Sorption of Ammonium and Nitrate to Biochars: Mechanisms and Impacts of Production Parameters.

Amendment of soils with biochar has been proposed as a means to reduce environmentally problematic nitrogen (N) losses from agricultural soils such as nitrate leaching and nitrous oxide (N₂O) emissions. Biochars also have potential for use as water filtration media for removal of reactive N (ammonium and nitrate). However, biochar’s interactions with reactive N remain poorly understood. Sorption of inorganic N to biochars is likely a key mechanism of interaction. It has been posited that reactive N can be chemically adsorbed to biochar surface moieties – including oxygen-containing functional groups and condensed aromatic structures – and/or physically absorbed into biochar micro- and nanopores. Here we quantify sorption rates of ammonium and nitrate to six biochars, distinguish between chemical adsorption and physical absorption mechanisms, and assess the relative impacts of pyrolysis temperature and feedstock composition on sorption. Ammonium sorption rates increased with increasing solution pH and with decreasing biochar pyrolysis temperature. On the other hand, nitrate sorption rates increased with decreasing solution pH and with increasing pyrolysis temperature. Sorption rates were also significantly affected by feedstock type. Sensitivity to initial sorbate concentration and solution pH indicates chemical adsorption mechanisms for both ammonium and nitrate. Thus, biochars can be effective ammonium or nitrate sorbents under the certain conditions, and soils must therefore be paired carefully to realize full benefits of inorganic N retention.