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

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+ (Qmax = 51.81 mg/g) compared with PAC (Qmax = 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.

Single and competitive adsorptions of micropollutants using pristine and alkali-modified biochars from spent coffee grounds.

This study investigated the single and competitive adsorption characteristics of micropollutants using the pristine and alkali-modified spent coffee grounds (SCG) biochars. The alkali modification substantially improved the physicochemical characteristics of the SCG biochars (specific surface area and pore volume), which may have led to differences in the adsorption behaviors of the micropollutants. The pseudo second order model (R2 ≥ 0.990) better described the single and competitive adsorption kinetics than the pseudo first order model (R2 ≥ 0.664). It is evident that chemisorption played a key role in the removal of the micropollutants by the pristine and alkali-modified SCG biochars. The single and competitive adsorptions of the micropollutants were highly dependent on the solution pH and ionic strength since the pore-filling effects, electrostatic and hydrophobic interactions governed their removal by the pristine and alkali-modified SCG biochars. The higher removal efficiencies of the micropollutants by the alkali-modified SCG biochars (≥ 44.5%) in the presence of dissolved organic matter compared to the pristine SCG biochars (≤ 18.5%) support the assumption that alkali modification could markedly reinforce the surface structural properties of the SCG biochars related to the adsorption capacities.