Elucidation of ammonium and nitrate adsorption mechanisms by water hyacinth biochar: effects of pyrolysis temperature

Numerous studies indicate biochar’s nitrogen (N) adsorption capacity plays a crucial role in soil N retention. However, there is limited understanding on inorganic N adsorption mechanisms in biochar derived from aquatic weeds such as water hyacinth (WH). This study investigated ammonium-N (NH₄⁺-N) and nitrate-N (NO₃^–-N) adsorption capacities and mechanisms of WH biochar pyrolyzed at different pyrolysis temperatures of 400 °C, 600 °C, and 800 °C (BC400, BC600, and BC800, respectively). Results showed NH₄⁺-N adsorption was maximized (1.07–1.09 mg g^–1) with BC400 at initial solution pH 7.0–9.0, while NO₃⁻-N adsorption peaked (0.80 mg g^–1) with BC800 at initial solution pH 5.0. Both NH₄⁺-N and NO₃^–-N followed well the Pseudo-second-order model in adsorption kinetics (R² = 0.990–0.997 and 0.962–0.992, respectively). The Sips model accurately described the adsorption isotherms for NH₄⁺-N (R² = 0.994–0.999) and NO₃^–-N (R² = 0.992–0.999). The calculated maximum adsorption capacity for NH₄⁺-N and NO₃^–-N using Sips model was 11.2–16.8 mg g^–1 and 0.693–4.99 mg g^–1, respectively. Co-existing cations and anions reduced NH₄⁺-N and NO₃^–-N adsorption capacity, respectively, with other ions with higher valence exhibiting higher inhibition effects (43%–97% and 44%–73%, respectively). Primary adsorption mechanism for NH₄⁺-N included cation exchange via oxygen-containing surface functional groups in BC400 and pore filling and surface struvite precipitation in BC800. Major adsorption mechanisms for NO₃^–-N included electrostatic interactions in BC400 and pore filling in BC800. These findings suggested that biochar derived from aquatic weeds possessed the same potential usefulness for soil N retention as biochar from other feedstocks, and that it might assist for further detailed considerations in other studies for biochar soil application.