Highly Efficient Adsorption of Sr²⁺ and Co²⁺ Ions by Ambient Prepared Alkali Activated Metakaolin

This study aimed to explore a low cost and sustainable adsorbent to remove Sr²⁺ and Co²⁺ ions, which are major radioactive ions in nuclear wastewater. The material properties of the alkali-activated metakaoline as a function of soaking time at ambient temperature from 1 day to 7 days were examined by XRD, XRF, SEM, and solid-state NMR. Adsorption isotherms were used to evaluate the appropriate soaking time for the optimal sorption performance for both Sr²⁺ and Co²⁺ ions. The alkali-activated metakaolin soaked for 3 days (BK3) presented the maximum adsorption capacities of 3.81 meq/g (167.5 mg/g) and 4.02 meq/g (118.5 mg/g) for Sr²⁺ and Co²⁺, respectively. The sorption mechanisms for Sr²⁺ and Co²⁺ in the BK3 sample were investigated, and the experimental results indicated that adsorption for Sr²⁺ was achieved via ion exchange. By contrast, surface complexation in combination with ion exchange contributed to the sorption mechanisms for the removal of Co²⁺ . Competitive adsorption experiments revealed that the alkali-activated metakaolin favored the adsorption for divalent ions (i.e., Sr²⁺ and Co²⁺), and it was less effective for Cs⁺ . Finally, the used adsorbent could be directly mineralized and vitrified by heat treatment to immobilize the Sr²⁺ and Co²⁺ ions.