Preparation Of Goethite-fulvic Acid Composites And Their Immobilization Performance On Lead And Cadmium In Co-contaminated Soil

Goethite and humus are both important components in the soil,which do not exist alone and form the complex by interaction with each other,which plays an important role in the migration of heavy metals in the soil.In this paper,goethite-fulvic acid composite?Ge-FA?was prepared by coprecipitation method.The adsorption properties and mechanism adsorption of Pb²⁺and Cd²⁺on Ge-FA were investigated.Taking lead-cadmium co-contaminated soil as the research object,through analyzing the morphology distribution of lead-cadmium in the soil before and after applying the compound material,the immobilization performance of the composite material with series of different dosage and time was evaluated.?1?The effects of mass ratio of FA on the structure and properties of Ge-FA were investigated by XRD,FTIR,SEM-EDS,specific surface area and the stability test.The results show that the FA mass ratio has a greater impact on the Ge-FA properties.At low FA mass ratio?<4%?,the XRD spectrum of Ge-FA is very similar to goethite.As the proportion of FA increases,the intensity of the peak gradually decreases.When FA is more than 5%,Ge-FA show hematite diffraction peak,and has a poor crystallinity at 15%.The addition of FA changes the morphology of the sample from the filamentous to block,which blocks the pores,decreases the mesoporous volume and pore size.It also affects the bonding of Fe-O in the material,making Ge-FA more stable than goethite.?2?The effects of Ge-FA on isothermal adsorption,adsorption kinetics,pH value and lead-cadmium coexistence of Pb²⁺and Cd²⁺showed that:the adsorption of Pb²⁺and Cd²⁺on Ge-FA fitted Langmuir isotherm model very well,and the maximum theoretical adsorption amounts were 161.29 mg/g and 78.25 mg/g,respectively.The kinetic adsorption process followed the pseudo-secondary equation,and the adsorption rate is mainly controlled by chemical adsorption.The presence of Pb²⁺reduces the adsorption of Cd²⁺on Ge-FA,but the presence of Cd²⁺can promote the adsorption of Pb²⁺.The adsorption mechanism is mainly surface complexation and electrostatic attraction.The groups participating in the complexation are mainly-COO^-groups and Fe-O bonds.?3?The exchangeable content of Pb and Cd in the simulated soil decreased rapidly with the increase of the dosage of Ge-FA.When the dosage was 5.0%,The exchangeable content of Pb and Cd decreased to 24.411².84 mg/kg and 5.22³.49mg/kg,respectively,which decreased by 42.42%⁹3.64%and 15.84%⁴3.67%,respectively.?4?The four kinds of FA mass ratio?0%,1%,5%and 10%?of Ge-FA all have a certain immobilization effect on Pb and Cd in soil,the immobilization efficiency?IE?of Pb increased with the extension time and was basically stable after 45 days,while the IE of Cd in soil was less affected by time.After 60 days,the IE of Pb in simulated soil was stable at 20.94%,31.30%,56.37%and 77.35%,respectively.And it was stable at 3.35%,11.63%,32.27%and 57.28%,respectively for Cd.While the IE of Pb in actual contaminated soil were 8.86%,3.39%,20.30%and 56.73%,respectively,and they were 11.58%,7.25%,29.36%and 71.81%for Cd,respectively.Tessier fractions show that the change of Pb/Cd morphological on Ge-FA is different,Ge-FA mainly reduced soil the exchangeable and carbonate content of Pb,increased residual Pb fraction;while Ge-FA mainly reduced exchangeable Cd content and significantly increased Fe-Mn oxide-bound Cd content for Cd in soil,thereby reducing the mobility of Pb and Cd in the soil.?5?After 60 days of Ge-FA application,the soil pH increased slightly,the effective phosphorus content ranged from-54.62%to 3.69%,and the ammonium nitrogen content ranged from-15.66%to 48.12%.Potassium content increased significantly by 2.95%to 462.17%.While Ge-FA acts as an immobilization,it can also increase the nutrient content of the soil to a certain extent.