Synergistic co-adsorption of phosphate and ammonium onto a dual-functional biochar composite: kinetics, isotherms, and mechanisms

Abstract

Novel composite adsorbents (C-1:4 and C-2:3) were developed by blending zinc-modified biogas residue biochar (Zn-BRB) and potassium-modified coconut shell biochar (KCBC_0.5) for synergistic co-adsorption of phosphate (PO₄³⁻) and ammonium (NH₄⁺) from simulated wastewater. Co-adsorption batch experiments were conducted for simultaneous PO₄³⁻ and NH₄⁺ removal, including the effects of pH, time, and initial concentrations. Physicochemical characterizations confirmed dual-active sites for capturing both nutrients. Adsorption kinetics followed the pseudo-second-order model (R² ≥ 0.977). C-1:4 fit the Langmuir isotherm (Q_m = 18.97 mg/g), indicating homogeneous chemisorption of PO₄³⁻ onto Zn-active sites. C-2:3 fit the Temkin model best, with a Langmuir-derived capacity of 10.23 mg/g, suggesting cation-exchange-driven NH₄⁺ capture. Thermodynamic analysis revealed positive ΔH and ΔS for all systems, indicating endothermic, entropy-driven processes. However, negative ΔG was only observed for PO₄³⁻ onto C-1:4; other combinations showed positive ΔG (288–318 K). Reusability over five cycles confirmed sustained performance with 80% PO₄³⁻ retention for C-1:4 and 60% NH₄⁺ retention for C-2:3, though further optimization of regeneration protocols is needed. In eutrophic lake water, C-1:4 achieves complete PO₄³⁻ removal at higher dosages, while C-2:3 removes > 90% NH₄⁺—consistent with synthetic water. XPS confirmed successful Zn/K doping and revealed adsorption mechanisms: new N 1 s peaks verified NH₄⁺ uptake, while shifts in P 2p and Zn 3 s signals indicated phosphate immobilization. The primary removal mechanisms are electrostatic attraction, ligand exchange, and ion exchange. This work demonstrates that strategic blending of biochar components creates tunable, dual-functional adsorbents, offering a practical strategy for co-adsorption of nutrients from wastewater.