Activation of dissolved molecular oxygen by ascorbic acid-mediated circulation of copper(II): Applications and limitations

Kinetic Modeling Assisted Analysis of Vitamin C-Mediated Copper Redox Transformations in Aqueous Solutions

The kinetics of oxidation of micromolar concentrations of ascorbic acid (AA) catalyzed by Cu(II) in solutions representative of biological and environmental aqueous systems has been investigated in both the presence and absence of oxygen. The results reveal that the reaction between AA and Cu(II) is a relatively complex set of redox processes whereby Cu(II) initially oxidizes AA yielding the intermediate ascorbate radical (A•-) and Cu(I). The rate constant for this reaction was determined to have a lower limit of 2.2 × 104 M-1 s-1. Oxygen was found to play a critical role in mediating the Cu(II)/Cu(I) redox cycle and the oxidation reactions of AA and its oxidized forms. Among these processes, the oxidation of the ascorbate radical by molecular oxygen was identified to play a key role in the consumption of ascorbic acid, despite being a slow reaction. The rate constant for this reaction (A • - + O 2 → DHA + O 2 • - ) was determined for the first time with a calculated value of 54 ± 8 M-1 s-1. The kinetic model developed satisfactorily describes the Cu/AA/O2 system over a range of conditions including different concentrations of NaCl (0.2 and 0.7 M) and pH (7.4 and 8.1). Appropriate adjustments to the rate constant for the reaction between Cu(I) and O2 were found to account for the influence of the chloride ions and pH on the kinetics of the process. Additionally, the presence of Cu(III) as the primary oxidant resulting from the interaction between Cu(I) and H2O2 in the Cu(II)/AA system was confirmed, along with the coexistence of HO•, possibly due to an equilibrium established between Cu(III) and HO•.

Efficient destruction of basic organo-nitrogenous compounds in liquid hydrocarbon fuel using ascorbic acid/H2O2 system under ambient condition

Due to the limitations of the conventional refinery methods, development of a new method such as oxidative denitrogenation (ODN) is highly desirable. This study described a novel ODN to remove organo-nitrogenous compounds (ONCs) in liquid fuel by ascorbic acid (AscH2) and H2O2 redox system under ambient conditions. Seven ONCs including pyridine, quinoline, acridine, 7,8-benzoquinoline, indole, N-methylpyrrolidone (NMP), and N,N-dimethylformamide (DMF) were chosen to assess the fuel-denitrified ability of the AscH2/H2O2 system. The results showed that the basic group of ONCs (pyridine, quinoline, and acridine) can be effectively removed (removal ratio > 95 %) while the removal efficiency of water-soluble compounds (7,8-benzoquinoline, NMP, and DMF) was moderate (61-68 %) under a mild temperature (30 °C) and atmospheric pressure. Free radical quenching and electron paramagnetic resonance experiments confirmed that hydroxyl and AscH2 radicals played a major role in the degradation of ONCs. The degraded products of quinoline were analyzed by gas chromatography-mass spectroscopy and ion chromatography. Based on the identified intermediate products, a putative reaction pathway majorly involving three steps of N-onium formation, transfer hydrogenation, and free radical oxidative ring-opening was suggested for the quinoline degradation. The presented approach can be performed at a normal temperature and pressure and will live up to expectations in the pre-denitrogenation and selective removal of basic ONCs in fuel oils.

Catalytic oxidation of 4-acetamidophenol with Fe3+-enhanced Cu0 particles: In-site generation and activation of hydrogen peroxide

Cu⁰ coupled with O₂ was used to degrade contaminant due to in-site generation and catalysis of H₂O₂, while the low reactivity and active dismutation reaction of Cu⁺ refrained the performance at acidic condition. In this study, the removal rate of 4-acetamidophenol increased from 27 % to 83.4 % with Fe³⁺ spiked into the Cu⁰ system within 60 min •OH was the primary reactive species in the Fe³⁺/Cu⁰ system. In the Fe³⁺/Cu⁰ system, Cu⁰ was corroded to form Cu⁺ by H⁺ and O₂, and then Cu⁺ interacted with O₂ generating H₂O₂, and meanwhile Fe³⁺ was reduced to Fe²⁺ by Cu⁺ and Cu⁰; Consequently, Cu⁺ and Fe²⁺ induced H₂O₂ to produce •OH, but Fe²⁺ was easier to catalyze H₂O₂ than Cu⁺ at acidic pH. Except for fulvic acid, common water matrix including sulfate ion, phosphate ion, chloride ion and nitrate ion had no inhibition effect on the degradation of 4-acetamidophenol in the Fe³⁺/Cu⁰ system. over 62 % of 4-acetamidophenol in tap water, Hou-lake water and well water was greatly oxidized by the Fe³⁺/Cu⁰ system. Furthermore, the amount of total dissolved copper decreased to 0.895 mg/L by the method of alkali precipitation in the Fe³⁺/Cu⁰ system. The study provided a theoretical direction to the Fe³⁺-enhanced Cu⁰ system for purifying wastewater.