Aqueous chemical bleaching of 4-nitrophenol brown carbon by hydroxyl radicals; products, mechanism, and light absorption

Abstract

The reaction of hydroxyl radicals (OH) with 4-nitrophenol (4NP) in an aqueous solution was investigated at pH = 2 and 9. The molar yield of the phenolic products quantified was ca. 0.2 at pH = 2 and 0.4 at pH = 9. The yield of 4-nitrocatechol (4NC) was higher at pH = 9. At the same time, a lower number of phenolic products was observed at pH = 9 due to irreversible reactions of some phenols formed at pH > 7. Mineralization investigated with a total organic carbon (TOC) analyzer showed that after 4NP was completely consumed, approximately 85 % of the organic carbon remained in the aqueous solution. Moreover, as inferred from the TOC measurements and the molar yields of the phenols formed, 65 % of the organic carbon that remained in the aqueous solution was attributed to the non-aromatic products. The light absorption of the reaction solution between 250 and 600 nm decreased as a result of the OH reaction with 4NP. However, the 4NP solution showed a noticeable resistance to the chemical bleaching reaction investigated due to the formation of light-absorbing by-products. This phenomenon effectively prolongs the timescales of the chemical bleaching of 4NP by OH by a factor of 3–1.5 at pH 2 and 9, respectively. The experimental data acquired indicated that both photolysis and the reaction with OH can be important processes for the removal of light-absorbing organic compounds from cloud water particles containing 4NP.