Effects of physicochemical properties of biochar derived from spent coffee grounds and commercial activated carbon on adsorption behavior and mechanisms of strontium ions (Sr2+)

Abstract

This study examined differences in the adsorption isotherms, kinetic equations, and thermodynamics of Sr²⁺ by biochar from spent coffee grounds (SCG) and powdered activated carbon (PAC). The specific surface area (957.6 m²/g) and pore volume (0.676 cm³/g) of PAC were much greater than those of SCG biochar (specific surface area = 11.0 m²/g, pore volume = 0.009 cm³/g). However, SCG biochar showed a higher maximum adsorption capacity of Sr²⁺ (Q_max = 51.81 mg/g) compared with PAC (Q_max = 32.79 mg/g) due to its abundance of O-containing functional groups. The negligible removal efficiencies of Sr²⁺ by SCG biochar and PAC under acidic conditions (pH = 1.0-3.0) are evidence that the electrostatic repulsion might hinder severely the adsorption of Sr²⁺ by the carbonaceous adsorbents. The higher R² values of the pseudo-second-order model (R² ≥ 0.999) compared with the pseudo-first-order model (R² ≥ 0.815) suggest that chemisorption governed the removal of Sr²⁺ using SCG biochar and PAC. Furthermore, the better description of the adsorption behavior of Sr²⁺ by the Langmuir isotherm model (R² ≥ 0.994) than the Freundlich isotherm model (R² ≥ 0.982) supports the assumption that the monolayer adsorption played critical roles in the removal of Sr²⁺ using SCG biochar and PAC. The thermodynamic studies revealed that adsorption of Sr²⁺ onto SCG biochar and PAC was endothermic and happened spontaneously. Despite the significant inhibitory effects of DOM, SCG biochar exhibited the higher removal efficiencies of Sr²⁺ compared with PAC. Hence, SCG biochar could be considered as an alternative to PAC for the removal of Sr²⁺ from aqueous solutions.

Keywords: Adsorption; Biochar; Commercial powdered activated carbons; O-containing functional groups; Spent coffee grounds; Strontium removal.