Dual roles of MWCNTs on plant growth and remediation efficiency during PGPR-assisted phytoremediation of Cd-Ni contaminated soil

As the main elements causing soil heavy metals (HMs) pollution, cadmium (Cd) and nickel (Ni) can cause irreversible harm to crops and ecosystems. Currently, research on plant growth-promoting rhizobacteria (PGPR)-assisted phytoremediation techniques for contaminated soil has received more attention. Meanwhile, multi-walled carbon nanotubes (MWCNTs) have also been explored for HMs pollution remediation in recent years due to their high specific surface area and strong adsorption. MWCNTs might improve phytoremediation efficiency of HMs-contaminated soil by directly immobilizing HMs and indirectly promoting plant growth, but less research has been devoted to the possible dual-enhancing roles of MWCNTs in PGPR-assisted phytoremediation process. Therefore, this study innovatively investigated the potential role of MWCNTs application in PGPR-assisted phytoremediation with Cd-Ni contaminated soil by rice. The results indicated that the presence of MWCNTs further improved the remediation efficiency of PGPR in assisting phytoremediation, reducing soil Cd^2 + and Ni²⁺ contents by 20.4 % and 13.7 %, respectively, superior to strain BT treatment individually. Scanning electron microscopy micrographs demonstrated that PGPR could attach to MWCNTs surface and be immobilized in MWCNTs. Meanwhile, MWCNTs application in PGPR-assisted phytoremediation could enhance rice survival under Cd and Ni stresses by stimulating antioxidant system and regulating Na⁺/K⁺ level in rice. Moreover, the increase in soil enzyme activities and relative abundance of beneficial microorganisms accelerated nitrogen and phosphorus nutrient cycling in the soil. In conclusion, MWCNTs might be used as adsorbent materials to directly immobilize Cd²⁺ and Ni²⁺ in soil, and also synergized with rice or PGPR to indirectly improve the phytoremediation efficiency of HMs.