| 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-. |
PDF Full Text Request