In this chapter, we discuss the design and synthesis of hydrogels and related polymeric materials with metal ion coordination properties, with the aim to review the main synthetic strategies used in the area. Then, we focus on the solid-state nuclear magnetic resonance (ss-NMR) spectroscopic technique due to its importance as a structural elucidation tool in both powdered and hydrated state, with emphasis on cross-polarization magic angle spinning (CP-MAS) and high-resolution magic angle spinning (HRMAS). Also, we explain different adsorption models, with the aim to present the methods most commonly used to analyze the uptake properties of hydrogel materials toward metal ions or organic compounds. Finally, we will discuss the applications of these materials for the removal of heavy metal ions and organic compounds, in terms of efficiency in the uptake of these ions and the different techniques commonly used to study the coordination process and the generation of reactive oxygen species (ROS) from hydrogen peroxide (H2O2). The main aim is to provide scientists with a review of the spectroscopic techniques most commonly used for bulk and surface characterization of non-soluble materials.
- metal complexes
- H2O2 activation
Various human activities lead to increase in the concentrations of heavy metal ions in the environment. For example, the effluents from electrical and plastic industries contain copper and cadmium ions, which are toxic and harmful, even at low concentrations, not only to humans but also to plants and animals because they are not biodegradable . An effective and versatile method to remove these heavy metal ions is adsorption . Thus, in the last years, many research groups have worked on the design of functionalized polymers for the development of effective and economic adsorbents for the removal of these toxics from effluents. In this context, polymeric materials with polyampholyte and polyelectrolyte characteristics have interesting properties such as acid–base behavior and coordination of inorganic and organic compounds . Polymeric materials with polyampholyte characteristics consist of monomers which can have positive and negative charges, whereas those with polyelectrolyte characteristics may possess electric charge of one sign. Both can be synthesized by conventional techniques of radical polymerization .
Regarding the textile industry, most of the pigments used today are poorly biodegraded or resistant to environmental conditions. Thus, there is a growing need to remove these pigments from its effluents and a growing demand for new physical, chemical, and/or biological methods to reduce their concentrations .
With the aim to fight this pollution panorama, scientists have developed different smart systems using copper complexes of inorganic or organic material for H2O2 activation, thereby generating reactive oxygen species (ROS) for oxidative degradation of the pollutant. There are numerous examples of Cu(II)/organic ligand complexes as homogeneous systems in which amino acids , carboxylic acid , and Schiff bases are used as chelating agents , since the coordination of the metal ion produces an increase in the catalytic activity of Cu(II). In this area, heterogeneous catalysts, obtained by modifying the surface of particulate materials (such as silica, clays, and diverse polymeric materials) with Cu(II), where its efficiency is strongly dependent on the synthetic method used in their preparations, are actively being used.
In turn, the macromolecular complexes generated between ligands and transition metal ions have been widely studied because they can model the complexation of metal ions with biological ligands present in the active site of enzymes that can be emulated with carboxylic, hydroxyl, and imidazole groups present in the polymer matrix [9, 10]. Particularly, copper proteins are a group of enzymes which have copper ion as a cofactor. According to the coordination mode in the copper centers, a subclassification is carried out taking into account the geometry of the complex or the number of Cu(II) ions .
Nowadays, the use of biohydrogels is preferred to reduce the amount of vinyl or acryl monomers used in the preparation of synthetic hydrogels, and thus to decrease the impact on the environment. However, in general, natural materials are modified with chemical crosslinking molecules to enhance their mechanical strength and applications. Furthermore, it is interesting to obtain biomimetic Cu(II) systems that are resistant to the attack by free radicals and adverse conditions of pH and temperature, which make their recovery and reutilization possible [12–15].
2. Synthesis of polyelectrolyte and polyampholyte hydrogels
Ionic polymers contain covalent and ionic bonds, the latter being responsible for their acid–base and coordination properties. This class of materials is divided into two groups: polyelectrolytes, which have anionic or cationic groups, and polyampholytes, which contain both groups. For polyampholytes, the charged or ionizable groups can be located in the same or different monomer units. From the chemical point of view, polyampholytes are copolymers consisting of weak acidic and basic monomers or strong acidic and basic monomers as well as combinations of them, where the net charge and the charge distribution along the polymer chain is mainly controlled by pH changes occurring in the solution wherein the polymeric matrix is dissolved or swelled depending on the soluble behavior of the material. The net charge and the effect of the different function groups in terms of acid–base properties can be studied through potentiometric titration and the determination of the Z-potential in different conditions . The first polyampholytes with weak basic and acidic groups were synthesized from acrylic acid (AA) or methacrylic acid (MAA) and 2-vinylpyridine (2VP) in the 1950s by the research group of Morawetz and Katchalsky as indicated below (AIBN: azo-bis-isobutyronitrile) (Scheme 1) :
Also, the vinyl pyridine compound can be replaced by any other vinyl basic monomers such as diethylamino-ethyl methacrylate. Using the same strategy, polymers containing sulfonic acids with vinyl or styrene residues with
More recently, Annenkov et al.  noted that the polymerization of AA or MAA with vinyl imidazole leads to a polymer contaminated with free imidazole monomers due to acid–base interactions between them . This reaction leads to a deviation from the classic mechanisms of polymerization because some of the monomers are coordinated with polymer chains, decreasing the concentration of the monomers and stimulating competing reactions, such as template polymerization. Regarding this point, our research group obtained polyampholyte hydrogels from MAA, ethylene glycol diglycidyl ether (EGDE, a diepoxy compound), and imidazole (IM) or 2-methylimidazole (2MI) monomers (
Chromatographic methods led to the observation that during the evolution of the synthetic procedure of
This evidence indicates that the best strategy to carry out the synthesis of copolymers by radical polymerization of vinyl monomers with acid–base properties is the use of the sodium salts of the corresponding acids . In this way, the reproducibility of the chemical reaction is achieved, and the sodium salts of the carboxylic acids are readily obtained after precipitation from ethanolic solutions, by the addition of equimolar amounts of sodium hydroxide. Another alternative is to carry out the synthesis of soluble polyampholytes in aqueous media using the copolymer prepared from 2-(1-imidazolyl)ethyl methacrylate (ImEMA) and tetrahydropyranyl methacrylate (THPMA), which is subsequently unprotected in the final step to generate the polyampholyte matrix as indicated below (MTS: 1-methoxy-1-trimethylsiloxy-2-methyl-1-propene, TBABB: tetrabutylammonium bibenzoate) (Scheme 3) :
Significantly, these procedures require the use of monomers where the acid function is protected. The combined use of imidazole derivatives and carboxylic acids allows obtaining polymers with targeting application, such as catalysts, resins, and ion exchange matrices for solid phase extraction, which are widely applied in analytical and organic synthetic chemistry. These are materials that respond to changes in pH and ionic strength of the medium, being suitable as matrices for the uptake of inorganic and organic compounds and for controlled release of drugs and proteins . Furthermore, the coordination processes may be studied during the uptake of Cu(II) ions in polyampholyte systems bearing 2-methyl-5-vinylpyridine and AA due to the catalase-like activity of the Cu(II) hydrogel toward H2O2 decomposition .
In turn, azole heterocyclic systems are interesting systems given their wide distribution in synthetic and natural compounds. In particular, our research group synthesized macromolecules containing imidazole in the structure due to the catalytic activity of this heterocyclic compound in a wide range of hydrolytic enzymes. The imidazole ring is present in most of the enzymes as part of the histidine amino acid residue, being partially responsible for catalytic activity with synergistic effect of other groups in the active site such as carboxylic acid, hydroxyl, or sulfhydryl residues. In addition, these imidazole-containing polymers have been used as anticorrosion agents , for protein separation , and as models to understand the biological activity of proteins involved in Alzheimer's disease or prion infections . Furthermore, the absence of toxicity in some of these materials makes them good candidates for their use in the engineering of artificial tissues. With this purpose, Casolaro et al. synthesized polyampholytes containing methacrylate-modified L-histidine (MA-His) as outlined below (TEA: triethylamine, EBA:
Continuing the development of new polymeric materials, monomers containing epoxy groups in its structure are widely used to cause thermosetting epoxy resins, given the high degree of crosslinking that can be obtained as the curing agent used. Furthermore, the amino or carboxyl residues present in the different monomers have the chemical ability to produce the opening of epoxy groups. The reason why the imidazole molecules are highly effective for use as curing agents when added to epoxy systems is due to the fact that they can catalyze the homopolymerization of epoxide groups via a
In this way, our research group prepared an interesting polyelectrolyte hydrogel
More traditional strategies for the design of macroporous ion exchange resins consist in the radical polymerization of glycidylmethacrylate (GMA) and ethylene glycol dimethacrylate (EGDMA) since the resulting material,
Finally, another strategy to synthesize hydrogels is the use of diepoxy (DE) with diamine (DA) compounds, which gives rise to hydrogels with high capacities for metal ion uptake. In this way, the linear chain containing --DE-DA-DE-DA-- is crosslinked by free DE molecules since the nitrogen site in the linear polymeric chain can still produce the opening of new oxirane rings . In particular,
2.1. Natural materials
Regarding biopolymers, chitosan is one of the most widely used for the treatment of wastewater containing heavy metal ions as well as starch and cellulose. Chitosan is a
Another natural polymer source to create hydrogels for the removal of pollutants together with the immobilization of enzymes is cellulose. Cellulose is a linear polymer of (1→4)-β-O-glucopyranose linkage between the glucose units and with the same chemical modifications as in chitosan. Cellulose can be grafted with acrylamide or AA, increasing the partition coefficient and retention capacity of the hydrogel if the grafted cellulose is hydrolyzed. The graft polymerization of acrylamide onto cellulose presents a metal ion uptake of around 80–90% for chrome, manganese, nickel, and lead .
3. Characterization techniques
3.1. Nuclear magnetic resonance (NMR)
Concerning the characterization of the chemical structure of the synthesized hydrogels or any other polymeric compound, only a few spectroscopic methods, such as FT-IR, Raman, and NMR, may bring substantial chemical information. Particularly, the non-soluble behavior of these compounds prevents studying them through the common spectroscopic techniques used in the structural elucidation of soluble organic compounds. Thus, solid-state NMR (ss-NMR) experiments are used to analyze in detail the chemical structure of hydrogels and non-soluble materials in general. This technique will be briefly explained below.
Conventional liquid or solution state 1H- and 13C-NMR spectra are formed by narrow and well-resolved signals containing molecular information that can be interpreted by chemists. However, similar experiments performed in solid samples produce very broad signals, which can be up to several kHz or MHz, which prevents obtaining accurate information by direct observation of the spectra. This broadening also implies loss of sensitivity, especially when low-abundance nuclei such as 13C (1.1%) are studied. The difference in the form of solid and liquid lines comes from the different mobility of the molecules. In the liquid state or in solution, molecules are reoriented very quickly, averaging anisotropic interactions, whereas in solid samples, this does not occur. Thus, special techniques should be applied to obtain high-resolution spectra of solids.
To resolve the structures of complex molecules, mainly chemical shifts (δ) together with the scalar coupling (
In fact, for spins
Additionally, the sensitivity of X nuclei that exhibit low isotopic abundance, such as 13C, 15N, or 29Si, which are typical nuclei studied through ss-NMR, can be improved by increasing the signal through the transfer of the magnetization of the abundant nuclei (1H in general) toward the X nuclei. In solids, this technique is known as cross-polarization (CP) , which is optimal under certain radiofrequency field, known as the Hartmann–Hahn condition. This condition is described as follows: (1H) = (X), where γ corresponds to the gyromagnetic ratio and
The presence of compounds containing naturally abundant nuclei, such as protons, usually generates strong interactions (either homonuclear or heteronuclear between the 1H and 13C, 15N, 29Si, and 31P). This leads to a broadening of the signals that cannot be completely averaged, making it necessary to carry out additional techniques, including homo- and heteronuclear decoupling sequences of radiofrequency pulses to average residual dipolar interactions under MAS conditions. The 1H-13C decoupling sequences most commonly used are SPINAL-64, Two-Pulse Phase-Modulation, and continuous-wave among others, used in the area of polymeric materials or pharmaceutical compounds . In summary, the combination of MAS, CP, and heteronuclear decoupling is used for the acquisition of the 13C-NMR experiment which is referred to as solid-state 13C CP-MAS (Figure 2D).
In addition, the 13C CP-MAS spectra can be edited to assist in the assignment of the NMR signals observed. Some of the edition techniques frequently used are the cross-polarization with polarization inversion (CPPI)  and non-quaternary suppression (NQS) experiments . In the former, the quaternary (>C<) and methyl (−CH3) carbons remain as positive signals, the methylene carbons are negative or have inverted signals, and methyne (>CH-) carbons remain in the baseline without being observed. In NQS experiments, only the quaternary and methyl carbons are visualized.
Chemists are also interested in obtaining quantitative information related to the monomer composition in copolymer materials or in any modification made in the polymer structure. However, this information is difficult to obtain because it is necessary to apply 13C direct polarization techniques (13C DP), which require the use of the NMR spectrometer for a long time, and because not all the polymeric powders give rise to an adequate signal-to-noise ratio. In contrast, 13C DP spectra can be very useful to provide quantitative information related to the crystalline and amorphous amounts in crystalline or semicrystalline polymers. For instance, our research group studied semicrystalline
Regarding 1H-NMR spectra, it is necessary to point out that these kinds of simple experiments are not determined as routine in the solid state because the dipolar coupling among protons is higher than the commonly spinning speed obtained by the commercial NMR probes (10–35 kHz). However, partially or well-resolved 1H spectra can be obtained at high spinning rate (>60 kHz) or with particularly high-power decoupling techniques at moderate spinning rate. In general, proton spectra of solid polymer samples consist in wide lines and in some cases an overlapping of wide and sharp lines at static conditions .
Fortunately, for hydrogels, there is an experiment called high-resolution magic angle spinning (HRMAS), where the 1H-1H dipolar interactions can be partially averaged, rendering liquid-like 1H-NMR spectra similar to those observed for liquids. In this technique, the material is swelled with deuterated solvents, making it possible to expand the analysis of hydrogel compounds and tissue samples [42, 43]. Another advantage of this technique is that the sample is not spun at high rates because at 2–4 kHz the residual 1H–1H dipolar couplings, which were partially averaged by the swelling produced by the solvent, are eliminated (Figure 2). The deuterated solvents that can be used are the same as in the liquid state since the HRMAS probe is designed with a deuterium channel (2H) to lock the NMR signal. The sample is placed in special zirconia rotors with different volumes as in ss-NMR, with the difference that they are designed with cylindrical spacers to contain the sample. In this way, HRMAS experiments allow chemists to study hydrogels or swelled samples as in liquid-state NMR (Figure 2A and B).
Although all these examples are very interesting, the ss-NMR technique has to achieve some sensitivity limits that make this spectroscopic tool not able to analyze small chemical modifications of materials or encapsulation of molecules among others. However, these sensitivity problems can be partially resolved with the use of dynamic nuclear polarization (DNP) experiments for solid samples [44–46]. In this technique, the powder sample is impregnated with a biradical solution and introduced in a zirconia rotor which will be spun at the magic angle at ~90–100 K. At this temperature, the polarization transfer is increased from the electron of the radical molecules to the proton network of the material under study, allowing the polarization of the spins in the material under study through spin diffusion. To polarize the electron of the radical molecules, microwave irradiation is on throughout the experiment. For protons, the maximum theoretical enhancement achievable is given by the gyromagnetic ratios (γe-/ γ1H), being around 660.
3.2. X-ray photoelectron spectroscopy (XPS)
Several spectroscopic analyses exist for surface characterization, but the most commonly used to conduct this experiment is based on the irradiation of the surface of the sample with monochromatic X radiation, called X-ray photoelectron spectroscopy (XPS), also known as electron spectroscopy for chemical analysis (ESCA) . This spectroscopic technique allows identifying all the elements of the periodic table, except hydrogen and helium, but more importantly it allows determining the oxidation state of an element and species to which it is attached, thus providing valuable information on the electronic structure of the molecules.
Due to the short penetration power of the electrons, this technique only provides information on a surface layer of a thickness of 20–50 Å. The most important and valuable application of XPS is for the qualitative analysis of the surfaces of solids, such as metals, alloys, polymers, semiconductors, and heterogeneous catalysts. It also allows quantifying each element on the surface, having into account their sensitivity factors.
XPS records the kinetic energy of the emitted electrons after a monochromatic X-ray beam of known energy (
As a result, an XPS spectrum is a graph of the number of emitted electrons (or electron beam power) based on
Simultaneously, Auger electron emissions are originated throughout the interaction with X-ray. The Auger process emissions are generated during the relaxation of the excited ion A+* after interacting with a beam of monochromatic X-ray photons, where an Auger electron is emitted at the same time as an ion A++ is generated on the surface of the material under study. These emissions are described according to the type of orbital transitions involved in the production of the electron (KLL, LMM, or MNN Auger transitions). The applications of XPS in hydrogels and copper–hydrogel complexes will be discussed in the last part of this chapter.
3.3. Other characterization techniques
Another technique for the study of heterogeneous catalysts or hydrogel-containing paramagnetic centers is electron paramagnetic resonance (EPR), also called electronic spin resonance (ESR). This technique is based on the absorption of electromagnetic radiation in the microwave region of a sample with paramagnetic electronic properties which is subjected to a magnetic field. However, this magnetic field is not as intense as in NMR. EPR allows obtaining information about the different geometries adopted by paramagnetic ions when they are part of either biological complexes or biomimetic systems, as well as of heterogeneous catalysts [10, 11]. An important difference with any other spectroscopic technique is that in the EPR spectra, the first derivative absorption line is plotted. The number and intensity of the EPR lines depend on the interaction between the unpaired electron spin (
Other complementary techniques for the study of surfaces include the following:
Scanning electron microscopy (SEM) and atomic force microscopy (AFM), which can provide information on the physical microstructure by images of morphology and topography.
Energy dispersive X-ray diffraction (EXD) experiments, which allow obtaining a semiquantitative elemental composition of the surface of the material observed by SEM microscopy and analyzing the distribution of a particular element such as spreading of a metal ion in a particular section of interest.
N2 adsorption isotherms, which allow measuring the specific surface area by using the nitrogen adsorption isotherm BET . The advantage of this approach is that it allows estimating the surface area and pore dimensions of different materials together with the interior texture of particles.
4. Metal ion uptake equilibria and characterization techniques
To analyze the sorption processes involved in the uptake of different inorganic (metal ions) or organic compounds (dyes, proteins, pollutants, among others), different models can be used, and these will be discussed in this section.
When a gas or solute in a solution affects a solid surface, it can either bounce or remain attached (
In physisorption, the molecules remain attached to the surface of the sorbent by means of van der Waals forces (dipolar interactions, dispersion, and/or induction). In chemisorption, the molecules remain attached to the surface, forming a strong covalent binding, and the chemisorption enthalpies are higher than the physisorption enthalpies and are generally exothermic processes that stop after monolayer formation on the surface. Chemisorption involves the breakdown and formation of bonds. This is why the chemisorbed molecule does not preserve the same electronic structure as in the former phase.
The equilibrium of metal ion uptake can be explored by analyzing the results of the adsorption isotherm for the polymeric adsorbent at a given temperature. Several isotherm models are generally used to fit the experimental data of the adsorption of ions or any other molecule on particles by nonlinear regression. These models include the Langmuir adsorption isotherm, the Freundlich equation, the Temkin model, and the Dubinin–Radushkevich isotherm, each of which is described below.
The Langmuir adsorption isotherm is derived from theoretical models, provides information on uptake capabilities, and reflects the usual equilibrium process behavior but does not provide information about the mechanistic aspects of adsorption. The equation is , where
The Freundlich equation is described as . This model assumes that the surface is heterogeneous in the sense that the adsorption energy is distributed, and the surface topography is patchwise . The sites with the same adsorption energy are grouped together into one patch (the adsorption energy here is the energy of interaction between the adsorbate and the adsorbent). Each patch is independent of each other (i.e., there is no interaction between patches), and the Langmuir equation is applicable for the description of equilibrium of each patch. In fact, this isotherm can be theoretically derived supposing that the surface has different types of adsorption sites.
The Temkin model is empirically derived and assumes that the heat of adsorption (which is a function of temperature) of all molecules in the layer will decrease linearly rather than logarithmically with coverage . It allows estimating the equilibrium constant and the adsorption heat: .
The Dubinin–Radushkevich isotherm is a semi-empirical equation which was originally developed for subcritical vapors in microporous solids, where the adsorption process follows a pore-filling mechanism:
Liquid-phase adsorption data can also be analyzed by the equation below, where the amount adsorbed corresponding to any adsorbate concentration is assumed to be a Gaussian function of the Polanyi potential (
The isotherm parameter sets with statistical support can be determined by nonlinear regression, using the algorithm based on the Gauss–Newton method. An error function can be used to evaluate the fit, the second-order corrected Akaike information criterion (AIC):
The Akaike weights provide information about the strengths of evidence supporting the two competing models. The ratio of the two Akaike weights,
Different kinetic models can be used to fit the experimental adsorption data by nonlinear regression. The Elovich equation is based on a general second-order reaction mechanism for heterogeneous chemisorption processes and is formulated as: where
The modified Freundlich model was originally developed by Kuo and Lotse: where
In addition, most metal ion sorption system models in the literature are based on reaction kinetics using pseudo first-order kinetics or pseudo second-order kinetics. The pseudo first-order model suggests that one ion sorbs to one active site. The integrated equation by applying the boundary conditions, for
Although all these analyses allow the complete characterization in terms of physicochemical aspects, they do not allow obtaining information related to example with the ligands involved in the uptake of metal ions. For that reason, complementing them with spectroscopic techniques can bring important information related to chemical aspects associated with the coordination process in different conditions. Our research group, for example, studied the coordination sphere of copper ions in the synthetic Cu(II) hydrogel complexes by using a combination of ss-NMR and EPR techniques [10, 66]. The EPR spectra of isolated Cu(II) centers with a minimum distance of 10–15 Å between each paramagnetic nucleus provide more information because the values of parallel hyperfine coupling constant (A||) and parallel
The uptake of Cu(II) ions allows that different ligands distributed in the same or different polymeric chains of the adsorbent material participate in the coordination process, modifying the polymer properties. Particularly, the glass transition temperatures (
Finally, the polymeric particles loaded with cations were cut into slices to see if these compounds had access to the entire network or were only attached to the surface. In all the cases, they reached the interior binding sites of these particles, and it was impossible to distinguish a core from the exposed surface. These results, together with the swelling properties of these materials, confirm the fact that they are hydrogels. The water molecules and solutes have access to the bulk of the swelled particles. The materials absorb water and dissolved ions. This was demonstrated by the fact that “water adsorption surface” determined for
5. Environmental application of hydrogels and metal ion hydrogel complexes
Both natural and synthetic polymeric materials can be used to remove organic and inorganic pollutants given their high load capacities associated with the functional groups present in their chemical structures. Adsorption is the easiest method to remove substances from effluents, but a second residue is generated because pollutants are adsorbed to the material used. Specifically, hydrogels can be used to concentrate different kinds of industrial dyes. Regarding inorganic contaminants adsorbed in different materials, heavy metal ions can be desorbed under acidic conditions from the matrix where they are retained. In these conditions, the metal ions are concentrated, which allows their recovery. Some of the materials that can be used for metal ion uptake have been described throughout this chapter.
On the contrary, organic pollutants can be adsorbed in the first instance, and then oxidative methods where ROS are generated from H2O2 can achieve the partial or complete mineralization to CO2 and H2O. These catalytic systems are usually called advanced oxidative technologies for wastewater treatment, and some of them will be covered in this section.
Hydrogen peroxide (H2O2) is a powerful oxidant used in the degradation of pollutants combined with catalysts and/or UV light to give rise to reactive species such as hydroxyl radical. With respect to the oxidation of aromatic hydrocarbons, Fenton-like reactions (Fe(III)/H2O2) are widely used but are only effective in acidic conditions . In contrast, Cu(II)/H2O2 systems can be used for similar purposes but in a broader pH range. In turn, the catalytic activity of the copper ion in the activation of H2O2 with concomitant generation of hydroxyl radicals is enhanced after coordination with pyridine, organic acids, and other chelating agents, but the recovery of these complexes is expensive because they are soluble in water. For these reasons, heterogeneous catalysts are an attractive alternative for decolorization, combining effectiveness, ease of recovery, and reuse potential. Also, transition metals supported on alumina and silica have proved to be more efficient for the activation of H2O2 than homogenous catalysts. In addition, complexes of Cu(II) with alumina or chitosan have been used to remove color from industrial waste [73, 74], Cu(II) complexes immobilized on silica particles have been used as catalysts in the hydroxylation of phenols from H2O2  and non-soluble Cu(II) chitosan/H2O2 systems have been used to degrade anthraquinone and azo dye compounds commonly found in the textile industry . Textile dyes are considered the most common industrial pollutants in waters. In addition, modern dyes are stable to the ineffective conventional treatment methods performed on wastewater (Scheme 7). This results in an intensely colored discharge that is released from the factory with a direct negative impact on the environment. Both Cu(II) chitosan and any other modified chitosan coordinated with copper ions can act as efficient heterogeneous catalysts for the activation of H2O2, in which radical species (mainly hydroxyl radicals, OH
However, even when these systems are very effective, some precautions must be taken into account to ensure the catalytic activity. To not affect the structure of the catalyst, the initial concentration of H2O2 ([H2O2]o) needs to be controlled. As an example, synthetic Cu(II)
There are some experimental techniques that can be done to do an exhaustive characterization of each Cu(II)-supported material or any other metal complex. Once the coordination behavior and the amount and distribution of the metal ion have been studied, the activation of H2O2 can be explored. However, it is also useful to explore the stability of the metal complexes in the experimental conditions in which the catalyst will be used (pH, ionic strength, temperature, etc.).
The reaction with 4-aminoantipyrine is usually used for the detection of free radical species and can be tested in different samples as an easy screening [15, 77]. Nevertheless, it is not able to discern which free radical species are generated through the activation process of H2O2 on the catalytic surface. For the correct identification of the radical species generated in diverse Cu(II) complex/H2O2 systems, EPR measurements should be made with a spin trap molecule. In this way, a stable radical is formed from the incubation of 5,5-dimethyl-1-pyrroline
It is important to note, that, in some cases, ROS can oxidize DMPO and/or the organic matrix, giving rise to nitroxide-like radical and/or carbon-centered radical, respectively, with a characteristic DMPO adduct depending on the reactivity of the system .
Then, when the generation of free radical species is identified, it is possible to access to the catalytic system which can describe the mechanism involved in the generation of these species. Particularly, the Cu(II) complexes have been extensively studied, and the most acceptable catalytic cycle involves the reduction of Cu(II) to Cu(I) induced by the action of the H2O2, which is converted to OH
In addition, the catalytic performance of the catalyst can be studied with the azo dye methyl orange as a model compound because its absorbance can be easily monitored as a function of time through UV-visible spectroscopy at 465 nm (Scheme 7). In general, the concentration of methyl orange in the solution where the catalyst and H2O2 are also present decreases as a consequence of two parallel processes: surface adsorption on the catalyst and oxidative degradation, both following pseudo first-order kinetics [5, 7, 12, 14, 15].
Regarding other metal ion complexes, cobalt complexes can be very useful for soft oxidative procedures in organic chemistry. There are only a few reports related to Co(II) complexes supported in non-soluble polymeric structures, although a Co(II)-crosslinked polyacrylamide can be mentioned as a selective catalyst for the oxidation of olefins and alkyl halides with H2O2 in aqueous media . The difference with copper complexes is that the metal ion is converted to Co(III) from the corresponding Co(II) in the presence of H2O2 [12, 13, 78].
Regarding environmental applications, our research group observed that the Co(II)
The coordination of a polymeric ligand by a transition metal ion is an efficient way to obtain processable materials with unique and valuable properties. Polymer networks offer new possibilities to scientists for the creation of artificial materials. In recent years, hydrogels with chelating ligands have attracted the attention of industrial applications. In particular, polyelectrolyte and polyampholyte hydrogels have become of great interest in the macromolecular chemistry area due to their versatility as excellent adsorbents of chemical compounds. Stimulus-sensitive hydrogels are used in a variety of novel applications, including controlled drug delivery, immobilized enzyme systems, separation processes, fuel cells, and sensor development.
The development of polymers containing nitrogen remains a growing area because of their applications in the destabilization of negative colloids in effluents and water clarification, electrophoretic depositions, recovery of heavy metal ions or exchange resins for ions, and the mimicking of active sites of enzymes.
Currently, the main goals for the material science community are the design and synthesis of new hydrogels containing ligand for the uptake of heavy metal ions to reduce the direct impact of this industrial waste on the environment. The characterization of the different non-soluble polymeric structures is limited to the spectroscopic techniques in the solid state, being ss-NMR the principal characterization tool for bulk analysis. However, some sensitivity problems can be resolved with new polarization techniques such as DNP-NMR. In addition, some other surface characterization techniques such as X-ray photoelectron spectroscopy can be used. However, the results provide only information limited to the interface area and not from the bulk content. Particularly, the activation of H2O2 from the corresponding Cu(II) and Co(II) hydrogels, obtained from the uptake of Cu(II) or Co(II) ions, can be successfully used with H2O2 for the degradation of azo dyes, to reduce the impact of both inorganic or organic pollutants.
Vijayaraghavan K, Jegan J, Palanivelu K, Velan M. Batch and column removal of copper from aqueous solution using a brown marine alga Turbinaria ornata. Chem Eng J. 2005;106(2):177–84.
Bailey SE, Olin TJ, Bricka RM, Adrian DD. A review of potentially low-cost sorbents for heavy metals. Water Res. 1999;33(11):2469–79.
Kudaibergenov SE. Polyampholytes: Synthesis, Characterization and Application. Springer US; 2002.
Lowe AB, McCormick CL. Synthesis and solution properties of zwitterionic polymers. Chem Rev. 2002;102(11):4177–89.
Janus M, Morawski A. New method of improving photocatalytic activity of commercial Degussa P25 for azo dyes decomposition. Appl Catal B Environ. 2007;75(1–2):118–23.
Lin T, Wu C. Activation of hydrogen peroxide in copper(II)/amino acid/H2O2 systems: Effects of pH and copper speciation. J Catal. 2005;232:117–26.
Shah V, Verma P, Stopka P, Gabriel J, Baldrian P, Nerud F. Decolorization of dyes with copper(II)/organic acid/hydrogen peroxide systems. Appl Catal B Environ. 2003;46(2):287–92.
Meng X, Zhu J, Yan J, Xie J, Kou X, Kuang X, et al. Studies on the oxidation of phenols catalyzed by a copper(II)-Schiff base complex in aqueous solution under mild conditions. J Chem Technol Biotechnol. 2006;81:2–7.
Bekturov EA, Kudaibergenov SE, Sigitov VB. Complexation of amphoteric copolymer of 2-methyl-5-vinylpyridine-acrylic acid with copper (ll) ions and catalase-like activity of polyampholyte-metal complexes. Polymer (Guildf). 1986;27:1269–72.
Lázaro-Martínez JM, Monti GA, Chattah AK. Insights into the coordination sphere of copper ion in polymers containing carboxylic acid and azole groups. Polymer (Guildf). 2013;54(19):5214–21.
Solomon EI, Sundaram UM, Machonkin TE. Multicopper oxidases and oxygenases. Chem Rev. 1996;96(7):2563–606.
Lombardo Lupano LV, Lázaro Martínez JM, Piehl LL, Rubín de Celis E, Torres Sánchez RM, Campo Dall’ Orto V. Synthesis, characterization, and catalytic properties of cationic hydrogels containing copper(II) and cobalt(II) ions. Langmuir. 2014;30(10):2903–13.
Lombardo Lupano LV, Lázaro Martínez JM, Piehl LL, Rubin de Celis E, Campo Dall’ Orto V. Activation of H2O2 and superoxide production using a novel cobalt complex based on a polyampholyte. Appl Catal A Gen. 2013;467:342–54.
Lázaro Martínez JM, Rodríguez-Castellón E, Sánchez RMT, Denaday LR, Buldain GY, Campo Dall’ Orto V. XPS studies on the Cu(I,II)–polyampholyte heterogeneous catalyst: An insight into its structure and mechanism. J Mol Catal A Chem. 2011;339(1–2):43–51.
Lázaro Martínez JM, Leal Denis MF, Piehl LL, de Celis ER, Buldain GY, Campo Dall’ Orto V. Studies on the activation of hydrogen peroxide for color removal in the presence of a new Cu(II)-polyampholyte heterogeneous catalyst. Appl Catal B Environ. 2008;82(3–4):273–83.
Lázaro Martínez JM, Chattah AK, Torres Sánchez RM, Buldain GY, Campo Dall’ Orto V. Synthesis and characterization of novel polyampholyte and polyelectrolyte polymers containing imidazole, triazole or pyrazole. Polymer (Guildf). 2012;53(6):1288–97.
Alfrey T, Morawetz H. Amphoteric polyelectrolytes. I. 2-Vinylpyridine—methacrylic acid copolymers. J Am Chem Soc. 1952;74(2):436–8.
Annenkov VV, Danilovtseva EN, Tenhu H, Aseyev V, Hirvonen SP, Mikhaleva AI. Copolymers of 1-vinylimidazole and (meth)acrylic acid: Synthesis and polyelectrolyte properties. Eur Polym J. 2004;40(6):1027–32.
Leal Denis MF, Carballo RR, Spiaggi AJ, Dabas PC, Campo Dall’ Orto V, Martínez JML, et al. Synthesis and sorption properties of a polyampholyte. React Funct Polym. 2008;68(1):169–81.
Krasia TC, Patrickios CS. Synthesis and aqueous solution characterization of amphiphilic diblock copolymers containing carbazole. Polymer (Guildf). 2002;43(10):2917–20.
Kumar A, Lahiri SS, Singh H. Development of PEGDMA: MAA based hydrogel microparticles for oral insulin delivery. Int J Pharm. 2006;323(1–2):117–24.
Jang J, Ishida H. A study of corrosion protection on copper by new polymeric agents: Silane-modified imidazoles. Corros Sci. 1992;33(7):1053–66.
Jilge G, Sébille B, Vidal-Madjar C, Lemque R, Unger KK. Optimisation of fast protein separations on non-porous silica-based strong anion exchangers. Chromatographia. 1993;37(11):603–7.
Kowalik-Jankowska T, Ruta-Dolejsz M, Wiśniewska K, Łankiewicz L, Kozłowski H. Copper(II) complexation by human and mouse fragments (11–16) of β-amyloid peptide. J Chem Soc, Dalton Trans. 2000:4511–9.
Casolaro M, Ito Y, Ishii T, Bottari S, Samperi F, Mendichi R. Stimuli-responsive poly(ampholyte)s containing L-histidine residues: Synthesis and protonation thermodynamics of methacrylic polymers in the free and in the cross-linked gel forms. Express Polym Lett. 2008;2(3):165–83.
Casolaro M, Bottari S, Cappelli A, Mendichi R, Ito Y. Vinyl polymers based on l-histidine residues. Part 1. The thermodynamics of poly(ampholyte)s in the free and in the cross-linked gel form. Biomacromolecules. 2004;5(4):1325–32.
Barton JM, Hamerton I, Howlin BJ, Jones JR, Liu S. Studies of cure schedule and final property relationships of a commercial epoxy resin using modified imidazole curing agents. Polymer (Guildf). 1998;39(10):1929–37.
Kwok AY, Qiao GG, Solomon DH. Interpenetrating amphiphilic polymer networks of poly(2-hydroxyethyl methacrylate) and poly(ethylene oxide). Chem Mater. 2004;16(26):5650–8.
van Berkel PM, Driessen WL, Reedijk J, Sherrington DC, Zitsmanis A. Metal-ion binding affinity of azole-modified oxirane and thiirane resins. React Funct Polym. 1995;27(1):15–28.
van Berkel PM, Punt M, Koolhaas GJAA, Driessen WL, Reedijk J, Sherrington DC. Highly copper(II)-selective chelating ion-exchange resins based ion bis(imidazole)-modified glycidyl methacrylate copolymers. React Funct Polym. 1997;32(2):139–51.
Wan Ngah WS, Endud CS, Mayanar R. Removal of copper(II) ions from aqueous solution onto chitosan and cross-linked chitosan beads. React Funct Polym. 2002;50(2):181–90.
Nada AAM, Alkady MY, Fekry HM. Synthesis and characterization of grafted cellulose for use in water and metal ions sorption. BioResources. 2008;3:46–59.
Blanc F, Copéret C, Lesage A, Emsley L. High resolution solid state NMR spectroscopy in surface organometallic chemistry: Access to molecular understanding of active sites of well-defined heterogeneous catalysts. Chem Soc Rev. 2008;37:518–26.
Harris RK. Nuclear Magnetic Resonance Spectroscopy: A Physiocochemical View. London: Logman Scientific and Technical; 1986.
Lowe IJ. Free induction decays of rotating solids. Phys Rev Lett. 1959;2(7):285–7.
Andrew ER, Bradbury A, Eades RG. Removal of dipolar broadening of nuclear magnetic resonance spectra of solids by specimen rotation. Nature. 1959;183(4678):1802–3.
Hartmann SR, Hahn EL. Nuclear double resonance in the rotating frame. Phys Rev. 1962;128(5):2042–53.
Fung BM, Khitrina K, Ermolaev K. An improved broadband decoupling sequence for liquid crystals and solids. J Magn Reson. 2000;142(1):97–101.
Lombardo Lupano LV, Lázaro-Martínez JM, Vizioli NM, Torres DI, Campo V, Orto D, et al. Synthesis of water-soluble oligomers from imidazole, ethyleneglycol diglycidyl ether, and methacrylic acid. An insight into the chemical structure, aggregation behavior and formation of hollow spheres. Macromol Mater Eng. 2016;301(2):167–81.
Wu XL, Zilm KW. Complete spectral editing in CPMAS NMR. J Magn Reson Ser A. 1993;102(2):205–13.
Lázaro-Martínez JM, Rodríguez-Castellón E, Vega D, Monti GA, Chattah AK. Solid-state studies of the crystalline/amorphous character in linear poly(ethylenimine hydrochloride) (PEI·HCl) polymers and their copper complexes. Macromolecules. 2015;48(4):1115–25.
Power WP. High resolution magic angle spinning – applications to solid phase synthetic systems and other semi-solids. Annu Reports NMR Spectrosc. 2003;51(03):261–95.
Higashi K, Yamamoto K, Pandey MK, Mroue KH, Moribe K, Yamamoto K, et al. Insights into atomic-level interaction between mefenamic acid and Eudragit EPO in a supersaturated solution by high-resolution magic-angle spinning NMR spectroscopy. Mol Pharm. 2014;11(1):351–7.
Maly T, Debelouchina GT, Bajaj VS, Hu K, Joo C, Mak-Jurkauskas ML, et al. Dynamic nuclear polarization at high magnetic fields. J Chem Phys. 2008;128(5):052211.
Lesage A, Lelli M, Gajan D, Caporini MA, Vitzthum V, Miéville P, et al. Surface enhanced NMR spectroscopy by dynamic nuclear polarization. J Am Chem Soc. 2010;132(44):15459–61.
Zagdoun A, Casano G, Ouari O, Lapadula G, Rossini AJ, Lelli M, et al. A slowly relaxing rigid biradical for efficient dynamic nuclear polarization surface-enhanced NMR spectroscopy: Expeditious characterization of functional group manipulation in hybrid materials. J Am Chem Soc. 2012;134(4):2284–91.
Wagner CD, Riggs WM, Davis LE, Moulder JF, Muilenberg GE. Handbook of X-ray photoelectron spectroscopy. In Muilenberg GE, editor. Minnesota: Perkin-Elmer Corporation; 1979.
Brunauer S, Emmett PH, Teller E. Adsorption of gases in multimolecular layers. J Am Chem Soc. 1938;60(1):309–19.
Langmuir I. The adsorption of gases on plane surfaces of glass, mica and platinum. J Am Chem Soc. 1918;40(9):1361–403.
Do DD. Adsorption analysis: Equilibria and kinetics, Vol. 2. London: Imperial College Press; 1998.
Aharoni C, Sparks DL. Rates of Soil Chemical Processes. In Sparks DL and Suarez DL, editors. Madison, WI; Soil Science Society of America 1991. 1–18 p.
Hsieh C-T, Teng H. Langmuir and Dubinin–Radushkevich analyses on equilibrium adsorption of activated carbon fabrics in aqueous solutions. J Chem Technol Biotechnol. 2000;75(11):1066–72.
Hobson JP. Physical adsorption isotherms extending from ultrahigh vacuum to vapor pressure. J Phys Chem. 1969;73(8):2720–7.
Gübbük IH, Güp R, Ersöz M. Synthesis, characterization, and sorption properties of silica gel-immobilized Schiff base derivative. J Colloid Interface Sci. 2008;320:376–82.
Hadi M, Samarghandi MR, McKay G. Equilibrium two-parameter isotherms of acid dyes sorption by activated carbons: Study of residual errors. Chem Eng J. 2010;160(2):408–16.
Burnham KP, Anderson DR, Huyvaert KP. AIC model selection and multimodel inference in behavioral ecology: Some background, observations, and comparisons. Behav Ecol Sociobiol. 2011;65(1):23–35.
Cheung C, Porter J, Mckay G. Sorption kinetic analysis for the removal of cadmium ions from effluents using bone char. Water Res. 2001;35(3):605–12.
Pérez-Marín AB, Zapata VM, Ortuño JF, Aguilar M, Sáez J, Lloréns M. Removal of cadmium from aqueous solutions by adsorption onto orange waste. J Hazard Mater. 2007;139(1):122–31.
Ho Y. The kinetics of sorption of divalent metal ions onto sphagnum moss peat. Water Res. 2000;34(3):735–42.
Loukidou MX, Zouboulis AI, Karapantsios TD, Matis KA. Equilibrium and kinetic modeling of chromium(VI) biosorption by Aeromonas caviae. Colloids Surfaces A Physicochem Eng Asp. 2004;242(1–3):93–104.
Copello GJ, Diaz LE, Campo Dall’ Orto V. Adsorption of Cd(II) and Pb(II) onto a one step-synthesized polyampholyte: Kinetics and equilibrium studies. J Hazard Mater. 2012;217–218:374–81.
Teng H, Hsieh C-T. Activation energy for oxygen chemisorption on carbon at low temperatures. Ind Eng Chem Res. 1999;38(1):292–7.
Kuo S, Lotse EG. Kinetics of phosphate adsorption and desorption by hematite and gibbsite. Soil Sci. 1973;116(6):400–406.
Bache BW, Williams EG. A phosphate sorption index for soils. J Soil Sci. 1971;22(3):289–301.
Ho Y., McKay G. Pseudo-second order model for sorption processes. Process Biochem. 1999;34(5):451–65.
Lázaro Martínez JM, Chattah AK, Monti GA, Leal Denis MF, Buldain GY, Campo Dall’ Orto V. New copper(II) complexes of polyampholyte and polyelectrolyte polymers: Solid-state NMR, FTIR, XRPD and thermal analyses. Polymer (Guildf). 2008;49(25):5482–9.
Peisach J, Blumberg WE. Structural implications derived from the analysis of electron paramagnetic resonance spectra of natural and artificial copper proteins. Arch Biochem Biophys. 1974;165(2):691–708.
Andersson M, Hansson Ö, Öhrström L, Idström A, Nydén M. Vinylimidazole copolymers: Coordination chemistry, solubility, and cross-linking as function of Cu2+ and Zn2+ complexation. Colloid Polym Sci. 2011;289(12):1361–72.
Sun Z, Jin L, Zhang S, Shi W, Pu M, Wei M, et al. An optical sensor based on H-acid/layered double hydroxide composite film for the selective detection of mercury ion. Anal Chim Acta. 2011;702(1):95–101.
Fang X, Chen R, Xiao L, Chen Q. Synthesis and characterization of Sm(III)–hyperbranched poly(ester-amide) complex. Polym Int. 2011;60(1):136–40.
Belfiore LA, Graham H, Uedat E. Ligand field stabilization in nickel complexes that exhibit extraordinary glass transition temperature enhancement. Macromolecules. 1992;25:2935–9.
Kang N, Lee DS, Yoon J. Kinetic modeling of Fenton oxidation of phenol and monochlorophenols. Chemosphere. 2002;47(9):915–24.
Gemeay AH, Salem MA, Salem IA. Activity of silica-alumina surface modified with some transition metal ions. Colloids Surfaces A Physicochem Eng Asp. 1996;117:245–52.
Salem IA. Kinetics of the oxidative color removal and degradation of bromophenol blue with hydrogen peroxide catalyzed by copper(II)-supported alumina and zirconia. Appl Catal B Environ. 2000;28(3–4):153–62.
Ray S, Mapolie SF, Darkwa J. Catalytic hydroxylation of phenol using immobilized late transition metal salicylaldimine complexes. J Mol Catal A Chem. 2007;267(1–2):143–8.
Šuláková R, Hrdina R, Soares GMB. Oxidation of azo textile soluble dyes with hydrogen peroxide in the presence of Cu(II)–chitosan heterogeneous catalysts. Dye Pigment. 2007;73(1):19–24.
Ettinger M, Ruchhoft C, Lishka R. Sensitive 4-aminoantipyrine method for phenolic compounds. Anal Chem. 1951;23(12):1783–8.
Tamami B, Ghasemi S. Modified crosslinked polyacrylamide anchored Schiff base–cobalt complex: A novel nano-sized heterogeneous catalyst for selective oxidation of olefins and alkyl halides with hydrogen peroxide in aqueous media. Appl Catal A Gen. 2011;393(1–2):242–50.