Stabilization Of Heavy Metals In Soil Using Palygorskite Modified By ZnSO₄

The rapid development of the industrial economy has brought severe challenges to the soil environment.Heavy metals are enriched in the soil environment.They migrated and passed through the food chain,and caused serious harm to people’s safe on the tip of the tongue.In order to minimize the bioavailability of heavy metals in soil,and repaired heavy metal-contaminated farmland soil.This study used palygorskite,a mineral resource with abundant reserves in Gansu,as the raw material.This is because palygorskite has a unique layer-chain structure,large specific surface area,and high porosity.The high adsorption performance plays an important role in the chemical stabilization of heavy metals.The inorganic zinc fertilizer-Zn SO₄widely used in agriculture.It is used as the composition modification material.Zn SO₄and palygorskite are respectively 0:1,1:3,1:5,1:7,1:9,1:11mass ratio.The first type of stabilizer is prepared by liquid immersion and ultrasonication.The second type of stabilizer was prepared by optimizing the conditions to prolong the reaction time and calcining at high temperature.The stabilizer was characterized by X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FTIR）and scanning electron microscope（SEM）.The two types of stabilizers before and after optimization were added to the simulated Cd-contaminated soil and the actual heavy metal-contaminated soil,respectively.Stabilization experiments were carried out to measure soil electric conductivity（EC）,pH,cation exchange capacity（CEC）and other physical and chemical indicators.The available state content of heavy metals are determined in soil by DTPA and TCLP.The BCR continuous extraction method could determine the acid soluble state,reduced state,oxidized state and residue state content of heavy metals.The pot experiments could determine growth conditions,such as stem length,root length,fresh mass and dry mass,and heavy metal content in crops.The germination experiments could determine seed germ,radicle length and germination rate in the water-soluble state of heavy metals.The above experiments were conducted to explore the stabilization remediation effect of Zn SO₄modified palygorskite on simulated soil Cd and various heavy metals in actual polluted soil.The results show:（1）The impregnation reaction time of Zn SO₄and palygorskite was extended from 4 h to12 h.At the same time,the surface of the stabilizer after calcination at high temperature（350℃-400℃）for 2 h was uniformly dispersed and the rod-like structure was disordered,which may have generated new chemical groups and enhanced coordination effect of palygorskite.（2）The simulated soil pH decreased by 0.21-0.62 units,compared with the control.The simulated soil EC value increased by 1.88-2.9 times compared with the control.The simulated soil CEC value increased by 0.71%-6.20%compared with the control.The mass ratio of Zn SO₄to palygorskite was 1:3,and the physicochemical properties have significant changes.The physical and chemical properties of the actual heavy metal soils were consistent with those of the simulated Cd soils.When the mass ratio of Zn SO₄to palygorskite was between1:9 and 1:11,the soil physical and chemical properties changed most significantly.（3）The available state on soil simulated Cd decreased significantly.The available state content of DTPA decreased largely by 32.77%,and the available state content of TCLP decreased largely by 35.65%.The stabilization effect on Cd is the best when the ratio of Zn SO₄and palygorskite is 1:3.The DTPA and TCLP available contents of heavy metals in the soil after stabilization were significantly reduced.The maximum reduction of Cd in the DTPA-extracted state was 12.69%.The maximum reduction of Pb was 11.98%.The maximum reduction of Cu was 13.20%.The maximum reduction of Cr was 41.53%.Ni has a maximum decrease of 20.97%,and Zn has a maximum decrease of 4.83%.The maximum reduction of Cd in TCLP extraction state is 31.11%.The maximum reduction of Pb is 15.49%.The maximum reduction of Cu is 18.70%.The maximum reduction of Cr is 6.47%.The maximum reduction of Ni is 28.93%,and the maximum reduction of Zn is 4.84%.When the mass ratio of Zn SO₄to palygorskite was 1:9,the effective content of five heavy metals such as Cd,Pb,Cu,Cr and Ni decreased most significantly.（4）The chemical speciation of soil simulated Cd after stabilization changed significantly.The residual state of Cd increased by 2.59%-31.52%,compared with the control.The chemical activity of Cd is inhibited obviously when the ratio of Zn SO₄and palygorskite is 1:3.The chemical speciation of most heavy metals in the stabilized actual soil changed from acid-soluble state with higher activity to residue state with lower activity.Compared with the control,the residual state of Cd increased by 42.12%-116.14%,the residual state of Pb increased by 4.14%-58.05%,the residual state of Cu increased by 20.83%-29.65%,the residual state of Cr increased by 35.92%-52.75%,and the residual state of Ni increased by5.97%-9.78%.The residual state of Zn decreased by 25.23%-33.59%compared with the control.When the mass ratio of Zn SO₄to palygorskite was 1:9,the chemical activity of five heavy metals other than Zn were the lowest.（5）The lengths of corn stems and roots in stabilization soil increased significantly.The fresh and dry weights of corn stems and roots increased significantly.When the mass ratio of Zn SO₄to palygorskite was 1:3,the enrichment of Cd in the stem of corn decreased by 60.06%,and the decrease in Cd in the root was 32.69%.This improved significantly the growth of corn under Cd stress.The seed germination rate,germ length and radicle length of the soil water solution after stabilization were significantly improved.When the mass ratio of Zn SO₄and palygorskite was 1:9,the seed germination rate reached 91.67%.The germ length and radicle length reached the maximum value,which increased by 115%and 75.71%respectively compared with the control.It can be seen that an appropriate amount of Zn SO₄is beneficial to promote the growth and development of seeds and reduce the toxic effect of heavy metals on seeds.（6）The stabilization efficiency of soil simulated Cd was significantly increased to about30%,and the ecological risk index was significantly reduced.The potential ecological risk index of Cd decreased by 30.44%,and the risk range of Cd was reduced from considerable risk to moderate risk.The stabilization efficiencies of Cd,Pb,Cr,Ni,and Cu in soil were all higher than those in the control group,and the ecological risk indices of these heavy metals were significantly lower than those in the control soil.The potential ecological risk values of Pb,Cu,Cr,and Ni in the soil are all in the low risk range.Cd is reduced from considerable risk to moderate risk range.Zn is still in low risk,and the addition of Zn SO₄does not cause its risk range to expand.By optimizing the reaction conditions,the material calcined at high temperature has less Zn SO₄and better stabilization effect,and can be used as one of the materials for stabilizing heavy metals in soil.Those can effectively reduce the risk of heavy metals in the soil environment and provide a guarantee for farmland safety.At the same time,it also provides a new way for the extensive utilization of palygorskite in the soil field.