pb(II) treated with 4 (50 mg).

2.13.5. Desorption of Pb(II)

For effective industrial applications, the regeneration and reuse of the adsorbent are of great importance. For this purpose, resin 4 sample (50 mg) was immersed in 10 ppm Pb(II) (20 mL) at pH 6 in a centrifuge tube for 24 h. The supernatant solution was analyzed for Pb(II) to determine its percent removal by the resin. The residue in the centrifuge tube was washed with deionized water and treated with 1 M HNO3 (5 mL) for 1 h. Desorbed Pb(II) in the supernatant solution was analyzed for percent desorption. The sequence was repeated for four cycles. The sorption efficiency in the range 94–98% and the desorption efficiency in the range 90–94% certify the stability of the resin.

2.14. SEM and EDX [of MB- and Pb(II)-Loaded Resin] and FTIR [of Pb(II)-Loaded Resin] Analyses

MB-loaded and Pb(II)-loaded resins were prepared by treating resin 4 with MB (3000 ppm) and Pb(II) (10 ppm) at pH 7 and 6, respectively. After sputter-coating the resins with a thin film of gold, they were scanned. The comparison of the EDX spectra of resin 4 (Figure 8a) and its MB-loaded and Pb(II)-loaded resins confirmed the presence of S (of MB) (Figure 8b) and Pb(II) (Figure 8c).

Figure 8.

Figure 8

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SEM and EDX analyses for resin 4 (a) unloaded, (b) MB-loaded, and (c) Pb(II)-loaded.

The Pb(II)-loaded resin was prepared in a similar fashion (as described in Section 2.6 for the MB-loaded resin) using 1000 ppm Pb(II) at pH 6. The P–O vibrations for resins 4 and 5 are expected to be in the region 900–1150 cm–1 (vide supra) (Figure 3b,c).73−75 The change in intensity and broadness of the phosphonate P–O vibrations in the Pb(II)-loaded resin 4 confirms the presence of Pb(II).76

3. Conclusions

Alternate copolymerization of the diallylammonium monomer [(H2C=CH–CH2)2N+CH2PO3H–], maleic acid (HO2C–CH=CH–CO2H), and a cross-linking monomer led to a new pH-responsive resin 4, which has been characterized by several physical techniques.

The resin containing CO2– and R2NCH2PO3= chelating motifs has been found to be a superadsorbent for MB with a qMB of 2445 mg g–1. The extraordinarily fast adsorption rate achieved 90.5, 96.5, and 98.5% MB removal within 10 min at 298, 313, and 328 K, respectively, from a 3000 mg L–1 MB solution. The kinetic data fit the second-order and Elovich kinetic model to demonstrate the adsorption as a chemical process. The nonlinear Langmuir adsorption model gave endothermic ΔHo and ΔSo values of (+)36.5 kJ mol–1 and (+)185 J mol–1 K–1, respectively. The resin gave stable efficacies for the adsorption/desorption process for over 4 cycles. The current resin performed outstandingly as compared to many recently reported sorbents (Table 4).

The resin was able to trap Pb(II) very effectively; the adsorption was very fast, achieving the equilibrium qe value within 5 min. The percent removal of Pb(II) from its 10 000 ppb solution was determined to be ≈97.5%. The nonlinear Langmuir adsorption model gave endothermic ΔHo and ΔSo values of (+)5.35 kJ mol–1 and (+)86.9 J mol–1 K–1, respectively.

The resin was remarkably successful in reducing the concentration of toxic metal ions Cr(III), Co(II), Pb(II), and Hg(II) in wastewater to ppb and sub-ppb levels. In wastewater spiked with Pb(II), the resin treatment reduced its concentration from 10 000 to 106 ppb (i.e., 99% Pb removal). The resin was also found to be effective in the simultaneous removal of MB and Pb(II).

4. Experimental Section

4.1. Materials

MB·3H2O (C16H24ClN3O3S: MW 373.9 g mol–1), ammonium persulfate (APS), and MA from Sigma-Aldrich and diallylamine, paraformaldehyde, and H3PO3 from Fluka Chemie AG were used. A stock solution of 1000 ppm Pb(II) was prepared using Pb(NO3)2 (Fischer Scientific Co, ACS-certified). The literature procedure was followed to synthesize Cross-linker 3.77 Millipore water (double-distilled, 18.2 MΩ·cm) was used to carry out the adsorption study.

4.2. Physical Methods

FTIR spectra were recorded in a Perkin Elmer Series II Model 2400. Elemental analyses were carried out in a Perkin Elmer (16F PC) spectrometer. Thermogravimetric analyses (TGA) were performed under a flow of N2 using an SDT analyzer (Q600: TA Instruments). pH measurements were performed by a Sartorius pH meter. Quattro ESEM-FEG integrated with an energy-dispersive X-ray spectrometer (EDX, Oxford Instruments) from ThermoFisherTM Scientific was utilized to examine the surface morphology of the samples. Brunauer–Emmett–Teller and Barrett–Joyner–Halenda methods were applied to characterize the resins. The MB concentrations were determined using a UV–vis spectrophotometer (GENESYS10S, Thermo Electron) for absorbance at λmax = 665 nm.

4.2.1. Analytical Method for Lead Analysis

A stock solution of 1000 ppm Pb(II) was prepared using Pb(NO3)2 and then diluted to the required concentrations. The adsorption efficiency of Pb(II) was assessed by the experiments that were carried out at 298 K in a series of screw-cap glass vials. Typically, 50 mg of resin 4 was added into a 20 mL solution with Pb(II) in the concentration range of 0.2–10 ppm. The solution pH was adjusted to 6.0 using 0.1 M NaOH and/or 0.1 M HNO3. The solutions were then stirred at 300 rpm using a magnetic stir-bar for 6 h. After the time elapsed, the Pb(II) solution was filtered and analyzed by the inductively coupled plasma-mass spectrometer (ICP-MS) (model: ICP-MS XSERIES-II) following the USEPA method 6020A.78 The ICP-MS (detection limit of 0.05 ppb) was calibrated using the blank and six different concentrations of Pb(II). The adsorption experiments for Pb(II) were repeated at 313 and 328 K.

4.3. Synthesis of Monomer 1

A modified procedure was followed to prepare Monomer 1 (≈100% purity).79,80

4.4. Synthesis of Resins

4.4.1. Polyzwitterionic Acid Resin (PZAR) 4

The following polymerization mixture contained monomers and water in a 60:40 weight ratio. The mixture of 1 (10.0 g, 43.9 mmol), MA 2 (8.12 g, 70 mmol), cross-linker 3 (2.66 g, 8.34 mmol), and water (13.9 g) in a condenser-fitted 100 mL RB flask under N2 was heated quickly to 102 °C followed by addition of APS (1.0 g). After 3 min, another portion of APS (0.75 g) was added. In 2–3 min after the second addition, the homogeneous mixture became cloudy and the exothermic process gave resin 4. After the elapse of 10 min, the resin was cooled to room temperature and immersed in water (24 h) with several exchanges of water. After filtration, the resin was dried in vacuo at 65 °C (14.0 g, 71%). Found: C, 44.9; H, 6.3; N, 4.8%. Monomers 1, 2, and 3 incorporated in a mol ratio of 38.9:53.7:7.4 require C, 45.36; H, 6.08; N, 4.69%.

Monomer 1 (43.9 mmol, 38.9 mol %) and cross-linker 3 (8.34 mmol, 7.4 mol %) can theoretically react with 60.58 mmol (53.7 mol %) of maleic acid. Note that the cross-linker can react with 2 equiv of maleic acid. Here, we have used maleic acid in excess (70 mmol). A calculation reveals that a sample of 412 mg of resin 4 contains 1.0, 1.38, and 0.19 mmol of the repeating units of monomers 1, 2, and 3, respectively.

4.4.2. Conversion of PZAR 4 to Polyanionic Resin (PAR) 5

Resin 4 (0.89 g, containing 5.96 mmol CO2H and 2.16 mmol each of NH+ and PO3H–) in water (5 mL) was treated under N2 with NaOH (1.2 g, 30 mmol) and stirred for 2 h. The mixture was filtered and washed with methanol to remove any trace of excess NaOH. Anionic resin 5 was then dried in vacuo at 60 °C (1.04 g, ≈94%). Found: C, 36.1; H, 4.3; N, 3.9%. Monomers 1 (in the dianionic form), 2 (in the dianionic form), and 3 (in the dihydroxy form) as depicted in resin 5 incorporated in a mol ratio of 38.9:53.7:7.4 require C, 36.67; H, 4.05; N, 3.79%.

A calculation reveals that 510 mg of 5 contains 1.0, 1.38, and 0.19 mmol of the anionic repeating units of monomers 1, 2, and 3, respectively.

4.5. Ion-Exchange Capacity (IEC)

A mixture of resin 5 (100 mg) and 0.1 M HCl (25 mL) was stirred overnight. A known aliquot of the supernatant (after centrifugation) was titrated with 0.1 M NaOH to determine the excess acid, and the IEC was calculated using eq 10

4.5. 10

where W represents the resin’s mass in g, while the HCli and HClf in mmol denote its initial and final amounts, respectively.

4.6. Swelling Coefficient (SC)

The volume change of resin 4 or 5 of 20- to 30-mesh fraction size in a buret covered with water was noted after 24 h. The final/initial ratio of volumes afforded SC values of 1.3 and 4.0 for resins 4 and 5, respectively.

4.7. Adsorption of MB

Experiments were carried out at 298 K in a series of vials containing resin 4 (50 mg) and MB in the concentration (Co) range 100–8000 ppm (20 mL). The solution pHs were adjusted to 7.5 by 0.1 M NaOH and stirred at 300 rpm using a magnetic stir-bar for 6 h. After appropriate dilution, the dye concentration at equilibrium (Ce) was determined by a UV–vis spectrometer at λ 665 nm and an adjusted pH of 7.0 using a calibration curve for MB in the concentration range 0.1–5.0 mg L–1. The experiments were repeated at 313 and 328 K.

The adsorption capacity (i.e., qe) (mg g–1) of resin 4 (in g) from an MB solution of volume (in L) was determined using eq 11.

4.7. 11

The percent MB removal (R) was calculated using eq 12.

4.7. 12

The kinetic runs at 298, 313, and 328 K were carried out in RB flasks (250 mL) using resin 4 (250 mg) in water (100-x mL) containing an appropriate amount of 0.1 M NaOH (x mL) to maintain the pH at 7.5 after addition of solid MB to make its concentration 3000 ppm. Several pretrials were conducted to determine the amount of NaOH (x mL) required to maintain the pH at 7.5. At a predetermined time, the stirring was briefly stopped and a small volume (≈0.2 mL) of the supernatant was used to measure the dye concentration after appropriate dilution at pH 7.0 using a UV–vis spectrophotometer. The qt (mg g–1) at various times t was determined using eq 13.

4.7. 13

A MATLAB environment was used to carry out isotherm fitting and statistical analyses.

4.8. Recycling Experiment

The qe for MB removal was determined by stirring resin 4 (100 mg) in a centrifuge tube with 500 ppm MB solution (40 mL) at pH 7.5 for 30 min at 298 K. The resultant supernatant was used to determine the remaining MB concentrations.

The residual resin was stirred with 1 M HCl (5 mL) for 15 min to carry out the desorption of MB from the loaded resin. After supernatant removal, the resin was further stirred with 0.1 M HCl (5 mL) for 10 min. The combined supernatants gave the amounts of desorbed MB.

The resin in the centrifuge tube was washed with deionized water to regenerate resin 4, and the above cycle was quadruplicated with the regenerated resin.

Acknowledgments

The facilities and financial assistance provided by King Fahd University of Petroleum & Minerals (KFUPM), Saudi Arabia, and the Dean of Scientific Research (KFUPM) under Project Number DUP19103 for “Distinguished University Professor Award” are gratefully acknowledged.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c02860.

  • Kinetic parameters for Pb(II) adsorption on PZAR 4 (Table S1), thermodynamic (ΔGo, ΔHo, ΔSo) data for Pb(II) adsorption on PZAR 4 (Table S2), plots of percent Pb removal by resin 4 versus time (t) and qt versus time; second-order and intraparticle diffusion; (Figure S1), Elovich kinetic model for fitting kinetic adsorption data of Pb(II) adsorption using 100 mL of 10 000 μg L–1 (Co) Pb(II) and resin 4 (250 mg) at pH 6.0 at various temperatures (Figure S2); plot of Arrhenius and initial Co (for Pb(II)) versus qe of resin 4 at 298, 313, and 328 K (Figure S3); nonlinear Langmuir isotherm curve for the adsorption of Pb(II) by resin 4 at 298 K, 313 K, and 328 K (Figure S4); plots of van’t Hoff and qe versus temperature (Figure S5) (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao0c02860_si_001.pdf (570.7KB, pdf)

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