Nitrate sorption and desorption in biochars from fast pyrolysis

• Chemical activation has effect on surface functional groups of biochars.
• Chemical activation has effect on surface charge of biochars.
• Chemical activation influences the nitrate sorption and desorption.
• Desorption is higher in chemical activated biochars.
• Herbaceous and woody biochars differ in their sorption and desorption potential.

Increasing the nitrate (NO3-) sorption capacity of Midwestern US soils has the potential to reduce nitrate leaching to ground water and reduce the extent of the hypoxia zone in the Gulf of Mexico. The objective of this study was to determine the sorption and desorption capacity of non-activated and chemically activated biochars from microwave pyrolysis using selected biomass feedstocks of corn stover (Zea mays L.), Ponderosa pine wood chips (Pinus ponderosa Lawson and C. Lawson), and switchgrass (Panicum virgatum L.). Surface characteristics such as surface area and net surface charge have shown significant effects on nitrate sorption and desorption in biochars. Freundlich isotherms performed well to fit the nitrate sorption data (R2 > 0.95) of biochars when compared to Langmuir isotherms. Nitrate sorption and desorption was significantly influenced by solution pH and presence of highly negative charged potential ions such as phosphate (PO43-) and sulfate (SO42-) in aqueous solution. Chemical activation with concentrated HCl had significant effect on surface characteristics of biochars and enhanced the nitrate sorption capacity. The first order model fit the nitrate desorption kinetics of biochars with a high coefficient of determination (R2 > 0.95) and low standard error (SE).

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