Uptake of selected organic ligands by hardened cement paste: Studies on proxy ligands for the degradation of polyacrylonitrile and general considerations on the role of different functionalities in the uptake process

Abstract

The uptake of α-hydroxyisobutyric acid (HIBA), 3-hydroxybutyric acid (HBA) and glutaric acid (GTA) by hardened cement paste (HCP) in the degradation stage II was investigated at various ligand concentrations (10−7 M ≤ [L]tot ≤ 0.1 M) and solid-to-liquid ratios (0.2 g⋅dm−3 ≤ S:L ≤ 50 g⋅dm−3). These organic ligands were previously identified as representative of the main degradation products of UP2W, a polyacrylonitrile-based material used as filter aid in nuclear power plants, under repository conditions. Sorption experiments were conducted with inactive (HIBA, HBA, GTA) and active (14C-labelled GTA) organic ligands. Sorption experiments show a weak uptake of HIBA and HBA by HCP, with distribution coefficients determined as Rd (HIBA) = (2.2 ± 1.3)⋅10−3 m3⋅kg−1 and Rd (HBA) = (1.6 ± 0.8)⋅10−3 m3⋅kg−1. A stronger uptake is observed for GTA, i.e. Rd (GTA) = (1.3 ± 0.5)⋅10−2 m3⋅kg−1, likely reflecting the contribution from the ligand’s second carboxylate group. GTA follows a linear sorption behaviour within 10−7 M ≤ [GTA]tot ≤ 0.1 M, which was successfully modelled with a one-site Langmuir isotherm. The adsorption capacity determined for the uptake of GTA by HCP is slightly higher but in line with the capacity previously reported for isosaccharinic acid (ISA), whereas the affinity constant derived for GTA is significantly lower than values reported for stronger binding sites in HCP for the uptake of ISA. HIBA and HBA have a minor impact on the surface charge of HCP up to [L]tot ≈ 0.1 M. On the contrary, GTA induces a clear decrease in the surface charge above [GTA]tot ≈ 10−3 M resulting in an isoelectric point at [GTA]tot ≈ 6⋅10−2 M. Comparison of sorption data obtained in this work and reported in the literature for organic ligands containing the functional groups -COOH and -OH underlines the key role of multiple functionalities as a factor strengthening the interaction with the HCP surface. The participation of alcohol groups is particularly strong when present in their deprotonated state. These results provide information to quantitatively assess the uptake by HCP of organic ligands relevant in the context of nuclear waste disposal, and to understand their impact on the surface properties of cement.

Impact of the degradation leachate of the polyacrylonitrile-based material UP2W on the retention of Ni(ii), Eu(iii) and Pu(iv) by cement

“…25 Beside the . Empty and grey symbols show the sorption data reported for GTA (Szabo et al 14 ) and ISA (Van Loon et al 19 ). See text for details.…”

“…A detailed description of the impact of the proxy ligands investigated in this work on the HCP is provided in our previous study dedicated to that binary system. 14…”

“…was prepared using the protocol described by Tasi et al and further used in the solubility and sorption studies with proxy ligands for UP2W degradation products. [12][13][14] The composition of the cement porewater is buffered by portlandite at pH ≈ 12.5 and [Ca] ≈ 0.02 M, and is characterized by a low content of Na and K (≤3 × 10 −4 M). A detailed description of the impact of the proxy ligands investigated in this work on the HCP is provided in our previous study dedicated to that binary system.…”

“…12 The present work further complements experimental studies by Szabo et al dedicated to the proxy ligands of UP2W, i.e., evaluating their effect on the solubility of selected radionuclides and assessing their uptake by HCP. 13,14 2. Experimental 15 but the choice of the +VI oxidation state for the spike of Pu aimed at avoiding high initial oversaturation conditions that may result in the formation of Pu(IV) colloids.…”

“…Data represented as log[DL] solid vs. log[DL] aq , with concentrations reported in ppm of C. Experiments performed at S : L = 20 g dm −3. Empty and grey symbols show the sorption data reported for GTA (Szabo et al14 ) and ISA (Van Loon et al19 ). See text for details.…”