The Combined Remediation Of Compound Heavy Metal-Contaminated Soils With Plant,microbe And Carbon-Based Nano-iron

With the continuous input and superposition of various sources of pollution such as industrial emissions from mining and metal smelting and chemical input,the compound heavy metals pollution in China’s soil is becoming increasingly serious.Multiple heavy metals exhibit environmental toxicity effects such as synergy,superposition or antagonism in the soil,which increases the difficulty of remediation.Single remediation techniques have weakened their effectiveness in the remediation of multiple heavy metal pollution due to their poor persistance and weak adaptability.Furthermore,the stability of residual heavy metals in the soil after remediation affects soil safety and subsequent utilization.Therefore,reducing the total amount of heavy metals and simultaneously reducing the activity of residual heavy metals in the soil is the target for the remediation of compound heavy metal contaminated soils.Therefore,developing multi-heavy metal pollution remediation products and innovative technology,revealing the interactions between different remediation techniques and the remediation mechanism,improving the combined remediation technology system are of great significance to remediate multi-heavy metal contaminated soil and ensure the quality and safety of agricultural products.In this paper,the typical heavy metals cadmium(Cd),lead(Pb),zinc(Zn)and mercury(Hg)are selected as the composite pollution research factors.The screening and effect evaluation of remediation plants and microbial strains are taken as the starting point,combined with the preparation and synthesis optimization of carbonbased nano-iron materials,to explore the applicability and remediation mechanism of the three materials to the multi-heavy metal pollution.Through quantitative analysis of the coupling relationship between plants,microorganisms and carbon-based nanoiron materials,we clarified the contribution pathways between the three materials and formed a joint remediation technology,which provides a theoretical basis and technical support for solving the environmental problems of multi-heavy metal pollution in soil.Based on the above basis,the application evaluation for the mining site is carried out to verify the practical feasibility of the combined remediation technology.The main research results are as follows:(1)Taking the germination rate and vigor of plants in the multi-heavy metal pollution system as tolerance indicators,we found that Solanum nigrum L.is effective against heavy metal compound pollution(Cd:30 mg/L;Pb:200 mg/L;Hg:30 mg/L;Zn:200 mg/L).The tolerance is the strongest,and its germination rate and vigor reach 74.00%and 30.0%,respectively.The tolerance of Lolium perenne L.,Festuca arundinacea,Cosmos bipinnatus and Medicago sativa decreased in turn.The combination of multiple heavy metal can increase the bioavailability of the single heavy metal,and the available content of Cd,Zn,and Hg in the compound system increased by 12.59%-21.04%,34.09%-47.93%,and 52.60%-56.21%,compared with the single system.Solanum nigrum L.can resist oxidative stress by increasing the activity of three antioxidant enzymes(superoxide dismutase,peroxidase,and catalase)and the content of photosynthetic pigments through internal regulation.Its cell wall can fix heavy metals and vacuoles can isolate heavy metals,which further reduce the damage of heavy metals to cells.In addition,Solanum nigrum L.enhances the resistance to heavy metals by increasing the richness,diversity and beneficial microorganisms in the rhizosphere.(2)Ten bacterial strains were screened and identified in the compound heavy metal contaminated soil,including three strains of Bacillus,three strains of Pseudomonas,three strains of Acinetobacter,and one strain of Klebsiella.Four highly tolerant microbial strains(Bacillus sp.HMR2,Bacillus sp.HMR3,Pseudomonas sp.HMR5,and Achromobacter sp.HMR8)were further established through resistance evaluation.Through the analysis of the influence of the strains on heavy metal speciation and plant growth promotion,it was shown that the four strains could promote the available contents of multiple heavy metals(15.24%-112.48%)and increase the biomass of Solanum nigrum L.(22.99%-37.36%).In addition,the combination of the four strains had a more significant effect on the biomass increase of Solanum nigrum L.(54.02%),which clarified the strengthening effect of microorganisms on the phytoextraction of heavy metals.(3)Typical carbon-based materials(CBMs)such as biochar(BC),fly ash(FA),carbon nanotubes(CNT),and graphene oxide(GO)are selected to synthesize and optimize loading sulfidized zero-valent iron nanoparticles(SnZVI)and nano-ferric oxide(nFe3O4).These materials showed certain passivation effects on Cd,Pb,Zn,and Hg,among which BC-SnZVI had the highest passivation rates on the four heavy metals,reaching 60.77%,48.96%,62.36%,and 53.70%,respectively.The immobilization mechanism of heavy metals by BC-SnZVI mainly includes electrostatic adsorption,coordinated adsorption,redox reactions,and precipitation.The immobilization mechanisms of different heavy metals are different and the coexistence of heavy metals produces promoting or competitive interactions.In the Cd(Ⅱ)and Cr(Ⅵ)compound system,Cd(Ⅱ)improves the electrostatic adsorption force of BC-SnZVI to Cr(Ⅵ),strengthens the ability of FeO,FeS,and-OH functional groups to reduce Cr(Ⅵ),and Cr(Ⅲ)was immobilized by BC-SnZVI through precipitation and coordinated adsorption.BC-SnZVI mainly immobilized Cd(Ⅱ)by precipitation and coordinated adsorption,but its immobilization effect was weakened by the competition of Cr(Ⅲ).(4)Solanum nigrum L.,composite strains(Bacillus sp.HMR2,Bacillus sp.HMR3,Pseudomonas sp.HMR5 and Achromobacter sp.HMR8)and BC-SnZVI are used as heavy metal remediation materials to study the combined remediation technology of plant-microorganism-carbon-based nano-iron,in which the treatment modes of adding microorganisms and carbon-based nano-iron simultaneously or in batches was set up.The combined remediation technique could significantly increase the activity of antioxidant enzymes(22.23%-39.93%)and the content of photosynthetic pigment content(15.98%-34.31%)in Solanum nigrum L.,as well as the soil organic matter content(17.81%-19.09%),available nitrogen-phosphoruspotassium content(170.12%-416.02%),soil enzyme activity(197.57%-468.48%)and bacterial community richness and diversity(65.81%-123.23%),among which microorganisms make the greatest contribution.Combined with the above factors,the biomass of Solanum nigrum L.increased by 54.05%and 63.78%,and the removal rate of heavy metals increased by 55.64%and 113.44%,and the activity of residual heavy metals decreased significantly.In particular,the treatment of adding microorganisms and carbon-based nano-iron in batches has a better effect on the stability of heavy metals.Compared with the simultaneous addition method,the available Cd,Pb,Zn,and Hg decreased by 34.99%,17.05%,34.72%,and 46.97%,which was mainly due to the fact that microorganisms promoted plant extraction by improving the availability of heavy metals in the early stage,and carbon-based nanoiron increased the stability of residual heavy metals through passivation in the later stage to improve the safety of the remediated soil.(5)Taking Solanum nigrum L.,Lolium perenne L.,Cosmos bipinnatus,Festuca arundinacea and Glycine max as remediation plants,the site application verification is carried out with the same microorganism and carbon-based nano-iron as above.The results showed that the heavy metal content was significantly reduced,the bioavailability of residual heavy metals was decreased and the soil fertility was increased in the soil of each combined remediation area.The effect of combined remediation depends on plant species,in which the combined remediation treatment with Solanum nigrum L.has excellent effect on the removal rate of heavy metals,the stability of residual heavy metals and the improvement of soil nutrients in the mining area.In summary,the combination of Solanum nigrum L.,composite strains,and BCSnZVI can comprehensively repair the contaminated soil and promote the health of soil environment.In this study,a green and sustainable combined remediation method is used to purify the polluted environment,which provides a new perspective for comprehensively improving the remediation efficiency of heavy metal cocontaminated sites,especially in large-scale and complex polluted environments.