| As a kind of inorganic pollutant with strong toxicity and non-biodegradation,heavy metals will cause serious damage to the ecosystem once accumulated in the soil.Therefore,it is of great practical significance to choose the low-cost and effective in-situ stabilization technology to reduce the ecological toxicity and environmental risk of heavy metals in soil.Palygorskite is a kind of hydrated magnesium aluminosilicate clay with layer chain structure,which can stabilize heavy metals in soil by ion exchange adsorption,isomorphous replacement,surface complexation and hydrolytic precipitation.However,the natural palygorskite clay contains a lot of impurities,which seriously affect its good performance.As an inorganic cationic polymer,poly-aluminum ferric chloride has the characteristics of strong charge neutralization ability,excellent adsorption performance and fast precipitation rate.At present,there are few reports on the preparation of new stabilization materials using palygorskite modified by poly-aluminum ferric chloride and on its application in the remediation of soil heavy metal pollution.Therefore,this research can provide new theoretical basis and data support for the current treatment of heavy metal in contaminated soil.In this study,poly-aluminum ferric chloride and Gansu Linze palygorskite were used as raw materials by impregnation method.Poly-aluminum ferric chloride was mixed into composites with different mass ratios(5%,10%,15%,20% and 25%)to prepare poly-aluminum ferric chloride modified palygorskite.Scanning electron microscopy(SEM),X-ray diffractometer(XRD)and Fourier transform infrared spectroscopy(FTIR)were used to characterize the micro-morphology,functional group characteristics and crystal phase composition of modified palygorskite.The artificial simulated Cd contaminated soil and the multi-heavy metals contaminated soil in actual mining area were collected and prepared.The palygorskite modified by poly-aluminum ferric chloride was applied into the target soil with4wt% for stabilization experiment,and the stabilization effect was further verified by pot experiment and seed germination experiment.After the process of stabilization,the stabilization effect and ecological toxicity of poly-aluminum ferric chloride modified palygorskite on soils were evaluated according to the test of basic physical and chemical properties of soils,the contents analysis of heavy metals in extraction with diethylenetriamine pentaacetic acid(DTPA)and toxicity characteristics(TCLP)methods,the chemical speciation distribution analysis of heavy metals with BCR method,and the verification of seed germination and plant growth index.The main results are as follows:(1)The result of scanning electron microscope,X-ray diffraction pattern and Fourier transform infrared analysis showed that the impurities such as quartz and carbonate in palygorskite can be removed and the pore structure is richer than raw materials.The rod of palygorskite modified by poly-aluminum ferric chloride is agglomerated and reunited in the form of aggregate and flocculent polymer.The area that can be contacted directly is larger,and the characteristic peak of hydroxyl / silicon hydroxyl group is enhanced,which indicates that there are common groups of palygorskite and poly-aluminum ferric chloride in the stabilization materials.(2)After the application of different poly-aluminum ferric chloride modified palygorskite into two kinds of heavy metal contaminated soils,the physical and chemical properties of soils can be significantly improved,and the effects of modified materials on different soils are different.Among them,when modified palygorskite was applied into the artificial simulated Cd contaminated soil,the soil p H value and electrical conductivity(EC)were significantly higher,and the soil cation exchange capacity(CEC)was significantly lower than those of the control group.When palygorskite modified by poly-aluminum ferric chloride was applied into the soil polluted by multi-heavy metals in the actual mining area,the soil p H value was significantly decreased,the soil electrical conductivity was significantly increased and the soil cation exchange capacity had no obvious change trend compared with the control group.(3)After the addition of palygorskite modified by poly-aluminum ferric chloride into the soils polluted by heavy metals,the bioavailable content and leaching toxicity of heavy metals in the soil can be effectively reduced,but the stabilization ability of the same material to different heavy metals is different.When the mass fraction of poly-aluminum ferric chloride was 25%,the stabilization effect of Cd,Cu,Zn and Ni was the best.When the mass fraction of poly-aluminum ferric chloride was 5%,the stabilization effect of Pb and Cr was the best.Among them,the content of DTPA extracted Cd decreased by 31.75% and the content of DTPA extracted Cr decreased by 66.01% compared with the control group.In addition,the treatment ability of the modified material to Pb was the weakest,and the two extractions of DTPA and TCLP decreased by 14.28% and 20.06% respectively compared with the control group.(4)The application of poly-aluminum ferric chloride modified palygorskite can promote the transformation of soil heavy metals from acid-soluble speciation with high activity to residual speciation with low activity.Similarly,when the mass ratio of poly-aluminum ferric chloride was 25%,the speciation transformation of Cd,Cu,Zn and Ni was the most obvious.When the mass ratio of poly-aluminum ferric chloride was 5%,the speciation transformation of Pb and Cr was the most obvious.Among them,the transformation rate of heavy metal Cr was the highest,the chemical speciation distribution of acid-soluble Cr in soil decreased from13.87% in the control group to 5.42%,and the distribution of residual Cr increased from9.73% to 25.27%.The stabilization ability of the modified material to Pb was weak.Compared with the control group,the content of acid-soluble Pb decreased by 17.24%,and the content of residual Pb increased by 10.7%.(5)The addition of modified palygorskite into contaminated soils can reduce the toxic effect of heavy metals on plants and promote the germination of corn seeds and the growth of corn plants.When the mass ratio of poly-aluminum ferric chloride was 25%,the root length,shoot length,fresh biomass of root and shoot,dry biomass of root and shoot and seed germination index of corn reached the maximum.Among them,the relative root elongation(RRE)and germination index(GI)of corn seeds were 83.69% and 83.68%,respectively.Palygorskite modified by poly-aluminum ferric chloride has large specific surface area,excellent ion exchange capacity and surface complexation properties,which reduced the biotoxicity and ecological risk of heavy metals in soils to a great extent.Therefore,it can be used in the practical stabilization and remediation of farmland soils polluted by heavy metals. |
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