Effects Of Heavy Metal Cd On The Decomposition Of Elodea Nattalii And The Migration Of Cd

Heavy metal pollution of water body has become the most serious water pollution at present,among which the heavy metal Cd was paid widespread attention owing to its strong toxicity and migration.Aquatic plants,especially submerged plants,have a strong absorption and enrichment effect on heavy metals and are widely used for the restoration of heavy metals in water.However,after the plant grows into a decline period,due to untimely harvesting or difficulty in salvaging,the plant debris will decay and decompose to release nutrients and heavy metals into the water body,which is most likely causing the secondary pollution of water body.Most of the existing researches focus on the law of decomposition of plant debris.The study on the migration of heavy metals during the decomposition of plant debris is still not very thorough.What's more,the influence of heavy metals on the decomposition of plant debris is rarely reported.In this thesis,the submerged plant Elodea nattalii was used as an experimental material to construct a small decomposing system of plant debris in a thermostatic laboratory.Two kinds of additive methods(plant debris and water body)and two concentrations(low and high concentrations)were used adding Cd to the system.By measuring the decomposition rate and the composition of plant residues in the process of decomposition of Elodea nattalii,the effects of Cd on the plant residue itself and its decay rate were analyzed;By determining the physical and chemical indicators of the water body and the sediment and the Cd content,the impact of Cd on the system environment and the migration of Cd during the decomposing process of Elodea nattalii residues were analyzed;Through comparing the changes of microbial community structure and diversity in the system,the microbiological mechanism of effects of Cd on the decomposing of aquatic plant residues were explored.Based on the above study,we look forward to providing a theoretical reference and guidance for the use of aquatic plants to carry out the restoration of heavy metals from sewage bodies.The main conclusions of the experiment are as follows:(1)The plant debris enriched of Cd,especially high concentration of Cd,whose initial P content and cellulose content were significantly lower than other treatments(p<0.01),and its decay decomposition rate was significantly reduced.The experimental results showed that the decomposing rate of the blank control group reached 59%on the 4th day,while the plant residue with high concentration of Cd had only 25.3%on the 32nd day.The effect of Cd in the water on the nutrient composition of plant residues is not significant,but it has an inhibitory effect on the decomposition of plant residues.The higher the concentration of Cd,the greater the inhibitory effect,indicating that Cd will influence the microorganisms related to decomposition.(2)The changes of the indicators of water bodies are consistent with the decay rate,and there is no significant difference in the indicators of the end of decomposition with their initial values,which shows that the submerged plants have a short-term impact on the organic matter content of water bodies.The content of TC in the sediment decreased significantly,and the contents of TN and TP increased.Indicating that the nutrient components of the plant debris were mainly enriched in the sediment and there was nosignificant difference among the treatment groups.(3)The accumulated Cd in plant residues will be released with the decomposition of plant residues,and the release rate will be very high,eventually reaching almost 100%.The released Cd will migrate into the sediment more quickly than into the water,and Cd in the water body will migrate in large quantities and enrich in the sediment.Therefore,there is a great risk of secondary pollution in aquatic plant debris enriched with Cd.In practical applications,it needs to be harvested in time.(4)The addition of heavy metal Cd changes the microbial community structure and significantly reduces its diversity.However,the effect of Cd in the plant debris on microorganisms is mainly concentrated in the late stage of decomposition,indicating that Cd exerts its effect after the release process.The effect of Cd in water on microorganisms in sediments was greater than that on microorganisms on the surface of plant debris.Thiseffect was weakened at the end of decomposition,indicating that the related microorganisms had a certain resistance to Cd stress.In addition,the Flaobacterium on the surface of plant residues was found to tolerate high concentrations of Cd,which are usually attached to the surface of plant debris or other organisms and can decompose organic macromolecules.In the follow-up study,a more in-depth exploration of the tolerance mechanism and stress resistance of Flavobacterium can be conducted,and it is expected to be applied to the restoration of Cd-contaminated water bodies.