Compositional characterization of nine agricultural waste biochars: The relations between alkaline metals and cation exchange capacity with ammonium adsorption capability

We converted different biochars from nine common feedstock were prepared for aqueous NH₄⁺ adsorption. The biochars were classified into three groups: wood-like (B@A: wood chips, white popinac wood, pinecone), shell-like (B@B: rice husk, longan shell, water caltrop shell), and other agricultural wastes (B@C: corncob, sugarcane bagasse, and coconut fiber). We aimed to characterize the cation exchange capacity (CEC), alkaline metal composition, and other physicochemical properties of biochars and to examine their relation to NH₄⁺ adsorption. Our as-prepared biochars were mesoporous with a large surface area of 142–371 m²/g and highly negatively charged surface (pH_zpc of 0.91–1.64). The ash contents and contact angle characterization distinguished the three biochar groups into: hydrophilic, low-ash B@A; super hydrophilic, high-ash B@B; and hydrophobic, mid-ash B@C. Alkaline metals (K, Na, Ca, and Mg) were found up to 40 g/kg, while CEC varied from 27.80 to 292.63 meq/kg. At [NH₄⁺]₀ of 15 mM, NH₄⁺ adsorption by biochars was mostly effective at pH∼7, following the order: B@B > B@C > B@A with sorption capacity q_e of 0.04–0.15 mmol/g. The adsorption isotherms followed Langmuir model (R² = 0.95–0.99), indicating the monolayer sorption process. From principle component analysis, we revealed that the ubiquitously-used physicochemical characterization, such as pH_zpc, wettability, surface-pore characteristics, carbon and ash contents, presented minor roles in the NH₄⁺-biochar adsorption, with statistically negative or insignificant correlations with q_e. By contrast, CEC and alkaline metal contents strongly correlated to q_e, with R² > 0.9, p < 0.05, indicating the predominance of the cation exchange mechanism in this study.