Mechanism of the photosensitizing action of a mixture humic acid–riboflavin in the degradation of water-contaminants

Kinetic and mechanistic aspects on the photooxidation of 3-hydroxypyridine (3-OHP) and phenol (PhOH), taken as model compounds for hydroxyaromatic water-contaminants, was studied. A mixture of the naturally occurring visible-light absorbers humic acid (HA) and vitamin B2 (riboflavin, Rf) was employed as a photosensitizing agent. The work was done in pH 7 aqueous solution, in the presence of 0.04 mM Rf and 50 μg/ml HA and employing photoirradiation wavelengths higher than 360 nm, a range where 3-OHP and PhOH are transparent.The mechanism involved in the photooxidation was mainly evaluated through oxygen consumption measurements. The mixture HA + Rf generates O2(1Δg) and O2−, the latter especially in the presence of electron donors such as the hydorxyaromatic compounds. The rates of oxygen consumption were taken as a measure of the overall oxidation rate of the contaminant model compounds. The photooxidation of 3-OHP at pH 7, sensitized by low Rf and HA concentrations, and standarized by comparison with the photooxidation rate of the known oxidizable target furfuryl alcohol, is an efficient process. PhOH is only degradable in the alkaline range of pH.A moderate decrease in the rate of oxygen consumption by 3-OHP upon Rf + HA sensitization was observed as compared to the simple addition of the oxygen uptake rates for 3-OHP upon individual Rf and HA sensitization. It is attributed to a catalytic decomposition of O2− by HA, which competes with 3-OHP by the oxidative species, inhibiting this oxygen-consumer channel, especially active in the presence of the pyridine derivative.Noticeable photofading of the sensitizers was detected when photolyzed in the absence of the contaminants, within typical irradiation times employed for the hydroxyaromatics degradation.