An MXene-Grafted Terpolymer Hydrogel for Adsorptive Immobilization of Toxic Pb(II) and Post-Adsorption Application of Metal Ion Hydrogel

Toxic metal ions present in industrial waste, such as Pb(II), introduce deleterious effects on the environment. Though the adsorptive removal of Pb(II) is widely reported, there is a dearth of research on the suitable utilization and disposal of the Pb(II)-adsorbed adsorbent. In this work, an MXene-grafted terpolymer (MXTP) hydrogel has been designed for the adsorption of Pb(II) under ambient conditions of pH and temperature. The hydrogel MXTP was synthesized by facile one-pot polymerization in aqueous solvent, and the detailed structural characterization of terpolymer (TP), MXTP, and Pb(II)-loaded MXTP, i.e., Pb(II)-MXTP, was carried out by a combination of proton nuclear magnetic resonance (1H NMR), Fourier-transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffractometric (XRD), thermogravimetric/differential thermogravimetric (TG/ DTG), and field emission scanning electron microscopic (FESEM) analyses. The specific capacitance and conductivities of Pb(II)-MXTP were studied with cyclic voltammetry (CV) and electrical impedance spectroscopy (EIS), which unambiguously indicate successful post-adsorption application. The specific capacitance of MXTP decreased after Pb(II) adsorption, whereas the conductivity increased significantly after Pb(II) adsorption, showing that MXTP can be successfully deployed as a solid electrolyte/anode after Pb(II) adsorption. This study covers the synthesis of a novel MXene-grafted terpolymer hydrogel for adsorptive exclusion of Pb(II) and assessment of the as-adsorbed Pb(II)-loaded hydrogel as a solid electrolyte/anode material and is the first demonstration of such post-adsorptive application.


Introduction
Dissolved metal ions in effluents of foundation industries, such as mining, metallurgy, steel, coal-based power, glass, electroplating and dyes and pigments carry severe environmental risks, especially when released into natural water resources without suitable treatment [1,2].Lead (Pb) is considered the second most toxic metal and is highly noxious and non-disintegrative while comprising only 0.002% of the Earth's crust [3,4].Pb has no known biological function and is hazardous for plants, animals, and human beings.Pb is toxic to plants beyond 30 mg/kg of soil concentration [4].In plants, Pb strongly inhibits seed germination and development, root elongation, plant growth, transpiration, chlorophyll production, and water/protein content [3][4][5].In human beings, Pb exerts detrimental effects on the brain, kidney, liver, and central nervous system, along Gels 2023, 9, 827 2 of 20 with anemia, dizziness, headache, irritability, and infertility [6].Therefore, research for removing Pb(II) via easy-to-use, effective, and sustainable technologies is of immediate environmental, societal and public health relevance.The commonly adopted techniques for Pb(II) removal include reverse osmosis, reduction, solvent extraction, coagulation, ion exchange, chemical precipitation, membrane-based separation, and adsorption [7,8].Of these, adsorption is extensively accepted as an efficient, cost-effective and easy-to-deploy method, whose operational cost can be controlled via the reusability of adsorbents [9][10][11].Hydrogels are a distinctive class of polymeric materials that exhibit variegated swelling and can be engineered with chemical functionalities to facilitate the adsorption of specific analytes [12].Accordingly, hydrogels are capable of detoxifying industrial effluents [13].
Poly[acrylic acid], i.e., PAA, is a low-cost biocompatible super-adsorbing pH-responsive anionic polymer, extensively studied for preparing pH-sensitive hydrogels [14][15][16][17][18].While the presence of easily ionizable -COOH functionalities of PAA make them pH-responsive, the higher water solubility, poor temperature responsiveness, and non-tunable mechanical properties limit their direct employability.Again, poly[2-hydroxyethyl methacrylate] (PHEMA)-based hydrogels are biocompatible, stable and possess high swelling abilities and tunable mechanical properties [19].The pendant O-H functionalities of PHEMA hydrogels serve as suitable sites for the adsorption of polar adsorbates, such as metal ions [20].Therefore, the copolymer of AA and HEMA, i.e., poly[AA-co-HEMA], should possess tunable mechanical properties accompanied by high pH responsiveness and adsorption capacity (AC) attributed to -COOH and O-H functionalities.Nevertheless, the high population of hydrophilic moieties in poly[AA-co-HEMA] results in appreciable water solubility constraining the repetitive adsorption-desorption during adsorptive recovery of metal ions including Pb(II) from industrial wastewater.Thus, while poly[AA-co-HEMA] may exhibit superior sustainability in dehydrated state, its sustainability of use in contact with aqueous solutions of pollutants is questionable.This challenge has triggered the need to synthesize a polymer composite with a reinforcing agent.Two-dimensional materials have shown promise in this regard based on the sustainability of use, mechanical properties, electrical conductivity, and specific capacitance [21].
Among different 2D materials, the family of transition metal carbides commonly known as MXenes are recently discovered materials possessing tunable metallic conductivity, solution processability, structural robustness, and aspect ratios [22][23][24][25].The general chemical formula of MXene is M n+1 X n T x (n = 1-4), where M, X, and T represent early transition metals (such as Zr, Ti, V, Ta, Nb, and Mo), carbon and/or nitrogen, and pendent termination functional groups (such as O-H and F), respectively [26][27][28].Among the discovered MXenes, Ti 3 C 2 T x is mostly studied to date [29].The two-dimensional MXene is generally synthesized by exfoliating the "A" layer from MAX-phase via HF-aided etching reaction at high temperatures.In MAX, the letter "A" stands for the interlaying Al 3+ layers, which are eliminated during etching [30].MAX phases can be denoted as alternately stacked "MX" layers and close-packed "A" atomic layers, represented by the general formula of M n+1 AX n , where M-X (ionic/covalent) bonds are more stable than those of M-A (metallic) bonds [16].MXene-incorporated polymeric hydrogels have been reported with versatile properties including photo-redox catalysis, photothermal behavior, and sensing [26].For instance, MXene/poly[AM-co-LMA]/PNIPA hydrogel, a nanomaterial containing a double network hydrogel, has been reported to exhibit attractive conductivity and capacitance [30].Again, MXene/poly[AM-co-PVA] hydrogel has been used to synthesize antifreeze material, which can be stored at −40 • C [31].An MXene/PAM hydrogel has been studied for the delivery of antimicrobial chloramphenicol drugs [32].
In this work, we have used an MXene-grafted hydrogel for the adsorption and reuse of Pb(II).Compared to adsorption itself, the post-adsorptive use of Pb(II) is not as widely reported.Desorption of the Pb(II) followed by recycling or reuse of the adsorbent is reported albeit for 5-6 cycles.The subsequent disposal of the adsorbent in landfills, often bearing non-trivial amounts of Pb(II), is fraught with risks.To solve this problem, we have tested a Pb(II)-adsorbed MXene-grafted hydrogel as a prospective solid electrolyte/anode material.The design of our MXene-grafted poly[AA-co-HEMA], i.e., MXTP, is based on the need to introduce conducting properties in the resulting adsorbent.The specific capacitance and conductivity of Pb(II)-MXTP highlight the hitherto unreported class of MXene-grafted hydrogels as promising Pb(II) adsorbents for sustainable post-adsorptive applications.

FTIR Analysis
In FTIR of MAX (Figure S1 and Table 1), peaks at 497/420 and 463 cm −1 could be related to Al-C and Ti-C bonds, respectively [34].Disappearance of Al-C bond-specific peaks after etching and simultaneous arrival of new peaks at 3455, 1350, and 528 cm −1 , attributed to O-H str., C-F band, and Ti-O band, respectively, envisaged the elimination of Al 3+ planes from MAX and formation of O-H and F − terminal functionalities in MXene [35][36][37][38].The presence of AA and HEMA in TP and MXTP could be justified from hydrogen-bonded >C=O str. of -COOH, >C=O asym.str. of -COO − , and -CH 2 -def.band of -CH 2 -CO-Ogroup of ester at 1703, 1554, and 1396 cm −1 in TP and 1675, 1540, and 1390 cm −1 (Figure 2a) in MXTP, respectively [36].The shifting in >C=O asym.str. of -COO − of TP after MXene incorporation could be related to the possible coordination of -COO − with Ti(IV) of MXene.The terminal O-H functionalities of MXene provided additional hydrogen bonding possibilities and hence stronger hydrogen bonding could be envisaged through a broader O-H str.peak in MXTP.The O-C coupled incorporation of the third comonomer, i.e., MBOEPA, was envisaged from C-O-C asym.str.and sym.str.peaks at 1186/1182 and 1111/1081 cm −1 in TP/MXTP, respectively [19,36].Such an observation was observed earlier from 1 H NMR analysis, in which the peak of -CH 2 CH(CH 3 )COOCH 2 CH 2 OCH 2 CH 2 COOH at 3.77 ppm in TP denoted the in situ O-C coupled incorporation of MBOEPA.During Pb(II) adsorption at pH > pH PZC (pH PZC = 5.93, Figure S2), considerable deprotonation of -COOH of AA and O-H of MXene occurred, resulting in a significant reduction in hydrogen bonding.Two distinct O-H str.peaks at 3341 and 3185 cm −1 were observed in Pb(II)-MXTP attributed to the O-H str. of terminal O-H and -COOH, respectively [35,39].The shifting of >C=O asym.str. of -COO − could be related to the coordination of -COO − with Pb(II) through various modes.

FTIR Analysis
In FTIR of MAX (Figure S1 and Table 1), peaks at 497/420 and 463 cm −1 could be related to Al-C and Ti-C bonds, respectively [34].Disappearance of Al-C bond-specific peaks after etching and simultaneous arrival of new peaks at 3455, 1350, and 528 cm −1 , attributed to O-H str., C-F band, and Ti-O band, respectively, envisaged the elimination of Al 3+ planes from MAX and formation of O-H and F -terminal functionalities in MXene [35][36][37][38].The presence of AA and HEMA in TP and MXTP could be justified from hydrogen-bonded >C=O str. of -COOH, >C=O asym.str. of -COO -, and -CH2-def.band of -CH2-CO-O-group of ester at 1703, 1554, and 1396 cm −1 in TP and 1675, 1540, and 1390 cm −1 (Figure 2a) in MXTP, respectively [36].The shifting in >C=O asym.str. of -COO -of TP after MXene incorporation could be related to the possible coordination of -COO -with Ti(IV) of MXene.The terminal O-H functionalities of MXene provided additional hydrogen bonding possibilities and hence stronger hydrogen bonding could be envisaged through a broader O-H str.peak in MXTP.The O-C coupled incorporation of the third comonomer, i.e., MBOEPA, was envisaged from C-O-C asym.str.and sym.str.peaks at 1186/1182 and 1111/1081 cm −1 in TP/MXTP, respectively [19,36].Such an observation was observed earlier     S3) closely resembled the standard JCPDS file No. 52-0875 suggesting the prevalent hexagonal MAX phases [41].After etching with (CaF 2 + HCl), the obsolescence of the peak at 2θ = 38.93• (Figure S3) in MXene indicated the removal of Al 3+ -planes from MAX [41], observed earlier from the vanishing of Al-C specific FTIR peaks at 497/420 cm −1 , respectively.Again, peaks at 2θ = 18.17, 27.37 and 61.12 • in MXene represented the crystalline planes of Ti 3 C 2 [41].Notably, the broadening of XRD peaks in MXene compared to those of MAX indicates the prevalence of exfoliation.
In the XRD profile of TP, peaks at 2θ = 14.95 and 22.07 • (Figure 2b) were attributed to properly organized polymer planes by virtue of hydrogen bonding.Notably, the arrival of the sharp MXene-specific peak at 2θ = 10.90 • reflected the introduction of MXene within the matrix of MXTP.The hydrogen-bonded orderly arrangement of polymer chains could also be observed in MXTP.Notably, the prevalence of hydrogen bonding in TP and MXTP was already described in FTIR analysis.After Pb(II) adsorption, the change in the environment of -COOH and -OH resulted in the shifting of the peak from 2θ = 22.59 of MXTP to 23.32 • in Pb(II)-MXTP.Surprisingly, the absence of Pb(II)-specific sharp peaks in the XRD spectrum of Pb(II)-MXTP indicated the superficial depositions of non-crystalline Pb(II) complexes.[42].The spin-orbit coupling of Ti2p 3/2 and Ti2p 1/2 peaks varied within 5.72-6.04eV.The presence of O-H and F − terminal groups in MXene could be envisaged from this fitting pattern, which was also deduced earlier from FTIR analysis.In MXTP, the presence of Ti could be inferred from the appearance of a Ti-specific peak at BE ~450 eV in the wide-scan survey plot of MXTP (Figure S4).Importantly, no such peak is present in the wide-scan survey plot of TP (Figure S4), which also supported that the peak ~450 eV in the survey plot of MXTP originated from Ti.After internalization within the matrix of TP, the Ti2p 3/2 /Ti2p 1/2 peaks shifted to 450.91/455.93 and 451.89/458.08 eV (Figure 3b), respectively [42].The shifting of Ti2p peaks towards the lower BEs indicated the coordinative attachment of Ti(IV) with TP during MXTP formation.The adsorption of Pb(II) could be justified by the presence of a Pb-specific peak at ~140 eV in the wide-scan survey plot of Pb(II)-MXTP (Figure 3e).No peak was present in the survey plots of TP and MXTP in this range (Figure S4).In fact, XPS played an important role in justifying the bonding pattern of Pb(II) during its adsorption onto MXTP.After Pb(II) adsorption at pH > pHPZC, the O1s BE of -COO -increased significantly from  In MXTP, the deconvoluted C1s spectrum was composed of four peaks at BE = 283.99,284.48, 285.60, and 287.38 eV (Figure 3c) because of metal carbide [43], -CH 2 -/-CH< [40,44], -C-O-C [11,20], and -COOH/-COO − [6,40,45,46], respectively.Of these, the metal carbide peak indicated incorporation of MXene in MXTP, whereas the BE for the C-O-C moiety substantiated the in situ O-C coupled formation of the new comonomer, i.e., MBOEPA, inferred earlier from -CH 2 CH(CH 3 )COOCH 2 CH 2 OCH 2 CH 2 COOH peak at 3.77 ppm of 1 H NMR and C-O-C asym./sym.str.peaks at 1182/1081 cm −1 of FTIR analyses.The deconvoluted O1s spectrum of MXTP possessed four peaks at BEs = 530.36,531.09, 532.06, and 532.88 eV (Figure 3d), attributed to -C(=O)-OH [9], -COO − [11,45], O-H termination group of MXene [47], and -C(=O)-OH [11,48,49], respectively.

XPS Analysis
The adsorption of Pb(II) could be justified by the presence of a Pb-specific peak at ~140 eV in the wide-scan survey plot of Pb(II)-MXTP (Figure 3e).No peak was present in the survey plots of TP and MXTP in this range (Figure S4).In fact, XPS played an important role in justifying the bonding pattern of Pb(II) during its adsorption onto MXTP.After Pb(II) adsorption at pH > pH PZC , the O1s BE of -COO − increased significantly from 531.09 to 532.37 eV (Figure 3f) indicating the existence of coordinate bonding between -COO − of MXTP and Pb(II) during adsorption, observed earlier from the shifting of >C=O asym.str. of -COO − from 1540 cm −1 of MXTP to 1535 cm −1 in Pb(II)-MXTP.This phenomenon was further supported by the decrease in Pb4f BEs of Pb4f 5/2 and Pb4f 7/2 from 143.80 and 139.30eV of Pb(NO 3 ) 2 to 143.16 and 138.25 eV, respectively (Figure 3g), in Pb(II)-MXTP [1,6].Again, Pb4f 5/2 and Pb4f 7/2 BEs at 144.35 and 139.66 eV in Pb(II)-MXTP envisaged the weaker interaction between Pb(II) and MXTP via the ion exchange mechanism [1].

TGA Explanation
From TG analysis, the faster rate of thermal degradation of TP than that of MXTP up to 120 • C (Figure 4a) could be related to the weaker hydrogen bonding in TP, observed earlier from FTIR and XRD analyses.Such an observation could be further justified by DTG peaks around 74 • C having degradation rates of 2.13 and 0.87 wt.% min −1 for TP and MXTP, respectively (Figure 4b).The presence of terminal O-H and F − functionalities in MXTP resulted in such enhanced hydrogen bonding.In the next major degradation range within 250-483 • C, the weight loss was associated with the evaporation of H 2 O and CO 2 because of the formation of acid into anhydride and its subsequent decomposition, respectively [50].In this degradation zone, the improved thermostability of MXTP compared to TP might be related to coordinate bonding between Ti(IV) and -COO − of TP.The enhanced thermostability of MXTP compared to that of TP beyond 517 • C indicated the reinforcement of the polymer backbone by MXene.Finally, the presence of thermostable inorganic Ti(IV) ions of MXene in MXTP resulted in a significantly higher residue amount (19.03 wt.%) compared to 12.02 wt.% of TP.After adsorption of Pb(II) at pH > pHPZC, the significant reduction in hydrogen bonding as reported from FTIR analysis would be expected to intensify the removal rate of vapor, resulting in the more rapid thermal degradation of Pb(II)-MXTP compared to MXTP.This expected behavior could be confirmed by the degradation rate of 4.69 wt.% min −1 of Pb(II)-MXTP compared to 2.13 wt.% min −1 of MXTP (Figure 4b).However, almost similar thermal stability was observed for Pb(II)-MXTP and MXTP up to 111 °C.Importantly, the improved thermostability of Pb(II)-MXTP compared to that of MXTP within 111-349 °C can be related to the formation of a Pb(II)-acetate type complex in Pb(II)-MXTP, which restricts acid to anhydride formation and its decomposition [1,2].Formation of coordinate bonding within Pb(II) and -COO -during adsorption has already been established from shifting of O1s of -COO -, Pb4f7/2, and Pb4f5/2 Bes from 531.09, 143.80, and 139.30eV of MXTP to 532.37, 143.28, and 138.49eV in Pb(II)-MXTP (Figure 3) in XPS as well as from shifting of >C=O asym.Str.Of -COO -from 1540 cm −1 of MXTP to 1535 cm −1 in Pb(II)-MXTP in FTIR.Moreover, the Pb(II)-acetate type complex contained crystal water, which could not be eliminated within 120 °C [51].Therefore, the expected initial poor thermostability originating from the lack of hydrogen bonding was made up by the retention of crystal water within the Pb(II)-acetate type complex in Pb(II)-MXTP.For MXTP, three distinct DTG peaks at 212, 286, and 314 °C were observed as having rates of mass loss of 1.74, 1.92, and 2.32 wt.% min −1 , respectively, within 111-349 °C (Figure 4b).However, the mere DTG peak at 262 °C with the rate of mass loss of 2.04 wt.% min −1 in Pb(II)-MXTP justified the elevated thermostability of Pb(II)-MXTP compared to that of MXTP within this temperature range.Nevertheless, the sharp weight loss (47.02 wt.%) within 350-388 °C and the corresponding sharp DTG peak at 361 °C (rate of mass loss = 56.14wt.% min −1 ) in Pb(II)-MXTP indicated the degradation of Pb(II)-acetate [52].In fact, the steady decomposition of Pb(II)-MXTP up to 800 °C was attributed to the multistage After adsorption of Pb(II) at pH > pH PZC , the significant reduction in hydrogen bonding as reported from FTIR analysis would be expected to intensify the removal rate of vapor, resulting in the more rapid thermal degradation of Pb(II)-MXTP compared to MXTP.This expected behavior could be confirmed by the degradation rate of 4.69 wt.% min −1 of Pb(II)-MXTP compared to 2.13 wt.% min −1 of MXTP (Figure 4b).However, almost similar thermal stability was observed for Pb(II)-MXTP and MXTP up to 111 • C. Importantly, the improved thermostability of Pb(II)-MXTP compared to that of MXTP within 111-349 • C can be related to the formation of a Pb(II)-acetate type complex in Pb(II)-MXTP, which restricts acid to anhydride formation and its decomposition [1,2].Formation of coordinate bonding within Pb(II) and -COO − during adsorption has already been established from shifting of O1s of -COO − , Pb4f 7/2 , and Pb4f 5/2 Bes from 531.09, 143.80, and 139.30eV of MXTP to 532.37, 143.28, and 138.49eV in Pb(II)-MXTP (Figure 3) in XPS as well as from shifting of >C=O asym.Str.Of -COO − from 1540 cm −1 of MXTP to 1535 cm −1 in Pb(II)-MXTP in FTIR.Moreover, the Pb(II)-acetate type complex contained crystal water, which could not be eliminated within 120 • C [51].Therefore, the expected initial poor thermostability originating from the lack of hydrogen bonding was made up by the retention of crystal water within the Pb(II)-acetate type complex in Pb(II)-MXTP.For MXTP, three distinct DTG peaks at 212, 286, and 314 • C were observed as having rates of mass loss of 1.74, 1.92, and 2.32 wt.% min −1 , respectively, within 111-349 • C (Figure 4b).However, the mere DTG peak at 262 • C with the rate of mass loss of 2.04 wt.% min −1 in Pb(II)-MXTP justified the elevated thermostability of Pb(II)-MXTP compared to that of MXTP within this temperature range.Nevertheless, the sharp weight loss (47.02 wt.%) within 350-388 • C and the corresponding sharp DTG peak at 361 • C (rate of mass loss = 56.14wt.% min −1 ) in Pb(II)-MXTP indicated the degradation of Pb(II)-acetate [52].In fact, the steady decomposition of Pb(II)-MXTP up to 800 • C was attributed to the multistage decomposition of Pb(II)-acetate to generate PbO [51].

FESEM Assessment
The etching of MAX (Figure 5a) by CaF 2 + HCl resulted in the formation of tightly bound parallel planes, which are clearly visible in the FESEM photomicrograph of MXene (Figure 5b).Distinct alternative planes of Ti(IV) and carbides were generated via etching with in situ generated HF.In TP, featureless dense morphology without the distinct phase boundary could be visualized (Figure 5c), because of the C-C/O-C coupled covalent attachment of monomers.However, in MXTP (Figure 5d), the characteristic comb-like structure was visualized, suggesting the internalization of MXene with TP.The protruded plane-like appearance of MXene could be observed in the zoomed morphology of MXTP (Figure 5e).The superficial deposition of irregularly shaped Pb(II)-acetate complexes could be visualized in the photomicrograph Pb(II)-MXTP (Figure 5f).The existence of such irregular/non-crystalline shapes of Pb(II)-acetate complexes might be the reason for the absence of sharp peaks in the XRD of Pb(II)-MXTP.

Adsorption of Pb(II) by MXTP
The equilibrium adsorption data were fitted to different isotherm models, such as Langmuir, Henry, Freundlich, Sips, and BET [1,42], of which the Langmuir model displayed the best fit due to the highest adjusted R 2 /F and lowest χ 2 values [6,9].The maximum AC, i.e., q max , varied as 175.36, 162.44, 154.21, and 145.41 mg g −1 at 298, 303, 308, and 313 K (Figure 6a and Table 3), respectively.Since the Langmuir model depends on the presumption of monolayer adsorption only, the adsorption of Pb(II) onto the surface of MXTP was also assumed to be monolayered [1,2].Again, the better fitting of kinetics data with the pseudo-second-order kinetics model compared to fitting with the pseudo-firstorder kinetics model (Figure 6b), and attainment of the activation energy of 66.54 kJ mol −1 (Figure 6d), indicated chemical bonding behind the adsorption of Pb(II) by MXTP.Pseudosecond-order rate constants, i.e., k 2 , followed an increasing trend with the increase in temperature because of the faster adsorption at elevated temperatures.The exothermic and spontaneous nature of chemisorption is justified by the negative values of ∆H 0 and ∆G 0 at the entire working temperature (Figure 6c and Table 4).ture was visualized, suggesting the internalization of MXene with TP.The protrud plane-like appearance of MXene could be observed in the zoomed morphology of MX (Figure 5e).The superficial deposition of irregularly shaped Pb(II)-acetate complex could be visualized in the photomicrograph Pb(II)-MXTP (Figure 5f).The existence such irregular/non-crystalline shapes of Pb(II)-acetate complexes might be the reason the absence of sharp peaks in the XRD of Pb(II)-MXTP.presumption of monolayer adsorption only, the adsorption of Pb(II) onto the surface of MXTP was also assumed to be monolayered [1,2].Again, the better fitting of kinetics data with the pseudo-second-order kinetics model compared to fitting with the pseudo-firstorder kinetics model (Figure 6b), and attainment of the activation energy of 66.54 kJ mol −1 (Figure 6d), indicated chemical bonding behind the adsorption of Pb(II) by MXTP.
Pseudo-second-order rate constants, i.e., k2, followed an increasing trend with the increase in temperature because of the faster adsorption at elevated temperatures.The exothermic and spontaneous nature of chemisorption is justified by the negative values of ΔH 0 and ΔG 0 at the entire working temperature (Figure 6c and Table 4).

Comparison of the Results
A brief literature study for the adsorption of Pb(II) by using various low-cost polymer materials, such as nanoparticles, clays, zeolites, chemically modified inorganic materials, and agricultural wastes, at varying initial concentrations (i.e., 5-1000), temperatures (i.e., 295-303 K), and pH o (i.e., 4.0-7.0) is detailed in Table 5.From this study, the maximum adsorption capacity of MXTP can be concluded as the highest among those of the previously reported adsorbents.pectin-g-(sodium acrylic acid-co-N-isopropylacrylamide), b guar gum-g-(acrylic acid-co-acrylamide-co-3acrylamido propanoic acid), c iron-coated zeolite, d aminated polyacrylonitrile, e iron oxide, f MXene-grafted poly[acrylic acid-co-2-hydroxyethyl methacrylate], and ˆpresent study.

Desorption of Pb(II) from Pb(II)-MXTP and Reusability Studies of MXTP
The primary driving force of the adsorption of Pb(II) by MXTP is coordinate bonding between -COO − /-O − of MXTP and Pb(II).Therefore, controlling the pH of the solution should be the key factor in desorbing the already adsorbed Pb(II).Actually, at pH < pH PZC , the electrostatic repulsion between positively charged MXTP and adsorbed Pb(II) would cause the detachment of Pb(II).Therefore, desorption was tried at pH 2, leading to ~95% desorption.Thereafter, MXTP was applied further for adsorption at pH 7, and again, desorption was tried at pH 2. Such iterative adsorption-desorption experiments were conducted to confirm the reusability of MXTP.Remarkably, MXTP achieved more than 92% adsorption even after the fifth adsorption-desorption cycle.

Capacitance Studies from CV Experiments
CV experiments of MXTP and Pb(II)-MXTP were carried out to study the effect of adsorbed Pb(II) on the specific capacitance of MXTP.From cyclic voltammograms of both MXTP and Pb(II)-MXTP (Figure 7a,b), current densities were found to increase with the increase in scanning rates from 5 to 100 mV sec −1 .However, since C p is inversely proportional to scanning rates (Text S1), C p values were found to decrease significantly with the increasing scanning rates from 5 to 100 mV sec −1 .The C p values of MXTP were found to be 1844.65,1579.02,1577.87,1392.79, and 760.88 F g −1 at 5, 10, 15, 25, 100 mV sec −1 , respectively, which are significantly higher than the other MXene-based polymers.Therefore, the as-synthesized MXene-grafted polymer system can be successfully employed as a supercapacitor.After Pb(II) adsorption, C p values were found to decrease, suggesting the increase in ion conductivity due to charge transfer (i.e., coordinative attachment)-type bonding of Pb(II) with MXTP.The variation in C p values of MXTP and Pb(II)-MXTP with the increasing scan rate is depicted in Figure 7c.

Conductance of MXTP and Pb(II)-MXTP
In order to confirm the increase in conductivity of Pb(II)-MXTP, EIS experiments of MXTP and Pb(II)-MXTP were carried out within the frequency range of 0.1-100,000 Hz. Figure 8a shows the Bode plot of MXTP and Pb(II)-MXTP.From the Nyquist plot (Figure 8b), the initial point of the semicircle, known as solution resistance (Rs, ohm), of MXTP and Pb(II)-MXTP were measured to be 49.37 and 28.76 ohm, respectively.By considering the area and length of GSE to be 7.10 × 10 −6 m 2 and 0.08 m, respectively, the conductivity values of MXTP and Pb(II)-MXTP were 228.23 and 391.78 S m −1 , respectively (Figure 8c).

Conductance of MXTP and Pb(II)-MXTP
In order to confirm the increase in conductivity of Pb(II)-MXTP, EIS experiments of MXTP and Pb(II)-MXTP were carried out within the frequency range of 0.1-100,000 Hz. Figure 8a shows the Bode plot of MXTP and Pb(II)-MXTP.From the Nyquist plot (Figure 8b), the initial point of the semicircle, known as solution resistance (Rs, ohm), of MXTP and Pb(II)-MXTP were measured to be 49.37 and 28.76 ohm, respectively.By considering the area and length of GSE to be 7.10 × 10 −6 m 2 and 0.08 m, respectively, the conductivity values of MXTP and Pb(II)-MXTP were 228.23 and 391.78 S m −1 , respectively (Figure 8c).The CV and EIS experiments indicate that MXTP possesses very high specific capacitance at room temperature, whereas after Pb(II) adsorption, MXTP transforms into a conducting material.The ability of Pb(II)-MXTP to behave as a conducting polymer clearly indicates the successful post-adsorptive application of Pb(II) on the MXene-grafted hydrogel.

Conclusions
In this work, we have developed a scalable and reusable multifunctional MXenegrafted terpolymer hydrogel for the adsorptive application of Pb(II).The hydrogel is prepared by polymerizing hydrophilic monomers, such as AA and HEMA, in the presence of MXene in its aqueous phase.Optimization of the synthesis parameters has been carried out by synthesizing a series of hydrogels with strategically varied compositions and temperatures.The as-obtained hydrogel has been characterized in detail by 1 H NMR, FTIR, XPS, XRD, TGA, DTG, and FESEM.The formation of MXene from the MAX phase, without using the conventional HF etching agent, has been confirmed from XRD, FTIR, and FESEM analyses.A strong coordinative association of MXene with TP is evident from XPS.Moreover, stronger coordinate bonding and weaker ion-exchange-type interactions of Pb(II) with -COO − /-O − of MXTP have been indicated by FTIR, XPS and XRD analyses and manifested in the fitting of Langmuir and pseudo-second-order models.After adsorption, the Pb(II)-MXTP hydrogel has been tested for desorption and re-adsorption, which clearly points towards the recyclability of the adsorbent.In addition, the Pb(II)-MXTP hydrogel exhibits an attractive balance between specific capacitance and conductivity.There is a dearth of reports on the direct post-adsorptive application of Pb(II)-adsorbed material.To the best of our knowledge, the present work represents the first demonstration of such postadsorptive application of Pb(II)-adsorbed material as a solid electrolyte/anode material.Enhancements in the scope of the use of such post-adsorptive materials could be sought based on further scrutiny of the polymer components and method of preparation.These efforts are currently underway in our laboratory.

Synthesis of MXene-Grafted Terpolymer Hydrogel 4.2.1. Synthesis of Mxene from MAX
We used the unconventional etching procedure of the MAX phase to avoid the involvement of high temperature and corrosive HF, the primary etching agent.Herein, HF was generated in situ by reacting CaF 2 with concentrated HCl.In brief, 1 g dried MAX powder was taken in a double neck round bottom flask, followed by the addition of 5.2 g CaF 2 .The flask was heated to 333 K in a water bath.One neck of the flask was sealed with a N 2 balloon and 30 mL conc.HCl was added dropwise through the other neck to remove the interlaying Al layers of MAX.After 30 min of etching, solid MXene was isolated by centrifugation, washed thoroughly by DW until acid-free, and finally dried in a vacuum oven.

Synthesis of TP and MXTP
The maximum adsorption capacity in relation to the structural integrity of hydrogel was ensured by optimizing the equilibrium swelling ratio (ESR, g/g) of a series of poly[AAco-HEMA] hydrogels obtained by varying synthesis parameters.The parameters varied include AA/HEMA (-/A) within 5-20; amounts of MBA (wt.%/B),K 2 S 2 O 8 + NaHSO 3 (wt.%/C),and total monomers (wt.%/D) in the range of 1.0-2.0,1.0-2.0,and 10-30 wt.%, respectively; and temperature (K/E) within the range of 288-308 K. Notably, despite having faster gelation beyond 308 K, such hydrogels have not been considered here to minimize the environmental impact of the use of synthetic constituents.The optimized conditions of synthesis were found to be 10, 1.5 wt.%, 1.5 wt.%, 20 wt.%, and 298 K for A, B, C, D, and E, respectively (Table S1).For the synthesis of TP (Scheme 1), 27.15 mL 8.66 (M) AA was neutralized to pH = 5.5 by NaOH, followed by the addition of 2.85 mL HEMA and 10 mL 0.97 (M) MBA solution.To this solution, 40 mL water was added and homogenized for 2 h on a magnetic stirrer.The homogeneous solution was purged with N 2 gas and polymerization was initiated by the addition of redox initiators (10 mL of 0.28 M K 2 S 2 O 8 and 10 mL of 0.72 M NaHSO 3 ).Gelation was observed after 34 min and the yield was 81.4%.The as-obtained TP was collected on a Petri dish, washed with 1:3 (v/v) water/methanol solution, and dried in a hot air oven for 48 h.MXTP was synthesized by an analogous protocol by the addition of 0.5 g MXene to the mixture containing partially neutralized AA, HEMA, and MBA solutions.Subsequent polymerization and work-up were achieved as described above.
Gels 2023, 9, x FOR PEER REVIEW 16 of 21 parameters varied include AA/HEMA (-/A) within 5-20; amounts of MBA (wt.%/B),K2S2O8 + NaHSO3 (wt.%/C), and total monomers (wt.%/D) in the range of 1.0-2.0,1.0-2.0,and 10-30 wt.%, respectively; and temperature (K/E) within the range of 288-308 K. Notably, despite having faster gelation beyond 308 K, such hydrogels have not been considered here to minimize the environmental impact of the use of synthetic constituents.The optimized conditions of synthesis were found to be 10, 1.5 wt.%, 1.5 wt.%, 20 wt.%, and 298 K for A, B, C, D, and E, respectively (Table S1).For the synthesis of TP (Scheme 1), 27.15 mL 8.66 (M) AA was neutralized to pH = 5.5 by NaOH, followed by the addition of 2.85 mL HEMA and 10 mL 0.97 (M) MBA solution.To this solution, 40 mL water was added and homogenized for 2 h on a magnetic stirrer.The homogeneous solution was purged with N2 gas and polymerization was initiated by the addition of redox initiators (10 mL of 0.28 M K2S2O8 and 10 mL of 0.72 M NaHSO3).Gelation was observed after 34 min and the yield was 81.4%.The as-obtained TP was collected on a Petri dish, washed with 1:3 (v/v) water/methanol solution, and dried in a hot air oven for 48 h.MXTP was synthesized by an analogous protocol by the addition of 0.5 g MXene to the mixture containing partially neutralized AA, HEMA, and MBA solutions.Subsequent polymerization and work-up were achieved as described above.

Characterization
Structural elucidation of TP was carried out by 1 H NMR (Ascend NMR-500 MHz, Bruker; spectra acquired in DMSO-d 6 solvent by 48 scans).Again, structures of TP, MXTP, and Pb(II)-MXTP were characterized by FTIR (Spectrum-2, PerkinElmer, Singapore; spectra acquired in solid state by using diamond plate and 16 scans), XPS (Omicron ESCA, Oxford Instrument Germany; spectra acquired in solid state and binding energy calibration was carrying out by taking adventitious carbon at 284.6 eV), TG (TGA 4000, PerkinElmer; thermal analysis was performed in N 2 -atmosphere within 20-850 • C and at the scanning rate of 10 • C min −1 ), XRD (D8 Discover, Bruker; data was acquired within 2θ = 5-75 • at the scanning rate of 0.005 • min −1 ), and FESEM (JSM7600F, Jeol, Tokyo, Japan), analyses.All chemical structures and graphical presentations were generated by ChemDraw Ultra 12.0 and Origin 9.0 software, respectively.Deconvolution of XPS plots was carried out by using the Gaussian function because of the higher adj.R 2 and lowest χ 2 values compared to other functions.Baseline corrections of XPS curves were carried out by using the Tougaard function.

Estimation of pH at Point of Zero Charge (pH PZC )
In total, 0.01 g dry MXTP was dipped into 50 mL buffered solutions of initial pH (pH i ) ranging from 2 to 10.After dipping for 72 h, the pH of these buffer solutions was measured (pH f ).The change in pH, i.e., ∆pH = pH f − pH i , was plotted against pH i to obtain pH PZC .

Electrochemical Measurements
Specific capacitance and conductivity values of MXTP/Pb(II)-MXTP were estimated by CV and EIS analyses, respectively.
CV analysis.The CV experiments were carried out using a three-electrode module containing (a) a glassy silicon electrode (GSE)-the working electrode-(b) the reference electrode (RE)-Ag(s)/AgCl(aq) half-cell electrode-and (c) the counter electrode (CE)-a clean Pt wire applied for maintaining electrical neutrality of GSE.The working solution was prepared by dispersing 5 mg of samples in 5 and 40 µL of nafion and IPA, respectively, with the help of a vortex and sonicator.After drop-casting 5 µL of the as-prepared suspension on GSE, all three electrodes were dipped into 10 mL of buffer solution (pH = 7.0) and connected to a potentiostat.The current (I, A) in GSE can be expressed as [60][61][62]: Here, C p (F g −1 ), m (g), and k (V s −1 ) represent specific capacitance, mass of electroactive material (drop-casted on GSE), and scanning rate, respectively.From Equation (1), C p can be calculated by using Equation (2) (Text S1): EIS analysis.EIS experiments were carried out using a similar three-electrode module, containing GSE, RE, and CE.During EIS, alternating potential is sent towards the sample on GSE, and the corresponding alternating current (i, A) signal is recorded.From EIS, values of real impedance (Z , ohm), imaginary impedance (Z , ohm), vector sum of Z and Z (Z mag ,

1 .
93 ppm attributed to -CH 2 -and -CH 3 of HEMA confirmed the inclusion of HEMA in TP.Similarly, the presence of MBA in TP was inferred from the characteristic -CH 2 -peak of MBA at 4.43 ppm.Importantly, the O-C coupled in situ attachment of the third comonomer, i.e., (3-(2-((2-methylbutanoyl)oxy)ethoxy)propanoic acid)/MBOEPA, could be inferred from -CH 2 CH(CH 3 )COOCH 2 CH 2 OCH 2 CH 2 COOH peak at 3.77 ppm in TP [19,20].Moreover, the characteristic N-H and O-H peaks of MBA and HEMA occurred at 7.57 and 4.07 ppm in TP, respectively.

Figure 1 .
Figure 1. 1 H NMR of TP performed in DMSO-d6 solvent at 500 MHz.Figure 1. 1 H NMR of TP performed in DMSO-d 6 solvent at 500 MHz.

Figure 1 .
Figure 1. 1 H NMR of TP performed in DMSO-d6 solvent at 500 MHz.Figure 1. 1 H NMR of TP performed in DMSO-d 6 solvent at 500 MHz.
)COOCH2CH2OCH2CH2COOH at 3.77 ppm in TP denoted the in situ O-C coupled incorporation of MBOEPA.During Pb(II) adsorption at pH > pHPZC (pHPZC = 5.93, Figure S2), considerable deprotonation of -COOH of AA and O-H of MXene occurred, resulting in a significant reduction in hydrogen bonding.Two distinct O-H str.peaks at 3341 and 3185 cm −1 were observed in Pb(II)-MXTP attributed to the O-H str. of terminal O-H and -COOH, respectively[35,39].The shifting of >C=O asym.str. of -COO -could be related to the coordination of -COO -with Pb(II) through various modes.
XPS was carried out with the primary objective of confirming the presence of MXene in MXTP.The binding pattern of MXene with TP was identified by observing changes in Ti2p BEs in MXene and MXTP.The deconvoluted Ti2p of MXene contained three sets of Ti2p 3/2 /Ti2p 1/2 peaks at BE = 453.65/459.69,454.91/460.95,and 457.76/463.48eV (Figure 3a, Table 2), attributed to Ti-C-T o , Ti-C-T o,F , and Ti-C-T F , respectively (Ti-C-T o , Ti-C-T F , and Ti-C-T o,F indicate terminal O-H, F − , and O-H + F − , respectively)

Gels 2023, 9 ,
x FOR PEER REVIEW 9 of 21 respectively[50].In this degradation zone, the improved thermostability of MXTP compared to TP might be related to coordinate bonding between Ti(IV) and -COO -of TP.The enhanced thermostability of MXTP compared to that of TP beyond 517 °C indicated the reinforcement of the polymer backbone by MXene.Finally, the presence of thermostable inorganic Ti(IV) ions of MXene in MXTP resulted in a significantly higher residue amount (19.03 wt.%) compared to 12.02 wt.% of TP.

Figure 7 .
Figure 7. (a,b) CV plots of MXTP/Pb(II)-MXTP and (c) variation in specific capacitances of MXTP/Pb(II)-MXTP.2.11.Conductance of MXTP and Pb(II)-MXTPIn order to confirm the increase in conductivity of Pb(II)-MXTP, EIS experiments of MXTP and Pb(II)-MXTP were carried out within the frequency range of 0.1-100,000 Hz.Figure8ashows the Bode plot of MXTP and Pb(II)-MXTP.From the Nyquist plot (Figure8b), the initial point of the semicircle, known as solution resistance (R s , ohm), of MXTP and Pb(II)-MXTP were measured to be 49.37 and 28.76 ohm, respectively.By considering the area and length of GSE to be 7.10 × 10 −6 m 2 and 0.08 m, respectively, the conductivity values of MXTP and Pb(II)-MXTP were 228.23 and 391.78 S m −1 , respectively (Figure8c).
Gels 2023, 9, x FOR PEER REVIEW 3 of 21 bearing non-trivial amounts of Pb(II), is fraught with risks.To solve this problem, we have tested a Pb(II)-adsorbed MXene-grafted hydrogel as a prospective solid electrolyte/anode material.The design of our MXene-grafted poly[AA-co-HEMA], i.e., MXTP, is based on the need to introduce conducting properties in the resulting adsorbent.The specific capacitance and conductivity of Pb(II)-MXTP highlight the hitherto unreported class of MXene-grafted hydrogels as promising Pb(II) adsorbents for sustainable post-adsorptive applications.

Table 5 .
Comparative behavior of reported adsorbents towards Pb(II).