Influence of co-milling oxide physical properties on the structural changes of natural clinoptilolite zeolites

3 Zeolites are a family of open-framework aluminosilicate minerals used in many diverse fields, including building materials, agriculture, water treatment, and catalysis. In this study, natural zeolites were mechano-chemically treated by co-milling with corundum and 6 cristobalite. The idea behind the study was that co-milling with high-hardness oxides would cause natural zeolite to undergo more structural distortion, potentially increasing its reactivity and sorption capabilities. Corundum has a density of 3.95 g/cm 3 and a hardness of 9, while 9 cristobalite has a density of 2.27 g/cm 3 and a hardness of 6-7, according to the Mohs hardness scale. In a planetary ball mill, the zeolites and 20 wt.% of various oxides were co-ground for 30 min.

A c c e p t e d m a n u s c r i p t INTRODUCTION 27 Natural zeolites are microporous aluminosilicate minerals that have many uses in industry, agriculture, medicine, and the environment. They are formed when volcanic rocks and alkaline groundwater interact. Natural zeolites are crystals that grow in the voids of sedimentary rocks 30 or basalt rocks that arise under various geological conditions [1,2]. Natural zeolites have received interest since the 1850s due to their base-exchange capabilities, which can be used in water softeners and agricultural applications [1,3]. Zeolites are used today primarily as 33 wastewater and gas pollution removers, catalysts, pesticides, and fertilizer carriers in food and agriculture, soil supplements, and animal feed additions [1,2,4]. Due to the contamination of industrial wastewater discharges, which contain many toxic heavy metals such as cadmium, 36 chromium, lead, and mercury, the levels of toxic heavy metals in surface and ground waters have been rising recently [5].
Chromium is regarded as a high-priority environmental pollutant among hazardous heavy metals. 39 The most prevalent chromium compounds have an oxidation state of (III) or (VI) and are considered hazardous to the environment [6]. Because of its solubility in practically the whole pH range, greater mobility than Cr(III), and chromium's carcinogenic state, Cr(VI) is more 42 dangerous than Cr(III). Cr(III) is less poisonous and less mobile than Cr(VI) [6].
Although natural zeolites are used for many different things, their use is restricted because of their low purity and small channel diameter, which precludes the adsorption of larger gas 45 molecules and organic compounds. Zeolites can be activated using a variety of techniques, such as mechanochemical activation, to improve their inherent qualities. The production of amorphous powders from elemental metals or powder mixes by ball milling, also known as 48 mechanical grinding (MG), has been shown to be an effective way of optimizing the properties of the powder [7][8][9]. Mechanical activation of natural zeolite leads to amorphous products with a higher surface area and adsorption properties than raw zeolite [8,9]. Previous references [10, 51 11] provide contradictory results in the sorption research of mechanically milled clinoptilolite. The mechanically amorphized clinoptilolite's cation exchange capacity (CEC) increased, according to Zolzaya et al. [10]. In contrast, Bohacs et al. discovered that methylene blue adsorption is 54 maintained in mechanically activated clinoptilolite [11]. Mechanical amorphization can be accomplished by milling the crystalline compounds alone or in conjunction with other crystalline compounds; however, in the latter case, the mechanochemical production of a new amorphous 57 or crystalline compound is also possible. Amorphization of the crystalline co-milling compounds is expected to rise if one of the compounds has a higher hardness than another. It was thought that the co-milled oxides' high hardness might also serve as a milling medium, accelerating the 60 amorphization of the softer co-milling compound. Therefore, a study of the structures of A c c e p t e d m a n u s c r i p t mechanically activated zeolite with the different oxides could clarify the influence of the hardness of the co-milled oxide on the sorption properties of the milled samples. This study aims to clarify 63 how co-milling oxide hardness affects the amorphization and adsorption characteristics of natural zeolite. In this study, X-ray diffraction (XRD), Fourier transform infrared (FTIR), and scanning electron microscopy (SEM) were used to analyze the mechanochemical effects on 66 natural zeolites caused by co-milling with different hardness oxides using a planetary ball mill.
The impact of co-milling natural zeolites with different hardness oxides on the reactivity of the original zeolite was checked using Cr (VI) adsorption tests.

EXPERIMENTAL
Materials: Natural zeolite was obtained from the Tsagaan tsav deposit in southern Mongolia.

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The zeolites were dried at room temperature before being pulverized by hand to reduce particle size with a ceramic pestle and mortar.
Mechanically activated zeolite preparation: Corundum and cristobalite oxides were added to the 75 zeolite for co-milling. Corundum has a density of 3.95 g/cm 3 and a Mohs hardness of 9.
Cristobalite has a density of 2.27 g/cm 3 and a Mohs hardness of 6. Cristobalite was created by calcinating quartz oxide for 4 hours at 1300 ° C. Quartz was converted to Cristobalite to decrease 78 the hardness for a better comparison with Corundum and to show the influence of hardness on the amorphization of natural zeolite. Some raw zeolites were ground alone and used as a reference.

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A planetary ball mill (NQM-0.4, China) was used to grind raw or mixed zeolites. Natural zeolite 80% w/w + oxides 20% w/w make up co-milled zeolite samples. The grinding was carried out in a hardened steel pot with a volume of 121 cm 3 . The ball-to-powder weight ratio was 20:1 and 84 the grinding media was hardened steel balls with diameters of 0.6 cm. The samples (5.5 g) were milled at 1500 rpm at room temperature for 30 minutes.
-amorphization%, -crystallinity%, Iact-X-ray diffraction intensity of activated sample, Iraw-X-ray diffraction intensity of the raw sample. For the calculation of the amorphization rate, the average 99 of the (020) and (131) peaks was used.
For approximate crystallite size determination was used Scherrer equation: D -mean size of crystallites, Kshape factor constant roughly 0.90, depends on the shape of crystallites, βfull width at half maximum in radians, λ -X-ray wavelength, The According to a review of the literature, acidic media are preferred for the adsorption of Cr(VI).
To conduct the adsorption test, we utilized pH 2. The pH variation and adsorption terms were not explored because the adsorption was not the main objective of the investigation. At 23°C 120 +/-2°C, batch studies were carried out in beakers at a batch rate of 0.5 g of zeolite per 50 mL of fluid. In glass beakers containing 50 mL of chromium standards, raw zeolite (clinoptilolite), milled zeolite, zeolite milled with corundum, and zeolite milled with cristobalite were added, 123 respectively.
All the reagents used were analytical grade. The 1000 mg/L chromium standards were prepared from K2Cr2O7 (Sigma). After 30 minutes of reaction time, the sorbents were removed by filtration 126 through a laboratory filter paper for qualitative analysis and the residual concentration of chromate ions was determined by the UV-Vis spectrophotometric method.
The samples were tested for adsorption of Cr (VI) and its removal %. Batch experiments were performed in duplicate and the average value was used.

RESULTS AND DISCUSSION
Characterization of zeolite: Table 1 shows the results of the chemical analysis of natural zeolite.

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According to while the cristobalite represents spherical morphology particles with sizes of 50 m.
The particle sizes of the samples were substantially smaller after grinding. Particles smaller than 100 m in size make up the milled samples. The larger particles with diameters of 100 m should 150 represent oxide particles, whereas the smaller particles should represent natural zeolites, because natural zeolites are considerably softer than oxide particles. This assertion might be supported by the fact that the milled zeolite contains smaller particles than those that were milled  The XRD patterns of raw, milled, and co-milled with oxides zeolite samples are shown in Fig. 2.

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Clinoptilolite is the primary crystalline phase in raw samples, with illite and feldspar as minor impurities. The composition of Tsagaan tsav natural zeolite is similar to that of natural zeolites from deposits in Slovakia and Ukraine [12].

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A c c e p t e d m a n u s c r i p t Grinding of natural zeolite alters the structure of zeolite and lowers XRD intensity (Fig.2B).

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The estimated crystallite size of the raw zeolite, the raw oxides and the co-milled oxide and zeolite samples determined by formula (2) is shown in In other words, virtually little mechanically induced zeolite amorphization occurred during milling.

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The amorphization of the oxide particles happens easier than that of the clinoptilolite particles, according to XRD patterns of the co-milled oxides zeolite samples. In other words, the structural integrity of the soft zeolite did not change significantly when it was co-milled with the high A c c e p t e d m a n u s c r i p t hardness oxides. Hard oxide particles experience preferential amorphization and a reduction in crystallite structure. Unexpected data showed that our first hypothesis was wrong.
189 Due to its extreme hardness, corundum is the mineral that is frequently used as an abrasive.
The following are the used milling media and powders hardness: clinoptilolite < cristobalite < hardened steel < corundum. The following are the densities of the same materials: zeolite < 195 cristobalite < corundum <hardened steel. The current study reveals that the density of the milling media possibly has a greater impact on the microstructure of the milled powders than the hardness of the powders being employed.

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The total alkali-silica (TAS) diagram, which is based on the total alkali (K2O+Na2O) and silica (SiO2) ratio of volcanic rocks, indicates (not shown here) that the used zeolite is found in the rhyolite area or is the same as the Idaho zeolite deposit of the tuff enriched in SiO2 [12].

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According to one theory, clinoptilolite typically develops from tuffs as a devitrified byproduct of volcanic glass or appears in rhyolite holes [13]. The TAS diagram supports this assertion.
In Fig. 3, the mid-FTIR spectra of raw, milled, and co-milled zeolite with corundum and   [11,14,15]. At 1040 cm -1 , a large broadband was seen. At 1635, 3443, and 3612 cm -1 , the bands are present. The hydroxyl of 213 illite [11], silanol, or aluminol (Si-O(H)-Al bridging hydroxyls) groups associated to Broensted acidity [14] are assigned to the band over 3600 cm -1 . The water molecules linked to the native zeolite structure are responsible for the bands at 3443 and 1635 cm -1 [11,15]. Near 1040, 785, Some authors [11] described the appearance of the shoulder at wave number 860-920 cm -1 as a result of mechanically induced crystallite size decrease in the zeolite and a broadening band 222 at 1060 cm -1 as a result of mechanical amorphization. Such alterations in the FTIR spectrum were not observed in the current study of mechanically milled clinoptilolite. The zeolites co-milled with the corundum and cristobalite (Fig. 3B) displays distinct bands for the zeolites, the typical 225 band of the corundum at 590 and 637 cm -1 , and the cristobalite at 793 cm -1 [17,18]. The water band centered at 1630, 3441, and 3661 cm -1 grew more in the zeolite sample co-milled with corundum than in the co-milled cristobalite sample. It implies that zeolite and corundum mixes 228 adsorb more water than zeolite and cristobalite blends. As a result, higher porosity and amorphization of zeolite in the co-milled with corundum than in the co-milled with cristobalite, which in agreement with the crystallite size data provided in Table 2. m a n u s c r i p t removal efficiency may indicate that structural changes in zeolite were not favorable in the mechanically milled zeolite's cation exchange capacity, confirming previous research [11]. The results of a preliminary study on the Cr(VI) adsorption of raw, milled and co-milled with corundum 243 and cristobalite zeolites show that co-milling with high hardness oxides is not the best method for enhancing the reactivity of natural zeolite in terms of adsorption effectiveness. In comparison to zeolite milled alone, agglomeration and microstructural changes in soft and low-density 246 natural zeolites that are co-milled with high hardness oxides are minimal. As shown by the Cr(VI) adsorption test, the reactivity of the co-milled natural zeolite was not improved. Co-milling oxides should have a higher density than high hardness if the natural zeolite is required to undergo 249 greater structural changes.

CONCLUSIONS
Natural zeolites milled alone undergo mechanical amorphization which reduces crystallite size.

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The rate of amorphization of the milled zeolites was not accelerated by co-milling with the highhardness oxides. However, the crystallite size of the milled samples has slightly decreased. The high-hardness corundum oxide initially becomes structurally distorted when they are co-milled 255 with soft zeolite using a high-density milling medium. It is preferred to mill with high-A c c e p t e d m a n u s c r i p t