Characteristics of biochar porosity by NMR and study of ammonium ion adsorption

Biochar produced from moderately slow pyrolysis of oilseed rape and mixed softwood at 550 and 700 °C were studied through NMR relaxation and cryoporometry analysis to determine the pore size distribution (micro-, meso-, macro-pores), surface area and liquid hydrocarbon content resident within the pores. In addition, the surface properties were also investigated to determine ability to adsorb aqueous ammonium ion. The micropore volume was always the lowest amongst pore diameters, but increasing pyrolysis temperature remarkably increased micropore volume in concentration by 1000 folds when temperature was raised to 700 °C. This supported the surface area increase with associated loss of surface functional groups. It was also accompanied with 50% and 33% reduction in condensable liquid hydrocarbon content in OSR and SWP biochars, respectively. This further reflected on the reduction in liquid hydrocarbon content and total PAH content from 0.5 to less than 0.1 mg Kg⁻¹ in OSR and from 4.4 to 0.2 mg Kg⁻¹ in SWP biochar, Nevertheless, OSR showed higher surface functionality compared to SWP biochars by adsorbing higher concentrations of ammonium ion within the mesopore region. This study revealed the relationship between temperature and feedstock on pore size distribution, liquid hydrocarbon content and ammonium adsorptive nature with NMR cryoporometry and relaxation analysis.