Chabazite: Stable Cation-Exchanger in Hyper Alkaline Concrete Pore Water

Abstract

To avoid impact on the environment, facilities for permanent disposal of hazardous waste adopt multibarrier design schemes. As the primary barrier very often consists of cement-based materials, two distinct aspects are essential for the selection of suitable complementary barriers: (1) selective sorption of the contaminants in the repository and (2) long-term chemical stability in hyperalkaline concrete-derived media. A multidisciplinary approach combining experimental strategies from environmental chemistry and materials science is therefore essential to provide a reliable assessment of potential candidate materials. Chabazite is typically synthesized in 1 M KOH solutions but also crystallizes in simulated young cement pore water, a pH 13 aqueous solution mainly containing K(+) and Na(+) cations. Its formation and stability in this medium was evaluated as a function of temperature (60 and 85 °C) over a timeframe of more than 2 years and was also asessed from a mechanistic point of view. Chabazite demonstrates excellent cation-exchange properties in simulated young cement pore water. Comparison of its Cs(+) cation exchange properties at pH 8 and pH 13 unexpectedly demonstrated an increase of the KD with increasing pH. The combined results identify chabazite as a valid candidate for inclusion in engineered barriers for concrete-based waste disposal.

Zeolite formation in the presence of cement hydrates and albite

“…The low Ca/Na ratio of the solution (Ca/Na <0.01) is expected to lead in all cases to the presence of mainly Na-zeolite X and Y. Also chabazite has been observed to take up other cations (Gottardi and Galli, 1985;Shim et al, 1999;Van Tendeloo et al, 2015). Ca-chabazite, is the most common form observed in nature and Δ f H° measurements indicate that Ca-chabazite could be more stable than Na-exchanged chabazite (Ogordova et al, 2004).…”

Alkali Activation of Fly Ash in the Presence of Sodium Nitrate

“…Zeolite X is considered to be metastable with respect to zeolite P ( [56,59]) and chabazite [60] and is often obtained at lower SiO 2 :Al 2 O 3 ratios in the pore solution [55]), whereas zeolite P is obtained when this ratio is higher ( [40,55,61]). The formation of zeolite X as the initial crystalline phase formed within geopolymer matrices was previously observed in low-silica MK-based geopolymers, where it was found to be metastable with respect to zeolite A [17].…”

“…For example, in presence of nitrate, zeolite X which is the initial crystalline phase formed at low alkalinities is metastable with respect to zeolite P [59], while the latter has a higher CEC (Cation Exchange Capacity) [58] and, therefore, a higher potential to incorporate cations of interest, such as hazardous metals and radionuclides. Chabazite was shown to be a good sorption medium for hazardous species ( [7,10,50,60,64]). The nitrate-cancrinite was shown to have a high immobilization e ciency towards Cs, a radionuclide often found in low-level radioactive wastes [13,63].…”

Investigation of chabazitic materials as Cs-137 sorbents from cementitious aqueous solutions

“…Leuven (abbreviated as SYN) [30]. The zeolitic materials were mineralogically characterized prior and after their use as sorbents.…”

“…Batch sorption experiments (S/L-ratio: 50 mg sorbent in 10 mL liquid) were performed using Cs-containing rain-based concrete pore water solutions (Csconcentration: 10 -9 to 10 -2 M) labelled with a small quantity of 137 CsCl in 0.1 M HCl supplied by EUROSTANDARD CZ s.r.o.. Two series of sorption experiments were performed using pulverized zeolitic materials of particle size between 5 and 50 μm and sorbate solutions of pH ~13 (Krich solution simulating the young concrete pore water (YCW) of state I) as well as its more neutral-pH counterpart (pH ~7 approaching the conditions of state IV pore water) [29,30] CHA, SYN and AT preloaded with cesium by contacting them with a 10 -2 M Cs YCWsolution for 2 weeks were subjected to leaching tests also using YCW State I as eluent. The total duration of the leaching tests was 100 days.…”