【作者】 张嵚；

【导师】 刘凡；

【作者基本信息】 华中农业大学 ， 土壤学， 2011， 博士

【摘要】 作为土壤的主要组成物质——土壤矿物,对土壤的物理性质、化学性质以及生物与生物化学性质均有深刻的影响。对土壤中矿物的形成、转化及其性质的研究是土壤矿物学研究的主要内容,此外土壤矿物的性质对环境中物质的循环和元素的地球化学过程也起着十分重要的影响。带有表面电荷、含有变价元素的氧化锰矿物是土壤中潜在的吸附剂、氧化剂和催化剂,它能吸附土壤中的重金属离子,催化氧化土壤中的变价元素和还原性的酚类化合物,从而改变这些物质形态和毒性。因此,开展氧化锰矿物的合成、转化及性质研究,对深入了解和认识土壤中氧化锰矿物的资源属性和环境属性,促进氧化锰矿物资源的开发与利用具有重要的理论和实践意义。本文研究了酸性条件下水钠锰矿形成的影响因素和反应机制,并以合成的水钠锰矿为前驱物,采用X-射线衍射(XRD)、透射电镜(TEM/SAED)、扫描电镜(SEM)、傅里叶红外光谱(FTIR)等测试技术,研究了水钠锰矿钾含量、锰氧化度、不同晶系、溶液pH、溶液中K+和Mn2+离子浓度和反应温度等对常压下对水钠锰矿转化的影响及其机制。在此基础上,提出了一步合成锰钾矿法并研究了反应中酸的浓度和类型对锰钾矿晶体大小的影响及其反应途径。此外,以苯酚作为研究对象,探讨了土壤中不同晶体结构类型氧化锰矿物光催化降解苯酚的特点及其反应机制,以期对自然环境中氧化锰矿物的环境光化学行为有一个详细的了解。本论文的主要研究内容、研究成果及创新点如下：1.不同体积和不同浓度盐酸与高锰酸钾反应均能得到单一矿相的酸性水钠锰矿,SEM下它们的晶体形貌相似,但化学分析表明它们的钾含量和锰氧化度(AOSMn)不同。通过对KCl、HCl与高锰酸钾反应产物以及高锰酸钾自身分解的研究表明,H+是反应体系中生成水钠锰矿的重要因素。四种其它类型的酸(硝酸、硫酸、高氯酸和乙酸)与高锰酸钾反应也均得到单一矿相的水钠锰矿,但SEM图片表明它们的晶体相貌和晶粒大小明显不同,其晶体大小顺序为HNO3>HCl≈HClO4>CH3COOH>H2SO4。酸性介质中,当体系中无还原剂(高氯酸、硝酸和硫酸体系)时,水钠锰矿主要通过MnO4-自身分解生成；当还原剂存在时(盐酸和乙酸),水钠锰矿通过MnO4-自身分解和被还原剂还原生成水钠锰矿。2.初步探明了回流条件下溶液pH对酸性水钠锰矿转化的影响,并考察了反应过程中pH、K+和Mn2+浓度的变化及产物微观形貌变化。溶液pH决定酸性水钠锰矿的矿相转变,当溶液pH≤5.60时,酸性水钠锰矿回流一定时间可转变为锰钾矿,而当pH≥7.14时,回流7天也不发生矿相转变。矿物转化过程中,pH≥5.60时,溶液中几乎没有Mn2+,而pH较低时(pH 0.83、1.36和2.26),Mn2+浓度随转化时间增加而升高,这主要是由于H+对氧化锰矿物的溶解导致的。矿物转化过程中K+的浓度变化与产物中矿物组成有关,当体系中为酸性水钠锰矿时,溶液中H+与矿物层间的K+交换使得溶液中K的浓度随着回流时间的增加而增加；但当体系中有锰钾矿生成时,锰钾矿的隧道结构吸附K+导致溶液中K浓度迅速降低。通过SEM和TEM观察反应不同时间产物的形貌表明,水钠锰矿向锰钾矿转化过程是由不规则形貌水钠锰矿逐渐向纳米线锰钾矿转变,这种转化是一个酸度控制的溶解-再结晶的过程。溶液pH低有利于加速水钠锰矿溶解,促进了锰钾矿形成。反应体系pH为0.83时,水钠锰矿回流3 h后便完全转化为锰钾矿；当pH上升至2.26时,回流6h水钠锰矿才完全转化；继续增加pH至5.60,矿物完全转化时间增加至24h。3.系统开展了回流条件下水钠锰矿的性质(如不同晶形、钾含量、锰氧化度)以及反应条件(如溶液中K、Mn2+浓度、温度等)对其转化的影响。在本实验体系中,水钠锰矿的钾含量和溶液中的K浓度都不影响水钠锰矿的转化,不同钾含量酸性水钠锰矿或溶液中K+浓度不同时,酸性水钠锰矿均转化为锰钾矿。酸性水钠锰矿的锰氧化度(AOSMn)影响其转变产物的矿相,当AOSMn≥3.83时,水钠锰矿转化为锰钾矿；而当AOSMn为3.67时,最终产物为拉锰矿和锰钾矿的混合物。溶液中Mn2+浓度对矿物转化的影响显著,当Mn2+浓度为0.01 mol/L时,转化产物为拉锰矿和锰钾矿的混合物；当Mn2+浓度大于0.1 mol/L时,转化产物为单一矿相的拉锰矿。反应温度不仅影响酸性水钠锰矿是否发生矿相转变,还影响着转化速率,当温度高于80摄氏度时,随着温度的升高,水钠锰矿向锰钾矿转变的速率增加,而当温度低于60℃时,回流7天矿相仍未发生变化。此外,碱性水钠锰矿和酸性水钠锰矿的晶体结构和形貌差异很大,使得它们在矿物转变规律上完全不同。当pH小于5.60时,酸性水钠锰矿经过一段时间回流均可转化为锰钾矿；而结晶较好的碱性水钠锰矿在pH低于1.25时,经一段时间回流转化为拉锰矿；当pH为2.15时,部分转变为拉锰矿；当pH增加至5.00以上时,其矿物结构不发生变化。4.采用一步回流法合成得到了锰钾矿,结合SEM、TEM技术研究了反应过程并分析酸的类型对产物晶体大小的影响。在回流条件下,高锰酸钾与无机酸溶液反应可以得到单一矿相的锰钾矿,但酸的浓度决定着反应产物的矿相。酸的浓度较低时,产物为酸性水钠锰矿；酸的浓度较高时,产物为锰钾矿。反应体系中锰钾矿通过两步形成：反应初期高锰酸钾与体系中的酸反应先生成水钠锰矿,其后在较高浓度H+的作用下,水钠锰矿通过溶解-重结晶作用转化为锰钾矿。不同类型酸形成锰钾矿的IR光谱与天然锰钾矿相似,且化学组成相近。对TEM图片中锰钾矿纳米线长度的统计分析表明,酸的类型对产物的晶体大小有较显著的影响,晶体大小顺序为：HCl (1104.4 nm)>HNO3 (441.5 nm)>H2SO4 (339.2 nm),这可能与反应过程中锰钾矿表面所带电荷量的大小及其吸附酸根离子所产生的位阻效应有关。5.首次在回流条件下通过改变与高锰酸钾反应的有机酸的碳链长度调控锰钾矿纳米线晶体的横向和纵向尺寸。有机酸烷基碳链长度从1增加至6,锰钾矿晶体的长度由1376 nm逐渐减小至35.64 nm,其宽度则由61.12 nm减小至8.22nm。通过SEM对反应过程中间产物晶体形貌观察表明,其形成途径与无机酸体系相似,这表明酸性环境是锰钾矿形成的一个重要条件。有机酸对锰钾矿晶体生长的影响机制与无机酸体系有所不同,其溶解度、疏水性及其分子大小可能是影响锰钾矿晶体生长的主要原因。研究了四种土壤中常见氧化锰矿物在光照和非光照条件下对苯酚的降解特点。在本反应体系中,无氧化锰矿物存在时,非光照条件下苯酚在通空气条件下基本不挥发；在光照下苯酚能被空气降解,反应12h后,溶液的苯酚降解率和TOC去除率分别为99.4%和12.3%。其降解机制主要是苯酚在紫外光照下发生直接光降解或溶液中的氧气吸收紫外光后生成臭氧等次生氧化剂氧化降解苯酚。暗反应下,四种氧化锰刊物对苯酚的降解作用均较弱,这与苯酚具有苯环结构,稳定性强不容易直接被氧化有关。反应12h后,氧化锰矿物的TOC降解率大小顺序为酸性水钠锰矿(11.5%)>锰钾矿(6.3%)>碱性水钠锰矿(4.6%)>钙锰矿(2.0%)。在光照条件下,供试氧化锰矿物均具有光催化氧化活性,施加光照能显著促进氧化锰矿物对苯酚的降解,且TOC去除率也显著增加。光照反应12h后,TOC降解率大小顺序为：锰钾矿(62.1%)>酸性水钠锰矿(43.1%)>钙锰矿(25.4%)>碱性水钠锰矿(22.5%)。锰钾矿良好的光催化氧化活性与其较大的表面积、较高的氧化度,特别是宽和强的光吸收带谱有关。光照下氧化锰矿物光化学降解苯酚主要存在三种机制：苯酚的直接光解,氧化锰矿物的氧化作用以及光催化作用。其中光催化降解机制起主导作用。更多还原

【Abstract】 As one of the main components of soils, minerals greatly affect the physical properties, chemical properties and biological and biochemical properties of soils, and influence material recycling and the elemental geochemical processes in surface environment. So the formation, transformation and properties of soil mineral is an interesting research area for soil researchers. For containing large surface charge and element with electrovalence change, Mn oxide minerals is kind of strong adsorbent, oxidant and catalyst in soils, and it could adsorb heavy metals, oxidize variable valence elements or reductive organic compounds. Hence, investigations on the syntheses, transformations and properties of them are of great significance to realize and understand the resource properties and environmental behaviors of soil Mn oxide minerals, and it also could promote exploration and utilization of Mn oxide minerals.In this study, syntheses conditions and reaction mechanism of birnessite in acid medium and transformation of birnessite were systematically investigated by using analysis techniques such as XRD, SEM, TEM/SAED, FTIR, ect. On this basis, a simple one-step method was developed to synthesize cryptomelane, and the type and concentration of acid and reaction pathway was also discussed. In addition, in order to have a thorough understanding on the environmental photochemistry behaviors of Mn oxide minerals in natural environment, degradation of phenol by several common Mn oxide minerals in soils with or without light irradiation was also studied.The main contents of dissertation were summarized as follows:1. Acid birnessite can be synthesized by reflux of hydrochloric acid (HCl) with KMnO4. The volume and concentration of hydrochloric acid (HCl) did not affect the phase and micro-morphology of acid birnessite, but affected the AOSMn and potassium content of the products. It is shown that H+ plays a very important role in reaction of KMnO4 with pure water, KCl solution and HCl solution. Four type of acids (HNO3, HClO4, CH3COOH, H2SO4) was used to replace HCl in the reaction, and the products are all acid birnessite. their crystal sizes are in order of HNO3>HCl ≈HCIO4>CH3COOH>H2SO4. The reaction mechanism of acid birnessite formation in different acid media was different. When the acid is not reducing acid (such as HClO4, HNO3 and H2SO4), acid birnessite is formed by decomposition of KMnO4. However, in present of reducing acid such as HCl and CH3COOH, the acid birnessite is formed by two reaction route:decomposition of KMnO4 or reduction of KMnO4 by Cl- or CH3COO-.2. Birnessite transformed to cryptomelane by reflux treatment in acidic medium at atmospheric pressure. The initial pH of suspensions significantly influences the phase transformation. When pH is below 5.60 birnessite transformed to cryptomelane, but no phase transformation is observed when pH is above 7.14. During the phase conversion, the release of Mn2+ is caused by dissolution of Mn oxide minerals in acid medium, but the release of K+ is the result of ion exchange effect of H+ with K+ in the layer of birnessite. The results of SEM, TEM and SAED showed that the growth of nanowires-like cryptomelane firstly occurs besides irregular-shaped birnessite. Then, birnessite completely converted to cryptomelane from the external to internal by a dissolution-recrystallization mechanism. These results will shed light on the phase transformation of manganese oxide and enrich our understanding of the growth mechanisms of relevant nanoparticles. In addition, the initial pH of the synthetic systems greatly affecte both phase conversion rate and the specific area of the produced cryptomelane.3. The factors governing formation of acid birnessite at atmospheric pressure, such as potassium content, type and the average manganese oxidation state (AOSMn) of acid birnessite, reaction temperature and the concentration of Mn2+ and K+ in solution, were investigated. The potassium content in minerals and the concentration of K+ in solution have little influence on the phase transformation, and the acid birnessite can be transform to cryptomelane in a wide range of K+ content and concentration. Reaction temperature not only determines the phase transformation but also impacts the conversion rate. When reaction temperature is below 60℃, the phase does not change. When the reaction temperature is above 80℃, birnessite transforms to cryptomelane and the conversion rate increases with the reaction temperature. The AOSMn in birnessite and the concentration of Mn2+ in solution influence the phase of the final products. When the AOSMn is above 3.83, birnessite transforms to cryptomelane; but when the AOSMn is 3.67, the final product is the mixture of ramsdellite and cryptomelane. If the concentration of Mn2+ is 0.01 mol/L, the final product is also ramsdellite and cryptomelane. However, if the concentration of Mn2+ is higher than 0.1 mol/L, birnessite converses to pure ramsdellite. In addition, different crystal structures of birnessite also affect the phase transformation. When pH is below 1.25, alkaline birnessite transforms to pure ramsdellite rather than cryptomelane. When the pH increases to 2.15, part of birnessite could transform to ramsdellite. If the pH is above 5.00, the phase of birnessite does not change.4. A simple one-step reflux method was developed to synthesize cryptomelane through the reaction of KMnO4 with three inorganic acids. The concentration of the inorganic acids plays a crucial role in determining the phases of the products (birnessite at low concentrations and cryptomelane at high concentrations). When the concentration of inorganic acid is 0.72 mol/L, the products for the three inorganic acids are cryptomelane. The type of inorganic acid has little impact on the phase of the products, but affects the particle size and size distribution. The mean particle sizes are in order of HCl (1104.4 nm)>HNO3 (441.5 nm)>H2SO4 (339.2nm), and the particles become more uniform. Cryptomelane nanowire formation can be divided into two steps. First, birnessite is formed through the reduction of MnO4-, and then transforms to cryptomelane under acidic conditions via a dissolution-recrystallization process.5. A facile method was developed to synthesize size-tunable cryptomelane nonmaterial using a series of organic acids as acid agents and regulators. The particle sizes, from 8.2 to 61.2 nm in width and 35.6 to 1376.1 nm in length, can be precisely controlled by decreasing alkyl chain lengths of organic acids from 6 to 1. This is the first time for the cryptomelane to be synthesized with controllable sizes in both longitudal and lateral dimensions of a broad range by tuning alkyl chain lengths of carboxylic acids. The differences in solubility, hydrophobic and molecular size of organic acids may be the controlling mechanism on the crystal growth of cryptomelane.6. Degradation of phenol with or without light irradiation by several synthesized manganese oxides, i.e. acidic birnessite (BIR-H), alkaline birnessite (BIR-OH), cryptomelane (CRY) and todorokite (TOD) were comparatively investigated. After 12 h of UV-Vis irradiation, the total organic carbon (TOC) removal rate reached 62.1%, 43.1%,25.4%, and 22.5% for cryptomelane, acidic birnessite, todorokite and alkaline birnessite, respectively. Compared to the reactions in the dark condition, UV-Vis exposure greatly improves the TOC removal rates by 55.8%,31.9%,23.4% and 17.9%. This suggests a weak ability of manganese oxides to degrade phenol in the dark condition, while UV-Vis light irradiation could significantly enhance phenol degradation. The manganese minerals exhibited photocatalytic activities in the order of CRY>BIR-H>TOD>BIR-OH.There may be three possible mechanisms for photochemical degradation:(1) direct photolysis of phenol, (2) direct oxidation of phenol by manganese oxides, and (3) photocatalytic oxidation of phenol by manganese oxides. Photocatalytic oxidation of phenol appeared to be the dominant mechanism.更多还原