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.