M88体育-明升M88体育讯(通讯员 严欣然）近日，资源与环境学院土壤矿物与环境化学团队殷辉副教授在极端气候下土壤矿物调控重金属环境行为研究方面取得新进展，相关成果以“Effect and fate of Ni during aging and thermal-induced phyllomanganate-to-tectomanganate transformation”为题在Geochimica et Cosmochimica Acta发表。研究成果揭示了在长期干旱和高温等引发野火的极端环境下，锰氧化物调控营养元素和重金属污染物Ni的赋存形式及其移动性，为气候变化背景下金属元素的环境行为提供了新认识。
人为活动导致全球气候变化加剧，干旱、高温等极端气候事件频率增加，亦导致全球森林大火灾害频发。在这些极端环境条件下，关于表生环境中活性矿物的结构和物理化学特性是否将发生改变，进而影响与之结合的重金属的地球化学行为的研究较为缺乏。
基于此，团队以表生环境中普遍存在的层状结构氧化锰矿物—水钠锰矿和金属元素Ni为研究对象，探究了在长期干旱 (8 年)和模拟山火产生的高温条件(500 摄氏度)下水钠锰矿物相的转变、Ni赋存形式及Ni在模拟溶解条件下的释放特征和规律。研究结果表明，水钠锰矿在室温干燥状态下老化8年后，矿物结构、Mn平均氧化态和Ni赋存形式(以同晶替代形式存在于水钠锰矿层内和吸附在矿物表面活性位点的比例)未发生明显变化。
图1 老化8年水钠锰矿样品(Ni10_8y)在不同温度加热后所得样品Ni K边 3次权重扩展X射线吸收精细结构光谱(EXAFS, A) 和对应傅里叶变化谱(FTs, B)。
图2 所得含Ni锰钾矿的球差电镜原子分布图
图3模拟酸还原溶解过程中，不同温度Ni10_8y（a）、200 摄氏度（b）、500 摄氏度（c）处理样品中Mn和Ni释放动力学及相应的溶解分数(χNi-χMn)曲线（d）。
相比之下，在水钠锰矿加热至200 摄氏度过程中，共边Ni-Ni(Mn)配位的比例随温度升高而增加；进一步加热到400 摄氏度不会改变这一比例，且矿物仍保持水钠锰矿层状结构 (图1)。在500 摄氏度时，水钠锰矿转化为锰钾矿，且原子级分辨球差电镜表明Ni进入锰钾矿骨架 (图2)。与初始水钠锰矿相比，进入锰钾矿骨架的Ni稳定性极大增加，在酸还原溶解过程中释放速率降低了~400倍 (图3)。这些结果揭示了在长期干旱和大火产生高温等极端环境条件下氧化锰矿物调控Ni等金属元素环境行为的过程和机制，为深入认识土壤等环境中金属元素的环境地球化学行为提供了新认识。
我校资源与环境学院殷辉副教授为论文第一作者，刘凡教授和殷辉副教授为论文共同通讯作者。该研究得到国家自然科学基金和M88体育-明升M88体育自主科技创新基金等项目资助。
审核人： 殷辉
【英文摘要】
Phyllomanganates are ubiquitous in a variety of environments and commonly enriched in transition metal elements, such as Ni. The effect of such foreign metal cations on phyllomanganate transformation is widely documented under aqueous conditions together with the induced modification of Ni geochemical behavior. A similar knowledge is lacking however on phyllomanganate transformation and on the induced fate of associated metal elements that may occur under dry conditions, that prevail in deserts and arid areas increasingly exposed to severe droughts or wildfires. The present study shows that crystallinity, morphology, Mn oxidation state, and Ni binding mechanisms are essentially unaffected when aging hexagonal birnessite (Mn oxidation state ~ 3.90 and Ni/Mn molar ratios of 0.00 and 0.13) in the dry state at room temperature for up to 8 years. In contrast, heating aged Ni-doped birnessite to 25–200 oC results in an increased proportion of edge-sharing Ni-Ni (Mn) pairs with increasing temperature induced by the migration of interlayer Ni to birnessite octahedral layers and/or by an increased sharing of coordination oxygens by interlayer Ni/Mn from adjacent layers. Further heating to 400 oC does not change this proportion, with birnessite layer structure being retained. Transformation of Ni-doped birnessite to cryptomelane is complete at 500 oC, while that of Ni-free birnessite is achieved at 400 oC, suggesting that Ni doping increases birnessite thermal stability. Birnessite-to-cryptomelane transformation comes with a strong increase of Mn oxidation state, whereas this parameter remains unchanged in heated birnessite samples. Ni incorporation in the cryptomelane framework, reduces its release during reductive acid dissolution by a factor of 396 ± 15 compared to initial birnessite. These results shed light on mineral transformation affecting layered manganates under dry conditions and on the fate of associated transition metal elements.
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