Screening tests for N sorption allow to select and engineer biochars for N mitigation during biomass processing

Biochar amendment during biomass processing can improve those processes and products, and reduce the emissions of greenhouse gases and NH₃, resulting in ecologic and economic benefits. The potential positive effects of biochar are related to NH₄⁺-N and NH₃ sorption, which in turn are depending on different biochar characteristics. By knowing the relationship between biochar characteristics and NH₄⁺-N and NH₃ sorption, biochar production can be steered towards a higher N sorption or existing biochars can be selected for targeted applications for high N sorption. Therefore, this study aims to develop fast screening tests to estimate the potential for both NH₄⁺-N and NH₃ sorption of biochar, before application in biomass processing. Further, the effects of feedstock, pyrolysis temperature, biochar characteristics and biochar treatments on N sorption are studied. The results show that NH₄⁺-N sorption varied between 0 and 1.54 mg NH₄⁺-N/g fresh biochar and was highest for manure-based biochars with a high nutrient content and cation exchange capacity, produced at lower temperatures (300-450 °C). For some biochars, the feedstock itself had a higher NH₄⁺-N sorption than the biochar. Grinding and washing increased the NH₄⁺-N sorption. In addition, a general linear model was proposed to predict the NH₄⁺-N sorption based on three chemical characteristics_· NH₃ sorption varied between 0 and 100 % of the negative control and showed a linear positive relationship with the NH₄⁺-N sorption, moisture retention factor and cation exchange capacity. Pyrolysis temperature and feedstock type did not significantly affect NH₃ sorption. NH₄⁺-N and NH₃ desorption varied amongst biochar type.