Adsorption characteristics and adsorption behavior of ammonium ions onto rice husk biochar pyrolyzed at different temperatures

Abstract

The low nitrogen use efficiency of applied fertilizers, often below 50% in most agroecosystems, is a significant concern. Biochar (BC) has large micropore structures, high surface area, and surface charges, which improve soil nutrient retention; these properties depend on pyrolysis conditions and the type of feedstock. This study investigated the adsorption of ammonium ion (NH4+) by rice husk BC (RHB) produced at three pyrolysis temperatures: 300 °C (RHB_300), 500 °C (RHB_500), and 600 °C (RHB_600) using a down draft double chamber (DDDC) pyrolyser (i.e., 500 °C) and a laboratory scale muffle furnace (i.e., 300 °C and 600 °C). A batch sorption experiment was conducted to evaluate NH4+ sorption onto BC and Langmuir and Freundlich sorption isotherms were fitted to the data. Additionally, adsorption kinetics were studied by fitting three models – pseudo first order, pseudo second order and intra particle diffusion model – to the experimental data to understand the adsorption behaviour of NH4+. A significantly higher ash content of 45% was observed in RHB_500, which could be due to the DDDC pyrolysis method increasing BC surface heterogeneity. The low-temperature BC (RHB_300) showed the highest capacity for NH4+ sorption at pH 7 (4.53 mg/g) and rapid adsorption kinetics. Among the tested BCs, RHB_300 exhibited the highest NH4⁺ sorption capacity suggesting that low-temperature rice husk-derived BC may be particularly suitable for enhancing ammonium retention in soil systems.