Study On The Enhanced Phytoremediation Of Cadmium Contaminated Sediments By Novel Materials And The Biomass Treatment After Phytoremediation

Heavy metals in the sediments are toxic to aquatic organisms and likely to cause water pollution.Therefore,it is an urgent task to remediate heavy metal contaminated sediment.Compared with other physical and chemical repair technology,phytoremediation is an ideal method for sediments remediation since it can remove heavy metals from sediments thoroughly without causing secondary pollution.However,the application of phytoremediation is limited by the low heavy metal bioavailability,long repair cycle length,few hyper accumulator species and biomass,and the large amounts of heavy metal contained biomass produced after phytoremediation.Thus,it is essential to improve the phytoremediation efficiency and find ways to handle with the biomass containing heavy metals.This dissertation firstly investigated the roles of three kinds of novel materials,tea waste biochar(TB),multiwall carbon nanotubes(MW)and starch stabilized nanoscale zerovalent iron(SN),in the enhancement of heavy metal accumulation and translocation and the alleviation of heavy metal stress in the plants,and then studied the effects of TB,MW,and SN on the change of metal behavior,enzymes activity,and microbial community structure and composition in the sediments.In addition,this dissertation also conducted a feasibility study on the stabilization of heavy metals in biomass obtained after phytoremediation by pyrolysis and the reuse of those pyrolysis products for wastewater treatment.According to these studys,we explored the comprehensive mechanisms of the enhanced phytoremediation by novel materials and provide a potential new method for the disposal of biomass produced after phytoremediation.The specific research work and creative achievements of this paper include the following four aspects.The first part emphasized on the accumulation and translocation of heavy metals in plants and the response of plants to heavy metal induced stress.(1)It was confirmed that ramie had the ability to accumulate Cd and it could be used to remediate Cd contaminated sediments.(2)The application of suitable concentrations of TB,MW and SN could increase the accumulation and translocation of Cd in ramie seedlings.Among them,MW significantly improved the transportation efficiency of Cd from ramie root to the aerial part,while SN was more prominent in upregulating Cd accumulation ability.Cd subcellular distribution results showed that the used materials could change the proportion of Cd in cell wall,cell soluble fraction and cell organelle part,thus enhancing the penetration of Cd into plant cells.However,TB and MW at high levels reduced the accumulation of Cd in ramie tissues.(3)The study of plant response to heavy metal induced stress found that appropriate levels of TB and MW could promote the growth of ramie seedlings under Cd stress.The appropriate levels of TB,MW and SN reduced the accumulation of reactive oxygen species(ROS)in ramie seedlings,thereby alleviating the oxidative stress.But high concentrations of materials inhibited plant growth and promoted ROS accumulation in ramie seedlings.(4)The effects of materials on the antioxidative defense system were investigated.Results showed that appropriate levels of MW and SN increased antioxidative enzymes activities and enhanced the antioxidant content,whereas the antioxidative enzymes activities and antioxidant content were reduced in ramie seedlings with the application of high levels of materials.The above results confirm that novel materials could enhance the phytoremediation by increasing the ability of plants to accumulate and translocate heavy metals,promoting plant growth and development,alleviating oxidative stress,and improving plant antioxidant capacity,but applying higher concentrations of materials would induce toxic effects to the plants,thus it is necessary to carefully select the applied concentrations when using novel materials to facilitate phytoremediation.As for this study,500 mg/kg TB,500 mg/kg MW,and 100 mg/kg SN were their optimum concentrations for enhancing the phytoremediation of Cd contaminated sediments using ramie seedlings.The second section investigated the influence mechanisms of novel materials on the heavy metal behaviors and sediments fertility during the phytoremediation process.(1)The speciation of Cd in the sediments was measured using the BCR sequential extraction procedure.It was found that the residual fraction of Cd decreased and the proportions of oxidizable and reducible Cd were increased during the phytoremediation process,indicating the change of heavy metals mobility and biological toxicity by plants.The application of TB,MW and SN could convert heavy metals bounding to organic matter or sulfide to metal oxides or hydroxides,which might be one of the reasons why novel materials could promote the accumulation of heavy metals in plants.(2)The sediments fertility was assessed by the enzymes activities.Results showed that the activities of urease and phosphatase were reduced due to the accumulation of N and P by ramie seedlings,while the removal of Cd by ramie seedlings up-regulated the activities of invertase and catalase in the sediments.The application of suitable concentrations of TB,MW and SN could increase enzymes activities,thus providing suitable conditions for the growth and development of ramie seedlings under Cd stress.However,higher concentrations of materials might reduce the activies of phosphatase and invertase,thus negatively influence the sediments fertility.The third section focused on the ecological effects of using novel materials to enhance phytoremediation.(1)The physical and chemical properties of the remediated sediments were determined.It was found that the applied novel materials changed the basic properties of sediments,such as pH and the organic matter content.The above change would not significantly reduce the sediment quality or cause damage to the sediments ecosystem,but it would be beneficial to the plant growth and the enhancement of heavy metals bioavailability.(2)The microbial diversity of the sediment was assessed by 16 S rRNA sequencing.It was found that 500 mg/kg TB increased the diversity of sediment bacteria,whereas 100 mg/kg SN slightly reduced the abundance and homogeneity of sediment bacteria.The applied MW at 500 mg/kg had little effect on the bacterial species,but it reduced the bacteria homogeneity.(3)The influence of novel materials on the composition and structure of the bacterial community was investigated.Results showed that novel materials increasd the abundance of sediment bacterial such as Nitrospira,Actinobacteria and Acidobacteria,which species had heavy metal tolerance or could promote plant growth and change heavy metal speciation.The abundance of Saccharibacteria was reduced with the application of novel materials,which specie contains lots of pathogenic bacteria.(4)Results of OUT cluster analysis and principal component analysis showed that the composition of bacteria in TB500 and MW500 treatments had high similarity.Bacteria in these two treatments were dominated by Proteobacteria,Acidobacteria and Gemmatimonadetes.However,the composition of the bacteria in the SN100 treatment was significantly different from that observed in CK,which increased the relative abundance of Nitrospira and Firmicutes in the sediment.TB and MW were carbon materials,so their effects on the microbial community were similar.SN,as a metal nanomaterial,its higher reactivity would significantly change the composition of microbial community in the sediment.(5)The interaction between the bacterial community structure and the sediment physical and chemical properties was discussed based on the redundant analysis.It was found that microbial community structure and composition were affected by the change of residual Cd proportion,pH values and organic matter content due to the application of novel materials.The above results confirmed that novel materials could enhance the phytoremediation by changing the physicochemical properties and the microbial community structure and composition of the sediment,and the application of suitable concentrations of novel materials would not adversely affect the sediments ecosystem.The fourth part investigated the treatment of biomass obtained after phytoremediation.(1)The biomass was treated by pyrolysis.Results indicated that pyrolysis reduced the quality of biomass,increased the concentration and the enrichment coefficient of Cd,Cr,Zn,Cu and Pb in the biomass,converted acid soluble and reducible heavy metals into oxidizable and residual forms,and significantly decreased the leachable metal contents.(2)The biomass pyrolysis products were used to adsorb cationic dyes from wastewater.The adsorption of methylene blue reached equilibrium at the 8 h,and the maximum adsorption capacity was 257.37 mg/g.In addition,the adsorbent had high recyclability with the adsorption efficiency higher than 70% at the fifth cycle.The above results indicated that pyrolysis could stabilize heavy metals in the biomass obtained after phytoremediation and reduce their potential hazards,and the biomass pyrolysis products could be reused for the remediation of environmental contamination.This dissertation revealed the mechanisms of using novel materials to enhance the phytoremediation of heavy metal contaminated sediments and explored methods for the treatment and reuse of biomass obtained after phytoremediation.It would provide new methods for improving phytoremediation efficiency and for the efficient treatment of heavy metal contaminated sediments,and provide a reference for the treatment and reuse of biomass containing heavy metals.Moreover,this paper also could provide new insights into the novel application of materials and their potential risk assessment.