Eco-Friendly Synthesis of Crown-Ether Functionalized Silica Nanospheres from Sorghum Waste for Thallium Adsorption

Background

An environmentally sustainable method for synthesizing aza-crown ether functionalized fibrous silica nanospheres was developed using sorghum waste as a biogenic silica source. The goal was to create a novel adsorbent for thallium [Tl(I)] removal from water.

Methods

A direct synthesis approach was used to anchor aza-crown ether onto the silica fibers. The material was characterized for its fibrous spherical morphology and surface area (242 m² g^–1), and the successful grafting of the functional groups was confirmed. Adsorption studies were conducted to optimize parameters for Tl(I) removal, and kinetic, isothermal, and thermodynamic analyses were performed.

Findings

The material's maximum adsorption capacity, based on the Langmuir model at 298 K, was 13.1 mg g^–1 under optimal conditions (pH 9.0, 25 mL solution, 60 min contact time, 0.2 g L^–1 adsorbent, 185 rpm stirring). Kinetic analysis showed a closer fit to the pseudo-first-order model, with three-stage diffusion. Thallium adsorption was endothermic ($\Delta H_{\text{Ads}.}^o$=38.72 kJ mol^–1), spontaneous ($\Delta G_{\text{Ads}.}^o$ ranging from –46.81 to –49.68 kJ mol^–1), accompanied by an increase in surface randomness ($\Delta S_{\text{Ads}.}^o$=0.287 kJ mol^–1 K^–1).