Immobilization Of Heavy Metals In Farmland Soil In The Lead-Zinc Mining Area

Human activities such as mining,smelting and petrochemical industry have been proved to be the main sources of heavy metal pollution,among which mining and smelting activities are major sources in heavy metal polluted farmland.To explore effective stabilization and remediation plan for heavy metal polluted farmland closed mine,survey analysis and risk assessment were conducted in a farmland around a lead-zinc waste mine in Shangyu.The self-made amendment--gypsum composite was used in laboratory simulated test and field,and the spatio-temporal variation law of heavy metal availability in the remediation was studied.The primary conclusions were summarized as follows:(1)The survey of paddy fields closed mine in Shangyu shows all paddy fields in the survey area were neutral to acidic.Taking the mine as the center,it extended outward along the three directions of northeast NE,northwest NW and south S,respectively.There were significant gradient differences in the contents of Cd,Pb and As in the soil.The closer to the mining area,the higher concentrations of heavy metals were,and the lower diversities of microbial community were.The pollution risk screening value of "soil environmental quality risk control standard for agricultural land pollution(GB15618-2018)" was taken as the evaluation criterion.The pollution in NE was most serious,75% samples belonged to severe contamination.75% samples in NW were moderately polluted;The heavy metal pollution in S was light,all samples were pollutionfree or mildly polluted.(2)In both laboratory and field experiments,gypsum composite had no significant effect on soil p H and decreased soluble organic carbon(DOC)content,but had little effect on soluble organic nitrogen(DON)content.Gypsum composite can reduce the biological availability of heavy metals in soil effectively,the content of available Cd decreases by 25.6-70.6%,the content of available Pb decreases by 45.8%-90.4%,and the content of Cd,Pb and As in soil solution decreases by 40.1%,70.7% and 33.6% at least,respectively.At the same time,the application of gypsum composite reduced the contents of Cd,Pb and As in rice grains by 65.1%,47.4-48.7% and 24.7-38.1%,respectively.Meanwhile the high dose(0.3%)of gypsum composite was better than the low dose(0.15%)for the stabilization of Pb(at all times)and Cd(at 25W)in the soil.In addition,field experiments showed that the stabilization effect of the amendment could last for two-seasons rice growth at least.(3)In the simulated column,the application of gypsum composite(S)played no significant effect on the p H of the surrounding soil,but elements S and Fe were significantly increased.Meanwhile,the migration and diffusion of gypsum composite affected the bioavailability of heavy metals in the surrounding soil,and the impacting range expanded gradually over time.In the horizontal column,the contents of Cd and Pb in soil solution within 8 cm from the boundary of the central soil column,were significantly decreased at 100 D in S compared with CK,indicating that the radius of influence of gypsum composite on the availability of heavy metals in the horizontal space was 8 cm at least.In the vertical column,the contents of available Cd and Pb in the soil and the contents of As in soil solution treated by S also decreased significantly compared with CK,which happened only on the soil surface or within 10 cm from the soil surface in 15 D,and in 100 D happened within 30 cm from the soil surface.