Characterisation, adsorption and desorption of ammonium and nitrate of biochar derived from different feedstocks

Abstract

Biochar is known to be a highly adsorptive material, especially when the biochar is altered by activation to further increase its sorption ability. Little information, however, is available on the potential reversibility of both ammonium (${\mathrm{NH}}_4^{+}$) and nitrate (${\mathrm{NO}}_3^{-}$) sorption on the inherent biochar pH. The objective of our study was to characterise biochars made using different pyrolysis conditions from five various plant materials and rubber tyre, and to use them to investigate the biochar properties responsible for ${\mathrm{NH}}_4^{+}$ and ${\mathrm{NO}}_3^{-}$ adsorption and desorption. The rubber tyre, maize stover and sugarcane pith were the weakest adsorbing biochars (5.7-7.8 mg g^-1) and best described by the Freundlich adsorption isotherm. The grape pip, grape skin and pine wood biochars had adsorption capacities in the range 8.3-9.4 mg ${\mathrm{NH}}_4^{+}$ g^-1 and best described by a linear adsorption isotherm at 100 mg L^-1. The ${\mathrm{NH}}_4^{+}$ adsorption results were associated with physisorption which implies that they can act as slow release ${\mathrm{NH}}_4^{+}$ fertilisers if ${\mathrm{NH}}_4^{+}$ is bioavailable. The six biochars had ${\mathrm{NO}}_3^{-}$ adsorption capacities in the range 15.2-15.9 mg g^-1 and were well fitted to the linear adsorption isotherm at 100 mg L^-1. All six biochars had a stronger ${\mathrm{NO}}_3^{-}$ removal affinity (82-89%) compared to ${\mathrm{NH}}_4^{+}$ (33-39%). Adsorbed nitrate was not desorbable (0.01-0.23%) compared to adsorbed ${\mathrm{NH}}_4^{+}$ which was 53-60% desorbable. The desorption result was possibly due to ${\mathrm{NO}}_3^{-}$ competing redox reactions or ${\mathrm{NO}}_3^{-}$ being too strongly adsorbed for extraction. Desorption of ${\mathrm{NH}}_4^{+}$ was associated with biochar net negative pH values and volatilisation of ammonia.

Keywords: Biochar; nitrogen; physicochemical characterisation; sorption capacity.