Preparation Of FeMnMg Layered Double Hydroxide And Its Remediation Effects On Environmental Lead And Cadmium Pollution

Along with the industrialization and urbanization,large mount of heavy metals have been put into natural environment though human’s activities,these heavy metals transformed amang air,water,soil and living things,but most of them eventually entered into the soil environment and lead to soil heavy metal pollution.The soil heavy metal pollution is seriously in China,about 1/5 of cultivated land is polluted by heavy metals,which poses a threat to the safety of agricultural products,people’s health and the ecological environment,especially,cadmium and lead pollution is the most common and the most harmful in cultivated land.The remediation of heavy metal pollution in soil has been paid great attention,and development of remediation technology and methods have become a hot topic in soil environmental protection.The remediation of heavy metal pollution in soil mainly comes from two ideas:One is to reduce the total amount of heavy metals in soil,and the other is to control the activity of heavy metals in soil and reduce their migration to the food chain.It is hard to reduce the total amount of heavy metals in soil by the existing technical methods economically and effectively,especially in cultivated land.Therefore,the regulation of heavy metal activity in soil has become an important direction of heavy metals remediation in cultivated land.The researches on the development of efficient soil amendments and their mechanisms in heavy metal inhibition are the basis for the regulation of soil heavy metals activation.Mean while,bio-organic fertilizer rich in humic substances is often used as the nutrient for rapid soil fertility,but whether humic substances which exist widely in the natural environment can lead to activation of soil heavy metals is still controversial.Therefore,the researches in regulation of heavy metal in soil by the key active components of humic acids have important significance in soil remediation of heavy metals.In this study,we prepared a kind of soil metal deactivator called FeMnMg-LDH which has good layered structure,and the preparation method of FeMnMg-LDH was optimized by adjusting a series of parameters.Then FeMnMg-LDH was used to remove the Pb²⁺and Cd²⁺from water,the adsorption capacity and removal mechanisms of FeMnMg-LDH for lead and cadmium also has been carried out by adsorption test and instrumental characterization.Simulating experiments together with chemical sequential extraction were carried out to understand the transformation and distribution characteristics of Pb and Cd in purple alluvial soil and grey brown purple soil,at the same time the influence of humic acid（HA）and fulvic acid（FA）on the bioavailability of Pb and Cd also has been discussed.And the last,FeMnMg-LDH also has been used as soil metal deactivator to understand the change of soil lead and cadmium in speciation,transformation and distribution before and after FeMnMg-LDH application,as well as the influence of FeMnMg-LDH on the bioavailability of lead and cadmium.All these studies above provid theoretical basis and technical support for soil lead and cadmium pollution remediation,and the main results are shown as follows:Through a series of exploratory experiments,the preparation method of FeMnMg-LDH material which we named Mg²⁺ion intervention co-precipitation method was established,the optimized technological parameters were determined,the preparation process is clean and stable and the composition and properties of the materials were systematically characterized.The best concentration ratio of metal reactant mixture is:0.5mol/L FeCl₃,0.5mol/L MnCl₂,1mol/L MgCl₂,pH≤3.0 which can be adjusted by HCl and NaOH.The concentration of alkali liquor used during the co-precipitation peocess is 3mol/L NaOH.The crystallization process lasts 12 hours and the temperature was set as 60℃.The resulting product was filtered and washed repeatedly,and then freezing dried by a freeze dryer for 96h to avoid the Mn²⁺oxidation during drying process.Well lamellar structure of the synthesized FeMnMg-LDH has been proved by its transmission electron microscope（TEM）images and the diffraction peaks are similar to the characteristic peaks of typical LDHs.The broken hexagonal plates and indistinct lattice fringe in TEM images also suggested the poor crystallinity of FeMnMg-LDH.The FeMnMg-LDH also showed strong thermal stability,it will not lose weight because of the thermal cleavage when the temperature is below200℃.The maximum adsorption capacity of FeMnMg-LDH for Pb²⁺and Cd²⁺in our study was421.42 mg/g 59.99 mg/g,respectively,much more higher than those for the most of the other similar kind of materials reported in the literatures,indicating that the FeMnMg-LDH synthesized in our study has good potential to be used as absorbent for the removal of heavy metals including Pb²⁺and Cd²⁺ion from the aqueous solution.FeMnMg-LDH showed the good chemical durability,and maintained the original shape of pristine LDH after Pb²⁺and Cd²⁺adsorption.The final equilibrium pH values were distributed in the range of 6.0 to 8.0 after Pb²⁺and Cd²⁺ion adsorption mainly due to the high pH buffering capacity.The pseudo-second-order equation is better in describing the adsorption kinetics of Pb²⁺and Cd²⁺as shown by the higher determination coefficient（R²）value,indicating that the chemisorption is the rate controlling step.versus the square root of time(_1/2)result in a linear relationship indicating that intra-particle diffusion is the rate-limiting step during the Pb²⁺adsorption,nonlinear relationship betweenand_1/2 indicating intra-particle diffusion was rate controlling step for Cd²⁺adsorption.The adsorption mechanisms of FeMnMg-LDH for Pb²⁺and Cd²⁺ion are mainly by surface adsorption,surface-induced precipitation and ion exchange.Self redox reaction occurred during the adsorption process,and this property may render the FeMnMg-LDH a potential to be used as catalyst.Pb and Cd in purple alluvial and grey brown purple soil all exist mainly in residual form,and the morphology distribution is different in the two kinds of soil.Pb has low biological activity and moderate potential release risk in purple alluvial,and has low biological activity high potential release risk in grey brown purple soil.Cd has relatively high biological activity and moderate potential release risk all in purple alluvial and grey brown purple soil.The adding of heavy metals greatly increased the environmental risk of heavy metals in soil.Pb showed higher biological activity in purple alluvial than that in grey brown purple soil under the equal lead adding level,however,Cd showed higher biological activity in grey brown purple soil.In the Pb and Cd combined pollution soil,Cd showed higher biological activity than Pb.The difference between（humic acid）HA and（Fulvic acid）FA and the differences of the soil humic acids dosage lead to the diffenent effections on the regulation of Pb and Cd by the key active components of humic acids.HA can passivate Pb and decrease the potential release risk of Pb all in the Pb polluted and Pb/Cd combined polluted purple alluvial as well as grey brown purple soil.Along with the increasing of content of HA,the passivation effect was significantly enhanced.The content of available Pb in purple alluvial and grey brown purple soil both significantly negatively correlated with the carbon content of HA,the correlation formula is y=-2.443x+17.52（R²=0.805）and y=-4.457x+20.03（R²=0.903）,respectively.HA can activate Cd and increase the potential release risk of Cd both in the Cd polluted and Pb/Cd combined polluted purple alluvial,but can passivate Cd and decrease the potential release risk of Cd both in the Cd polluted and Pb/Cd combined polluted grey brown purple soil.Along with the increasing of content of HA,the passivation effect and activation effect were decreased.The content of available Cd significantly negatively correlated with the carbon content of HA in purple alluvial,the correlation formula is y=-0.0009x+0.1482（R²=0.999）and significantly positively correlated with the carbon content of HA in grey brown purple soil,the correlation formula is y=0.033x+0.2124（R²=0.937）.Except for in the Pb polluted purple alluvial,FA passivate Pb and decrease the potential release risk of Pb,FA can activate Pb and increase the potential release risk of Pb all in Pb/Cd combined polluted purple alluvial and Pb polluted and Pb/Cd combined polluted grey brown purple soil.Along with the increasing of content of FA,the passivation effect was decreased while activation effect was significantly enhanced.The content of available Pb in purple alluvial and grey brown purple soil both significantly positively correlated with the carbon content of FA,the correlation formula is y=2.584x+23.37（R²=0.983）and y=10.72x+6.061（R²=0.976）,respectively.Except for in the Pb/Cd combined polluted grey brown purple soil,FA activate Cd and increase the potential release risk of Cd,FA can passivate Cd and decrease the potential release risk of Cd all in Pb polluted and Pb/Cd combined polluted purple alluvial and Pb polluted grey brown purple soil.Along with the increasing of content of FA,the passivation effect was enhanced.The content of available Pb in purple alluvial and grey brown purple soil both significantly negatively correlated with the carbon content of FA,the correlation formula is y=-0.0058x+0.159（R²=0.984）and y=-0.0138x+0.2429（R²=0.999）,respectively.The effects of FA/HA mixture in different soil types and different pollution in soil on regulation of Pb,Cd in soil are not exactly the same,which has a direct relationship with the speciation of soil properties,Pb and Cd particles in the soil.In the Pb/Cd combined pollution soil,the strong competitive effect of Pb leads to the activation of soil CdFeMnMg-LDH has significant passivation for Pb and Cd both in purple alluvial and grey brown purple soil.The Pb passivation priority of FeMnMg-LDH was higher than that of Cd.Under the same addition level,the highest inhibitory percentage of available Pb in purple alluvial and grey brown purple soil by FeMnMg-LDH could reach 64.1%and 64.1%,respectively.The inhibitory percentage of Pb by FeMnMg-LDH in purple alluvial is higher than that in grey brown purple soil,indicating FeMnMg-LDH has stronger passivation effect on Pb in purple alluvial.The highest inhibitory percentage of available Cd in purple alluvial and grey brown purple soil by FeMnMg-LDH could reach 35.0%and 36.2%,respectively.The inhibitory percentage of Cd by FeMnMg-LDH in purple alluvial is lower than that in grey brown purple soil,indicating FeMnMg-LDH has stronger passivation effect on Pb in grey brown purple soil.Fe MnMg-LDH treatment decreased the percentages of exchangeable lead（Exc-Pb）,carbonate bound lead（Carb-Pb）and exchangeable cadmium（Exc-Cd）,and increased the percentages of Fe-Mn oxide bonded state-Pb（FeMnO_x-Pb）and Fe-Mn oxide bonded state-Cd（FeMnO_x-Cd）greatly which caused by the increasing content of Fe-Mn minerals after the FeMnMg-LDH added and the transformation of Pb and Cd from other forms transformed to FeMnO_x-Pb and FeMnO_x-Cd.FeMnMg-LDH has significant passivation for Pb and Cd both in the purple alluvial and grey brown purple soil after humuic acids treated.Under the same HA addition level,the highest inhibitory percentage of available Pb in purple alluvial and grey brown purple soil by FeMnMg-LDH could reach 52.5%and 42.8%,respectively.The inhibitory percentage of Pb by FeMnMg-LDH in purple alluvial is lower than that in grey brown purple soil,indicating FeMnMg-LDH has stronger passivation effect on Pb in grey brown purple soil,but Pb in purple alluvial has higher biological activity.The highest inhibitory percentage of available Cd in purple alluvial and grey brown purple soil by FeMnMg-LDH could reach 35.4%and 27.7%,respectively.The inhibitory percentage of Cd by FeMnMg-LDH in purple alluvial is higher than that in grey brown purple soil,indicating FeMnMg-LDH has stronger passivation effect on Pb in purple alluvial.Under the same FA addition level,the highest inhibitory percentage of available Pb in purple alluvial and grey brown purple soil by FeMnMg-LDH could reach 57.8%and 53.0%,respectively.The inhibitory percentage of Pb by FeMnMg-LDH in purple alluvial is higher than that in grey brown purple soil,indicating FeMnMg-LDH has stronger passivation effect on Pb in purple alluvial,but Pb in purple alluvial has higher biological activity.The highest inhibitory percentage of available Cd in purple alluvial and grey brown purple soil by FeMnMg-LDH could reach 32.1%and 21.0%,respectively.The inhibitory percentage of Cd by FeMnMg-LDH in purple alluvial is higher than that in grey brown purple soil,indicating FeMnMg-LDH has stronger passivation effect on Cd in purple alluvial.Cd in grey brown purple soil has higher biological activity.The highest inhibitory percentage of available Pb and Cd in FA/HA treated purple alluvial and grey brown purple soil by FeMnMg-LDH could reach 41.9%,58.3%and 21.9%,33.2%,respectively.FeMnMg-LDH has stronger passivation effect on Pb and Cd in FA/HA treated purple alluvial.FeMnMg-LDH did not change the transformation trend of Cd in humic acids treated purple alluvial and grey brown purple soil,but decreased the percentages of exchangeable lead（Exc-Pb）,carbonate bound lead（Carb-Pb）and exchangeable cadmium（Exc-Cd）,and increased the percentages of Fe-Mn oxide bonded state-Pb（FeMnO_x-Pb）and Fe-Mn oxide bonded state-Cd（FeMnO_x-Cd）greatly which caused by the increasing content of Fe-Mn minerals after the FeMnMg-LDH added and the transformation of Pb and Cd from other forms transformed to FeMnO_x-Pb and FeMnO_x-Cd.Inaddition,FeMnMg-LDH treatment decreased the potential release risk of Pb both in humuic acids treated soil.The potential release risks of Pb both in FA treated purple alluvial and grey brown purple soil were decrease from very high level to moderate level or high level,while that in HA treated purple alluvial and grey brown purple soil were still at the high level after FeMnMg-LDH treated.The potential release risks of Cd in FA and HA treated purple alluvial as well as grey brown purple soil were all at the high level after FeMnMg-LDH treated.In all,exogenous lead and cadmium can lead to the increasing of biological activity of Pb and Cd in soil.The key active components of humic acid FA and HA have some differences in the activation and passivation effects of lead and cadmium in two kinds of soil.FeMnMg-LDH not only has good ability in Pb and Cd adsorption in water,but also has good ability in Pb and Cd passivation in soil.FeMnMg-LDH can reduce the biological activity and the available content of Pb and Cd in soil effectively.These results show that FeMnMg-LDH has a good potential for the removal of heavy metals in water and the passivation of heavy metals in soil.