Chemical and morphological modification of complex manganese oxides with different sizes of structural tunnels

Abstract

The methods for the synthesis of protonated forms of complex manganese oxides Mg x Mn 4 O 8 •H 2 O and Ba 6 Mn 24 O 48 (1h and 2h, respectively) in the form of whisker crystals possessing tunnel crystal structures of different sizes were optimized. The microstructural features and physicochemical properties of the synthesized materials were studied. The treatment with nitric acid leads to the formation of the Mn-O-H hydroxy groups due to the insertion of protons into the structural tunnels. The protonated forms of the 1h and 2h phases are active in ion exchange processes in aqueous solutions, the tunnel size being the determining factor for the sorption of heavy metal cations.At present, researchers are greatly interested in inor ganic ion exchange materials with the tunnel structure, because they possess thermal, chemical, and radiation stability and, in some cases, unique sorption selectivity with respect to heavy metal cations. 1-5 One of the repre sentatives of similar materials, which are important from the viewpoint of practical use, is the family of com plex manganese oxides and their protonated forms. For instance, the size of tunnels in todorokite Mg x Mn 4 O 8 •xH 2 O (1) is record large among the tun nel manganese oxides: ~10 Å. This predetermines its unique characteristics as a sorbent for nuclear waste dis posal 2,6 and a catalyst for heterophase reactions ("mo lecular sieves"). 7-10 The Ba 6 Mn 24 O 48 phase (2) is characterized by the composite tunnel structure. 11,12The protonated form of this material manifests the prop erties of a one dimensional ionic conductor and can be used as a cathodic material 13,14 in chemical current sources.In this work, we optimized the methods for the syn thesis of the protonated forms of phases 1 and 2 (as whisker crystals) possessing diverse tunnel sizes. The microstructural peculiarities and physicochemical prop erties of the synthesized materials were studied in com parison in context of sorption activity with respect to a series of toxic heavy metal cations. ExperimentalTo synthesize the starting Mg todorokite (1), Na birnessite, pre pared by an earlier described procedure, 15 was stirred for 12 h with a 1 M solution of Mg(NO 3 ) 2 . The Mg buserite that formed was sub jected to the hydrothermal treatment at 160 °C for 24 h followed by cooling down to ~20 °C. The final reaction product was crystalline compound 1. H Todorokite (1h) was prepared by keeping a suspension of compound 1 in a 1.0 M solution of HNO 3 for 48 h with magnetic stirring. To carry out the ion exchange with heavy metal cations, compound 1h was placed in 0.33 M solutions of CsNO 3 , Pb(NO 3 ) 2 , or Ba(NO 3 ) 2 , and the suspensions were stirred for 5-10 days at ~20 °C.Whisker crystals (whiskers) of Ba 6 Mn 24 O 48 (2) were grown according to a previously described procedure. 16 The H form of the whiskers (2h) was prepared by storing the whiskers in concentrated HNO 3 , and the duration of the action at ~20 °C was 1-10 days. To study the sorbability of compound 2h, 1 М solutions of Ag I , Zn II...

Efficient removal of Pb(ii) ions using manganese oxides: the role of crystal structure

“…Luo et al 16 studied adsorptive removal of lithium by 3D-MnO 2 from wastewater and found that 3D-MnO 2 with high degree of porosity and large pores exhibited a higher adsorption capacity than 1D-MnO 2 . Kozlova et al 17 prepared complex manganese oxides with different sizes of structural tunnels, and concluded that the tunnel size was the determining factor for the sorption of heavy metal cations. In addition, Korshin et al 18 and Abdullah et al 19 found that heavy metal ions could be complexed at different adsorption sites on birnessite and a-MnO 2 using EXAFS spectroscopy.…”