Oxidation Of Cr(â…¢)-Organic Acid Complex And Its Adsorption And Mobility In Soils

Chromium(Cr), a transition metal element, is an important material in industrial production and people’s lives because of its special physical and chemical properties. However, chromium pollution has become increasingly serious due to the unreasonable emissions of chromium-containing wastes (waste gas, waste water, waste residue). Cr(Ⅲ) and Cr(Ⅵ) are the two mainly forms that exist in environment. Cr(Ⅲ) has low reductivity, weak toxicity and is easily adsorbed by soil, while Cr(Ⅵ) is highly soluble, toxic and mobile, posing serious environmental hazards. As a result, reduction of Cr(Ⅵ) has become the focus on the management of Cr pollution. Small molecular organic acids, which are ubiquitous in soil, are common reductants for Cr(Ⅵ), but the formation of complex betweenCr(Ⅲ), a reduction product, and small molecular organic acid reduces the adsorption of Cr(Ⅲ) by soils. However, more attention should be paid on the reoxidation of reduced Cr(Ⅲ) to Cr(Ⅵ) which will re-enter into the soil. Manganese oxides are known to be the only one type of nature oxidants resulting in transformation of Cr(Ⅲ) to Cr(Ⅵ) in soils, and it is an important pathway for Cr(Ⅲ) oxidation to Cr(Ⅵ). Another important pathway for Cr(Ⅲ) transformation to Cr(Ⅵ) is photo-oxidation. As the formation of Cr(Ⅲ)-organic acids greatly reduces the adsorption of Cr(Ⅲ) by soils, which virtually enhanced the hazards of Cr(Ⅲ) oxidation to Cr(Ⅵ). Therefore, it is important to study the oxidation of Cr(Ⅲ)-organic acid and its adsorption and mobility in soils. In this study, three common small molecular organic acids were selected to synthesize the Cr(Ⅲ)-organic complexes as models in order to systematically investigate the oxidation, adsorption and mobility of Cr(Ⅲ)-organic complexes in soils. The dissertation includes three parts.In part Ⅰ:Cr(Ⅲ)-citrate (Cr(Ⅲ)-cit) and Cr(Ⅲ)-tartrate (Cr(Ⅲ)-tar) complexes were synthesized and purified, and then their stability in the presence of δ-MnO2were further investigated in batch experiments under different conditions to predict the potential oxidation behaviors of Cr(Ⅲ)-organic acid complexes in environments. Lower pH and higher concentration of δ-MnO2markedly enhanced the production of Cr(Ⅵ).The reaction was primarily affected by pH in a way of the forms of Cr(Ⅲ)-cit and Cr(Ⅲ)-tar affected by pH. Three anions, NO3-、Cl-and SO42-had a little positive promotion role in the oxidation of Cr(Ⅲ)-cit and Cr(Ⅲ)-tar in comparison with control. Ammonium ion significantly improved the oxidation of Cr(Ⅲ)-cit and Cr(Ⅲ)-tar by the formation of [Cr(NH3)6]3+, but phosphate ion demonstrated an opposite effect due to form more stable CrPO4. The oxidation amounts of Cr(Ⅲ)-cit and Cr(Ⅲ)-tar increased with the temperature increasing during the process. The oxidation process of Cr(Ⅲ)-cit and Cr(Ⅲ)-tar over δ-MnO2could be divided into two phases. At the initial phase, a relatively rapid reaction obeyed to first-order model, and then was followed by a very slow one with a characteristic of zero-order. The results indicated that although the rates and extents of Cr(Ⅲ)-cit and Cr(Ⅲ)-tar oxidation by δ-MnO2were much lower than those of aqueous Cr(Ⅲ),Cr(Ⅵ) could be gradually released through the whole reaction. It is observed that in the all cases the extent of Cr(Ⅲ)-cit oxidation was lower than Cr(Ⅲ)-tar. Thus, it is concluded that the stability of Cr(Ⅲ)-cit was higher than Cr(Ⅲ)-tar.In part Ⅱ:Cr(Ⅲ)-malate complex(Cr(Ⅲ)-mal) was synthesized and purified and its species in different pHs were analyzed by High Performance Liquid Chromatography (HPLC). Batch photo-oxidation experiments were conducted to reveal the potential pathway of Cr(Ⅲ) to Cr(VI). The results indicated that in Cr(Ⅲ)-malate complex the chelating molar ratio of Cr(Ⅲ) and malic acid was2:3.[Cr2(Ⅲ)-mal3] and [Cr2(Ⅲ)-mal3-OH]-are the two main species of complex at pH6.0-12.0,[Cr2(Ⅲ)-mal3-OH2]2-and [Cr2(Ⅲ)-mal3-OH3]3-appeared and then coexisted in solution at pH8.0and pH12.0, respectively. The photo-oxidation activity was speculated in this order:[Cr2(Ⅲ)-mal3-OH3]3->[Cr2(Ⅲ)-mal3-OH2]2->[Cr2(Ⅲ)-mal3-OH]->[Cr2(Ⅲ)-mal3]. Higher pH, temperature and stronger light intensity promoted the conversion process. Cr(Ⅱ) and·OH, two intermediates, produced through a ligand-metal charge-transfer(LMCT) path of Cr(Ⅲ)-malate complex were the two pre-condition for Cr(Ⅵ) production. The introduction of H2O2, considered as a direct source of hydroxyl radicals (·OH) with irradiation, markedly enhanced the yield of Cr(Ⅵ). But the malic acid released from the photoexcitation could reduce Cr(Ⅵ), and the oxidation of Cr(Ⅲ)-malate complex took place only at pH>7.0. The photo-oxidation of Cr(Ⅲ)-malate complex obeyed to first-order kinetics at the initial stage of the reaction and then a zero-order one was followed. In part Ⅲ:Cr(Ⅲ)-tar and Cr(Ⅲ)-mal complex were synthesized and purified and batch experiments were conducted to investigate their adsorption and mobility in red soil, yellow-brown soil and chernozem. The results indicated that the adsorption of Cr(Ⅲ)-tar and Cr(Ⅲ)-mal by the three soils increased with the soil concentration(ratio of soil to solution by weight) increasing at pH4.0and25℃, but far less than that of free Cr(Ⅲ)-under the same condition. Almost no effect of pH in a range of4.0to9.0on the adsorption of Cr(Ⅲ)-tar by the three soils was found, but a weak role of pH in adsorption of Cr(Ⅲ)-mal occurred. The adsorption of both Cr(Ⅲ)-tar and Cr(Ⅲ)-mal by the three soils was in the order:red soil>yellow-brown soil>chernozem, and the adsorption amount of Cr(Ⅲ)-mal was larger than that of Cr(Ⅲ)-tar. The adsorption of both Cr(Ⅲ)-tar and Cr(Ⅲ)-mal by the three soils increased when the organic matter was removed from the soils. The adsorption amount of Cr(Ⅲ)-mal, however, was still higher than that of Cr(Ⅲ)-tar. The mobility of Cr(Ⅲ)-tar in the three soils was slightly affected by pH when the initial pHs of leaching solution were4.0,6.5and9.0. In contrast, Cr(Ⅲ)-mal was greatly affected by pH and precipitation occurred in Cr(Ⅲ)-malate solution at pH9.0. The mobility of Cr(Ⅲ)-mal in the three soils is weaker than that of Cr(Ⅲ)-tar. The mobility of Cr(Ⅲ)-tar/mal in yellow-brown soil and chernozem is stronger than that in red soil.