Abstract

In this study β-zeolite, ferrierite and partially delaminated PREFER (precursor of ferrierite) zeolites with several chemical and textural properties were synthesized for the removal of zinc and lead ion metals from their respective solutions. Adsorption experiments involving the suspension of tiny amounts of these solids in aqueous solutions containing either Zn2+ or Pb2+ showed that the removal of these metals at a considerable extent may be attained. Among the studied materials, β-zeolite displayed the better performance in metal ion removal, which may be ascribed to its higher aluminum content, surface area and external surface area, that allows a greater density and availability of ion adsorption active sites. Kinetic data from a pseudo-second-order model indicate that the chemical interaction among metal ions and active sites is the rate-limiting step. Furthermore, the better performance of the β-zeolite displayed in reusability testing makes it a potential adsorbent for future applications in the treatment of effluents containing toxic metals.

INTRODUCTION

It is a known fact that human activities may generate a huge amount of waste, which should undergo proper treatment and disposal procedures in order to minimize their hazardous effects to the environment. Among these, the industry-generated aqueous effluents containing dissolved heavy metal ions are one of the most attention deserving, because their improper disposal contaminates soils and natural waters at a rate that tends to be in phase with the ever increasing economic growth (Omidvar-Hosseini & Moeinpour 2016; Zhang et al. 2017).

Due to the fact that many of these heavy metals are not essential nutrients for the vast majority of organisms, they are hardly metabolized or eliminated from them. Their main fate once contaminating any organism is to be either incorporated into cell proteins and nuclear material, potentially causing cell damage, or plainly segregated into some cell corpuscles. This implies that it ends up being bioaccumulated upwards in the food chain, for instance the amount of Pb present in a fish living in contaminated waters may be orders higher than that found in lichens. Therefore, heavy metal bioaccumulation contribute to the increase of the number of diseases caused by the contamination of water, air and soil and the control of the concentration of these hazardous substances at very low limits is obliged by law in most countries (Misaelides 2011; Malamis & Katsou 2013). In this way, many technologies have been employed in order to minimize the presence of heavy metals in the environment. Studies indicate that among several remediation techniques, those based on the pollutant removal by the use of an adsorbent are promising because they promote extensive removal of these heavy metals at relatively lower costs (Fu & Wang 2011; Misaelides 2011; Díaz 2017). In this context, promising examples are inorganic aluminosilicates, specifically zeolites, which have been shown to be highly efficient in the adsorption process (Fu & Wang 2011; Misaelides 2011).

Zeolites are highly porous aluminosilicates, with crystalline, three-dimensional structure, formed by the linkage of TO4 (where T = Si and/or Al) oxygen sharing tetrahedral units (Ramos et al. 2013). Although it may occur naturally from the recombination of clay particles under particular geothermal conditions, the zeolites also may be synthetic, in that a number of different structures have been obtained by the careful control of preparation conditions (Cundy & Cox 2005). The combination of the inherent porosity of the zeolites, in addition to a negatively charged three-dimensional network, result in a successful interaction with the metal cations present in solution (Ramos et al. 2013; Díaz 2017).

It is noteworthy that there are a few works in the literature that describe the use of synthetic zeolite applications for metal recovery (or for the remediation of heavy metal contamination in bodies of water). Nevertheless, the use of these solids for environmental remediation may open up interesting perspectives in this segment. The use of natural zeolites may be a problem because these natural solids generally contain impurities and small pore systems that can hinder the adsorption process (Perego et al. 2013). On the other hand, the use of synthetic zeolites results in a controllable pore structure and chemical composition, via the control of the relative amounts the Si4+ and Al3+ ions in the structure. Furthermore, the isomorphous replacement of Si4+ by Al3+ ions in the structure gives rise to a negative charge in the framework, which must be balanced by cations (usually ion exchangeable sodium and potassium). Thus, structures with both high Al content and large pore sizes are potentially desirable for ion exchange operations (Cundy & Cox 2005; Perego et al. 2013).

Based on this information, β-zeolite (also known as zeolite beta) and the PREFER (precursor of ferrierite) lamellar precursor may be suitable for the process. The β-zeolite has a three-dimensional network that displays large pores when compared to other microporous solids and high Al insertion capacity. This structure comprises the arrangement of two ring types of approximately 12-MR (membered ring), with pore diameters of 0.76 × 0.64 nm, in the a- and b-axes directions and 0.55 × 0.55 nm in the channel parallel to the c-axis. These diameters are similar to other large pore molecular sieves such as faujasite (FAU) (Sazama et al. 2014; Van Oers et al. 2014).

The precursor of the ferrierite zeolite, PREFER, is a lamellar solid, which is synthesized for the first time by Schreyeck et al. (1996). This structure has attracted the attention of researchers because it may be transformed into various materials, the zeolite ferrierite itself, ITQ-36 (Instituto de Tecnología Química) and ITQ-6 (Pietre et al. 2011; Ramos et al. 2013).

The structure of the ferrierite zeolite may be obtained through the calcination of the PREFER at relatively high temperatures (>500 °C). The ferrierite network is composed of 10-MR (4.3 × 5.5 Å) parallel to the c-axis interconnected perpendicularly by channels defined by 8-MR (3.4 × 4.8 Å) parallel to the b-axis (Ramos et al. 2013).

The complete and/or partial delamination of the PREFER can be carried out by its suspension in a solution containing cetyltrimethylammonium (CTA+) salts and tetrapropylammonium hydroxide (TPAOH), followed by ultrasound treatment (Pietre et al. 2011). Such treatment causes the expansion of the layer by the insertion of the bulky organic cations between them. In some cases it is desirable to prepare the partially exfoliated solid to minimize both structural collapse and loss of Al (Chica et al. 2009).

In this way, the present contribution presents the synthesis of the following materials: β-zeolite and the layered precursor, PREFER. The PREFER-based solids used in this study were obtained from (i) its transformation into ferrierite zeolite and (ii) from its partial delamination. The latter PREFER-based material was synthesized to ensure more accessibility of its pores to dissolved adsorbates. Those materials were then tested for adsorption of Zn2+ and Pb2+ ions from aqueous solutions containing them. Adsorption experiments using both fresh and recycled solid adsorbents were carried out in order to verify the ability of these adsorbents to remove toxic metal ions, as well as their reusability.

METHODS

Synthesis of β-zeolite

The synthesis of the β-zeolite was adapted by Zhang et al. (2011). Initially, 0.19 g of NaOH and 0.76 g of NaAlO2 were added into 32.50 g of tetraethylammoniumhydroxide aqueous solution (TEAOH 20%, Aldrich). The suspension was mechanically stirred for approximately 10 min until total dissolution. Afterwards, 32.50 g of tetraethylorthosilicate (TEOS, Aldrich) was slowly added. The mixture was kept under constant stirring for 4 h at room temperature before being transferred to a stainless steel autoclave lined with Teflon and maintained at 140 °C for 48 h. The solid was washed, filtered and dried overnight in a desiccator before calcination at 560 °C at 1 °C·min−1 in O2 atmosphere for 5 h. This solid was labeled β-zeolite.

Synthesis of PREFER and its derivatives

The synthesis of layered precursor, PREFER, was adapted from Pietre et al. (2011). The procedure consists of adding 9.32 g of NH4F (Vetec into 8 mL of 11% HF aqueous solution followed by 10.04 g of fumed silica (Aldrich) and 1.12 g of NaAlO2. The mixture was mechanically stirred until a homogeneous gel was obtained. The structure directing agent (4-amino-2,2,6,6-tetramethylpiperidine, Fluka), was slowly added with the aid of a Pasteur pipette. After complete addition, the gel was mechanically stirred for 180 min and transferred into a Teflon-lined stainless steel autoclave, which was maintained for 5 days at 175 °C. The resulting solid was washed, filtered and dried overnight in a desiccator before further treatment.

After preparation, the PREFER was transformed into two distinct derivatives. The first treatment corresponds to the calcination of a part of the layered precursor under the same conditions as already described for the β-zeolite synthesis, creating the zeolite ferrierite (three-dimensional solid). The second procedure consisted of the partial exfoliation of another part of the PREFER solid, according to a procedure modified from Pietre et al. (2011) which is intended to avoid extensive destruction of the structure. The latter mentioned procedure was carried out as follows: 8 g of an aqueous suspension containing 25%wt. of cetyltrimethylammonium bromide (Aldrich) was mixed with 8 mL of tetraethylammonium hydroxide aqueous solution (TEAOH 20%, Aldrich). The mixture was then stirred at room temperature for 2 h and added to a suspension containing 10 mL of distilled water and 2 g of the layered material. The reaction mixture was refluxed at 60 °C for 20 h.

After this period, water was added to the reaction mixture until pH reached 13.0. Then the system was sonicated (50 W, 40 kHz) for 1 h at room temperature, keeping the temperature below 50 °C. The PREFER partially delaminated was obtained after calcination of the sample at 560 °C and 1 °C min−1 under O2 atmosphere for 5 h. This sample will hereafter be labeled as PREFER DEL.

Characterization

The solids were characterized by X-ray diffraction (XRD). For the analysis, Rigaku MiniFlex equipment with CuKα radiation (40 KW, 40 mA) was used and scanned from 5 to 50° (2θ), with a scanning speed of 2° min−1.

The porosity of the samples was analyzed by N2 physisorption at −96 °C on a Micromeritics ASAP2420 equipment. The samples were previously treated under vacuum at 200 °C for 4 h before the measurements. From isotherm data, the specific surface area using Brunauer, Emmett and Teller (BET) formalism (SBET), the average pore size using the Barrett–Joyner–Halenda (BJH) method (desorption branch), the external surface area (Sext, by t-plot method), total volume (VTotal) and micropore volume (Vmicro by t-plot method), were determined.

The semi-quantitative elemental analysis was performed by X-ray fluorescence spectrometry (XRF) using the Bruker S8 TIGER.

Adsorption experiments

Aqueous solutions containing Zn2+ and Pb2+ concentrations of 100 mg·L−1 were prepared, respectively, from zinc nitrate hexahydrate (Zn(NO3)2.6H2O) salt (Dinâmica) and diluting a lead nitrate at 1,000 ppm standard solution (SpecSol). Adsorption studies were carried out at 25 °C, under constant stirring by varying the pH (3.5–6.5), contact time (5–120 min) and adsorbent dosage (50 or 100 mg).

In a typical experiment, 0.05 g of the solids were suspended in 60 mL of either zinc or lead solutions at 100 mg·L−1. The first set of experiments was performed at different pH values, processed over pH range 3.5–6.5. The pH was adjusted using HNO3 or NaOH dilute solutions.

After the uptake of Pb2+ or Zn2+ on the zeolites, solution aliquots were withdrawn in duplicate and quantified by atomic absorption spectrometry (Varian, model 55B SpectrAA). The removal efficiency (%) of the ions adsorbed at equilibrium per unit mass were determined according to the equations:  
formula
(1)
 
formula
(2)
where C0 and Ce are the initial and the equilibrium concentrations of Pb2+ and Zn2+, V(L) is the solution volume, m(g) is the mass of the zeolite and Qe is the calculated amount of metal adsorbed onto the zeolite.

In relation to the effect of the pH on metal uptake, the evaluation of contact time was performed on time intervals of 5 to 120 minutes. The amount of adsorbent, volume and concentration of the solutions were the same as stated above. The total amount of aliquots withdrawn each time from the experiment did not exceed more than 5% of the initial volume. Kinetic studies using the best performing adsorbent materials were also carried out and the parameters values obtained were used to predict the adsorption process.

Experiments to determine the number of cycles necessary to decrease metal concentration to that required by Brazilian environmental law was carried out. These experiments consisted of cycles that involved (i) metal removal by a small amount of the adsorbent, followed by solution filtering and then (ii) another metal removal step with new, fresh zeolite aliquots. This cycle was repeated until reaching the metal concentrations required by legislation. The final metal retention by the filter paper was assumed to be negligible throughout these experiments.

The tests were carried out under the same conditions stated above, i.e. 60 mL of the metal solution (approximately 100 mg·L−1) and with the mass and zeolite type that presented the best performance in adsorption tests, under appropriate pH during 30 min.

In order to investigate the reusability of the adsorbent, regeneration tests using the same zeolite with the best performance in adsorption experiments were performed three times. Before use, the solid was ion exchanged for 24 h in NaNO3 1.0 molL−1 solution, followed by filtration and dried in a desiccator for 24 h.

RESULTS AND DISCUSSION

Characterization

As already mentioned, the zeolite ferrierite may be prepared from the PREFER precursor at the calcination step. Alternatively, the partially exfoliated product (from now on referred to as PREFER DEL) may also be prepared by a controlled delamination procedure. Figure 1(a) displays the XRD patterns of the PREFER precursor and its modified derivatives.

Figure 1

XRD of the (a) PREFER and its derivatives, calcined ferrierite and PREFER DEL; (b) calcined β-zeolite.

Figure 1

XRD of the (a) PREFER and its derivatives, calcined ferrierite and PREFER DEL; (b) calcined β-zeolite.

Agreement between the XRD profile of the PREFER and zeolite ferrierite samples and that obtained in previous studies indicated that the phases were successfully obtained with non-detectable impurities (Schreyeck et al. 1996; Rakoczy et al. 2002; Pietre et al. 2011).

It is common to find in literature reported diffraction profiles of ferrierite samples with slight differences in both 2θ and relative intensities. This may be related to preferential crystalline plane growth, as well as eventual interplanar distance contraction/expansion (Rice et al. 1994; Isobe et al. 2012; Pan et al. 2014). Furthermore, differences in the shapes and average crystallite sizes can also be found in the literature regarding this material. All these structural variations may be related to different crystallization rates of the solids depending on the particular physicochemical conditions and reagents used in such reported preparations. Nevertheless, the most intense peaks at 0 2 0 and 0 4 0 (Figure 1) were found in several reported XRD data (Schreyeck et al. 1996).

Regarding the solid obtained from the delamination procedure, PREFER DEL, a structural disorder in the material was verified through peak broadening and low intensity (Pietre et al. 2011). Thus, it is possible that the final product corresponds to the partially exfoliated solid with some expressive amount of ferrierite phase because intense and defined peaks corresponding to this zeolitic structure are still observed.

As for the β-zeolite (Figure 1(b)), a well-defined diffraction pattern with non-detectable impurities was verified, in accordance with that presented by Zhang et al. (2011), characteristic of the β-structure.

The ion-exchange character in these zeolitic materials is closely related to the Si/Al atomic ratio. We therefore sought to determine the Si/Al ratio by FRX analysis. The results are displayed in Table 1.

Table 1

Textural analysis of the solids and Si/Al molar ratios for the synthesis gel (theoretical) and final solid (FRX)

Structure Si/Al (gel-theoretical) Si/Al (solid−FRX) SBET (m2 g−1Sext (m2 g−1VTotal (cm³ g−1Vmicro (cm³ g−1
BETA 44 34 522 125 0.097 0.187 
FERRIERITE 50 107 143 26 0.080 0.056 
PREFER DEL 50 133 340 80 0.237 0.134 
Structure Si/Al (gel-theoretical) Si/Al (solid−FRX) SBET (m2 g−1Sext (m2 g−1VTotal (cm³ g−1Vmicro (cm³ g−1
BETA 44 34 522 125 0.097 0.187 
FERRIERITE 50 107 143 26 0.080 0.056 
PREFER DEL 50 133 340 80 0.237 0.134 

SBET = total surface area, VTotal = total volume of pores (BJH desorption branch), SEXT = external surface area, SEXT = SBET – SMICRO, Vmicro = micropore volume by t-plot method.

The lower Si/Al molar ratio (higher Al content) found in the β-zeolite when compared to the synthetic gel indicates that not all Si could be incorporated into the solid but remained soluble in the supernatant during the hydrothermal synthesis.

On the other hand, approximately half of the total Al in the synthetic gel (high Si/Al molar ratio) was inserted into the zeolite ferrierite. Furthermore, the Al content is almost three times higher in β-zeolite compared to ferrierite sample. For the partially exfoliated solid, as expected, more Al extraction was observed as a consequence of the delamination procedure.

Regarding the textural property values of the solids displayed in Table 1, β-zeolite shows both the highest specific surface (BET) and external surface area compared to the PREFER derivatives samples, which indicate more accessible adsorption sites.

When comparing PREFER derivatives samples, the exfoliated solid presents both much larger total area (BET) and total volume of pores (VTotal-BJH method) than the ferrierite material (Table 1), with a slightly larger external area (Sext), as verified by the t-plot (not shown). Since the amounts of micropores detected by t-plot are expressive (Vmicro), some non-delaminated ferrierite remained in the final product as also observed by the XRD pattern of the PREFER DEL solid. Therefore, based on textural values and elemental analysis, it can be assumed that β-zeolite may be more active for the adsorption of heavy metals ions from aqueous solutions due to its higher aluminum content and the largest total/external surface area, ensuring a solid with high density of accessible ion exchange sites.

Adsorption experiments

Effect of initial pH

The pH of aqueous solutions plays an important role in the ion-exchange process, influencing the character of both the exchangeable ions and the zeolite itself.

Under very acidic solutions (usually pH < 3.0) there is great competition between metal cations and the protons present in solution by the zeolite exchange sites. On the other hand, the precipitation of metal ions eventually occurs when alkaline solutions are used (usually pH > 7.0), which means that any adsorptive processes should occur in a well determined pH range (Çoruh et al. 2010; Yang et al. 2016). In general, at optimal pH conditions the adsorbate (metal ion) and adsorbent (zeolitic surface) must have opposite charges for a greater electrostatic interaction. In order to find optimized adsorption conditions, a study involving the effect of pH on the amount of metal ion adsorption was undertaken.

Figure 2 displays the effect of pH on the adsorption of Zn(II) and Pb(II) ions on β-zeolite, as well as ferrierite, respectively. Considering the zeolite ferrierite, changing the initial pH of zinc solution does not significantly influence the adsorption efficiency of the solid. On the other hand, the adsorption with β-zeolite in acidic medium (pH = 3.5) results in a greater competition between H3O+ ions and the metal ions, reducing the uptake of Zn2+ on the zeolite surface. Even though the solid has not been characterized after adsorption, another possibility for the lower efficiency of β-zeolite at acidic medium is that in such conditions some Al3+ may be leached out of the zeolite network, which reduces the amount of available ion exchange sites (Ören & Kaya 2006).

Figure 2

The effect of pH on zinc and lead adsorption. Conditions: t = 30 min, adsorbent mass = 50 mg, V = 60 mL and metal concentration = 100 mg·L−1.

Figure 2

The effect of pH on zinc and lead adsorption. Conditions: t = 30 min, adsorbent mass = 50 mg, V = 60 mL and metal concentration = 100 mg·L−1.

As shown in Figure 2, the adsorption capacity of β-zeolite reached a maximum at pH values 4.5 and 5.5. Taking into account that the pH of the solution containing Zn(II) was approximately 5.5, this value was adopted for the adsorption tests involving as-prepared Zn(II) solutions, without further pH adjustment.

The results for lead adsorption using ferrierite zeolite under different pH values are also presented in Figure 2. Again, it is verified that the pH control is fundamental in the adsorption studies, where the best performance was observed for pH 5.5. As for pH 4.5, there is probably a greater competition between the metal ions and the protons of the medium, which prevents the adsorption to be effective, decreasing its performance. At pH 6.5, a lower uptake of lead was also observed. Based on the speciation studies involving lead species in aqueous solutions (Huang et al. 2012), Pb2+ species is rather unstable and in fact hydrolyzes to form Pb(OH)+. The formation of this specie may be related to the efficacy of lead adsorption because Huang et al. (2012) also found a decrease in the lead adsorption at higher pH.

The pH studies on lead with β-zeolite was not used because, as will be shown later (effect of adsorbent dosage), increasing the adsorbent mass leads to a decrease in the pH of the system and, consequently, a lower uptake of Pb2+ is observed. In the same way, it would be reasonable to suppose that for higher pH values a decrease in adsorption is expected due the formation of Pb(OH)+. Based on this information, the optimum pH value at which the maximum metal uptake could be achieved was 5.5. Thus, for all subsequent adsorption studies for both zinc and lead, this pH value was used.

Effect of contact time and adsorbent dosage

Figure 3(a)3(c) display the uptake of zinc ions as a function of time. In this study, different dosages of adsorbents were used. The contact time ranges from 5 to 120 min and the results showed that for 50 mg of all adsorbents there was a rapid uptake within 5 min when the equilibrium of the system was achieved.

Figure 3

Effect of contact time and adsorbent dosage on zinc ions adsorption: (a) ferrierite; (b) PREFER DEL and (c) β-zeolite. Conditions: 50 or 100 mg of adsorbent, V = 60 mL and pH = 5.5.

Figure 3

Effect of contact time and adsorbent dosage on zinc ions adsorption: (a) ferrierite; (b) PREFER DEL and (c) β-zeolite. Conditions: 50 or 100 mg of adsorbent, V = 60 mL and pH = 5.5.

Figure 3 shows that the increase of zeolite mass from 50 to 100 mg exerted little or no effect when the PREFER derivatives samples were used (Figure 3(a) and 3(b)). More specifically, no difference was noticed for zinc ion removal with increasing mass of PREFER DEL sample. Additionally, the improvement in adsorption for 100 mg of zeolite ferrierite did not exceed 10%. These results suggests that a further increase in the adsorbent dosage would not provide a great availability of exchangeable sites and no significant enhancement in zinc ion removal efficiency were observed for PREFER derivatives. Particle aggregation at high solid dosage may have occurred and resulted in a decrease in the surface area of the material (Çoruh et al. 2010). It may also result in electrostatic interactions and cause interferences between biding sites at higher adsorbents densities (Çoruh et al. 2010; Alswata et al. 2017).

Figure 3(c) shows the effect of β-zeolite content in the zinc ion removal. A considerable increase in the Zn2+ uptake percentage with adsorbent dosage from 40% at 50 mg to 58% at 100 mg (in 30 min) is observed. However small, this observed increase in Zn2+ uptake with further amount of adsorbent is more significant than what was observed in the PREFER-derived samples. This effect can be attributed to the higher BET and external surface areas, besides the larger size of the micropores observed in the β-zeolite when compared to ferrierite and PREFER DEL solids.

The results obtained from the studies on Pb(II) adsorption are displayed in Figure 4(a)4(c). A decrease in lead ions removal with increasing adsorbent mass for all adsorbents was found. The increase in zeolite mass (50 mg to 100 mg) resulted in a decrease in the pH of the medium from 5.5 to 4.2, respectively. As already verified in Figure 2, in low pH values the Pb(II) uptake percentage significantly decreases.

Figure 4

Effect of contact time and adsorbent dosage on Pb(II) adsorption: (a) ferrierite; (b) PREFER DEL and (c) β-zeolite. Conditions: 50 or 100 mg of adsorbent, V = 60 mL and pH = 5.5.

Figure 4

Effect of contact time and adsorbent dosage on Pb(II) adsorption: (a) ferrierite; (b) PREFER DEL and (c) β-zeolite. Conditions: 50 or 100 mg of adsorbent, V = 60 mL and pH = 5.5.

Figures 3 and 4 show an improved performance of β-zeolite for removal of both Zn(II) and Pb(II) ions when compared with PREFER derived adsorbents. This better performance displayed by the β-zeolite may be explained in terms of (i) a higher density of Al-related sites, which is responsible for the ion exchange event, (ii) wider pore system dimensions of 7.6 × 6.4 Å (a- and b-axes) and 5.5 × 5.5 Å (in the channel parallel to c-axis), in comparison to ferrierite pore system of 3.5 × 4.8 Å and 4.2 × 5.4 Å in the 8- and 10-MR channels (Mintova et al. 2006; Ramos et al. 2013; Sazama et al. 2014), and (iii) much larger surface area/external surface area than others adsorbents.

The fact that Pb(II) was removed in a higher extent than Zn(II) may be related to the relative sizes of its hydrated ions. The hydrated ionic radius of Pb2+ and Zn2+ is 4.01 Å and 4.30 Å, respectively (Minceva et al. 2007). Because it is smaller, the Pb2+ ion can then diffuse with lesser restrictions throughout the pores and hence is adsorbed more easily at the active sites.

Based on this information, high levels of structural Al, as well as a solid with elevated surface area/pore size, are required for a better performance of the adsorbents. The ion-exchange event can be understood as metal ion diffusion through the pores of zeolite and the substitution of exchangeable available cations (generally sodium). Consequently, the access and diffusion is faster for structures with larger pores and is retarded when the ions move through smaller channels. In this way, new synthesis that allows the design of β-zeolite with these characteristics for an improvement of adsorption efficiency in the ion-exchange systems is important.

The knowledge about the kinetics of the adsorption process for the treatment of heavy metal is important because optimum operational conditions for full-scale metal removal processes may be achieved. A kinetics study for the Zn2+ and Pb2+uptakes was conducted using the β-zeolite. In doing so, the amount of adsorbed ions in the β-zeolite as a function of time was determined. The adsorption process usually takes place following a pseudo-second-order kinetics, according to the following equation, if the metal ion adsorption on the zeolite surface active sites is the rate limiting step.  
formula
(3)
where Qt (mg·g-1) and Qe (mg·g-1) are the amounts of the Zn2+ and Pb2+ adsorbed on β-zeolite at any time t(min) and at equilibrium, respectively. K2(g·mg−1·min−1) is the pseudo-second-order rate constant of adsorption (Çoruh et al. 2010).

Figure 5 displays the linear plot t/Qt versus t and the correlation coefficient (R2) is close to 1, indicating that a pseudo-second-order model can describe the experimental data very well. The values of Qe and K2 were obtained from the intercept and slope. Respective experimental Qe values observed in either Zn2+ (45.2 mg·g−1) or Pb2+ (86.6 mg·g−1) agree with those calculated by the pseudo-second-order model, 46.0 and 88.3 (mg·g−1), respectively (Table 2). The rate constants of adsorption (K2) were found to be 0.850 g·mg−1min−1 for Zn2+ and 0.056 (g·mg−1·min−1) for Pb2+. Therefore, these kinetic studies indicated that the adsorption process follows second-order kinetics, where the chemical interaction among metal ions and active sites is the rate-limiting step.

Table 2

Comparative kinetics adsorption parameters data for Pb2+ and Zn2+ on zeolite

Structure Metal Qe(mg·g−1)a K2(g·mg−1·min−1)a Reference 
Mixture of zeolite Pb2+ 90.9 0.009 Chen et al. (2017)  
Combination of natural kaolin-bentonite Pb2+ 27.3 0.086 Salem & Sene (2011)  
Zeolite A Zn2+ 117.4 600 Nibou et al. (2010)  
Natural clinoptilolite Zn2+ 6.7 5 × 10−4 Stojakovic et al. (2011)  
Natural zeolite Zn2+ 5.4 0.290 Varank et al. (2014)  
β-zeolite Pb2+ (Zn2+88.3 (46.0) 0.056 (0.850) This work 
Structure Metal Qe(mg·g−1)a K2(g·mg−1·min−1)a Reference 
Mixture of zeolite Pb2+ 90.9 0.009 Chen et al. (2017)  
Combination of natural kaolin-bentonite Pb2+ 27.3 0.086 Salem & Sene (2011)  
Zeolite A Zn2+ 117.4 600 Nibou et al. (2010)  
Natural clinoptilolite Zn2+ 6.7 5 × 10−4 Stojakovic et al. (2011)  
Natural zeolite Zn2+ 5.4 0.290 Varank et al. (2014)  
β-zeolite Pb2+ (Zn2+88.3 (46.0) 0.056 (0.850) This work 

aData from the pseudo-second-order model.

Figure 5

Linearized pseudo-second-order kinetic plot for β-zeolite for Zn2+ and Pb2+ solutions. Conditions: 50 mg of β-zeolite, V = 60 mL and pH = 5.5.

Figure 5

Linearized pseudo-second-order kinetic plot for β-zeolite for Zn2+ and Pb2+ solutions. Conditions: 50 mg of β-zeolite, V = 60 mL and pH = 5.5.

The results of zinc and lead removal in this study were compared to the literature for the adsorption systems of zinc and lead metals involving different zeolites (Table 2). Similar to that described in this work, zeolites generally present a pseudo-second-order adsorption mechanism. As verified in Table 2, in comparison with some analogous works, excellent kinetics adsorption parameters data were found for the β-zeolite and, from these results, it is possible to verify that this material can act as an effective adsorbent in the metal removal process.

Cycles required for cleaning the contaminated solution

According to the Brazilian law, Zn(II) and Pb(II) concentrations allowed to be tolerated in effluent waters are 5 mg·L−1 and 0.5 mg·L−1, respectively (CONAMA 2011). We performed a series of experiments in order to determine the number of adsorption cycles that are necessary to bring Zn(II) and Pb(II) concentrations (from the initial 100 mg·L-1) down to below the tolerated limits. In doing so, after the adsorption experiment was performed under the same procedure as indicated in the experiment section, another adsorption cycle was performed using a fresh adsorbent.

For this study, the experiment was carried out with β-zeolite because it displayed the best performance in the previous studies with both Zn(II) and Pb(II) solutions. It can be seen from Figure 6 that the numbers of cycles required for metal concentrations to reach the limit of legislation for disposal are four for Pb(II) and five for Zn(II), respectively. From these results, it is possible to verify that β-zeolite can be potentially applied in the industrial segment using few cycles of fresh β-zeolite to clean the contaminated solution.

Figure 6

Number of cycles required to reach the limits permitted by Brazilian legislation for disposal. Conditions: β-zeolite (50 mg), time 30 min and initial pH = 5.5.

Figure 6

Number of cycles required to reach the limits permitted by Brazilian legislation for disposal. Conditions: β-zeolite (50 mg), time 30 min and initial pH = 5.5.

Regeneration experiments

Due to its better performance, the β-zeolite was also studied in terms of the capacity of regeneration because this parameter is evidently an important variable in the development for adsorption process. The cost effectiveness of an adsorbent is fundamentally dependent on the cycles of adsorption–desorption experiments that it could withstand whilst maintaining an admissible performance. In these experiments, the solid was regenerated after each run by ion-exchange in a 1.0 mol·L−1 NaNO3 solution for 24 h before sequential reuse. Figure 7 shows the behavior of the solid after three adsorption cycles. Regarding the Pb(II) adsorption, it was found that after decreasing the amount of Pb2+ adsorbed from 68% to 48%, the adsorption capacity becomes constant throughout further adsorption cycles. For Zn(II) ions, the adsorption percentage decreases slowly after each experiment (46%, 33% and 26%).

Figure 7

Adsorption–desorption experiments with β-zeolite (50 mg). V = 60 mL, metal concentration =100 mg·L−1, initial pH = 5.5 and time = 30 min.

Figure 7

Adsorption–desorption experiments with β-zeolite (50 mg). V = 60 mL, metal concentration =100 mg·L−1, initial pH = 5.5 and time = 30 min.

The decrease in the adsorption capacity is explained as a result of the likely saturation of the zeolite pores. Alternatively, the total removal of either Zn(II) or Pb(II) ions from the adsorbent upon ion-exchange with sodium ions (which could enable active sites with labile Na+ for further heavy metal adsorption in subsequent cycles) might not occur.

Even with a slight decrease in the adsorptive potential, the β-zeolite displayed excellent reusability in the treatment of contaminated solutions by Zn2+ and Pb2+ ions, which indicates that it could be regarded as an efficient adsorbent for practical applications.

CONCLUSIONS

Zeolites with different chemical and textural properties were successfully synthesized. The solids were found to be active for Zn(II) and Pb(II) removal from their aqueous solutions. Under the conditions presented in this work, the β-zeolite was the most promising, displaying rather high efficiency in heavy metal removal. The best performance of β-zeolite in relation to the PREFER derivatives may be regarded in terms of (i) a higher aluminum content, responsible for the ion exchange sites and (ii) a higher surface area and external surface area. Furthermore, the wider pores of the β-structure (vs ferrierite solid) promote easier access of heavy metal ions to the internal active sites. The initial pH is an important parameter to evaluate the maximum metal removal in that maximum adsorption capacities were attained within pH range 4.5 to 5.5 for all adsorbents examined in this study. It was also verified that the adsorbent mass increase resulted in an unexpected decrease in the percentage of lead adsorbed, probably due to a significant drop in the initial pH solution. However, regarding the testing using Zn2+-containing solutions, the increase amount of either β-zeolite or ferrierite resulted in a slight improvement in the amount of adsorbed Zn2+ ions. No difference in the amount of zinc adsorbed was detected for the mass increase using the PREFER DEL sample. An interesting fact is that the PREFER DEL and ferrierite solids presented similar performance in the adsorption experiments. As expected, the partial delamination procedure resulted in a solid with higher surface area than zeolite ferrierite. On the other hand, the exfoliation process also resulted in some extraction of structural aluminum, responsible for the adsorption sites. In this way, probably the increase of surface area (more accessibility) may have been compensated by the removal of structural Al (less active sites) for PREFER DEL sample. It was also observed that the numbers of cycles required for metal concentrations to fall within the limits of the legislation for disposal are four for lead and five for zinc. Furthermore, the β-zeolite displayed a good performance even after several regeneration cycles. These results point to the β-zeolite as a promising candidate for the use as a practical adsorbent in heavy metal remediation in aqueous bodies.

ACKNOWLEDGEMENTS

The authors acknowledge the Fundação de Amparo à Pesquisa no Estado do Rio de Janeiro (FAPERJ) for financial support to this work, to Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for the fellowships, to Instituto Nacional de Tecnologia (INT) for the samples characterization and to Professor Mauro Celso Ribeiro (UFF) for the support in this work.

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