The Interaction Of Birnessite With Cu,V And The Coordination Stability Of Adsorbed Zn

Manganese oxide minerals widely exist in the natural environment,with small particles,high chemical reactivity and negative charge etc.,which usually adsorb and enrich other metal ions(e.g.Pb²⁺,Zn²⁺,Cu²⁺etc.)then affect their migration and transformation in the environment.As the metal ions enter the manganese oxide mineral,the structure,morphology and physical and chemical properties of manganese oxide minerals also can be changed,which will impact on the environmental effect of other metal ions.At present,manganese oxides adsorption mechanism of metal ions have been more,but the effect and isomorphous substitution mechanism of metal ions on birnessite is not fully clear.Less study is on the effect of ambient conditions on the release behavior of metal ions from oxide manganese to environment.Furthermore,comparison with high valent oxide manganeses,the low valent oxide manganeses have less attention.Therefore,hexagonal birnessite with layer structure and manganite with tunnel structure are as the research objects of this paper.Cu²⁺or V³⁺doped hexagonal birnessite by coprecipitation synthesis are prepared to study the coordination form and spatial distribution of Cu²⁺or V³⁺in hexagonal birnessite,effect of Cu²⁺or V³⁺doping on the crystal structure of hexagonal birnessite and the release behaviors of Cu²⁺and/or Zn²⁺with different coordinations in hexagonal birnessite and manganite,combined with X-ray diffraction?XRD?,specific surface area?SSA?,field emission scanning electron microscopy?FESEM?,high resolution transmission electron microscopy?HRTEM?and thermogravimetric analysis?TGA?and Fourier Transform infrared spectroscopy spectroscopy?FTIR?,X-ray photoelectron spectroscopy?XPS?,X-ray absorption structure?XAS?analysis,atomic pair distribution function?PDF?,dissolution and/or desorption methods.The main conclutions were listed as following:1.Cu-doped hexagonal birnessites with Cu/Mn mole ratio of 0?HB?,0.05?Cu5?,0.10?Cu10?and 0.20?Cu20?were synthesized by coprecipitation.With Cu content,the crystallinity and cell parameters of hexagonal birnessites were both decreased,but the manganese average oxidation states?AOSs?and SSA were both increased.Cu-doping had a little effect on the structure and micromorphology of hexagonal birnessites.The distances of Mn-O and Mn-Mn in the first shell were gradual decrease.The analysis results of Cu K-edge XAS indicated that a part of Cu?<20%?were entered into the layer,the most adsorbed on the vacancy and part of Cu as cluster at edge site,iii especially for high Cu content.With increasing the content of Cu,the maximum adsorption capacities of hexagonal birnessite for Pb²⁺and Zn²⁺were both decreased,which indicated that the vacancy and edge sites were gradually occupied.The results of dissolution at HCl and desorption at H₂SO₄ of Cu-doping hexagonal birnessites indicated that the rapidly Cu²⁺was from the polynuclear Cu clusters and the highly distorted Cu octahedron and other Cu²⁺released with the dissolution of mineral structure,which supported our assumption that Cu²⁺in our doped birnessite samples has an octahedron geometry.2.V-doped hexagonal birnessites with V/Mn mole ratios of 0?HB?,0.02?V2?,0.05?V5?,0.08?V8?,0.10?V10?,and 0.15?V15?were synthesized by addition of V³⁺cation during hexagonal birnessite crystallization.Doping V had significantly effect on the crystalline and micromorphology.The sizes along c axis decreased from10.98 nm to 2.63 nm,SSAs increased first and then decreased,and three-dimensional?3D?hierarchical microspheres composed of two-dimensional?2D?nanoplates both decreased.V valence in hexagonal birnessite was determined as V⁵⁺.The coordination structure of V had not V=O bond and only the content of Mn⁴⁺decreased,which indicated that a small part of V entered into or adsorbed on the vacancy,based on FTIR vibrating peak of Mn-OH decreasing and FTIR vibrating peak of Mn-O-V increasing.Furthermore,PDF results showed that most V as anion clusters adsorbed on the edge sites.It increased the negative chager of hexagonal birnessite and kept K+content.During isothermal adsorption,with increasing V content,the maximum adsorption capacity of hexagonal birnessite for Pb²⁺increased with less V released,while that for Zn²⁺decreased with more V released.It indicated that Pb²⁺could form precipitation but not for Zn²⁺.3.The released behaviors of octahedral(^VIZn)and tetrahedral(^IVZn)triple corner-sharing?TCS?complexes on synthesized hexagonal birnessite under different conditions have been explored by solution chemistry combined with spectroscopic analysis.The results indicate that the three-dimensional?3D?hierarchical microsphere structure of nano-birnessite is stable in acidic environment.In addition,the dissolution process of birnessite with adsorbed Zn²⁺could be divided into two stages:at the first stage,Zn²⁺?⁶0%?is desorbed fast while the Mn dissolution proportion is only⁸%,and the mole ratio of^VIZn/^IVZn on birnessite increases with time;at the second stage,the residual Zn²⁺is released slowly and majority of them is ^VIZn,and its released rate almost stays constant until the complete dissolution of birnessite.For Zn²⁺desorption by Pb²⁺,the percentage of ^IVZn on birnessite decreases while percentage of ^VIZn keeps almost constant,indicating that the adsorbed Pb²⁺can also replace a large amount of ^IVZn.However,based on that ^IVZn-TCS is more stable than ^VIZn-TCS,it suggests that part of remaining ^IVZn-TCS on birnessite is transformed to ^VIZn-TCS immediately when ^VIZn-TCS is replaced by H⁺or Pb²⁺.In addition,it is also possible that the change of structure and composition of birnessite is due to the distribution of different Zn complexes.These results provide new insights into the migration behavior of Zn²⁺on nano-minerals.4.Manganite samples?M1 and M24?were synthesized by different aging time?1h and 24h?.M1 sample had characteristics of weak crystallinity,large SSA,small particle size,low thermal stability and low zero charge point in comparison with M24.At same Zn²⁺concentration,the small size sample of M1 had higher adsorption density and lower desorption rate for Zn²⁺than big size sample of M24.Although the content of Zn²⁺had a great influence on the adsorption density,had no significant effect on the desorption rate.During the adsorption and desorption,the Zn coordinations had different changes for M1 and M24.As increasing the content of Zn and decreasing the particles size,the adsorption results indicated that the content of four-fold coordination Zn increased;and the desorption results indicated the content of four-fold coordination Zn with corner-sharing was gradually increasing.