As an emerging carbonaceous material, biochar has been extensively studied especially in the field of soil remediation11121314151617. The carbonaceous residue and entrained minerals (ash) are two main components of biochar18, they perform different functions in soil remediation. The carbonaceous residue can adsorb and retain water19 and some organic pollutants20212223; the ash can provide nutrient elements and improve soil pH24. In terms of heavy metal immobilization, both of the two components could be involved in the process. Plenty of studies have reported the immobilization behavior of Zn on biochars derived from different biomass (such as hardwood2526, corn straw25, sugarcane straw27, dairy manure28, and meat and bonemeal29), and some of them have proposed the immobilization mechanisms which implied more important role of ash on biochar on Zn immobilization272830. Xu et al. studied the sorption behavior of Zn on biochar derived from dairy manure, and found that large amounts of PO43− and CO32− were released from biochar and reacted with Zn2+ to form Zn phosphate and Zn carbonate28. With the help of P K-edge XANES spectroscopy, Wegner et al. found that hopeite (Zn3(PO4)·2H2O) or a similar Zn-P phase were formed, when biochar was used to immobilize Zn in the sewage field soils30. In addition to forming Zn precipitates, the formation of Zn complexes on biochar surface also led to Zn immobilization29. By using the method of extended X-ray absorption fine structure (EXAFS) spectroscopy, Betts et al.29 found that Zn bound to phosphate groups in meat and bonemeal biochar in a monodentate inner-sphere surface complex.