The hydrothermal synthesis of nano-faujasite has been successfully performed and the effects of some crystallization parameters were investigated, along with the use of this material as a heavy-metal ion adsorbent. X-ray diffraction patterns have shown that the structure of the nano-faujasite is strongly dependent on both the crystallization time and the alkalinity of the synthesis medium. According to N2 physisorption, X-ray fluorescence, SEM/EDS, and solid state 29Si and 27Al NMR data, the produced nano-faujasite consists of a solid with low molar Si/Al ratio (1.7), with high availability of ion exchange sites and high surface area/small particle size, allowing easy diffusion of metal ions to adsorbent active sites. As a consequence, an excellent performance on removal of Cd2+, Zn2+ and Cu2+ ions was found for this solid. The adsorption capacity followed the order Cd2+ (133 mg·g−1) > Zn2+ (115 mg·g−1) > Cu2+ (99 mg·g−1), which agrees with the order of increasing absolute values of the hydration energy of the metal ions. Kinetic studies and adsorption isotherms showed that the metal ion removal takes place by ion exchange on the monolayer surface of the nano-faujasite. The electrochemical recovery of copper in metallic form exhibited an efficiency of 80.2% after 120 min, which suggests that this process can be adequately implemented for full-scale metal removal.

  • Nanostructured faujasite for high metal retention.

  • Considerable reusability capacity.

  • Electrochemical recovery.

The environmental contamination with heavy metals has attracted the attention of researchers worldwide because these heavy metals are not essential nutrients for the vast majority of organisms, are highly toxic and not biodegradable (He et al. 2016; Nagy et al. 2017; Sobhanardakan & Zandipak 2017; Gong & Tang 2020). The major concern is that most industrial effluents are not appropriately treated prior to their discharge (Stojakovic et al. 2011).

In order to minimize the effects of these contaminants, several remediation techniques are used, such as precipitation, phytoextraction, ultrafiltration and reverse osmosis. Among these methods, the use of low cost adsorbents for water treatment is promising especially when extensive heavy metals uptake take place (Sobhanardakan & Zandipak 2017).

Heavy metal removal with activated carbon can be an excellent option at first; however, due to the high cost of its production and regeneration, the large-scale process becomes practically unfeasible (Erden et al. 2004). On the other hand, the use of natural zeolites is not an interesting alternative, since neither structural pores nor chemical composition can be controlled (Perego et al. 2013; Oliveira et al. 2017). Natural solids generally contain impurities and small pore systems that can hinder the adsorption process.

In this sense, synthetic zeolites have been shown to be attractive for heavy metal removal, since their chemical and textural properties can be properly designed for adsorption processes and catalysis (Pietre et al. 2012; Irannajad et al. 2016). The use of zeolites in both catalysis and adsorption processes is favored by aspects such as: (1) high specific surface area and a 3D framework of pores with diameters compatible with the size of molecules present in substances used in many industrial processes; (2) control of the Si/Al molar ratio (Pietre et al. 2011). The isomorphous replacement of Si4+ by Al3+ gives rise to a framework with negative charge that is compensated by alkali or alkali earth metal cations. Therefore, an excellent performance can be expected for adsorbents with high Al content, high specific surface area and large pore sizes in ion exchange operations.

Based on this information, faujasite-type zeolites display superior performance in different segments because they have a 3D-framework comprised of large pores, high specific surface area and low Si/Al molar ratio (i.e. high Al content) when compared to other microporous solids (Nibou et al. 2010; Ghrib et al. 2016). In addition, their synthesis is carried out in the absence of organic structural directing templates, thus eliminating the calcination step at the end of the hydrothermal synthesis, which makes the process more attractive from both the economic and the environmental points of view.

Few works have reported the synthesis of nano-faujasite for applications in some important areas such as catalysis (Vuong et al. 2010) and adsorption processes (Humelnicu et al. 2017), obtaining interesting results. Smaller particles tend to display larger surface area and, therefore, more active sites may be available for adsorbate interaction with less diffusional restrictions in relation to micro-sized zeolites (Chaves et al. 2012).

Adsorption studies onto zeolites generally are concerned only with parameters influencing the adsorption event; that is, the effect of pH, contact time, adsorbent content, adsorbate concentration, etc. Unfortunately, the role played by the chemical and textural properties of the zeolite on the efficiency for metal removal is not frequently investigated. In addition, there are few studies that adequately deal with the recovery of the metal ions contained in the regenerated solution.

Considering the increasing importance of nanostructures for applications in environmental remediation, the present work aims to produce nano-faujasite without an organic directing template by varying the experimental conditions (i.e. the hydrothermal treatment time and the alkalinity of the synthesis reaction medium), along with the use of this material for metal ion removal from aqueous solutions. Furthermore, electrochemical deposition was used to promote the recovery of copper present in the regenerated solution. The proposed route can contribute to open new perspectives in terms of zeolite design for applications in environmental remediation, which can strongly impact the industrial segments that try to reduce environmental impacts and establish sustainable processes.

Faujasite synthesis

The syntheses of the nano-faujasite samples were conducted starting from a gel with the following fixed molar composition: 7.8 SiO2: Al2O3 : 207 H2O: 9.4 Na2O. Two systems were prepared from this gel: System A consists of NaOH (Dinâmica, purity >97%), sodium aluminate (Aldrich, as 50–56% Al2O3 and 37–45% Na2O, purity ≥99.5%), sodium metasilicate (Dinâmica, as 27.5–29.5% SiO2, 28–30% Na2O and 40–50% H2O, pure) and distilled water. The gel was aged for 24 h under constant magnetic stirring. System B was obtained by dissolving NaOH, sodium aluminate, sodium metasilicate and distilled water. Subsequently, 12.25 g of gel A was added to gel B. The resulting mixture was aged for 24 h under mechanical agitation before hydrothermal treatment at 100 °C for different crystallization times. The produced solids were washed, filtered and dried overnight in a desiccator. Also, in a second set of experiments, the effect of changing the alkalinity of the synthesis reaction medium (i.e. the NaOH amount) on the characteristics of the final products was investigated by decreasing the NaOH content in the synthesis gel by 10 and 50%, respectively.

Characterization

The zeolitic phase was characterized by X-ray diffraction (XRD) using a Shimadzu XRD600 powder diffractometer with the diffraction angle (2θ) scanned from 5 to 40° at a scanning speed of 2° min−1, with Cu-Kα radiation (40 kW, 30 mA).

The porous structure of the produced solids was analyzed by N2 physisorption at 77 K using a Micromeritics ASAP2020 instrument. The samples were outgassed under vacuum at 150 °C for 2 h before the measurements. From the adsorption/desorption isotherms, the specific surface area (SBET) was determined using the Brunauer, Emmett and Teller (BET) method (Lowell et al. 2004). The micropore volume (VMICRO), micropore surface area (SMICRO) and the external surface area (SEXT) were calculated by the t-plot method. The total pore volume (VTotal) was directly measured from the amount of adsorbed gas at relative pressure (P/P0) close to 1 (Lowell et al. 2004).

The semi-quantitative elemental analysis was performed by X-ray fluorescence spectrometry (XRF) using a Bruker S8/Tiger instrument. Scanning electron microscopy (SEM) images were recorded in a Shimadzu SSX-550 microscope, under vacuum and 15 kV acceleration voltage, equipped with an accessory for energy dispersive X-ray spectroscopy (EDS) analysis.

The local chemical environment was investigated by solid-state 27Al and 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) experiments. The experiments were performed at room temperature using a Varian/Agilent VNMR 400 MHz spectrometer operating at a magnetic field of 9.4 T (27Al and 29Si NMR frequencies of 104.16 MHz and 79.41 MHz, respectively).

Regarding the 27Al NMR experiments, single pulse excitation (SPE) experiments were conducted using a short excitation pulse with duration of 1.0 μs (corresponding to a nutation angle of ca. π/10), a recycle delay of 1.0 s and a MAS rate of 14 kHz. The spectra were obtained by Fourier transform of the free induction decays (FIDs), after accumulation of 200 transients. The frequency shifts (ppm) were externally referenced to the single resonance peak observed for an aqueous solution of Al(NO3)3. The 29Si NMR spectra were obtained after accumulation of 400 transients, with an acquisition time of 0.032 s, a pulse duration of 4 μs, a recycle delay of 20 s and a MAS rate of 10 kHz; the spectra were externally referenced to tetramethylsilane (TMS), using the 29Si NMR signal of kaolinite (at −91.2 ppm).

Adsorption experiments

The experiments were performed using 0.05 g of the nano-faujasite suspended in 120 mL of single metal ion aqueous solutions (containing Zn2+, Cd2+ or Cu2+ ions), at the optimal pH conditions, ion concentration and contact time determined in preliminary experiments. After the uptake of the metals, solution aliquots were withdrawn in duplicate and quantified by atomic absorption spectrometry (Varian, model 55B SpectrAA). The removal efficiency (R) of the ions adsorbed at equilibrium and the adsorption capacity (qe in mg·g−1) were determined according to Equations (1) and (2), respectively.
formula
(1)
formula
(2)
where C0 and Ce are the initial and the equilibrium concentrations (in mg·L−1) of the metal ions, respectively, V is the solution volume (in L) and m is the zeolite mass (in g).

For the recording of the adsorption isotherms, a sample of 0.05 g of the zeolite was suspended in 120 mL of the single solution containing Zn2+, Cd2+ or Cu2+ ions at different concentrations in the range from 20 to 160 mg·L−1 during 40 min. After determining the best concentration of metal uptake, kinetic studies were performed and aliquots were collected at different periods (5–120 min).

The non-linear kinetic data corresponding to the adsorption of Zn2+, Cu2+ and Cd2+ ions were studied according to the pseudo-first order, pseudo-second order and intra-particle diffusion (Morris-Weber) models (Equations (3)–(5)) (Thanos et al. 2017), whereas the adsorption isotherm studies were investigated by Langmuir and Freundlich models in their non-linear forms (Equations (6) and (7)) (Mthombeni et al. 2016).
formula
(3)
formula
(4)
formula
(5)
formula
(6)
formula
(7)

In these expressions, qt and qe (both in mg·g−1) are the amounts of ions adsorbed on the zeolite at any time t (min) and at equilibrium, respectively; K1 (min−1) and K2 (g.mg−1·min−1) are the pseudo-first-order and pseudo-second-order rate constants of adsorption, respectively; and Kipd is the rate constant of intra-particle diffusion. Also, qmax is the maximum adsorption capacity (mg·g−1), KL (L·mg−1) is the Langmuir equilibrium constant, KF (mg·g−1) is the Freundlich constant (related to the adsorption capacity of the adsorbent) and 1/n is the empirical constant associated with surface heterogeneity.

From Equation (6), the dimensionless Langmuir parameter RL can be defined, according to:
formula
(8)
where for 0 < RL < 1 the adsorption process is considered favorable and for RL > 1 the adsorption process is considered unfavorable (Pandey et al. 2015).

In order to assay the reusability of the adsorbent, regeneration studies were conducted by ion exchange of the preliminarily used nano-faujasite, for 48 h in NaNO3 2.0 mol L−1 solution. The solid was filtered and dried in a desiccator for 24 h before reuse.

Electrochemical experiments aimed at analyzing copper recovery were conducted on Solartron ModuLab XM ECS equipment. The working electrode was composed of a copper rod (0.5 cm2), the counter electrode was composed of vitreous carbon and the reference electrode was a silver chloride electrode (Ag/AgCl/saturated KCl). Cyclic voltammetry (CV) measurements were performed over a potential range from 0.1 to −0.6 V vs Ag/AgCl at a scan rate of 25.0 mV s−1 for 120 min. The removal efficiency (R) of the recovered Cu2+ ions was calculated according to Equation (1).

For adsorption on competitive system studies, a sample of 0.05 g of the zeolite was suspended in 120 mL of the individual and multi-metal systems (Zn2+ + Cd2+ + Cu2+) with approximately 40 mg L−1 each, at 25 °C for 40 min.

Characterization of the samples

The XRD patterns of the nano-faujasite samples showed only broad and weak peaks after 3 h of crystallization time (Figure 1(a)). After 5 h of crystallization, both the intensity and the sharpness of the diffraction peaks were considerably enhanced, indicating a fast crystallization rate. The XRD patterns of these two samples agree with the literature results, suggesting that the synthesized solids are single-phase faujasite (Zhan et al. 2002; Chaves et al. 2012; Zhang et al. 2015; Motta et al. 2018).

Figure 1

XRD patterns recorded for faujasite solids prepared using different conditions: (a) samples prepared with different crystallization times; (b) effect of NaOH content. The symbol * indicates contamination with GIS phase.

Figure 1

XRD patterns recorded for faujasite solids prepared using different conditions: (a) samples prepared with different crystallization times; (b) effect of NaOH content. The symbol * indicates contamination with GIS phase.

Close modal

On the other hand, after 7 and 10 h of hydrothermal synthesis, an appreciable decrease in peak intensity was observed, suggesting that zeolite crystallization was complete within 5 h. Besides, the presence of XRD peaks due to the zeolite phase NaP1 (GIS type) was detected after 10 h (Chaves et al. 2012; Zhu et al. 2008). The peaks relative to this contaminant phase were more distinguishable for the solid obtained after 20 h of crystallization. Similar changes in XRD peak intensity accompanied by GIS phase formation were also reported by Zhu et al. (2008). Apparently, the decrease in intensity of the faujasite diffraction peaks may be related to the GIS phase transformation.

The nanocrystalline nature of the faujasite structure was evaluated from the XRD patterns shown in Figure 1, with the average crystallite size determined according to the Scherrer equation (Muniz et al. 2016). The value found for the average crystallite size was 15 nm for the sample Fau 5 h, consistent with previous reports involving nano-faujasite synthesis (Zhan et al. 2002; Chaves et al. 2012). However, the particle size estimated by N2 physisorption (Dext) is around 45 nm (Table 1). The smaller value of the average crystallite size compared to the particle size is an indication of the polycrystalline nature of the zeolite particles, in agreement with previous reports (Chaves et al. 2012).

Table 1

Textural analysis results and Si/Al molar ratios for the synthesis gel (nominal composition) and final solid product (from XRF)

SampleSi/AlgelSi/AlsolidSBET (m²·g−1)SEXT (m²·g−1)SMICRO (m². g−1)VMICRO (cm³·g−1)VTOTAL (cm³·g−1)Dext (nm)a
Fau 5 h 17.6 1.7 668 90 578 0.26 0.46 45 
SampleSi/AlgelSi/AlsolidSBET (m²·g−1)SEXT (m²·g−1)SMICRO (m². g−1)VMICRO (cm³·g−1)VTOTAL (cm³·g−1)Dext (nm)a
Fau 5 h 17.6 1.7 668 90 578 0.26 0.46 45 

SBET = SEXT + SMICRO, SBET = BET surface area, SEXT = external surface area, SMICRO = micropore surface area (t-plot), VMICRO = micropore volume (t-plot), Vtotal = total pore volume.

aCalculated from the equation Dext = 4061/Sext (Chaves et al. 2012).

The XRD patterns in Figure 1(b) depict the samples produced after 5 h of hydrothermal treatment by changing the NaOH content; that is, the alkalinity. The patterns are appreciably changed after reducing the NaOH content by both 10 and 50%, where contamination with the GIS phase is observed. Based on these results, it was found that changes in both the hydrothermal treatment time and the NaOH content can lead to the formation of the GIS phase. Thus, the sample prepared with 5 h of crystallization (named Fau 5 h) was chosen for the other studies to be described in the sequence.

The particle morphology of the faujasite sample (Fau 5 h) is depicted in Figure 2(a). The SEM image reveals the presence of particles with spheroidal shape, most in the submicron size range (500–650 nm). The particle sizes observed by SEM are considerably larger than the average crystallite sizes determined by XRD, evidencing once more the polycrystalline nature of the zeolite particles. Also, EDS analyses (omitted results) show that Si, Al, O and Na are the major elements present in their chemical composition, as expected.

Figure 2

SEM image, 14,000 times magnification (a) and N2 physisorption isotherm of nano-faujasite (Fau 5 h).s

Figure 2

SEM image, 14,000 times magnification (a) and N2 physisorption isotherm of nano-faujasite (Fau 5 h).s

Close modal

The Si/Al molar ratio was determined by both EDS and XRF analyses. The value obtained by EDS (1.5) was found to be close to that obtained by XRF (1.7); both these values are much smaller than the molar ratio of the synthesis gel (Table 1). Probably, due to the greater solubility of Si species in the alkaline medium, a considerable amount of these species remains dissolved in the supernatant during the crystallization process, while most Al atoms are preferably inserted into the zeolite framework (Chaves et al. 2012, 2015).

The N2 physisorption isotherm of Fau 5 h sample reveals features of a type I isotherm (Figure 2(b)), typical of microporous materials, with high N2 uptake at low relative pressure. However, the amount of N2 adsorbed with rising pressure is not constant, as is usually found for microporous solids due to the monolayer formation. Instead, the adsorption increases gradually with increasing P/P0. Since this solid is essentially microporous, as can be seen in Table 1 (high SMICRO), this unexpected behavior is likely to be a consequence of N2 uptake in inter-particle voids formed by the aggregation of the particles (Zhan et al. 2002; Chaves et al. 2012; Reinoso et al. 2018). In addition, a H4 hysteresis loop is observed, characteristic of solids containing mesopores and consisting of non-rigid aggregates of particles (Leofanti et al. 1998).

Regarding the textural results given in Table 1, the specific surface area and the external surface area are typical of nanostructured faujasite, which is consistent with the reduced crystallite size observed by XRD and by N2 physisorption analysis and also in agreement with other recent reports dealing with the synthesis of nanostructured faujasites (Zhan et al. 2002; Chaves et al. 2012; Zhang et al. 2015).

The local arrangements of Si and Al atoms were investigated by 29Si and 27Al MAS NMR experiments, respectively (Figures 3(a) and 3(b)). The 29Si NMR spectrum shown in Figure 3(a) reveals well-defined peaks at −81, −85 and −90 ppm, corresponding to chemical environments Si (4Al), Si (3Al) and Si (2Al), respectively; a weak shoulder is also observed, which can be described as a superposition of signals at −96 and −101 ppm, attributed to Si (1Al) and Si(0Al) species, respectively. The low-intensity signal relative to the Si (0Al) species indicates a solid with high aluminum content. The Si/Al molar ratio calculated from the 29Si NMR spectra (Zhan et al. 2002; Motta et al. 2018) was 1.4, which is consistent with the results obtained by EDS and XRF analyses.

Figure 3

Solid-state single pulse 29Si MAS NMR (a) and 27Al MAS NMR (b) spectra of nano-faujasite (Fau 5 h).

Figure 3

Solid-state single pulse 29Si MAS NMR (a) and 27Al MAS NMR (b) spectra of nano-faujasite (Fau 5 h).

Close modal

It is worth noting here that the presence of well-defined signals in the 29Si NMR spectrum indicates a high degree of local ordering of Si atoms, contrasting with the presence of relatively broad diffraction peaks verified by XRD. Whereas XRD gives information on long-range structural ordering, the signals observed in the NMR spectra are sensitive to local ordering of atoms. Thus, the observation of broad XRD peaks and well-defined 29Si NMR signals is indicative of the occurrence of a nanostructured arrangement in the nano-faujasite material.

The 27Al MAS NMR spectrum obtained for the Fau 5 h sample (Figure 3(b)) is composed of an intense signal centered at 58 ppm, due to tetra-coordinated Al in the zeolitic framework (Pietre et al. 2012) .These structural Al sites are responsible for the negative charge in the network and, therefore, are important for ion exchange processes. The 27Al MAS NMR results thus suggest that nearly all Al atoms are inserted into the nanostructure, ensuring maximum efficiency in metallic ions removal.

Thus, based on the presented characterization results, a high metal ion removal capacity by the nano-faujasite sample can be anticipated, since this material exhibits high aluminum content (i.e. low Si/Al molar ratio) and considerably large surface area and pore volume, providing a solid with high density of accessible ion exchange sites.

Adsorption experiments

Effect of initial pH

The adsorption behavior of Zn2+, Cd2+ and Cu2+ ions on the nano-faujasite sample was obviously affected by the initial solution pH (Figure 4). The experiments were carried out in the pH range of 3.0–6.2. Under very acidic solutions (pH < 4.0), the great competition between H3O+ and metal cations for the faujasite active sites led to the lowest metal removal efficiency. In other words, both H3O+ and M2+ ions compete for the negative charge present in the zeolite framework.

Figure 4

Effect of initial solution pH (a), Effect of adsorbent mass (b) and initial metal ion concentration (c) on uptake of Zn2+, Cd2+ and Cu2+ ions by nano-faujasite (Fau 5 h). Conditions: solution volume = 120 mL; metal concentration ≈ 40 mg·L−1; adsorbent mass = 50 mg; pH = 5.0 (Zn2+) or 5.2 (Cd2+ and Cu2+) and contact time = 40 min.

Figure 4

Effect of initial solution pH (a), Effect of adsorbent mass (b) and initial metal ion concentration (c) on uptake of Zn2+, Cd2+ and Cu2+ ions by nano-faujasite (Fau 5 h). Conditions: solution volume = 120 mL; metal concentration ≈ 40 mg·L−1; adsorbent mass = 50 mg; pH = 5.0 (Zn2+) or 5.2 (Cd2+ and Cu2+) and contact time = 40 min.

Close modal

Alternatively, some Al3+ ions may be leached out from the zeolitic framework in very acidic conditions, which reduces the availability of ion exchange sites (Moller & Bein 2013). On the other hand, both metal oxide and metal hydroxide precipitation may take place at pH values above the range used in the experiments shown in Figure 4(a), which could mask the results; thus, no adsorption experiment was performed under these conditions. The maximum removal efficiency was achieved at pH 5.0 (for Zn2+) and 5.2 (for Cd2+ and Cu2+), which corresponds to the pH of the as-prepared ion solutions, without further pH adjustment. In this sense, this result indicates that the electrostatic interaction between the negatively charged surface of faujasite and the metal cations is favored by using natural solution pH. Similar behavior was detected recently by our group in studies involving the uptake of cadmium, copper and zinc ions by other synthetic zeolites (Oliveira et al. 2017; Pratti et al. 2019).

Effect of adsorbent dosage

Another important parameter for identification of the optimal operating conditions in the uptake process is the amount of adsorbent, as it might influence the availability of adsorption sites. As verified in Figure 4(b), for all studied metals, an increase in the metal uptake by increasing the zeolite dosage is observed for small dosage values, due to an increased number of active sites available for adsorption. However, the amount of adsorbed metal decreases when larger amounts of the adsorbent are used. Such effect is probably associated with a decrease in the amount of available adsorption sites. This behavior may result from the electrostatic interactions, interference between binding sites, and reduced mixing at higher adsorbent densities (Çoruh et al. 2010). Also, particle aggregation may take place, reducing the effective surface area available for adsorption.

The maximum metal retention occurs at 50 mg of faujasite (for zinc and cadmium) and 75 mg for copper. Regarding Cu2+ removal, it is worth mentioning that, after the addition of 75 mg of zeolite, the solution immediately became turbid, probably due to copper hydroxide formation, as a consequence of the pH increase. Based on these results, the most appropriate amount of adsorbent chosen for the next batch experiments of metal removal was 50 mg.

Effect of initial metal ion concentration and adsorption isotherms

Figure 4(c) displays the metal uptake on nano-faujasite as a function of initial metal ion concentration. It can be observed that the metal ion capture decreases with the increase in the solution concentration. This behavior can be attributed to the variations in the diffusion rates (driving forces to overcome the mass transfer resistance) of Cu2+, Cd2+ and Zn2+ ions towards the zeolite active sites in less concentrated solutions (Pandey et al. 2015). The maximum removal efficiencies are observed at the lowest metal ion concentrations (20–30 mg·L−1), but considerable metal ion uptake is detected up to approximately 40 mg·L−1. Thus, an initial metal ion concentration of 40 mg·L−1 was chosen for the subsequent tests.

The study of equilibrium adsorption isotherms is an important tool to evaluate the adsorption capacity of a specific adsorbent. From the equilibrium adsorption data, fundamental parameters to design a practical operating procedure may be obtained. In this study, Langmuir and Freundlich models were investigated to analyze the data for the adsorption of Zn2+, Cu2+ and Cd2+ ions on nano-faujasite (Fau 5 h); the adsorption data and the corresponding nonlinear fittings are shown in Figure 5 and the parameters obtained from these fittings are reported in Table 2.

Table 2

Parameters obtained from the Langmuir and Freundlich models used to analyze the adsorption of Zn2+, Cd2+ and Cu2+ ions onto nano-faujasite (Fau 5 h)

ModelsParameterFau 5 h
Zn2+Cd2+Cu2+
Langmuir qmax (mg·g−1114.6 ± 2.3 132.8 ± 3.2 99.2 ± 2.5 
KL (L.mg−10.051 0.070 0.042 
RL 0.099–0.489 0.050–0.324 0.043–0.180 
R2 0.995 0.994 0.993 
RSS 5.947 8.886 4.216 
Freundlich KF (mg·g−118.8 26.6 15.2 
1/n 0.373 0.345 0.358 
R2 0.950 0.970 0.952 
RSS 62.313 43.225 97.336 
ModelsParameterFau 5 h
Zn2+Cd2+Cu2+
Langmuir qmax (mg·g−1114.6 ± 2.3 132.8 ± 3.2 99.2 ± 2.5 
KL (L.mg−10.051 0.070 0.042 
RL 0.099–0.489 0.050–0.324 0.043–0.180 
R2 0.995 0.994 0.993 
RSS 5.947 8.886 4.216 
Freundlich KF (mg·g−118.8 26.6 15.2 
1/n 0.373 0.345 0.358 
R2 0.950 0.970 0.952 
RSS 62.313 43.225 97.336 
Figure 5

Equilibrium adsorption isotherm data for the adsorption of Zn2+, Cu2+ and Cd2+ ions onto nano-faujasite (Fau 5 h), with the corresponding nonlinear fittings using the Langmuir and Freundlich isotherm models.

Figure 5

Equilibrium adsorption isotherm data for the adsorption of Zn2+, Cu2+ and Cd2+ ions onto nano-faujasite (Fau 5 h), with the corresponding nonlinear fittings using the Langmuir and Freundlich isotherm models.

Close modal

The coefficients of determination (R2) associated with the Langmuir isotherm models for all studied ion metals (Table 2) were slightly higher (R2 ≥ 0.99) than those of Freundlich isotherm models, evidencing an adsorption of the ion metals on the zeolite via the formation of a monolayer surface. This behavior can be attributed to the fact that the nano-faujasite combines high surface area and small particle size for metal adsorption.

The faujasite structure is comprised of a supercavity whose internal diameter is approximately 1.20 nm and a pore diameter of approximately 0.74 nm (Zhang et al. 2013). On the other hand, the hydrated ionic radii of the divalent ions Cd2+ (0.429 nm), Cu2+ (0.419 nm) and Zn2+ (0.430 nm) are all smaller than these values (Álvarez-Ayuso et al. 2003; Ibrahim et al. 2010). Apparently, all of the hydrated ions can easily diffuse through the zeolite pores and channels and be adsorbed on the monolayer surface of the nano-faujasite.

The dimensionless Langmuir parameter RL ranges from 0 to 1, which means a favorable process of adsorption of all metal ions on the nano-faujasite active sites. The value of the maximum adsorption capacity (qmax) is associated with the total capacity of the monolayer coverage for a given metal ion. According to Table 2, the monolayer capacity follows the selectivity sequence Cd2+> Zn2+> Cu2+. This trend follows exactly the order of increasing absolute values of the hydration energy of the metal ions. In fact, Cu2+ (−2,100 kJ·mol−1) and Zn2+ (−2,044 kJ·mol−1) ions present the highest hydration energy absolute values, whereas Cd2+ (−1,806 kJ·mol−1) ions have the lowest (Caputo & Pepe, 2007; Liu et al. 2019). This means that Cd2+ ions have less affinity to water molecules and can favorably interact with the nano-faujasite active sites more easily in relation to Cu2+ and Zn2+ ions, which tend to remain in solution.

Regarding the Freundlich parameters, 1/n is less than 1 in all cases, suggesting a favorable adsorption process. In addition, KF represents the adsorption capacity on a heterogeneous surface and the values found for each metal ion indicate a high affinity for the nano-faujasite surface (Karapinar & Donat 2009).

The nanostructured faujasite sample produced in this work is thus a solid with high adsorptive capacity (qmax) for the removal of Cd2+, Cu2+ and Zn2+ ions, when compared with some analogue systems involving different synthetic or natural zeolites (Table 3). The excellent metal uptake obtained relative to other zeolites can be understood as a consequence of: (i) low molar ratio Si/Al, which can provide high availability of ion exchange sites and (ii) high surface area and small particle size, allowing easy access of the metal ions to the nano-faujasite active sites. These features indicate the nano-faujasite sample as a feasible candidate for wastewater treatment.

Table 3

Comparative literature data on the maximum adsorption capacity (qm) for Cd2+, Cu2+ and Zn2+ ions on aluminosilicates

AluminosilicateIons (single system)qm (mg·g−1)aReferences
Faujasite-NaX Zn2+ and Cd2+ 199 and 144 Izidoro et al. (2013)  
Faujasite-NaX, NaA Zn2+ 108 and 119 Nibou et al. (2010)  
Faujasite nano-NaX Cu2+ 126 Ansari et al. (2015)  
Natural clinoptilolite Zn2+ 21.2 Çoruh (2008)  
Natural clay Zn2+ and Cu2+ 80.6 and 44.8 Velis & Alyuz (2007)  
Natural zeolite Cd2+ and Cu2+ 25.9 and 14.3 Taamneh & Sharadqah (2017)  
β-Zeolite Cd2+; Zn2+; Cu2+ 29.5; 29.5; 17.3 Pratti et al. (2019)  
Faujasite-NaX-from coal gangue Cu2+ 45.5 Lu et al. (2017)  
Nanostructured faujasite Cd2+; Zn2+; Cu2+ 132.8 ± 3.2; 114.6 ± 2.3; 99.2 ± 2.5 This work 
AluminosilicateIons (single system)qm (mg·g−1)aReferences
Faujasite-NaX Zn2+ and Cd2+ 199 and 144 Izidoro et al. (2013)  
Faujasite-NaX, NaA Zn2+ 108 and 119 Nibou et al. (2010)  
Faujasite nano-NaX Cu2+ 126 Ansari et al. (2015)  
Natural clinoptilolite Zn2+ 21.2 Çoruh (2008)  
Natural clay Zn2+ and Cu2+ 80.6 and 44.8 Velis & Alyuz (2007)  
Natural zeolite Cd2+ and Cu2+ 25.9 and 14.3 Taamneh & Sharadqah (2017)  
β-Zeolite Cd2+; Zn2+; Cu2+ 29.5; 29.5; 17.3 Pratti et al. (2019)  
Faujasite-NaX-from coal gangue Cu2+ 45.5 Lu et al. (2017)  
Nanostructured faujasite Cd2+; Zn2+; Cu2+ 132.8 ± 3.2; 114.6 ± 2.3; 99.2 ± 2.5 This work 

aqm values derived from the Langmuir model.

Effect of contact time and kinetic studies

In order to exanimate the kinetics of the adsorption of metal ions onto nano-faujasite, three models were tested to describe the experimental data: pseudo-first order, pseudo-second order and intra-particle diffusion; the fittings are exhibited in Figure 6(a)–6(d), while the parameters obtained from these analyses are summarized in Table 4.

Table 4

Kinetic data for the adsorption of Zn2+, Cd2+ and Cu2+ ions onto nano-faujasite (Fau 5 h)

Pseudo-first-order
Pseudo-second-order
Weber and Morris intra-particle diffusion parameters
qe expqeK1R2RSSqeK2R2RSSKipdR2
mg·g−1mg·g−1min−1mg·g−1g.mg−1.min−1mg·g−1.min−0.5
Fau 5 h Zn 64.6 62.6 0.244 0.991 4.3 65.6 0.007 0.993 3.2 1.491 0.257 
Cd 62.1 59.4 0.266 0.997 8.7 62.5 0.009 0.998 5.6 1.331 0.344 
Cu 47.1 45.0 0.189 0.982 6.2 48.7 0.006 0.996 4.5 1.816 0.617 
Pseudo-first-order
Pseudo-second-order
Weber and Morris intra-particle diffusion parameters
qe expqeK1R2RSSqeK2R2RSSKipdR2
mg·g−1mg·g−1min−1mg·g−1g.mg−1.min−1mg·g−1.min−0.5
Fau 5 h Zn 64.6 62.6 0.244 0.991 4.3 65.6 0.007 0.993 3.2 1.491 0.257 
Cd 62.1 59.4 0.266 0.997 8.7 62.5 0.009 0.998 5.6 1.331 0.344 
Cu 47.1 45.0 0.189 0.982 6.2 48.7 0.006 0.996 4.5 1.816 0.617 
Figure 6

Kinetic plots for metal ion adsorption onto nano-faujasite (Fau 5 h) (Figures 8(a)–8(c) = Pseudo-first-order and Pseudo-second-order fittings for Zn2+, Cu2+ and Cd2+). Figure 8(d) = intra-particle diffusion fittings for all metals studied). Conditions: adsorbent mass = 50 mg; solution volume = 120 mL; pH = 5.0 (Zn2+) or 5.2 (Cd2+ and Cu2+); metal concentration ≈ 40 mg·L−1.

Figure 6

Kinetic plots for metal ion adsorption onto nano-faujasite (Fau 5 h) (Figures 8(a)–8(c) = Pseudo-first-order and Pseudo-second-order fittings for Zn2+, Cu2+ and Cd2+). Figure 8(d) = intra-particle diffusion fittings for all metals studied). Conditions: adsorbent mass = 50 mg; solution volume = 120 mL; pH = 5.0 (Zn2+) or 5.2 (Cd2+ and Cu2+); metal concentration ≈ 40 mg·L−1.

Close modal

The plots shown in Figure 6(a)–6(c) show that the metal uptake increases rather sharply at the initial stages of adsorption (i.e. the first 0–10 min), due to the availability of active sites located on the external surface of the nano-faujasite; in the sequence, the adsorption proceeds more gradually up to 40 min, likely as a consequence of metal ions' intra-particle diffusion through the nano-faujasite pores to the inner adsorption active sites. Finally, the evidence for the full occupation of all adsorption sites can be evidenced by a plateau formation. Similar behavior has been recently observed in systems involving metal adsorption onto other aluminosilicates (Visa 2016; Oliveira et al. 2017). Thus, the interval of 40 min can be considered an industrially feasible period to reach the equilibrium for Cd2+, Zn2+ and Cu2+ ions uptake by the nano-faujasite sample.

According to Table 4, the experimental data were better fitted by the pseudo-second order model than the pseudo-first order model, by comparing the corresponding values of the coefficients of determination (R2). The comparison of the values of the residual sum of squares (RSS), also given in Table 4, confirms that the pseudo-second order model is the one that best fits the experimental data. This indicates that the metal ion uptake was not diffusion controlled, but instead the rate-limiting step is probably controlled by the ion exchange between the heavy metal cations and the Na+ ions located on the zeolite surface (Pratti et al. 2019). Also, the qe values obtained by the pseudo-second order model agree better with the qe exp values for all studied metal ions.

The intra-particle diffusion fittings for the metal ions uptake onto nano-faujasite are displayed in Figure 6(d), whereas Table 4 exhibits the rate constant of intra-particle diffusion (Kipd) and the respective R2 values. All the plots show a first step with a linear increase of qt followed by a practically horizontal plateau. It is noticed that low R2 values were found for this model.

The first step, with higher slope, corresponds to the external surface adsorption, where the metal ions diffuse through the solutions to the active sites, probably, on the external surface of the nano-faujasite sample (boundary layer diffusion) (Dinu & Dragan 2010; Stojakovic et al. 2011; Alver & Metin 2012). Deviations of the straight lines from the origin give an idea about the thickness of the boundary layer and are attributed to the difference in the rate of boundary layer diffusion in the initial stage of adsorption. The almost horizontal plateaus are related to intra-particle diffusion, as the final equilibrium stage is reached and the adsorption rate becomes very slow, with the observation of the maximum amount of adsorbed metal ions.

Thus, it can be verified that the external surface adsorption, observed at the first stage, is faster and also more important than the intra-particle diffusion, which occurs at the second stage. This apparently means that the intra-particle diffusion is not the only rate-limiting step, and the adsorption process occurs through external surface adsorption and intra-particle diffusion.

Regeneration studies and electrochemical recovery

After the adsorption process, it is fundamental to be able to restore the used adsorbents for further metal ion uptake cycles. This investigation is crucial to evaluate the cost-effectiveness ratio of an adsorbent, which becomes dependent on the number of adsorption-desorption cycles that the zeolite can perform while keeping a reasonable performance (Oliveira et al. 2017). Batch adsorption results are expressed in Figure 7. Due to the extensive loss of adsorbent during the tests (≈15 wt. % after three reuse cycles), no more than three regeneration cycles, with similar preparations, were performed. This experiment was carried out only for Cd2+ and Cu2+ ions because of their high toxicity according to the Brazilian law (CONAMA 2011).

Figure 7

Reusability experiments of adsorption of Cd2+ and Cu2+ ions onto nano-faujasite (Fau 5 h). Conditions: adsorbent mass = 50 mg; solution volume = 120 mL; pH = 5.2; metal concentration ≈ 40 mg·L−1; contact time = 40 min.

Figure 7

Reusability experiments of adsorption of Cd2+ and Cu2+ ions onto nano-faujasite (Fau 5 h). Conditions: adsorbent mass = 50 mg; solution volume = 120 mL; pH = 5.2; metal concentration ≈ 40 mg·L−1; contact time = 40 min.

Close modal

As can be inferred from Figure 7, the removal efficiency did not change by a considerable amount after three reuse cycles for Cd2+, resulting in a reduction of approximately 12 mg·L−1. However, this reduction was more pronounced for Cu2+, decreasing by around 16 mg·L−1 after three adsorption-desorption cycles.

According to previous studies (Oliveira et al. 2017; Pratti et al. 2019), the decrease in the adsorption capacity could be explained as a consequence of the reduction in the effectiveness of the ion exchange of Na+ ions with Cd2+ and Cu2+ ions during regeneration in subsequent cycles. However, even if a slight decrease in the adsorption capacity of the nano-faujasite sample was found with an increasing number of cycles, this adsorbent displayed admissible reusability in the treatment of contaminated solutions containing Cd2+ and Cu2+ ions, since high metal retention is still observed, suggesting that this solid can act as a promising adsorbent for practical effluent treatment processes.

To properly dispose of the Cu2+ ions contained in the regenerated solution, electrochemical deposition was investigated. The cyclic voltammogram (not shown) showed a cathodic peak at −0.3 V, corresponding to the reduction of Cu (II) on the surface of the copper electrode. An efficiency of 80.2% after 120 min was achieved for the recovery of copper in metallic form. This result shows that it is feasible to recover Cu2+ ions and that this is a promising route for the industrial segment, which means that significant progress can be achieved and strongly impact the industrial sector to reduce environmental impacts and establish sustainable processes.

Adsorption on multi-metal system

The knowledge about multi-metal system adsorption is crucial because industrial effluents generally contain more than one kind of metal ion. Figure 8 depicts the Cd2+, Cu2+ and Zn2+ ions' uptake in an individual and in a competitive system. In all cases, the adsorption capacity corresponding to each ion in a single system was found to be higher than in the multi-metal solution. This effect is associated with the competition for the adsorption sites between the various metal ions present in the solution (Nguyen et al. 2015).

Figure 8

Individual and multi-metal adsorption data for the adsorption of Zn2+, Cd2+ and Cu2+ ions onto nano-faujasite (Fau 5 h). Conditions: adsorbent mass = 50 mg; solution volume = 120 mL; metal concentration ≈ 40 mg·L−1; contact time = 40 min.

Figure 8

Individual and multi-metal adsorption data for the adsorption of Zn2+, Cd2+ and Cu2+ ions onto nano-faujasite (Fau 5 h). Conditions: adsorbent mass = 50 mg; solution volume = 120 mL; metal concentration ≈ 40 mg·L−1; contact time = 40 min.

Close modal

As stated earlier, the selective order in an individual system follows the order of increasing hydration energy absolute values of the metal ions (Cd2+ > Zn2+ > Cu2+). However, a different behavior was observed in the case of the multi-metal system. The selectivity order in the studied multi-metal system was: Cd2+ ≈ Cu2+ > Zn2+. In this case, differently to that found for the individual systems, no direct correlation with any particular physicochemical parameter was identified and the amounts of adsorbed Cd2+ and Cu2+ ions were very close to each other. This means that, besides the hydration energy, other parameters may also be influencing the adsorption behavior of the metal ions, such as the hydrated ionic radius of the metal ions. As Cu2+ ions have the smallest hydrated ionic radius among the studied metals, it can be assumed that Cu2+ ions might have easier access to the zeolite ion exchange sites, thus leading to larger qe values for the adsorption of Cu2+ ions. Thus, different behaviors are expected in single and competitive metal ion systems (Pratti et al. 2019).

The synthesis of nanostructured faujasite was successfully performed in the present work. The crystallization time of 5 h was found to be the ideal period for the formation of the nanocrystalline structure. The presence of a contaminant phase (zeolite phase NaP1, GIS type) was identified after 10 h of hydrothermal treatment. The contamination with the GIS phase was also observed after reducing the NaOH content by 10%. These results indicated that the formation of the nano-faujasite structure is strongly dependent on both the crystallization time and the alkalinity of the reaction synthesis medium. According to several characterization techniques used in this work, the produced nano-faujasite sample consists of a solid with particles in the submicron size range, high surface area, large micropore volume and low Si/Al molar ratio. These characteristics provided a solid with excellent ability to remove Cd2+, Zn2+ and Cu2+ ions from aqueous solutions. According to kinetic studies and adsorption isotherms, the metal ions' uptake takes place by ion exchange on the monolayer surface of the adsorbent. The copper electrochemically recovered in metallic form showed an efficiency of 80.2% after 120 min, which is indicative that this process can be adequately used for full-scale metal removal.

The authors are grateful to FAPERJ (E-26/211.776/2015) for financial support to this work. The support from CNPq, FAPES and CAPES is also gratefully acknowledged. The authors are thankful to Dr Gilmar Clemente (UFF) for his help with the electrochemical experiments and to UFRRJ for the N2 physisorption experiments.

Mariana B. Gonçalves: Investigation; Djanyna V. C. Schmidt: Investigation; Fabiana S. dos Santos: Investigation; Formal analysis; Writing – Review & Editing; Daniel F. Cipriano: Investigation; Gustavo R. Gonçalves: Investigation; Formal analysis; Jair C. C. Freitas: Funding acquisition; Formal analysis; Writing – Review & Editing; Mendelssolm K. de Pietre: Conceptualization; Funding acquisition; Formal analysis; Writing – Original Draft; Writing – Review & Editing; Project administration.

All relevant data are included in the paper or its Supplementary Information.

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