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

Biochars are potentially effective sorbents for NH and NO in water treatment and soil applications. Here we compare NH and NO 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 NH and NO sorption, as well as Ca and Cl displacement for corn stover biochars. Solution pH curves showed that NH sorption was maximized (0.7-0.8 mg N g) with low pyrolysis temperature (400 °C) biochar at near neutral pH (7.0-7.5), whereas NO sorption was maximized (1.4-1.5 mg N g) with high pyrolysis temperatures (600 °C) and low pH (3.5-4). The Langmuir (r = 0.90-1.00) and Freundlich (r = 0.81-0.97) models were good predictors for both NH (pH 7) and NO (pH 3.7) sorption isotherms. Lastly, NH and NO displaced Ca and Cl, respectively, from previously CaCl-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 NH and NO sorption capacities of biochar.