Chlorine dioxide oxidation pathways

Wang P, He Y-L, Huang C-H (2010) Oxidation of fluoroquinolone antibiotics and structurally related amines by chlorine dioxide reaction kinetics, product and pathway evaluation. Water Res 44 5989-5998... 

The kinetics of the oxidation of l-phenyl-2-thiourea by chlorite, in aqueous acidic media, are strongly influenced by the pH and show a complex acid dependence. The proposed mechanism involves HOC1 as a major intermediate whose autocatalytic production determines the observed kinetics of the reaction. The oxidation involved the formation of two stable intermediates, the sulfinic acid and the sulfonic acid, on the pathway towards total desulfurization to form phenylurea. A comprehensive 29-reaction scheme has been proposed to describe the observed complex kinetics.96 The oxidation of trimethylthiourea (TMTU) by chlorite in slightly acidic media is very fast. The oxidation of TMTU proceeds through the formation of sulfinic acid then to the sulfoxylate anion. The direct reaction of chlorine dioxide and TMTU is autocatalytic and is also inhibited by acid. A series of 28 reactions have been proposed to describe the mechanism.97... 

The oxidation pathways of chlorine dioxide under actual conditions are complex because a number of species including chlorine, hypochlorous, chlorous, and chloric acids are formed as intermediates. A rapid conversion of chlorine dioxide to chloride and chlorite (chlorous acid, pK 2.0) may first take place, followed then by a slow phase during which mainly the chlorite reacts with the pulp components. However, continuous generation of chlorine dioxide during bleaching takes place, for example, by the reaction between chlorite and chlorine (or hypochlorous acid) ... 

Under the oxidative pathway, chlorinated aliphatic hydrocarbons act as an electron donor. The presence of an electron acceptor, such as Fe, NOi, S04, or carbon dioxide, is required. According to Dolling et al (2006), only dichloromethane was known to have been dechlorinated via this pathway under anaerobic conditions therefore, this will not be evaluated further. 

PCDD/F and other chlorinated hydrocarbons observed as micropollutants in incineration plants are products of incomplete combustion like other products such as carbon monoxide, polycyclic aromatic hydrocarbons (PAH), and soot. The thermodynamically stable oxidation products of any organic material formed by more than 99% are carbon dioxide, water, and HCl. Traces of PCDD/F are formed in the combustion of any organic material in the presence of small amounts of inorganic and organic chlorine present in the fuel municipal waste contains about 0.8% of chlorine. PCDD/F formation has been called the inherent property of fire. Many investigations have shown that PCDD/Fs are not formed in the hot zones of flames of incinerators at about 1000°C, but in the postcombustion zone in a temperature range between 300 and 400°C. Fly ash particles play an important role in that they act as catalysts for the heterogeneous formation of PCDD/Fs on the surface of this matrix. Two different theories have been deduced from laboratory experiments for the formation pathways of PCCD/F ... 

In the case of semiconductor assisted photocatalysis organic compounds are eventually mineralized to carbon dioxide, water, and in the case of chlorinated compounds, chloride ions. It is not unusual to encounter reports with detection of different intermediates in different laboratories have been observed. For example, in the degradation of 4-CP the most abundant intermediate detected in some reports was hydroquinone (HQ) [114,115,123], while in other studies 4-chloro-catechol, 4-CC (3,4-dihydroxychlorobenzene) was most abundant [14,116-118, 121,163]. The controversy in the reaction intermediate identification stems mainly from the surface and hydroxyl radical mediated oxidation processes. Moreover, experimental parameters such as concentration of the photocatalyst, light intensity, and concentration of oxygen also contribute in guiding the course of reaction pathway. The photocatalytic degradation of 4-CP in Ti02 slurries and thin films... 

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