Study On Microcosmic Mechanism Of Pb²⁺ Adsorption On Birnessite

Lead is an environmental heavy metal that has always attracted extensive attention.An exploration of the mechanisms that determine lead and soil component interactions is crucial to understand the fate of lead in soils,sediments,and water.A greater understanding of Pb interaction mechanisms could also aid in water purification and lead-contaminated soil remediation efforts.In this study,two series of birnessites with ranges in Mn average oxidation state(AOS) were synthesized.The chemistry states of Mn and O on the surface of birnessites,the relation between d₁₁₀-spacing,maximum Pb²⁺ adsorption and the AOS in the birnessites before and after treatments with Na₄P₂O₇,Zn²⁺ or Mn²⁺ solution,characterization of Mn²⁺,H⁺ and K⁺ release from birnessites during Pb²⁺ adsorption,and complex configuration of Pb²⁺ adsorbed on the birnessites with different AOS were analyzed by XRD,TEM,HRTEM,IR,XPS,XAFS,BET.The effect of variance of birnessite substructure on the amount of Pb²⁺ adsorbed was discussed.1.One birnessite series was prepared in acidic media(49.6%to 53.6%Mn) and the other in alkaline media(50.0%to 56.2%Mn).The correlations of synthetic birnessite Pb²⁺ adsorption capacity to d(110)-interplanar spacing,AOS by titration,and release of Mn²⁺, H⁺,and K⁺ during Pb²⁺ adsorption were investigated.Maximum Pb²⁺ adsorption to the birnessites synthesized in acidic media ranged from 1320 to 2457 mmol/kg with AOS values that ranged from 3.67 to 3.92.For birnessites synthesized in alkaline media, maximum Pb²⁺ adsorption ranged from 524 to 1814 mmol/kg and AOS values ranged from 3.49 to 3.89.Birnessite AOS values and Pb²⁺ adsorption increased as Mn content decreased.Maximum Pb²⁺ adsorption to the synthetic birnessites calculated from a Langmuir fit of the Pb adsorption data was linearly related to AOS.Birnessite AOS was positively correlated to Pb²⁺ adsorption,but negatively correlated to d(110) spacing. Vacant Mn structural sites in birnessite increased with AOS and resulted in greater Pb²⁺ adsorption,leading to the increase of the total amount of Mn²⁺,H⁺ and K⁺ released,and the increased likelihood for two Pb²⁺ adsorbed in the region of one side of a vacant site. Birnessite AOS values apparently reflect the quantity of vacant sites that largely account for Pb²⁺ adsorption.Therefore,birnessite Pb²⁺ adsorption capacity was largely determined by Mn site vacancies,from which Mn²⁺,H⁺,and K⁺ released during adsorption were mostly derived.2.The investigations on birnessites with different AOS by XPS indicated that there were two chemistry states of Mn,staturated and undersaturated coordinately Mn,whose relative contents were 83.79%-91.69%and 8.31%-17.21%in the structure,respectively. -OH located at vacant Mn octahedral site increased with the increase of AOS in birnessite accounting for the aggregation of crystal grains,which led to the decrease in relative content of undersaturated coordinately Mn.There were three chemistry states of oxygen, lattice oxygen,hydroxide and H₂O,whose relative contents were 50.44%-65.05%, 24.90%-39.27%and 8.07%-12.63%in the structure,respectively.-OH located at vacant Mn octahedral site increased with the increase of AOS in birnessite,which resulted in the increase in relative content of chemistry state of hydroxide in the structure.Structure models of birnessites were presumed,and the distribution of vacant sites in the Mn³⁺-rich MnO₆ rows was discussed,which provided fundamental for further study on the substructure of birnessite.3.The structural features of birnessites with different Mn average oxidation state(AOS) before and after Pb²⁺ adsorption were characterized by fourier transform infrared(FTIR), and the adsorbent band centres of FTIR were determined by second derivatives of these spectra.The band at 899-920 cm^-1 was assigned to the bending vibration of -OH located at vacant Mn octahedral sites.The lower AOS in birnessite,the more Mn⁴⁺ coordinated to -OH located at vacant Mn octahedral sites displaced by Mn³⁺,which led to the position of -OH bending vibration shifting to a relative low wave number.The bands at 1059-1070, 1115-1124 and 1165-1171 cm^-1 were assigned to the vibration of Mn³⁺-OH located in the layer,and the intensity of these bands increased with the decrease of AOS in birnessite. The bands at 990 and 1023-1027 cm^-1 were assigned to the vibration of Mn³⁺-OH located in the interlayer.A partial of Mn³⁺ in the layer migrated to the interlayer during the Pb²⁺ adsorption inducing the intensity of the band at 990 cm^-1 to increase and its position shifting to a relative high wave number.The band at 564-567cm^-1 was assigned to the vibration of Mn-O located at vacant Mn octahedral sites,and would split due to the effect of coupling on vibration when Pb²⁺ or Mn³⁺ was adsorbed above or below vacant sites. The space between the bands at 610-626 and 638-659 cm^-1 reflected the symmetry of Mn³⁺ distribution in the birnessite,and increased after Pb²⁺ was adsorbed on the birnessite.4.Acid birnessite was treated with Na₄P₂O₇ at pH 2,4,5 respectively.After the treatments, the species and content of manganese ion in the complex solution,and the variation of average oxidation state(AOS) of Mn in birnessite,and the amount of Pb²⁺ adsorbed and Mn²⁺,H⁺ released during the Pb adsorption were investigated.The results indicated that after acid birnessite(AOS=3.67) was treated by Na₄P₂O₇ at different pH,Mn³⁺ located in the layer-structure-edge and partial of Mn³⁺ located in the interlayer were released to the solution through complexation with Na₄P₂O₇.The Mn AOS of birnessites after treatments were increased to 3.78(pH 2),3.78(pH 4),3.82(pH 5) respectively.While the crystal structure of birnessite didn't change after treatments,the amount of Mn³⁺ located above or below vacant sites decreased,and the amount of H⁺ located above or below vacant sites went up in the structure.The amount of vacant sites responsible for Pb²⁺ adsorption increased,which led to the increase of the maximum amount of Pb²⁺ adsorbed. Additional,the distribution of Mn³⁺ in the strucuture of acid birnessite was deduced. About one sixths of Mn³⁺ located in layer-structure-edge,and five sixths of Mn³⁺ located in the interlayer and non-layer-structure-edge.5.The association of vacant Mn octahedral sites with Pb²⁺ adsorption was deeply elucidated from the variances of AOS,d(110)-interplanar spacing,maximum Pb²⁺ adsorption,maximum Zn²⁺ and Mn²⁺ release during the Pb²⁺ adsorption for the birnessites before and after treatments.The AOS and d(110)-interplanar spacing of the birnessites remained almost unchanged as the concentration of the treating Zn²⁺ increased,indicative of unchanged amount of vacant Mn octahedral sites.Whereas maximum Pb²⁺ adsorption decreased from 3190 to 2030 mmol/kg due to occupancy of the treating Zn²⁺ on adsorption sites.However,the AOS of the Mn²⁺-treated birnessites decreased,and most of the treating Mn²⁺ were oxidized to Mn³⁺ and located below or above vacant Mn octahedral sites or migrated into vacant Mn octahedral sites,when the concentration of the treating Mn²⁺ increased from 1 to 2.4 mM.The d(110)-interplanar spacings of the treated birnessites were found to increase from 0.14160 to 0.14196 nm,indicative of the decrease in the amount of vacant Mn octahedral sites,mainly due to the increase of the produced Mn³⁺ migrating into vacant Mn octahedral sites.Moreover,the maximum Pb²⁺ adsorption of the Mn²⁺-treated birnessites was observed to decrease from 3190 to 1332 mmol/kg.The results suggest that birnesstie Pb²⁺ adsorption capacity is largely determined by the number of Mn site vacancies.6.The investigations on birnessites with Pb²⁺ by XAFS indicated that two Pb-O and two Pb-Mn shells were found in the birnessites after Pb²⁺ adsorption.For the same birnessite, at low amount of adsorption,Pb²⁺ coordination numbers were 2.7 and 9.1,respectively,in the two Pb-O shells at distances of 0.226 and 0.397 nm,respectively,and Pb²⁺ coordination numbers were 2.8 and 6.6,respectively,in the two Pb-Mn shells at distances of 0.356 and 0.375 nm,respectively.At high amount of adsorption,in the two Pb-O shells, Pb²⁺ coordination numbers decreased to 1.5 and 3.4,respectively,the distances between Pb and O increased to 0.227 and 0.398 nm,respectively,and in the two Pb-Mn shells, Pb²⁺ coordination numbers decreased to 1.2 and 3.6,respectively,the distances between Pb and Mn increased to 0.357 and 0.376 nm,respectively on account of the distortion of the Pb²⁺ coordination environment.The same three bonding mechanisms in Pb²⁺ adsorption on the bimessites with different AOS:a single-corner-sharing complex on particle edges along a axis,a double-corner-sharing complex on particle edges along b axis,and a triple-corner-sharing complex in the interlayer above or below vacant sites. The amount of Pb²⁺ adsorbed increased with the increase in AOS of birnessite,which led to the decrease of Pb²⁺ coordination numbers and the increase of the distances between Pb and O in Pb-O and Pb-Mn shells due to the distortion of the Pb²⁺ coordination environment.