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

Abstract

Abstract. The reaction of hydroxyl radicals (OH) with 4-nitrophenol (4-NP) in the aqueous solution was investigated at pH = 2 and 9. As a result, the molar yield of the phenolic products was measured to be 0.20 ± 0.05 at pH = 2 and 0.40 ± 0.1 at pH = 9. The yield of 4-nitrocatechol (4-NC) was higher at pH = 9; at the same time, a lower number of phenolic products was observed due to the hydrolysis and other irreversible reactions at pH > 7. Mineralization investigated with total organic carbon (TOC) technique showed that after 4-NP was completely consumed approx. 85 % of the organic carbon remained in the aqueous solution. Hence, up to 65 % of the organic carbon that remained in the aqueous solution accounted for the open-ring non-phenolic products. The light absorptivity of the reaction solution between 250 and 600 nm decreased as a result of OH reaction with 4-NP. At the same time, 4-NP solution showed some resistance to chemical bleaching due to the formation of the light-absorbing by-products. This phenomenon effectively prolongs the time-scale of chemical bleaching or 4-NP via reaction with OH by a factor of 3–1.5 at pH 2 and 9, respectively. The experimental data acquired indicated that both photolysis and reaction with OH can be important removal processes of the atmospheric brown-carbon from the aqueous particles containing 4-NP.

Aging of α-Pinene Secondary Organic Aerosol by Hydroxyl Radicals in the Aqueous Phase: Kinetics and Products

“…The steady-state concentration of OH in the aqueous phase ([OH] aq ) in the absence of organic reactants was approximately 6 × 10 −13 M, as estimated by fitting the measured rate of H 2 O 2 photolysis into a kinetic model. 61,62 This value is within the range of [OH] aq in atmospheric aqueous particles. 63,64 2.3.…”

Continuous Measurement and Molecular Compositions of Atmospheric Water‐Soluble Brown Carbon in the Nearshore Marine Boundary Layer of Northern China: Secondary Formation and Influencing Factors

“…Phillips et al (2017) found that absorption increased 10% per pH unit in the range of 2-12, and the resulting change in the spectral shape caused the absorption Ångström exponent (AAE) to decrease by 0.18 per unit increase of pH. Besides, the time-scale of chemical bleaching of BrC chromophores was prolonged by a factor of 1.5-3 at pH = 2 or pH = 9 (Witkowski et al, 2021), and the formation of secondary BrC was even suppressed under pH < 2 (Kampf et al, 2012;Sedehi et al, 2013).…”