Role Of Nano-zero-valent Iron And Arbuscular Mycorrhizal Fungi In Phytoremediation Of Heavy Metal Contaminated Soil

Increasing heavy metal pollution in soil has promoted the development of new soil remediation technologies.In recent years,green,economical,and ornamental phytoremediation technology has attracted increasing attention,but the shortcomings such as low efficiency and time-consuming restrict its development.Therefore,it needs to be combined with other methods to strengthen phytoremediation.In addition to large specific surface area,strong activity and high adsorption,nano-zero-valent iron(nZVI)has higher reducibility,reactivity and processing efficiency.It can adsorb and precipitate heavy metals in the soil,and can also reduce the toxicity of Cr through redox reactions.Arbuscular mycorrhizal(AM)fungi can promote the growth of plants and their tolerance under heavy metal stress.They are considered beneficial microorganisms to assist phytoremediation.nZVI and AM fungi may synergistically enhance phytoremediation,but there are few related studies,and the effects and mechanisms need to be further explored.In this study,the energy plant sweet sorghum was used as the test plant,and the cadmium,lead,and zinc compound contaminated soil and Cr(Ⅵ)contaminated soil were used as remediation targets.Pot experiments were conducted to study the the role of bulk nZVI(B-nZVI)and starch-stabilized nZVI(S-nZVI)and AM fungus(Acaulospora mellea ZZ)in phytoremediation of heavy metal contaminated soil.X-ray diffraction(XRD),scanning electron microscope-mapping(SEM-mapping)and metabolomics were used to explore the remediation effects and mechanisms to provide basis and reference for demonstrationg the role and mechanism of nZVI and AM fungi in phytoremediation of heavy metal contaminated soil.In compoud heavy metal contaminated soil,AM fungi significantly reduced the bioavailability of Cd,Pb and Zn in the soil and the concentrations of Cd,Pb and Zn in sweet sorghum,indicating that AM fungi effectively enhanced the immobilization of heavy metals.Both B-nZVI and S-nZVI in the concentration range of 50 mg/kg to1000 mg/kg did not inhibit the growth of plants.XRD and mapping analyses showed that S-nZVI was easier to oxidize than B-nZVI,and stabilized heavy metals more efficiently.Low concentrations of nZVI(such as 100 mg/kg)and AM fungi had a synergistic effect on heavy metal immobilization.They reduced the content of Pb and Cd in roots and enhanced the accumulation of Zn in roots.Although both S-nZVI and B-nZVI inhibited AM fungal colonization,A.mellea ZZ still successfully colonized the roots of sweet sorghum.In general,AM fungi and nZVI could jointly promote the phytoremediation of compound heavy metal contaminated soil.In Cr(Ⅵ)contaminate soil,A.mellea ZZ still successfully colonized the roots of sweet sorghum,showing a strong tolerance to Cr(Ⅵ)and nZVI.AM fungal inoculation significantly reduced the availability of Cr in the soil and the content of Cr in the roots,enhanced the stabilization of Cr in the soil,and increased the content of S and Mn in the roots of sweet sorghum.nZVI showed no obvious plant toxicity,but led to decreased abundance of amino acids regulating the oxidative stress of plants,and enriched phenylalanine metabolic pathway,suggesting that the defense response of sweet sorghum was induced.Cr(Ⅵ)activated the defense system of sweet sorghum and improved the ability to respond to oxidative stress.Addition of nZVI reduced the content of Cr in the roots.AM fungi and nZVI further changed the relative abundance of metabolites(such as soluble sugar,salicylic acid,and abscisic acid)and metabolic pathways in sweet sorghum,enhanced the ability of sweet sorghum to respond to oxidative stress,and further activated the defense system.AM fungi,nZVI and Cr(Ⅵ)had significant interactions.nZVI and Cr(Ⅵ)might alleviate each other’s toxic effects on AM fungi and hence increased mycorrhizal colonization rate;AM fungi and S-nZVI synergistically promoted the stabilization of Cr(Ⅵ)and alleviated the pressure of abiotic stress on sweet sorghum.In general,A.mellea ZZ had strong tolerance to heavy metals and nZVI,and enhanced heavy metal stabilization;nZVI was not toxic to sweet sorghum and reduced the content of heavy metals in the roots.AM fungi and nZVI synergistically promoted the phytostabilization of sweet sorghum,showing a potential in strengthening phytoremediation of heavy metal contaminated soil.