The LCF was subsequently performed on the samples using the Zn species selected by TT. The main components and their fractions in the samples were obtained and shown in Fig. 3 and Supplementary Table S6. From the fitting results (Fig. 3), we find that Zn precipitates/minerals (i.e. hydrozincite, Zn(OH)2, and hemimorphite) accounted for a large share of Zn species (>70%) in raw biochars, which indicated the indispensable role of ash in biochar on Zn immobilization. High proportions of hydrozincite (>40%) and Zn(OH)2 (>10%) were observed in most of the raw biochars. This result indicated that CO32− and OH− were two common ions for Zn immobilization. It is noteworthy that, unlike other raw biochars, WS550 contained large fraction of hemimorphite (66%). This was probably caused by the large amount of dissolved Si species released from WS550, which implied the crucial role of dissolved Si species in Zn immobilization. The species Zn(NO3)2 aq represents the Zn adsorbed on oxygen-containing functional groups (i.e. carboxyl and hydroxyl) on biochar. In PN350 and WS350, the fractions of Zn(NO3)2 aq were respectively 14% and 26%; while in PN550 and WS550, the two values dropped to 6% and 2%. These changes were due to the increase in ash contents and loss of oxygen-containing functional groups in biochars with pyrolysis temperature. In de-ashed biochars, only two main Zn species, hydrozincite and Zn(NO3)2 aq, existed in de-ashed biochars. Although hydrozincite is Zn mineral, the formation of small amounts of hydrozincite in de-ashed biochars may be caused by the dissolution of CO2 in the air. The pHPZC of de-ashed biochars were 4.3–5.5 (Table 1), thus when the solution pHs were controlled to 7, the de-ashed biochars could easily adsorb few amount of Zn2+.