Factors Governing The Chemical Formation Of Todorokite At Atmospheric Pressure

Todorokite is a common manganese oxide in soils,sediments and other Earth surface environments.However,the formation of todorokite in Earth surface environments has little been documented,and the related geochemistry experiments have seldom been carried out.In this work,the factors governing formation of todorokite at atmospheric pressure,such as type and amount of interlayer cation of buserite precursor,the amount of Mn(â…¢) in the structure of layered precursor,the average manganese oxidation state and crystallinities of the precursors,reaction temperature,pH and clay minerals,were investigated by using XRD,TEM/ED, HRTEM,SEM/EDS,IR,XPS TGA,BET and chemical analysis,and the realtionship between migration of Mn(â…¢) in the structure of layered precursor and formation of tunnel structure,interaction of interlayer cation and MnO₆ layers of precursor were discussed.1.The type and amount of interlayer cation of buserite precursor obviously affected the formation of todorokite at atmospheric pressure.Weak bonding between interlayer cation and MnO₆ layer of buserite was favorable to todorokite formation.More Na-buserite would transform to crystal todorokite gradually with the increase of interlayer Cu²⁺ and Co²⁺,but no todorokite could be obtained when over 4 mmol of Cu²⁺ and Co²⁺ were added.The transformation of buserite to todorokite depended on the form and amount of the exchangeable cations and their bonding sites in the interlayer of buserite.The hydration rate of Cu²⁺ or Co²⁺ and occupation of interlayer sites above or below layer vacancies sites in the interlayer varied with the amount of added Cu²⁺ or Co²⁺.When the hydrated form of Cu²⁺ or Co²⁺ is dominant,the buserites can convert into todorokites,and the formation rate of todorokite would be promoted with the increase of percentage of hydrated form.While most of exchangeable Cu²⁺ or Co²⁺ located above or below octahedral layer vacancies sites,they can be bound with MnO₆ layers tightly by Cu-O or Co-O bond,and then the transformation from layer to tunnel structure would be interrupted.2.The pH in the system slightly influenced the todorokite formation.Todorokite can be prepared within pH5-9,but the rate of conversion and crystallinity of todorokite followed this sequence:neutrality^alkali＞acidity.Cu-todorokite was successfully synthesized at atmospheric pressure by refluxing treatment of Cu -buserites,which were preparcd using Na-buserite ion-exchanging with Cu²⁺ by controlling the pH value of reaction solution.TEM and HRTEM images reveal that this material has a needle-phase crystal with the thickness 0.1-1 um,and the spacing of the lattice fringes is 1.0 nm,corresponding to the[100]planes of the todorokite structure.Such morphological and intergrowth characteristics were similar to those of hydrothermally synthesized todorokites.The Cu-todorokite with a chemical composition of Cu_0.34MnO_2.19·1.11H₂O,was stable below 400℃.The BET surface area was found to be 62.5 m²/g,and a major micropore size distribution peak centered at 0.70 nm for Cu-todorokite by the Horvath-Kawazoe(HK) method.3.Pyrophosphate,which is known to form strong complexes with Mn(â…¢) at a pH range of 1-8,was added to the suspension of Na-buserite in order to sequester available Mn(â…¢) of Na-buserite.Thus the Mn(â…¢) availability is critical on the transformation from layered to tunnel structure of manganese oxide at atmospheric pressure. Significant transformation reductions were observed for pyrophosphate concentrations between 0 and 0.05 mol/L at pH7.The Na-buserite treated with 0.10 mol/L pyrophosphate at pH7,was hardly converted into todorokite at atmospheric pressure. The transformation from Na-buserite to todorokite decreased gradually with the prolongation of treatment time from 1 to 24 h when treated with 0.05 mol/L pyrophosphate solution at pH7.4.The transformation from the birnessites to todorokite at atmospheric pressure indicated that the formation of todorokite decreased with the increase of AOS of Na(K)-birnessites and Na-birnessites in the range of 3.51-3.80,and no todorokite would be obtained when using the Na-birnessite with AOS of 3.87 as precursor.When the Na(K)-birnessites were treated with 0.01 mol/L Na₄P₂O₇ solution at pH7,Mn(â…¢) migrated from MnO₆ layers and formed complexes with pyrophosphate in the solutions, and the transformation from birnessite to todorokite was restricted,and the Na(K)-birnessite with higher AOS was more difficult to convert into todorokite.No Na(K)-birnessite could transform to todorokite when they were treated with 0.02 mol/L Na₄P₂O₇ solution at pH7.The interlayer K⁺ of initial Na(K)-birnessites could not be completely ion-exchanged with Mg²⁺,which is unavailable to the formation of todorokite at atmospheric pressure.When the K-birnessites were used to prepare todorokite,no todorokite could be obtained even if the AOS of K-birnessite is low. 5.Todorokite could be obtained by treating Mg-buserite in range of 100-40℃at atmospheric pressure.However,the conversion rate of todorokite decreased with the temperature falling,and Mg-buserite could not be completely transformed to todorokite at 40℃.The poor crystalline precursor could be converted into todorokite more easily than high crystalline precursor.The crystallinity of synthetic todorokite could be further enhanced with the increase of reaction temperature for high crystalline precursor,but almost no obvious change to that of products of poor crystalline precursor.6.The aged precursors more easily convert into todorokite than the unaged one at atmospheric pressure.Montmorillonite or goethite could slow down the formation reaction of todorokite in the refluxing process,and the reaction time would be prolonged with an increase of them.7.Layered manganese oxides-doped with different amounts of Co have been synthesized,and Co could be doped into the framework of the precursor by substitute the Mn(â…¢) of MnO₆ layer of the precursor as a Co(â…¢) form.These Co-doped precursors could transform to todorokite under refluxing condition after ion-exhanging with Mg²⁺.The doped precursors with small amount of Co,such as 5Co-Na-bus and 10 Co-Na-bus,could not convert into todorokite under refluxing condition after ion-exhanging with Co²⁺,but the 20Co-Na-bus sampe could completely transform to todorokite under refluxing condition.The synthetic Co-todorokite with a chemical composition of Co_0.70MnO_2.19·0.76H₂O,was stable above 400℃.The TEM images revealed that this material consist of plate-phase with the size of 0.5 -1 um and needle-phase crystal with the thickness 0.1-1 um.