Production and characterization of tea waste-based biochar and its application in treatment of Cd-containing wastewater

Biochar was prepared from tea waste that had been heated to 300 degrees C, 500 degrees C, or 700 degrees C (TWBC300, TWBC500, and TWBC700, respectively), and was characterized by elemental analysis, scanning electron microscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. Then, the Cd removal characteristics and mechanism for the biochar were investigated. The results showed that as the pyrolysis temperature increased, the hydrophilicity and polarity of the biochar became poor, and some functional groups (oxygen-containing groups) decreased. Furthermore, pH and the aromatic structure of the biochar were enhanced, but the carbon structural defects of the biochar also increased. The Langmuir isotherm model and the pseudo-second-order kinetic model accurately described the adsorption process. The maximum adsorption capacity for Cd by TWBC300, TWBC500, and TWBC700 was 8.90 mg g(-1), 4.73 mg g(-1), and 7.26 mg g(-1), respectively. The adsorption capacities of the biochars greatly decreased after acid washing, which indicated that the soluble minerals in biochars played a key role during the Cd removal process. The characterization approach (FTIR, XPS, and Raman spectroscopy analysis) combined with measurements of the pH value, and the Ca and Mg in the supernatant after Cd adsorption inferred that the mechanisms between tea waste-based biochar (TWBC) and Cd were surface complexation, cation exchange, the cation-pi interaction, and precipitation. The functional groups in TWBC300 could participate in Cd removal, and the aromatic structures in TWBC700 may also play an important role during Cd adsorption. The results suggest that all of the examined TWBC can act as an adsorbent to remove Cd. But, the best TWBC was TWBC300.