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

초록

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 m(2)/g) and pore volume (0.676 cm(3)/g) of PAC were much greater than those of SCG biochar (specific surface area=11.0 m(2)/g, pore volume=0.009 cm(3)/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 R-2 values of the pseudo-second-order model (R-2 >= 0.999) compared with the pseudo-first-order model (R-2 >= 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 (R-2 >= 0.994) than the Freundlich isotherm model (R-2 >= 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.

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