Impacts of pyrolysis temperature on lead adsorption by cotton stalk-derived biochar and related mechanisms

Impacts of pyrolysis temperature of biochar on lead adsorption from aqueous solution and related mechanisms were investigated in this work. Biochars derived from cotton stalk were prepared under temperature from 250 to 650 ℃. The physicochemical properties and the Pb2+ adsorption capacity (Qe) of biochar were characterized. The results revealed that Qe was positively correlated (p < 0.01) with ash content, pH, C content and average pore diameter, and negatively correlated (p < 0.01) with O content, O/C ratio, (N + O)/C ratio and yield of biochar, respectively. Batch adsorption were performed using the optimal biochar sample (CS550), and the effects of ambient temperature, initial pH, biochar dose, adsorption time and initial concentration of Pb2+ on adsorption were explored. The results suggest that the pseudo-second-order model and Freundlich model fit the adsorption data well, and the maximum sorption capacity calculated by Langmuir model was 146.78 mg/g. SEM-EDS, FTIR and XRD were used to characterize the biochar before and after Pb2+ adsorption. The results indicate that there are six adsorption mechanisms participate in the adsorption process, and precipitation, ion exchange with AAEMs, Pb2+-π interactions and complexation were found to be the main adsorption mechanisms. The regeneration experiment results indicate that biochar could be reused to remove Pb2+ for at least 3 runs with a slight decrease in the adsorption capacity. The conclusion is that cotton stalk biochar is an environment-friendly adsorbent for the purification of wastewater containing heavy metals.