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

Rivka B Fidel,David A Laird,Kurt A Spokas

doi:10.1038/s41598-018-35534-w

Rivka B Fidel, David A Laird + Show 1 more

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https://doi.org/10.1038/s41598-018-35534-w

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Abstract

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.

Highlights

Anthropogenic perturbations to the nitrogen cycle have resulted in elevated concentrations of inorganic N in natural water bodies, threatening human health and aquatic ecosystems
A decrease in O content with increasing pyrolysis temperature is consistent with increased aromatization, and higher Si in corn stover biochar is consistent with higher Si content of corn stover compared to wood[30,39,40]
At the time of writing, this is the first NH4+ and NO3− sorption study to control for both solution pH and competitive ions

Summary

Introduction

Anthropogenic perturbations to the nitrogen cycle have resulted in elevated concentrations of inorganic N in natural water bodies, threatening human health and aquatic ecosystems. Biochar produced from the pyrolysis of biomass feedstocks has the potential to be a low cost and efficient sorbent for NH4+ and NO3−, through both soil applications and use in water treatment[6,7,8]. Gai et al.[26] and Yang et al.[25] investigated NH4+-N sorption on slow pyrolysis wheat straw biochars produced at 400–700 °C, and 350 & 550 °C, respectively – but reported substantially different model parameters for Langmuir and Freundlich isotherms. From these studies we can compile some interesting insights into sorption mechanisms. No single study has systematically examined the effects of biochar feedstock and pyrolysis temperature on sorption, while controlling for solution pH and competing cations

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