Biochar functionalization with layered double hydroxides composites: Preparation, characterization, and application for effective phosphate removal

In this study, rice straw-derived biochar was used as a raw material for the synthesis of biochar functionalized with layered double hydroxides (BC-LDHs) as efficacious phosphate adsorbents. Many types of BC-LDH composites were successfully fabricated by either the precipitation of layered double hydroxides on pristine biochar or the pyrolysis of feedstock loaded with LDHs; successful synthesis of composites was corroborated via characterization analyses. Experimental investigations including those for pH effects, coexisting anion effects, reaction times, and initial concentrations of phosphate were performed with two samples, namely, BC-LDHs 6 and pure LDHs. An optimum pH of 3.0 was observed for both samples. The kinetic performance of BC-LDHs 6 (24 h) was slower than that of pure LDHs (3 h); however, a maximum adsorption capacity of the composite of up to 192 mg/g was achieved, which exceeded that of pure LDHs by nearly 1.2 times. This was considered to be related to the superior properties of the composite acquired when biochar and LDHs were combined. In both cases, the kinetics of phosphate adsorption followed the pseudo-second-order model, whereas the adsorption isotherms were consistent with the Freundlich model. The composite also exhibited specific selectivity toward phosphate in the presence of highly competing anions and pH changes. The results obtained demonstrate that amalgamation of both starting materials shows promise for the development of innovative adsorbents for phosphate removal.