KOH-Activated Geopolymer Microspheres Recycle Co(II) with Higher Adsorption Capacity than NaOH-Activated Ones

A new type of absorbent with high efficiency was synthesized by KOH-activated slag-based geopolymer microspheres (K-SGM), which exhibited higher adsorption capacities for recycling Co­(II) (Q e,K‑SGM = 192.31 mg/g, Q e,Na‑SGM = 91.21 mg/g) than NaOH-activated ones (Na-SGM). During the Co­(II) adsorption process, these two kinds of geopolymeric adsorbents could be combined with heavy metal ions to optimize each other and form heavy metal-grown aid adsorbents. The morphology of Na-SGM and K-SGM was different which varied from coarse pores to nanonetwork or nanosheets after Co­(II) adsorption, and the Brunauer–Emmett–Teller (BET) surface areas of Na-SGM (10.46 m2/g) and K-SGM (22.96 m2/g) increased to 117.38 and 228.73 m2/g after Co­(II) adsorption, respectively. The BET surface area of K-SGM is twice that of Na-SGM whether before or after Co­(II) ion adsorption. The hydrated ionic radius of K and Na, the alkalinity degree of K+ and Na+, the electronegativity of Na-SGM and K-SGM surface, the BET surface area and Fourier transform infrared changes of CO3 2– and OH before and after Co­(II) adsorption, and X-ray photoelectron spectroscopy analysis like the relative content of geopolymer gel and bridging oxygen bonds in the Na-SGM and K-SGM are the fundamental reasons for the obvious differences in Co­(II) adsorption between Na-SGM and K-SGM.