含Ni六方水钠锰矿的表征及其对Pb 2+ (Zn2+)环境行为的影响

六方水钠锰矿是土壤中普遍存在、活性最强的氧化锰矿物。它常常富集各种过渡金属如Ni等，对其地球化学行为具有重要影响。在六方水钠锰矿形成过程中加入Ni²⁺，Ni以+2价存在于矿物中。进入水钠锰矿结构中的Ni大部分以NiO₆ ]八面体形式存在于层内；仅有小部分Ni存在于八面体空位上下方。含Ni水钠锰矿沿c轴方向堆叠锰氧八面体层数逐渐减小，而a-b板面晶体大小没有明显变化，即层片状晶体逐渐变薄，比表面积显著增大。随着Ni含量的增加，水钠锰矿结构中锰氧八面体空位数减少，而层边面吸附位点数基本保持不变，其对重金属离子(Pb²⁺/Zn²⁺)吸附去除能力逐渐降低。本文为明确过渡金属离子(Ni)对土壤中氧化锰矿物的形貌、结构及其性质的影响提供了参考。

Characterization of Ni-rich hexagonal birnessite and its effect on behavior of aqueous Pb2+(Zn2+) in environment

Powder X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) techniques were used to investigate variations of the structures of hexagonal birnessites as related to average oxidation states of manganese therein (Mn AOS). As the synthesized series of hexagonal birnessites from HB1 to HB6 decrease from 3.92 to 3.67 in Mn AOS, but increase from 2.838 ? to 2.848 ? in unit cell parameter b. The series also decrease from 12.0 nm to 7.0 nm in the coherent scattering domains (CSD) in the a -b layer plane, forming a stack of 10.6~13.4 layers of manganese oxide octahedron along axis c. With the decrease in Mn AOS, the content of octahedral voids declines from 18% to 8%. Mn AOS and vacancy content have a very significant positive linear relationship, and they both show negatively linear relationships with unit cell parameter b. Linear fitting of X-ray absorption near edge structure spectroscopy (XANES) shows that Mn in these samples is dominated by Mn⁴⁺ and the proportion of Mn^2+/3+ increases with the decreasing content of octahedral voids. Fitting analysis of Mn K -edge extended X-ray absorption fine structure spectroscopy (EXAFS) with a full multiple scattering model demonstrates that the samples, though different in Mn AOS, are similar in crystal structure and Mn local coordination environment. With the decrease in content of octahedral voids and particle size, apparent Mn site occupancy (f_occ) decrease from 0.74 for HB1 to 0.66 for HB6, and with the decrease in Mn AOS, the average bond-length of Mn-Mn(O) shells increase.