Valorization of agricultural, forestry and nut-shell biomass residues into functional biochars for phosphorus removal: Feedstock screening and process optimization

Agricultural, forestry, and nut-shell biomass residues were valorized into functional biochars for phosphorus removal from water, aiming to elucidate the role of feedstock characteristics in governing modification efficiency and phosphorus adsorption performance. In these three separate categories, ML screening identified rice straw, walnut shell, and bamboo as the optimal biomass sources, while also determining the ranges for process parameters, including pyrolysis temperature (600–800 °C), time (2–4 h), and La 3 + concentration (2–4 mol/L). Subsequently, RSM was employed to experimentally validate the ML results and optimize pyrolysis conditions, revealing that the phosphorus adsorption capacity of La-modified biochars decreased in the order of rice straw > walnut shell > bamboo. The La-modified rice straw biochar exhibited the highest phosphorus adsorption capacity of 247.77 mg/g under the optimized conditions (727.57 °C, a La³⁺ concentration of 3.18 mol/L, and 2.89 h). Comprehensive characterization (XPS, XRD, XRF, and SEM-EDS) revealed that the abundant silicon dioxide in rice straw biochar not only formed a robust skeletal framework but also provided active sites facilitating the uniform dispersion and stabilization of La. In contrast, inadequate minerals in walnut shell and bamboo biochars hindered stable La-O-P phase formation, thus yielding weaker adsorption capacity. Finally, density function theory calculations elucidated the adsorption mechanisms of La-modified biochars for different phosphate species, indicating that phosphate and hydrogen phosphate tend to bind primarily via chemisorption, while dihydrogen phosphate favors physisorption. This study highlights the critical role of feedstock composition in determining phosphorus adsorption performance and demonstrates the potential of agricultural biomass residues, particularly rice straw, for the development of functional biochars for phosphorus removal. • Agri, forest, nutshell biomass screened for P adsorption. • Feedstock characteristics influenced La-modified biochar performance. • La-rice straw biochar best: 247.77 mg/g P adsorption capacity. • SiO₂ facilitated La dispersion and stable La–O–P bond formation. • DFT verified spontaneous phosphate adsorption on La–biochar surfaces.