SYNTHESIS OF MAGNETITE-MODIFIED NATURAL ZEOLITE USING COPRECIPITATION AND PHYSICAL MIXING TECHNIQUES

ABSTRACT


Introduction
Zeolite is a hydrated aluminosilicate material composed of tetrahedral alumina (AlO4 5-) and tetrahedral silica (SiO4 4-), which are interconnected via oxygen atoms [1] .The typical chemical formula for zeolite is: Mx/n {(Al2O)x(SiO2)y.zH2O}Where: M = cations (Na, K, Li) or (Ca, Mg, Ba, Sr) n = charge of the cation x,y = number of tetrahedral per unit cell z = number of water molecules per unit cell Zeolites are classified into two types based on their origin i.e., natural zeolites and synthetic zeolites.Natural zeolites are formed primarily from volcanic ash and can be found as crystals in Indonesian Journal of Pure and Applied Chemistry journal homepage: http://jurnal.untan.ac.id/index.php/IJoPACIndo.J. Pure App.Chem.6 (3), pp.122-131, 2023  igneous and metamorphic rocks.Natural zeolite can also be found in small granules which accumulate in rock sediments [2] .There are two main categories of natural zeolites, and they are [3] : a. Zeolite, which grows between the fractures of rock layers, allows for admixtures of other minerals such as calcite, quartz, granite, chlorite, fluorite, and sulfide minerals.b.Zeolite occurs naturally as rocks.Several examples of these zeolites are clinoptilolite, laumontite, mordenite, phillipsite, erionite, chabazite, and heulandite.Meanwhile, synthetic zeolite is a material that has been produced by humans, typically by a hydrothermal process, using natural or synthetic silica as the raw material, to have features and characteristics similar to natural zeolite.Zeolites such as zeolite X, zeolite Y, zeolite A, LTL zeolite, and ZSM-5 zeolite have been successfully synthesized.
Zeolite has various distinguishing features, some of which are significant i.e. its low density and sizeable void space volume, possess channel and pore spaces with varied and specific dimensions, high hydration levels, high degree of crystallinity, molecular and ion absorption ability, ion exchange capacity and catalytic properties [4] .These unique characteristics have attracted researchers' interest, and they have been used in various applications.Zeolite has been used in the environmental field to absorb various heavy metal liquid pollutants such as chromium, manganese, selenium, nickel, cobalt, and iron [5] .Then, zeolite tuffs were employed as fillers in warm mix asphalt in civil engineering [6] .In the energy field, zeolite captures and stores thermal energy [7] .Furthermore, zeolite is employed as a catalyst in the hydrodeoxygenation reaction of oleic acid to produce renewable diesel [8] .
Many researchers have modified zeolites to improve their properties and capabilities.One of the most basic zeolite modifications is washing with an acid solution.Acid washing can remove contaminants that block pores, allowing more molecules or cations to enter or be absorbed by the zeolite.Another modification strategy is to use organic surfactants to modify the surface properties of the zeolite.Quaternary amine surfactants such as HDTMA, CTMA, ODMBA, CPD, BDTDA, and SDBAC are employed to produce a bilayer layer on the surface of the zeolite, allowing it to absorb anions and non-polar molecules [9] .Zeolite has also been modified with several transition metals to improve its catalytic ability.
Among zeolite modifications mentioned above, one that has attracted attention is modifying the zeolite's nature to become magnetic.Magnetic zeolites are well recognized for their ease of separation from liquids.This updated method, known as magnetic separation, allows the zeolite to quickly and rapidly separate from the liquid phase using an external magnetic field.Such a simple approach is crucial for improving operational efficiency and lowering costs, particularly in environmental applications such as water purification, sewage treatment, catalysis, and gas separation.
Iron oxide (Fe3O4, γ-Fe2O3) is one of the materials used to modify magnetic zeolites.Iron oxide is advantageous since it is both abundant and inexpensive.It is also biocompatible and environmentally friendly [10] .Adding iron oxide (Fe3O4) particles to the zeolite structure affects the zeolite's nature, transforming it into a superparamagnetic material [11] .This superparamagnetic property causes the zeolite to be easily and quickly separated using an external magnetic field.The preparation of Fe3O4-modified zeolite adsorbents can be carried out through several techniques, such as hydrothermal [12] , coprecipitation techniques [13], and physical mixing techniques [11] .However, coprecipitation and physical mixing procedures are considered more promising due to their technical simplicity, economy, and ease of implementation.The coprecipitation technique is based on the reaction of Fe3O4 precipitation on the surface of the zeolite from a solution containing a combination of Fe(II) and Fe(III) ions in NH4OH solution.The reaction for the formation of natural zeolite/Fe3O4 by coprecipitation is as follows [14] : Zeolite (s) + 2Fe 3+ (aq) + Fe 2+ (aq) + 8OH -(aq) → Zeolite/Fe3O4 (s) + 4H2O (l) The physical mixing technique, on the other hand, involves physically interacting with zeolite powder and Fe3O4 powder by stirring [14] or grinding [11] .The objective of this study was to investigate the effect of preparing magnetite-modified natural zeolite utilizing coprecipitation and physical mixing techniques on the character of the resulting composite material.Furthermore, the effect of increasing Fe3O4 fraction on natural zeolite by 25.0% w/w, 33.3% w/w, and 50.0%w/w on the crystallinity, pore characteristics, and recovery capabilities of produced natural zeolite/Fe3O4 composites was also evaluated.

Materials and Instrument
The natural zeolite used originates from the Bayat Klaten region.Iron (III) chloride hexahydrate, Iron (II) chloride tetrahydrate, and ammonia solution were purchased by Merck.Both Na2EDTA and hydrofluoric acid are technical grade.All chemical was used without further purification.As a reagent solvent, deionized water is used.XRD spectra were obtained using X-ray Diffraction Rigaku Miniflex.Nitrogen adsorption-desorption measurements were performed using a BELSORP-mini.The turbidity measurement of the filtrate was carried out using Lovibond Turbicheck.

Natural Zeolite Washing
Natural zeolite was washed with HF and Na2EDTA solutions.Initially, the natural zeolite was sieved using a 250 mesh sieve.The sieved zeolite was then washed in 25 mL of 0.05% HF solution (v/v) for 3 hours with stirring.The natural zeolite was then filtered using Whatman 42 paper and washed with distilled water to a neutral pH.The resulting natural zeolite was dried in a 110°C oven for 4 hours.After being washed in HF solution, natural zeolite that weighed as much as 1 g was subjected to a second washing in 200 mL EDTA 0.075 M solution for 50 hours while being stirred.After that, the natural zeolite was filtered on Whatmann 42 paper and rinsed with distilled water to neutralize the pH.The washed natural zeolite was dried in an oven at 110 °C for 4 hours.The washed natural zeolite is used for the preparation of magnetite-modified natural zeolite.

Magnetization of Natural Zeolite Using Coprecipitation Technique
After being washed, 1.5 g of natural zeolite was added to 400 mL of 0.5 M NH4OH solution.The mixture was thoroughly mixed and deoxygenated with an N2 gas stream.The mixture was then heated to 70 °C.In an N2 gas atmosphere, 100 mL of a solution containing Fe(II) and Fe(III) ions with a molarity ratio of 1:2 was added dropwise to the mixture until a black suspension formed.The black suspension precipitate (that is 50.0%w/w Fe3O4 fraction stoichiometrically) was then rinsed with deionized water until the pH was neutral and dried in an oven at 80 °C.The same procedure was carried out to get 33.3% w/w and 25.0% w/w Fe3O4 fraction by adjusting the amount of natural zeolite used i.e. 3 g and 4.5 g respectively.

Magnetization of Natural Zeolite Using Physical Mixing Technique
As much as 0.5 g of washed natural zeolite was suspended in 40 mL of deionized water.The mixture was stirred until homogeneous under an N2 gas atmosphere.Then, gradually, 0.5 g of magnetite powder (prior synthesized using coprecipitation method) was added to the zeolite suspension.The mixture was stirred until homogenous and left for 1 hour.The black suspension (that is 50.0%w/w Fe3O4 fraction by mass ratio) that formed was then filtered and dried in an oven at 80 °C.The same procedure was carried out to get 33.3% w/w and 25.0% w/w Fe3O4 fraction by adjusting the amount of natural zeolite used i.e. 1 g and 1.5 g respectively

Zeolite/Fe3O4 Characterization
The crystallinity of natural zeolite/Fe3O4 was determined using the Rigaku Miniflex X-Ray Diffraction instrument.Measurements were conducted at a diffraction angle (2θ) = 0°-90°, using Cu-Kα light with a wavelength of 1.5418 Å and operated at 45 kV and 200 mA.Nitrogen Adsorption-desorption measurements of natural zeolite/Fe3O4 were performed using a BELSORP-mini.Prior to measurement, 0.1 g of the sample was degassed for 2 hours at 110 °C.The sample was then fed with N2 gas at 77 K for 24 hours.To evaluate the recovery ability of natural zeolite/Fe3O4, the filtrate obtained from the separation of the composite in water medium using an external magnetic field was measured by Lovibond Turbicheck turbidimeter using a calibration curve.

The effect of zeolite modification techniques on its physical appearance 3.1.1 Washing treatment
The washing treatment with HF solution aims to remove non-framework SiO2 impurities, whereas the Na2EDTA solution is useful for removing metal oxides such as MgO, Fe2O3, and CaO, which commonly contaminate natural zeolite.It has been reported that HF solutions are able to dissolve SiO2 effectively [15] .On the other hand, EDTA is able to bind Mg, Fe, and Ca from their oxide or mineral forms through chelation so that they are dissolved as M-EDTA (M=Mg, Fe, Ca) [16] .The natural zeolite appears to be a brighter color after being washed, as shown in Figure 1.This color shift reflects the elimination of impurities as a result of the washing procedure.

Coprecipitation technique
The coprecipitation method (COP) is based on the precipitation reaction of Fe3O4 on the surface of the zeolite from a solution containing a combination of Fe(II) and Fe(III) ions in NH4OH.The following is the proposed reaction for the synthesis of natural zeolite/Fe3O4 using the coprecipitation technique [14] : Zeolit (s) + 2Fe 3+ (aq) + Fe 2+ (aq) + 8 OH -(aq) → Zeolit/Fe3O4 (s) + 4H2O (l) This preparation involved varying the percentage of Fe3O4 in natural zeolite as much as 25.0% w/w, 33.3% w/w to 50.0% w/w.The percentage was calculated from the mass ratio of natural zeolite and Fe3O4 produced stoichiometrically through the coprecipitation reaction.The end product by this technique was labelled as natural zeolite/Fe3O4 COP 25.0% w/w, 33.3% w/w, and 50.0%w/w.The appearance of the produced natural zeolite/Fe3O4 COP powder is shown in Figure 2. The color of natural zeolite/Fe3O4 COP ranges from brown to blackish brown, depending on the Fe3O4 fraction.The natural zeolite/Fe3O4 COP color becomes darker as the Fe3O4 proportion increases because Fe3O4 particles are typically black.

Physical mixing technique
Physical mixing (PHY) was performed by mixing zeolite powder and Fe3O4 powder in a deionized water medium.The Fe3O4 fraction in natural zeolite was 25.0% w/w, 33.3% w/w, and 50.0%w/w.The percentage was calculated from the mass ratio of natural zeolite and Fe3O4 powder which prior synthesized using coprecipitation method.Additionally, the material is labelled as natural zeolite/Fe3O4 PHY 25.0%, 33.3%, and 50.0%w/w. Figure 3 shows the appearance of the prepared natural zeolite/Fe3O4 PHY powder.The colors of natural zeolite/Fe3O4 PHY range from brown to blackish brown.It can be observed that increasing the Fe3O4 fraction darkens the natural zeolite/Fe3O4 PHY color.Although increasing the Fe3O4 fraction produces the same color-shifting behavior, the natural zeolite/Fe3O4 COP is often darker in color than the natural zeolite/Fe3O4 PHY.This could be due to the precursor producing Fe3O4 being in the form of a solution, allowing it to distribute evenly throughout the natural zeolite.Meanwhile, because the Fe3O4 precursor in the physical mixing approach is in the form of a powder, it is more difficult to distribute evenly over natural zeolite.

The effect of preparation techniques and Fe3O4 fractions on the characteristics of natural zeolite/Fe3O4 3.2.1 Crystallinity
The diffractograms natural zeolite/Fe3O4 COP are shown in Figure 4 (a), (b) and (c).There are six diffraction peaks (blue triangle dot) that appeared at 2θ= 9.62°, 13.18°, 19.40°, 22.05°, 25.43° and 27.41°.This diffraction pattern indicates mordenite-type zeolite based on JCPDS data No. 5-0490.These results are consistent with previous study that the zeolite from the Bayat region of Klaten is mainly of the mordenite type [17] .In the diffraction pattern of natural zeolite/Fe3O4 COP, four Fe3O4 peaks (red rounded dot) were observed (2θ= 30.87°, 35.64°, 57.28°, and 62.81°).This peak belongs to Fe3O4 which has been successfully embedded in the natural zeolite surface.
From natural zeolite/Fe3O4 COP diffraction pattern, it can also be observed that increasing the Fe3O4 fraction generates an increase in Fe3O4 peak intensity but a decrease in natural zeolite peak intensity.The decreasing strength of the zeolite peaks was proportional to the amount of Fe3O4 present.This is because the proportion of natural zeolite in the composite has decreased.The diffractograms natural zeolite/Fe3O4 PHY are shown in Figure 4 (d), (e) and (f).Five Fe3O4 diffraction peaks can be identified in diffraction pattern.This demonstrates that Fe3O4 has formed on the natural zeolite/Fe3O4 surface.Identical to previous findings, the peak intensity of natural zeolite decreased as the Fe3O4 fraction increased.The higher the Fe3O4 content, the greater the decrease in the intensity of the zeolite peaks.The formation of Fe3O4 particles possibly only happens on the natural zeolite's surface.As a consequence of it, the decrease in zeolite peak intensity is not attributable to structural degradation but rather to a smaller zeolite fraction.

Pore Characteristics
The N2 adsorption-desorption isotherms curve of natural zeolite/Fe3O4 COP are shown in Figure 5 (a), (b) and (c).The curve was a mixture of types II and IV which contain a hysteresis loop.This loop hysteresis is caused by the capillary condensation phenomenon, which is typical for mesoporous isotherm materials [11] .The hysteresis loop shape was found to be of the H3 type, with a lower desorption shoulder and a closing point located about 0.42 P0 on adsorption N2 at 77 K [19] .This hysteresis loop type corresponds with a modest degree of pore curvature [19] .This is most likely due to Fe3O4 particles occupying the pores of natural zeolite.
The N2 adsorption-desorption isotherms curve of natural zeolite/Fe3O4 PHY are shown in Figure 5 (d), (e) and (f).The curve consists of a mix of types II and IV, and also observed hysteresis loops.The hysteresis loop on natural zeolite/Fe3O4 PHY 25.0% w/w, showed type H3, which was similar to the natural zeolite/Fe3O4 COP.In natural zeolite/Fe3O4 PHY 33.3% w/w and 50.0%w/w (Fig. 5e and 5f), the N2 adsorption-desorption isotherm curve indicated a mixture of types II and IV, but the hysteresis loop shape was changed.The observed loop hysteresis is of type H2(b).This type is associated with the occurrence of pore blockage [20] .This event was probably caused by pore blockage by Fe3O4 particles.The quantity of Fe3O4 particles in natural zeolite is abundant for fraction 33.3% w/w and 50.0%w/w, making pore blockage easier to occur.Figure 6a and 6b shows that the pore sizes of washed natural zeolite, zeolite/Fe3O4 COP and zeolite/Fe3O4 PHY are mesoporous type.Both natural zeolite/Fe3O4 COP and zeolite/Fe3O4 PHY has smaller pore diameters than washed natural zeolite.This is likely because Fe3O4 has filled the pores of natural zeolite, making them smaller.Furthermore, as the Fe3O4 ratio increases, the pore diameter of the natural zeolite increases.This can be attributed at the low Fe3O4 fraction, there are fewer Fe3O4 particles, thus they can easily enter the zeolite pores and reduce the zeolite pore diameter.Meanwhile, the amount of Fe3O4 in the higher Fe3O4 fraction is increased, as a result, Fe3O4 tends to form aggregates with large sizes, making it difficult to enter the zeolite pores and only coat the surface of the zeolite.

Recovery capability
Recovery demonstrates the ability of natural zeolite/Fe3O4 composites to rapidly separate from the water medium.This capability was determined by measuring the turbidity of the solution after separation using an external magnetic field, as shown in Figure 7. Figure 8 shows the filtrate's turbidity after the natural zeolite/Fe3O4 separation process.It can be observed from the figure that the filtrate obtained by separating natural zeolite/Fe3O4 COP has a lower turbidity (is cleaner) than the filtrate obtained by natural zeolite/Fe3O4 PHY separation.In other words, the ability to recover as well as its stability in water of natural zeolite/Fe3O4 COP is superior to natural zeolite/Fe3O4 PHY.Furthermore, it can be observed that increasing Fe3O4 fraction in natural zeolite/Fe3O4 reduces the filtrate turbidity (making it cleaner).As a result, increasing Fe3O4 fraction often increases zeolite/Fe3O4 ability to recover as well its stability in water.

Conclusion
The results showed that the natural zeolite/Fe3O4 prepared by coprecipitation had better dispersion of Fe3O4 (darker color composite) and recovery capabilities than natural zeolite/Fe3O4 prepared by physical mixing.The pore space of natural zeolite was filled with Fe3O4 particles resulting both natural zeolite/Fe3O4 has smaller pore diameters than washed natural zeolite.In general, an increase of Fe3O4 fraction in natural zeolite reduces the intensity of the natural zeolite XRD peaks but improves the composite's recovery capabilities.

Figure 7 .Figure 8 .
Figure 7. Recovery capability of natural zeolite/Fe3O4 using an external magnetic field