In-depth understanding of local soil chemistry reveals that addition of Ca may counteract the mobilisation of 226Ra and other pollutants before wetland creation on the Grote Nete river banks

• Published in: The Science of the total environment Vol. 823; p. 153703
• Main Authors: Impens, Nathalie R.E.N., Jensen, Karl A., Skipperud, Lindis, Gompel, Axel Van, Vanhoudt, Nathalie
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-06-2022
• Subjects: 226Ra; Inundation; Mechanistic understanding; Phosphate legacy; Trace metals; CEC; OPC; FTIR-PAS; FC1000; a.u; Ksp; UV-VIS; (P)ACS; Cu-TRIEN; Inundation; NOR; DW; Mechanistic understanding; Phosphate legacy; FC150; Trace metals; NORM; Kd; QQQ-ICP-MS; FC; 226Ra
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2022.153703

The “Sigma plan” https://www.sigmaplan.be/en/ aims to create in Belgium inundation zones along the Grote Nete river to prevent Antwerp from flooding in extreme weather conditions. The riverbanks of the Grote Nete are at some hotspots historically contaminated by the phosphate industry resulting in Naturally Occurring Radionuclides (NOR) legacy. 226Ra is from a radiation protection point of view one of the most important radionuclides present at the hot spot under study, with a local soil activity concentration higher than 3000 Bq/kg 226Ra. In this paper, we identify the most relevant mechanisms governing the mobility of 226Ra. We selected for this study the role of CaSO4.2H2O, clay minerals and humic acids as the main contributors determining the speciation of Ra, due to their presence at the hot spot, their cation exchange capacity and their functional group density, respectively. Various novel analytical chemistry approaches were developed to study the prevailing reaction mechanisms that impact the solid-liquid distribution of 226Ra. We show that 226Ra coprecipitates in a (Ca,Ra)SO4 solid solution due to the high Ca2+ and SO42− concentrations in the local hot spot. If CaSO4.2H2O is not saturated in the soil solution, 226Ra adsorption to clay minerals counteracts the tendency of 226Ra partitioning to the liquid phase by interactions with humic and fulvic acids. Interactions between different soil compounds may further alter the partitioning of Ra. As, Cd, Pb and Zn in the hot spot are significantly above background values in Flemish sediments. Pb may be coprecipitated as sulphate salts, whereas Cd and Zn are most probably partially present as arsenate salts. The excess of Zn may interact with humic acids. The observed reaction mechanisms suggest that Ca2+ might play a key role in the immobilisation of Ra. The role of Ca2+ as immobilisation agent of the other contaminants is discussed.
[Display omitted]
•Legacy site conversion into wetland requires thorough understanding of mechanisms.•Precipitation of (Ca,Ra)SO4 significantly affects the Kd of Ra before flooding.•Clay mineralogy correlates positively to Ra mobility at inundation conditions.•Humic/fulvic acids may stimulate the mobility of Ra at inundation conditions.•Addition of Ca may immobilise Ra and possibly As, Cd, Pb and Zn before flooding.