Mechanistic Understanding of Superior Methylene Blue Adsorption Capacity in a Novel g-CN Modified Amorphous Na-Ca-Mg Silicate Adsorbent: Insights from Multinuclear Solid-State NMR Spectroscopy.

Silicate-based adsorbents offer significant advantages over traditional materials, particularly due to their superior thermal and chemical stability, enhanced regenerability, and the ability to endure more rigorous operating conditions. In this study, an amorphous Na-Ca-magnesium silicate adsorbent (SAAM) and its g-CN-modified counterpart (gCN-SAAM) were synthesized via alkali activation and a subsequent thermal process, respectively. The g-CN modification resulted in a novel hybrid adsorbent with a remarkable methylene blue (MB) adsorption capacity of 420 mg g, four times higher than the unmodified sample, setting a new benchmark. Solid-state Si (MAS and CP/MAS), H MAS, and C CP/MAS NMR spectroscopy were used to investigate the complex structures of these adsorbents and their interactions with MB. The local structure of SAAM primarily consists of Q Si units, with minor Q and Q Si species, structural water, and Mg-OH sites. Exposure to MB caused an upfield shift in the Si CP/MAS spectrum and enhanced resonances in the high-field region, indicating MB interaction with Si sites. H MAS NMR spectra revealed significant interactions between water molecules in the geopolymer-like framework of SAAM and MB. The thermal treatment of SAAM with urea to produce gCN-SAAM enhanced the polymerization of Q Si species and increased the relative fraction of Q Si sites. This treatment also reduced the intensity of some Mg-OH units, showing interaction with g-CN. After MB adsorption on gCN-SAAM, NH groups of g-CN disappeared, and shifts in the C and C sites indicated their involvement in adsorption, while Si sites remained intact. This thermal method creates a sustainable, cost-effective and efficient adsorbent for MB removal from wastewater. Multinuclear NMR spectroscopy provides detailed insights into the adsorbent's complex structure and MB interactions, potentially guiding the design of improved future adsorbents.