Modification of natural zeolite with NaOH for removal of manganese in drinking water

    Modification of natural zeolite with NaOH for removal of manganese in drinking water Ayten Ates, G¨okc¸en Akg¨ul PII: DOI: Reference:

S0032-5910(15)30110-8 doi: 10.1016/j.powtec.2015.10.021 PTEC 11287

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Powder Technology

Received date: Revised date: Accepted date:

29 January 2015 3 August 2015 11 October 2015

Please cite this article as: Ayten Ates, G¨ok¸cen Akg¨ ul, Modification of natural zeolite with NaOH for removal of manganese in drinking water, Powder Technology (2015), doi: 10.1016/j.powtec.2015.10.021

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ACCEPTED MANUSCRIPT Modification of natural zeolite with NaOH for removal of manganese in

Ayten Ates1*, Gökçen Akgül2

Cumhuriyet University, Engineering Faculty, Department of Chemical Engineering, 58140

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Sivas, TURKEY

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*1

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drinking water

Recep Tayyip Erdoğan University, Engineering Faculty, Department of Energy Systems

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Engineering, 53100 Rize, TURKEY

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Corresponding author. (Ayten Ates) Tel.: +90 346 219 1010/2248,

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ACCEPTED MANUSCRIPT Abstract Natural zeolite (NZ) obtained from the Manisa-Demirci region of Turkey was modified by

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NaOH aqueous solutions (0.5- 2.0 mol /dm3) and the adsorption capacity of natural and modified forms was determined for manganese removal. The characterisations of the zeolites

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were carried out by XRD, N2 sorption, FTIR, NH3-TPD and SEM-EDS. Treatment of the NZ

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with NaOH leads to a significant decrease of its silica content by desilication and an increase of its Na content by formation of hydroxysodalite. It was seen from the NH3- TPD results that NaOH treatment alters the acidity of zeolite.

While the number of weak and medium

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Brønsted/ Lewis acid sites decreases, the number of strong acid sites increases through the introduction of Na+ and the removal of silica. Treatment with NaOH has the effect of

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multiply by two the manganese adsorption capacity of the natural zeolite. The maximum manganese adsorption capacity was achieved with the zeolite treated with 1.5 M of NaOH.

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More than this concentration of NaOH leads not only to a decrease in adsorption capacity of natural zeolite for manganese, but also to a significant deformation of the zeolite structure. The Langmuir isotherm model fits well with the results obtained from manganese adsorption

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on natural zeolite, while the Freundlich isotherm model fits well with the results obtained on NaOH modified zeolites. Keywords:

Natural

zeolite;

Adsorption;

Manganese;

Desilication;

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NaOH;

Dealumination;

ACCEPTED MANUSCRIPT 1. Introduction Zeolites are the crystalline microporous aluminosilicates of elements such as Na, K, Mg, Ca,

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[1-3]. Given their porous structure, molecular sieve and absorbing ability for small molecules such as water, zeolites are good adsorbents for the removal of metal ions, and therefore also Compared to

metal oxides, another adsorbent,

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for contamination/pollution control.

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unmodified natural zeolites have low adsorption capacity for the removal of metals [4]. To allow them to be used as effective adsorbents , the characteristics of natural zeolites have to be improved by various methods such as ion exchange with alkali bases, acid-steam and/or

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high temperature treatments and aluminium addition [5-15].

The cation exchange capacity of a zeolite is a function of its Si/Al ratio, which is defined as

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the number of cations per unit mass or volume. Also, depending on its position in the zeolite,

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the type of cation may affect the pore diameter. If the zeolite includes one of potassium (K+), sodium (Na+) and calcium (Ca2+) cations, their effective pore diameters will be approximately 3Å, 4Å , and 5Å, respectively. The resulting material can then be defined

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according to same terms i.e. 3A, 4A and 5A, respectively. All of these materials have been used as ionic exchangers (water softeners) and adsorbents [16-18].

Steaming or acid-leaching may lead to the formation of additional pores due to defects in zeolites. However, such treatments may affect the acidic properties of zeolites [19, 20]. Desilication of zeolite is an effective post-treatment method for improvement of its pore structure. This works through selective silicon extraction from the framework without significant changes in the acidity and crystal structure [21, 22]. The creation of mesoporosity by desilication has been most closely studied in the case of synthetic zeolites such as FER[23], beta[23-25], ZSM-22[26], mordenite [27] and ZSM-5 [23, 28-36]. Reported results showed that the ratio of Si/Al affects the mesopore formation. The optimum Si/Al range is 253

ACCEPTED MANUSCRIPT 50 [23, 29, 30]. Lower values of Si/Al result in insignificant extraction of silicon, while higher values lead to dissolution of the zeolite. Furthermore, the effectiveness of mesopore

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formation depends on the type of zeolite. For example, zeolite beta is more susceptible to mesoporosity development than MFI, FER and MOR, since the framework aluminium in

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zeolite beta is less stable [24, 37]. When zeolite beta with Si/Al ratio of 35 was treated by

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0.2 M NaOH solution at 338 K for 30 min, which is the optimal condition for mesoporous ZSM-5, the crystallinity was severely damaged [24]. Similar to zeolite beta, natural zeolites are less stable compared with synthetic zeolites [6].

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Summarising all these investigations, although desilication of several different synthetic zeolites by NaOH solution has been studied, desilication of natural zeolites has not been

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studied in detail so far. Therefore, the present work will focus on detailed investigation of natural zeolite treated with NaOH. In this study, the natural zeolite obtained from Demirci

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region, Manisa/Turkey, was selected to investigate its structural modification by NaOH treatment. The effect of NaOH concentration on its manganese adsorption was also

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investigated.

2. Experimental 2.1. Material

Natural zeolite (NZ) obtained from the Demirci region, Manisa/Turkey, was examined. The zeolite was first ball- milled to a particle size ranging from 0.25 mm to 0.5 mm, and dried in an oven at 120 °C overnight. After washing and drying, 20 g of the NZ was mixed with 500 cm3 of sodium hydroxide (NaOH) with varying concatenations (0.5 M (NZ(0.5)), 1.0 (NZ(1.0)), 1.5 (NZ(1.5)), and 2.0 M (NZ(2.0))) at 90 °C for 1 h, and a mixing rate of 500 rpm. Washed and NaOH-treated zeolites were denoted as NZ-W and NZ (X), respectively, where X indicates the concentration of NaOH. 4

ACCEPTED MANUSCRIPT 2.2. Characterisation of samples The chemical composition of the zeolites was analysed using Energy –dispersive X- Ray

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spectroscopy (EDS) (OXFORD INSTRUMENTS INCA X-Act/51-ADD0013) on a scanning electron microscope (SEM) (JEOL/ JSM-6610).

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Microscope (SEM) (JEOL/ JSM-6610).

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The morphology of the natural and modified zeolites was examined by a Scanning Electron

X-ray powder diffraction (XRD) patterns of the zeolites were recorded on a Rigaku SmartLab X-ray diffractometer using non-monochromotographic Cu Kα1-radiation (40 kV, 40 mA,

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λ = 1.5). Scanning was in the range 5–65 °C of 2θ.

The specific surface area and micropore volume of the samples were measured using

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N2 adsorption–desorption (AUTOSORB 1C) at − 196 °C. Prior to adsorption, the samples

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were evacuated until a pressure of 66.6 Pa at room temperature was reached, then heated up to 300 °C and evacuated until a pressure of 1.3 Pa was reached. This condition was maintained overnight. The surface area, total pore volume and micropore volume were determined by

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multipoint BET, t-plot and DR (Dubinin–Radushkevic), respectively. Infrared absorption measurements were carried out using a Fourier Transform Infrared (FTIR) spectrophotometer (Bruker Optics- Alpha). The FTIR spectra were obtained in the wavenumber range 650–4000 cm− 1 using single bounce ATR with diamond crystal. The temperature-programmed desorption (TPD) with ammonia (NH3-TPD) was carried out in Autochem II-2920, Micromeritics. The samples were saturated with a flow of 15 (v/v)% NH3 in He at 50 °C. Subsequently, NH3 was desorbed in a He flow of 25 cm3 min− 1 up to a temperature of 900 °C with ramp rate of 10 K min− 1.

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ACCEPTED MANUSCRIPT 2.3. Adsorption experiments Batch adsorption experiments were carried out in glass flasks (0.1 L) using a magnetic shaker at 25 °C at a constant agitation of 200 rpm. In the kinetic studies, suspensions containing a

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range of 25–250 mg L− 1 of Mn2 + were stirred for different periods of time at initial pH of 6.

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After the reaction, suspensions were centrifuged at 5000 rpm for 3 min in order to separate

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the solution and the solids. The initial and non-adsorbed concentrations of Mn2 + in supernatants were determined by atomic absorption spectroscopy (AAS). Adsorption studies of Mn2 + onto NZ and NaOH- treated NZ were conducted using the same procedure in sufficient time for attaining equilibrium - 8 h for all samples, for varying feed

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solution concentrations (25–200 mg L− 1). All results were expressed as averaged values of duplicate tests.

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using Eq.2.1 and 2.2.

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The adsorption capacity (qe, mg g− 1) and removal percentage (%) of Mn2 + were determined

(2.1)

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(2.2)

where C0 and Ce are the initial and final concentrations of Mn2 + (mg L− 1), V is the volume of solution (L) and m is the amount of adsorbent (g).

Adsorption isotherms The Langmuir model essentially describes the monolayer type of adsorption. It is expressed as follows:

(2.3)

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ACCEPTED MANUSCRIPT where Qm (mg·g-1) is the maximum adsorption capacity and b (L · mg− 1) is the Langmuir constant.

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The Freundlich isotherm is derived from a multilayer heterogeneous adsorption model. The

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Freundlich isotherm is as follows:

(2.4)

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qe=kFC e 1 / n

where kF (L g− 1) is the Freundlich adsorption constant related to adsorption capacity and n is the adsorption intensity. The 1/n value was between 0 and 1, indicating that the adsorption

3. Results and Discussion

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3.1. EDS analysis of samples

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was favourable at the studied conditions.

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EDS analyses of the natural and modified zeolites are listed in Table 1. After alkaline treatment with 0.5M NaOH solution, the composition of the zeolite is changed insignificantly. Increasing the NaOH concentration from 0.5 to 2.0 M

leads to significant decationisation,

dealumination and desilication. The percentage of desilication is higher than dealumination. It

O

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was reported that the Si

O

Si bond is relatively easier for cleavage compared to the Si

Al bond in the presence of OH− , because of the negative charge of the AlO4− tetrahedron

[38, 39].

The desilication process is clearly dependent on the concentration of NaOH

solution, and higher pH values are favourable for the extraction of Si from the zeolites.

3.2. XRD study and SEM images of samples

A comparison of XRD patterns of natural and modified zeolites is shown in Fig. 1. NZ contains mainly clinoptilolite ((Na, K, Ca)2–3Al3(Al, Si)2Si13O36·12(H2O)) and dolamite (CaMg(CO3)2 ) as well as quartz (SiO2), feldspar (KAlSi3O8–NaAlSi3O8–CaAl2Si2O8) and clay. Washing of the zeolite led to the disappearance of biotite (K(Mg,Fe)3AlSi3O10(F,OH)2) 7

ACCEPTED MANUSCRIPT phase and a reduction of clay phase. Although the treatment with NaOH affects its phase composition up to 1.0 M insignificantly,

increasing the NaOH concentration to 2.0 M

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resulted in a slight decrease in the intensities of the diffraction peaks of clinoptilolite and

seen in Table 1 and reported by Kang et al.[11].

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feldspar, and a peak at 35° of 2θ appeared because of the formation of hydroxysodalite, as

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The SEM micrographs of the samples are shown in Fig. 2. The NaOH treatment results in crystal de-agglomeration as reported in references [40, 41]. Above 1M of NaOH, the particles of the alkaline-treated sample have a slightly rougher surface, and this accord with the

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adsorption isotherms in Fig. 3a. Treatment with 1.5 and 2.0 M of NaOH led to the severe crystal de-agglomeration of the zeolite and their particles became very irregular, indicating

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XRD pattern [17].

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that the severe desilication may have damaged the structure to some extent as seen in the

3.3. Surface area and pore size distribution of samples Adsorbents with a large surface area are preferable for ensuring large adsorption capacity.

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However, large surface area in a limited volume inevitably gives rise to large number of small size pores between adsorption surfaces. The size of pore determines the transport of adsorbate molecules from the outer adsorption surface to the adsorption surface inside the particle, so the pore size width is one of the most important properties for characterizing adsorptive of the adsorbent. Several experimental methods are available to characterize adsorbent pore structure. Among them, the most common method of measuring surface areas involves the principles of physical adsorption by van der Waals’ electrostatic forces and the surface area is determined by measuring the amount of gas adsorbed in a monoloayer. Therefore, the change in pore size and surface area of adsorbent with treatment was determined by N2-physisorption.

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ACCEPTED MANUSCRIPT The N2-physisorption isotherms and the corresponding BJH pore size distributions of natural and modified samples are shown in Fig. 3. The corresponding textural properties obtained

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from N2-physisorption isotherms are given in Table 2. As reported previously [42], the shape of NZ is consistent with Type IV which showed a noticeable hysteresis at high p/p0, and

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which relates to the presence of larger pores and more open surface area mesopores, as well

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as micropores [43]. After NaOH treatments, a decrease in surface area and micropore volume is observed, which must be a result of mesopore formation with desilication and dealumination, based on EDS and XRD results. Furthermore, although the NZ sample has the

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pore size distribution centered around 50 Å in a wide range in Fig. 3b, the pore diameters of the zeolites treated with NaOH shift higher particle diameters from 50 to 100 Å, depending on the concentration of NaOH. It is also confirmed by an increase in the average pore diameter

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of the treated samples from 120 to 350 Å (Table 2). Here, the dissimilar pore structures of the

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zeolites with alkali treatment can be distinguished by gas sorption. However, recently the most powerful methods such as identical- location scanning transmission and secondary electron microscopy and positron annihilation lifetime spectroscopy (PALS) to assess the

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pore structure have been reported by Perez-Ramirez et al.[35, 36]. The results can be further supported by these techniques. 3.4. NH3-TPD of the samples The effects of desilication on the acidity of the natural zeolite were studied by TPD of NH3 as shown in Fig. 4. It has been generally accepted that the TPD peak position is directly related to the strength of the acid sites. Peaks with maxima above 300 °C are due to strong acid sites (Brønsted and/or Lewis) [41, 44-46]. However, the peaks below 300 °C are strongly debated. These have been attributed to the fact that NH3 is weakly adsorbed on the Brønsted/Lewis acid sites [44, 45], and associated with Na+[46] or extra-framework Al [45].

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ACCEPTED MANUSCRIPT NH3 was desorbed on the natural zeolites in three regions. Two distinct desorption peaks were observed at 100 oC and 650 oC, and a shoulder around 250oC in the NH3 -TPD profile. The

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peak at 100 oC and the shoulder around 250oC are related with weak acid sites and Lewis acid sites, respectively [47, 48]. The peak at temperatures higher than 500 oC is assigned to strong

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acid sites. After NaOH treatment, the acid site distribution on the samples changes

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significantly due to the exchange of zeolitic protons by sodium cations. Esquivel et al.[49] reported that Na (I) exchanges protons on β-zeolite and decreases the number of medium and strong acid sites. Here, on the other hand, NaOH treatment of natural zeolite causes an increase in strong Brønsted acid sites above 500 oC and a decrease in medium or

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weak Brønsted/ Lewis acid sites below 500 oC. This difference can be explained by the different nature of natural zeolite from synthetic zeolites,

formation of hyroxysodalite and

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formation of extra-framework aluminium by desilication. NaOH treatment causes not only

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desilication and formation of hydroxysoldalite but also affects composition and structure of crystal phase as seen in XRD, EDS and N2-physisorption results. 3.5. FTIR study

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The FTIR spectrum of natural and modified zeolites is shown in Fig. 5. The IR absorption band at 3610 cm−1 is typical for Brønsted acidity, and remained present in natural and modified samples. The total acidity increases in correlation with the higher aluminium content relative to the silicon in the alkaline-treated zeolite [29]. The hydroxyl groups are observed in the range 3610-3780 cm−1of the FTIR spectrum, in which intensities changed insignificantly with NaOH treatment. The signal at 1544 cm−1 indicates the Al sites on the surface. The peak intensity is increasing with NaOH treatment because of desilication.

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ACCEPTED MANUSCRIPT The band at ~ 1635 cm− 1 shows the bending mode of water molecules, appearing in all samples.

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In the range of 600–800 cm− 1, the bands are attributed to exchangeable cations due to pseudocrystallinic vibrations [6]. The bands are clear in NZ and NZ (0.5), but their intensities

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decrease with increasing NaOH concentration due to decationisation as well as desilication

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and loss of crystallinity.

3.6. Adsorption of manganese by natural and modified zeolites Adsorption capacities of natural and modified zeolites with contact time are shown in Fig. 6.

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Treatment of the zeolite with NaOH increased by a factor of 2 its Mn2+ adsorption capacity. However, treatment with higher concentrations of NaOH decreased its manganese adsorption capacity. Alkali treatment varies the ratio of Si/Al through desilication which gives rise to a

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significant increase in the adsorption capacity of the zeolite. High NaOH concentration leads

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to structural deformation and serious desilication and dealumination of the zeolites, causing the surface area to decrease even though the pore diameter increases. This effect can be seen in Table 2 and Fig. 3.

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The equilibrium contact time of Mn2 + was determined as 60 min for NZ, 120 min for NZ(0.5), NZ(1.5) and NZ(2.0) and 240 min for NZ(1.0). The extension of equilibrium time with treatment in 1.0 M NaOH may be due to the formation of additional micropores through desilication and partial dealumination. Furthermore, based on the results of the power of positron annihilation lifetime spectroscopy (PALS) used to characterize to pore connectivity in hierarchical MFI zeolites [34], this may be related to critical mesopore size of the zeolite treated with NaOH. A further increase in NaOH concentration enhances the formation of mesopores which in turn facilitates diffusion of Mn2+ as seen in Table 2 and Fig. 3. Based on the results in Fig. 6, the Langmuir and Freundlich models were fitted to the adsorption isotherms and adsorption constants obtained from the isotherms were given 11

ACCEPTED MANUSCRIPT in Table 3. Higher regression coefficients suggest that the Langmuir model is suitable for manganese removal with NZ and NZ (0.5), whereas the Freundlich model is better suited to

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modified NZ than the Langmuir model. While the Langmuir models describe adsorption on the adsorbent with a homogenous surface, the Freundlich models describe adsorption on

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adsorbents with a heterogeneous surface. As explained above, the multilayer adsorption

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mechanism of the treated zeolite occurs largely due to the increasing heterogeneity of the surface through treatment of the zeolite with NaOH.

Qm calculated from the Langmuir parameters represents the monolayer saturation at

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equilibrium and b indicates the binding affinity for Mn2 +. The high b value indicates the high affinity of natural and modified zeolites.

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The value of the kF constant found from the Freundlich isotherms changes depending on the type of adsorbent. 1/n values of all samples are in the range 0 to 1, showing the strong

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adsorption capacity as reported in reference [50] .

Based on results calculated from the Langmuir model (Fig. 7), higher manganese adsorption

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capacity was found to be 66.1 mg·g− 1 and 51.5 mg·g− 1 for NZ (1) and for NZ (1.5), respectively. In Fig. 7, the points were easily analysed by 3rd order polynomial fitting. One plausible explanation for the high adsorption capacities of these samples is provided by the optimum ratio of Si/Al , the structure of the zeolite and its natural composition. A comparison of results with the literature as given in Table 3 shows that the adsorption capacity of NZ varies with source and composition of the zeolite. The manganese adsorption capacity of the NZ obtained from Manisa-Demirci (51 % of clinoptilolite , 12 % of dolomite, 4% of quartz, 17 % of feldspar and 9 % of biotite ) is higher than that of the Chilean zeolite [51] (36% of clinoptilolite, 33% of mordenite, 26% of quartz and 5% of montmorillonite) and the Sivas-Yavu zeolites [18](30 % of clinoptilolite 40% of mordenite, 10% of quartz,

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ACCEPTED MANUSCRIPT 10% of feldspar and 5% of clay) but lower than that of the Brazilian natural zeolite [52] (natural scolecite (Na0.26Ca0.95Al2.07Si3.00O10.3H2O)). Clinoptilolite-rich zeolites showed

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higher manganese adsorption. The NaOH treated zeolites showed higher adsorption capacity than natural zeolites and Na-clinoptilolite [53]. These differences may be related to NaOH

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concentration as well as the source and composition of the NZ.

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4. Conclusion

Natural zeolite obtained from the Manisa-Demirci region of Turkey was modified using various concentrations of NaOH. The influence of the concentration of NaOH on the chemical

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and physical properties of natural and modified zeolites was studied by various techniques such as XRD, N2 sorption, FTIR, NH3-TPD, and SEM-EDS. The manganese adsorption

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capacities of the zeolites were determined. The data obtained were applied to isotherm

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models. We conclude that the adsorption of manganese depends on textural properties such as composition, the size and distribution of pores and the crystal structure of the adsorbate. NaOH treatment up to 1.5 M increases Mn2+ adsorption capacity which gives rise to optimum Si/Al ratio. While the Langmuir isotherm fitted well with results obtained from the

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manganese adsorption of natural zeolite, the Freundlich isotherm model is more appropriate for the results obtained on NaOH-modified zeolites.

Acknowledgments I gratefully acknowledge the financial support given for this study by the research fund of Cumhuriyet University (M-492) and The Scientific and Technological Research Council of Turkey (TUBİTAK) (113M813).

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directing agent, Chem-Eur J, 11 (2005) 4983-4994. [30] J.C. Groen, J.A. Moulijn, J. Perez-Ramirez, Desilication: on the controlled generation of

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mesoporosity in MFI zeolites, J Mater Chem, 16 (2006) 2121-2131.

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[31] J.C. Groen, W.D. Zhu, S. Brouwer, S.J. Huynink, F. Kapteijn, J.A. Moulijn, J. PerezRamirez, Direct demonstration of enhanced diffusion in mesoporous ZSM-5 zeolite obtained via controlled desilication, J Am Chem Soc, 129 (2007) 355-360.

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[32] V. Gomez, P. Ramirez, J. Cervera, S. Nasir, M. Ali, W. Ensinger, S. Mafe, Charging a Capacitor from an External Fluctuating Potential using a Single Conical Nanopore, Sci RepUk, 5 (2015).

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[33] D. Verboekend, S. Mitchell, M. Milina, J.C. Groen, J. Perez-Ramirez, Full

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Compositional Flexibility in the Preparation of Mesoporous MFI Zeolites by Desilication, J Phys Chem C, 115 (2011) 14193-14203. [34] M. Milina, S. Mitchell, D. Cooke, P. Crivelli, J. Perez-Ramirez, Impact of Pore

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Connectivity on the Design of Long-Lived Zeolite Catalysts, Angew Chem Int Edit, 54 (2015) 1591-1594.

[35] M. Milina, S. Mitchell, P. Crivelli, D. Cooke, J. Perez-Ramirez, Mesopore quality determines the lifetime of hierarchically structured zeolite catalysts, Nat Commun, 5 (2014). [36] S. Mitchell, N.L. Michels, K. Kunze, J. Perez-Ramirez, Visualization of hierarchically structured zeolite bodies from macro to nano length scales, Nat Chem, 4 (2012) 825-831. [37] J.C. Groen, L.A.A. Peffer, J.A. Moulijn, J. Perez-Ramirez, On the introduction of intracrystalline mesoporosity in zeolites upon desilication in alkaline medium, Micropor Mesopor Mat, 69 (2004) 29-34.

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ACCEPTED MANUSCRIPT [38] S.R. Taffarel, J. Rubio, Removal of Mn2+ from aqueous solution by manganese oxide coated zeolite, Miner Eng, 23 (2010) 1131-1138.

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[39] A.R. Loiola, J.C.R.A. Andrade, J.M. Sasaki, L.R.D. da Silva, Structural analysis of

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zeolite NaA synthesized by a cost-effective hydrothermal method using kaolin and its use as

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[41] I. Melian-Cabrera, S. Espinosa, C. Mentruit, F. Kapteijn, J.A. Moulijn, Alkaline leaching for synthesis of improved Fe-ZSM5 catalysts, Catal Commun, 7 (2006) 100-103. [42] A. Ates, Characteristics of Fe-exchanged natural zeolites for the decomposition of N2O

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and its selective. catalytic reduction with NH3, Appl Catal B-Environ, 76 (2007) 282-290.

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[43] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Reporting Physisorption Data for Gas Solid Systems with Special Reference to the Determination of Surface-Area and Porosity (Recommendations 1984), Pure Appl

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[44] G.I. Kapustin, T.R. Brueva, A.L. Klyachko, S. Beran, B. Wichterlova, Determination of the Number and Acid Strength of Acid Sites in Zeolites by Ammonia Adsorption Comparison of Calorimetry and Temperature-Programmed Desorption of Ammonia, Appl Catal, 42 (1988) 239-246. [45] T. Suzuki, T. Okuhara, Change in pore structure of MFI zeolite by treatment with NaOH aqueous solution, Micropor Mesopor Mat, 43 (2001) 83-89. [46] N.Y. Topsoe, K. Pedersen, E.G. Derouane, Infrared and Temperature-Programmed Desorption Study of the Acidic Properties of Zsm-5-Type Zeolites, J Catal, 70 (1981) 41-52.

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ACCEPTED MANUSCRIPT [47] Y.T. Kim, K.D. Jung, E.D. Park, Gas-phase dehydration of glycerol over ZSM-5 catalysts, Micropor Mesopor Mat, 131 (2010) 28-36.

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[48] G.L. Woolery, G.H. Kuehl, H.C. Timken, A.W. Chester, J.C. Vartuli, On the nature of framework Bronsted and Lewis acid sites in ZSM-5, Zeolites, 19 (1997) 288-296.

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[49] D. Esquivel, A.J. Cruz-Cabeza, C. Jimenez-Sanchidrian, F.J. Romero-Salguero, Local

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environment and acidity in alkaline and alkaline-earth exchanged beta zeolite: Structural analysis and catalytic properties, Micropor Mesopor Mat, 142 (2011) 672-679. [50] Y.S. Tao, H. Kanoh, K. Kaneko, Developments and structures of mesopores in alkaline-

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treated ZSM-5 zeolites, Adsorption, 12 (2006) 309-316.

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[52] S.M. Dal Bosco, R.S. Jimenez, W.A. Carvalho, Removal of toxic metals from

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wastewater by Brazilian natural scolecite, J Colloid Interf Sci, 281 (2005) 424-431. [53] N. Rajic, D. Stojakovic, S. Jevtic, N.Z. Logar, J. Kovac, V. Kaucic, Removal of aqueous manganese using the natural zeolitic tuff from the Vranjska Banja deposit in Serbia, J Hazard

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Mater, 172 (2009) 1450-1457.

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Figure captions

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Fig. 1. XRD patterns of natural and modified zeolites.

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Fig. 2. SEM images of natural and modified samples.

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Fig. 3. N2 adsorption–desorption isotherm (a) and differential pore size distribution (b) of natural and modified zeolites

Fig. 4. NH3-TPD results of natural and modified zeolites.

(b) spectrum between 4000-2600 cm-1

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Fig. 5. FTIR spectra of natural and modified zeolites. (a)spectrum between 1850-650 cm-1,

Fig. 6. The adsorption capacity of NZ –W , NZ(0.5) , NZ (1.0) , NZ(1.5) , and NZ(2.0) and

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comparison of capacities of samples with time as a function of initial Mn2 + concentration (100 mg L− 1) at ambient temperature .

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Fig. 7. A comparison of adsorption capacity of natural and modified zeolites for Mn2+

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ACCEPTED MANUSCRIPT Figures Figure 1.

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NZ (2.0)

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Hs

Cy: Clay Bt: Biotite Clp: Clinoptilolite Qtz: Quartz Fel: Feldispare Dol: Dolomite Hs : Hydroxysodalite

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NZ (1.5)

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35 40 2 CuK

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NZ(1.5)

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a

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NZ NZ-W NZ (0.5) NZ (1.0) NZ (1.5) NZ (2.0)

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0.0

0.2 0.4 0.6 0.8 o Relative pressure [P/P ] [-]

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7.0E-03

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NZ NZ-W NZ(0.5) NZ(1.0) NZ(1.5) NZ(2.0)

Desorbed NH3 [%]

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NZ NZ-W NZ (0.5) NZ (1.0) NZ(1.5) NZ(2.0)

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Figure 7.

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ACCEPTED MANUSCRIPT Tables Table 1. Composition of natural and modified zeolies determined by EDS.

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NZ (1.5) 24.4 7.5 56.1 1.9 3.4 2.2 1.3 3.1

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NZ(1.0) 26.3 7.8 51.2 2.9 3.9 2.6 1.9 3.4

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NZ (0.5) 29.6 6.0 53.5 2.2 3.2 1.6 1.5 -

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NZ-W 30 5.8 57.5 2.4 2.0 1.4 0.9 -

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NZ 29.5 6.3 56 2.8 2.7 1.6 1.1 -

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Sample Si Al O K Ca Mg Fe Na

NZ(2.0) 13.9 4.6 49.9 1.1 2.1 1.2 1.0 26.1

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V Total b(cm3·g-1) 0.33 0.28 0.24 0.17 0.37 0.43

Dp (Å) 119 132 153 320 223 346

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Multipoint BET method ; bVolume adsorbed at p/p0 = 0.99.; cMicropore volume calculated by DR method

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SBETa (m2·g-1) 112 86.4 63.0 26.0 66.7 49.8

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Sample NZ NZ-W NZ (0.5) NZ (1.0) NZ(1.5) NZ(2.0)

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ACCEPTED MANUSCRIPT Table 3. Adsorption isotherm constants of adsorbents.

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Freundlich isotherm kF 1/n R2 (L·g-1) 0.8 0.64 0,99 13.2 0.20 0.86 13.1 0.26 0.99 2.9 0.58 0.99 19.3 0.08 0.97 2.0 0.33 0.98 112.2 0.13 0.85 0.02 0.92 0.84 -

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NZ NZ(0.5) NZ (1.0) NZ (1.5) NZ(2.0) NZ Na-NZ NZ Brazilian NZ Na-Clinoptilolite

Langmuir isotherm Qmax b R2 (mg·g-1) (L·mg-1) 31.2 0.01 0.99 31.8 0.26 0.89 51.5 0.06 0.98 66.1 0.01 0.94 28.9 0.40 0.94 7.6 0.154 0.97 232.6 0.151 0.84 7.1 0.08 0.99 109.9 0.0014 0.95 10.0 0.0182 0.98

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Adsorbent

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Reference

This study This study This study This study This study [4] [4] [47] [48] [49]

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights Adsorption of Mn2+ on natural and modified zeolites was studied.

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Alkali treatment of the zeolite changed its textural and structural properties

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The concentration of NaOH affects greatly pore size distribution of the zeolite

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Alkali treatment of the zeolite increased adsorption capacity of Mn2+ twice.

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

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