Mechanism And Remediation Effect Of Mineral Amendment On Heavy Metals Contaminated Red Soil

Because of the long-term intensive cropping,overuse of chemical fertilizers and acid deposition,soil in Southern China has experienced a huge loss of essential cations and available silicon.The evolution process of clay minerals was accelerating and soil colloidal structure was destroyed,which eventually formed the red soil that predominant minerals are kaolinite,iron and aluminum oxides with low buffering and controlling ability of ecological environment and poor purification capacity.Heavy metal pollution caused by the fast development of industrial mining enterprises further deterioration of the red soil.This paper proceeds from mineral evolution,red soil formation and crop nutrition.Use potash feldspar?framework structure?and illite?layer structure?as the main raw material respectively,silicates and carbonate minerals as auxiliary material.Two mineral amendments were prepared by mechanically ball milling and activation at medium-high temperature.The mineral amendments were full hydrated with immersion treatment,then analysis of its chemical compositions and mineral components,specific surface area,pore size,functional group,surface characteristics and ion exchange capacity by XRF,XRD,BET,FTIR,SEM,XPS and other chemical analysis methods.The potential of two mineral amendments for the removal of heavy metals from aqueous solution was investigated by static adsorption and desorption experiment.The removal mechanism of heavy metals by mineral amendments was studied.Then heavy metal contaminated red soil was selected as the research object.The effects of amendments on the improvement of physical and chemical properties,supplement of mineral nutrients for red soil,crops yield increase and quality improving,increasement of2:1 type clay minerals,remediation of heavy metal pollution by field remediation experiment were studied,which provide the technique theories for using minerals in the remediation of heavy metal pollution and restore the original ecological essence of soil.All raw materials in the two system of potash feldspar–silicate–limestone–dolomite and illite–silicate–limestone–dolomite were caculated and mixed.After the mixtures being mechanical ball milled,dried,thermal activation for 1 h at a temperature of 1000?,quenching,two mineral amendments?MA-?and MA-??were finally prepared.The main mineral components in two mineral amendments are gehlenite,grossular and dicalcium silicate and some of glass phases or amorphous substances.The contents of available mineral constituents?2%citric acid?in MA-?like K₂O,CaO,MgO and SiO₂ were 4.46%,30.53%,5.42%and 19.8%,respectively.The contents of available mineral constituents?2%citric acid?in MA-?like K₂O,CaO,MgO and SiO₂ were 2.33%,33.22%,4.61%and 17.2%,respectively.The contents of available K,Mg and Si in MA-?were higher than MA-?.But the content of available Ca in MA-?were lower than MA-?.After the mineral amendments being full hydrated with immersion treatment,some typical enviromental remediation mineral materials such as montmorillonite,laumontite and gismondine and silicon aluminum oxide colloids were found.The leavings after hydration were characterized with small size,large specific surface area,high cation exchange capacity,which made them have the material basis of remediation of heavy metal pollution.Through static adsorption and desorption experiment,it can be found that the removal capacities of Cd²⁺and Pb²⁺by mineral amendment increased with pH,dosage,contact time,initial concentration of metal ions and temperature.The maximum removal capacity of Cd²⁺by MA-?and MA-?was 25.6 mg/kg and 47.0 mg/kg,respectively.The maximum removal capacity of Pb²⁺by MA-?and MA-?was 235.9 mg/kg and 143.3 mg/kg,respectively.To avoid forming hydroxide precipitation,the optimal pH for Cd²⁺and Pb²⁺removal was 4.0 and3.0,respectively.The contact time is only 30 min.The process of Cd²⁺and Pb²⁺removal were found to follow pseudo-second-order kinetic model and Freundlich isotherm model,which indicated that the chemical reaction was the rate-limiting step.The surfaces of mineral amendments are made up of small heterogeneous adsorption patches.The maximum removal capacity of mineral amendments in this study is higher than most mineral materials in the literature.Moreover,the adsorbed Cd²⁺and Pb²⁺onto mineral amendments can hardly be desorbed at pH 3.0.Ion exchange of heavy metals for Ca²⁺and Mg²⁺was principal mechanism in the removal of Cd²⁺and Pb²⁺from aqueous solution,followed by adsorption and precipitation.Due to the addition of mineral amendments to red soil,the pH,EC,CEC and available mineral elements like K,Ca,Mg and Si in soil were were greatly increased.Because the K in the mineral amendments exists in soluble salt,and Ca,Mg and Si exist mainly in the form of citrate soluble SiO₂-undersaturated minerals or feldspathoids.Therefore,Except for K,Ca,Mg and Si have the function of control and long release.The crops yield and quality of enhancements can be attributed to the increased pH and available mineral nutrients in soil.Meanwhile,the microbe quantity also increased as well as microbial biomass carbon and nitrogen.The yield of rice,treated with the applied MA-?at 750-1500 kg/ha,was decreased by 0.6-17.4%.The imperfect rate and chalkiness were all reduced,and the gel consistency of brown rice was increased.The yield and vitamin C of Lactuca sativa L.,treated with the applied MA-?at 1500-4500 kg/ha,was increased.The yield of Lactuca sativa L.increased by 8.4-23.6%.Due to the addition of mineral amendments into red soil,the content of 2:1 type clay minerals were increased.There are two factors contribute to this trends.Firstly,the mineral amendments that added into red soil formed montmorillonite after hydration.Secondly,By applying the mineral amendments,the essential mineral elements lacking in red soil can be adequately replenished,then the 1:1 type clay mineral?e.g.kaolinite?,under the soil rich in K,Ca,Mg and Si,gradually transformed into 2:1 type clay mineral?e.g.montmorillonite,illite and chlorite?,which improved the function of nutrient storage,eco-environmental buffer and control and purification capacity of pollutantsThe observed montmorillonite,laumontite and gismondine and silicon aluminum oxide colloids in mineral amendments after hydration as well as the increasing 2:1 type clay mineral in soil play an important role in the immobilization of heavy metals by ion exchange,adsorption and surface precipitation.Application of mineral amendment was effective in reducing the bioavailability of soil heavy metals such as Cd,Pb,Cr and Hg except for As by converting them from more labile fractions to stable fractions and migrating them to downward in the treated soil.This process significantly decreased the up-taking and accumulation of Cd,Pb,Cr and Hg in Lactuca sativa L.tissues?p<0.05?.Compared with the control,the uptke of Cd,Pb and Hg in brown rice treated with the applied MA-?at750-1500 kg/ha decreased by 2.8-28.5%,56.7-72%and 91.6%,respectively.Except for As,the concentration of Cd,Pb,Cr and Hg in different parts of Lactuca sativa L.decreased by6.1-53.4%,15.4-57.7%,13.3-38.5%and 8.8-27.1%,respectively.