Immobilizing and stabilizing effect of nano-MnFe2O4@DE composite on heavy metal vanadium

Abstract: In order to obtain a stabilizing material for treatment of soil vanadium pollution, based on the functional complementarity between different materials, iron-manganese oxide @ diatomite composite (MnFe2O4@DE) was synthesized. Through XRD, SEM and BET characterization, it can be seen that MnFe2O4 nanoparticles were uniformly deposited on the surface of diatomite, resulting in larger specific surface area and pore volume of composite. Besides, MnFe2O4@DE presented much larger specific surface area (112.3m2/g) and pore volume (0.244cm3/g) than DE and MnFe2O4, respectively.. This was mainly because the MnFe2O4 nanoparticles can disperse well on the surface of diatomite support restraining the self-agglomeration, which can effectively reduce the grain size of MnFe2O4 nanoparticles. Generally, smaller grain size would lead to a larger specific surface area of materials. The larger specific surface area as well as pore volume could provide more surface active sites for adsorption of the contaminant molecules, which was beneficial to improve the removal performance of the composite for pollutants. The stabilizing effect and mechanism of the composite on soil vanadium were studied by simulation experiments. Results indicated that as-synthesized MnFe2O4@DE composite had better soil vanadium stabilizing effect than the monomer stabilizer (DE, MnFe2O4) at the optimal dosage (7%), and its stabilizing rate of vanadium was about 16.87and 1.24 times as much as that of diatomite (DE) and MnFe2O4, respectively. MnFe2O4@DE (7% dosage, 30 days of stabilizing) had the best stabilizing effect on vanadium in soil, and the stabilization rate was 94.81%±2.5%. Stabilizing time and pH experiments proved that the stabilizing effect of MnFe2O4@DE composite on soil vanadium can be maintained for a long time, and acidic as well as neutral conditions were beneficial to the stability of vanadium by composite. The stabilizing rate of vanadium in soil by MnFe2O4@DE decreased with the increase of pH value. This phenomenon could be attributed to the greater pH value, which increased in the electrostatic repulsion between the negative charge on the surface of the composite and the vanadium anion in the soil. The XPS and vanadium form analysis showed that the composite had excellent stabilizing effect on vanadium in soil, which related to the larger specific surface area and pore volume as well as the reduce of vanadium valence. Compared with the form of vanadium without added stabilizer, the content of residual state of vanadium in soil increased after adding the MnFe2O4@DE stabilizer, which increased from 19.70% to 30.69%. XPS analysis demonstrated that vanadium could be reduced to low valence vanadium with low toxicity by MnFe2O4@DE composite. Effects of the composite on the bioavailability of soil vanadium were also investigated through the plant experiments. Plant germination experiment indicated that the germination rate, plant height and root length of rapeseed (Brassica napus) seedlings in vanadium-contaminated group with the addition of composite were higher than those of the other two monomer stabilizers, and which had no significant difference compared with the control group without vanadium. The vanadium content of plants in the vanadium-contaminated group after adding the composite was also lower than that in the vanadium-contaminated group with the addition of only monomer. Besides, the composite reduced the vanadium content in the plants by about 80% compared to the vanadium-contaminated group without stabilizers. Therefore, MnFe2O4@DE composite had a good application prospect in the remediation of heavy metal contaminated soil.