环境中Cr（Ⅲ）的MnO_2氧化以及光化学氧化研究
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摘要
重金属元素铬(Cr)常见于土壤和水环境中,有些源于自然的作用,有些来自人为的排放。铬有多种氧化态,但主要以Cr(Ⅵ)和Cr(Ⅲ)两种形态存在于自然环境中。在土壤和水环境中,Cr(Ⅲ)通常以氢氧化物形态沉淀或以配合离子形态吸附于矿物质表面；在生物体系中,Cr(Ⅲ)是葡萄糖耐量因子的重要组分,可加强胰岛素的作用而成为维持正常的血糖和血脂代谢的必须微量元素。而Cr(Ⅵ)在环境中的溶解性、迁移性大,并且具有高毒性,对人体、动物和植物均有危害性。所以,目前国内外对铬污染控制措施研究主要集中在Cr(Ⅵ)的还原作用上,而对Cr(Ⅲ)在土壤中可能存在的再氧化途径及其反应机理仍缺乏较全面的了解和深入的研究。事实上,土壤中Cr(Ⅲ)转化为Cr(Ⅵ)的现象确实是存在的,人们应该引起足够的重视并去关注它。到目前为止氧化锰矿物是已知唯一可氧化Cr(Ⅲ)的天然氧化剂,是将土壤环境中的Cr(Ⅲ)氧化为Cr(Ⅵ)的重要途径。本研究选择人工合成的水钠锰矿(6-Mn02)对水溶态的Cr(Ⅲ),有机结合态的Cr(Ⅲ)以及沉淀态的Cr(Ⅲ)的氧化做了全面的研究。除了氧化锰矿物之外,揭示了光照下诱发的氧化反应可能是Cr(Ⅲ)转化为Cr(Ⅵ)另一条潜在途径。很多研究证实了Fe(Ⅲ)-OH配合物(尤其是Fe(OH)2+)在光照下通过有机基团向金属的电子转移的方式(LMCT)断裂生成活性中间体-OH和Fe(Ⅱ),·OH可氧化降解多种有机物,同时Fe(Ⅱ)被氧化成Fe(Ⅲ),Fe(Ⅲ)和Fe(Ⅱ)之间的循环在Fe(Ⅲ)-有机配体体系中更为高效。与Fe(Ⅲ)类似,Cr(Ⅲ)可与EDTA、柠檬酸、酒石酸等有机配体配合。柠檬酸和酒石酸常用来还原Cr(Ⅵ),随即与还原生成的Cr(Ⅲ)形成可溶性的Cr(Ⅲ)-柠檬酸和Cr(Ⅲ)-酒石酸配合物,文献表明以前的研究主要是关于前期有机酸对Cr(Ⅵ)的还原,很少涉及后期Cr(Ⅲ)-柠檬酸和Cr(Ⅲ)-酒石酸配合物的再氧化动力学研究。本研究选择Cr(Ⅲ)-柠檬酸和Cr(Ⅲ)-酒石酸配合物作为模型深入研究其中Cr(Ⅲ)的光化学氧化机制。本论文共分为两部分：
第一部分：在15℃,25℃,35℃和pH 2-8条件下,通过批式试验研究了δ-Mn02对水溶态的Cr(Ⅲ),有机结合态的Cr(Ⅲ) (Cr(Ⅲ)-EDTA, Cr(Ⅲ)-柠檬酸和Cr(Ⅲ)-酒石酸)以及沉淀态的Cr(Ⅲ)(Cr(OH)3, CrFe(OH)6和CrP04)的氧化。结果表明δ-Mn02对水溶态Cr(Ⅲ)的氧化遵循先快后慢的规律；温度的升高能使其氧化反应速率增加而并不改变其反应机制；氧化反应速率和氧化程度受不断还原出的Mn(Ⅱ)在δ-Mn02表面吸附覆盖因素的限制；pH值的降低以及初始δ-Mn02浓度的增加能有效地提高Cr(Ⅲ)的反应速率以及氧化程度,当pH值上升到8时,Cr(Ⅲ)的氧化量受溶液中其他离子影响比较大,NH4+的存在能显著提高δ-Mn02对Cr(Ⅲ)的氧化速率和氧化量；P043-的存在会与Cr(Ⅲ)形成更为稳定的CrP04,对Cr(Ⅲ)的氧化有抑制作用。δ-Mn02对Cr(Ⅲ)-EDTA的氧化与Cr(Ⅲ)和EDTA的配位比,配合时间以及配位温度有关。相对于水溶态的Cr(Ⅲ),Cr(Ⅲ)-EDTA配合物难于被Mn02氧化,当pH值达到5时就没有Cr(Ⅵ)的生成；温度的升高使得Cr(Ⅲ)/EDTA配位更完全,不利于δ-Mn02对Cr(Ⅲ)的氧化。δ-MnO2对三种有机结合态的Cr(Ⅲ)的氧化量的大小顺序为Cr(Ⅲ)-酒石酸>Cr(Ⅲ)-柠檬酸>Cr(Ⅲ)-EDTA。在pH 2-4条件下,δ-Mn02对Cr(OH)3和CrFe(OH)6的氧化速率和氧化量都随pH的上升而有所降低。当pH上升到4,CrFe(OH)6体系就检测不到Cr(Ⅵ)的生成。而在CrPO4的体系中始终没有检测到Cr(Ⅵ)的生成。表明δ-Mn02存在下,这三种沉淀态铬的溶解度遵循CrPO4> CrFe(OH)6> Cr(OH)3的顺序。
第二部分：在接近自然环境的条件下制备出Cr(Ⅲ)-柠檬酸和Cr(Ⅲ)-酒石酸配合物,使用732型阳离子交换树脂提纯了这两种配合物,并用高效液相色谱法分离检测出配合物在不同pH条件下的存在形态。结果表明,Cr(Ⅲ)与柠檬酸配位摩尔比为1：1,而Cr(Ⅲ)与酒石酸配位摩尔比为1：2。HPLC图谱显示在pH 3-5范围内,Cr(Ⅲ)-柠檬酸配合物主要以[Cr(Ⅲ)-cit-H]+和[Cr(Ⅲ)-cit]这两种形态存在；在pH 6-8之间仅以[Cr(Ⅲ)-cit]形态存在；在pH 9-11之间的形态为[Cr(Ⅲ)-cit]和[Cr(Ⅲ)-cit-OH]-；当pH达到12的时候,Cr(Ⅲ)-柠檬酸配合物仅以[Cr(Ⅲ)-cit-OH]-形态存在。Cr(Ⅲ)-酒石酸配合物在pH 3时以[Cr(III)-tar2-H], [Cr(III)-tar2]和[Cr(III)-tar2-OH]2三种形态共存于溶液中；在pH 4-10之间的形态为[Cr(Ⅲ)-tar2]-和[Cr(III)-tar2-OH]2-;当pH达11时仅以[Cr(III)-tar2-OH2]3形态出现。25℃和pH 5-12条件下,在不同功率的中压汞灯以及氙灯照射下,研究了无机Cr(Ⅲ)和Cr(Ⅲ)-柠檬酸/酒石酸配合物光照引发的Cr(Ⅲ)光化学氧化的动力学。结果表明在不同光源照射下,Cr(Ⅲ)-柠檬酸/酒石酸配合物光解产生的Cr(Ⅱ)和柠檬酸自由基结合溶解氧生成羟基自由基(·OH),双氧水(H202)等氧化物可实现Cr(Ⅱ)的逐步氧化到Cr(Ⅵ),Cr(Ⅲ)-酒石酸配合物中Cr(Ⅲ)的氧化量略高于Cr(Ⅲ)-柠檬酸中的。光照同样能引发无机Cr(Ⅲ)断裂生成Cr(Ⅱ)和羟基自由基(·OH)实现自身氧化,但其氧化速率和氧化程度均远远低于Cr(Ⅲ)-柠檬酸/酒石酸配合物的。Cr(Ⅲ)-柠檬酸配合物中Cr(Ⅲ)的氧化过程符合零级反应动力学规律；Cr(Ⅲ)-酒石酸配合物中Cr(Ⅲ)的氧化可进行分段拟合,第一阶段遵循一级反应动力学规律,第二阶段遵循零级反应动力学规律。光照强度的增加和pH的升高均有利于不同形态Cr(Ⅲ)的氧化。pH决定了无机Cr(Ⅲ)和Cr(Ⅲ)-柠檬酸/酒石酸配合物的存在形态,从而决定了各自的光化学活性的大小,并最终决定了其中Cr(Ⅲ)的氧化速率及氧化量。pH 7-9时变化不敏感,但在pH>9时,三者的氧化速率和氧化量都随pH的升高而大幅提升。苯作为捕获剂确证了Cr(Ⅲ)和Cr(Ⅲ)-柠檬酸/酒石酸配合物光解均能产生羟基自由基,并且[Cr(III)-OH2-tar2]3这种形态的光化学活性最强,能够释放出更多的羟基自由基(·OH), [Cr(Ⅲ)-cit-OH]形态次之。厌氧条件下不同形态Cr(Ⅲ)的氧化都大大减弱,低pH条件下检测不到Cr(Ⅵ)的生成,高pH条件下存在的[Cr(III)-OH-tar2]2-、[Cr(III)-OH2-tar2]3和[Cr(III)-cit-OH]-光照直接断裂生成的·OH,可实现部分Cr(Ⅲ)的氧化。H202结合紫外光照可快速实现Cr(Ⅲ)-柠檬酸/酒石酸配合物的氧化；TiO2, Fe(Ⅲ),苯酚,甲醇,异丙醇对Cr(Ⅲ)-柠檬酸/酒石酸配合物的氧化有不同程度的抑制；Ti02在预吸附了磷酸根基团之后对Cr(Ⅲ)-酒石酸配合物的氧化显示出极强的催化加速作用。
Chromium is commonly found in soil and water from both natural sources and anthropogenic discharge. Although chromium has multiple oxidation states, Cr(Ⅲ) and Cr(Ⅵ) are the two most stable states in natural environments. In soils and aquatic environments, Cr(Ⅲ) can be readily precipitated as hydroxide solids or adsorbed onto mineral surfaces as complex ions. In biological systems, Cr(Ⅲ) is an important component of glucose tolerance factor and is considered a necessary micro-nutrient in normal carbohydrate and lipid metabolism by potentiating the action of insulin. In contrast, most Cr(Ⅵ) compounds are highly toxic, soluble, and mobile, posing as hazards to plants, animals, and humans. As a result, reduction of Cr(Ⅵ) has been widely investigated. But, there is an increasing concern about Cr(Ⅲ) because of the threat of its reoxidation to Cr(Ⅵ) in the presence of certain oxidants. Manganese oxides are considered to be the only one nature oxidants resulting in transformation of Cr(Ⅲ) to Cr(Ⅵ) in soils. So, oxidation of aqueous Cr(Ⅲ), chelated Cr(Ⅲ) and insoluble Cr(Ⅲ), such as Cr(OH)3 byδ-MnO2 were investigated in this study to predict the potential for Cr(Ⅲ) oxidation in soil environment. Photo-oxidation is another potential pathway to transform Cr(Ⅲ) to Cr(Ⅵ). Many studies have recognized that the photoexcitation of Fe(Ⅲ)-OH complexes (especially Fe(OH)2+) can lead to the formation of·OH and Fe(Ⅱ) through a ligand-to-metal charge-transfer path. Then, OH causes decomposition of organic compounds as well as reoxidation of Fe(Ⅱ) to Fe(Ⅲ), respectively. This cycling of Fe(Ⅲ) to Fe(Ⅱ) is even faster in Fe(Ⅲ)-organic systems. Similar to Fe(Ⅲ), Cr(Ⅲ) can be chelated with many kinds of organic ligands, such as EDTA, citric acid and tartrate acid. Also citric acid and tartaric acid are often used as reductants for Cr(Ⅵ) with subsequent formation of Cr(III)-cit complexes during the reduction process. Past investigations reported in the literature primarily focused on Cr(Ⅵ) reduction, and few attempts have been made to study the kinetics of Cr(Ⅲ)-cit reoxidation. In this study, we selected Cr(Ⅲ)-cit and Cr(Ⅲ)-tar as the model complexes for further investigation of their photoredox behavior. The thesis includes two parts.
In PartⅠ:at 15℃,25℃,35℃and pH 2-8, oxidation of aqueous Cr(Ⅲ), chelated forms of Cr(Ⅲ), such as Cr(Ⅲ)-EDTA, Cr(Ⅲ)-cit and Cr(III)-tar, and insoluble forms of Cr(Ⅲ), such as Cr(OH)3, CrFe(OH)6 and CrPO4, byδ-MnO2 were investigated in batch reaction systems to predict the potential for Cr(Ⅲ) oxidation in soil environment. Results indicate that Cr(Ⅲ) can be rapidly oxidized to Cr(Ⅵ) at the beginning of the reaction. However, Mn(II) is produced and fills the adsorption sites on the manganese oxide surface. As a result, produced Mn(Ⅱ) greatly slows Cr(Ⅲ) oxidation byδ-MnO2. Lower pH, higher temperature and higher concentration ofδ-MnO2 markedly enhance the rate and extent of aqueous Cr(Ⅲ) oxidation. When pH value raised to 8, the oxidation of Cr(Ⅲ) is greatly affected by the other ions that co-existed in the reaction system. NH4+can significantly enhance the oxidation rate and extent of Cr(Ⅲ), H2PO4- restrained the oxidation of Cr(Ⅲ) due to the formation of CrPO4.The oxidation of Cr(Ⅲ)-EDTA byδ-MnO2 is significantly affected by the chelating time between Cr(Ⅲ) and EDTA and the molar ratio of EDTA to Cr(Ⅲ). The formed complex ions of Cr(Ⅲ)-EDTA are hardly oxidized byδ-MnO2 and no Cr(VI) was detected at all when pH was above 5. Raising temperature is benefit to the chelation between Cr(Ⅲ) and EDTA and causes the decrease of Cr(III) oxidation. The oxidation extent order of three chelated forms of Cr(Ⅲ) byδ-MnO2 is Cr(Ⅲ)-tar> Cr(Ⅲ)-cit> Cr(Ⅲ)-EDTA. The rate and extent of oxidation of Cr(OH)3 and CrFe(OH)6 byδ-MnO2 decrease with pH increasing from 2 to 4. No release of Cr(Ⅵ) was observed in the suspension of CrFe(OH)6 andδ-MnO2 at pH 4 and in the suspension of CrPO4 andδ-MnO2 at all pH levels tested. The results demonstrate that the order of stability of Cr(Ⅲ) in these precipitates is CrPO4> CrFe(OH)6> Cr(OH)3 in the presence ofδ-MnO2.
In PartⅡ:Cr(Ⅲ)-citrate (Cr(Ⅲ)-cit) and Cr(Ⅲ)-tartrate (Cr(Ⅲ)-tar) complexes were prepared and purified by 732 cation innovatively. Results indicate that the mole ratio of Cr(Ⅲ)/citrate in Cr(Ⅲ)-cit complex was 1:1 and the mole ratio of Cr(Ⅲ)/tartrate in Cr(Ⅲ)-cit complex was 1:2. Their forms were analyzed by HPLC. The analyses show that Cr(Ⅲ)-cit exists in [Cr(Ⅲ)-H-cit]+and [Cr(Ⅲ)-cit] forms in a pH range of 3 to 5, in [Cr(Ⅲ)-cit] only from pH 6-8, in [Cr(III)-cit] and [Cr(Ⅲ)-OH-cit]- from pH 9-11, and only in [Cr(Ⅲ)-OH-cit]-at pH 12. And Cr(Ⅲ)-tar exists in [Cr(Ⅲ)-H-tar2], [Cr(Ⅲ)-tar2]- and [Cr(Ⅲ)-OH-tar2]2- forms at pH 3, in [Cr(Ⅲ)-tar2]- and [Cr(Ⅲ)-OH-tar2]2-in a pH range of 4 to 10, and the formation of [Cr(Ⅲ)-OH2-tar2] 3- has been established when pH reaches 11. In the batch reaction systems at pHs of 5 to 12 and 25℃, aqueous Cr(Ⅲ), Cr(Ⅲ)-cit and Cr(Ⅲ)-tar with different initial concentrations were fully exposed to light from medium pressure mercury lamps or a xenon lamp to mimic solar light irradiation. It appears that the innersphere electron transfer happens to Cr(Ⅲ)-cit/tar complexes after irradiation, resulting in the generation of Cr(Ⅱ) and cit·/tar-by a ligand-to-metal charge-transfer (LMCT) pathway. The accompanied decomposition of cit·/tar-, together with O2, lead to the formation of hydroxyl radical (·OH), hydrogen peroxide (H2O2) and other reactive oxygen species, and ultimately realizes the oxidation of Cr(Ⅱ) to Cr(Ⅵ) step by step. Both dissolved oxygen and the hydroxyl radical (·OH), an intermediate, serves as oxidants, and Cr(Ⅱ) was a precursor of oxidation of Cr(Ⅲ) to Cr(Ⅵ). The oxidation of Cr(Ⅲ) in Cr(Ⅲ)-cit is a little slower than that in Cr(Ⅲ)-tar but is much faster than that of aqueous Cr(Ⅲ). The oxidation rate of Cr(Ⅲ) increases with the initial concentration. The oxidation of Cr(Ⅲ) in Cr(Ⅲ)-cit obeyed to zero order kinetics; piecewise fitting method was adopted in the oxidation of Cr(Ⅲ) in Cr(Ⅲ)-tar, the initial stage obeyed to first order kinetics, and the later stage obeyed to zero order kinetics. In general, higher pH enhance the rates of Cr(Ⅲ) oxidation. In aqueous Cr(Ⅲ), Cr(Ⅲ)-cit and Cr(Ⅲ)-tar, photo-oxidation rates of Cr(Ⅲ) oxidation are not sensitive to pH in the range of 7 to 9, but increase significantly as pH further ascends, which is highly consistent with the distributions of Cr(Ⅲ) forms. It appears that both [Cr(Ⅲ)-OH2-tar2]3- and [Cr(Ⅲ)-cit-OH]-are photochemically active form. The photoproduction of·OH was determined by HPLC using benzene as a probe to support the reaction mechanism. Exposure of aqueous Cr(Ⅲ), Cr(Ⅲ)-cit and Cr(Ⅲ)-tar to full light of a 500 W mercury lamp in the absence of O2 was also investigated. Results demonstrate that dissolved oxygen plays an important role in the photo-oxidation of Cr(Ⅲ). The oxidation rates of Cr(Ⅲ), Cr(Ⅲ)-cit and Cr(Ⅲ)-tar under anoxic conditions are all slower than those under oxic conditions. Aqueous Cr(Ⅲ) oxidation rate under anoxic conditions is slightly slower than that under oxic conditions. But, the discrepancy is obviously enlarged in Cr(Ⅲ)-cit and Cr(Ⅲ)-tar, where Cr(Ⅵ) is not even detected until the forms [Cr(Ⅲ)-OH-tar2]2-, [Cr(Ⅲ)-OH2-tar2]3- and [Cr(Ⅲ)-cit-OH]-appeared. That is to say [Cr(Ⅲ)-OH-tar2]2-, [Cr(Ⅲ)-OH2-tar2]3- and [Cr(Ⅲ)-cit-OH]-can yield·OH directly and result in Cr(Ⅲ) oxidation even under anoxic conditions. It is affirmed that the generation of hydroxyl radical (·OH) is the key step, which is further affirmed by the result of obvious increase of Cr(Ⅲ) oxidation when H2O2 was introduced into the reaction system. However, some organic compounds, such as methanol,2-propanal, benzene, phenol, naphthaline and semiconductors, such as TiO2, and Fe(Ⅲ) resulted are·OH scavengers or electron donors, resulting in a decrease of Cr(Ⅲ) oxidation and an increase of Cr(Ⅵ) reduction simultaneously. However, a strong promotion of Cr(Ⅲ) oxidation occurred when TiO2 pre-adsorbed PO43-.
第一部分：在15℃,25℃,35℃和pH 2-8条件下,通过批式试验研究了δ-Mn02对水溶态的Cr(Ⅲ),有机结合态的Cr(Ⅲ) (Cr(Ⅲ)-EDTA, Cr(Ⅲ)-柠檬酸和Cr(Ⅲ)-酒石酸)以及沉淀态的Cr(Ⅲ)(Cr(OH)3, CrFe(OH)6和CrP04)的氧化。结果表明δ-Mn02对水溶态Cr(Ⅲ)的氧化遵循先快后慢的规律；温度的升高能使其氧化反应速率增加而并不改变其反应机制；氧化反应速率和氧化程度受不断还原出的Mn(Ⅱ)在δ-Mn02表面吸附覆盖因素的限制；pH值的降低以及初始δ-Mn02浓度的增加能有效地提高Cr(Ⅲ)的反应速率以及氧化程度,当pH值上升到8时,Cr(Ⅲ)的氧化量受溶液中其他离子影响比较大,NH4+的存在能显著提高δ-Mn02对Cr(Ⅲ)的氧化速率和氧化量；P043-的存在会与Cr(Ⅲ)形成更为稳定的CrP04,对Cr(Ⅲ)的氧化有抑制作用。δ-Mn02对Cr(Ⅲ)-EDTA的氧化与Cr(Ⅲ)和EDTA的配位比,配合时间以及配位温度有关。相对于水溶态的Cr(Ⅲ),Cr(Ⅲ)-EDTA配合物难于被Mn02氧化,当pH值达到5时就没有Cr(Ⅵ)的生成；温度的升高使得Cr(Ⅲ)/EDTA配位更完全,不利于δ-Mn02对Cr(Ⅲ)的氧化。δ-MnO2对三种有机结合态的Cr(Ⅲ)的氧化量的大小顺序为Cr(Ⅲ)-酒石酸>Cr(Ⅲ)-柠檬酸>Cr(Ⅲ)-EDTA。在pH 2-4条件下,δ-Mn02对Cr(OH)3和CrFe(OH)6的氧化速率和氧化量都随pH的上升而有所降低。当pH上升到4,CrFe(OH)6体系就检测不到Cr(Ⅵ)的生成。而在CrPO4的体系中始终没有检测到Cr(Ⅵ)的生成。表明δ-Mn02存在下,这三种沉淀态铬的溶解度遵循CrPO4> CrFe(OH)6> Cr(OH)3的顺序。
第二部分：在接近自然环境的条件下制备出Cr(Ⅲ)-柠檬酸和Cr(Ⅲ)-酒石酸配合物,使用732型阳离子交换树脂提纯了这两种配合物,并用高效液相色谱法分离检测出配合物在不同pH条件下的存在形态。结果表明,Cr(Ⅲ)与柠檬酸配位摩尔比为1：1,而Cr(Ⅲ)与酒石酸配位摩尔比为1：2。HPLC图谱显示在pH 3-5范围内,Cr(Ⅲ)-柠檬酸配合物主要以[Cr(Ⅲ)-cit-H]+和[Cr(Ⅲ)-cit]这两种形态存在；在pH 6-8之间仅以[Cr(Ⅲ)-cit]形态存在；在pH 9-11之间的形态为[Cr(Ⅲ)-cit]和[Cr(Ⅲ)-cit-OH]-；当pH达到12的时候,Cr(Ⅲ)-柠檬酸配合物仅以[Cr(Ⅲ)-cit-OH]-形态存在。Cr(Ⅲ)-酒石酸配合物在pH 3时以[Cr(III)-tar2-H], [Cr(III)-tar2]和[Cr(III)-tar2-OH]2三种形态共存于溶液中；在pH 4-10之间的形态为[Cr(Ⅲ)-tar2]-和[Cr(III)-tar2-OH]2-;当pH达11时仅以[Cr(III)-tar2-OH2]3形态出现。25℃和pH 5-12条件下,在不同功率的中压汞灯以及氙灯照射下,研究了无机Cr(Ⅲ)和Cr(Ⅲ)-柠檬酸/酒石酸配合物光照引发的Cr(Ⅲ)光化学氧化的动力学。结果表明在不同光源照射下,Cr(Ⅲ)-柠檬酸/酒石酸配合物光解产生的Cr(Ⅱ)和柠檬酸自由基结合溶解氧生成羟基自由基(·OH),双氧水(H202)等氧化物可实现Cr(Ⅱ)的逐步氧化到Cr(Ⅵ),Cr(Ⅲ)-酒石酸配合物中Cr(Ⅲ)的氧化量略高于Cr(Ⅲ)-柠檬酸中的。光照同样能引发无机Cr(Ⅲ)断裂生成Cr(Ⅱ)和羟基自由基(·OH)实现自身氧化,但其氧化速率和氧化程度均远远低于Cr(Ⅲ)-柠檬酸/酒石酸配合物的。Cr(Ⅲ)-柠檬酸配合物中Cr(Ⅲ)的氧化过程符合零级反应动力学规律；Cr(Ⅲ)-酒石酸配合物中Cr(Ⅲ)的氧化可进行分段拟合,第一阶段遵循一级反应动力学规律,第二阶段遵循零级反应动力学规律。光照强度的增加和pH的升高均有利于不同形态Cr(Ⅲ)的氧化。pH决定了无机Cr(Ⅲ)和Cr(Ⅲ)-柠檬酸/酒石酸配合物的存在形态,从而决定了各自的光化学活性的大小,并最终决定了其中Cr(Ⅲ)的氧化速率及氧化量。pH 7-9时变化不敏感,但在pH>9时,三者的氧化速率和氧化量都随pH的升高而大幅提升。苯作为捕获剂确证了Cr(Ⅲ)和Cr(Ⅲ)-柠檬酸/酒石酸配合物光解均能产生羟基自由基,并且[Cr(III)-OH2-tar2]3这种形态的光化学活性最强,能够释放出更多的羟基自由基(·OH), [Cr(Ⅲ)-cit-OH]形态次之。厌氧条件下不同形态Cr(Ⅲ)的氧化都大大减弱,低pH条件下检测不到Cr(Ⅵ)的生成,高pH条件下存在的[Cr(III)-OH-tar2]2-、[Cr(III)-OH2-tar2]3和[Cr(III)-cit-OH]-光照直接断裂生成的·OH,可实现部分Cr(Ⅲ)的氧化。H202结合紫外光照可快速实现Cr(Ⅲ)-柠檬酸/酒石酸配合物的氧化；TiO2, Fe(Ⅲ),苯酚,甲醇,异丙醇对Cr(Ⅲ)-柠檬酸/酒石酸配合物的氧化有不同程度的抑制；Ti02在预吸附了磷酸根基团之后对Cr(Ⅲ)-酒石酸配合物的氧化显示出极强的催化加速作用。
Chromium is commonly found in soil and water from both natural sources and anthropogenic discharge. Although chromium has multiple oxidation states, Cr(Ⅲ) and Cr(Ⅵ) are the two most stable states in natural environments. In soils and aquatic environments, Cr(Ⅲ) can be readily precipitated as hydroxide solids or adsorbed onto mineral surfaces as complex ions. In biological systems, Cr(Ⅲ) is an important component of glucose tolerance factor and is considered a necessary micro-nutrient in normal carbohydrate and lipid metabolism by potentiating the action of insulin. In contrast, most Cr(Ⅵ) compounds are highly toxic, soluble, and mobile, posing as hazards to plants, animals, and humans. As a result, reduction of Cr(Ⅵ) has been widely investigated. But, there is an increasing concern about Cr(Ⅲ) because of the threat of its reoxidation to Cr(Ⅵ) in the presence of certain oxidants. Manganese oxides are considered to be the only one nature oxidants resulting in transformation of Cr(Ⅲ) to Cr(Ⅵ) in soils. So, oxidation of aqueous Cr(Ⅲ), chelated Cr(Ⅲ) and insoluble Cr(Ⅲ), such as Cr(OH)3 byδ-MnO2 were investigated in this study to predict the potential for Cr(Ⅲ) oxidation in soil environment. Photo-oxidation is another potential pathway to transform Cr(Ⅲ) to Cr(Ⅵ). Many studies have recognized that the photoexcitation of Fe(Ⅲ)-OH complexes (especially Fe(OH)2+) can lead to the formation of·OH and Fe(Ⅱ) through a ligand-to-metal charge-transfer path. Then, OH causes decomposition of organic compounds as well as reoxidation of Fe(Ⅱ) to Fe(Ⅲ), respectively. This cycling of Fe(Ⅲ) to Fe(Ⅱ) is even faster in Fe(Ⅲ)-organic systems. Similar to Fe(Ⅲ), Cr(Ⅲ) can be chelated with many kinds of organic ligands, such as EDTA, citric acid and tartrate acid. Also citric acid and tartaric acid are often used as reductants for Cr(Ⅵ) with subsequent formation of Cr(III)-cit complexes during the reduction process. Past investigations reported in the literature primarily focused on Cr(Ⅵ) reduction, and few attempts have been made to study the kinetics of Cr(Ⅲ)-cit reoxidation. In this study, we selected Cr(Ⅲ)-cit and Cr(Ⅲ)-tar as the model complexes for further investigation of their photoredox behavior. The thesis includes two parts.
In PartⅠ:at 15℃,25℃,35℃and pH 2-8, oxidation of aqueous Cr(Ⅲ), chelated forms of Cr(Ⅲ), such as Cr(Ⅲ)-EDTA, Cr(Ⅲ)-cit and Cr(III)-tar, and insoluble forms of Cr(Ⅲ), such as Cr(OH)3, CrFe(OH)6 and CrPO4, byδ-MnO2 were investigated in batch reaction systems to predict the potential for Cr(Ⅲ) oxidation in soil environment. Results indicate that Cr(Ⅲ) can be rapidly oxidized to Cr(Ⅵ) at the beginning of the reaction. However, Mn(II) is produced and fills the adsorption sites on the manganese oxide surface. As a result, produced Mn(Ⅱ) greatly slows Cr(Ⅲ) oxidation byδ-MnO2. Lower pH, higher temperature and higher concentration ofδ-MnO2 markedly enhance the rate and extent of aqueous Cr(Ⅲ) oxidation. When pH value raised to 8, the oxidation of Cr(Ⅲ) is greatly affected by the other ions that co-existed in the reaction system. NH4+can significantly enhance the oxidation rate and extent of Cr(Ⅲ), H2PO4- restrained the oxidation of Cr(Ⅲ) due to the formation of CrPO4.The oxidation of Cr(Ⅲ)-EDTA byδ-MnO2 is significantly affected by the chelating time between Cr(Ⅲ) and EDTA and the molar ratio of EDTA to Cr(Ⅲ). The formed complex ions of Cr(Ⅲ)-EDTA are hardly oxidized byδ-MnO2 and no Cr(VI) was detected at all when pH was above 5. Raising temperature is benefit to the chelation between Cr(Ⅲ) and EDTA and causes the decrease of Cr(III) oxidation. The oxidation extent order of three chelated forms of Cr(Ⅲ) byδ-MnO2 is Cr(Ⅲ)-tar> Cr(Ⅲ)-cit> Cr(Ⅲ)-EDTA. The rate and extent of oxidation of Cr(OH)3 and CrFe(OH)6 byδ-MnO2 decrease with pH increasing from 2 to 4. No release of Cr(Ⅵ) was observed in the suspension of CrFe(OH)6 andδ-MnO2 at pH 4 and in the suspension of CrPO4 andδ-MnO2 at all pH levels tested. The results demonstrate that the order of stability of Cr(Ⅲ) in these precipitates is CrPO4> CrFe(OH)6> Cr(OH)3 in the presence ofδ-MnO2.
In PartⅡ:Cr(Ⅲ)-citrate (Cr(Ⅲ)-cit) and Cr(Ⅲ)-tartrate (Cr(Ⅲ)-tar) complexes were prepared and purified by 732 cation innovatively. Results indicate that the mole ratio of Cr(Ⅲ)/citrate in Cr(Ⅲ)-cit complex was 1:1 and the mole ratio of Cr(Ⅲ)/tartrate in Cr(Ⅲ)-cit complex was 1:2. Their forms were analyzed by HPLC. The analyses show that Cr(Ⅲ)-cit exists in [Cr(Ⅲ)-H-cit]+and [Cr(Ⅲ)-cit] forms in a pH range of 3 to 5, in [Cr(Ⅲ)-cit] only from pH 6-8, in [Cr(III)-cit] and [Cr(Ⅲ)-OH-cit]- from pH 9-11, and only in [Cr(Ⅲ)-OH-cit]-at pH 12. And Cr(Ⅲ)-tar exists in [Cr(Ⅲ)-H-tar2], [Cr(Ⅲ)-tar2]- and [Cr(Ⅲ)-OH-tar2]2- forms at pH 3, in [Cr(Ⅲ)-tar2]- and [Cr(Ⅲ)-OH-tar2]2-in a pH range of 4 to 10, and the formation of [Cr(Ⅲ)-OH2-tar2] 3- has been established when pH reaches 11. In the batch reaction systems at pHs of 5 to 12 and 25℃, aqueous Cr(Ⅲ), Cr(Ⅲ)-cit and Cr(Ⅲ)-tar with different initial concentrations were fully exposed to light from medium pressure mercury lamps or a xenon lamp to mimic solar light irradiation. It appears that the innersphere electron transfer happens to Cr(Ⅲ)-cit/tar complexes after irradiation, resulting in the generation of Cr(Ⅱ) and cit·/tar-by a ligand-to-metal charge-transfer (LMCT) pathway. The accompanied decomposition of cit·/tar-, together with O2, lead to the formation of hydroxyl radical (·OH), hydrogen peroxide (H2O2) and other reactive oxygen species, and ultimately realizes the oxidation of Cr(Ⅱ) to Cr(Ⅵ) step by step. Both dissolved oxygen and the hydroxyl radical (·OH), an intermediate, serves as oxidants, and Cr(Ⅱ) was a precursor of oxidation of Cr(Ⅲ) to Cr(Ⅵ). The oxidation of Cr(Ⅲ) in Cr(Ⅲ)-cit is a little slower than that in Cr(Ⅲ)-tar but is much faster than that of aqueous Cr(Ⅲ). The oxidation rate of Cr(Ⅲ) increases with the initial concentration. The oxidation of Cr(Ⅲ) in Cr(Ⅲ)-cit obeyed to zero order kinetics; piecewise fitting method was adopted in the oxidation of Cr(Ⅲ) in Cr(Ⅲ)-tar, the initial stage obeyed to first order kinetics, and the later stage obeyed to zero order kinetics. In general, higher pH enhance the rates of Cr(Ⅲ) oxidation. In aqueous Cr(Ⅲ), Cr(Ⅲ)-cit and Cr(Ⅲ)-tar, photo-oxidation rates of Cr(Ⅲ) oxidation are not sensitive to pH in the range of 7 to 9, but increase significantly as pH further ascends, which is highly consistent with the distributions of Cr(Ⅲ) forms. It appears that both [Cr(Ⅲ)-OH2-tar2]3- and [Cr(Ⅲ)-cit-OH]-are photochemically active form. The photoproduction of·OH was determined by HPLC using benzene as a probe to support the reaction mechanism. Exposure of aqueous Cr(Ⅲ), Cr(Ⅲ)-cit and Cr(Ⅲ)-tar to full light of a 500 W mercury lamp in the absence of O2 was also investigated. Results demonstrate that dissolved oxygen plays an important role in the photo-oxidation of Cr(Ⅲ). The oxidation rates of Cr(Ⅲ), Cr(Ⅲ)-cit and Cr(Ⅲ)-tar under anoxic conditions are all slower than those under oxic conditions. Aqueous Cr(Ⅲ) oxidation rate under anoxic conditions is slightly slower than that under oxic conditions. But, the discrepancy is obviously enlarged in Cr(Ⅲ)-cit and Cr(Ⅲ)-tar, where Cr(Ⅵ) is not even detected until the forms [Cr(Ⅲ)-OH-tar2]2-, [Cr(Ⅲ)-OH2-tar2]3- and [Cr(Ⅲ)-cit-OH]-appeared. That is to say [Cr(Ⅲ)-OH-tar2]2-, [Cr(Ⅲ)-OH2-tar2]3- and [Cr(Ⅲ)-cit-OH]-can yield·OH directly and result in Cr(Ⅲ) oxidation even under anoxic conditions. It is affirmed that the generation of hydroxyl radical (·OH) is the key step, which is further affirmed by the result of obvious increase of Cr(Ⅲ) oxidation when H2O2 was introduced into the reaction system. However, some organic compounds, such as methanol,2-propanal, benzene, phenol, naphthaline and semiconductors, such as TiO2, and Fe(Ⅲ) resulted are·OH scavengers or electron donors, resulting in a decrease of Cr(Ⅲ) oxidation and an increase of Cr(Ⅵ) reduction simultaneously. However, a strong promotion of Cr(Ⅲ) oxidation occurred when TiO2 pre-adsorbed PO43-.
引文
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