High-efficiency remediation of Hg and Cd co-contaminated paddy soils by Fe–Mn oxide modified biochar and its microbial community responses

Fe–Mn oxide modified biochar (FMBC) was produced to explore its potential for remediation of Hg–Cd contaminated paddy soils. The results showed that the application of FMBC decreased the contents of bioavailable Hg and Cd by 41.49–81.85% and 19.47–33.02% in contrast to CK, while the amount of labile organic carbon (C) fractions and C-pool management index (CPMI) was increased under BC and FMBC treated soils, indicating the enhancement of soil C storage and nutrient cycling function. Dry weight of different parts of Oryza sativa L. was enhanced after the addition of BC and FMBC, and the contents of Fe and Mn in root iron–manganese plaques (IMP) were 1.46–2.06 and 6.72–19.35 times higher than those of the control groups. Hg and Cd contents in brown rice under the FMBC treatments were significantly reduced by 18.32–71.16% and 59.52–72.11% compared with the control. FMBC addition altered the composition and metabolism function of soil bacterial communities, especially increasing the abundance of keystone phyla, including Firmicutes, Proteobacteria and Actinobacteria. Partial least squares path modelling (PLSPM) revealed that the contents of Na₂S₂O₃–Hg, DTPA–Cd and IMP were the key indicators affecting Hg and Cd accumulation in rice grains. These results demonstrate the simultaneous value of FMBC in remediation of Hg and Cd combined pollution and restoring soil fertility and biological productivity.

Graphical Abstract