| Cadmium?Cd?have been identified as a highly toxic heavy metal and is widely present in the environment due to natural and man-made emissions.At present,light pollution of arable land is common in China,and cadmium is the highest content of inorganic pollutants in soil,which has a huge impact on economic and social development.Therefore,there is an urgent need to develop an effective and economical method to repair cadmium contaminated soil.Palygorskite is a kind of non-toxic natural layered chain aluminum-magnesium silicate clay mineral.Its unique crystal structure makes it have fast and efficient adsorption process for heavy metals and no harsh requirements for adsorption conditions.But the disadvantage is that it contains impurities.Palygorskite in Linze is used as raw material in this paper.According to five different volume fractions of sulfuric acid?5%,7.5%,10%,12.5%,15%?,five different mass ratios of palygorskite and alkaline CaO?1:1,2:1,3:1,4:1,5:1?,five different temperatures?250°C,300°C,350°C,400°C,450°C?,palygorskite is modified by acid,alkali and thermally,respectively.The effects of passivation and remediation of Cd-contaminated soil?2.0 mg/kg CdCl2 simulation?by adding three kinds of modified palygorskite with acid,alkali and heat are also studied.The experiments include soil passivation experiments and pot experiments.Modified palygorskite is added to the contaminated soil at an addition amount of 20 g/kg,and soil pH,electrical conductivity?EC?,cation exchange capacity?CEC?,organic matter?OM?,Cd-TCLP and Cd-DTPA content,and the chemical form of heavy metal Cd are measured after passivation for 10 d,20 d,30 d,50 d,80 d,and 120 d.After 35 days of sowing,the corn growth indexes and Cd accumulation are measured.The passivation effect of modified palygorskite on Cd-contaminated soil is analyzed from physical and chemical properties of soil,content and chemical form of Cd,biomass and concentration of corn.The results shows:?1?Through electron microscope scanning analysis,the grains of palygorskite clay minerals are in the form of fine rods.Compared with the original ores,after acid,alkali and heat,the pores between the rod-shaped single crystals and rod beams of palygorskite increase and the channels are wider,which increase the effective specific surface area and the adsorption capacity of palygorskite.According to X-ray diffraction analysis,the palygorskite is mainly composed of attapulgite?A?,quartz?Q?,dolomite?D?,muscovite?M?,chlorite?C?,feldspar?F?and other minerals.After acid,alkali and heat modification,the position of characteristic peak value of palygorskite do not change obviously,which indicates that the crystal structure of palygorskite do not change after acid-base activation and heat activation,and the modification do not destroy the structure of palygorskite.?2?Soil pH,EC,and CEC showed an upward trend with increasing passivation days,and soil organic matter showe a downward trend with increasing passivation days.The pH and EC of the soil modified with palygorskite added with acid and alkali are lower than those in the CK group,and the pH and EC of the soil with added thermally-modified palygorskite are higher than the control.The soil pH shows a tendency to increase first and then decrease with the increase of sulfuric acid volume fraction,the mass ratio of palygorskite and CaO,and heat reforming temperature,which are higher under AP10,JP3,and RP350 respectively.However,soil EC decreased with the increase of sulfuric acid volume fraction,the mass ratio of palygorskite and CaO,and increased first and then decreased with the increase of temperature;soil organic matter and CEC changed obviously under AP10,JP3 and RP300,respectively.?3?After the application of various passivators,the extraction content of TCLP and DTPA of Cd in the soil decreased firstly and then increased slightly with the increase of passivation days,but still show a downward trend as a whole,basically decreased to the lowest level in 50 and 80 days of passivation.The addition of three modified palygorskite passivators of acid,alkali,and heat all significantly reduce the Cd-TCLP and Cd-DTPA content in the soil,and with the increase of the volume fraction of sulfuric acid,the mass ratio of palygorskite to CaO and the increase of the temperature,the content of TCLP and DTPA in the soil with AP10,JP3 and RP350 is the lowest,respectively.?4?During the whole passivation process,the acid soluble and reduced states of heavy metal Cd in soil decrease with the increase of passivation time,and the oxidized and residual states of Cd increase with the increase of passivation time,in which the acid soluble and residual states of Cd changed greatly,while the reduced and oxidized states changed little.Compared with the control,the application of modified palygorskite significantly promote the mutual transformation of chemical forms of Cd.With the increase of volume fraction of sulfuric acid-modified palygorskite,the mass ratio of palygorskite to CaO of alkali-modified palygorskite and the increase of the temperature,the content of four chemical forms of Cd decrease first and then increase.?5?The addition of modified palygorskite passivator significantly promotes the growth of corn.The plant height,root length,fresh and dry weight of the above-ground and underground corn seedlings under each treatment are significantly higher than the control.Different modified palygorskite treatments significantly affect the growth indexes of corn.The growth indexes of corn seedlings increased first and then decreased with the increase of volume fraction of sulfuric acid-modified palygorskite,the mass ratio of palygorskite to CaO of alkali-modified palygorskite and the temperature of thermally-modified passivator.?6?The addition of modified palygorskite significantly inhibits the accumulation of heavy metals in corn,and the application of modified palygorskite significantly reduce the accumulation coefficient and transport coefficient of heavy metals Cd in corn seedlings.The accumulation coefficient and transport coefficient of Cd of corn seedling are less than 1,and the accumulation coefficient of the above ground part is less than the underground part,that is,the accumulation coefficient of Cd of the root is easier to the stem and leaf.The transport coefficient of Cd of corn seedling is the smallest under the treatment of AP10,JP3 and RP350,respectively. |
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