Adsorption mechanism of Cs+ and Pb2+ on the N-A-S-H surface under the different Si/Al ratio and temperature

The application of low-calcium alkali-activated geopolymers has significantly enhanced the removal of Pb2+ and Cs+ from the contaminated soils and water. However, the adsorption mechanisms of Pb2+ and Cs+ influenced by the temperature and Si/Al at the molecular scale remain unclear, which were the two critical factors influencing the adsorption capacity. This study employed molecular dynamics simulation to elucidate the effects of temperature and Si/Al ratio on the adsorption mechanism and behavior of Pb2+ and Cs+ on the N-A-S-H surface. Research findings demonstrate that the adsorption of Pb2+ and Cs+ on the N-A-S-H surface is attributed to chemical adsorption. Increasing temperature can regulate the removal ratio of heavy metal ions by enhancing the quantity of cation exchange (correlation coefficient of 0.88). Temperature also influences the removal ratio by changing in adsorption energy, potential of mean force, and controlling the maximum adsorption capacity, hydration, and diffusion ability of the heavy metals (correlation coefficient of 0.67, 0.55, 0.6, and 0.57). While the Si/Al ratio influences the adsorption of heavy metals by enhancing the negative charge density of N-A-S-H gel and strengthening the interaction between heavy metals and oxygen atoms on the N-A-S-H surface (correlation coefficient of 0.96 and 0.52). Additionally, temperature exhibits a more pronounced effect on enhancing in the removal ratio of heavy metals. The heavy metal removal was maximally increased by 435 % (for Cs+) and 439 % (for Pb2+) via elevated temperature, while it was increased by 63 % (for Cs+) and 98 % (for Pb2+) via decreased Si/Al ratio. This study can not only provide a new perspective on the removal of heavy metals by N-A-S-H gel, but also can contribute to the achievement of green and sustainable development goals.