Biochar composites with nano zerovalent iron and eggshell powder for nitrate removal from aqueous solution with coexisting chloride ions

Biochar (BC) was produced from date palm tree leaves and its composites were prepared with nano zerovalent iron (nZVI-BC) and hen eggshell powder (EP-BC). The produced BC and its composites were characterized by SEM, XRD, BET, and FTIR for surface structural, mineralogical, and chemical groups and tested for their efficiency for nitrate removal from aqueous solutions in the presence and absence of chloride ions. The incidence of graphene and nano zerovalent iron (Fe⁰) in the nZVI-BC composite was confirmed by XRD. The nZVI-BC composite possessed highest surface area (220.92 m² g⁻¹), carbon (80.55%), nitrogen (3.78%), and hydrogen (11.09%) contents compared to other materials. Nitrate sorption data was fitted well to the Langmuir (R ² = 0.93–0.98) and Freundlich (R ² = 0.90–0.99) isotherms. The sorption kinetics was adequately explained by the pseudo-second-order, power function, and Elovich models. The nZVI-BC composite showed highest Langmuir predicted sorption capacity (148.10 mg g⁻¹) followed by EP-BC composite (72.77 mg g⁻¹). In addition to the high surface area, the higher nitrate removal capacity of nZVI-BC composite could be attributed to the combination of two processes, i.e., chemisorption (outer-sphere complexation) and reduction of nitrate to ammonia or nitrogen by Fe⁰. The appearance of Fe-O stretching and N-H bonds in post-sorption FTIR spectra of nZVI-BC composite suggested the occurrence of redox reaction and formation of Fe compound with N, such as ferric nitrate (Fe(NO₃)₃·9H₂O). Coexistence of chloride ions negatively influenced the nitrate sorption. The decrease in nitrate sorption with increasing chloride ion concentration was observed, which could be due to the competition of free active sites on the sorbents between nitrate and chloride ions. The nZVI-BC composite exhibited higher nitrate removal efficiency compared to other materials even in the presence of highest concentration (100 mg L⁻¹) of coexisting chloride ion.