Hydrothermal synthesis of hardened diatomite-based adsorbents with analcime formation for methylene blue adsorption

A facile hydrothermal method has been developed to synthesize natural diatomite into hardened diatomite-based adsorbents with zeolite (analcime) formation for methylene blue (MB) adsorption. The results showed that the initial and final strengths were provided with the formed C-S-H gel and zeolite (analcime), respectively. Due to the low temperature synthesis, the formed analcime and retained diatomite were also found to exert a synergistic effect on MB adsorption. The NaOH concentration had a significant effect on the C-S-H and analcime formations, and a lower NaOH concentration (<= 9 M) was favorable for C-S-H gel formation, while analcime formed readily at a higher NaOH concentration (>= 12 M). The curing temperature and time also influenced the formation of analcime, a long curing time (>= 12 h) or a high temperature (>= 473 K) was favorable for analcime formation, while an over-long time (>= 24 h) or over-high temperature (>= 493 K) had a negative effect on the strength of the specimens. The adsorption of MB in this study followed pseudo-second-order kinetics, with a maximum adsorption capacity of 129.87 mg g(-1) at 308 K, according to the Langmuir model. Thermodynamic studies also showed that the adsorption process was spontaneous and endothermic. As such, tough diatomite-based adsorbents with analcime formation could be synthesized hydrothermally, and could be used to capture MB in wastewater efficiently.