Effects of modified distiller’s grains biochar on the adsorption characteristics of ammonia nitrogen in water

Ammonium nitrogen (NH₄⁺-N)-laden wastewater has been discharged from agricultural and industrial activities in recent years. Excessive discharge has posed a serious threat to aquatic ecosystems. It is often required to develop efficient and low-cost adsorbents. Fortunately, biochar derived from agricultural residues can be expected to serve as a sustainable remediation. Yet its pristine form has suffered from the limited adsorption capacity. In this study, a pre-impregnation with slow pyrolysis was proposed to fabricate two modified distillers’ biochars. Among them, the feedstock was taken as the modified distillers, which were widely available, underutilized by-products from the liquor industry. Phosphoric acid (H₃PO₄) and zinc chloride (ZnCl₂) were employed as impregnation agents to produce phosphoric acid-modified biochar and zinc oxide modified biochar respectively. Physicochemical properties of the biochar were 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, and temperature). Both synthetic NH₄Cl solutions and real swine wastewater were selected to evaluate adsorption kinetics, isotherms, and the interference of coexisting ions. The results showed that both PBC and ZBC exhibited enhanced O/C and (O+N)/C atomic ratios (0.48-0.51 vs. 0.09; 0.49-0.56 vs. 0.15), compared with unmodified biochar (BC), indicating greater hydrophilicity and surface polarity. FTIR spectra revealed that there was the successful grafting of phosphorus-containing groups (P-O, P-O-P, PO₄^3-) on PBC and Zn–OH moieties on ZBC. BET analysis showed that PBC and ZBC were achieved in the 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 the average pore diameters of 2.20-2.25 nm characteristic of mesoporous structures; XRD patterns further confirmed that there was the presence of phosphate crystals (PDF#25-0408) on PBC and ZnCl₂ phases (PDF#74-0517) on ZBC, thus 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 then reached equilibrium at 240 min. The pseudo-second-order model provided superior fits (R²> 0.95) compared with the pseudo-first-order model. Equilibrium adsorption capacities (231.35 mg/g for PBC, and 218.31 mg/g for ZBC) were calculated to closely match experimental values (232.66 and 224.18 mg/g, respectively): 50-fold higher than that of BC (4.51 mg/g) thereby confirming chemisorption as the dominant rate-controlling mechanism. Real swine wastewater was also applied to contain coexisting cations (Na⁺, K⁺, Ca²⁺, and 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 the 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 indicated that the PBC was enhanced NH₄⁺ adsorption via: (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 (4) cation–π interactions with the aromatic carbon skeleton, whereas ZBC primarily relied on: (1) ion exchange between Zn–OH and NH₄⁺; (2) improved physical adsorption, due to ZnCl₂-induced pore development; (3) supplementary cation–π interactions. A preliminary cost assessment showed that the unit cost of modified biochar with 62.85-64.85 CNY/kg at laboratory scale was significantly lower than that of the commercial activated carbon (200-300 CNY/kg) and ion-exchange resins (>1 000 CNY/kg), thereby highlighting its economic viability. A highly efficient, low-cost, and environmentally sustainable adsorbent was obtained for NH₄⁺ removal from both synthetic and real wastewater. The modified distillers’ grains-derived biochar was also prepared after H₃PO₄ or ZnCl₂ impregnation. A waste-to-resource strategy was provided to reduce the aquatic nitrogen pollution from industrial by-products. Future research should focus on column adsorption performance, adsorbent regeneration, and field-scale validation in integrated wastewater systems.