Fig. 3.

The strategies of source identification of HMs. (a) Contributions of PM sources in Lanzhou based on the PMF analysis (Reprinted from ref.[47]. Copyright (2021), with permission from Elsevier); (b) breakpoint corrected spectra of 20 samples with high and low Pb content; and scatter plot of the measured and predicted soil Pb content from optimized model evaluated by coefficient of determination (R²), residual prediction deviation (RPD), and root mean square error (RMSE) (Reproduced from ref.[51]); (c) ²⁰⁸Pb/²⁰⁶Pb versus ²⁰⁶Pb/²⁰⁷Pb ratios in fly ash samples investigated in this study compared to other Pb sources in the United States (Reprinted with permission from ref.[55]. Copyright (2019) American Chemical Society); (d) the mechanical, biological, and physicochemical migration behaviors of HMs.