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

The adsorption of NH4+ and PO43 by sesame straw biochars (C-300, C-500, and C-700) prepared under different temperatures (300, 500, and 700 degrees 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 NH4+ adsorption capacity of 3.45 mg/g because of its abundant surface functional groups at low pyrolysis temperature. C-700 achieved the optimal PO43- 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 NH4+ and PO43- adsorption values, indicating the multiple adsorption processes of nutrients on biochars. The maximum NH4+ adsorption capacity was 93.61 mg/g on C-300, whereas the maximum PO43- adsorption capacity was as high as 116.58 mg/g on C-700. Kinetic study showed that NH4+ adsorption on C-300 was mainly controlled by intraparticle diffusion, and the pseudo-second-order model could well describe PO43- adsorption on C-700.