Elsevier

Ceramics International

Volume 45, Issue 6, 15 April 2019, Pages 7309-7317
Ceramics International

Preparation and characterization of nano SiO2@CeO2 extracted from blast furnace slag and uranium extraction waste for wastewater treatment

https://doi.org/10.1016/j.ceramint.2019.01.014Get rights and content

Abstract

Nowadays, ceramic oxides such as SiO2, CeO2 and their hybrid SiO2@CeO2 are considered as interesting materials for many applications. In the present work, SiO2 nanoparticles (NPs) were extracted from blast furnace slag (waste material derived from iron industry) by chemical method. On the other hand, CeO2 NPs were also extracted from radioactive nitrate solution waste using tris(2-ethylhexyl) phosphate solvent (TEHP). A simple precipitation and calcination method was used to prepare silica functionalized by ceria i.e. SiO2@CeO2 core-shell nanoparticles. In order to examine phase composition, structural characteristics, morphological, surface area and physical properties, the prepared NPs were characterized by various techniques. These techniques were XRF, XRD, Raman spectroscopy, Nitrogen adsorption–desorption isotherms, DLS, TEM, and Zeta potential. The activity of prepared SiO2 and hybrid SiO2@CeO2 materials towards the adsorption and photocatalytic degradation of methylene blue dye (MB) was investigated in absence (dark) or presence of UV–visible light. The results revealed that CeO2, SiO2 and SiO2@CeO2 nanoparticles having high surface area, high pore volume and small particle sizes were successfully prepared using the aforementioned waste materials. SiO2 NPs exhibited higher absorption capacity for MB in dark due to its higher negative surface charge as indicated by its zeta potential result. On the other hand, enhanced photocatalytic activity was obtained after coating CeO2 on SiO2 nanoparticles as compared with that of single SiO2. The overall adsorption efficiencies for MB in dark followed by UV light using SiO2 and SiO2@CeO2 were 94% and 99.1%, respectively. The kinetic studies indicated that the reaction proceeded as a pseudo-second order reaction.

Introduction

Recently, the disposal of industrial wastes by safe and economical ways is considered as one of the main targets for modern and even poor countries. Recycling, waste management and conversion of industrial wastes into new products by practicable procedures in marketable levels are serious issues for researchers and industrial sector. The extraction of active ceramic oxides from industrial wastes or utilizing the industrial wastes themselves as alternative sources (precursors) for production of new products have been investigated by many researchers [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Silica and clay nanoparticles with high surface area have drawn extensive consideration and been applied in different fields, for example, electronic, catalysts, sensors and adsorbents [11], [12], [13]. It is well known that the precursors used in preparation of silica nanoparticles are tetraethyl orthosilicate (TEOS), fumed silica, water glass or sodium silicate. The high cost and toxicity of some of these precursors restrict their utilization for reasonable purposes, so less expensive choices such as rice husk ash, fly ash and blast furnace slag became more attractive [14], [15], [16]. Among different classes of commercial wastes, blast furnace slag (BFS) has been drawing much attention in regards to its production amount, which exceeds some of other waste materials [17]. BFS is a by-product generated in the iron making process, since about 300–350 kg of BFS is delivered per ton of pig iron [18]. These huge amounts of BFS opened the door and encouraged the researchers to find advanced recycling processes which could conceivably open another market for BFS. Furthermore, energetic utilization of slag wasted in industry as precursors for various materials is invaluable in reduction of waste generation and manufacturing costs. There are many tools and methods for reducing and recycling wastes in the industry. These include photodegradation [19], [20] oxidation [21], [22], precipitation, [23], [24] condensation [25], cryogenic distillation [26], [27], membrane separation [21], [23], [27], catalytic reduction [28] evaporation, ion exchange [23], [29], and adsorption [21], [22], [23], [25], [26], [27].

Cerium dioxide (CeO2) is a one of most searched oxides among the rare earth metal oxides. It has drawn a remarkable interest as catalyst, sensor and electrode material in solid oxide fuel cells [30], [31]. Extraction of CeO2 form radioactive waste material is an interested research issue and needs focusing of researchers. The hydrometallurgical treatment of crude yellowcake in uranium refining processes produces a Raffinate solution that contains recyclable CeO2.

Many researchers have studied preparation and utilization of hybrid nano ceramic metal oxides which have high photocatalytic activity as a photo-catalyst. This is attributed to their high surface area and high portability of charge barriers. On the other hand, there are a few published scientific articles on synthesis of SiO2 @CeO2 core–shell structure. The incorporation of semiconductor photo catalyst as CeO2 on SiO2 to form SiO2@CeO2 core-shell improves its activity and makes new catalytic active center having high thermal and chemical stability, good mechanical value and high surface area [32], [33], [34]. It is well known that the stability, dispersion, catalytic and optical properties could be enhanced by covering the fine particles with a thin layer [35]. Sukon et al. prepared CeO2 shell on mesoporous SiO2 by homogeneous precipitation and used it as a photicatalyst for Rhodamine B degradation in water [36].

In the present study, we reported the recovery of SiO2 and CeO2 with high yields from blast furnace slag and radioactive wastes, respectively. These ceramic metal oxides were used in synthesizing nano silica particles coated by ceria (SiO2@CeO2 core-shell) which is investigated by different suitable techniques. The photocatalytic activity of synthesized SiO2@CeO2 core-shell towards decomposition of methylene blue (MB) in absence (dark) or the presence of UV–visible light was also investigated.

Section snippets

Synthesis of CeO2 from radioactive solution waste

The hydrometallurgical treatment of crude yellowcake in uranium refining processes produces a sulfate solution waste that contains many rare earth elements; this solution called Raffinate solution. In the present study, collected sulfate solution was subjected to complete precipitation of its total rare earth content using 10% oxalic acid at pH of 1.0 [37]. The rare earth oxalate precipitates were settled down for 6 h, filtered, washed with 1% oxalic acid solution, dried at 110 °C, and then

Characteristics of radioactive solution waste and synthesized CeO2

Fig. 2 shows XRF analysis of the original radioactive waste used as a precursor for CeO2 extraction. As indicated from the analysis, the total percent of rare earth oxides in the cake is about 14.15% with about 4.37% CeO2. The remaining oxides are Na2O 14.3%, SO3 68.17%, CaO 0.923% and Fe2O3 2.41%. As mentioned before, a nitrate stock solution containing 0.224% cerium (IV) was prepared by dissolving 100 g of precipitated rare earth oxides (by oxalic acid) in 1 L of 6 M nitric acid. This nitrate

Conclusion

SiO2, CeO2 and SiO2@CeO2 ceramic oxides were successfully synthesized by economic method and cost effective ingredients (blast furnace slag and radioactive waste). The synthesized materials have porous structure with large surface area and small particle sizes in the nano range. The coating of silica with ceria (SiO2@CeO2) improved its catalytic degradation activity towards methylene blue as compared with single SiO2, when the adsorption conducted under UV radiation. In dark, single SiO2 was

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