Surface engineering of biomass-derived carbon material for efficient water softening

[Display omitted] •A novel biomass-derived carbon adsorbent is reported.•Surface properties are modulated through a simple process of phosphorylation.•Oxygen-containing functional groups promoted the complexation with Ca2+.•Surface functionalization greatly improves the performance of water softening. Adsorption treatment has been considered an attractive technique for water softening due to its simple operation and environmental friendliness. The application of this technique, nevertheless, is limited by the lack of materials with low price, easy synthesis, and high absorption efficiency. In this work, we synthesize biomass-derived carbon (BDC) adsorbents with a flake-like structure and modulate the surface properties of the obtained BDC through a simple process of phosphorylation (BDC-P). After surface modification, the adsorption capacity of Ca2+ increased from 26.83 mg/g for the pristine sample to 42.23 mg/g for the BDC-P, and the adsorption kinetics does not change and still follow the pseudo-second-order kinetics model. Thermodynamic analysis reveals that the adsorption process is a spontaneous endothermic process, with an increase in entropy at the solid–liquid interface. As revealed by spectroscopic techniques, the adsorption mechanism is dominated by surface complexation. The enhanced adsorption performance is attributed to the presence of abundant oxygen-containing functional groups (phenolic hydroxyl, carboxyl, and phosphate groups) on the surface, which promoted the complexation process of Ca2+. Our results demonstrate that surface functionalization is an efficient strategy for strongly promoting the adsorption capacity of biomass-derived materials, which can be applied to constructing carbon-based functional materials for other environmental devices.