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Precipitation of secondary minerals potentially affects the long-term barrier performance of bentonite, including hydraulic conductivity and diffusivity. In this regard, diffusion pathways of anions are expected to be preferentially clogged with secondary minerals, that may contribute to the containment of anionic radionuclides. However, experimental evidence is lacking because of the slow transport of reactants and complex alterations in the compacted bentonite. In this study, bentonite sample clogged with CaCO 3 was prepared by an electrochemical technique. Then, through-diffusion experiments of HDO (deuterium water) and I− ions were conducted to elucidate the effect of CaCO 3 on their effective diffusivities. Overall effective diffusivities were determined from through-diffusion experiments, and local effective diffusivities at the CaCO 3 layer were determined from concentration gradients of tracers inside bentonite after reaching steady states of through-diffusion. A CaCO 3 layer consisting of vaterite and calcite was precipitated in Na-bentonite; correspondingly, total porosity was maximally reduced from 0.57 to 0.35 at the CaCO 3 layer. The overall effective diffusivity slightly decreased in the presence of CaCO 3 , while the local effective diffusivity at the CaCO 3 layer considerably decreased for both HDO and I− ions. The impact of CaCO 3 on the local effective diffusivity of I− ions was greater than that of HDO. The results suggest that long-term precipitation of secondary minerals originating from alterations of bentonite may decrease the diffusivity of radionuclides, particularly anions. • The through-diffusion of HDO and I− ions was conducted using bentonite clogged with CaCO 3. • Not only overall D e , but local D e at CaCO 3 were determined from internal concentrations. • The local D e of both HDO and I− ions considerably decreased in the presence of CaCO 3. • The impact of CaCO 3 on the diffusion of I− ions was greater than HDO owing to anion exclusion. • Long-term mineral precipitation may contribute to a lower diffusivity of anions. [ABSTRACT FROM AUTHOR]

A comparative study of arsenic enrichment in the Bengal Delta (BD) was carried out in three alluvial aquifers in south-central Bangladesh. Investigated sites included Sonargaon in Narayanganj, Chandina in Comilla and Sirajdikhan in Munshiganj districts. At all sites samples from different depths were collected, and water chemistry and redox status vs. depth trends were determined. The concentrations of DOC and HCO3 − were highest at Sirajdikhan site, while at the Sonargaon and Chandina sites the concentrations were lower. On the contrary, the NH4 + concentration was high at the Chandina site as compared to the other sites. There was a good match between dissolved As and Fe at the Sirajdikhan and Sonargaon sites, but not at the Chandina site. The dissolved aqueous concentration of Mn was low at the Chandina site, which suggested that the Mn(IV) redox buffering step was missing. Speciation modeling indicated a possibility of siderite precipitation at all sites, but precipitation of rhodochrosite only at the Sonargaon and Sirajdikhan sites. At the Sirajdikhan site, the log $$ {\text{P}}_{{{\text{CO}}_{ 2} }} $$ values were very high (−1.37), which revealed the production of CO2 in redox processes. Principal component analysis (PCA) indicated an impact of sea water and redox status of different samples. These results suggest that the dissolved As is de-coupled from dissolved Mn because when released, As is re-adsorbed onto the Fe(III) minerals in solid phase, as well as from dissolved Fe when precipitation of Fe(II) minerals controls the aqueous concentrations of Fe. In addition, several other concurrent redox processes may exert kinetic constraints depending on refractory characteristics of Fe(III) minerals. [ABSTRACT FROM AUTHOR]