(120d) Selective Separation of Cesium Contaminated Clays from Pristine Clays By Froth Flotation

The release of cesium-137 (¹³⁷Cs) into the environment poses a substantial safety and environmental concern due to its high fission yield and significant half-life (t_1/2 ~ 29 years)^1,2. Once delivered into the biosphere there is a high potential for the fission products to undergo a series of chemical and geochemical reactions that ultimately impact human health. Following the nuclear accidents at Chernobyl and Fukushima, cesium remains the key contributor to radioactive contamination³, along with nuclear sites such as Hanford and Sellafield, where leakage into ground water can be of high cesium concentration⁴. Hence, the selective capture of mobile ¹³⁷Cs⁺ from effluent streams is crucial for remediation, although the treatment and reduction of large volumes of contaminated environments remains a challenging task. In the current study, based on a colloidal clay system, froth flotation, an industrial technique commonly used in mining to separate target minerals from the waste gangue⁵, was considered as a new approach to recover the Cs-loaded particles from uncontaminated particles thus significantly reducing the volume of waste for further treatment.

Firstly, cesium adsorption onto montmorillonite clay particles was investigated over a wide concentration range (10^-5 to 0.5 M), where a multi-stage sorption process was observed and best described using a multi-site model. At low concentration (c.a. < 10 mM), the adsorption was dominated by the typical cation exchange in the interlayer with a site capacity being consistent with the CEC arising from the exchangeable cations in the interlayer. Moreover, in this regime, the adsorption isotherm can be well described by the Langmuir model with a maximum capacity (Q_m) of ~ 103 mg/g. At high Cs⁺ concentrations (c.a. > 5mM), adsorption onto a second site of the planar surface with a low selectivity but high capacity becomes more important. This was confirmed from both modelling and experimental evidence including XPS and zeta potential data. As the surface Cs⁺ enrichment increases the particles’ intrinsic negative charge gradually becomes neutralized. As a result, when ethylhexadecyldimethylammonium bromide (EDAB) was used as a cationic collector for flotation, the threshold to compensate the surface charge and promote particle flocculation for enhanced flotation was shown to be dependent on the degree of Cs⁺ contamination. I.e., the higher the Cs⁺ loading, the lower the EDAB concentration required to induce particle aggregation. This modulation provided a means to selectively separate the Cs⁺ contaminated clays from uncontaminated clays using froth flotation.

Secondly, a laboratory-scale column setup was used to perform froth flotation experiments with the dosed concentration of EDAB optimized for the critical separation regime, ~ 0.18mM EDAB (relative to 1g/L suspension). It was observed that the flotation recovery percentage of Cs-MMT gradually increased with increasing Cs⁺ contamination, reaching a value as high as 80% recovery for Cs 50mM, while the recovery of the uncontaminated particles was less than 25%. Considering a binary mixture of MMT-Cs50mM and uncontaminated MMT, excellent selectivity of the contaminated MMT was achieved with the overall particle recovery increasing proportionally with the fraction of MMT-Cs50mM. The preferential recovery of the contaminated particles was confirmed by EDX data, which showed increased Cs distribution in the collected particles, thus demonstrating froth flotation as a method to selectively separate caesium-contaminated clays from uncontaminated clays.

To better understand the mechanism for the preferential flotation of Cs-MMT compared to MMT, particle characterization techniques including zeta potential, size, contact angle and quartz crystal microbalance with dissipation (QCM-D) were utilized to elucidate the effect of cesium contamination on the interaction between EDAB molecules and the MMT particles. It was shown that preferential flotation is governed by particle aggregation of the Cs-MMT, while at the same collector concentration the MMT particles remained dispersed, as evidenced by particle size analysis. Cesium contamination not only enhanced the ability to aggregate particles but also increased the particle hydrophobicity, another factor contributing to the enhanced recovery of Cs-MMT particles. Hence, the mechanism for enhanced flotation is as follows: without Cs⁺ contamination, the dosed EDAB is ion-exchanged with the cations in the MMT interlayer with few EDAB molecules compensating the particle surface charge. However, following Cs⁺ contamination (Cs-MMT), the clay interlayer becomes saturated and the planar surface sites partially occupied with Cs⁺, hence a low concentration of EDAB is required to induce particle aggregation. This mechanism was indirectly confirmed by QCM-D experiments which confirmed the more favourable adsorption of EDAB on the MMT clay compared to Cs-MMT particles.

In conclusion, this research has demonstrated that following cesium contamination of clay particles, these particles can be preferentially separated from a clay matrix of uncontaminated particles by the concentration dependent flocculation and successful flotation. This engineering approach to recover radioactive contaminated soils appears promising to rapidly process and significantly reduce the volume of nuclear waste for further treatment before ultimate disposal.

References

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