Effect Of Carbon Quantum Dots On Dissimilatory Iron Reduction And Cadmium Transformation In Soil And Its Microbial Mechanism

Carbon quantum dots(CQDs),as a new type of carbon nanomaterial,are widely used in biological and environmental fields because of their excellent fluorescence properties,high biocompatibility,and good water solubility.During the production and use of CQDs products,the amount entering the environment is bound to increase.Soil is an important part of the geochemical elemental cycle and the home of nanomaterials.The release of nanomaterials affects soil enzyme activity as well as microbial community structure.Microbial-mediated reduction of dissimilatory iron reduces Fe(?)to Fe(?)and stores energy required for life from this process,which is closely related to the elemental cycle of soil,the transformation of organic pollutants,and the transport and transformation of heavy metals.It has been reported that the abundant functional groups on the surface of water-soluble organic carbon can bind to metals to form complexes and promote the dissolution of iron oxides in soil through ligandenhanced solubilization to facilitate the occurrence of dissimilatory iron reduction.However,this process may also promote the release of heavy metals.In recent years,Cd contamination in agricultural soils has become a serious challenge for humans due to its high mobility and biological effectiveness.In this thesis,the effects on the reduction of microbial dissimilatory iron and the morphological transformation of Cd in soil were investigated,and the main research contents and results are as follows:1.The effects of CQDs on the reduction of dissimilatory iron and the morphological transformation of Cd.Soils from Dongguan(DG),Nan'an(NA)and artificially enriched Cdcontaminated soils were used for the study.Different amounts of CQDs were applied and incubated in flooded water for 60 days.Fe(?)concentration,Total Fe release,pH and Cd morphology were dynamically monitored.The results showed that the application of CQDs significantly increased the maximum potential for Fe(?)reduction compared to the CK group.The promotion effect of N-CQD was stronger than that of CQD in DG soil and NA soil,while the opposite was true in Cd-contaminated soil.The pH of the incubation process system increased with time,CQDs was beneficial to raise the pH of the system.Cd in Cdcontaminated soil existed mainly as the acid extractable(F1)and reducible(F2)states.The content of oxidizable(F3)and residual states(F4)increased and F2 decreased with increasing incubation time.After 60 days of incubation,the application of 1.5% CQD increased the soil pH by about 1.5 units,which may account for the 1% higher relative percentage content of F4 compared to the CK group.2.Study on soil enzyme activity and microbial community structure.Urease,dehydrogenase and catalase activities as well as microbial community structure were measured during the incubation process.Alpha-diversity analysis,beta-diversity analysis,species composition analysis,linear discriminant analysis effect size(LEfse)and redundancy analysis(RDA)were performed based on the results of 16 S r RNA high-throughput sequencing.The results showed that soil urease,dehydrogenase and catalase activities were significantly reduced by Cd stress.The addition of CQDs had an activating effect on all three enzymes.CQDs altered soil microbial community abundance as well as diversity in a concentration-and time-dependent manner.At the phylum level,CQDs significantly increased the relative abundance of Firmicutes in DG and Cd contaminated soil and Proteobacteria in NA soil,and reduced the inhibitory effect of Cd on sensitive bacteria such as Actinobacteriota.Among them,both Firmicutes and Proteobacteria have been reported to have iron-reducing activity.Combined with the RDA results,it is clear that the first dominant flora of all three soils were positively correlated with the maximum potential for Fe(?)reduction,pH and enzyme activity,indicating that the addition of CQDs may alter the microbial community structure by increasing the altered soil pH and enzyme activity.In addition,Firmicutes in Cd-contaminated soils were positively correlated with F4 and negatively correlated with F1,indicating that Firmicutes may be an important flora in reducing Cd mobility as well as immobilizing Cd.In this thesis,we investigated the effects of CQDs on soil microbial dissimilatory iron reduction,Cd morphological transformation,enzyme activity,and microbial community structure,aiming to investigate the role of CQDs on geochemical element cycling and remediation of heavy metal-contaminated soils,and to provide reference data and information to explain the environmental behavior of CQDs in soils.