Effects Of Nitrilotriacetic Acid On Lead Accumulation In Athyrium Wardii And Its Migration Risk In Soil

Phytostabilization is widely used in the restoration and vegetation of heavy metal contaminated soils in tailings because of its environmentally friendly and economic efficiency.The mining ecotype of Athyrium wardii,a perennial herb with great lead?Pb?accumulation ability in roots,shows great potential in the phytostabilization of Pb-contaminated soils in tailings.Chelant-assisted phytoremediation shows great potential in enhancing phytoremediation efficiency,thus been widely promoted.In this study,with the object of the mining ecotype of A.wardii,an incubation experiment and a subsequent soil-column experiment were conducted to investigate:?1?the effects of the application time and application mode of 2 mmol kg^-1 of nitrilotriacetic acid?NTA?on Pb availability in soils,?2?the effect of NTA application on the rhizosphere soil characteristics and Pb accumulation in A.wardii,?3?the effect of NTA application on Pb leaching in soils and environmental risk.The main results obtained are as follows:?1?After the NTA application,the soil pH decreased first and then increased with the increasing NTA application time,and the lowest soil pH was 5.07 at the 5^th day.The soil pH tended to be no changes after the 14-day application of NTA.The application of NTA significantly enhanced Pb availability in soils,leading to the greatest Pb availability at the 7^th day,which was 3.33 times more than that of the soils without NTA application.After the application of NTA,the exchangeable Pb and carbonate bound Pb in soils increased generally and reached the maximum at the 3th and 7^th day,which were 1.33 and 1.51 times of that of the soils without NTA application.Pb bound to Fe-Mn oxides in soils showed the greatest decrease of 35.6%at the 3^rd day of NTA application,compared with that of the soils without NTA application.According to the NTA application mode dependent experiment,the application of NTA showed the greatest efficiency in enhancing Pb availability when the quantitive amount of the 2mmol kg^-1 of NTA was divided into single doses and applied into soils at once,for which the Pb availability in soils were 1.22 and 1.33 times of those was divided into double and three successive doses and applied into soils at twice and three times.?2?The application of NTA showed no significant influence on plant biomass of A.wardii grown in 800 mg kg^-11 of Pb-contaminated soils.The NTA application significantly enhanced Pb concentration and Pb accumulation in underground parts as well as Pb accumulation in whole plant of A.wardii,which increased by 23.8%,14.3%and 14.5%when compared with that without NTA application.The rhizosphere soil pH decreased by 0.37 units,while the amount of dissolved organic carbon?DOC?and the activities of urease,acid phosphatase and catalase in rhizosphere soils increased by11.0%,105.0%,20.0%and 27.0%after the NTA application,compared with that without NTA application.The soil respiration,microbial biomass carbon amount in soils and soil invertase activity in rhizosphere soils decreased significantly?with 8.9%,16.2%and 18.9%of reduction?after the NTA application.There were no significant changes for rhizosphere soil microbial metabolic after the NTA application.?3?According to the soil-column experiment(with 40 cm of column length,and filled with 800 mg kg^-1 Pb-contaminated soils in 0-20 cm surface soils and uncontaminated soils in 20-40 cm soils)with the plantation of A.wardii,the concentrations of available Pb and exchangeable Pb in 0-5,5-10,10-15,15-20 cm soil layers increased by 11.1%-23.4%and 17.4%-44.4%after the NTA application,compared with that without NTA application.After the NTA application,the concentrations of Pb bound to carbonate and Fe-Mn oxides significantly decreased by9.7%-13.0%and 16.3%-19.1%compared with the treatment without NTA application.These indicate that the application of NTA promoted the transformation of Pb from low availability fractions to high availability fractions.After the NTA application,the available Pb and exchangeable Pb in 0-5,5-10,10-15,15-20 cm soils without plantation were 1.10-1.25 and 1.10-1.29 times more than those with the plantation of A.wardii.The exchangeable Pb amount in 20-30 cm soils after the NTA application was significantly more than that of the soils without NTA application.It indicates that the application of NTA promoted Pb leaching in soil profiles.According to the toxic leaching experiment?TCLP?,the TCLP extracted Pb in 20-40 cm uncontaminated soils without plantation was up to 1.52 mg·L^-1 and did not exceed the international limit value(5 mg·L^-1).According to the Environmental Risk Coding?RAC?evaluation,the pollution degrees of the 20-30 cm and 30-40 cm soils after NTA application were moderate and low risk level,indicating the application of NTA posed some risk to environment.