Mechanistic Understanding of Superior Methylene Blue Adsorption Capacity in a Novel g-C3N4 Modified Amorphous Na-Ca-Mg Silicate Adsorbent: Insights from Multinuclear Solid-State NMR Spectroscopy.

Abstract

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-C₃N₄-modified counterpart (gCN-SAAM) were synthesized via alkali activation and a subsequent thermal process, respectively. The g-C₃N₄ modification resulted in a novel hybrid adsorbent with a remarkable methylene blue (MB) adsorption capacity of 420 mg g^-1, four times higher than the unmodified sample, setting a new benchmark. Solid-state ²⁹Si (MAS and CP/MAS), ¹H MAS, and ¹³C 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 Q³ Si units, with minor Q⁰ and Q¹ Si species, structural water, and Mg-OH sites. Exposure to MB caused an upfield shift in the ²⁹Si CP/MAS spectrum and enhanced resonances in the high-field region, indicating MB interaction with Si sites. ¹H 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 Q³ Si species and increased the relative fraction of Q⁴ Si sites. This treatment also reduced the intensity of some Mg-OH units, showing interaction with g-C₃N₄. After MB adsorption on gCN-SAAM, NH₂ groups of g-C₃N₄ disappeared, and shifts in the C_2N-NHx and C_3N 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.