Phosphate and ammonium adsorption of sesame straw biochars produced at different pyrolysis temperatures

Abstract

The adsorption of \( \mathrm{N}{\mathrm{H}}_4^{+} \) and \( {\mathrm{PO}}_4^{3-} \) by sesame straw biochars (C-300, C-500, and C-700) prepared under different temperatures (300, 500, and 700 °C) was investigated in this study. The physicochemical properties of the biochars were characterized using Brunauer–Emmett–Teller method, X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectrometry. In batch experiments, C-300 showed the best \( \mathrm{N}{\mathrm{H}}_4^{+} \) adsorption capacity of 3.45 mg/g because of its abundant surface functional groups at low pyrolysis temperature. C-700 achieved the optimal \( {\mathrm{PO}}_4^{3-} \) adsorption capacity of 34.17 mg/g because of its high Ca, Mg, and Al contents and high surface area at high pyrolysis temperature. The isothermal study showed that the Langmuir–Freundlich model could sufficiently describe the \( \mathrm{N}{\mathrm{H}}_4^{+} \) and \( {\mathrm{PO}}_4^{3-} \) adsorption values, indicating the multiple adsorption processes of nutrients on biochars. The maximum \( \mathrm{N}{\mathrm{H}}_4^{+} \) adsorption capacity was 93.61 mg/g on C-300, whereas the maximum \( {\mathrm{PO}}_4^{3-} \) adsorption capacity was as high as 116.58 mg/g on C-700. Kinetic study showed that \( \mathrm{N}{\mathrm{H}}_4^{+} \) adsorption on C-300 was mainly controlled by intraparticle diffusion, and the pseudo-second-order model could well describe \( {\mathrm{PO}}_4^{3-} \) adsorption on C-700.