Preparation and Characterization of Plaster Kiln Dust-Fe3O4 Magnetic Nanoparticles

Magnetic plaster kiln dust (MPKD) was synthesized as a unique, low-cost composite reused of byproduct plaster kiln dust (PKD), which is considered a source of air pollution. The FESEM, EDS, XRD, FTIR, VSM, and BET tests were used to characterize the MPKD. The characterization revealed that the MPKD was nanotubes non-agglomerated and super-paramagnetic with a high specific surface area (102.7 m/g). Compared with the specific area of other materials (composites), the MPKD could be considered a promising substance in the field of water/wastewater treatment.


INTRODUCTION
With the rapid development of nanotechnology in recent years, considerable attention to synthesizing various types of magnetic nanoparticles was conducted to find a solution of environmental issues like adsorption of dyes by magnetic halloysite ( Ostensibly, the advantages of magnetic nanoparticles are high magnetic properties, a high number of surface-active sites, and large surface area resulting in increased uptake (high removal rate of contaminants) and easy, rapid separation of adsorbent from solution by the magnetic field. Then, the harmful components can be removed from the magnetic particles then can be reused. Several essential aspects have been taken into account in improving the physical and chemical properties of the materials by controlling the chemical composition, size, shape, and morphology ( Plaster kiln dust is solid and white powdered matter extracted and collected from the large gas emission through smoke chimney or smokestacks that are found at the ending of the plaster kiln (or gypsum kiln) production facility. The sources of emissions in plaster (gypsum) kiln plants are often controlled through fabric filters and electrostatic precipitators as exhibited. Gypsum is considerd the source material for plaster (Juss) industries, and it is used as a retarder in the cement industry. Gypsum (CaSO4. 2H2O) is the calcium sulfate dihydrate, a gray or white natural mineral occurrence. The gypsum must be dehydrated partly, to produce plaster, or calcined to manufacture calcium sulfate half hydrate (CaSO4. ½H2O), usually calling stucco. In most plants, the calcination to produce stucco takes place in roughly 250 to 300 °F (120 to 150 °C), at that 1 ton of gypsum calcines to about 0.85 ton of stucco, and the rest is emissions as particulate matter disposal of the byproduct from gypsum plants is frequently the biggest problem. In Europe, the byproduct gypsum method of disposal includes discharge into the sea or estuaries, dumping on land, and dumping in the excavated area and old mines The present study aims to prepare and characterize composite from byproduct low-cost plaster kiln dust (PKD) and magnetic nanoparticles Fe3O4.
The byproduct PKD, Fig. 1 (a) was collected from local factories located in Iraq (the Modern Investment Mechanical Plaster Factory). The PKD was coated by the Fe3O4 magnetic nanoparticles (Merck USA; 10-20 nm in diameter) to produce magnetic plaster kiln dust (MPKD) through the wet impregnation method represented in (Makarchuk, et al., 2016;Daneshfozoun, et al., 2017). The Fe3O4, magnetic nanoparticles powder, Fig. 1 (b), was scattered and sonicated in deionized water for 3 min using sonication (1200W Ultrasonic Homogenizing and Mixing Liquid Chemicals -MSK-USP-12N). Then the PKD was added to the magnetic fluid to achieve a percent of Fe3O4 to PKD as (20 %). The new mixture was sonicated again for 10 min and stirred for one hour using a magnetic stirrer (SH-3) to adsorb magnetite onto the PKD. The synthesized adsorbent (MPKD) was separated by an external magnetic field and dried at 100 ºC for one day and then milled for 60 min at a speed of 500 rpm, and the final composite was shown in Fig. 1 (c). The characterization study of the PKD and MPKD included a description of morphology, shape, and size before and after coating to explore changes in surface topology via field emission scanning electron microscope (FESEM MIRA3 TESCAN, HV: 10 kV). Elemental analysis for the above materials performed by energy dispersive spectroscopy (EDS) operated at an accelerating voltage of 10 kV. The metal structure of both PKD and MPKD was detected by X-ray diffraction (XRD, Phillips Xpert) at room temperature with CuKα radiation source (λ= 0.154 nm wavelength) generated at 40 kV/40 mA. The functional group of the above materials was also examined using Fourier transform infrared spectroscopy (FTIR, Shimadzu) by mixing samples with KBr at a 1:1 ratio. Physical properties such as specific surface area and pore size distribution were calculated by the Brunauer-Emmett-Teller (BET) model and the Barrett-Joyner-Halenda (BJH) method, respectively. The magnetization of samples was carried out using a vibrating sample magnetometer (VSM, EZ7 model Microsense) at room temperature. The sample was put in a uniform magnetic field to analyze the magnetic properties.

FESEM and EDS
The FESEM analysis was shown in Fig. 1 with different scales. The appearance of the PKD particles was agglomerated nanotubes crystal with an average diameter of 53 nm of different sizes, as shown in Fig. 1 (a) and (b). The surface of PKD was highly porous textural with cracks and fissures (dark parts). These configurations increased the possibility of the Fe3O4 nanoparticles to adhere to the PKD particle's surface. After the coating process, the cracks on the surface of PKD particles disappeared and covered by the Fe3O4 nanoparticles that appeared as a bright spot, as shown in Fig. 1 (c) and (d). Also, the agglomeration of nanotubes no longer existed. Besides, new cavities were formed. Thus, the coating of PKD to produce MPKD was successful. Another verification of the coating was achieved by the EDS spectrum, as shown in Fig. 2. The presence of the Fe3O4 nanoparticles onto the PKD particle surface appeared in new peaks of the iron element (Fe) and represented by a percent of 2.81 % in Fig. 2 (b).

XRD and FTIR analysis
The XRD patterns (according to the International Centre for Diffraction Data, ICDD) of the PKD and MPKD were exhibited in Fig. 3. Based on the XRD analysis of the PKD, the noticeable peaks referred to several compounds of gypsum (the main constituent), quartz, calcium carbonate, and calcium sulfate. After coating, new diffraction peaks at 2ϴ= 29.76º, 35.98º, 36.64º, 42.35º, 43.36º, 47.82º, 54.17º, 63.07º and 72.85º could be identified as magnetite (Fe3O4). These results referred that the magnetic nanoparticles successfully adhered to the surface of the PKD. FTIR spectra for PKD and MPKD were scanned in the range of 4000-400 cm -1, as shown in Fig.  4 (a) and (b), respectively. From Fig. 4 (a), the bands located around 3547.09, 3493.09, and 3406.29 were typically ascribed to stretching vibration O-H groups due to H2O molecules in gypsum constituent. The band located at 3246.2 cm -1 was attributed to N-H stretching. The bands around 1799.59 cm -1 were imputed to C-H bending. The band around 1683.86 cm -1 was due to C=N stretching. The trough at 1622.13 cm -1 was seen for C=C stretching. The peak observed around 1433.11 and 1369.46 cm -1 corresponded to C-H bending. The bands located at 1138 cm -1 and 1118.71 were typically attributed to C-O stretching. The band around 1039.63 cm -1 was due to S=O stretching. The peak at 987.55 cm -1 was assigned to C=C bending. Carboxylic acids (general formula R-COOH) was indicated by a peak of 875.68 cm -1 (Socrates, 2004). The presence of silica in this sample was observed by 785.03 cm -1 . The band at 704.02 cm -1 was attributed to the C-C stretching absorptions. The peaks at 669.3 and 601.79 cm -1 were assigned to the stretching and bending modes of sulfate of the gypsum spectrum. Magnetometry (by field -7.02≤H≤7.02 kOe) of the MPKD was represented as a hysteresis loop and displayed in Fig. 5. The results indicated that this material was a soft magnet or superparamagnetic due to the value of remanence (Mr) and coercivity (Hc) closed to zero. The saturation magnetization was 29.12 emu/g, so the MPKD possessed a good response to an external magnetic field. Similar results were found by (Hu, et al., 2014).

BET analysis
The BET analysis of PKD, MPKD, and Fe3O4 was determined and reported in Table 1. As a whole, the area of particles surface and entire pores volume of the MPKD were greater than any of the PKD and Fe3O4 as many researchers like (Orolínová and Mockovčiaková, 2009; Yan et al., 2016) who discovered the same results after coating the raw materials by iron oxide, which can provide more effective sites for the uptake of pollutants. This can be described by the growth (development) of the micropores and mesopores composition with the creation of a magnetite layer of nanosized on the pore's surfaces of the PKD, as verified by FESEM in Fig. 1b, where new cavities and pores and appear after coating. The PKD and MPKD have an average pore diameter, which shows that both materials are mesoporous (between 2 and 50 nm). The data of Table 1 Journal of Engineering Volume 26 November 2020 Number 11 119 clearly indicate that the Fe3O4 magnetic nanoparticles easily agglomerate together. Therefore, the more Fe3O4 nanoparticles, the smaller the BET values. These results were in a good consensus with some publications (Chang, et

CONCLUSIONS
The byproduct PKD was coated by magnetic nanoparticles Fe3O4, and the new composite (MPKD) characterized by many tests. The morphology and the elemental structure proved successful development of the MPKD; besides, it acquired higher surface area (by double) with new effective functional groups as verification by the FTIR test. Preparation and characterization of the MPKD were not mentioned previously, as shown in the literature survey. So, it could be considered as an efficient composite compared to other publications in Table 2.