Biomass-Derived Carbonaceous Adsorbents for Trapping Ammonia

The preparation of low-cost carbonaceous adsorbents for nitrogen recovery is of interest from agricultural and waste management perspectives. In this study, the gaseous ammonia (NH3) and aqueous ammonium (NH4+) sorption capacities have been measured for different types of carbonaceous chars produced under different conditions. The study includes a comparison of an oak-based hydrochar produced from hydrothermal carbonisation (HTC) at 250 °C with two biochars produced from slow pyrolysis at 450 °C and 650 °C, respectively. The chars were also chemically modified with H2SO4, H3PO4, H2O2, and KOH to investigate the potential for sorption enhancement. The highest sorption capacities for NH3 were observed for the hydrochars with typical uptake capacities ranging from 18–28 mg g−1 NH3. Sorption capacity for oak biochars is significantly lower and ranges from 4–8 mg g−1 for biochars produced at 450 °C and 650 °C, respectively. Hydrochar showed a substantially higher sorption capacity for NH3 despite its lower surface area. The CaCl2 extractable NH4+ following ammonia adsorption is incomplete. Typically, only 30–40% of the N is released upon washing with CaCl2 in form of NH4+. Post chemical modification of the chars resulted in only limited enhancement of char NH3 and NH4+ sorption. H3PO4 treatment showed the greatest potential for increasing NH3/NH4+ sorption in biochars, while KOH and H2O2 treatment increased NH3 sorption in the hydrochar. As only marginal increases to char surface area were observed following char treatment, these findings suggest that char surface functionality is more influential than surface area in terms of char NH3/NH4+ sorption.