Impact of alkali cations on properties of metakaolin and metakaolin/slag geopolymers: Microstructures in relation to sorption of 134Cs radionuclide

Highlights: • Four geopolymeric sorbents were prepared and characterized from MK and MK/BFS blend. • Alkali cations (Na⁺, K⁺) divided sorbents into Al- and Si-rich groups. • Comparative equilibrium sorption studies of ¹³⁴Cs radionuclides were carried out. • Sorption capacities and mechanisms were differed from one sorbent to another. • Ion exchange mechanism governed all sorption systems, except in ¹³⁴Cs/K-MK GP one. - Abstract: Radio-cesium constitutes major environmental threats. Sorption of hazardous species onto geopolymeric sorbents is relatively recent and may give information about the retention mechanisms when geopolymers are applied to immobilize radwastes. Here, Na-MK, K-MK, Na-MKBFS, and K-MKBFS geopolymeric sorbents were synthesized from metakaolin (MK) and blast furnace slag (BFS) and were characterized using XRD, XRF, FT-IR, DTA/TGA and SEM. FT-IR/XRF results clarified the impact of mono-valent alkali cation (M⁺) in dividing the sorbents into Al-rich (sodium-based) and Si-rich (potassium-based). All sorbents were amorphous to semi-crystalline containing mica-phyllosilicates (greater in Si-rich), tobermorites (greater in MKBFS-based), gehlenite, calcite, quartz, hematite and hydrotalcite. Isotherms of ¹³⁴Cs radionuclide sorption were constructed, being regular with a positive temperature effect. Al-rich sorbents gave higher sorption capacities than Si-rich ones. Na-MK sorbent recorded the more distinctive sorption capacity (74.95 mg/g; at 333 K). Langmuir, Freundlich and D-R models were used to disclose the capacities and mechanisms governing the sorption processes. Sorption of Cs⁺ onto the examined sorbents was favorable. All systems were controlled by ion exchange mechanism, except ¹³⁴Cs/K-MK system which was controlled by physi- sorption mechanism. ¹³⁴Cs/Na-MK GP system was the only spontaneous among all. The endothermic natures were the common denominator between the tested systems.