Influence of Intrinsic Colloid Formation on Migration of Cerium through Fractured Carbonate Rock

Abstract

Migration of colloids may facilitate the transport of radionuclides leaked from near surface waste sites and geological repositories. Intrinsic colloids are favorably formed by precipitation with carbonates in bicarbonate-rich environments, and their migration may be enhanced through fractured bedrock. The mobility of Ce(III) as an intrinsic colloid was studied in an artificial rainwater solution through a natural discrete chalk fracture. The results indicate that at variable injection concentrations (between 1 and 30 mg/L), nearly all of the recovered Ce takes the form of an intrinsic colloid of >0.45 μm diameter, including in those experiments in which the inlet solution was first filtered via 0.45 μm. In all experiments, these intrinsic colloids reached their maximum relative concentrations prior to that of the Br conservative tracer. Total Ce recovery from experiments using 0.45 μm filtered inlet solutions was only about 0.1%, and colloids of >0.45 μm constituted the majority of recovered Ce. About 1% of Ce was recovered when colloids of >0.45 μm were injected, indicating the enhanced mobility and recovery of Ce in the presence of bicarbonate.

Mobility of Radionuclides in Fractured Carbonate Rocks: Lessons from a Field-Scale Transport Experiment

“…In the tracer solution prior to injection, most of the Ce (93 ± 0.5%), some of the Sr (34 ± 4%), and very little of the Cs and Re (6.0 ± 3 and 1.3 ± 9%, respectively) were associated with the particulate fraction ( Table S3 ). In the presence of carbonates, Ce readily precipitates out of the solution as Ce 2 (CO 3 ) 3 ·8H 2 O, 11 and these have been shown to heteroaggregate with bentonite. 27 Similar bentonite–Ce heteroparticles were measured using dynamic light scattering in a filtered groundwater pumped from the same location as the present experiment, and sizes were found to range from 2.9 to 3.5 μm.…”

Current knowledge on transport and reactivity of technology-critical elements (TCEs) in soil and aquifer environments

“…However, colloids can be remobilised in case of heavy rain and the impact of varying flow rates should be investigated. In summary, while Tran et al (2015) demonstrated that inorganic cerium colloids are not particularly mobile in a carbonated fractured rock, Tran et al (2016) found that cerium mobility can be significant owing to organic mineral colloid associations (see the section Colloid-facilitated transport) in the same carbonated fractured rock.…”

“…Cerium has been studied because of its analogy to trivalent actinides. Tran et al (2015Tran et al ( , 2016 investigated the transport of cerium in fractured carbonate rocks, focusing mostly on how the intrinsic formation of cerium carbonate colloids, and the presence of humic acid and bentonite, can affect cerium mobility (the influence of carbonates is discussed in Sorption processes). Tran et al (2016) used a chalk core to study the impact of bentonite and humic acid on the transport of cerium.…”

“…Cerium can precipitate with carbonates that are ubiquitous in the environment to form carbonated colloids that can be mobile in fractured rocks. Tran et al (2015) performed cerium transport experiments in a chalk core containing a natural fracture, with a CeCl 3 salt dissolved in artificial rainwater similar to that in the Negev Desert of Israel. The solution contained carbonates, so that Ce-carbonate complexes were expected to form in solution and/or to precipitate.…”

Molecular dynamics simulations of Y(iii) coordination and hydration properties

“…Tran 9 investigated the geological migration of Ce in fractured carbonate rock formations and found that in a groundwater environment with pH 7-9, the main rare-earth carbonate complex ions are RECO 3 + and RE(CO 3 ) 2…”