Sorption of ammonium and nitrate to biochars is electrostatic and pH-dependent

Biochars are potentially effective sorbents for NH₄⁺ and NO₃⁻ in water treatment and soil applications. Here we compare NH₄⁺ and NO₃⁻ sorption rates to acid-washed biochars produced from red oak (Quercus rubra) and corn stover (Zea mays) at three pyrolysis temperatures (400, 500 and 600 °C) and a range of solution pHs (3.5-7.5). Additionally, we examined sorption mechanisms by quantification of NH₄⁺ and NO₃⁻ sorption, as well as Ca²⁺ and Cl⁻ displacement for corn stover biochars. Solution pH curves showed that NH₄⁺ sorption was maximized (0.7-0.8 mg N g⁻¹) with low pyrolysis temperature (400 °C) biochar at near neutral pH (7.0-7.5), whereas NO₃⁻ sorption was maximized (1.4-1.5 mg N g⁻¹) with high pyrolysis temperatures (600 °C) and low pH (3.5-4). The Langmuir (r² = 0.90-1.00) and Freundlich (r² = 0.81-0.97) models were good predictors for both NH₄⁺ (pH 7) and NO₃⁻ (pH 3.7) sorption isotherms. Lastly, NH₄⁺ and NO₃⁻ displaced Ca²⁺ and Cl⁻, respectively, from previously CaCl₂-saturated corn stover biochars. Results from the pH curves, Langmuir isotherms, and cation displacement curves all support the predominance of ion exchange mechanisms. Our results demonstrate the importance of solution pH and chemical composition in influencing NH₄⁺ and NO₃⁻ sorption capacities of biochar.