Abstract
Understanding the influence of colloids on radionuclide migration is of significance to evaluate environmental risks for radioactive waste disposals. In order to formulate an appropriate modelling framework that can quantify and interpret the anomalous transport of Strontium (Sr) in the absence and presence of colloids, the continuous time random walk (CTRW) approach is implemented in this work using available experimental information. The results show that the transport of Sr and its recovery are enhanced in the presence of colloids. The causes can be largely attributed to the trap-release processes, e.g. electrostatic interactions of Sr, colloids and natural sediments, and differences in pore structures, which gave rise to the varying interstitial velocities of dissolved and, if any, colloid-associated Sr. Good agreement between the CTRW simulations and the column-scale observations is demonstrated. Regardless of the presence of colloids, the CTRW modelling captures the characteristics of non-Fickian anomalous transport (0 < β < 2) of Sr. In particular, a range of 0 < β < 1, corresponding to the cases with greater recoveries, reveal strongly non-Fickian transport with distinctive earlier arrivals and tailing effects, likely due to the physicochemical heterogeneities, i.e. the repulsive interactions and/or the macro-pores originating from local heterogeneities. The results imply that colloids can increase the Sr transport as a barrier of Sr sorption onto sediments herein, apart from often being carriers of sored radionuclides in aqueous phase. From a modelling perspective, the findings show that the established CTRW model is valid for quantifying the non-Fickian and promoted transport of Sr with colloids.
Highlights
Due to potential radio-ecological impacts and environmental risks to human health, the safe disposal of spent nuclear fuel (SNF), high-level radioactive waste (HLW) and low-and intermediatelevel radioactive waste (LILW) in underground geological media is widely recognized as a grand engineering challenge affecting present and future generations (Faybishenko et al, 2016)
Liu et al / Environmental Pollution 252 (2019) 1491e1499 advection-dispersion models based on a constant dispersivity and a linear sorption process (Kersting, 2012; Kosakowski, 2004; Kosakowski and Smith, 2005) nor in thermodynamic methods based on the solubility and attachment potential (Za€nker and Hennig, 2014), as they commonly assume that sorbing radionuclides can attach to the stationary solid phase and underestimate the transport domain in real scenarios
Our investigation reveals the importance of quantifying nonFickian processes including the effect of colloids on the Sr migration
Summary
Due to potential radio-ecological impacts and environmental risks to human health, the safe disposal of spent nuclear fuel (SNF), high-level radioactive waste (HLW) and low-and intermediatelevel radioactive waste (LILW) in underground geological media is widely recognized as a grand engineering challenge affecting present and future generations (Faybishenko et al, 2016). Modelling of colloid-facilitated species transport mainly refers to the topics of colloid stability, colloid-species stability, their transports, and various interactions among colloids, species and porous media (Flury and Qiu, 2008; Yuan and Shapiro, 2012; Z€anker and Hennig, 2014). It is beyond the scope of this article to give a comprehensive overview and detailed discussion of those topics
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