| Thallium(Tl)is a trace metal element widely present in the natural environment,and its biological toxicity is much greater than that of heavy metals such as mercury(Hg),copper(Cu),zinc(Zn),lead(Pb)and cadmium(Cd).Tl mainly has two valence states of Tl(Ⅰ)and Tl(Ⅲ)in nature,and the toxicity of Tl(Ⅲ)is much higher than that of Tl(Ⅰ).Tl(Ⅰ)mainly exists in the form of free and bioavailable ions in the water environment,its solubility is higher than that of Tl(Ⅲ),and it is easy to migrate and transform in the environment.In addition,Tl+ ions and K+ions have similar ionic radii and are more easily absorbed,transported and accumulated by organisms.Birnessite is a layered manganese oxide found in almost all near-surface environments.Due to its large specific surface area,good oxidation performance,and small PZC value,birnessite regulates the fate and transport of metal elements in the environment through adsorption,co-precipitation,and redox reactions,especially for many heavy metals(such as Pb,Cu,Cd)and plays an important role in the biogeochemical cycle.Previous studies have shown a high geochemical affinity between Tl and birnessite.Natural soil or water sedimentary environment often contains a variety of transition metal ions,such as nickel(Ni),cobalt(Co),vanadium(V),Cu and Zn,etc.These metal ions often occur in birnessite,which will cause changes in the structure and physical and chemical properties of birnessite(specific surface area,surface groups,redox properties,etc.),and will enhance or weaken the adsorption and oxidation reactions of birnessite to other substances.At present,there have been many reports on the adsorption of traditional heavy metals on birnessite,but there are few studies on the adsorption mechanism of birnessite and transition metal-doped birnessite to heavy metal Tl.In addition,the competitive adsorption mechanism for Tl between birnessite and transition metal-doped birnessite is not clear.In this study,acid birnessite and acid birnessite doped with transition metals Ni and Co were taken as the research object,and combined with various characterization methods,adsorption kinetics,adsorption isotherms,and competitive adsorption experiments were carried out to study the effect of birnessite on the adsorption mechanism of Tl(Ⅰ),and explore its adsorption rule for Tl(Ⅰ)in complex environment.The key finding are as follows:(1)Using concentrated hydrochloric acid and potassium permanganate solution,adding a certain molar ratio of Ni(Ⅱ)and Co(Ⅱ)during the reaction process,a total of 5 kinds of manganese oxide materials were prepared.The analysis results of morphology,phase and composition confirmed that the synthesized material is birnessite with hexagonal symmetry structure.The XRD analysis results show that the reflection intensity of the diffraction peak of Ni-doped birnessite(001)decreases,the interlayer spacing becomes wider,and the crystallinity decreases;the crystal structure of Co-doped birnessite changes little.Nitrogen isothermal elution and adsorption experiments show that the specific surface area of the prepared acid birnessite is 21-42m2/g.XPS and manganese AOS test results show that the prepared acid birnessite contains high Mn(Ⅳ)content(AOS close to 4)and strong oxidation performance.(2)The experimental results of adsorption kinetics of acid birnessite and acid birnessite doped with Ni and Co on Tl(Ⅰ)showed that the adsorption quickly reached equilibrium.Within 1h after the reaction,Tl(Ⅰ)quickly occupied highly active adsorption sites(mainly octahedral cavities).As the surface adsorption sites were gradually saturated,the repulsion between Tl(Ⅰ)increased,the steric hindrance effect strengthened and the rate of adsorption slows down.According to the isotherm experiment results,the Langmuir model is suitable for the isotherm experiment.The higher the doping molar ratio,the lower the adsorption capacity.When the molar ratio of Co/Mn is 0.015,the adsorption capacity is the highest(qmax is 393 mg/g),which is slightly higher than that of acid birnessite(qmax=343 mg/g)and birnessite with other doped molar ratios.When the molar ratio of Ni/Mn is 0.015,the adsorption capacity for Tl(Ⅰ)is 322 mg/g;when the molar ratio of Ni/Mn is 0.2,the maximum adsorption capacity is 280 mg/g;when the molar ratio of Co/Mn is 0.2,the maximum adsorption capacity is 328mg/g.(3)After the birnessite was loaded with Tl(Ⅰ),SEM and TEM results showed that the particle size decreased,the edge of the layered structure was blurred,and the particles were broken.XRD results show that the reflection intensity of diffraction peak(001)decreases or even disappears with the increase of Tl(Ⅰ)solution concentration,indicating that the crystallinity of acid birnessite decreases.The XPS results showed that the loading of Tl(Ⅰ)was partly oxidized to Tl(Ⅲ),and(1)Using concentrated hydrochloric acid and potassium permanganate solution,adding a certain molar ratio of Ni(Ⅱ)and Co(Ⅱ)during the reaction process,a total of 5 kinds of manganese oxide materials were prepared.The analysis results of morphology,phase and composition confirmed that the synthesized material is birnessite with hexagonal symmetry structure.The XRD analysis results show that the reflection intensity of the diffraction peak of Ni-doped birnessite(001)decreases,the interlayer spacing becomes wider,and the crystallinity decreases;the crystal structure of Co-doped birnessite changes little.Nitrogen isothermal elution and adsorption experiments show that the specific surface area of the prepared acid birnessite is 21-42m2/g.XPS and manganese AOS test results show that the prepared acid birnessite contains high Mn(Ⅳ)content(AOS close to 4)and strong oxidation performance.(2)The experimental results of adsorption kinetics of acid birnessite and acid birnessite doped with Ni and Co on Tl(Ⅰ)showed that the adsorption quickly reached equilibrium.Within 1 h after the reaction,Tl(Ⅰ)quickly occupied highly active adsorption sites(mainly octahedral cavities).As the surface adsorption sites were gradually saturated,the repulsion between Tl(Ⅰ)increased,the steric hindrance effect strengthened and the rate of adsorption slows down.According to the isotherm experiment results,the Langmuir model is suitable for the isotherm experiment.The higher the doping molar ratio,the lower the adsorption capacity.When the molar ratio of Co/Mn is 0.015,the adsorption capacity is the highest(qmax is 393 mg/g),which is slightly higher than that of acid birnessite(qmax=343 mg/g)and birnessite with other doped molar ratios.When the molar ratio of Ni/Mn is 0.015,the adsorption capacity for Tl(Ⅰ)is 322 mg/g;when the molar ratio of Ni/Mn is 0.2,the maximum adsorption capacity is 280 mg/g;when the molar ratio of Co/Mn is 0.2,the maximum adsorption capacity is 328mg/g.(3)After the birnessite was loaded with Tl(Ⅰ),SEM and TEM results showed that the particle size decreased,the edge of the layered structure was blurred,and the particles were broken.XRD results show that the reflection intensity of diffraction peak(001)decreases or even disappears with the increase of Tl(Ⅰ)solution concentration,indicating that the crystallinity of acid birnessite decreases.The XPS results showed that the loading of Tl(Ⅰ)was partly oxidized to Tl(Ⅲ),and with the increase of Tl/Mn,the percentage of Mn(Ⅳ)decreased accordingly.(4)Competitive adsorption experiments between acid birnessite and acid birnessite doped with Ni and Co showed that in the initial stage of the reaction,the adsorption rate of acid birnessite doped with Ni and Co was much higher than that of Tl(Ⅰ),which is higher than acid birnessite undoped with transition metals.However,with the progress of the reaction,the Tl(Ⅰ)adsorbed by the acid birnessite doped with transition metals appeared to be desorbed,and was then adsorbed by the acid birnessite without doped transition metals.The desorption rate of acid birnessite with low doping ratio is slower than that of high doping ratio,and it takes longer to reach adsorption equilibrium.Doped Ni and Co will replace K(Ⅰ)in birnessite,and the higher the doping ratio,the higher the substitution ratio.In addition,K(Ⅰ)was released when the adsorbed Tl(Ⅰ)was oxidized to Tl(Ⅲ),indicating that Tl(Ⅰ)replaced the interlayer K(Ⅰ)during the adsorption process. |
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