The k3-weighted χ(k) functions of raw biochars and de-ashed biochars are shown in Fig. 3. To obtain the types and fractions of Zn species on these biochars, Principal Component Analysis (PCA) accompanied with Target Transformation (TT) and Linear Combination Fitting (LCF) were performed on χ(k) k3-spectra ranging from 3–11 Å−1 for all the samples. PCA was used to determine the number of primary components which may be present in the spectra of biochars; TT was used to evaluate which species may be contained in these biochars; and LCF was applied to quantitatively analyze the Zn species on biochars. (The details on PCA, TT and LCF were shown in S1 in SI).