Passivation effects of different ratios of fly ash and straw on heavy metals in coal gangue-contaminated soil in Guizhou Province, China

To simultaneously achieve the stabilization of heavy metals in coal gangue- contaminated soil and the resource utilization of solid waste, fly ash (FA) and corn straw (CS) were adopted as passivators to construct a low-cost and sustainable remediation system. Through pot experiments, FA, CS and their mixture (FC) (mass ratio 1:1) with different addition ratios (1, 3, 6%) were applied to contaminated soil, and ryegrass was planted to evaluate the passivation effect. By analyzing the morphological transformation of heavy metals and their accumulation in plants, the passivation effects on various heavy metals were evaluated. Results showed that FA converted exchangeable iron (Fe), copper (Cu), zinc (Zn) and cadmium (Cd) to a residual state, and CS reduced the reducible Cu and Zn concentrations. The FC transformed Fe, manganese (Mn), lead (Pb) and Cd from exchangeable and reducible states to oxidizable and residual states. At the 6% addition rate, the passivation efficiencies of FC for Fe, Mn, Pb and Cd reached 34.33, 9.98, 14.92 and 17.32%, respectively. Meanwhile, FC inhibited the enrichment of Cu, Zn, Pb, and Cd in ryegrass. Research indicates that FC is more conducive to passivating heavy metals in gangue-contaminated soil, with a 6% mixture ratio being the optimal recommended proportion.