The Mechanism Of Antimonite(Sb(?)) And Antimonate(Sb(?)) Removal From Water Using Fe-based Metal-organic Frameworks

Antimony is a kind of amphoteric rare metal.It has extensive industrial applications and will enter into the environment in the future continuely.Adsorption is an efficient and simple process for the treatment of heavy metal pollution in water.However,traditional activated carbon and other adsorbents don't have good adsorption ability to antimony.Metal organic frameworks(MOFs)is formed by the coordination of metal ions or metal clusters with organic ligands.is a class of nano materials with enormous internal surface areas and controlled pore size,which make MOFs a adsorbent for potential application in the field of wastewater treatment.The article aims to select one or more adsorbents among many MOFs,which is highly efficient for antimony,and explain the removal mechanism,and evaluat the impact of environmental factors,laying the foundation for similar sorbent application development in the future.Hydrothermal synthesis method was used to prepare metal organic framework materials.Batch experiments were conducted to remove antimony from water using metal organic framework.First,the removal efficiency of Sb(?)and Sb(?)onto four MOFs with different metal center was compared.The results showed that UiO-66(Zr)and MIL-101(Fe)materials have good adsorption capacity for Sb(?)and Sb(?),especially MIL-101(Fe),which can remove 98.1%Sb(?)within two hours.Cu-BTC and MIL-96(Al)almost have no adsorption capacity for Sb(?)and Sb(?).The reason is Cu-BTC has extremely strong absorption for water.The interaction between water molecules and unsaturated sites of Cu is stronger than that of Sb and unsaturated sites of Cu.However,MIL-96(Al)has no ability to remove antimony because its window size of cages is so small that Sb(?)and Sb(?)molecules cann't enter into the internal structure.To further understand the adsorption mechanism of Sb onto MOFs,the removal efficiency of Sb(?)and Sb(?)by five Fe-based MOFs were compared.The results showed the adsorption capacity of MIL-101(Fe)towards Sb(?)and Sb(?)is much higher than the other four MOFs.The calculated maximum adsorption capacities of MIL-101(Fe)for Sb(?)and Sb(?)were 151.81 and 472.83 mg/g respectively when initial pH is 6.0.The windows size of MOFs estimated from the crystal structure can explain the difference in adsorption capacities of five MOFs.The molecule size of Sb(OH)3 is 3.05 ?,which can enter into the internal pores of the above five kinds of MOFs.The molecular size of Sb(OH)6-is 7.36 ?,it can only enter the interior of its cage through the surface windows of MIL-101(Fe)and MIL-88C(Fe).Namely,Sb(?)just can penetrate into the internal structures of MIL-101(Fe)and MIL-88C(Fe)and occupy unsaturated metal sites of Fe(?).The increasing pH can promote the Sb(?)adsorption.But it inhibited Sb(?)adsorption.Which is related to the existing form of antimony and surface charge of adsorbent.NO3-had slight influence on the adsorption of Sb(?)and Sb(V).PO43could destroyed the structure of MOFs and the presence of Fe3+ in the water confirmed this fact.So Sb(?)and Sb(?)adsorption were affected greatly when PO43-exist.SO42-could promote the Sb(?)adsorption onto MIL-101(Fe)greatly.Because the adding SO42-could increases the ionic strength of the solution.Magnetic MOFs composites were synthesized by combining MOFs with Fe3O4,and they can be rapidly separated from water under the action of an external magnetic field.Fe3O4/FeBTC(10:3)can remove more Sb(?)and Sb(?)than the two single materials.The possible reason is Fe-BTC acted as a template in the synthesis process of composites.Thus the particle size of Fe3O4 nanoparticles was smaller than Fe3O4 obtained by ordinary methods.The small size of Fe3O4 nanoparticles has more hydroxyl groups on the surface,which can provide more adsorption sites.However,the adsorption capacity of Fe3O4/MIL-101(Fe)for Sb(?)and Sb(?)is much smaller than that of MIL-101(Fe).This is due to the fact that both the specific surface area of the composite and the adsorption sites contained are much smaller than MIL-101(Fe).Cl-,NO3-and SO42-almost had no effect on the adsorption of Sb(?)and Sb(?)onto the magnetic MOFs composites,while PO43-had little effect on the removal of Sb(?),but can influence Sb(?)adsorption greatly.Because PO43-and Sb(OH)6-are similar in chemical structure.So PO43-will compete the adsorption site on adsorbent with Sb(OH)6-and thus affecting the Sb(V)adsorption.The stability of MOFs and magnetic MOFs composites in water was explored by exposing the material in water for a certain period of time.MOFs and magnetic MOFs composites had slight effect on Sb(?)adsorption but had large effect on Sb(?)adsorption.The MOFs and magnetic MOFs composites have strong adsorption performance,good separation ability,and good application prospect.