Mechanism of oxidation of 3-hydroxy-2,7-naphthalenedisulfonic acid disodium salt with oxygen in subcritical water

•Process of wet oxidation of R-salt with oxygen at pH = 10 was studied.•Stable salts of 4-sulfophthalic acid are formed during that process.•At temperature 569 K only 60% reduction of TOC was obtained.

The article presents the results of studies on the oxidation mechanism of 3-hydroxy-2,7-naphthalenedisulfonic acid disodium salt (R-salt) with oxygen in subcritical water. To this aim, a series of experiments were carried out which showed that at a temperature of 413 K and pH > 9 the oxidation reaction of a substrate with oxygen was relatively quick and after approximately 40 min the R-salt oxidation yield exceeded 95%. In an acidic medium (pH < 7), the rate of R-salt oxidation is small.In order to identify the mechanism of R-salt oxidation, experiments were carried out at 413-569 K in solutions with pH = 10.0 and at partial oxygen pressure pO2 = 1.73 MPa. As a result of these experiments, a stable oxidation product was isolated from the reaction mixture and subjected to spectroscopic analysis. The analysis of HNMR of this product proved that a stable intermediate product of R-salt oxidation was 4-sulfophthalic acid sodium salt.The results of the experiments have shown that destructive oxidation of R-salt can easily be obtained at a temperature of 413 K, but satisfactory reduction of TOC in wastewater containing this substrate requires the use of very high temperature: at 569 K only 60% reduction of TOC was achieved.

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