Tetrapolymer Network Hydrogels via Gum Ghatti-Grafted and N–H/C–H-Activated Allocation of Monomers for Composition-Dependent Superadsorption of Metal Ions

Abstract

Herein, gum ghatti (GGTI)-g-[sodium acrylate (SA)-co-3-(N-(4-(4-methyl pentanoate))acrylamido)propanoate (NMPAP)-co-4-(acrylamido)-4-methyl pentanoate (AMP)-co-N-isopropylacrylamide (NIPA)] (i.e., GGTI-g-TetraP), a novel interpenetrating tetrapolymer network-based sustainable hydrogel, possessing extraordinary physicochemical properties and excellent recyclability, has been synthesized via grafting of GGTI and in situ strategic protrusion of NMPAP and AMP during the solution polymerization of SA and NIPA, through systematic multistage optimization of ingredients and temperature, for ligand-selective superadsorption of hazardous metal ions (M(II)), such as Sr(II), Hg(II), and Cu(II). The in situ allocation of NMPAP and AMP via N–H and C–H activations, grafting of GGTI into the SA-co-NMPAP-co-AMP-co-NIPA (TetraP) matrix, the effect of comonomer compositions on ligand-selective adsorption, crystallinity, thermal stabilities, surface properties, swellability, adsorption capacities (ACs), mechanical properties, and the superadsorption mechanism have been apprehended via extensive microstructural analyses of unloaded and/or loaded GGTI-g-TetraP1 and GGTI-g-TetraP2 bearing SA/NIPA in 8:1 and 2:1 ratios, respectively, using Fourier transform infrared (FTIR), ¹H/¹³C/DEPT-135 NMR, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis, differential scanning calorimetry, X-ray diffraction, field emission scanning electron microscopy, rheological analysis, and energy-dispersive X-ray spectrometry, along with measuring % gel content, pH at point of zero charge (pH_PZC), and % graft ratio. The thermodynamically spontaneous chemisorption has been inferred from FTIR, XPS, fitting of kinetics data to pseudo-second-order model, and activation energies. The chemisorption data have exhibited excellent fitting to the Langmuir isotherm model. For Sr(II), Hg(II), and Cu(II), ACs were 1940.24/1748.36, 1759.50/1848.03, and 1903.64/1781.63 mg g^–1, respectively, at 293 K, 0.02 g of GGTI-g-TetraP1/2, and initial concentration of M(II) = 500–1000 ppm.

Introduction

An interpenetrating polymer network (IPN)-type hydrogel is produced via grafting of a natural polymer into a co-, ter-, and tetrapolymer network, synthesized via polymerization of two, three, and four synthetic monomers, followed by cross-linking the matrix.¹ The swelling of hydrogels depends on several synthetic parameters, such as pH, temperature, monomer ratio, wt % of natural polymer, cross-linkers, and initiators.^2,3 Hydrogels, especially magnetic hydrogels, are widely employed for the removal of waste contaminants, drug delivery, manufacturing of contact lenses, and biosensing of glucose, hepatitis B antigen, cholesterol, hemoglobin A1c, uric acid, and human metabolites.⁴⁻⁶ Additionally, hydrogels are also effectively used in size exclusion chromatography, ion exchangers, and membrane-based applications.^7,8

The waste effluents of several industries contain toxic Sr(II), Hg(II), and Cu(II), of which Sr(II) causes anemia, abnormal bleeding, and deficiencies in immunity power/bone growth. Hg(II) is extensively used in gold mining, gold recovery from electronic waste, chlor-alkali, and vinyl chloride production. Alongside, all forms of Hg cause gastrointestinal disorder, neurologic malfunction, and renal problem and impose detrimental effects on lungs, kidneys, digestive system, brain, and reproductive system in human beings.⁹ Cu(II) influences the alteration in chemical parameters of water, such as hardness, pH, alkalinity, natural organic matter, and freshwater organisms.¹⁰ Besides, Cu(II) contamination imparts physiological complications, such as destruction of liver and kidney, hemolysis, demolition of living tissues, occurrence of influenza syndrome, damage of aquatic food chain, acute vomiting, convulsions, cramps, and carcinogenicity. Several methods, such as ion exchange, biological treatment, chemical oxidation, photocatalytic degradation, coagulation/flocculation, membrane-based separation, and adsorption, have been employed, of which adsorption is the most widely employed technique for the detoxification of polluted wastewater, owing to the impulsive versatility, simple instrumentation, flexible design, cost-effectivity, accuracy, selectivity, and reusability of the adsorbent.

Though polyacrylic acid (PAA)-based hydrogels are frequently used for targeted drug delivery, poor mechanical stability and high water solubility limit the usability of the PAA hydrogels. Despite having a better thermomechanical property, the poor pH sensitivity restricts the widespread applications of poly-N-isopropylacrylamide (PNIPA) hydrogels. Additionally, the homopolymers of acrylamido derivatives show high water solubility and poor sustainability. Hence, IPN hydrogels were prepared via grafting of gum ghatti (GGTI) and combining the properties of sodium acrylate (SA), NIPA, 3-(N-(4-(4-methyl pentanoate))acrylamido)propanoate (NMPAP), and 4-(acrylamido)-4-methyl pentanoate (AMP). GGTI, an anionic high-molecular-weight complex polysaccharide, possesses excellent biodegradability, pH stability, and gelling characteristics. The complex structure of GGTI is primarily composed of α-l-arabinofuranose (α-l-Araf, 49.69–72.85%), β-d-galactopyranose (β-d-Galp, 16.43–37.27%), β-d-glucoronopyranose (β-d-GlcpA, 2.03–14.31%), α-l-rhamnopyranose (α-l-Rhap, 2.10–8.03%), and proteins (1.99–3.78%). The hairy region of GGTI is constructed by 1,6-linked β-d-Galp residues at O-3 and O-4, whereas the smooth region possesses →2)-α-l-Araf-(1→4)-β-d-GlcpA-(1→6)-β-d-Galp-(1→6)-β-d-Galp-(1→ linkage of backbone. Introduction of GGTI enhanced the population of −COO^– in GGTI-g-TetraPs, which made these more suitable for M(II) adsorption. In this regard, several works were devoted to produce natural polymer-based hydrogels containing pectin,² guar gum,^1,3 chitosan,¹¹ xanthan gum,¹² agarose,¹³ wheat bran,¹⁴ and GGTI.¹⁵ Similarly, several homo-, co-, ter-, and tetrapolymeric hydrogels were already reported for drug delivery and water treatment.¹⁶⁻²⁰ The mechanical weakness and fragility of homo-/copolymers could be surmounted by using ter-/multipolymer hydrogels, which offer improvement in the hydrogel structure and provide high mechanical strength and enhanced sensitivity toward physical and chemical stimuli. In this context, few researchers have synthesized tetrablock polymers by ex situ incorporation of four monomers.²¹⁻²⁴ For the first time, the synthesis of a natural polymer-grafted tetrapolymeric IPN using only two monomers and in situ auxiliary protrusion of NMPAP and AMP monomers, via N–H and C–H activations during solution polymerization, has been reported. Till date, very few works have reported the transformation of quaternary sp² carbon into quaternary sp³ carbon during free-radical polymerization.^13,25−28 However, the transformation of tertiary vinylic and isopropyl carbon into quaternary sp³ carbon via C–H activation during solution polymerization is still to be explored. Furthermore, the synthesis of a tailor-made interpenetrating tetrapolymeric network superadsorbent by varying comonomer ratios via C–H and N–H activations to impart two new monomers, such as NMPAP and AMP; evaluation of the ligand-selective superadsorption mechanism of Sr(II), Hg(II), and Cu(II); and extensive microstructural analyses of unloaded and/or loaded hydrogels, along with the estimation of adsorption isotherm, kinetics, and thermodynamics parameters are not reported anywhere.

Experimental Section

Materials

AA, NIPA, GGTI, N,N′-methylenebisacrylamide (MBA), potassium persulfate (PPS), sodium bisulfite (SBS), Sr(NO₃)₂, Cu(NO₃)₂·3H₂O, and HgCl₂ of analytical grades were purchased from Sigma-Aldrich and used without any further modification.

Synthesis of Hydrogels

GGTI-g-TetraP hydrogels were synthesized by free-radical solution polymerization via grafting of GGTI into the tetrapolymer network and in situ auxiliary protrusion of NMPAP and AMP into SA-co-NIPA using MBA and PPS/SBS as redox initiators in N₂ atmosphere. A series of hydrogels were synthesized via successive incorporation of varied dosages of SA, NIPA, PPS/SBS, and MBA at different temperatures and pH. From the measurements of swelling of the as-prepared hydrogels, conditions for obtaining the hydrogel bearing maximum equilibrium swelling ratio (ESR) were determined (Scheme S1). Two different GGTI-g-TetraPs, one possessing the optimum conditions (GGTI-g-TetraP1, SA/NIPA = 8:1) and the other containing different SA/NIPA ratios (GGTI-g-TetraP2, SA/NIPA = 2:1), were synthesized. Initially, two homogeneous GGTI solutions, each containing 0.5 g of GGTI in 30.00 mL of H₂O at 313 K, were prepared using an ultrasonicator to ensure complete dissolution of GGTI. Such GGTI suspension was then taken in a round-bottomed flask, followed by the dropwise addition of the SA solution (0.23/0.16 mol prepared in 24.92/21.64 mL water) at 30 drops min^–1 for GGTI-g-TetraP1/2 at pH = 5.50 and NIPA solution (0.03/0.08 mol prepared in 27.08/30.36 mL water for GGTI-g-TetraP1/2) with constant stirring at 300 rpm. Thereafter, MBA solution (0.52 mmol prepared in 10.00 mL water) was added at 298 K, and the resultant solution was allowed to homogenize under N₂ atmosphere for 8 h. Thereafter, polymerization was initiated via gradual addition of redox initiators, prepared in 8.00 mL water, by the amounts 0.74 and 1.9 mmol, respectively (Scheme 1). The prepared GGTI-g-TetraPs were allowed to swell in 1:3 methanol/water solution (v/v) and washed several times for the complete removal of unreacted monomers and water-soluble oligomers. Finally, GGTI-g-TetraPs were air-dried for 3 days, followed by drying in a vacuum oven at 323 K.

Scheme 1. Synthesis of GGTI-g-TetraP.

Characterization

GGTI-g-TetraP1 was characterized using techniques as given in Table S1. Additionally, GGTI-g-TetraPs were characterized by measuring % gel content (% GC), % graft ratio (% GR), % −COOH, pH at point of zero charge (pH_PZC), and ESRs at different pH_i and temperatures. All graphics-based analyses and chemical structures were done by Origin 9.0 and ChemDraw Ultra 12.0 software.

Methodology

M(II) solutions of varying concentrations, that is, 10–60 and 1000–2000 ppm, were prepared by the exact dilution of 5000 ppm stock solutions. In the present study, 0.02 g of dry GGTI-g-TetraP was added to 50 mL buffered solutions of M(II) with constant stirring at 300 rpm. The progress of adsorption was monitored using an atomic absorption spectrometer (PerkinElmer A-ANALYST 100). From the precalibrated equation, the concentration (C_t) of M(II) was calculated and q_t (mg g^–1) was determined using eq 1.

1

Results and Discussion

Fourier Transform Infrared (FTIR) Analysis

GGTI. (KBr pellet, wavenumber in cm^–1): 3425 (weak H-bonded O–H str.), 2925 (asym. −CH₂– str.), 2352–2330 (strong H-bonded O–H str.), 1736/1660–1615 (C=O str. of −COOH), 1548 (asym. C=O str. of −COO^–), 1448/1412 (sym. C=O str. of −COO^–), 1385 (C–H def.), 1156 (glycosidic linkage), 1088 (C–O str. of −CH₂OH), 1026 (−CH₂– twisting), and 518 (pyranose ring) (Figure S1a and Table S2).

TetraP1. (KBr pellet, wavenumber in cm^–1): 3451 (weak H-bonded O–H str.), 2927 (asym. −CH₂– str.), 2351–2319 (strong H-bonded O–H str.), 1718 (C=O str. of −COOH), 1703 (C=O str. of −COOH dimer), 1651 (amide-I), 1550 (amide-II), 1562 (asym. C=O str. of −COO^–), 1455/1403 (sym. C=O str. of −COO^–), 1385/1368 (C–H def. of −CH₃), 1320 (amide-III), 869 (asym. C–N–C str. of −CON<), 659 (N–C=O def. of −CONH−), 609 (N–C=O in-plane def. of −CON<), and 501 (C–C=O def.) (Figure S1a and Table S2).

GGTI-g-TetraP1. (KBr pellet, wavenumber in cm^–1): 3450 (weak H-bonded O–H str.), 3166 (>NH⁺−), 2962 (asym. −CH₂– str. of −OCH₂−), 2928 (asym. −CH₂– str.), 2857 (sym. −CH₂– str. of −OCH₂−), 2355–2321 (strong H-bonded O–H str.), 1720 (C=O str. of −COOH), 1700 (C=O str. of −COOH dimer), 1654 (amide-I), 1545 (amide-II), 1560 (asym. C=O str. of −COO^–), 1458–1401 (sym. C=O str. of −COO^–), 1385/1353 (C–H def. of −CH₃), 1313/1266 (amide-III), 1195/1159 (glycosidic linkage), 1123 (−H₂C–O–CH₂−), 874/827 (asym. C–N–C str. of −CON<), 657 (N–C=O in-plane def. of −CONH−), 618 (N–C=O def. of −CON<), 533 (C–C=O def.), and 519 (pyranose ring) (Figure S1a and Table S2).

Sr(II)-GGTI-g-TetraP1. (KBr pellet, wavenumber in cm^–1): 3440 (weak H-bonded O–H str.), 3180 (>NH⁺−), 2356–2323 (strong H-bonded O–H str.), 1723 (C=O str. of −COOH), 1708 (C=O str. of −COOH dimer), 1656 (amide-I), 1550 (amide-II), 1620/1564/1533 (asym. C=O str. of −COO^–),1458/1403 (sym. C=O str. of −COO^–), 1385/1355 (C–H def. of −CH₃), 1315/1242 (amide-III), 830 (asym. C–N–C str. of −CON<), and 536 (Sr–O) (Figure S1b and Table S2).

Hg(II)-GGTI-g-TetraP1. (KBr pellet, wavenumber in cm^–1): 3585/3526 (weak H-bonded O–H str.), 3190 (>NH⁺−), 2352–2320 (strong H-bonded O–H str.), 1714 (C=O str. of −COOH), 1704 (C=O str. of −COOH dimer), 1633 (amide-I), 1538 (amide-II), 1614/1558/1530 (asym. C=O str. of −COO^–), 1447/1400 (sym. C=O str. of −COO^–), 1385/1370 (C–H def. of −CH₃), 1336/1310/1260 (amide-III), 861 (asym. C–N–C str. of −CON<), 667/645 (N–C=O in-plane def. of −CONH−), 592 (N–C=O in-plane def. of −CON<), and 517 (Hg–N) (Figure S1b and Table S2).

Cu(II)-GGTI-g-TetraP1. (KBr pellet, wavenumber in cm^–1): 3514–3343 (weak H-bonded O–H str.), 3182 (>NH⁺−), 2350/2345 (strong H-bonded O–H str.), 1722 (C=O str. of −COOH), 1702 (C=O str. of −COOH dimer), 1650 (amide-I), 1550 (amide-II), 1619/1572/1560 (asym. C=O str. of −COO^–),1452/1409 (sym. C=O str. of −COO^–), 1385/1370 (C–H def. of −CH₃), 1320/1286 (amide-III), 855 (asym. C–N–C str. of −CON<), 529 (Cu–N), and 437 (Cu–O) (Figure S1b and Table S2).

In GGTI-g-TetraP1, grafting of GGTI into TetraP1 could be realized from the arrival of a new peak at 1123 cm^–1, attributed to the formation of a new −H₂C–O–CH₂– linkage via polymerization between −CH₂Ȯ of GGTI and ĊH₂–(C_q)– of TetraP1 (Figure S1a). The formation of −H₂C–O–CH₂– in GGTI-g-TetraP1, originally absent in GGTI/TetraP1, could also be substantiated from the disappearance and simultaneous arrival of GGTI specific −CH₂– twisting/C–O str. of −CH₂OH and asym./sym. −CH₂– str. of −OCH₂– at 1026/1088 and 2962/2857 cm^–1, respectively. Additionally, the formation of −H₂C–O–CH₂– could also be apprehended by the arrival of a new peak at 810 cm^–1 because of coupling of −CH₂– rocking and C–O str. of −O–CH₂–. Moreover, during the synthesis of TetraP1, the in situ protrusion of NMPAP, via conversion of secondary to tertiary amides, could be apprehended by the appearance of tertiary amide specific asym. C–N–C str. and N–C=O in-plane def. at 869 and 609 cm^–1, respectively. Significant alterations of such peaks were noted in GGTI-g-TetraP1 because of grafting-driven massive changes in both O–H···O–H and O–H···N–H H-bonds influencing C–H def. of gem-dimethyl groups of NIPA, asym. −CH₂– str., amide-I/-II bands, C–C=O def., and N–C=O in-plane def. of secondary amides (Table S2). In this context, several GGTI specific peaks, such as glycosidic linkage of the carbohydrate chain and pyranose ring, altered markedly in GGTI-g-TetraP1 (Table S2). Notably, the appearance of the amide-III band at 1320 and 1313/1266 cm^–1 for TetraP1 and GGTI-g-TetraP1, respectively, indicated the prevalence of sterically favored trans configuration for NIPA moieties.

Sr(II)-GGTI-g-TetraP1. Sr(II) interacted with −COO^– of GGTI-g-TetraP1 via ionic and various coordination modes, such as BB, BC, and M (Scheme S2), as realized from the variegated Δν values of Sr(II)-GGTI-g-TetraP1 (Table 1 and Figure S1b). Ionization of −COOH into −COO^– and preferential coordination of Sr(II) with −COO^– of GGTI-g-TetraP1 led to several changes in both O–H···O–H and O–H···N–H H-bonds, resulting in diversified alterations of C=O str. of H-bonded dimeric −COOH, −H₂C–O–CH₂– linkage, C–C=O def., amide-I/-II/-III, glycosidic linkage, C–H def. of gem-dimethyl groups of NIPA, asym. C–N–C str., and N–C=O in-plane def. of tertiary amide (Table S2). Indeed, lowering of weak O–H···N–H H-bonding peak in Sr(II)-GGTI-g-TetraP1 was ascribed to the relatively fewer availability of O–H, owing to the preferential attachment of Sr(II) with O-donors, supported by the formation of the Sr–O bond at 536 cm^–1. Notably, the peaks at 1463/1089 and 862/690 cm^–1 in Sr(II)-GGTI-g-TetraP1 might be responsible for asym./sym. C–O str. and out-of-plane/in-plane def. of isolated planar CO₃^2–.²⁹

Table 1. Various Interacting Modes of GGTI-g-TetraP1 and Sr(II)/Hg(II)/Cu(II)-GGTI-g-TetraP1.

sample	νas(−COO–) – νs(−COO–) = Δν (cm–1)	mode(s) of interaction
GGTI-g-TetraP1	1560 – 1401 = 159	I
Sr(II)-GGTI-g-TetraP1	(1533 – 1403)/(1533 – 1458) = 130/75	I, BC
	(1564 – 1403)/(1564 – 1458) = 161/106	I, BB
	(1620 – 1403)/(1620 – 1458) = 217/162	M, I
Hg(II)-GGTI-g-TetraP1	(1530 – 1400)/(1530 – 1447) = 130/83	I, BC
	(1558 – 1400)/(1558 – 1447) = 158/111	I, BB
	(1614 – 1400)/(1614 – 1447) = 214/167	M, I
Cu(II)-GGTI-g-TetraP1	(1560 – 1409)/(1560 – 1452) = 151/108	I, BB
	(1572 – 1409)/(1572 – 1452) = 163/120	I, BB
	(1619 – 1409)/(1619 – 1452) = 210/167	M, I

Hg(II)-GGTI-g-TetraP1. The preferential attachment of Hg(II) with amide could be ascertained via the appearance of Hg–N covalent bond at 517 cm^–1. Additionally, the characteristic peak of C–C=O def. shifted from 533 to 460 cm^–1 (Figure S1b). The notable reduction in mutual O–H···N–H H-bonds in Hg(II)-GGTI-g-TetraP1 due to the involvement of N–H into >N–Hg–N< cross-links led to the appearance of relatively sharp O–H str. at 3585 and 3526 cm^–1. Though Hg(II) was relatively reluctant to interact with the O-donor, the possible coordination with −COO^– occurred preferably through M and BC modes (Scheme S2, Table 1), realized from the appearance of a very sharp peak at 1614 cm^–1 and broad intense shoulders at 1558 and 1530 cm^–1. Besides, interaction of Hg(II) with amides and −COO^– led to several changes in H-bonds, imparting diversified alterations of several peaks.

Cu(II)-GGTI-g-TetraP1. In Cu(II)-GGTI-g-TetraP1, the predominant BB and M modes were evidenced from the characteristic Δν values (Table 1). Nevertheless, the coordination between amides and Cu(II) was realized from Cu–O- and Cu–N specific peaks at 437 and 529 cm^–1, respectively. As a consequence of such a coordinate bonding, possible existence of the malachite-type Cu(II) complex onto Cu(II)-GGTI-g-TetraP1 was confirmed from the appearance of a peak at 1341 cm^–1.³⁰ Similar to other M(II), ionization of −COOH and associated changes in H-bonds were manifested in the alteration of several characteristic peaks of various functional groups in Cu(II)-GGTI-g-TetraP1 (Table S2).

¹H NMR Analysis

The formation of −H₂C(β)–CH₂(α)–C=O and −H₂C(β)–CH(C=O)– saturated backbone moieties from H₂C=CH–C=O was inferred by the appearance of all possible (−CH₂(α)–, −CH−)/–CH₂(β)– within 2.00–2.36/0.86–1.58 and 2.01–2.62/0.85–1.58 ppm in TetraP1 (Figure S3) and GGTI-g-TetraP1 (Figure 1), respectively. In fact, the simultaneous obsolescence of vinyl proton specific peaks within 6.03–6.59, 5.60–6.22, and 5.59–6.13 ppm for SA, NIPA, and MBA, respectively, confirmed the conversion of H₂C=CH–C=O into −CH₂–CH₂–C=O and −H₂C–CH(C=O)– in TetraP1 and GGTI-g-TetraP1.⁴ Again, the inclusion of NIPA and MBA in the network of both TetraP1/GGTI-g-TetraP1 was ascertained from the arrival of characteristic peaks for −CH₃ of NIPA and −CH₂– of MBA at 1.27/1.25 and 4.32/4.34 ppm, respectively. The characteristic peak at 4.00 ppm for −CH– of −CH(CH₃)₂ in NIPA³¹ was receded from view in both TetraP1 and GGTI-g-TetraP1 because of the formation of quaternary carbon moieties, such as −CH₂(C_q)CONH–((C_q)(Me₂)–CH₂(C_q)COOH) (Figure 2b) and −CH₂(C_q)CON((C_q)(Me₂)−)CH₂(C_q)COOH (Figure 2b) via C–H activation. However, free-radical propagation between −COṄ((C_q)(Me₂)−) of NIPA and ĊH₂(C_q)CO– of SA resulted in the formation of −CH₂(C_q)CON((C_q)(Me₂)−)CH₂(C_q)COOH (Figure 2b) moiety via N–H activation, which was inferred from the arrival of new broad N–CH₂ specific peaks within 3.49–3.51 and 3.49–3.53 ppm in TetraP1 (Figure S3) and GGTI-g-TetraP1, respectively (Figure 1). Furthermore, the characteristic N–H peaks of NIPA/MBA were observed at 6.99/7.50/7.75/8.61 ppm in GGTI-g-TetraP1.

Figure 1.

¹H NMR of GGTI-g-TetraP1.

Figure 2.

¹³C NMR of GGTI-g-TetraP1.

GGTI is a complex polysaccharide, comprising T-α-l-Araf, β-d-Galp, β-d-GlcpA, and α-l-Rhap. The ¹H NMR spectrum of GGTI (Figure S2) showed the characteristic peaks of H-1, H-2/H-4, H-3, and H-5 of T-α-l-Araf at 5.25, 4.18, 3.62, and 3.80 ppm, respectively.³² The peaks at 4.78, 3.41, and 3.85 ppm were designated as H-1, H-2/H-3, and H-4/H-5/H-6 of β-d-Galp, respectively.³³ Moreover, the anomeric protons of β-d-GlcpA and α-l-Rhap appeared at 4.48 and 5.03 ppm, respectively.³³ The GGTI specific peaks at 5.34, 4.15, 3.65, and 3.88 ppm of H-1, H-2/H-4, H-3, and H-4/H-5/H-6 of T-α-l-Araf and β-d-Galp, respectively,³³ were found in GGTI-g-TetraP1. Additionally, the anomeric protons of β-d-GlcpA and α-l-Rhap occurred at 4.43 and 5.05 ppm, respectively, in GGTI-g-TetraP1. In fact, grafting of GGTI onto TetraP1 was inferred from the noteworthy shifting of the characteristic −CH₂OH peak of T-α-l-Araf from 3.80 ppm in GGTI to the −CH₂–O–CH₂– specific peak at 3.75 ppm in GGTI-g-TetraP1 via −CH₂Ȯ formation,³³ realized earlier by FTIR analyses (Table S2).

¹³C NMR Analysis

From Figures S4a and 2a, complete disappearance of the characteristic H₂C(β)=C(α)H–C=O peaks at 127.87(α)/132.97(β), 125.49(α)/131.30(β), and 125.53(α)/130.88(β) ppm for SA, NIPA, and MBA, respectively, in both TetraP1 and GGTI-g-TetraP1 inferred the occurrence of free-radical polymerization between SA and NIPA, followed by subsequent cross-linking via MBA. Again, the prevalent intense and downfield peaks, at 161.10/176.22/178.62 and 160.04–169.84/171.68/175.65 ppm, inferred the presence of −CONHCH₂NHCO– of MBA/–CH₂(C_q)CON((C_q)(Me₂)−)CH₂(C_q)COOH (Figure 2b)/–CH₂(C_q)CONH–((C_q)(Me₂)–CH₂(C_q)COOH) (Figure 2b) in TetraP1 and GGTI-g-TetraP1, respectively.^4,34 Moreover, the peaks at 181.09/185.96 and 180.12/184.57 ppm were ascribed to the existence of −(C_q)–COOH/–(C_q)–COO^– in TetraP1 and GGTI-g-TetraP1, respectively.³⁵ In this context, the appearance of a peak at 176.96 ppm was related to the presence of β-d-GlcpA in GGTI-g-TetraP1.³⁶ On another note, the sharp peaks at 23.86 and 23.00 ppm were ascribed to −CH₃ of NIPA in TetraP1 and GGTI-g-TetraP1, respectively, inferred from the appearance of a positive peak in DEPT-135 (Figure S4b). Moreover, the characteristic −CH₂– peaks of MBA, −CH₂(C_q)COOH, −CH₂(C_q)COO^–, −CH₂(C_q)CONH((C_q)(Me₂)−), −CH₂(C_q)CON((C_q)(Me₂)−)CH₂(C_q)COOH (Figure 2b), and −CH₂(C_q)CONH–((C_q)(Me₂)–CH₂(C_q)COOH) (Figure 2b) appeared at 42.74, 32.58, 33.43, 30.18, 39.66, and 37.79 ppm in TetraP1 and 42.38, 32.35, 34.83, 30.19, 39.75, and 37.56 ppm in GGTI-g-TetraP1,^35,37 respectively, inferred from negative signals in DEPT-135 (Figure S4b). However, the absence of the characteristic peak for −CH(CH₃)₂ of NIPA at 43.40 ppm³⁸ and the absence of any positive peak in DEPT-135 confirmed the abstraction of the −CH– proton from −CH(CH₃)₂ of NIPA to produce a quaternary carbon. In fact, the characteristic peaks at 46.67/51.38 and 46.15/52.03 ppm^28,39 in ¹³C NMR of TetraP1 and GGTI-g-TetraP1, respectively, confirmed the formation of −CH₂(C_q)COOH, −CH₂(C_q)COO^–, −CH₂(C_q)CONH((C_q)(Me₂)−), and −CH₂(C_q)CON((C_q)(Me₂)−)CH₂(C_q)COOH (Figure 2b) moieties via C–H activation. Indeed, the appearance of a quaternary carbon was further justified from the absence of such peaks in DEPT-135 (Figure 2b).

The presence of GGTI in GGTI-g-TetraP1 was confirmed by the prevalence of several GGTI specific peaks, such as 17.27, 61.46, 62.79, 67.14, 68.95, 70.11, 71.32, 72.26, 73.48, 74.50, 75.90, 76.48, 77.78, 77.93, 78.03, 79.40, 83.30, 85.00, 101.10, 104.98, 105.05, and 108.87 ppm (Figure 2a). In fact, the peaks at 108.87–111.18, 104.98–106.05, 101.10–102.35, 95.15–98.59, and 89.19–93.04 ppm were associated with the anomeric carbons of T-α-l-Araf, β-d-Galp/β-d-GlcpA, α-l-Rhap, α-galactose, and α-d-glucose, respectively.^32,33,39,40 Moreover, the characteristic peaks of C-2, C-3, C-4, and C-5 of T-α-l-Araf and β-d-GlcpA appeared at 83.30, 78.03, 85.00–87.00, and 62.79 ppm³² and 73.48, 75.90, 79.40, and 77.93 ppm,³³ respectively. However, the symbolic peaks of C-2, C-3, C-4, C-5, and C-6 of β-d-Galp and α-l-Rhap appeared at 72.26, 74.50, 68.95, 76.48, and 61.46 ppm,³⁶ and 67.14, 77.78, 71.32, 70.11, and 17.27 ppm,³³ respectively (Table 2).

Table 2. ¹³C NMR Analysis of GGTI-g-TetraP1.

δ (ppm)	assignment	refs
160.04/163.70/168.65/169.84	–CONHCH2NHCO– of MBA	(4,34)
171.68	–CH2(Cq)CON((Cq)(Me2)−)CH2(Cq)COOH
175.65	–CH2(Cq)CONH–((Cq)(Me2)–CH2(Cq)COOH)
180.12/184.57	–(Cq)–COOH/–(Cq)–COO–	(35)
176.96	–COOH of β-d-GlcpA	(36)
23.00	–CH3 of NIPA	(4)
42.38	–CH2– peaks of MBA	(4)
32.35	–CH2(Cq)COOH	(35,37)
34.83	–CH2(Cq)COO–
30.19	–CH2(Cq)CONH((Cq)(Me2)−)
39.75	–CH2(Cq)CON((Cq)(Me2)−)CH2(Cq)COOH
37.56	–CH2(Cq)CONH–((Cq)(Me2)–CH2(Cq)COOH)
46.15/52.03	–CH2(Cq)COOH, −CH2(Cq)COO–, −CH2(Cq)CONH((Cq)(Me2)−), and −CH2(Cq)CON((Cq)(Me2)−)CH2(Cq)COOH	(28,37)
108.87–111.18	anomeric carbons of T-α-l-Araf of GGTI	(32,39,33)
104.98–106.05	anomeric carbons of β-d-Galp and β-d-GlcpA of GGTI
101.10–102.35	anomeric carbons of α-l-Rhap of GGTI
95.14–98.59	anomeric carbons of α-galactose of GGTI	(40)
89.19–93.04	anomeric carbons of α-d-glucose of GGTI
83.30, 78.03, 85.00–87.00, and 62.79	C-2, C-3, C-4, and C-5 of T-α-l-Araf of GGTI	(32)
73.48, 75.90, 79.40, and 77.93	C-2, C-3, C-4, and C-5 of β-d-GlcpA of GGTI	(33)
72.26, 74.50, 68.95, 76.48, and 61.46	C-2, C-3, C-4, C-5, and C-6 of β-d-Galp of GGTI	(39)
67.14, 77.78, 71.32, 70.11, and 17.27	C-2, C-3, C-4, C-5, and C-6 of α-l-Rhap of GGTI	(33)

X-ray Photoelectron Spectroscopy (XPS) Analysis

The deconvoluted spectra of C 1s, N 1s, and O 1s orbitals of GGTI-g-TetraP1 and Sr(II)-/Hg(II)-/Cu(II)-GGTI-g-TetraP1 (Table 3) showed the prevalence of indirect interaction between M(II) and C-center of GGTI-g-TetraP1 during adsorption. In fact, because of active participation of O-/N-center adjacent to C-atoms, relatively lesser shifting in the binding energies (BEs) of C 1s than N 1s and O 1s was observed in M(II)-GGTI-g-TetraP1 (Figure 3). The deconvoluted O 1s peaks at 530.32, 532.35, 534.10, 537.96, and 541.13 eV in GGTI-g-TetraP1 (Figure 3b) were attributed to >C=O,² −COO^–,¹ −COOH,¹ HOH of liquid water clusters containing more than 1000 water molecules,¹ and shake-up satellite band of the O atom in the −CH₂(C_q)CON((C_q)(Me₂)−)CH₂(C_q)COOH segment of GGTI-g-TetraP1, respectively.¹ Moreover, the N 1s peak at 401.02 eV (inset of Figure 3h) was attributed to −CH₂(C_q)CON⁺H–((C_q)(Me₂)−)CH₂(C_q)COOH and −CH₂(C_q)CON⁺H₂–((C_q)(Me₂)–CH₂(C_q)COOH) moieties.

Table 3. XPS Analyses of GGTI-g-TetraP1 and Sr(II)-/Hg(II)-/Cu(II)-GGTI-g-TetraP1.

	peaks (eV)
orbitals	GGTI-g-TetraP1	Sr(II)-GGTI-g-TetraP1	Hg(II)-GGTI-g-TetraP1	Cu(II)-GGTI-g-TetraP1	significance
C 1s	(i) 283.67	(i) 283.96	(i) 284.17	(i) 284.01	(i) −CH3 of NIPA
	(ii) 285.23	(ii) 285.12	(ii) 285.32	(ii) 285.30	(ii) diamond-like Cq(41)
	(iii) 286.50	(iii) 286.63	(iii) 286.76	(iii) 286.69	(iii) C–N/C–O/C–O–C42
	(iv) 289.21	(iv) 288.02	(iv) 289.53	(iv) 288.94	(iv) –COOH/–COO–/–CONH–/–CON<43,44
O 1s	(i) 530.32	(i) 531.17	(i) 531.01	(i) 529.79/531.97	(i) >C=O2
	(ii) 532.35	(ii) 532.87	(ii) 532.66/533.50	(ii) 533.81	(ii) −COO–1
	(iii) 534.10	(iii) 534.82	(iii) 534.58	(iii) 536.11	(iii) −COOH1
	(iv) 537.96	(iv) absent	(iv) 537.33	(iv) absent	(iv) HOH of liquid water clusters containing >1000 water molecules1
	(v) 541.13	(v) 541.15	(v) 541.32	(v) 540.69	(v) shake-up satellite band of the O atom in –CH2(Cq)CON((Cq)(Me2)−)CH2(Cq)COOH1
N 1s	(i) 401.02		(i) 397.89		(i) –CH2(Cq)CON+H–((Cq)(Me2)−)CH2(Cq)COOH and –CH2(Cq)CON+H2–((Cq)(Me2)–CH2(Cq)COOH) types moieties formed in slightly acidic medium (i.e.,pHi = 5.5) during synthesis and then decrease to 397.89 eV because of the formation of a strong Hg–N covalent bond in pHi > pHPZC
			(ii) 401.45/408.92		(ii) weak ionic/coordinate bonding
Sr 3d5/2: 134.20 eV of Sr(NO3)2		(i) 133.49/134.93			(i) coordinate bonding, also surface deposition of SrCO3
Hg 4f7/2: 102.58 eV, Hg 4f5/2: 106.68 eV of Hg(II)			(i) 102.09		(i) weak ionic interaction
			(ii) 103.47		(ii) covalent bonding
			(iii) 105.80		(iii) ionic bonding
			(iv) 107.43		(iv) covalent bonding
Cu 2p3/2: 935.50 eV, Cu 2p1/2: 953.70 eV of Cu(NO3)2				(i) 933.07	(i) coordinate bonding between Cu and O
				(ii) 944.16	(ii)shake-up satellite peak of Cu 2p3/2
				(iii) 953.02	(iii) coordinate bonding

Figure 3.

XPS of (a and c/f/j) C 1s and (b and d/g/k) O 1s of GGTI-g-TetraP1 and Sr(II)-/Hg(II)-/Cu(II)-GGTI-g-TetraP1, respectively; (h) N 1s of GGTI-g-TetraP1 and (inset of h) N 1s of Hg(II)-GGTI-g-TetraP1; (e) Sr 3d_5/2, (i) Hg 4f_7/2,5/2, and (l) Cu 2p_3/2,1/2 for Sr(II)-, Hg(II)-, and Cu(II)-GGTI-g-TetraP1, respectively.

Sr(II)-GGTI-g-TetraP1. The O 1s spectra of Sr(II)-GGTI-g-TetraP1 were deconvoluted into four peaks (Figure 3d), of which the peak at 532.87 eV was assigned to −COO^–,¹ coordinated preferably via BC and BB modes. In addition, the enhancement in the BEs of >C=O/–COOH specific peaks in Sr(II)-GGTI-g-TetraP1 from 530.32/534.10 to 531.17/534.82 eV (Table 3) emphasized the participation of lone pair electrons in bonding with Sr(II). In this context, the strong interaction between Sr(II) and −COO^– could also be realized from the significant reduction in the characteristic Sr 3d_5/2 peak from 134.20 eV of pure Sr(II) to 133.49 eV in Sr(II)-GGTI-g-TetraP1 (Figure 3e), indicating the deposition of SrCO₃ onto Sr(II)-GGTI-g-TetraP1.

Hg(II)-GGTI-g-TetraP1. The significant reduction in the N 1s BE of GGTI-g-TetraP1 from 401.02 to 397.89 eV in Hg(II)-GGTI-g-TetraP1 (Figure 3h) inferred the formation of covalent bonds between Hg(II) and N-center of −CH₂(C_q)CON((C_q)(Me₂)−)CH₂(C_q)COOH and −CH₂(C_q)CONH–((C_q)(Me₂)–CH₂(C_q)COOH)³ in pH_i > pH_PZC, also evidenced from FTIR (Figure S1b). The formation of the Hg–N covalent bond was inferred from the substantial enhancement in the BEs of Hg 4f_7/2/Hg 4f_5/2 from 102.58/106.68 eV of Hg(II) to 103.47/107.43 eV in Hg(II)-GGTI-g-TetraP1 (Figure 3i).³ However, the formation of weak ionic bonds between Hg(II) and N-donors was also substantiated from the significant increase in N 1s BE from 401.02 eV of GGTI-g-TetraP1 to 401.45 eV and at the same time decrease in BE from 102.58/106.68 eV to 102.09/105.80 eV for Hg 4f_7/2/Hg 4f_5/2 in Hg(II)-GGTI-g-TetraP1. Moreover, shifting of GGTI-g-TetraP1 specific O 1s peaks from 530.32, 532.35, and 534.10 to 531.01, 532.66/533.50, and 534.58 eV in Hg-GGTI-g-TetraP1 (Figure 3g) inferred the occurrence of coordinate bonds between O-center and Hg(II), realized from the prevalent Hg–O str. at 460 cm^–1 in FTIR. In fact, the appearance of the O 1s peak at 532.66/533.50 eV supported the formation of BB/BC complexation with Hg(II). Moreover, the significant change in low-dimensional H-bonded water cluster was understood from the change in BE from 541.13 to 541.32 eV in Hg(II)-GGTI-g-TetraP1, supported from the FTIR analysis.

Cu(II)-GGTI-g-TetraP1. The −COO^– specific O 1s peak of GGTI-g-TetraP1 at 532.35 eV was shifted to 533.81 eV (Figure 3k) in Cu(II)-GGTI-g-TetraP1, indicating the prevalence of BB and BC interactions.¹ Additionally, participation of −COOH/–COO^– in strong bonding was also ascertained from the shifting of peaks from 530.32 (>C=O) and 534.10 eV (−COOH) to 529.79/531.97 and 536.11 eV, respectively, during Cu(II) adsorption (Figure 3k).² In fact, Cu–O bond formation could also be rationalized from the significant lowering of the characteristic Cu 2p_3/2/Cu 2p_1/2 peaks from 935.50/953.70 eV of pure Cu(NO₃)₂ to 933.07/953.02 eV in Cu(II)-GGTI-g-TetraP1 (Figure 3l). In this regard, the shake-up satellite band at 541.13 eV of O-atom (Table 3) was shifted to 540.69 eV, which indicated the possible alteration in O 1s BE of −CH₂(C_q)CON((C_q)(Me₂)−)CH₂(C_q)COOH, predominantly through the formation of strong H-bonds to induce slightly bigger water clusters in Cu(II)-GGTI-g-TetraP1. Additionally, the shake-up satellite peak of Cu 2p_3/2 was obtained at 944.16 eV.

Thermogravimetric Analysis (TGA)

The initial thermal decomposition of GGTI-g-TetraP2 from 51 °C was comparatively more rapid than GGTI-g-TetraP1 (Figure S5a), attributed to the faster elimination of loosely adhered water molecules from more hydrophobic GGTI-g-TetraP2. In addition, greater conversion of amides to imides played the pivotal role in drastic deterioration of the amide-populated GGTI-g-TetraP2. Nevertheless, the thermoresistant secondary amides hindered the conversion of amides to imides in GGTI-g-TetraPs. Notably, slightly accelerated decarboxylation of GGTI-g-TetraP1 within 346–378 °C could be attributed to the decomposition of higher extent of anhydrides, produced via dehydration of more frequently available −COOH in GGTI-g-TetraP1. However, the relatively inferior thermal stability of GGTI-g-TetraP2 above 378 °C could be anticipated from the decomposition of imide intermediates. Finally, the higher residue (i.e., 30.68 wt %) for GGTI-g-TetraP1 than for GGTI-g-TetraP2 (i.e., 25.02 wt %) (Figure S5a) was related to the presence of a relatively higher amount of thermostable Na⁺ salts.

Sr(II)-GGTI-g-TetraPs. At pH_i > pH_PZC, considerable conversion of −COOH to −COO^– destroyed the existing H-bonds of GGTI-g-TetraPs that resulted in slightly deteriorated thermal stabilities of Sr(II)-GGTI-g-TetraPs up to 210 °C (Figure S5b). However, beyond this, the thermal stabilities of Sr(II)-GGTI-g-TetraPs exceeded GGTI-g-TetraPs because of the formation of SrCO₃-type crystal chelates, leading to the restricted formation and decomposition of anhydrides because of the lack of free −COO^–/–COOH in GGTI-g-TetraPs. The strong binding affinity of Sr(II) with −COO^– was realized from the elevated thermal stability of Sr(II)-GGTI-g-TetraP1, as GGTI-g-TetraP1 was immensely populated with −COO^–. The huge residues of 44.29 and 39.97 wt % for Sr(II)-GGTI-g-TetraP1 and Sr(II)-GGTI-g-TetraP2 (Figure S5b), respectively, were attributed to the formation of extremely thermostable SrCO₃ crystals.

Hg(II)-GGTI-g-TetraPs. Though Hg(II) adsorption brought about significant alterations in the thermograms of GGTI-g-TetraPs, the initial decomposition pattern of Hg(II)-GGTI-g-TetraPs resembled GGTI-g-TetraPs up to 100 °C (Figure S5c) because of the lesser tendency of Hg(II) to interact with water. The greater mass loss up to 169 °C from Hg(II)-GGTI-g-TetraP2 was ascribed to the higher hydrophobic character of GGTI-g-TetraP2. However, sharp decomposition of Hg(II)-GGTI-g-TetraP1 than GGTI-g-TetraP1 within 100–317 °C could be explained by the rapid rupture of thermolabile Hg–N bonds, accompanied by the liberation of volatile Hg(II) components. Furthermore, the preferential interaction of Hg(II) with N-donating NIPA moieties and deprotonation of −COOH destroyed the mutual O–H···N–H H-bonds to restore free N–H of NIPA, which were converted into imides in the bulk of GGTI-g-TetraP1. Notably, as compared to Hg(II)-GGTI-g-TetraP2, the drastic degradation of Hg(II)-GGTI-g-TetraP1 could be explained by the relative enhancement of N-donors in GGTI-g-TetraP2 that produced more Hg–N bonds and >N–Hg–N< cross-links in Hg(II)-GGTI-g-TetraP2, as observed from FTIR analyses. Finally, the sharp decomposition of Hg(II)-GGTI-g-TetraPs above 324 °C was mainly associated with the vaporization of adsorbed Hg(II) components and decomposition of imides, anhydrides, and polymeric backbone. In fact, vaporization of Hg(II) components significantly lowered the amount of residues, that is, 0.005 and 9.17 wt % for Hg(II)-GGTI-g-TetraP1 and Hg(II)-GGTI-g-TetraP2 (Figure S5c), respectively, as Hg(II) eliminated a larger amount of Na⁺ from GGTI-g-TetraPs. Moreover, the higher residue content for Hg(II)-GGTI-g-TetraP2 could be attributed to the delayed detachment of volatile Hg(II) components from the relatively well cross-linked GGTI-g-TetraP2.

Cu(II)-GGTI-g-TetraPs. Deprotonation of a significant amount of −COOH into −COO^– during the adsorption of Cu(II) at alkaline pH_i led to the appreciable destruction of mutual O–H···N–H H-bonds in Cu(II)-GGTI-g-TetraPs. Thus, Cu(II)-GGTI-g-TetraPs were thermally less stable than GGTI-g-TetraPs (Figure S5d). Moreover, the higher mass loss of Cu(II)-GGTI-g-TetraP1 over Cu(II)-GGTI-g-TetraP2 within 100 °C could be attributed to the removal of a higher number of water molecules from malachite-type crystals, deposited significantly onto GGTI-g-TetraP1. In fact, such deposition was also realized from the extended loss of coordinated water up to 400 °C bearing one-step mass loss of Cu(II)-GGTI-g-TetraPs. The amount of residue was found to be higher for Cu(II)-GGTI-g-TetraP2 (i.e., 18.73 wt %) than for Cu(II)-GGTI-g-TetraP1 (i.e., 13.93 wt %) (Figure S5d), though adsorption should be higher onto GGTI-g-TetraP1. This obscurity could be contributed by the greater elimination of moisture during the initial stage.

Differential Scanning Calorimetry (DSC) Analysis

The prevalence of a broad and intense peak at 72 °C of GGTI-g-TetraP1 (Figure S5e) was related to the vaporization of moisture and loosely adhered volatile components. However, comparatively more intense and sharp endothermic peak at 77 °C in GGTI-g-TetraP2 could be attributed to the easier removal of moisture and volatile components from the more hydrophobic surface of GGTI-g-TetraP2 (Figure S5f), evidenced earlier from TGAs (Figure S5a). Such a broader peak in GGTI-g-TetraP1, constituted of a shoulder at 202 °C, could be ascribed to the greater extent of anhydride formation via dehydration of frequently available neighboring −COOH in GGTI-g-TetraP2. The absence of a shoulder at 202 °C was related to the lower population of −COOH in GGTI-g-TetraP1, whereas the exclusive appearance of an endothermic peak at 291 °C was corroborated with the conversion of secondary amides into imides. Consequently, another sharp peak at 357 °C, found exclusively in GGTI-g-TetraP1, was related to the decarboxylation of anhydrides. Moreover, the broad peak centered at 432 °C, originated by the decomposition of imides, was more intense for GGTI-g-TetraP2, owing to the greater relative population of poly-NIPA-type moieties, which showed a characteristic sharp degradation peak at 371 °C. Consequently, the fewer availability of such moieties resulted in the absence of such peaks in GGTI-g-TetraP1. Nevertheless, disappearance of the TetraP specific endothermic peak at 134 °C in both GGTI-g-TetraPs indicated the existence of strong H-bonds in GGTI-g-TetraPs, realized earlier from FTIR analyses.

Sr(II)-GGTI-g-TetraPs. The greater hydrophobicity of Sr(II)-GGTI-g-TetraP2 resulted in a more rapid removal of moisture as compared to that of Sr(II)-GGTI-g-TetraP1 (Figure S5e,f). Though the DSC thermogram of Sr(II)-GGTI-g-TetraP1 is constituted of distinct peaks at 70, 195, 258, and 350 °C, which are assigned to moisture removal, anhydride formation, amide formation, and amide to imide transition, respectively, the thermogram of Sr(II)-GGTI-g-TetraP2 exhibited a broad peak centered at 95 °C and a shoulder extended up to 350 °C because of the decomposition of Sr(II) complexes containing both carboxyl and amide ligands. A broad and intense peak centered at 405 °C was ascribed to the breakdown of imides in thermally less stable Sr(II)-GGTI-g-TetraP2.

Hg(II)-GGTI-g-TetraPs. In comparison to Hg(II)-GGTI-g-TetraP1, the relatively intense and broad endothermic transition in Hg(II)-GGTI-g-TetraP2 at 105 °C (Figure S5e,f) could be related to the rapid moisture loss from the relatively hydrophobic surface of Hg(II)-GGTI-g-TetraP2 than Hg(II)-GGTI-g-TetraP1, as manifested in TGA (Figure S5c). Moreover, the existence of a distinct endothermic peak at 176 °C in Hg(II)-GGTI-g-TetraP1 was associated with dehydration during anhydride formation. The two new exothermic peaks within 200–220 °C in Hg(II)-GGTI-g-TetraPs were originated because of the possible displacement reaction between adsorbed HgCl₂/Hg(OH)₂ and Al pan. However, within 235–350 °C, the relatively less intense and broader endothermic transition for Hg(II)-GGTI-g-TetraP2 could be assigned to the higher population of Hg–N bonds and >N–Hg–N< cross-links, produced through the higher amount of N-donating NIPA in GGTI-g-TetraP2. Thus, the sharp endothermic peak at 279 °C in GGTI-g-TetraP1 became broader and shifted to 327 °C in GGTI-g-TetraP2, indicating restricted amide to imide transition in Hg(II)-GGTI-g-TetraP2. Thus, greater involvement of adsorbed Hg(II) in generating Hg–N and >N–Hg–N< restricted the extent of the displacement reaction between Hg(II) and Al pan, to produce a relatively truncated exothermic peak within 200–220 °C.

Cu(II)-GGTI-g-TetraPs. The characteristic intense and broad peak of GGTI-g-TetraP1 at 72 °C was significantly enhanced to 91 °C in Cu(II)-GGTI-g-TetraP1 (Figure S5e), suggesting restricted removal of loosely held moisture and volatile components from the relatively ordered and cross-linked network of Cu(II)-GGTI-g-TetraP1. Such an endothermic peak was further broadened and shifted to 144 °C in Cu(II)-GGTI-g-TetraP2 that could be explained by the relatively easier removal of bound water from adsorbed malachite-type crystals and amorphous Cu(II) components (Figure S5f). The higher population of −COO^– encouraged higher accumulation of Cu(II) components onto GGTI-g-TetraP1 via complexation of O-donors with Cu(II) in various coordination modes (Table 1), leading to the exclusive arrival of a small endothermic peak at 258 °C in Cu(II)-GGTI-g-TetraP1. Furthermore, the greater amide content in GGTI-g-TetraP2 and lesser coordination of Cu(II) with N-donors resulted in relatively intense amide decomposition-related transitions at lower temperature. Accordingly, the exclusive peak of Cu(II)-GGTI-g-TetraP2 at 300 °C, originated through amide to imide transformation, was not prominent in Cu(II)-GGTI-g-TetraP1. Furthermore, the peaks at 379 and 424 °C for Cu(II)-GGTI-g-TetraP1 could be compared with the glass-transition temperatures of 349.8 and 412.2 °C for Cu(II)-adsorbed polyacrylate-based IPN, respectively. However, such peaks were merged together with the enthalpy change related to the decomposition of intermediate imide to produce a broad and intense peak centered at 397 °C in Cu(II)-GGTI-g-TetraP2.

X-ray Diffraction (XRD) Analysis

The characteristic peaks at 2θ = 21.13°/23.48° and 19.29°/21.04°/22.09° (Figure S6a) for TetraP1 and TetraP2, respectively, broadened in GGTI-g-TetraP1 and disappeared in GGTI-g-TetraP2. In spite of GGTI incorporation in the network, the decrease in crystallinity of GGTI-g-TetraPs indicated the extensive loss of orderliness during grafting. In fact, such loss in the crystallinity of GGTI after grafting indicated the intimate mixing between GGTI and TetraPs. In this context, extensive surface reconstruction for GGTI-g-TetraPs was understood via the appearance of new peaks at 2θ = 22.85°/22.16° in GGTI-g-TetraP1/2 (Figure S6a), respectively. Again, the appearance of a GGTI specific weak shoulder at 2θ = 42.32°/40.08° in GGTI-g-TetraP1/2, respectively, supported the incorporation of GGTI. The better intermixing of GGTI with more hydrophilic TetraP1 was ascertained through the enhancement of crystallinity of TetraP2/GGTI-g-TetraP2 than TetraP1/GGTI-g-TetraP1.

Sr(II)-GGTI-g-TetraPs. The appearance of numerous highly crystalline peaks and the loss of GGTI-g-TetraP1 specific peaks in Sr(II)-GGTI-g-TetraP1 supported extensive surface accumulation of Sr(II) components. The peaks at 2θ = 26.34°, 30.54°, 45.69°, 47.11°, and 51.96° (Figure S6c) for (1,1,1), (0,0,2), (2,0,2), (1,3,2), and (1,1,3), respectively, were similar to the characteristic planes of highly crystalline SrCO₃.⁴⁵ Again, the crystallographic bond lengths closely matched with c1-, c2-, d1-, and d2-type bond lengths of SrCO₃ (Scheme S3),⁴⁶ indicating that surface deposition of SrCO₃-type crystals occurred because of the conversion of Sr(NO₃)₂ to SrCO₃ in an alkaline medium.⁴⁷ In addition, peaks at 2θ = 28.52°, 33.01°, 40.47°, 51.96°, 59.74°, 64.69°, 71.59°, 75.39°, and 80.63° (Figure S6c) supported direct surface deposition of Sr(NO₃)₂. Again, the formation and surface occupation of Sr(II) carboxylate-type crystals were inferred from the arrival of new characteristic peaks at 2θ = 24.03°, 33.90°, 38.19°, 42.62°, and 44.73° (Figure S6c). Several new crystalline peaks at 2θ = 27.34°, 30.34°, 33.05°, and 44.49° (Figure S6d) appeared in the XRD of Sr(II)-GGTI-g-TetraP2, along with the retention of GGTI-g-TetraP2 specific peaks. However, the lower intensity of such peaks than Sr(II)-GGTI-g-TetraP1 indicated lesser surface deposition of Sr(NO₃)₂, SrCO₃, and Sr(II) carboxylate because of lowering of −COO^– in GGTI-g-TetraP2.

Hg(II)-GGTI-g-TetraPs. The crystalline peaks of both GGTI-g-TetraPs disappeared in Hg(II)-GGTI-g-TetraPs, indicating substantial alteration of the hydrogel surface (Figure S6a,b). Though the XRD diagram of Hg(II)-GGTI-g-TetraPs did not contain any characteristic peaks, the overall intensities of Hg(II)-GGTI-g-TetraP2 were slightly higher than that of Hg(II)-GGTI-g-TetraP1 (Figure S6b). This observation indicated the prevalence of a higher amount of Hg(II) on the surface of GGTI-g-TetraP2 because bulky NIPA moieties predominantly resided at the surface of GGTI-g-TetraP1, realized from FTIR analyses.

Cu(II)-GGTI-g-TetraPs. The diffractogram of Cu(II)-GGTI-g-TetraPs showed several new peaks of variable intensities at 2θ = 15.03°, 19.02°, 21.83°, 24.10°, and 40.93° (shoulder) for Cu(II)-GGTI-g-TetraP1 and 20.87°, 22.06°, 23.87°, and 39.65° (shoulder) for Cu(II)-GGTI-g-TetraP2 (Figure S6b) and envisaged the presence of malachite-type crystals at the surface of Cu(II)-GGTI-g-TetraPs.³⁰ In fact, the surface deposition of Cu(II) onto GGTI-g-TetraPs was apprehended by the significant increase in the crystallinity of GGTI-g-TetraPs.

Field Emission Scanning Electron Microscopy (FESEM) and Energy-Dispersive X-ray Spectrometry (EDX) Analyses

The FESEM microphotographs of TetraP1 showed distinct phase boundaries, whereas the surface of GGTI-g-TetraP1 was devoid of such distinct phase boundaries (Figure S7a,b) and possessed featureless, smooth, well-organized, and compact morphology because of effective intermixing of grafted GGTI with TetraP1. The FESEM images of Cu(II)-GGTI-g-TetraP1 (Figure S7e) showed the deposition of tabular and prismatic crystals of malachite Cu(II), apprehended earlier from FTIR and XRD analyses. From the EDX spectrum (inset of Figure S7e), the appearance of different intense peaks supported variegated interactions with GGTI-g-TetraP1 through ionic/coordinate bonding. Again, the appearance of uneven and heterogeneous surface of Hg(II)-GGTI-g-TetraP1 (Figure S7d) indicated either bulk diffusion or surface occupation of amorphous Hg(II) salts. The formation of covalent, coordinate, and/or ionic bonds between GGTI-g-TetraP1 and Hg(II) was eventually realized from the appearance of variegated Hg(II) peaks in EDX (inset of Figure S7d). For Sr(II)-GGTI-g-TetraP1, extensive surface deposition of various crystals was observed (Figure S7c). In fact, the existence of SrCO₃, Sr(NO₃)₂, and Sr(II) carboxylate types of crystals on the surface of GGTI-g-TetraP1 was already established by XRD analyses.

Rheological Analysis

From Figure S8, a significant increase in the storage modulus with the increase in frequency from 0 to 40 rad s^–1 was noted for GGTI-g-TetraP1, followed by the gradual decrease in storage modulus within 40–100 rad s^–1. In contrast, the loss modulus increased up to 4 rad s^–1 and then reduced marginally with further increase in frequency. However, no crossover between storage and loss moduli was observed from 0 to 100 rad s^–1. The higher storage as compared to loss modulus indicated stable, compact, and strong GGTI-g-TetraP network, generated via extensive physical and chemical cross-linking.

Adsorption Isotherm, Kinetics, and Thermodynamics Studies

The interaction between the adsorbate and the adsorbent was realized by fitting of the experimental data with different isotherm models (eqs S1–S3), of which the Langmuir model (Figure S9a–f) fitted the best (Table 4). Thus, the prevalence of monolayer adsorption on the structurally homogeneous adsorbent surface was realized. Alongside, the nonprevalent interaction between unadsorbed and already adsorbed M(II) was understood via the presence of electrostatic repulsion between the positively charged M(II).² The enhancement of q_max with the increasing temperature suggested chemisorption between M(II) and GGTI-g-TetraPs, supported from the inferences of FTIR and XPS analyses. The ligand selectivity of GGTI-g-TetraPs for M(II) adsorption was rationalized from the q_max values in the following order: Sr(II) (1940.24 mg g^–1) > Cu(II) (1903.64 mg g^–1) > Hg(II) (1759.50 mg g^–1) for GGTI-g-TetraP1, but Hg(II) (1848.03 mg g^–1) > Cu(II) (1781.63 mg g^–1) > Sr(II) (1748.36 mg g^–1) for GGTI-g-TetraP2. The higher binding affinity of Hg(II) with N-center elevated the adsorption capacity (AC) for NIPA-populated GGTI-g-TetraP2. In this regard, separation factor (R_L) and a dimensionless quantity (eq S4), measuring the feasibility of adsorption (R_L > 1, 0 < R_L < 1, R_L = 1, and R_L = 0 indicate unfavorable, favorable, linear, and irreversible adsorptions, respectively), were found to vary within 0–1, indicating favorable adsorption.

Table 4. Adsorption Isotherm and Kinetics Parameters.

temperature (K)
models/parameters	293	303	313	323
Sr(II)
Langmuir (GGTI-g-TetraP1)
qmax (mg g–1)/pHi/C0 (ppm)	1940.24/7/500–1000	2216.01/7/500–1000	2358.28/7/500–1000	2453.52/7/500–1000
kL (L mg–1)	0.1086	0.0407	0.0283	0.0233
R2/F	0.9951/3768.34	0.9986/13 184.50	0.9977/7652.67	0.9981/9429.39
Langmuir (GGTI-g-TetraP2)
qmax (mg g–1)/pHi/C0 (ppm)	1748.36/7/500–1000	1752.06/7/500–1000	1753.54/7/500–1000	1754.34/7/500–1000
kL (L mg–1)	0.0671	0.0532	0.0419	0.0361
R2/F	0.9980/9478.93	0.9996/54 177.84	0.9992/23 451.02	0.9971/6446.39
Langmuir (GGTI-g-TetraP1)		Langmuir (GGTI-g-TetraP2)
qmax (mg g–1)/pHi/C0 (ppm)	114.32/7/5–55	qmax (mg g–1)/pHi/C0 (ppm)	92.36/7/5–55
kL (L mg–1)	0.5762	kL (L mg–1)	0.4044
R2/F	0.9979/4453.42	R2/F	0.9998/55 107.27
Pseudo-Second-Order (GGTI-g-TetraP1)
qe,cal (mg g–1)/pHi/C0 (ppm)	1231.27/7/500	1174.55/7/500	1165.12/7/500	1140.39/7/500
qe,exp (mg g–1)	1215.43 ± 36.46	1178.74 ± 35.36	1162.58 ± 34.88	1151.76 ± 34.55
k2 (g mg–1 min–1)	1.17 × 10–4	1.53 × 10–4	1.96 × 10–4	2.44 × 10–4
R2/F	0.9963/16 383.23	0.9950/12 625.01	0.9951/12 669.36	0.9958/15 502.39
Pseudo-Second-Order (GGTI-g-TetraP2)
qe,cal (mg g–1)/pHi/C0 (ppm)	1186.15/7/500	1155.47/7/500	1136.76/7/500	1113.56/7/500
qe,exp (mg g–1)	1174.55 ± 35.24	1159.91 ± 34.80	1141.18 ± 34.24	1125.06 ± 33.75
k2 (g mg–1 min–1)	9.94 × 10–5	1.22 × 10–4	1.56 × 10–4	1.87 × 10–4
R2/F	0.9942/10 853.23	0.9849/4509.02	0.9972/24 505.70	0.9949/14 338.01
Pseudo-Second-Order (GGTI-g-TetraP1)		Pseudo-Second-Order (GGTI-g-TetraP2)
qe,cal (mg g–1)/pHi/C0 (ppm)	99.79/7/55	qe,cal (mg g–1)/pHi/C0 (ppm)	82.60/7/55
qe,exp (mg g–1)	100.33 ± 3.12	qe,exp (mg g–1)	81.17 ± 2.78
k2 (g mg–1 min–1)	0.0035	k2 (g mg–1 min–1)	0.0024
R2/F	0.9977/33 825.37	R2/F	0.9962/21 072.57
Hg(II)
Langmuir (GGTI-g-TetraP1)
qmax (mg g–1)/pHi/C0 (ppm)	1759.50/7/500–1000	1765.68/7/500–1000	1786.11/7/500–1000	1881.51/7/500–1000
kL (L mg–1)	0.2992	0.1304	0.0818	0.0403
R2/F	0.9949/3824.20	0.9908/2101.03	0.9918/2350.91	0.9944/3157.50
Langmuir (GGTI-g-TetraP2)
qmax (mg g–1)/pHi/C0 (ppm)	1848.03/7/500–1000	1974.62/7/500–1000	2054.91/7/500–1000	2153.18/7/500–1000
kL (L mg–1)	0.1121	0.0586	0.0394	0.0282
R2/F/χ2	0.9956/1779.38	0.9948/3594.45	0.9975/7493.61	0.9962/4756.72
Langmuir (GGTI-g-TetraP1)		Langmuir (GGTI-g-TetraP2)
qmax (mg g–1)/pHi/C0 (ppm)	84.97/7/5–55	qmax (mg g–1)/pHi/C0 (ppm)	113.35/7/5–55
kL (L mg–1)	1.9219	kL (L mg–1)	0.7219
R2/F	0.9939/1729.04	R2/F	0.9926/1276.45
Pseudo-Second-Order (GGTI-g-TetraP1)
qe,cal (mg g–1)/pHi/C0 (ppm)	1286.69/7/500	1262.33/7/500	1206.38/7/500	1207.64/7/500
qe,exp (mg g–1)	1227.42 ± 36.82	1200.19 ± 36.01	1177.92 ± 35.34	942.79 ± 28.28
k2 (g mg–1 min–1)	3.12 × 10–5	4.86 × 10–5	8.14 × 10–5	1.28 × 10–4
R2/F	0.9973/19 176.96	0.9917/6672.73	0.9967/17 308.20	0.9962/17 068.16
Pseudo-Second-Order (GGTI-g-TetraP2)
qe,cal (mg g–1)/pHi/C0 (ppm)	1269.69/7/500	1236.43/7/500	1185.92/7/500	1170.63/7/500
qe,exp (mg g–1)	1202.71 ± 36.08	1178.31 ± 35.35	1159.71 ± 34.79	1141.30 ± 34.24
k2 (g mg–1 min–1)	3.66 × 10–5	5.21 × 10–5	7.88 × 10–5	9.23 × 10–5
R2/F/χ2	0.9973/18 045.27	0.9961/13 602.89	0.9947/10 162.66	0.9981/27 319.17
Pseudo-Second-Order (GGTI-g-TetraP1)		Pseudo-Second-Order (GGTI-g-TetraP2)
qe,cal (mg g–1)/pHi/C0 (ppm)	88.22/7/55	qe,cal (mg g–1)/pHi/C0 (ppm)	105.42/7/55
qe,exp (mg g–1)	86.93 ± 2.12	qe,exp (mg g–1)	103.61 ± 2.78
k2 (g mg–1 min–1)	0.0029	k2 (g mg–1 min–1)	0.0034
R2/F	0.9969/21 819.36	R2/F	0.9951/11 993.88
Cu(II)
Langmuir (GGTI-g-TetraP1)
qmax (mg g–1)/pHi/C0 (ppm)	1903.64/7/500–1000	1916.52/7/500–1000	1931.81/7/500–1000	1967.01/7/500–1000
kL (L mg–1)	0.1228	0.0947	0.0702	0.0561
R2/F	0.9954/4066.81	0.9991/19 809.10	0.9982/10 404.60	0.9970/7229.00
Langmuir (GGTI-g-TetraP2)
qmax (mg g–1)/pHi/C0 (ppm)	1781.63/7/500–1000	1826.32/7/500–1000	1870.47/7/500–1000	1903.21/7/500–1000
kL (L mg–1)	0.0589	0.0464	0.0368	0.0301
R2/F	0.9986/13 532.90	0.9981/10 127.60	0.9971/6579.52	0.9968/5950.62
Langmuir (GGTI-g-TetraP1)		Langmuir (GGTI-g-TetraP2)
qmax (mg g–1)/pHi/C0 (ppm)	110.15/7/5–55	qmax (mg g–1)/pHi/C0 (ppm)	89.27/7/5–55
kL (L mg–1)	0.6384	kL (L mg–1)	0.4712
R2/F	0.9977/4125.01	R2/F	0.9994/16 065.12
Pseudo-Second-Order (GGTI-g-TetraP1)
qe,cal (mg g–1)/pHi/C0 (ppm)	1268.07/7/500	1258.45/7/500	1225.25/7/500	1201.23/7/500
qe,exp (mg g–1)	1217.21 ± 36.51	1205.95 ± 36.22	1194.19 ± 34.12	1181.66 ± 32.57
k2 (g mg–1 min–1)	3.91 × 10–5	5.77 × 10–5	7.97 × 10–5	1.06 × 10–4
R2/F	0.9986/44 816.26	0.9951/12 787.33	0.9951/12 507.16	0.9927/5701.03
Pseudo-Second-Order (GGTI-g-TetraP2)
qe,cal (mg g–1)/pHi/C0 (ppm)	1229.05/7/500	1206.24/7/500	1173.81/7/500	1150.22/7/500
qe,exp (mg g–1)	1164.94 ± 32.25	1152.12 ± 31.19	1138.41 ± 30.94	1123.21 ± 30.28
k2 (g mg–1 min–1)	3.79 × 10–5	4.54 × 10–5	7.46 × 10–5	9.88 × 10–5
R2/F	0.9956/12 792.24	0.9979/28 413.02	0.9981/31 026.85	0.9991/61 229.11
Pseudo-Second-Order (GGTI-g-TetraP1)		Pseudo-Second-Order (GGTI-g-TetraP2)
qe,cal (mg g–1)/pHi/C0 (ppm)	102.67/7/55	qe,cal (mg g–1)/pHi/C0 (ppm)	82.68/7/55
qe,exp (mg g–1)	102.02 ± 3.48	qe,exp (mg g–1)	82.37 ± 2.54
k2 (g mg–1 min–1)	0.0049	k2 (g mg–1 min–1)	0.0043
R2/F	0.9944/13 350.66	R2/F	0.9998/423 526.29

The adsorption mechanism, that is, physisorption or chemisorption, was studied through fitting of kinetics data to the pseudo-first-/-second-order kinetics models (Figure S9g–l). The extent of adsorption deteriorated gradually with advancement of adsorption because of gradual reduction in the number of free sites onto hydrogels. Better fitting of pseudo-second-order kinetics (eq S6) than pseudo-first-order (eq S5) indicated the prevalence of chemisorption⁴⁸ (Table 4) through ionic and variegated coordinative interactions (i.e., M, BB, and BC) between −COO^– of GGTI-g-TetraPs and M(II). The pseudo-second-order rate constants (k₂) for both M(II) were found to increase with the rise in temperature in GGTI-g-TetraPs, indicating faster adsorption at the relatively higher temperatures. The activation energies (E_a) of adsorption, calculated using the Arrhenius-type equation (eq S7), were as follows: 37.35/21.28, 16.85/20.51, and 26.24/25.29 kJ mol^–1 for Sr(II), Hg(II), and Cu(II) adsorption onto GGTI-g-TetraP1/2, respectively, indicating the prevalence of chemisorption.⁴⁹

The spontaneity of the chemisorption was inferred by observing negative ΔG⁰ (eq S8) for all M(II).⁵⁰ Again, the exothermic nature of adsorption was comprehended by the negative ΔH⁰ (Table 5), as calculated by van’t Hoff’s eq S10, whereas the positive ΔS⁰ suggested fair affinity of M(II) for both GGTI-g-TetraPs and the decrease in randomness at the solid–solution interface during adsorption (Figure S10).

Table 5. Adsorption Thermodynamic Parameters.

concentration (ppm) of M(II)/temperature (K)	–ΔG0 (kJ mol–1) of Sr(II)/Hg(II)/Cu(II) for GGTI-g-TetraP1(GGTI-g-TetraP2)	–ΔH0 (kJ mol–1) of Sr(II)/Hg(II)/Cu(II) for GGTI-g-TetraP1(GGTI-g-TetraP2)	ΔS0 (J mol–1 K–1) of Sr(II)/Hg(II)/Cu(II) for GGTI-g-TetraP1(GGTI-g-TetraP2)
500/293	10.90(8.92)/11.97(10.12)/11.04(8.61)	27.95(14.55)/36.79(22.93)/19.99(11.41)	–59.52(−19.20)/–85.89(−44.19)/–30.71(−9.55)
500/303	9.38(8.75)/10.32(9.36)/10.65(8.52)
500/313	9.12(8.50)/9.65(9.03)/10.36(8.43)
500/323	9.07(8.36)/9.99(8.77)/10.11(8.32)
600/293	8.81(7.68)/11.35(9.54)/9.40(8.02)	8.07(8.42)/38.32(19.96)/8.93(8.59)	2.41(−2.59)/–92.77(−35.92)/1.53(−1.94)
600/303	8.75(7.65)/9.86(8.92)/9.36(8.01)
600/313	8.81(7.53)/9.33(8.69)/9.41(7.99)
600/323	8.88(7.64)/8.46(8.44)/9.44(7.97)
700/293	8.40(6.93)/9.38(8.49)/8.53(6.73)	13.16(11.17)/26.29(14.52)/11.28(5.19)	–16.27(−14.56)/–57.86(−20.69)/–9.59(5.34)
700/303	8.28(6.73)/8.69(8.25)/8.33(6.84)
700/313	7.89(6.57)/8.16(7.91)/8.16(6.86)
700/323	7.99(6.50)/7.64(7.93)/8.28(6.89)
800/293	7.30(5.75)/6.61(6.76)/7.20(5.57)	5.30(9.46)/7.28(5.93)/10.86(3.08)	6.81(−12.76)/2.29(2.98)/–12.71(8.54)
800/303	7.38(5.56)/6.56(6.84)/6.96(5.69)
800/313	7.35(5.45)/6.59(6.96)/6.79(5.77)
800/323	7.55(5.36)/6.52(6.81)/6.84(5.82)
900/293	6.13(4.69)/4.99(5.26)/5.98(4.85)	–5.21(3.81)/2.91(−5.16)/4.63(1.68)	38.79(3.07)/7.06(35.73)/4.62(22.30)
900/303	6.56(4.77)/5.02(5.73)/6.05(5.08)
900/313	6.97(4.75)/5.10(6.04)/6.06(5.31)
900/323	7.28(4.80)/5.19(6.34)/6.13(5.52)
1000/293	4.82(3.84)/4.16(4.35)/4.65(4.01)	–9.49(−0.65)/2.35(−5.07)/–2.72(2.34)	49.18(15.35)/6.17(32.32)/25.14(21.68)
1000/303	5.55(4.00)/4.22(4.78)/4.89(4.23)
1000/313	5.97(4.16)/4.28(5.06)/5.14(4.44)
1000/323	6.30(4.29)/4.34(5.33)/5.41(4.65)

Swelling and pH Reversibility of GGTI-g-TetraPs

The swelling studies of hydrogels were performed in solutions of pH_i = 2, 4, 7, 9, and 12 at 313 K (Figures S11a,b and S12a,b). The gradual increment of ESR was observed from pH_i = 2 to 9 because of the increasing electrostatic repulsion between the relatively higher population of −COO^– in the macromolecular network. Therefore, in acidic pH_i, shrinking and associated poor swelling were observed because of the predominant population of −COOH. The maximum swelling was observed at pH_i = 9 for both TetraPs and GGTI-g-TetraPs. Moreover, at very high pH_i, that is, pH_i = 12, instead of complete dissociation of functional groups, formation of a remarkably high concentration of counterions acted as a screening layer because of the reduction in ion–ion repulsion, and thus, lower ESR was noted. Interestingly, in spite of getting the maximum swelling at pH_i = 9, M(II) adsorption was carried out at pH_i = 7 for avoiding the precipitation of M(OH)₂. The higher ESR of GGTI-g-TetraPs than TetraPs was attributed to the incorporation of GGTI bearing −COOH/–COO^– and O–H/O^–. The swelling experiments were also carried out at 293, 303, 313, 323, and 333 K at pH_i = 7 (Figures S11c,d and S12c,d), and ESR was found to increase with the rise in temperature. The pH reversibility of both TetraPs and GGTI-g-TetraPs was understood via performing repeating swelling–deswelling studies at pH_i = 9 and pH_i = 2 (Figure 4a). A 0.01 g xerogel was first dipped into pH_i = 9 to attain the equilibrium, followed by immersing into pH_i = 2 for 1 h to effect deswelling. Such swelling–deswelling studies were continued for several cycles. Notably, TetraPs became fragile after three complete cycles, whereas GGTI-g-TetraPs exhibited the retention of network even up to five complete cycles (Figure 4a).

Figure 4.

(a) pH reversibility swelling and (b) reusability of TetraPs and GGTI-g-TetraPs.

Calculation of % GC, % GR, % −COOH, and pH_PZC of GGTI-g-TetraPs

The % GC, % GR, % −COOH, and pH_PZC of GGTI-g-TetraPs were estimated using the methods reported elsewhere.¹⁻³ However, the values were found to be 78.89/76.32%, 3.07/3.12%, 9.23/5.54%, and 6.38/5.88 (Figure S13), respectively.

Desorption and Reusability

The recyclability of GGTI-g-TetraPs was studied by carrying out repetitive adsorption/desorption studies at pH_i = 9/2 (Figure 4b). The extent of desorption was approximately 90% for Sr(II)-/Cu(II)-GGTI-g-TetraPs, whereas the extent of desorption was within 80–85% for Hg(II)-GGTI-g-TetraPs. The excellent reusability of GGTI-g-TetraPs could be explained by observing brilliant ACs in the successive rates of adsorption even after five cycles.

Comparison of the Results

Several low-cost natural, physically and/or chemically modified micro-/nanomaterials, blends, homo-/co-/terpolymers, IPN, and composite hydrogels have been employed for the adsorptive remediation of Sr(II), Hg(II), and Sb(III) at varying initial concentrations (i.e., 5–3800 ppm), temperatures (i.e., 293–323 K), and pH_i (i.e., 1.0–10.6) (Table S3). According to Table S3, the maximum reported ACs were 303, 192.50, and 70.92 mg g^–1 for Sr(II), Hg(II), and Sb(II), respectively. However, the maximum ACs of GGTI-g-TetraPs are greater than 1700 mg g^–1, in spite of the identical experimental conditions (Table S3), which are significantly larger than the previously reported adsorbents. Such marked enhancement of ACs has been called as superadsorption.

Conclusions

The present work describes the unusual development of a series of versatile and sustainable GGTI-g-TetraP-based superadsorbents with a tetrapolymeric network constituting of in situ generated NMPAP and AMP moieties by N–H and C–H activations, along with the formation of CH₂–O–CH₂ linkage via grafting of GGTI onto TetraP. The inclusion of NMPAP in the TetraP was confirmed from the arrival of N–CH₂ specific ¹H/¹³C NMR peaks at 3.49–3.53/39.75 ppm, along with the appearance of the shake-up satellite band of −CH₂(C_q)CON((C_q)(Me₂)−)CH₂(C_q)COOH. In situ generation of AMP, through the conversion of the tertiary carbon of −CH(CH₃)₂ into the quaternary carbon of −(C_q)(CH₃)₂–, was comprehensively ascertained from the quaternary carbon specific ¹³C NMR peaks at 46.15 and 52.03 ppm in GGTI-g-TetraP and subsequent obsolescence of these peaks in DEPT-135. Extensive deposition of Sr(NO₃)₂, SrCO₃, and Sr carboxylate-type crystals onto the Sr(II)-GGTI-g-TetraP1 surface was established from the appearance of distinct XRD peaks, also substantiated from the respective FESEM and abnormally high residue content (i.e., 44.29 wt %) in TGA, whereas formation of malachite-type crystals on Cu(II)-GGTI-g-TetraP1 was detected from the characteristic XRD peaks and FTIR. The introduction of this new pathway can be adopted for the synthesis of NMPAP and AMP-based tetrapolymers, without orthodox ex situ addition of NMPAP, AMP, or derivatives. GGTI-g-TetraP, showing excellent recyclability, performance characteristics, and outstanding adsorption efficiency, has shown the novelty in a kinetically fast decontamination process.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b01218.

• Characterization of GGTI-g-TetraP1; optimization scheme; FTIR of GGTI, TetraP1, and GGTI-g-TetraP1 and Sr(II)-/Hg(II)-/Cu(II)-GGTI-g-TetraP1; FTIR analyses of GGTI-g-TetraP1 and Sr(II)-/Hg(II)-/Cu(II)-GGTI-g-TetraP1; different modes of interactions; H NMR of GGTI; bond lengths of Sr(CO₃)₂; ¹H NMR of TetraP1; ¹³C NMR of TetraP1 and DEPT-135 of GGTI-g-TetraP1; TGA of GGTI-g-TetraPs, Sr(II)-GGTI-g-TetraPs, Hg(II)-GGTI-g-TetraPs, and Cu(II)-GGTI-g-TetraPs; DSC of GGTI-g-TetraP1 and Sr(II)-/Hg(II)-/Cu(II)-GGTI-g-TetraP1 and GGTI-g-TetraP2 and Sr(II)-/Hg(II)-/Cu(II)-GGTI-g-TetraP2; XRD of TetraP1, TetraP2, GGTI-g-TetraP1, and GGTI-g-TetraP2, Hg(II)-/Cu(II)-GGTI-g-TetraPs, Sr(II)-GGTI-g-TetraP1, and Sr(II)-GGTI-g-TetraP2; FESEM of Sr(II)-, Hg(II)-, and Cu(II)-GGTI-g-TetraP1 and EDX of Sr(II)-, Hg(II)-, and Cu(II)-GGTI-g-TetraP1 in the insets; rheological analysis of GGTI-g-TetraP1; adsorption isotherm, kinetics and thermodynamics models; Langmuir fitting of Sr(II)-/Hg(II)-/Cu(II)-GGTI-g-TetraP1 and Sr(II)-/Hg(II)/Cu(II)-GGTI-g-TetraP2 and pseudo-second-order fitting of Sr(II)-/Hg(II)-/Cu(II)-GGTI-g-TetraP1 and Sr(II)-/Hg(II)-/Cu(II)-GGTI-g-TetraP2; ln k_d vs 1/T plots of Sr(II)-/Hg(II)-/Cu(II)-GGTI-g-TetraP1 and Sr(II)-/Hg(II)-/Cu(II)-GGTI-g-TetraP2; swelling of TetraP1 and GGTI-g-TetraP1 at different pH_i and different temperatures; swelling of TetraP2 and GGTI-g-TetraP2 at different pH_i and different temperatures; comparison table; and pH_PZC of GGTI-g-TetraPs (PDF)

The authors declare no competing financial interest.