Location: Soil and Water Management ResearchTitle: Sorption of ammonium and nitrate to biochars is electrostatic and pH-dependent
|FIDEL, RIVKA - Iowa State University|
|LAIRD, DAVID - Iowa State University|
Submitted to: Nature Scientific Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/27/2018
Publication Date: 12/4/2018
Citation: Fidel, R.B., Laird, D.A., Spokas, K.A. 2018. Sorption of ammonium and nitrate to biochars is electrostatic and pH-dependent. Nature Scientific Reports. 8(1):1-10. https://doi.org/10.1038/s41598-018-35534-w.
Interpretive Summary: Excess nutrient transport from agricultural fields is an environmental issue that requires a solution. Biochar has been proposed as one soil amendment that could improve nutrient retention in soils. Laboratory and field results have shown statistically significant impacts in the availability of soil nutrients and the production of some greenhouse gases following biochar additions. There have been several hypotheses into the potential mechanisms causing these effects. This paper carefully examines two of these potential pathways: pH alteration and ion exchange mechanisms. Data collected in this experiment show the importance of these factors in determining NH4+ and NO3- sorption capacities. These results are significant to farmers and policy makers and will assist scientists and engineers in developing improved biochars based on properties to improve soil carbon management and accurate predictions of the impact of biochars on net greenhouse gas exchanges following soil addition.
Technical 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 and corn stover at three pyrolysis temperatures (400, 500 and 600°C) and over 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 (0.74-0.84 mg N g-1) with low temperature (400°C) biochars at high solution pHs (7.0-7.5), whereas NO3- sorption was maximized (1.35-1.49 mg N g-1) with high pyrolysis temperature (600°C) and low pH (3.5-4). The Langmuir model provided the best predictor for both NH4+ (pH 7) and NO3- (pH 3.7) sorption isotherms (r2 = 0.90-0.99). Lastly, NH4+ and NO3- displaced Ca2+ and Cl-, respectively, from previously CaCl2-saturated corn stover biochars. Data from pH curves, Langmuir isotherms and solution chemistry assessments synthesize to support an ion exchange site mechanism, in contrast with chemisorption or precipitation. In addition, results emphasize the importance of solution pH and solution chemical composition as well as biochar production parameters in determining NH4+ and NO3- sorption capacities.