Preparation of biochar with high absorbability and its nutrient adsorption-desorption behaviour

To study the formation of biochar with high absorbability, experiments were carried out at different carbonization temperature (300, 400, 600, and 800 °C) and under different carbonization atmosphere (activating gases (steam and CO₂) and inert gas (N₂)) to prepare biochar. In this paper, the effects of the carbonization atmosphere on the biochar pore structure were studied, and the influence of the biochar pore structure on the adsorption-desorption behaviour of nutrients (NH₄⁺-N, NO₃^--N, P, and K) was investigated. Experimental results: (1) The activating gases (steam and CO₂) can catalytically crack activated carbon atoms and tar blocking the biochar pores at high temperatures (T > 600 °C), and the activating gas promotes the formation of microporous biochar (d < 2 nm). (2) Micropores with a pore diameter distribution of 0.6-2 nm in biochar have the strongest nutrients adsorption, and pores with a diameter below 0.6 nm cannot adsorb hydrated ions of nutrients. (3) Biochar prepared at 600 °C and CO₂ atmosphere has the best adsorption effect on nutrients. The adsorption kinetic was well described by Pseudo-second-order model. (4) After 5 cycles of biochar, the adsorption of the nutrients is still >40% of the first adsorption. Biochar has relatively high reusability.