Abstract: Excessive discharge of ammonium nitrogen (NH₄⁺-N)-laden wastewater from agricultural and industrial activities has become a critical threat to aquatic ecosystems and human health, necessitating the development of efficient, low-cost adsorbents. Biochar derived from agricultural residues offers a sustainable remediation strategy, yet its pristine form suffers from limited adsorption capacity. This study proposes a novel pre-impregnation coupled with slow pyrolysis strategy to fabricate two modified biochars using distillers’ grains—a widely available, underutilized by-product of the liquor industry—as the feedstock, with phosphoric acid (H₃PO₄) and zinc chloride (ZnCl₂) employed as impregnation agents to produce PBC and ZBC, respectively. The physicochemical properties of the biochars were comprehensively characterized by elemental analysis, Fourier transform infrared spectroscopy (FTIR), Brunauer–Emmett–Teller (BET) surface area measurement, X-ray diffraction (XRD), scanning electron microscopy (SEM), and zeta potential analysis, while batch adsorption experiments were conducted under varying conditions (contact time, initial concentration, pH, temperature) using both synthetic NH₄Cl solutions and real swine wastewater to evaluate adsorption kinetics, isotherms, and the interference of coexisting ions. Compared with unmodified biochar (BC), both PBC and ZBC exhibited substantially enhanced O/C and (O+N)/C atomic ratios (0.48–0.51 vs. 0.09; 0.49-0.56 vs. 0.15), indicating greater hydrophilicity and surface polarity, with FTIR spectra revealing the successful grafting of phosphorus-containing groups (P-O, P-O-P, PO₄³⁻) on PBC and Zn-OH moieties on ZBC. BET analysis showed that PBC and ZBC achieved specific surface areas of 781.34 and 557.46 m²/g, respectively—77 and 55 times higher than that of BC (10.09 m²/g)—while total pore volume increased from 0.03 cm³/g (BC) to 0.43 cm³/g (PBC) and 0.31 cm³/g (ZBC), with average pore diameters of 2.20-2.25 nm characteristic of mesoporous structures; XRD patterns further confirmed the presence of phosphate crystals (PDF#25-0408) on PBC and ZnCl₂ phases (PDF#74-0517) on ZBC, providing structural bases for electrostatic attraction and ion exchange. Adsorption kinetics revealed that NH₄⁺ uptake on both modified biochars proceeded rapidly within the first 60 min and reached equilibrium at 240 min, with the pseudo-second-order model providing superior fits (R² > 0.95) compared to the pseudo-first-order model and yielding calculated equilibrium adsorption capacities (231.35 mg/g for PBC, 218.31 mg/g for ZBC) that closely matched experimental values (232.66 and 224.18 mg/g, respectively)—approximately 50-fold higher than that of BC (4.51 mg/g)—thereby confirming chemisorption as the dominant rate-controlling mechanism. When applied to real swine wastewater containing coexisting cations (Na⁺, K⁺, Ca²⁺, Mg²⁺), the NH₄⁺ adsorption capacities of PBC and ZBC decreased by 19.22% and 39.66%, respectively, under optimal conditions (35℃, pH 7, 360 min), with competitive adsorption and surface site coverage by coexisting ions—particularly the precipitation of phosphates with Ca²⁺/Mg²⁺ for PBC and the preferential exchange of Na⁺/K⁺ at Zn–OH sites for ZBC—identified as the primary inhibitory mechanisms via ion concentration monitoring and speciation analysis. Mechanistic analysis indicates that PBC enhances NH₄⁺ adsorption through: 1) physical pore filling enabled by its highly developed porosity; 2) electrostatic attraction between negatively charged phosphate groups (PO⁻) and NH₄⁺;3) ion exchange via H⁺ release from –OH and –COOH groups; and (iv) cation–π interactions with the aromatic carbon skeleton, whereas ZBC primarily relies on: 1) ion exchange between Zn-OH and NH₄⁺; 2) improved physical adsorption due to ZnCl₂-induced pore development; and 3) supplementary cation–π interactions. A preliminary cost assessment based on laboratory-scale production estimates the unit cost of modified biochar at 62.85–64.85 CNY/kg, significantly lower than commercial activated carbon (200-300 CNY/kg) and ion-exchange resins (>1000 CNY/kg), highlighting its economic viability. This study demonstrates that distillers’ grains-derived biochar modified via H₃PO₄ or ZnCl₂ impregnation is a highly efficient, low-cost, and environmentally sustainable adsorbent for NH₄⁺ removal from both synthetic and real wastewater, providing a waste-to-resource strategy that valorizes an industrial by-product while addressing the pressing challenge of aquatic nitrogen pollution; future research should focus on column adsorption performance, adsorbent regeneration, and field-scale validation in integrated wastewater treatment systems.