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 (957.6m2/g) and pore volume (0.676cm3/g) of PAC were much greater than those of SCG biochar (specific surface area = 11.0m2/g, pore volume = 0.009cm3/g). However, SCG biochar showed ahigher maximum adsorption capacity of Sr2+ (Qmax = 51.81mg/g) compared with PAC (Qmax = 32.79mg/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 theadsorption 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.