Study On The Removal Of Thallium From Wastewater Using δ-MnO₂ Material And Its Mechanism

Thallium is a highly toxic metal that exists mainly as Tl（I）and Tl（III）in the environment.Tl⁺often forms surface complexes with water molecules,which can become pollutants and cause water contamination.The lethal dose of Tl⁺for humans is only 10-15mg kg^-1.Therefore,strict emission standards have been established in various countries to control thallium emissions.δ-MnO₂is considered an excellent adsorbent for thallium removal due to its large surface area,porosity,and strong interaction between pollutants and adsorbents.This paper uses a one-pot method to prepareδ-MnO₂and optimize the thallium removal process.The mechanism of thallium removal byδ-MnO₂is also revealed,and the effect of Mn（III）on the adsorption process and the Mn leaching during the adsorption process are investigated to provide feasible treatment measures to prevent secondary pollution during the removal of thallium.The main research contents of this paper are as follows:（1）Firstly,δ-MnO₂birnessite-type manganese dioxide was prepared at different temperatures using a hydrothermal method,and the optimal conditions were determined.The proportion of Mn（III）/Mn（IV）inδ-MnO₂birnessite-type manganese dioxide was adjusted by changing the hydrothermal temperature,and the role of Mn（III）/Mn（IV）in the adsorption process was revealed,providing a theoretical basis for better utilization of manganese dioxide in wastewater purification.Transmission electron microscopy（TEM）and X-ray diffraction spectroscopy（XRD）analysis showed that the synthesized material was a typical layered MnO₂.The isotherm data was consistent with the Freundlich adsorption model,and the adsorption capacity Q_ereached 400.41mg g^-1.Kinetic studies confirmed that the adsorption process ofδ-MnO₂birnessite-type manganese dioxide for thallium（I）followed a pseudo-first-order kinetic process（R2=0.99）.By introducing in-situ Fourier transform infrared spectroscopy（FTIR）and X-ray photoelectron spectroscopy（XPS）,the influence of Mn（III）in the oxidation of Tl（I）during the adsorption process was investigated.The results showed that Mn（III）played an important role in the adsorption process.As the Mn（III）content increased,the affinity sites inδ-MnO₂birnessite-type manganese dioxide increased,and the Mn（III）in the structure was more likely to oxidize Tl（I）to Tl（III）.In addition,in-situ diffuse reflectance FTIR showed that Tl（III）interacted with the surface hydroxyls ofδ-MnO₂,indicating that the mechanism of Tl（I）removal byδ-MnO₂included not only adsorption but also oxidation,hydroxyl complexation,and transformation to Tl₂O₃.It is worth mentioning that the removal rate of Tl reached more than 98%in the p H range of 3.5-8,indicating thatδ-MnO2 had great potential for removing Tl from acidic wastewater.Finally,considering thatδ-MnO₂is rich in Mn elements,which is a toxic substance with strict emission standards,the leaching of Mn was determined by atomic absorption spectroscopy（AAS）and inductively coupled plasma mass spectrometry（ICP-MS）in the adsorbed solution.The Fe-Mn composites were prepared by adding Fe element,which inhibited the crystallization of MnO₂and reduced the dissolution of Mn by introducing Fe ions and forming a solid solution.The experimental results showed that the addition of Fe not only reduced the amount of Mn leaching to a safe level but also enhanced the adsorption effect of the material on Tl（I）.This may provide a new idea for how to reasonably solve the problem of Mn leaching.Based on these results,δ-MnO₂was used to treat industrial wastewater containing thallium,and the concentration of thallium in the treated solution was much lower than the national standard(5μg L^-1).At the same time,the concentration of other heavy metal ions in the wastewater can also be effectively controlled during the adsorption process.The experimental results show thatδ-MnO₂has an excellent thallium removal effect and will have a high application prospect in thallium pollution prevention and control.（2）Compared with single manganese oxide compounds,binary metal oxide composite materials usually have superior performance.Considering thatδ-MnO₂-type manganese dioxide is rich in Mn,which has strict emission standards,iron-manganese composite materials were prepared by introducing iron ions to form a solid solution and suppress the crystallization of manganese dioxide to reduce Mn leaching.Atomic absorption spectroscopy（AAS）and inductively coupled plasma mass spectrometry（ICP-MS）were used to determine the Mn leaching in the adsorbed solution.The experiment proved that the addition of Fe can not only reduce the amount of Mn leaching to a safe range but also enhance the material’s adsorption effect on Tl（I）(the adsorption capacity of the material increased to 539.4 mg g^-1after the introduction of iron),which will provide a new idea for solving the problem of Mn leaching.