水铁矿的结构、组成及环境地球化学行为

水铁矿广泛分布于水、土壤、沉积物和生物体中，它具有大比表面积和高表面活性，控制和影响着环境中某些污染物质和营养元素的形态、迁移和转化。此外，它也是环境中其他晶质铁(氢)氧化物形成的前驱体。然而，人们对水铁矿在环境中的作用和重要性的认识还存在不足，主要表现在：(1)水铁矿通常被混淆为无定形铁氢氧化物或水合铁氧化物，环境中的水铁矿因难以分离和进行定量分析而使其含量常被低估；(2)水铁矿结晶弱，结构表征困难，对其在环境中的分布、组成、结构及性质的认识较欠缺。水铁矿以弱晶质的纳米颗粒形式存在，其XRD(X射线衍射)线少且宽、强度弱。根据衍射线的条数可将水铁矿分为结晶较弱的2 线水铁矿(2LFh)和结晶较好的6 线水铁矿(6LFh)。目前，普遍接受的水铁矿化学式为Fe5HO8·4H2O，但大量实验表明H2O不是水铁矿结构的必要成分，且容易被吸附的外来物质取代。有人提出水铁矿是一种二元结构，中心为八面体配位Fe，表面以四面体配位Fe为主，这有待论证。最近，通过PDF(配对分布函数)研究水铁矿的结构表明，理想水铁矿结构包含20%四面体配位Fe和80%八面体配位Fe，化学式为Fe10O14(OH)2。然而, 不少学者对这一新的水铁矿结构模型提出了质疑。对近年来水铁矿的分布、组成、结构和环境行为方面的研究和存在的问题进行了综述和讨论，并且对水铁矿的未来研究方向进行了展望。

The structure and composition of ferrihydrite and its environmental geochemical behaviors

Ferrihydrite occurs widely in nature,such as in water body,soils,sediments and organisms.It has large surface area and high surface activity,controlling and influencing the morphology,migration and transformation of some contaminations and nutrient elements in the environment.It is also the important precursor form of many crystalloid iron(hydr)oxides.However,environmental geochemical behaviors of ferrihydrite and its implications for environments are not fully understood,which mainly presents in following aspects:(1)Ferrihydrite is always confused with amorphous iron(hydr)oxides or hydrate iron oxides and it is difficult to obtain experimental ferrihydrite samples in the environment and make quantitative analyses for them,whose contents are always underestimated.(2)Ferrihydrite is poorly crystallized,which makes the determination of its structure more difficult.The distribution,composition,structure and nature are not yet very clear.The XRD diffraction lines of ferrihydrite are wide,few and weak in intensity.Based on the number of diffraction lines,ferrihydrite can be classified into 2LFh with poorer crystallization and 6LFh with better crystallization.At present,generally accepted chemical formula of ferrihydrite is Fe5HO8·4H2O.However,lots of experiments have shown that H2O is not necessarily in its structure and it is easy to be replaced by external adsorbed matters.Some people think that ferrihydrite has a dualistic structure,i.e.with both octahedral coordination of its central Fe and tetrahedral coordination of surface Fe,which needs to be proved.Recently,structure study with use of PDF(Pair Distribution Function) shows that ideal ferrihydrite structure contents 20% tetrahedral coordination Fe and 80% octahedral coordination Fe and the chemical formula is Fe10O14(OH)2.However,quite a few researchers are doubtful of this new structure model.In this paper,the distribution,composition,structure and environmental geochemical behaviors of ferrihydrite are reviewed.The prospective research areas on ferrihydrite are also proposed.