Comparison of properties, adsorption performance and mechanisms to Cd(II) on lignin-derived biochars under different pyrolysis temperatures by microwave heating

The application of biochar derived from various biomass to treat heavy metal pollution has received particular attention in recent years. Lignin is an abundant, highly aromatic byproduct from the paper-making industry with a low utilization rate, which is a potential feedstock for biochar preparation. In this study, lignin was carbonized at different temperatures by microwave heating to obtain biochars (LBx), and the effect of pyrolysis temperature on biochar properties, Cd(II) adsorption behavior, and mechanisms was comprehensively investigated. It was found that the biochar-inherent minerals gradually enriched, the surface functional groups decomposed, and more structural defects formed at the edges of graphite-like structures with the increasing pyrolysis temperature. Adsorption experiments demonstrated that the adsorption capacity of lignin improved significantly after carbonization. And as the temperature increased from 200 °C to 500oC, the qm of LBx increased from 573.27 to 635.44 mg⋅g−1, which were vastly superior to common reported biochars. The adsorption mechanisms of Cd(II) on the LBx can be classified into the interaction with dissolved fractions (biochar-inherent minerals co-precipitation and DOM interaction) and bulk biochar (surface complexation and ionic exchange). The quantitative mechanisms indicated that the dissolved matters contribute more to Cd(II) adsorption than bulk biochar, but the contribution decreased from 89.48% to 77.38% with increasing temperature. Surface complexation and ionic exchange are two dominant mechanisms on bulk biochar, and the former has a more significant advantage for higher-temperature prepared biochar. Taken together, lignin was an efficient and promising feedstock to prepare biochar used for Cd(II) removal.