Effects of the presence of isosaccharinate on nickel adsorption by calcium silicate hydrate (CSH) gels: Experimental analysis and surface complexation modelling

Abstract

The presence of organics may enhance contaminant mobility; thus, the evaluation of radionuclide-organiccement interactions is a topic of particular interest for the safety assessment of low-intermediate level radioactive waste (LILRW) disposals. The objective of this work is to analyse nickel adsorption on the most abundant phases of hydrated cement paste, calcium silicate hydrate (CSH), in the presence of isosaccharinic acid (Isa), the principal degradation product of cellulose under alkaline conditions. Two different CSHs were studied (Ca/Si ratios of 0.8 and 1.4) to cover a range of chemical conditions representative of different cement degradation stages. To gather information on the interactions in the ternary Ni/Isa/CSH system, batch sorption tests, electrophoretic measurements, and spectroscopic analyses (FT-IR) were combined. The presence of Isa at a concentration up to 1⋅10-2 M slightly decreased the surface charge (- 10 mV) of the two CSH phases analysed, and moderately affected Ni adsorption, causing a diminution of Kd(Ni) by less than 1 order of magnitude. To interpret the experimental dataset, surface complexation modelling was successfully used in this system for the first time. The modelling was based on a double layer approach, which accounts for the properties of the binary Ni/CSH system and includes the formation of Ni-Isa aqueous complexes as well as Isa adsorption on the CSH, under the specific chemical conditions generated by the solids.