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Home > An indole-based aerogel for enhanced removal of heavy metals from water via the synergistic effects of complexation and cation–p interactions†
An indole-based aerogel for enhanced removal of heavy metals from water via the synergistic effects of complexation and cation–p interactions†
Peng Yang,a Li Yang, Yan Wang,b Lixian Song,a Junxiao Yanga and Guanjun Chang
Preparation and characterization
Traditional indole-based microporous polymers were synthesized by the catalyst-free nucleophilic substitution poly- condensation via C–N and C–O coupling reactions,31 which means the sacriftce of phenolic-OH. As a metal-philic active site, the phenolic-OH is crucial for achieving a high metal loading. Keeping this in mind, in this paper, we describe a simple, highly efficient way to fabricate a OH-containing indole-based porous architecture by polycondensation curing of the mixed acetonitrile solution of 4-hydroxyindole and formaldehyde (see the Experimental part for details) (Scheme 1). The resultant sample is in the form of an aerogel with a brown color (inset in Fig. 1c). The successful formation of the indole-based aerogel was conftrmed by Fourier Transform Infrared (FT-IR) spectrometry and solid-state cross-polarization magic-angle-spinning (CP/ MAS) NMR spectrometry. The FTIR spectrum of the 4-HIFA is shown in Fig. 1a, with bands at 2927 and 1458 cm—1 associated with the CH2 stretching and bending vibrations, whereas the broadband at 3409 cm—1 is indexed to the NH and the aromatic OH groups of 4-hydroxyindole, respectively.33 The band at 1352 cm—1 comes from O–H bending vibrations.23 The band at 1637 cm—1 comes from aromatic ring stretches, whereas medium to weak absorption bands at 1256 and 1046 cm—1 indicate that methylene ether linkages between 4-hydroxyindole rings are present but not dominant.33 There are no bands at about 2128 cm—1 associated with the stretching vibrations of the terminal cyano groups of CH3CN,32 which indicates the complete removal of CH3CN from the ftnal products (Fig. 1a). The RFA was also characterized by Fourier transform infrared spectroscopy, and the results were in good agreement with the proposed structures (Fig. S3†). The structural information of the prepared 4-HIFA was also obtained by 13C CP/MAS NMR spec- troscopy (Fig. 1b). There are three broad peaks at 155–98, 68, and 25 ppm. The peak at about 147 ppm is ascribed to the phenolic carbons of the substituted 4-hydroxyindole (Fig. 1b; 6), and the broad peaks at 155–98 ppm are ascribed to the indole group carbons (Fig. 1b; 3, 4, 5, 7, 8, 9, and 10). Consistent with the IR spectrum, the smaller peak at 68 ppm is assigned to the small amount of CH2–O–CH2 bridges, and the broad peak at about 25 ppm is assigned to the different types of CH2 bridges, which is perfectly consistent with a previous study about acid- catalyzed resorcinol-formaldehyde aerogels.33 In conclusion, the characterization data can conftrm that the desired aerogel has been synthesized successfully.
Porosity parameters of the resulting material were investi- gated by sorption analysis using nitrogen as the sorbate molecule. The nitrogen adsorption–desorption isotherms of the 4-HIFA measured at —196 ○C are shown in Fig. 1d, displaying a typical type IV curve.35 There are hysteresis loops at relative pressures of 0.7–0.95, suggesting the presence of mesopores and macropores in the sample,36 in good agreement with those observed with the SEM technique (Fig. 1c). Correspondingly, the pore size is distributed at 46 nm, as calculated by the nonlocal density functional theory method (NLDFT). The BET speciftc surface area and pore volume of the 4-HIFA are esti- mated to be 130 m2 g—1 and 1.01 cm3 g—1, respectively (inset in Fig. 1d). As shown in Fig. S5,† the RFA exhibits a similar nitrogen adsorption–desorption isotherm and pore size distribution (inset in Fig. S5†). The BET speciftc surface area of the RFA was estimated to be 151 m2 g—1, and the pore size was distributed at 40 nm.
Heavy metal removal using the 4-HIFA
It was expected that the resulting 4-HIFA with an OH-containing indole-based porous architecture may attract small heavy metal ions through an improved non-covalent interaction. In addition to the complexation of hydroxyls with metal ions, the high charge density of the indole units of the aerogel might facilitate the cation–p interactions between the heavy metal ions and the adsorbent, which inspired us to investigate its heavy metal ion adsorption capacity.
The amount of 4-HIFA is one of the most important parameters affecting the sorption process. As shown in Fig. S1a,† notably, the removal of Ni2+, Cu2+, Cr3+ and Zn2+ ions was increased with the increase of the amount of 4-HIFA from
0.002 to 0.02 g. This phenomenon could be explained by the fact that the increase of the amount of 4-HIFA would increase the total surface area and the number of adsorbent sites, correspondingly, also increasing their chance to come into contact with the heavy metal ions in the solution, resulting in high adsorption capacity. The removal of Ni2+, Cu2+, Cr3+ and Zn2+ ions was practically unchanged with the increase of the amount of 4-HIFA from 0.02 to 0.03 g, which could be explained by the fact that the concentration of Ni2+, Cu2+, Cr3+ and Zn2+ gradually decreased with the adsorption proceeding, resulting in reaching equilibrium. 0.02 g 4-HIFA was selected as the optimal amount for the following batch experiments.
The pH of a solution is one of the most important parame- ters affecting the sorption process. Fig. S1b† shows the depen- dencies of the adsorption capacity for Ni2+, Cu2+, Cr3+ and Zn2+ on the pH value (2.0–6.0) over the 4-HIFA in order to eliminate the effect of precipitation at higher pH values. Notably, it can be observed that the adsorption of Ni2+, Cu2+, Cr3+ and Zn2+ ions was increased with the increase of pH value from 2.0 to 6.0. This phenomenon could be explained by the fact that at lower pH, the relatively high H+ concentration would strongly compete with metal ions for adsorption sites, resulting in low adsorption capacity. Meanwhile, the hydroxyls of the 4-HIFA would be protonated to form –OH2+ groups, which lead to electrostatic repulsion between the metal cations and the protonated groups and prevent the uptake of the metal ions. With the increase of pH, the competition between H+ and other cations could be neglected, and the hydroxyl groups would be deprotonated to form –O— groups, thereby enhancing the electrostatic attraction between the adsorbent sites and meal cations. This justiftcation was also in accordance with surface complex formation theory, according to which an increase in the pH decreases the competition between metal ions and protons favouring the metal ion adsorption.37 In view of the fact that the partial hydrolysis of M+ (e.g., Ni2+, Cu2+, Cr3+, Zn2+ etc.) takes place resulting in the formation of MOH+ and M(OH)2 at pH > 6,19 pH 6.0 was selected as the optimum pH for the following batch experiments to eliminate the effect of hydrolysis.
The uptake of heavy metal ions by the 4-HIFA from aqueous solutions was studied with the batch method at room temper- ature. The affinity of the 4-HIFA for Mn+ ions can be expressed in terms of the distribution coefficient KM (for deftnition, see the Experimental section). The data of the adsorption behavior toward single ions of Ni2+, Cu2+, Cr3+, Zn2+, Hg2+, Cd2+ and Pb2+ (at z10 ppm initial concentration) are shown in Table 1. The removal ability is poor for Hg2+, Cd2+ and Pb2+ but excellent for Ni2+, Cu2+, Cr3+ and Zn2+. For Cr3+ and Zn2+, 96.74% and 97.23% removal rates were reached, respectively, and for Ni2+ and Cu2+, 99.61% and 99.53% removal rates were achieved, all of which reect high capture ability for these ions.
The adsorption kinetics of the Ni2+, Cu2+, Cr3+ and Zn2+ ions by the 4-HIFA was also investigated in order to study the adsorption rate and pathways of adsorption until equilibrium was reached. The results (Tables 2 and S2–S4†) and sorption kinetics curves (Fig. 2) show rapid uptake rates and high removal efficiencies. Within 5 min, the 4-HIFA achieved $97% removal rates and Kd values of >104 mL g—1 for Ni2+ (Table 2) and Cu2+ (Table S2†). Within 30 min, the 4-HIFA achieved $98% removal rates and Kd values of >104 mL g—1 for Ni2+ (Table 2) and >105 mL g—1 for Cu2+ (Table S2†). Within 1 h, the 4-HIFA achieved $99% removal rates and Kd values of >105 mL g—1 for Ni2+ (Table 2) and for Cu2+ (Table S2†). For the Cr3+ and Zn2+ ions (Tables S3 and S4†), the adsorption is slightly
slow but still 96.7% and 97.8% removal rates are achieved in 1 h, respectively. The adsorptions for all of the four ions reach equilibrium within ~5 min (Fig. 2b and c).
A Langmuir isotherm is used to describe the experimental data of Ni2+, Cu2+, Cr3+ and Zn2+. In this model, the adsorbate moieties (Ni2+, Cu2+, Cr3+ and Zn2+) are assumed to undergo monolayer type coverage of the sorbent on an adsorbent surface. Once an adsorption site is occupied, no further where q (mg g—1) is the equilibrium adsorption capacity of Ni2+, Cu2+, Cr3+ and Zn2+ adsorbed, Ce (mg L—1) is the concentration of Ni2+, Cu2+, Cr3+ and Zn2+ at equilibrium, and qm (mg g—1) is the theoretical maximum sorption capacity. The equilibrium adsorption isotherm is shown in Fig. 3, with the equilibrium concentration of Ni2+, Cu2+, Cr3+ and Zn2+ ranging from 0.38 to 425 ppm. The experimental data of uptake capacity for Ni2+, Cu2+, Cr3+ and Zn2+ are fttted well with the Langmuir isotherm model of eqn (3) (see Fig. 3). According to the Langmuir isotherm model, the expected capacity qm is 240.4 mg g—1 for Ni2+, 264.6 mg g—1 for Cu2+, 92.2 mg g—1 for Cr3+ and 126.6 mg g—1 for Zn2+, which are consistent with the experimental value of 240.0 mg g—1 for Ni2+, of 265.0 mg g—1 for Cu2+, of 92.5 mg g—1 for Cr3+ and of 125.0 mg g—1 for Zn2+. The large correlation coefficient (R2 > 0.97) shows a good ftt with the Langmuir isotherm, suggesting a monolayer adsorption53 on the 4-HIFA (Table S9†).
Competitive adsorption of metal ions on the 4-HIFA was investigated using binary, ternary and quaternary mixed solu- tions (Fig. 4). It can be seen that the affinities of the 4-HIFA for the four metal ions followed the order of Ni(II) > Cu(II) > Cr(III) >
Zn(II). This order was a positive correlation with metal electronegativity [metal electronegativity was 1.92, 1.90, 1.66 and 1.65 for elements of Ni, Cu, Cr and Zn, respectively],54 which can be explained by the fact that the higher electronegativity of Ni, Cu, Cr and Zn leads to the lower electronegativity of Ni(II), Cu(II), Cr(III) and Zn(II), respectively, and the lower electronegativity of Ni(II), Cu(II), Cr(III) and Zn(II) leads to the higher adsorption affinities. So the higher electronegativity of Ni, Cu, Cr and Zn leads to the higher adsorption affinities. This order was a negative correlation with effective ionic radius [effective ionic radius was 55, 57, 61.5 and 60 pm for Ni(II), Cu(II), Cr(III) and
Zn(II), respectively].54 The complexation of metal ions with the functional group –OH was considered as one reason for the metal ion adsorption. Besides, the electronegativity of the elements and ionic radius contributed to the adsorption of metal ions because metal cations were attracted to the nega- tively charged surface of the 4-HIFA to form cation–p interac- tions, and the smaller the radius of metal ions, the better the metal ion adsorption.
Conclusions
In summary, a novel indole-based aerogel (4-HIFA) has been obtained by the condensation of 4-hydroxyindole and formal- dehyde via sol–gel technology. Taking advantage of the syner- gistic effects of the complexation and cation–p interactions of the hydroxyl and indole groups with heavy metals, the 4-HIFA exhibited enormous capacities for Ni2+ ( 240 mg g—1), Cu2+ ( 265 mg g—1), Cr3+ ( 92 mg g—1) and Zn2+ ( 125 mg g—1), a selectivity order of Ni2+ > Cu2+ > Cr3+ > Zn2+, and a very high distribution coefficient (Kd) of ~105 mL g—1, which place it at the top of materials known for such removal. Furthermore, the adsorptions for all of the four ions reach equilibrium within~5 min, which makes it promising to be used for the processing of large amounts of sewage.
Conflicts of interest
There are no conicts to declare.
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