Application of Flax (<i>Linum Usitatissimum L.</i>) Straw Derived Activated Carbon Using Phosphoric Acid (H<sub>3</sub>PO<sub>4</sub>) for the Removal of Nitrate (NO&

Significant health and environmental hazards are associated with nitrate (NO<sub>3</sub>-) contamination in water, which calls for economical and environmentally friendly treatment techniques. This study investigates using phosphoric acid (H<sub>3</sub>PO<sub>4</sub>) activation to turn flax straw (Linum usitatissimum L.), an underutilized agricultural residue, into activated carbon (FS-AC) for the removal of nitrate from aqueous solutions. The FS-AC was prepared by chemical activation under varying conditions (temperature: 400-600°C, H<sub>3</sub>PO<sub>4</sub> concentration: 1-3 M, impregnation ratio: 1:4-1:6) and optimized using response surface methodology (RSM). Characterization via Scanning Electron Microscopy (SEM) revealed a highly porous morphology (surface area: 798.23 m2/g), while X-ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR) confirmed an amorphous carbon structure with oxygen-rich functional groups and residual crystalline phases. Under optimal conditions (532°C, 1 M H<sub>3</sub>PO<sub>4</sub>, impregnation ratio 1:5), FS-AC achieved 92.79% nitrate removal at pH 4, 0.25 g/L dose, and 60 min contact time. Adsorption followed the Langmuir isotherm (R2=0.9895), indicating monolayer adsorption, and pseudo-second-order kinetics (R2=0.9408), suggesting chemisorption. Thermodynamic analysis revealed spontaneity (ΔG°: −1.27 to −0.18 kJ/mol) and exothermicity (ΔH°=−14.19 kJ/mol). Generally, the study highlights FS-AC’s competitive performance against biomass-derived carbons, with fixed carbon content up to 34.89%. By converting flax straw waste into an efficient adsorbent, this study addresses dual challenges of agricultural residue management and water pollution, aligning with circular economy principles. Thus, future research should explore scalability, regeneration, and application in real wastewater systems to further validate its industrial viability.