Polypyrrole Aerogels: Efficient Adsorbents of Cr(VI) Ions from Aqueous Solutions

Three-dimensional and porous polypyrrole (PPy) aerogels were prepared using a facile two-step procedure in which cryogels were synthesized via the cryopolymerization of pyrrole with iron (III) chloride in the presence of supporting water-soluble polymers (poly(N-vinylpyrrolidone), poly(vinyl alcohol), gelatin, methylcellulose or hydroxypropylcellulose), followed by freeze-drying to obtain aerogels. The choice of supporting polymers was found to affect the morphology, porosity, electrical conductivity, and mechanical properties of PPy aerogels. PPy aerogels were successfully used as adsorbents to remove toxic Cr(VI) ions from aqueous solutions.


Introduction
Recently, the rapid growth of the world's population has led to increased industrialization, which has unfortunately influenced the environment negatively through the contamination of surface-and groundwater with heavy metal ions (Cr(VI), Pb(II), Cd(II), Hg(II), etc.) [1][2][3]. Among these, hexavalent chromium, Cr(VI), is widely produced by various industries, including chrome plating, dye manufacturing, the leather and textile industries, wood preservation, the paint industry, etc. [4][5][6]. Additionally, chromium occurs in groundwater naturally due to the erosion of natural chromium deposits. The World Health Organization has stated that the total amount of chromium in drinking water should not exceed 0.05 mg L −1 . Due to the harmful nature of Cr(VI) to humans (as a carcinogenic, skin irritant, allergen, etc.) [5,7], many techniques are applied to remove it from water nowadays, for example, electrochemical [8] or catalytic reduction [6], photoreduction [9], ultrafiltration [10][11][12], and adsorption [4,5,7].
Among these methods, adsorption has become the most popular owing to its low cost, easy performance, and high efficiency. However, the preparation of adsorbents with good selectivity, stability, and reusability, as well as easy removability from wastewater, is still a challenge [13]. Currently, adsorbents based on conducting polymers have become popular candidates for the removal of wastewater pollutants [4,14]. Polypyrrole (PPy), polyaniline (PANI), poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(p-phenylenediamine) are some of the most common examples of conducting polymers, which, either pristine or as a part of composites, have been widely used for the adsorption and/or photocatalysis of organic anionic and cationic dyes (Reactive Black 5, methyl orange, safranin, etc.), heavy metal ions (Cr(VI), Pb(II), etc.), drugs (tetracycline, diclofenac, etc.), personal care products (e.g., caffeine), etc. from wastewater [4][5][6][7][8][9][13][14][15][16]. Gupta et al. synthesized magnetoconductive PEDOT/maghemite composites and applied them to the removal of an anionic dye, Reactive Black 5, from the aqueous medium, with a removal efficiency of 95 ± 5% [13]. In addition to their excellent removal efficiency, the magnetic properties of these composites have allowed for the easy separation of adsorbents from water using an external magnet.

Characterization of Polypyrrole Aerogels
The polymerization of pyrrole in the presence of water-soluble polymers at room temperature led to the formation of colloidal particles [21]. It was observed that the choice of the water-soluble polymer had an impact on the obtained particles' size and shape. When the same polymerization was carried on in frozen conditions, conducting and porous cryogels were obtained. In the present study, PPy aerogels prepared in the presence of various stabilizing polymers were successfully produced using a facile cryopolymerization technique. The formation of PPy was confirmed with FTIR spectroscopy (Figure 1 785 cm −1 (C-H out-of-plane deformations), 670 cm −1 (out-of-plane C-C deformations of the pyrrole ring and C-H rocking), and 615 cm −1 (out-of-plane N-H deformations), were observed in all PPy aerogels [22][23][24]. These spectra corresponded to partially deprotonated PPy and are consistent with previously analyzed PPy cryogels [25]. The stabilizing polymers were reflected in the FTIR spectra as well. Gelatin was manifested, with its carbonyl stretching at 1640 cm −1 [26]. PVP displayed a carbonyl stretching band at 1655 cm −1 , a group of C-H deformation-related bands around 1450 cm −1 , CH 2 wagging coupled with C-N stretching at 1285 cm −1 , and C-C stretching, which contributed as a shoulder of the band at 910 cm −1 [26]. PVAL was observed via its C-O stretching vibrations and contributed as a shoulder of the band at 1170 cm −1 with its C-C-O stretching at 845 cm −1 [26]. Cellulose derivatives showed a weak band of carbonyl stretching (overoxidation) at 1710 cm −1 , an O-H deformation band at 1640 cm −1 , C-H deformation bands at 1400 and 1370 cm −1 , and C-O stretching contributes in the region 820-1150 cm −1 (klucel has a defined band at 1130 cm −1 ) [26,27]. of-plane C-C deformations of the pyrrole ring), 910 cm −1 (C-H out-of-plane deformations in PPy base), 785 cm −1 (C-H out-of-plane deformations), 670 cm −1 (out-of-plane C-C deformations of the pyrrole ring and C-H rocking), and 615 cm −1 (out-of-plane N-H deformations), were observed in all PPy aerogels [22][23][24]. These spectra corresponded to partially deprotonated PPy and are consistent with previously analyzed PPy cryogels [25]. The stabilizing polymers were reflected in the FTIR spectra as well. Gelatin was manifested, with its carbonyl stretching at 1640 cm −1 [26]. PVP displayed a carbonyl stretching band at 1655 cm −1 , a group of C-H deformation-related bands around 1450 cm −1 , CH2 wagging coupled with C-N stretching at 1285 cm −1 , and C-C stretching, which contributed as a shoulder of the band at 910 cm −1 [26]. PVAL was observed via its C-O stretching vibrations and contributed as a shoulder of the band at 1170 cm −1 with its C-C-O stretching at 845 cm −1 [26]. Cellulose derivatives showed a weak band of carbonyl stretching (overoxidation) at 1710 cm −1 , an O-H deformation band at 1640 cm −1 , C-H deformation bands at 1400 and 1370 cm −1 , and C-O stretching contributes in the region 820-1150 cm −1 (klucel has a defined band at 1130 cm −1 ) [26,27]. Scanning electron microscopy (SEM) revealed that all the PPy aerogels possessed uniform, interconnected 3D macroporous networks ( Figure 2); however, similar to the formation of colloidal particles, the sizes of the macropores and micropores could vary through the use of water-soluble polymers. Generally, it is well known that the porous structure of cryogel formed during cryopolymerization is mainly determined by the formation of ice crystals [14]; however, clearly, it can be seen that the nature of the stabilizer not only modified the ice crystal formation rate and size but also influenced the pore structure of the final PPy aerogels (  Scanning electron microscopy (SEM) revealed that all the PPy aerogels possessed uniform, interconnected 3D macroporous networks ( Figure 2); however, similar to the formation of colloidal particles, the sizes of the macropores and micropores could vary through the use of water-soluble polymers. Generally, it is well known that the porous structure of cryogel formed during cryopolymerization is mainly determined by the formation of ice crystals [14]; however, clearly, it can be seen that the nature of the stabilizer not only modified the ice crystal formation rate and size but also influenced the pore structure of the final PPy aerogels (   As can be seen from Table 1, the porosity of PPy aerogels obtained after freeze-drying was very similar in all cases (91 to 95%), excluding the PPy-PVP aerogel reaching just 63%. This might be connected to the compact wall structure of PPy-PVP ( Figure 2b) and low pore volume (1.3 cm 3 g −1 ) when compared to the other PPy aerogels (Table 1). Similarly, the effect of water-soluble polymers (used during cryopolymerization) on the mechanical properties (Young's modulus, tensile strength, and strain at break) of PPy cryogels could be strongly noticed ( Table 1). The Young's modulus of the PPy cryogels swollen with water was the highest for the PPy-gelatin (461 kPa) and the lowest for PPy- As can be seen from Table 1, the porosity of PPy aerogels obtained after freeze-drying was very similar in all cases (91 to 95%), excluding the PPy-PVP aerogel reaching just 63%. This might be connected to the compact wall structure of PPy-PVP ( Figure 2b) and low pore volume (1.3 cm 3 g −1 ) when compared to the other PPy aerogels (Table 1). Similarly, the effect of water-soluble polymers (used during cryopolymerization) on the mechanical properties (Young's modulus, tensile strength, and strain at break) of PPy cryogels could be strongly noticed ( Table 1). The Young's modulus of the PPy cryogels swollen with water was the highest for the PPy-gelatin (461 kPa) and the lowest for PPy-PVAL cryogels (19 kPa) ( Table 1). In the case of the PPy cryogel, which was prepared Gels 2023, 9, 582 5 of 16 in the presence of a culminal, the mechanical properties were not evaluated due to its unitability in a swollen state. It is worth noticing that PPy-culminal aerogel possessed very poor mechanical properties in its swollen form (cryogel); however, after freeze-drying, it maintained its integrity, and the handling properties were good enough for the investigation of the PPy-culminal aerogel as an adsorbent.
The DC conductivity values, measured on PPy aerogels compressed to pellets, were also dependent on the water-polymer used for the preparation of the PPy cryogels ( Table 1). The highest DC conductivity was found for the PPy-PVAL and PPy-gelatin cryogels, 6.1 and 2.4 S cm −1 , respectively. This seems to be connected to the morphology. Both PPy-PVAL and PPy-gelatin cryogels had the most uniform network without the presence of large pores (Figure 2a,d). Such morphology guarantees the better connectivity of the conducting PPy phase in the aerogel, which improves the transport of charges and consequently increases the conductivity compared to the PPy aerogels with larger pores. The lowest DC conductivity of 3×10 −2 S cm −1 was obtained for PPy-klucel aerogels. The DC conductivity of PPy aerogels prepared in this study was in good agreement with the conductivity previously reported (10 −2 to 10 −5 S cm −1 ) in the literature for similar aerogels [14,20]. Figure 3 shows the effect of the contact time on the removal capacity and efficiency of the Cr(VI) ions by various PPy aerogels. It is clearly visible that the amount of Cr(VI) ions adsorbed onto the PPy aerogels increased with time, and the adsorption rate was much faster at the beginning of the process before gradually slowing down until equilibrium was attained. The fast adsorption at the early stage was due to the availability of the surface binding sites, which were gradually occupied during the adsorption process. Various PPy aerogels showed a similar adsorption behavior toward the Cr(VI) ions. The continuous and smooth Q t vs. time adsorption curves predicted a monolayer adsorption process of Cr(VI) ions onto the surface of the PPy aerogels [28]. PPy-PVAL had the highest removal efficiency of 96.2%, while PPy-PVP showed the lowest efficiency at 87.8% removal. This could be explained based on the fact that PPy-PVAL aerogel had the highest porosity while PPy-PVP had the lowest porosity and the smallest pore volume (Table 1). Additionally, PPy-klucel showed 95.2%, PPy-gelatin showed 94.4%, and PPy-culminal showed 90.9% removal efficiencies.

Effect of Contact Time
PVAL cryogels (19 kPa) ( Table 1). In the case of the PPy cryogel, which was prepared in the presence of a culminal, the mechanical properties were not evaluated due to its unitability in a swollen state. It is worth noticing that PPy-culminal aerogel possessed very poor mechanical properties in its swollen form (cryogel); however, after freeze-drying, it maintained its integrity, and the handling properties were good enough for the investigation of the PPy-culminal aerogel as an adsorbent.
The DC conductivity values, measured on PPy aerogels compressed to pellets, were also dependent on the water-polymer used for the preparation of the PPy cryogels ( Table  1). The highest DC conductivity was found for the PPy-PVAL and PPy-gelatin cryogels, 6.1 and 2.4 S cm −1 , respectively. This seems to be connected to the morphology. Both PPy-PVAL and PPy-gelatin cryogels had the most uniform network without the presence of large pores (Figure 2a,d). Such morphology guarantees the better connectivity of the conducting PPy phase in the aerogel, which improves the transport of charges and consequently increases the conductivity compared to the PPy aerogels with larger pores. The lowest DC conductivity of 3×10 −2 S cm −1 was obtained for PPy-klucel aerogels. The DC conductivity of PPy aerogels prepared in this study was in good agreement with the conductivity previously reported (10 −2 to 10 −5 S cm −1 ) in the literature for similar aerogels [14,20]. Figure 3 shows the effect of the contact time on the removal capacity and efficiency of the Cr(VI) ions by various PPy aerogels. It is clearly visible that the amount of Cr(VI) ions adsorbed onto the PPy aerogels increased with time, and the adsorption rate was much faster at the beginning of the process before gradually slowing down until equilibrium was attained. The fast adsorption at the early stage was due to the availability of the surface binding sites, which were gradually occupied during the adsorption process. Various PPy aerogels showed a similar adsorption behavior toward the Cr(VI) ions. The continuous and smooth Qt vs. time adsorption curves predicted a monolayer adsorption process of Cr(VI) ions onto the surface of the PPy aerogels [28]. PPy-PVAL had the highest removal efficiency of 96.2%, while PPy-PVP showed the lowest efficiency at 87.8% removal. This could be explained based on the fact that PPy-PVAL aerogel had the highest porosity while PPy-PVP had the lowest porosity and the smallest pore volume (Table 1). Additionally, PPy-klucel showed 95.2%, PPy-gelatin showed 94.4%, and PPy-culminal showed 90.9% removal efficiencies.

Adsorption Kinetics
Adsorption kinetics describe the adsorption rate and the interactions between the adsorbate and the adsorbent (adsorption mechanism). Pseudo-first-order and pseudo-second-order models were used to study the adsorption kinetics ( Figure 4). Table 2 presents the kinetic parameters and correlation coefficient (R 2 ) values. The R 2 values for the pseudosecond-order model were very close to the unit (0.999) for all the PPy aerogels and were always higher than R 2 for the pseudo-second-order model (0.960-0.986). In addition, the calculated equilibrium adsorption capacities of the pseudo-second-order model agreed well with the empirical results. This implies that the adsorption of Cr(VI) ions onto all PPy aerogels followed pseudo-second-order kinetics, indicating that the adsorption of Cr(VI) ions onto the PPy aerogels was controlled by chemisorption behavior. This chemisorption behavior might be due to the electrostatic interaction and anion exchange process between the counter-ions (Cl − ) and HCrO 4− ions. The obtained data were in good agreement with the previously published results of Cr(VI) ions adsorption onto PPy composites [29,30].

Adsorption Kinetics
Adsorption kinetics describe the adsorption rate and the interactions between the adsorbate and the adsorbent (adsorption mechanism). Pseudo-first-order and pseudosecond-order models were used to study the adsorption kinetics ( Figure 4). Table 2 presents the kinetic parameters and correlation coefficient (R 2 ) values. The R 2 values for the pseudosecond-order model were very close to the unit (0.999) for all the PPy aerogels and were always higher than R 2 for the pseudo-second-order model (0.960-0.986). In addition, the calculated equilibrium adsorption capacities of the pseudo-second-order model agreed well with the empirical results. This implies that the adsorption of Cr(VI) ions onto all PPy aerogels followed pseudo-second-order kinetics, indicating that the adsorption of Cr(VI) ions onto the PPy aerogels was controlled by chemisorption behavior. This chemisorption behavior might be due to the electrostatic interaction and anion exchange process between the counter-ions (Cl − ) and HCrO 4− ions. The obtained data were in good agreement with the previously published results of Cr(VI) ions adsorption onto PPy composites [29,30].

Intraparticle Diffusion
The adsorption process of a solute into a porous adsorbent was controlled by three types of mechanisms; surface diffusion onto the external surface of the adsorbents, intraparticle diffusion into the interior of pores of adsorbents, and the sorption of the adsorbate onto the interior binding sites of the adsorbent [31]. The adsorption of Cr(VI) ions onto the PPy aerogels was analyzed using the intraparticle diffusion model by plotting Q t versus t 1/2 ( Figure 5) in order to find out if intraparticle diffusion was the rate-limiting step of the adsorption process. The plots of all PPy aerogels showed multi-linearity (Figure 4), which indicated that the adsorption of Cr(VI) took three stages to process [32]. The first Gels 2023, 9, 582 7 of 16 stage is attributed to external surface adsorption, while the second stage describes the gradual diffusion process, where intraparticle diffusion is rate-limiting, and the third stage is attributed to the slow final equilibrium process. The intraparticle diffusion rate constant (k i ) and the c values were determined from the slope and the intercept of the second linear sections ( Table 2). The higher the intercept (c) value, the bigger the role of interparticle diffusion was as a rate-limiting step. Among all the prepared PPy aerogels, PPy-PVAL was the one onto which the adsorption process was most controlled by intraparticle diffusion (with the highest c value of 45.9 mg g −1 ).

Adsorption Kinetics
Adsorption kinetics describe the adsorption rate and the interactions between the adsorbate and the adsorbent (adsorption mechanism). Pseudo-first-order and pseudo-second-order models were used to study the adsorption kinetics ( Figure 4). Table 2 presents the kinetic parameters and correlation coefficient (R 2 ) values. The R 2 values for the pseudosecond-order model were very close to the unit (0.999) for all the PPy aerogels and were always higher than R 2 for the pseudo-second-order model (0.960-0.986). In addition, the calculated equilibrium adsorption capacities of the pseudo-second-order model agreed well with the empirical results. This implies that the adsorption of Cr(VI) ions onto all PPy aerogels followed pseudo-second-order kinetics, indicating that the adsorption of Cr(VI) ions onto the PPy aerogels was controlled by chemisorption behavior. This chemisorption behavior might be due to the electrostatic interaction and anion exchange process between the counter-ions (Cl − ) and HCrO 4− ions. The obtained data were in good agreement with the previously published results of Cr(VI) ions adsorption onto PPy composites [29,30].

Intraparticle Diffusion
The adsorption process of a solute into a porous adsorbent was controlled by three types of mechanisms; surface diffusion onto the external surface of the adsorbents, intraparticle diffusion into the interior of pores of adsorbents, and the sorption of the adsorbate onto the interior binding sites of the adsorbent [31]. The adsorption of Cr(VI) ions onto the PPy aerogels was analyzed using the intraparticle diffusion model by plotting Qt ver-

Effect of Initial Cr(VI) Concentration
The amount of Cr(VI) adsorbed onto the PPy aerogels (Q e , mg g −1 ) was found to increase with the increasing Cr(VI) ions' initial concentration (Figure 6a). However, by increasing the Cr(VI) initial concentration, the removal efficiency decreased (Figure 6b). At higher Cr(VI) ion concentrations, the available binding sites on the surface of the PPy aerogels saturated fast, and within a short time, no more available free binding sites for all Cr(VI) ions present in the solution were available. Among the aerogels, PPy-PVAL always had the highest removal efficiency, while PPy-PVP had the lowest removal efficiency, especially at high Cr(VI) initial concentrations ( Figure 6). This, again, could be attributed to the lowest porosity, pore size, and volume of the PPy-PVP aerogel (Table 1), which could be directly connected to the lowest specific surface area.

Effect of Initial Cr(VI) Concentration
The amount of Cr(VI) adsorbed onto the PPy aerogels (Qe, mg g −1 ) was found to increase with the increasing Cr(VI) ions' initial concentration (Figure 6a). However, by increasing the Cr(VI) initial concentration, the removal efficiency decreased (Figure 6b). At higher Cr(VI) ion concentrations, the available binding sites on the surface of the PPy aerogels saturated fast, and within a short time, no more available free binding sites for all Cr(VI) ions present in the solution were available. Among the aerogels, PPy-PVAL always had the highest removal efficiency, while PPy-PVP had the lowest removal efficiency, especially at high Cr(VI) initial concentrations ( Figure 6). This, again, could be attributed to the lowest porosity, pore size, and volume of the PPy-PVP aerogel (Table 1), which could be directly connected to the lowest specific surface area.

Equilibrium Adsorption Isotherms
To estimate the maximum adsorption capacity of the various PPy aerogels for the removal of Cr(VI) ions from aqueous solutions, the equilibrium isotherms were studied (Figure 7). The Langmuir isotherm describes the homogenous monolayer adsorption of chromium ions onto the surface of the adsorbents. The Freundlich isotherm expresses heterogeneous multilayer adsorption, and the Temkin isotherm describes the heat of adsorption due to the adsorbate (metal ions) and adsorbent (PPy aerogels) interactions [28]. The calculated parameters of isotherms and linear coefficient R 2 are listed in Table 3. The results show that the Langmuir isotherm model has the highest R 2 values for all PPy aerogels, implying that the adsorption of Cr(VI) ions onto the PPy aerogels is better described by the Langmuir model than by other models. This indicates that the adsorption took place as a homogenous monolayer on the surfaces of PPy aerogels. The calculated maximum adsorption capacities are presented in Table 3. PPy-PVAL has the ultimate adsorption capacity of 497.5 mg g −1 , which was relatively high compared to the majority of PPy-based adsorbents (Table 4), as previously reported in the literature [7,29,30,33] [35]. Shao et al. prepared the PPy/bacterial cellulose composites with a nanofiber structure, which possessed a maximum adsorption capacity of 555.6 mg g −1 at pH 2 [36]. The highest adsorption capacity of PPy-PVAL aerogels could be attributed to the highest porosity (95.2%) and pore volume (15.4 cm 3 g −1 ) ( Table 1) among all the investigated PPy aerogels. The Freundlich model predicted a favorable adsorption process for all the PPy aerogels with the values of 1/n < 1. The divergence of the slope from 0.5 indicated that intraparticle diffusion partially affected the rate-limiting step, besides other processes controlling the overall adsorption process [37]. The greater the 1/n value, the higher the favorability of adsorption. PPy-PVAL showed the most favorable process ( Table 3). The small values of heat for the adsorption-related parameter (B), as obtained from the Temkin model, indicated that there were weak interactions between the Cr(VI) ions and PPy chains through the electrostatic interaction or ion-exchange mechanisms [38].

Equilibrium Adsorption Isotherms
To estimate the maximum adsorption capacity of the various PPy aerogels for the removal of Cr(VI) ions from aqueous solutions, the equilibrium isotherms were studied (Figure 7). The Langmuir isotherm describes the homogenous monolayer adsorption of chromium ions onto the surface of the adsorbents. The Freundlich isotherm expresses heterogeneous multilayer adsorption, and the Temkin isotherm describes the heat of adsorp- limiting step, besides other processes controlling the overall adsorption process [37]. The greater the 1/n value, the higher the favorability of adsorption. PPy-PVAL showed the most favorable process ( Table 3). The small values of heat for the adsorption-related parameter (B), as obtained from the Temkin model, indicated that there were weak interactions between the Cr(VI) ions and PPy chains through the electrostatic interaction or ionexchange mechanisms [38].    Further analysis of the Langmuir isotherm could be performed based on a dimensionless equilibrium parameter called separation factor (R L ), which is defined as follows: The R L value indicates the nature of the adsorption process: unfavorable (R L > 1), linear (R L = 1), favorable (0 < R L < 1), or irreversible (R L = 0). K L is the Langmuir constant, and C i is the initial concentration of Cr(VI) ions. A lower R L value indicated a more favorable adsorption process. Figure 7d shows that all R L values for all the PPy aerogels were in the range of 0.02 to 0.4, which provided an indication that the adsorption of Cr(VI) onto PPy aerogels is a highly favorable process. Additionally, R L was found to decrease by increasing the Cr(VI) ions' initial concentrations. PPy-klucel aerogels had the lowest R L values among the PPy aerogels, which implied their highest affinity toward the Cr(VI) uptake, while PPy-gelatin had the highest R L values and the lowest affinity toward Cr(VI) ions. Table 4. Cr(VI) ion adsorption capacity of various materials based on PPy reported in the literature.

Adsorbent
Conditions Adsorption Capacity (mg g −1 ) Ref. After the adsorption of Cr(IV) (Figure 8), the pyrrole ring C-C stretching band shifted to 1550 or even 1570 cm −1 , the pyrrole ring C-N stretching band formed a maximum at 1475 cm −1 , C-H or C-N in the in-plane deformation band shifted to 1320 cm −1 , the pyrrole ring breathing band shifted to 1190 cm −1 , the N-H + deformation shifted to 1100 and decreased in intensity, the C-H and N-H in-plane deformation band shifted to 1045, the band out of plane C-C deformations of the pyrrole ring decreased, the PPy out-ofplane C-H deformation band shifted to 920 cm −1 with a shoulder at 935 cm −1 , and the C-H out-of-plane deformation band shifted to 790 cm −1 . These changes corresponded to the decreased protonation of PPy [22,23]. They are more or less pronounced in all the aerogels except the one stabilized with klucel, which was less protonated from the beginning. Additionally, a small peak appeared at 853 cm −1 . This band, together with a band at 905 cm −1 , which was not observed probably due to an overlap with a PPy C-H deformation, could be attributed to HCrO 4− , which was adsorbed by an electrostatic (outer-sphere) interaction [41,42]. Other forms of chromium were not detected with FTIR spectroscopy. Based on the adsorption kinetic study and FTIR spectroscopy, the proposed adsorption mechanism is presented in Figure 9.   Figure 9. Adsorption mechanism of Cr(VI) ions onto PPy-PVAL aerogel. Figure 9. Adsorption mechanism of Cr(VI) ions onto PPy-PVAL aerogel.

Conclusions
Mechanically stable, porous, and conducting PPy cryogels/aerogels were obtained by the cryopolymerization of pyrrole in the presence of various water-soluble polymers in a frozen state. These obtained physicochemical properties could be easily varied by the choice of the stabilizer used. The highest DC conductivity (6 S cm −1 ), porosity (95%) and pore volume (15 cm 3 g −1 ) were found for the PPy-PVAL aerogel. While the PPy-PVP aerogel had the lowest porosity (63%), and pore volume (1.3 cm 3 g −1 ). The best mechanical properties were possessed by the PPy-gelatin aerogel, which reached Young's modulus of Gels 2023, 9, 582 13 of 16 461 kPa and tensile strength of 32.7 KPa. All the PPy aerogels were applied as adsorbents of Cr(VI) ions from an aqueous solution, and the adsorption efficiency was investigated in detail. The adsorption process followed pseudo-second-order kinetics for all the PPy aerogels, which indicates that the adsorption was controlled by chemisorption behavior. PPy-PVAL possessed an adsorption capacity of 497.5 mg g −1 for the removal of Cr(VI) ions from the aqueous solution and a removal efficiency of 96.2%, pointing to the high potential of such materials as efficient adsorbents in water-pollution treatment. Additionally, such porous and conducting materials could also be applied as electrode materials for energy conversion and storage applications, as well as in biological applications.

Characterization
PPy formation was confirmed by Fourier transform infrared spectroscopy (FTIR) in region 4000-400 cm −1 using a Thermo Nicolet NEXUS 870 FTIR Spectrometer (DTGS TEC detector; 64 scans; resolution 2 cm −1 ) in a transmission mode on samples that were ground to potassium bromide pellets. The spectra were corrected for the carbon dioxide and humidity in the optical path.
Static mechanical properties of PPy cryogels (diameter 3 mm; length 60 mm) in the deionized water at room temperature were studied with an electromechanical testing device Instron 6025/5800R (Instron, Norwood, MA, USA) equipped with a 10 N load cell and with a cross-head speed of 10 mm min −1 .
DC electrical conductivity was evaluated by a Van der Pauw method on pellets (diameter 13 mm; thickness ≈1 mm) made of compressed PPy aerogels at 530 MPa by a hydraulic press Trystom H-62 (Olomouc, Czech Republic). The pellets were inserted in a home-made sample holder and equipped with four gold-plated spring-loaded equidistant electrodes that made contact at the boundary of the pellet. A Keithley 230 Programmable Voltage Source was connected serially with a Keithley 196 System DMM, which supplied the current: a Keithley 617 Programmable Electrometer was used to estimate the potential difference. The value is averaged from the measurements in two perpendicular directions (room temperature; relative humidity 35 ± 5%). To avoid heat dissipation in the sample, the current was kept below 1 mA.
An internal porous structure of PPy aerogels was measured on a mercury porosimeter Pascal 140 and 440 (Thermo Finigan, Rodano, Italy) which operated in two pressure intervals, 0-400 kPa and 1-400 MPa, and enabled the determination of pore size from 0.004 to 116 µm. Program Pascal by means of Washburn's equation using a cylindrical pore model was used to calculate the pore volume and the most frequent pore diameter. Porosity (p) was gained by the formula: where V is the cumulative pore volume, and ρ is the sample density.

Removal of Hexavalent Chromium Ions
For a typical adsorption experiment, 10 mg of the PPy aerogel was immersed in 25 mL of Cr(VI) aqueous solution (50 mg L −1 ) under mild stirring (150 rpm) at room temperature (20 ± 2 • C) and pH 2. Cr(VI) removal was followed by a UV-Vis measurement (Lambda 950 spectrometer, Perkin Elmer, Coventry, UK).
Adsorption capacity, Q e (mg g −1 ), and removal efficiency, R (%), were calculated according to the equations: where C i and C e (mg L −1 ) are the initial and equilibrium concentrations of Cr(VI), respectively, V (L) is the volume of the Cr(VI) solution and m (g) is the mass of the aerogel. The kinetics of Cr(VI) ions' uptake was analyzed using pseudo-first-order, pseudosecond-order, and intraparticle diffusion models as follows: Pseudo-first-order: log(Q e − Q t ) = logQ e − k 1 2.303 t Pseudo-second-order: Intraparticle diffusion: where Q e and Q t (mg g −1 ) are the quantity of Cr(VI) ions' uptake at equilibrium and time t (min), respectively, k 1 (min −1 ), k 2 (g mg −1 min −1 ), and k i (mg g −1 min −1/2 ) are the pseudo-first-order, pseudo-second-order, and intraparticle diffusion kinetic rate constants, respectively, and c is the intraparticle diffusion constant. The equilibrium adsorption isotherms were studied by varying the initial concentration of the Cr(VI) solution (15-150 mg L −1 ), which was brought into contact with the PPy aerogel (5 mg) at pH 2 under mild stirring (200 rpm) until equilibrium was achieved (24 h) at room temperature. Langmuir, Freundlich, and Temkin models were used to explore the experimental data as follows: Langmuir Freundlich lnQ e =lnK F + 1 n lnC e Temkin Q e = Bln K T +Bln C e where Q max (mg g −1 ) is the maximum adsorption capacity at saturation, K L , K F, and K T are constants in the Langmuir, Freundlich, and Temkin isotherm models, respectively, and n is a constant in relation to the heterogeneity of the adsorption. Adsorption is favorable for 0.1 < 1/n < 1 (the bigger the 1/n, the higher the favorability). B is a Temkin constant in relation to the heat of adsorption. The models' validity was confirmed by a correlation with the experimental data and the best fit of the kinetic and isotherm models was predicted based on the linear regression coefficient (R 2 ) value.