Role of Ionic Moieties in Hydrogel Networks to Remove Heavy Metal Ions from Water.

A variety of methods for removing heavy metal ions from wastewater have been developed but because of their low efficiency, further production of toxic sludge or other waste materials, high expense, and lengthy procedures, limited progress has been achieved to date. Polymeric hydrogel has been attracting particular attention for the effective removal of heavy metal ions from wastewater. Here, ionogenic polymeric hydrogels were prepared by free-radical copolymerization of a neutral acrylamide (AAm) monomer with an ionic comonomer in the presence of a suitable initiator and a cross-linker. Different types of ionic comonomers such as strongly acidic: 2-acrylamido-2-methylpropane sulfonic acid, weakly acidic: acrylic acid (AAc), and zwitterionic: 2-methacryloyloxy ethyl dimethyl-3-sulfopropyl ammonium hydroxide with varying amounts were incorporated into the poly(AAm) networks to fabricate the hydrogels. The heavy metal ions (Fe3+, Cr3+, and Hg2+) removal capacity of the fabricated hydrogels from an aqueous solution via electrostatic interactions, coordination bond formation, and a diffusion process was compared and contrasted. The poly(AAm) hydrogel containing weakly acidic AAc groups shows excellent removal capacity of heavy metal ions. The release and recovery of heavy metal ions from the hydrogel samples are also impressive. The compressive strength of hydrogels was found to be significantly high after incorporating heavy metal ions that will increase their potential applications in different sectors.

Introduction

Heavy metal ions and many other toxic elements originating from various industries are continuously contaminating pan class="Chemical">water, soil, and air. They are prevailing in water from various industries such as electroplating, tanning mills, steel production, wood processing, plastic manufacturing, metallurgical and mining operations, nuclear power plants, dyes and pigments, ceramic, paints, and fertilizer industries.[1,2] Iron, lead, cadmium, arsenic, mercury, aluminum, antimony, chromium, cobalt, copper, manganese, selenium, gold, thallium, uranium, and so forth are the most common heavy metal ions responsible for the poisoning. The main problem with heavy metal ion contamination is that they are not biodegradable and thus persist in living organisms’ bodies, causing dangerous diseases and severe cell abnormalities. Excessive vomiting, abdominal pain, reduced sense of touch, sight, vision, and taste, fatigue or lack of physical stamina, tremors and incoordination, anemia, autoimmune disorders, weakened or inefficient renal function, hyperallergenic symptoms, and compromised metabolism of vitamin D are the common effects of heavy metal ion poisoning.[3,4] Various heavy metal ion removal techniques have been widely employed to date, for example, chemical precipitation (hydroxide precipitation, sulfide precipitation, and chelating precipitation), ion exchange, adsorption, membrane filtration (ultrafiltration, reverse osmosis, nanofiltration, and electrodialysis), coagulation and flocculation, electrochemical treatment, and so on.[5−9] While all the above-mentioned techniques could be used for the treatment of wastewater-containing heavy metal ions, the selection of the most suitable treatment techniques based on the initial metal ion concentration, removal efficiency, hazardous sludge production, material recovery, wastewater component, capital expenditure and operating cost, the durability of the plant, reliability, and environmental impacts, and so forth are not yet materialized.[10] The adsorption process is usually favored to eliminate heavy metal ions because of its high performance, ease of handling, availability of various adsorbents, and cost-effectiveness.[11−13]

Hydrogels, cross-linked polymeric materials that absorb significant volumes of water without dissolving in any solvent, can be treated as unique, incredibly versatile, and high-capacity adsorbent materials for extracting heavy metal ions from wastewater.[14,15] They have many advantages over conventional methods, including hydrophilic composition, suitable for task-specific functional modification, greater chemical penetration due to their three-dimensional structure, controllable dimensional synthesizability, differences in the functional group, eco-friendliness due to rapid biological decomposition, and the possibility of reuse due to the controlled desorption process.[16−19] Metal ion-chelating polymers, referred to as polychelatogens, contain one or more electron donor atoms, such as N, S, O, and P, which may form coordinate bonds with most toxic heavy metal ions. Amide, amine, carboxylic acid, sulfonic acid, and/or ammonium moieties containing hydrogels can chelate metal ions and be powerful polychelatogens for wastewater treatment. Various hydrogel synthesis and their adsorption activity for the elimination of heavy metal ions have been investigated. The poly(ethyleneglycol dimethacrylate-co-acrylamide) hydrogel beads have the following sequence of elimination of heavy metal ions Pb(II) > Cd(II) > Hg(II).[20] Essawy and Ibrahim reported the poly(vinylpyrrolidone-co-methylacrylate) hydrogel with the following order of heavy metal ion removal Cu(II) > Ni(II) > Cd(II).[21] The poly(3-acrylamidopropyl)trimethyl ammonium chloride hydrogels are capable of removing As(V).[16] Hydrogels comprised of acrylic, vinyl, and other functional monomers such as acrylic acid (AAc), acrylamide (AAm), 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS), hydroxyl ethyl methacrylamide (HEMA), N-vinyl imidazole (NI), and 4-vinyl pyridine have proven to be strong adsorbents to heavy metal ions.[22−27] Magnetic hydrogels based on poly(AMPS-co-NI) for the removal of heavy metal ions have also been reported.[28] Recently, Morán-Quiroz et al. synthesized superabsorbent poly(AAc-co-AAm) hydrogels using a redox initiator and used them to remove Cu(II) ions from aqueous solutions.[29] The poly(AAm-co-sodium methacrylate) hydrogels prepared by free-radical copolymerization using poly(ethylene glycol) diacrylate as a cross-linker can remove Cu(II) ions and Cd(II) ions at pH 5.0.[30] Evren et al. fabricated poly(AMPS-co-itaconic acid) hydrogels to remove heavy metal ions.[31] The hydrogels made of poly(AAm-AAc) could bind metal ions such as Cu(II) and Cd(II) stronger than alkali or alkaline earth ions.[32] After a subtle change in the pH values of the medium, the ionogenic hydrogels containing ionizing side groups may contribute to major changes in the degree of swelling. The side groups ionize and electrostatic repulsive forces occur between polymer chains in the aqueous medium to increase the degree of adsorption and swelling.[33−35] The nature and dissociation ability of ionizing side groups should strongly influence their heavy metal ion removal capacity. It is obvious from the literature that the use of anionic moiety, polyampholyte, and polyzwitterion have been studied to prepare hydrogels as heavy metal ion adsorbents by various research groups. Still, unfortunately, the effects of ionic moieties (strongly acidic, weakly acidic, and zwitterionic) present in the same polymer networks on heavy metal ion removal capacity have been neglected or scarcely explored.

Here, we report the fabrication of pan class="Chemical">poly(AAm) hydrogels by incorpn>orating different typn>es of weakly dissociating, strongly dissociating, and zwitterionic moieties into n>an class="Chemical">polymer networks to precisely control the removal of heavy metal ions (Fe3+, Cr3+, and Hg2+) from aqueous samples. The heavy metal ion removal capacity and kinetics, release, and recovery have been studied extensively. In addition, the substantial increment of compressive strengths after adsorbing heavy metal ions has been investigated.

Results and Discussion

Poly(AAm-AAc) hydrogel, pan class="Chemical">poly(AAm-AMPS) hydrogel, poly(AAm-AAc-AMPS) hydrogel, and poly(AAm-MEDSA) hydrogel are prepared by free-radical polymerization, one of the most common and facile polymerization techniques for hydrogel synthesis. The free-radical polymerization of the AAm main monomer along with one or more from AAc, AMPS, and MEDSA as comonomers in the presence of the N,N-methylenebisacrylamide (BIS) cross-linker and potassium persulfate (KPS) initiator easily produces the hydrogels. The weakly dissociating AAc and strongly dissociating AMPS can effectively remove various types of heavy metal ions as the anionic charges have a strong affinity toward positively charged heavy metal ions. AAc is a weakly dissociable acid, and its pKa is 4.25 at 25 °C.[36] If the pH increases above the pKa, the carboxyl group of the AAc becomes deprotonated, leading to a strong, attractive interaction with heavy metal ions. AMPS contains a strongly ionizable sulfonate group with a pKa value of 1.5, which gives its hydrogel’s high chelating ability and dissociates entirely in the whole pH range.[37,38] MEDSA contains both anionic and cationic moieties used to synthesize electrically neutral polyzwitterionic hydrogels to remove heavy metal ions and their counterparts from water.[39,40] The importance of zwitterionic comonomers is their selectivity because of the interaction between ions and positive and negative charges within the chain.[41,42]

In the Fourier transform infrared (FT-IR) spectra of poly(AAm-AAc), pan class="Chemical">poly(AAm-AMPS), poly(AAm-AAc-AMPS), and poly(AAm-MEDSA) hydrogels, the absorbance at 2930 cm–1 is originated from −CH2 stretching frequency, and the broad absorption band around 3100–3500 cm–1 is ascribed to the overlapping peaks of the −NH and −OH groups (Figure ). The presence of a carbonyl group in the hydrogels is confirmed from a band observed at 1665 cm–1. The symmetric stretching vibrations of the carboxylate group indicated by a band observed at around 1451 cm–1 confirm the successful incorporation of AAc into the poly(AAm-AAc) hydrogel network. The S=O asymmetric stretching is observed at 1390 cm–1 for poly(AAm-AMPS), poly(AAm-AAc-AMPS), and poly(AAm-MEDSA) hydrogels. The absorption bands around 1700–1600 cm–1 are because of amide I, amide II, and amide III from AMPS. The presence of the absorption bands at 1039 and 1370 cm–1 corresponding to sulfonate (SO3–)-stretching vibration and the quaternary ammonium group of MEDSA, respectively, confirms that the fabrication of the poly(AAm-MEDSA) hydrogel was successful.[43]

Figure 1

FT-IR spectra of poly(AAm-AAc), poly(AAm-AMPS), poly(AAm-AAc-AMPS), and poly(AAm-MEDSA) hydrogels.

FT-IR spectra of pan class="Chemical">poly(AAm-AAc), n>an class="Chemical">poly(AAm-AMPS), poly(AAm-AAc-AMPS), and poly(AAm-MEDSA) hydrogels.

The swelling behavior of an ionic hydrogel mainly depends on the concentration of the ionic groups, the degree of ionization, the pH, the ionic strength of the surrounding media, the valence and the nature of the counterion, and the compn>osition of the pan class="Disease">swelling medium. If the content of ionic groups (AAc, AMPS) in the hydrogel system increases, the hydrophilicity of the polymer network increases. The swelling ratio of the poly(AAm-AMPS) (1.6/0.4) M hydrogel in water is found to be higher than that of the poly(AAm-AMPS) (1.9/0.1) M hydrogel (Figure ). The strong dissociation power of the ionic groups (−SO3H) of AMPS provides more electrostatic repulsion of negative charges leading to increased swelling. The poly(AAm-AAc) hydrogel does not show any prominent temperature sensitivity, but the poly(AAm-AMPS) hydrogel containing much amount of AMPS shows detectable temperature sensitivity. At higher temperatures, the ionic groups (−SO3H) present in the poly(AAm-AMPS) (1.6/0.4) M hydrogel prefer to interact among themselves than associating with water molecules, forcing the polymer chains to contract in their size and repelling water molecules.

Figure 2

Swelling behaviors with varying temperatures for (a) poly(AAm-AAc) (1.9/0.1) M, poly(AAm-AMPS) (1.9/0.1) M, poly(AAm-AAc-AMPS) (1.9/0.05/0.05) M, and poly(AAm-MEDSA) (1.9/0.1) M hydrogels. (b) Poly(AAm-AAc) (1.6/0.4) M, poly(AAm-AMPS) (1.6/0.4) M, poly(AAm-AAc-AMPS) (1.6/0.2/0.2) M, and poly(AAm-MEDSA) (1.6/0.4) M hydrogels.

pan class="Disease">Swelling behaviors with varying tempn>eratures for (a) n>an class="Chemical">poly(AAm-AAc) (1.9/0.1) M, poly(AAm-AMPS) (1.9/0.1) M, poly(AAm-AAc-AMPS) (1.9/0.05/0.05) M, and poly(AAm-MEDSA) (1.9/0.1) M hydrogels. (b) Poly(AAm-AAc) (1.6/0.4) M, poly(AAm-AMPS) (1.6/0.4) M, poly(AAm-AAc-AMPS) (1.6/0.2/0.2) M, and poly(AAm-MEDSA) (1.6/0.4) M hydrogels.

The pan class="Disease">swelling ratio of the n>an class="Chemical">poly(AAm-AAc-AMPS) hydrogel does not show any temperature sensitivity and it is an intermediate swelling ratio between poly(AAm-AMPS) and poly(AAm-AAc) hydrogels. Incorporation of both AAc and AMPS into the poly(AAm-AAc-AMPS) networks averages the hydrophilicity of the polymer chains to exhibit an intermediate swelling ratio. The swelling ratio of the poly(AAm-MEDSA) hydrogel is the lowest and slightly responds to temperature as the ionic moieties of the MEDSA are undissociated under most environmental conditions.

The pan class="Chemical">poly(AAm-AAc) hydrogel shrinks considerably as n>an class="Chemical">carboxylate groups are protonated in a highly acidic environment (Figure ). However at pH > 4.5, the dissociation of carboxylic acids occurs and increases the swelling ratio of the hydrogel. The poly(AAm-AAc) hydrogel shows the maximum swelling ratio at pH 9. AMPS remains in the dissociated state in the pH range of 2–12. Consequently, because of a strong swelling driving force created by electrostatic repulsion between the ionized sulfonate groups at a wide pH range, the poly(AAm-AMPS) hydrogel swells quickly. The sulfonic acid groups are associated, collapsed, and have a relatively low swelling ratio at pH values lower than the pKa value. In contrast, because of the dissociation of the sulfonic groups and the destruction of hydrogen bonding, the swelling ratio increased at pH values greater than the pKa value. The increase in the AMPS content increased the dissociated groups, thus increasing electrostatic repulsion leading to an extension of the hydrogel network.[44] At pH < pKa of AMPS and AAc, the excess hydrogen cations shield the repulsive pendant anions to reduce the swelling capacity.[45] The swelling ratio of poly(AAm-AAc) and poly(AAm-AMPS) hydrogels at a particular pH increases with increasing AAc and AMPS concentration.

Figure 3

Swelling behaviors with varying pH for (a) poly(AAm-AAc) (1.9/0.1) M, poly(AAm-AMPS) (1.9/0.1) M, poly(AAm-AAc-AMPS) (1.9/0.05/0.05) M, and poly(AAm-MEDSA) (1.9/0.1) M hydrogels. (b) Poly(AAm-AAc) (1.6/0.4) M, poly(AAm-AMPS) (1.6/0.4) M, poly(AAm-AAc-AMPS) (1.6/0.2/0.2) M, and poly(AAm-MEDSA) (1.6/0.4) M hydrogels.

pan class="Disease">Swelling behaviors with varying pH for (a) n>an class="Chemical">poly(AAm-AAc) (1.9/0.1) M, poly(AAm-AMPS) (1.9/0.1) M, poly(AAm-AAc-AMPS) (1.9/0.05/0.05) M, and poly(AAm-MEDSA) (1.9/0.1) M hydrogels. (b) Poly(AAm-AAc) (1.6/0.4) M, poly(AAm-AMPS) (1.6/0.4) M, poly(AAm-AAc-AMPS) (1.6/0.2/0.2) M, and poly(AAm-MEDSA) (1.6/0.4) M hydrogels.

The pan class="Disease">swelling ratio of the n>an class="Chemical">poly(AAm-AAc-AMPS) hydrogel is found to be an intermediate between those of poly(AAm-AAc) and poly(AAm-AMPS) hydrogels, and it increases with increasing pH. The poly(AAm-AAc-AMPS) hydrogel contains both AAc and AMPS moieties in the network; therefore, the polymer networks experience greater extensibility because of the repulsive force of ionic groups to increase the swelling ratio. The poly(AAm-MEDSA) hydrogels do not show any noticeable pH sensitivity. With increasing pH, anionic groups become deprotonated and cationic groups become protonated states. Consequently, there is no net change of repulsive force on polymer networks with varying pH to exhibit any pH sensitivity.[46−49]

The heavy metal ion removal capn>acity of hydrogels was analyzed by cyclic voltammetry. The observed cathodic peak current was normalized with the initial peak current and plotted against time for different hydrogels in Figure . A decrease in the normalized peak current of Fe3+ with time has been observed for the presence of all types of hydrogels in the aqueous solution of Fe3+. The decrease in the value of peak current indicates the elimination of Fe3+ from the solution. The poly(AAm-AAc) hydrogel shows the fastest current decrement compared with others. The hydrogel networks containing high AAc and AMPS contents demonstrated good removal capacity of Fe3+ from water (Figures S1–S4). The presence of the anionic carboxylate (−COO–) group of AAc and the sulfonate (−SO3–) group of AMPS in the polymer network enhances the binding capacity with the metal ions. The coordination bond and electrostatic interactions might be responsible for these strong binding capacities. The poly(AAm-AAc-AMPs) hydrogel shows moderate Fe3+ removal capacity compared to poly(AAm-AAc) and poly(AAM-AMPS) hydrogels. The Fe3+ removal capacity of the poly(AAm-MEDSA) hydrogel is the lowest compared with those of the other two hydrogels. MEDSA contains both anionic and cationic moieties, the anionic part in the poly(AAm-MEDSA) hydrogel can effectively form a complex with Fe3+ by electrostatic attraction but simultaneously experience repulsion of bound Fe3+ with cationic parts of the network. A calibration curve of concentration (mM) versus current (A) is plotted in (Figure S5). The concentration of the Fe(III) ion solution at different times can be determined by observing the peak current values, applying the same scan rate in a fixed potential window with a fixed three-electrode system. The removal capacity of Fe3+ for the poly(AAm-AAc) (1.6/0.4) M hydrogel is 276 mg/g, poly(AAm-AMPS) (1.6/0.4) M hydrogel is 92 mg/g, poly(AAm-AAc-AMPS) (1.6/0.2/0.2) M hydrogel is 210 mg/g, and poly(AAm-MEDSA) (1.6/0.4) M hydrogel is 55 mg/g. The hydrogel containing large AAc shows the highest capacity to remove Fe3+ as carboxylic (−COOH) functional groups can effectively bind with Fe3+ than other ionic moieties of monomers. An excess of hydrogen ions can compete effectively with metal ions for binding sites at a lower pH, resulting in a lower degree of metal ion uptake. Owing to the Fe(III) hydroxide’s insolubility formed via hydrolysis of Fe(III) ions, the adsorption experiment could not be carried out at high pH. The maximum binding of all metal ions occurred within the first 3 h and remained unchanged over the entire 24 h. The removal process of the Fe(III) ions will be more complex at higher pH. It will be difficult to differentiate between the adsorption and precipitation of Fe(III) ions removed from solutions.[50]

Figure 4

Fe3+ removal kinetics of poly(AAm-AAc) (1.6/0.4) M, poly(AAm-AMPS) (1.6/0.4) M, poly(AAm-AAc-AMPS) (1.6/0.2/0.2) M, and poly(AAm-MEDSA) (1.6/0.4) M hydrogels from aqueous solutions by cyclic voltammetry.

pan class="Chemical">Fe3+ removal kinetics of n>an class="Chemical">poly(AAm-AAc) (1.6/0.4) M, poly(AAm-AMPS) (1.6/0.4) M, poly(AAm-AAc-AMPS) (1.6/0.2/0.2) M, and poly(AAm-MEDSA) (1.6/0.4) M hydrogels from aqueous solutions by cyclic voltammetry.

A significant depan class="Chemical">crease in the peak current of Hg2+ for all types of hydrogels is observed after 24 h (Figure ). Similar to Fe3+ ions, the poly(AAm-AAc) hydrogel shows the most and the poly(AAm-MEDSA) hydrogel shows the least removal capacity of Hg2+ ions. The poly(AAm-AMPS) hydrogels and poly(AAm-AAC-AMPS) hydrogels also show good capacity because of the presence of AAc and AMPS anionic monomers. The hydrogels containing more anionic moieties showed good Hg2+ removal capacities.

Figure 5

Cyclic voltammograms of the HgCl2 solution at a scan rate of 0.1 Vs–1 after equilibrating with a fixed amount of different types of hydrogels for 24 h.

Cyclic voltammograms of the pan class="Chemical">HgCl2 solution at a scan rate of 0.1 Vs–1 after equilibrating with a fixed amount of different typn>es of hydrogels for 24 h.

pan class="Chemical">Chromium has a characteristic UV–vis peak at 300 nm. The n>an class="Chemical">poly(AAm-AAc), poly(AAm-AMPS), and poly(AAm-AAc-AMPS) hydrogels containing more AAc or AMPS show more removal capacity of Cr3+, similar such results were also obtained in the cyclic voltametric study. In an aqueous solution, the deprotonation of anionic moieties gives more negatively charged ions, and thereby, Cr3+ can easily bind with them by attractive electrostatic forces. The Cr3+ removal capacity of the poly(AAm-MEDSA) hydrogel also increases with increasing MEDSA content in the polymer network. For a quantitative analysis of the amount of Cr3+, a calibration curve of concentration (M) versus absorbance (au) has been plotted. The Cr3+ removal capacity of the poly(AAm-AAc) (1.6/0.4) M hydrogel, poly(AAm-AMPS) (1.6/0.4) M hydrogel, poly(AAm-AAc-AMPS) (1.6/0.2/0.2) M hydrogel, and poly(AAm-MEDSA) (1.6/0.4) M hydrogels is 139, 129, 129, and 84 mg/g, respectively. The poly(AAm-AAc) hydrogel shows the quickest and poly(AAm-MEDSA) hydrogel shows the slowest Cr3+ removal capacities (Figure ).

Figure 6

Decay of the Cr3+ concentration with time for poly(AAm-AAc) (1.6/0.4) M, poly(AAm-AMPS) (1.6/0.4) M, poly(AAm-AAc-AMPS) (1.6/0.2/0.2) M, and poly(AAm-MEDSA) (1.6/0.4) M hydrogels by UV–vis spectra analysis.

Decay of the pan class="Chemical">Cr3+ concentration with time for n>an class="Chemical">poly(AAm-AAc) (1.6/0.4) M, poly(AAm-AMPS) (1.6/0.4) M, poly(AAm-AAc-AMPS) (1.6/0.2/0.2) M, and poly(AAm-MEDSA) (1.6/0.4) M hydrogels by UV–vis spectra analysis.

Generally, hydrogels remove heavy pan class="Chemical">metal ions by diffusion, adsorption, and forming a coordination complex with metal ions. The hydrogels containing anionic–COOH and −SO3H functional groups dissociate and electrostatically attract metal ions to show adequate removal capacity. The zwitterionic poly(AAm-MEDSA) hydrogel shows poor removal capacity as its anionic part forms a complex with Cr3+ by electrostatic force, but its cationic part simultaneously repulses with heavy metal ions.

The prepared hydrogel exhibits greater chelating ability at low pan class="Chemical">metal ion concentration, which means that to adsorb this, electrostatic attraction (physical adsorption) is employed. On the other hand, the adsorption is relatively low at high metal ion concentrations, indicating that the chelating interaction (chemical adsorption) occurs in the adsorption process. This may be because of the interaction with the external transport paths of highly concentrated metal ions through the boundary layer from the solution to the adsorbent and the adsorption of metal ions to the pores and voids of the inner surfaces of hydrogels.[51] Langmuir isotherm is most commonly employed for measuring the chelating ability of heavy metal ions by the polymeric hydrogel.

A variety of kinetics models are suggested to establish and interpret the dynamics of metal ion adsorption processes and the key parameters governing sorption kinetics. The kinetic model of the pseudo-first-order suggests that the rate of adsorption is proportional to the number of adsorption sites. In comparison, the kinetic model of the pseudo-second-order assumes that the adsorption rate is proportional to the square of the available number of adsorption sites. Figure shows the fitting for the adsorption of Fe3+ on poly(AAm-AAc), poly(AAm-AMPS), poly(AAm-AAc-AMPS), and poly(AAm-MEDSA) hydrogels using the pseudo-first-order kinetics and pseudo-second-order kinetics, respectively.

Figure 7

Fitting curves for the adsorption of Fe3+ on different types of hydrogels using (a) pseudo-first-order kinetic model and (b) pseudo-second-order kinetic model.

Fitting pan class="Chemical">curves for the adsorption of Fe3+ on different types of hydrogels using (a) pseudo-first-order kinetic model and (b) pseudo-second-order kinetic model.

The pseudo-first-order expression of Lagergren is a fundamental kinetic model proposed for the sorption process in solid/liquid systems. The adsorption kinetics described by a pseudo-first-order equation is as followswhere qe and q (mg/g) represent the amount of Fe3+ adsorbed at equilibrium and at any adsorption time t, and k1 (min–1) is the pseudo-first-order constant. The adsorption rate constant k1 can experimentally be determined from the slope of linear plots of log(qe – q) versus t.

The pseudo-second-order kinetics model of Ho can also interpret the kinetic data. This model is based on the assumption that the second-order chemisorption precedes by the sorption. The adsorption kinetics described using a pseudo-second-order equation is as followswhere qe (mg/g) and q (mg/g), k2 (min–1) represent the amount of Fe3+ adsorbed at equilibrium, the amount of Fe3+ adsorbed at equilibrium after time t, and pseudo-first-order rate constant, respectively. From the slope of the linear plot t/q versus t, the adsorption rate constant k2 was experimentally calculated. Table S5 summarizes all kinetic parameters obtained by linear regression of the kinetic models. The higher values of the coefficient of correlation (R2) and the good agreement between the measured qe,exp and the calculated qe,cal suggest a better fit of the results obtained using the pseudo-first-order kinetic model. When adsorption occurs on the hydrogel’s surface at the beginning of the process, a rapid adsorption rate was observed after that; when adsorption takes place on the inner surface of the polymer networks, the adsorption rate was slow.

The recovery of pan class="Chemical">heavy metal ions under acidic conditions was excellent (Figure S10). At low pH, the n>an class="Chemical">polymer–metal complex becomes unstable because of the competing affinity of H+ to the anionic segment of the network. H+ replaces the metal ions and thereby releases them from the hydrogel sample. All fabricated ionogenic hydrogels release about 90% of heavy metal ions at pH 1 within 3 h, at room temperature.

Thermal analysis was carried out to measure the thermal stability and pyrolysis behavior of poly(AAm-AAc-AMPS) and pan class="Chemical">poly(AAm-AAc-AMPS)@Fe3+ hydrogels (Figure S11). All the samples show a slight weight loss at low temperatures (<150 °C) because of the release of weakly adsorbed water molecules from the hydrogels. In the case of the poly(AAm-AAc-AMPS) hydrogel, the initial degradation starts at 225 °C (25%), and a drastic change appears up to 365 °C (27%) because of the degradation of oxygen-containing groups. The final degradation of poly(AAm-AAc-AMPS) hydrogels starts at 460 °C, and after that temperature, only carbon residue exists. However after Fe3+ adsorption, the initial degradation starts at 210 °C. With increasing temperature, degradation occurs uniformly and slowly in the region of 210–740 °C where oxygen-containing groups are degraded, and about 37% weight loss has occurred in that region. The poly(AAm-AAc-AMPS) (1.6/0.2/0.2) M hydrogel after equilibrium incorporation of Fe3+ shows better thermal resistance, and no drastic degradation pattern is observed. The high ash residue at high temperatures is because of the presence of much amount of nondecomposed Fe3+ in the hydrogel.

The as-prepared pan class="Chemical">poly(AAm-AAc) hydrogel exhibits poor compn>ressive strength. After incorpn>orating n>an class="Chemical">Fe3+ into the poly(AAm-AAc) hydrogel, the compressive strength is incredibly increased (Figure and Table ). The poly(AAm-AAc-AMPS)@Fe(III) hydrogel also shows good compressive strength, but poor compressive strengths are observed for poly(AAm-AMPS) and poly(AAm-MEDSA) hydrogels. The acrylate group may form a strong complex with Fe3+, which acts as an additional physical cross-linking to the hydrogel network to give a strong compressive response. However, in the case of poly(AAm-AMPS) hydrogel and poly(AAm-MEDSA) hydrogels, the polymer networks form a complex with a lower amount of Fe3+ to give relatively poor compressive strength. Thus, a tougher hydrogel may be obtained by increasing the degree of cross-linking, but it compromises the percent of elongation. Therefore, in order to obtain a sufficiently strong and ductile hydrogel, the optimization of the degree of cross-linking is prerequisite.

Figure 8

Uniaxial compressive test of different types of poly(AAm-AAc) (1.6/0.4) M, poly(AAm-AMPS) (1.6/0.4) M, poly(AAm-AAc-AMPS) (1.6/0.2/0.2) M, and poly(AAm-MEDSA) (1.6/0.4) M hydrogels. (a) Stress–strain curves, (b) photographs of fragile poly(AAm-AAc) (1.6/0.4) M (without Fe3+), strong poly(AAm-AAc) (1.6/0.4) M (with Fe3+), and fragile poly(AAm-MEDSA) (1.6/0.4) M (with Fe3+) hydrogels during compressive tests.

Table 1

Young’s Modulus, Compressive Strength, and Toughness Values of Poly(AAm-AAc) (1.6/0.4) M, Poly(AAm-AMPS) (1.6/0.4) M, Poly(AAm-AAc-AMPS) (1.6/0.2/0.2) M, and Poly(AAm-MEDSA) (1.6/0.4) M Hydrogels before and after Adsorption of Fe3+

	Young’s modulus (MPa)		compressive strength (MPa)		toughness J/m3
name of the hydrogel	before	after	before	after	before	after
poly(AAm-AAc) (1.6/0.4) M	1.30	10.02	27.72	70.33	0.12	3.96
poly(AAm-AMPS) (1.6/0.4) M	1.54	0.59	26.24	53.56	0.011	0.71
poly(AAm-AAc-AMPS) (1.6/0.2/0.2) M	0.86	3.77	29.22	69.74	0.09	13.07
poly(AAm-MEDSA) (1.6/0.4) M	0.90	0.70	26.08	42.40	0.07	0.59

Uniaxial compressive test of different types of pan class="Chemical">poly(AAm-AAc) (1.6/0.4) M, n>an class="Chemical">poly(AAm-AMPS) (1.6/0.4) M, poly(AAm-AAc-AMPS) (1.6/0.2/0.2) M, and poly(AAm-MEDSA) (1.6/0.4) M hydrogels. (a) Stress–strain curves, (b) photographs of fragile poly(AAm-AAc) (1.6/0.4) M (without Fe3+), strong poly(AAm-AAc) (1.6/0.4) M (with Fe3+), and fragile poly(AAm-MEDSA) (1.6/0.4) M (with Fe3+) hydrogels during compressive tests.

Conclusions

We have successfully synthesized different ionogenic hydrogels containing weakly and strongly dissociating anionic moieties and zwitterionic moieties via a simple free radical polymerization method. pan class="Chemical">AAm is used as a main monomer; one or more from AAc, AMPS, and MEDSA are used as comonomers to make poly(AAm-AAc), poly(AAm-AMPS), poly(AAm-AAc-AMPS), poly(AAm-MEDSA) hydrogels with varying monomer and comonomer concentrations. Synthesized hydrogels are confirmed by FT-IR spectral analysis. The swelling ratios of synthesized hydrogels increase with incorporating the ionic monomer into the network because of their electrostatic repulsion forces. The hydrogels exhibit temperature and pH sensitivity. The reported hydrogels can remove heavy metal ions (Fe3+, Hg2+, and Cr3+) from aqueous samples, which are analyzed and compared by UV–visible spectroscopy and cyclic voltammetry. The weakly dissociating AAc containing hydrogels show great heavy metal ion removal capacities. Hydrogels remove metal ions from the samples by electrostatic attraction forces and entrap inside polymer networks. The adsorbed heavy metal ions from hydrogels can be recovered at low pH so that they can be further used for water treatment. The enhanced compressive strength and thermal stability of hydrogels with the incorporation of metal ions will expand their area of applications and open up windows to further investigate the mechanical property improvement of conventional hydrogels using metal ions as an additional cross-linker. The synthesized hydrogels can possibly be used for the treatment of industrial wastewater, water purification, and so on.

Experimental Section

Chemicals and Reagents

The chemicals, which were mainly used in this work are as follows pan class="Chemical">AAm (Sigma-Aldrich, Germany), n>an class="Chemical">AAc (Scharlab, Spain), 2-methacryloyloxy ethyl dimethyl-3-sulfopropyl ammonium hydroxide (MEDSA) (Sigma-Aldrich, Germany), 2-acrylamido-2-methylpropane sulfonic acid (AMPS) (Sigma-Aldrich, Germany), BIS (Acros Organics, USA), KPS (Acros Organics, USA), tetramethylethylenediamine (TEMED) (Sigma-Aldrich, Germany), iron trichloride (FeCl3) (Sigma-Aldrich, Germany), mercuric chloride (HgCl2) (Sigma-Aldrich, Germany), and chromium(III) nitrate (Cr(NO3)3) (Sigma-Aldrich, Germany). Analytical standard chemicals and reagents were directly used without further purification. Deionized water was used as a solvent, unless otherwise stated, to prepare most of the solutions.

Fabrication of Hydrogels

The following four types of hydrogels (i) pan class="Chemical">poly(AAm-AAc), (ii) n>an class="Chemical">poly(AAm-AMPS), (iii) poly(AAm-AAc-AMPS), and (iv) poly(AAm-MEDSA) were prepared using AAm as a major monomer, one or more comonomers from AAc, AMPS, and MEDSA, BIS as a cross-linker, KPS as an initiator, TEMED as an accelerator, and deionized water as a solvent. The recipes of all hydrogels with varying concentrations of monomers and other gel precursors (e.g., cross-linker, initiator, and solvent) are shown in Tables S1–S4, and the detailed procedure has been reported elsewhere.[52,53] Briefly, a solution of monomer, BIS, and TEMED in H2O was prepared into a test tube, and nitrogen (N2) gas was bubbled for 30 m to remove any dissolved oxygen. In another container, a 2.96 mM KPS solution was prepared and N2 bubbled. The two solutions were then mixed inside an ice bath, and free-radial polymerization was carried out at room temperature for 24 h. The fabricated hydrogel was then immersed in distilled water and washed for 3 days to eliminate any unreacted monomer, cross-linker, and initiator. The distilled water was changed every 6 h.

Swelling Study of Hydrogels

The temperature-dependent pan class="Disease">swelling of hydrogels was observed in the tempn>erature range from 10 to 60 °C. A piece of preweighed dry hydrogel was immersed in a jacketed glass cell containing distilled n>an class="Chemical">water, and the cell was attached to a circulating water bath to control the temperature. The hydrogel sample was kept 3 h at a certain temperature to reach an equilibrium swelling state, and weight was measured. The swelling ratio was calculated using the following equationhere, Wwet is the weight of the swollen hydrogel after equilibrium, and Wdry is the weight of the dry hydrogel. The pH dependency of hydrogels was observed in buffer solutions of pH 2, 5, 7, and 9 at 30 °C.

Cyclic Voltammetry Analysis

The cyclic voltametric analysis was carried out using an electrochemical analyzer, 797 VA Computrace, Metrohm, Switzerland. To investigate heavy metal ion removal capn>acity, 1–2.5 g dry hydrogel was immersed in an aqueous solution of 0.1 M pan class="Chemical">heavy metal ion at room temperature. To maintain homogeneity, the solution was stirred at a fixed 150 rpm. After a certain interval, 0.5 mL heavy metal ion solution was withdrawn from the solution and taken for cyclic voltametric analysis. Three electrode cells with disk shape Pt as the working electrode having a surface area of 0.071 cm2, Ag|AgCl (aq) as a reference electrode, and Pt wire as the counter electrode were used for the cyclic voltametric analysis. A concentration of 0.1 M KCl solution was maintained for the experiments. Calibration curves of cathodic peak current versus concentration were drawn with different concentrations of heavy metal ion solution at a fixed scan rate of 0.1 Vs–1. Removal capacities of heavy metal ions were calculated from the following expressionwhere Co and Cf are the initial and final concentrations (mg/L) of heavy metal ions, V is the volume of the solution (L), and m is the mass (g) of hydrogel samples used as an adsorbent. The cyclic voltammograms of Fe3+ and Hg2+ were obtained with the aqueous solution of FeCl3 and HgCl2.

UV–Visible Spectrophotometric Analysis

Heavy pan class="Chemical">metal ion removal capn>acities were analyzed using a UV–visible spn>ectropn>hotometer, Shimadzu-1800, Japn>an. A carefully dried 10 mg hydrogel sampn>le was transferred into 10 mL of 0.01 M n>an class="Chemical">heavy metal ion solution at room temperature. Deionized water was chosen as the reference or baseline. The concentrations of remaining heavy metal ion solutions after adsorption were measured from the calibration curve.

Mechanical Strength Test

The mechanical strength of hydrogels was tested using a universal testing machine (UTM Model-100P250-12, TestResources Inc., USA) at room tempn>erature. The cylindrical-shapn>ed hydrogel with a 10 mm diameter was immersed into a 100 mL 0.1 M heavy pan class="Chemical">metal ion solution and was kept undisturbed for 24 h to reach equilibrium swelling state. The equilibrium swelled gel was placed in the UTM base for the compressive test. The crosshead speed was 50 mm/min. To maintain reproducibility, each specimen was tested at least three times. From the recorded force and area data, the compressive stress (σ) of the hydrogels was obtained, while the strain (%) was computed from the ratio of the change in length (Δl) and the original length (l0) of the sample (ε = Δl/l0 × 100). The Young’s modulus was calculated from the initial 10% deformation slope of the stress–strain curves. The toughness of each specimen was also calculated from the integral area under the stress–strain curves.[54]

FT-IR Spectral Analysis

The infrared spectra were studied using a FT-IR spectrophotometer (FT-IR 8400, Shimadzu, Japan) in the region of 4000–400 cm–1. The hydrogel samples were oven dried at 50 °C and ground well to obtain powder form. The samples were then uniformly mixed with KBr pan class="Chemical">crystals. A sampn>le pellet was made using a hand-press pellet maker and placed carefully in the path of the IR beam for analysis.[55]

Thermogravimetry and Differential Thermal Analysis

The thermal stability of hydrogels was analyzed using a thermogravimetric analyzer (DT/TGA 7200, HITACHI, Japan). Approximately, 5 mg of the previously dried and the powdered sample was taken in a pan class="Chemical">platinum pan for each test and analyzed from 25 to 800 °C at a heating rate of 10 °C/m under a constant flow of n>an class="Chemical">nitrogen at a flow rate of 10 mL/m. Before the data acquisition, the sample was equilibrated at 25 °C for 5 m to obtain an isothermal condition.