Cadmium removal from aqueous solution by biochar obtained by co-pyrolysis of sewage sludge with tea waste

Resource utilization is a critical pathway for sustainable solid waste treatment. Biochar was prepared from the co-pyrolysis of sewage sludge and tea waste. Brunauer-Emmett-Teller measurement, scanning electron microscopy and Fourier transform infrared analysis were employed to characterize the biochar. Then, the interface behavior between biochar and Cd from aqueous solution was investigated. The effect of adsorbent dose and pH on Cd adsorption was evaluated. Adsorption kinetics and the adsorption isotherm were studied, and the adsorption mechanism was explored. The results showed that the suitable adsorbent dose was 4 g L-1 and the optimal pH of the Cd solution remained at 6.0. Cadmium sorption on the biochar could be well described by the pseudo-second order kinetic model (R (2) > 0.98). The adsorption process was described using the Langmuir (R (2) > 0.86), Freundlich (R (2) > 0.86), Temkin (R (2) > 0.84) and Dubinin-Radushkevich (R (2) > 0.86) isotherm models. The proportion of organic constituents in biochar was 69.2-72.4%. Minerals that originated in biochar played an important role during the Cd adsorption process, and the contribution of minerals accounted for 27.6-30.8% of the total adsorption. The main mechanism of the Cd adsorption process by biochar involved ion exchange, surface complexation, electrostatic interaction, surface co-precipitation, and other mechanisms. Therefore, biochar created by the co-pyrolysis of sewage sludge and tea waste could be used as an adsorbent for the removal of metal ions from contaminated water.