Immobilization of strontium in geopolymers activated by different concentrations of sodium silicate solutions

Sodium silicate is always used as an activator for the synthesis of geopolymer. However, the effect of sodium silicate concentration on the geopolymer used as adsorbent was still unclear. Therefore, the immobilization of Sr2+ in geopolymers activated by different concentrations of sodium silicate was studied through kinetic and isotherm modeling and solid characterizations including XRD, FTIR, TG, SEM-EDS, and N-2 adsorption-desorption isotherm. The adsorption amount of Sr2+ decreased with the sequence of S1, S2, and S3. According to the kinetic and isotherm modeling results, these sorption processes fitted better with pseudo-second-order, mainly governed by film diffusion. However, the diffusion mode was gradually closed to particle diffusion as for the sequence of S3, S2, and S1. Besides, the Langmuir model can be more befitting to sorption data than the Freundlich model, and the free energies decreased with the order of S1, S2, and S3. In addition, the specific surface areas did not change regularly with S1, S2, and S3. Thus, the distribution of Al tetrahedrons has a decisive role in the sorption process of Sr2+, even though the specific surface area is also a critical factor. More Al tetrahedrons can be formed under the activation of sodium silicate with higher concentration, leading to the low Si/Al molar ratio of the as-synthesized geopolymer.

Automated summary

The study investigated the immobilization of Sr2+ in geopolymers activated by different concentrations of sodium silicate, revealing that the adsorption amount of Sr2+ decreased with higher concentration of sodium silicate. The sorption process was mainly governed by film diffusion, with the diffusion mode shifting towards particle diffusion at higher sodium silicate concentration. Additionally, the Langmuir model was found to be more suitable for sorption data, and more Al tetrahedrons were formed under higher sodium silicate concentration, impacting the sorption process of Sr2+.