Characteristic and Mechanism of Methylene Blue Adsorption by Pepper Stem Biochar
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Abstract:
In this study, pepper stem, one of wastes from preliminary processing of pepper, was used as raw material to produce pepper stem biochar (PB-200) by pyrolysis in a nitrogen atmosphere under 200 ℃. Adsorption characteristics of PB-200 on methylene blue (MB) was researched by batch adsorption experiment under different pH, adsorption time, temperature and initial concentration conditions; and it was characterized by scanning electron microscopy (SEM), surface area and porosity analyzer, particle size analyzer, zeta potential analyzer, Fourier-transform infrared (FT-IR) spectrometer, elemental analyzer, etc to investigate the mechanism of the biochar adsorption. The results showed that PB-200 resulted in good adsorption performance at pH=3~11, and as the pH increased from 3 to 11, the adsorption of PB-200 increased from 72.34 to 144.91 mg/g. The kinetic studies showed that the experimental data was fitted by the pseudo second-order model well (R2≥0.99); and the isothermal adsorption process of MB can be better described by the Langmuir adsorption isotherms model than the Freundlich model (R2≥0.99). The adsorption capacity of MB was up to 115.08 mg/g under the condition of an initial MB concentration of 100 mg/L when the biochar (20 mg) was mixed with MB solution (40 mL, pH 7.0) for 24 h at 25 ℃. It can be seen that PB-200 is an efficient adsorbent for removing MB in wastewater. Compared with pepper stems, the average particle size and pore size of PB-200 were reduced from 334.58 μm and 20.30 nm to 233.81 μm and 16.95 nm, respectively, and the specific surface area was increased from 0.70 m2/g to 0.96 m2/g; rich pore structure and oxygen-containing functional groups were demonstrated by the SEM, FT-IR and elemental analysis. The physical properties of PB-200 were significantly improved compared with Pepper stem; the adsorption of MB was chemisorption (electrostatic adsorption and and monalayer adsorption). PB-200 is an efficient adsorbent for removing methylene blue.
