Microwave-assisted pyrolysis of waste lignin to prepare biochar for Cu2+ highly-efficient adsorption: Performance, kinetics and mechanism resolution

Alkali lignin is the major byproduct of the paper-making industry and biorefining processes. However, due to its complex structure and low reactivity, most alkali lignin is still discharged as "black liquor" or directly incinerated, resulting in significant waste of bioresources and severe environmental pollution. In this study, biochar was prepared from alkali lignin using microwave-assisted pyrolysis, and its adsorption performance and mechanism for Cu2+ were investigated. The experimental findings demonstrate that the adsorption of Cu2+ onto biochar follows the pseudo-second-order kinetic model and the Langmuir isotherm model, suggesting that the adsorption process is predominantly governed by the chemisorption mechanism. Furthermore, the qm ranged from 405.55 to 492.75 mg·g−1, which is significantly higher than many other reported biochars. The adsorption mechanism includes mineral co-precipitation, DOM (mainly humic substances) adsorption, surface complexation, as well as ion exchange. The contribution of co-precipitation and surface complexation reaches 86.6 %, indicating that this biochar poses a low environmental risk. Thus, this study shows that alkali lignin could be a valuable bioresource for mitigating the environmental impact arising from Cu2+ pollution.