The Substructural Variation Of Birnessite Caused By Different Treatments And Its Influences On The Adsorption Of Pb/Cd

In this paper, bimessites were synthesized with the different AOS (denoted as H series), the comparative analyses were carried out among them in term of total quantity of different positive ions released during Pb²⁺and Cd²⁺ absorption. Two series of modified bimessites (denoted as M series and Z series) were prepared by treating the birnessite (HB0, has a Mn AOS of 4.0) with adsorption of different amount of Mn²⁺ and Zn²⁺ ion. X-ray diffraction spectrum (XRD), Infrared spectrophotometre (IR), Rapid potentiometric titration (RPT) and BET-N2 analyses were used to characterize the structural and surface properties of the manganese oxide. At the same time the structure characteristic, AOS, superficial electric charge, as well as Pb²⁺ adsorption characteristic were also investigated.The results in this study were listed as follows:1. Isothermal adsorption of Pb/Cd with H series birnissites(1) Mn AOS for H series bimessites are in the range of 3.67 to 4.0, (PZC) are from 1.3to 2.3. Their surface area increased with the decrease of AOS. X-ray diffraction (XRD) analysis indicated that they were all single phased bimessite .(2) For the H series bimessites, Pb²⁺ adsorption capacity was larger than that of Cd²⁺, It was found that that the max adsorption amount of both Pb²⁺ and Cd²⁺ was increased with increase of AOS.(3) There was a significant positive correlation between Pb²⁺ adsorption capacity and the total amoumt of released Mn²⁺, H⁺and K⁺.2. Pb adsorption with modified bimessites of M and Z series(1) X-ray diffraction (XRD) and infrared (IR) analysis indicated that both M and Z series bimessites were single phased bimessite. Transmission Electron Micro scope (TEM) indicate the morphology of bimessite had no obvious change after a dsorbing Mn²⁺and Zn²⁺.(2) AOS of M series bimessites was in the range of 3.75 to 3.85, decreasing with increase of Mn²⁺ added. There was no obvious difference in AOS for Z series after being treated.(3) PZCs or M series bimessites were in the range of 1.41 to 2.35, and it was from 3.4 to 3.7 for Z series birnesssites. The values were all larger than that of HBO without treatment, and the increments of Z seriers bimessites was much larger than that of M series birnessites. It was suggested that the surface variable charge were increased with the increase of Mn²⁺and Zn²⁺ adsorbed by birnessite, and the variable charge of Z series birnessites increased much more than that of M series birnessites.(4)As for M series birnessites, surface area were increased to 26.2, 27.6, 39.0, 44.5, 50.3,56.7,67.0 (m²/g), respectively with the increase of the adding Mn²⁺.(5)To M series birnessites, d₁₁₀ spacing had a negative correlation with their Mn AOS, which indicated that partial Mn²⁺ possibly entered into the octahedral vacancies and filled vacant site or Mn²⁺and Mn⁴⁺ had redox reaction to produce a couple of Mn³⁺ ions, which entered into the vacancies and filled the vacant sites.(6)As for M series the adsorption amount of Pb²⁺ was increased with the increase of AOS. The Pb²⁺ adsorption quantity of Z series birnessites was much greater than that of M series birnessites when the Pb solution of the same concentration was used. This order is contrary with the order of competitive adsorption abilities of Mn²⁺ and Zn²⁺, which implied that the AOS of M series birnessites decreased after treatment, and the quantity of vacant sites was reduced. Therefore the adsorption amount of Pb²⁺ decreased.For Z series birnessites, the total amount of Pb²⁺ adsorption and Zn ²⁺ retention (taking out the released Zn²⁺ of the added Zn) was close to Pb adsorption amount of HBO. For M series birnessites, the sum amount of adsorbed Pb ²⁺ and remained Mn ²⁺ (taking out Mn²⁺release of the added Mn²⁺ ) was less than Pb²⁺ adsorption of HBO. During the Pb²⁺ adsorption of HBO, there was no Mn²⁺ releasing, but the amount the released Mn²⁺ was increased with the decrease of AOS during the Pb adsorption for M series, but the amount of released Mn²⁺ were much less than that of Mn²⁺ involving in the adsorption reaction. This also indicated that a part of Mn²⁺ ions were adsorbed, the other parts might enter into crystal lattice, and filled vacant sites, forming MnO₆ octahedra. The variation of this kind of sub-structure for birnessite, not only the reduction of the AOS but also decrease of the amount of octahedral vacant sites, resulting in the decrease of the amount of adsorbed Pb²⁺.