Study on the synergistic remediation of heavy metal contaminated soil by micro-nano bubbles-ryegrass enhanced system

Abstract

Abstract Conventional phytoremediation of heavy metal-contaminated industrial soils often fails to achieve desired outcomes due to limited metal accumulation capacity and low plant biomass. To overcome these limitations, we developed a synergistic micro-nano bubbles (MNBs)-ryegrass (Lolium perenne L.) remediation system. MNBs supply reactive oxygen species to mobilize heavy metals, thereby optimizing the rhizosphere microenvironment. The results demonstrated that MNBs treatment significantly stimulated both aboveground growth and biomass accumulation in ryegrass. Under high-polluted condition, the shoot length and dry weight increased by 41% and 88%, respectively, compared to the control group, accompanied by a 43% reduction in the root-to-shoot ratio. Under low-polluted condition, MNBs enhanced the plant content of Pb, Cr, Zn, and Cu by 1.9-, 1.9-, 1.2- and 1.2-fold, respectively, reducing the soil residual rates of Cr, Zn, and Cu to below 30%. Furthermore, principal component and correlation analyses revealed that soil heavy metal retention was positively correlated with total dissolved solids (TDSs), and these parameters inversely affected ryegrass growth. Membership evaluation showed the MNBs-treated group scored 0.83, much higher than the control group’s 0.17 under metal stress. Ultimately, the synergistic MNBs–ryegrass system emerges as a highly efficient and promising strategy for the practical remediation of industrial soils. Graphical AbstractMulti-panel diagram comparing growth levels, heavy metal accumulation, and soil removal rates in control and bubble-treated plants.The figure is a multi-panel diagram illustrating plant growth and heavy metal dynamics under Control (0.17) and Bubble (0.83) treatments. The Control group shows growth levels between 28.1-40.6 cm/plant and heavy metal accumulation for Pb (67-588 mg/kg), Cr (51.9-926 mg/kg), Zn (126.4-354.4 mg/kg), and Cu (31.6-120.3 mg/kg). The Bubble group has growth levels of 31.2-50.3 cm/plant with higher metal accumulations (Pb 411.1-1262 mg/kg, Cr 57-332.2 mg/kg, Zn 146.7-368.1 mg/kg, Cu 43.7-172 mg/kg). Soil removal rates are also higher in the Bubble group, indicating improved translocation of metals from roots to shoots. A conceptual model highlights differences in biomass allocation and metal retention. NOVELTY STATEMENT To the best of our knowledge, this is the first systematic investigation of micro-nano bubble (MNB)-coupled ryegrass as an intensified phytoremediation system for multi-heavy-metal contaminated soil remediation. Unlike conventional single plant remediation or chemical activation methods, the system relies on MNB irrigation to reshape rhizosphere conditions, building an ROS-driven matrix rich in oxygen and bioavailable heavy metals. Continuous ROS generation from MNBs not only converts inert heavy metals into plant-absorbable forms but also boosts ryegrass root growth and biomass, raising overall metal uptake efficiency. Comprehensive rhizosphere tests combined with BCR extraction, biomass measurement and gradient pot trials reveal the core remediation pathway: ROS-mediated metal speciation transformation facilitates plant metal accumulation. The work proves that MNB-enhanced ryegrass remediation delivers a low-cost, eco-friendly, and field-adaptable method, offering a reliable strategy for heavy metal remediation of farmland and industrial soils. HIGHLIGHTS Bubble treatment increased ryegrass growth by 41%. Metal contents increased 1.2–1.9-fold under bubble treatment. MNBs activated soil heavy metals effectively. Bubble-treated group scored membership of 0.83.