Studies On Preparation And Performance Of Effective Adsorbents For Removal Of Lead Ions From Water

With the deterioration of the ecological environment,the removal of heavy metal ions in wastewater has become a hot spot of current concern.Nanomaterials have attracted a lot of attention by scientists because of their a plentiful of merits such as large surface area,sufficient surface-active sites and good structural stability.Designing and synthesizing nanomaterials with specific functional groups to explore their removal of heavy metal ions in wastewater have an important scientific signirficance.In this paper,the molybdenum disulfide and biochar composites(MoS2@Biochar)and metal-organic framework materials(Tb-MOFs)with terbium ions as metal ions have been synthesized.Characterization techniques including scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD),Fourier transform infrared spectroscopy(FT-IR),Raman analysis,Thermogravimetric analysis(TGA),Brunauer-Emmett-Teller(BET),X-ray photoelectron spectroscopy(XPS),and Zeta analysis have been employed to explain the physicochemical properties of the materials.Static adsorption experiments were also used to investigate the influence factors during the adsorption process,such as pH,contact time,ionic strength,solid-liquid ratio,temperature,coexisting ions.XPS,density functional theory(DFT)and other techniques have been also applied to investigate the adsorption mechanism between the adsorbents and lead ions(Pb2+)at the molecular and atomic levels.The sulfur-containing adsorbents have a strong adsorption capacity for Pb2+·.We synthesized MoS2@Biochar nanocomposites with selective adsorption capacity by hydrothermal method.By loading MoS2 on the surface of biochar,most of the sulfur atoms are exposed on the surface of MoS2@Biochar with a flower-like morphology,which can improve the adsorption capacity of Pb2+ because of the sufficient contact between the sulfur atoms and the Pb2+.With the increase of pH,the concentration of hydrogen ions reduces,which is beneficial to the adsorption of Pb2+ by MoS2@Biochar.The adsorption mechanism is attributed to the inner-sphere surface complexation,the combination of sulfur atoms with Pb2+.The adsorption process is monolayer adsorption on the surface of MoS2@Biochar.The maximum adsorption capacity of MoS2@Biochar is 189 mg/g,which is higher than that of other materials.The adsorption process is a spontaneous endothermic process.The structural stability and recycling performance of the MoS2@Biochar are high after adsorption-desorption experiments for many times.Under the interference of coexisting ions,such as calcium,cobalt and cadmium ions,the MoS2@Biochar still maintains a very high adsorption performance.This work proves that the MoS2@Biochar is a selective adsorbent for Pb2+removal from contaminated water.The nitrogen atoms with a lone pair of electrons can be complexed with Pb2+.We synthesized the nanotube-like Tb-MOFs by solvothermal method.The structure of Tb-MOFs(monoclinic crystal P21/c)is determined by means of characterization and theoretical calculation.The carboxyl group in the ligand coordinates with the Tb metal ions,while the nitrogen-containing functional group on the ligand does not participate in the coordination,which can offer a plethora of active sites.Tb-MOFs have a very good structural stability in the acid-base range for adsorption Pb2+.The increase of pH is beneficial for adsorption Pb2+on the surface of Tb-MOFs.The second-order reaction kinetics model can fit the whole adsorption process well,and the main rate-limiting step of the adsorption process is chemistry adsorption.The adsorption process of Tb-MOFs accords with the Langmuir model.The maximum adsorption capacity is 547 mg/g.The adsorption process is a spontaneous and endothermic process.The adsorbent still has a strong adsorption capacity after repeated cycles.XRD analysis shows that the structure of Tb-MOFs does not change significantly during the recycling process.XPS analysis shows that the mechanism of the adsorption process is the complexation of the nitrogen-containing functional groups with Pb2',i.e.,(C-/N...Pb).Density functional theory(DFT)calculation confirms that the deprotonation of nitrogen-containing functional groups on Tb-MOFs can further improve adsorption capacity,and bidentate complexation is the most important complexation form.This work provides a theoretical basis for designing and synthesizing highly stable MOFs materials and exploring their adsorption properties.