Hydrothermal processing, characterization and leaching toxicity of Cr-added “fly ash-metakaolin” based geopolymer
Introduction
Chromium slag is a highly toxic solid waste discharged from the chromium salt and the ferrochrome alloy plants during the production process. In the past decades, thousands of tons of chromium slag accumulate increasingly in the natural environment and need to be disposal, which is a great concern to the ecological environment and human being health. Chromium exists as the form of Cr3+, CrO42− and Cr2O72− ions in chromium slag, of which the valence state of chromium in the latter two is hexavalent [Cr(VI)] [1]. Studies show that Cr(VI) is highly mobile and easily carcinogenic to humans comparative to Cr3+ [2], [3]. The toxicity of chromium is mainly derived from the water-soluble Cr(VI) in the chromium residue. In order to minimize the toxicity of chromium, the treatment of chromium slag is mainly focused on the detoxification and solidification of Cr(VI) [4], [5], [6]. The detoxification and solidification can be processed in sequence or synchronously. The detoxification is most easily achieved through redox reaction, for example, Cr(VI) in Cr2O72- and CrO42- can be reduced by Fe2+ as following Eqs. (1), (2):
Commonly, the detoxification and solidification of Cr(VI) is carried out at the same time, and the detoxified chromium could be directly solidified in a geopolymer matrix [7]. However, the simultaneous process for the two cannot fully achieve the effective detoxification of Cr(VI), and there is still a potential risk of highly leaching toxicity.
Geopolymer is a gelling material which can be easily prepared by using aluminosilicate active materials as raw materials under alkaline conditions. Due to the three-dimensional zeolite-like structure, geopolymers can be used for the solidification of toxic heavy metals [8], [9]. In addition, it was reported that the geopolymer structure still remains stable when heavy metals (including chromium) were solidified in geopolymer [10], [11]. And the addition of a small amount of reducing agent can effectively reduce the leaching rate of chromium from geopolymer [12], [13].
Metakaolin and fly ash are the most two kinds of aluminum-silicon active materials for preparing geopolymer. The solidification of heavy metals in fly ash or metakaolin based geopolymer is an inexpensive and efficient way to achieve the purpose of waste treatment, which has widespread application prospect in the field of waste management [14], [15]. Generally, the fly ash, metakaolin, fly ash-metakaolin based geopolymers all could be prepared at lower temperature by alkaline activation [16]. While, when fly ash-metakaolin mixtures were alkali-activated and followed by a hydrothermal processing at higher temperatures (90–210 ℃), the hydroceramics [17] and zeoceramics [18] could be synthesized. These hydroceramics and zeoceramics are mainly composed of inorganic sodium aluminosilicate gel as well as crystalline zeolite phases, such as zeolite A/NaP1, sodalite and analcime etc. Compared with geopolymer fabricated under ambient condition, hydroceramics and zeoceramics can accommodate large amounts of alkali and ions because of their special composition and structure. Consequently, the hydroceramics have been used as a potential candidate to the immobilization of radioactive nuclide wastes [19], [20], [21]. However, as far as we know, there are almost no reports on the structure and properties of geopolymer for solidifying Cr after hydrothermal processing. Whether the phase compositions of the Cr-added geopolymer after hydrothermal process can also develop a zeolite-like structure? How does the new structure affect the curing efficiency of Cr in geopolymer? For these purposes, it is necessary to ascertain the influences of hydrothermal conditions on the phase compositions, mechanical strength as well as the leaching toxicity of Cr-added FMB geopolymer.
In the present study, to achieve the detoxification of Cr(VI) effectively, the Cr(VI) was reduced in advance in solution with a reducing agent of Fe2+ and was so called ‘detoxified waste solution’. Immediately, various contents of detoxified Cr-added FMB geopolymers were fabricated from fly ash, metakaolin and the detoxified waste solutions by an alkaline-activated hydrothermal processing. The influences of hydrothermal temperature, content of Cr on the phase compositions, mechanical strength and morphology of the samples were investigated systematically. Furthermore, the Cr(VI) leaching toxicity of the samples was also measured and evaluated.
Section snippets
Raw materials
Type F fly ash used in this work was supplied from Bashu power station at Jiangyou (Sichuan Province, China). Metakaolin was produced by calcining kaolin at 800 °C for 3 h (Kaolin was taken from Lianjiang county, Guangdong Province, China, purity> 88.5%). The water glass was used as alkali activator [the modulus (SiO2/Na2O molar ratio) was 3.18], and NaOH was used to adjust the modulus of water glass to obtain composite chemical activator [the final modulus was 1.0]. The chemical compositions
XRD analysis
XRD patterns of the samples obtained by varying content of Cr and hydrothermal temperature are shown in Fig. 1, Fig. 2, respectively. It is shown that the crystalline phases of the samples are quartz, gismondite and sodium chloride. Quartz is introduced from the raw materials, and crystalline sodium chloride phase is produced from NaOH, water glass and excessive FeCl2·4H2O. While gismondite phase originates from some silica-alumina gel after alkaline activation and hydrothermal processing [25],
Conclusions
In this paper, the detoxified Cr-added FMB geopolymers were prepared by alkaline activation and hydrothermal processing. The phase compositions, microstructure, compressive strength and Cr(VI) leaching toxicity of the samples were systematically investigated. It was shown that the phase compositions of the detoxified Cr-added FMB geopolymer were mainly composed of amorphous phase and the crystalline phases of quartz, gismondite and sodium chloride. Also, the SEM images verified that the
CRediT authorship contribution statement
Yufeng Wei: Methodology, Formal analysis, Investigation, Writing - original draft. Jin Wang: Conceptualization, Resources, Supervision. Junxia Wang: Validation, Supervision. Lei Zhan: Data curation, Formal analysis. Xin Ye: Investigation, Formal analysis. Hongbin Tan: Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We gratefully thank the projects supported by the National Natural Science Foundation of China (No. 11705153), the Scientific Research Fund of Sichuan Provincial Education Department of China (No. 15ZA0113) and the Research Fund of the Sichuan Science and Technology Program of China (19ZDYF2817).
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