Eggshell incorporated agro-waste adsorbent pellets for sustainable orthophosphate capture from aqueous media

Abstract

In this study, granular adsorbents containing varying ratios of torrefied wheat straw (TWS), eggshells (ES), and chitosan (Chi) were prepared, which are referred to as ternary wheat straw composites (TWCs). The TWCs were assessed for mechanical stability during handling in aqueous media and their orthophosphate (P_i) adsorption properties were studied at equilibrium. The characterization of the TWCs employed spectroscopy (IR, solids ¹³C NMR, PXRD), TGA, and surface area/pore size analysis via N₂ gas and dye (4-nitrophenol) adsorption. The BET surface area for the composites increased with greater ES/Chi content from 0.26 m² g⁻¹ for C72 (80% TWS content) to 2.2 m² g⁻¹ for C22 (20% TWS content; 40% each ES and Chi content). The P_i adsorption properties of selected TWC adsorbents were evaluated via the Langmuir, Freundlich, and Sips isotherms at variable pH (4.5, 8.5) and 295 K. The TWCs showed moderate P_i uptake (23–30 mg g⁻¹) at pH 4.5 with a slight decrease (9–12 mg g⁻¹) at pH 8.5 for elevated P_i concentrations. Environmentally relevant P_i concentrations (<5 mg L⁻¹) revealed that TWCs with 20–80% TWS content observed similar uptake (ca. 1 mg g⁻¹). This study demonstrates that sustainable composite adsorbents that contain TWS, Chi and ES were modified to yield mechanically stable systems with tailored orthophosphate adsorption properties, especially at low concentrations for neutral or slightly alkaline pH. The proof-of-concept for this adsorbent technology reveals the role of synergistic effects, along with its overall sustainability and scalability, according to a facile synthetic strategy that includes support based on a preliminary cost analysis for these granular adsorbents.