Figure 1. Conversion of blast furnace slag to high quality sulfonated mesoporous silica
Figure 1. Conversion of blast furnace slag to high quality sulfonated mesoporous silica
Figure 2. Proposed adsorption mechanism of Uranium ion species on sulfonated mesoporous silica adsorbent
Figure 2. Proposed adsorption mechanism of Uranium ion species on sulfonated mesoporous silica adsorbent

In order for countries to fulfil their obligations to the Paris agreement, coal powered and other anthropogenic energy sources have been discouraged in favour of renewable, high energy output processes. A fission nuclear reactor using Uranium is 36 million times more energy efficient by weight than conventional coal sources, making it an extremely attractive fuel source for countries with a high energy demand. The use of nuclear power plants has gained momentum since the mid 1900’s due to the incredible amount of energy generated by the process, however has always been subject to scrutiny due to the difficulty in treating and storing generated radioactive waste.

 A new method of storing radioactive Uranium has been proposed by researchers [El-Magied et al., Journal of Nuclear Materials (2018), doi: https://doi.org/10.1016/j.jnucmat.2018.06.034 who synthesised a high quality mesoporous sulfonated silica from an existing silica-rich by-product called blast furnace slag, produced during the manufacture of iron. The conversion of blast furnace slag to mesoporous sulfonated silica starts with a pre-treatment process to remove impurities by successive dissolution in acid and activation in boiling sulfuric acid to yield the high quality silica product (see Figure 1).

Millions of metrics tons of Blast Furnace slag is produced in the US per year, predominantly used in the construction industry as an aggregate building material due to its favourable porosity and insulation characteristics, while being readily available and cost effective.

Treatment of radioactive Uranium salts usually involve separation from the waste stream using energy intensive methods such as membrane separation, isotype-dilution precipitation and liquid-liquid extraction. Solid-liquid adsorption is recognised as the most effective processing method due to the green-chemistry aspect of the process; target species are removed from the waste stream by adsorption to active sites in the matrix while the adsorbent material can be regenerated. Researchers have shown that the synthesised mesoporous sulfonated silica is capable of selectively adsorbing multiple charged species of uranium due to the material containing both protonated silica sites (positively charged) and anionic sulfonate sites (negatively charged) present within the adsorbent matrix (see Figure 2).

An essential component in the effectiveness of an adsorbent is the speed in which the target ion can be absorbed into the matrix; most of the Uranium ions are adsorbed rapidly in the first 20 minutes exhibiting second order kinetics. Once adsorbed, the Uranium can remain in the adsorbent matrix for transportation, recovery or long-term storage. Regeneration of the sulfonated mesoporous silica after use was demonstrated with dilute nitric acid, maintaining stability over 3 cycles. This process is ideal for large scale operations compared to current treatment methods due to the abundance of the starting material, superior selectivity and the green-chemistry aspect, enabling the absorbent to be reused again and again after regeneration.

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