Biochar derived from traditional Chinese medicine residues: An efficient adsorbent for heavy metal Pb(II)

Biochar (BC) is widely used in the remediation of soil and wastewater polluted by heavy metals, but there are few reports on the characteristics of biochar derived via pyrolysis from different traditional Chinese medicine residues (TCMRs). In this study, biochars were prepared by slow pyrolysis of five common Chinese medicine residues, namely, Salvia miltiorrhiza (DNS), Ligusticum striatum (CX), Angelica sinensis (DG), Codonopsis pilosula (DGS), and Astragalus membranaceus (HQ). The biochars were systematically investigated by determining their physicochemical properties and using common characterization techniques. The Spearman correlation matrix between factors was used to examine relationships between properties of different biochars. Batch adsorption experiments were carried out to investigate the adsorption characteristics of biochar on Pb(II) and the mechanisms involved. The results showed that the physicochemical properties and adsorption performance of biochar were related to the type of its pharmaceutical residue. Biochar produced from materials with higher lignin content showed a better adsorption of the heavy metal Pb(II). All biochars were alkaline, with yields ranging from 29.30–38.65%, and the main structure comprised of mesopores and macropores. The FT-IR and Boehm experiments revealed that the various TCMR biochars contained comparable functional groups, but their content varied. XRD and TEM results show that all biochar is amorphous with a crystalline structure, with the surface dominated by cellulose crystals and graphitic carbon. The O/C ratio (<0.2) and H/C ratio (<0.6) of biochars suggested that it had a desirable half-life. The TGA results confirmed its stability in the environment. The results of mineral experiments confirm the enrichment of the mineral content of biochar during biomass pyrolysis.The biochars' adsorption kinetics followed pseudo-second-order models (R2 > 0.99), and their isotherms were consistent with the Langmuir model (R2 > 0.99), indicating a monolayer chemisorption process. The biochar samples exhibited varying adsorption capacities for Pb(II), with the highest capacity observed for BDNS (36.42 mg/g). The adsorption mechanism mainly involved precipitation, complexation with oxygen-containing functional groups, and ion exchange. This study indicates that biochars from herbal residues exhibit promising potential for adsorbing heavy metal Pb(II), suggesting biochar production as a viable method for recycling herbal residues.