Ac ce pt ed m an us cr ip t Mongolian Journal of Chemistry Page 1 of 13 Influence of co-milling oxide physical properties on the structural changes of natural clinoptilolite zeolites Narantsogt Natsagdorj1, Narangarav Lkhagvasuren1, Bolortuya Munkhjargal1 and Jadambaa Temuujin2* 1 School of Mathematics and Natural Sciences, Mongolian National University of Education, 14191, Ulaanbaatar, Mongolia 2 CITI University, Denver Street, 14190, Ulaanbaatar, Mongolia *Author to whom correspondence should be addressed Jadambaa Temuujin CITI University Denver Street, 14190 Ulaanbaatar, Mongolia E-mail: temuujin.jadamba@citi.edu.mn ORCID: https://orcid.org/0000-0003-0930-7271 This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, and proofreading process, which may lead to differences between this version and the official version of record. Please cite this article as: Narantsogt N., Narangarav L., Bolortuya M, and Jadambaa T. Influence of co- milling oxide physical properties on the structural changes of natural clinoptilolite zeolites. Mongolian Journal of Chemistry, 24(50), 2023, xx-xx https://doi.org/10.5564/mjc.v24i50.1250 mailto:temuujin.jadamba@citi.edu.mn Ac ce pt ed m an us cr ip t Mongolian Journal of Chemistry Page 2 of 13 Influence of co-milling oxide physical properties on the structural changes of natural clinoptilolite zeolites Narantsogt Natsagdorj1, https://orcid.org/0000-0002-1040-1276 Narangarav Lkhagvasuren1, Bolortuya Munkhjargal1, Jadambaa Temuujin2*, https://orcid.org/0000-0003-0930-7271 1 School of Mathematics and Natural Science, Mongolian National University of Education, 210648 Ulaanbaatar, Mongolia 2 CITI University, Denver Street, 14190, Ulaanbaatar, Mongolia ABSTRACT 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/cm3 and a hardness of 9, while 9 cristobalite has a density of 2.27 g/cm3 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. 12 The grinding media used were hardened steel balls with a weight ratio of 20:1 between the balls and the minerals. Raw minerals and milled products were evaluated using X-ray diffraction, Fourier-transform infrared spectroscopy and scanning electron microscopy. 15 It revealed that co-milling with different hardness oxides had a minor effect on the structural distortion of raw zeolite. Crystallite size reduction and amorphization were observed in high- hardness oxides rather than in zeolite particles. After milling, the amorphization of natural zeolite 18 milled alone was 30.4%, while no significant amorphization was observed when co-milled with corundum and cristobalite. Preliminary results of Cr(VI) adsorption tests on raw and milled zeolites indicate that co-milling with high-hardness oxides is not the preferred method to 21 enhance the activity of natural zeolite. Keywords: Natural zeolite, co-milling, mechano-chemical treatment, corundum, cristobalite, 24 FTIR, XRD, adsorption, Cr(VI) https://orcid.org/0000-0002-1040-1276 https://orcid.org/0000-0003-0930-7271 Ac ce pt ed m an us cr ip t Mongolian Journal of Chemistry Page 3 of 13 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 [7-9]. 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 Ac ce pt ed m an us cr ip t Mongolian Journal of Chemistry Page 4 of 13 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. 69 EXPERIMENTAL Materials: Natural zeolite was obtained from the Tsagaan tsav deposit in southern Mongolia. 72 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/cm3 and a Mohs hardness of 9. Cristobalite has a density of 2.27 g/cm3 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. 81 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 cm3. 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. Characterization: Chemical analysis (XRF): Chemical analyzes of the zeolite samples were 87 performed by XRF (Shimadzu, Primini-X-ray fluorescence, Japan) using the pressed tablet. SEM analysis: Scanning electron microscope (SEM) TM 1000 (Hitachi, Japan) was used to study the surface morphology of the raw and activated zeolites. Before the analysis, the 90 aluminum stubs were coated with an adhesive. The samples were strewn across the stubs. To prevent static charge, the samples were gold coated with an Emscope SC500 Sputter coater. XRD analysis: Mineralogical characterization was performed using powder XRD instruments 93 (Shimadzu, MAXima-X XRD-7000, Japan). Measurements were carried out with Cu Kα radiation wavelength of 1.54056 Å, angle of 2 θ (5-60°) and scanning step size of 0.02°. After milling to determine the degree of amorphization, the following equation was used: 96 Ac ce pt ed m an us cr ip t Mongolian Journal of Chemistry Page 5 of 13 𝐴 = 100 − 𝐾, %; 𝐾 = 𝐼𝑎𝑐𝑡 𝐼𝑟𝑎𝑤 ∙ 100, % (1) -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 = 𝐾 𝜆 𝛽 𝑐𝑜𝑠𝜃 (2) 102 D - mean size of crystallites, K – shape factor constant roughly 0.90, depends on the shape of crystallites, β - full width at half maximum in radians, λ - X-ray wavelength, The non-overlapping (020, 131, 151) peaks were used to determine the crystallite size of the 105 zeolite. For the cristobalite (111, 102, 200) peaks and for corundum (012, 104, 113) peaks were used. FTIR analysis: The Shimadzu, Fourier transform infrared spectrometer (FTIR 8200PC, Japan), 108 was used for the measurements by using 100 scans at 4 cm−1 resolutions, over the IR region of 400–4000 cm−1. An air background spectrum was collected at the start of the sample analysis. Samples were diluted with spectroscopic purity KBr at powder weight ratio to KBr 1:200. The 111 zeolite samples were measured three times and averaged for further processing. A background spectrum was measured before every sample to compensate for atmospheric conditions around the FT-IR instrument. 114 Adsorption test of Cr(VI): Only to determine whether the reactivity of the zeolites milled alone and in combination with other materials changed, a sorption test was conducted. A batch adsorption experiment was performed to determine the effectiveness of removing Cr(VI). 117 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 %. 129 Ac ce pt ed m an us cr ip t Mongolian Journal of Chemistry Page 6 of 13 𝑄𝑒 = (𝐶𝑖 − 𝐶𝑒 ) 𝑤 ∗ V (3) 𝐶𝑟 (𝑉𝐼) 𝑟𝑒𝑚𝑜𝑣𝑎𝑙 % = (𝐶𝑖 − 𝐶𝑒 ) 𝐶𝑖 ∗ 100 (4) Where Qe is amount of adsorbed Cr(VI), Ci and Ce are the initial and equilibrium Cr(VI) 132 concentrations of the test solution (mg/L), V is the test solution at volume (L), and W is the amount of adsorbent (g). Batch experiments were performed in duplicate and the average value was used. 135 RESULTS AND DISCUSSION Characterization of zeolite: Table 1 shows the results of the chemical analysis of natural zeolite. 138 According to Table 1, the main elements of natural zeolite are SiO2, Al2O3, K2O, Na2O, and Fe2O3. Clinoptilolite is the primary crystalline phase of raw zeolite, according to the XRD pattern (Fig. 141 2A) of the material. The Si to Al molar ratio is 5.44. K+>Na+>Ca2+>Mg2+ are the primary exchangeable cations. Illite and feldspar are examples of aluminosilicate minerals that may also be present based on chemical analysis. 144 Table 1. Chemical composition of natural zeolite, weight% SiO2 Al2O3 K2O Na2O Fe2O3 CaO Cl MgO TiO2 SrO P2O5 MnO SO3 72.6 14.8 4.2 3.54 2.16 1.51 0.0307 0.709 0.207 0.125 0.042 0.0094 0.066 SEM micrographs (Fig. 1) show that natural zeolite consists of particles with the main sizes of 50 to 300 m. The raw corundum represents chunky particles with varying sizes of 50-400 m, 147 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 153 along with the oxide particles. Ac ce pt ed m an us cr ip t Mongolian Journal of Chemistry Page 7 of 13 (A) (B) (C) (D) (E) (F) Fig. 1. SEM images of raw zeolite (A), activated zeolite (B), raw corundum (C), raw cristobalite (D), zeolite activated (co-milled) with corundum (E) and zeolite activated with cristobalite (F) at 156 different magnification. The XRD patterns of raw, milled, and co-milled with oxides zeolite samples are shown in Fig. 2. 159 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]. 162 Ac ce pt ed m an us cr ip t Mongolian Journal of Chemistry Page 8 of 13 Fig. 2. XRD pattern of zeolite (A-raw zeolite/clinoptilolite; B-milled zeolite; C- zeolite co-milled 165 with corundum; D- zeolite co-milled with cristobalite), (Cl – Clinoptilolite; Co – Corundum; Cr – Cristobalite) 168 Grinding of natural zeolite alters the structure of zeolite and lowers XRD intensity (Fig.2B). Formula (1) determined that the amorphization rate of clinoptilolite milled alone was approximately 30.4%. However, there are many overlapped peaks, making it difficult to 171 accurately characterize them. However, almost no amorphization of clinoptilolite was observed in the co-milled corundum and cristobalite oxide particles. Milling, in general, reduces crystallite size while increasing strain to a particular limit of crystallite and strain. Then, as the milling time 174 increases, the crystallite size is usually observed to increase as a result of agglomeration of the milled particles. It can be argued that the soft natural zeolite particles are agglomerating, consequently their crystallite size change is insignificant. 177 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 Table 2. The crystallite size of the milled zeolite was dramatically reduced from 22.58 to 15.84 nm. However, the crystallite sizes of the 180 zeolites in co-milled samples were almost identical to those of the raw zeolites. The average crystallite size of zeolite co-milled with corundum was 22.63 nm and 18.02 nm with cristobalite. In other words, virtually little mechanically induced zeolite amorphization occurred during milling. 183 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 186 Ac ce pt ed m an us cr ip t Mongolian Journal of Chemistry Page 9 of 13 hardness oxides. Hard oxide particles experience preferential amorphization and a reduction in crystallite structure. Unexpected data showed that our first hypothesis was wrong. 189 Table 2. Approximate crystallite size of the raw and milled samples determined by Scherrer equation Samples The Miller indices (hkl) Average D (nm) Raw zeolite 020 22.58 131 151 Milled zeolite 020 15.94 131 151 Raw corundum 012 52.20 104 113 Crystallite size of the corundum milled with zeolite 012 46.94 104 113 Raw cristobalite 111 29.73 102 200 Crystallite size of the cristobalite milled with zeolite 111 22.67 102 200 192 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