Synthesis of value-added zeolites and tobermorite from waste glass cullet using conventional and microwave-assisted hydrothermal treatments
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Date
2021-02-16
Authors
Majdinasab, Alireza
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Abstract
The ever-increasing volume of the waste glass cullet (WGC) is becoming a global environmental concern due to the majority of it being discarded in landfills. In US, only, 10 million tons of WGC were generated in 2013 with roughly 26% recycling for the post-consumer applications. As a result, the estimated WGC landfilling in North America is above 60% while landfilling could not potentially be an environmentally friendly alternative. Hence, reusing the massive amounts of WGC towards the synthesis of value-added products like zeolites and tobermorite could be a greener option to overcome this challenge.
In terms of zeolitization of WGC, a great attention was paid in this research to utilizing novel sources of energy such as microwave (MW) radiation to accelerate the induction period and shorten the crystallization time. Results showed that 10min of radiation was sufficient for the synthesis of zeolite at 150ºC with almost similar crystallinity than that of the one which was treated for 24h using a conventional heating oven. However, specific radiation conditions should be taken in to account especially if the synthesis is conducted under autogenous pressure. Experimental data obtained in this work verified the adverse effect of high temperature/ pressure environment on the nucleation and crystallinity of zeolites. This overshot in pressure and temperature was found to be relevant to the ionic conductivity of the reaction mixture, which could affect the MW interaction with the gel components. Landfill leachate was also found to be a promising alternative aqueous media for the zeolitization of WGC and produced zeolites with higher crystallinity compared to the deionized water. This approach could pave the way for the facile reusing of a hazardous waste stream such as leachate towards the production of value-added zeolites, due to high cost and complexity of the processes are currently used for its treatment.
Synthesis of Al substituted 11Å tobermorite using WGC as the precursor was also studied in the current work. The key parameters to prepare a well crystalline pure 11Å tobermorite were as follows: 180ºC reaction temperature, 5h of reaction time, 0.5M of activator concentration, and Ca/Si ratio of 0.83. Various analytical techniques confirmed that Al substitution occurred preferentially for the bridging and branching sites within the dreierketten silica chains resulting in an enhanced polymerization degree of the silica chains. This improvement within the microstructure by the impact of the Al substitution could broaden the application of 11Å tobermorite particularly in the environmental and construction sectors.
Overall, the results obtained in this study showed that MW radiation could significantly reduce the zeolite crystallization time due to fast and homogenous nucleation. However, increasing the power of radiation in the reaction vessel without efficient monitoring the temperature and pressure overshoot could result in the synthesis of poorly crystalline phases. The introduction of a fusion pretreatment prior to the crystallization was found to be significantly influential on the improvement of the WG zeolites properties including the crystallinity and cation exchange capacity compared to the direct hydrothermal synthesis. The reussage of the landfill leachate towards the synthesis of zeolites also was a promising alternative for the aqueous phase and provided zeolites with comparable properties as deionized water. WGC was found as a cheap suitable alternative for the synthesis of pure well crystalline 11Å tobermorite. The Al content within the WGC matrix successfully substituted within the layered microstructure of the synthesized 11Å tobermorite. This would increase the polymerization degree of the silica chains in the drierketten structure, which could potentially improve the mechanical properties, ion exchange properties, and stability of the prepared 11Å tobermorite.
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Keywords
Waste glass cullet, Zeolite, Microwave radiation, Conventional heating, Tobermorite, Constant temperature mode, Constant power mode