Introduction

Modification of clay structure has attracted great attention due to the possibility of obtaining promising materials to new applications. Various materials based on clay minerals, are extensively used in many fields of life and industry as nanocomposite fillers, thickeners in paints, oil-base drilling mud, adsorbents of heavy metals, and organic pollutants as well as catalysts (e.g., for Fischer–Tropsch synthesis, catalytic cracking, Friedel–Crafts reaction, combustion of volatile organic compounds, oxidation of H2S) [1–10].

One of the most often investigated and abundant layered aluminosilicate is montmorillonite (MMT), belonging to the smectite group. In the MMT structure, the sheet containing Al3+ in octahedral coordination with O2− and OH− ions is sandwiched between two sheets with Si4+ in tetrahedral coordination with O2− ions. The layers are interconnected by sharing O2− at polyhedral corners and edges [11]. The partial substitution of Al3+ cations by lower valency ions (mainly Mg2+) leads to the generation of negative charge on the clay layers, which is neutralized by hydrated, exchangeable cations (e.g., Ca2+, Mg2+, Na+, K+) located in the interlayer spaces. Such a structure causes that the interlamellar space of the natural clay can be functionalized through the replacement of interlayer ions by other species, for instance transition metal cations (or clusters), cationic surfactants, or monomers [12–15].

MMT can be used as a component of polymer–clay nanocomposites, which exhibit interesting functional and structural properties [16]. Superabsorbent composites are one group of such materials, in which the polymer matrix consists of hydrophilic polymers, mainly of poly(acrylic acid) (PAA) [17] and its salts [18], poly(acrylamide) [19, 20], 2-methylpropane sulfonic acid (AMPS), and their copolymers [21, 22]. These hybrid materials, due to their functional groups, are efficient adsorbents of heavy metals cations (e.g., Pb2+, Cd2+, Cu2+) or dyes [22–24] often tested in water purification.

The aim of this article was the synthesis and characterization of PAA/MMT composites designed as metal adsorbents. The influence of hydrogel content in a composite on the Fe3+ adsorption capacity as well as the effect of the presence of iron species on the thermal stability of the synthesized materials were investigated. Since clays modified with Fe turned out to be active catalysts of various processes (e.g., selective reduction of NO with ammonia [25], oxidation of phenol [26], alkylation of arenes [27], and dehydrogenation of alkanes [28]), the hydrogel/clay composites with adsorbed Fe3+ are expected to be a novel type of catalyst precursors for the mentioned reactions. Therefore, investigation into their thermal behavior is crucial to optimize the conditions of calcination, which could result in obtaining Fe-doped aluminosilicate materials with highly dispersed transition metal oxide phase.

Experimental

Materials

Wyoming MMT with cation exchange capacity (CEC) of 90 meq/100 g and the chemical composition as follows: 47.8 % O, 32.3 % Si, 13.6 % Al, 2.5 % Fe, 1.9 % Mg, 0.9 % Na, 0.7 % Ca, 0.2 % K, was used to the synthesis. Acrylic acid (AA) was purchased from Arkema (France). Ammonium persulfate (APS) and N,N′-methylenebisacrylamide (MBA) were supplied by Sigma-Aldrich. Iron(III) nitrate nonahydrate and potassium thiocyanate, used in adsorption studies, were purchased from POCh (Poland).

Synthesis of composites

The series of PAA/MMT composites was prepared by in situ polymerization. The AA monomer was dissolved in deionized water to obtain 10 mass% solution. In the prepared solution, various amounts of clay were dispersed. MMT was introduced at MMT/AA mass ratios of 1.0, 1.5, 2.3, and 4.0. Subsequently, the MBA crosslinking agent was added at a MBA/monomer molar ratio of 0.01. The mixture was stirred at 400 rpm at room temperature to obtain homogenous suspension. The slurry was purged with argon for 10 min to remove oxygen and then the APS initiator (at a APS/monomer molar ratio of 0.01) was added. The mixture was heated to 338 K and kept at this temperature for 3 h until the complete polymerization of monomers. Such obtained material was cut to small pieces, dried at 333 K and ground to fine powder. The composites were denoted as MMT-PAA-20, MMT-PAA-30, MMT-PAA-40, and MMT-PAA-50, where the number indicates the polymer content in composite.

Methods

X-ray diffraction (XRD) analysis was performed by means of a Bruker D2 Phaser X-ray powder diffractometer (30 kV, 10 mA) with Cu anode (λ = 0.15406 nm) at room temperature. The patterns were collected in the 2θ range of 2–50° with step size of 0.02° and scan rate of 1 s.

The structure of composites and raw MMT was additionally investigated using a Thermo Scientific Nicolet 6700 FTIR spectrometer equipped with a DRIFT (EasyDiff) accessory and a MCT-A detector. The dried samples of clay and composites were ground with dried potassium bromide powder (10 mass%). FTIR spectra were recorded from 650 to 4,000 cm−1 collecting 200 scans with 2 cm−1 resolution.

The kinetics of Fe3+ cations adsorption from aqueous solution was examined in batch experiments. Each sample (1 g) was immersed in 300 mL of 0.01 M iron(III) nitrate solution. The solutions were stirred at 303 K for 27 h with an agitation rate of 200 rpm. During the adsorption tests, pH of a solution was maintained at 2.5 using 0.1 M HNO3 and 0.1 M KOH. The changes in the metal ion concentration were determined by means of a Merck Spectroquant Pharo 100 VIS spectrophotometer using thiocyanate method with measurements of absorbance at λ max = 480 nm. The amount of adsorbed metal ions was calculated from the following equation,

$$ q_{{\text{t}}} = \frac{{\left( {C_{0} - C_{{\text{t}}} } \right)}}{m} \cdot V $$

where q t is the amount of Fe3+ cations adsorbed by a unit amount of composite after time t (mg g−1), C 0 and C t are the concentrations of Fe3+ ions in the initial solution before adsorption and after time t, respectively (mmol mL−1), V is the volume of the solution containing Fe3+ cations (mL), and m is the mass of composite (g).

Thermal stability studies of dry samples before and after adsorption of Fe3+ ions were performed by means of a TA Instruments SDT Q600 thermoanalyzer in the temperature range of 303–1,273 K at a heating rate of 10 K min−1 using a dry air purge at a flow rate of 100 mL min−1 (or 20 mL min−1 in the case of TG–FTIR study). The initial sample mass was about 20 mg in each run. The FTIR spectral maps of evolving gaseous products were measured with a Nicolet 6700 FTIR (Thermo Scientific) spectrometer equipped with a FTIR–TG (Thermo Scientific) accessory. Spectra were recorded with 4 cm−1 resolution collecting 8 scans for each spectrum.

Results and discussion

Structure of PAA/montmorillonite composites

The structure of clay and PAA/clay composites was investigated by XRD. The XRD patterns are shown in the Fig. 1. The basal spacing in raw clay and composites was calculated according to the Bragg equation based on the position of (001) diffraction peak. The (001) interplanar distance of raw MMT was 1.22 nm. After the modification with various amounts of PAA the lamellar structure of starting mineral was retained. Moreover, the (001) peak was shifted to the lower 2θ angles (5.08°–5.37°). This indicates an increase in the d 001 spacing of the clay by about 0.43–0.52 nm. For the MMT-PAA-20 sample containing the lowest amount of polymer, the d 001 spacing increased to a lesser degree (1.64 nm) compared to other composites, which resulted from the intercalation of the lower amount of polymer into the interlayer space. In the case of the materials with higher content of PAA, the differences in the distance between the layers slightly varied (in the range of 1.72–1.74 nm). Thus, one can conclude that only limited amount of hydrogel can be intercalated into the interlayer gallery of MMT. The remaining part of polymer probably forms on the external surface of MMT particles.

Fig. 1
figure 1

XRD patterns of raw montmorillonite and the series of poly(acrylic acid)/montmorillonite composites

Full size image

The deeper insight into the structure of MMT and PAA/clay composites was possible due to FTIR measurements. The FTIR spectra of these materials are presented in Fig. 2. In the spectrum of MMT, characteristic absorption peak of hydroxyl group bound with Al3+ cations is observed at 3,630 cm−1. The stretching and bending vibrations of the hydroxyl groups of water molecules present in the clay can be noticed at 3,430 and 1,645 cm−1, respectively. The most intensive band at 1,015 cm−1 is attributed to Si–O in-plane stretching, while the shoulder at 1,115 cm−1 comes from Si–O out-of-plane stretching vibrations. The three peaks below 1,000 cm−1 originate from bending vibrations of Al–Al–OH (920 cm−1), Al–Fe–OH (887 cm−1), and Al–Mg–OH (844 cm−1). Furthermore, Si–O stretching of SiO2 admixture phase (most probably quartz detected by XRD) appears at 798 cm−1 [29].

Fig. 2
figure 2

DRIFT spectra of clay and PAA/MMT composites

Full size image

Beside the characteristic bands of MMT, absorption bands attributed to the PAA part are found in the FTIR spectra of composites. The peaks at 2,945–2,950 cm−1 (CH2 stretching), 1,453 cm−1 (CH2 deformation), and 1,316 cm−1 (C–C stretching) are associated with a carbon backbone chain. Other three distinct peaks at 1,717, 1,540, and 1,410 cm−1 can be assigned to the C=O stretching mode in the protonated carboxylate group as well as to the asymmetric and symmetric C–O stretching modes in the COO− ions, respectively [30]. For all the composites, the wide absorption band observed between 2,700 and 3,500 cm−1 indicates a significant amount of absorbed water. The presence of undissociated COOH groups is confirmed by the maximum between 2,500 and 2,600 cm−1, attributed to hydrogen bonds involving polymeric species [31].

Simultaneous thermogravimetric (TG) and differential thermal analyses (DTA) were performed in order to study the mechanism of thermal decomposition of the polymer part present in the MMT structure. In the case of the MMT sample, only two mass loss effects are observed. The first one (up to 433 K) is attributed to the dehydration of the material and the next one (above 823 K) to dehydroxylation of MMT layers. The results obtained for the MMT-PAA composites are presented in Fig. 3. As seen, the physically adsorbed and interlayer water was removed to 443 K. Above this temperature, the complex decomposition of organic matrix is observed. At first, the carboxyl side groups underwent decomposition to 588–600 K which was detected in the DTG curves as mass loss effects and in the DTA curves as endothermic peaks. A further increase in temperature resulted in oxidation of carbon backbone chains, confirmed by two step exothermic maximum in the temperature range of 600–900 K [20]. Above 900 K, the dehydroxylation of MMT was observed. The evaluated mass losses attributed to the polymer part in composites (compared in Table 1) are close to the intended values.

Fig. 3
figure 3

TG, DTG, and DTA curves recorded for raw montmorillonite and the series of PAA/MMT composites before Fe3+ ions adsorption

Full size image
Table 1 Comparison of adsorption parameters of the raw montmorillonite and the PAA/MMT composites
Full size table

Adsorption capacity of PAA/montmorillonite

The efficiency of raw MMT and composites in the adsorption of Fe3+ cations from aqueous solution was studied. Adsorption kinetics was examined to determine the adsorption capacity and to explain the adsorption mechanism. The experimental data were fitted using the pseudo-first-order kinetic equation [32]:

$$ \ln \left( {q_{\text{e}} - q_{{\text{t}}} } \right) = \ln q_{\text{e}} - k_{\text{f}} t $$

where k f is the adsorption rate constant for the first-order adsorption, q t (mmol g−1) is the amount of Fe3+ cations adsorbed at time t, and q e (mmol g−1) is the amount of Fe3+ ions adsorbed at equilibrium; and pseudo-second-order kinetic equation [33]:

$$ \frac{t}{{q_{{\text{t}}} }} = \frac{1}{{k_{\text{s}} {q_{\text{e}}}^{2} }} + \frac{t}{{q_{\text{e}} }} $$

where k s is the adsorption rate constant for the second-order adsorption.

The experimental q t values collected during the measurements are presented in Fig. 4, and the calculated kinetic parameters are collected in Table 1. The kinetic results of adsorption of Fe3+ ions suggest that in the case of composite materials the adsorption occurred according to the model of pseudo-second-order, whereas for the MMT sample the pseudo-first-order kinetic model describes better this process. It can be noticed that an increase in the content of PAA in composite has a positive impact on the adsorption capacity of Fe3+ cations. This effect can be attributed to the high efficiency of carboxyl groups in the adsorption of Fe3+ cations by ion exchange with protons.

Fig. 4
figure 4

Adsorption kinetics of Fe3+ ions for raw montmorillonite and composites

Full size image

Structural characterization and thermal stability of PAA/montmorillonite composites after Fe3+ adsorption

The structure of the studied materials after adsorption of Fe3+ cations was examined using XRD analysis. The diffraction patterns of the clay and the composite materials containing Fe3+ ions are presented in Fig. 5. The position of main (001) diffraction peak and basal spacing (d 001) for all examined materials before and after the adsorption of Fe3+ cations as well as amounts of adsorbed metal ions are presented in Table 2. The Fe3+ ions’ content in samples was calculated based on the results of adsorption experiments. After adsorption the insignificant changes in the d 001 spacing of all materials were observed which resulted from the introduction of metal ions into the interlayer spaces. Furthermore, a decrease in the intensity of (001) diffraction line was the result of partial delamination of clay structure caused by adsorption of Fe3+ cations between aluminosilicate layers.

Fig. 5
figure 5

XRD patterns of raw montmorillonite and the series of poly(acrylic acid)/montmorillonite composites after Fe3+ cations adsorption

Full size image
Table 2 Position of d 001 diffraction peak and basal spacing of the clay and the PAA/MMT composites before and after adsorption of Fe3+ ions as well as amounts of Fe3+ ions eliminated by adsorption
Full size table

The effect of Fe3+ ions present in the structure of PAA/MMT composites on the mechanism of their thermal decomposition was studied using thermal analysis techniques. The TG, DTG, and DTA curves recorded for the composite materials with adsorbed Fe3+ cations are presented in Fig. 6. The example results of TG–FTIR experiments, testing thermal decomposition of the MMT-PAA-30 sample before and after adsorption of Fe3+ cations, are shown in Fig. 7.

Fig. 6
figure 6

TG, DTG, and DTA curves recorded for the series of PAA/MMT composites and montmorillonite containing Fe3+ cations

Full size image
Fig. 7
figure 7

TG–FTIR results for MMT-PAA-30 composite before and after adsorption of Fe3+ cations

Full size image

In the case of the iron-free material, the elimination of carboxyl groups in the temperature range of 460–640 K was the first step of PAA decomposition. It was confirmed by two peaks at 1,765 and 1,136 cm−1 attributed to vibrations of C=O and C–O bonds, respectively [34]. From 505 to 1,030 K, two intensive and wide bands (at 2,250–2,400 and 670–750 cm−1) originating from carbon dioxide indicate the combustion of carbon backbone. In this temperature range, three exothermic maxima are observed on the DTA curves. The first peak (505–720 K) is most likely connected with the thermal decomposition of polymer deposited on external surface of clay particles, while the second effect (735–890 K) is related to the oxidation of polymer located in the interlayer spaces. The last low intensive peak (above 890 K) can be attributed to the removal of carbon deposit formed between layers during the incomplete oxidation of hydrogel. The formation of the carbon deposit is confirmed by characteristic bands of carbon monoxide observed on the FTIR maps at 2,110–2,220 cm−1 in the temperature range of 596–968 K. Moreover, this last stage of the polymer decomposition overlaps with the dehydroxylation process starting at 900 K. The bands in the wavenumber ranges of 3,400–3,750 and 1,250–2,000 cm−1, attributed to water formed during the oxidation of hydrogel, are observed in the temperature range of 500–1,010 K.

For the material after Fe3+ adsorption, the starting temperature of the thermal decomposition of polymer matrix did not change, however the temperature range of this process is narrower—from 443 to 760 K. Moreover, in the DTA curve only one exothermic peak is visible, which indicates fast oxidation of organic matrix. The gaseous products of this process formed in the temperature ranges of 533–673 and 505–795 K were water and carbon dioxide, respectively. The position of the maximum of this exothermic effect is different and depends on the PAA content and its location in the composite structure. In the case of the composites containing to 30 mass% of PAA, the Fe3+ cations are mainly adsorbed by hydrogel introduced between clay layers. In this case, the high concentration of Fe3+ in the interlayer gallery causes that during thermal treatment Fe3+ oxide particles are easily formed, which catalyze the oxidation of polymer. For the composites with 40 and 50 mass% of PAA, the decomposition temperature of hydrogel part is 30–35 K higher than for other materials. This effect can be explained by higher dispersion of adsorbed Fe3+ cations in PAA deposited on both external and internal surface of MMT. Thus, the formation of Fe oxide species, responsible for faster polymer oxidation, requires higher temperature than for the MMT-rich composites.

Conclusions

PAA can be introduced into the MMT structure by in situ polymerization. Only limited amount of PAA is intercalated between the clay layers. The excess of the formed polymer is deposited on the external surface of the clay particles. PAA/MMT composites are effective adsorbents of metal cations from aqueous solution. The modification of the MMT structure with hydrogel significantly increases the adsorption capacity of Fe3+ ions. The adsorbed metal cations strongly influence the thermal stability of the composite. During heating, iron oxide species form, which catalyze the oxidation of polymer and it is decomposed much faster than in the iron-free material. Therefore, for complete transformation of PAA/MMT composite into a final oxide-type catalyst, thermal treatment at a temperature even below 700 K can be considered.