Study On Adsorption Properties Of Molybdenum Compounds And Their Composites For Cr(?) In Water

In the past years,with the accelerating process of China's industrialization,the industry of discharging heavy metals has gradually increased,and the problem of heavy metal pollution has become increasingly serious and cannot be ignored.A large amount of wastewater rich in heavy metal ions emission into the environment and migrate with water or accumulate in the soil,making heavy metal pollution become the major environmental problem threatening human health.Heavy metals can not be biodegraded after entering the environment,and participate in the food chain cycle,eventually accumulating in the organism,destroying the normal physiological metabolism of the organism,thereby causing many environmental problems Among them,hexavalent chromium has been valued by researchers in various countries because of its long-lasting environmental pollution and great harm.The hexavalent chromium produced in the industrial and agricultural production process infiltrate into the river,groundwater and lakes,thereby damaging the growth of crops,animals,plants and the health of the human body.The oxidized hexavalent chromium is easily absorbed by the body and acts through the biofilm When the human body comes into contact with hexavalent chromium,it will irritate and corrode the skin and mucous membranes,causing skin allergies,respiratory tracts and other diseases,and then entering the human body,causing defects in genes and causing cancer.The various adsorption marerials have been studied to removing Cr(?)in water,but conventional adsorbents such as activated carbon,diatomaceous earth and the like don't have good selectivity and are difficult to regenerate.Therefore,the development and application of new adsorbents is of great significance.Nanomaterials such as magnetic nanoparticles and metal compounds have attracted much attention due to their large specific surface area,high activity and easy desorption process.In this study,N/LC-MOS2,Fe3O4-MoO2 and LaCoO3-MoO2 with good adsorption properties and specific adsorption of Cr(?)were successfully synthesized by simple coprecipitation method and hydrothermal method.,which have important practical significance for the treatment and disposal of chromium-containing wastewater.(1)Nitrogen-doped lignocellulose was treated by high-temperature calcination,and then added to the hydrothermal synthesis of molybdenum sulfide.Then the materials were characterized by SEM,TEM and XPS,the results showed that N/LC-MoS2 with a high specific surface area was successfully prepared.Through a series of experiments,the optimum pH of the material was pH 2,but considering the actual operation,further experiments were carried out under the condition of pH 4,which proved that the specificity and the repeated utilization ratio of the material were good.All experimental concentrations reached equilibrium within 180 min,reaching a maximum adsorption capacity at an initial concentration of 25mg/L.The kinetic fitting of Cr(?)adsorption on N/LC-MoS2 proved that the adsorption process is more in line with the second-order kinetic model.The second-order kinetic regression coefficient(R2=0.996)is greater than the first-order kinetics(R2=0.960),which indicates that the adsorption is controlled by the chemical mechanism.The thermodynamic fitting of the Langmiur isotherm regression coefficient(R2=0.983)is higher than the Freundlich isotherm(R2=0.911),indicating that the Langmuir model has a better fitting effect,further suggesting that the adsorption process is monolayer adsorption.The qmax is calculated to be 101.32mg/g according to the Langmuir model.(2)A new type of Fe3O4-MoO2 nanocomposite was directly synthesized by hydrothermal method.The composite material is superior in adsorption performance to Fe3O4 and MoO2 monomers.SEM and TEM showed that MoO2 exhibited a uniform one-dimensional morphology,and Fe3O4 particles adhered uniformly to the surface.Both XRD and XPS proved that the two monomers are successfully combined.Magnetic test indicated the material was magnetic,which is convenient for separation and recycling.At room temperature,the adsorption effect is best at pH 4.The experimental data were fitted to 5ppm and 20ppm Cr(?)solution.The results show that the adsorption process is more in line with the second-order kinetic model and the Langmuir model,indicating that the adsorption is monolayer chemical adsorption.After the material was recycled for 5 times,the efficiency is reduced from 100%to 96.5%,indicating that its regeneration performance is good,and the specificity experiment prove that it is specific to the adsorption of Cr(?)in the presence of coexisting ions.(3)LaCoO3 synthesized by co-precipitation and calcination was added to the synthesis system of MoO2,and LaCoO3-MoO2 was successfully prepared.The composite have a high adsorption capacity of 172.8mg/g.The SEM characterization results show that MoO2 has a special spatial structure,similar to the hollow rod formed by irregular nano-plates,the cross-section shows that it is a hollow hexagon.When combined with LaCoO3,the irregular LaCo03 nanoparticles grow on the surface of MoO2.XRD results show that molybdenum dioxide in the compound is MoO2 with monoclinic symmetry(JCPDS 65-5787),and lanthanum cobaltate is diamond-shaped LaCoO3(JCPDS 01-084-0848).The XPS results show that the LaCo03 nanoparticles are successfully on the surface of MoO2.The optimum pH of the composite material is 2,but further experimental investigation was carried out under the condition of pH 4 in consideration of practical application.The kinetic fitting results show that the pseudo-secondary model(R2=0.989)is more suitable than the pseudoufirst-order model(R2=0.944).The thermodynamic fitting results show that the Langmuir model better described the adsorption process of Cr(?)on LaCo03-MoO2,because it has a higher regression coefficient value(R2=0.997)compared to the Freundlich isotherm(R2=0.983).