KOH-Activated Iron-Modified Corn Straw Biochar for Nitrate Removal: Preparation, Adsorption Performance, and Mechanism

Nitrate contamination of groundwater requires low-cost adsorbents with strong affinity for anionic species. This study developed an iron-modified corn straw biochar (Fe-BC) by coupling KOH activation with FeCl3 impregnation and secondary pyrolysis, thereby integrating a porous carbon framework with iron-derived adsorption sites. The optimal Fe-BC was obtained at an iron-to-biochar mass ratio of 1:3 and a secondary pyrolysis temperature of 600 °C. It contained uniformly dispersed crystalline Fe3O4, had a Brunauer–Emmett–Teller (BET) surface area of 550.20 m2/g, and exhibited a point of zero charge of 7.3. Fe-BC maintained an adsorption capacity above 5 mg/g at pH 2–7 and reached a Langmuir maximum capacity of 12.74 mg/g at 313 K, nearly fivefold that of pristine biochar. The pseudo-second-order model showed higher R² values (0.978–0.980) than the pseudo-first-order model, while the Langmuir model yielded R² values of 0.970–0.986. Thermodynamic parameters (ΔG = −17.77 to −19.47 kJ/mol, ΔH = 7.11 kJ/mol, and ΔS = 84.89 J/(mol·K)) indicated spontaneous and mildly endothermic adsorption. Phosphate produced the strongest competitive inhibition, whereas chloride and sulfate had smaller effects. Characterization results support a combined mechanism involving pH-dependent electrostatic attraction, association with Fe–OH sites, and pore filling. The principal contribution is the deliberate coupling of KOH-generated porosity and Fe3O4 functionalization in an agricultural-waste matrix, which provides a balanced improvement in accessible surface area, near-neutral-pH performance, and resistance to common anions. Regeneration and flow-through performance require further verification.